Table 3.3: Applications of various filters
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
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
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
Notch filter Need to check stability and transient response.
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