Design and Control of an anthropomorphic thumb robot prototype

Abstract

The complexity of mechatronic systems is barrier for their diffusion. That is why robots have to be anthropomorphic to facilitate interaction with humans. Or, it is not possible to modify every objects in a house so robots must be adapted to the usual human environment. This explains why the importance of anthropomorphic hands is growing. Currently, several models are under research but none are deeply investigating the special role of the thumb. In this report, a thumb design, based on medical analysis, is proposed. With the help of a thumb surgeon, numerous medical articles dealing with thumb surgery have been merged. Not only the lengths and the forces must be carefully chosen but the stiffness must be adjustable. The tendon displacement, required for a motion under a specifc stiffness, can be calculated as a non linear optimisation problem. The tendon stiffness range can be optimised if a joint stiffness range is provided. An extension of the usual metrics shows that it is possible to compare thumb designs. The term of workspace has to be defined carefully otherwise the global results will not have much meaning. The use of the global metrics as optimisation parameters has to be done with care since they do not include technological constraints. The control of an intrinsically compliant mechanism differs from the classical manipulator control. To take advantage of the intrinsic stiffness behaviour, the control cannot be based on link side measurements. A link side position controller can be used to tackle the calibration and zeroing problems. The validity of a single block stiffness controller is exhibited through several experiments and simulations. The reports consolidates the fondations of thumb design. It helps the designer by providing standard values for forces, stiffness and lengths. It reduces the constraints by sorting biological and functional ones. It is now possible to move one step ahead, to concentrate on the configuration of the fingers in the hand and to develop new grasping strategies.