An optical measurement technique for dynamic stiffness and damping of precision spindle system

The dimensional measurement technique utilizing curved-edge diffraction is applied for the dynamic system identification of precision spindle system. Both static and dynamic behavior including stiffness, damping ratio, and parasitic motion of a precision ball bearing spindle system is characterized by impact response, while the spindle displacement is measured with the curved-edge-type sensors (CES). The CES effects of spindle radius and surface quality were theoretically and empirically investigated. The capacitive-type sensors (CS) were used for a baseline comparison with the CES outputs. Unlike CS, CES is not sensitive to the spindle radius, surface quality, and coupling motions, shows high bandwidth and low noise, and allows for the dimensional measurement at a localized area. These results indicate that CES can be a good alternative to CS for spindle metrology. In the experiment, natural frequency (56 Hz (linear) and 680 Hz (angular)), stiffness (460-790 N/µm) and damping ratio (0.04-0.08) of precision spindle system were measured by CS and CES at the same time. The dynamic model of ball bearing spindle system was also discussed.