Hardware Reference
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Figure 3.60: Feedforward compensation scheme for disk flutter.
the transfer function is simple to design and easy to implement. From Fig-
ure 3.60, the feedforward compensator F(s) whose output is added to the
normal feedback controller G c (s) can be obtained by letting:
S h (s)F(s)=S(z) 1
s K,
1+G P (s)G c (s) ,
is the sensitivity transfer function and
G p (s)
1+G p (s)G c (s) ,
S h (s)=
is the shock transfer function. Substituting equations 3.136 and 3.137 into
equation 3.135, we immediately have
F(s)= S(s)
S h (s)
s K = P den (s)
s K,
P num (s)
provided F(s) is realizable.
For the system described above, G ( s)= P num (s)
P den (s) has a relative degree of
2. Therefore, is a first order low pass filter added to the right hand side
of equation 3.138 makes F(s) realizable. A feedforward compensator can be
designed as
2π× 12000
s +2π× 12000 ,
which contains a second low pass filter. The feedforward controller is dis-
cretized at 25.28 kHz which is two times as fast as the feedback controller
F(s)= 1.4596 × 10 −8 s
s +2π× 4000
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