Hardware Reference
In-Depth Information
Although the electrostatic actuator has a low resonant frequency at around
1.5 kHz, it has a very smooth and clean transfer function similar to a single
spring mass system up to above 40 kHz. Such microactuator can be considered
as having little uncertainty except uncertainty in the gain of its model. The
displacement range of such an actuator is about 1 µm with a typical driving
voltage of less than 40 Volts.
When a PZT actuator is used to drive the slider or head, the resonant
frequency can move up to above 12 kHz [179] with a displacement range of
about 0.5 µm, or even above 25 kHz with a displacement range of about 0.2
µm [134]. The later design is able to support a servo bandwidth of above 4
3.6.3 Actuated Head
Instead of driving the entire slider, one can choose to use a microactuator to
drive a small part of the slider that holds the read-write head. This scheme
has the lightest moving mass among all possible placements of the secondary
microactuator. A silicon electrostatic microactuator fabricated on the trailing
edge of an AlTiC slider [136] is shown in Figure 3.68. Figure 3.69 shows a plan
view of this microactuator design.
Figure 3.68: Schematic of microactuator-slider assembly attached on suspen-
The microactuator consists of stationary structures and movable structures
with a solid head plate. All the movable structures are suspended by two
springs. The read/write element is attached to the head plate of the movable
structure via four flexure of wires with pads on the slider which are station-
ary. Comb drives are used to generate the electrostatic force. When voltages
are applied across one pair of the metal pads, the movable structures together
with the head plate and the head element are driven by electrostatic attraction.
Only half of the comb finger pairs are used to generate the attractive force;
the remaining pairs can be used as capacitive sensors during this period to
measure displacement that can be used for feedback control. Applying alter-
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