Hardware Reference
In-Depth Information
be greater than 0, resulting in more attenuation of vibration than amplification
with a continuous time controller. Additionally, we note that Clegg integrator
has 39 degree phase lag instead of the usual 90 degree phase lag. As a result, it
provides the necessary integral action but does not add the amount of phase lag
found in the usual linear controls. Henceitispossibletoavoidthewaterbed
effect. Nevertheless, with limited sampling frequency, a digital servo loop will
have more vibration amplification than a continuous time servo loop of the
same bandwidth.
3.3.3 Bandwidth Limitations
Assumingatypicalservoloopshape(Section 3.2) and stability margin require-
ments, the two major factors thet limit the achievable servo bandwidth are (1)
the uncertainties related to the actuator, and (2) various delays in the control
system.
The phase margin of a control system defines the extent of additional phase
lag that can be tolerated before the stability is lost, and the gain margin defines
the amount of increase in gain that makes the loop unstable. So the servo loop
can be pushed to the point that enough margin is left for the actuator's phase
and gain uncertainty. Since the control systems are typically designed to have
6 dB gain margin, the servo bandwidth attainable is limited to the frequency
where the uncertainty of actuator gain is equal to 6 dB. In general, a rule of
thumb states that a servo bandwidth of 1/4 of the critical resonant frequency
(beyond which, the frequency response will show uncertain behavior at different
excitation level) can be achieved [144].
The PES sampling frequency and the achievable servo bandwidth are in-
terrelated. It is generally preferred to have PES sampling frequency roughly
10 times the open loop servo bandwidth or higher for effective suppression of
vibration. The sampling frequency depends on the number of servo sectors
per revolution which, in turn, is limited by the space on the disk allocated
for servo bursts. Increasing the rotating speed of disk increases the sampling
frequency, but it also increases the level of internal vibrations and therefore
demands for better servo design, which in turn requires a even higher PES
sampling frequency. Similar to the limitations on control performance due to
the sampling frequency, the computation delay and more importantly the de-
lay introduced by the ADC limit the achievable performance by adding extra
phase lag. These delays must be kept less than a fraction of the PES sampling
period.
The success in achieving the objective of the HDD servo control to meet the
requirements on tracking accuracy and response time demanded by the system
depends on many factors including the limitations of actuator's performance
(plant uncertainty), lack of accurate disturbance model, and insuļ¬ƒcient feed-
back information due to limited sampling frequency. It is obvious that no single
solution exists that can tackle all these limitations, and optimization plays an
 
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