PID FB41 (Siemens) to regulate Motor

Hi,

I use an S7-315 CPU

Used the OB35 and placed the FB41 in it. The cycle time is on default (100ms)

I have humidity measurement in g/kg. We Set the value 70 g/kg for the SV (set value).

We control a frequency drive on profibus, so the PID output connected to the drive like this:

L PID
L 100.0
/R
L 16384.0
*R
RND
T PQW326


My problem is:

The PID works waaaaay to fast

THese are my seTTings:


The regulation must be reaeaeaeaeaeal slow..., like changing the output every 10 minuts,
The output only has to change earlier if the measured value changes too much.
So wouldn't it be better to write something myself instead of the PID ?

Hello Combo;

First, you are using a Proportional gain of 4, which is quite high, your controller will try to correct an error (SP-PV) with 4 times correction, this probably means high speed of reaction and overshoot. We generally try our PID loops with a gain of 1 and tune it y hand (up or down) from there, when we are doing it manually. Once the values of the P, I and D gains are optimized, and if your process is controllable and self-regulating, the controller should act without you having to intervene, as if in the background.
If you can get hold of some more information on open-loop reaction times (a graph of controller output versus step input for your process, with Auto mode off) you could at least look into Ziegler-Nichols or Cohen-Coon tables for suggested values of P, I and D gains in standard applications.
Better still, if you can use the transfer function of your process, you could calculate the optimized gain values for this process and implement them without to much adjustments. Others on this site have written on this (Peter Nachtwey and Pandiani come to mind, search on their posts) and you could have a look at Controlguru.com

Hope this helps,
Daniel Chartier