Research And Application On Technology Of Precise PSD Micro-displacement Online Measurement

As a kind of device for detecting the position of incident light, PSD (positionsensitive device) have many advantages compared CCD and CMOS. Therefore PSDwill have a good prospect of application in many kinds of fields on measurement.Firstly, the PSD characteristics and laser triangulation method are analyzedsystematically in this dissertation, and a micro-displacement measurement programsis developed combined the amplitude modulation and demodulation principle.Secondly, through the study of the impacts of light source, beam quality, measuredsurface characteristics and speckle on the measurement, we determine the importantparameters of the optical system. Then, based on the analysis of influence ofinterference signal, asymmetric and nonlinear on the PSD signal processing, amodem-based and PSD signal single-channel processing module is designed. Incombination with optical part, we make a micro-displacement measuring systemprototype, and give an experimental validation and analysis of system performance.Finally, we have designed the experimental prototype and industrial prototype ofprecision sheet thickness measurement system by using PSD micro-displacementsensors and differential triangulation，and achieved good results.The major innovations of the dissertation mainly include five parts, as follows:1. A spot shaping method based on the slit or pinhole spatial filtering technologyand dual-beam compensation principle is proposed. The method improves the qualityof laser spot, and greatly reduces the measurement error caused by drift of the centerof spot intensity which is mainly caused by high-frequency noise and spotdisplacement.2. An adaptive method based on PWM&PFM(pulse-width modulation andpulse-frequency modulation) laser modulation was applied to the measurementsystem。This method allows the system to be adaptive as the spot intensity variationcaused by different measured surface, and solves the problems, such as integrationdifficutlty and measurement error increases, which are brought about by theconventional methods such as using digitally controlled potentiometer to adjusting thesignal strength and changing the laser current to adjust the intensity of the laser.3. A PSD signal single-channel processing method based on many signalprocessing techniques including modulation, band-pass filter, peak detection, etc, isproposed. In case of no filter the method eliminates the impact of background lightand dark current, reduces the cost and significantly suppresses the temperature drift. 4. Combined with the PSD output characteristics, a dual nonlinear correctionmethod is proposed to correct the non-linearity of PSD and peak detection, and thealgorithm including segmentation processing and least squares method is applied tothe method. The method not only guarantees the measurement accuracy of the system,but also make the system easy to handle, and is suitable for embedded technology.5. A micro-displacement measurement method based the combination of PSDsignal single-channel processing and the laser triangulation is proposed, and the signalprocessing techniques include modulation, band-pass filter, peak detection, etc. Alaser micro-displacement measuring system is made according the method, withmeasurement frequency up to2KHz, resolution up to0.35‰, stability error betterthan0.90‰, linearity better than12‰.