s7224xp pir loops

Why does the actual value on my PID controller jump sporadically?

Description of the problem:
The PID controller which is created using the wizard attempts to adapt the actual value in line with the setpoint value. The actual value sporadically jumps away from the setpoint value, but then returns close to it in accordance with the controller parameter settings. This process is repeated at different intervals depending on the user program. Cause:
The POE created by the "PID controller" equation in the S7-200 operation wizard is an interrupt routine which is called periodically at the controller's scanning frequency. Since no synchronization takes place between an interrupt routine and the main routine, particular care is required when exchanging data between them.
The problem described here generally originates from repeatedly writing a variable in the cyclic program within a cycle and from accessing the same variable within the interrupt routine. Therefore, it isn't a specific controller problem, it is a system property of interrupt-controlled subroutines. You can find a detailed explanation and possible remedies in Entry ID 7792431

Click to expand...