Path Planning – Wavefront algorithm

REPORT 4

Introduction:

One of the main achievements for functioning robots is to perform interesting tasks on its own. Some robots have been programmed to fold napkins; others have been able to help assemble various car parts, and recently, NASA launched their newest rover, Curiosity, to Mars on an exciting exploration mission, to tell whether there could be life on Mars and so on. The world of robotics seems to get bigger and interesting each day and this is as a result of the different components and studies that come together to make robotics what it is today. One of the studies is motion planning, and in this report we will discuss one of the various motion planning algorithms – Wavefront algorithm – and relate it to a particular task that was given to us in class.

Task 4: Application of Wavefront Algorithm:

In the task that was given, our goal was to program a robot to find its way from a starting point to a goal area given a map. In this map, obstacles are placed in various points and the robot must prevent itself from crossing any path that had an obstacle in the way. In other words, the robot must find an optimal path from the starting point to the goal point without moving into the path of obstacles.    The map is represented in a 2D array with a starting point (x and y coordinates), which has been defined already for us. Below is a simple file demonstrating the map

 

#define X_SIZE 5

#define Y_SIZE 4

#define MAP_AREA 20

#define FREE 0

#define OBST 1

#define GOAL 2

 

int world_map[] = {{0, 0, 0, 0, 0},

{0, 0, 1, 1, 0},

{0, 0, 1, 0, 0},

{0, 1, 0, 0, 0}};

int start_x = 1;

int start_y = 2;

int goal_x = 4;

int goal_y = 3;

 

The world_map[] is  a representation of a given area in which the robot can traverse and below is the declaration of the coordinates of the start position and goal position in the map.

Implementation and Observation:

Path Following:

We divided the task into two major models. The first part was to plan a path for the robot(path following) to follow in a given map. Before we were able to complete this model, we created different methods for specific tasks. Some of the methods are spinLeft(), spinRight(), moveForward(), moveBackward(). As the names of the methods suggest, they perform exactly the task given it. Also, we implemented another method known as orientation() and direction(). The orientation method defines the robot’s position at a given time in a particular cell. So the robot could be facing north, south, east or west depending on the direction it is heading. Now this takes us to the direction method. The direction method only helps the robot to move into the next cell after processing all its neighbouring cells. So we had four different direction points labeled 1, 2, 3, and 4 and at every orientation the robot must determine which direction it is heading with respect to the cell it must move to.

Wavefront algorithm:

The next model was the application of the wavefront algorithm. This time the robot wasn’t given a path to follow but was given a map, a starting point, a goal point and obstacle cells. The work of the robot was to apply the wavefront algorithm to the map, extract the path from the map and follow the extracted map.

Implementing:

We were required to repeatedly process cells and this was done by implementing queues to store the cells. Now we had to add elements to the queue and so we added the goal cell to the queue. As long as the queue is not empty and the start cell has not been processed, and this is done over and over so far as a cell hasn’t been processed. Then we take the next cell from the queue, set their values by adding one (1) of the current cell.

Observation:

We faced series of challenges when implementing the wavefront algorithm. One major challenge was that whenever the robot made a right turn or a left turn, it couldn’t determine its direction as defined earlier in the report. Another was how to extract the path after the robot has gone through the map. Also, we experienced oscillation between two cells, in other words, when the robot was going to make a left turn or right turn, the robot keeps going back and forth between its current cell and initial cell.

Conclusion:

This task gave us a real experience in motion planning, and made us understand the various principles that come together to complete this task with respect to wavefront planning.

 

Keith and Frank

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