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'[EE]: PID controllers (was PWM motor control)'
2001\02\22@071712 by Bob Ammerman

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I help to build hydroelectric plant control systems for one of my customers
and we use PID controllers to control the gates that let the water into the
generators (which controls the power output of the generator).

Tuning these controllers is always a pain in the neck, and unfortunately any
given tuning applies to a relatively narrow band of opeation. Temperature
changes, load level, wear and aging and, I swear, the phase of the moon,
affect the unit's response dramatically.

We've thought about trying to build an adaptive controller of some sort.
This controller would analyze the response of the unit to the control
signals we send it, and then adjust the algorithm to result in smoother
control (ie: quick response without overshoot).

One possibility would be to dynamically create a function of all the
appropriate variables (temperature, load, phase of moon, etc.) to modify the
PID loops constants. This function would be refined over time (days, months,
years).

Alternately, and the technique that I feel has the best chance of working,
we can simply watch the unit over a midrange timer interval (perhaps an hour
or so), and adjust the algorithm based on 'recent' behavior.

One additional issue with these controllers is that 'overcontrol' is a very
bad thing because you cause the mechanics of the gates to wear quickly. Its
not like we're just changing a PWM input into a motor. The controller
actually generates pulses that operate hydraulic valves that move a huge
cylinder that slews the gates around.

Does anybody have any good references/sources for such an adaptive system?

Bob Ammerman
RAm Systems
(contract development of high performance, high function, low-level
software)

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2001\02\22@074500 by Simon Nield

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bob:
>Does anybody have any good references/sources for such an adaptive system?

"no, but..." (sorry!)
you might want to look for a fuzzy logic control system. it was suggested by my lecturer back in
university that there was a good chance that fuzzy logic systems may well end up as superior
replacement for PID systems, due to the difficulties in tuning PID controllers.
i am not sure if this was just part of the generally unbased enthusiasm for fuzzy logic for its own
sake, or if the guy was right. what i do know is he was doing some research into it so he must have
at least believed his own argument ... prehaps an avenue for you to investigate anyway, i don't know
enough about it to recommend it or otherwise.

Regards,
Simon

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2001\02\22@080412 by Alan B. Pearce

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>One possibility would be to dynamically create a function of all the
>appropriate variables (temperature, load, phase of moon, etc.) to modify the
>PID loops constants. This function would be refined over time (days, months,
>years).

......

>Does anybody have any good references/sources for such an adaptive system?

Is this not what fuzzy logic is supposed to be about? Never tried to do anything
in fuzzy logic, its just one of those things to do "one day".

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2001\02\22@080813 by Michael Rigby-Jones

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{Quote hidden}

I'd love a really good reference to fuzzy control systems.  Most of the
stuff I have seen suggests that Fuzzy control is just a grand name for a
heavily quantized control system  e.g.

if(water_temp >= very_cold)
       heater_output = 100;
else if(water_temp < very_cold  && water_temp >= cold)
       heater_output = 70;
else if(water_temp < cold  && water_temp >= cool)
       heater_output = 30;
else if(water_temp < cool  && water_temp >= tepid)
       heater_output = 5;

etc..etc...

I'm obviously missing the pint totaly a I can't see the advantage of having
discreet control steps over a continuous system such as a PID controller.

Someone care to enlighten this me?

Mike

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2001\02\22@085018 by Bob Ammerman

picon face
> >Does anybody have any good references/sources for such an adaptive
system?
>
> Is this not what fuzzy logic is supposed to be about? Never tried to do
anything
> in fuzzy logic, its just one of those things to do "one day".

I was hoping for something a bit more solid :-)

Bob Ammerman
RAm Systems
(contract development of high performance, high function, low-level
software)

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2001\02\22@095425 by Spehro Pefhany

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At 07:16 AM 2/22/01 -0500, you wrote:
>
>Does anybody have any good references/sources for such an adaptive system?

Bob,

There are probably a lot more references than working systems. This kind
of continously adaptive (stochastic, fuzzy-neural, model-adaptive)
control has been bandied about in the literature for over 20 years.
(as opposed to the type which deliberately perturbs the system and
uses Z-N or other methods to tune the controller, which is much
easier to accomplish).

From what I've seen, what's out there in the literature won't work
well without a LOT of work, and the few that do work are proprietary
or only work with a very limited problem domain, or both.

You might get better results by focusing on the elements of the
problem, improving the performance (and eliminating variations in)
the individual parts of your system. This could involve smarts, and
maybe additional sensors (a la fuzzy control) or it could just involve
mechanical improvements.

<anecdote>
One time I had a car in which the steering got gradually looser and
looser, to the point where the steering wheel could be turned 1/4 turn
almost and nothing would happen at the front wheels. Being run by a
superb adaptive control system (me), I hardly noticed the change.
Then someone else tried to drive my car, and could barely keep in
the proper lane. A crank of a wrench and the requirements for the
control system were drastically reduced. </anecdote>

Best regards,

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Spehro Pefhany --"it's the network..."            "The Journey is the reward"
speffspamKILLspaminterlog.com             Info for manufacturers: http://www.trexon.com
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2001\02\22@100718 by Bob Ammerman

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----- Original Message -----
From: Spehro Pefhany <.....speffKILLspamspam.....INTERLOG.COM>
To: <EraseMEPICLISTspam_OUTspamTakeThisOuTMITVMA.MIT.EDU>
Sent: Thursday, February 22, 2001 9:56 AM
Subject: Re: [EE]: PID controllers (was PWM motor control)


> At 07:16 AM 2/22/01 -0500, you wrote:
> >
> >Does anybody have any good references/sources for such an adaptive
system?
{Quote hidden}

Unfortunately, since we are only responsible for the control system we have
_no_ control over the rest of the system (the generator, gate, hydraulics,
etc.).

We do have inputs for such thing as temps, load, etc.

> <anecdote>
> One time I had a car in which the steering got gradually looser and
> looser, to the point where the steering wheel could be turned 1/4 turn
> almost and nothing would happen at the front wheels. Being run by a
> superb adaptive control system (me), I hardly noticed the change.
> Then someone else tried to drive my car, and could barely keep in
> the proper lane. A crank of a wrench and the requirements for the
> control system were drastically reduced. </anecdote>

Yikes!

> Best regards,
> Spehro Pefhany

Bob Ammerman
RAm Systems
(contract development of high performance, high function, low-level
software)

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2001\02\23@065008 by Peter L. Peres

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A fuzzy control is more like a PID control coupled with a state machine.
The state machine adjusts the coefficients in the PID dynamically
depending on the present state and on the previous state. Typically fuzzy
controls use at least one more input term than the equivalent linear or
PID control. F.ex. if a PID control uses the input value, the feedback
value, and time, then an equivalent fuzzy control would also use the
history of the error of the system (in a way more complex than the I term
in PID, f.ex. switching D gain down and zeroing I if the previous 3
direction changes seemed to be oscillation -- i.e. error changed sign
without input changing).

When a set of fuzzy control rules is implemented in a microprocessor it
undergoes defuzzification. This turns the complex set of equations into a
simple set of decisions, similar to those described in your posting.

Peter

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2001\02\26@050159 by Bob

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Don't know if this will help, but here is a link titled "PID Without a PhD", I
found it just tonight.

http://www.hugoordonez.com/pid.html




{Original Message removed}

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