When many people mention PLC debugging, complex circuits, obscure codes, and elusive troubleshooting may come to mind. However, PLC debugging is not an insurmountable challenge. In fact, as long as you master a few key steps, even a complete beginner can handle PLC debugging in a short amount of time.
Next, let's explore these steps and see how we can make learning PLCs both fun and efficient.
Input/Output Loop Commissioning
Analog Input (AI) Loop Debugging
The first step is to ensure that the I/O modules are assigned the correct addresses. This is similar to making sure your mailing address is accurate. Next, check that the loop power supply matches the field instrumentation, which is critical to ensuring proper power supply. Finally, use a signal generator to test each channel, typically selecting three points: 0%, 50%, and 100%. For AI loops with alarm and interlock values, also check the accuracy of the alarm and interlock statuses.
Analog Output (AO) Loop Commissioning
Here, you can check the response of the actuator, such as valve opening, using manual outputs. Test at three points: 0%, 50%, and 100%. Additionally, use closed-loop control to ensure that the output meets the requirements. For AO circuits with alarms and interlock values, carefully check the alarm and interlock statuses as well.
Discrete Input (DI) Loop Commissioning
This section is like switching a light on and off. By shorting or disconnecting the corresponding field terminals, check the LEDs of the discrete input modules for changes and observe the on/off status of the channels.
Discrete Output (DO) Loop Commissioning
The output points can be checked using the forcing function provided by the PLC system. By using this function, you can observe the changes in the LEDs of the discrete output module and monitor the on/off status of the channels.
Control Logic Function Debugging
Next is control logic function debugging, which is akin to programming a computer to ensure it runs correctly according to instructions. This should be done in collaboration with design and process representatives and project management. Utilize the processor's test functions to set input conditions and verify that output states change correctly to confirm the system's control logic functionality. For all interlocking circuits, simulate the interlocking process conditions, carefully verify the accuracy of the interlocking actions, and maintain debugging records and confirmations.
Processor Performance Testing
Processor performance testing is a key step in ensuring the stability and reliability of the entire system. This must be carried out according to the system manual's requirements, including checks for system communications, backup batteries, and other special modules. For systems with redundant configurations, redundancy testing is essential. For example, if you cut off one power supply, the system should continue normal operation. If you cut off the main processor’s power or switch its operating switch, the hot standby processor should automatically become the main processor with no system disturbance. Check I/O redundancy and communication redundancy as well.
Power Redundancy
If one power supply is cut off, the system should continue to operate normally without disturbance. The system should resume normal operation once the power supply is restored.
Processor Redundancy
If you cut off the power supply of the main processor or switch its operating switch, the hot standby processor should automatically take over as the main processor, and the system should continue to run normally with no disturbance in output. The disconnected processor should return to normal and be in standby mode once power is restored.
I/O Redundancy
Select mutually redundant input and output points with corresponding addresses. Apply the same input signals to the input modules and connect the output modules to status indicator instruments. Turn the redundant input and output modules on and off separately to check if their status remains consistent.
Communication Redundancy
Check whether the system can communicate and operate normally by cutting off the power supply of one of the communication modules or disconnecting a network. After resetting, the status of the corresponding module should automatically return to normal.
Precautions for Loop Commissioning
Remember the following points during loop debugging:
For discrete input/output loops, maintain consistency in state principles. Typically use positive logic, meaning when the input and output are energized, they are in the “ON” state with a data value of “1”; otherwise, they are in the “OFF” state with a data value of “0”.
For discrete input and output modules with high loads, isolate them from the field using relays and avoid direct connections to the input and output modules whenever possible.
When using the PLC’s forcing function, restore the state after testing. Avoid forcing too many points simultaneously to prevent module damage.
After reading this article, do you find that PLC debugging isn’t as difficult as it seems? By mastering these key steps, even beginners can get started easily. Remember, debugging not only ensures the system operates correctly but also helps identify and solve potential issues, making the system more stable and reliable.
