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CS 733/833 Introduction to Mobile Robotics

Fall 2022

Lab Assignment 3: Mobile Robot Kinematics and Odometry [10 points]

Introduction:

In  this  lab  you  will  use  odometry  to  control  the  trajectory  of  the  Turtlebot.  A  complete understanding  of  the  code  in  Pre-Lab  3  and  the  ability  to  move  the  robot  using  the  code  is  a requirement for this lab.

Lab Work:

1. Subscribe to Odometry messages via the ‘/odom’ topic. Investigate the Odometry message

to determine its contents.

2. A starter code is provided for this lab (lab3_skeleton.py). Use the starter code to implement the functions listed in the “Functions” section of this manual. Each function returns two objects:

a. A Twist message that indicates the speed the robot should travel at.

b. A ‘finish_condition’ function that returns a Boolean indicating if the robot is at the goal or not

3. Write a launch file to bring up your node. We will use ONLY this launch file to run and test your code. Therefore, without a launch file, you will not get any credit for your work.

Functions:

All units should be in meters, degrees, and meters/second

velocity_from_wheels(phi_left, phi_right)

Convert the velocities of a differential drive robot to the linear and angular components of a Twist message. Hint: Consider what the linear velocity will be given different combinations of wheel velocities. Then do the same for the angular velocity. If you execute this function using the ‘exeute_trajectory’ function it will continue to execute the given Twist messages until the node is killed.

drive_straight_no_odometry(speed, distance)

distance=speed x time; Evaluate the time required to travel the given distance and move the robot for that duration. Since the robot will not achieve the given speed instantaneously, you have to find the correct time on a trial and error basis.

drive_straight_odometry(speed, distance)

Subscribe to odometry message and obtain the starting position. Keep driving the robot at the specified velocity until you travel the specified distance. You have to evaluate the Euclidean distance between the starting position (obtained from odometry) and current position (obtained from odometry) to determine when you have traversed the specified distance.

rotate(angle)

Use  the  odometry  message,  as  discussed  above.  Remember  that  odometry  message supplies orientation as a Quaternion. You should convert it to Euler angle to conveniently measure the distance between two orientations. Assume an angular velocity of 1.0.

Note: if needed you can easily convert radians to degrees and degrees to radians using math.degrees(rad) and math.radians(deg) respectively.

Note: we have also included a normalize_angle function and an angular_distance function to help you in manipulating angles. Refer to the code for further descriptions.

drive_arc(radius, speed, angle)

You may use any combination of the above approaches/functions to perform this functionality.

execute_trajectory()

Consider inputs to be in degrees (clockwise is negative, and counterclockwise is positive)

- Drive forward 60cm

- Turn 90°

- Drive -180° arc with a radius of 15cm

- Turn -135°

- Drive forward 42cm

Deliverables and Deadlines

This lab is due on December 2. Make sure to push your launch file along with codes. We will use ONLY this launch file to run and test your code. Therefore, without a launch file, you will not get any credit for your work.

In addition to your code, you will submit a .txt file that briefly discusses the following:

a. Compare the performance of “drive_straight_no_odometry” and “drive_straight_odometry”