Expt. 1 - Level Control

Essentials of Process Control Level Control Process

Instruction Manual

PCT50

ISSUE 2

June 2012

Table of Contents Copyright and Trademarks ...................................................................................... 1

General Overview ....................................................................................................... 2

Equipment Diagrams................................................................................................... 4

Important Safety Information....................................................................................... 7

Introduction.............................................................................................................. 7

Electrical Safety....................................................................................................... 7

Water Borne Hazards .............................................................................................. 7

Description .................................................................................................................. 9

Overview.................................................................................................................. 9

PCT50 Level Control Process ................................................................................. 9

Software ................................................................................................................ 11

Installation................................................................................................................. 13

Advisory................................................................................................................. 13

Electrical Supply .................................................................................................... 13

Connecting the PCT50 to a PC ............................................................................. 13

Filling the Sump Tank............................................................................................ 13

Installing the Software ........................................................................................... 13

Installing the Equipment ........................................................................................ 18

Operation .................................................................................................................. 22

Operating the Software.......................................................................................... 22

Operating the Equipment....................................................................................... 34

Equipment Specifications.......................................................................................... 38

Overall Dimensions ............................................................................................... 38

IFD Connections.................................................................................................... 38

Other Specifications .............................................................................................. 40

Environmental Conditions...................................................................................... 41

Routine Maintenance ................................................................................................ 42

Responsibility ........................................................................................................ 42

General.................................................................................................................. 42

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Table of Contents

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Cleaning the Equipment ........................................................................................ 42

Accessing the electrical circuits inside the electrical enclosure............................. 42

Laboratory Teaching Exercises................................................................................. 43

Index to Exercises ................................................................................................. 43

Exercise A - Manual control of water level (Open loop) ............................................ 44

Exercise B - On/off control of water level (Closed loop)............................................ 47

Exercise C - Proportional control of water level (Closed loop, P only and P + I) ...... 51

Exercise D - Optimising proportional control of water level (Closed loop, P + I + D) 57

Exercise E - Manual control of water level (Open loop) ............................................ 61

Exercise F - On/off control of water level (Closed loop)............................................ 64

Exercise G - Proportional control of water level (Closed loop, P only and P + I) ...... 68

Exercise H - Using a PID controller (PCT54) ............................................................ 75

Exercise I - Using a PLC controller (PCT55)............................................................. 76

Contact Details for Further Information ..................................................................... 77

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Disclaimer This document and all the information contained within it is proprietary to Armfield Limited. This document must not be used for any purpose other than that for which it is supplied and its contents must not be reproduced, modified, adapted, published, translated or disclosed to any third party, in whole or in part, without the prior written permission of Armfield Limited.

Should you have any queries or comments, please contact the Armfield Customer Support helpdesk (Monday to Thursday: 0830 – 1730 and Friday: 0830 - 1300 UK time). Contact details are as follows:

United Kingdom International

(0) 1425 478781 (calls charged at local rate)

+44 (0) 1425 478781 (international rates apply)

Email: [email protected]

Fax: +44 (0) 1425 470916

Copyright and Trademarks Copyright © 2012 Armfield Limited. All rights reserved.

Any technical documentation made available by Armfield Limited is the copyright work of Armfield Limited and wholly owned by Armfield Limited.

Brands and product names mentioned in this manual may be trademarks or registered trademarks of their respective companies and are hereby acknowledged.

General Overview Four independent but complementary process units provide a compact, hands-on introduction to the fundamental principles of process control engineering. Each fluid-based process provides a clear demonstration of a different single PID loop using Level, Flow, Temperature or Pressure as the measured variable.

Each process is supplied complete with software that allows it to be controlled using a suitable PC via a USB connection. The effect of making changes to the system or to the controller configuration can be quickly investigated by applying repeatable disturbances to the process, clearly demonstrating the need for correct matching of system characteristics and controller settings. The software allows all appropriate measurements and controller settings to be displayed continuously and recorded for later analysis, if required by the user. Alternatively the process can be operated using an external controller such as the optional Industrial PID controller (PCT54) or Programmable Logic Controller (PCT55). If required, a PC can be used to record the responses of the process when using an external controller.

A Sensor Conditioning and Calibration Trainer (PCT56) completes this range of process control products.

The range of products comprises the following small-scale processes, optional controllers and sensor calibration trainer:

PCT50 Level Control Process

PCT51 Flow Control Process

PCT52 Temperature Control Process

PCT53 Pressure Control Process

PCT54 Industrial PID Controller

PCT55 Programmable Logic Controller

PCT56 Sensor Conditioning and Calibration Trainer

This instruction manual describes the operation of the PCT51 Level Control Process when used with a PC to control and record the process.

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General Overview

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PCT50 Level Control Process

Equipment Diagrams

Figure 1: Front View of the PCT50 Level Control Process

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Equipment Diagrams

Figure 2: Schematic diagram of the PCT50 Level Control Process (Inflow Control)

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Armfield Instruction Manual

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Figure 3: Schematic diagram of the PCT50 Level Control Process (Outflow Control)

Important Safety Information

Introduction All practical work areas and laboratories should be covered by local safety regulations which must be followed at all times.

It is the responsibility of the owner to ensure that all users are made aware of relevant local regulations, and that the apparatus is operated in accordance with those regulations. If requested then Armfield can supply a typical set of standard laboratory safety rules, but these are guidelines only and should be modified as required. Supervision of users should be provided whenever appropriate.

Your PCT50 Level Control Process has been designed to be safe in use when installed, operated and maintained in accordance with the instructions in this manual. As with any piece of sophisticated equipment, dangers exist if the equipment is misused, mishandled or badly maintained.

Electrical Safety The equipment described in this Instruction Manual operates from a mains voltage electrical supply via a 24V DC adapter. It must be connected to a supply of the same frequency and voltage as marked on the equipment or the mains lead. If in doubt, consult a qualified electrician or contact Armfield.

The equipment must not be operated with any of the panels removed.

Water Borne Hazards The equipment described in this instruction manual involves the use of water, which under certain conditions can create a health hazard due to infection by harmful micro-organisms.

For example, the microscopic bacterium called Legionella pneumophila will feed on any scale, rust, algae or sludge in water and will breed rapidly if the temperature of water is between 20 and 45°C. Any water containing this bacterium which is sprayed or splashed creating air-borne droplets can produce a form of pneumonia called Legionnaires Disease which is potentially fatal.

Legionella is not the only harmful micro-organism which can infect water, but it serves as a useful example of the need for cleanliness.

Under the COSHH regulations, the following precautions must be observed:

Any water contained within the product must not be allowed to stagnate, ie. the water must be changed regularly.

Any rust, sludge, scale or algae on which micro-organisms can feed must be removed regularly, i.e. the equipment must be cleaned regularly.

Where practicable the water should be maintained at a temperature below 20°C. If this is not practicable then the water should be disinfected if it is safe and appropriate to do so. Note that other hazards may exist in the handling of biocides used to disinfect the water.

A scheme should be prepared for preventing or controlling the risk incorporating all of the actions listed above.

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Further details on preventing infection are contained in the publication “The Control of Legionellosis including Legionnaires Disease” - Health and Safety Series booklet HS (G) 70.

Description Where necessary, refer to the drawings in the Equipment Diagrams section.

Overview The PCT50 Level Control Process consists of a combined sump tank / level vessel assembly together with an electrical connection box, mounted on a common base plate. It belongs to a family of four processes having common features, as follows:

All four processes are free standing, without the need for a permanent water supply or drain connection, and suitable for locating on a work bench alongside a PC.

Each process incorporates an electronic interface with sensor conditioning, drive circuits and integrated USB connection. This means that the user only needs to fill the process with water (where appropriate), connect the mains adaptor and connect the supplied USB cable to a suitable PC for the equipment to be operational.

The use of transparent materials gives clear visibility of the process in operation and the location of control components such as sensors and control elements.

The facility to vary the operating characteristics to allow optimization of the PID settings to suit the characteristics and to demonstrate the differing responses of both optimum and non-optimum controller configuration.

Supplied with software and USB lead allowing control and data acquisition using a PC.

Disturbances (step changes) can be applied to the process, remotely from the PC, to test the stability of the system with the controller settings under investigation. The magnitude of the disturbance can be changed in fixed steps for repeatability or varied continuously to achieve a specific response in the system.

Manual operation via the PC will allow open loop testing of each process to determine the system response prior to assigning appropriate values for the P, I and D terms, cycle time etc in a closed loop configuration.

In addition to control of the process and initiation of step changes, the software supplied will also allow recording of the responses, graphing etc. when testing any of the control loops.

The electrical interface, mounted alongside the process, incorporates additional front panel connectors that allow the input and output signals to be connected to an alternative PID controller, Programmable Logic Controller or similar control equipment if required by the end user.

The electrical supply is provided by a universal 24V DC in-line adaptor with interchangeable leads to suit the local electrical supply.

PCT50 Level Control Process Refer to the equipment diagram for details about the PCT50 Level Process. The front view shows the layout of the PCT50 and the circuit diagrams show the process in operation with inflow to the process vessel (controlled by varying pump speed) or

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outflow from the process vessel (controlled by opening and closing a solenoid valve on a time cycle). These alternatives are described in detail later.

PCT50 is a level control process using water as the working fluid for safety and convenience in use.

Water stored in the lower sump tank is transferred to the upper process vessel via a submerged, variable speed centrifugal pump (10). A quick release connector (9) allows the flexible pump outlet tube to be disconnected to aid priming of the pump after filling the sump tank with water. A vertical inlet arrangement in the process vessel allows visualisation of the water entering the vessel, independent of water level and an integral non-return valve (8) prevents water from draining back into the sump vessel when the pump speed is reduced or stopped. An inline ball valve (CV1) above the quick release connector allows the flow of water entering the process vessel to be varied to suit a particular demonstration.

The water level inside the process vessel is measured using an electronic pressure sensor (2) mounted through the floor of the vessel. One side of the pressure sensor is connected to the inside of the process vessel and the other is open to atmosphere, allowing the pressure in the process vessel to be measured relative to atmosphere. This sensor therefore measures the level of water inside the process vessel. The level is indicated (1) on the scale on the side of process vessel.

Water drains from the process vessel back into the lower sump tank via two outlets in the floor of the process vessel. Flow through the main outlet is continuous. Flow through the second outlet can be started and stopped by a solenoid valve (SOL) under remote control. Both outlets incorporate a hand operated ball valves (CV2 and CV3) that allows the flow of water to be varied continuously to suit a particular demonstration. Both outlets also incorporate an interchangeable orifice (3 and 5) that allows the flow to be fixed at predetermined values, varying only with the size of the orifice and the head in the process vessel. The orifice size is changed by unscrewing the plastic fitting containing the orifice and replacing it with the required alternative. The fitting incorporates an ‘O’ ring seal and only requires hand tightening. The alternative sizes of orifice (12) are stored in threaded holes at the front of the base plate when not in use.

An overflow (6) in the process vessel returns water to the sump tank, preventing the process vessel from flooding in use.

The pressure sensor measuring the level, the centrifugal pump and the solenoid valve are connected to an electrical interface (4) that incorporates the necessary signal conditioning, allowing the process to be operated directly from a PC using a single USB port. Alternatively the process can be operated using a controller such as PCT54 (PID Controller) or PCT55 (Programmable Logic Controller) connected to the electrical interface.

The computer software supplied with PCT50 allows control of the level process and data logging of the responses using a PC. Alternatively the software allows data logging only while operating the process remotely using a PID controller or PLC.

Two process control loops are available for demonstration, namely:

Level control by proportionally varying the pump speed (Inflow control)

Level control by time-proportioned opening of a solenoid valve (Outflow control)

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Description

When filled with water, the PCT50 is self contained requiring only a mains electrical supply to the in-line DC converter and connection to either a PC via the USB port or to a process controller such as PCT54 (PID Controller) or PCT55 (PLC with PID controller).

The unit is drained using the ball valve (11) located at the rear.

Software The PCT50 is supplied with an educational software package with a wide range of facilities and functions. The computer is the primary interface between the user and the equipment. The software displays a real time process mimic diagram with readings of the relevant sensor outputs and controls for the system inputs. Manual, On/off, time proportioned and PID control loops can be configured using two predetermined student exercises:

Ex 1 Inflow control (PC)

Exercises demonstrating Inflow Control by varying pump speed:

Exercise A - Manual control of water level (Open loop)

Exercise B - On/off control of water level (Closed loop)

Exercise C - Proportional control of water level (Closed loop, P only and P + I)

Exercise D - Optimising proportional control of water level (Closed loop, P + I + D)

Ex 2 Outflow control (PC)

Exercises Demonstrating Outflow Control by time proportioned cycling of a solenoid drain valve:

Exercise E - Manual control of water level (Open loop)

Exercise F - On/off control of water level (Closed loop)

Exercise G - Proportional control of water level (Closed loop, P only and P + I)

Two alternative exercises are included that allow the process to be monitored and recorded on the PC while using an external controller such as PCT54 or PCT55:

Ex 3 Inflow control (External Control)

Exercise H - Using a PID controller (PCT54)

Exercise I - Using a PLC controller (PCT55)

Ex 4 Outflow control (External Control)



When controlling using a PC all control and sensor signals including the setpoint, and controller configuration can be logged continuously using the PC. Disturbances of fixed magnitude can be introduced remotely using the PC to evaluate the stability of a particular process / controller configuration.

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When using the PC to monitor the process the input and output signals can be monitored and logged. Disturbances are introduced remotely from the external controller.

This eliminates the need for a separate process recorder or chart recorder for analysis of the process control responses. This software is compatible with PCs using Microsoft WindowsTM 98, 2000 and XP. The computer communicates to the PCT50 using a standard universal serial bus (USB) interface. The software includes a comprehensive online Help Text.

Installation

Advisory Before operating the equipment, it must be unpacked, assembled and installed as described in the steps that follow. Safe use of the equipment depends on following the correct installation procedure.

Electrical Supply Connect the lead from the 24V dc power supply to the socket marked 24V DC IN at the rear of the electrical enclosure. Connect the mains lead with appropriate plug to the mains inlet socket on the power supply.

The universal adapter supplied with PCT50 requires connection to a single phase, fused electrical supply. Leads are supplied allowing connection to:

220V / 50 Hz via European style 2-pin plug

220V / 50 Hz via UK style 3-pin plug

120V / 60Hz via USA 3-pin plug (125V style)

220V / 60Hz via USA 3-pin plug (250V style)

Connecting the PCT50 to a PC After installing the PCT50 software and driver on the PC the PCT50 is connected to the PC using the USB lead supplied with the PCT50. The USB socket is located on the rear panel of the electrical enclosure.

Filling the Sump Tank No permanent supply of water is required when operating the PCT50.

Ensure that the drain valve at the rear of the sump tank is fully closed before filling the sump tank.

The sump tank can be filled using a hose or by pouring water through the cut out at the front or via the process vessel at the top. The water level should be approximately 30 mm below the cut out in normal use. For safe handling the sump tank should be drained before moving the equipment to a different location.

Note: To ensure reliable priming of the water pump after filling for the first time or after draining and refilling the equipment, the flexible tube from the pump outlet should be temporarily disconnected from the system by pressing the latch on the quick release connector. After running the pump briefly, with the tube disconnected to expel any trapped air, the tube can be reconnected for normal operation.

Installing the Software The -306 version of the software supplied with this product is designed to operate on PC’s with either 32 or 64 bit operating systems provided that the equipment carries a label stating 32/64 bit os compatible alongside the USB port on the equipment.

If -306 software is to be used with equipment not displaying this label then the software can be operated using a PC with 32 bit operating system. If it is required to

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use a PC with 64 bit OS then it will be necessary to contact Armfield Ltd to upgrade the hardware.

To enable the -306 version of the software to communicate with the equipment the USB port on the PC utilizes a virtual serial driver that must be enabled each time the software is operated. Correct installation and operation of the -306 software is achieved as described below.

Install the -306 software supplied with the product

To successfully install the driver associated with the Armfield software it will be necessary for the user to be logged on to the computer as administrator or as a user that has been granted equivalent administrator rights. If any problems are experienced when installing the driver as described below then refer to the leaflet supplied with the CD-ROM that describes the detailed procedure for installing the driver.

Insert the CD-ROM in the CD/DVD drive. If autorun is disabled or installation does not start automatically choose the file Setup.exe on the CD-ROM.

The Welcome screen will be displayed showing the available options:

Choose Install Software from the options, wait for the welcome screen to clear then follow the onscreen instructions to install the software on the PC. All of the default options can be accepted unless installation in a particular location is required.

When the Welcome screen is displayed again choose Exit the CD but leave the CD-ROM in the CD/DVD drive of the PC to enable the USB driver to be installed later.

Note: It is not appropriate to choose Install Driver unless it is intended to operate the equipment using software other than that supplied by Armfield.

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Installation

Connect the USB port on the equipment to the USB port on the PC using the lead supplied. A message will be displayed showing that new hardware has been found and the appropriate driver must be installed:

When the location of the driver is requested, browse to find the CD/DVD drive containing the -306 Armfield CD-ROM then follow the onscreen instructions to install the driver.

Correct installation of the driver will be confirmed by the illumination of the green ‘Active’ LED alongside the USB connector on the equipment.

If there are any problems locating or installing the driver then refer to the leaflet supplied with the CD-ROM that describes the detailed procedure for installing the driver.

The CD-ROM can be removed from the CD/DVD drive.

Load the software

Locate the appropriate Armfield Folder using Start \ Programs then choose the required Armfield product, and accessory (if appropriate).

The opening screen will display the range of experiments available. Choose the appropriate experiment to undertake (choose any experiment if not sure which).

A Welcome screen will be displayed with a toolbar across the top.

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Enable the USB virtual serial COM Port

At the right hand end of the top menu bar click the Start COM Session icon as shown to enable the port:

The Select Port window will open. Display the available COM ports by pulling down the menu:

Highlight the last COM port in the list (highest number) then click OK to enable the port:

Confirm that the Serial Port has been enabled

If the Serial Port has been correctly enabled then the virtual COM Port number followed by SE or Diff will be displayed in the bottom right hand corner of the screen:

If the wrong Serial Port has been selected then the number of the port will be displayed but SE or Diff will not be displayed:

Repeat the above procedure and select the last COM port in the list.

If the serial port has not been enabled or if the device driver has not been installed correctly then IFD: Device Error will be displayed:

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Installation

If the USB connection between the PC and the equipment has been broken then a warning message will be displayed:

Clearing this message will result in a message in the bottom right hand corner indicating an error, typically:

Check that the USB lead is connected between the USB ports on the Armfield equipment and the PC. Reload the software then enable the virtual serial COM port as described above.

If the problem persists check that the device driver has been correctly installed as described above.

Note: Where the equipment has two USB ports the messages for both ports will be displayed in the bottom right hand corner of the screen. Each connection will use a different virtual COM port and the two ports must be configured separately as described above. The master port will include m in the message and the slave connection will include s to differentiate between the two. When installed correctly, typical messages will be:

Similarly, a connection or driver error with either or both ports will be indicated:

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Operate the -306 software supplied with the product

From the Welcome to Armsoft screen either choose the View icon in the top menu bar or choose View then Diagram to display a diagram showing the equipment and any relevant measured variables.

For details about operating the software refer to the Help facility in the software (accessible using the Help icon in the top menu bar then choosing Operating the Software). Alternatively refer to the section Operating the Software in the product instruction manual.

Installing the Equipment To install the equipment:

1. Carefully remove the PCT50 from its packing and remove any loose protective materials. Retain the power supply, connecting leads and CD-ROM in a safe place until required for use.

2. Place the PCT50 on a firm, level bench top or table where the equipment is to be operated with access to a mains electrical supply.

Note: No permanent connection is required to a water supply but it will be necessary to fill the sump tank before use with approximately 8 litres of clean cold water. The equipment should be drained before moving to a different location.

3. Wipe the equipment, especially the inside surfaces of the sump tank and process vessel, with a soft damp cloth to remove any dust etc. before proceeding.

4. Before operating the equipment ensure that a 4 mm diameter orifice is fitted below outlet valve CV2. The orifice size is changed by unscrewing the plastic fitting containing the orifice and replacing it with the required alternative. The fitting incorporates an ‘O’ ring seal and only requires hand tightening.

The size of the orifice can be changed to suit the demonstration. Alternative sizes of orifice are stored at the front of the support plinth.

5. Also ensure that a 2 mm diameter orifice is fitted below the solenoid valve SOL.

6. Close the outlet valve CV2 in the base of the process vessel then pour approximately 10 mm of water into the process vessel to prime the level sensor.

7. Open outlet valve CV2 and allow the water to drain from the process vessel into the sump tank so that the process vessel is empty. The level sensor will then be primed when zeroing the level sensor in a later step.

8. Ensure that the drain valve at the rear of the sump tank is fully closed then fill the lower sump tank with clean water to a level approximately 30 mm below the cut out in the front of the tank. If the local water is hard or contains impurities then demineralised or deionise water is recommended to minimise any scaling on the sump tank or process vessel walls.

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Installation

9. Ensure that the flow control valve CV1 is fully open at the inlet to process vessel so that water can flow into the process vessel when the pump is running.

10. Ensure that outlet valve CV2 is fully open in the base of the process vessel to allow water to return to the sump tank when the process vessel fills with water. With CV2 fully open the outflow is determined by the size of the orifice and the head of water in the process vessel giving repeatable system characteristics.

11. Ensure that valve CV3 is fully open above the solenoid valve SOL to allow water to return to the sump tank when the solenoid valve is open and the process vessel contains water. With CV3 fully open the outflow is determined by the size of the orifice and the head of water in the process vessel allowing repeatable disturbances to be applied to the process.

12. Switch on the PC and connect the USB lead from the rear of the PCT50 to the PC.

13. Load the PCT50 software then choose experiment ‘Ex1: Inflow control (PC control)’ from the menu.

For detailed information about operating the software refer to ‘Operating the Software’.

14. Display the mimic diagram by choosing View then Diagram or via the mimic

diagram icon .

15. Enable the virtual COM port using the red telephone icon in the top toolbar. Refer tot he section Operating the Software if necessary.

16. The mimic diagram will be displayed as follows:

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17. Connect the lead from the 24V DC power supply to the socket marked 24V DC IN at the rear of the electrical enclosure. Connect the mains lead with appropriate plug to the mains inlet socket on the power supply.

18. Click Power on (in controls) to allow the PCT51 to be operated via the PC.

19. Each time the software starts the PID controller is reset to manual control, with the pump off and the solenoid valve closed. Default values for the Set point and control parameters are restored unless a previous set-up has been saved. The level measured by the pressure sensor is indicated directly in engineering units i.e. mm depth of water in the process vessel.

Correct operation of the PCT50 can be checked using the manual controls in the software before using the equipment for automatic control exercises.

20. With the process vessel empty click on the zero button adjacent to the level indication on the mimic diagram to zero the reading of the water level with water at its minimum level inside the process vessel.

21. Disconnect the flexible tube from the pump outlet by pressing the latch on the quick release connector (this allows the pump to prime by expelling any trapped air). On the mimic diagram (in controls), gradually increase the pump speed to 50 % by clicking on the up arrow until 50 % is displayed.

When water is flowing steadily from the flexible tube return the pump speed to 0% by clicking the down arrow then reconnect the quick release connector. Gradually increase the pump speed again, briefly to 100% then back to 50% to ensure that the pump is fully primed. Check that the pump operates smoothly

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Installation

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and the process vessel starts to fill with water via the vertical inlet arrangement with integral non-return valve. As the vessel fills with water check that it returns to the sump tank via the outlet valve CV2. Check that the rising water level inside the process vessel is displayed on the mimic diagram (in controls). The sensor output should approximately match the level reading from the scale on the vessel side but exact correlation is not necessary (if recalibration is necessary refer Operating the Software).

Adjust outlet valve CV2 so that the level remains at mid height in the vessel with the pump operating at 50 % speed.

22. Check that the ball valve above the solenoid is fully open then open the solenoid valve SOL from the mimic diagram (in controls).

The switch is represented as a button on the screen, and is operated by clicking on it. The switch will display a ‘0’ when the valve is off (closed) and a ‘1’ when the valve is on (open). Check that water returns to the sump tank via the solenoid valve when the switch indicates ‘1’ and stops when the switch indicates ‘0’.

Check that the level falls slowly with the solenoid valve open due to the additional outflow from the vessel. Close the solenoid valve then check that the level stops falling.

23. Set the pump speed to 0%, and allow the contents of the process vessel to drain into the sump tank.

24. The operation of the PCT50 has been confirmed and the equipment is ready for use.

The sump tank should be drained to prevent fouling or scaling if the PCT50 is not to be used immediately.

Operation Where necessary, refer to the drawings in the Equipment Diagrams section.

Operating the Software NOTE: The diagrams in this section are included as typical examples and may not relate specifically to the individual product described in this instruction manual.

The Armfield Software is a powerful Educational and Data Logging tool with a wide range of features. Some of the major features are highlighted below, to assist users, but full details on the software and how to use it are provided in the presentations and Help text incorporated in the Software. Help on Using the Software or Using the Equipment is available by clicking the appropriate topic in the Help drop-down menu from the upper toolbar when operating the software as shown:

Before operating the software ensure that the equipment has been connected to a USB port on the PC (not supplied) using the USB lead supplied with the equipment. Where a separate USB interface (E.g. IFD5) has been supplied ensure that the equipment is connected to the interface and the interface is connected to a USB port on the PC. For further information about these actions refer to the sections Installing the Software and Installing the Equipment.

Load the software. If multiple experiments are available then a menu will be displayed listing the options. Wait for the presentation screen to open fully as shown:

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Operation

Check that the green Active LED (alongside the USB port on the equipment or IFD interface as appropriate) is illuminated to confirm that the USB port is connected and the USB driver is correctly installed. If the green LED is not illuminated refer to the section Installing the Software.

Before operating the software it will be necessary to enable the USB virtual serial COM port. At the right hand end of the top menu bar click the Start COM Session icon as shown to enable the port:

The Select Port window will open. Display the available COM ports by pulling down the menu:

Highlight the last COM port in the list (highest number) then click OK to enable the port:

Confirm that the Serial Port has been enabled

If the Serial Port has been correctly enabled then the virtual COM Port number followed by SE or Diff will be displayed in the bottom right hand corner:

If the wrong Serial Port has been selected then the number of the port will be displayed but SE or Diff will not be displayed. Refer to the section Installing the Software if there are any problems.

Presentation Screen - Basics and Navigation

As stated above, the software starts with the Presentation Screen displayed. The user is met by a simple presentation which gives them an overview of the capabilities

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of the equipment and software and explains in simple terms how to navigate around the software and summarizes the major facilities complete with direct links to detailed context sensitive ‘help’ texts.

To view the presentations click Next or click the required topic in the left hand pane as appropriate. Click More while displaying any of the topics to display a Help index related to that topic.

To return to the Presentation screen at any time click the View Presentation icon

from the main tool bar or click Presentation from the dropdown menu as shown:

For more detailed information about the presentations refer to the Help available via the upper toolbar when operating the software.

Toolbar

A toolbar is displayed at the top of the screen at all times, so users can jump immediately to the facility they require, as shown:

The upper menu expands as a dropdown menu when the cursor is placed over a name.

The lower row of icons (standard for all Armfield Software) allows a particular function to be selected. To aid recognition, pop-up text names appear when the cursor is placed over the icon.

Mimic Diagram

The Mimic Diagram is the most commonly used screen and gives a pictorial representation of the equipment, with continuously updated display boxes for all the various sensor readings, calculated variables etc. directly in engineering units.

To view the Mimic Diagram click the View Diagram icon from the main tool bar or click Diagram from the View drop-down menu as shown:

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Operation

A Mimic diagram is displayed, similar to the diagram as shown:

The details in the diagram will vary depending on the equipment chosen if multiple experiments are available.

In addition to measured variables such as Temperature, Pressure and Flowrate (from a direct reading flowmeter), calculated data such as Motor Torque, Motor Speed and Discharge / Volume flowrate (from pressure drop across an orifice plate) are continuously displayed in data boxes with a white background. These are automatically updated and cannot be changed by the user.

Manual data input boxes with a coloured background allow constants such as Orifice Cd and Atmospheric Pressure to be changed by over-typing the default value, if required.

The data boxes associated with some pressure sensors include a Zero button alongside. This button is used to compensate for any drift in the zero value, which is an inherent characteristic of pressure sensors. Pressing the Zero button just before

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starting a set of readings resets the zero measurement and allows accurate pressure measurements to be taken referenced to atmospheric pressure. This action must be carried out before the motor is switched on otherwise the pressure readings will be offset.

The mimic diagram associated with some products includes the facility to select different experiments or different accessories, usually on the left hand side of the screen, as shown:

Clicking on the appropriate accessory or exercise will change the associated mimic diagram, table, graphs etc to suit the exercise being performed.

Typical Control Facilities in the Mimic Diagram

A Power On button allows the motor to be switched off or on as required. The button always defaults to off at startup. Clicking this button switches the power on (1) and off (0) alternately.

A box marked Pump Speed allows the speed of the motor to be varied from 0 to 100% either stepwise, by typing in values, or using the up / down arrows as appropriate. It is usual to increase the motor speed gradually until the required operating speed is reached. On some products it is usual to operate the equipment with the motor set at 100% while gathering performance data, and then reduce the setting as required to investigate the effect of reduced speed on performance of the equipment.

When the software and hardware are functioning correctly together, the green LED marked Watchdog Enabled will alternate On and Off. If the Watchdog stops alternating then this indicates a loss of communication between the hardware and software that must be investigated.

Details on the operation of any automatic PID Control loops in the software are included later in this section.

Data Logging Facilities in the Mimic Diagram

There are two types of sampling available in the software, namely Automatic or Manual. In Automatic logging, samples are taken regularly at a preset but variable interval. In Manual logging, a single set of samples is taken only when requested by the operator (useful when conditions have to be changed and the equipment allowed to stabilize at a new condition before taking a set of readings).

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Operation

The type of logging will default to manual or automatic logging as appropriate to the type of product being operated.

Manual logging is selected when obtaining performance data from a machine where conditions need to stabilize after changing appropriate settings. To record a set of set

of data values from each of the measurement sensors click the icon from the

main toolbar. One set of data will be recorded each time the icon is clicked.

Automatic logging is selected when transients need to be recorded so that they can

be plotted against time. Click the icon from the toolbar to start recording, click

the icon from the toolbar to stop recording.

The type of logging can be configured by clicking Configure in the Sample drop-down menu from the upper toolbar as shown:

In addition to the choice of Manual or Automatic sampling, the parameters for Automatic sampling can also be set. Namely, the time interval between samples can be set to the required number of minutes or seconds. Continuous sampling can be selected, with no time limit or sampling for a fixed duration can be set to the required number of hours, minutes or seconds as shown:

Tabular Display

To view the Table screen click the View Table icon from the main tool bar or click Table from the View dropdown menu as shown:

27


The data is displayed in a tabular format, similar to the screen as shown:

As the data is sampled, it is stored in spreadsheet format, updated each time the data is sampled. The table also contains columns for the calculated values.

New sheets can be added to the spreadsheet for different data runs by clicking the

icon from the main toolbar. Sheets can be renamed by double clicking on the sheet name at the bottom left corner of the screen (initially Run 1, Run 2 etc) then entering the required name.

For more detailed information about Data Logging and changing the settings within the software refer to the Help available via the upper toolbar when operating the software.

Graphical Display

When several samples have been recorded, they can be viewed in graphical format.

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Operation

To view the data in Graphical format click the View graph icon from the main tool bar or click Graph from the View drop-down menu as shown:

The results are displayed in a graphical format as shown:

(The actual graph displayed will depend on the product selected and the exercise that is being conducted, the data that has been logged and the parameter(s) that has been selected).

Powerful and flexible graph plotting tools are available in the software, allowing the user full choice over what is displayed, including dual y axes, points or lines, displaying data from different runs, etc. Formatting and scaling is done automatically by default, but can be changed manually if required.

To change the data displayed on the Graph click Graph Data from the Format dropdown menu as shown:

29


The available parameters (Series of data) are displayed in the left hand pane as shown:

Two axes are available for plotting, allowing series with different scaling to be presented on the same x axis.

To select a series for plotting, click the appropriate series in the left pane so that it is highlighted then click the appropriate right-facing arrow to move the series into one of the windows in the right hand pane. Multiple series with the same scaling can be plotted simultaneously by moving them all into the same window in the right pane.

To remove a series from the graph, click the appropriate series in the right pane so that it is highlighted then click the appropriate left-facing arrow to move the series into the left pane.

The X-Axis Content is chosen by default to suit the exercise. The content can be changed if appropriate by opening the drop down menu at the top of the window.

The format of the graphs, scaling of the axes etc. can be changed if required by clicking Graph in the Format drop-down menu as shown:

30

Operation

For more detailed information about changing these settings refer to the Help available via the upper toolbar when operating the software.

PID Control

Where appropriate, the software associated with some products will include a single or multiple PID control loops whereby a function on the product can be manually or automatically controlled using the PC by measuring an appropriate variable and varying a function such as a heater power or pump speed.

The PID loop can be accessed by clicking the box labelled either PID, Control, Flow or Heater depending on the particular product software:

A PID controller window is then displayed as shown:

31


The Mode of operation always defaults to Manual control and 0% output when the software is loaded to ensure safe operation of the equipment. If appropriate, the operator can retain manual operation and simply vary the value from 0 to 100% in the Manual Output box, then clicking Apply.

Alternatively, the PID loop can be changed to Automatic operation by clicking the Automatic button. If any of the PID settings need to be changed from the default values then these should be adjusted individually before clicking the Apply button.

The controller can be restored to manual operation at any time by clicking the Manual button. The value in the Manual Output box can be changed as required before clicking the Apply button.

Settings associated with Automatic Operation such as the Setpoint, Proportional Band, Integral Time, Derivative Time and Cycle Time (if appropriate) can be changed by the operator as required before clicking the Apply button.

Clicking Calculations displays the calculations associated with the PID loop to aid understanding and optimization of the loop when changing settings as shown:

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Operation

Clicking Settings returns the screen to the PID settings.

Clicking OK closes the PID screen but leaves the loop running in the background.

In some instances the Process Variable, Control variable and Control Action can be varied to suit different exercises, however, in most instances these boxes are locked to suit a particular exercise. Where the variables can be changed the options available can be selected via a drop-down menu.

Exporting Data

All logged data can be saved as a Formula 1 .vts data file that can be for read back into the software at a later date using the Open tab. Alternatively the data can be exported in Microsoft Excel format that can be loaded into a spreadsheet. To save data choose File / Save, enter the required location and filename then choose the required data format from the drop down menu as shown.

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Advanced Features

The software incorporates advanced features such as the facility to recalibrate the sensor inputs from within the software. Other features for use with specific products include an integral Oscilloscope for fast date capture and a Scheduler to vary parameters with time.

For more detailed information about these advanced functions within the software refer to the Help available via the upper toolbar when operating the software.

Operating the Equipment Note: Before operating the equipment ensure that it has been correctly installed in accordance with the section Installing the Equipment and that you have read the section Important Safety Information.

Operating the Water Pump (Outflow Control or Manual operation)

The speed of the pump can be varied by the operator to change the inflow to the process vessel.

On the mimic diagram (in controls) set the pump speed to the required percentage of maximum speed by typing in the value or clicking on the up arrow until the required percentage is displayed. For example, to achieve half speed type 50 or click the appropriate arrow until 50 is displayed.

Operating the Water Pump (Inflow Control)

The speed of the pump is varied via the PID window on the mimic diagram.

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Operation

When set to Manual Mode the PID controller gives a fixed pump speed that is set by the operator. The fixed pump setting is also indicated on the mimic diagram. In Manual mode the pump speed can also be varied from the mimic diagram.

When set to Automatic Mode the PID controller varies the pump speed in an attempt to maintain the required level in the process vessel. The output from the controller will depend on the current measured level in the vessel, the setpoint in the PID controller and the P, I and D settings of the controller.

In Automatic mode the pump speed is indicated on the mimic diagram but cannot be changed on the mimic diagram.

Operating the On/Off Solenoid Valve (Inflow Control)

In the inflow control exercise the solenoid valve is used to inject a small step change to the outflow from the process vessel to test the stability of the control loop.

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The solenoid valve SOL is opened and closed as and when required by the operator from the mimic diagram (in controls).

The switch is represented as a button on the screen, and is operated by clicking on it. The switch will display a ‘0’ when the valve is off (closed) and a ‘1’ when the valve is on (open).

The magnitude of the step change can be varied in steps by changing the size of the orifice fitted below the solenoid valve. The orifice size is changed by unscrewing the plastic fitting containing the orifice and replacing it with the required alternative. The fitting incorporates an ‘O’ ring seal and only requires hand tightening. The alternative sizes of orifice are stored in threaded holes at the front of the base plate when not in use.

Alternatively, the magnitude of the disturbance can be varied continuously to any intermediate value using the hand operated valve CV3 above the solenoid valve. However, the magnitude of the step cannot be replicated accurately on different runs when adjusting the flow in this manner.

Operating the On/Off Solenoid Valve (Outflow Control)

In the outflow control exercise the solenoid valve is used to maintain the level in the process vessel by varying the outflow. Because the solenoid valve is either open or closed, the flow is varied by time proportioned opening of the valve on a time cycle that can be varied by the operator. In this exercise the solenoid valve is controlled via the PID control window and the switch displayed onscreen gives an indication of whether the valve is closed (0) or open (1).

In Manual operation the required percentage of opening is entered in the Manual Output box of the PID controller (0% or 100%).

Changing the size of a flow orifice

The flow of water draining from the process vessel via the outlet valve or solenoid valve is determined by the size of the orifice fitted. The orifice size is changed by unscrewing the plastic fitting containing the orifice and replacing it with the required alternative. The fitting incorporates an ‘O’ ring seal and only requires hand tightening. The alternative sizes of orifice are stored in threaded holes at the front of the base plate when not in use.

Connecting the PCT50 to an external controller

The water pump, solenoid valve and level sensor on PCT50 are permanently connected to the electrical enclosure.

The 0-5 Volt output from the level sensor, the 0-5Volt input to control the pump speed and the digital input to open the solenoid valve are available via connectors on the front of the electrical enclosure. This allows a PID controller such as PCT54 or a PLC such as PCT55 to operate the process. The connections between PCT50 and the external controller should be made using the leads supplied with PCT54 or PCT55.

Using Guest (Push) Fittings

Tubes that may be connected to a Guest fitting require a corresponding rigid tube coupling on the end of the tube. Align the parallel section of the rigid tube with the loose collet on the Guest fitting and push firmly until the tube stops.

36

Operation

37

To remove the tube, push the loose circular collet against the body of the Guest fitting while pulling the tube firmly. The tube will slide out from the fitting. The tube/fitting can be assembled and disassembled repeatedly without damage.

Calibration of the Level sensor

The software supplied with the PCT50 includes the facility to eliminate any offset in the level reading using a zero button on the mimic diagram. Before zeroing the level sensor the process vessel should be partly filled with water then allowed to drain fully before pressing the zero button.

The software also includes the facility to recalibrate the level sensor should this become necessary. Choose Options, Calibrate IFD channels then choose the pressure sensor measuring water level. The procedure for calibrating the sensor is described in Operating the Software.

Equipment Specifications

Overall Dimensions Height - 575 mm

Width - 425 mm

Depth - 350 mm

IFD Connections The electrical interface on PCT50 incorporates a PCB that provides the USB connection to a PC. The pin-outs of the 50 way connector on this PCB are as follows:

Pin No

Channel No PCT Function Signal Eng Unit

1 Channel 0 Analog Input Not used




5

Channel 4 Analog Input

Water Level L1

(Pressure sensor)

0-5 V

0-150 mm


7

Channel 6 Analog Input

Drive 1 monitor

(Pump speed)

0-5 V

0-100 %








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17 Ground 0V

18 Ground 0V

19 Analog Output 2 Not used

20 Analog Output 3 Not used

21 Ground 0V

22 Channel 0 Analog Output

Water pump speed 0-5 V 0-100%

23 Channel 0 Analog Ground

0V

24 Channel 1 Analog Output

Not used

25 Channel 1 Analog Ground

0V

26 Ground 0V

27 Ground 0V

28 Channel 0 Digital Input Not used

29 Channel 1 Digital Input Solenoid valve monitor 0/5V



32 Ground 0V





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37 Ground 0V

38 Digital Output Line 0 Power enable 0/5V

39 Digital Output Line 1 Watchdog 0/5V

40 Digital Output Line 2 Remote / Local operation 0/5V

41 Digital Output Line 3 Output 1 (Pump On/Off) 0/5V

42 Digital Ground 0V

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Digital Output Line 4

Output 1 Solenoid Valve

(On/Off)

0/5V

44 Digital Output Line 5 Not used



47 Ground 0V

48 Ground 0V

49 Not used

50 Not used

Other Specifications

Level sensor L1

Piezoelectric pressure sensor

0 - 150 mm H2O (gauge)

Solenoid valve Internal orifice 4 mm diameter

Orifice sizes 2 mm, 3 mm, 4 mm

Water Pump Flow rate 0 to 3 litres/min (nominal)

Process Vessel Capacity (To overflow) 6 litres (nominal)

Sump tank capacity 8 litres (nominal)

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41

Environmental Conditions This equipment has been designed for operation in the following environmental conditions. Operation outside of these conditions may result reduced performance, damage to the equipment or hazard to the operator.

a. Indoor use;

b. Altitude up to 2000 m;

c. Temperature 5 °C to 40 °C;

d. Maximum relative humidity 80 % for temperatures up to 31 °C, decreasing linearly to 50 % relative humidity at 40 °C;

e. Mains supply voltage fluctuations up to ±10 % of the nominal voltage;

f. Transient over-voltages typically present on the MAINS supply;

NOTE: The normal level of transient over-voltages is impulse withstand (over-voltage) category II of IEC 60364-4-443;

g. Pollution degree 2.

Normally only nonconductive pollution occurs.

Temporary conductivity caused by condensation is to be expected.

Typical of an office or laboratory environment

Routine Maintenance

Responsibility To preserve the life and efficient operation of the equipment it is important that the equipment is properly maintained. Regular maintenance of the equipment is the responsibility of the end user and must be performed by qualified personnel who understand the operation of the equipment.

General The equipment should be disconnected from the electrical supply when not in use.

The process vessel and sump tank should be drained after use.

In areas of hard water, the amount of scale in the process vessels and pipework can be reduced by using de-ionised water when filling the sump tank.

Cleaning the Equipment Build-up of scale on the inside of the process vessel can be minimised by drying the vessel when it is to be left unused for some time. Scale that does develop within the process vessel may be cleaned by hand.

It will be necessary to clean the filter in the pressure regulator at regular intervals. If the filter is visibly obstructed or the inlet flow rate appears reduced then the filter may need cleaning. Remove the filter from the equipment and clean by flushing with clean water flowing in the opposite direction to normal flow. The frequency of cleaning will depend on the cleanliness of the water supply.

Any build up of scale in the pipe work or process vessels can be removed by passing a mild descaler through the system then flushing thoroughly with clean water. Any stubborn deposits in the large process vessel can be eliminated by manual cleaning. Remaining deposits in the smaller vessel may need longer soaking in descaling solution. Always follow the instructions provided with the descaler regarding suitable dilution of the chemical.

The water in the process vessel should be drained into the sump tank after use. The water should be changed at regular intervals to prevent the growth of algae that may present a hazard to health.

Accessing the electrical circuits inside the electrical enclosure Maintenance of the PCT50 does not require access to the electrical circuits or components located inside the electrical enclosure. However, in the event of an electrical problem it may be necessary for a competent electrician to gain access to the inside of the enclosure as follows:

Ensure that the equipment is disconnected from the electrical supply (not just switched off).

The electrical circuits inside the enclosure are accessible after unscrewing the front panel then carefully sliding the loose side panel forwards, but only as far as the flexible ribbon cable will allow. It will be necessary to release the latches on the ribbon cable connecter (accessible from the front after unscrewing the front panel) to allow the side panel to slide forwards.

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Laboratory Teaching Exercises

Index to Exercises

Index to Exercises demonstrating Inflow Control by varying pump speed





Index to Exercises Demonstrating Outflow Control by time proportioned cycling of a solenoid drain valve




Index to Exercises Demonstrating Inflow Control or Outflow Control using an external controller



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Objective

To control the level of water in the process vessel by manually varying the speed of a pump supplying water to the process vessel (inflow control).

To determine the characteristics of the level process with different ranges of inflow and outflow to / from the process vessel.

Method

Using a PC to operate the process, the pump speed (water flow into the process vessel) will be varied by the operator in an attempt to maintain a steady water level in the process vessel.

With outflow valve CV2 fully open water will drain from the process vessel continuously, the rate determined by the size of the orifice fitted and the depth of water in the vessel.

Disturbances will be applied to the process by opening and closing the solenoid valve. This changes the outflow from the process vessel resulting in a change in the level inside the vessel. The magnitude of the disturbance is determined by the size of the orifice fitted below the solenoid valve and the depth of water in the vessel.

A pressure sensor measures the water level in the process vessel and indicates the value in mm depth of water on the mimic diagram.

Equipment Required

PCT50 Level Process

PC with PCT50 software loaded

Theory

For the level to remain constant in the process vessel the flowrate into the vessel supplied by the pump must match the flowrate leaving the vessel via CV2 and the solenoid valve, if open. Since the flow of water entering the vessel is proportional to pump speed, the speed must be increased to raise the level and decreased to lower the level.

Equipment set up

Ensure that the apparatus has been set up according to the Installation section, the sump filled with clean water and the power supply connected to the socket marked 24V IN at the rear of the electrical enclosure.

Before starting this exercise fully open outlet valve CV2 and fit a 4 mm diameter orifice below it. Fit a 2 mm orifice below the solenoid valve.

Connect the USB socket at the rear of the electrical enclosure a suitable PC on which the PCT50 software has been installed using the USB cable supplied. Check that the PC is switched on then run the PCT50 software and select Experiment Ex1: Inflow Control (PC Control).

Ensure that the Virtual COM port has been enabled using the red telephone icon in the top toolbar.

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Exercise A

Before starting the exercise it is advisable to adjust inlet valve CV1 to give appropriate flow into the process vessel as follows:

Fully open outlet valve CV2 and fully open inlet valve CV1.

Choose the icon (or View \ Diagram) to display a mimic diagram of the process.

Click Power On (in Controls).

If the level indicated on the mimic diagram is not 0mm with the vessel empty click the zero button to zero the reading.

Set the pump speed to 50%, by typing in the value or by clicking the up arrow, so that water is pumped into the process vessel.

As the water level rises adjust inlet valve CV1 until the level remains steady at approximately mid height in the process vessel (flow into the vessel via CV1 matches flow out of the vessel via CV2).

Reset the pump speed to 0 % on the mimic diagram and allow the vessel to drain via outlet valve CV2.

Procedure

Gradually increase the speed of the pump and observe that the level rises in the process vessel when the flow into the vessel exceeds the flow out. Observe that the level only remains constant if the pump speed is adjusted to match the outflow. If the pump speed is too high then the level will continue to rise, if the pump speed is too low then the level will continue to fall.

With the level constant in the process vessel choose the icon to begin data logging and note the level reading L1 in mm.

Open the solenoid valve by clicking the button on the mimic diagram (1 indicates that the valve is open). Observe that the level falls slowly due to the additional outflow via the solenoid valve. When the level has fallen by at least 20 mm note the current pump speed indicated on the mimic diagram then increase the pump speed to restore the original level.

Close the solenoid valve by clicking the button on the mimic diagram (0 indicates that the valve is closed).

Observe that the level rises very slowly because the flow from the pump now exceeds the flow from the base of the process vessel. When the level has risen by at least 20 mm note the current pump speed indicated on the mimic diagram then reduce the pump speed to restore the original level.

Choose the icon to finish data logging.

Choose the icon (or View \ Graph) to display a graph of the response obtained. Observe the rising and falling water level as the solenoid is opened and closed and action is taken to restore the level.

Return to the mimic diagram, reduce the pump speed and allow the process vessel to drain.

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46

Create a new results sheet by selecting the icon in the tool bar of the software

then choose the icon to begin data logging.

Gradually increase the pump speed until the level just starts to rise. Leave the pump at this speed and allow the vessel to fill. When the level reaches 150 mm stop the pump and allow the vessel to drain.

When the vessel is empty start the pump at 75 % speed and allow the vessel to fill. When the level reaches 150 mm stop the pump and allow the vessel to drain.


View the graph and observe the different rates of filling at different pump speeds. Also note that the vessel drains at the same rate, defined by the outlet orifice below valve CV2. These characteristics will affect the control of the process.

If time permits repeat the exercise with different sizes of orifice fitted to the outlet valve CV2 and solenoid valve, E.g. 2 mm at CV2 and 4 mm at the solenoid valve (smaller outflow with larger disturbance). The filling characteristics with different settings of CV1 could also be investigated.

Create a new results sheet for each run by selecting the icon in the tool bar of the software.

Results

Save a copy of the results obtained so that the data can be viewed in graphical or tabular format at a later date.

Conclusion

The level in a vessel will only remain constant when the flow into the vessel matches the flow leaving the vessel.

Excess water entering the vessel will cause the level to rise. Insufficient water entering the vessel will cause the level to fall.

When level is stable in a vessel, with inflow matching outflow, any disturbance such as additional flow entering or leaving the vessel will result in a change in level, requiring variation of the inflow to restore the level.

It is important that the flow from the pump is suitable over the available range of pump speeds to match the outflow from the process.


Objective

To control the level of water in the process vessel using an on/off controller to automatically start and stop the feed pump as required to maintain the required level in the process vessel (inflow control).

To determine the variations in water level due to the dead band inherent in an on/off controller.

To change the level inside the process vessel by changing the set point on the on/off controller and to determine the effect of disturbances to the process.

To investigate the effect of different ranges of inflow and outflow to / from the process vessel.

Method

Using a PC to operate the process, the pump (water flow into the process vessel) will be started and stopped by the controller in an attempt to maintain a steady water level in the process vessel.

With outflow valve CV2 fully open water will drain from the process vessel continuously, the rate determined by the size of the orifice fitted.

Disturbances can be applied to the process by opening and closing the solenoid valve. This changes the outflow from the process vessel resulting in a change in the level inside the vessel. The magnitude of the disturbance is determined by the size of the orifice fitted below the solenoid valve.

A pressure sensor measures the water level in the process vessel and indicates the value on the PC.

Equipment Required

PCT50 Level Process.


Theory

An on/off controller is a simple and effective way of controlling many processes but does have disadvantages because its output can only be on or off. In the case of the level process in this exercise the pump is started and stopped.

An on/off controller incorporates a dead band to avoid rapid switching of the controlled variable when at the setpoint i.e. in this case the level must rise above the setpoint by a fixed amount before the pump switches off and the level must fall below the setpoint by a fixed amount before the pump starts again.

Note: On PCT50 an On/off controller with fixed dead band is created by setting the Proportional Band to 0 % in the PID controller. In a commercial On/off controller the dead band can be varied to suit the process. This allows the choice of less frequent switching but larger variations in the process variable or closer control of the process variable but more frequent switching with attendant wear etc. of the components.

47


Equipment set up

Ensure that the apparatus has been set up according to the Installation section, the sump filled with clean water and the power supply connected to the socket marked 24 V IN at the rear of the electrical enclosure.

For the start of this exercise check that outflow valve CV2 is fully open and fitted with a 4 mm diameter orifice. Fit a 2 mm orifice to the solenoid valve.






Click Power On (in controls).


Set the pump speed to 50 %, by typing in the value or by clicking the up arrow, so that water is pumped into the process vessel.



Procedure

Choose the PID box on the mimic diagram, set the Proportional Band, Integral Time and Derivation time to 0 then set the Set Point to 75 mm (default at start up). Click Apply to enter the changes to the settings.

Choose the Automatic Mode of Operation.

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Exercise B

Select the icon to begin data logging.

The pump will switch on, the level will gradually rise and water will drain from the vessel via outlet valve CV2. The level will continue to rise until it reaches the level set on the controller I.e. the setpoint of 75 mm. When the level rises above the setpoint by a fixed amount (the dead band) the pump will switch off and the level will fall as water drains via outlet valve CV2. When the level falls below the setpoint by a fixed amount (the dead band) the pump will switch on again and the cycle will continue.

In the PID controller adjust the set point to 50 mm, click Apply then observe the pump remains off until the level falls to the new set point then switch on and off as before to maintain the new level.

Adjust the set point to 1000 mm and observe the response of the process.

Effect of Disturbances

Open the solenoid valve by clicking the button on the mimic diagram (1 indicates that the valve is open). Observe that the level falls faster due to the additional outflow via the solenoid valve then the controller switches on the pump to restore the original level.

Close the solenoid valve by clicking the button on the mimic diagram (0 indicates that the valve is closed). Observe that the process returns to the original characteristics.

Effect of differences in Inflow Rate and Outflow rate

Close inlet valve CV1 so that the level only just rises when the pump is running. Allow the level to rise and fall through several cycles then open the solenoid valve to disturb the process. If the combined outflow exceeds the inflow then the level will continue to fall with the pump operating and the required level cannot be maintained. Close the solenoid valve again.

49


50

Open inlet valve CV1 so that the level rises rapidly when the pump is running. Allow the level to rise and fall through several cycles then open the solenoid valve to disturb the process.




Results

Choose the icon (or View \ Graph) to display a graph of the responses obtained. Observe the rising and falling water level about each of the set points as the pump starts and stops to maintain the required level. Observe the change in the rate of filling when the solenoid valve is opened to disturb the process and when the inflow is reduced by partly closing valve CV1.

From the graph or table of results ( icon or View\Table) determine the variation in level about the set point to determine the dead band of the controller. In a commercial controller the dead band would usually be adjustable to suit the process but too small a variation would lead to wear of components switching rapidly on and off.

From the graph observe that the rate of filling when the pump is on is not necessarily the same as the rate of emptying when the pump is stopped.

Conclusion

Where a low cost solution is required, small variations in process variable are acceptable and the response of the process is slow enough then an on\off controller can be used to control a process. The level process on PCT50 is such a process. Note: a faster process such as flow control (demonstrated using PCT51) is not suitable for on\off control.

The device controlled by an on\off controller (in this case a water pump) is simply on or off as required to maintain the process variable.

With inflow control on process vessel the pump will reduce the flow or even stop the flow if the outflow reduces or is interrupted. If the outflow exceeds the maximum inflow then the vessel will continually drain despite the pump operating (Inflow to the process vessel must always exceed the outflow from the vessel with inflow control).


Objective

To control the level of water in the process vessel using a proportional controller to automatically vary the speed of the feed pump (inflow control).

To determine the response of a level process when using a P only controller to vary the pump speed.

To determine the response of a level process when using a P+I controller to vary the pump speed.

To change the level inside the process vessel by changing the set point on the P or P+I controller and to determine the effect of disturbances to the process.


Method

Using a PC to operate the process, the pump speed (water flow into the process vessel) will be varied automatically by a proportional controller in an attempt to maintain a steady water level in the process vessel.

The effect of the Proportional Band setting (P) can be demonstrated by configuring a P only controller and observing the response of the process to different settings of P.

The contribution of the of the Integral Time setting (I) can be demonstrated by configuring a P+I controller and observing the response of the process to different settings of I with P fixed then different combinations of P+I.




Equipment Required



Theory

Proportional term (P)

The Proportional Band setting on a process controller makes a change to the output (pump speed on PCT50) that is proportional to the current error value (the difference between the measured level and the set point on the controller). The proportional response can be adjusted by multiplying the error by a constant Kp, called the

51


Proportional Gain. This is related to the Proportional Band setting on the controller as follows:

Proportional Gain (Kp) = 100 % / Proportional Band %

i.e. 100 % PB means unity gain (change in controller output = error at input)

and 50 % PB means a gain of 2 (change in controller output = 2x error at input)

A low setting of the Proportional Band (large gain) results in a large change in the output for a given change in the error. If the Proportional Band is too low, the system can become unstable. In contrast, a large setting of the Proportional Band (low gain) results in a small output response to a large input error, and a less responsive or less sensitive controller. If the Proportional Band is too high, the control action may be too small when responding to system disturbances resulting in slow response and offsets of the resulting process variable from the set point.

A Proportional-only controller will not always settle at the set point, but may retain a steady-state offset. Offset can be reduced in Proportional-only control by reducing the Proportional Band setting. However, if the PB is set too small then hunting or oscillating will occur. The offset can be minimised by adding a bias to the set point (setting the set point above or below the required value to compensate for the offset) but this technique is only appropriate if the system characteristics are known and fixed. A better solution is to remove the offset by adding Integral action to the controller (P+I) as described below.

Integral term (I)

The contribution from the integral term is proportional to magnitude and duration of the error. The Integral term in a PID controller is the sum of the instantaneous error over time and gives the accumulated offset that should have been corrected previously. The resulting controller output is the sum of the contribution from the Integral term and the contribution from the P term.

When the I term is correctly adjusted any residual offset in the process variable due to the P term will be gradually reduced by the Integral term until the offset is eliminated. If the time setting of the I term is too long then correction to any offset will be very slow.

However, since the integral term responds to accumulated errors from the past, it can cause the present value to overshoot the setpoint value or to make the process completely unstable if the time setting of the I term is too short. If this occurs the I term makes adjustments to the controller output faster than the process can respond. I.e. the I term winds up the controller output so that the process overshoots considerably, hence the term Integral Wind-up or Integral saturation.

Careful selection of the I term in combination with the P term will give efficient response to changes in the system.

Derivative term (D)

The derivative term slows the rate of change of the controller output. Derivative control is used to reduce the magnitude of the overshoot produced by the I term and improve the combined controller-process stability. However, the derivative term slows the response of the controller. Also, differentiation of a signal amplifies noise and thus this term in the controller is highly sensitive to noise in the error term, and can

52

Exercise C

cause a process to become unstable if the noise and the derivative gain are sufficiently large.

Equipment set up










Set the pump speed to 50%, by typing in the value or by clicking the up arrow, so that water is pumped into the process vessel.


Reset the pump speed to 0% on the mimic diagram and allow the vessel to drain via outlet valve CV2.

Procedure - P only controller

Choose the PID box on the mimic diagram, set the Proportional Band P to 200%, the Integral Time I to 0 and the Derivation time D to 0 then set the Set Point to 75 mm (default at start up). Click Apply to enter the changes to the settings.


53



The pump will start, the level will gradually rise and water will drain from the vessel via outlet valve CV2. The level will continue to rise until it reaches a steady level but this will not correspond with the setpoint of 75 mm on the controller because of the offset inherent in a P only controller.

Open the solenoid valve by clicking the button on the mimic diagram (1 indicates that the valve is open) to disturb the process. Observe that the level falls slightly, the pump speeds up slightly due to the increased error but a large offset remains. When the level has settled close the solenoid valve and allow the level to settle.

Adjust the set point to 100 mm then click apply and observe the response to a requested increase in water level. When the level has settled, return the set point to 75 mm and observe the response.

Adjust the P term to 100 % in the PID controller then click Apply. Observe that the pump speeds up and the offset reduces slightly. Open then close the solenoid valve as before to disturb the process and observe the response, the stability of the level and any reduction in offset.

Adjust the P term to 50 % and repeat the above steps.


Adjust the P term to 10 % and repeat the above steps



54

Exercise C


Results – P only controller

Choose the icon (or View \ Graph) to display a graph of the responses obtained.

Choose Format \ Graph Data and Plot Level L1 and Set point on 1 axis, and Controller Output on the second axis.

Observe the changes in water level and the stability of the responses for each change in P setting with a disturbance or change in set point.

Choose a setting for P that gives stable control with a reasonably small offset that can be used in the next part of the exercise introducing Integral action.

Procedure - P+I Controller

Create a new results sheet for this run by selecting the icon in the tool bar of the software.

Choose the PID box on the mimic diagram, set the Proportional Band P to the value selected above, the Integral Time I to 0 seconds and the Derivation time D to 0 then set the Set Point to 75 mm. Click Apply.


Choose the Automatic mode of operation and allow the water level to stabilise.

Adjust the I term to 100 seconds in the PID controller then click Apply. Observe that the pump speed varies to reduce the offset. When the level is settled at the set point open the solenoid valve to disturb the process. Observe that the level falls slightly, the pump speed varies and the level gradually returns to the set point. When the level has settled close the solenoid valve and allow the level to settle.

Adjust the set point to 100 mm then click Apply and observe the response to a requested increase in water level. When the level has settled return the set point to 75 mm and observe the response.

Adjust the I term to 50 seconds in the PID controller then click Apply. Observe that the level returns to the set point faster than before. When the level is settled at the set point open then close the solenoid valve as before to disturb the process.


Continue reducing the Integral time setting and repeating the disturbance and set point changes until the level becomes unstable.

Results – P+I controller


Choose Format \ Graph Data and Plot Level L1 and Set point on 1 axis, and Controller output on the second axis.

55


56

Observe the changes in water level and the stability of the responses for each change in I setting with a disturbance or change in set point.

Choose a setting for I that gives stable control and reduces any offset swiftly without causing the system to become unstable.

Since the effect of Integral Action is related to the Proportional Band setting (reduced PB gives greater effect for any setting of I) the above exercise should be repeated at a completely different value of PB to determine the necessary change in I for effective control of the level and to demonstrate the relationship between P and I.

If time permits repeat the exercise with different sizes of orifice fitted to the outlet valve CV2 and solenoid valve, E.g. 2 mm at CV2 and 4 mm at the solenoid valve (smaller outflow with larger disturbance). The filling characteristics with different settings of CV1 could also be investigated e.g. partly closing valve CV1 to reduce the flow or increasing the flow by opening the valve.

Conclusion

A large proportional band setting will result in very stable control but will result in slow changes and large offsets in the process variable.

Reducing the proportional band will improve the speed of response and give smaller offsets but the process may become unstable if the proportional band in reduced too far.

Optimum results are obtained with the lowest proportional band setting that gives stable control but with the addition of Integral action to eliminate any offset.

Reducing the Integral time setting increases the speed at which any offset is reduced but settings that are too short will result in instability because the process is unable to respond quickly enough to the changes from the controller. The effect is called integral saturation of integral wind up and must be avoided.

Increasing the PB reduces the effect of Integral Action so shorter setting of I is required to eliminate offset swiftly.


Objective

To control the level of water in the process vessel using a proportional PID controller to automatically vary the speed of the feed pump (inflow control).

To determine the optimum settings for the P, I and D terms appropriate to the level process and to test the stability of the system by applying step changes and changing the set point.


Method

Using a PC to operate the process, the pump speed (water flow into the process vessel) will be varied automatically by a proportional controller in an attempt to maintain a steady water level in the process vessel.

Appropriate settings for the Proportional Band setting (P), the Integral Time (I) and the Derivative Time (D) will be determined by applying a step change to the process and observing the response of the system (the Reaction Curve Method).

The parameters determined using the Reaction Curve Method can be refined by testing the response of the system with these parameters set.




Note: This exercise assumes a basic knowledge of the equipment and the software from experience gained performing previous exercises. Refer to previous exercises if further information is required.

Equipment Required



Theory

REACTION CURVE METHOD

The controller output is operated manually so that a step change is applied to the process without corrections being applied by the controller.

57


The start of the step change is determined from the graph of controller output, the size of the step change is recorded as M % (change in controller output) and the shape of the level curve is recorded.

A typical response curve is shown below:

Typical response with the Reaction Curve method

Draw a straight line through the point of maximum slope so that the line intersects the time axis.

Measure the dead time L in minutes. (Time at which step change is applied to time where straight line intersects time axis.)

Calculate the maximum slope R.

Determine R1 using the equation

The optimum settings for the controller may be calculated as follows:

P only PB = R1 x L

P + I PB = 1.1 (R1 x L) IAT = 3.0L

P + I + D PB = 0.5 (R1 x L) to 0.8 (R1 x L) IAT = 2.0L to 2.5L DAT = 0.3L to 0.5L

ULTIMATE PERIOD METHOD (Ziegler-Nichols)

This alternative technique utilises the response of the system with the controller configured for Proportional control only. It cannot be applied to relatively stable processes because the system must have the ability to oscillate continuously.

The Proportional Band is gradually reduced in steps and at each step a step change is applied to test the stability of the system. This is repeated until the process variable continually oscillates. The correct point is achieved when the amplitude of the oscillations remains constant. If the oscillations gradually die away then the PB should be decreased slightly. Conversely if the oscillations gradually increase then the PB should be increased until the amplitude remains constant.

58

Exercise D

Note the setting of the Proportional Band, PBc, at which continual cycling occurs. From the trace obtained on the recorder, measure the period of the oscillation, Pc, in minutes.

Typical response with the Ultimate Period method

The optimum settings for the controller may be calculated as follows:

P only PB = 2 PBc

P + I PB = 2.2 PBc IAT = 0.83Pc

P + I + D PB = 1.7 PBc IAT = 0.5Pc DAT = 0.13Pc

Equipment set up




Before starting the exercise it is advisable to adjust inlet valve CV1 to give approximately 75 mm depth of water with the pump operating at 50 % speed.

Procedure

REACTION CURVE METHOD

Set the controller for Manual Mode of operation.

Adjust the output from the controller to give a steady level at mid-height (75 mm) in the process tank i.e. process variable = 50 %. Adjust valve CV1 if necessary to achieve 75 mm height with the controller output at 50 %.


Leave the controller in manual operation and apply a step disturbance to the process by increasing the output slightly. Note the magnitude of the step change applied (delta = M %).

59


60

The step change will result in a new level in the process tank. The open loop response may be analysed from the graph to determine the optimum settings for P, I and D using the equations listed in the theory above.

Having calculated the starting values apply these to the controller then click Apply.

Test the stability of the process by applying disturbances using the solenoid valve or by changing the set point. (Refer to previous exercises if necessary for information about this).

ULTIMATE PERIOD METHOD (Ziegler-Nichols)

Set the controller for Proportional control only, initially with a P value of 100 %. Set the controller for Automatic Mode of operation.


Allow the process variable to settle, then apply a step change to the process by opening or closing the solenoid valve SOL.

If the process variable settles to a steady new value, decrease the Proportional Band and re-apply a step change (by opening or closing SOL).

Continue adjusting the Proportional Band and applying a step change until the process variable continually oscillates.

Note the setting of the Proportional Band, PBc, at which continual cycling occurs.

From the graph obtained measure the period of the oscillation, Pc, in minutes.

The closed loop response may be analysed from the graph to determine the optimum settings for P, I and D using the equations listed in the theory above.

Having calculated the starting values apply these to the controller then click Apply.

Test the stability of the process by applying disturbances using the solenoid valve or by changing the set point. (Refer to previous exercises if necessary for information about this).

Results

Retain graphs for each test showing the response of the system using Open Loop and close Loop tuning techniques.

Conclusion

Starting values for P,I and D terms on a process controller can be determined by testing the response of the system using Open Loop or Closed Loop tuning techniques.

The values obtained using these techniques are only a starting point and minor adjustments may be necessary to obtain optimum control of a process.


Objective

To control the level of water in the process vessel by manually opening and closing a solenoid valve that allows water to drain from the process vessel (outflow control).

To determine the characteristics of the level process with different ranges of inflow and outflow to / from the process vessel.

Method

Using a PC to operate the process, the solenoid valve (water flow out from the process vessel) will be varied by the operator in an attempt to maintain a steady water level in the process vessel.

With outflow valve CV2 fully open water will drain from the process vessel continuously, the rate determined by the size of the orifice fitted and the depth of water in the vessel.

Disturbances will be applied to the process by varying the pump speed. This changes the flow into the process vessel resulting in a change in the level inside the vessel. The magnitude of the disturbance is determined by the step change in pump speed.

A pressure sensor measures the water level in the process vessel and indicates the value in mm depth of water on the mimic diagram.

Equipment Required

PCT50 Level Process


Theory

For the level to remain constant in the process vessel the flowrate into the vessel supplied by the pump must match the flowrate leaving the vessel via CV2 and the solenoid valve. When the level rises above the required value, the solenoid valve must be opened to allow more water to flow out and reduce the level. When the level falls below the required value, the solenoid valve must be closed to reduce the water flowing out and allow the level to rise.

Equipment set up



Connect the USB socket at the rear of the electrical enclosure to a suitable PC on which the PCT50 software has been installed using the USB cable supplied. Check that the PC is switched on then run the PCT50 software and select Experiment Ex2: Outflow Control (PC Control).


61





Click Power On (in Controls).


Set the pump speed to 50% by typing in the value or by clicking the up arrow, so that water is pumped into the process vessel.


Reset the pump speed to 0% on the mimic diagram and allow the vessel to drain via outlet valve CV2.

Procedure

Set the pump speed to 50% and allow the level to stabilise in the process vessel. Observe that the level only remains constant if the pump speed is adjusted to match the outflow. If the pump speed is too high then the level will continue to rise, if the pump speed is too low then the level will continue to fall.

With the level constant in the process vessel choose the icon to begin data logging and note the level reading L in mm.

Adjust the pump speed to 60 % and observe that the level rises in the process vessel because the inflow exceeds the outflow via valve CV2.

Open the solenoid valve by clicking the button on the mimic diagram (1 indicates that the valve is open). Observe that the level falls slowly due to the additional outflow via the solenoid valve. When the level has fallen below the original level close the solenoid valve and observe that the water level starts to rise again.

Continue to open and close the solenoid valve for several cycles as required to maintain the original level

Attempt to control the water level at a different value e.g. 50mm or 100mm.

Continue to open and close the solenoid valve for several cycles as required to maintain the required level.

Increase the pump speed in steps of 5% until the level cannot be maintained despite the solenoid valve remaining open.

Reduce the pump speed to 55% and attempt to maintain the original flow.

Reduce the pump speed to 40% and observe that the water level cannot be maintained despite the solenoid valve remaining closed.

62

Exercise E

63


Choose the icon (or View \ Graph) to display a graph of the response obtained. Observe the rising and falling water level as the solenoid is opened and closed and action is taken to restore the level. These characteristics will affect the control of the process.

If time permits repeat the exercise with different sizes of orifice fitted to the outlet valve CV2 and solenoid valve, E.g. 2 mm at CV2 and 3 mm at the solenoid valve (smaller outflow with smaller flow from the solenoid valve disturbance). The filling characteristics with different settings of CV1 could also be investigated.

Results

Save a copy of the results obtained so that the data can be viewed in graphical or tabular format at a later date.

Conclusion

The level in a vessel will only remain constant when the flow into the vessel matches the flow leaving the vessel.

Excess water entering the vessel will cause the level to rise. Insufficient water entering the vessel will cause the level to fall.

When level is stable in a vessel, with inflow matching outflow, any disturbance such as additional flow entering the vessel will result in a change in level, requiring variation of the outflow to restore the level.

It is important that the flow from the solenoid valve is suitable over the available range of pump speeds to match the change in inflow to the process.


Objective

To control the level of water in the process vessel using an on/off controller to automatically open and close the solenoid valve as required to maintain the required level in the process vessel (outflow control).

To determine the variations in water level due to the dead band inherent in an on/off controller.

To change the level inside the process vessel by changing the set point on the on/off controller and to determine the effect of disturbances to the process.


Method

Using a PC to operate the process, the solenoid valve (water flow out from the process vessel) will be opened and closed by the controller in an attempt to maintain a steady water level in the process vessel.

The speed of the pump and valve CV1 will be adjusted to give a suitable flow into the vessel to suit the demonstration.

Outflow valve CV2 will be fully open to allow water to drain from the process vessel continuously, the rate determined by the size of the orifice fitted.

Disturbances can be applied to the process by step changing the speed of the pump. This changes the inflow to the process vessel resulting in a change in the level inside the vessel. The magnitude of the disturbance is determined by the step in pump speed. Valve CV2 can be opened and closed manually to create an additional disturbance.


Equipment Required



Theory

An on/off controller is a simple and effective way of controlling many processes but does have disadvantages because its output can only be on or off. In the case of the level process in this exercise the solenoid valve is opened and closed.

An on/off controller incorporates a dead band to avoid rapid switching of the controlled variable when at the setpoint i.e. in this case the level must rise above the setpoint by a fixed amount before the solenoid valve opens and the level must fall below the setpoint by a fixed amount before the solenoid close again.

Note: On PCT50 an On/off controller with fixed dead band is created by setting the Proportional Band to 0 % in the PID controller. In a commercial On/off controller the dead band can be varied to suit the process. This allows the choice of less frequent

64

Exercise F

switching but larger variations in the process variable or closer control of the process variable but more frequent switching with attendant wear etc. of the components.

Equipment set up


For the start of this exercise check that outflow valve CV2 is fully open and fitted with a 4 mm diameter orifice. Fit a 4 mm orifice to the solenoid valve.

Connect the USB socket at the rear of the electrical enclosure a suitable PC on which the PCT50 software has been installed using the USB cable supplied. Check that the PC is switched on then run the PCT50 software and select Experiment Ex2: Outflow Control (PC Control).







Open the solenoid valve by clicking the button on the mimic diagram (1 indicates that the valve is open).


As the water level rises adjust inlet valve CV1 until the level remains steady at approximately mid height in the process vessel (flow into the vessel via CV1 matches flow out of the vessel via the solenoid valve and valve CV2).

Reset the pump speed to 0% on the mimic diagram and allow the vessel to drain via.

Close the solenoid valve on the mimic diagram.

Procedure

Choose the PID box on the mimic diagram, set the Proportional Band, Integral Time and Derivation time to 0 then set the Set Point to 75 mm (default at start up). The cycle time is not relevant when operating as an on/off controller so can be left at the default value of 10 seconds. Click Apply to enter the changes to the settings.


65



The solenoid valve will remain closed because there is no water in the vessel.


The level will gradually rise and water will drain from the vessel via outlet valve CV2 only. The level will continue to rise until it reaches the level set on the controller I.e. the setpoint of 75 mm. When the level rises above the setpoint by a fixed amount (the dead band) the solenoid valve will open and the level will fall as water drains via the solenoid valve and outlet valve CV2. When the level falls below the setpoint by a fixed amount (the dead band) the pump will switch on again and the cycle will continue.

In the PID controller adjust the set point to 50 mm, click Apply then observe that the solenoid valve closes until the level falls to the new set point then opens and closes as before to maintain the new level.

Close outlet valve CV2 and observe that the level continues to rise in the process vessel because the inflow is greater than the outflow via the solenoid valve. Open CV2 and allow the controller to restore the level.

Effect of Disturbances

Allow the level to rise and fall through several cycles then change the pump speed to 45 % to reduce the flow into the vessel. Observe that the rise in level is slower than before but the level oscillates about the set point as before. Change the set point to 75 mm and observe that the controller maintains the required level.

Change the pump speed to 40 % to reduce the flow into the vessel further. Observe that the rise in level is even slower than before but the level in maintained as before.

66

Exercise F

67

Effect of differences in Inflow Rate and Outflow rate

Change the pump speed to 60 % to increase the flow into the vessel. The level will rise quickly in the process vessel and the solenoid will open when the level exceeds the set point. Observe that the level continues to rise despite the solenoid valve being open because the inflow exceeds the outflow and the vessel cannot drain quickly enough. Open valve CV2 to increase the outflow from the vessel and observe that control is resumed about the set point.




Results

Choose the icon (or View \ Graph) to display a graph of the responses obtained. Observe the rising and falling water level about each of the set points as the solenoid valve opens and closes to maintain the required level. Observe the change in the rate of filling when the pump speed is changed to disturb the process. Observe the change in the rate of draining when the outflow is reduced by closing valve CV2.

From the graph or table of results ( icon or View\Table) determine the variation in level about the set point to determine the dead band of the controller. In a commercial controller the dead band would usually be adjustable to suit the process but too small a variation would lead to wear of components switching rapidly on and off.

From the graph observe that the rate of filling is not necessarily the same as the rate of emptying when the solenoid valve is opened.

Conclusion

Where a low cost solution is required, small variations in process variable are acceptable and the response of the process is slow enough then an on\off controller can be used to control a process. The level process on PCT50 is such a process. Note: a faster process such as flow control (demonstrated using PCT51) is not suitable for on\off control.

The device controlled by an on\off controller (in this case a solenoid valve) is simply on or off as required to maintain the process variable.

With outflow control on the process vessel the solenoid valve will remain closed until the water entering the vessel reaches the set point. This system will operate satisfactorily provided that the inflow cannot exceed the outflow. If the inflow exceeds the maximum outflow then the vessel will continually fill and the level cannot be controlled. (Inflow to the process vessel must always be less than the outflow from the vessel with outflow control).


Objective

To control the level of water in the process vessel using a proportional controller to automatically vary the opening and closing of a solenoid valve in a time-proportioned cycle (outflow control).

To determine the response of a level process when using a P only controller to vary the opening of a drain valve.

To determine the response of a level process when using a P+I controller to vary the opening of a drain valve.

To change the level inside the process vessel by changing the set point on the P or P+I controller and to determine the effect of disturbances to the process.


Method

Using a PC to operate the process, the opening of a solenoid drain valve (water flow out from the process vessel) will be varied automatically by a proportional controller in an attempt to maintain a steady water level in the process vessel.

The effect of the Proportional Band setting (P) can be demonstrated by configuring a P only controller and observing the response of the process to different settings of P.

The contribution of the of the Integral Time setting (I) can be demonstrated by configuring a P+I controller and observing the response of the process to different settings of I with P fixed then different combinations of P+I.

With outflow valve CV2 fully open water will drain from the process vessel continuously, the rate determined by the size of the orifice fitted. When solenoid valve CV3 opens, the rate of the additional outflow is determined by the size of the orifice fitted.

Disturbances can be applied to the process by varying the speed of the water pump that feeds the process vessel. This changes the inflow to the process vessel resulting in a change in the level inside the vessel. The magnitude of the disturbance is determined by the change in pump speed.


Equipment Required



68

Exercise G

Theory

Time Proportioned Control

Although the solenoid valve is an on/off device and therefore either fully open or fully closed, an average flow of water can be achieved by opening and closing the valve in a cycle. Having set a cycle time on the controller, the valve will open and close for different periods depending on the error measured and the settings of the Proportional and Integral settings of the controller as described below. For example if the cycle time is set to 10 seconds then at 50% controller output, the valve will open for 5 seconds and close for 5 seconds. This will repeat until the controller output changes. At 25% controller output, the valve will open for 2.5 seconds and close for 7.5 seconds.

If the cycle time is set to 20 seconds then at 50% controller output, the valve will open for 10 seconds and close for 10 seconds.

In practice the cycle time should be as long as practicable to minimise wear of the components (in this case a relay and solenoid valve) but short enough that the sequence of opening and closing does not affect the stability of the process.

Proportional term (P)

The Proportional Band setting on a process controller makes a change to the output (time proportioned solenoid valve on PCT50) that is proportional to the current error value (the difference between the measured level and the set point on the controller). The proportional response can be adjusted by multiplying the error by a constant Kp, called the Proportional Gain. This is related to the Proportional Band setting on the controller as follows:

Proportional Gain (Kp) = 100 % / Proportional Band %

i.e. 100 % PB means unity gain (change in controller output = error at input)

and 50 % PB means a gain of 2 (change in controller output = 2x error at input)

A low setting of the Proportional Band (large gain) results in a large change in the output for a given change in the error. If the Proportional Band is too low, the system can become unstable. In contrast, a large setting of the Proportional Band (low gain) results in a small output response to a large input error, and a less responsive or less sensitive controller. If the Proportional Band is too high, the control action may be too small when responding to system disturbances resulting in slow response and offsets of the resulting process variable from the set point.

A Proportional-only controller will not always settle at the set point, but may retain a steady-state offset. Offset can be reduced in Proportional-only control by reducing the Proportional Band setting. However, if the PB is set too small then hunting or oscillating will occur. The offset can be minimised by adding a bias to the set point (setting the set point above or below the required value to compensate for the offset) but this technique is only appropriate if the system characteristics are known and fixed. A better solution is to remove the offset by adding Integral action to the controller (P+I) as described below.

Integral term (I)

The contribution from the integral term is proportional to magnitude and duration of the error. The Integral term in a PID controller is the sum of the instantaneous error over time and gives the accumulated offset that should have been corrected

69


previously. The resulting controller output is the sum of the contribution from the Integral term and the contribution from the P term.

When the I term is correctly adjusted any residual offset in the process variable due to the P term will be gradually reduced by the Integral term until the offset is eliminated. If the time setting of the I term is too long then correction to any offset will be very slow.

However, since the integral term responds to accumulated errors from the past, it can cause the present value to overshoot the setpoint value or to make the process completely unstable if the time setting of the I term is too short. If this occurs the I term makes adjustments to the controller output faster than the process can respond. I.e. the I term winds up the controller output so that the process overshoots considerably, hence the term Integral Wind-up or Integral saturation.

Careful selection of the I term in combination with the P term will give efficient response to changes in the system.

Derivative term (D)

The derivative term slows the rate of change of the controller output. Derivative control is used to reduce the magnitude of the overshoot produced by the I term and improve the combined controller-process stability. However, the derivative term slows the response of the controller. Also, differentiation of a signal amplifies noise and thus this term in the controller is highly sensitive to noise in the error term, and can cause a process to become unstable if the noise and the derivative gain are sufficiently large.

Equipment set up



Connect the USB socket at the rear of the electrical enclosure a suitable PC on which the PCT50 software has been installed using the USB cable supplied. Check that the PC is switched on then run the PCT50 software and select Experiment Ex2: Outflow Control (PC Control).







70

Exercise G




Procedure - P only controller

Choose the PID box on the mimic diagram, set the Proportional Band P to 200 %, the Integral Time I to 0, the Derivation time D to 0 and the Cycle time to 10 seconds then set the Set Point to 75 mm (default at start up). Click Apply to enter the changes to the settings.



Set the pump speed to 60% on the mimic diagram, by typing in the value or by clicking the up arrow, so that water is pumped into the process vessel.

When the level rises above the set point the solenoid valve will start to open and close in on a 10 second cycle to allow water to drain via solenoid valve CV?. The final level will not correspond with the setpoint of 75 mm on the controller because of the offset inherent in a P only controller.

Increase the pump speed to 65 % on the mimic diagram. Observe that the level rises slightly, the solenoid valve opens for a longer part of the cycle due to the increased

71


error but a large offset remains. When the level has settled return the pump speed to 60% and allow the level to settle.


Adjust the P term to 100 % in the PID controller then click Apply. Observe that the open time of the solenoid valve increases and the offset reduces slightly. Increase then decrease the pump speed as before to disturb the process and observe the response, the stability of the level and any reduction in offset.








Results – P only controller


Choose Format \ Graph Data and Plot Level L1 and Set point on 1 axis, and Controller Output on the second axis.

Observe the changes in water level and the stability of the responses for each change in P setting with a disturbance or change in set point.

Choose a setting for P that gives stable control with a reasonably small offset that can be used in the next part of the exercise introducing Integral action.

Procedure - P+I Controller

Create a new results sheet for this run by selecting the icon in the tool bar of the software.

Choose the PID box on the mimic diagram, set the Proportional Band P to the value selected above, the Integral Time I to 0 seconds and the Derivation time D to 0 then set the Set Point to 75 mm. Click Apply.


Choose the Automatic mode of operation and allow the water level to stabilise.

Adjust the I term to 100 seconds in the PID controller then click Apply. Observe that the opening time of the solenoid valve gradually increases to reduce the offset. When the level is settled at the set point increase the pump speed to 65 % to disturb

72

Exercise G

the process. Observe that the level rises slightly, the opening time of the solenoid valve varies and the level gradually returns to the set point. When the level has settled return the pump speed to 60 % and allow the level to settle.


Adjust the I term to 50 seconds in the PID controller then click Apply. Observe that the level returns to the set point faster than before. When the level is settled at the set point increase then decrease the pump speed as before to disturb the process.


Continue reducing the Integral time setting and repeating the disturbance and set point changes until the level becomes unstable and is no longer maintained at the required set point.

Results – P+I controller


Choose Format \ Graph Data and Plot Level L1 and Set point on 1 axis, and Controller output on the second axis.

Observe the changes in water level and the stability of the responses for each change in I setting with a disturbance or change in set point.

Choose a setting for I that gives stable control and reduces any offset swiftly without causing the system to become unstable.

Since the effect of Integral Action is related to the Proportional Band setting (reduced PB gives greater effect for any setting of I) the above exercise should be repeated at a completely different value of PB to determine the necessary change in I for effective control of the level and to demonstrate the relationship between P and I.

If time permits repeat the exercise with increasing settings of cycle time until the effect of the cycle time is observed as instability in the process.

Different sizes of orifice fitted to the outlet valve CV2 and solenoid valve, E.g. 2 mm at CV2 and 4 mm at the solenoid valve (smaller outflow). The filling characteristics with different settings of CV1 could also be investigated e.g. partly closing valve CV1 to reduce the flow or increasing the flow by opening the valve.

Conclusion

A large proportional band setting will result in very stable control but will result in slow changes and large offsets in the process variable.

Reducing the proportional band will improve the speed of response and give smaller offsets but the process may become unstable if the proportional band in reduced too far.

Optimum results are obtained with the lowest proportional band setting that gives stable control but with the addition of Integral action to eliminate any offset.

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Reducing the Integral time setting increases the speed at which any offset is reduced but settings that are too short will result in instability because the process is unable to respond quickly enough to the changes from the controller. The effect is called integral saturation of integral wind up and must be avoided.

Increasing the PB reduces the effect of Integral Action so shorter setting of I is required to eliminate offset swiftly.


Objective

To control the level of water in the process vessel using a Commercial PID controller to automatically vary the speed of a water pump to vary the flow into the process vessel (inflow control).

To control the level of water in the process vessel using a Commercial PID controller to automatically vary the opening and closing of a solenoid valve in a time-proportioned cycle to vary the flow out from the process vessel (outflow control).

To demonstrate the operation of a commercial controller and the use of menus to configure the instrument.

To configure the controller to indicate process variables in engineering units (default setting 0 – 100% input).

Method

The PCT54 PID Controller, or equivalent PID controller, can be used to control the process by connecting the signals from the controller to the electrical sockets on the front of the electrical console.

Equipment Required

PCT50 Level Process

PCT54 PID Controller or alternative PID controller with appropriate input and output signals:

For Inflow control: 0 – 5 Volt input and 0 – 5 volt output

For Outflow control: 0 – 5 Volt input and relay output

PC with PCT50 software loaded (only required for logging the response of the process).

Theory

Any of the previous exercises performed using a PC to control the process can be repeated using a commercial PID controller, typically PCT54. A PC can be used to log the response of the process by selecting the appropriate exercise when loading the software:

Choose Ex 3: Inflow Control (External Control) when controlling the water pump in a PID loop

Choose Ex 4: Outflow Control (External Control) when controlling the solenoid valve in a PID loop

Step changes can be applied to the process using the front panel controls on PCT54.

Equipment set up / Procedure

Refer to the instruction manual supplied with PCT54 for details about connections to the electrical console on the process and operating the PID controller itself.

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Objective

To control the level of water in the process vessel using a Commercial Programmable Logic controller to automatically vary the speed of a water pump to vary the flow into the process vessel (inflow control).

To control the level of water in the process vessel using a Commercial Programmable Logic Controller to automatically vary the opening and closing of a solenoid valve in a time-proportioned cycle to vary the flow out from the process vessel (outflow control).

To demonstrate the operation of a commercial Programmable Logic Controller with touch panel interface.

To demonstrate how the configuration of a PLC can be changed using downloadable software.

Method

The PCT55 Programmable Logic Controller, or equivalent PLC, can be used to control the process by connecting the signals from the controller to the electrical sockets on the front of the electrical console.

Equipment Required

PCT50 Level Process

PCT55 Programmable Logic Controller or alternative PLC with appropriate input and output signals:

For Inflow control: 0 – 5 Volt input and 0 – 5 volt output

For Outflow control: 0 – 5 Volt input and relay output

PC with PCT50 software loaded (only required for logging the response of the process).

Theory

Any of the previous exercises performed using a PC to control the process can be repeated using a commercial Programmable Logic Controller, typically PCT55. A PC can be used to log the response of the process by selecting the appropriate exercise when loading the software:

Choose Ex 3: Inflow Control (External Control) when controlling the water pump in a PID loop

Choose Ex 4: Outflow Control (External Control) when controlling the solenoid valve in a PID loop

Step changes can be applied to the process using the front panel controls on PCT55.

Equipment set up / Procedure

Refer to the instruction manual supplied with PCT55 for details about connections to the electrical console on the process and operating the Programmable Logic Controller itself.

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Contact Details for Further Information

Main Office: Armfield Limited

Bridge House West Street Ringwood Hampshire England BH24 1DY

Tel: +44 (0)1425 478781 Fax: +44 (0)1425 470916 Email: [email protected] [email protected] Web: http://www.armfield.co.uk

US Office: Armfield Inc.

436 West Commodore Blvd (#2) Jackson, NJ 08527

Tel: (732) 928 3332 Fax: (732) 928 3542 Email: [email protected]

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Documents

Expt. 1 - Level Control