Exploiting body redundancy to control supernumerary robotic limbs in human augmentation

In the last decades, supernumerary robotic limbs (SRLs) have been proposed as technological aids for rehabilitation, assistance, and functional augmentation. Whether they are in the form of wearable devices or grounded systems, SRLs can be used to compensate for lost motor functions in patients with disabilities, as well as to augment the human sensorimotor capabilities. By using SRLs, users gain the ability to perform a wide range of complex tasks that may otherwise be challenging or even impossible with their natural limbs. Designing effective strategies and policies for the control and operation of SRLs represents a substantial challenge in their development. A key aspect that remains insufficiently addressed is the formulation of successful and intuitive augmentation policies that do not hinder the functionality of a person's natural limbs. This work introduces an innovative strategy based on the exploitation of the redundancy of the human kinematic chain involved in a task for commanding SRLs having one degree of freedom. This concept is summarized in the definition of the Intrinsic Kinematic Null Space (IKNS). The newly developed procedure encompasses a real-time analysis of body motion and a subsequent computation of the control signal for SRLs based on the IKNS for single-arm tasks. What sets our approach apart is its explicit emphasis on incorporating user-specific biomechanical and physiological characteristics and constraints. This ensures an efficient and intuitive approach to commanding SRLs, tailored to the individual user's needs. Towards a complete evaluation of the proposed system, we studied the users' capability of exploiting the IKNS both in virtual and real environments. Obtained results demonstrated that the exploitation of the Intrinsic Kinematic Null Space allows to perform complex tasks involving both biological and artificial limbs, and that practice improves the ability to accurately manage the coordination of human and supernumerary artificial limbs.