Robot

Damper

Supernumerary

Magnetorheological fluid

Engineering

Neuroscience

Biology

Robotic hand

Control engineering

Exoskeleton

Actuator

Ankle

Computer science

Swing

Classical mechanics

Mechanical engineering

Gait

Thermodynamics

Physics

Torque

Trajectory

Rehabilitation

Simulation

Anatomy

Astronomy

Physical therapy

Artificial intelligence

Structural engineering

Kinematics

Medicine

Physical medicine and rehabilitation

Gait training

Ground reaction force

Pathology

IEEE Robotics and Automation Letters

Supernumerary robotic limbs are emerging to augment human function. Unlike exoskeletons, these robots provide additional kinematic structures to the user that enable novel human-robot interactions. To assist walking, a supernumerary leg should be compliant to impacts, minimize efforts on users, move quickly when swinging and exert large assistive forces on the ground. Here, we study the potential of a supernumerary leg powered by delocalized magnetorheological clutches (MR leg) to assist walking with three different gaits. Simulations show that the MR leg's low actuation inertia reduces the impact impulse by a factor 4 compared to geared motors and that delocalizing the clutches reduces by half the inertial forces transmitted to the user during swing. An impedance controller receives a reference trajectory based on each ankle's position to move the MR leg in synchrony with the gait cycle. Experiments show that the MR leg can comfortably contact the ground and swing at 3.9 m/s for a 1.4 m/s walk. The MR leg tracks the ankle within 5% of the gait cycle for the leader-follower gait, alternately tracks both ankles for the double gait and contacts the ground in between each step for the three-legged gait. A theoretical upper limit suggests that the average transmitted power in a gait cycle could be 84 W for the leader-follower gait, which is 4 times higher than autonomous ankle exoskeletons.

A Supernumerary Robotic Leg Powered by Magnetorheological Actuators to Assist Human Locomotion