Energy density and hysteresis comparison in natural rubber tube springs for wearable exoskeleton applications

Wearable exoskeletons show promise as a means for compensating lost function as well as for providing optimal assistance for maximal therapeutic benefit during everyday tasks. Development of lightweight spring systems for efficient storage and return are proposed as a key component in the successful deployment of wearable exoskeletons for individuals with neurological deficits. Both spring steel and natural rubber are common materials used in energy storage, but have not been directly compared by metrics such as energy storage density, energy storage efficiency, and hysteresis. In this work, we perform cyclic loading tests on spring steel extension springs of varying wire diameter and natural rubber tubing of varying wall thicknesses. We then use measured load-extension profiles to illustrate and compute metrics to better quantify the energy storing capabilities of each material and their appropriateness for use as energy storing and returning components in wearable robotic applications. Results show that natural rubber has a higher capacity for energy storage per unit weight in comparison to steel springs. Hysteresis is also higher in natural rubber and can be dramatically reduced by applying adequate pre-strain at levels greater than the anticipated strain during use.