I have created a PID compact block that I want to use exclusively as a P controller with feedforward bias (disturbance variable).

However, even though I deactivate all PID parameters (0.0) except for 'Sampling time of PID algorithm', 'Proportional Gain', and 'Proportional Weighting' I still seem to get Integral control in my PID output (the MV value drifts even though the error (SP-PV) remains constant).

Why is that?

Thanks,
David 

AutoGen_4904513

I have created a PID compact block that I want to use exclusively as a P controller with feedforward bias (disturbance variable).

However, even though I deactivate all PID parameters (0.0) except for 'Sampling time of PID algorithm', 'Proportional Gain', and 'Proportional Weighting' I still seem to get Integral control in my PID output (the MV value drifts even though the error (SP-PV) remains constant).

Why is that?

Thanks,
David 

Hi David.

How about we take it from basic level. Roughly the PID algorithm would look at follows.(https://en.wikipedia.org/wiki/PID_controller#Pseudocode) 

Hello Qwazee,

I have no SCL knowledge. Only LAD and FBD. 

I am already sort of aware on how a PID controller works. I just thought that you could deactivate the ID part of the PID compact controller (leaving a P controller only) by setting things such as integral action time to 0.0. Is this not true?

Other than that, the PID compact block seems to work fine with the feedforward bias using the disturbance variable.

Hello;

You are right. Based on the S7-1200 S7-1500 PID control manual, setting the integral action time to 0.0 and the derivative action time to 0.0 deactivates the integral and derivative actions. The integral gain should not be affect your controller output with those settings.

https://support.industry.siemens.com/cs/us/en/view/108210036/127445416971

Hope this helps,

Daniel Chartier 

AutoGen_4904513

I have no SCL knowledge. Only LAD and FBD.

I understand if you are reluctant to change what you have now.
Just as background, the LAD code for the SCL is:

dchartier

Hello;

You are right. Based on the S7-1200 S7-1500 PID control manual, setting the integral action time to 0.0 and the derivative action time to 0.0 deactivates the integral and derivative actions. The integral gain should not be affect your controller output with those settings.

https://support.industry.siemens.com/cs/us/en/view/108210036/127445416971

Hope this helps,

Daniel Chartier 

 Exactly, but how come my output still slowly drifts (with integral action time deactivated) even though my PV and SP remain constant?

qwazee

 

AutoGen_4904513

I have no SCL knowledge. Only LAD and FBD.

I understand if you are reluctant to change what you have now.
Just as background, the LAD code for the SCL is:

 Thanks for the example. I am merely an electrician apprentice so this is all new to me.

I understand the proportional control in in the format of K * e + U0 (what you call Kf). In other words proportional gain * (SP-PV) + proportional bias.

The proportional bias (U0) would represent the constantly updated feedforward variable, right?

In this case, why do you multiply Kf with the setpoint? I don't see what you gain from that. Care to elaborate?

Thanks