A capacitance probe-based system is developed for measurement carriage straightness and roll error motions. The system which is added to a conventional laser interferometer for yaw and pitch measurement, consists of a probe-unit and an auxiliary flat surface. The probe-unit, which is kept stationary, has three capacitance probes. The three probes are aligned in such a way that two aligned along the X-axis (X-probes) and two aligned along the Y-axis (Y-probes). The profile error of the flat surface, which is moved by the carriage, is first measured with the X-probes accurately through separating the carriage lateral straightness error (Z-error). The auxiliary flat surface is then used as an accurate reference for carriage lateral straightness error measurement. The roll error of the carriage can be measured by dividing the difference of the Y-probes by the probe distance between the Y-probes. The developed system has been applied to the measurement of an ultra-precision aspheric grinding machine. The straightness error and roll error were measured to be 140nm and 2.4arcsec over a stroke of 80mm, respectively.
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