Modeling and System-level Simulation of Force-balance MEMS ...

Sensors & Transducers Journal, Vol. 127, Issue 4, April 2011, pp. 88-101
element and interface electronics. Simulation results are obtained from COVENTOR and MATLAB in
Section III. The conclusions are reported in Section IV.
2. Theoretical Principles of MEMS Comb Accelerometers
The MEMS comb accelerometer system consists of two main parts: the sense element and the interface
electronics. As shown in Fig. 1, the sense element comprises a proof mass suspended by two folded
beams on each end, differential sensing and feedback capacitances. Movable fingers are mounted on
the mass, and stator fingers are attached to substrate. The movable fingers and the stators establish
differential sensing capacitances that can be evaluated by a signal pick-off circuit. In the initial state,
there is no acceleration and the proof mass rests in the null position. The differential capacitances are
equal, and the output voltage is zero. If an external acceleration is applied on the proof mass in the x
direction, the balance breaks and position variation of the proof mass is sensed, causing relative
changes of differential capacitances ( C ). With proper interface electronics, C can be converted to
a voltage signal, which is proportional to the magnitude of the external acceleration. In order to
improve the system stability and widen the measurement range, we use a closed loop structure, and
this can be realized by applying the feedback voltage signal to the proof mass, which can pull back the
deflected proof mass to the null position.
Anchor
F EXT
Anchor
C a1
C b1
y
x
Proof Mass: M
Air Damping: B
Spring Constant: K
Movable fingers
Stator fingers
C a2
C b2
Anchor
X
Anchor
Fig. 1. Simplified model of the MEMS comb accelerometer.
2.1. Sense Element
The structure design of a MEMS comb accelerometer is shown in Fig. 2. Lateral accelerometers have
been developed using comb electrodes and differentially detecting parallel electrodes to obtain linear
output. The movable parts of this sense element consist of four U-shape folded beams, a proof mass
and some movable fingers. The fixed parts include four anchors and stator fingers. The proof mass is
connected to four anchors through four folded beams. The stator fingers and the movable fingers form
four capacitances, which are C a1 , C a2 , C b1 and C b2 respectively.
The specific structure parameters of the sense element are shown in Table 1.
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