Although industrial control computers and programmable logic controllers (PLCs) are inherently highly reliable, errors in the digital input signals to the PLC, significant deviations in the analog input signals, and failure of the actuators controlled by the PLC output ports to operate as required can all lead to control errors and cause irreparable economic losses.
The main reasons for errors in the signals input to the PLC from the field are:
1) Causes short circuits or open circuits in the transmission signal line (due to mechanical pulling, aging of the line itself, especially rodent damage). When the transmission signal line fails, the field signal cannot be transmitted to the PLC, causing control errors.
2) Mechanical contact jitter: Although the field contacts only close once, the PLC thinks they have closed multiple times. Although the hardware has added a filter circuit and the software has added a differential instruction, the PLC scan cycle is too short, and errors may still occur in the counting, accumulating, shifting and other instructions, resulting in incorrect control results.
3) Faults in the field transmitter or mechanical switch itself, such as poor contact, large deviations in the non-electrical quantities reflected by the transmitter, or failure to work properly, will also cause the control system to malfunction.
The main reasons for errors in the execution mechanism are:
1) The control load contacts cannot operate reliably. The PLC issues an action command, but the actuator does not operate as required.
2) Controlling the start of the frequency converter: Due to a fault in the frequency converter itself, the motor driven by the frequency converter did not work as required;
3) Various electric and solenoid valves failed to open when they should have, or failed to close completely when they should have. Because the actuators did not operate according to the PLC's control requirements, the system could not function properly, reducing its reliability. To improve the overall reliability of the control system, the reliability of input signals and the accuracy of actuator actions must be improved. Otherwise, the PLC should be able to detect problems promptly and alert operators using audible and visual alarms to quickly troubleshoot and ensure the system operates safely, reliably, and correctly.
