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

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Sensors & Transducers Journal, Vol. 127, Issue 4, April 2011, pp. 88-101 K electrical 1 V 2 2 DC n hl d f 2 1 , (4) where E is the Young’s modulus of silicon in the sense direction; b, h, l b are the width, height and length of the U-shape folded beams, respectively; n is the total number of movable fingers; is the dielectric constant; the initial distances between movable finger and fixed are shown as d 1 and d 2 (d 1
2.2. Interface Electronics Sensors & Transducers Journal, Vol. 127, Issue 4, April 2011, pp. 88-101 n order to measure the capacitance change, it’s not only to apply the excitation signal on the sense element, but also need to pick up signals from it for subsequent conditioning circuit’s processing. The excitation signal can be either a voltage or current signal, and also are the picked signals. When using the current signal as the excitation signal, the magnitude of the current signal through the sense element is determined by the admittance of the excitation node, which means it’s related to the magnitude of the parasitic capacitance. Hence, it’s better to use the voltage signal as the excitation signal. If the picked signals are voltage signal, it’s also affected by the parasitic capacitance, so it’s more often used to pick up current signals from the sense element. To sum up, it’s a suitable way to use a voltage signal to excite the sense element and pick up current signals from it. Based on the above reasons, we build the interface electronics in Fig. 3. The capacitances changes ( C1, C2 ) can be converted into two voltage signals at the input terminals of the high pass filter. The signals are filtered and then sent to the differential operational amplifier, and the output of the differential operational amplifier is demodulated, amplified, and then filtered by a low pass filter. The output of the low pass filter is the output signal of circuit, which we can use a PID controller to improve the dynamic characteristics of the whole system. Proportional to the output signal, the feedback voltage is continuously applied to the proof mass, and then the deflected is pulled back to the original position. C 1 U drive C 2 Fig. 3. The schematic diagram of interface electronics in a force-balance MEMS comb accelerometer. The voltage signals coming out of the high pass amplifier can be expressed as: C U () t - Acos( t) (11) 1 1 0 CCA C U () t - Acos( w t) (12) 2 2 0 CCA where C CA is the magnitude of feedback capacitance in charge amplifier; U drive is the excitation signal, which frequency is w 0 and amplitude is A. The differential operational amplifier can translate the two voltage signals (U 1 and U 2 ) into one voltage signal, which is U pos . It can be expressed as: 93
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