Study Of High Precision Weak Capacitance Measurement Techniques

Spherical electrode capacitive displacement sensor based on the work principle that mesuring the change of electrode capacitance to measure the displacement. Because of the small size and the special shape of the electrode, the sensor capacitance is weak. It can be easily disturbed by the Stray capacitance in the Surrounding environment which is much larger than the sensor capacitance. So how to measure the sensor capacitance through the large interfering signal is the key technique of the sensor. And also it is a main research problem to improve the displacement measurement precision of the sensor.This paper focuses on the problem mentioned above, with the deep research of the characteristic of the sensor capacitance, a AC based method is used as the principle of the weak capacitance measurement. And on the basis of the study of key technology unit in the conventional capacitance measurement system, a improved capacitance measurement system is designed.The main research topics are as follows:1. Research on the characteristic of spherical electrode capacitive displace-ment sensor capacitance:Starting from the electrostatic field theory, different sensor capacitance model is developed when the sensor is aiming at different target. On the basis of the developed model, the simulation analysis work of the sensor capacitance is done to provide theoretical guidance for the design of the weak capacitance measurement system.2. Design of Precision weak capacitance measurement system:In response to the thermal noise caused by the large value feedback resistor in AC based C/V conversion circuit, which is difficult to measure, a method based on T-shaped network structure is put forward. In this method, three low value resistors are used to substitute for the large value feedback resistor, and then reduced the negative impact of thermal noise. In response to the negative impact of stray capacitance, drive cable technique is combined with AC based C/V conversion circuit, to ensure capacitance signal convert to AC voltage signal accurately.According to the principle of phase sensitive demodulation, using an analog multiplier and a 8-order Butterworth filter to achieve the cross-correlation operation between the modulated signal and a reference signal. With this method, the influence of the random noise error of the measurement results can be reduced, and the SNR of system can be significantly improved.According to the basic principle and the characteristics of direct digital synthesis(DDS), a signal generate system based on specific integrated DDS chips are designed to provide high-accuracy carrier signal and reference signal for the AC based C/V conversion unit and the signal demodulation unit. With this method to overcome the shortcomings of the conventional signal generate unit, further improve the low capacitance measurement accuracy.Based on the design work above, the hardware system is designed and developed, the relevant experimental systems are designed to test the performance of the weak capacitance measurement system. Experimental results show that, this system has a measurement resolution of 0.25fF, has achieved a measuring an stability of 0.7mV/ 30min, and an linearity of 0.18%.