Measures to be taken on PLC anti-jamming software

Fault detection: plc itself has a very complete self-diagnosis function, but in engineering practice, the failure rate of plc i/o components such as limit switches, solenoid valves, contactors, etc. is much higher than the failure rate of plc itself, these After a component fails, the PLC will generally not be aware of it and will not shut down immediately. This will cause multiple failures to occur one after another. In order to facilitate the detection of faults, it can be realized by the ladder diagram program. Here is a logic combination judgment method: when the system is running normally, there is a definite relationship between the input and output signals of the plc. Therefore, according to the state of the output signal and the logic between the control process relationship to determine whether the device is functioning properly.
Information protection and recovery: When sporadic fault conditions appear, the information inside the PLC will not be destroyed. Once the fault conditions disappear, it can resume normal work and continue the original work. Therefore, when the plc detects a fault condition, it immediately stores the current state into the memory, and the software cooperates to close the memory and prohibit any operation on the memory to prevent the memory information from being washed out. Once it detects that the external environment is normal, it can be restored. To the state before the failure, continue the original program work.
Set the warning clock wdt: the action time of mechanical equipment is generally unchanged, and these times can be used as a reference. When the plc sends a control signal, the corresponding mechanical action is executed, and a timer is started at the same time. The set value of the timer The action time of mechanical equipment is 20% longer than that under normal conditions. If the time is up and the plc has not received the signal of the end of the action of the actuator, it will start an alarm.
Improve the reliability of the input signal: Due to the influence of electromagnetic interference, noise, analog signal error and other factors, it will cause the error of the input signal, cause the error of the program judgment, and cause accidents, such as the shaking of the button, the instant of the relay contact Jumping will cause system malfunction, and software delay can be used to debounce. For the influence of the analog signal error, the analog signal can be continuously sampled three times. The sampling interval is determined according to the a/d conversion time and the change frequency of the signal. The three data are stored in different data registers successively, and the middle value is taken after comparison. or the average value as the current input value.

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