> -----Original Message-----
> From: Simon Nield [SMTP:simon.nield@QUANTEL.COM]
> Sent: Thursday, February 22, 2001 9:59 AM
> To:   PICLIST@MITVMA.MIT.EDU
> Subject:      Re: [PIC] PWM motor control
>
> vasile:
> >P, PI but never PID. Real application demonstrate ( for me ) that PID is
> >almost pure theoretical. So could you point just one *real* example in
>
>
> This website has a great engineering style motion control tutorial (i.e.
> it's not just a load of
> abstact math) which I personally found a good refresher to the stuff I
> studied at university. I
> really can't recommend this document enough:
> http://www.newport.com/Motion_Control/Tutorial/
> or grab the pdf:
> http://www.newport.com/file_store/Motion_Control/Tutorial/PDF_Files/motion
> _tutorial.pdf
>
>
> in order of increasing complexity of control systems, I would suggest:
> P - gets near there quite quickly. the quicker you get the more (chance)
> of overshoot
> PD - adding some derivative lets you crank up the P term some more to get
> faster response without
> too much overshoot
> PID - the integral term is the icing on the cake allowing you to remove
> steady state error
> (depending on your application you may not actually care about steady
> state error it it is already
> 'small enough', in which case you may end up with the integral
>  term at zero to avoid the possible decrease in stability.
>
> PI systems would require physical damping of some sort in order to avoid
> instability, so maybe if
> you have used this type of system with success in the past it is because
> your physical system is
> already realtively heavily damped?
>
>
> Regards,
> Simon
>
I would have to agree with Vasile.  Certainly all the controllers I have
built have been PI, the derivative term just dosen't seem to give much in
the way of a performance gain, certainly not enough to justify the extra
code overhead.
Having said that, all the sytems I have worked with have been temperature
controllers that have a long thermal time constant (maybe up to 10 seconds).
I do get a slight overshoot with as step input, but adding even quite large
amounts of D into my controller didn't seem to help overshoot very much at
all.

Mike

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