Absolute Profile-Measurement Using Software Datum

For the profile measurement using software datum, it is not avoidable to suffer the error due to zero adjustment of multiple sensors. This error causes a parabolic error to the software datum line. Previously this zero adjustment is performed by measuring a given reference straightness. Theoretically the length of reference surface must he as long as the measured surface. This requirement makes the applications of software datum for the ultra-precision measurement narrow because the reference with sufficient accuracy can not be obtained. In this paper a new method is proposed to estimate the difference among zero points of multiple sensors by using only the data of profile measurement of the workpiece. This new method does not require any reference surface or any reversal measurement process. Experimental results with differential laser autocollimation method show that the reproducibility of the measurement of a mirror of 50 mm long was better than 30 nm which is equivalent to the highest accuracy expected from the thermal drift and the SN ratio of the sensor output.