Lab 4: Advanced Sensing

Using reactive control to follow lines on the ground

Introduction

Finite State Machines (FSM for short) were introduced to you in the previous lab. In this lab, you will be using FSM in order to to complete more complicated tasks using more advanced sensors on the iRobot.

Preliminaries

This lab will require you to make use of FSM again in order to program the iRobot to follow colored lines on the ground. Use the instructions from the Getting Started lab in order to connect to the create base and launch the drivers. Do not launch the keyboard driver.

Sensing

Similar to the last lab, we will be observing sensor data. The last lab made use of the bumper sensors on the iRobot. In this lab, we will be using the more advanced cliff sensors. The cliff sensors are located beneath the iRobot:

The iRobot has a total of four Cliff sensors, two near the front center of the robot and two more near the sides of the robot. To make this lab easier we will only be using the center two sensors. Of course nothing is preventing you from using all four to complete the tasks in this lab if you wish.

These sensors detect the reflectivity of the surface beneath them which allows us to use them to detect when the robot drives over a black line on white ground or a white line on black ground. In cases where the ground is the right shade of grey (Like our lab!) we can detect both black and white lines.

Examining the Sensor Data

To start off we need to understand what kind of data the Cliff sensors provide. The return Integer values between 0 and 1400 with lower values representing less reflective surfaces (Black) and higher values representing more reflective surfaces (White).

Here are some sample readings from the iRobot Cliff sensors:

The top two graphs represent the two front center Cliff sensors with one being offset slightly to the left the other to the right. The “Far” Cliff sensors are the ones near the sides of the robot.

The graphs represent the data from the Cliff sensors over a four seconds period while driving over white and black lines on the gray ground of the lab. An upward spike in the graph represents the robot driving over a white line (More reflective, higher sensor values) and a downward plunge in the graph represents the robot driving over a black line (Less reflective, lower sensor values).

Notice that while all the graphs have the same range of 0 to 1400 they all have different values for the same piece of ground. This is an inferent discrepency between the individual Cliff sensors on the iRobot. The Cliff sensor values for the same piece of ground will also be different for two different iRobots so it’s a good idea to try and use the same one for the duration of this lab. You will have to account for these discrepencies in your code.

Viewing Live Cliff Sensor Data

The ROS driver for the iRobot Create publishes a stream of sensor data whenever it is connected to the robot. To start, we’ll use ROS’s command-line tools in order to see the data stream that ROS is providing. These “data streams” we’ve been using are known as topics in ROS terminology.

• At your Terminal run rostopic to see all the different ROS provided commands related to topics.

    $ rostopic
  

• In particular, run rostopic list to see all the topics available at the momment. Which are the ones you are interested in.

    $ rostopic list
  

• If you do not see the topic mobile_base/sensors/core, try reconnecting your iRobot following the lab 1 commands.

• To print the data flowing through Terminal, we will be using echo command.

    $ rostopic echo mobile_base/sensors/core
  

• Try putting different colored item under the cliff sensors and observe the topics printed on your Terminal.

• The sensors are found under the robot. It is labeled as “Cliff Sensor Openings” on the picture.

Sensor Subscribers

Similar to previous lab, we will be using subscriber once again. However, this lab will need to access the data of cliff sensors. Compare the codes to see how the subscribers are different for different sensors.

Line-following

The idea of this lab is:

• First, place the iRobot so that it’s cliff sensors straddle in a line

• Using FSM, your iRobot will have to follow the line provided

• When the cliff sensor detects the line, make appropriate state to get back on track

• The picture will guide you the task a bit more clearly.

To begin, you must be able to find the threshold the colors using the cliff sensor. Use the front-left and front-right sensors only. Be careful of the classroom ground color with the tapes. To test, start with straight lines and slow curves. Make sure your line-following codes are written using FSM.