Roundness measuring system for small balls

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An aerostatic bearing with active inherent restrictors is developed for the main spindle of a roundness measuring system planned to measure small balls. The active inherent restrictor (abbreviated AIR) was invented by the head investigator, and incorporated into several aerostatic bearings and guideways for improving their performances. The AIR consists of a piezoelectric actuator having a through hole, one end of the hole is small enough to function as an orifice when the actuator is embedded in the bearing surface. When a sensor detects the change in the air gap on the bearing surface, a computer of the AIR-control system calculates the supply voltage for the piezoelectric actuator to change the pressure on the bearing surface, and to reduce the displacement and vibration of the main spindle. The major specification of the aerostatic bearing for the main spindle is as follows : the outer diameter is 140mm and the height is 86mm. There are eight AIRs on each radial and thrust bearing surface.Without using the active control system, the rotational accuracy of the main spindle is about 0.2mm. Active control using the AIR improves the rotational accuracy to be 20-30nm. The frequency response of the bearing with the AIR shows that the effect of the AIR on the dynamic stiffness of the bearing can be seen in the low frequency range (less than 20Hz). However, the measured rotational accuracy is not enough for the main spindle of the roundness measuring machine because the planned resolution of the roundness is 1nm. The main reason of the lack in the rotational accuracy is the geometrical error of the bearing surface on the spindle. Several efforts have been performed to improve the geometrical accuracy, however, the rotational accuracy of the main spindle can not be improved.