Analysis and Preliminary Design of a Passive Upper Limb Exoskeleton

This article reports the analysis and preliminary design of a passive, wearable, upper limb exoskeleton to support workers in industrial environments in a vast range of repetitive tasks, offering an effective strategy to reduce the risk of injuries in production lines. The system primary purpose is to compensate for gravity loads acting on the human upper limb. The proposed exoskeleton is based on 6 Degrees-of-Freedom (DoFs) kinematics with 5-DoFs for the shoulder joint (two displacements plus three rotations) and 1-DoF for the elbow. Gravity compensation is implemented with passive elastic elements to minimize weight and reduce cost. A detailed analytical tool is developed to support the designer in the preliminary design stage, investigating the exoskeleton kinetic-static behaviour and deriving optimal design parameters for the springs over the human arm workspace. By defining specific functional requirements (i.e., the user’s features and simulated movements), computationally efficient optimization studies may be carried out to determine the optimal coefficients and positions of the springs, thus, maximizing the accuracy of the gravity balancing. Two different solutions for the arrangement of the elastic elements are investigated, and obtained results are validated with a commercial multi-body tool for some relevant movements of the user’s arm.

I. Introduction

Nowadays, manufacturing companies are increasingly introducing automation technologies to comply with the high requests of modern markets [1], [2]. Nevertheless, industrial operators are still involved in many repetitive manual tasks [3]. The use of an ExoSkeleton (ES) can provide muscle fatigue relief and reduce the risk to undergo work-related musculoskeletal disorders [4]. As known, an ES is an assistive device that can be used to enhance the user’s physical capabilities by supporting the body’s gravitational load during movements. This paper presents a novel 6 Degrees-of-Freedom (DoFs), passive, upper limb ES for the industrial sector. By pursuing features such as lightness and accuracy, the proposed device may become useful also in the medical field, e.g., for treating patients with neuromuscular diseases and for rehabilitation [5], [6]. Referring to existent prototypes, important considerations can be made based on their field of application. Different ES types have been conceived and presented in past researches (see [7] for a review). These recent, commercially available industrial ESs are designed to be light and manoeuvrable to not interfere with worker’s movements, although they may be constrained in accuracy because of their limited motion capabilities. A simple structure with a reduced number of parts is preferred since additional components (e.g., motors, sensors and cables) may affect the overall weight reducing the ES comfort. Conversely, ESs for medical applications are designed to reach high motion capabilities in the workspace with the drawback of being heavy, due to the use of lots of components [5]. In any case, the ES design is strongly influenced by the biomechanics of the human body, especially for upper limbs ESs, owing to the small size and wide range of motion of the limb itself [8]. A kinematic chain that correctly reproduces the human limb DoFs may be employed, allowing the ES to properly follow the human body movements avoiding the generation of undesired forces [9], [10]. The choice on how to represent the kinematics of the shoulder need to be evaluated in detail. In the literature, several configurations are studied, ranging from the simple ball and socket joint [11] to serial chains of 3 Revolute (R) joints [5], [12], 4R or 5R [10], [13], [14]. A serial chain use allows for accurately reproducing the motion trajectory of the shoulder’s center of rotation, whereas the inclusion of extra DoFs overcomes possible singularity configurations [13], [14].