PWM inside feedback loop

Hi
I'm attempting to control a brushed DC motor using an analogue PWM chip inside a feedback loop. I'm currently doing some modelling in simulink with
a proportional controller, a pwm stage, a motor's electrical component I=(V-E/(Ls+R) and a current as the feedback. My input demand is therefore torque or current.
I have a problem such that when I increase the proportional loop gain so that I get a decent step response, the input to the pwm module tends to follow the internal sawtooth reference signal, causing the pwm output to do stange things......It tends to output a much higher frequency signal as the pwm input makes multiple transitions above and below the sawtooth waveform during one cycle.
Now, is this just a practical limit when a pwm module is placed in the feedback loop (control action limited to 0-100% pwm)? or am I overlooking something somewhere???
Thanks for any advice in advance
Cheers
Steve Minshull
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On Thu, 18 Nov 2004 20:31:17 GMT, "Steve Minshull"

It sounds like the bandwidth of the feedback signal is way too high. You obviously cannot take any action in less than one PWM cycle, so don't try. Slew-rate-limit or low-pass filter that feedback signal so it cannot make multiple transitions in the PWM signal.
-Robert Scott Ypsilanti, Michigan (Reply through this forum, not by direct e-mail to me, as automatic reply address is fake.)
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