International Journal of Dynamics and Control https://doi.org/10.1007/s40435-019-00548-3 Design and control of an ergonomic robotic shoulder for wearable exoskeleton robot for rehabilitation Md. Rasedul Islam 1 · Md. Assad-Uz-Zaman 1 · Mohammad Habibur Rahman 1 Received: 26 February 2019 / Revised: 2 May 2019 / Accepted: 17 May 2019 © Springer-Verlag GmbH Germany, part of Springer Nature 2019 Abstract In this paper, we present an ergonomic robotic shoulder for upper limb exoskeleton. Most exoskeletons have been designed considering shoulder joint instantaneous center of rotation (ICR)—often termed as the center of glenohumeral joint—as fixed joint. In reality, shoulder joint ICR does move during shoulder abduction–adduction and vertical flexion–extension. The abduction–adduction causes shoulder joint ICR to move in the frontal plane, resulting from a combined motion of shoulder depression–elevation and horizontal translation, while vertical flexion–extension produces a movement of shoulder protraction–retraction. If these additional motions are not compensated in exoskeleton design, they can produce discomfort and/or pain to the robot’s wearer. While in most of the exoskeleton, shoulder joint articulation is considered as a 3-DOFs spherical joint, this research proposes a novel shoulder joint mechanism of robotic exoskeleton with two additional passive DOFs to provide ergonomic shoulder movement. The mechanisms in proposed robotic shoulder comprises of two sliders that allow above-mentioned passive movements of shoulder joint ICR. In this research, we demonstrated that the slider-1 enables shoulder joint ICR to move in the frontal plane with a quarter circular arc of 60 mm during abduction–adduction, while slider-2 allows movement of shoulder protraction–retraction as needed. Proposed mechanism has been designed to be used eventually in a 7 DOF modular robotic exoskeleton for upper limb rehabilitation. We have also applied computed torque control (CTC) to the designed shoulder. The performance of CTC shows stability and effectiveness. Keywords Ergonomic shoulder · Robotic exoskeleton · Instantaneous center of rotation (ICR) · Shoulder joint · Shoulder mechanism · Rehabilitation · CTC 1 Introduction Upper limb (UL) dysfunction (ULD) is constantly growing, resulting from traumatic brain injury (TBI), spinal cord injury (SCI), neurological (e.g., stroke, cerebral palsy, etc.) and orthopedic etiologies. American heart association reports, annually in the United States, approximately 785,000 indi- viduals experience a new or recurrent stroke [1], and which becomes leading cause of serious long-term disability [2]. Rehabilitation programs are the main method of promoting B Md. Rasedul Islam islam4@uwm.edu Md. Assad-Uz-Zaman assaduz2@uwm.edu Mohammad Habibur Rahman rahmanmh@uwm.edu 1 Bio Robotics Lab, Mechanical Engineering Department (Room # USR 281), University of Wisconsin-Milwaukee, 4090 N 1st Street, Milwaukee, WI 53212, USA functional recovery in individuals with upper limb dysfunc- tion, requiring a long commitment by both the clinician and patient. Having said that ULD is constantly growing, and require long duration of rehabilitation, to date, a number of exoskeletons have been designed and developed with aim to assist in rehabilitation [3–7]. Researchers are still looking for better solutions in delivering UL rehabilitation therapy. The upper limb is involved in doing wide variety of daily activities, and most of which require movements of shoul- der complex. Given that there are multiple articulations in shoulder complex, its movement results from combination of its articulations. Designing kinematic structure for shoulder complex is a key design requirement in building an exoskele- ton. Particularly, there are two things that need to be taken into consideration. First, actuation of shoulder complex must provide full range of motion (ROM). Second, it should not cause any discomfort to exoskeleton’s wearer while maneu- vering. Usually, the shoulder joint is modelled as a spherical (ball and socket) joint with intension to provide arm’s three 123