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

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