The Research On Detection Circuit For Micro-machined Capacitive Accelerometer

As today's MEMS technology is becoming increasingly mature,MEMS has been well applied in the production of complex micron structures, devices and systems.The potential impact of MEMS and the extensive use of a variety of transmission mechanisms, make a variety of new MEMS products progressive realizing commercialization.Micro-mechanical acceleration sensor is one particularly important of MEMS products.Because of small size, high sensitivity and low power consumption,it has become a very important device to measure the acceleration. Micro-acceleration sensor has broad range of applications, such as medicne, automotive, industrial control and other areas,and it has become a research hotspot around the world. In the future, MEMS accelerometer will also play an important role in the field of aerospace.However, the capacitance variation of micro-accelerometer is very small.So in the real applications,it must be used together with the detection circuit.Only in this way,it can perform its proper function.Therefore, the demand for the design of the detection circut is increasingly critical , the detection accuracy and circuit performance is critical to all of the acceleration sensors.The upper and lower plates of differential capacitive accelerometer are composed of differential capacitance, and the capacitance variation directly reflects the acceleration, the acceleration is linear proportional relationship with the capacitance variation. This thesis is the study of differential capacitive micro-accelerometer detection circuit,and it includes the following sections:At the beginning of thesis, we make a simple introduction to the MEMS technology.Based on this ,we discuss the research status of micro accelerometer and its application in the automotive, military and other fields.According to the working mechanism of the sensor ,micro accelerometers are classified,and the performance characteristics and working principle of various sensors are introduced. At the same time, the performance indexes of accelerometer are introduced.Next we discuss the measurement and control principle of capacitive micro-accelerometer, and the working principle of the acceleration sensors is introduced. Based on this, acceleration sensors are divided into two types including gap variation and area variation.We highlight the differential capacitive acceleration sensor made by laboratory,the sensitive element use a four-beam model.Four forms of detection circuits including switched-capacitor, AC bridge, resonant amplification and charge amplification detection circuit,are analyzed and compared in detail. The charge amplification circuit is chosed as the detection circuit, because its sensitivity and resolution are ideal for testing. In this circuit, the parasitic capacitance and stray capacitance will not affect the capacitance change detection, and we do not have to consider the problem of potential shielding.Then combining with the inherent characteristics of micro accelerometer and the use of charge amplification detection circuit, we developed an overall program of the detection circuit. This is a key part of the thesis, the program mainly includes seven modules, named power supply circuit ,carrier generation circuit,charge amplifier,band-pass filter circuit, phase-shifting circuit, phase-sensitive demodulation circuit, low pass filter circuit, voltage conversion circuit and data acquisition circuit.Charge amplifier,band-pass filter circuit and phase-shifting circuit are called the front-end charge amplifier. The design of power supply circuit,carrier generation circuit and front-end amplifier is emphasized.The positive and negative output voltage regulator composed of LM7805 and LM7905 is used as the power supply module.Sine wave is generated by DDS, mainly using the VHDL to describe hardware circuits, and finally generate the sine wave of 10kHz. Charge amplification circuit realize the conversion from capacitance to voltage, two carriers with the same amplitude,same frequency and contrast phase are its input. The output voltage variation only associated with capacitance variation. The output waveform of charge amplifier circuit has glitches and interference.The waveform will be improved after band-pass filter circuit.The role of phase-shifting circuit is to ensure the phases of demodulation inputs completely inverting. The role of low-pass filter is to preserve the useful DC signal of demodulation,while the voltage conversion circuit is to make the voltage range become benefitly observed. After the last module circuit,acceleration change which is in the form of a digital voltage will be displayed in the LCD.After that,to verify the design of the circuit is right or not, we make simulations for the detecton circuit,mainly on the power supply circuit, front-end amplifier.After the simulation of key circuits,we find the design of evry circuit module is reasonable. On this basis, we have made the circuit PCB layout and the real-time measurement of evry module in the PCB.Finallly we find the output waveforms coincide the simulation results of each module.Finally we have design the detection system which consist of the DC speed controller, DC motor and tachometer.And using this detection system,the voltage were measured and recorded when acceleration is not loaded or loaded on the accelerometer.And after data were processed ,the voltage variation curves with the acceleration were fitted out.From the fitting curve,we obtained key performance indicators of the MEMS accelerometer,their results are as follows: sensitivity is about 80mV / g, range is about and the precision of about 1.64%.±12g...