hi alll!! just a quick one. You know when you design a controller - you go and plot the open loop transmission, L(jw) on a nichols plot, where L(jw) = C(jw)H(jw) P(jw) where P and C are the plant and controller respectively. You then 'manipulate' this plot (i.e. vary your controller) until your design specs are met. My question is once you have a gain margin - what does this actually tell us - say we have gain margin of -5dB. does this tell us that our controller gain can only increase by 5dB before the system bcomes unstable (assuming the system was already stable). The thing i don't get is that our controller gain is fixed and doesn't change after we design it! so which gain (the controller or plant or H(hw) ) does the gain margin put a limit on? think i might be missing something... thanks!!!