Hardware Reference
In-Depth Information
where R is the distance from the centroid of the plate to the center of rotation
of the rotor, x
n
is the nominal capacitive gap with zero rotation, A is the
area of each plate and is the permittivity of air. The comb drive structure,
originally proposed by Tang [194], increases the area and hence generates a
larger force or torque for rotary actuator.
Because the torque is inversely proportional to the square of the gap of the
parallel plate pair, typically a small gap is desirable provided the structure
can be manufactured. Deep Reactive Ion Etching (DRIE) technology [111]
enabled the fabrication of high aspect ratio MEMS structures and especially
the comb drive structure. Typically, the microactuator has electrically isolated
microstructureswithanaspectratioof20:1withafinger width of 2 µmand
gap width of 2 µm. Novel mechanical designs that are supported by the micro
fabrication process have been studied to achieve larger deflections, low driving
voltages, better area-efficiency and improved lateral stability.
The actuator can be driven with differential driving scheme to linearize the
quadratic voltage nonlinearity of the electrostatic force. The slider, driven by
the rotor for fine positioning of the magnetic head, is bonded on the top of the
rotor using ultraviolet (UV) curable adhesive [57], [51], [195], [25], [85], [197].
The transfer function of a MEMS actuated slider is modeled as a second-
order system
k
s
2
+2ζωs +ω
2
,
P
M
(s)=
(3.141)
with a typical set of parameter as ω=2π× 1500, ζ =0.03 and k =2.5 ×ω
2
.
Figure 3.67: MEMS micro-actuator frequency response.
