Determination of the maximal singularity-free workspace of 3-DOF parallel mechanisms with a constructive geometric approach

• A geometric approach to obtain the maximal singularity-free circle
• The algorithm is applicable to planar parallel mechanisms.
• It does not depend on the degree of the singularity expression.
• Offset curve algorithm is used to find the center and radius of the circle.
• As a case study, a 3-PRR PPM, with singularity degree of 20, is investigated.

This paper proposes a novel approach to obtain the maximal singularity-free regions of planar parallel mechanisms, which is based on a constructive geometric reasoning. The proposed approach consists of two algorithms. First, the borders of the singularity-free region associated with an arbitrary starting point of the moving platform are obtained. Then, the second algorithm finds the center of the maximal singularity-free circle, which is obtained using the so-called offset curve algorithm. The procedure is applied to a 3-PRR planar parallel mechanism as an example and the obtained results illustrate graphically the effectiveness of the proposed algorithm. The proposed approach can be directly applied to obtain the maximal singularity-free circle of similar parallel mechanisms, which is not the case for other approaches proposed in the literature that are limited to a given parallel mechanism, namely, the 3-RPR planar parallel mechanism. Moreover, as the main feature of the proposed approach, it can be implemented both in a CAD system and in a computer algebra system where non-convex and reentrant curves can be considered.