| Summary: | Existing strategies for coordinating a group of drones to follow a moving circular path while maintaining a specified distance between them (not necessarily identical) have generally involved circumnavigation around a single target or tracking an ellipse around multiple targets. The main drawback of tracking an ellipse is the difficulty of maintaining a constant velocity due to the variation in curvature around the ellipse. In this study, a novel strategy was developed so that drones track a tilted circle in the air whose projection becomes an ellipse on the ground that encloses multiple moving targets. By tracking a tilted circle, the drones can maintain an almost constant velocity while changing altitudes slightly with no inter-vehicle collision, which is technically much easier than tracking an ellipse directly in the air. To complete the tilted circumnavigation task, a vector-field guidance law followed by integral sliding-mode control was designed so that the following three conditions are satisfied under bounded disturbances: 1) all drones reach the plane of the tilted circle in the steady state; 2) each drone turns around the tilted circle’s center with a time-varying radius in the steady state; and 3) drones avoid colliding with each other at all times. Numerical simulations showed that the proposed strategy was effective in situations mimicking real-life scenarios.
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