Abstract: A study of a class of 2 rotational degrees-of-freedom (DOF) parallel pointing mechanism. Such mechanisms are comprised of at least three identical 4-revolute jointed serial linkages, and are endowed with a plane of symmetry which the mobile platform and the base platform, the upper link planes and the under link planes are symmetrical about during the movement. The compact and unified kinematic model is obtained based on the geometric constraints by using the tilt-torsion angle for describing the attitude (pointing) of this class of mechanism. The nature of parasitic motions, workspace and singularity is discussed on this basis, and the impact of design variables to kinematics performance is summarized. Finally the isotropy of the mechanism is analyzed by means of the first-order kinematic influence coefficient, and architecture optimization of the angle between actuate linkages is undertaken for the best of isotropy. The conclusions could be used to guide the design of this type of parallel pointing mechanism.

Key words: parallel mechanism;pointing mechanism;kinematics;parasitic motion