Theoretical Distinction between Self-locking Locus and Kinematic Singularity Locus in Closed Kinematic Chains

Self-locking analysis in closed kinematic chains with more than one degree of freedom is usually assimilated to kinematic singularity analysis. Although in singular configurations a mechanism is surely locked-up since joints constraints reactions and friction forces raise to infinite, this approach leads to identify only a sufficient condition for mechanism self-locking: in fact, self-locking may occur also outside kinematic singularity locus and the extension of its domain strictly depends on friction coefficients. The present work formalizes self-locking analysis in a more rigorous way, describing an algorithm to compute the geometrical locus corresponding to the a specific self-locking configuration. The proposed approach has been applied to a simple parallel kineamtic mechanism with two degrees of freedom. The predicted self-locking behavior has been verified by numerical multi-body simulations.