Summarize the experience in PLC debugging process

Debugging is the key work to check whether the PLC control system can meet the control requirements. It is an objective and comprehensive evaluation of the system performance. Before the system is put into use, it must be strictly debugged for all system functions until it meets the requirements and is signed and confirmed by relevant user representatives, supervisors and designers before it can be put into use. The debugging personnel should have received special training on the system and be familiar with the composition of the control system, the use and operation of hardware and software.

If the debugging personnel find any problems during debugging, they should contact the relevant designers in time. Modifications can only be made after the designers agree. Detailed records should be made for the modifications, and the modified software should be backed up. The documents of the debugging modifications should be organized and archived. The debugging content mainly includes input and output functions, control logic functions, communication functions, processor performance testing, etc.

Input and output circuit debugging

(1) Analog input (AI) circuit debugging. Carefully check the address assignment of the I0 module; check whether the circuit power supply mode (internal power supply or external power supply) is consistent with the field instrument; use a signal generator to add a signal to each channel at the field end, usually taking 0, 50% or 100% for inspection. For AI circuits with alarm and interlock values, the alarm interlock value (such as high alarm, low alarm and interlock point and accuracy) must also be checked to confirm the correctness of the relevant alarm and interlock status.

(2) Analog output (AO) loop debugging. According to the requirements of loop control, the actuator (such as valve opening, etc.) can be checked by manual output (i.e. directly set in the control system). Usually, 0, 50% or 100% are used for inspection; at the same time, through closed-loop control, check whether the output meets the relevant requirements. For AO loops with alarm and interlock values, the alarm interlock values ​​(such as high alarm, low alarm, interlock point and accuracy) must also be checked to confirm the correctness of the relevant alarm and interlock status.

(3) Debugging of the digital input (DI) circuit: Short-circuit or disconnect the corresponding field end, check the changes of the LED corresponding to the channel address of the digital input module, and check the on and off changes of the channel.

(4) Debugging of the switch output (DO) circuit. The output point can be checked through the forced function provided by the PLC system. Through forced operation, the changes of the LED corresponding to the channel address of the switch output module are checked, and the on and off changes of the channel are checked at the same time.

Notes on loop debugging

(1) For the switch input and output circuit, attention should be paid to the principle of maintaining the consistency of the state. Usually, the positive logic principle is adopted, that is, when the input and output are energized, they are in the "ON" state and the data value is "1"; conversely, when the input and output are de-energized, they are in the "OFF" state and the data value is "0". This makes it easier to understand and maintain.

(2) Switching input and output modules with large loads should be isolated from the site through relays, that is, the site contacts should not be directly connected to the input and output modules as much as possible.

(3) When using the forced function provided by the PLC, it is important to restore the status after the test is completed; do not force too many points at the same time to avoid damaging the module.

Control logic function debugging

Control logic function debugging needs to be completed together with the design, process representatives and project management personnel. The test function of the processor should be used to set the input conditions, and the output state changes should be checked according to the processor logic to confirm the control logic function of the system. For all interlocking circuits, the process conditions of the interlocking should be simulated, the correctness of the interlocking action should be carefully checked, and the debugging records should be made and signed for confirmation.

Inspection is the process of checking and accepting the design control program software. It is the most complex, technically demanding and difficult task in the debugging process. Especially in the case of patented technology applications and special software, the correctness of the control should be carefully checked, and a certain operating margin should be left to ensure the normal operation of the process operation and the safety, reliability and flexibility of the system.

Processor performance test

Processor performance testing should be carried out in accordance with the requirements of the system manual to ensure that the system has the functions described in the manual and is stable and reliable, including inspections of system communications, backup batteries and other special modules. Redundancy testing must be carried out on systems with redundant configurations. That is, a comprehensive inspection of the redundant design parts, including power redundancy, processor redundancy, I0 redundancy and communication redundancy, etc.

(1) Power redundancy. If one of the power supplies is cut off, the system should be able to continue to operate normally without disturbance; the power supply that was cut off can be restored to normal after being powered on.

(2) Processor redundancy. When the power supply of the main processor is cut off or the operation switch of the main processor is switched, the hot standby processor should automatically become the main processor, the system should operate normally, and the output should be unperturbed; the processor that was powered off should be able to return to normal and be in standby mode after power is restored.

(3) I0 redundancy. Select redundant input and output points with corresponding addresses. Apply the same input signal to the input module and connect the output module to the status indicator. Switch on and off (or hot-swap, if allowed) the redundant input module and output module respectively to check whether their status can remain unchanged.

(4) Communication redundancy. You can 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.

Redundancy testing should be carried out on all modules with redundant design according to design requirements. In addition, the system function check includes system self-check, file search, file compilation and downloading, maintenance information, backup and other functions. For more complex PLC systems, the system function check also includes logic diagram configuration, loop configuration and special I0 functions.