Design And Noise Analysis Of Electrostatic Space Accelerometer Based On Mems Technology

In traditional electrostatically levitated accelerometer, the position and orientation of the proof mass is detected by measuring the differential capacitance between the proof mass and electrodes. And closed-loop control is used to suspend the proof mass stably in six degrees of freedom (DOF). The sensitivity, stiffness, bandwidth and measurement range can be adjusted flexibly, because of no mechanical constraint of the proof mass. Both the linear accelerations and the angular accelerations each in three DOFs can be measured by one accelerometer.The technology of MEMS fabricataion is applied on the electrostatically levitated micromechanical accelerometer which not only has the advantages of the traditional electrostatically levitated accelerometer, but also has the advantages of small volume, light weight, low power consumption and high integration. These advantages make the accelerometer much more suitable to be used on the micro-satellites which have rigidly requirement on volume and weight. Two structures of proof mass are designed to be respectively used in ground environment and space environment, and some key techniques are studied in this paper. The main contents of this research are as following: The mathematical model for the motion of proof mass in six DOFs is developed.This model describes the capacitance and the electrostatic forces/torques in six DOFs. First-order equations are derived from the linear approximation of the models. The error is less than 4.0%, when the displacement of the proof mass reaches 20% of nominal gap. Finite element analyzing is used to calculate the squeeze film damping. Because the thickness of the proof mass is limited by the bulk silicon processes, the squeeze film damping between axial electrodes and the proof mass is two orders greater than that between radial electrodes and the proof mass.The force/torques balanced loops of the proof mass are designed. The axial and radial measurement range of the accelerometer used in ground environment can reach to 7.7g and 2.6g; closed-loop bandwidth can reach to 727Hz and 1.886 kHz. And the measurement range of the accelerometer used in space environment can reach to 0.71mg and 0.9mg with bandwidths of 47Hz and 24Hz, respectively.The acceleration noise is analyzed on the aspects of space environment, sensor structure and displacement detection circuit. The overall noise level of the space accelerometer is 2.73×10-7ms^-2/Hz^1/2.Two capacitive displacement detection schemes, transformer bridge and charge amplifier, are designed. Both carrier frequencies of 100 KHz and 1 MHz are applied in each scheme. Theoretical simulation and experiment evaluation are used to optimize structure and parameters of the circuit. Simulation and experimental results show that the low-frequency (0.1Hz or less) noise of displacement detection is dominated by the low-pass filter and analog multiplier, and the intermediate-frequency (0.1Hz to position sensor bandwidth) noise is dominated by the charge amplifier. The performance of each circuit on noise, output stability and rejection to stray capacitance is compared. Experimental results show that the charge amplifier based sensing scheme with high carrier frequency is preferred for the micromechanical electrostatic accelerometer.