Strategies for Damage Prevention after Cable Breaks in Parallel Cable Robots (StraDaC)

• Applicant: Professor Dr.-Ing. Tobias Bruckmann
• Subject Area: Automation, Mechatronics, Control Systems, Intelligent Technical Systems, Robotics
• Term: since 2022
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 506498608

Cable robots use a system of tensioned cables in a parallel topology to move the end effector - e.g. a platform with a payload. The cables can be wound onto drums to practically any extent and therefore allow enormously large workspaces. It must be taken into account, however, that cables can only pull, but never push. Fully tensed cable robots therefore use at least one more cable than the robot's tool to be guided has degrees of freedom. This leads to a so-called redundancy, which in particular complicates a cable force calculation: The corresponding force equilibrium leads to an underdetermined system of equations with an infinite number of solutions if seven or more cables are used for a total of six degrees of freedom. At the same time, a continuous course of the cable forces is desired for motions, which must be identified among all possible solutions. Solving this problem is considered computationally intensive, especially since the cable forces must be determined in real time for control purposes. Although cable forces can be monitored during operation, failures of individual cables sometimes occur: As has been observed repeatedly in the past, the failure of a cable of a cable robot, e.g. due to a cable break, is a very critical event that can cause damage to people, equipment and the environment. Examples include a crash of the cable-guided SkyCam in December 2011, which was operating a camera over a stadium, and the catastrophic damage to the cable-guided space telescope at Arecibo in December 2020.As long as cables are still connected to the actuators after one or more cables have failed, there is still a possibility of intervention to influence the cable robot with the remaining cables. If, however, the platform of the robot is outside the workspace of the remaining rope system at the moment of the cable failure, a determination of valid cable forces by means of common methods is no longer guaranteed. An uncontrolled movement of the end effector then follows outside the workspace. Therefore, strategies are needed to deal with this case.Here, the cable breaks themselves are usually less threatening than the potential consequential damage resulting from collisions of the end effector with other objects in the workspace, e.g. products or production equipment. Furthermore, it is desirable to bring the cable robot to a standstill in a recovery position. The applicants have already developed and simulated basic strategies within the scope of their own preliminary work, which are to be generalized, extended by new strategies and transferred to experimental testing within the scope of the present application. In addition, the applicants have expertise in the field of machine learning, on the basis of which alternatives to model-based methods and strategies are to be developed and evaluated.

• DFG Programme: Research Grants
• Co-Investigator: Professor Dr.-Ing. Dieter Schramm