Exercise 4-2 EXERCISE OBJECTIVE To describe the operation of an electrical counter; To assemble and test a continuous reciprocation system; To extend and retract a cylinder a definite number of times using an electrical counter; To describe the operation of a photoelectric switch; To measure the rotation speed of a motor using an electrical counter. DISCUSSION Electrical Counters Electrical counters are used in electrically controlled pneumatic systems when parts of the system must be activated or deactivated after a definite number of events have occurred. They are widely used in industry to count quantities produced during process and control operations. They are also used in machine maintenance scheduling to control the number of machine operations. A typical application is an automated packaging system that stacks and counts production items into groups. The usual method is for one cylinder to continuously extend and retract, picking and stacking one item on each cycle, and for a counter to count the number of cycles which have been made by the cylinder. When the required count is reached, the counter generates a switching signal which causes another cylinder to push the stack away. Basic Counter Operation An electrical counter basically consists of one or more NO and NC contacts, a control terminal to receive input count pulses, a reset terminal to which a momentary pulse can be applied to reset the counter to zero, and some means to preset the counter value, usually thumbwheel switches. The counter preset value specifies the value which the counter must reach before activating its output contacts. Each time a pulse is received at the control terminal, the count is incremented by one. Additional input pulses continue to increment the count. Momentary activation of the reset input deactivates the counter contacts and resets the count to zero. 4-15
Pneumatic Trainer Counter As was said in Exercise 3-4, the Time-Delay Relay / Counter supplied with your trainer can be programmed for the counting function by configuring the thumbwheel switches on top of the unit accordingly. To program the Time-Delay Relay / Counter for the counting function, three parameters must be configured: the counting mode, the preset count, and the counting interval, as shown in Figure 4-7. RESET CONTROL DISPLAY AREA R C 1 ON NC NO 4 2 5 C A 0 0 5 O 3 6 7 8 + - THUMBWHEEL SWITCHES COUNTING INTERVAL A, D TO J: NORMAL B: DIVIDE BY 10 C: DIVIDE BY 100 PRESET COUNT COUNTER MODE 001 TO 999 CO: OPERATE AT PRESET COUNT CR: RELEASE AT PRESET COUNT Figure 4-7. Time-Delay Relay / Counter in Counting Mode. The "counter mode" switch selects between the following operation modes: Operate at Preset Count (switch set to CO): each time the CONTROL (C) input is switched to common ( ) and then disconnected from common, the counter value is incremented by one count. When the preset count is reached, both sets of relay contacts are shifted to their activated state. Additional input pulses continue to increment the displayed count. Momentarily switching the RESET (R) input to common deactivates the relay contacts and resets the counter value to zero. 4-16
Release at Preset Count (switch set to CR): momentarily switching the RESET input to common activates the relay contacts. Operation is similar to CO mode except that the relay contacts are deactivated when the preset count is reached. The "preset count" switches specify the value which the counter must reach before the relay contacts are activated (CO operation) or deactivated (CR operation). Can be set between 001 and 999. The "counting interval" switch selects the interval counted by the counter. Selectable as Normal, Divide by 10, or Divide by 100: Normal (switch set to A, or D to J): the counter value is incremented each time an input signal is received at the CONTROL input. Divide by 10 (switch set to B): the counter value is incremented for every 10 input signals for a maximum count of 9990. Divide by 100 (switch set to C): the counter value is incremented for every 100 input signals for a maximum count of 99 900. Continuous Reciprocation of a Cylinder Many automated processes involving machining operations require that a cylinder be extended and retracted indefinitely after the process is started. This is called "continuous reciprocation", and a simple electrical control circuit can be used to control this action. Continuous reciprocation systems are usually started manually. The cylinder then extends and retracts continuously until a signal stops the cycling process. This signal could come from an operator, a counter, a timer, or an emergency circuit. As an example, Figure 4-8 shows a continuous reciprocation system featuring single-cycle reciprocation and counting functions. The "single-cycle reciprocation function" allows the cylinder to extend and retract only one time, allowing initial set-up and testing of the machine. The counting function allows the cycling process to be stopped after a pre-definite number of cycles. 4-17
SOL-A DV1 PX1 PX2 FCV1 PNEUMATIC DIAGRAM (+) START PB1 PX1, NO PX2, NC ( ) 1 CR1 CONTINUOUS CR2-A DV1-SOL-A HOLD CR1-A L1 2 CR1-B CONTINUOUS MODE PB2 STOP PB3 CR2 CR2-B CT1-A CR1-C 3 + C CT1 R CT1-B LADDER DIAGRAM Figure 4-8. Single-Cycle and Continuous Cylinder Reciprocation System. Depressing the START pushbutton will cause the cylinder to extend and retract one time, allowing initial set-up and testing of the system. Depressing the START pushbutton PB1, and then the CONTINUOUS MODE pushbutton PB2 while the cylinder is extending will cause the cylinder to reciprocate continuously until the STOP pushbutton is depressed. Detailed circuit action is as follows: Depressing the START pushbutton PB1 causes relay coil CR1 to energize. Holding relay contact CR1-A closes to lock in relay coil CR1 so as to energize solenoid DV1-SOL-A and extend the cylinder. NO contact CR1-C closes and the counter is incremented by one count. 4-18
When the cylinder becomes fully extended, magnetic proximity switch PX2 opens, thus deenergizing relay coil CR1. This deenergizes solenoid DV1-SOL-A, causing the cylinder to retract. When the cylinder becomes fully retracted, it activates magnetic proximity switch PX1. If the CONTINUOUS MODE pushbutton PB2 has been depressed during cylinder extension, NO contact CR2-A in rung 1 will be closed and the cylinder will reciprocate until the pre-definite number of cycles set on the counter is executed, NC contact CT1-A will then open and the cylinder will stop. If PB2 has not been depressed, rung 1 will be open and the cylinder will stop. The NO contact CT1-B allows the counter to be reset by connecting the RESET (R) input to common when the pre-definite number of cycles is attained. Photoelectric Switches A photoelectric switch is a sensing element that uses a light beam to sense the presence or motion of an object. Unlike a mechanical limit switch, it can perform these functions without physical contact. Photoelectric switches have several useful applications, including cylinder position sensing, level sensing, product detection and counting, and speed monitoring. The photoelectric switch consists of a light source, a receiver, and one or more sets of NO and NC contacts. The light source and receiver can be in the same casing or separate casings. The light source projects a light beam, which may be visible or infrared. The receiver picks up the light from the source, but ignores ambient light. Figure 4-9 shows the Diffuse Reflective Photoelectric Switch supplied with your trainer. This switch is of the proximity type. It consists of a visible light source and a receiver combined in the same casing. When powered by a 24-V dc voltage, the light source projects a beam of red light. When no object is within the detection zone of the switch, the receiver sees dark and keeps the switch contacts deactivated. When a reflective object such as a cylinder rod enters the detection zone, light reflects off the object back to the receiver. This causes the receiver to activate the switch contacts and to keep them activated until the object is removed from the detection zone. A red LED at the rear of the switch indicates the status of the contacts. When it is on, the LED indicates that the switch contacts are activated. 4-19
LIGHT-SENSING RECEIVER VISIBLE LIGHT SOURCE PNEUMATIC DIAGRAM SYMBOL NO TERMINAL DETECTION ZONE DETECTION RANGE COMMON TERMINAL NC TERMINAL LADDER DIAGRAM SYMBOL Figure 4-9. Photoelectric Switch. The trainer Diffuse Reflective Photoelectric Switch has a detection range of 10.2 cm (4 in), which means that it is able to detect objects located within 10.2 cm (4 in) of it. This range will vary slightly depending upon the reflectance, or ability to reflect light, of objects. The more reflective an object, the farther the detection range. The reflectance of an object depends on its surface material, color, and texture. Procedure Summary In the first part of the exercise, you will test the operation of an electrical counter. In the second part of the exercise, you will connect and test a system that uses an electrical counter to extend and retract a cylinder a definite number of times. In the third part of the exercise, you will test the operation of a photoelectric switch. In the fourth part of the exercise, you will use an electrical counter to measure the rotation speed of a pneumatic motor. 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. 4-20
PROCEDURE Operation of an Electrical Counter * 1. Connect the circuit shown in Figure 4-10. (+) COUNT ( ) PB1 COUNTER 1 CONTROL (C) + CT1 RESET (R) RESET PB2 2 CT1-A L1 Figure 4-10. Testing the Operation of an Electrical Counter. * 2. On the Time-Delay Relay / Counter, set the thumbwheel switches to A005, This will program the Time-Delay Relay / Counter for the CO 2 (Operate at Preset Count) counting function, set the preset value to 5, and select a normal counting interval. * 3. Turn on the DC Power Supply. * 4. What is the count value shown on the Time-Delay Relay / Counter display? 4-21
* 5. Depress momentarily the COUNT pushbutton PB1. What happens to the displayed count? Why? Explain by referring to the ladder diagram in Figure 4-10. * 6. While observing pilot lamp L1, depress pushbutton PB1 several times until the displayed count reaches the preset value of 5. What happens to lamp L1 when the displayed count reaches 5? Why? Explain by referring to the ladder diagram in Figure 4-10. * 7. Again depress pushbutton PB1 several times while observing the displayed count. Do additional input pulses continue to increment the displayed count? * Yes * No * 8. Momentarily depress the RESET pushbutton, PB2. What happens to the displayed count? To pilot lamp L1? Why? * 9. While observing the displayed count, depress and hold pushbutton PB1 for a few seconds, then release PB1. Is the displayed count incremented immediately when PB1 is depressed or after PB1 is released? * 10. Based on the observation you made in the previous step, is the counter value incremented each time the CONTROL (C) input is switched to common ( ) and then disconnected from common? * Yes * No * 11. Turn off the DC Power Supply. Disconnect all leads and components. 4-22
Continuous Reciprocation of Cylinders * 12. Connect the circuit shown in Figure 4-8. * 13. Mount the Magnetic Proximity Switches on the cylinder so that PX1 is activated when the cylinder rod is fully retracted, and PX2 activated when the cylinder rod is fully extended. Refer to exercise 2-3 if necessary. * 14. On the Time-Delay Relay / Counter, make sure the thumbwheel switches are set to A005. 2 * 15. Verify the status of the trainer according to the procedure given in Appendix F. * 16. Close Flow Control Valve FCV1 by turning the control knob fully clockwise. Then open the valve by turning the control knob two turns counterclockwise. * 17. 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. * 18. Turn on the DC Power Supply, and start the system by depressing the START pushbutton momentarily. The cylinder rod should automatically extend and retract 1 time, then it should stop at the home position. Record whether you observe this or not. * Yes * No * 19. Restart the system several times to become familiar with the operation. What causes the cylinder rod to extend when the START pushbutton is depressed? Explain by referring to the ladder diagram in Figure 4-8. 4-23
* 20. Depress the START pushbutton and observe the count value on the Time- Delay Relay / Counter display. Is the displayed count incremented by one each time the cylinder rod becomes fully extended? Why? * 21. Is the counter value automatically reset to zero at the moment when the cylinder completes its fifth extension stroke? Why? * 22. Depress simultaneously the START pushbutton and the CONTINUOUS MODE pushbutton while the cylinder is extending. Observe the operation of the system in continuous mode. Does the cylinder rod extend and retract five times, then stop at the home position? Record whether you observe this or not. * Yes * No * 23. Depress the STOP pushbutton while the cylinder rod is extending. Does the rod stop immediately? Explain why. * 24. Try to start the cylinder rod by depressing the CONTINUOUS MODE pushbutton. What happens? Explain why. * 25. Turn off the DC Power Supply. * 26. On the Conditioning Unit, close the shutoff valves, and turn the regulator adjusting knob completely counterclockwise. 4-24
* 27. Disconnect all leads and components. Operation of a Photoelectric Switch * 28. Clamp the Diffuse Reflective Photoelectric Switch to the work surface. Connect the () and ( ) terminals of the switch to the corresponding terminals of the DC Power Supply. Turn on the DC Power Supply, which will power the light source inside the switch. * 29. Pass your hand in front of the photoelectric switch. Does the red LED at the rear of the switch turn on, indicating the switch contacts are activated? * Yes * No * 30. Slowly move your hand away from the photoelectric switch. Does the switch LED turn off, indicating the switch contacts are deactivated? * Yes * No * 31. Turn off the DC Power Supply. Counting of Motor Revolutions * 32. Position the pneumatic motor so that it is perpendicular to the photoelectric switch at a distance of 10 cm (4 in) (2 rows of perforations) as shown in Figure 4-11. The beam of the photoelectric switch must be pointing in the direction of the white sticker on the motor shaft. Clamp the motor in place. Note: Ensure that a white sticker is present on the motor shaft. Ask your instructor to install a new one if it is not the case. The reflective properties of the sticker activates the receiver of the photoelectric switch each time the sticker enters the detection zone. * 33. Turn on the DC Power Supply. * 34. Manually turn the motor shaft to check that the photoelectric switch is activated when it senses the white sticker of the motor shaft and deactivated when it senses the grey. 4-25
PNEUMATIC MOTOR PHOTOELECTRIC SWITCH DC POWER SUPPLY Figure 4-11. Photoelectric Switch Positioning. * 35. Turn off the DC Power Supply. Connect the circuit shown in Figure 4-12. Be careful not to modify the mounting positions of the motor and photoelectric switch. * 36. On the Time-Delay Relay / Counter, set the thumbwheel switches to B000. This will program the Time-Delay Relay / Counter for the 2 CO counting function, set the preset value to 0, and select a divide-by-10 counting interval. * 37. Open the shutoff valve and the branch shutoff valves at the manifold. Open Flow Control Valve FCV1 by turning the control knob fully counterclockwise. * 38. Turn on the DC Power Supply, and depress the START pushbutton PB1 to energize valve solenoid A and start the motor. Increase the regulated pressure until the motor turns at a constant speed. Note: If the LED on the photoelectric switch seems to skip, reduce the rotation speed of the motor by decreasing the air flow with FCV1. * 39. Observe the count value on the Time-Delay Relay / Counter display. Since a divide-by-10 counting interval has been selected, the displayed count is incremented by one for every 10 revolutions of the motor. 4-26
SOL-A DV1 PE1 FCV1 PNEUMATIC DIAGRAM (+) START PB1 STOP PB2 1 CR1 ( ) CR1-A DV1-SOL-A L1 PE1,NO 2 C CT1 + R RESET PB3 LADDER DIAGRAM Figure 4-12. Counting of Motor Revolutions. * 40. Using a stopwatch, measure the rotation speed of the motor by performing the following steps: Depress the RESET pushbutton, PB3, and simultaneously start the stopwatch. Let the motor turn for 60 s, then stop the motor by depressing the STOP pushbutton, PB2. 4-27
Multiply the displayed count by 10 to obtain the motor speed, in revolutions per minute (r/min). Record the motor speed. Motor speed: r/min * 41. Again depress pushbutton PB1 to start the motor. Place a tachometer on the motor shaft and measure the motor speed. Does this speed approximately correspond to the speed recorded in the previous step? * Yes * No * 42. Turn off the DC Power Supply. * 43. On the Conditioning Unit, close the shutoff valves, and turn the regulator adjusting knob completely counterclockwise. * 44. Disconnect and store all leads and components. CONCLUSION In the first part of the exercise, you tested the operation of an electrical counter placed in the CO (Operate at Preset Count) mode of operation. You saw that the electrical counter does not shift its contacts to the activated state until a preset count is reached. It returns them to the deactivated state when the accumulated count is reset. In the second part of the exercise, you learned that an electrical counter can cause a cylinder to repeat a cycle a definite number of times. This function is often used when parts of the system must be activated or deactivated after a definite number of events have occurred. In the third part of the exercise, you tested the operation of a photoelectric switch. You saw that the photoelectric switch detects the presence of objects within a specific range and zone. In the fourth part of the exercise, you determined the speed of rotation of a pneumatic motor by counting the number of revolutions performed in one minute. 4-28
REVIEW QUESTIONS 1. Give two ways an electrical counter is used in electrically controlled pneumatic systems. 2. Describe the basic operation of an electrical counter. 3. What is meant by "preset value"? 4. Once the preset value is reached, do additional input pulses continue to increment the counter? 5. How can the counter value be returned to zero? 4-29
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