Spectral Confocal Displacement Sensor Design Technology Research

Displacement sensor technique is one of the critical important fundamental techniques which implement precise measurement of geometric sense. The chromatic confocal sensor, which appeared in recent years, is based on wavelength-displacement modulation technique. It has submicron precision and KHz working frequency, which guarantee its wide application prospect. Based on theoretical analysis of chromatic confocal principle, important parameters and key design techniques were studied systemically with numerical simulation and experiment research.Aiming at actualization of wavelength-displacement modulation, the transducer element chromatic dispersion lens was designed with Zemax optical design software. In order to get a linear dependence of longitudinal chromatic aberration on the wavelength of light, we distributed initial focal power for elements of different glasses in the lens according to paraxial aberration theory. For the sake of eliminating the broading effect on aixal confocal response caused by lens aberration, a method for monochromatic spherical aberration correction was described which used ray tracing calculation and multi-configuration function in Zeamx. After optimization, aberration analysis and evaluation were carried out. The working spectral region of lens was486nm-656nm, and the axial dispersion displacement range of about91μm. The result showed a good linear relationship between the axial chromatic aberration and wavelength, and the linear regression coefficient of determination R2=0.99. The radius of disc of confusion on each monochromatic paraxial image plane was less than airy disc radius of the same wavelength. The lens aberration has been corrected to the diffraction limit. This design method of lens has some reference value for selecting sensor’s design-scheme and optimum design.Confocal pinhole size is one of the most important factors which affect the resolution and signal-to-noise ratio of the sensor system. To improve the signal of ratio of light exiting from confocal pinhole, a optimum design of confocal pinhole size was carried out. According to classical confocal theory, a numerical simulation of spectral distribution of light which exited from confocal pinhole was done. The relationships between confocal pinhole size and peak intensity of spectral distribution and that with FWHM were stuied respectively. Based on the analysis before, a optimization method using the ratio of peak intensity and spectral bandwidth as an evaluation criterion was proposed. The optimized design results were also described.Calculation of peak spectral line which corresponds to the modulation displacement is the key to ensuring sensor measurement accuracy. Analysis of the factors that cause peak shift was carried out. A pre-processing method of spectral signal was proposed and its feasibility was verified by experiments with Micro-Epsilon OptoNCDT-2401chromatic confocal sensor. We choosed centroid and Gaussian fitting algorithm as positioning algorithm and compared the displaement output bias in two cases, removing and not removing the light source spectral characteristics from spectral signal. Experimental results showed that removing the system light source spectral characteristics from the original spectral data can mprove the positioning accuracy of spectral peak and this method did have certain engineering reference value.