If your first attempt at
does not achieve everything you wanted, you will need to readjust
your target desired loop shape
Gd. Here are some
basic design tradeoffs to consider:
Stability Robustness. Your target loop
have low gain (as small as possible) at high frequencies where typically
your plant model is so poor that its phase angle is completely inaccurate,
with errors approaching ±180° or more.
Crossover and Roll-Off. Your desired loop shape
have its 0 dB crossover frequency (denoted ωc)
between the above two frequency ranges, and below the crossover frequency
ωc it should roll off with a negative
slope of between –20 and –40 dB/decade, which helps
to keep phase lag to less than –180° inside the control
loop bandwidth (0 < ω < ωc).
Other considerations that might affect your choice of
the right-half-plane poles and zeros of the plant
which impose ffundamental
limits on your 0 dB crossover frequency ωc . For instance,
your 0 dB crossover ωc must be greater
than the magnitude of any plant right-half-plane poles and less than
the magnitude of any right-half-plane zeros.
If you do not take care to choose a target loop shape
conforms to these fundamental constraints, then
loopsyn will still compute the optimal
K for your
but you should expect that the optimal loop
have a poor fit to the target loop shape
consequently it might be impossible to meet your performance goals.