The development of precision manufacturing puts forward higher requirements for surface flaw detection technology in industrial testing. Because precision devices are easy to scratch, the contact measurement technology based on mechanical probe is no longer suitable for the detection needs of such devices, while the non-contact measurement technology based on optical method begins to highlight its advantages. At present, the commonly used high-precision displacement sensor includes the laser triangle ranging sensor. It has the advantages of simple structure and convenient operation, but the measurement accuracy is influenced by the size and shape of the light spot, environmental light and other factors. Besides, the laser triangle ranging sensor depends on its sample surface scattering light, so it is not suitable for the measurement of high reflection and inclined surface. The chromatic confocal technology has the advantages of high resolution and insensitivity to surface texture, tilt, stray light and other factors, so it is more suitable for the detection needs of precision devices. In order to meet the requirement of a large measurement range in industrial testing, we design a large-range dispersion objective lens. In order to reduce the cost, a traditional spherical lens is adopted. We design a four-piece objective lens and optimize the aberration using the Zemax software. A chromatic confocal displacement measurement system is built with the designed dispersive objective lens. Experiments are designed and carried out to evaluate the displacement measurement error of the system and the measurement performance on different material samples. The results show that the system has certain adaptability to different material samples.

A dispersion objective lens is designed by using the traditional spherical lens. According to the linear dispersion condition, we choose suitable materials, compute the initial structure of the objective lens, and then use the Zemax software to optimize the spherical aberration, dispersion range, and light spot size. In the process of optimization, the weight of the large dispersion is increased, and the demand for dispersion linearity is reduced. At the same time, the small spot size and spherical aberration are ensured, and finally the structure of the dispersion objective lens with large axial chromatic aberration is obtained. Using the designed dispersion objective lens, the chromatic confocal displacement measuring system is established. A spectral characteristic normalizing method is designed to eliminate the undesired effects on the spectral signals, resulting from the optical fiber attenuation characteristics, light source spectral distribution characteristics and spectrometer detector responsivity. The evaluation experiment is designed and carried out to figure out the displacement measurement error of the system. In addition, the displacement measurement experiment of different materials commonly used in industrial application is carried out, and the results show that the system has certain adaptability to the measurement of different materials.

Most of the existing studies on the design of chromatic confocal objective lens based on traditional spherical lens have a small measurement range and are greatly limited in the industrial testing. The dispersion objective lens designed in this study has an axial chromatic aberration of nearly 10 mm in the working band, and the spherical aberration of monochromatic wave is well corrected(Fig. 5). The chromatic confocal displacement measurement system built with this objective lens can obtain a measurement range of 10 mm, which can better meet the demand for industrial detection with large range(Fig. 9). Aiming at the problem that the spectral signal in the system is subjected to extra modulation, a normalized processing method of spectral characteristics is designed(Fig. 2), which reduces the adverse effects of the spectral distribution characteristics of light source, the attenuation characteristics of fiber and the response characteristics of spectrometer detector on the spectral signal received by the system and obtains the spectral signal with good characteristic peak(Fig. 8). The experimental results show that the system has good adaptability to different material samples(Table 3).

In this study, according to the basic principle of chromatic confocal displacement sensor and linear dispersion condition, the initial structure of the dispersion objective lens is designed. The light spot size at each wavelength and the monochromatic spherical aberration of the initial structure are optimized by using the Zemax software, and the axial chromatic aberration is controlled at the same time. Finally, a dispersion objective lens with an axial chromatic aberration of 9.962 mm in the 500-700 nm band is designed. A chromatic confocal displacement measurement system is designed by using the dispersion of the objective. In order to eliminate the effects of fiber attenuation characteristics , spectral distribution characteristics of the light source, and the spectrometer detector responsivity, the spectral characteristic normalizing processing method is designed and then the relationship between the peak wavelength and the position is obtained. The built displacement measurement system obtains the 10 mm measuring range, and the standard deviation of measurement is 0.5 μm and the mean absolute error of measurement is 0.6 μm. In addition, different material samples are measured, and the results show that the system has certain adaptability to different material samples. Compared with the laser triangle ranging sensor, the system is more suitable for high reflection samples.