Seite - 41 - in Proceedings of the OAGM&ARW Joint Workshop - Vision, Automation and Robotics
Bild der Seite - 41 -
Text der Seite - 41 -
Fig. 3. Grid selection for the localization, together with the topological
map [5].
tions. Each vertex is specified as a full 2D pose in the global
reference frame allowing to specify the difference from the
robots location to any frame in the graph. Furthermore, each
vertexcontainsagridmaprepresenting the localenvironment
at this position. It is ensured that every position within the
grid map can reach the center. To ensure proper connections
of vertices a connection is only made if the combination of
both grid maps allow the robot to reach one vertex from the
other.Let’sconsider thesimpleexampleofa topologicalmap
as it isdepicted inFigure3.Grid11 isclose togrid3,butdue
to the wall between these two grids, no connection between
grid 11 and grid 3 is made. Thus, the robot knows which
traversals are possible in the environment with the help of
the grid map. We will exploit this knowledge to select the
right grid for localization if the robot moves beyond the area
of one grid.
Initially, the robot knows its starting location, this is done
through the input of the user. After the robot has selected
the initial position the vertex which is the closest to the
current initial location is chosen. Additionally, the robot
should check if it can move between the initial pose to the
grid map vertex. Using this vertex, the robot can use the grid
map of the vertex to localize itself. This is done with the help
of a particle filter [7]. The particle filter uses the grid map to
align the current laser measurements with the occupied cells
in the grid map. Additionally, the robot uses the GPS signal
to localize itself. This is done by anchoring each vertex with
a GPS position. Thus, by using the current GPS signal the
robot can estimate its position relative to the currently used
vertex in the topological map. Using the grid map and the
GPS the robot derives an estimation of its current location.
If the robot is moving in the grid map the localization can be
done with the current grid map. But as we assume a large
space of the outdoor environment the robot will at some
point reach the border of the grid map. In such a case the
robot needs to decide which vertex in the topological map is
the next one to localize itself. This is done by checking the
distance to each vertex in the topological map which has a
connection to the currently used vertex. The vertex with the
smallest distance to the current robot pose is used for future localization. Thus, the robot will switch the vertex and the
occupancy grid only if it is closer to that vertex than any
other vertex which could be reached from the robot.
Let’s consider the situation in Figure 3. If the robot is
moving from grid 3 to grid 4. It checks the distance from
the vertex of grid 3 and the distance of vertex of grid 4. But
the robot does not check the distance to the vertex of grid 11
as the robot already knows that there is no possibility that it
has traveled from grid 3 to grid 11. If the distance of grid 4
is larger than the distance to grid 3 the robot will use grid
3 for future localization.
Due to the use of the connections within the topological
map one saves the effort to check all nearby vertexes if they
should be used for localization. Furthermore, more important
is that the robot will not select a vertex which cannot be
reached. Let’s consider the map in Figure 3. Grid 3 and 4
can be on the outside of the building whereas grid 11 is the
inside of the building. Thus, if the robot is localized outside
of the building it does not make sense that the robot jumps
suddenly through the wall inside the building. As we don’t
consider grid 11 as an alternative such a jump is not possible.
This also allows the robot to move close to the wall of the
building without being incorrect localized.
IV. NAVIGATION
With thehelpof the topologicalmap, the robotcan localize
itself. Using this localization, the robot can plan its path
to the destination. To do so, the robot uses a hierarchical
planningapproach.Asweconsidera large-scaleenvironment
the robot is not able to use a grid map of the complete
environment. Thus, the robot uses a road map to plan the
overall navigation. This allows the robot to plan for the large
environment in a fast manner. After a plan is found using the
road map the robot use the current grid to search for a mid-
level plan within this map to move between different nodes
in the roadmap. Finally, the robot uses a local planner to
move along the mid-level plan and avoid obstacles which
are not present in the grid map.
The road map which is used by the robot to generate a
high-level plan consists of a graph of nodes which specify
locations and edges which describe possible traversals be-
tween these nodes. A roadmap together with the high-level
plan is depicted in Figure 4. The road map is constructed by
considering the distance between the nodes and if the node
is collision free. To check if a node is collision free the
footprint of the robot and the local grid map of the position
to check is used. As the robot, has not specified a complete
description of the environment traversability, one uses the
positions used during mapping as a seed for the road map
calculation. This allows that the robot uses the positions and
traversals which were created during mapping.
To plan a path within the road map the robot first deter-
mines theclosestnodeof the roadmap to its current location.
Afterward, the closest node to the destination is determined.
The node close to the current position is the start node of
the search and the node close to the destination is the goal
node of the search. After determining these two nodes the
41
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