Quartz Crystal Capacitive Sensor For Micro Displacement Measurement

Micro displacement measurement is widely applied in many fields, such as precision measurement, precision positioning, micro fabrication, microelectronic manufacture and so on. Many sensors have been used for measuring micro displacement, including inductive type, optical type, capacitive type, and so on. They have their own advantages and disadvantages. Among them, capacitive micro displacement sensor has been widely used due to the advantages of low cost, high resolution, simple structure. In this paper, a novel quartz crystal capacitive sensor was proposed for micro displacement detection. A series of theoretical research and experimental analysis concentrate on the novel sensor. The main points of this dissertation can be summarized as follows:(1) Based on the working principle and characteristics of the quartz crystal resonator and planar capacitance, the research design a plan which measuring micro displacement by a novel quartz crystal capacitive sensor. The novel quartz crystal capacitive sensor consists of quartz crystal resonator and planar capacitance in series. Capacitance value of planar capacitance changes because the displacement variety of the measured object and this change influence the resonance frequency of quartz crystal resonator. This sensor has advantages of high sensitivity, better stability, small volume, convenient test, low cost and easy to implement digital signal output.(2) We built the model of quartz crystal resonator using finite element analysis method. Quartz crystal resonator resonance response is analyzed through the simulation. The effect of the thickness of the electrode on the quartz crystal resonator resonance frequency is analyzed firstly.(3) The equivalent circuit of quartz crystal resonator is analyzed, and the series resonant frequency and parallel resonant frequency of quartz crystal resonator is derived. Through the analysis about the resonant frequency, we get the relation among micro displacement of measured object, external series capacitor and the series resonant frequency of the quartz crystal capacitive sensor.(4) The feasibility of the novel quartz crystal capacitive sensor for measuring micro displacement test and verify through a series of experiment of the quartz crystal capacitive sensor measure micro displacement. When the measured object is metal, such as copper or aluminum, the series resonant frequency response curve exhibited piecewise linearization. The series resonant frequency of the sensor tends to decrease with decreasing the micro displacement in the range of20μm-1000μm. Further increasing the distance beyond1000μm, the serial resonant frequency goes nearly to steadiness. The sensitivity of the sensor decreases with increases in micro displacement. The sensitivity of the sensor is lower when the measured object is medium. And the sensitivity is in relation to the dielectric constant of material. The quality factor of sensor exhibits an increase with decreases in micro displacement, which shows that the sensor is more reliable and more stable.