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Clinical potential and design of programmable mechanical impedances for orthotic applications

William S. Harwin a1, Lee O. Leiber a2, Gregory P. G. Austwick a1 and Chryssa Dislis a3
a1 Department of Cybernetics, University of Reading, Reading RG6 6AY, UK
a2 Applied Science and Engineering Laboratories, University of Delaware and the Alfred I. duPont Institute, Wilmington, Delaware 19899, USA
a3 Abstract Hardware, Brunel Science Park, Uxbridge, UB8 3PQ, UK


A person with a moderate or severe motor disability will often use specialised or adapted tools to assist their interaction with a general environment. Such tools can assist with the movement of a person's arms so as to facilitate manipulation, can provide postural supports, or interface to computers, wheelchairs or similar assistive technologies. Designing such devices with programmable stiffness and damping may offer a better means for the person to have effective control of their surroundings.

This paper addresses the possibility of designing some assistive technologies using impedance elements that can adapt to the user and the circumstances. Two impedance elements are proposed. The first, based on magnetic particle brakes, allows control of the damping coefficient in a passive element. The second, based on detuning the P-D controller in a servo-motor mechanism, allows control of both stiffness and damping. Such a mechanical impedance can be modulated to the conditions imposed by the task in hand. The limits of linear theory are explored and possible uses of programmable impedance elements are proposed.

Key Words: Mechanical impedance; Clinical potential; Motor disability; Orthotic application.