  A low-cost 1-DOF hand exoskeleton for neuromuscular rehabilitation has been
designed and assembled. It consists of a base equipped with a servo motor, an
index finger part, and a thumb part, connected through three gears. The index
part has a tri-axial load cell and an attached ring to measure the finger
force. An admittance control scheme was designed to provide intuitive control
and positive force amplification to assist the user's finger movement. To
evaluate the effects of different control parameters on neuromuscular re-sponse
of the fingers, we created an integrated exoskeleton-hand musculo-skeletal
model to virtually simulate and optimize the control loop. The exo-skeleton is
controlled by a proportional derivative controller that computes the motor
torque to follow a desired joint angle of the index part, which is obtained
from inverse kinematics of a virtual end-effector mass driven by the finger
force. We conducted parametric simulations of the exoskeleton in action, driven
by the user's closing and opening finger motion, with different proportional
gains, end-effector masses, and other coefficients. We com-pared the
interaction forces between the index finger and the ring in both passive and
active modes. The best performing assistive controller can re-duce the force
from around 1.45N (in passive mode) to only around 0.52N, more than 64% of
reduction. As a result, the muscle activations of the flex-ors and extensors
were reduced significantly. We also noted the admittance control scheme is
versatile and can also provide resistance (e.g. for strength training) by
simply increasing the virtual end-effect mass.
