limits of a servo control system are discussed in the next section.
3.3 Factors Limiting Servo Performance
It is generally expected to have the performance of servo system as high as
possible. However, the performance can not be improved to any arbitrary
level. Therefore, knowing the bounds on the limit of performance in a control
system is of great interest to the designer.
Let us consider the read-write head positioning servomechanism shown in
Figure 3.31 with actuator model G p (s), controller transfer function G c (s),
input and output disturbances I D (s)andO D (s), and noise N P (s). Then true
Figure 3.31: HDD servo loop block diagram.
PES (PES t (s)) is,
PES t (s)=−G p (s)S(s)I D (s) −S(s)O D (s)−T (s)N P (s),
where the sensitivity transfer function S(s) and the complementary sensitivity
transfer functions T (s)aredefined by
1+G p (s)G c (s) ,