A close-to-body 3-spring configuration for gravity balancing of the arm

A close-to-body arm support is needed to meet the need of patients for an inconspicuous arm support that is not stigmatizing. At the moment, these arm supports do not exist. All commercially available arm support use springs with a parallelogram structure that needs auxiliary links to balance the arm. Recent literature presents a 2-spring configuration without auxiliary links with multi-articular springs. A restriction to this spring configuration is that it cannot be attached close to the body. In this paper a new 3-spring configuration and a concept for attachment to the body is proposed. One bi-articular spring that spans the shoulder and elbow joint, one mono-articular spring that spans the elbow, and one mono-articular spring that spans the shoulder are used to balance the arm in its complete 3D workspace. The spring that spans the shoulder joint is rotated about the shoulder to bring the configuration closer to the body. To study the effect of this modification, the system was evaluated for the 9 most important positions for activities of daily life and the potential energy values of the whole system are compared. The energy values show no large difference as compared to the ideal 2-link system. We conclude therefore that the addition of an extra spring makes it possible to bring the spring configuration closer to the body. This extra spring does not introduce significant balancing errors, on top of the error of extra mass of the arm support and other alignment errors.