Capacitive pressure sensor is based on the variable gap type parallel plate capacitor. When the movable plate is subjected to pressure,a certain deformation occurs, so the distance between the upper and lower electrodes changes, thus the capacitance between them is also changed.
However, the relationship between pressure and capacitance is non-linear. In order to eliminate nonlinearity, the designer will design the movable plate of capacitive sensor according to the theory of small deflection plate,so that it can decrease and even change linearly in a certain pressure range.
In addition, the output capacitance can be compensated by non-linear measurement circuit with compensating function. In the range of the pressure,the measurement object of the sensor is absolute pressure. The initial capacitance C0 and Shang (the ratio of γ to movable plate shape variable and initial plate spacing) are the key factors affecting the sensitivity of capacitive sensors.
In order to obtain a larger capacitance output and higher sensitivity, the plate area should be as large as possible, the spacing between the plates should be as small as possible,the deformation of the movable plates should be as large as possible, at the same time using the material dielectric materials to do dielectric layer.
The large and thin plates not only increase the C0 of the sensor's initial capacitance, but also make the movable plate more deformed when subjected to the same pressure, which will increase the capacitance output value of the sensor.But the large and thin plates will cause the linear performance of the device to deteriorate, and the design concept of MEMS miniaturization is contrary. Also, too thin plates are brittle and fragile.
The reduction of plate spacing can also significantly improve the C0. But when the plate spacing is very small, the dynamic range of the sensor is not only too narrow,and the presence of any trace gas in the cavity will affect the sensor performance, so the vacuum of the sensor cavity must be very high.
In addition, within the small pressure range, the effect of internal stress on the working performance of the movable plate is particularly large, the internal stress will hinder the deformation of the movable plate, reduce the sensitivity of the sensor and increase the non-linear error. At present,the difficulty of measuring micro-capacitance is the problem of the complexity and precision of the detection circuit.
Because the output capacitance of the small range sensor is too low, so the detection circuit is extremely complex, compared with the small capacitance measured,stray capacitance is generally much higher, and stray capacitance will vary with many factors, the result is often the interference signal far greater than the measured signal, it is difficult to get accurate results.
Therefore, in order to realize the small-range pressure measurement, it is necessary to change the capacitive sensor structure to improve the capacitance output,to weaken the stray capacitance interference and reduce the complexity of the test circuit, and to optimize the measuring circuit and increase its resolution and precision.
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