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Where 'ʵ' is the permittivity of the material between the fingers, 'A' is overlapping
area and 'd' is the distance between the fingers. At stationary the capacitance value is
maximum, its value decreases as there exist some oscillation or displacement. Finger
overlapping area is given by
In the proposed model the overlapping length is always constant, change in area is
always because of change in overlapping width. Let the change in width be ∆
w
, and
then change in area is given by
∆
Changed capacitance is given by
∆
It is important to note that the model is only to sense the change in capacitance, we
need to add an additional digital circuitry to convert that capacitance value into
voltage to determine proper acceleration.
The force sensed by the accelerometer is the only force applied on the proof mass
that oscillates it. Proof mass oscillation results displacement in comb attached to it
and hence change in capacitance between interdigitated fingers.
4
Accelerometer Design and Simulation
Accelerometer sensitivity, resonance frequency etc. depends upon the proof mass,
comb structure and folded beam. In the present paper we are trying to design and
simulate the structure to obtain high resonance frequency.
4.1
Folded Beam Design
Designing of folded beam is important in the terms that proof mass support and
resonance frequency depends only upon it. We have designed and simulate the folded
beam structure for different flexure length, width and height. Our motive is to obtain a
structure that can provide maximum resonance frequency and high strength. We set the
initial parameters of folded beam as flexure width 1[µ m], flexure length 10[µ m] and
flexure height 1[µ m]. We now calculate the displacement of the free edge that connects
to the proof mass for a varying range of frequencies and varying structural parameters.
The folded beam structure indicating different parameters is shown in fig 4.
Fig. 4.
Folded beam structure. 'H' is height, 'W' is width and 'L' is length of folded beam.
