Laser Differential Confocal Freeform Surface Shape Measurement Based on Monitoring and Compensation of Translational Motion Error

The manufacturing accuracy of a freeform surface is limited by its detection accuracy. In this article, a laser differential confocal freeform surface shape measurement (DCFSM) method based on translational motion error monitoring and compensation is proposed. This method realizes helical scanning measurements of a freeform surface via sensor translation and freeform surface rotation. Based on the normal tracking principle, a position-sensitive detector (PSD) is used to track the normal direction, solving the problem of large inclination freeform surface measurements limited by the maximum measurable inclination of the sensor. The laser differential confocal sensor (LDCS) is used to precisely focus on the surface, realizing a high-spatial-resolution measurement of the surface normal vector, with an axial resolution of up to 1 nm. By introducing a reference frame into the system, an independent translational motion reference is provided for freeform surface measurement. By monitoring, decoupling, and compensating the 2-D translational motion errors, the impact of these errors on measurement accuracy is greatly reduced. High-accuracy freeform surface shape measurement is finally realized. The experimental results show that after error compensation, the PV repeatability is superior to ±50 nm (3σ ), rms repeatability is superior to ±14 nm (3σ ), and the measurement results are close to the standard values, enabling high-accuracy measurement of freeform surface shapes.