Systematically designing local interaction rules to achieve collective behaviors in robot swarms is a challenging endeavor, especially in micro robots, where size restrictions imply severe sensing, communication, and computation limitations. In such robot swarms, performing useful functions is often preconditioned on the formation of high-density aggregations which can facilitate collective signaling and information sharing. In this article, we present a systematic approach to control aggregation behaviors by leveraging the physical interactions in a swarm of 300 3-mm vibration-driven micro bristle robots that we designed and fabricated. We demonstrate the ability to control the degree of aggregation by varying the motility characteristics of the robots through global vibration frequency and amplitude inputs, after comprehensive characterization, modeling, and simulation of the locomotion dynamics and robot interactions. To quantify the degree of aggregation, we also introduce a new metric, the motility-induced phase separation index index, which unlike many existing methods does not require a scenario-specific tuning of parameters. Our investigations reveal how physics-driven interaction mechanisms can be exploited to achieve desired behaviors in minimally equipped robot swarms and highlight the specific ways in which hardware and software developments aid in the achievement of collision-induced aggregations.

This paper presents a novel steering mechanism, which leads to frequency-controlled locomotion demonstrated for the first time in micro bristle robots. The miniaturized robots are 3D-printed, 12 mm × 8 mm × 6 mm in size, with bristle feature sizes down to 400 µm. The robots can be steered by utilizing the distinct resonance behaviors of the asymmetrical bristle sets. The left and right sets of the bristles have different diameters, and thus different stiffnesses and resonant frequencies. The unique response of each bristle side to the vertical vibrations of a single on-board piezoelectric actuator causes differential steering of the robot. The robot can be modeled as two coupled uniform bristle robots, representing the left and the right sides. At distinct frequencies, the robots can move in all four principal directions: forward, backward, left and right. Furthermore, the full 360◦2D plane can be covered by superimposing the principal actuation frequency components with desired amplitudes. In addition to miniaturized robots, the presented resonance-based steering mechanism can be applied over multiple scales and to other mechanical systems.