Hardware Reference
In-Depth Information
of one of the modes. Choice of time optimal or near time-optimal controller
for the seek mode is explained in chapter 2. The PTOS ([54], [209]) discussed
there is a solution that ensures smooth transition between a nonlinear seek con-
troller and a linear state feedback controller for track-following. Since all states
of the head positioning actuator are not available as measured variable, the
states are estimated using an observer. Many well defined and eļ¬cient algo-
rithms are available for calculating the parameters of state feedback controller
and the observer. However, state feedback is only one of the many standard
and well studied methods available for designing a linear controller. Tradi-
tionally, the industry finds itself more comfortable with PID(Proportional-
Integral-Derivative) controller or PID-type compensators. One can realize the
track-following servomechanism using one such controller. A simple feedback
controller does not always provide the best solution to meet design objectives
of the HDD servomechanism. Feed-forward control or piggy-back correction
algorithms in addition to the usual feedback control are widely used for better
performance of the HDD servomechanism.
3.1 Review of Design Methods
Design of controller for a plant with known model involves two steps - se-
lecting the structure of the controller and determining the parameters such
that the given robustness and perfomance objectives are fulfilled. While de-
ciding the structure of the controller, one must consider different factors such
as knowledge about the plant dynamics, type of reference signals, nature of
disturbances and desired performances. For example, integral action should
be included in the controller for a first order plant if it is desired to have zero
steady state error in presence of constant input disturbance. There are sev-
eral approaches that can be used to find the appropriate parameters once the
structure is decided.
In a mechatronic system, the plant is a continuous-time system but the
controller is usually implemented using a digital processor. In such case, two
approaches can be taken to design and realize the controller - (1) by discretiz-
ing an analog prototype controller or (2) by direct digital design applied to a
discretized model of the plant. The choice between these two approaches de-
pends on the relative rate of sampling with respect to the desired closed-loop
bandwidth. One particular concern in the design of discrete-time controller
for continuous-time plant is the delay introduced by the discretization process
and implementation of control algorithm. The inter-dependence of sampling
frequency, desired bandwidth and choice of design approach is explained in the
following.
