Commissioning is a crucial step in verifying whether the PLC control system meets the control requirements; it provides an objective and comprehensive evaluation of the system's performance. Before being put into operation, the entire system must undergo rigorous commissioning until it meets the requirements and is confirmed by the relevant user representatives, supervisors, and designers. Commissioning personnel should have received specialized training in the system and be familiar with its structure, hardware, and software operation.
Any problems discovered during debugging should be promptly reported to the relevant designers. Modifications can only be made after the designers' approval, and detailed records must be kept of all modifications. The modified software must be backed up. All debugging modifications should be properly documented and archived. Debugging primarily includes testing input/output functions, control logic functions, communication functions, and processor performance.
1. Input/output circuit debugging
(1) Analog Input (AI) Loop Debugging. Carefully verify the address allocation of the I0 module; check whether the loop power supply method (internal or external power supply) is consistent with the field instruments; use a signal generator to add a signal to each channel at the field end, usually checking at three points: 0%, 50%, or 100%. For AI loops with alarm and interlock values, also check the alarm and interlock values (such as high alarm, low alarm, interlock point, and accuracy) to confirm the correctness of the relevant alarm and interlock status.
(2) Analog Output (AO) Loop Debugging. The actuator (such as valve opening) can be checked manually (i.e., directly set in the control system) according to the loop control requirements. Typically, three points (0%, 50%, or 100%) are used for checking. Simultaneously, closed-loop control is used to check whether the output meets the relevant requirements. For AO loops with alarm and interlock values, the alarm and interlock values (such as high alarm, low alarm, interlock point, and accuracy) should also be checked to confirm the correctness of the alarm and interlock status.
(3) Debugging of digital input (DI) circuit. Short-circuit or disconnect the corresponding field terminals and check the changes in the LEDs of the corresponding channel address of the digital input module, while also checking the on/off changes of the channel.
(4) Debugging of digital output (DO) circuit. The output points can be checked using the forced function provided by the PLC system. By forcing, check the changes in the LEDs of the corresponding channel address of the digital output module, and at the same time check the on/off changes of the channel.
2. Precautions for circuit commissioning
(1) For digital input/output circuits, it is important to maintain the consistency of the state. Generally, the positive logic principle is adopted, that is, when the input or output is energized, it is in the "ON" state and the data value is "1"; conversely, when the input or output is de-energized, it is in the "OFF" state and the data value is "0". This makes it easier to understand and maintain.
(2) For high-load digital input/output modules, they should be isolated from the field through relays, that is, field contacts should not be directly connected to the input/output modules as much as possible.
(3) When using the forced function provided by the PLC, be sure to restore the state after the test is completed; do not perform forced operation on too many points at the same time to avoid damaging the module.
3. Debugging of control logic functions
Debugging the control logic function requires collaboration with design, process representatives, and project management personnel. The processor's test functions should be used to set input conditions, and the output state changes should be checked against the processor logic to verify the correctness of the system's control logic function. For all interlocking loops, the interlocking process conditions should be simulated, the correctness of the interlocking actions carefully checked, and debugging records should be kept and jointly signed off.
The inspection process involves accepting the designed control program software, and it is the most complex, technically demanding, and challenging task during the debugging process. Especially when patented technologies or specialized software are used, it is crucial to carefully check the correctness of the controls, ensuring sufficient operational margins while guaranteeing the normal operation of the process and the system's safety, reliability, and flexibility.
4. Processor performance test
Processor performance testing must be conducted according to the system manual to ensure the system functions as described in the manual and is stable and reliable. This includes checking system communication, backup batteries, and other special modules. Redundancy testing is mandatory for systems with redundant configurations. This involves a comprehensive check of the redundant design components, including power redundancy, processor redundancy, I/O redundancy, and communication redundancy.
(1) Power redundancy. If one of the power supplies is disconnected, the system should continue to operate normally without disturbance; the system should be able to return to normal after the disconnected power supply is restored.
(2) Processor redundancy. When the main processor power is cut off or the main processor operation switch is switched, the hot standby processor should be able to automatically become the main processor, the system should operate normally, and the output should be undisturbed; the processor that was powered off should be able to return to normal and be in standby state after being powered on.
(3) I0 Redundancy. Select mutually redundant input and output points with corresponding addresses. Apply the same input signal to the input modules and connect the output modules to status indicators. Turn on and off (or hot-swap, if allowed) the redundant input and output modules respectively and check whether their states can remain unchanged.
(4) Communication redundancy. The system can be checked for normal communication and operation by cutting off the power to one of the communication modules or disconnecting a network; after resetting, the corresponding module status should automatically return to normal.
Redundancy testing requires performing redundancy checks on all modules with redundant designs, according to design requirements. Furthermore, system function checks include system self-tests, file search, file compilation and download, maintenance information, and backup functions. For more complex PLC systems, system function checks also include logic diagram configuration, loop configuration, and special I/O functions.
