Research On The Detecting Technology Of Tiny Capacitance

Micrometer technology of capacitance is widely used in the measurement ofdisplacement, level, pressure and other parameters, because its low powerconsumption, high precision and dynamic performance, high stability andnon-contact measurement. With the development of ultra-precision manufacturingand modern industrial production, high accuracy detection is demanded. In the fieldof micro-displacement measurement, the detection accuracy has reached thenanometer scale. Displacement changes in Nanometer scale correspond tocapacitance changes in fF magnitude and to interference of parasitic capacitance inpF magnitude. To increase the detecting accuracy, detection of weak signal in highbackground noise is a basic problem need to be solved immediately.To solve the above problem, four capacitive measuring principles and theadvantages and disadvantages of them were investigated. The AC excitationcapacitance measurement was chosen as the main method of the project at last. TheAC excitation capacitive measurement system mainly includes a preceding stagecircuit, a signal generating unit and a signal demodulation unit. The system wasdesigned on the basis of analyzing and optimizing the implementation of eachtraditional unit.The main research tasks of this dissertation are as shows1The index analysis of Capacitance measurement circuit of capacitivedisplacement sensor and the determination of system inputThe index of the noise, resolution, bandwidth and others which are used Tomeasure the performance of capacitance measurement system in terms of definition,principle and measuring method were introduced and analyzed. Capacitance range is the input of the capacitance measuring system. The capacitive sensor model wasestablished by using electromagnetic simulation software CST and the system inputrange was obtained through simulation.2. The design of tiny capacitance measurement systemPreceding stage circuit plays as a Converter in the C/V (capacitance-voltage)conversion and a decisive role in resolution of the system. A new design ofcapacitive bridge circuit which includes a measuring arm and a reference arm wasproposed on the basis of previous studies. Capacitance sensor voltage measured bymeasuring arm can be led to reference arm through the1:1amplifier, and thenfeedback to the excitation source after multiplied by appropriate times. By doing this,the ability of anti-interference of system can be improved and the range of outputvoltage of the preceding stage can be increased.Former bridge circuit consists of two sources, a reference capacitance and acapacitance sensor. The bridge circuit can be balanced by adjusting the excitationvoltage, the ratio of capacitive sensors and reference capacitance equals to the ratioof two excitation voltage. The value of the capacitive sensors can be obtained byproportional relationship. Capacitance bridge circuit has a simple structure, gooddynamics and strong anti-interference ability when it compares to bridge circuit.Signal generating unit was realized by using the high precision and performanceDDS chip of ADI. Signal demodulating unit includes detecting circuit and low-passfilter. The switch full-wave detection was used in the former and FDNR filter waschose in the latter.On the basis of the above, tiny capacitance measuring system was designed andexperimental platform for performance test was established. Experimental resultsindicate that the system resolution is30nm in the range of150~650um, thestability of indication is0.1mV/30min.