Design Optimization Of Butterfly Micro-accelerometer

Micro-accelerometers are sensors for linear acceleration measurement.They are important parts of inertial navigation element and widely used on many military and civilian areas as for the advantage of small size,small mass,low cost and easily fabricated in batches.Our laboratory firstly put forward the butterfly torsional accelerometer structure and finished the design,fabrication and testing of the first version accelerometer system.Although we have gotten the good experiment results,there are still some disadvantages such as asymmetric sensing structure,complex fabrication process and poor temperature performance.This paper makes some optimizations on the structure design,fabrication,readout circuit and temperature compensation based on the previous research and the testing results show that the optimizations can improve the performances greatly.The main research works of this paper are following:1.Optimization of the sensing structure.This paper made some optimizations aiming at the asymmetrical problem of the sensitive torsional beam and supporting frame in the previous version accelerometer.In the new design,the parallelogram beam is changed to V-shape beam,and the supporting frame in which one side is parallelogram beam and the other side is trapezoid beam is changed to the supporting frame with two trapezoid beams,realizing the fully symmetry of the fame structure.The simulation results show that the cross axis error and the temperature coefficient are both decreased.FEM results show that the sensitivity of the new design is 69.11fF/g,x axis cross axis error is0.00012%,y axis cross axis error is0.0018%,resonant frequency is 1618.4Hz,drift in the full temperature range is 5.13 mg and the temperature coefficient is 0.051mg/??Also the fabrication errors have less influence for the V-shaped beam,so the robustness of the structure can be greatly increased.2.Fabrication technology optimization.Four masks are needed in the previous design,the complex fabrication process will result in a larger cumulate fabrication errors so the precision will be decreased.In this paper,the fabrication process is optimized,which reduces the masks number from four masks to two masks,greatly reduces the fabrication complexity.Also,as for the self-stopped characteristics of V-shaped structure,the over-etching problem can be effectively avoided,which can increase the fabrication precision greatly.3.Readout circuit optimization.In the previous readout circuit,which is anti-phase symmetrical driving circuit,the reference capacitor is set on PCB.As the temperature coefficient of the sensing capacitor and the reference capacitor are different and also the temperatures out of the package and in the package are different,the accelerometer has a poor temperature performance.This paper moves the reference capacitor into the package and set it in the sensing structure.By this way,the temperature environment and temperature coefficient differences are decreased,which can improve the system's temperature performance.4.Testing results of typical prototype show that,by using the integrated capacitor,the 1g stability is improved from 0.37 mg to 0.26 mg,the output bias in the full temperature range is decreased from 182 mg to 61 mg,and the temperature sensitivity of the scale factor is improved from 120.4 ppm/? to 79.9 ppm/?.For further increase the stability and sensing precision,this paper designs the temperature compensation for the accelerometer system.The test result after compensation indicated that the 1g stability among full temperature range can be reduced 74%.