  Pediatric exoskeletons offer great promise to increase mobility for children
with crouch gait caused by cerebral palsy. A lightweight, compliant and
user-specific actuator is critical for maximizing the benefits of an
exoskeleton to users. To date, pediatric exoskeletons generally use the same
actuators as adult exoskeletons, which are heavy and resistive to natural
movement. There is yet no easy way for robotic exoskeletons to accommodate the
changes in design requirements that occur as a child ages. We developed a
lightweight (1.65 kg unilateral mass) and compliant pediatric knee exoskeleton
with a bandwidth of 22.6 Hz that can provide torque assistance to children with
crouch gait using high torque density motor. Experimental results demonstrated
that the robot exhibited low mechanical impedance (1.79 Nm average backdrive
torque) under the unpowered condition and 0.32 Nm with zero-torque tracking
control. Root mean square (RMS) error of torque tracking result is less than
0.73 Nm (5.7% with respect to 12 Nm torque). To achieve optimal age-specific
performance, we proposed the first optimization framework that considered both
motor and transmission of the actuator system that can produce optimal settings
for children between 3 and 18 years old. The optimization generated an optimal
motor air gap radius that monotonically increases with age from 0.011 to 0.033
meters, and optimal gear ratio varies from 2.6 to 11.6 (3-13 years old) and
11.6 to 10.2 (13-18 years old), leading to actuators of minimal mass.
