The Design And Research Of The Detection Circuit For A New Micromachined Capacitive Accelerometer

Micro-electro-mechanical systems (MEMS) have broad application prospects in motor vehicles, medical, industrial control, scientific measurement, military and space systems, and other fields. Microaccelerometer is a typical micro-mechanical systems. It is widely used in automobile safety airbags, Vibration compensation and anti-skid systems, for the increase in vehicle maneuverability, safety and comfort in the mid-1980s. Currently the main application of MEMS sensors in automobiles conclude balloon acceleration sensor, wheel speed sensor rotation, tire pressure sensors, pressure sensors for cooling, engine oil pressure sensors, pressure sensors and braking from speed sensors. In the next few years MEMS accelerometer will be largely applied to automobiles.Microaccelerometer represent the trend of current sensors and it is urgent to do research in this field. Capacitive accelerometer is sensitive to Acceleration when the capacitance in it changes. So it requires the measurement circuit, which is an important component of the capacitive accelerometer. For MEMS accelerometer sensor, because of the micro-structure effects, its structure capacitance is very small, about only a few pF. And its differential capacitance variation is even smaller. So it is very difficult to detect. But the detection circuit will have a direct relationship to the such parameters as resolution, bandwidth and the stability of signals. Therefore the design of detection circuit which has low noise and great ability of anti - Weak signals is not only one of the key technologies of microaccelerometer, but also is the major research objectives.In this paper, we have deep research about the structure of microaccelerometer of variable area capacitive. And overall program of the corresponding detection circuit is displayed. We have simulated these circuits by simulation software computer. Based on the simulations, then we do experiment about them.There are two different forms of Capacitive MEMS accelerometer, they are area changing and gap changing. The capacitance change in The gap changing microacclerometer area changing microacclerometer is nonlinear. Usually we restrict the displacement of the Mass to obtain approximate linear relationship. Finally the change of differential capacitance must be very small, which brings great difficulties to the following processing circuit. Generally, we need to add feedback signal to make sure that it has high linearity. The changes of differential capacitance of area changing microacclerometer have linear relationship to acceleration. So it is easy to detect the signal.The detection of capacitive microacclerometer is very complicate. Generally there are several methods such as capacitance - frequency conversion, switched capacitor, capacitor AC bridge, enlarging charge. According to the special requirements of sensors, Only capacitance AC bridge and enlarging charge are suitable for us. And because enlarging charge method have many advantages that other methods don't have, we have chosen this method to produced a suitable overall program. The detection circuits we have designed not only contain preamplifier, but also contain signal-handling part. The main task of Preamplifier is to change the small capacitance to high-frequency voltage signal. We use single-carrier and two-way integration structure, which includes capacitance - voltage transformation part and subtraction amplifier. Signal handling circuit will handle weak and high frequency voltage to DC signal, then we can receive DC voltage output. So this completed the acceleration signal detection. It includes three main parts: band-pass filter and amplifier, the multiplier phase-sensitive detection circuit and a low pass filter circuit. For band-pass filter circuit, we select a second-order band-pass and positive feedback filter in according to our own needs. It not only has high quality factor, but also has higher gain. As for Low-pass filter, we select infinite gain low-pass filter circuit, which not only can adjust its gain easily, but also have no self-excited oscillation.We first used a simulation software in computer to test the circuits we designed, which simplified the debugging process. We Used the Multisim 8.0 of EWB to simulate. By the contrast of simulation results and the experimental results, we can see that the circuits we designed meet our requirements.Finally, in the paper, there are some performance analysis such as measurement accuracy, circuit gain, anti-interference capabilities and the error. As for measurement accuracy, it can reach , that is, when the differential input capacitance is greater than , we can get satisfactory test results. For circuit gain, the entire detection circuit gain is . That is, when the differential input capacitance 10?16F3. 6×10?16F2 .73×1015 is , we can get 0.273 V DC voltage output. As for the anti-interference capabilities, the Q value of the band-pass filter is 18.2.That is if the center frequency is 40KHz,the bandwidth is only 1.5 KHz, so noise outside of 39 KHz ~ 41KHz all can be good suppressed and filted. As for error, the experimental error corresponding to the equivalent input capacitance measured actual difference is about one-tenth of the differential capacitance, so it has little affect on the correctness of the results.The work of this thesis is one of the main tasks of improving the capability of microaccelerometer. With the accomplishment to this thesis, it will be beneficial to succedent research and the practicality of the microaccelerometer.