| Although a number of dextrous hands exist in research laboratories today, the application of such hands in practical manipulation tasks is essentially unexplored. There are several reasons for this situation, of which two of the most important are an insufficient understanding of manipulation processes and an inability to construct robust control strategies that will enable the hands to respond in a versatile way to unexpected manipulation events. In my work, we first understand the mechanics of contact when soft fingertips are used, use these models in the dynamics of manipulation, and then study the problem of contact force control.; Soft fingertips generally perform better than the kinematic idealizations proposed in recent research would indicate, allowing smaller gripping forces, providing stabler grasps, and dissipating energy from the system. These soft fingertips are created either by using urethane rubber or by filling robot fingers with powders or plastic fluids. Contact models, adapted from solid mechanics theory, have been developed to represent such fingertips more realistically. The nature of these fingertips affects the dynamics of manipulation by dissipating energy. This dissipation of energy has an effect similar to that of adding damping or velocity feedback to the finger joints. The interest in this effect is in the context of simulating the dynamics, both steady state and transient, of manipulation.; The issue of controlling the contact forces generated when the fingers of a dextrous device, such as the Stanford/JPL hand, first grasp an object is then addressed. "Semi-active" fingertips are proposed as a means of enhancing the system performance and of solving the contact instability problem resulting from non-collocated sensors and actuators. The issue is studied in the context of a simple manipulation device, and analytical and simulation studies are provided to identify an "ideal" fingertip to achieve a desirable performance. Optimizing the performance of more realistic fingers is investigated and preliminary experimental results that explore the use of Electro-Rheological fluids as prototypical materials for building such active fingertips are provided. It is suggested that while soft fingertips are better than hard ones, "semi-active and active" soft fingertips, whose damping can be varied, perform even better. |
