I'm sure this is a naive question but I have been using software ( provided by a kind gentleman on this NG, was it Jerry? ) called Rootloc, which plots out root locii and Bode plots etc, but only for unity gain feedback systems, i.e. the Open Loop forward function is G(s) so Rootloc calculates G(s)/[1+G(s)].
If I cascade a pole-zero compensator Gc(s) with the process function, then take unity gain feedback to the summing point this is fine. The closed loop function is G(s)Gc(s)/[1+G(s)Gc(s)]
Surely though this will not always be possible, for instance where the process function is mechanical, and the compensator is electronic, the only sensible place to put the compensator is in the feedback loop.
I then get a different closed loop response, G(s)/[1+G(s)Gc(s)].
This gives me the same denominator ( characteristic equation as before ) but a different numerator ( different zero ). Therefore the root locus plot is the same in both cases ( based on characteristic equation ), but the frequency response will be altered ( as the zero influences the magnitudes of the poles )
Thus a compensation network for a process will be different depending on whether it is in forward cascade or in the feedback path. Can anyone confirm my feedback 101 analysis, and suggest a source ( er, free would be nice of course ) for a program that can handle frequency response plots for CLOSED LOOP transfer function inputs, rather than one that accepts the open loop transfer function and assumes unity gain feedback?
I've had a look, but haven't found anything that actually works,
cheers,
Andy.