PPR-PMs (parallel manipulators) are the parallel counterparts of the 3-DOF PPR serial robots, which are composed of two P (prismatic) joints and one R (revolute) joint. For a PPR-PM, the moving platform can rotate arbitrarily about an axis undergoing a planar translation. This paper deals with the type synthesis of 3-DOF PPR-PMs. At first, virtual chains are introduced to represent the motion patterns of 3-DOF motions and relevant results from screw theory are recalled. A method is then proposed for the type synthesis of 3-DOF PPR-PMs. Using the proposed approach, the type synthesis of 3-DOF PPR-PMs is performed in three steps, namely, the type synthesis of legs for PPR-PKCs (parallel kinematic chains), the type synthesis of PPR-PKCs, and the selection of actuated joints of PPR-PMs. The three steps are dealt with in detail consequently. The characteristics of the proposed approach is that the type synthesis of legs for PPR-PKCs is reduced to the type synthesis of 3-DOF overconstrained single-loop kinematic chains and thus easy to perform. In addition to all the 3-DOF PPR-PKCs and 3-DOF PPR-PMs proposed in the literature, a number of new 3-DOF PPR-PKCs and 3-DOF PPR-PMs are identified. It is also found that there are no PPR-PMs with identical types of legs.

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