CABLE DRIVEN PARALLEL ROBOTS WITH RECONFIGURABLE BASE FRAME

This thesis focuses on the study of Cable-Driven Parallel Robots (CDPRs) with particular interest on Reconfigurable Cable-Driven Parallel Robots (RCDPRs). CDPRs are parallel robots that exploit several cables to displace an end-effector throughout the robot workspace. Cable lengths are set by servo controlled winches. Cables are routed in the robot workspace through guiding pulleys fixed onto a base frame, which define the so-called exit-points. RCDPRs, unlike CDPRs, present the possibility of displacing the exit-points along a prescribed direction or in the three-dimensional space: examples are CDPRs combined with moving agents such as drones or ground vehicles. Generally speaking, the displacement of exit-points allows either increasing the robot workspace, both in terms of translational and rotational capabilities of the end-effector, or optimizing the robot performances within the workspace. Such benefits arising from reconfigurability, can only be exploited if reconfiguration strategies are developed and this, in turn, imposes the availability or the development of accurate dynamic models capable of reproducing the complex behaviour of RCDPRs. Therefore, the goals of this thesis are, firstly, to develop a modelling framework capable of managing any CDPR topology at almost any useful levels of component representation complexity, and secondly, to exploit the dynamic model to investigate limits and opportunities of RCDPR reconfigurability and to suggest reconfiguration strategies. More in details, the developed dynamic model manages either lumped mass or rigid body end-effectors, cables can be represented as rigid and massless elements, or as simply axially flexible elements or even considering the full cable flexibility and distributed mass. Additionally, the pulleys guiding cables in the robot workspace can been modelled as purely geometrical entities or accounting for both their geometry and inertia. Such flexibility in the modelling of RCDPRs has been achieved by overcoming the traditional modelling approach proposed in the literature on CDPRs, which relies on obtaining the set of minimal Ordinary Differential Equations (ODEs) representing the dynamics of the end-effector of a CDPR. Differently, in this thesis, the nonlinear dynamic model of the multibody system comprising the end-effector, the motors, the cables, the guiding pulleys, and, if present, the reconfigurable exit-points has been obtained by employing a redundant set of Differential-Algebraic Equations (DAEs). A relevant part of the thesis provides insight into the modelling of all the typical components of RCDPRs and provides evidence of the dynamic model effectiveness in simulating standard tasks (e.g., the handling of a payload) or unconventional conditions (e.g., the response in case of cable failures). Then it is shown how the model can be employed for the synthesis of an Extended Kalman Filter (EKF) first validated on a CDPR with a lumped mass end-effector and then on a RCDPR. The last part of the thesis focuses on the development of reconfiguration strategies for underactuated RCDPRs and introduces an original method for solving the inverse dynamics problem for this robot topology. The theoretical achievements are finally experimentally validated by means of a laboratory test rig used as demonstrator.