Development of a Novel Shoulder Exoskeleton Using Parallel Actuation and Slip

This paper presents a 5 degree-of-freedom (DoF) low inertia shoulder exoskeleton that was developed using two novel technologies with a broad range of application. The first novelty is a 3-DoF spherical parallel manipulator (SPM) that uses three linear actuators. Each actuator is designed using a method of motion coupling such that the pitch and linear stroke DoF are dependent. By using an SPM, this shoulder exoskeleton takes advantage of the inherent low effective inertia property of parallel architecture. The second novelty is a 2-DoF passive slip mechanism that couples the user’s upper arm to the SPM. This slip mechanism increases system mobility and prevents joint misalignment caused by the translational motion of the user’s glenohumeral joint from introducing mechanical interference that could affect the device’s kinematic solution or harm the user. An experiment to validate the kinematics of the SPM was performed using motion capture. A computational slip model was created to quantify the slip mechanism’s response for different conditions of joint misalignment. In addition to offering a low inertia solution for the rehabilitation or augmentation of the human shoulder, the presented device demonstrates the technologies of actuator motion coupling and passive slip for use in exoskeletal systems. The use of motion coupling could be applied to other types of parallel actuated architectures in order to constrain the kinematics or improve stiffness characteristics. Passive slip mechanisms could have application in either serial or parallel actuated systems as a means of negating the adverse effects of joint misalignment.