Tuning AB SLC PID block for noisy flow rate
Tuning AB SLC PID block for noisy flow rate
(OP)
I am trying to determine how to tune the PID variables for a case where the flow rate jumps around up to 10%. I have not been able to try bumps yet, due to process use.
The symptoms of the problem are:
-Auto has a "sweet spot" and only works at that pressure difference.
-Any jog in pressure causes it to begin oscillating all the way to 0-100.
Current PID vars:
P - 1
I - .2 (tc)
D - .1 (tc)
The symptoms of the problem are:
-Auto has a "sweet spot" and only works at that pressure difference.
-Any jog in pressure causes it to begin oscillating all the way to 0-100.
Current PID vars:
P - 1
I - .2 (tc)
D - .1 (tc)





RE: Tuning AB SLC PID block for noisy flow rate
Derivative is not a good thing in loops with noisy measurements, like flow. Does your PID controller have the option to have the derivative act on the measured parameter, or a filtered derivative, instead of directly acting on the error signal? If not, you may want to set D = 0.
Also, it's hard to tell you what to do without knowing the PID algorithm that your controller is using. Can you post your PID math function? It sounds like your system may have too much integral effort. Integral tends to make a closed-loop system less stable, which would explain the oscillations you see.
Obviously, pressure fluctuations are a disturbance input into your system. Can you measure these disturbances and use feedforward control to compensate for them in your flow loop? Can you reduce your pressure transients by controlling pressure?
Finally, have you mathematically modeled your system? Most manual tuning requires a parameterized model that you use to calculate your tuning values.
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RE: Tuning AB SLC PID block for noisy flow rate
I have read alot of things that say what you said, that PI control is better for a flow like this. It is currently set up as a PID, one which I do not know the algorithm for at the moment.
The pressure fluctuations are caused by a sudden drop in pressure due to reactor venting, so they are hard to get rid of.
I am in the process of modeling it, but I will definitely look into setting the derivative to 0.
RE: Tuning AB SLC PID block for noisy flow rate
RE: Tuning AB SLC PID block for noisy flow rate
RE: Tuning AB SLC PID block for noisy flow rate
RE: Tuning AB SLC PID block for noisy flow rate
(1) A noisy process variable.
(2) and process disturbance.
1) Noisy process variable:
First I suggest you to trend the variable (most recorders can output a text like file which data can easily be manipulated with Excel) .... If despite of the spikes and wiggles that the signal could present, its average describes the behaviour of the process variable … good news! .. The noisy can be isolated.
You could use for instance, a low pass filters (process people called ‘lag’ filter) of first order or higher. If you are using an AB SLC 5/04 or 5/05, I suggest you to implement the digital filter in the ‘STI’ (periodic execution routine) …
Use an exponentially weighted moving average to implement the low pass filter:
This site explains very well how to implement the discrete filter in software:
http://lorien.ncl.ac.uk/ming/filter/fillpass.htm
If a higher order is required, then feed the output of the filter again through the filter a second time (2nd order) or a third time (3rd order) and so on …. The cut frequency (or Tf – see link) has to be chosen to get rid of or damp the noise as much as possible. Again, I found Excel quite useful for testing and simulating the filter operation.
Note: The cut frequency you choose for your filter will also be the interval between executions of your STI routine.
Another technique is characterizing the controller’s error. In order to do this you need to know it better.
2) Process disturbance:
I agree with a comments made earlier in this forum … please get rid of the derivative. Depending on the algorithm you are using, you could initially set to ZERO the integral value and tune the proportional and after you are happy with that you could then, start tunning the integral
Where process disturbance are a serious problem, it is common practice to de-tune the controller to make it less sensible.
I hope this help,
Cheers
RE: Tuning AB SLC PID block for noisy flow rate
Final Settings:
P .5
I .1
D .01
Coriolis meters are hard to work with sometimes, but when they work, they are great, and just as long as this valve is not trying to "micro manage" the flowrate, everything seems to smooth out and not vary more than 2%/s.