simatic s7 PID Control Output Swing from Min to Max

[Galaniz]

I am working with a FB 58 TCONT_CP with sample time set to 1 second P GAIN set to 10, I to .13, D to .79.

I am controlling a process heater/incinerator by varying combustion air/gas with a VFD. I am able to maintain my setpoint within 1 to 2 percent either way, but within that temp range, my VFD swings from 4-20 mA constatnly. I have tried auto tune and manual tuning and I can slow it down a little, but I am unable to get the VFD to settle. I have varied sample time and the coefficients to no avail.

The osciallating VFD would be okay, however, I am trying to maintain O2 concentration and the osciallations are making this difficult.

Any suggestions for slowing down the output to the VFD so that it could settle down to semi constant speed.

Addtional info: the PID coefficients were originally taken from an old Partlow + controller that was previously controlling the temperature and which supposedly maintained a realtively constant output speed.

Thanks.

 

[BryanG]

Not all controllers use the P, I and D components in the same way, you can't really copy the settings from one to another. So first you need to find if increasing or decreasing q component increases or decreases the component action, the manual is a good place to start. Assuming that Gain does increase as you increase P, then to me, a Gain of 10 seems huge, try bringing it down to maybe 0.5 and see if that helps. After you have a smooth output but lazy output you can adjust P,I and D to get quicker reactions.

I haven't had much experience with auto tuning, so I played with it on a simple speed control system. The system had averaging on the speed and the auto tune choice was to have the system constantly go between stopped and full speed. OK, the averaged output was near what I wanted but the method was appalling. I went back to manual tuning.

 

[metwally.mustafa]

you can use FPDT controller that was shown here by pandiani and MR. petter

that is very good to deal with dead time in your process and you can use rate limiter (first order filter ) to improve the preformance of your system