The instantaneous forward problem (IFP) singularities of a parallel manipulator (PM) must be determined during the manipulator design and avoided during the manipulator operation, because they are configurations where the end-effector pose (position and orientation) cannot be controlled by acting on the actuators any longer, and the internal loads of some links become infinite. When the actuators are locked, PMs become structures consisting of one rigid body (platform) connected to another rigid body (base) by means of a number of kinematic chains (legs). The geometries (singular geometries) of these structures where the platform can perform infinitesimal motion correspond to the IFP singularities of the PMs the structures derive from. In this paper, the singular geometries of the structures with topology SX-YS-ZS (S stands for spherical pair, whereas X, Y and Z stand for three generic one-dof pair which may be or may not be of the same type) are studied with a unified approach. The presented approach leads to obtain an analytic condition which allows all the singular geometries of these structures to be determined. Moreover, the geometric interpretation of the found singularity condition and the exhaustive enumeration of the types of singular geometries is provided. Finally, the use of the presented results in the design of the manipulators which become one structure with topology SX-YS-ZS when the actuators are locked is discussed.

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