Hardware Reference
In-Depth Information
Figure 2.9: Circuit representation of a typical VCM driver.
This model represents only the rigid body dynamics of the actuator. The
actual dynamics, however, is more complex and includes higher order dynamics
representing different torsional and sway modes of the arm, lightly damped
flexible modes of the gimbal-slider assembly, and modes of coil structure etc.
It may need a transfer function of order as high as 40 to accurately model the
dynamic behavior of the head positioning actuator [54]. Frequency response of
a typical HDD actuator is shown in Figure 2.10. The response of an identified
model that includes a double integrator plus 10 poles and 10 zeros is also drawn
on the same figure [6].
2.2.1 Measurement of Frequency Response
In the head positioning servomechanism of HDD, position feedback is obtained
from the readback signal produced by sensing special magnetic patterns written
on the disks. These patterns, which are explained later in section 2.3, are
created by a process known as servo-writing
§
. This position signal is available
only in an assembled and servo-written HDD. However, to obtain a dynamic
model of the head positioning actuator, one may use other means to measure
the displacement. The use of interferometry to measure changes in position
is well known [181],[18]. The interferometer optics split the laser light into
a reference path and a measurement path. These lights are reflected using
two retroreflectors - the reference beam from a stationary reflector but the
measurement beam from a retroreflector attached to the object whose change
in position is to be measured. Recombination of the two reflected beams creates
an interference signal. A difference of one wavelength between the lengths of
§
Servo-writing and related issues are discussed in chapter 5
