Design Of Analog Servo Loop For Quartz Flexure Accelerometer

This dissertation focuses on the design of an analog servo loop (rebalance loop) applied to Quartz Flexure Accelerometer. The quartz accelerometer utilizes capacitive sensing to detect the pendulum swing to sense the acceleration. We developed a capacitive detection circuit based on singal-carrier modulation and a servo drive circuit, and carried out the experiment to test the function of the rebalance loop. Through analyzing the parameters influencing the capacitive detection and noise performance of the circuit, selecting suitable circuit structure and analog electronic elements, a capacitive detection circuit with good performance was achieved. As the output signal of the capacitive system is rather weak, and cannot directly drive the Torquer coil in the accelerometer gauge header, a power amplifier circuit is needed with the purpose of providing large current with the torquer, and composing a closed loop detetion system. In order to make the acclerometer work stably and improve its dynamic and static performance, we also added correction function to the closed-loop based on control theory. The capacitive detection circuit of the closed-loop includes a signal source producing modulated wave, modulated signal amplifier, phase shifter, demodulating circuit, and low pass filter. The drive circuit includes a correction cuicuit and power amplifier.We tested the function of the sevo loop on two working positions (0 g and 1 g), including noise test and stability test. In the noise testing, we changed the value of the sampling resistor of the closed-loop to observe its influence on the output noise level. The experimental results show that the noise power spectral density of designed servo loop is up to order of 10-6g/ Hz when the sampling rate is 44 Hz and order of 10-7g/ Hz when the sampling rate is 4.4 Hz. Regarding the stability test, we recorded the output of the servo loop once in every 10 s, and maintaining this for 1 h, the corresponding stability is up to order of 10-5g, moreover, the order decreased to 10-6g after further filtering processing.Finally, we carried out the roll test for the accelerometer in gravitational field, and applied four-point method to calibrate the accelerometer and gain its output expression. The roll test shows that the closed-loop works stably, and has a good repeatability, and the scale factor lies in the range recommended by the manufacturer.