Exercise 7. Controlling a Filling Line with a PLC EXERCISE OBJECTIVE

Transcription

1 Exercise 7 Controlling a Filling Line with a PLC EXERCISE OBJECTIVE To create a ladder program for controlling an industrial line filling boxes with detergent. To test program operation through the completion of verification steps. DISCUSSION Automative control of processes is widely used in industry to perform repetitive and sequential tasks, as in assembly lines and production lines. Automatic process control can be provided through the use of PLC timer, counter, and sequencer instructions. Examples of PLC-controlled processes include canning and bottling lines, automated punching and cutting processes, automated painting operations, etc. With automatic control, an operator is not needed to start a machine or operation. Control is performed through the use of automatic control devices, which take the place of a human operator. These devices are designed to provide an electrical signal when a predetermined condition is obtained. The condition can be a specific position for a machine part, a critical temperature or pressure, or the level of a liquid. An example of automatic control device is a level sensor. The contacts on a level sensor are operated by the upward and downward movement of a float placed in a tank or in a hopper. As the liquid level changes, the float causes a rod to open or close electrical contacts. Another example of automatic control device is a photoelectric sensor. Photoelectric sensors usually consist of a phototransmitter and a photoreceiver. When an object breaks the light beam transmitted by the phototransmitter, the photoreceiver produces an electrical signal indicating that an object has been detected. Typical applications for photoelectric sensors include detecting the absence or presence of objects, detecting flaws, and reading printed codes to guide items on conveyors. A solenoid-operated valve is also an automatic control device. This type of valve is used to control the flow of fluids and gases. It consists of a solenoid, a plunger, and a valve containing an orifice where a plug is placed to regulate the flow. The valve is operated by the movement of the magnetic plunger being drawn into the solenoid when the coil is energized. 7-1

2 PROCEDURE SUMMARY In the first part of this exercise, you will connect the Lab-Volt PLC Trainer to the computer used to run the P-SIM and RSLogix 500 softwares. You will run P-SIM and select the Silo Simulator option, which simulates an industrial line filling boxes with detergent. You will then create a ladder program in order to control the filling line. You will test program operation through the completion of verification steps. Notice that the Silo Simulator can be controlled in a same manner using ladder programs that are quite different. The student is encouraged to improve the ladder program wherever possible. Fewer programming steps are advisable as PLC's have a limited amount of memory used to store program instructions. 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 Setup G 1. Connect the PLC Trainer, Model , to the computer used to run RSLogix 500 and P-SIM through the P-SIM to PLC Interface, Model Then, connect the computer to the PLC communications port, through a 1761-CBL cable. Note: If you are using the former PLC Trainer Model , connect the PLC Trainer to the computer used to run RSLogix and P-SIM, through the Voltage Converter/Isolator, Model 3242, and the P-SIM to PLC Interface, Model Then, connect the computer to the communication port of the PLC on the PLC Trainer, through the PC Interface, Model (Refer to Figure E-2 of the manual for the detail of the connections to make). G 2. On the PLC Trainer, Model , make sure the V1 and V2 toggle switches of the PLC output jacks are set to 24 VDC (toggle switches upward). Note: If you are using the PLC Trainer Model , make sure to connect both PLC output groups to the trainer 24-V DC source, not to the 120 (240)-V AC source, which could damage the interface modules, Models 3243 and G 3. Turn on the computer. Do not start RSLogix 500 for now. 7-2

3 G 4. On the computer, start the P-SIM Simulator program. In the entry window of P-SIM, select the Serial Port command of the main menu, then select the serial port (COM 1, 2, 3 or 4) that corresponds to the computer serial port to which the P-SIM to PLC Interface module, Model 3243, is connected. G 5. Turn on the PLC Trainer. In the entry window of P-SIM, select the Initialize IO Board command of the main menu, which will bring up a window asking you if you want to reset the IO board now. Click OK to close the box and reset the board. In the entry window of P-SIM, select the Silo Simulator option of P-SIM. This will bring up the Silo Simulation window (see Figure 7-1). This window allows control of an industrial line that fills boxes with detergent. 7-3

4 Figure 7-1. The P-SIM Silo Simulation window. Creating a Ladder Program for Controlling a Filling Line G 6. The P-SIM Silo Simulation window allows control of an industrial line that fills boxes with detergent. Observe that the addresses of the input devices in the Silo Simulation window correspond to the PLC input bit addresses listed in Table

5 SILO INPUT DEVICES Momentary-action, normally-closed pushbutton switch S1: I:1/0 Momentary-action, normally-open pushbutton switch S2: I:1/1 Momentary-action, normally-open pushbutton switch S3: I:1/2 Photoelectric sensor: I:1/3 Level sensor: I:1/4 CORRESPONDING PLC INPUT BIT ADDRESS I:0/0 I:0/1 I:0/2 I:0/3 I:0/4 Table 7-1. Addresses of the Silo input devices and their corresponding PLC input bit address. G 7. Now, observe that the addresses of the output devices in the Silo Simulation window correspond to the PLC output bit addresses listed in Table 7-2. SILO OUTPUT DEVICES Conveyor motor: O:1/0 Solenoid valve: O:1/1 RUN lamp: O:1/2 STANDBY lamp: O:1/3 FULL lamp: O:1/4 CORRESPONDING PLC OUTPUT BIT ADDRESS O:0/0 O:0/1 O:0/2 O:0/3 O:0/4 Table 7-2. Addresses of the Silo output devices and their corresponding PLC output bit address. G 8. On the computer, start RSLogix 500 (leave the P-SIM Silo Simulation window open). In RSLogix, create a new project having the following processor name: ECS7. G 9. Based on Tables 7-1 and 7-2, write a ladder program for the P-SIM Silo which will control the following sequence of operations: Upon power up, the conveyor is stopped and the RUN, STANDBY, and FULL lamps in the control panel of the industrial filling line are off. Pressing the START pushbutton (momentary-action, normally-open switch S2 I:1/1), by clicking it using the mouse, starts the conveyor motor and turns the RUN lamp on. 7-5

6 Starting the conveyor motor moves empty boxes towards a silo containing detergent. As the right edge of a box is detected by a photoelectric sensor I:1/3, the conveyor motor is stopped. This stops the empty box while it is under the silo gate. Once the conveyor motor has been stopped, a solenoid-operated valve (O:1/1) is energized to open the silo gate and dispense detergent into the box. Meanwhile, the STANDBY lamp in the control panel is lit. When the box is full, level sensor I:1/4 is activated. This de-energizes the solenoid-operated valve (thereby closing the silo gate), turns the STANDBY lamp off, turns the FULL lamp on, and restarts the conveyor motor. When photoelectric sensor I:1/3 no longer detects the box that has just been filled with detergent, the FULL lamp turns off. When photoelectric sensor I:1/3 detects the right edge of another box, the filling process is repeated. Pressing the STOP pushbutton (momentary-action, normally-open switch S3 I:1/2) stops the system at any sequential step of the filling process and turns the RUN lamp off. At this point, pressing the START pushbutton causes the filling process to resume from the step reached when the system was stopped, and the RUN lamp to turn on. Write your ladder program on paper. Note that the photoelectric and level sensors are software controlled during the animated process. Thus, photoelectric sensor I:1/3 sends a signal to terminal 3 of the PLC input interface within the PLC Trainer when a box is detected. Level sensor I:1/4 sends a signal to terminal 4 of the PLC input interface within the PLC Trainer when the box is full. G 10. Enter your ladder program in program file LAD 2 of project ECS7. Verify each rung, then save the project in a project file having the same name as the processor name. Download the project to the PLC. Go online and set the PLC in the Run mode. Testing the Ladder Program with the P-SIM Silo Simulator G 11. Test program operation by completing the verification steps below. If the program fails to function, do not proceed to the next step. Go offline to modify the program as required, then go back online to complete the verification steps. a. Upon power up, the conveyor should be stopped and the RUN, STANDBY, and FULL lamps in the control panel of the industrial filling line should be off. 7-6

7 b. Press the START pushbutton S2 by clicking it using the mouse. The conveyor motor should start and the RUN lamp in the filling-line control panel should turn on. c. When the right edge of a box reaches photoelectric sensor I:1/3, this sensor should be activated. This should stop the conveyor motor, energize the solenoid-operated valve (thereby opening the silo gate and dispensing detergent into the box), and turn on the STANDBY lamp in the control panel. d. When the box is full, level sensor I:1/4 should be activated. This should de-energize the solenoid-operated valve (thereby closing the silo gate), turn off the STANDBY lamp, turn on the FULL lamp, and restart the conveyor motor. e. When photoelectric sensor I:1/3 no longer detects the box that has just been filled with detergent, the FULL lamp should turn off. f. The filling process should repeat whenever photoelectric sensor I:1/3 detects the right edge of another box. g. Press the STOP pushbutton S3 while the motor conveyor is in operation. The conveyor motor should stop immediately and the RUN lamp should turn off. h. Press the START pushbutton. The motor conveyor should restart and the RUN lamp should turn on. i. Press the STOP pushbutton while a box is being filled with detergent. The solenoid-operated valve should be de-energized immediately to close the silo gate and stop filling the box with detergent. The RUN and STANDBY lamps should turn off. j. Press the START pushbutton. The filling operation should resume and the RUN and STANDBY lamps should turn on. G 12. Have your instructor verify the operation of the program. G 13. When you have finished, set the PLC in Program mode, and clear the PLC memory. G 14. Close RSLogix 500, then close P-SIM. G 15. Turn off the computer. 7-7

8 G 16. On the PLC Trainer, make sure that all PLC inputs are deactivated. Turn off the PLC Trainer, remove all connection leads, if any, and return all the equipment. CONCLUSION In this exercise, you created a ladder program for controlling an industrial line filling boxes with detergent. You tested program operation through the completion of verification steps. You saw that automated systems can be controlled through the use of automatic control devices, which take the place of a human operator. These devices provide an electrical signal when a predetermined limit or condition is obtained. REVIEW QUESTIONS 1. Briefly describe the operation of a level sensor. 2. Suggest typical practical applications for photoelectric sensors. 3. What type of valve can be used to control the flow of fluids and gases? 7-8