When I have a PID loop to control a process where there is a delay between setting the set point value and when it can be seen to have an effect on the measure point. which of the three inputs adjusts for that, proportional, integral, or differential? How would I tune my PID loop factors to account for the delay?

 

You are talking about dead time. I am assuming you are talking about independent gains so the simple answer is that all the gains are reduced.

You didn't say what kind of system you have so here is an example FOPDT heat exchanger system

http://www.controlguru.com/wp/p71.html

This example assumes the gains are dependent but you can see the overall controller gain is reduced as the dead time increases.

The formulas provided for Kc and Ti are correct for a FOPDT system.

The formulas for a SOPDT system are similar in that the controller gain, Kc, is reduced as the dead time increases.

If the dead time is too long then a simple PID by itself will not work because the controller gain will be too small. In this case one needs to use a Smith Predictor but using a Smith Predictor requires that one has a reasonable estimate of the plant gain, time constant and dead time.

Read the heat exchange example at the controlguru website.

 

First of All is it your PID loop controller is

--> PLC Based

or

--> Electronic Controller Based

Because PLC PID loop is much slower than Dedicated Electronic Controller. PLC SCAN TIME makes the response slow. However Dedicated electronic card PID loop is usaully near to real time.

Secondly, Making response fast, you have to tune proportional part of PID loop more efficiently. Proportional Part of PID Loop setting affects the stability of output response. But this may be tackled by by rate of Change of process value for PID loop through Integral tunning.

 

To the extent that delays in feedback and PID loop processing get smaller, the better off you are. You can't have a high performance PID system with long delays in the feedback. So, your tuning will have to get looser, or more specifically you will have to lower your gains.

Deadband can help as someone already mentioned.

KEJR

 

This is software based controller to control the rate of packet transmission for network. The delay is the round trip time for the packet to reach the other end and a response to come back.