Hello All, I have been delving into control theory for the first time since university and am slowly making progress. Until last night, I thought I had pole-zero compensation cracked: if your system has 180 degrees of phase shift with a loop gain of more than one, due to poles causing phase lag as frequency increases, then add some lead phase shift to the system around that point with a pole-zero compensator, right? Gives you a bit of phase margin.
So, I now come across a system with a G(s) of k/s^2, and a feedback H(s) =
- We now have a system that suffers from 180 degrees at ALL frequencies. Yet my book shows it being compensated around the frequency where the gain falls to 1 with a standard pole zero compensator.
My question is ( and I'm sure the answer is staring me in the face ) why won't such a compensated system oscillate at frequencies far above or below the compensator's region of operation? OK, say sytsem gain falls below 1 at HF, ruling that out, why won't it oscillate at low frequency, where there is plenty of gain and 180 degrees of phase shift from the double pole at the origin?
Andy.