Research On The Precision Assembly Of Miniature Accelerometers And Its Influencing Factors Of Performance

As one of the key devices of the inertial navigation and inertial guidance system, accele-rometers have great influence on the performance and navigation accuracy. High performance miniature accelerometers assembled manually, and that results in some problems, such as low quality rate, low efficiency and high labor intensity. Therefore, the manual way is unsuitable for mass-production of high performance miniature devices. In this thesis, the precision as-sembly technology and the uncertain factors in the assembly process were studied on basis of the miniature flexure accelerometer.According to the structural characteristics and the operational principle of the miniature flexure accelerometer, the mathematical model of input-output was constructed. Based on the model, and the analysis of assembly process, it was found that the bias error of the accelero-meter was mainly influenced by three kinds of disturbance inputs, including the disturbance torque AM, elastic recovery angleβand capacitor error△C. The influence mechanisms of the disturbance inputs were analyzed, and it was found that the disturbance torque AM caused by the conductive floating threads, the capacitor error△C caused by the distortion of the pendu-lum chip were the main factors influencing the bias error of the accelerometer. The assembly error and the shift of centre-of-gravity of the pendulum part were the main factors that leaded to the cross-coupled errors and nonlinear errors, respectively. The stability of accelerometer bias was analyzed theoretically. It was concluded that there were three factors which influ-enced the stability of the accelerometer bias greatly, including the stability of disturbance torque AM caused by the conductive floating threads, the stability of capacitor error△C caused by the distortion of the pendulum chip, and the change of the gap between the elec-trodes of capacitor, which is induced by the unstable preload.Finite-Element-Method was used to analyze the bias error caused by the deformation of the conductive floating threads in the process of welding, and the temperature stability of the bias error. According to the analytical results, the welding processing method was optimized. According to the analysis of the bias error and the distortion of the pendulum chip caused by the mismatched thermal expansion coefficient, the adhesive joining technique was improved. By establishing the mathematical relationship between the geometric error and the cross-coupled sensitivity, and analyzing the machining and assembly process of the accele- rometer and its parts, reasonable tolerance requirements for the machining and assembly were proposed.The contact stiffness model between the electrode plates of the capacitance was estab-lished on basis of the assumptions of the Majumdar-Bhushan model. Both of the theoretical analysis and the experiments were carried out to demonstrate the influence of the preload on the capacitance between the parallel-plate electrodes. As to the miniature bolt joints of the accelerometer, disc springs with zero stiffness characteristics were used to improve the stabil-ity of the bolt joints preload. FEM was employed to analyze the stiffness characteristics of the combined disc springs and the single disc springs with different sizes. The experiments were conducted to verify the results of FEM analysis. According to the assembly of the miniature bolt joints with zero stiffness characteristics, an improved torque method for preload control was proposed, which can be used to control the preload in the miniature bolt joints assembly of the accelerometer. Assembly experiments were performed for demonstration with the de-veloped experimental setup.An automatic assembly system based on microscopic machine vision was developed to meet the requirements on the structural characteristic and the assembly process of the minia-ture flexure accelerometer. According to trans-scales miniature parts measurement by using microscopic vision, a local feature splicing positioning method was proposed. Based on the analysis of the image quality, a light controlling method was proposed to regulate the light intensity automatically. The mixed control strategy of position and impedance was adopted for precisely control of the micro assembly force in the assembly process of the pendulum component. Finally, assembly experiments on multiple randomly selected sets of parts were conducted to verify the reliability and the precision of the assembly system.The research works in this paper are about the precision assembly of miniature accele-rometers and its influencing factors of performance. The research results include uncertain factors in the assembly process and the mechanisms influencing on the performance of acce-lerometers, and the automatic assembly system for the miniature flexure accelerometers. All the research results have been applied in the assembly production of the miniature flexure ac-celerometers.