Exercise 8 Controlling a Batch Mixing Process EXERCISE OBJECTIVE To create a ladder program for controlling a batch mixing process. To test program operation through the completion of verification steps. DISCUSSION Today's PLC's are used in many branches of modern industry, such as rubber and plastic, chemical and petrochemical, power, metals, pulp and paper, materials handling, machining and manufacturing. Automation of industrial processes through the PLC capabilities has brought a substantial improvement in production. Safer and more comfortable working conditions, increased production, efficiency, minimal downtime, higher quality products, are only a few examples of the innumerable system benefits provided by PLC installations. In most applications, PLC installations provided investment payback and additional savings. A good example of industrial process where the PLC can be very beneficial is chemical batch mixing. The PLC controls the ratio of two or more chemicals, determines the mixing time, and monitors the heating temperature, as well as the low and high tank limit levels. This PLC application provides faster batching and consistent formulations from batch to batch. 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 softwares. You will run P-SIM and select the Batch Simulator, which simulates an automated system controlling chemical batch mixing. You will then create a ladder program in order to control the batch mixing process. You will test program operation through the completion of verification steps. Notice that the Batch 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, keeping in mind that fewer programming steps are advisable. 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. 8-1
PROCEDURE Setup G 1. Connect the PLC Trainer, Model 3240-2, to the computer used to run RSLogix 500 and P-SIM through the P-SIM to PLC Interface, Model 3243. Then, connect the computer to the PLC communications port, through a 1761-CBL cable. Note: If you are using the former PLC Trainer Model 3240-1, 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 3243. Then, connect the computer to the communication port of the PLC on the PLC Trainer, through the PC Interface, Model 3246-1. (Refer to Figure E-2 of the manual for the detail of the connections to make). G 2. On the PLC Trainer, Model 3240-2, make sure the V1 and V2 toggle switches of the PLC output jacks are set to 24 VDC (togle switches upward). Note: If you are using the PLC Trainer Model 3240-1, 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 3242. G 3. Turn on the computer. Do not start RSLogix 500 for now. 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 Batch Simulator option of P-SIM. This will bring up the Batch Mixing Simulation window (see Figure 8-1). 8-2
Figure 8-1. The P-SIM Batch Mixing Simulation window. Controlling a Batch Mixing Process G 6. The P-SIM Batch Mixing Simulation window allows control of an industrial batch mixing process. The purpose of this process is to combine two reactants (A and B) to produce a third product (C). Reactants A and B are first charged into the tank in proportions of two to one. The reactants are then mixed and the temperature is raised to a certain value (temperature setpoint). Then the mixing operation continues for 30 seconds. The temperature is automatically kept at setpoint during this period. The reaction product (C) is then withdrawn from the tank. Observe that the addresses of the input devices in the Batch Mixing Simulation window correspond to the PLC input bit addresses listed in Table 8-1. 8-3
BATCH MIXING INPUT DEVICES Momentary-action, normally-closed pushbutton switch S1: I:1/0 Momentary-action, normally-open pushbutton switch S2: I:1/1 Temperature switch: I:1/2 Low-level switch: I:1/3 High-level switch: I:1/4 Flowmeter FL1: I:1/5 Flowmeter FL2: I:1/6 Flowmeter FL3: I:1/7 CORRESPONDING PLC INPUT BIT ADDRESS I:0/0 I:0/1 I:0/2 I:0/3 I:0/4 I:0/5 I:0/6 I:0/7 Table 8-1. Addresses of the Batch Mixing devices and their corresponding PLC input bit address. G 7. Now, observe that the addresses of the output devices in the Batch Mixing Simulation window correspond to the PLC output bit addresses listed in Table 8-2. BATCH MIXING OUTPUT DEVICES Mixer: O:1/0 Pump P1: O:1/1 Pump P2: O:1/2 Pump P3: O:1/3 Heater: O:1/4 RUN lamp: O:1/5 STANDBY lamp: O:1/6 FULL lamp: O:1/7 CORRESPONDING PLC OUTPUT BIT ADDRESS O:0/0 O:0/1 O:0/2 O:0/3 O:0/4 O:0/5 O:0/6 O:0/7 Table 8-2. Addresses of the Batch Mixing devices and their corresponding PLC output bit address. G 8. On the computer, start RSLogix 500 (leave the P-SIM Batch Mixing Simulation window open). In RSLogix, create a new project having the following processor name: ECS8. 8-4
G 9. Based on Tables 8-1 and 8-2, write a ladder program for the P-SIM Batch Simulator which will control the following sequence of operations: Upon power up, the STANDBY lamp in the control panel of the batch mixer is lit. The pumps, the mixer, and the heater are off. The RUN and FULL lamps in the batch-mixer control panel are also off. Pressing the START pushbutton (momentary-action, normally-open switch S2, I:1/1), by clicking it using the mouse, starts pump P1, turns the RUN lamp on, and turns the STANDBY lamp off. This causes reactant A to be pumped into the tank. Once flowmeter FL1 has generated 10 pulses, pump P1 is stopped and pump P2 is started. This causes reactant B to be pumped into the tank. Once flowmeter FL2 has generated 10 pulses, pump P2 is stopped, the FULL lamp turns on, the mixer is started, and the heater is turned on. Both reactants are then mixed and heated up to the temperature setpoint. Once the temperature of the reactants reaches the setpoint value, the contacts of temperature switch I:1/2 close. The mixing operation then continues for 30 seconds. During this time interval, the reactant temperature is automatically kept very close to the setpoint value (by switching the heater on and off). After the 30-second mixing interval, pump P3 is started to withdraw the product from the tank, the mixer is stopped, the heater is turned off, and the FULL lamp turns off. Once the level of product in the tank has decreased below the minimum-level limit, the contacts of the LO-LEVEL switch (I:1/3) open. This causes pump P3 to stop and the batch mixing process to repeat (through the starting of pump P1 and so on). Pressing the STOP pushbutton (momentary-action, normally-closed switch S1, I:1/0) stops the system at any sequential step of the mixing process, turns the RUN lamp off, and turns the STANDBY lamp on. At this point, pressing the START pushbutton causes the mixing process to resume from the step reached up to when the system was stopped, the RUN lamp to turn on, and the STANDBY lamp to turn off. The LO-LEVEL switch prevents the mixer and heater from being turned on as long as the level of reactants in the tank is lower that the minimum-level limit. In fact, this switch contacts remain open as long as the level of reactants is lower than the minimum-level limit. When the level of reactants exceeds the maximum-level limit, the contacts of the HI-LEVEL switch (I:1/4) close. This causes pumps P1 and P2 to stop, the RUN lamp to turn off, and the FULL lamp to turn on, 8-5
while the STANDBY lamp remains off. This situation remains until the START pushbutton is pressed and kept pressed, thereby starting pump P3 (to withdraw the product from the tank) and turning the RUN lamp on. When the FULL lamp turns off, the START pushbutton can be released without causing pump P3 to stop. Once the level of the product in the tank has decreased below the minimum-level limit, the contacts of the LO-LEVEL switch open. This causes pump P3 to stop and the batch mixing process to repeat (through the starting of pump P1 and so on). Write your ladder program on paper. Note that pulse generation from flowmeters FL1 through FL3 is automatically controlled by the software during the animated process, as well as the opening and closing of the HI- and LO-LEVEL switch contacts. G 10. Enter your ladder program in program file LAD 2 of project ECS8. 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 Batch 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 STANDBY lamp in the control panel of the batch mixer should be ON. The pumps, the mixer, and the heater, should be off. The RUN and FULL lamps in the batch-mixer control panel should also be off. b. Press the START pushbutton S2. Pump P1 should start, the RUN lamp should turn on, and the STANDBY lamp should turn off. This causes reactant A to be pumped into the tank. c. Once flowmeter FL1 has generated 10 pulses, pump P1 should stop and pump 2 should start. This causes reactant B to be pumped into the tank. d. Once flowmeter FL2 has generated 10 pulses, pump P2 should stop, the FULL lamp should turn on, the mixer should start, and the heater should turn on. This causes the reactants to be mixed and heated up to the temperature setpoint. e. Once the temperature of the reactants reaches the setpoint value, the mixing operation should continue for 30 seconds. During this time interval, the heater should turn on and off to keep the reactant temperature very close to the setpoint value. 8-6
f. After the 30-second mixing interval, pump P3 should start to withdraw the product from the tank, the mixer should stop, the heater should turn off, and the FULL lamp should turn off. g. Once the level of product in the tank has decreased below the minimum-level limit, pump P3 should stop and the batch mixing process should repeat (through the starting of pump 1 and so on). Observe circuit operation through a few cycles to verify that proper control is performed by the PLC. h. Press the STOP pushbutton S1 while reactant A is being pumped into the tank. Pump P1 should stop immediately, the RUN lamp should turn off, and the STANDBY lamp should turn on. i. Press the START pushbutton. Pump P1 should restart, the RUN lamp should turn on, and the STANDBY lamp should turn off. j. Press the STOP pushbutton while reactant B is being pumped into the tank. Pump P2 should stop immediately, the RUN lamp should turn off, and the STANDBY lamp should turn on. k. Press the START pushbutton. Pump P2 should restart, the RUN lamp should turn on, and the STANDBY lamp should turn off. l. Press the STOP pushbutton while the reactants are being mixed and heated. The mixer should stop, the heater should turn off immediately, the RUN lamp should turn off, and the STANDBY lamp should turn on. m. Press the START pushbutton. The mixer should restart, the heater should turn on, the RUN lamp should turn on, and the STANDBY lamp should turn off. n. Press the STOP pushbutton while the reaction product is being withdrawn from the tank. Pump P3 should stop, the RUN lamp should turn off, and the STANDBY lamp should turn on. o. Press the START pushbutton. Pump P3 should restart, the RUN lamp should turn on, and the STANDBY lamp should turn off. p. Stop system operation while reactant A is being pumped into the tank. Force PLC input bit I:0/3 off to simulate a low level of reactant in the tank. Press the Start pushbutton to restart the batch mixing process. Once the required quantity of reactants A and B has been pumped into the tank and the FULL lamp has turned on, the mixer should not start and the heater should remain off because the sensed (detected) level of reactant in the tank is below the low-level limit. Remove the force installed on PLC input bit I:0/3. System operation should resume. q. Stop system operation while reactant B is being pumped into the tank. Force PLC input bit I:0/6 off to simulate a failure of flowmeter FL2. Press the Start pushbutton to restart the batch mixing process. When the level of reactant reaches the high-level limit, pump P2 should stop 8-7
immediately, the RUN lamp should turn off, and the FULL lamp should turn on. The STANDBY lamp should remain off. r. Press and hold the START pushbutton (by pressing and holding the mouse left button). Pump P3 should start immediately to withdraw the reactants from the tank and the RUN lamp should turn on. When the FULL lamp turns off, release the START pushbutton. Pump P3 should remain on until the reactants have been completely withdrawn from the tank, and the batch mixing process should restart automatically. s. Stop system operation while reactant A is being pumped into the tank. Remove the force installed on PLC input bit I:0/6. Press the START pushbutton. The batch mixing process should resume. 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. 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 a batch mixing process. You tested program operation through the completion of verification steps. Industrial processes being composed of dynamic and static elements, PLC programmation can be difficult since the combined effects of these elements are hard to visualize. However, a good understanding of the system requirements, coupled to a systematic, step-by-step system approach, minimize the programmation errors and the time required to troubleshoot and tune the system. 8-8