Hybrid Electric-Pneumatic Actuator (EPA) for legged locomotion

image1 A better understanding of how actuator design supports locomotor function may 
help design and develop novel and more functional powered assistive or robotic legged
 systems. Legged locomotion can be described as a composition of locomotor
 sub-functions, namely axial leg function, leg swinging and balancing.

In this 
project, we focus on the axial leg function (e.g., spring-like hopping) based on a novel concept of a hybrid electric-pneumatic actuator (EPA). This principal locomotor sub-function determines 
the movement of the body center of mass. We will design and manufacture EPA prototypes 
as enhanced variable impedance actuators (VIA). In contrast to other VIAs, the EPA provides not only adaptable compliance (e.g. an adjustable spring) 
but with the pneumatic artificial muscle (PAM) also 
an additional powerful actuator with muscle-like properties, which can be
arranged in different configurations (e.g., in series or parallel) to the electric motor (EM). This novel hybrid actuator
 shares the advantages of EM and PAM combining precise control with compliant
 energy storage required for efficient, robust and versatile human-like leg motions via simple control 
laws.

Based on human experiments, the EPA design will be optimized to minimize
 energy consumption and maximize robustness against perturbations within a
 desired operational range. We consider human hopping in place as a simple movement concentrating on the axial leg function.

A simulation model of human muscle-skeletal function reproducing human hopping experiment results will be used to identify the objective function for the biological actuators (muscles) through “inverse
 optimal control”. This biologically inspired cost function will then help us to 
identify the most appropriate EPA actuator design. A robotic setup of the MARCO-2 hopping robot will be equipped with EPA to demonstrate and evaluate the actuator design and control. Based on its mechanical properties and its flexible arrangement in 
multi-segment-systems, the EPA provides a novel actuator that mimics human 
muscle function and is able to mechanically adapt to different gaits and 
conditions (e.g. locomotion speed). Preliminary experimental and simulation
 studies in our group show evidence of expected advantages of adding PAM to EM.

We expect that only limited exchange of sensory information between the different locomotor sub-function controllers will be required enabling the envisioned modular architecture of the locomotor control system. With EPA technology, new versatile, efficient and robust locomotor
 systems for a wide range of applications can be designed.

BioAct Workshop at BioRob 2018