Abstract: At present, five hundreds meter aperture spherical radio telescope (FAST) what is termed "China's Sky Eye" has been put into operation. The innovative optical-electromechanical-integrated feed supporting design makes the structure weight reduced by 2 orders of magnitude. On the basis that the preliminary positioning and orientating of the feed cabin is provided by the cable-driven subsystem, the AB-axis mechanism and fine-tuning Stewart platform in the feed cabin tackling with the positioning and orientation error compensation real time to realize the high precision tracking. In order to overcome the fact that the elastic deformation and joint clearance are neglected in the position and orientation computation of the feed platform with feedback information of the actuators, this paper presents a straight measurement method for the FAST feed platform based on wire-driven parallel mechanisms. This method has the advantages of real-time performance, high precision and strong ability to reject disturbance. By studying the measurement model, the forward solution algorithm of the wire-driven parallel mechanism is simplified. The comparison result indicates the forward solution algorithm simplified has the advantage and effectiveness of fast convergence. The paper then gives an analysis of the sources of systematic error and study the mapping between the error of draw-wire displacement sensors and the position and orientation error. The results of simulation indicate that the wire-driven parallel mechanism measurement method has the characteristic of averaging-error effect. With appropriate selection of workspace, this measurement system will be able to provide 0.5 mm and 0.025° precision in position and orientation.

Key words: error analysis, FAST, forward kinematics, parallel mechanism, position and orientation measurement, Tracking of the feed