Constraint- and synergy-based specification of manipulation tasks

This work aims to extend the application field of the constraint-based control framework called iTaSC (instantaneous task specification using constraints) toward manipulation tasks. iTaSC offers two advantages with respect to other methods: the ability to specify tasks in different spaces (and not only in Cartesian coordinates as for the Task Frame Formalism), and the treatment of geometric uncertainties. These properties may be very useful within a manipulation context, where tasks are executed by robots with many degrees of freedom, which calls for some degree of abstraction; by choosing a suitable set of coordinates, it is possible to reduce the complexity and the number of constraints that fully describe such tasks; in addition, controlling only the subspace that is needed to fulfil a task allows us to use the remaining degrees of freedom of the robot system to achieve secondary objectives. This paper discusses the instruments and techniques that can be employed in manipulation scenarios; in particular it focuses on aspects like the specification of a grasp and control of the stance of the robotic arm. iTaSC offers the possibility of specifying a grasp. While this approach allows for very fine control of a grasping task, in most cases a less fine-grain specification suffices to guarantee a successful execution of the grasping action. To this end synergy-based grasp specification is formulated within iTaSC. We also show how to take into account secondary objectives for the arm stance. In particular we consider, as an example, the manipulability index along a given direction. Such indexes are maximised by exploring the null space of the other tasks. The proposed approach is demonstrated by means of simulations, where a robotic hand grasps a cylindrical object.