Hardware Reference
In-Depth Information
tor parameter calibration and/or adaptive implementation of the phase design
might mitigate the robustness issue. Designing the actuator with certain phase
property for achieving higher performance using simpler low order controllers
is an interesting research topic. Interested readers may refer to [10], [122] and
the references therein for further information.
3.2.5
Inserting a Peak Filter
Following methods of designing servo controller of HDD have been explained
so far.
1. Design a PI controller using a nominal rigid body model of the actuator:
The resulting controller produces satisfactory results as long as the fre-
quencies of actuator resonant modes are a few order of magnitude higher
than the servo bandwidth.
2. Design via gain stabilization: Gain stabilization is achieved by cancelling
the resonant modes with a notch filter. It works well if the resonant
properties of the actuator are well known and there exist a few prominant
resonant modes. If the frequency and damping of resonances vary from
actuator to actuator or if they are changed over time, the implemented
filter must have the provision to adapt with changing parameters. The
order of the compensator increases with increasing number of resonant
modes having significant magnitude.
3. Design based on phase stabilization: This approach retains the actuator
resonance but shapes its phase to maintain a high loop gain and hence
increased servo bandwidth for vibration rejection.
We can expand the bandwidth using gain stabilization at the cost of reduced
gain in certain frequencies. The phase stabilization method, on the contrary,
helps to expand the servo bandwidth and retains the actuator's high gain
at certain frequency providing vibrationrejectionatthatfrequency. Inall
these methods explained so far, the objective is to enhance the servo loop gain
at desired frequencies or band of frequencies so that the sensitivity transfer
function meets the requirements for good vibration rejection.
In HDD servomechanism, there exist both broad band and narrow band
noise and vibrations. Sensor noise and windage induced vibrations are broad
band, while the narrow band vibrations are contributed mainly by (1) struc-
tural vibrations of disks ([141], [140], [79]), (2) structural vibrations of spindle
[175] and (3) actuator resonance. Moreover, imperfections of the shape of data
tracks contribute to RRO whose frequency spectrum consists of frequencies
which are integer multiple of the spinning frequency of disks. The servo loop
must follow these variations of track.
According to the internal model principle [53] for dealing with external
disturbance, a suitable copy of the exosystem should be included in each control
