Search

To solve the problem of inaccurate peak demodulation resulted from the system uncertain phase shift in Amplitude Modulation(AM) for a capacitive displacement sensor,an AM capacitive displacement sensor based on an improved delayed feedback peak detector circuit was proposed and its applied functional blocks and circuits were investigated.Firstly,the measurement principles of the AM capacitive displacement sensor and its detection circuit were analyzed.Based on generation principle of phase shift with the AM signal,an improved delayed feedback peak detector was presented against the problem of inaccurate peak demodulation.Then,the AM capacitive displacement sensor was prototyped using printed circuit boards and its performance indicators were tested as well.Finally,experimental results and errors were discussed.It indicates that the linearity of the delayed feedback peak detector circuit is better than 0.05%,the data shift of prototyped sensor is less than 10 nm/30 min,and the maximum deviation of sensor is 36 nm within the measuring range of 0~25 μm.It concludes that the delayed feedback peak detector circuit can effectively solve the problem of inaccurate peak detector resulted from the system uncertain phase shift,and the proposed AM capacitive sensor can satisfy the requirements of high precision measurement.

Based on the phenomena of double refraction and double reflection in crystals,phase characteristics of p-polarized light reflecting and refracting at the boundary between an isotropic and a uniaxial crystal are studied by numerical evaluation.Results show that the direction of optical axis has important influence on phase properties.When light is incident from an optically thinner isotropic medium upon a crystal,the direction of optical axis affects the phase shift of reflected p-polarization light but has no effect on the phase of refracted light.When light is incident from an optically denser isotropic medium upon a crystal with the incident angle less than angle of total reflection,the direction of optic axis affects both the phase shift of reflected and refracted p-polarization light.If the incident angle is larger than critical angle,the optical axis has influence on the curve of phase change of reflected p-polarization light.For the case of light enters into an optically thinner isotropic medium from a crystal with the incident angle less than angle of total reflection,the direction of optical axis will change the way of phase shift of reflected p-polarization light.And the refracted p-polarization light undergoes a phase shift near the Brewster's angle when the angle between the optical axis and crystal boundary is small.After the incident light is reflected totally,the curve of phase change of reflected p-polarization light varies with the direction of optical axis.