Pid ?

We have an air regulation systems that uses a PI loop with no derivitive at this point. The control loop has been operating fine althought we would like to make it a bit more aggressive. Currently P= .03 and I= .055. Can anyone let me know which values to tweak to make it more aggressive. Thanks

If you increase the P gain, you will increase the ouput in Proportion to the error. If you change the I gain, you will affect the output with regard to the error over time. Depending on which form of the equation in use by your PLC, decreasing the I gain may increase the reaction.

The Derivative is designed to react to the rate of change in the PV.

Ron Beaufort has some excellent white papers on this subject. They are aimed at Allen-Bradley users:

http://www.ronbeaufort.com/sample_lessons.htm

Paul

jcraft said:

We have an air regulation systems that uses a PI loop with no derivitive at this point. The control loop has been operating fine althought we would like to make it a bit more aggressive. Currently P= .03 and I= .055. Can anyone let me know which values to tweak to make it more aggressive. Thanks

Click to expand...

If it operates just fine maybe it will continue to do so if you leave it alone??

Dan Bentler

Pressure controller loops tend to have substantial proportional gain and fast integral time. without knowing what the setpoint pressure is and how much disturbance is in your system with demand and such like it is hard to give advice,

As a general rule though, increasing proportional gain will make your system more responsive but may cause overpressure if there is a rush of demand on air. Likewise, decreasing the Intergral Action time (IAt) will make the system more responsive just as if the gain were being increased but will introduce more phase lag so too much will cause instability.

just google PID tuning and all will become clear.

Okay, I am just getting confused by the integral portion. In our system I is in repeats/sec., so if you wanted to increase the intergral(reset) time would you decrease the repeats/sec.? If you can clarify that I'll be all set. Thanks

Yes, if you want a stronger Integral Action then you will decrease the Integral Action Time. I noticed that you have a very strong integral action hence why your gain is so low.

I disagree. If his units are truly repeats per second, then increasing the value increases the integral action.

In my A/B PLCs, the units are inverted: minutes per repeat, so decreasing the value increases the action.

Maybe I am wrong, but that is how I interpret it.

Paul

If the integral is repeats / second, increasing the number will increase the action
1 repeat per second will increase the output by the error every second
2 repeats per second will increase the output by the error every 1/2 second
Thats assuming the integral is independent of gain setting which is anybodies guess with most PLCs

Okie, i get what youre saying but he has not said whether he is using a PLC or a standalone controller?

regards

LH