Programmable logic controllers (PLCs) have taken the basic role in large-scale industrial setups. Therefore, having the basic knowledge of their troubleshooting has become more important than ever. The process of troubleshooting starts with the correct identification of the problem. Inexperienced troubleshooters are prone to make mistakes at this step by mistakenly taking one or more symptoms of the problem for the problem itself. Doing so only delays the problem to a later date, causing it to grow meanwhile.
It is often seen that a process develops multiple small issues over time and as a result continues to function but fails to match the standard efficiency level. Then, the whole system breaks down due to a small problem. The system breakdown itself should not be considered as the basic problem; but those small glitches, which add up to reduce the overall efficiency and eventually lead to the breakdown.
Troubleshooting programmable logic controllers
The troubleshooting steps have been written below in bullets for better and easy understanding.
- Keep the main power at 120 VAC
- Ensure 24V power is available
- Inspect the problem area
- Ensure sensor operation is good in the problem area
- Note sensor inputs to the PLC
- Observe if altering the sensor state changes the input LED on the PLC to switch on or off
- Find the input on PLC ladder diagram that controls the output
- Check that output LED is switching on and off with corresponding input
- Ensure output voltage is accurate and responds correctly to the input
- Identify the output device and check voltage reaches the device and fluctuates with input
- Make sure that output device is operating accurately
- The performance of an input or output module may be poor in one circuit while perfect in other circuits
- The modular internal circuit becomes a fuse and can be affected by a field short circuit if all the field circuits are not fuse protected
- Replace the modular circuit if it is demaged
- Shut down PLC main power and 24V power before changing any module
- Identify the problem in the field circuit and replace wires if a short circuit is found
- Even if the problem is found with the wires, check the device for short circuit and electrical and mechanical operation. Change the defective device and ensure the whole circuit is working perfectly. Place the circuit in operation and check again to ensure module and circuit are working fine.
- Replace supply module if there is no output in power supply module.
- Replace backplane as it may go bad.
- Occasionally, the Reset key switch can be used to reset PLC. Make sure switching the PLC off does not affect other sub-set programs. Enable a restart by turning the key to the extreme right. Hold the key for 15 seconds and then, turn it to extreme left and wait. Return to the middle position.
- The PLC may have a latch relay with no reset. In such a situation, the key switch reset does not affect the latch. Switch the power on and off to reset the latch and restart the program.
- Sometimes, turning the main 480V power on and off is necessary to reset all the circuits.
Generally, the PLC works as a part of a control circuit and a complex system that contains motors, safety controls, controllers, and other controls. The system is supplied with 120VAC by a 460V/120V by a transformer. Some of the elements that are critical to solve maintenance related issues include a ladder diagram, wiring diagram, manufacturer’s Manual and troubleshooting skills.
PLCs may prove to be difficult when it comes to maintenance or repair; but it is due to their complex nature that in turn contributes to their functionality.