Towards high-speed on-machine measurement of freeform surfaces: a dynamic error calibration method

Metrology challenges have increasingly been the bottleneck in the manufacturing of freeform optics.On-machine metrology has emerged as a potential solution to bridge the gap between measurement and machining processes.It is widely recognised that metrology systems integrated with manufacturing platforms are affected by the dynamic characteristics of the host platform.However, there is a limited understanding of how these dynamic disturbances contribute to measurement error, primarily due to a lack of comprehensive system-level analysis.This paper aims to depict the relationship between mechanical vibration and measurement error in an on-machine surface measurement (OMSM) system and proposes a method to mitigate this error through system calibration.A dynamic error model of the OMSM system is developed, incorporating feed disturbances, machine dynamics, and the compliance of the measurement units.Our analysis identifies phase lag in the measurement unit as a primary contributor to the distortion of the measured surface figure.To address this issue, a calibration procedure is performed based on system transfer function identification.The proposed calibration method effectively reduces the residual error in high-speed measurements.Experimental results demonstrate a reduction of the peak-to-valley (PV) error from 6 m to 0.5 m, and a decrease in the root mean square (RMS) error from 2.3 m to 20 nm.Using this system, a high-density point cloud of 290,000 points was measured in 6 minutes, successfully meeting the challenges of measuring freeform surfaces on the manufacturing platform with both high accuracy and high efficiency.The proposed calibration method, which compensates for the absence of an independent metrology frame, can be adapted for other similar on-machine systems.This approach offers a cost-effective and time-efficient solution for the rapid prototyping of freeform optical components using integrated surface metrology.