PT Unknown
AU Boumann M. Sc., R
TI Damage Prevention After Cable Failure in Redundant Parallel Cable Robots
PD 04
PY 2024
DI 10.17185/duepublico/81869
LA en
DE Seilroboter; Sicherheit; Resilienz; Seilriss; Redundanz
AB The goal of this thesis is to increase safety and reliability of redundant cable-driven parallel robots (CDPRs). A CDPR uses a set of computerized winches to move a common platform carrying a robotic tool. Towards industrial application of such robots, e.g. in automated masonry work, potential cases of failure need to be considered. A rare but hazardous fault is the failure of a cable, which is focused in this dissertation. Initially, it is elaborated how this failure affects a redundant CDPR. Besides a drastic reduction of the workspace, the failure can lead to an uncontrolled platform movement. This may result in a loss of payload or a crash of the platform, potentially causing system or environmental damage and harm to people nearby. To prevent subsequent damage, two new emergency strategies are developed, utilizing the remaining cables of a redundant CDPR to bring the post-failure system into a safe state. Various emergency strategies for cable failure in CDPRs exist. However, to the authors best knowledge, none of them has been validated on a real spatial redundant CDPR with six degrees of freedom. In addition, most of the known strategies use trajectory planning along pre-defined paths, which might be time critical in case of failure. The two strategies developed, on the opposite, recreate a reflexive behavior that does not require pre-defined trajectories. The first strategy presented in this work aims at minimizing the systems kinetic energy, which leads the robot into a static force equilibrium pose. It is based on a nonlinear model predictive control forecasting the platform's movement that follows from set-point cable forces. The second strategy presented uses potential fields in the workspace that introduce virtual forces and torques to the platform. While an attractive field guides the platform into a static force equilibrium pose, repulsive fields avoid collisions with obstacles in the workspace. As the required platform wrench might be unfeasible, the so-called Nearest Corner Method is developed to obtain approximated cable force distributions in real-time outside the wrench-feasible workspace. Furthermore, a conventional CDPR controller ignoring the failed cable as well as the usage of motor brakes are considered. For low-risk testing of the developed strategies, a multibody simulation representing the SEGESTA prototype owned by the Chair of Mechatronics at the University of Duisburg-Essen is set up and a variety of simulations is conducted. Simulation results show that the conventional controller with fault tolerance and the use of motor brakes mainly stabilize the platform only in the post-failure workspace. Conversely, the two proposed emergency strategies can rescue the platform from outside of it in various scenarios while preventing it from crashing. As both methods have many parameter setting options, their influences are carefully elaborated. Based on the successful simulative assessment of the strategies, a practical examination is conducted with experiments on the SEGESTA prototype. To mimic a cable failure, a mechatronic cable decoupling device is developed and tested based on a requirement analysis. Furthermore, a simple yet effective and reliable failure detection algorithm to determine a cable failure is introduced and tested. Both emergency strategies are subsequently validated in successful rescue scenarios on the SEGESTA prototype. Additionally, the robot operation with a reduced set of cables in the post-failure workspace after successful stabilization is demonstrated. Finally, practical examples of CDPRs in the construction industry are considered, including large scale 3D-printing of concrete as well as automated masonry work. For both applications, full scale prototypes are realized within the research group during the path of this work. An approach for generating optimal trajectories based on cost functions and penalty terms is further introduced and demonstrated. In a simulated cable failure scenario of a CDPR performing automated masonry work, the emergency strategies are also applied. It is demonstrated that both approaches can prevent the platform from crashing into the already erected building structures. Moreover, a vertical pulley reconfiguration feature is used to extend the capability of the Kinetic Energy Minimization Method. This feature can also recover lost workspace after a cable failure, increasing the operational range of the robot. This allows for more effective operation until the robot is repaired and maintained. In summary, it can be stated that the developed emergency strategies for damage prevention after cable failure in redundant CDPRs are successfully validated in simulation as well as on a real prototype. Both approaches are not limited to an emergency situation and may also serve for regular CDPR control. Especially their practical validation emphasizes their benefit for upcoming industrial usage of cable-driven parallel robots and empowers the transfer of knowledge from research into practice.
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