Re: PID Controller

On Sep 22, 4:02 am, Vladimir Vassilevsky <antispam_bo...@xxxxxxxxxxx>
wrote:
Peter Nachtwey wrote:
Yes. And PID is a short for "pidor", which is the Russuan bad word for
"pederast".

This reminds of Chekhov in Star Trek where Chekhov thinks everything
was 'inwented' in Russia.

Star Trek - Red Neck.
A. P. Chekhov is a classic dramatist.http://en.wikipedia.org/wiki/Anton_Chekhov

Often you can use more than one integrator provided you
stabilise the loop properly.
Can you provide an example?

Well, not a pure double integrator, of course. The double integrator
can't be stable. The regulator action should be the integrator + lowpass
filter, so the phase shift at DC will be 90, not 180.

PID is just a quick fix but not the best
solution.

Why?

Because there is no such things like the ideal LTI systems in reality.
Because PID is inefficient for the systems with big amount of plain delay.

I still use PIDs very effiectively.

No doubt about that. However PID approach has its limitations.

I sometimes add feed forwards, a
2nd derivative and/or an output filter. I would love to see your solution
for the Bode plot below.
ftp://ftp.deltacompsys.com/public/NG/Bode%20Plot.rtf

Isn't a compensation of a single peak by zero in PID loop a scholar problem?

For the best solution you need a Bode plot and design lag-
lead compensators along with integrators.

Unfortunately, the Bode plot makes sense only for LTI systems with the
monotonic response.

Fine, where is your example filter? I provided one.

As I said above the double integrator needs a phase-advance for
stability and sometimes two phase advances depending on whatever there
is in there. There are a number of applications where this can work.
One simple one is a phase-locked loop where there is one
integrator(the VCO) and a lag-lead. The lag is like another integrator
over a limited frequency range and the lead gives stbility. So the
Bode plot is -20,-40,-20 (through unity gain) then finally -40dB/
decade.
The first roll-off can be -40 instead of -20 but this would decreas
the phase margin.

Fitlike

.

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