On-Machine Calibration of Pitch Deviations of a Linear Scale Grating by Using a Differential Angle Sensor

A differential angle sensor is newly developed to calibrate the pitch deviations of a linear scale grating with a nominal pitch of 1.6 µm on an ultra-precision lathe. The angle sensor is composed of two angle detection units based on the laser autocollimation method. A collimated laser beam with a diameter of 1 mm, which is output from a laser diode with a wavelength of 685 nm, is projected onto the linear scale grating. The positive and the negative first-order diffracted beams from the scale are received by the two angle detection units, respectively. The X-slide of the ultra-precision lathe is employed to generate the necessary scanning motion for the calibration. Based on the fact that the pitch deviations will cause changes in the positive and the negative first-order diffraction angles, which are equal in magnitude and opposite in sign, the pitch deviations can be obtained from the differential output of the angle sensor. The tilt error motion of the X-slide, which is a major error factor in on-machine calibration, can also be removed in the differential output. The robustness of the developed angle sensor for on-machine calibration has been confirmed by testing the basic performances of the sensor on the machine tool. The feasibility of the on-machine calibration result of pitch deviations has been verified through comparing with the off-machine calibration result.