Abstract: Absolute coordinates of spherical joints on movable platform are chosen as unknowns ,a direct displacement analysis which is based on resultant eliminatiom method for a 4-DOF spatial parallel mechanism is presented, 7 degrees input-output closed form equation with one unknown is obtained,and all of the roots for all unknowns are gained. Numeric-symbolic technique is introduced in the procedure of resultant elimination, procedure of number-symbol manipulation is transformed into that of matrix manipulation.Because joint variables are treated as symbols and all structure parameters are treated as real num-bers,the complexity of derivation for input-output equation is reduced.The solution is verified with a numerical example,its rsults without extraneous roots agree with the original equations.

Key words: Direct displacement solution, Parallel mechanism, Resultant elimination Numeric-symbolic approach