Toward Motor-Unit-Recruitment Actuators for Soft Robotics

Among the many features of muscles, their softness,
(the ability to deform to accommodate uncertainty in
the environment), and their ability to continue functioning
despite disturbances, even partial damage, are qualities one
would desire to see in robotic actuators. These properties
are intimately related to the manner in which muscles work
since they arise from the progressive recruitment of many
motor units. This differs greatly from current robotic actuator
technologies. We present an actuation platform prototype that
can support experimental validation of algorithms for muscle
fiber recruitment-inspired control, and where further ways to
exploit discretization and redundancy in muscle-like control
can be discovered. This platform, like muscles, is composed of
discretely activated motor units with an integrated compliant
coupling. The modular, cellular structure endows the actuator
with good resilience in response to damage. It can also be
repaired or modified to accommodate changing requirements in
situ rather than replaced. Several performance metrics particular
to muscle-like actuators are introduced and calculated for
one of these units. The prototype has a blocked force of 2.51 N,
a strain rate of 21.1 %, and has an input density of 5.46 103
motor units per square meter. It consumes 18 W of electrical
power during a full isometric contraction. The actuator unit
is 41.0 mm3 in size. The force during isometric contractions
as it varies with activation is evaluated experimentally for two
configurations of modules.