Mechanical resonance induced measurement uncertainty of ultra-precision on-machine surface measurement system

Mechanical vibration is an inevitable factor for on-machine surface measurement (OMSM), and it can significantly impact the measurement results, particularly when measuring high-sag surfaces. A great number of OMSM system has been reported in recent decades, however, the contribution of the mechanical resonance on the measurement result has never been revealed. This paper investigates the source of measurement uncertainty arising from mechanical resonance errors in the OMSM system. The system dynamic model of the whole measurement platform is modelled by an equivalent two degree of freedom (2-DOF) oscillating system. The frequency response property of on-machine measurement device fixture is evaluated experimentally by hammer excitation test. A case study on evaluation of large off axis spherical (OAS) with 380 mm diameter aperture is performed. Resonance condition is identified by a joint time-frequency analysis on the slide acceleration and the chromatic sensor displacement. A net-zero-force criteria is applied to locate the moments resonance happens. The on-machine measurement result indicates that time-variant force fluctuation of the precise slide is the major excitation source for the integrated measurement device. It shows that the disturbance induced by system resonance can reach up to 300 nm, which is 100 times greater than the ground truth amplitude in a quasi-static measurement. This research provides a potential solution for evaluating the real-world performance of optical measurement devices integrated with manufacturing equipment.