Siemens S7 FB58 PID Control

Hi
Sorry for the long explaination

I am commisioning a new install with these PID control blocks which are supposedly auto tuning.

The process is cooling, and is a 1 shot process via a heat exchanger, with a modulating valve controlling the cooling medium.

I have FB58 within OB35 on a 1 second cycle time and i have coded the gains to be negative.

I need to cool the product from 85 to 35C.

With P only I get a swing of about +/- 10C over a time period of about a minute. I used the ziegler nicholls idea and used 20 seconds for the I term and 8 seconds for the D term ... useless

So, I tried to use the autotuning.... Now the options are to use a step change or to try and tune around the setpoint.

I enabled the process and enabled auto tune (step change) and altered the setpoint by 30 Degres but the tuning block just sat their scanning for over 10 minutes eithout moving from step 0.

I gave up and tried the around the setpoint idea but this also didnt do much apart from give an output of 40% and sit like that for about 10 minutes.

So

A -- Am i to impatient... does the tuning take ages. Based on instrumentation i have used in the past this would take typically 30s-1 minute.

B -- Any better stragy is more than welcome. i am thinking of trying a PI control tomorrow

C Is the self tuning of FB58 aactually amy good

Cheers

Auto Tuning can take a while

You need to make a step in the CO and then wait for the controller to respond and then settle at the PV. The time it takes to do that is 5 time constants. Set your integrator time constant to 1 time constant.
The gain should be set to ΔPV/ΔCO.

There is a good example of how to do this at the www.controlguru.com website.

Can you make a trend and save the data in a text format that can be loaded into Excel?

Hi thanks for the reply. excuse my ignorance, what is CO.... control output ?

I see what your say regarding the I and D terms. I need to do 5 of these in a 4 hour slot tomorrow but they are broadley similar so if i do 1 i would say copying the parameters would suffice until i return afer christmas.

So how would i make a change at the output? Change the SP ? Or select manual mode force an output and back to Auto

cjd1965 said:

Hi thanks for the reply. excuse my ignorance, what is CO.... control output ?

Click to expand...

Yes, control output.

I see what your say regarding the I and D terms. I need to do 5 of these in a 4 hour slot tomorrow but they are broadley similar so if i do 1 i would say copying the parameters would suffice until i return afer christmas.

Click to expand...

If they are similar then you should be able to copy now and fine tune later.

So how would i make a change at the output? Change the SP ? Or select manual mode force an output and back to Auto

Click to expand...

Use manual mode and change the control output directly.

Hi Peter
I did a manual output of 9% and the process settled at 82C. I then did a manual output of 12% and the process settled at 43C.

I then removed cooling and the product returned to 85C as expected.

I then initiated a 30C process with the PID manual output at 100% and it took 22 seconds to achieve 55C (.63 of 85-30)

I am know uncertain of how to convert these into the true terms and ran out of time on site, although i am controlling to about +/-8C at the moment but trying to achieve at least +/-5C finally.

That control guru site got me started but i cant seem to find (or identify in my brain) the next step
The FB58 block seems to want to be in Auto to do a tune but even if i fire off the process so the setpoint changes from 85 to 30 it still doesnt go into tune mode.

I would be more interested in learning how to tune the loop with the delta OP and PV data than spending time learning the FB58 so i could apply the technique to anything else in the future

Thanks