  Supernumerary Robotics Device (SRD) is an ideal solution to provide robotic
assistance in overhead manual manipulation. Since two arms are occupied for the
overhead task, it is desired to have additional arms to assist us in achieving
other subtasks such as supporting the far end of a long plate and pushing it
upward to fit in the ceiling. In this study, a method that maps human muscle
force to SRD for overhead task assistance is proposed. Our methodology is to
utilize redundant DoFs such as the idle muscles in the leg to control the
supporting force of the SRD. A sEMG device is worn on the operator's shank
where muscle signals are measured, parsed, and transmitted to SRD for control.
In the control aspect, we adopted stiffness control in the task space based on
torque control at the joint level. We are motivated by the fact that humans can
achieve daily manipulation merely through simple inherent compliance property
in joint driven by muscles. We explore to estimate the force of some particular
muscles in humans and control the SRD to imitate the behaviors of muscle and
output supporting forces to accomplish the subtasks such as overhead
supporting. The sEMG signals detected from human muscles are extracted,
filtered, rectified, and parsed to estimate the muscle force. We use this force
information as the intent of the operator for proper overhead supporting force.
As one of the well-known compliance control methods, stiffness control is easy
to achieve using a few of straightforward parameters such as stiffness and
equilibrium point. Through tuning the stiffness and equilibrium point, the
supporting force of SRD in task space can be easily controlled. The muscle
force estimated by sEMG is mapped to the desired force in the task space of the
SRD. The desired force is transferred into stiffness or equilibrium point to
output the corresponding supporting force.
