Development Of High Precision Digital Ultra-low Frequency Quartz Flexible Accelerometer

Quartz flexible accelerometer is widely used in the low-frequency vibration, inertial navigation and other acceleration measurement systems, because of its high resolution, accuracy and stability, etc. The output of traditional quartz flexible accelerometer is in the form of analog current, which leads that accelerometer can’t be directly compatible with the digital system, and the IV conversion circuits and analog-digital conversion circuits should be utilized. However, there is extra-precision loss when the conversion units are utilized. On the other hand, parameters of an analog servo system can’t be easily adjusted after its completion and the mechanical structure of quartz flexible pendulum has processing errors. That means parameters of different quartz flexible pendulum systems are unknown and they perform quietly different in both static and dynamic characteristics. It’s difficult to adjust parameters of an analog servo system according to different quartz flexible pendulum.In this paper, a full-digital quartz flexible accelerometer is designed, which can solve above problems of analog quartz flexible accelerometer. First, the key mechanical structure of a quartz flexible pendulum is analyzed and optimized. Then a full-digital servo system is designed to realize directly digital output. The mathematical model of a quartz flexible pendulum is obtained by the system model identification method. According to the system model of a quartz flexible pendulum, a compensation network is designed to compensate and calibrate accelerometer system. Finally, the performance of full-digital quartz flexible accelerometer is experimentally tested.The main research works are as followings:A cantilever model of the quartz flexible pendulum is built, and its mechanical structure is optimized, using a commercial software ANSYS, to enhance the stability of quartz flexible accelerometer and to suppress the cross torsion by increasing the frequency of the first order torsion mode of the quartz flexible pendulum. Then, a torque electromagnetic field model is established to run the Maxwell simulation on the distribution of the magnetic field. According to the simulation results, the torque structure is further optimized to achieve a uniformity distribution of the magnetic field in the working air gap of the torque coil and to decrease its influence on the stability of quartz flexible accelerometer.A full-digital servo system is proposed to solve problems of the conventional analog servo system: extra-precision loss of the transformation system during matching an analog servo system with a digital measuring system and the performance of the digital-pulse servo system is limited by the precision, stability and symmetry of discrete components. Wherein, an AC unbalanced bridge is employed to detect the differential capacitance in quartz flexible accelerometer and decrease parasitic capacitance disturbance; a phase-sensitive detection method is utilized to extract the acceleration signal and signal-to-noise ratio is simultaneously improved; the AD and DA circuits make the servo system digital and the measurable acceleration can be directly read in a digital measuring system; finally, the driving ability of the feedback loop is improved through a power amplification design.To enhance the dynamic characteristics of quartz flexible accelerometer, the electric stimulation method is used to accurately identify the open loop transfer function of the quartz flexible pendulum system. According to the actual requirements of the dynamic characteristics, a compensation network in the servo system, consisting of integral, lead, lag and other units, is designed to improve the static and dynamic characteristics of quartz flexible accelerometer.Static and dynamic characteristics of full-digital quartz flexible accelerometer are experimentally tested. Experimental results indicate that the acceleration resolution is 20 μg, the stability is better than 100 μg, nonlinerity is less than 0.37%, and bandwidth is 198 Hz through frequency sweep experiments.The research achievements of this paper improve the form of output signal and of quartz flexible accelerometer and solve the problem of compatibility between the quartz flexible pendulum system and the servo system. Therefore, every flexible pendulum system can be identified through the system model identification method and be compensated to achieve optimal control design to enhance the performance of the quartz flexible accelerometer.