Abstract: Attitude description of the parallel robotic mechanism with at least two orientational degrees of freedom (DOFs) is one of the important contents in kinematic analysis. The description method plays a key role in the performance analysis of such mechanisms. The [PP]S type of parallel mechanism, which is such a mechanism that when the mobile platform moves the center points of three spherical joints in the mobile platform are always in three vertical planes, is one of the important members in the family of parallel mechanisms. Its pose description is also the research emphasis at home and abroad. A method using two angles is introduced to describe the attitude of this type of parallel mechanism. By using this method, the description parameters of the [PP]S type parallel mechanism can be reduced to 5. There is no parasitic motion about the perpendicular of the mobile platform plane, and the kinematic analysis of this type of parallel mechanism can be simplified. Compared with the description by using three Euler angles, the introduced method has the advantages in terms of simple description of kinematics, and convenient description of the parasitic motion and attitude workspace. This method fulfils the requirement of industrial application, and is worthy to be popularized.

Key words: Attitude, Parallel mechanism, Parallel robot