Re: Limiting PID Loop Overshoot


"Peter Nachtwey" <peter@xxxxxxxxxxxxxxxx> wrote in message
news:1145891582.330300.54770@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
Tam, your new plant has 3 poles. So how do you place the poles with
your simple lead-lag controller? You keep saying the same thing over
and over but saying the same thing over and over doesn't make it so.
Show me a close loop transfer function that can be verified.
In any case you are changing the topic which is about integrator
limiting. My threads were just to refute the statement that
traditional integrators always cause the PV to over shoot the SP. I
have shown this isn't necessarily so with type 0 plants.

The closed loop poles can be placed to there is no overshoot even
without a integrator limit. The controller is a PID with an extra
higher order derivative gain.

For your plant I assume
G=1
omega = 1
zeta=.5
alpha=1 The real pole

My desired response has four real poles at -0.63 and it critically
damped.


Doesn't mean anything - what is your BANDWIDTH!!? We can easily get a greta
response if you have very little bandwidth.

My gains are
Ki = 0.157
Kp=0
Kd=.381
Kdd=.52

Tam, now it is your turn to provide your proof.

Peter Nachtwey

Who says I am using a lead-lag controller? For your plant I would use the
following

(1+sT1)/s - that's what you would call PI I think using your terminology.

Then I would add a second integrator making the system unstable. I would add
a lead to stabilise this out but would have to be careful of the
resonance.We also have to maximise the bandwith without letting the resonant
peak show through.

So we have

(1+sT1)/s^2.(1+sT3)/(1+sT4)

However, I would probably add a second phase advance. To see what's going on
you must draw a Bode Plot.
All this time domain stuff and placing poles is crap - you have no concept
of bandwidth. The basic Bode Plot will have a -12dB/octave roll off
then -6dB/Octave then -12 then the resonance with teh peak at least 6dB down
below 6dB.I mean this is basic stuff why the mystery? Every disk-drive in
the world is controlled this way.It is true that there is an effect you can
get from integrators which I believe you call wind-up. I get around this by
putting diodes back to back around the integrators (or several as the case
may be).You can do the same in software.

Now, this PID stuff is crap, it's for amateurs who cannot draw a Bode Plot
and don't understand phase-margins.In some large plants this is quite hard
to do and the PID methods are best however but not for let's say basic speed
controllers and position servos.

Tam


.

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