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In-Depth Information
In the present work, piezoresistive type transduction has been explored in a specific
type of mass-spring system. In piezoresistive type accelerometers; there is elongation
or shortening of suspension beam when the proofmass moves relative to the
reference/support frame under external acceleration. The piezoresistors are placed at
the fixed ends of the suspension beams at the maximum stress regions. As the proof-
mass moves under applied acceleration the induced stress changes the resistance of the
piezoresistors. The main advantage of piezoresistive accelerometers is that they are
simple in structure, easy to fabricate and less susceptible to parasitic capacitance or
electromagnetic interference (EMI) [2, 3]. The major drawback is that they have large
temperature drift.
The type of design discussed in this paper is aimed for an acceleration of ± 10g. In
the current design, four piezoresistors are placed in a Wheatstone bridge configuration
as shown in Fig. 1. In case of zero acceleration the bridge is balanced and output
voltage is zero, as all the four resistors have the same resistance value. Under
acceleration, the proofmass moves in opposite direction of the acceleration and there
is elongation or shortening of suspension beam which causes maximum stress at the
fixed ends of the beams. This stress is sensed by the piezoresistors and this causes
change in the value of its resistance. Due to this change in resistance, the bridge is no
longer balanced and there is an output voltage corresponding to the acceleration. Fig.
1 shows the arrangement of piezoresistors and corresponding increase and decrease in
resistances. The resistances of R
1
and R
3
increase, whereas R
2
and R
4
decrease, as a
result of applied stress under the acceleration.
Fig. 1.
Wheatstone bridge configuration of piezoresistors
2
Device Configuration
Top and bottom view of the proofmass and suspension system (supporting beam) is
shown in Fig. 2. The structure is to be realized using bulk micromachining of a silicon
(100) substrate. Polysilicon piezoresistors are placed along [110] direction on the
substrate. The die size is 6 mm × 6 mm. The central proof-mass is trapezoidal with
height of 310 µ m and size of 2.8 mm × 2.8 mm (top) and 2.362 mm × 2.362 mm
(bottom). The flexures or beams dimensions are (800×150×20) µ m
3
and piezoresistors
have dimensions of (70×10×0.5) ʼm
3
. Detail design parameters of the accelerometer
are given in Table 1.
