Hardware Reference
In-Depth Information
servo loop gain at the disturbance frequencies by inserting a narrow band fil-
ter. This method can be applied successfully in single-stage actuator (e.g. [48],
[168], [184], and the references therein) as well as for dual-stage actuator [225],
[113]. Use of dual-stage actuator, which is discussed in details in section 3.7,
enables higher bandwidth of the servo system. With the help of this increased
gain in designated frequencies, the runout signals can be rejected more effec-
tively. However, significant amount of disk flutter energy lies in the frequencies
around 500 Hz and above while the bandwidth of the servo systems nowadays
ranges from a few hundred Hz to 2 kHz. Inserting a narrow band filter to
suppress disk flutter, therefore, affects the stability of the loop. Besides, such
feedback control systems typically show poor settling performance [48]. The
prolonged settling caused by the insertion of narrow band filter can be reduced
by initializing the states of controller to make the transient of the filter mini-
mal [184], [213]. If it is necessary to insert multiple narrow band filters then
the initialization of the controller becomes very complicated, if not impossible.
Modifying the feedback control for addressing the issues of disturbances of
specific nature is relatively diļ¬cult. However, mitigating with the basic feed-
back servo loop by including additional sensors and hence applying the concept
of multi-sensing servo (e.g., instrumented suspension [89], active damping of
actuator vibration [90], and acceleration feedback [112]) is a viable option for
achieving better rejection of NRRO. Multi-sensing can improve the bandwidth
by up to a few hundred hertz only and hence the improvement in vibration
rejection is limited.
The concept of multisensing servo has been widely studied by HDD servo
engineers and researchers [99], [161], [2], [126], and [64]. The vertical motion of
diskscanbemodeledasequivalentoff-track motion of the head slider [185], [66].
These developments inspire a new approach using feedforward control for re-
ducing TMR induced by disk flutter. In order to establish the applicability of
this approach, we first show that the disk's vertical vibration signal picked from
the slider has a fairly good correlation with the track misregistration at the
frequencies of disk flutter. Once this correlation is established, we shall show
how a feedforward controller can be designed to cancel the effects of disk flut-
ter. The feedforward controller is an approximated differential element whose
input is the vertical component of the velocity of disk. It was proven through
experiment that the feedforward controller reduced the TMR induced by the
first four modes of disk flutter by an average of about 56%. One potential
application of this method is the high TPI servo track writers (STW), where
this approach can reduce the written-in RRO. This, in turn, will have positive
influence on the performance of the HDD servomechanism. Another method
presented in [110] overcomes the disk flutter problem by using a flexure that is
optimized to cause less off-track error at locations where the disk vibration is
maximum, e.g., the OD. Compared to this method, the feedforward controller
given in the following paragraphs works for wider operating conditions. An
experimental setup for measurement of disk flutter is presented first, followed
