Basic problems in artificial prehension.

An attempt is made to identify those factors associated with artificial prehension that influence the selection of prehension control laws. A study is made of the complex requirements of artificial prehension and it is proposed that these may best be satisfied by a system composed of six independent sub-control systems, defined as prehension elements. It is conjectured that complex actions, such as writing, may be realised from combinations of these prehension elements. Synthesis shows that, in general, control is a function of the modulus of the object state variables and that some prehension elements require no conscious involvement. The need to consider the object within the control loops, and in some instances the elimination of conscious effort, is a departure from traditional concepts. In an attempt to separate the problem of control from the many other factors that influence prehension device design the latter is realised by a two-dimensional calijDfci' system having a pneumatic power drive. Test objects are realised in a two-dimensional form moving in a direction normal to the caliJ)C.;system. To evaluate theability of the prehension elements to attain the required forms of object control, such as balancing, holding, etc., common tests are conducted with the prehension elements together with a simple object-open-loop pressure-feedback system representing a conventional prosthesis. A "figure of merit" associated with each test reflects the success with which the required control action is achieved by each prehension element. The range of object disturbances for the tests is limited to sinusoid, ramp and step functions. Experimental results in general confirm the need for multiple control systems, and of the importance of object position and velocity in the feedback structure, to satisfy artificial prehension. The results further indicate improved prehension control over that given by traditional systems.