Tethers are an underutilized tool in multi-robot systems: tethers can provide power, facilitate retrieval and sensing, and be used to manipulate and gather objects. Starting with the simplest possible configuration, our work explores how agents linked in series by flexible, passive, fixed-length tethers, can use those tethers as sensors to achieve distributed formation control. In this study, we extend upon previous work to show the applicability of strain-coordinated formation control for encapsulation and migration along a global gradient as well as the trade-offs between formation control and taxis in an obstacle-laden environment. Our results indicate significant potential for tethered robot collectives: versatile behaviors that can work on simple, resource-constrained robots or serve as a fallback mechanism in case more sophisticated means of coordination fail.

Tethers have great potential in multi-robot systems from enabling retrieval of deployed robots and facilitating power transfer, to use by the robots as a net or partition. In this paper, we show in simulation that tethers can also be used to do distributed formation control on very simple robots. Specifically, our simulated agents are connected in series by un-actuated, flexible, fixed-length tethers and use tether angle and strain, in conjunction with the physical constraints of the tethers, to adjust their position with respect to their neighbors. This presents a significant simplification over traditional formation control which, at a minimum, requires exteroceptive sensors to perceive bearing and/or distance to nearby agents. We present and evaluate an algorithm on a large set of transitions between formations with 5 agents and an example transition with 35 agents. The convergence time grows with the number of agents, however, the memory and computation time per agent remain constant. Future work will investigate the ability to use tethers and strain for reactive behaviors and more diverse tasks.