  Powered lower limb exoskeletons are a viable solution for people with a
spinal cord injury to regain mobility for their daily activities. However, the
commonly employed rigid actuation and pre-programmed trajectories increase the
risk of falling in case of collisions with external objects. Compliant
actuation may reduce forces during collisions, thus protecting hardware and
user. However, experimental data of collisions specific to lower limb
exoskeletons are not available. In this work, we investigated how a variable
stiffness actuator at the knee joint influences collision forces transmitted to
the user via the exoskeleton. In a test bench experiment, we compared three
configurations of an exoskeleton leg with a variable stiffness knee actuator in
(i) compliant or (ii) stiff configurations, and with (iii) a rigid actuator.
The peak torque observed at the pelvis was reduced from 260.2 Nm to 116.2 Nm as
stiffness decreased. In addition, the mechanical impulse was reduced by a
factor of three. These results indicate that compliance in the knee joint of an
exoskeleton can be favorable in case of collision and should be considered when
designing powered lower limb exoskeletons. Overall, this could decrease the
effort necessary to maintain balance after a collision and improved collision
handling in exoskeletons could result in safer use and benefit their usefulness
in daily life.
