Research on the Orientation Error of the Translational Cable-Driven Parallel Robots

Abstract

Parallel cables are widely used in cable-driven parallel robots (CDPR) to provide constraints to the end-effector and to realize translational degrees-of-freedom(DOFs). However, when there are structural errors, parallel cables become no longer parallel and will cause orientation errors of the end-effector, which cannot be compensated by kinematic calibration. In this paper, the orientation assurance method is studied considering a 3DOFs translational CDPR. First, the kinematic model and error mapping model of the CDPR are established by using the closed-loop method, considering the pulley radius. Second, the influence of the structural parameters errors on the error of the end-effector is analyzed with the sensitivity index, which establishes a theoretical basis for the simplification of the accuracy synthesis process. Third, the design tolerances of the cable connection points are determined through accuracy synthesis, which is implemented with the genetic algorithm considering the optimal manufacturing cost and the orientation constraints of the end-effector. Finally, to reduce the influence of cable length error, the method of adjusting the initial pose was proposed and studied, which is verified as an effective approach to reduce the orientation error.