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Fig. 2. System overview of the transport robot [5].
safely navigate between buildings. The robot is depicted in
Figure 1 and is based on a pioneer 3-AT platform which
allows the robot to navigate indoors as well as outdoor.
Additionally, the robot has three laser scanners to detect
obstacles. Two laser scanner are mounted horizontally to
detect obstacles, like cars or people. Furthermore, these
two lasers are used to localize the robot. The third laser
is mounted tilted down to scan the ground in front of the
robot. This laser is used to build a map of the local terrain.
Besides the laser scanners, the robot has a GPS sensor for
the localization and mapping. To improve the accuracy of the
odometer of the robot an inertial measurement unit (IMU)
is used which is mounted on the robot. To perform the
transportation task, the robot uses the system architecture
depicted in Figure 2.
The robot uses its sensors to estimate the current location.
This is done using the robots odometer, the IMU, GPS and
the horizontal laser scanners. Due to this redundancy, the
estimation of the current location is stable in areas where one
sensor may yield wrong results, e.g. the GPS sensor near tall
buildings. To perform the estimation, the robot matches the
sensor readings with the information of a topological map.
The topological map consists of several small maps which
are linked to each other to allow the robot to only keep small
maps to be localized.
Using the estimation of the current location together with
a road map, the robot plans a high-level path for naviga-
tion. The roadmap describes possible traversal routes in the
environment on a higher level. Due to this abstraction, the
planning can be done very efficiently even in the case of
large environments. After generating a high-level plan, the
plan is passed to the lower-level planner which tries to find
a valid path in the environment for each path segment in
the high-level plan. This is done by considering the current
small local map of the environment as well as the sensor data
which are used to build a cost map. If a valid path is found
the robot tries to follow this path as accurate as possible.
To incorporate the information of the terrain the robot uses
the tilted laser to perform a terrain analysis. Afterward, the
resultsof this terrainanalysisareused toupdate thecostmap. Thus, holes, as well as small objects which are below the
horizontal laser scans but bigger than the robots clearance,
are added to the cost map as obstacles. This allows the robot
to consider the terrain in the low-level planning.
In the following two sections, we will discuss the local-
ization as well as the navigation in more detail.
III. LOCALIZATION
Starting from an initial known position the robot needs
to know its location during the entire delivery. This is done
throughonepartof the robot systemwhich isused to localize
the robot. This localization should ensure that the robot has
an estimation of its global position. First, the robot corrects
its odometer to get a good estimation of its 2D position using
dead reckoning. Afterward, it uses the created topological
map to localize itself.
To correct the odometer of the robot we use an unscented
Kalman filter (UKF) [6]. The Kalman filter uses the raw
odometer of the robot to perform a prediction of the robot
pose.Thisprediction is formedinaprobabilisticmannerwith
a position and a covariance matrix specifying the uncertainty.
The covariance matrix is defined in such a way that the linear
speed has a higher accuracy as the rotational speed, as the
rotation is badly estimated through the raw odometry due
to the slippage of the wheels during rotation. To correct the
prediction the IMU data are used. The IMU data is used to
provide an additional estimation of the robots velocity in all
three axes as well as the global orientation the robot has in
space. As in the case of the raw odometry, the IMU data
update the estimate in a probabilistic manner with the help
of a covariance matrix. The covariance matrix for the IMU
data is formed in such a manner that the rotational speed is
estimated more accurately than with the raw odometry. Due
to the use of the Kalman filter, we have a better estimation
of the robot pose instead of the very noisy raw odometer of
the robot.
After the odometer is corrected the robot can perform its
estimation on the topological map. The topological map is
a graph with vertices which represent positions in the world
and edges which represent connections between those posi-
40
Proceedings of the OAGM&ARW Joint Workshop
Vision, Automation and Robotics
- Titel
- Proceedings of the OAGM&ARW Joint Workshop
- Untertitel
- Vision, Automation and Robotics
- Autoren
- Peter M. Roth
- Markus Vincze
- Wilfried Kubinger
- Andreas Müller
- Bernhard Blaschitz
- Svorad Stolc
- Verlag
- Verlag der Technischen Universität Graz
- Ort
- Wien
- Datum
- 2017
- Sprache
- englisch
- Lizenz
- CC BY 4.0
- ISBN
- 978-3-85125-524-9
- Abmessungen
- 21.0 x 29.7 cm
- Seiten
- 188
- Schlagwörter
- Tagungsband
- Kategorien
- International
- Tagungsbände