Online Energy Restoration by a Mobile Robot in a Ring of Sensors

As most existing sensors are powered by batteries, the coverage provided by a sensor network degrades over time and eventually disappears if energy is not restored. A common approach to energy restoration is to use a robot acting as a mobile battery charger/changer. The goal is to maintain the best level of coverage; that is, to constantly minimize the number of coverage holes and their duration. Depending on the strategy employed by the robot, clearly different results can be obtained. In this paper, we focus on sensors arranged in a ring topology and we consider the intuitive online ON-DEMAND strategy where: the robot visits the sensors in clockwise direction when aware of a pending request; a sensor whose battery is about to become depleted originates a recharging request and waits for the robot; the request is forwarded along the ring in counter-clockwise direction until it reaches either the robot or another sensor waiting for the robot. We also consider the simplest possible online strategy, BLIND, where no requests are sent and the robot automatically moves along the ring looking for needing sensors. We experimentally study the efficiency of the two strategies, and we make the counter-intuitive discovery that BLIND, which does not require communication nor computation, is as efficient as On Demand