The following technical report is available from
http://aib.informatik.rwth-aachen.de:
Experimental Validation and Mathematical Analysis of Cooperative
Vehicles in a Platoon
Hilal Diab
AIB 2015-07
The infrastructure of roads in many countries is congested because of
the rapid increase of the amount of traffic flow in the past few years.
A significant part of this is due to the increase of freight transport,
which is continuing to grow. Nowadays, safety on the roads and saving
fuel should also be taken into consideration while finding the solutions
to road congestion. One suggested solution to the problem is to increase
the capacity of the highways.
This can be realized by forming platoons of vehicles and automatically
controlling the vehicles in order to maintain short, but at the same
time, safe distances between the different vehicles within the platoon.
The development of intelligent sensors and electronic control systems
within vehicles make autonomous driving in platoons possible. Enabling
vehicular wireless communication between the items of the platoon
improves the performance of the controlled platoon significantly.
Furthermore, enabling the platoon to communicate with other traffic
members, such as other vehicles, intelligent traffic lights and
infrastructure road units, allows the platoon to perform more complex
driving scenarios for autonomous vehicles, such as crossing intersections.
The first part of this thesis investigates the behavior of the platoon
when only communication between its items is possible. The safety of the
vehicles within the platoon is investigated, taking the effects of the
communication faults on the behavior into account. This analysis could
be helpful in the process of controller design, where the developed
controller should ensure stability despite network failures and should
achieve an optimal performance in every situation. Therefore, the
verification of the controller behavior was investigated by formal
verification methods: a reachability analysis of a dynamic and hybrid
system. The safety of practical relevant scenarios was checked.
In addition, a hardware platform was set up to test the platoon's
behavior under the influence of hardware shortage, such as noises and
time delays caused by hardware components. A 1:14 scaled platoon of four
trucks equipped with sensors and WiFi modules was designed. This
platform was used for testing different cooperative vehicle platoon
controllers by examining their performance and influence on the safety
in case of communication problems within the platoon.
The second part of this thesis studies the behavior of the platoon when
communication between the platoon and the intersection road unit is
enabled. The following scenario has been considered: When a platoon of
autonomous vehicles following a leader approaches an intersection, the
platoon should change its highway mode to other modes in order to cross
the intersection safely and efficiently. To realize that, information
about the actual position of the platoon together with information of
other vehicles in the intersection area are needed. Based on this
information the platoon has to decide which mode should be performed.
Therefore, we extended our platform with an indoor positioning system
which is able to provide the position to the objects in a test
environment independently. An intersection management system was
implemented in order to test different scenarios related to different
crossing modes of the platoon. Results showed that platoons can be
controlled efficiently and safely while crossing the intersection.
The following technical report is available from
http://aib.informatik.rwth-aachen.de:
Reachability Analysis of Non-Linear Hybrid Systems Using Taylor Models
Xin Chen
AIB 2015-09
With the ubiquitous use of computers in controlling physical systems, it
requires to have a new formalism that could model both continuous flows
and discrete jumps. Hybrid systems are introduced to this purpose. A
hybrid system, which is modeled by a hybrid automaton in the thesis, is
equipped with finitely many discrete modes and continuous real-valued
variables. A state of it is then represented by a mode along with a
valuation of the variables. Given that the system is in a mode l, the
variable values are changed continuously according to the Ordinary
Differential Equation (ODE) associated to l, or discretely by a jump
starting from l. The thesis focuses on the techniques to compute all
reachable states over a bounded time horizon and finitely many jumps for
a hybrid system with non-linear dynamics. The results of that can then
be used in safety verification of the system.
Although a great amount of work has been devoted to the reachability
analysis of hybrid systems with linear dynamics, there are few effective
approaches proposed for the non-linear case which is very often in
applications. The difficulty is twofold. Firstly, it is not easy to find
an over-approximation with acceptable accuracy for a set of the
solutions of a non-linear ODE. Secondly, to detect and compute the
reachable states under a jump requires solving non-linear real
arithmetic problems which is also difficult in general. In the thesis,
we present our approaches to deal with the above difficulties. For the
first one, we present the use of Taylor models as the over-approximate
representations for non-linear ODE solutions. Our work can be viewed as
a variant of the Taylor model method proposed by Berz et al., such that
we are able to efficiently deal with some examples with more than $10$
variables. Besides, we also extend the work of Lin and Stadtherr to
handle the ODEs with bounded time-varying parameters. For the second
difficulty, we present two techniques: (a) domain contraction and (b)
range over-approximation to compute an enclosure for the reachable set
from which a jump is enabled. They can be seen as Satisfiability Modulo
Theories (SMT) solving algorithms which are specialized for the
reachability analysis of hybrid systems. In order to reduce the
computational cost, we also propose different heuristics for aggregating
Taylor models. Besides the above contributions, we describe a method to
fast generate Taylor model over-approximations for linear ODE solutions.
Its performance is demonstrated via a comparison with the tool SpaceEx.
To make our methods accessible by other people, we implement them in a
tool named Flow*. To examine the effectiveness, we thoroughly compare it
with some related tools which are popularly used, according to their
functionalities, over a set of non-trivial benchmarks that are collected
by us from the areas of mechanics, biology, electronic engineering and
medicine. From the experimental results, the advantage of Flow* over the
other tools becomes more apparent when the scale of the system grows. On
the other hand, it also shows that Flow* can be applied to analyzing
realistic systems.