Time-Delay Electropneumatic Applications

Transcription

1 Exercise 3-4 EXERCISE OBJECTIVE & & & To introduce time delays; To describe the operation of a time-delay valve; To describe the operation of a time-delay relay. DISCUSSION Time-Delays Time delays are commonly used in machining operations to delay the start-up of a cylinder, start a series of cylinders one at a time, or hold a cylinder in a predetermined position for some period of time. For example, a metal drilling operation may require the drill bit to remain in the hole temporarily to clean up the cut after drilling through the metal. This function, called "cylinder dwell", is accomplished through the use of a time-delay valve or by a time-delay relay that prevents the drill cylinder from retracting immediately after reaching the end of the extension stroke. Time-Delay Valves Time delay valves involve bleeding air into or out of a volume chamber. Figure 3-18 shows a pneumatic circuit using a typical time-delay valve. In this circuit, solenoid DV1-SOL-A controls the extension of the cylinder rod and DV1-SOL-B controls the retraction. When solenoid DV1-SOL-B is actuated, compressed air flows through flow control valve FCV1 to the accumulator, and to the pilot port of directional valve DV3. When the pressure in the accumulator reaches a certain point, it shifts the spool of directional valve DV3. This causes the spool of DV2 to shift and the cylinder rod to retract. 3-39

2 DV2 SOL-A LS1 LS2 ACCUMULATOR DV3 FCV2 FCV1 DV1 SOL-B TIME-DELAY VALVE PNEUMATIC DIAGRAM (+) START PB1 LS1, NO CR2-A 1 CR1 ( ) CR1-A DV1-SOL-A L1 2 LS1, NC LS2, NO CR2 CR2-B DV1-SOL-B L2 LADDER DIAGRAM Figure Pneumatic Circuit Using a Time-Delay Valve. Time-Delay Relays Time-delay relays are also designed to create a delay between two operations of a work cycle. They basically consist of a solenoid coil, an internal timer, and one or more sets of NO and NC relay contacts. They can be designed for either ON-delay or OFF-delay operation. 3-40

3 In an ON-delay circuit, the relay is energized after a preset time period has elapsed. In an OFF-delay circuit, the relay energizes immediately, and deenergizes immediately when the preset time elapses. The Time-Delay Relay / Counter supplied with your trainer is shown in Figure It can be programmed for either timing or counting function by configuring the thumbwheel switches on the unit. RESET CONTROL DISPLAY AREA R C 1 ON NC NO A S THUMBWHEEL SWITCHES TIMER MODE A : ON-DELAY B : OFF-DELAY PRESET TIME TIME BASE 001 TO 999 S : SECONDS M : MINUTES H : HOURS Figure Time-Delay Relay / Counter. To program the Time-Delay Relay / Counter for the timing function, three parameters must be configured: the "timer mode", the "preset time", and the "time base". The "timer mode" switch selects between the following timer modes: & ON-delay (switch set to A). In this mode, the internal timer starts when the CONTROL (C) input is switched to common ( ). When the preset time has elapsed, both sets of relay contacts are activated. Momentarily switching the RESET (R) input to common deactivates both sets of relay contacts and resets the timer value. 3-41

4 & OFF-delay (switch set to B). In this mode, the internal timer starts and both sets of relay contacts are activated immediately when the CONTROL (C) input is switched to common ( ). When the preset time has elapsed, both sets of relay contacts are deactivated. Momentarily switching the RESET input to common deactivates both sets of relay contacts and resets the timer value. The "preset time" switches specify the value (between 001 and 999) the timer must reach before the relay contacts are activated (ON-delay operation) or deactivated (OFF-delay operation). The "time base" switch determines the time base. The time base is a measure of the interval counted by the timer. Selectable as 0.1 second (switch set to.1 S), 1 second (S), 0.1 minute (.1 M), 1 minute (M), 0.1 hour (.1 H), 1 hour (H), or 10 hours (10 H). Procedure Summary In the first part of the exercise, you will test the operation of a time-delay relay. In the second part of the exercise, you will test the operation of a time-delay valve. In the third part of the exercise, you will replace the time-delay valve by a time-delay relay in the circuit of second part. EQUIPMENT REQUIRED Refer to the Equipment Utilization Chart, in Appendix A of this manual, to obtain the list of equipment required to perform this exercise. PROCEDURE Operation of a Time-Delay Relay * 1. Connect the circuit shown in Figure Use the trainer Time-Delay Relay / Counter as timer TD

5 (+) ( ) TIME PB1 1 RESET PB2 + C TD1 R - 2 TD1-A L1 3 TD1-B L2 Figure Testing the Operation of a Time-Delay Relay. * 2. On the Time-Delay Relay / Counter TD1, set the thumbwheel switches to A010S. This will program the Time-Delay Relay / Counter for ON-delay operation, and set the preset time to 10 s. * 3. Turn on the DC Power Supply. Note: When pushbutton PB1 is in the normal (released) condition, control (C) input is deactivated and relay contacts TD1-A and TD1-B are in their normal state. * 4. Record the status of the pilot lamps in the appropriate cell in Table 3-3. * 5. What is the timer value shown on the Time-Delay Relay / Counter display? * 6. While observing the displayed time and the status of pilot lamps L1 and L2, depress pushbutton PB1 momentarily to activate the CONTROL input of time-delay relay TD

6 CIRCUIT CONDITIONS LAMP L1 LAMP L2 Control Input Deactivated Immediately After Activation of the Control Input 10 s After Activation of the Control Input After Reset Table 3-3. Lamp Status Versus Control Input Condition. * 7. Do the pilot lamps change state immediately after the CONTROL input is activated? Or do they change state 10 s after the CONTROL input has been activated? * 8. Record the status of the pilot lamps in the appropriate cell in Table 3-3. * 9. Momentarily depress the RESET pushbutton, PB2, which will remove the supply voltage at the () input of the time-delay relay and will reset the timer value. Do the pilot lamps return to their initial state when the timer value is reset? * Yes * No * 10. Record the status of the pilot lamps in the appropriate cell in Table 3-3. * 11. Based on the data recorded in Table 3-3, explain how the time-delay relay operates in the ON-delay mode. * 12. On the Time-Delay Relay / Counter (TD1), set the thumbwheel switches to B010S. This will program the Time-Delay Relay / Counter for OFF-delay operation, and set the preset time to 10 s. 3-44

7 * 13. While observing the displayed time and the status of pilot lamps L1 and L2, depress pushbutton PB1 momentarily to activate the CONTROL input of time-delay relay TD1. * 14. Do the pilot lamps change state immediately after the CONTROL input is activated or do they change state 10 s after the CONTROL input has been activated? * 15. Based on your observations, explain how the time-delay relay operates in the OFF-delay mode. * 16. Turn off the DC Power Supply, and disconnect your circuit. Operation of a Time-Delay Valve * 17. Connect the circuit shown in Figure Use the AND Function Valve as air-piloted directional valve DV4. Refer to Exercise 2-4 to identify the ports of the valve if necessary. * 18. Ensure that the cylinder rod is retracted. * 19. Verify the status of the trainer according to the procedure given in Appendix F. * 20. Close Flow Control Valves FCV1 and FCV2 by turning the control knob fully clockwise. Then open the valves by turning the control knobs two turns counterclockwise. * 21. Open the shutoff valve and the branch shutoff valves at the manifold and set the pressure at 400 kpa (or 60 psi) on the regulated pressure gauge. 3-45

8 DRILL CYLINDER (SIMULATION) DV1 DV3 FCV2 ACCUMULATOR DV4 DV2 FCV1 Figure Testing the Operation of a Time-Delay Valve. * 22. Start the system by depressing the control button of directional valve DV1 momentarily. The cylinder rod should extend to full stroke. * 23. Depress and hold the control button of directional valve DV2 to retract the cylinder rod. * 24. Does the rod retract immediately? Explain. * 25. Operate the system several times to become familiar with its operation. * 26. Without modifying the regulated pressure, use Flow Control Valve FCV1 to set the time delay at approximately 4 s. Note: Opening FCV1 causes the pressure in the Accumulator to increase more rapidly. Directional valve DV3 shifts earlier, and the time delay is shorter. 3-46

9 * 27. Set the pressure at 600 kpa (or 80 psi) on the regulated pressure gauge. * 28. Restart the system several times to observe the time delay variation. Explain what happens to the time delay when the pressure increases. * 29. On the Conditioning Unit, close the shutoff valves, turn the regulator adjusting knob completely counterclockwise, and disconnect your circuit. Note: Be careful when disconnecting your circuit. The Accumulator may contain compressed air. Dwell Period Control Using a Time-Delay Relay * 30. Modify your circuit as shown in Figure * 31. On the Time-Delay Relay / Counter TD1, set the thumbwheel switches to A004S. This will program the Time-Delay Relay / Counter for ON-delay operation, and set the preset time to 4 s. * 32. Turn on the DC Power Supply. * 33. Referring to the ladder diagram in Figure 3-22, describe the operation of the circuit. * 34. On the Conditioning Unit, open the shutoff valves and set the pressure at 300 kpa (or 40 psi) on the regulated pressure gauge. * 35. Start the system by depressing the START pushbutton momentarily. 3-47

10 SOL-A LS1 LS2 FCV1 DV1 SOL-B PNEUMATIC DIAGRAM (+) START PB1 LS1, NO CR2-A 1 CR1 ( ) CR1-A DV1-SOL-A L1 2 LS1, NC LS2, NO CR2 CR2-B CR2-C C TD1 + R 3 TD1-A DV1-SOL-B L2 LADDER DIAGRAM Figure Testing the Operation of a Time-Delay Relay. 3-48

11 * 36. Restart the system several times to become familiar with the operation of the system. What causes the cylinder rod to dwell when it becomes fully extended? * 37. Set the pressure at 600 kpa (or 80 psi) on the regulated pressure gauge. * 38. Restart the system several times to observe the dwell period variation. Explain what happens to the dwell period when you increase the pressure. * 39. Turn off the DC Power Supply. * 40. On the Conditioning Unit, close the shutoff valves, and turn the regulator adjusting knob completely counterclockwise. * 41. Disconnect and store all leads and components. CONCLUSION In the first part of the exercise, you tested the operation of a time-delay relay. You saw that a time-delay relay can be used in the ON-delay and OFF-delay modes. In the second part of the exercise, you tested the operation of a time-delay valve. You saw that time-delay valves involve bleeding air into or out of a volume chamber. You saw that the delay can be adjusted using a flow control valve. You also saw that pressure variations cause delay variations. In the third part of the exercise, you replaced the time-delay valve by a time-delay relay. You saw that with a time-delay relay, the delays can be set very accurately and do not vary with pressure variations. 3-49

12 REVIEW QUESTIONS 1. What is the difference between an ON-delay and OFF-delay relay? 2. What is meant by "preset time"? 3. What is meant by "cylinder dwell"? 4. In the pneumatic circuit of Figure 3-22, determine how the time delay will vary if the volume of the accumulator is increased? 5. What is the main disadvantage of a time-delay valve? 3-50

13 Unit Test 1. The main function of a sequencing circuit is to a. operate actuators in a particular order. b. cycle a cylinder many times. c. cycle a cylinder one time. d. monitor system pressure. 2. Similar to the pressure relief valve, the sequence valve is a. normally open and it senses the pressure downstream. b. normally closed and it senses the pressure upstream. c. normally open and it senses the pressure upstream. d. normally closed and it senses the pressure downstream. 3. Multi-pressure systems are primarily used to a. synchronize two actuators. b. reciprocate a cylinder. c. sequence the operation of actuators. d. provide different pressures during different stages of operation. 4. Which setting of a pressure switch prevents minor pressure drops in a sensing line from deactivating the switch after it has activated? a. Set pressure. b. Differential pressure. c. Actuation pressure. d. Reset pressure. 5. What is the purpose of a limit switch in an electrically controlled cycling system? a. Maintaining the position of a cylinder; b. Monitoring the position of a cylinder; c. Adjusting the position of a cylinder; d. Energizing and deenergizing valve solenoids. 6. Which of the following can be used as a pressure selector valve? a. Sequence valve; b. Relief valve; c. Check valve; d. Directional control valve. 3-51

14 Unit Test (cont'd) 7. The difference between the actuation pressure and the reset pressure of a pressure switch is called the a. proof pressure. b. pressure differential. c. ambient pressure. d. actuation point. 8. Which characteristic of a double-air-pilot directional valve allows to maintain the spool position without maintaining pilot pressure? a. There are two pilot ports; b. There is no return spring; c. Because they are air-pilot operated; d. None of the above. 9. Cylinder dwell is a function used to a. allow a cylinder to retract immediately after contacting the workpiece. b. allow a cylinder to retract immediately after reaching the end of the extension stroke. c. prevent a cylinder from extending immediately after reaching the end of the retraction stroke. d. prevent a cylinder from retracting immediately after reaching the end of the extension stroke. 10. Which one of the following describes the operation of a solid-state ON-delay relay? a. It immediately shifts its contacts to the activated state when the coil is energized. It returns them to the normal state after a preset time has passed; b. It shifts its contacts to the activated state after a preset time has passed once the coil has been de-energized. It returns them to the activated state immediately when the coil is energized; c. It does not shift its contacts to the activated state until a preset time has passed after the coil has been energized. It returns them to the normal state immediately when the coil is de-energized; d. It does not shift its contacts to the deactivated state until a preset time has passed after the coil has been energized. It returns them to the activated state immediately when the coil is de-energized. 3-52