The Magazine of IEEE-Eta Kappa Nu October 2017 - 18

FEATURE
robots, few have addressed achieving a desired timevarying connectivity profile. Introducing a controller
that adjusts the relative position of UAVs to track a
communication connectivity profile provides greater
flexibility in cooperative tasks, where the environment
may require UAVs to have differing levels of connectivity
for a successful mission. For example, the required
connectivity for shared information during a search
and rescue mission may be lower for an open field
compared to that of an urban environment where much
more information is captured by sensors.

III. UAV Control

This article describes a leaderless, distributed approach
for a network of cooperative UAVs to track a single ground
target while maintaining desired network connectivity. In
our approach, a UAV autonomously adjusts its position
to establish a formation around a mobile ground target
while simultaneously tracking a desired algebraic
connectivity profile.

A. Target Tracking

II. Representation of Swarm
A swarm of UAVs can be considered as a multi-agent
graph with point-mass agents as graph nodes and
inter-agent communication links as graph edges. In our
previous work [18], [19], we assumed an undirected
multi-agent graph where agents communicate through
bidirectional links. The communication exchange
topology among agents is considered as time-varying.
In a two-dimensional (2D) realization, each agent has
a circular communication range of equal radius, and an
agent is connected to its neighboring agents situated in
its circular neighborhood. The inter-agent connection
strength decreases as the corresponding relative
distance increases and the connection is lost when the
relative distance exceeds a threshold value.
The inter-agent connections in a graph can be captured
mathematically using the Laplacian matrix [8]. The
matrix describes the connections among UAVs including
weights that show the strengths of those connections.
To model actual inter-agent communication limits,
time varying weights are added to the graph edges to
simulate decreasing signal strength relative to increasing
inter-agent distances. We use an exponential decay
model, where the communication capability reduces at
an exponential rate with increasing distance between
agents. As the Fiedler value of the Laplacian matrix gives
a global connectivity measure of the network, a high
value of connectivity is typically desired when there is
a need for a large amount of inter-agent information
sharing capability.
18

In the cooperative target tracking mission, UAVs aim to fly
in a symmetric formation surrounding a moving target.
Figure 1 shows a visualization of a group of UAVs flying
in a target-centric formation. Starting from an initially
connected, but arbitrary, topology, all UAVs move toward
desired formations by utilizing appropriate control
commands. A necessary condition for the success in a
cooperative mission is to maintain the global connectivity
throughout the formation process.

The target tracking goal is to keep the mobile target at
the center of the UAV formation, thus the target tracking
goal is coupled with the formation controller. This is
accomplished by ensuring there is a distance δ and angle
ψ between each UAV and the target as shown in Figure
1. The majority of target tracking algorithms use a camera
for visual detection and extraction of the target location.
This requires the use of image processing algorithms, and
filtering techniques to estimate the 3D position of the
target using 2D information provided by the camera. As
the focus of this article is formation control, we assume
the target location can be obtained by at least one UAV.
In our experiments, the target moves in a predefined
pattern while the UAVs continually adjust their relative

Fig. 1: A desired formation of four UAVs around a target.
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http://www.brightcopy.net/allen/brid/113-3
http://www.brightcopy.net/allen/brid/113-2
http://www.brightcopy.net/allen/brid/113-1
http://www.brightcopy.net/allen/brid/112-3
http://www.brightcopy.net/allen/brid/112-2
http://www.brightcopy.net/allen/brid/112-1
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