Acceleration and Deceleration Control

Transcription

1 Exercise 2 Acceleration and Deceleration Control EXERCISE OBJECTIVE To learn how to eliminate abrupt starting and stopping of an actuator with acceleration and deceleration control. DISCUSSION Acceleration and Deceleration Control Conventional directional control valves have only two conditions: open (passing) and closed (non passing). For this reason, they cause sudden increase, stoppage, or reversal of the air flow, which, in turn, may result in a jerky motion of the actuator. Servo control valves, by contrast, allow smooth variation of the air pressure. Smooth variation is achieved by using a ramp generator that converts the rapid transitions in the valve control voltage into ramps with slopes m 1 and m 2, as Figure 2-1 shows. Slopes m 1 and m 2 are usually adjusted independently by means of separate potentiometers. Slope m 1 controls the rate of change of positive ( ) voltages and, therefore, the acceleration and deceleration rates of the actuator. In the case of directional control valves where the shifting direction of the spool is determined by the polarity of the control voltage, slope m 2 controls the rate of change of negative ( ) voltages and, therefore, the acceleration and deceleration rate of the actuator in the other direction of motion. The Servo Control Valve supplied with your trainer is of the pressure type and does not require negative control voltages. The steeper the slope is, the quicker the acceleration and deceleration rates of the actuator will be. 2-1

2 Figure 2-1. Voltages at the input and output of a ramp generator. The RAMP GENERATOR of the Trainer PID Controller The trainer PID Controller comprises a RAMP GENERATOR section that can be used to "ramp" the control voltage applied to the Servo Control Valve, as Figure 2-2 shows. Potentiometer 1 of the RAMP GENERATOR allows adjustment of the ramp time for positive control voltages. The ramp time can be set between 0.5 s and 3 s for a 10-V variation of the positive control voltage. If, for example, the ramp time is set at 3 s, it will take 3 s for the control voltage to increase from 0.0 V to 10.0 V or decrease from 10.0 V to 0.0 V, but it will take only 1.5 s for this voltage to increase from 3.0 V to 8.0 V or decrease from 7.0 V to 2.0 V; Potentiometer 2 of the RAMP GENERATOR allows adjustment of the ramp time for negative control voltages. The ramp time can be set between 0.5 s and 3 s for a 10.0-V variation of the negative control voltage. If, for example, the ramp time is set at 1.5 s, it will take 1.5 s for the control voltage to increase from 0.0 V to 10.0 V or decrease from 10.0 V to 0.0 V, but it will take only 0.75 s for this voltage to increase from 5.0 V to 10.0 V or decrease from 6.0 V to 1.0 V. 2-2

3 Figure 2-2. The RAMP GENERATOR section of the PID Controller. Procedure summary In the first part of the exercise, Ramping of a Control Voltage, you will learn how to ramp the control voltage applied to the trainer Servo Control Valve. In the second part of the exercise, Acceleration and Deceleration Control of a Pneumatic Cylinder, you will control the acceleration and deceleration of a cylinder rod. 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 Ramping of a Control Voltage G 1. On the PID Controller, connect the SETPOINT output 1 to the RAMP GENERATOR input and to a DC voltmeter, as Figure 2-3 shows. G 2. Turn on the PID Controller. 2-3

4 Figure 2-3. Ramping of a control voltage. G 3. Select the SETPOINT potentiometer 1 by setting the toggle switch S1 to the position A. Set the SETPOINT potentiometer 1 to obtain 10.0 V at the SETPOINT output 1. G 4. Select the SETPOINT potentiometer 2 by setting the toggle switch S1 to the position B. Set the SETPOINT potentiometer 2 to obtain 0.0 V at the SETPOINT output 1. G 5. While monitoring the voltage at the SETPOINT output 1, select the SETPOINT potentiometer 1. Does the voltage instantly rise to 10.0 V? G Yes G No G 6. While monitoring the voltage at the SETPOINT output 1, select the SETPOINT potentiometer 2. Does the voltage instantly return to 0.0 V? G Yes G No G 7. Connect the DC voltmeter to the RAMP GENERATOR output. G 8. Set the RAMP GENERATOR potentiometer 1 to MAX. This selects the maximum ramp time for positive control voltages. 2-4

5 G 9. While monitoring the RAMP GENERATOR output voltage, select the SETPOINT potentiometer 1. Does it take about 3 s for the voltage to reach 10.0 V? Explain. G 10. While monitoring the RAMP GENERATOR output voltage, select the SETPOINT potentiometer 2. Does it take about 3 s for the voltage to return to 0.0 V? Explain. G 11. Set the RAMP GENERATOR potentiometer 1 to MIN. Alternately select the SETPOINT potentiometers 1 and 2 to observe the effect of decreasing the ramp time on the RAMP GENERATOR output voltage. Record your observations. G 12. Set the RAMP GENERATOR potentiometer 1 to MAX. Set the SETPOINT potentiometer 1 to obtain 10.0 V at the SETPOINT output 1. Leave the SETPOINT potentiometer 2 set at 0.0 V. G 13. Set the RAMP GENERATOR potentiometer 2 to MAX. This selects the maximum ramp time for negative control voltages. G 14. Alternately select the SETPOINT potentiometers 1 and 2 and observe what happens to the RAMP GENERATOR output voltage. Try various settings for the RAMP GENERATOR potentiometer 2 and record your observations. G 15. Turn off the PID Controller. 2-5

6 Acceleration and Deceleration Control of a Pneumatic Cylinder G 16. Connect the circuit shown in Figure 2-4. Note: The setting of the RAMP GENERATOR potentiometer 1 will determine the speed at which the force generated by the cylinder will be applied to the load. G 17. Verify the status of the trainer according to the procedure given in Appendix B. G 18. Turn on the DC Power Supply and PID Controller. G 19. Set the SETPOINT potentiometers 1 and 2 to obtain 0.0 V and 10.0 V, respectively, at the SETPOINT output 1. Then select the SETPOINT potentiometer 1. G 20. Set the RAMP GENERATOR potentiometer 1 to MIN. G 21. 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 630 kpa (90 psi) on the regulated pressure gauge. G 22. While monitoring the force generated by the cylinder on the load device, select the SETPOINT potentiometer 2. Alternately select potentiometers 1 and 2 to observe the speed at which the force generated by the cylinder increases. Does the force rapidly increase to the maximum value? G Yes G No 2-6

7 Figure 2-4. Ramp control of a pneumatic cylinder. G 23. Select the SETPOINT potentiometer 1, then set the RAMP GENERATOR potentiometer 1 to MAX. G 24. While monitoring the force generated by the cylinder on the load device, select the SETPOINT potentiometer 2. Alternately select potentiometers 1 and 2 to observe the speed at which the force generated by the cylinder increases. 2-7

8 With the RAMP GENERATOR potentiometer 1 set to MAX., do you observe that the force generated by the cylinder increases more slowly, due to the action of the ramp circuit? G Yes G No G 25. On the Conditioning Unit, close the shutoff valves, and turn the regulator adjusting knob completely counterclockwise. G 26. Turn off the PID Controller and the DC Power Supply. G 27. Disconnect and store all leads and components. CONCLUSION In this exercise, you learned how to ramp the control voltage applied to the trainer Servo Control Valve. Without the ramp generator, the control voltage changed instantly when you changed the setpoint. With the ramp generator, the control voltage gradually changed more or less rapidly, depending on the setting of the RAMP potentiometers. In the second part of the exercise, you used ramped control voltage to control the speed at which the force generated by a cylinder is applied to a load. REVIEW QUESTIONS 1. Why do conventional on/off switching directional control valves often result in a jerky motion of the actuator and load? 2. How is smooth shifting of a valve achieved? 2-8

9 3. What happens to the acceleration and deceleration rates of a pneumatic actuator when the slope of the control voltage applied to a servo control valve is increased? 2-9