Measurement of Six-Degree-of-Freedom Absolute Postures Using a Phase-Encoded Pattern Target and a Monocular Vision System

A vision based method for six-degree-of-freedom (6-DOF) absolute posture measurement is proposed for evaluation of multi-axis stages or robots. It obtains 6-DOF information from an acquired image of a target pattern plate fixed at a moving object. By introducing a phase-encoded absolute position pattern (PEAPP), the proposed method can overcome the limitation of conventional target patterns carrying additional fiducial marks, and measure wider range of 6-DOF posture with higher precision. A robust data-processing algorithm was devised to decode absolute position codes from the PEAPP images, and to calculate 6-DOF posture efficiently. Performance of a prototype measurement system was evaluated at various aspects including readout stability, nonlinearity, and crosstalk. The measurement results of 6-DOF postures agreed with the reference values within +/- 15 mu m and +/- 0.05 degrees for linear and angular positions, respectively. The limitation of the proposed method and the effect of camera matrix calibration on the performance were also discussed.