Hardware Reference
In-Depth Information
sensitivity functions that T (s)+S(s) = 1. It is obvious from this identity that it
is not possible to achieve simultaneously the objectives of rejecting disturbance
and keeping the influence of noise on PES low. Increased servo bandwidth
means low magnitude of S(jω) for wider range of frequencies ensuring better
rejection of disturbances (I
D
(s)andO
D
(s) in Figure 3.31). But it also im-
plies higher magnitude of T (jω) and, as a result, more effect of measurement
noise on PES. To reduce the measurement noise in PES, one can choose to
reduce the servo bandwidth such that T (jω) has smaller magnitude in high
frequencies. But this increases the magnitude of S(jω) in those frequencies
causing less effective rejection of disturbances, and the main contribution to
PES comes from various disturbances.
The challenge of designing a tracking controller is thus to pick the struc-
ture and parameters of the controller that balance the impact of disturbance
sources and measurement noise. This must be achieved inspite of the limita-
tions of actuator bandwidth and PES sampling frequency, and without using
unreasonable knowledge of the disturbance (or vibration) models and noise
models.
High bandwidth actuator has always been considered as necessity for achiev-
ing higher positioning accuracy. This is especially true when the sensing noise
level is low, or the disturbance is concentrated mainly in the low frequency
band. It is easily deduced from equation 3.36 that increasing the bandwidth
always reduces the measured PES. However, depending on the spectrum of
noise, a lower bandwidth servomechanism may achieve higher positioning ac-
curacy compared to that obtained by simply pushing the servo bandwidth
higher [121]. When the sensing noise level is high, one must differentiate be-
tween the measured PES and the true PES while designing and optimizing the
servo controller.
The MEMS actuators reported so far in the published literature show con-
stant magnitude response up to 40 kHz, beyond which, the resonant modes
appear. Using the rule of thumb that the bandwidth is limited to 1/4 of the
high frequency actuator resonance, these MEMS actuators make it possible to
extend servo bandwidth to minimum 10 kHz, leaving all mechanical vibrations
under 10 kHz compensated by the MEMs actuator. The MEMS actuators
have very limited range of movement and, therefore, must be used together
with the VCM making it a dual-stage actuator. The single stage actuators
(just the VCM actuator) can support servo bandwidth up to 2 kHz only. How-
ever, the single stage actuator costs less than the dual stage and is widely
used in the HDD industry. Moreover, the 10 kHz servo bandwidth supported
by MEMS actuator is not achievable with the current state of the HDD tech-
nologies as the PES sampling frequency available so far does not support such
bandwidth. Designing a servo controller which is bandwidth restricted and yet
effective in rejecting vibrations is of prime interest in the HDD industry.
For better rejection of vibration via feedback control, the loop gain G
p
(s)G
c
(s)
should be elevated to higher magnitude at the frequencies where the distur-
