Abstract
A highly precise vertical turntable provided with hydrostatic bearings and radial runout error compensator has been developped. The compensator has two functions of a closed loop differential hydrostatic actuator for positioning a spindle to a nanometer accuracy and a datum roundness error measuring instrument with multiple position sensors. By separating the datum error from the runout error readings, the compensator can overcome the limitation of geometrical datum accu racy. Design philosophy of the turntable spindle system is as follows : (1) Isolation of disturbance sources within the system. (2) Design procedure of bearing pads taking into consideration of the performance of servd valve. (3) Introduction of microprocessor for separating the datum error in rounout error measurment from the runout error readings. Several test results are concluded as follows : (1) The bearing pad dimensions are determined in connecting with servo valve characteristics. (2) The positioning resolution is at least 1 nm and the bandwidth is 250 Hz. (3) Two point roundness measurement method has proved that the compensation accuracy is ± 4.5nm (3 σ). However, correcting the datum error in accordance with the measurement principle, compensation accuracy improved to 6.1nm (2LSB, 5rpm) or 9.15nm (3LSB, 30rpm) in peak to valley.
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