PID for PLC

PID blocks are ready for immediate selection and use within the programming systems for Allen Bradley PLC-5, SLC500 and Controllogix 5000 processors (usual disclaimer). Other makes of PLC have them too.

Do you mean something like this? >> http://radhesh.files.wordpress.com/2008/05/pid.jpg

It doesn't have set-point weighting, nor is it in discrete form, but the format is pretty self evident.

hope that helps

Hi there,

You will have to provide more information. It would be nice to know the PLC make and PID block you are using.

A few quick pointers though:

1. What PID algorythm is used - is it a serial; parallel or ideal controller?

2. Does the controller use Gain or Proportional?

3. How does the integral work - repeats per second or seconds per repeat?

If you do not understand the above mentioned points, and/or you do not know how your actual controller works - you will not be able to apply laplace, Ziegler Nichols or even WAG (SWAG) - Scientific Wild Ass Guess - methods of tuning.

A good site to have a look at (it has case studies) is: www.controlloop.co.za

The guy who presents these courses and does this sort of work is brilliant. Read through the tutorials and you will have a much better understanding.

The next thing you would need to get hold of is the PID block diagram with it's own algorythm. Try www.automation.semens.com You need to look for the manual for the PCS7 function blocks, and the block you are looking for is called CTRL_PID. It has all you need and more if you are using this for homework.

I would also recommend that you look at a few PLC makes with different algoryhtms. You should try Allen Bradley, Siemens and Honeywell (DCS). All three of these PLC's employ different algorythms to get their control done.

And if you do your homework properly, you will notice that a parallel algorythm is rubbish. Maybe you can come back and tell us why? (Hint - the derivative action).

Regards,

Craig