Development of a 2-DOF Singularity-Free Spherical Parallel Remote Center of Motion Mechanism With Extensive Range of Motion

In this letter, we report the development of an innovative two-degrees-of-freedom (2-DOF) spherical parallel remote center of motion mechanism (SPRCMM), which can offer a wide range of movement in both DOFs without encountering singularities. To facilitate the design process, the letter briefly reviews the existing spherical joints, including serial and parallel structures with and without the remote center of motion (RCM). Aiming at combining the advantages of these existing spherical joints, this letter proposes a novel design that utilizes the parallelogram mechanism to form a parallel RCM mechanism without using universal or spherical joints. Forward and inverse kinematics were constructed using the product of the exponentials. Moreover, space and closed Jacobians were derived, accompanied by manipulability in the available workspace for the mechanism. The prototype of the 2-DOF SPRCMM was built and experimentally evaluated. The experimental results confirm that the singularity-free motion of the two DOFs of the mechanism in a wide range is feasible, and the root mean squared errors in the trajectory tracking of the mechanism in most states were less than 10% of the motion range.

I. Introduction

Animals have dexterous and agile motion because of their well-integrated joint system, which provides multiple degrees of freedom (DOF) in limited spaces. In humans, the wrist has two DOFs (flexion/extension and ulnar/radial deviation) as well as the subtalar (talocalcaneal) joint, which is part of the ankle (inversion/eversion and dorsiflexion/plantar flexion). The ball-and-socket joint (i.e., shoulder and hip) allows limbs to move three DOFs in a compact form [1]. The common feature of these joints is the movement of a spherical workspace for the connected bone by utilizing at least two degrees of freedom. In artificial multi-DOF joints, achieving such angular movement requires a combination of multiple one-DOF actuators. The arrangement of actuators and linkages determines whether the mechanism is serial or parallel type. The following paragraphs will briefly review these two types of mechanisms and then focus on the mechanism with only two DOFs.