Using the parallel or independent PID equation with e=sp-pv for temperature control of a electrically heated furnace. Ki is 1/sec and Kd is secs. Dead time for process is about 2 minutes. Using Allen Bradley hardware (plc5/40e) and software and now have PID block with update at 1 second. The STI is not configured and the plc scan time is about 34 ms. Have question on the changing the update time for the PID block. If the time was changed from 1 second to 2 second would the output related to the integral part of the equation change given the slow changing temperature loop? Or if it was changed to 10 seconds would the integral related output change much? In the Allen Bradley PID equation I don't see the scan time so I am thinking would not change if scan time was to change and don't see this changing much the area under the error/time curve.

 

You should decide on a PID execute time between 200 ms. and 1 sec. and stick to it. PID scan time is chosen based on process time delays.

More importantly is your integral time, with a deadtime of > 2 min, it should be 2 min or 0.5 repeats/min or 0.0056/sec or possibly slower. In our DCS we choose 250 ms. as the default for all our PID loops; unless you are really CPU cycle limited it is not worth the time to customize it for each loop.

Your PLC has excess scan time available.