I tried to implement a low pass filter with my PD controller today,
but that drastically failed. I looked at a work that talked about
practical implementation of PD control, and it talked about preceding
the controller with a low pass filter to reduce the noise in the error
signal. Using Routh's stability criteria, I determined that my new
controller would be stable if the pole of my filter was greater than the
ratio of my proportional and derivative gains.
Unfortunately,
the system wasn't stable. The pitch angle just oscillated wildly with
increasing amplitude until the plant blew up. The pitch angle response
plot was "smooth", however, unlike the response for the system without a
filter.
The weirdest thing was, the pitch angle
oscillated with increasing magnitude and blew up when I had a step
reference for my x-position. I don't know why that is happening.
The
only thing I can think of doing is to actually simulate my control
system in Simulink and see if I am getting the same response. I am
thinking I have done something wrong at this point.
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