Distorted laser interferometric angle measurements of a disk drive pivot

A laser interferometric measurement sensor is used for angle measurements of rotating ultralight structures in order to avoid the mechanical interaction between the system and the sensor. The laser interferometer detects a linear displacement which is a distorted image of the angle displacement. The distortion depends strongly on the amplitude and the laser misalignment angle. In this paper, a measurement procedure is presented based on an analytical expression that describes the nonlinear relationship between the measured displacement and the actual angle displacement. Furthermore, a distortion map is presented to quantify the nonlinear effects and to indicate the necessity for compensation of these effects. Finally, a calibration procedure for identifying the geometrical parameters, i.e., the arm length and laser misalignment angle, is included. The method is applied to the pivot of a Kitty Hawk, a miniature hard disk drive.