使用A*算法完成移动机器人的路径规划

49274 Advanced Robotics (Spring 2019) Assignment 2: A* Path Planning

• Due date: 02/09/2019 (week 7) at 6 pm

• Total marks: 10 (10% of the final mark for the subject)

Introduction

Path planning is essential for mobile robotics. Given a known map, a start position, and a goal position, the aim of path planning is to find a path from the start to goal. Dijkstra’s algorithm and A* are two important optimal path planning algorithms.

More generally Dijkstra’s algorithm and A* are known as graph traversal algorithms. A graph is a set of vertices (also called nodes) and edges that connect them. Dijkstra’s algorithm and A* are used to search through the graph to find the optimal (shortest) path between two vertices/nodes.

In mobile robotics, a (2D) map of an environment is typically represented by an occupancy grid. With a holonomic vehicle (i.e. differential drive) we can inflate the occupancy grid so that the robot can be represented by a point (shown in Figure 1a). Finally we can discretise the occupancy grid at a lower resolution to reduce the amount of computation for path planning (shown in Figure 1b).

By assuming that each cell is connected to any unoccupied adjacent cell, we have a graph that we can search with Dijkstra’s algorithm or A*. Each unoccupied cell is a node in the graph, and the weight of the edge between connected nodes is the distance between the centres of each cell. We can allow only Manhattan movement (up, down, left, and right), or both Manhattan and diagonal movement.

Dijkstra’s algorithm

Dijkstra’s algorithm can be implemented using two sets of nodes, an open set and a closed set. Each node has a cost (the distance from the start node), and a parent node. Initially the open set contains only the start node. In each iteration we remove the node with the lowest cost from the open set, put it on the closed set, and put any nodes connected to it onto the open set. This process is referred to as expanding the node.

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(a) Initial (black) and inflated (grey) occupancy grid (b) Discretised at a lower resolution (red: occu- pied, green unoccupied)

Figure 1: An example occupancy grid

Before placing a new node onto the open set we first check if the node is already on the closed or open sets. If the node is already on either the closed or open set we discard the new node, since the cost of the node already on the closed or open set will be less than the cost of the new node.

The search finishes when we place the goal node onto the closed set. The closed set gives us the shortest path from the start node to the goal node. First we find the goal node on the closed set, then we find it’s parent, and then the next parent, and so on until the start node is found. The list of parent nodes is the shortest path from the goal to the start, and reversing this gives us the shortest path from start to goal.

Dijkstra’s algorithm is shown in Algorithm 1, and the process of extracting the path from the closed set is shown in Algorithm 2.

A*

A* is a modification of Dijkstra’s algorithm that can significantly reduce the number of nodes expanded. It does this using a heuristic, a function that provides an estimated value for the quality of a node. In mobile robotics each node in the graph represents a real position in the map, and the heuristic we use is the real distance between the node and the goal node. If a heuristic is both admissible (it doesn’t overestimate the cost of reaching the goal) and consistent (the estimate is always less than the estimate of any neighbour, plus the cost of reaching the neighbour), A* is guaranteed to give an optimal result. Real world distance is

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Data: Graph, start node, goal node

Result: Set of expanded nodes (ClosedSet) OpenSet contains start node;

ClosedSet is empty;

while Goal node not found do

Remove lowest cost node from OpenSet; Put lowest cost node onto ClosedSet;

if Lowest cost node is goal node then

Goal node found;

end

Get neighbours of lowest cost node;

for Each neighbour do

if Neighbour node not in ClosedSet or OpenSet then

Add neighbour node to OpenSet;

end end

end

Algorithm 1: Dijkstra’s algorithm

Data: Set of expanded nodes from Dijkstra’s algorithm (ClosedSet), start node, goal node Result: Path from start to goal

CurrentNode is goal node;

Path is empty;

while Start node not found do

Add CurrentNode to Path;

if CurrentNode is start node then

Start node found;

end

CurrentNode is the parent of CurrentNode;

end

Reverse Path;

Algorithm 2: Exracting the path from the set of expanded nodes (closed set)

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both admissible and consistent.

There are two modifications to Dijkstra’s algorithm to implement A*. First, when selecting the lowest cost node to remove from the open set, a combined cost is used:

cost + heuristic × weight (1) The weight is used to modify the behaviour of the algorithm. A weight of 0 will behave like

Dijkstra’s algorithm, while a weight of 1 is a traditional implementation of A*.

The second modification to Dijkstra’s algorithm is a change to what happens when checking

if a new node is already on the closed or open sets:

• If the node is already on the closed set, we do nothing.

• If the node is already on the open set, we check if the new cost is less than the existing

cost. If the new cost is less we replace the cost and the parent node of the node in the

open set.

• If the node is not on the closed or open sets, we add it to the open set.

A* is shown in Algorithm 3. As with Dijkstra’s algorithm, Algorithm 2 is used to extract the path from the closed set.

Your Task

Your task is to complete the code in 6 methods/functions:

• pop in the OpenSet class (0.5 marks)

• update in the OpenSet class (1 mark)

• getAdjacentCells in the OccupancyGrid class (3 marks)

• heuristicCost in the “astar_path_planner.cpp” file (0.5 marks) • planPath in the PathPlanner class (3 marks)

• getPath in the ClosedSet class (1 mark)

The locations where you should insert your code are marked with a comment “YOUR CODE HERE”

There is one final task: describe what happens when the heuristic cost weight is above 1.0. You should experiment with various start and goal positions, and various weight values, and describe what you see. Answer this question in the “QUESTION.txt” file, found in the root folder of the package.

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Data: Graph, start node, goal node

Result: Set of expanded nodes (ClosedSet) OpenSet contains start node;

ClosedSet is empty;

while Goal node not found do

Remove lowest combined cost node from OpenSet; Put lowest cost node onto ClosedSet;

if Lowest cost node is goal node then

Goal node found;

end

Get neighbours of lowest cost node;

for Each neighbour do

if Neighbour node is in ClosedSet then

Do nothing;

end

else if Neighbour node is in OpenSet then

if Neighbour node has lower cost than node already in OpenSet then Replace node in OpenSet with neighbour node;

end end

else

Add neighbour node to OpenSet;

end end

end

Algorithm 3: A* algorithm

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Overview of the node

The node starts with the main function in “src/astar_path_planner.cpp”. The main function sets up the node, creates an instance of the PathPlanner class, and “spins” to process callbacks.

The PathPlanner class makes use of three other classes:

• OccupancyGrid • OpenSet

• ClosedSet

OccupancyGrid class

The OccupancyGrid class is declared in “include/astar_path_planner/occupancy_grid.h” and implemented in “src/occupancy_grid.cpp”. The constructor of the class takes a ROS occupancy grid message, inflates it, and stores it.

In the header file (“occupancy_grid.h”) a number of data structures are declared:

• WorldPosition • GridPosition • Cell

• AdjacentCell

WorldPosition and GridPosition both contain the member variables x and y. In WorldPosition these variables are doubles (real numbers) and refer to a position in the map in metres. In GridPosition these variables are ints (integers), and refer to a position in the map in cells.

The Cell structure is used to return information about a cell in the grid. It contains the member variables id, occupied, grid_position, and world_position. world_position is the position of the centre of the cell.

The AdjacentCell structure is used to return information about cells adjacent to a particular cell. It contains the member variables id, cost, and world_position. cost is the cost (distance in metres) of moving from the parent cell to the adjacent cell, and world_position is the position of the centre of the cell.

The constructor of the class takes a ROS occupancy grid message and an inflation radius (in metres), and:

• Copies the given occupancy grid message 6

• Creates an image that is used to access the data in the occupancy grid message • Creates a structuring element of the correct size for the given inflation radius

• Dilates the image with the structuring element

• Sets variables that store the limits of the map

The class has methods for querying the occupancy grid, such as:

• isOutOfBounds

• isOccupied

• getGridPosition • getWorldPosition • getCellId

• getCell

There are a number of different methods with the same name, which differ by type of argument they accept. You should also keep in mind that the private methods (indicated in the header file) are only accessible within the class. You should not need to use these from outside of the class (i.e. in the planPath method of the PathPlanner class), but they are accessible in the getAdjacentCells method.

getAdjacentCells method

The getAdjacentCells method takes a cell ID and a bool indicating whether or not diagonal movement is allowed, and returns a vector of AdjacentCell. Completing the method is one of the tasks for this assignment.

The cell ID argument has been converted into a grid position for you (named grid_position). From this grid position you can find adjacent cells, e.g. the cell to the right is(x+1, y)andthecelltotheupperrightis(x+1, y+1).

You need to use isOutOfBounds and isOccupied to ensure that you return only valid unoccupied cells, and you should only return diagonal cells if diagonal_movement is true.

The AdjacentCell structure contains the variables id, cost, and world_position. You should use getCellId and getWorldPosition to get the ID and world position respectively. The cost can be determined by the resolution of the map, given by the map_.info.resolution variable. This is the horizontal or vertical distance between cells, remember that the cost value is different for diagonal cells.

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Node structure

The Node structure is declared in the file “include/astar_path_planner/Node.h”, and is used by the OpenSet and ClosedSet classes. It contains the member variables id, parent_id, cost, and heuristic_cost. cost is the cost (distance in metres) of the node from the start node. heuristic_cost is the Euclidean distance (in metres) of the node from the goal node.

OpenSet class

The OpenSet class stores a vector of Node structures in nodes_, and provides a number of

methods:

• push: adds a node to the open set.

• pop: removes the lowest combined cost node from the open set.

• contains: returns true if the open set contains the given node ID.

• update: replaces a node in the open set, if the cost of the given node is less than the

node already in the open set.

• empty: returns true if the open set is empty.

• getNodes: returns a reference to the nodes_ vector, which is used for publishing the

open set markers.

Completing the pop and update methods are tasks for your assignment.

pop method

In the pop method you want to find the node with the lowest cost combined cost

(cost + heuristic × weight). Set the value of the index variable to be the index of the lowest cost node in the nodes_ vector, and it will be removed and returned.

update method

The update method takes a node as an argument. You want to find the same node already in the nodes_ vector and, if the cost of the new node is less than existing node, replace the existing node.

ClosedSet class

The ClosedSet class stores a vector of Node structures in nodes_, and provides a number of methods:

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• size: returns the number of nodes in the closed set.

• push: adds a node to the closed set.

• contains: returns true if the closed set contains the given node ID.

• getPath: returns a vector of node IDs (integers) from the given start id to the given

goal id.

• getNodes: returns a reference to the nodes_ vector, which is used for publishing the

closed set markers.

Completing the getPath method is a task for your assignment.

getPath method

The getPath method should return a vector of node IDs which is the shortest path from the

start ID to goal ID. You want to fill the path vector, which is returned by the method.

To extract the path from the closed set, you find the goal node on the nodes_ vector, then find it’s parent, and then find it’s parent, and so on until you find the start node. The path created will be from the goal node to the start node, so it should be reversed before it is returned.

PathPlanner class

The constructor of the class:

• Reads parameters.

• Acquires a map (ROS occupancy grid message) from the “static_map” service.

• Creates an instance of the OccupancyGrid class with the map.

• Publishes the map created by the OccupancyGrid class.

• Advertises topics for a number of markers: start and goal positions, nodes in the open

and closed sets, and the path.

• Subscribes to the “initialpose” and “move_base_simple/goal” topics that are published

by RViz.

• Advertises the “plan_path” service, which is used to initiate path planning.

The planPath method is where the A* path planner is implemented. The req variable is the request message that is used to call the method. req contains two variables:

• heuristic_cost_weight: The weight value for the heuristic cost function

• diagonal_movement: a Boolean variable that indicates that diagonal movement should

be enabled

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The template code provided will:

• Create a Cell for the start and goal positions (named start_cell and goal_cell)

• Create an empty OpenSet and ClosedSet (named open_set and closed_set)

• Create a start node (named start_node), which is put onto the open set

• Create a Boolean variable that indicates that the goal has been found (named

goal_found), which is initially false

Your task is to complete the code within the while loop to finish the implementation of A*

path planning. Your code should:

1. Remove the lowest cost node from the open set with the pop method (remember to use req.heuristic_cost_weight).

2. Put the lowest cost node onto the closed set.

3. Exit the while loop if the lowest cost node is the goal node (remember to set

goal_found to true).

4. Get the cells adjacent to the lowest cost node (remember to use

req.diagonal_movement).

5. Then, for each adjacent cell create a new node and:

(a) If the new node is already on the closed set, do nothing.

(b) If the new node is already on the open set, update it if it’s better.

(c) If the new node is not on the closed or open sets, add it to the open set.

The code after the part you are expected to write (within the while loop) will publish markers for the open and closed sets, and delay to loop to achieve a certain update rate.

After the while loop, the code will:

• Display an error if the goal has not been found

• Call the getPath method to get the path from the closed set

• Convert the path of node/cell IDs into a vector of WorldPosition

• Publish markers for the path

• Set the variables length_of_path and number_of_nodes_in_closed_set in the

response message

Compiling and Running the Node

You will have already created a Catkin workspace for your previous assignment, place the “astar_path_planner” package into the “src” directory.

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Compiling the node

Open a terminal.

If you are using the UTS FEIT computer labs, first run:

singularity shell /images/singularity_containers/ros-melodic-ar.sif

Navigate to the “catkin_ws” folder, e.g.:

cd ~/catkin_ws

Compile all nodes in the workspace with:

catkin_make

Running the node

Open a terminal.

If you are using the UTS FEIT computer labs, first run:

singularity shell /images/singularity_containers/ros-melodic-ar.sif

Set up your environment (you only need to do this once when you open a new terminal):

source ~/catkin_ws/devel/setup.bash

Run the A* path planner node and other required nodes with:

roslaunch astar_path_planner astar_path_planner.launch

To set the start and goal positions, use the “2D Pose Estimate” and “2D Nav Goal” buttons in RViz. Click on the button, and then click on somewhere in the map. A green or red sphere will indicate the start and goal positions (shown in Figure 2).

To initiate path planning you need to send a service call. First open a terminal.

If you are using the UTS FEIT computer labs, first run:

singularity shell /images/singularity_containers/ros-melodic-ar.sif

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Figure 2: RViz

Set up your environment (you only need to do this once when you open a new terminal):

source ~/catkin_ws/devel/setup.bash

You need to source the setup script before you can send the service call. To send a service call run:

rosservice call plan_path false 0.0

“plan_path” is the name of the service. The next argument is diagonal movement: false to disable diagonal movement, true to enable diagonal movement. The last argument is the heuristic cost weight: 0 will behave like Dijkstra’s algorithm, 1 will behave like a conventional A* algorithm.

When the service call finishes it will print out the length of the path, as well as the number of nodes expanded (number of nodes in the closed set). The final task is to describe what happens when the heuristic cost weight is above 1.0. You will need to use the service call with different values for the heuristic cost weight, and observe difference in the length of path and number of nodes expanded. You should also try various start and goal positions.

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Tips for developing your code

You can print a node, or the whole open or closed sets at using ROS_INFO_STREAM. For example:

ROS_INFO_STREAM(“\n\nNode: \n” << node);

ROS_INFO_STREAM(open_set);

ROS_INFO_STREAM(closed_set);

This will print to the console where “astar_path_planner” is running. Note the \n is just a line break to improve readability.

You can also pause execution with:

waitForKey();

To resume execution press “Enter” in the console where “astar_path_planner” is running.

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