IEEE - Aerospace and Electronic Systems - August 2022 - 30

Cooperative Control of UAVs Over an Unreliable Communication Network
give-way object detects that the stand-on object deviated
from its trajectory as pSðtkÞ 6¼ wSðtkÞ or it changed its
future trajectory wSðtÞ, ðt>tkÞ or the give-way object
changed its future trajectory wGðtÞ, ðt>tkÞ,the give-way
object determines new future communication time instants
tc;kþj and sends them as a request RGðtc;kþ1Þ to the stand-on
object, which responds by sending its data SSðtc;kþ1Þ at the
next predicted communication time instant.
SIMULATION RESULTS OF THE CONTROL METHOD
OVERVIEW
This section provides a simulation study with two quadrotors
to show the effectiveness ofthe method. Quadrotors are
used due to their complex dynamics, which makes them difficult
to control. A quadrotor has six degrees of freedom,
which makes it able to move in 3D airspace [see Figure 7
(left)]. With a variation of the individual rotor speeds ni,
ði ¼ 1; .. . ; 4Þ ofthe four motors the movement can be controlled.
The schematic structure of an UAV is illustrated in
Figure 7 (right). The model of the quadrotor and its twodegrees-of-freedom
controller is derived in [1].
Three cases are investigated, where the quadrotors are
Figure 6.
Communication flow of the event-based method.
including the initial trajectory. The give-way object confirms
the receipt ofthe data by sending an ACKmessage.
In the execution phase the give-way object determines
based on the initial information the future communication
time instants tc;kþj, ðj ¼ 1; ...;NÞ at which communication
must be invoked and sends them with a request
RGðtc;kÞ (12) to the stand-on object at the communication
time instant tc;k. If it does not respond within the time
span 2 ~tmaxðdðtkÞÞ the request is considered to be lost and
is sent again. After the reception of a request the stand-on
object sends its current data SSðtc;kÞ automatically at the
time instants tc;kþj and waits for an ACK message from
the give-way object. The give-way object waits the time
delay ~tmaxðdðtkþjÞÞ for the information of the stand-on
object. If it does not receive the data within this time span
the data are considered to be lost. As the give-way object
does not send an ACK message, the stand-on object sends
its information again after the time span 2 ~tmaxðdðtkþjÞÞ
after the communication time instant tc;kþj.
After reception of the data before or at an event time
instant tk the give-way object checks whether the stand-on
object is on its communicated trajectory as pSðtkÞ¼ wSðtkÞ
and if it did not change its future trajectory. If both is the
case, the give-way object sends an ACK message and waits
for the information of the stand-on object at the next predicted
event time instant tkþ1. On the other, hand if the
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connected by a network that induces transmission delays
and packet losses and that has the parameters stated in
Table 1.
Case 1: It is shown that the collision avoidance cannot
be ensured if the occurrence ofpacket losses is not considered
in the control method.
Case 2: Collision avoidance is ensured when packet
losses are taken into account in the control method.
Case 3: It is illustrated that the requirement on the
maximum separation can only be fulfilled with a certain
probability.
The control parameters method are given in Table 3. The
thresholds are derived based on the dynamics of the UAVs,
which also cause the rotor limitations and the angle limitations.
The safety distance is chosen so as to avoid the collisions
with respect to the size of the UAVs. The trajectories
of the quadrotors need to be planned so as to fulfill the
dynamic limitations (3) and (4). In the simulation the UAVs
move in a 16m 13m	5m region, which is covered by a
fixed coordinate system. More information about the
Figure 7.
Schematic structure of the quadrotor.
IEEE A&E SYSTEMS MAGAZINE
AUGUST 2022
IEEE - Aerospace and Electronic Systems - August 2022

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