Drones build a walkable bridge

The pursuit is multi-disciplinary, requiring the development of nonstandard material systems, advanced digital design and construction processes and adaptive strategies for controlling the aerial robots as they interact with their environment and cooperate in the assembly task. Because the structures produced in this framework will be less constrained by conventional assembly parameters (such as, for example, the need of scaffolding to build from the ground upward), ETH expect that its work will foster new forms of architecture and construction methods.

The research carried out at the Institute for Dynamic Systems and Control focuses on physical human-quadrocopter interaction and contact with the environment and aerial assembly of tensile structures.

Flying machines offer a number of advantages compared to traditional construction machines. Specifically, they can reach any point in space and fly in or around existing objects. However, they also have drawbacks, such as limited payload and accuracy. Tensile structures fit this combination of characteristics and constraints very well.  

Eventually, a rope bridge that can support the crossing of a person was assembled by quadrocopters. The rope bridge acts as a demonstrator, showing for the first time that small flying machines are capable of autonomously realising load-bearing structures at full-scale and proceeding a step further towards real-world scenarios. Except for the required anchor points at both ends of the structure, the bridge consists exclusively of tensile elements and its connections and links are entirely realised by flying machines. Spanning 7.4m between two scaffolding structures, the bridge consists of nine rope segments for a total rope length of about 120m and is composed of different elements, such as knots, links and braid.

The vehicles are equipped with a motorised spool that allows them to control the tension acting on the rope during deployment. A plastic tube guides the rope to the release point located between two propellers. The external forces and torques exerted on the quadrocopter by the rope during deployment are estimated and taken into account to achieve compliant flight behaviour. The rope used for these experiments is made out of Dyneema, a material with a low weight-to-strength ratio and thus suitable for aerial construction.

Aerial construction requires UAVs to physically interact with their environment. One possibility for achieving this is hybrid force-position control strategies. These are often used in assembly tasks with robotic manipulators, such as the peg-in-hole problem. Generally speaking, force control is used whenever compliance with the environment is necessary. Furthermore, force control enables physical human-quadrocopter interaction.

Physical human-robot interaction has been largely treated in the context of humanoid/mobile robots, however, physical interaction between a person and a UAV has seen little research activity. Reliable and safe physical interaction with UAVs will be of great importance in real world applications, where robots will tightly cooperate with humans, such as, for example, on a construction site. The company use a control strategy based on admittance control that can be used for both human-vehicle interaction and force tracking and makes use of a force estimator.