7: Wander Bot slides |

Simulating behavior in simulation

NOTE! All the book instructions make reference to turtlebot_xxx. We will be using turtlebot3_xxx

We will modify the instructions from the book so follow along here!
Code will be part of cosi119_src package which we have been using: cosi119_src


$ cw
$ cd src/cosi119_src/samples/src
$ roslaunch turtlebot3_gazebo turtlebot3_world.launch
$ ./red_light_green_light.py

Reading Sensor Data

TB3 has a lidar
Lidar returns and array of 360 doubles (and some other stuff)
0 degrees is straight ahead
Each element has distance to nearest obstacle (in M)
To compute the distance to the obstacle immediately ahead (0 degrees)
- msg.ranges[0]
To compute the nearest obstacle:
- nearest_obstacle = min(msg.ranges)
But of course those won’t be very useful (why?)
Experiment by running laser_scan.py program and understanding it
Experiment by seeing how the scan data changes based on where the simulated robot is


$ roslaunch turtlebot3_gazebo turtlebot3_world.launch
$ rostopic echo scan

Program will perform a pretty stupid trick
It will drive in a straight line for some seconds
Then it will spin in place for some more seconds
If it is too near an obstacle then it will spin in place
It turns out this is a pretty terrible algorithm
It gets stuck all the time
Take a look at the annotated code: Wander Bot Code
rospy.Time.now()
- returns a time structure with
- fields secs and nsecs
- Comparison operator is defined
- rospy.Duration(x) is an interval, in seconds
Now run the program


$ roslaunch turtlebot3_gazebo turtlebot3_stage_3.launch
$ chmod +x  wander.py
$ ./wander.py