Industrial control computer and programmable controller itself has a high degree of reliability, but if an error occurs the switch signal input to the PLC, analog signals deviate, PLC outputs control of actuators are not required, these may make the process go wrong, causing irreparable economic loss.
Affect the signal input to the PLC error the main causes are:
1) transmission line short circuit or open circuit (due to mechanical pull, aging line itself, especially rodents), when the transmission line failure, signal cannot be transferred to the PLC, resulting in error control;
2) mechanical contact jitter field contact closed only once, PLC was convinced that closed several times, while hardware and filter circuits, differential instruction of software, but due to the PLC scanning cycle is too short, there may be errors in the counting, accumulating, and shift instructions, error control results;
3) transmitter, mechanical switch itself failed, contact poor contact transmitter reflect non-electric large deviation or is not working properly, which will also make the control system is not working properly.
Influence of actuator error the main causes are:
1) control load contact cannot be reliable, PLC has issued an action directive, implementing agencies and no action as required;
2) control inverter starting, due to the fault inverter, frequency converter the motor does not work according to the requirements;
3) electrically operated valve, solenoid valve open can not open, not locked in place of the closed, due to implementing agencies did not press PLC control requirements, the system does not work, reduces system reliability. To improve the reliability of the entire control system must improve the reliability and accuracy of the implementing agencies of the input signal, otherwise the PLC should be able to find, sound and light alarm approach gives operators as soon as possible, troubleshooting, and system safety, reliability and work correctly.