A unified approach for different tasks on rings in robot-based computing systems

International audienceA set of autonomous robots have to collaborate in order to accomplish a common task in a ring-topology where neither nodes nor edges are labeled. We present a unified approach to solve three important problems: the exclusive perpetual exploration, the exclusive perpetual search and the gathering problems. In the first problem, each robot aims at visiting each node infinitely often; in perpetual graph searching, the team of robots aims at clearing the whole network infinitely often; and in the gathering problem, all robots must eventually occupy the same node. We investigate these tasks in the Look-Compute- Move distributed computing model where the robots cannot communicate but can perceive the positions of other robots. Each robot is equipped with visibility sensors and motion actuators, and it operates in asynchronous cycles. In each cycle, a robot takes a snapshot of the current global configuration (Look), then, based on the perceived configuration, takes a decision to stay idle or to move to one of its adjacent nodes (Compute), and in the latter case it eventually moves to this neighbor (Move). Moreover, robots are endowed with very weak capabilities. Namely, they are anonymous, oblivious, uniform (execute the same algorithm) and have no common sense of orientation. In this setting, we devise algorithms that, starting from an exclusive rigid (i.e. aperiodic and asymmetric) configuration, solve the three above problems in anonymous ring-topologies

"Semi-Asynchronous": A New Scheduler in Distributed Computing

The study of mobile entities that based on local information have to accomplish global tasks is of main interest for the scientific community. Classic models for the activation and synchronization of mobile entities are the fully-synchronous (FSync), semi-synchronous (SSync), and asynchronous (Async) models, where entities alternate between active and inactive states with different timing. According to the assumed synchronization model, very different results have been achieved in the field of distributed computing. One of the main outcomes is the big gap between the Async and the other models in terms of manageability and algorithm design. In fact, there are still many problems for which it is not known whether synchronicity is crucial for designing resolution algorithms or not. In order to better understand the Async case, here we propose a further model referred to as the semi-asynchronous (SAsync). This slightly deviates from SSync. In fact, like in SSync (and FSync), the duration of the activation of an entity is kept of fixed time whereas, like in Async, the starting instant of the activation is not fully synchronized with the possible activation of other entities. We show that for entities moving on graphs, the SSync model allows accomplishing more tasks than the SAsync that in turn allows accomplishing more tasks than the Async. Furthermore, our results show that, especially to tackle problems in the Euclidean plane, the SAsync model is already quite challenging, therefore there is no need to get involved with complications arising in the Async model

Rendezvous on a Known Dynamic Point on a Finite Unoriented Grid

In this paper, we have considered two fully synchronous $\mathcal{OBLOT}$ robots having no agreement on coordinates entering a finite unoriented grid through a door vertex at a corner, one by one. There is a resource that can move around the grid synchronously with the robots until it gets co-located along with at least one robot. Assuming the robots can see and identify the resource, we consider the problem where the robots must meet at the location of this dynamic resource within finite rounds. We name this problem "Rendezvous on a Known Dynamic Point". Here, we have provided an algorithm for the two robots to gather at the location of the dynamic resource. We have also provided a lower bound on time for this problem and showed that with certain assumption on the waiting time of the resource on a single vertex, the algorithm provided is time optimal. We have also shown that it is impossible to solve this problem if the scheduler considered is semi-synchronous

Gathering of Mobile Robots in Anonymous Trees

Gathering problem of mobile robots is a class of graph problem that has a lot of relevance in everyday life. The problem requires a set of mobile robots, initially located at different nodes of a graph, to gather at the same location in the graph, which is not decided before. This report considers the gathering problem of mobile robots in anonymous trees. The robots considered here are identical, do not communicate directly with other robots and also, all the robots execute the same algorithm to achieve gathering. Robots are assumed to have minimal capabilities with respect to the memory associated with them as well as their visibility capability. In this report, three models have been proposed for solving gathering problem under three different scenarios. Possible solutions in each of these models have been described. The current work that has already happened and the future work that can be done in each model have also been mentioned