vasile:
>  I don't know who is the teacher at your control systems course but it
...
>relay and a simple comparator. ( or a 12C509 to be in topics...)


sorry, I obviously didn't explain that last remark clearly enough.

what I am talking about is the design of the control system, not the design of the plant you are
trying to control.
if the plant is badly designed then obviously the control system is going to have a hard/impossible
time getting useful output.

i am suggesting that the 'classical method' of plant control, whereby the plant is analysed and
described in terms of a mathematical transfer function, the parameters of which are then measured,
and an inverse transfer function created and used as the control system, is almost never used in
real life.
it's too complicated a process: analysing the plant to create the transfer function generally
involves a whole bunch of approximations, and measuring the parameters of that transfer function is
difficult and often inaccurate. The transfer function has to be rationalised to avoid too many
levels of differentiation to avoid problems with noise in the error signal... and the performance
obtained after such a complicated process is often no better (and sometimes worse) than can be
acheived with a simple PID system.

i am suggesting that most often the plant is designed, then an off-the-self PID control system is
configured to suit the job... this is not just my opinion by the way, it was also what the lecturer
happily told us after we had a grounding in the subject - it's good background stuff, but not used
very often in the real world.


Regards,
Simon

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