On the Electrical Energy Consumption of Active Ankle Prostheses with Series and Parallel Elastic Elements

With series and parallel elastic elements, considerable reductions in the mechanical peak power and energy consumption of active ankle prostheses can be obtained. Very few works, however, evaluate the electrical energy consumption of these devices. In this work, we analyze and discuss the differences between the mechanical and electrical energy consumption of these actuators. Design optimizations based on mechanical and electrical energy consumption are compared for a series elastic actuator, parallel elastic actuator and series elastic actuator with unidirectional spring. The results are then analyzed by means of torque-angle plots, power flow graphs and motor efficiency maps. The analysis highlights the impact of drivetrain inertia on the peak power and energy efficiency of the system. Moreover, interaction between the series spring and unidirectional parallel spring is identified as a potential cause of reduced actuator bandwidth. A parallel elastic actuator is found to be the most compact and energy-efficient solution overall as it makes the most efficient use of the electric motor.