Feedback system for compensating of joint runout and thermal deformation in parallel kinematic machine

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This study proposes 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 Super-Invar 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 effecter 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. The conclusions are drawn as follows ; (1) The displacement sensors built in the joints and even the linear scale unit can measure the joint motion errors and the thermal expansion of the link, (2) Connecting the scale unit and the joints by Super-Invar rods compensates both the thermal expansion and the elastic deformation of the link, (3) Measuring the changes of the distances between the surface plate and the joint supports can compensate the frame deformation caused by the temperature fluctuation.