The effect of series elasticity on actuator power and work output: Implications for robotic and prosthetic joint design

Evidence from biomechanics research suggests that tendon series elasticity allows muscle to act in an optimal range of its force–length and force–velocity curves to achieve work and power amplification. In this investigation we put forth a simple model to quantify the capacity of series elasticity to increase work and power output from an actuator. We show that an appropriate spring constant increases the energy that an actuator can deliver to a mass by a factor of 4. The series elasticity changes the actuator operating point along its force–velocity curve and therefore affects the actuator work output over a fixed stroke length. In addition, the model predicts that a series spring can store energy and deliver peak powers greater than the power limit of the source by a factor of 1.4. Preliminary experiments are performed to test model predictions. We find qualitative agreement between the model and experimental data, highlighting the importance of series elasticity for actuator work and power amplification across a fixed stroke length. We present several non-dimensional relations that can aid designers in the fabrication of robotic and prosthetic limbs optimized for work and power delivery.