Calibration of a hexapod machine tool using a redundant leg

Parallel configurations are recently being applied to the machine tool with the hopes of greater rigidity, stability, and accuracy than conventional m ulti-axis structures allow. However, the many calibration methods presently available for serial machine tools are not applicable to hexapod type stru ctures. A calibration method is presented that uses a ball-bar or other sim ple length measuring device to act as an 'extra leg,' allowing calibration of the hexapod's true kinematic parameters. This method utilizes a total le ast squares minimization, does not require any special hexapod configuratio n or difficult six degree of freedom pose measurements, and is effective wi th as few as one additional length sensor. Selection of calibration pose se ts is briefly discussed, as well as the influence of measurement noise on c alibration accuracy. Simulations show the potential for this algorithm to s ignificantly reduce errors to the point where machining errors are within 5 -10 times the measurement errors. (C) 1999 Elsevier Science Ltd. All rights reserved.