  This paper presents design and control innovations of wearable robots that
tackle two barriers to widespread adoption of powered exoskeletons, namely
restriction of human movement and versatile control of wearable co-robot
systems. First, the proposed quasi-direct drive actuation comprising of our
customized high torque density motors and low ratio transmission mechanism
significantly reduces the mass of the robot and produces high backdrivability.
Second, we derive a biomechanics model-based control that generates biological
torque profile for versatile control of both squat and stoop lifting
assistance. The control algorithm detects lifting postures using compact
inertial measurement unit (IMU) sensors to generate an assistive profile that
is proportional to the biological torque produced from our model. Experimental
results demonstrate that the robot exhibits low mechanical impedance (1.5 Nm
resistive torque) when it is unpowered and 0.5 Nm resistive torque with
zero-torque tracking control. Root mean square (RMS) error of torque tracking
is less than 0.29 Nm (1.21% error of 24 Nm peak torque). Compared with
squatting without the exoskeleton, the controller reduces 87.5%, 80% and 75% of
the of three knee extensor muscles (average peak EMG of 3 healthy subjects)
during squat with 50% of biological torque assistance.
