Approximate spring balancing of linkages to reduce actuator requirements

The potential benefit of applying gravity balancing to orthotic, prosthetic and other wearable devices is well recognized, but practical applications have been elusive. Although existing methods provide exact gravity balance, they require additional masses or auxiliary links, or all the springs used originate from the ground, which makes the resulting device bulky and space-inefficient. This work presents a new method that is more practical than existing methods to provide approximate gravity balancing of mechanisms to reduce actuator loads. Current balancing methods use zero-free-length springs or simulate them to achieve balancing. Here, non-zero-free-length springs can be used directly. This new method allows springs to be attached to the preceding parent link, which makes the implementation of spring balancing practical. The method is applicable to planar and spatial, open and closed kinematic chains. Applications of this method to a lower-limb orthosis and a manually-operated sit-to-stand wheelchair mechanism are presented. Results show considerable reduction in actuator requirements with practical spring design and arrangements.