Exercise 8 Closed-Loop Pressure Control, EXERCISE OBJECTIVE To understand open and closed-loop pressure control; To learn how to sense the pressure in a pneumatic circuit; To control the pressure in a closed-loop mode. DISCUSSION Pressure Control Pressure control is normally used either to limit the pressure applied to a cylinder piston or to maintain a specific level of pressure in a circuit branch. In some applications, for example, it is necessary to limit the pressure applied to the cylinder piston to prevent distorting or crushing the workpieces; In other applications, it is necessary to maintain the pressure applied to the cylinder piston at a specific level on successive cylinder cycles to exert a very precise force against the workpieces. Control of cylinder pressure can be done either with a conventional pressure regulator, or with a servo control valve of the pressure type (electropneumatic pressure regulator). However, a servo control valve of the pressure type can be controlled remotely with a potentiometer on an operator s panel. Moreover, the servo control valve allows precise adjustment of the pressure level. As with position control, pressure control can be accomplished by using either an open-loop or a closed-loop system. With the open-loop system, the pressure is controlled by a setpoint only, and the actual pressure is not taken into account. This type of system cannot provide and maintain accurate pressure control. With the closed-loop system, the pressure is controlled with a controller and a feedback loop. Most systems of pressure control are closed-loop because they provide accurate control of pressure and they improve repeatability (the ability to provide the same output every time for a same input), for a given setpoint voltage. 8-1
Closed-Loop Pressure Control Systems Figure 8-1 shows the block diagram of a closed-loop system used to control the pressure in a pneumatic system. The controller operates in the Proportional-Plus- Integral mode. A pressure transducer senses the pressure at the output of the servo control valve and generates a proportional voltage which is fed back to the controller. The setpoint is a voltage that corresponds to the desired pressure level. Figure 8-1. Closed-loop pressure control system. Detailed system operation is as follows: the error detector of the controller continuously compares the setpoint to the measured pressure. When there is a difference between these two signals, the error detector generates an error signal. The error signal is then amplified by the proportional amplifier and time integrated by the integral amplifier. Finally the output signals of the proportional and integral amplifiers are added at a summing point to produce the controller output signal. When the error is positive (measured pressure lower than the setpoint), a positive control voltage is applied to the servo control valve to increase the pressure in the circuit. When the error is negative (measured pressure higher than the setpoint) a negative control voltage is applied to the servo control valve to decrease the pressure in the circuit. As the circuit pressure increases or decreases as required, the difference between the setpoint and measured pressure becomes smaller until finally, the error becomes null. The higher the proportional and integral gains are, the faster the circuit pressure will reach the desired level, but if the gains are too high, the system may start to oscillate. It is important that the air supply pressure at the inlet port of the servo control valve be kept above that called for by the setpoint at all times while the system is 8-2
functioning. Otherwise, the servo control valve will not be able to bring or maintain the pressure at setpoint. Pressure Transducer Your trainer comes with a Pressure Transducer, model 6471, that provides a voltage proportional to air pressure. The Pressure Transducer is of the solid state type. It consists of a silicon sensing diaphragm capable of mechanical flexing. Four diffused resistors form a bridge circuit on the diaphragm. When the pressure is applied, the diaphragm is deflected causing the diffused resistors to change resistance (piezoelectric effect). As the pressure increases, sensor output voltage becomes larger. A 24-V DC voltage must be applied to the supply terminals of the transducer in order for the transducer to operate. The pressure to be measured is applied to the pneumatic port of the transducer. Transducer operation is as follows: the pressure applied to the pneumatic port of the transducer is transmitted to the sensing diaphragm, causing the diaphragm to deflect. This results in a resistance change and in a voltage increase at the transducer output. The higher the applied pressure, the higher the resistance change and, therefore, the higher the transducer output voltage. The transducer output voltage increases from 1.0 to 5.0 V when the applied pressure increases from 0 to 1000 kpa (0 to 145 psi). The transducer output voltage can be converted into a standard 0.0-10.0 V voltage for utilization with the trainer PID Controller. To do so, the transducer output voltage must be applied to the 0-5 V input of the [0-5 V] E/E converter on the Signal Conditioners module, and the converted voltage must be taken at the 0-10 V output of this converter (see Figure 8-2). 8-3
Figure 8-2. Pressure transducer. You will notice that the [0-5 V] E/E converter of the Signal Conditioners module has two potentiometers, labeled Z (zero) and S (span). These potentiometers are used to set the converter input voltage range within which the converter output voltage will pass from minimum to maximum: The zero potentiometer sets the converter input voltage for which the converter output voltage will be minimum; The span potentiometer sets the converter input voltage for which the converter output voltage will be maximum. As an example, the zero and span potentiometers can be adjusted so that when the converter input voltage varies between 1.0 and 3.5 V, the voltage will vary between 0.0 and 5.0 V at the [0-5 V] E/E converter output, or between 0.0 and 10.0 V at the [0-10 V] E/E converter output. When using the [0-5 V] E/E converter to convert the Pressure Transducer output voltage into a 0-10 V voltage, the span and zero potentiometers must be adjusted as follows: With the minimum operating pressure applied to the pneumatic port of the Pressure Transducer, the potentiometer Z is adjusted so that the voltage at the 0-10 V output of the [0-5 V] E/E converter is 0.0 V; With the maximum operating pressure applied to the pneumatic port of the Pressure Transducer, the potentiometer S is adjusted so that the voltage at the 0-10 V output of the [0-5 V] E/E converter is 10.0 V. 8-4
Procedure summary In the first part of the exercise, Open-Loop Pressure Control, you will observe the relationship between flow and pressure in open-loop control. In the second part of the exercise, Sensing the Pressure Using the Trainer Pressure Transducer, you will learn how to sense the pressure in a circuit using a pressure transducer. In the third part of the exercise, Closed-Loop Pressure Control, you will perform closed-loop pressure control. You will observe the relationship between flow and pressure in closed-loop control. In the last part of the exercise, System Repeatability, you will determine if the closedloop system is able to arrive at the same pressure level on successive cycles for a given setpoint voltage. EQUIPMENT REQUIRED Refer to the Equipment Utilization Chart, in Appendix A of the manual, to obtain the list of equipment required to perform this exercise. PROCEDURE Open-Loop Pressure Control Setting up the system G 1. Connect the circuit shown in Figure 8-3. Figure 8-3. Open-loop control of pressure. G 2. Verify the status of the trainer according to the procedure given in Appendix B. 8-5
G 3. Close the Flow Control Valve FCV1 by turning the control knob fully clockwise, and open the Flow Control Valve FCV2 by turning the control knob fully counterclockwise. G 4. On the Conditioning Unit, open the main shutoff valve and the required branch shutoff valve at the manifold. Set the main pressure regulator to obtain 140 kpa (20 psi) on the regulated pressure gauge. G 5. Set the Flow Control Valve FCV1 to obtain 30 l/min (1 SCFM) on the Flowmeter. Readjust the setting of the main pressure regulator and Flow Control Valve FCV1 to obtain exactly 140 kpa (20 psi) and 30 l/min (1 SCFM). Note: The Flow Control Valve FCV1 is set at 30 l/min (1 SCFM) in order to correspond to the flow characteristics of the Servo Control Valve which will be used later in the exercise. G 6. Close Flow Control Valve FCV2 by turning the control knob fully clockwise. Record the pressure indicated by Pressure Gauge PG2 in the "0 turn" row of the column OPEN LOOP in Table 8-1. G 7. Gradually open Flow Control Valve FCV2 by turning the control knob counterclockwise as indicated in Table 8-1. Refer to the mark on the control knob to help you set the correct position. For each setting, record the pressure indicated by Pressure Gauge PG2 in the appropriate cell of Table 8-1. G 8. Referring to the data in Table 8-1, what relation is there between the pressure indicated by Pressure Gauge PG2 and the flow rate? 8-6
FLOW CONTROL VALVE OPENING (FCV2) 0 turn 1 turn 2 turns 3 turns 4 turns 5 turns 6 turns 7 turns 8 turns 9 turns 10 turns OPEN-LOOP PRESSURE CLOSED-LOOP CONTROL VOLTAGE IN THE CLOSED-LOOP MODE Table 8-1. Comparison between open-loop and closed-loop pressure control. G 9. Do your observations confirm that the open-loop pressure control makes it possible to maintain the pressure constant when the flow rate varies? G Yes G No G 10. On the Conditioning Unit, close the main shutoff valve. Sensing the Pressure Using the Trainer Pressure Transducer G 11. Connect the circuit shown in Figure 8-4. Note: Do not connect the 0-10 V output of the [0-5 V] E/E converter of the Signal Conditioners module to the negative input of the ERROR DETECTOR of the PID Controller at this time. Also, do not connect the integral amplifier output to the summing point of the PID Controller at this time. G 12. Make the following settings on the PID Controller: PROPORTIONAL (P) GAIN range...................... LOW PROPORTIONAL (P) GAIN............................ MIN. 8-7
G 13. Turn on the DC Power Supply and PID Controller. Do not open the main shutoff valve on the Conditioning Unit at this time. G 14. On the PID Controller, set the SETPOINT potentiometer 1 to obtain 10.0 V at the SETPOINT output 1. Set the PROPORTIONAL GAIN to obtain 12.0 V approximately at the LIMITER input. Set the UPPER LIMIT potentiometer to obtain 10.0 V at the LIMITER output. Set the SETPOINT potentiometers 1 and 2 to obtain 0.0 V at the SETPOINT output 1, then select SETPOINT potentiometer 1. Set the PROPORTIONAL GAIN at ½ of MAX. G 15. On the Signal Conditioners module, set the Z (zero) and S (span) potentiometers of the [0-5 V] E/E converter as follows: Zero (Z).................................... fully clockwise Span (S)............................. fully counterclockwise G 16. Close Flow Control Valve FCV2 by turning the control knob fully clockwise. G 17. On the Conditioning Unit, open the main shutoff valve. Set the main pressure regulator to obtain 630 kpa (90 psi) on the regulated pressure gauge. G 18. Connect a DC voltmeter to the 1-5 V output of the Pressure Transducer. With a setpoint value of 0.0 V, the pressure indicated by the Pressure Transducer display should be approximately 0 kpa (0 psi). Is this your observation? G Yes G No Note: If the pressure units indicated at the bottom of the Pressure Transducer display do not correspond to your unit system, refer to Appendix D to modify the setting of the Pressure Transducer. 8-8
Figure 8-4. Closed-loop pressure control. Record below the voltage generated at the 1-5 V output of the Pressure Transducer for that pressure. Pressure transducer output voltage = 8-9
G 19. Connect the DC voltmeter at the 0-10 V output of the [0-5 V] E/E converter on the Signal Conditioners module. While observing the voltmeter reading, slowly turn the knob of the potentiometer Z (zero) of the [0-5 V] E/E converter counterclockwise and stop turning as soon as the voltage reaches 0.0 V. G 20. Increase the system pressure by increasing the SETPOINT potentiometer 1 value to obtain 350 kpa (50 psi) on the Pressure Transducer display. Observe that this causes the voltage at the 1-5 V output of the Pressure Transducer to increase. Record below the voltage generated at the 1-5 V output of the Pressure Transducer for that pressure. Pressure transducer output voltage = G 21. With the DC voltmeter connected to the 0-10 V output of the [0-5 V] E/E converter on the Signal Conditioners module, set the potentiometer S (span) of this converter to obtain 10.0 V on the voltmeter display. Closed-Loop Pressure Control G 22. Make the following settings on the PID Controller: PROPORTIONAL (P) GAIN range....................... LOW PROPORTIONAL (P) GAIN....................... a of MAX. INTEGRAL (I) GAIN............................. ½ of MAX. INTEGRATOR ANTI-RESET.............................. I G 23. Place the system in the closed-loop mode. To do so, connect the 0-10 V output of the [0-5 V] E/E converter on the Signal Conditioners module to the negative input of the ERROR DETECTOR. G 24. On the P.I.D. Controller, connect the integral amplifier output to the summing point. Set the SETPOINT potentiometer 1 to obtain 140 kpa (20 psi) on the Pressure Transducer display. Record the pressure value in the "0 turn" row of the column CLOSED- LOOP. Record also the voltage control at the 0-10 V input of the Servo Control Valve in the appropriate cell in Table 8-1. G 25. Gradually open Flow Control Valve FCV2 by turning the control knob counterclockwise as indicated in Table 8-1. For each setting, record the pressure indicated on the Pressure Transducer display and the voltage 8-10
control at the 0-10 V input of the Servo Control Valve in the appropriate cells in Table 8-1. Note: Reduce the PROPORTIONAL GAIN and/or INTEGRAL GAIN if the system becomes unstable and starts to oscillate. G 26. Compare the pressure values obtained in the closed-loop mode of control to that obtained in the open-loop mode. In the closed-loop mode, the pressure should be maintained constant for many turns of opening of the Flow Control Valve while it rapidly starts to decrease in the open-loop mode of control. Is this your observation? G Yes G No G 27. Referring to the data in Table 8-1, describe what happens to the control voltage of the Servo Control Valve as the opening of the Flow Control Valve increases in the closed-loop mode of control. System Repeatability G 28. Close Flow Control Valve FCV2 by turning the control knob fully clockwise, then open the valve four turns. G 29. Now test the repeatability of the system by alternately selecting the SETPOINT potentiometers 1 and 2 and watching the Pressure Transducer display. Is the system capable of arriving at the same pressure level (140 kpa (20 psi)) on successive cycles? Explain. G 30. Close Flow Control Valve FCV2 by turning the control knob fully clockwise. On the P.I.D. Controller, select the SETPOINT potentiometer 1. On the Conditioning Unit, use the main pressure regulator to decrease the regulated pressure by decrements of 70 kpa (10 psi) until it attains 0 kpa (0 psi). For each pressure setting, enter the pressure displayed by the Pressure Transducer in Table 8-2. 8-11
REGULATED PRESSURE (SERVO CONTROL VALVE INPUT) OUTPUT PRESSURE (SERVO CONTROL VALVE OUTPUT) 630 kpa (90 psi) 560 kpa (80 psi) 490 kpa (70 psi) 420 kpa (60 psi) 350 kpa (50 psi) 280 kpa (40 psi) 210 kpa (30 psi) 140 kpa (20 psi) 70 kpa (10 psi) 0 kpa (0 psi) Table 8-2. Relationship between the input and output pressure of the Servo Control Valve G 31. Referring to the data in Table 8-2, explain the relationship you observe between the input and output pressure of the Servo Control Valve. G 32. On the Conditioning Unit, close the shutoff valves, and turn the regulator adjusting knob completely counterclockwise. G 33. Turn off the PID Controller and the DC Power Supply. G 34. Disconnect and store all leads and components. CONCLUSION In this exercise, you controlled the system pressure with an open-loop and a closedloop system. In the open-loop system, the pressure decreased rapidly as the opening of the flow control valve was increased. You saw that in the closed-loop system, the system was able to maintain constant the pressure for many turns of opening of the flow control valve. You also saw that the system is able to arrive at the same pressure level on successive cycles as long as the supply pressure remains above that called for by the setpoint. 8-12
REVIEW QUESTIONS 1. What are the advantages of using a servo control valve of the pressure type to control the pressure in a system? 2. What happens in a closed-loop pressure control system when a positive error exists between the setpoint and measured pressure? 3. Explain why it is important that the supply pressure at the inlet port of the servo control valve be kept above that called for by the setpoint at all times while the system is functioning. 4. Briefly describe the operation of the trainer Pressure Transducer. 8-13