Energy-Free Adjustment of Gravity Equilibrators Using the Virtual Spring Concept

People with neuromuscular diseases often have limited muscle force, and many rely on mobile arm supports to move their arms. Most of these make use of static balancing, a useful concept to reduce the operating effort of mechanisms, where spring mechanisms are used to achieve a constant total potential energy, thus eliminating any preferred position. Once statically balanced, the mechanism can be moved virtually without operating energy. This allows the patients using an arm support to move their arms freely. In some cases it is desirable to adjust the balancer characteristic, for instance due to a change of payload when picking up an object. In all available arm support mechanisms this adjustment is associated with considerable mechanical effort, while clearly this application would benefit greatly from an energy-free adjustment. Recently several methods have been invented to adjust spring and linkage-based static balancers with no need for external energy, by conserving the total energy in the system. The technique discussed in this paper is based on substituting an initial spring by two substitute springs. The two substitute springs generate a virtual spring with the same spring properties as the initial spring, with one unique difference: the virtual spring can be elongated or shortened while no work is done. Consequently, no external energy is needed for the adjustment. A table-top demonstrator has been developed, showing the feasibility of the concept.