Abstract
This thesis addresses the position tracking control of Cable-Driven Parallel Robot (CDPR) within the framework of the European H2020 project named Hephaestus. The main goal of this project is to develop a robotic solution for the installation of curtain wall modules on building facades. An essential requirement is that the CDPR should safely operate close to the system constraints. It was observed that state-of-the-art control schemes do not cope with this requirement. The control strategies used in the design of such schemes are not able to consider system constraints as an integral part of the main controller.Since Model Predictive Control (MPC) is one of the few control strategies able to explicitly handle the system constraints, this thesis is focused on the design and analysis of MPC schemes for position tracking of CDPRs. Two approaches are then proposed: a linear MPC and a nonlinear MPC (NMPC).The proposed linear MPC is based on a linear approximation of the CDPR dynamic model. The Experimental tests proved that the linear MPC may safely operate close to system constraints. This capability is validated by applying a desired trajectory that cannot be performed without violating the cable tension limits. In this case, the proposed linear MPC scheme is able to perform a trajectory tracking as best as possible while satisfying the cable tension bounds. Conversely, state-of-the-art control schemes are not able to suitably respond under such conditions. Comparing the behavior obtained with the proposed linear MPC and a state-of-the-art control scheme, one may conclude that the capability to operate close to the system constraints represents an important result related to the safety of the operation of CDPRs.Nevertheless, it was noted that the proposed linear MPC may be sensitive to increased nonlinearities. The precision of positioning tracking may be deteriorated for trajectories presenting relatively high velocities. Accordingly, an NMPC able to consider the system nonlinearities is proposed. In contrast to its linear counterpart, the stability of the resulting closed-loop system could be analyzed. Details on its numerical implementation are presented and the improved performance is validated through simulations.In addition to the design of MPC control schemes, this thesis also presents contributions related to the kinematic model of CDPRs and the control of cable tensions. A Forward Kinematic (FK) algorithm considering the pulley kinematics is proposed. An explicit expression for the differential kinematics enabled the implementation of a numerical solution of the nonlinear least-squares system representing the FK problem. Its convergence capabilities are evaluated experimentally and numerically.It is worth noting that the algorithms and control schemes proposed in this thesis were implemented in an industrial software, which demonstrates the applicability of the proposed solutions in commercial applications.