Viscoelastic model for redundancy resolution of the human arm via the swivel angle: Applications for upper limb exoskeleton control

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One of the key research efforts associated with a redundant seven degree of freedom (7-DOF) upper limb exoskeleton robot that is mechanically coupled to the human body is to develop high and low level control algorithms that enable the system to become a natural extension of the human body. Improving the synergistic relationship between the exoskeleton and the operator is manifested in part by decreasing the force exchange between the two entities. Such a reduction is accomplished in part by developing criteria for resolving the human arm redundancy. The redundancy may be represented by a swivel angle which is defined as the angular rotation of the elbow around an axis that passes through the shoulder and wrist joints. The proposed criteria for defining the swivel angle takes into account the dynamics of the human arm along with a viscoelastic muscle-like model with variable damping. The swivel angle is estimated using the pseudo-inverse of the Jacobian with a secondary objective function that estimates the desired joint angles during human arm movement. The result is then fed to the muscle model to create a more realistic human motion. The estimated swivel angle is then compared with the actual swivel angle measured experimentally by a motion capture system. Results indicate that the average error between the estimated and measured swivel joint angle is 4.4 degrees (in the range [3.7-6] degrees), which are lower than the kinematically based redundant resolution criterion.