Hardware Reference
In-Depth Information
Table 3.3: Applications of various filters
HDD actu-
Lag section to elevate low frequency gain.
Lead section for achieving -20 dB/decade
slope for the open loop transfer function in the
range of frequencies around the 0-dB crossover
for stability.
Notch filter
Smooth output in response to step change in
reference command. Shock transfer function
may still show a peak indicating inadequate
suppression of oscillation induced by input dis-
turbance. Relatively robust to actuator reso-
nance variation compared with phase stable
Phase sta-
ble design
Shock transfer function has lower peak. Good
for reducing oscillation induced by narrow
band input disturbance at resonant frequency.
Response to reference command shows oscil-
latory output, but can be mitigated via input
shaping. Not robust to changes in actuator
band noise
Notch filter Need to check stability and transient response.
band dis-
Peak filter
Loop gain elevated at center of peak filter.
Transient response of the loop degraded. Need
to check the stability.
results in a lower sensitivity transfer function, and hence rejects the process
disturbances more effectively making more accurate tracking control possible.
Though a simple lag-lead controller can be used as the nominal controller
for HDD servomechanism, such design results in low servo bandwidth and
therefore poor disturbance rejection capability. Increase in bandwidth is possi-
ble with proper compensation of actuator resonances and various narrow-band
and broad-band noise and disturbances. These compensation techniques often
include special purpose filters. Table 3.3 summarizes these basic control filters
used in the HDD servo loop.
In a practical servomechanism, a combination of the above mentioned filters
is used. Selection of parameters involves more complex procedure when these
filters are combined. Optimal control theory or on-line optimization is useful
tool for finding the parameters for such complex compensator. The application
of optimal control is discussed in section 3.4, but before that, the fundamental
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