Hardware Reference
In-Depth Information
achieve better steady-state response. The poles in the track-following con-
troller that are desirable for steady-state properties but not for transient can
now be canceled by proper design of the matrix K ivc .
2.6.5 Suppression of Residual Vibration
It explained earlier in this chapter that the lightly damped resonant modes of
the head positioning actuator are inevitable. The problem is further aggravated
by the present trend of making actuators lighter. Reduction of mass is an
important consideration during designing the actuator as it makes movement
of the read/write head faster using the available torque. Power consumption,
an important issue in HDD for mobile and consumer electronics applications,
is another factor that demands reduced moving mass. As a result, a light and
sleek actuator always remains in the wish list of the designer.
Reduction of actuator mass however comes with a cost to pay - its vi-
bration modes become more prominent. The achievable bandwidth of the
servomechanism is upper bounded by the frequencies of these lightly damped
modes. The mechanical vibrations are usually damped modes and, therefore,
eventually decay to zero; but exciting these modes increases the seek time. No
matter how fast the seek controller brings the read/write head to the desti-
nation track, operation of reading or writing can not be initiated unless the
residual vibration is sufficiently low in amplitude. Suppression of the resid-
ual vibrations is an important step towards realizing a very high speed access
mechanism.
Notch Filter or Band-reject Filter
A notch filter or band-reject filter in series with the VCM actuator can solve
the problem of residual vibration. Such filter inhibits the band of frequencies
around the resonant frequency from reaching the read/write head. If an ana-
log filter is used, it is often integrated into the VCM driver circuit. But such
filter, once designed, becomes a filter with fixed properties. Unfortunately, the
actuators being mass produced, the frequencies of resonant modes vary from
actuator to actuator. Resonant frequencies may also change due to many other
factors, for example, changes in operating conditions, age of the components
etc. To address this issue, a notch filter that can accommodate such variations
in the resonant frequencies should be used. Digital filter is preferred over its
analog counterpart as it comes with better flexibility to accommodate changes.
Notch filters not only attenuates the frequencies around the frequency of res-
onance but also modifies the phase of the open loop transfer function. Care
must be taken while including these filters in series with the plant. Design of
notch filters for the HDD servomechanism is further explained in section 3.2.2.
 
Search WWH ::




Custom Search