UCP- Process Control Manual-1

8/11/2019 UCP- Process Control Manual-1

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Unit Ref.: UCP Date: October 2010 Pg: 1 / 115

7 PRACTICES MANUAL ...........................................................................................................................................................2

7.1 GENERAL DESCRIPTION OF THE SYSTEM ...........................................................................................................27.1.1 General description of the system ............................................................................................................................2

7.1.2 Operation of the subsystems......................................................................................................................................5

7.2 THEORETICAL BASIS.....................................................................................................................................................77.2.1 Modeling, simulation and control process .............................................................................................................7

7.2.2 Dynamics and control ................................................................................................................................................8

7.2.3 Dynamic simulation of the control systems .........................................................................................................12

7.2.4 Operation and calibration of the process equipment and control elements..................................................15

7.3 LABORATORY PRACTICES ........................................................................................................................................187.3.1 Practice 1: Flow control loops (manual) ............................................................................................................18

7.3.2 Practice 2: Flow control loops (on/off) ................................................................................................................ 20

7.3.3 Practice 3: Flow control loops (proportional) ...................................................................................................22

7.3.4 Practice 4: Flow control loops (Proportional + Integral) ...............................................................................24

7.3.5 Practice 5: Flow control loops (Proportional + Derivative) ........................................................................... 267.3.6 Practice 6: Flow control loops (Proportional + Derivative + Integral) .......................................................28

7.3.7 Practice 7: Adjustment of the flow controller constants (Ziegler-Nichols) ................................................... 30

7.3.8 Practice 8: Adjustment of the flow controller constants (Reaction Curves) ................................................. 32

7.3.9 Practice 9: Level control loops (manual) ............................................................................................................35

7.3.10 Practice 10: Level control loops (on/off) .............................................................................................................37

7.3.11 Practice 11: Level control loops (proportional) ................................................................................................38

7.3.12 Practice 12: Level control loops (Proportional + Integral) ............................................................................41

7.3.13 Practice 13: Level control loops (proportional + derivative) .........................................................................42

7.3.14 Practice 14: Level control loops (Proportional + Derivative + Integral) .................................................... 43

7.3.15 Practice 15: Adjustment of the constants of a flow controller (Ziegler-Nichols) ......................................... 45

7.3.16 Practice 16: Adjustment of the constant of a flow controller (Reaction Curves) ......................................... 47

7.3.17 Practice 17: Temperature control loops (manual) ............................................................................................50

7.3.18 Practice 18: Temperature control loops (on/off) ................................................................................................527.3.19 Practice 19: Temperature control loops (proportional) ...................................................................................53

7.3.20 Practice 20: Temperature control loops (Proportional + Integral) ............................................................... 55

7.3.21 Practice 21: Temperature control loops (Proportional + Derivative) ..........................................................56

7.3.22 Practice 22: Temperature control loops (Proportional + Derivative + Integral). ...................................... 58

7.3.23 Practice 23: Adjustment of the constant of a controller of temperature (Ziegler-Nichols) ........................ 59

7.3.24 Practice 24: Adjustment of the constants of a temperature controller (Reaction Curves) ......................... 61

7.3.25 Practice 25: pH control loops (manual) ..............................................................................................................64

7.3.26 Practice 26: pH control loops (on/off) .................................................................................................................66

7.3.27 Practice 27: pH control loops (proportional) .....................................................................................................67

7.3.28 Practice 28: pH control loops (Proportional + Integral) ................................................................................69

7.3.29 Practice 29: pH control loops (Proportional + Derivative) ............................................................................70

7.3.30 Practice 30: pH control loops (Proportional + Derivative + Integral) ........................................................72

7.3.31 Practice 31: Adjustment of the constant of a pH controller (Ziegler-Nichols) ............................................. 73

7.3.32 Practice 32: Adjustment of the constant of a pH controller (Reaction Curves) ...........................................75

7.3.33 Practice 33: Conductivity control loops (manual) .............................................................................................79

7.3.34 Practice 34: Conductivity control loops (on/off) ................................................................................................81

7.3.35 Practice 35: Conductivity control loops (proportional) ...................................................................................82

7.3.36 Practice 36: Conductivity control loops (Proportional + Integral) ............................................................... 84

7.3.37 Practice 37: Conductivity control loops (Proportional + Derivative)...........................................................86

7.3.38 Practice 38: Conductivity control loops (Proportional + Derivative + Integral) .......................................88

7.3.39 Practice 39: TDS control loops (manual) ...........................................................................................................90

7.3.40 Practice 40: TDS control loops (on/off) ...............................................................................................................92

7.3.41 Practice 41: TDS control loops (proportional) ..................................................................................................93

7.3.42 Practice 42: TDS control loops (Proportional + Integral) .............................................................................. 95

7.3.43 Practice 43: TDS control loops (Proportional + Derivative)..........................................................................96

7.3.44 Practice 44: TDS control loops (Proportional + Derivative + Integral) ...................................................... 98

7.4 ANNEX ................................................................................................................................................................................1007.4.1 Annex 1: Flow sensor calibration .......................................................................................................................100


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7.4.2 Annex 2: Temperature sensor calibration .........................................................................................................104

7.4.3 Annex 3: Level sensor calibration ......................................................................................................................108

7.4.4 Annex 4: pH sensor calibration ...........................................................................................................................112

7 PRACTICES MANUAL

7.1 GENERAL DESCRIPTION OF THE SYSTEM

In this section, the process unit and the control loops used in measuring

experiments for level, flow, temperature and pH regulation are described, togetherwith the different equipment and necessary instruments for the physical simulation of

the corresponding dynamic systems.

7.1.1 General description of the system

This unit consists on a hydraulic circuit, with an bottom tank (1) and a

superior process tank (2), both dual ones, two pumps of centrifugal circulation (3),two flowmeters with a manual control valve (4), three on/off solenoid valves (5) and

a motorized proportional valve (infinitely variable) (6). Of course, together with the

tubes, the union elbows, connections, feedthroungh, main valve and the appropriate

drainage for the circuit operation. All the above-mentioned is set on a designed

support structure so, that it is placed on a work table (7).

As additional fixed elements, there is also a turbine flow sensor that is

installed in one of the upward lines of flow (8), and a temperature sensor located in a

lateral bottom of the process tank (9) together with a serpentine with electric heating

(11).

The interchangeable additional elements are an agitator (10), the

immersion level sensor should be located in the process tank (12) and the pH sensor(solenoid), can be in the process tank or also in the second tank (13), to study the


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effect of the time out.

Figure 1.1.1 Main diagram of the equipment

The UCP elements indicated in the diagram are:

1. A main tank and collectorwith an orifice in the central dividing

wall (2 x 25 dm), and drainage in both compartments (made in

methacrylate).

2. A dual process tank (2 x 10 dm), interconnected through an

orifice and a ball valve and an overflow in the dividing wall

(methacrylate); a graduate scale and a threaded drain of adjustable

level with bypass (metallic).

3. Two centrifugal pumps.


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4. Two variable area flowmeters (0.2-2 l/min, and 0.2-10 l/min),

and with a manual valve.

5. Line of on/off regulation valves (solenoid). Usually one is

opened (AVS-1), and the other two are usually closed with

different Cv (AVS-2 and AVS-3); and manual drainage valves of

the superior tank.

6. A motorized control valve (AVP-1; piston type) with rotationindicator.

7. Structures, panels, pipes and connections made in stainless

steel and methacrylate.

8. A flow sensor, fixed, turbine type.

9. A temperature sensor.

10.A helix agitator.

11.An electric resistor(0.5 KW), fixed.

12.Level sensor0-300 mm (of capacitive immersion, 4-20 MA), can

be dismantled.

13.PH sensor(glass electrode, ddp(V)), collapsible (tank).

14. On/off level sensor. This sensor determines the performance of

the immersion resistor.

15.Control loops : interface, controller, monitor and keyboard (PC

and cards), electric connections.


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7.1.2 Operation of the subsystems

For the level, flow and temperature control test, the liquid (water) is

impelled from the tank by the pump, located to the left of the front of the equipment,

going through the flowmeter, the solenoid valve (usually open), the motorized valve,

the turbine (flow sensor) and the process tank. It is possible to use the second pump

in the level tests, as it will be indicated.

The pH control test of requires a second parallel line of flow (right),

provided only with pump and a flowmeter. The compartments of the inferior tank

should be loaded with diluted solutions of an acid and a base, respectively.

The process tank is divided in two halves, with an orifice between them

that allows their communication or isolation.

The right compartment has an overflow of variable level (that it prevents

the complete overflow of the tank, and it allows to modify its effective liquid

volume), two drains with solenoid valves with different Cv (normally closed), and a

third one with a normal drainage valve.

The left compartmentis only connected to a drainage valve.

The level control tests require all the elements of the circuit and of the

tank, besides the sensor located in it. In some experiments, it is required the second

pump placed to the right-hand side of the equipment.

The TEMPERATURE CONTROL tests, in these cases, as we will see later

on, can be carried out with experiments in closed circuit or in open circuit. In the

close circuit case, fill the superior tank with the right pump 1 (AB-1) and carry outthe experiment. In open circuit, keep a constant water flow using the pump 1, this


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way, a small water flow is adjusted and the superior overflow is used as a drainage

system. In this case, it is necessary to use the agitator to guarantee a goodtemperature uniformity.


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7.2 THEORETICAL BASIS

Modeling, simulation, design and optimization require a series of processes that

will be explained in the following paragraphs. The understanding of these theoretical

concepts will help the student to follow the suitable instructions for the practices

procedure.

7.2.1 Modeling, simulation and control process

The development, and good operation of the industrial plants, requires the

right selection of the equipment and process parameters. This election is

supplemented with the instrumentation and the control, as well as with the dexterity

to adjust and to manipulate them correctly.

For it, the industrial engineering processes use different tools and technical

aspects based on:

1.- The modeling of the systems.

2.- The simulation of the stationary and dynamics response.

The control process is the objective of this modeling and simulation that

guarantees that the dynamic behavior is efficient and precise.

The design and the process optimization is based on calculations in

stationary state to specify, in a first approach, the operation conditions of the

equipment in the process plants. These design calculations in the stationary state

don't say anything about the dynamic response of the system, they only tell us wherewe begin and where we end up, but anything about the process behavior. This type of


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information is the one that informs about the study of the dynamics procedure.

The dynamics and the control study the non-stationary behavior of the

processes and the design of the control systems in function of the interferences. With

this, the design of the process systems is finished, that include the own process and

its control loops.

In a first approach, the process control unit has been designed for the study

of the dynamic behavior of the different control loops. The modeling and stationarysimulation of the processes is not deal with in this equipment.

7.2.2 Dynamics and control

Once the freedom degrees of the process are selected and the design

variables good values are calculated, these values should remain unaffected with the

deviations that take place during the real operation of the systems during thestationary state.

The factors that cause such deviations are denominated interferences.

These can be internal or external to the process, random or programmed. The random

interference type are those that produce deviations from the programmed regime due

to some process input variables fluctuations. In the programmed interferences, it is

necessary to consider the transitory periods of starting-up, stop or changes in the

stationary state of the process.

The number of variables that you can fix coincides with the freedom

degrees (although they don't have to coincide with the free variables that were chosen

during the design phase). The variables that are more easily controlled are: level,

flow, pressure, temperature and composition. We will denominate them controlledvariables and their values we will be called set point. In a controller, the input


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magnitude of the set point is fixed in a constant value. In a programmer controller

the inlet magnitude is a value that varies in function of the time, according to theprogrammed law.

The variable used to regulate the set values is denominated manipulated

variable. The function of the regulating device of the system is to activate

automatically the control elements that allow to modify the value of the manipulated

variables, so that the controlled variables are the next to the set point.

7.2.2.1Dynamics in open loop

The systems dynamics studies the non-stationary situations of the process.

This study is always previous to the design of the control that avoids the non-

reversibility of some deviations of the stationary state. In the process, the

manipulated variable acts changing the value of the controlled variables so, in the

simplest cases, it can be predicted using of a mathematical model. So, it is

fundamental to have an exhaustive knowledge of the process to will be studied.

The dynamic simulation of a process consists on:

1. Developing the material and energy balance equations, the

balances, the physical laws or other independent sources of

information.

2. Selecting the values of the necessary physical-chemical

magnitudes and the values of the preset points.

3. Obtaining the differential expressions of the variables determining

their temporary evolution for an interference in the stationary

regime starting from a defined initial state.


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As a particular case, the design equations in the stationary state can be

obtained (null accumulation) and, starting from them, calculate the values of thedesign and state variables in this regime.

The system dynamic in open loop represents the process behavior in the

absence of controllers. The velocity and the tolerance of the response of the process

in function of the interferences can be studied, this is the necessity of the control

system.

7.2.2.2Feedback Control

The feedback control system consists, essentially, on measuring the

controlled variable, to compare its value with the one wanted (set point) and,

according to the error, to act on the control element of the manipulated variable. This

is a feedback system, in which the corrector action persists while the error is not null.

The corresponding signals spread in the circuit, making comparison cycles, of

calculation and of correction (in closed loop) with a certain time of response.

If a closed control is turned into manual operation, the operator can govern

directly the control valve and the value of the regulated variable obtained from the

transmitter can be observed in the controller. In this case, we are operating in open

loop, since the output signal of the controller is off.

The main elements of a control system in closed loop are:

Sensor: Device that is able to measure the value of the controlled

variable (it is also denominated primary measure element).

Transducer: Device that transforms the measures in normalized

equivalent signals, pneumatic or electric, according to the distance

between the process and control rooms, which are sent to the


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comparator (also denominated signal conditioner element).

Controller: Device that receives the error from the comparator,

interprets it, and it acts on the final element of control. There are

three types of corrective actions:

- Proportional: the signal sent to the final element is

proportional to the error. If the proportional constant is very

big the control behaves as an on/off control.

- Integral: the corrective signal sent is proportional to the

error accumulated with the time (integral error).

- Derivative: the signal is proportional to the velocity of

variation of the error.

These last two actions are usually combined with the primary signal

(proportional) to improve the control quality and even, they usually

combine the three in complex processes. These combined processes

are known as P.I.D. control (proportional-integral-derivative).

Final element of control (actuator): Device that, according to the

signal of the controller, acts on the manipulated variable regulatingthe inlet material or energy flow to the process (valve, volumetric

pump, compressor, rheostat, etc.).

The control systems are effective with small interferences. In the case of

big interferences, the final element can end up being totally open or totally closed,

and, above a certain value, the control would not act appropriately (saturated). In

these cases, to correct it, you should act manually on the parameters of the system.


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The sensors, the transmitters and the final control elements are inserted in

the process in a logical way. The control devices are usually located in a controlroom, and they are in normalized panels with graphic instruments for the variables

and a diagram of flow (in a simple pilot plant it can be a simple PC computer with a

monitor and a printer).

In the control diagrams, the variables are designated with the letters F

(flow), L (level), P (pressure), T (temperature), followed by the indicative letters of

the service and of the functions of the instruments: T (transmitter), C (controller), I

(indicator), R (register), A (alarms).

7.2.2.3Forward control

The forward control makes the corrective action in the moment in which

the interference is detected, instead of waiting to its spread through the process, as it

happened in the feedback process. The action is independent of the value of the

controlled variable, making it depend on another, according to a calibrated preset.

This system is used with simple processes that don't require a great accuracy, or in

those processes in which the closed loop doesn't give good results, for example, in

processes in which that the measure and the corrective action take place in a great

period of time.

7.2.3 Dynamic simulation of the control systems

The dynamic simulation of the control systems allows studying the

response of the system facing up diverse interferences, such as changes in the input

variables, in the set point, or in the controller adjustments. The system consists of a

block that represents the process, and of another that corresponds the controller

connected to the first one forming a feedback loop.


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Figure 2.3.1

The process consists basically of an input (q), a manipulated variable, alsocalled feedback (m) and an output (c) (controlled variable). The controlled variable is

subtracted to the set point (r) to obtain the error (e) (input to the controller). The

controller calculates an output signal by means of an appropriate control algorithm.

The characteristic response of a controller on/off comes determined by its hysteresis,

also called dead band. In a PID controller (proportional-integral-derivative) the signal

comes determined by the gain (Kc) or its percentage inverse (BP=100/Kc,

proportional band) and the integral (I) and derivative (D) times.

[1][ ]

++=

=

=

)(

,

t

eDteeKcm

cre

mqKpc

In open loop, the state variables (outputs) are calculated starting from the

initial values and from the process pattern. It is defined in function of differential

equations for the transitory regime, or by state equations for the stationary regime

(invariable with the time).

These last ones are the techniques employed during the design phase of the

experiment and they allow calculating the values of the necessary input variables to

reach the stationary regime. The first ones allow studying the transitory periodsduring the setting phases in setting-in, stops or changes in the stationary regime of


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the process.

During the real operation of the process, the variables are modified with

the time as a consequence of diverse interferences in the input variables or in the set

point (programmed). In these cases, the process can only work correctly using a

closed loop control.

The dynamic behavior of the system is calculated by an iterative algorithm

at discreet intervals of time (Eulers method). The first phase of the program is thedata input: fixed variables, initial values of the manipulated variables, set point of the

controlled variables, adjustment of the controller (hysteresis, parameters PID) and

interference variables.

Qt RT H,KC, I,D

m Ct eI mt

Figure 2.3.2

The mathematical pattern of the process calculates the outlet (Ct) in

function of the inlets (q t, mt). This value is compared with the set point (rt) and the

resulting error (et) is calculated (the accumulated error (integral) or its derivative with

the time, the trapezoidal rules and finite increments are used in this case). Finally,

with these values, the controller output (st) and the manipulated variable (mt) are

calculated making use of the adjustments of the control actions (H, Kc, I, D) and of

the calibration of the actuator, respectively.

PROCESO COMPARACIN CONTROL


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The relationship between these last two variables can be settled down on

the base of a function (lineal) between the width of the actuator (m) and the signal of

the controller (s).

Also, automatic graph representation commands of response can be

included (controlled and manipulated variables), time outs in the process (always

superior or similar to the time of sampling, tt) and sampling time of the controller

similar to those of the process (analogical control) or superiors (digital control).

In case a graphic register is used, this should be connected to the electric

signs of the sensor/conditioner (controller input) and to its output (actuator input) to

obtain a register of the response of the process.

7.2.4 Operation and calibration of the process equipment and control elements

The verification and calibration of all the sensors you can find in the

equipment can be carried out in two different ways. The Saced System that is

supplied with the equipment, has a calibration window specially designed for such a

purpose (see Calibration Manual).

However, and due to the importance that the calibration the sensors has inthe control process, the system offers two text windows in which the gain and the

zero can be introduced of each one of the transducers that the equipment has: flow,

level, temperature and pH.


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Figure 2.4.1

In the same way, the verification of the operation of the pumps, solenoid

valves, agitator, etc. it can be carried out from the calibration window in the Saced

System. This process of operation test should be carried out with extreme care andunder the introduction of the professor's PASSWORD. This Password has been

indicated in the software manual. Once the professor's password is introduced, the

program allows you to select the channels included in the equipment for the digital

outputs as well as for the analogical ones. Verify each one of the elements assembled

in the equipment and carry out a good calibration of the sensors. We should point out

that, when storing the calibration values in the file UCP.EDB, these will be recoveredevery time that you enter in the program, independently from the values introduced

by the students in the different practice sessions.

When selecting the different outputs that has the acquisition card

(analogical and digital outputs, and analogical and digital inputs), you can verify the

operation of all devices of the equipment. As an example we have the following

table:

Name of the sensor for

calibration.


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Analogical port (Inputs) Digital Port (Outputs)

Channel Action/Sensor Channel Action/SensorChannel 0 pH sensor Channel 0 AVS-1Channel 1 Level sensor Channel 1 AVS-2Channel 2 Flow sensor Channel 2 AVS-3Channel 3 Temperature sensor Channel 3 Resistor On/Off Channel 4 Alarm of Level Channel 4 Agitator On/Off Channel 5 Proportional Valve Channel 5 Pump 1 On /Off

Channel 6 Pump 2 On/Off

Table 2.4.1

To outputs and inputs it is necessary to add the analogical output that

corresponds to the motorized valve.


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7.3 LABORATORY PRACTICES

It is very important for the formation of the student, the calibration of the

sensors, hence is recommendable that the students see the Annex 1 to Annex 4.

7.3.1 Practice 1: Flow control loops (manual)

7.3.1.1Objectives

The objective of this experiment is to control the flow that circulates

through a conduction of water by a manual procedure. We assume that the manual

control works as:

Manual regulation of the adjustable valve placed under the area

flowmeter.

Manual control of the elements used in the equipment as the

motorized valve, solenoid valves, etc.

7.3.1.2Required material

To make this practice, the following elements are necessary:

UCP-F

Water

SACED Software.

7.3.1.3Experimental procedure

1.- Connect the interface of the equipment and execute the program


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SACED UCP-F.

2.- Inside the program, select the option Configuration and connect pump 1

(AB-1) (see Software Manual for a more detailed operation).

3.- In the manual regulation (no controller) the flow can be regulated by the

manual adjustable valve VR1, placed in the inferior part of the flowmeter. Vary its

position and observe the adjustment of the flow in function of its position.

4.- Select the option Manual Control of the software supplied with the

equipment.

5.- Connect pump 1 (AB-1) and vary the position of the motorized valve by

the Slip bar or the command associated to this action. Check a fixed position, the

flow is regulated.

6.- Vary the position of the valve and repeat the values to observe the

reproduction of the flow control.

7.- Use the controls prepared in the software for controlling the solenoid

valves AVS-1 and the on/off button of the pump. Observe how an on and off button

also produces a flow control of the liquid.


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7.3.2 Practice 2: Flow control loops (on/off)

7.3.2.1Objectives

Figure 3.6.1

The objective of this practice is to carry out a closed loop control by an

on/off controller. For it, the student will select the value wanted for the flow and the

controller will adjust this control by the closing and opening of the solenoid valve

AVS-1.

ControlOn/off


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1.- Connect the interface of the equipment and the control software.

2.- Select the control option on/off.

3.- By a double click on the on/off control, select the flow wanted. By

defect there is certain flow, a tolerance and a performance time. It allows the students

to play with these parameters and they can see the influences of each one of them.

4.- It calculates the inertia of the system before an on/off response and

determines the limit time for an exact control.

Figure 3.6.2

7.3.2.3Conclusions

From the results obtained by the on/off control on the flow variable we can

affirm that this controller is not the most appropriate due to the quick variation of this

magnitude before a small interference. Only with small values in the performance

times and the tolerances we can obtain a flow control next to the set value, but in any

case we will have a stable value of the flow.

Results of the on/offaction.


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7.3.3 Practice 3: Flow control loops (proportional)

7.3.3.1Objectives

This configuration allows studying the system dynamics and the response

to the control actions in closed loop. The object of the experiment is to regulate the

set point (flow) by the employment of controllers that operate automatically on the

final element of the loop (control valves).

You can control the flow in the tank by a sensor and a controller

configured for proportional outputs to the actuator without the typical oscillations of

the on/off control. The response of the control loop can be studied compared to the

interferences in the variables of the process (flow) or variations in the set point (the

flow is changed fixing different set points).

Modifying the set point in a remote way, the flow changes can beobserved, oscillating around the new value. We can have the case that the set point is

not reached if the range of the actuator (manipulated variable) is not enough as to

control the interferences or the changes in the set point, so it will be stabilized only

until the maximum that the available water allows. In our case, the manipulated

variable is the water flow that circulates through a motorized valve, managed

automatically from the controller (0-10V signal), by means of superimposed actionsof proportional, integral and derivative type.


To make the practice the following material is needed:

UCP-F

Control and Acquisition Software.


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Water.


1.- Connect the Interface and execute the control software (For more

details about the control software, see the Software Manual M4)

2.- Select the Option Control PID on the capture screen.

Figure 3.7.1

3.- Select a set point, PID controller and a proportional constant (For more

information about the meaning of each parameter, see the Software Manual M4).


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4.- Indicate a value of 0 for the integral and derivative performance. In this

experiment we want to observe the effects of a proportional action.

Figure 3.7.2

6.- Activate the PID controller and start and go out and save the values.

The student will observe that the motorized valve begins to act.

7.- Connect pump 1 (AB-1).

8.- The controller will modify the position of the AVP (Proportional Valve)

to adjust the flow to the set value.

7.3.4 Practice 4: Flow control loops (Proportional + Integral)

7.3.4.1Objectives

This practice supplements the previous one. The objective is to observe the

effect that an integral performance superimposed to a proportional action, in an

actuator, has.


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The following material is necessary for the practice development:

UCP-F


Water.



details about the control software, see the Software Manual, M4)

2.- Select the Option Control PID on the capture screen. (For more

information about the meaning of each parameter, see the Software manual M4).

3.- Select a set point, PID controller and a proportional constant and an

integral value. The value for the integral constant should be big so that the error

accumulation is carried out smoothly and it doesnt generate an on/off performance

in the actuator.

5.- Indicate a value of 0 for the derivative performance. In this experiment,

we want to observe the effects of a proportional action plus an integral action.

6.- Activate the PID controller, go out, and save the values. The student

will observe that the motorized valve begins to work.


8.- The controller will modify the position of the AVP-1 (ProportionalValve) to adjust the flow to the set value.


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7.3.5 Practice 5: Flow control loops (Proportional + Derivative)

7.3.5.1Objectives


effect that a derivative performance superimposed to a proportional action, in an

actuator, has.


It is required for the realization of the practice:

UCP-F


Water.



details about the software control, see the Software Manual M4)

2.- Select the option Control PID on the capture screen. (For more

information about the meaning of each parameter, see the Software Manual, M4).

3.- Select a set point, PID controller and a proportional and derivative

constant. The value for the derivative constant should be small so that the

performance is small and it does not generate an on/off performance in the actuator.

4.- Indicate a value of 0 for the integral performance. In this experiment,we want to observe the effects of a proportional action plus a derivative action.


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will observe that the motorized valve begins to act.


7.- The controller will modify the position of the AVP-1 (Proportional

Valve) to adjust the flow to the set value.


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PRACTICES MANUAL


7.3.6 Practice 6: Flow control loops (Proportional + Derivative + Integral)

7.3.6.1Objectives

This practice supplements to the previous one. The objective is to observe

the effect that has a derivative performance superimposed to an integral performance

and a proportional action in an actuator.


The following material is required for the realization of the practice:

UCP-F


Water.



details about the control software, see the Software Manual M4)



3.- Select a set point, PID controller and a proportional constant, derivative

and integral. The value for the derivative constant should be small and the integral

constant should be big so that the performance is small and it does not generate an

on/off performance in the actuator.

4.- Activate the PID controller, go about, and save the values. The student


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PRACTICES MANUAL





Valve) to adjust the flow to the set value.


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7.3.7 Practice 7: Adjustment of the flow controller constants (Ziegler-Nichols)

7.3.7.1Objectives

To follow the optimization process of a controller of three terms (PID), for

a given process.

When you optimize the PID control values you will have to take into

account several initial considerations:

1.-The process has slow or quick response.

2.-The process reaction goes very retarded of the action.

3.-The sensors and controllers response is immediate or they need a

time out to reach the balance.

The objective of this practice is to get familiarized with the most usual

methods of optimizing the variables of a PID controller starting from the

characterization of the process.

For such a purpose the following methods will be used:

- Ziegler-Nichols (or closed loop).

- Reaction curve (or open loop).


The data to be analyzed will be obtained configuring the controller only

with the Proportional Band or the proportional action. The Integral and Derivative

Actions should be at zero.


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The objective of the experiment is to maintain the conduction of the system

with a flow of 1 l/m using a P controller for the control of the motorized valve.

With the motorized valve at the 50% of its complete way, regulate the

needle valve manually VR-1, until the flow of the system is at 1 l/min.

7.3.7.2.1Method of the minimum period (Ziegler-Nichols)

Pass now to an automatic control and observe how the flow stays constant

at the 50% of the process variable. Change the process variables for a partial opening

of the needle valve, VR-1. As the process will become stable, increase the value of the

proportional constant and close the needle valve partially, VR-1, observing the

behavior of the process.

Continue increasing the value of the proportional constant and applying

every time a step interference (closing or opening VR-1), until the variable of theprocess oscillates continually. Write down the value of the proportional constant

(Limit Proportional Band, L.P.B.) when this happens, to measure the oscillation time

of the process (O.T.).

The optimum values, depending on the control type that we are going to

make on our process are:

Type of Control B.P. I.T. D.T.P 2 (L.P.B.) -- --

P+I 2.2 (L.P.B.) O.T. / 1.2 --P+I+D 1.7 (L.P.B.) O.T. / 2.0 O.T. / 8.0

Table 3.11.1

A variant of the gain limit method is the method of the minimum overflow

of the set point. Once the self-maintained oscillation of Oscillation Time O.T. for a


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Limit Proportional Band L.P.B. is obtained, the control action values are the

following ones:

B.P (%) = 1.25 L.P.B.

I.T. (MIN/REP) = 0.6 O.T.

D.T. (MIN) = 0.19 O.T.

7.3.8 Practice 8: Adjustment of the flow controller constants (Reaction Curves)

In this method of open loop, the general procedure consists on opening the

regulation closed loop before the valve, that is to say, the valve must be operated

directly with the manual controller and to create a small and quick change in step in

the input process. From the registration of the signal and from their graphicrepresentation the control values of the PID will be obtained. The graphic

representation of the controlled variable versus the time is a sigmoid. In the inflection

point of the sigmoid a tangent straight line is traced and the values ofRand Lcan be

measured.Ris the slope of the tangent in the inflection point of the curve and Lis the

retard time of the process. That is, the time (in minutes) between the instant of the

change in the step and the point in which the tangent straight cuts the sigmoid in theinflexion point crosses with the initial value of the controlled variable. DP is the

percentage (%) of position variation of the control valve that introduces the step in

the process, see figure 3.12.1.


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Figure 3.12.1 representation of the reaction curve.

From this representation we can obtain the slope of the sigmoid, R, and the time of retard, L.

The optimum values, depending on the control type that we will make on

our process, are

Type of Control B.P. I.T. D.T.P P -- --

P+I 110RL/DP L/0.3 --P+I+D 83RL/DP L/0.5 0.5L

Table 3.12.1.

Optimum values that are going to be used in function of the type of control used.L: Time of retard,R.: Slopeof the sigmoid in the inflection point.D: Derivative,P: Proportional.

Compare the values obtained by the two methods.

7.3.8.1Other experiments to carry out

7.3.8.1.1Evaluation of the PID controller calibration

Once the PID values are entered to the controller, adjust in manual way,

with the motorized valve positioned at 50% of their way, the needle valve VR-1, until

the flow of the system is at the 50% of the maximum flow provided by the pump. Go

to automatic control of the process and apply an interference, as the solenoid valve

AVS-1. Observe the temporary behavior of the process. Repeat the process for the


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PID control values obtained by the other method.

7.3.8.2Conclusions

- There are techniques to obtain the different values of the variables of a

PID controller and they should be determined for any particular process.

- The values obtained by any of the different methods differ and they

should be treated as start values for the good regulation of the process; they should beslightly modified by the operator, carrying out, this way, their fine adjustment until

you obtain the good values.

- There are methods of automatic adjustment, in which the instrument has

an algorithm of self adjustment of the control actions that allows it to tune in with a

wide range of industrial processes. The application of a test signal to the process and

the analysis of the obtained response and its mathematical modeling leads to thecontroller analytic design (Nishikawa, Sannomiya, Ohta and Tanaka, 1984). Or you

can use an iterative process to the method of the gain limit (Chindambara, 1970, and

Kraus and Myron, 1984):

The error signal obtained is analyzed, in the case of changes in the set point

or in the load of the process, and iterating the new PID values can be determined.

Controller P: B.P.N+1 = B.P.N / (0.5 + 2.27 R)

Controller PI: same B.P.n+1 and I.T. = P / (1.2 *sqr(1+R2)) min/rep

Controller PID: same B.P.and I.T. = P / (2 *sqr(1+R2))

D.T. = P / (8 *SQR(1+R2)).

being R = 1/(2*3.14) * Ln(a/b) and P the period of the oscillation muffled, in


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minutes. Where aand b are the widths of the first two oscillations introduced the

after the interference.

If, when applying these methods, the process enters into oscillation, the

interference can invalidate the application, in case the process does not allow it.

7.3.9 Practice 9: Level control loops (manual)

7.3.9.1Objectives

The objective of this experiment is to control the level in a tank of water by

a manual procedure. We understand that the manual control works as:


flowmeter.

Manual control of the elements prepared in the equipment;

motorized valve, solenoid valves, etc.


The following elements are required for the realization of this practice:

UCP-L

Water

SACED Software.


1.- Connect the interface of the equipment and execute the programSACED UCP-L.


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PRACTICES MANUAL


2.- Inside the program, select the option Manual.

3.- In the manual regulation (no controller) the flow can be regulated by the

manual adjustable valve VR1, placed in the inferior part of the flowmeter. This

regulation, together with the opening of the tank manual valves, to notice a level of

water. Change its position and observe the adjustment of the level in function of their

position.

4.- Select the option Manual Control of the software supplied with theequipment.

5.- Connect pump 1 and vary the position of the motorized valve in the slip

bar or the command associated to this action. Open AVS-1 or AVS-2 and check how,

for a given position, the level of water in the tank fixes.

6.- Change the position of the valve and repeat the values to observe thereproducibility of the level control.

7.- Use the controls prepared in the software for the control of the solenoid

valves AVS-1, AVS-2 and AVS-3 and the switch on/off button of the pump. Observe

how an on/off of it also produces a level control of the liquid.


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PRACTICES MANUAL


7.3.10Practice 10: Level control loops (on/off)

7.3.10.1 Objectives

Figure 3.14.1


on/off controller. For it, the student will select a value wanted for the level and the

controller will adjust this control by the closing and opening of the solenoid valve

AVS-1, AVS-2, AVS-3 and the activation of pump 2.

7.3.10.2 Experimental procedure


2.- Select the control on/off option.

On/Offcontroller


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3.- Make a double click on the on/off control, select the wanted flow. By

defect, there is a certain flow, tolerance and performance time. It allows the studentsto play with these parameters and see the influences of each one.

4.- The level control can be carried out by the activation of a single

actuator, or of several ones, to which different tolerances are allowed. These

controllers work as security system measures when the controlled variable exceeds in

a tolerance the set value. To activate or to disable each one of these controllers you

have to make a double click on each one of them and press the button PAUSE.

5.-Calculate the inertia of the system before an on/off response and

determine the limit time for an exact control.

7.3.10.3 Conclusions

From the results obtained by the on/off control on the variable level, wecan affirm that this controller has an acceptable behavior due to the fact that the

variation of this magnitude under a small interference is slow. If we also take small

values in the performance times and in the tolerances, we can obtain a level control

next to the set value.

7.3.11Practice 11: Level control loops (proportional)

7.3.11.1 Objectives

This configuration allows studying the dynamics of the system and the

response to the control actions in closed loop. The object of the experiments is to

regulate the set point (LEVEL) by the use of the controllers that automatically

operate on the final element of the loop (control valves).

You can control the level in the tank by a sensor and a controller


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PRACTICES MANUAL


information about the meaning of each parameter, see the Software Manual M4).

3.- Select a set point, PID controller and a proportional constant.


experiment, we want to observe the effects of a proportional action.

5.- Activate the PID controller, go out and save the values. The student will

observe that the motorized valve begins to work.


7.- Activate the solenoid valve AVS-2.


Valve) to adjust the flow that controls the level from the water tank to the set value.


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PRACTICES MANUAL


7.3.12Practice 12: Level control loops (Proportional + Integral)

7.3.12.1 Objectives


effect that an integral performance superimposed to a proportional action in an

actuator has.

7.3.12.2 Required material


UCP-L


Water.





information about the meaning of each parameters, see the Software Manual, M4).



accumulation is carried out smoothly and it doesnt generate an on/off performance

in the actuator.

4.- Indicate a value of 0 for the derivative performance. In this experiment


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PRACTICES MANUAL


we want to observe the effects of a proportional action plus an integral action.


will observe that the motorized valve begins to act.

6.- Connect pump 1.

7.- Open the solenoid valve AVS-1.

8.- The controller will modify the position of the AVP-1 (ProportionalValve) to adjust the flow that controls the set value.

7.3.13Practice 13: Level control loops (proportional + derivative)

7.3.13.1 Objectives

This practice supplements the previous one. The objective is to observe theeffect that has a derivative performance superimposed to a proportional action in an

actuator.



UCP-L


Water.




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PRACTICES MANUAL





3.- Select a set point, PID controller and a proportional constant and

derivative. The value for the derivative constant should be small so that the

performance is small and it doesnt generate an on/off performance in the actuator.

4.- Indicate a value of 0 for the integral performance. In this experiment we

want to observe the effects of a proportional action plus a derivative action.

5.- The activated PID controller and start and save the values. The student


6.- Connect pump 1.

7.- Open valve AVS-2.

8.- The controller will modify the position of the AVP-1 (Porportional

Valve) to vary the flow to adjust the level to the set value.

7.3.14Practice 14: Level control loops (Proportional + Derivative + Integral)

7.3.14.1 Objectives


effect that a derivative performance superimposed to an integral performance and a

proportional action in an actuator has.




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PRACTICES MANUAL


UCP-L


Water.






3.- Select a set point, PID controller and a proportional constant, derivative

and integral. The value for the derivative constant should be small and the integral

constant should be big so that the performance is small and doesnt generate an

on/off performance in the actuator.


observe that the motorized valve begins to act.

5.- Connect pump 1.

6.- Open the solenoid valve AVS-2.


Valve) to vary the flow to adjust the level to the set value.


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PRACTICES MANUAL


7.3.15Practice 15: Adjustment of the constants of a flow controller (Ziegler-Nichols)

7.3.15.1 Objective of the experiment

To follow the optimization process of a controller of three terms (PID), for

a given process.

When the PID control values of a process are optimized, you will have to

take into account several initial considerations:

1.-The process is of slow or quick response.

2.-The reaction of the process goes very retarded of the action.

3.-The response of the sensors and controllers is immediate or theyneed a time out to reach the balance.

The objective of this practice is to get familiarized with the most usual

methods of optimizing the variables of a PID controller starting from the

characterization of the process.



- Reaction Curves (or open loop).


The data to be analyzed will be obtained configuring only the controller

with the Proportional Band or the proportional action. The integral and derivative


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PRACTICES MANUAL


actions should be at zero.

The objective of the experience is to maintain the system with a constant

level using a controllerPfor the control of the motorized valve.

With the motorized valve at the 50% of its way, regulate the needle valve

manually VR-1, until getting that the level of the tank is constant.

7.3.15.3 Method of the minimum period (Ziegler-Nichols)

Pass now to an automatic control and observe how the level stays constant

at the 50% of the process variable. Change the variables of the process for partial

opening of the needle valve, VR-1. As the process will become stable, increase the

value of the proportional constant and close the needle valve, VR-1, partially

observing the behavior of the process.

Continue increasing the value of the proportional constant, applying each

time an interference in step (closing or opening VR-1), until the variable of the process

oscillates continually. Note down the value of the proportional constant (Limit

Proportional Band, L.P.B.) when this happens, measure the oscillation time of the

process (O.T.).

The optimum values, depending on the control type we will make on ourprocess are:

Type of Control B.P. I.T. D.T.P 2 (L.P.B.) -- --

P+I 2.2 (L.P.B.) T.O/1.2 --P+I+D 1.7 (B.P.L) O.T. / 2.0 T.O/8.0

Table 3.19.1


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A variant of the gain limit method is the method of the minimum overflow

of the set point. Once the self-maintained oscillation of the Time of Oscillation O.T.is obtained for a Limit Proportional Band L.P.B., the values of the control actions are

the following ones:

B.P (%) = 1.25 L.P.B.


D.T. (MIN) = 0.19 O.T.

7.3.16Practice 16: Adjustment of the constant of a flow controller (Reaction

Curves)

In this open loop method, the general procedure consists on opening the

closed loop of regulation before the valve, that is to say, the valve must be directly

operating with the controller manually and to create a small one and quick change in

the step in the input process. From the signal registration and from their graphic

representation the PID control values can be obtained. The graphic representation of

the controlled variable versus the time is a sigmoid. In the inflection point of the

sigmoid a tangent straight line is traced and the valuesRandLare measured.Ris the

slope of the tangent in the inflection point of the curve and L is the retard time of the

process. That is, time (in minutes) that takes place between the instant of the change

in step and the point in which the straight line, tangent to the sigmoid, cuts the initial

value of the controlled variable. DP is the percentage (%) of the position variation of

the control valve that introduces the step in the process, see figure 3.20.1


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PRACTICES MANUAL


Figure 3.20.1:

Representation of the reaction curve.

From this representation we can obtain the slope of the sigmoidRand the time of retard L.

The optimum values, depending on the control type that we will be made

on our process are:


P+I 110RL/DP L/0.3 --P+I+D 83RL/DP L/0.5 0.5LTable 3.20.1:

Optimum values necessary to use in function of the type of control used.

L: Time of retard, R.: Slope of the sigmoid in the inflection point. D. Derivative, P.: Proportional.

Compare the values obtained by the two methods.

7.3.16.1 Other experiments to carry out

7.3.16.1.1 - Evaluation of the calibration of the PID controller

Once the PID values are introduced in the controller in a manual way, with

the motorized valve positioned at the 50% of their way, regulate the needle valve

manually VR-1, until obtaining that the flow of the system is at the 50% of the

maximum flow provided by the pump. Go to the automatic control of the process and

apply an interference, as the solenoid valve AVS-1. Observe the temporary behavior


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PRACTICES MANUAL


of the process. Repeat the process for the PID control values obtained by the other

method.




- The values obtained by any of the different methods differ, and theyshould be treated as starting values for the good regulation of the process, which

should be slightly modified by the operator, carrying out this way their fine

adjustment until obtaining the optimum values.

- There are methods of automatic adjustment, in which the instrument has

an algorithm of self-adjustment of the control actions that allows him to tune in with

a wide range of industrial processes. The application of a test signal to the processand the analysis of the obtained response and its mathematical modeling leads to the

controller analytic design (Nishikawa, Sannomiya, Ohta and Tanaka, 1984). Or you

can use an iterative process to the method of the gain limit (Chindambara, 1970 and

Kraus and Myron, 1984):

The obtained error signal is analyzed in the case of changes in the set point

or in the load of the process, and by iteration the new values PID can be determined.

Controller P: B.P.N+1 = B.P.N / (0.5 + 2.27 R)

Controller PI: same B.P.n+1 and I.T. = P / (1.2 *sqr(1+R2)) min/rep

Controller PID: same B.P.and I.T. = P / (2 *sqr(1+R2))

D.T. = P / (8 *SQR(1+R2)).


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PRACTICES MANUAL


BeingR= 1/(2*3.14) * Ln(a/b) and P the period of the oscillation muffled in

minutes. Where aand bare the widths of the first two oscillations introduced afterthe interference.

If, when applying these methods, the process enters into oscillation, the

interference can invalidate the application, in case the process does not allow it.

7.3.17Practice 17: Temperature control loops (manual)

7.3.17.1 Objectives

The objective of this experiment is the temperature control in a tank of

water by a manual procedure. We understand that the manual control works as:


flowmeter.

Manual control of the equipment elements: motorized valve, solenoid

valves, relay of activation / deactivation of the resistor, etc.


The following material is required for the realization of this practice:

UCP-T

Water.

SACED Software.


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PRACTICES MANUAL



1.- Connect the interface of the equipment and execute the program

SACED UCP-T.

2.- Inside the program, select the option Configurationand connect pump 1

(see Software Manual for more operation details).

3.- The temperature regulation of a tank of water can be carried out by two

different procedures that we will identify as:

a. - Static; it consists on filling the left superior tank above the level

alarm.

b. - Continuous or Dynamic; it consists on fixing a water level in the left

superior tank but with an inlet and outlet of constant water. In this

second procedure, it is required that the incoming and outcoming water

flows are small in order to establish a thermal balance in the tank.

Under these conditions, it is also necessary to maintain the water level

above the level alarm.

3.- In the manual regulation (no controller) the temperature can be

regulated by the on and off immersion resistor placed in the tank.

4.- Select the option Manual Control of the software supplied with the

equipment.

5.- Connect pump 1

a) Fill the tank above the level alarm. Disconnect pump 1 and close

the valve VR1manually.


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PRACTICES MANUAL


b) Connect pump 1 and fill the tank until getting the level alarm.

Once surpassed this, AVS-1 opens, and using VR1 fix a constantinlet and outlet water flow in the tank.

6.- Connect, anyway, the agitator given with the equipment.

7.- By the connection and disconnection of the resistor, fix a temperature

for the water. In the case b, if it is necessary, fix the temperature varying the inlet and

outlet of flow. So, open or close valve VR1or the AVS-1.

7.3.18Practice 18: Temperature control loops (on/off)

7.3.18.1 Objectives


on/off controller. For it, the student will select a value wanted for the temperature and

Off/On Control


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PRACTICES MANUAL


the controller will adjust this control by the on/off switch of the resistor.



2.- Select the control option on/off.

3.- Select the wanted temperature (Set point). By defect, There is a set

point, tolerance and performance time determined. It allows the students to play with

these parameters and to see the influences of each of them.

4.- It calculates the inertia of the system before an on/off response and

determines the limit time for an exact control.


From the results obtained by the on/off control on the variable of

temperature, we can affirm that this controller has an acceptable behavior due to the

fact that the variation of this magnitude before a small interference is slow. If we also

take small values in the performance times and in the tolerance we can obtain a

TEMPERATURE CONTROL next to the set value.

7.3.19Practice 19: Temperature control loops (proportional)

7.3.19.1 Objectives

This configuration allows studying the dynamics of the system and the

response to the control actions in closed loop. The object of the experiments is toregulate the set point (TEMPERATURE) by using the controllers that operate


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PRACTICES MANUAL


automatically on the final element of the loop (control ACTIONS).

You can control the temperature in the tank by a thermal probe and a

controller configured for proportional outputs to the actuator without the typical

oscillations of the on/off control. You can study the response of the control loop in

front of interferences in the process variables (flow) or variations in the set point (the

temperature is changed fixing different set points).

Modifying the set point in a remote way the temperature changes can beobserved oscillating around the new value. It can happen that the set point is not

reached if the range of the actuator (manipulated variable) it is not enough to control

the interferences or the changes in the set point, so it will be stabilized only up to the

maximum that allows the water available. In our case, the manipulated variable is the

water temperature of the tank that, at the same time, is determined through a PWM

actuator, which acts as a temporizer, whose performance time is proportional to thevalue of 0-10 volt. A bigger performance time of the resistor means a bigger energy

given by the resistor to the liquid.



UCP-T


Water.




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PRACTICES MANUAL



2.- Select the option Control PID on the capture screen.


experiment we want to observe the effects of a proportional action. (For more




7.3.20Practice 20: Temperature control loops (Proportional + Integral)

7.3.20.1 Objectives


effect that an integral performance superimposed to a proportional action in an

actuator has.



UCP-T


Water.


1.- Connect the Interface and execute the control software (For moredetails about the control software, see the Software Manual, M4)


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PRACTICES MANUAL






accumulation is carried out smoothly and doesnt generate an on/off performance in

the actuator.

4.- Indicate a value of 0 for the derivative performance. In this experimentwe want to observe the effects of a proportional action plus an integral action.

5.- Activate the PID controller, go out, and save the values. You will

observe that the resistor begins to work.

7.3.21Practice 21: Temperature control loops (Proportional + Derivative)

7.3.21.1 Objectives


effect that a derivative performance superimposed to a proportional action in an

actuator has.



UCP-T


Water.


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PRACTICES MANUAL






information about the meaning of each parameter see the Software Manual, M4).

3.- Select a set point, PID controller and a proportional and derivative

constant. The value for the derivative constant should be small so that the

performance is small and it doesnt generate in a on/off performance in the actuator.

4.- Indicate a value of 0 for the integral performance. In this experiment we

want to observe the effects of a proportional action plus a derivative action.




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PRACTICES MANUAL


7.3.22Practice 22: Temperature control loops (Proportional + Derivative +Integral).

7.3.22.1 Objectives


effect that a derivative performance superimposed to an integral performance and a

proportional action in an actuator has.



UCP-T


Water.




2.- Select the Option Control PID on the capture screen. (For more

information about the meaning of each parameters, see the Software Manual, M4).

3.- Select a set point, PID controller and a proportional, derivative and

integral constant. The value for the derivative constant should be small and the

integral constant should be big so that the performance is small and it doesnt

generate an on/off performance in the actuator.


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PRACTICES MANUAL




7.3.23Practice 23: Adjustment of the constant of a controller of temperature

(Ziegler-Nichols)

7.3.23.1 Objective

To follow the process of optimization of a controller of three terms (PID),

for a given process.

When the values of a PID process are optimized you have to take into

account several initial considerations:

1.-The process is of slow or quick response.

2.-The reaction of the process goes very retarded of the action.

3.-The response of the sensors and controllers are immediate or they

need a time out to reach the balance.

The objective of this practice is to familiarize with the most usual methods

of optimizing the variables of a PID controller starting from the characterization of

the process.



- Reaction Curves (or open loop).


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PRACTICES MANUAL



The data to be analyzed will be obtained configuring the controller only

with the Proportional Band or the proportional action. The Integral and Derivative

Actions should be at zero.

The objective of the experience is to maintain the system with a constant

temperature using a controller P for the control of the resistor.

With the motorized valve at the 50% of its way, regulate the needle valve

manually VR-1, until getting that the level of the tank is constant.

7.3.23.2.1 Method of the minimum period (Ziegler-Nichols)

Pass now to an automatic control and observe how the temperature stays

constant at the 50% of the process variable. Change the process variables for partial

opening of the needle valve VR-1. As the process will become stable, increase the

value of the proportional constant and close the needle valve, VR-1, partially,

observing the behavior of the process.

Continue increasing the value of the proportional constant and applying

each time an interference in step (close or opening of VR-1), until the process variable

oscillates continually. Write down the value of the proportional constant (Limit

Proportional Band, L.P.B.). When this happens, measure the oscillation time of the

process (O.T.).

The optimum values, depending on the control type that we will make in

on our process are:


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Type of Control B.P. I.T. D.T.

P 2 (L.P.B.) -- --P+I 2.2 (L.P.B.) O.T. / 1.2 --

P+I+D 1.7 (L.P.B.) O.T. / 2.0 T.O/8.0

Table 3.27.1

A variant of the limit gain method is the method of minimum overflow of

the set point. Once the self-maintained oscillation of Time of Oscillation O.T. for a

Limit Proportional Band L.P.B., the values of the control actions are the followingones:

B.P (%) = 1.25 L.P.B.


D.T. (MIN) = 0.19 O.T.

7.3.24Practice 24: Adjustment of the constants of a temperature controller

(Reaction Curves)

In this open loop method, the general procedure consists on opening the

regulation closed loop before the valve, that is to say, the valve must operated

directly with the controller in manual and create a small and quick change in step in

the input process. From the registration of the signal and from its graphic

representation the control values of the PID will be obtained. The graphic

representation of the controlled variable versus the time is a sigmoid. In the inflection

point of the sigmoid a tangent straight line is traced and the values R and L are

measured.Ris the slope of the tangent in the inflection point of the curve and Lis the

retard time of the process. That is, time (in minutes) that takes place between the

instant of the change in step and the point in which the tangent straight line crossesthe initial value of the controlled variable. DP is the percentage (%) of position


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variation of the control valve that introduces the step in the process. See figure

3.28.1.Fig. 3.28.1

Representation of the reaction curve. From this representation we can obtain the slope of thesigmoidRand the time of retardL.

The optimum values, depending on the control type that we will make on

our process are


P+I 110RL/DP L/0.3 --P+I+D 83RL/DP L/0.5 0.5L

Table 3.28.1.

Optimum values good necessary to use in function of the type of control used.L: Time of retard,R: Slope ofthe sigmoid in the inflection point.D:Derivative,P: Proportional.

Compare the values obtained with the two methods.

7.3.24.1 Other experiments to carry out

7.3.24.1.1 Evaluation of the calibration of the PID controller

Once the PID values have been entered to the controller, adjust, in a

manual way, with the motorized valve positioned at the 50% of their way, regulate


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the needle valve manually, VR-1, until getting that the flow of the system is at the

50% of the maximum flow provided by the pump. Go to the automatic control of theprocess and apply an interference, as the solenoid valve AVS-1. Observe the

temporary behavior of the process. Repeat the process for the PID control values

obtained by the other method.




- The values obtained by any of the different methods differ, and they

should be treated as starting values for the good regulation of the process that should

be slightly modified by the operator, carrying out, this way, their fine adjustment

until obtaining the good values.

- There are methods of automatic adjustment, in which the i

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UCP- Process Control Manual-1