Strange PID adjustment technique (Experts Only Please)
Strange PID adjustment technique (Experts Only Please)
(OP)
Hi
I've worked as a system integrator for a few years and have run across an interesting setup at a local plant. One of the electricians has been doing most of the programming over the years and he has an interesting technique of tuning his loops.
Essentially what he does is change the kp value depending on the current process value eg
kp=(pv-A)/B+C
where A and B are gain values and C is an offset
His value of ki is dependant on the value of kp e.g.
ki=kp/D+E
And for the derivative term he scales this depending on the value of ki e.g
if ki>F then kd = G
if ki<F then kd = H
where G and H are also constants
I have never seen this before and was wondering if any of you have and if you could comment on whether this idea has any merit at all. He also has other code where the PID values are different depending on whether the PV is higher or lower than the SP.
Personally I don't like it because it makes the loops harder to tune (since you have to think about how the PID parameters are going to change depending on where the pv is).
I could see this being useful if the process was inherently nonlinear but is it really necessary for lime addition into a tank to control pH? (simple, first order system with a time constant of about 3 min)
Is this some advanced control technique or just a lot of hot air?
Thanks in advance
I've worked as a system integrator for a few years and have run across an interesting setup at a local plant. One of the electricians has been doing most of the programming over the years and he has an interesting technique of tuning his loops.
Essentially what he does is change the kp value depending on the current process value eg
kp=(pv-A)/B+C
where A and B are gain values and C is an offset
His value of ki is dependant on the value of kp e.g.
ki=kp/D+E
And for the derivative term he scales this depending on the value of ki e.g
if ki>F then kd = G
if ki<F then kd = H
where G and H are also constants
I have never seen this before and was wondering if any of you have and if you could comment on whether this idea has any merit at all. He also has other code where the PID values are different depending on whether the PV is higher or lower than the SP.
Personally I don't like it because it makes the loops harder to tune (since you have to think about how the PID parameters are going to change depending on where the pv is).
I could see this being useful if the process was inherently nonlinear but is it really necessary for lime addition into a tank to control pH? (simple, first order system with a time constant of about 3 min)
Is this some advanced control technique or just a lot of hot air?
Thanks in advance





RE: Strange PID adjustment technique (Experts Only Please)
xnuke
"Live and act within the limit of your knowledge and keep expanding it to the limit of your life." Ayn Rand, Atlas Shrugged.
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RE: Strange PID adjustment technique (Experts Only Please)
This form of parameter adjustment is being used for flow controllers, level controllers and pressure control loops. All of which are have parameters and gains that can be linearized around the operating point. Any further thoughts?
RE: Strange PID adjustment technique (Experts Only Please)
xnuke
"Live and act within the limit of your knowledge and keep expanding it to the limit of your life." Ayn Rand, Atlas Shrugged.
Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips.
RE: Strange PID adjustment technique (Experts Only Please)
I'm afraid you blew it. That attitude is not working well for you.
Gunnar Englund
www.gke.org
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: Strange PID adjustment technique (Experts Only Please)
If you substitute deviation for pv, kp increases on either side of setpoint which makes more sense.
Sometimes just PID is not the best control e.g. pH or in some levels where the gain in one direction is completely different to the gain in the other (I am thinking of float cell level control).
Its not a method I would use
Roy
RE: Strange PID adjustment technique (Experts Only Please)
RE: Strange PID adjustment technique (Experts Only Please)
To start one should identify the plant transfer function Gp(s). Then one can compute the controller transfer function Gc(s) by using the equation
Gp(s)*Gc(s)/(1+Gp(s)*Gc(s))=CLTF(s) where CLTF is the desired closed loop transfer function. Now one can solve for Gc(s), the controller gains.
There has been no mention of what the plant is. Know that would help a lot, but like I said, the system identification is the hardest and the most important part of tuning. How can you control what you don't understand?