Modelling and Control for Soft Finger Manipulation and Human-Robot Interaction (2010)

One of the greatest challenges of humanoid robotics is to provide a robotic systems with autonomous and dextrous skills. Dextrous manipulation skills, for personal and service robots in unstructured environments, are of fundamental importance, in order to accomplish manipulation tasks in human-like ways and to realize a proper and safe cooperation between humans and robots. The contributions presented in this thesis are aimed at modeling and controlling multifingered robotic hands with soft covers for manipulation tasks. The control issue of a hand-arm robotic system involved in grasping tasks, which can interact with the environment or a human, is also addressed. A port-Hamiltonian model of a multifingered robotic hand, with soft-pads on the finger tips, grasping an object has been developed. The port-Hamiltonian framework is based on the description of systems in terms of energy variables, and their interconnection in terms of power ports. Any physical systems can be described by a set of elements storing kinetic or potential energy, a set of energy dissipating elements, and a set of power ports interconnected by power preserving interconnections. The viscoelastic behavior of the contact is described in terms of energy storage and dissipation. Using the concept of power ports, the dynamics of the hand, the contact, and the object are described. The algebraic constraints of the in- terconnected systems are represented by a geometric object, called Dirac structure. This provides a powerful way to describe the non-contact to contact transition and contact viscoelasticity, by using the concept of energy flows and power preserving interconnections. Using the port based model, an Intrinsically Passive Controller (IPC) is used to control the internal forces and the motion of the object. In grasping tasks, in the case that also interaction with the environment or a human is involved, the control issue of a hand-arm robotic system, is addressed. Thecontrol law adopted for the arm is a compliance object-level control, which aims to reduce the interaction forces. The control action is based on the reconstruction of the external load applied to the object, using the force sensors measurement at the fingertips. Force sensing is also used to compute in real time the desired contact forces, able to guarantee the stability of the grasp. The regulation of the grasping forces is in charge of the hand control. In detail, the contents of the thesis are organized as follows. Chapter 1 provides an introduction on grasping and manipulation applications in the context of advanced robotics where the robot has to operate in unstructured environment. Here the relevance of dexterous manipulation skills in performing many di®erent tasks is emphasized. The framework of the research work in this section is introduced, i.e., the activities in the European project DEXMART. A brief description of the research objectives and the key innovations carried out within the DEXMART project are given. Chapter 2 contains an overview on the relations between the designing features of a robotic hand and its anthropomorphism and dexterity. Then the robotic hand built within the DEXMART project is introduced. A detailed description of the mechanical structure and of the actuation system by means of tendons is provided. Moreover, the kinematics, the statics and the dynamics of the hand are derived. The control structure and the control of the interaction in presence of soft contact is analyzed. Chapter 3 presents a port-Hamiltonian model of a multifingered robotic hand, with soft-pads, while grasping and manipulating an object. An introduction to the port-based formulation is provided. For the validation of the model, a simple example modeled in 20-sim simulation software is considered. Simulation results are presented to validate the model and to show the behavior of the system when an IPC based controller is applied. In Chapter 4 the control issue of a hand-arm robotic system involved in grasping tasks, which can interact with the environment or a human, is addressed. An introduction on the combined control of hand-arm systems is given. The proposed control action is based on the reconstruction of the forces appliedto the object, using the measurement at the fingertips, in order to obtain a compliant behavior of the arm and to reduce the interaction forces. A detailed simulation model of the robotic hand has been developed with the aim of testing the control strategies, using the SimMechanics toolbox of MATLAB. Simulation tests in MATLAB/SimMechanics environment demonstrate the effectiveness of the proposed approach. Chapter 5 contains concluding remarks and proposals for further investigations.