Research On Micro-displacement Measurements With Thick-film Capacitive Pressure Sensors

In the late 1980s, thick-film capacitive pressure sensors is used to measure fluid pressure sensor, which is made by ceramic materials with special process. It has advantage of wide working temperature range, resistance to overloads strong, creep and little hysteresis, direct contact with corrosive liquids. It is widely used in petroleum, chemical, food, power machinery, biomedical engineering, meteorology, geology, seismic surveys, etc.Normally, the thick-film capacitive pressure sensors is used to measure pressure. For example, for atmospheric,water and sound pressure measurement, etc. In these applications, the sensor's diaphragm is working under uniform loading conditions, the uniform force model used to analyze and study. There is also an important application that can be used to measure displacement, for instance, PZT piezoelectric ceramic precision displacement measurement. The piezoelectric ceramic PZT probe is directly contact with the sensor flexible diaphragm. Through giving voltage to the two ends of the piezoelectric ceramic PZT with deforming, it pushes the elastic deformation of the sensor diaphragm with the output capacitance changing. Then, PZT piezoelectric ceramics sensor's probe is contact with the center of diaphragm, if we use the uniform load model to calculate and analyze, it will cause error in micro displacement detection. For this reason, the concentrate load model is presented. Through analyzing the uniform load model and concentrate load model, we can improve the thick-film capacitive pressure sensor's sensitivity and precision.For this reason, the author of paper made research. The main works are as follows:1. Researched the status and progress of precision displacement measurement technology at home and abroad. The thick-film capacitive pressure sensor is used to measure micro displacement of PZT locator by the model of force, displacement, capacitance of the transmission.2. Researched parallel plate capacitance sensor and the PZT displacement precision positioning device working principle and structure and studied the principles and application of piezoelectric effect.3. With the thick film capacitive pressure sensor diaphragm bending, uniform load model, concentrated load model, concentric load model are presented.4. Give capacitance formula of three models and membrane bending curve by using MATLAB. Calculated the capacitance of three models under small deflection. The study results show that the diaphragm center deflection of less than 4μm, the capacitance error is less than 1.10%. In this case, Uniform load models can be used to calculate the capacitance. When the diaphragm center deflection is greater than 4μm, the reality is to be considered, then we should use concentrate load model to calculate the capacitance and improve its measurement precision and accuracy.