The singularity research of a class of 4-DOFs parallel mechanism and configuration analysis

4-SPS parallel mechanism(PM) adding a constraint driven limb, a new kind of parallel mechanism which possesses four degrees of freedom(DOFs) is built. These mechanisms are composed of fixed and moving platform by means of four parallel driving-limbs and one constraint driven-limb. Moving character is analysed by using screw theory and the different DOFs of PMs are calculated by revised Kutzbach-Grübler formula. The moving platform of this mechanism may have two translational and two rotational (2T2R) DOFs, 1T3R DOFs and 3T1R DOFs. The special position of PM leads to determinant of constraint screws being equal to zero by line geometry method, that is to say, this class of PM is clear at singular configuration. This analysis lays a foundation for further research on kinematics and dynamics.