Modeling and control of flexible-link robotic systems

The scientic activity presented in this Ph.D. thesis deals with the modeling and control of flexible-link robotic systems. Nowadays, the industrial demand for high performances, high speeds and low energy consume has highlighted the need to develop lightweight manipulators and robots. However, their design and control result more difficult and challenging with respect to traditional rigid-link robotic systems mainly due to the flexibility of the arms. In the first part of my Ph.D., the research activity has been focused on the modeling and simulation of flexible-link mechanism, using an Equivalent Rigid-Link System (ERLS) formulation. In recent years, the ERLS approach, firstly implemented together with a Finite Element Method (FEM) formulation, has been extended through a modal approach and, in particular, a Component Mode Synthesis (CMS) technique. This novel formulation allows a reduced-order system of equations to be maintained even when a fine discretization is needed. After an analysis of the state of the art about dynamic modeling of flexible-link mechanisms, a numerical comparison between the ERLS-FEM and the ERLS-CMS approaches has been conducted. A benchmark manipulator has been implemented and the results have been compared in terms of accuracy and computational effort under different input conditions. The discretization of the mechanism and the number of considered vibrational modes have been as well discussed. In the CMS approach, a classical Craig-Bampton reduction has been adopted. However, this is not the only technique capable of reducing the number of degrees of freedom of flexible-link mechanisms. For this reason, further developments of the work have seen the implementation and comparison of different Model Order Reduction Techniques, which can be applied to different benchmark robotic systems in order to highlight their advantages and disadvantages. The second part of this thesis is focused on cable-driven parallel robots, which are a special class of flexible-link mechanisms in which flexible cables, rather than rigid links, are employed to actuate the end-effector. A particular class of cable-driven robots is given by cable-suspended parallel robots, which rely on gravity to maintain the cables taut. These mechanisms are characterized by large workspaces, hig velocities and payload-to-weight ratios and can be employed for several different tasks such as handling and moving loads, pick-and-place and building tasks. In collaboration with University of Trieste (Italy), a novel design of cable-suspended parallel robot based on variable radius drums has been developed and experimentally validated. A variable radius drum is characterized by the variation of the radius along the spool. This device is used in the cable-driven manipulator to move the end-effector through a planar working area, using just two actuated joints. Experimental results demonstrate a good agreement with the theoretical model. Another example of cable-driven robot has been studied during the months that I spent at the Wearable Robotic Systems (WRS) Laboratory, Department of Mechanical Engineering, Stevens Institute of Technology (Hoboken, NJ, USA). The device consists of a 3-degree-of-freedom, under-actuated, pendulum-like robot. The mechanism is capable of performing planar point-to-point motions in its dynamic workspace by means of two actuated joints only, using parametric excitation in a way similar to playground swings. The control system is based on a feedback linearization that allows the dynamics of the variable-length pendulum to be decoupled from the dynamics of the rotation of the end-effector. Adaptive Frequency Oscillators have been introduced to estimate the phase of the pendulum-robot in real-time and without delay. The device has been experimentally validated showing the feasibility of the design and good performances of the control architecture.