This article introduces common PLC faults and component replacement methods, hoping to be helpful to friends engaged in automation control.
I. PLC Fault Symptoms and Troubleshooting
1. Fault Symptom: The PLC stops at certain outputs that are energized (usually in an intermediate state). Troubleshooting: Locate the signal (input, timer, line, drum controller, etc.) that triggers the next operation, and set the programmer to display the ON/OFF state of that signal.
2. Fault Symptom: The status displayed on the programmer after inputting a signal is inconsistent with the LED indication of the input module. Troubleshooting: Replace the input module; if multiple modules need to be replaced on the expansion rack, check the I/O expansion cable and its connections before replacement.
3. Fault Symptom: The input status matches the LED indication on the input module, but the status of the LED and the input device (button, limit switch, etc.) differs. Troubleshooting: Measure the input module. If a problem is found, replace the I/O device, field wiring, or power supply; otherwise, replace the input module.
4. Fault Symptom: The signal is connected, but there is no output or the output state is different from the connection state. Troubleshooting and Handling: Use a programmer to check the output drive logic and the program listing. The check should be performed from left to right, finding the first non-connected contact. If the non-connected contact is an input, check that input point. Finally, confirm that the main control relay does not affect the logic operation.
5. Fault symptom: The signal is a timer and is stuck on a non-zero value less than 999.9. Troubleshooting: Replace the CPU module.
6. Fault Symptom: This signal controls a counter. Troubleshooting: Check the logic controlling the reset, and then check the counter signal.
II. Replacement of Major Components in PLC Assembly
1. Replace the frame
(1) Disconnect the AC power supply;
(2) If a programmer is installed, unplug the programmer.
(3) Remove the plastic cover from the terminal block on the right side of the frame and disconnect the power supply wiring.
(4) Disconnect all I/O modules. If there were multiple circuits during the original installation, note the location of each module in the frame to avoid messing up the I/O wiring, so that you can reconnect them accordingly.
(5) If it is a CPU frame, remove the CPU components and filler modules and place it properly to avoid damage.
⑹ Remove the two screws securing the bottom frame and loosen the two screws at the top, but do not remove them.
(7) Push the frame up slightly, then pull it down and place it to the side.
(8) Slide the new frame onto the top screw, then install the bottom screw and tighten it evenly.
(9) Insert the I/O module according to the recorded position to avoid incorrect module insertion causing malfunctions in the control system.
⑽ Reinsert the removed CPU module and filling module.
11 Reconnect the power supply to the terminal block on the right side of the frame and replace the plastic cover on the power supply terminal block.
12. Check that the power wiring is correct before turning on the power.
13. Debug the entire control system to ensure that all I/O modules are functioning properly and that the program has not changed.
2. Replacement of CPU module
(1) Disconnect the power supply.
(2) If it has a programmer, unplug the programmer.
(3) Squeeze the upper and lower fasteners on the CPU module panel to disengage it from the latch.
(4) Pull the CPU module vertically out of the slot.
(5) If the original CPU has an EPROM memory, remove the EPROM and then install it on the new CPU.
⑹ Align the printed circuit board with the bottom guide slot before inserting the new CPU module into the bottom guide slot.
(7) Carefully move the CPU module to align it with the top guide slot.
(8) Insert the CPU module into the frame and lock the fastener into the bayonet.
(9) Plug in the programmer.
(10) Connect the power supply.
11. Initialize the system programming and reload the program recorded on the tape.
12. Debug the entire system operation.
3. Replacement of I/O modules
(1) Disconnect the power supply to the frame
(2) Disconnect the power supply to the I/O system.
(3) Disconnect the wiring on the I/O module.
(4) Depending on the type of module, disconnect the field wiring of the I/O terminals or remove the detachable wiring socket, and label each wire with its corresponding marking.
(5) Press the upper and lower elastic latches of the I/O module towards the center to disengage them from the bayonet.
(6) Pull the I/O module vertically upwards.
(7) Insert the I/O module to be replaced.
(8) Lock the I/O module into the bayonet.
(9) Connect the I/O module wiring according to the record label and corresponding marking.
(10) Connect the power supply to the frame and the power supply to the I/O module system.
11. Debug the I/O module and confirm that it functions normally.
