METERED FLOW LOOP PROCESS CONTROL LAB …edge.rit.edu/content/P13630/public/FinalDocuments... · Web viewA poorly designed control system could cost its owner considerably in terms

Multidisciplinary Senior Design IIKate Gleason College of Engineering

Rochester Institute of TechnologyProject Number P13630

METERED FLOW LOOP PROCESS CONTROL LAB CART

Andre Berwin (Project Manager), Nathan Fulcher, Andrew Watson, Travis Bardsley, Anthony Parker, Peter Dunning, James Mazza

Abstract

This project is to create a mobile apparatus for the RIT Chemical Engineering Department’s Chemical Engineering Principles Lab II to demonstrate PID (Proportional-Integral-Derivative) control and the effects of noise. Variables can be manipulated and results viewed with LabVIEW. The cart is to be used in tandem with the Process Control class taught for fourth year chemical engineering students. This loop consists of a pump and flow meter connected to a PC utilizing LabVIEW control via a custom rigged microcontroller. This cart and the proposed lab outline which accompanies it are intended to be completed by three fourth year chemical engineering students in three three-hour laboratory periods. All design constraints were tested and found to be sufficient. Recommend considering a new pump to achieve higher flows if required in the future and replacing the wheels on the cart.

Introduction

Today's industries deliver a number of chemicals for use in the production of most man-made objects. To meet the demands of a growing population, the processes used to create these chemicals are conducted on a massive scale, requiring sophisticated methods of control to ensure that safe temperature and pressure conditions are maintained along with product quality. A poorly designed control system could cost its owner considerably in terms of ruined product or failure due to exceeding operating parameters. Designing these systems requires an in depth look at the nature and response of the components, as well as the system of equations that describe the process. This allows for a programmable response that can keep the process in control as effectively as possible.

In order to design the chemical plant systems of tomorrow, chemical engineering students must understand how process control systems function. To this end, students may supplement theoretical knowledge of process control with practical labs that expose the students to common components from industry, such as pumps, control valves, heat exchangers, and PID control. This will produce chemical engineers with more complete knowledge that will aid them in future designs down the road.

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Lab Plan

The lab is designed to take place over three consecutive lab sessions, with each session being three hours in duration. The first session is designed to be a basic introduction to the system, LabVIEW, PID control, and noise. The objective of this session is to demonstrate the power of control systems and their advantages over manual controls. Students attempt to control a scenario manually while collecting data. They then repeat this process with P, PI, and PID control and compare this to their manual control data. The students repeat this process with disturbances in the system. The disturbances vary between one of four sources or a combination of these sources. The various disturbance sources include head pressure from switching tanks, pipe length with a pressure drop, control valve, or needle valve.

The second session is designed to provide a deep understanding of PID control, and each of its individual elements. The complete PID equation is shown and defined. Each parameter’s effect on the control of the system will also be explained. The objective of this session is for the complete PID equation to be understood, as well as understanding how to manipulate its components to achieve a well controlled system. An analytical analysis of the PID equation will ultimately lead to the introduction of the proportional constant, integration constant, and derivative constant. Students took data and are told to note differences between responses from P, PI, and PID control for a given scenario. This process was then repeated with noise. Moreover, students are presented with an open ended portion near the end of the lab. They are then tasked with varying the levels of noise in order to find the limit of the control system. Next, they are instructed to vary the PID constants to learn of their practical effects on the system and their own limitations.

Lastly, the third session is designed to focus on noise management and to provide a final test for the students. The objective is to provide insight regarding real-world methods of managing noise. Methods of eliminating noise are discussed along with their trade-offs. Students are made to run a scenario in which they average data to reduce noise and improve their signal to noise ratio. Their final task requires one of two criteria to be met; students could either develop another method to eliminate noise and demonstrate its effectiveness; or they could be given a complex scenario that had to be controlled with no outside guidance. Time permitting; the students would present their findings to the rest of the class.

Process Control

This project utilizes a fluid flow process that incorporates a pump to move fluid, a control valve that restricts flow, a pressure sensor and a flow sensor. Through the application of process control theory, the pressure and flow variables can be controlled via closed loop control utilizing the process operation, process sensor, and microcontroller.

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Physical Layout

A modular and adaptable cart was modeled after and consistent with existing flow carts. The cart was designed to house the process control system; prioritizing usability, safety and reliability for the students. The cart was designed to be portable, easily moved, and easily connected and disconnected to lab utilities. The cart was augmented with stainless steel framing to mount and support all of the system components. All of the system components may be removed and rearranged if needed. In order to avoid the previous balance issues, the frame was designed to be located in the center of the cart. In addition, the system components weights were evenly distributed across the whole cart, keeping the bottom of the cart weighted heavily. The motor, flow tube, and the primary feed tank were located on the bottom rack of the cart, which provided the required mass to stabilize the cart. The flow tube was mounted on the uprights of the framing that go through the center of the cart. These uprights provide structural stability, and help the load of the cart to be centralized. The core components on the top rack were split up between the components that the students needed to utilize on the “front” facing direction, with the electrical box, containing the electrical components, balancing out the weight on the opposite side of the framing. Overall, the design should require minimal maintenance and cleaning while using water as the primary fluid.

Figure 1: Picture of Completed Cart

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Figure 2: P&ID

Processor

The processor chosen for controlling the components in the cart design was the Texas Instruments MSP430. This device is an ultra-low power 16-bit Reduced Instruction Set Computer (RISC) mixed-signal processor complete with several clocks, serial data ports, and analog to digital converters. The processor was being used in several different applications in this particular project.

The first use for the processor was to take the analog signals present in the circuit and convert them to digital signals to be transported to the computer. This is accomplished using the internal 10 bit Analog to Digital Converter (ADC) present in the microprocessor. A 10 bit ADC

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converts an analog signal to a 10 bit digital number. The resolution of the conversion depends on the number of bits and the reference voltage. For example, a 10 bit ADC using 1 volt as a reference will be able to convert values in discrete steps of 1 volt divided by 210 levels, or roughly 1 millivolt.

The next use for the processor was to transmit the data using the Universal Asynchronous Receive/Transmit (UART) communication standard. Two outputs of the processor send digital data serially to a connected PC using a USB interface. Sending data serially means once every clock cycle one new bit of data is transferred along the UART lines to the computer. Once 8 bits are received by the PC this data is stored as one byte of information and can be used for a number of applications such as signal processing. Similarly, data can be received by the processor from the computer to be used for process control in the system. The signals being sent to the computer include the pressure data from the system and the flow data. The signals being sent to the processor include the new pump speed as well as the control signal for the I/P (current to pressure) component.

Lastly, the processor can control the various components in the circuit using another form of serial data transfer called Serial Peripheral Interface (SPI.) This is very similar to the UART transfer of data except the number of bits being sent is not limited to 8 at a time. In the case of this specific application 16 bits were sent to a digital to analog integrated circuit that converts the digital signal to an analog output. The DAC output can then be conditioned to be used as a 4-20 mA signal or a 0-10 volt signal. These signals are used to power the drive for the motor or the I/P for the control valve.

The processor was chosen for its low price, ease of use, and versatility. The processor can be programmed via computer using TI software and a USB cable. The software written for the MSP430 was written in C for this project but could also be written in assembly for very specific applications. Texas Instruments provides a free Integrated Development Environment (IDE) in which to program the device. The serial communication protocols, ADC devices, and integration with various peripherals make it an excellent choice for the project.

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Figure 3: Electrical Loop

Control Valve

The control valve regulates flow rate or pressure of a stream by changing the valve position by the following relation:

FV=C v (x ) √∆ P

The Cv(x) parameter is a function of the valve position, which is regulated between 0-100%. The valve position is controlled by sending a pressurized air signal to the valve, which moves a diaphragm connected to the valve stem. The air signal is regulated by a current to pressure (I/P) converter, which takes an analog current signal being sent by a processor (4-20mA) and converts it linearly to a (3-15psi) air signal. The converter is supplied with pressurized air via a regulator, which converts the pressure from 80 psi to 30 psi. An additional 30 psi air line is sent to the control valve to speed the valve dynamics.

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Pump: MicroPump GJ.N-25

The positive displacement pump also regulates stream flow and pressure by driving a pump rotor via a motor, which is controlled via voltage supplied by a drive. The voltage sent by the drive is controlled via an analog current signal sent from the processor. The characteristic of the pump is the following:

FP= (−α+ βω ) ∆ P+γω ≈ γω

Where ω is the speed (angular speed, RPM) of the motor and α , β, γ are fitting parameters characteristic to the pump.

Motor and Drive

The speed of the motor is regulated by the following equation which relates the frequency of the drive and the angular speed of the pump:

ω=kfWhere f is the frequency of the drive, which ranges between 0-60 Hz and is directly controlled via a mA signal sent by the microprocessor. k is the ratio between drive frequency and motor speed, which is set to 1800RPM/60Hz.

Process Control via Pump and Control Valve

The system serves to exercise process control on two variables, flow rate and pressure, via two process units, pump and control valve. The following is a control loop diagram showing the logic of the control system. Also shown is a flow diagram for reference and process equations that govern the system variables.

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Diagram 1: MIMO Control Loop

The following shows the region of allowable operation for the two set-points of flow and pressure with the currently installed pump and control valve.

Plot 1: Pump Region of Operation

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Results

All customer constraints were collected and organized in a table. We created a test plan for each parameter and recorded our results in the following table:

Parameter Target Test Plan results Reference

Min Flow Rate 158 g/min Check flow rate at lowest pump settings

Yes Pump testing,

11/8/2013Max Flow Rate 1500 g/min Check flow rate at highest

pump settingsYes, highest is 1504

g/minPump

testing, 11/8/2013

System Pressure 5 to 80 psi Check Pressure during operation

Yes, pressure can span 0.5-30psi with

supply

Pump testing,

11/8/2013Pump Response

Time< 1 second Check time to new steady

state after change is madeYes, pump response time can be set via

the PowerFlex drive

Operating manual pg. 3-12

Flow Accuracy < 1% error Compare flow meter value against another flow

meter or total volume per time

Yes, dependant on flow rate, ranged between 0.1-1%

innacuracy

Pump testing,

11/8/2013

Control Valve Response Time

< 5 seconds Check time to new steady state after change is made

< 1 second

Leakage No Leaks Check for leaks under normal operating

procedures

Yes

Operator 2 Minimum, 3 Maximum

Attempt to operate the system with 2 people

Yes

Automatic Control

Functioning PID control with ability to alter constants

Implement new set-points and observe system

response

Yes

Manual Control Can adjust flow manually

Adjust flow Yes

Automatic Data Collection

Labview records data that can be

graphed or exported to excel

Ensure it exists and check for accuracy

Yes

Manual Data Collection

Real time data is displayed

Ensure it exists and check for accuracy

Yes

Modularity Can alter system to put different comonents in

series or parallel

Reconfigure system to put different components in

alternate order

Yes

Portability Easy to move, connect to and

disconnect from utilities

Connent and disconnect system from utilities. Push

cart around.

Yes but wheels need to be replaced

Noise 2 Sources Test if our sources effect the data

Yes

Table 1: Test Plan

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In the spring we have estimated our expenses to be $984.45 with the microprocessor. We estimated our expenses to be $1569.45 by replacing the microprocessor with the National Instruments controller. We decided to save money and use a microprocessor; however, we did not anticipate having to buy a drive which greatly changed out expenses. Ultimately we spent $1760.62 which was still in our allotted budget, but greatly exceeded our expectations.

Predicted (Low) Predicted (High) Actual Budget0

500

1000

1500

2000

2500

984.45

1569.451760.62

2000

Team Spending

Spending

Amou

nt ($

)

Plot 2: Budget & Expenses

Conclusions

Fortunately, even though we had to make unexpected purchases, we were able to stick within our budget. We made a few purchases for building the frame that became obsolete once we welded it. Some of our budget also needlessly went towards purchasing an inadequate cart. Neither the framing or extra cart purchases will be a true waste, as they will be useful to the department in the future. If we had chosen to go with our expensive build instead of our cheap build, we would have gone over budget. Luckily we decided to save as much money as we could and it paid off.

We were able to meet all design constraints as specified in the PRP. The only concern we have is that the pump may not be adequate. While it achieved a maximum flow of 1504g/min and the target was 1500g/min, we feel this is not a large enough safety margin. This is not an issue if the 1500g/min target was made while already considering a safety margin. We did not push the system past 30 psi in fear of damaging the pump. The original constraint was up to 80 psi which we are not willing to test. The final recommendation we have is to replace the wheels on the cart with heavy duty wheels that are lockable. The current wheels cannot lock in place, and seem inadequate for the weight of the cart.

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References1. “Rubbermaid Heavy-Duty Tray Shelf Service & Utility Cart Beige 44 x 25" Global

Industrial. N.p., n.d. Web. 04 Nov. 2013. <http://www.globalindustrial.com/p/material-handling/trucks-carts/plastic-shelf-carts/cart-utility-26x45x33-bge>.

2. "PowerFlex 40 AC Drives." PowerFlex 40 AC Drives. N.p., n.d. Web. 04 Nov. 2013. <http://ab.rockwellautomation.com/Drives/PowerFlex-40>.

3. "MSP430G2553(ACTIVE)MSP430G2x53, MSP430G2x13 Mixed Signal Microcontroller."MSP430 Ultra-Low Power 16-bit MCUs. N.p., n.d. Web. 04 Nov. 2013. <http://www.ti.com/product/msp430g2553>.

4. "TLC5615(ACTIVE)10-Bit, 12.5 Us DAC, Serial Input, Low Power." Digital to Analog Converter. N.p., n.d. Web. 04 Nov. 2013. <http://www.ti.com/product/tlc5615>.

5. "NDS7002A − N-Channel Enhancement Mode Field Effect Transistor." NDS7002A PDF Datasheet N-Channel Enhancement Mode Field Effect Transistor. N.p., n.d. Web. 04 Nov. 2013. <http://www.fairchildsemi.com/pf/ND/NDS7002A.html>.

6. "LT1490A - Dual Over-The-Top Micropower Rail-to-Rail Input and Output Op Amps." - Linear Technology. N.p., n.d. Web. 04 Nov. 2013. <http://www.linear.com/product/LT1490A>.

7. "Vishay - IRF9Z24, SiHF9Z24 - Power MOSFET." Vishay - IRF9Z24, SiHF9Z24 - Power MOSFET. N.p., n.d. Web. 04 Nov. 2013. <http://www.vishay.com/product?docid=91090>.

Acknowledgments

Steve PossanzaPaul Gregorius

Christiaan Richter

Special Thanks to: RIT Chemical Engineering Department & Eastman Kodak for their generous donations

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