Design of a Low Cost Trainer for Flow Control

Transcription

1 Design of a Low Cost Trainer for Flow Control James A. Rehg Assistant Professor, Engineering The Pennsylvannia State University Altoona, PA Abstract: Process control laboratories in most colleges and universities include a single process trainer for control exercises in temperature, level, flow, and pressure. Single trainers are present because of the size and cost of the systems; however, the use of single systems makes it difficult to use small laboratory teams without reducing the enrollment limit placed on the laboratory section. Ideally, the process laboratory should have multiple trainers in at least two of the four primary process variables to learn the basics of process control. Then the students could move to the larger more robust systems to study control of a large industrial process. This paper describes a low cost flow trainer that was developed with the support of a National Science Foundation (NSF) Instructional Laboratory Instrumentation (ILI) grant to permit training in basic control concepts to multiple student teams. Construction cost for the flow system is less than $500, and all of the components are available off-theshelf from a number of industrial sources. The system can be controlled by any single-loop process controller, a programmable logic controller, or a microcomputer using a control software product, such as LabView. The system has been tested at several colleges and universities including Trident Technical College with excellent results. This paper describes the system design, construction requirements, example laboratory exercises, and test results. A web site that includes a complete set of drawings for the trainer and other supporting information is available at Additional information is also available by contacting James Rehg at jar14@psu.edu. performance of these systems is satisfactory, problems occur when these large systems are used in introductory control theory laboratories. The problems include: The high trainer cost prohibits the purchase of multiple student stations. The small number of student stations requires large student teams if standard laboratory section sizes and traditional laboratory scheduling practices are followed. The large student teams generate uneven learning among the team members. A single large trainer for each control variable (temperature, pressure, flow, and level) dictates that student teams work on different laboratory exercises. Students struggle to learn process control basics because of the complex nature of large trainers. The combination of long setup time and slow system response makes it difficult to complete a laboratory in a standard two- or three-hour period. Exposure to industrial level process systems is important in a controls laboratory, but controls exercises should start on smaller process control systems. To satisfy this training need, an NSF ILI grant was used to develop the process control flow trainer pictured in Figure 1. Introduction Manufacturers interested in improved quality have increased the number of closed loop controls in continuous, repetitive, and line type production systems. As a result, engineering technology programs at the two- and four-year level have added control courses and laboratories to prepare the graduates for the systems awaiting them in industry. Process control laboratories that use standard industrial control elements are costly and require a large laboratory area. Traditionally, process control laboratories have used large system trainers to teach the control of material level, flow, temperature, and pressure. While the Figure 1 Hugo I Process Control Flow Trainer Trainer Design Criteria

2 The NSF-ILI grant supported the development of two low cost process control trainers. The flow system described in this paper and a second trainer for temperature process control exercises [1]. The design criteria for both trainers included the ability to reach the desired steady state condition in a reasonable amount of time; to provide relatively quick response to set point changes or system disturbances; to use off-the-shelf industrial components; to interface easily to a variety of control systems; to fit easily on a bench top; and to cost less than $500. System Overview The system includes a 100 cfm shaded-pole centrifugal blower connected to one end of a 2.5 x 14 inch clear plastic tube with a flow sensor positioned at the opposite end. A damper valve was placed between the blower and the sensor to control the level of air flow through the tube. The system uses 120 volt ac power, and a standard toggle switch controls power to the unit. The system is mounted on a 21 x 16 inch vertical panel with a base that is 19 x 16 inches. Three configurations of the flow trainer have been built and tested. The design and operation of Hugo I, Hugo II, and Hugo-Stepper are described in the following sections. Hugo I Process Flow System The Hugo I flow trainer, pictured in Figure 1, uses a current to pressure (I/P) converter to translate the 4 to 20 milliamp control variable (CV) signal into a 3 to 15 psi pressure signal which drives a proportional linear actuator. The mechanical linkage between the linear actuator and the damper valve changes the linear motion to angular valve positions. The flow sensor, mounted on the top of the flow tube, is a 12 volt dc muffin fan that is used to generate a signal proportional to the rate of air flow through the tube. The generator output is passed through an active second-order low-pass Butterworth filter with a corner frequency of 25 hertz. This process variable (PV) signal conditioning removes the low frequency noise that is introduced from the fan generator. The PV signal range from the filter is 0 to 5 volts dc for a minimum (damper closed) to maximum (damper open) flow change. The control diagram for the system is illustrated in Figure 2. The controller in the control diagram can be any control device that accepts a PV in the 0 to 5 volt dc range and provides a CV with a 4 to 20 milliamp range. Hugo II Process Flow System The Hugo II system was built with the same specifications as the Hugo I trainer, but additional hardware was added to generate the PV. The muffin fan was modified to operated like a turbine flow sensor by adding reflective tape to one of the fan blades and installing a retroreflective type photoelectric sensor in the airflow above the fan. The small fiber-optic sensor head detects every rotation of the fan and generates a discrete pulse. This pulsed output is converted to a proportional analog signal by a Red Lion pulse rate to analog converter. The Red Lion unit has three dc output ranges - 0 to 10 volts, 0 to 1 milliamp, and 4 to 20 milliamps - that are programmed by switches in the unit. The unit requires 120 volt ac for power and provides the 12 volts dc power required by the photoelectric sensor. The control diagram in Figure 2 does not change when the Hugo II configuration is used, but the range and/or type of signal for the PV changes. Hugo-Stepper Process Flow System In the Hugo-Stepper system the pneumatic actuator is replaced with a stepper motor and a belt and pulley system to change the angular position of the damper. The drive linkage includes a 10 groove 1/5 pitch double flange pulley on the stepper shaft, a 30 groove 1/5 pitch double flange pulley on the damper shaft, and a _ inch wide neoprene belt with 80 grooves and 1/5 pitch. The stepper motor used in the design has a resolution of 0.25 degrees and is connected to a stepper motor driver card mounted on the back of the system. The stepper motor driver accepts either a 0 to 10 volt dc or a 4 to 20 milliamp dc signal from the controller. Both of the flow sensor systems described previously were used with this configuration. Figures 3 and 4 illustrate a system block diagram for the Hugo I and Hugo-Stepper configurations. Figure 2 Flow System

3 2) it illustrates the operation of a turbine type flow sensor used frequently in process control; and 3) it illustrates a second type of parameter conversion from pulses to analog values. Figure 3 Hugo I System Block Diagram Design Considerations An air or gas flow trainer design was chosen because it is less costly to build when compared to a fluid type of flow system, and it could be operated in any laboratory environment with only compressed air (Hugo I and II only) and 120 volt ac power required. In addition, the air-type system provides a quick and visual response to set-point and load changes. The I/P converter and linear actuator were chosen because 1) they are a low cost solution; 2) they are commonly found on industrial process systems; 3) they provide an opportunity to study set up techniques for mechanical drive linkages; and 4) they provide an example for the calculation of a mechanical transfer function. The Hugo-Stepper configuration with a stepper motor used as the control device was implemented because: 1) it provides an interesting application for stepper motor use; 2) it provides an opportunity to study the limiting factors in system response; and 3) it provides an opportunity to introduce programming of stepper motor drivers. The size and geometry of the mounting surfaces were chosen to permit the trainers to be easily transported, stored, and used on standard student benches. The wiring for all system components is terminated at a set of terminal blocks on the front of the trainer so that all connections are made in one location. Also, maintenance on the system component is minimized because students do not make connections to terminals on the control system components. System Operation The Hugo I and II were tested with Allen Bradley programmable logic controllers (PLCs), including the SLC 504 and the Series 5 systems. In addition, they were used with analog to digital conversion boards in microcomputers running LabView software [2] and with Opto-22 Smart Boards using a microcomputer BASIC program. Hugo-Stepper was tested with the SLC 504 Allen Bradley programmable logic controller. Figure 4 Hugo-Stepper System Block Diagram The tachometer type of flow sensor, using a dc motor to generate the PV value, was chosen for the Hugo I configuration because 1) it is low cost; 2) it provides an opportunity to study signal conditioning for noise and gain; and 3) it provides a smooth transition to the turbine type of flow sensor used in Hugo II. Despite the added cost, the second type of flow sensor, added to the Hugo II system, provides a number of training advantages: 1) it provides an opportunity to study a common type of photoelectric sensor used in industry; In a current laboratory configured for the Bachelor of Science in Electro-Mechanical Engineering Technology program at Penn State Altoona, three systems are used that have the capability to function as either a Hugo I or a Hugo II type system. Three additional systems, built with the Hugo-Stepper control configuration, are used in the controls laboratory. Students start laboratory exercises on a Hugo I configuration and then move to the Hugo II type of control system. The Hugo-Stepper exercises are performed last. PID software and hardware modules in programmable logic controllers (PLCs) are used to control the flow trainer. In addition the system is used with stand-alone single loop controllers and a microcomputer running LabView software with a-to-d input and d-to-a output boards installed.

4 The objectives for five experiments for the trainer are: Experiment 1 1. Identify all of the control components in the flow system and describe the function and operation of each. 2. Connect the flow system to the Allen Bradley SLC 504 programmable logic controller and develop a program for manual control. 3. Adjust the span and scale for all of the control devices and set up the analog input and output module scaling. 4. Use the Visual Basic process monitor software to Experiment 2 1. Set up a manual control system using the PID command. 2. Manually change the control variable (CV) value to achieve steady state values of the process variable (PV) at several operating points. 3. Test the system response to set-point and load changes. 4. Set up and test the system for on/off control. Experiment 3 1. Set up automatic control of the flow system using the PID command. 2. Use the open-loop control method to find the process gain, dead-time, and time constant for the system with the pneumatic actuator. 3. Apply the Ziegler-Nichols closed-loop method to determine the proportional gain (P), integral reset (I), and derivative (D) tuning parameters for the pneumatic actuator. 4. Test the system response to set-point and load changes with PID control applied. Experiment 4 1. Set up automatic control of the flow system using the PID command. 2. Use the open-loop control method to find the process gain, dead-time, and time constant for the system with the stepper actuator. 3. Apply the Ziegler-Nichols closed-loop method to determine the proportional gain (P), integral reset (I), and derivative (D) tuning parameters for the stepper actuator. 4. Test the system response to set-point and load changes with PID control applied. Experiment 5 1. Optimize the control characteristics of both the pneumatic and stepper actuator. 2. Test the response of the optimized system for setpoint and load changes. 3. Use the Visual Basic process monitor software to Performance Results The trainers have been used with good results in several different laboratory settings. At Penn State Altoona they are used to introduce the concepts of PID control using a PLC and LabView type controllers. At Trident Technical College they are used with a PLC controller to introduce PID control [5]. Students follow that experience with work on a large industrial PID control system. The flow trainers offer students the following advantages: A visual look at control concepts because students can see optimum and non-optimum performance by watching the movement of the valve. A small system that provides fast response with a dead time of less than a second and a response time in the 2 second range. An easy interface to use with few wires to connect to the controller. A non-threatening size and straight forward operation that invites students to experiment with different PID parameters and see how the system responds. Conclusions The process control flow systems were built and used at a number of colleges with a variety of controllers. In one laboratory implementation eight student teams with two students per team worked through the laboratory exercises to learn how to fine tune a PID controller and a control process. The students found the control problem presented by the smaller trainer easy to understand. The size and complexity of the smaller system is not intimidating, and the level of understanding of process control techniques exceeded the level previously attained when students started on the larger system trainer. A web site that includes a complete set of drawings for the trainer and other supporting information is available at

5 Additional information is also available by contacting James Rehg at References 1. Rehg, J. Low Cost Process Control Trainers, Proceedings of the 1998 ASEE Annual Conference, June 30 July 2, 1998, Seattle, WA: Session Papannareddy, R. New Laboratory Experiments in Analog Electronics Courses Using Microcomputer- Based Instrumentation and LabVIEW, Proceedings of the 1996 ASEE Annual Conference, June 23-26, 1996, Washington, DC: Session Henry, J. Controls Laboratory Teaching via the World Wide Web, Proceedings of the 1996 ASEE Annual Conference, June 23-26, 1996, Washington, DC: Session Leybourne, A. A Cost Effective Technique for Incorporating Distributed Control Systems Concepts into the Undergraduate Curriculum, Proceedings of the Twenty-Fifth Frontiers in Education Conference, November 6-9, Beaufort, R., Student Exercises for Process Control System, unpublished laboratory experiments, Trident Technical College; Charleston, SC, Biography JAMES A. REHG James Rehg received a B. S. and M. S. in Electrical Engineering from St. Louis University and has completed additional graduate work at the University of South Carolina and Clemson University. Since August 1995, Jim has been working as an assistant professor of engineering and Program Coordinator of the B. S. program in Electro-mechanical Engineering Technology at Penn State Altoona. He is the author of five texts, including the following books published by Prentice Hall: Introduction to Robotics in CIM Systems and Computer Integrated Manufacturing.