Kinematic Error Modeling of a Parallelogram Arm of the Delta Robot and Its Dimensional Optimization

Delta robots are a subset of a class of robots called parallel robots. These are widely used in pick-and-place and assembly tasks involving high motion accuracy. The basic idea for the design is the development of parallelogram structures attached to the output platform. Three such parallelogram mechanisms are used to ensure that the orientation of the moving platform is restricted to only translational motion relative to the base. This paper aims to improve the robot’s motion accuracy by performing a study on the parallelism error and verify its influence on accuracy. Previous studies on the error analysis in the motion of Delta robots mainly consider dimensional tolerance, joint clearance and driving error of each component, mainly focusing on the position error of the moving platform. However, the analysis of the parallelogram arm is also equally important to ensure minimum posture error to reduce the pose error(orientation + position) of the mobile platform. Any posture error in the mechanism results in the posture error of the moving platform. This article approaches to reduce this posture error by considering the main influencing factors-length error in the connecting rods and joint errors in the parallelogram mechanism. The work is divided into two sections. The first section deals with the kinematic analysis and estimation of optimum dimensions. The second section deals with the error analysis using the kinematic approach. This section also relates the errors of pose and structural types. And, due to its analytical nature, this error model can be used for sensitivity analysis to further enhance manipulator accuracy.