Abstract: Overconstrained structures of lower-mobility parallel robot are considered more advantageous in simple structure and big rigidity. However certain special geometric constraints, such as joint axes parallel or perpendicular to each other or intersecting at one point, are necessary in realizing the end-effecter motion. Such mechanisms have the essential drawback of sensitivity with respect to the geometrical conditions, and therefore constraint errors can produce a series of bad effects on the mechanisms. Due to this fact, based on overconstraints analysis, the deformation compatibility conditions of planar 5R parallel robot, considering geometric constraint errors of joint axes, are studied. The additional internal force of mechanism and deformation of link are calculated by using the matrix force method, and then parasitic motions of moving platform are analyzed. Moreover, the strain energy fluctuations are investigated. For a specific planar 5R parallel robot, simulation is performed. The results reveal the sensitivity with respect to the constraint errors of overconstrained mechanism, and provide an insight into the effects of constraint errors and overconstraints on the performance of parallel robot mechanisms.

Key words: Constraint errors, Matrix force method, Overconstraint, Planar 5R parallel robot