Hardware Reference
In-Depth Information
the feedback system and fulfilment of the specifications for performance and
robustness. The control structure is shown in Figure 3.82.
Figure 3.82: PQ method compensated system block diagram
In the first step of PQ design method, the controllers C
1
(s)andC
2
(s)are
chosen simultaneously to address the issues of stable zeros, relative contribution
of the two actuators to the combined output, and the interference between the
two contributions. Let us define the ratio R(s) between the transfer functions
of the VCM path and the transfer function of the microactuator path,
P
V
(s)C
1
(s)
P
M
(s)C
2
(s)
= R(s).
(3.162)
This ratio determines the relative share of contribution between the two ac-
tuators. The VCM actuator should have the dominant role for correcting
errors in the low frequency range whereas the microactuator should contribute
more for correcting high frequency errors. This division of task defines the
desired shape of the ratio R(s) which should have large magnitude ( 1) at
low frequency and small magnitude ( 1) at high frequency. In the range of
frequencies where |R(s)| is close to 1, the outputs from VCM and microac-
tuator have comparable magnitudes. Then the phase of R(s) determines the
relative phase between the outputs of P
M
(s)andP
V
(S), i.e., the amount of
interference between the outputs of VCM and microactuator, and hence the
overall magnitude of the output.
The transfer functions C
1
(s)andC
2
(s) are selected such that open loop
transfer functions for the VCM (C
1
(s)P
V
(s)) and microactuator (C
2
(s)P
M
(s)),
and an acceptable frequency response for R(s) are obtained. After selecting
these two compensators, the two parallel branches are combined to form a
single-input single-output model P
SISO
:
P
SISO
(s)=C
1
(s)P
V
(s)+C
2
(s)P
M
(S).
(3.163)
In the second stage of the PQ method, a controller is designed for the SISO
plant so that the overall system achieves the stability and performance of the
