CS/ME/ECE/AE/BME 7785 Lab 3


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Lab 3

1 Overview
The objective of this lab is to explore PID control and combining multiple sensor inputs. The goal
is to have the robot chase a desired object observed in its local coordinate frame. Specifically, this
means you must make the robot always face the object and maintain a desired distance from the
object. You can track any object you would like as long as it can be moved by one of the instructors.
If you want, you can use the object tracking script you developed in previous labs. We have seen in
class that it is not possible to determine an objects distance from a robot at all times from camera
data alone so you will have to use the LIDAR as well.
As you did with lab 2, you should run the roscore and all files on-board the robot. To move
folders from your computer to the robot, use the scp (secure copy) command. For example,
scp -r <Directory on your cpu to copy> burger@<ip-of-burger>:<Directory to copy to
on robot>

2 Lab Instructions
Create a package called TeamName_chase_object (refer back to the ROS tutorials from Lab 1 if
needed). For your Team Name you can make one up or use the Team Number you were assigned with
your lab teams. Note, package names must begin with a letter so if your Team Number was 1 you
could name your package team1_object_follower or one_object_follower. Similar to the last
lab, useful dependencies include rospy, roscpp, sensor_msgs, std_msgs, and geometry_msgs. You
can add as many nodes as you like. An example structure would be:
detect_object: This node should be similar to the find_object you created in last lab. It
should subscribe to the raspberry pi camera node and publish the location of the of the object
you’re tracking.
Note: unlike previous labs, you will now be coordinating your camera with your LIDAR so they
will have to agree on reference frames and units. This means you should convert your pixel values
to degrees or radians and know the relation between the reference frame of your camera and your
get_object_range: This node should subscribe to receive the location of the object published
from your find_object node and publish the angular position and distance of the object using some
combination of the LIDAR data and camera data. To get the LIDAR data, this node should also
subscribe to the \scan topic.
LIDARS produce a lot of data and are one of the more complicated sensors to use. For more
information on the LIDAR used on the turtlebot3 please look at:
You can also echo the topic to view the data that is being produced.
chase_object: This node should subscribe to the data being published by get_object_range.
Based on this data you should create two PID (or some variant) controllers to follow your tracked
object at a desired distance (meaning the robot drives forward and backwards if needed). One
controller should act on the angular error to make the robot face the object while the other acts on
the linear error to make the robot maintain a distance from the object. This node should publish
the velocity commands for the robot to follow in the same manner you did in lab 2.
Note there are PID controllers already acting on the motors of the turtlebot3. If this was a
custom robot, or you did not like the motor controllers of the robot, you would have to design a
PID controller for the motors of the robot as well that would have a settling time much faster than
your high level controller!
3 Questions
These questions must be answered in a writeup submitted with your code. This does not have to
be a formal lab report, just answer the questions directly. Include supplemental material where you
think it would help. These questions should also be considered while developing your code. You
may be asked to answer these questions in person during the demo.
1. What is the sampling time of your system(hint: how fast can you get sensor data)? How does
this matter with a subscriber based controller?
2. What variant of PID control did you use? Why?
3. If you use an integral term, how do you deal with windup? If you use a derivative term
how do you deal with noise/fast changes in the object’s location? If you just used a purely
proportional control, how do you deal with disturbances?
4. What does it mean for this system to be unstable? A helicopter/plane will fall out of the sky
if it uses an unstable controller, what does your robot do when your controller is unstable?
5. Describe your algorithm to determine where the object is relative to the robot. Specifically,
how do you use the camera and LIDAR data to produce a desired velocity vector? Include
mathematical expressions used and supportive figures where appropriate.
4 Grading
The lab is graded out of 100 points, with the following point distribution:
Consistently determine the location of your object (distance and angle relative to
the robot)
Rotate the robot to face the object 20%
Drive the robot to maintain a distance from the object 20%
Robot stops at desired distance and heading of the stationary object within 5 seconds 10%
Question responses 30%
5 Submission
1. Perform a live demonstration of you running the robot to one of the course staff by the listed
2. Put the names of both lab partners into the header of the python script. Put your python
script, your writeup, and any supplimentary files, in a single zip file called and upload on Canvas under Assignments–Lab 3.
3. Only one of the partners needs to upload code.
We will set aside class time and office hours on the due date for these demos, but if your code is
ready ahead of time we encourage you to demo earlier in any of the office hours (you can then skip
class on the due date). Office hour times are listed on the Canvas homepage.

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