1. Kinematic Geometry of Mechanisms;Hunt,1978
2. Singularity analysis of closed-loop kinematic chains;Gosselin;IEEE Transactions on Robotics and Automation,1990
3. The rigidity of compound spatial grids;Dandurand;Structural Topology,1984
4. J.P. Merlet, Parallel Manipulator, part 2: singular configurations and Grassmann geometry, INRIA, Report No. 791, 1988.