Modeling and detection of cable collisions for collaborative Cable-Driven Parallel Robots

Abstract

This Master’s Thesis studies the problem of cable collisions with the environment for Cable-Driven Parallel Robots (CDPR) in the context of collaborative robotics. When operators are working in the same workspace as a CDPR, they are very likely to be in contact with cables at some point because these span a large portion of the workspace. Hence, this work aims at defining a way to tolerate light collisions so as to maintain a continuous workflow in a safe manner by proposing a collision detection method. In case dangerous collisions are detected, appropriate actions to mitigate their impact need to be defined.

An overview of the work present in the literature regarding CDPR safety is first presented, focusing on cable collisions and post-failure recovery. Then, collisions between the cables of a Cable-Driven Parallel Robot (CDPR) and the environment are modeled using a simple punctual contact model with a lateral force. This model returns a tension increase inside the collided cable that can be determined knowing its initial tension and the collision force. The lateral collision force can be bounded by values excerpted from international safety standards, allowing to define thresholds for the tension increase in the cables due to a collision. This model is then validated experimentally using an adapted test bench.

Then, by monitoring the tension in the cables, it is possible to determine whether the robot dangerously collided with the environment if a cable saw its tension increase over the admissible threshold determined with the collision model. An operational implementation of this detection method on a CDPR prototype is introduced and evaluated for two different collision types. A safe mitigation strategy is also proposed to limit the damage that could result of such dangerous collisions, relying on finding alternative tension distributions able to maintain the equilibrium of the platform while releasing an over-taut collided cable if the robot pose allows it. Several possibilities are also proposed if the alternative equilibrium cannot be found on the current pose, relying on previous work in the literature conducted on cable failures.