Accuracy Improvement of Parallel Kinematic Machine

This paper describes compensation methods for deformations of both the parallel kinematic mechanism and the frame supporting the mechanism. In general, these deformations are caused by the heat, the internal forces and the external forces during the processes. First, runouts and deformations of spherical joints and revolutionary joints are measured and compensated by nine displacement sensors built in above joints or by linear scale units built in the prismatic joints. Second, the longitudinal deformation of the prismatic joints is mechanically compensated by Super-Invar rod connected to the joints and the linear scale units. Moreover, nine displacement sensors with nine SuperInvar rods measure the variation of distances between the surface plate and three spherical joint supports. Consequently, the forward kinematics of Hexapod mechanism calculates the displacement and the attitude variations of the mechanism during operating from measured data. Last, the coordinates of the end effector are compensated by these displacements and attitude of the mechanism. Experimental results show reduction of the influence of the temperature fluctuation and variation of measured values.