CONTROL OF 3-JOINT AND 4-JOINT ARM MOVEMENT - STRATEGIES FOR A MANIPULATOR WITH REDUNDANT DEGREES OF FREEDOM

Control of arm movements when the number of joints exceeds the degrees of freedom necessary for the task requires a strategy for selecting s pecific arm configurations out of an infinite number of possibilities. This report reviews strategies used by human subjects to control the shoulder, elbow, and wrist (three degrees of freedom) while moving a p ointer to positions in a horizontal plane (two degrees of freedom). An alysis of final arm configurations assumed when the pointer was at the target showed the following: (a) Final arm configurations were virtua lly independent of the configuration at the start of the pointing move ment, (b) subjects avoided configurations subjectively felt to be unco mfortable (e.g., those with extreme flexion or extension of the wrist) , and (c) the results could be simulated by assigning hypothetical cos t functions to each joint and selecting the arm configuration that min imized the sum of the costs. The fitted cost functions qualitatively a greed with psychophysically determined comfort; they appeared to depen d on joint angle and on muscular effort. Simple neural networks can le arn implicit representations of these cost functions and use them to s pecify final arm configurations. The minimum cost principle can be ext ended to movements that use the fingers as a fourth movable segment. F or this condition, however, experiments showed that final configuratio ns of the arm depended upon initial configurations. Analysis of moveme nt trajectories for arms with three degrees of freedom led to a contro l model in which the minimum cost principle is augmented by a mechanis m that distributes required joint movements economically among the thr ee joints and a mechanism that implements a degree of mass-spring cont rol.