Since the vibration of the actuator is a big hurdle in achieving good perfor-
mance, elimination of structural vibration is a very important issue. We can
use a feedforward control to shape the reference command, which is otherwise
a step function, to inhibit actuator vibration. The settling performance of
the HDD servomechanism is affected by non-smooth handover between track
seek and track-following controllers. Application of initial value compensation
(IVC) eliminates the undesirable transient response induced by initial value.
Similarly an RRO compensator can be used in addition to the feedback con-
troller so that performance of the closed loop is improved. Two other methods,
use of multirate control and multi-sensing servo, give additional freedom to the
designer in his/her effort to achieve improved performance.
Input Command Shaping
The methods described in the previous sections address primarily the issues
related to accurate track following controller. The step responses are examined
for all design examples, but the main purpose of these examples is to see how
oscillatory the system is when subjected to a change in one of the inputs.
These linear feedback controllers are not suitable for large reference commands
expected in track seek mode of operation as the fixed gain controllers makes
the VCM driver saturate. As a result, the output goes through excessive
overshoot and undershoot. Use of nonlinear control for track seek, such as
PTOS described in chapter 2, can eliminate such problems. However, the
linear part of the PTOS is simple state feedback and the tracking performance
is not very satisfactory particulalrly in presence of external disturbances such
as RRO and NRRO. The controllers explained in this chapter so far are better
suited for tackling the issues of track following, but transition between such
controller and PTOS with smooth handover becomes a challenging issue.
It is possible to meet the contradicting objectives of track seek and track
following satisfactorily if the controller structure has two degree-of-freedom.
In such case, the track following issues are handled by the linear feedback
controller such as a PID-type control law in cascade with suitable filter for
enhancement of performance. Since this feedback control is not suitable for
large changes in external signals, the command input should not be allowed
to enter the loop directly. In stead, shaping the command input using a two-
degree-of-freedom structure can be used.
Let us consider the closed-loop system shown in Figure 3.37. In this figure,
G c and G p represent the transfer functions of the controller and the plant,
respectively. The overall output of the system is,
G c G p
1+G c G p r.
Let the command signal be r . Assuming that the feedback controller G c has
been designed for accurate track following, we can design an input shaper I s