Hardware Reference
In-Depth Information
Figure 2.22: Write-to-Write and Write-to-Read Track Misregistration.
2.5.2 Pivot Friction
The friction of the actuator pivot has detrimental effect on the performance
of the HDD servo control loop. This problem, which is particularly serious in
small form factor drives, demands attention of mechanical engineers as well as
control engineers. For the drives in early days, the inertia of the actuator arm
was larger than that of today's actuators and the specifications for position
error during track following was less stringent; such specifications could be
met without paying much attention to the problem of friction. Inertia plays
the dominant role in the dynamics of large actuators, and therefore effect of
friction is negligible. With the continuous development of HDD, the actuator
technology experienced an appropriate evolution. To meet the demand for
smaller size of drives and faster response, the inertia of the actuators has to
be reduced and the issue of friction becomes more noticeable. The friction of
the pivot produces a torque that opposes the torque generated by VCM. The
friction torque is a nonlinear function of actuator's velocity and position. In
the frequency domain, the effect of friction is manifested as reduction in gain
at low frequency, below 100 Hz in most of the actuators used. Reduction in
gain depends on the amplitude of the excitation signal used to measure the
frequency response.
Performance of the track-following servo controller can be significantly en-
hanced by proper compensation of the pivot friction. The disturbance torque
arising from pivot friction can be recreated from the knowledge of inertial
torque and actual acceleration of the actuator. Inertial torque is proportional
to the VCM coil current. By including two additional sensors, one for coil cur-
rent and the other for acceleration, friction torque can be estimated. In [99], the
difference between the coil current and acceleration of the actuator is filtered
through a disturbance filter. The output of this filter is an approximation
ofthefrictiontorqueandcanbeusedtocanceltheeffect of pivot friction.
