D. Shuo, A. Narayan, F. Anaya Reyes, S. Han, S. Bhattacharya, H. Yu
National University of Singapore,
Keywords: Shoulder injuries, Exoskeleton, SEA, Force control, Multimodal assist, Overhead work
Summary:Shoulder injuries are the second most common work-related musculoskeletal disorder (MSD) and represent the third leading cause of sick leave, with an average of 23 days of absence from work. Over the past two decades, much research has been conducted to develop devices, namely shoulder exoskeletons, that can assist in overhead work. However, current shoulder exoskeletons face the challenge that their force generators are often installed parallel to the human arm, which adds a significant amount of inertia to the user's arms, resulting in a large energy expenditure for the user. In addition, the shoulder is one of the most complex human joints and aligning robotic devices with its complex anatomical and geometric motion is a major challenge. To facilitate overhead work, shoulder exoskeletons must have good portability (low weight, including power supply) to move with the user. More importantly, structures that move with the human arm should be compact and lightweight to reduce motor power and improve user comfort. In this work we present a lightweight and portable industrial exoskeleton designed to support the upper extremities during occupational work. It is powered by serial elastic actuators (SEA) and Bowden cables, which provide a flexible connection to the human body and allow the device to adapt to complex shoulder joint movements to assist in overhead work. In total, the exoskeleton reduces the muscle forces used when lifting a 5 kg load above shoulder height by up to 40%. The device comes in two variants, one passive (without motor) and one active (with motor), and the choice between them depends largely on the intended use. In the passive design, a unique spring and cam mechanism is intended to satisfy the nonlinear force requirements of the shoulder joint. In the active design, the assist force is given by a servomotor and is modulated by a multimode assist strategy in which three control modes (i.e., free mode with zero force control when no assist is needed; assist mode during overhead work; and safe mode when a sudden impact load is detected) are automatically switched according to different working conditions. In each state, the shoulder support exoskeleton compensates different percentages of the gravitational load according to the user's needs, and sensor data (e.g., arm angle, speed of movement, weight lifted, etc.) are collected that can be useful for improving safety, productivity, and quality of work. These innovations make the shoulder support exoskeleton significantly advantageous over its competitors in today's industrial market. Overall, our novel devices provide a lighter and safer solution capable of overcoming shoulder joint misalignment problems at a lower cost. To verify the effectiveness of the technology, we have conducted usability tests based on muscle activations of healthy individuals during typical overhead work simulations. The results suggest an unloading effect on the shoulder muscles, which is a promising indicator that our shoulder exoskeletons might be a good solution to help workers perform overhead tasks.