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An Efficient Path Planner for Large Mobile Platforms in Cluttered Environments

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  • Dubai Futue Labs

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This paper presents a one step smooth and efficient path planning algorithm for navigating a large robotic platform in known cluttered environments. The proposed strategy, based on the generation of a novel search space, relies on non-uniform density sampling of the free areas to direct the computational resources to troubled and difficult regions, such as narrow passages, leaving the larger open spaces sparsely populated. A smoothing penalty is also associated to the nodes to encourage the generation of gentle paths along the middle of the empty spaces. Collision detection is carried out off-line during the creation of the configuration space to speed up the actual search for the path, which is done on-line. Results prove that the proposed approach considerably reduces the search space in a meaningful and practical manner, improving the computational cost of generating a path optimised for fine and smooth motion
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An Efficient Path Planner for Large Mobile
Platforms in Cluttered Environments
Tarek Taha, Jaime Valls Mir´
o and Dikai Liu
ARC Centre of Excellence for Autonomous Systems
Mechatronics and Intelligent Systems Group
University of Technology Sydney
NSW2007, Australia
{t.taha, j.vallsmiro, d.liu}@cas.edu.au
Abstract This paper presents a one step smooth and efficient
path planning algorithm for navigating a large robotic platform
in known cluttered environments. The proposed strategy, based
on the generation of a novel search space, relies on non-uniform
density sampling of the free areas to direct the computational
resources to troubled and difficult regions, such as narrow
passages, leaving the larger open spaces sparsely populated. A
smoothing penalty is also associated to the nodes to encourage the
generation of gentle paths along the middle of the empty spaces.
Collision detection is carried out off-line during the creation of
the configuration space to speed up the actual search for the path,
which is done on-line. Results prove that the proposed approach
considerably reduces the search space in a meaningful and
practical manner, improving the computational cost of generating
a path optimised for fine and smooth motion.
I. INTRODUCTION
The problem of computing a collision-free path for a
moving object among obstacles is well known in the field of
robotics, and has been an active research topic for decades [4].
The majority of the proposed algorithms transform the problem
into a pure geometric path planning problem by defining the
search in what is known as configuration space, or C-space,
an approach originally introduced by Lozano-Perez [2], [5].
Here, a robot with kdegrees of freedom can be described by
kvalues, which can in turn be considered as a single point
in a k-dimensional C-space of the robot. This configuration is
considered free if two parts touch and blocked when two parts
overlap. For mobile robots operating in flat ground, C-space is
usually defined as a set of all possible configurations encoding
the position and orientation of the vehicle. A collision free
feasible path is that connecting the start and goal point
configurations. Also, a holonomic characteristic is normally
assumed, which holds for the case of differential-drive robots
like a wheelchair.
The exact construction of the C-space is however a com-
putationally expensive solution to the path planning problem.
The need to move away from complete path planning algo-
rithms inspired the development of sampling-based techniques.
Hence, the majority of techniques make further assumptions
and construct approximate representation of the C-space using
sampling-based techniques. These techniques provided a faster
practical solution by sacrificing completeness, in which a set
of sampling points are used to represent the C-space that is
used in constructing solutions. Traditionally, sampling-based
algorithms are based on uniform sampling which considers
the whole environment as uniformly complex and thus the
overall sampling density will be equivalent to the density
needed by the most complex region. The result is that every
region in the C-space has the same computational complexity,
reaching its worst case when narrow passage areas exist in the
environment [1]. Furthermore, paths produced by randomised
planners usually contain non-smooth segments because of this
randomness and the absence of optimisation criteria.
For the problem of navigating large robots in narrow and
cluttered environments, such as as ”intelligent” wheelchairs
in the average home surroundings, conventional path planning
algorithms based on free C-space construction also tend to fail:
in order to be able to consider the robot as a k-dimensional
point, they generally expand the obstacles in an over-simplistic
manner by the length of the larger robot dimension, which very
often will prevent reaching a solution even when it exists [5].
In this paper we propose a hybrid path planning algorithm
inspired by the C-space approach, where we avoid the com-
putational complexity of generating a denser search area by
employing a non-uniform sampling density: this is increased
in complex areas, leaving simple areas with lower resolution
density, hence directing computational resources towards the
complex areas, also know as narrow passages. A reduction of
the information embedded in the C-space, and a smoothing
cost function are also introduced to generate smoother paths
in an efficient manner. The algorithm takes further advantage
of techniques like the bridge test [3] and an optimised obstacle
expansion method to further reduce the number of samples and
the points to be check for obstacle collision. A modified A*
search is then implemented to find suitable paths on this space.
The remainder of this paper is organised as follows: latest
proposals to the path planning problem and where our ap-
proach represents an improvement for the problem at hand is
analysed in-depth in Section II. The proposed methodology
for the creation of the search space is presented in Section III,
with Section III-A.5 explaining the non-uniform random dis-
cretisation. Section III-B summarises the customisations to the
A* graph search technique to take advantage of the search
framework proposed here. Detailed experimental setup and
c
2006 IEEE RAM 2006
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connecting distance, and the computational complexity: shorter
distances between the nodes will result in fewer angular and
position discretisations (fewer neighbours) and less impact
on computation, but might decrease the possibility of finding
a path. On the other hand, longer distances will produce a
larger number of connections, hence increasing the chances of
finding a solution at the expense of complexity. A reasonable
compromise is to connect nodes with a distance equivalent
to the distance used to sample the uniform regular grid. This
ensures continuity in node connections and at the same time
results in a faster search.
6) Collision Detection: Finally, those connections that
cause a collision of the platform with the obstacles are elimi-
nated. The connections between nodes determines the possible
orientations of the robot should it follow that path. The center
of the circles that describe the area of the robot along that path
can be rotated and translated accordingly. Hence we can then
determine if any of them falls into an occupied area or not,
removing those conecting nodes that will cause a collision.
The result will be a collision free search space of connected
nodes in which to efficiently carry out the on-line search for
a path.
B. On-line Path Planning
The A* path planning algorithm [13] is a well known
technique, well regarded for its accuracy and calculation speed
in searching for an optimal solution. A* works by exploring
nodes based a cost function which is the sum of g(n), the
cost from the start node to node n, and the estimated cost
from node n to the goal h(n). It uses an heuristic search to
estimates the cost to the goal node and minimises the cost of
the path so far. A* is optimal if the estimated cost to the goal
is always underestimated. Since the shortest distance between
two points is a straight line, euclidean distance serves as an
excellent estimated cost to goal, making A* well suited for fast
computations. In the algorithm proposed here, the cost function
J(d)combines the sum of the partial path distances d,the
sum of all the distances travelled as a result of changing
orientation θx where xis the length of the axis of the rear
wheels, the sum of the clearance penalties previously computed
offline - which is directly proportional to the distance d- and
the number of reversals (backward motion) in the path nrev.
The cost function, defined as follows:
J(d)=d+θx+dC
p+nrev (3)
encourages the robot to avoid whenever possible turns and
reversing actions, while at the same time directing it towards
the middle of free space. The result is a smooth and secure
path - in the context of the obstacles around the platform -
efficiently generated in a single step.
IV. EXPERIMENTAL RESULTS
In order to compare the efficiency of the proposed al-
gorithm, we first compare the performance of a traditional
uniform regular sampling C-space with our random non-
uniform sampling approach for a simple navigation problem.
Fig. 4. Uniform sampling: the whole environment is equally mapped with a
constant density of search nodes
Fig. 5. Non uniform sampling: density is increased around tight places,
leaving open spaces with a lower number of search nodes
The simulations were carried out in a PC with a 1.8 GHz
Pentium IV processor and 512 Mb RAM. A map of size
45x20mwas discretised uniformly by 0.1mto generate the
search space, a detailed section of which is shown in Figure 4,
where white spaces represent the obstacles in the map after
having bee expandedn. This is the same density employed
to discretised the tight passages with the non-uniform sample
method proposed here, where a bridge test with 2mlength was
exercised. The resulting search map is that depicted in Fig-
ure 5, where a 0.2muniform sampling density was employed
for the open spaces. In both cases the obstacle expansion was
set to 0.2m, and the starting and goal configurations were the
same. Search technique was also the same in both cases, so that
the final paths produced were expectedly similar, as seen by
the sequence of rectangles which represent the configurations
of the wheelchair at each step on the planned paths based
on the wheelchair footprint. However, the comparison results
in table I clearly show the computational advantages of the
random sampling technique proposed here.
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results with simulation and a real wheelchair robot are pro-
vided in Sections IV and IV-A respectively. Finally, Section V
summarises the contribution of this paper.
II. BACKGROUND
In general terms, the sampling algorithms developed to
construct an approximate representation of the free space
currently available can be divided into two: single-query and
multiple-query approaches. Multiple-query approaches starts
with a pre-processing step that usually takes a large amount
of time but makes solving path planning problems in the
same environment faster. Probabilistic Roadmaps (PRMs) [6]
is an example of a multi-query approach that initially used
uniform sampling in constructing the path. This method was
problematic because the entire C-space will be sampled with
a density required by the most complex area of the environ-
ment, such as a narrow passage area. Nowadays, PRMs are
moving into a non-uniform methods for sampling such as the
Gaussian sampling method [10], and the bridge test to insure
that most of the configurations in C-space are actually close
to obstacles or inside a narrow passage, thus reducing the
unnecessary samples and decreasing the computational time.
Single-query methods were developed to avoid the large pre-
computational time that the multi-query methods take, and
they have been proved to be efficient [15], [16]. Randomly-
exploring Random Trees (RRTs) [12], [18] are mainly based on
single-query methods. They have gained popularity from their
good performances, which has lead to a number of extensions
specifically targeting the solution to complicated geometrical
problems [17], such as the deterministic resolution-complete
alternatives that have been proposed to replace the random
sampling methods in [19].
In many cases, an optimal and not just a feasible path is
required. As a result of randomness, the paths generated by the
execution of the above planners are very often sub-optimal and
non-smooth. A two-phase approach was proposed in [8] to op-
timise paths generated in the special case where the first-phase
path planned is made up of straight line segments connected
by way-points. Another two-phase planning algorithm based
on RRT was developed in [9]. This algorithm can compute
low cost paths given a desired cost function by a numerical
gradient descent algorithm that minimises the Hamiltonian of
the entire path.
The approach proposed here is based on a simple multi-
query one-phase planner that addresses the optimality and
smoothness weaknesses of probabilistic path planning algo-
rithms, a requirement for the general case of fine motion
platforms, such as wheelchairs. The planner uses an a-priory
map of the environment to calculate an offline, minimal free
search space, where and additional smoothness cost function is
used to address the issues associated with smoothly navigating
a large robot in an environment with narrow passages and
obstacles.
III. PATH PLANNING ALGORITHM
The proposed algorithm starts by generating the search
space to containing information about the node position, the
(a) Before Expansion (b) After Expansion
Fig. 1. Largest robot dimension obstacle expansion method
connections to neighbouring nodes, a path smoothing penalty
which will be used later to fine-tune the path during the on-line
planning, and a collision detection method. The on-line path
planning step consists of a path search using the A* algorithm
with a modified cost function to favour smooth fine motions.
The result is an optimal and smooth path that can be quickly
generated in one step.
A. Generating the search space
The pre-processing step aims at minimising the on-line
computation by pre-generating a search space to contain all the
information that will be used during the on-line path planning,
while at the same time avoid generating an unnecessarily
complete and complex space. The steps used during the search
space creation can be defined as follows:
Algorithm 1 C-space generation
Input: map, robot dimensions
1. Expand Obstacles.
2. Generate Regular Grid with low resolution.
3. Apply bridge test to add dense narrow passages.
4. Penalise nodes by adding a smoothing cost.
5. Connect nodes to form search space discretisation.
6. Eliminate those that cause collision.
Output: free search space.
1) Obstacle expansion: In this step obstacles are enlarged
with a radius Rto simplify the on-line collision detection by
reducing the number of points on the robots to be checked
for collision. It also reduces the number of nodes in the
search space thus increasing the on-line path planning perfor-
mance. The traditional approach [14] is based on expanding
the obstacles by a radius requivalent to the robots largest
dimension, hence planning as if the robot could navigate as a
point in the environment, as depicted in Figure 1. This over-
simplification, however, is not suitable for the case of large
robots in constrained spaces, as expanding the obstacles along
narrow passages will effectively block the passage, as shown
in Figure 2.
A more suitable solution is proposed by finding the largest
possible expansion radius Rthat allows the robot to pass
through the narrowest path and then divide the area of the
robot into circles of that radius, as depicted in Figure 3. The
centre points of those circles will then be used to check for
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Fig. 2. Narrow passage blocked as a result of largest robot dimension obstacle
expansion
Fig. 3. The area of the robot is covered by circles of radius R, the centres
of these circles will be the points to be checked for collision
obstacle collision. The expansion radius Ris determined based
on the a-priory knowledge of the environment: suppose that
the narrowest passages is of width land the largest robot
dimension is rthen the largest expansion that allows the robot
to pass through can be determined by:
R=lε
2if l<2r
rotherwise (1)
where εis a minimal safety distance to make sure the platform
does not get uncomfortably close to the obstacles.
2) Regular grid discretisation: The C-space is then popu-
lated with nodes using a low resolution regular grid. This will
help in maintaining the connectivity of the graph by defining a
minimum discretisation for the open spaces. The discretisation
density is adjusted to suit the environment, selecting as sparse
a grid as possible. Up to this stage the nodes hold only position
information.
3) Bridge test: Narrow passages in free space are small
regions critical in preserving the connectivity of a path during
the path planning process. Any attempt to sample the narrow
passages using a uniform distribution based on volume will
fail precisely because of their small volumes. The bridge test
[3] was introduced to boost the sampling density inside narrow
passages using only a simple test of the local geometry. Narrow
passage can be usually defined as a space where the motion
of the robot is restricted in at least on direction and any small
changes in the robots configuration in that direction may result
in a collision with obstacles. In these passages, robot motion is
limited to those directions perpendicular to the restricted ones.
A short line segment of length dcan sample randomly through
a point min the free space such that the end points of the line
segment lie in obstacles. This line segment is what we call a
bridge because it acts like a bridge across the narrow passage
with its endpoints in an occupied location and the point min
a free space. If we are able to build a bridge through point m,
then the bridge test is successful at this point and point mis
added to the search space.
Algorithm 2 Bridge Test
1. repeat
2. Pick a point pfrom the regular-grid map
3. If pis in an occupied location then
4. Pick a point pthat is ddistance away from p
5. If pis in an occupied location then
6. Let mbe the midpoint of ppline segment
7. If mis in a free location then
8. Insert minto the search space as a new node
Building short bridges is easier in narrow passages than
in free space and by favouring short bridges we increase the
chance of getting point in the narrow passages. The off-line
test increases the density of free-space sample points to our
search space where it matters most, in the narrow passages,
instead of the whole region. Figure 5 (further described later
in Section IV) shows the result of the bridge test on a map
with narrow passages.
4) Clearance (smoothness) penalty: Costs are added to the
nodes in C-space to indicate how far they are from an obstacle.
The cost Cpis a normalized cost that is inversely proportional
to this distance d, so that the closer the point is from an
obstacle the higher its cost, according to:
Cp=Dd
D(2)
where Dindicates the clearance distance beyond which the
node will be assigned a zero cost. This cost will be used during
the on-line path planning process to plan smoother paths in one
step, and no further smoothing step will be necessary after the
path is generated. The end results are paths which tend more
towards the middle of empty spaces and are within a safe
distance from the obstacles.
5) Node connections: In order to find a path among the
resulting nodes, these need to be connected together. This is
done by establishing a link between each node and its neigh-
bouring nodes a certain distance away. The more neighbours
a node is linked to, the more discrete poses (position and
orientation) with be available during the search for a viable
path. However, there should be a compromise between the
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Fig. 6. A case where the wheelchair robot has to go through two narrow
doors
TAB L E I
COMPARISON OF UNIFORM AND NON-UNIFORM C-SPACE GENERATION
Uniform Sampling Random Sampling
no. nodes 32364 15634
no. connections 691155 129479
time 1.8589 sec 0.3228 sec
A more challenging path planning problems where the
mobile robot had to manouvre to pass through two tight and
narrow passages is depicted in Figure 6. It can be seen how
the addition of search nodes where it really matters not only
makes finding a collision-free path feasible, but the additional
penalties also tend to direct the robot as far as possible from
obstacles along a smooth path.
A. Testing on a hardware platform
The real-time feasibility and smoothness of the approach
was also tested in a real mobile platform, a commercial
electric-powered wheelchair. The platform has two differen-
tially driven wheels at the rear, and two passive casters at
the front, and can travel at speeds of up to 15km/h. The
wheelchair, depicted in Figure 7, was instrumented with a
computer (attached behind the back rest), wheel encoders
and a laser range finder used for localisation. The functional
architecture of the system is described by the block diagram
of Figure 8.
The computing platform comprises of a 1 GHz Pentium
III processor with 256Mb RAM that communicates with the
two optical wheel encoders through the serial port. A laser
rangefinder is located on the foot rest at the front of the
wheelchair and also communicates via a serial link with
the computer. The actual control of the wheelchair motion
takes place through an interface DAC box that is attached
to the joystick and simulates the standard command signals
that control the normal funcioning of the wheelchair as if
a user was controlling it, such as activating the motors,
increasing/decreasing the gears or sounding the horn amongst
others. The wheelchair measures 1.2x0.7m, by all accounts a
Fig. 7. Instrumented autonomous wheelchair platform
Fig. 8. Mobile platform navigational architecture
large robot when driving around a typical office environment
with narrow passages, long corridors and cluttered static ob-
stacles. Tests were fully autonomous in that the wheelchair
was commanded to plan a path in the given map from a start
configuration to a goal configuration. A simple linear controller
based on displacement and orientation error was implemented
to traverse the path.
Figure 9 shows the result of navigating a similar path to that
depicted in Figure 5. Velocity of the wheelchair was constant
during the experiment at 0.2 m/sec. Localising the robot was
done through the Adaptive Monte Carlo Localization (AMCL)
algorithm [7] with the aid of the laser range-finder. A true
estimation of the location was provided every 2 seconds, and
dead-reckoning based on odometry and the kinematic vehicle
model was employed in between these updates to establish
the location of the vehicle. The goal was reached as shown in
Figure 9 shows how close the path was followed, achieving
the goal position within 3 cm and 5 degrees linear and angular
error. Similar results were obtained for other paths generated
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Fig. 9. Result of navigating a path generated by the planner, where the thick
line represents the feedback position as estimated by the localizer, and the
thin line represents the actual planned path
in this environment.
V. C ONCLUSION AND FURTHER WORK
This paper has presented a new approach for generating
optimized smooth paths afor large platforms in constrained
spaces using a novel C-space creation method. A non-uniform
sampling technique has been proposed to efficiently target
the narrow passage problem, of particular relevance for such
large mobile platforms in cluttered environments. Results from
simulation and a real-time implementation in an automated
wheelchair have shown that the path planner was able to plan
one-stage smooth feasible paths, quickly and efficiently due to
the simplicity of the search space method prposed.
While these preliminary results have shown the feasibilty
of the motion planner to generate and navigate suitable paths
in what represents a challenging, if static, environment for
a large mobile platform, the next step is the introduction of
dynamic obstacles, which is by no means trivial. Further, the
current cost function gives preference to on-the-spot rotations,
circumventing non-holonomic constraints which could also be
taken into account. Also, efforts are being directed to apply
the advances presented here to the fine motion planning and
control of a user-driven wheelchair, effectively allowing the
user think he/she is a better driver than he/she really is.
ACKNOWLEDGMENT
This work is supported by the Australian Research Council
(ARC) through its Centre of Excellence programme, and by
the New South Wales State Government. The ARC Centre of
Excellence for Autonomous Systems (CAS) is a partnership
between the University of Technology Sydney, the University
of Sydney and the University of New South Wales.
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... In our previous work [2] we addressed the problem of designing an efficient near optimal global planner for mobile platforms. The proposed solution was proved to be time efficient and feasible even in the case of large robots in narrow passages, and compared favourably to currently available static path planning algorithms such as RRTs and PRMs. ...
... The main contribution of this paper lies with the framework proposed to ensure safe and optimal navigation of a robot in uncertain dynamically changing scenarios. Given a partially known model of the environment, a higher-level planner [2] is employed for describing an optimised path from source to destination, and also for constantly monitoring the traversibility of the waypoints to trigger a global re-planning when the feasible path gets obstructed. An innovative strategy is then presented to handle underlying local navigation issues in between waypoints. ...
... In this section an overview of the proposed path planner algorithm is given. Further details can be obtained from [2]. ...
Conference Paper
Full-text available
A novel method which combines an optimised global path planner with real-time sensor-based collision avoidance capabilities in order to avoid moving obstacles (e.g. people) in a complex environment is presented. The strategy is based on a time efficient one step path planning algorithm for navigating a large robotic platform in indoor environments. The planner, which has been proved to compare favourably to currently available path planning algorithms such as Randomly-exploring Random Trees (RRTs) and Probabilistic Road Maps (PRMs) in known static conditions, is enhanced here with a modified Variable Speed Force Field (V SF 2 ) mechanism to accommodate for dynamic changes of the environment. The basic concept of the modified DV SF 2 is to generate a continually changing parameterised familiy of virtual force fields for the robot based on characteristics such as location, travelling speed, heading and dimension of all the objects present in the vicinity, static and dynamic. The interactions among the repulsive forces associated with the various obstacles provide a natural way for local collision avoidance and situational awareness. This is harnessed here by locally modifying the planned behaviour of the moving platform in real time, whilst preserving as much as possible the optimised nature of the global path. Furthermore, traversability of the path is continually monitored by the global planner to trigger a complete re-planning from the robot’s current location in the case of major changes to the environment, most notably when the path is completely blocked by an obstacle. Overall, a complete solution to the navigational problem in partially known cluttered environments is provided.
... In this paper we present a time efficient one step path planning algorithm for navigating a large robotic platform in indoor environments. The proposed strategy, based on the generation of a novel search space [1], relies on non-uniform density sampling of the free areas to direct the computational resources to troubled and difficult regions, such as narrow passages, leaving the larger open spaces sparsely populated. A smoothing penalty is also associated to the nodes to encourage the generation of gentle paths along the middle of the empty spaces. ...
... An overview of the proposed algorithm is given below. Further details can be obtained from [1]. ...
... It can be seen how the addition of search nodes where it really matters not only makes finding a collision-free path feasible, but the additional penalties also tend to direct the robot as far as possible from obstacles along a smooth path. Further details of the actual implementation on a real autonomous wheelchair can be found in [1]. ...
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In this paper we present a time efficient one step path planning algorithm for navigating a large robotic platform in indoor environments. The proposed strategy, based on the generation of a novel search space [1], relies on non-uniform density sampling of the free areas to direct the computational resources to troubled and difficult regions, such as narrow passages, leaving the larger open spaces sparsely populated. A smoothing penalty is also associated to the nodes to encourage the generation of gentle paths along the middle of the empty spaces. Collision detection is carried out off-line during the creation of the configuration space to speed up the actual search for the path, which is done on-line. Results compared to currently available path planning algorithms such as Randomly-exploring Random Trees (RRTs) and Probabilistic Road Maps (PRMs) proved that the proposed approach considerably reduces the searching time and produces smoother paths with less jagged path segments than those from randomized planners.
... In our previous work, Taha et al. (2006b), we addressed the problem of designing an efficient near optimal global planner for mobile platforms. The proposed solution was proved to be time efficient and feasible even in the case of large robots in narrow passages, and compared favourably to currently available static path planning algorithms such as RRTs and PRMs. ...
... The main contribution of this paper lies with the framework proposed to ensure safe and optimal navigation of a robot in uncertain dynamically changing scenarios. Given a partially known model of the environment, a higher-level planner (Taha et al., 2006b) is employed for describing an optimised path from source to destination and also for constantly monitoring the traversability of the way-points to trigger a global re-planning when the feasible path gets obstructed. An innovative strategy is then presented to handle underlying local navigation issues in between way-points. ...
... In this section, an overview of the proposed path planner algorithm is given. Further details can be obtained from Taha et al. (2006b). ...
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... Traditional sampling-based algorithms have a sampling density as required by the most complex region of the map. Non-uniform sampling has also been proposed to maximize eciency and completeness [41,42]. In Probabilistic Roadmaps [43], the environment is sampled and a roadmap is constructed, then the given start and goal congurations are connected to two of the nodes of the roadmap. ...
... In this study the strategy is to use a variable steering set for any arbitrary map. This is similar to the concept of non-uniform sampling [41,42] used in C-Space path planners. ...
... An example of a navigation task on a real wheelchair platform is shown in Fig. 5. The wheelchair used was the one described in [19] and it measures 1.2x0.7m. The wheelchair's size is considered large compared to the environment and driving it in such a constrained environment can be a challenging task for inexperienced users or users with severe tremors. ...
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