Dynamic Stable Grasp and Object-Manipulation by Multi-Fingered Hands with Soft Tips Based on Sensory Feedback

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This research project started from noting that human can grasp an object stably and manipulate it dexterously while it is quite difficult to let multi-fingered robot hands to do the same, even though their mechanisms resemble human hands and each of their finger joints is equipped with a superior actuator. We also noted that there is a vast literature concerning the problem of form closure or immobilizability of 2D or 3D objects by using a number of frictionless fingers. It is known that three frictionless fingers suffice to immobilize any 2D object with triangular shape but four fingers are necessary for immobilizing a parallelepiped. Just before starting this research we found that only two fingers are enough to realize secure grasp of a rigid object with parallel flat surfaces in a dynamic sense if finger ends have a hemispherical shape with appropriate radius and thereby rollings are induced between finger ends and object surfaces. The principal reason lies on the fact that a singl … More e rolling contact generates two contact constraint forces, one in the direction normal to the object surface at the contact point and another in the direction tangent to the object surface.In this research project, we firstly verified theoretically that a pair of multi-degrees of freedom robot fingers with hemispherical finger ends can grasp stably a rigid object with non-parallel or parallel flat surfaces and control its orientation angle by means of sensory-motor coordination based on sensory feedback signals constructed on the basis of information of only finger kinematics and measurements of finger joints and the rotational angle of the object. It is also shown that the overall fingers-object system is under-actuated and the total d.o.f. of the system is redundant. Notwithstanding such underactuation and redundancy of the dynamics, it is shown that a new concept of "stability on a manifold" plays a crucial sole in realizing a dynamic force/torque closure (stable grasp) together with concurrent control of the object orientation. Secondly, simulation results on the basis of the CSM (Constraint Stabilization Method) verified the effectiveness of the proposed method of sensory-motor coordination and present guidelines for determining feedback gains in various feedback terms. Finally, experimental results show that concurrent grasp and orientation control of a rigid object can be realized without knowing the size of the object and location of the object mass center. The theory is now under extending to the case of grasp and manipulation of 3D polygonal objects by using two or three multi-dof fingers with spherical ends. Less