Research On Frequency Discrimination Of Frequency Modulated Capacitive Displacement Sensor

With the advantages of simple structure, fast response, high resolution, and can be used in non-contact measurement, capacitive displacement sensor has a wide application in fields of high precision processing and measurement. It can be used to measure displacement, flatness, micro vibration and so on. FM capacitive displacement sensor usually use a LC oscillator as the FM circuit, which has a weak output signal and contains certain nonlinearity. Digital frequency detector can be very precise and achieve a most high resolution, however, at the cost of more time. Analog frequency discriminator has much faster response, but lower resolution and stability. If the nonlinearity is calibrated by digital means, the frequency response of the sensor will be further reduced. Therefore how to quickly attain the FM signal and implement effective non-linearity calibration and, at the meantime, ensure the stability and resolution, has become the key ways to improve the performance of the sensor.Researching on the key technologies in frequency discrimination of FM capacitive sensor, this paper solves kinds of critical technical problems such as stability, repeatability, as well as the difficulty to gain high resolution and high speed at the same time. The main research is as follows:1. High resolution and fast speed as the frequency discriminator of the sensor requires, this paper studies and prepares several digital and analog methods and then presents the quadrature detector method. Quadrature detector has a similar non-linearity with the FM signal, hence the final output of the sensor achieves very good linearity automatically. The core device is a multiplier, which can be easily integrated. Therefore, the quadrature detector can achieve both high resolution and fast speed, moreover, it's much easier to perform due the smaller and simpler structure compared with traditional methods.2. According to the characters of the FM signal, the down-conversion module is designed which carries the center frequency of the FM signal from about 3.7MHz down to 12.5kHz. It consists of a constant-frequency signal generator which is based on the DDS techniques, and the down-conversion circuit. As the FM signal and the frequency discrimination curve have similar but converse non-linearity, the linearity of the sensor is effectively improved.3. Automatically non-linearity calibration happens only when the frequency of the DDS-generated signal is higher than the FM signal. To make sure that at the beginning of the measurement the DDS-generated signal has a 12.5kHz higher frequency than the FM signal, the frequency -adjustment module is designed, which measures the frequency of the FM signal first by the multiple period synchronization frequency measuring principles, and then adjusts the frequency of the DDS-generated signal correspondingly.4. Experiments on the frequency discrimination module and the capacitive displacement sensor are carried out based on PI PZT.The results show that in the measurement range of 90~110μm, resolution of 5nm, linearity of 0.835% and stability of 20nm/h can be achieved. Dynamic experiments show that the sensor has frequency response of more than 1.1kHz. Frequency response of the sensor is theoretical 7kHz.