Build a Sound-Tracking Search and Rescue Robot

Abstract

HELP! Locating survivors trapped under rubble is a difficult and dangerous task. After a natural disaster, like an earthquake, rescuers must act quickly to save as many lives as possible. They can use robots with different types of sensors to help find survivors. In this project you will build a sound-tracking robot that can use two microphones to drive toward a sound source. Designing the robot's algorithm will be up to you.

Summary

Areas of Science

Robotics

Difficulty

Method

Engineering Design Process

Time Required

Short (2-5 days)

Prerequisites

Previous experience with Arduino is recommended. See our How to Use an Arduino page.

Material Availability

Bluebot 4-in-1 robotics kit and Electronics Kit for Arduino are available from our partner Home Science Tools®. See the Materials section for details.

Cost

High ($100 - $150)

Safety

No issues

Credits

Ben Finio, PhD, Science Buddies

Science Buddies is committed to creating content authored by scientists and educators. Learn more about our process and how we use AI.

https://www.youtube.com/watch?v=AUguvwpnJ5M

Objective

Build a sound-tracking robot and write a program to make it drive toward a sound source.

Introduction

Robots can go places that are dangerous for humans, like damaged buildings that are at risk of collapse or areas contaminated by radiation or severe pollution. This makes robots useful in search and rescue situations, especially after natural disasters like earthquakes, hurricanes, and landslides. Robots like the one in Figure 1 can work alongside human rescue crews to help find survivors.

Image Credit: Wikimedia Commons user Mike1024 / Public domain

Figure 1. A robot at an indoor competition designed to simulate finding survivors after an earthquake.

Just as humans use different senses, like sight, sound, and touch, robots can use many types of electronic sensors to gather information from their environment. They can use cameras and computer vision to recognize objects. But what about environments where it is difficult to see, like the rubble of a collapsed building after an earthquake? A robot might have to rely on another type of sensor to locate people trapped under the rubble—like a microphone to listen for the sound of a person calling for help.

Most humans can naturally use their two ears to tell which direction a sound is coming from. This is called binaural hearing. A robot can do the same thing by recording sound in stereo using two microphones. This works because sound waves travel radially outward from the sound source. As the waves expand outward, they get quieter. This means that the sound will seem louder to the microphone closer to the sound source, as shown in Figure 2. The sound will also reach that microphone slightly before it reaches the farther microphone.

Image Credit: Ben Finio, Science Buddies / Science Buddies

A dot in the top right corner of the image is the sound source. Sound waves, represented by quarter-arcs, radiate out from the source toward the bottom left of the image. Two microphones are placed just to the left and right of the center of the image. Microphone #1 is on the left and microphone #2 is on the right.

Figure 2. A sound wave expands radially outward from a source toward two microphones. The sound wave will reach microphone #2 first and will seem louder to microphone #2 than to microphone #1.

While they are represented by lines in Figure 2, sound waves are actually traveling pressure waves. Air pressure fluctuates above and below the baseline atmospheric pressure as air particles bump into each other (Figure 3). Larger amplitude fluctuations in pressure result in louder sound, and higher frequency fluctuations result in higher-pitched sound. Microphones convert this changing air pressure into an electrical voltage, which can be read by a microcontroller like an Arduino.

Image Credit: Ben Finio, Science Buddies / Science Buddies

The first segment of the line showing pressure is horizontal at a y value of zero, representing the baseline atmospheric pressure. Farther to the right, the line moves up and down, showing the fluctuation above and below the baseline pressure. The amplitude of the wave is the vertical distance from the baseline to a peak or trough.

Figure 3. Graph showing an example sound wave. The x axis of the graph is time, and the y axis is pressure (which can be measured either as absolute pressure or as a change from baseline atmospheric pressure). Section 1 represents a period of silence and section 2 represents a period of sound. The arrow labeled 3 represents the baseline atmospheric pressure, and the arrow labeled 4 shows the amplitude of the wave (the change from atmospheric pressure). The graph of a microphone's output would look the same, but the y-axis would be voltage instead of pressure. (Image credit: Wikimedia Commons user CLI, CC BY-SA 4.0)

There are different ways to process the sound recorded by multiple microphones in order to determine the direction of the source. One method is to compare the amplitudes of the two sound waves and assume that the louder wave was detected by the microphone closer to the source. If you can record data fast enough, you can also check to see which microphone recorded a sound earlier, as shown in Figure 4.

Image Credit: Ben Finio, Science Buddies / Science Buddies

Figure 4. Sound recorded by a stereo microphone as a car drove by, showing the time difference between left and right. The x axis of the graph is time and the y axis is voltage. (Image credit Wikimedia commons user Sophie means wisdom, CC BY-SA 3.0)

In this project you will use the first method. You will mount two microphones on an Arduino robot, use them to record sound, and compare the resulting measurements to find the one with the greater amplitudes. We will provide example code and a circuit diagram to get you started, but it is up to you to decide what to do with this information. When should your robot decide to turn and drive toward a sound? When should it drive straight? When should it consider the sound "noise" and just ignore it? You will design and code your own algorithm to control the robot's behavior. Before you get started, you may wish to watch this video to learn more about the microphones you will use in this project:

https://www.youtube.com/watch?v=bMs5J4bJOD0

Terms and Concepts

• Search and rescue
• Sensor
• Microphone
• Binaural
• Stereo
• Air pressure
• Amplitude
• Frequency
• Voltage

Questions

• What are some uses for search and rescue robots?
• How does binaural hearing let humans determine the direction of a sound source?
• How can robots use stereo microphones to determine the direction of a sound source?

Materials and Equipment

Recommended Project Supplies

Get the right supplies — selected and tested to work with this project.

View Kit

Materials for this project:

• BlueBot 4-in-1 Robotics Kit, available from our partner Home Science Tools®. The kit includes a robot chassis, wheels, motors, breadboard, batteries, and circuit parts to build four different robots.
• Electronics Kit for Arduino, available from our partner Home Science Tools®.
  • Note: This project will work with the Arduino UNO R3, UNO R4 Minima, UNO R4 WiFi, and compatible third-party boards.
• Windows or Mac computer. See this page if you have a Chromebook. Your computer will need:
  • Access to the Arduino IDE, either installed local version or web-based editor (note that Chromebooks can only use the web version). Watch this video for a comparison of the two options.
  • USB port. The Science Buddies kit comes with a USB-A to B cable. The "B" end plugs into the Arduino and the "A" end plugs into your computer. You will need an adapter or different cable if your computer only has USB-C ports. Watch this video to learn about the different types of cables and adapters.
• Electret microphone with amplifier (2, not included in kit). These are available from various vendors. Most require soldering to attach the pins, so you will need access to a soldering iron. Examples: Adafruit, SparkFun, Amazon
• Some additional tools are recommended for working with circuits:
  • A digital multimeter is strongly recommended for help with troubleshooting
  • Alligator clip leads make it easier to connect your multimeter probes to your circuit
  • Needle nose pliers or tweezers make it easier to handle small parts
  • Wire strippers allow you to cut wires to custom lengths, keeping your circuit neater
• Double-sided foam tape
• Lab notebook

You may wish to purchase additional parts to customize or add features to your robot. See Variations section for ideas.

This project follows the Engineering Design Process. Confirm with your teacher if this is acceptable for your project, and review the steps before you begin.

Before you begin: Review How to Use an Arduino Tutorials 1-3.

1. Follow the instructions in this video to assemble your Bluebot chassis. However, instead of mounting the 4xAA battery pack on top of the robot, mount it on the lower plate. Then mount your Arduino next to the breadboard on the top plate. This will make it easier to connect the circuit to the Arduino.

https://www.youtube.com/watch?v=SBeGl_IgWwY

2. If needed, solder header pins to your microphone breakout boards.
3. Build the circuit shown in Figures 5 and 6. Be careful and note that there are three different power supplies involved in this circuit: 5 V and 3.3 V from the Arduino, and 6 V from the 4xAA battery pack. All parts of your circuit should have a common ground connection, but you must be careful not to short-circuit the different positive voltages to each other. Also note that the Arduino pins used in this diagram match the pins used in the example code, but you could choose to use different pins.
   1. Place the H-bridge in the breadboard, straddling the center gap. Refer to the H-bridge's datasheet from the Bibliography for the pinout. Going counterclockwise from the top left, starting with pin 1, connect the pins as follows:
      1. Pin 1 to Arduino pin 10
      2. Pin 2 to Arduino pin 3
      3. Pin 3 to the right motor negative wire
      4. Pin 4 to ground
      5. Pin 5 to ground
      6. Pin 6 to the right motor positive wire
      7. Pin 7 to Arduino pin 2
      8. Pin 8 to the 6 V from the AA battery pack
      9. Pin 9 to Arduino pin 9
      10. Pin 10 to Arduino pin 4
      11. Pin 11 to the left motor positive wire
      12. Pin 12 to ground
      13. Pin 13 to ground
      14. Pin 14 to the left motor negative wire
      15. Pin 15 to Arduino pin 5
      16. Pin 16 to 5 V from the Arduino
   2. Place a slide switch in the breadboard to use as an on/off switch for the 4xAA battery pack. Connect the battery pack's positive wire to the switch, and the negative wire to a ground bus.
   3. Connect your microphones using male-female jumper wires. Make sure you read the pin labels on your microphones, as they may be in a different order than the ones shown in the diagram.
      1. Microphone ground to Arduino ground
      2. Microphone VCC to Arduino 3.3 V (not 5 V). The documentation for the microphone recommends using the 3.3 V supply since it is less noisy than the 5 V supply.
      3. Each microphone out to one of the Arduino's analog inputs, such as A0 and A1.
   Image Credit: Ben Finio, Science Buddies / Science Buddies
   Figure 5. Breadboard diagram for the circuit. A larger version is available for download.
   
   
   Image Credit: Ben Finio, Science Buddies / Science Buddies
   Figure 6. Schematic for the circuit. A larger version is available for download.
4. Mount the microphones to your robot chassis. Experimenting with microphone placement and orientation is part of what you can try for this project. In general, you do not want the microphones too close together. Figure 7 shows one example, but you could put your microphones somewhere else, or even use other supplies (like popsicle sticks) to extend them.
   
   Image Credit: Ben Finio, Science Buddies / Science Buddies
   Figure 7. Microphones mounted on the robot chassis.
5. Download the example code. This code contains functions to make the robot drive and steer. It also samples the two microphones for a set period of time and calculates the maximum amplitude for each microphone. It does not, however, contain an algorithm that tells the robot what to do with this information—that part is your job! Make sure you read through all the comments and understand how the code works. If you do not know what an Arduino command does, you can look it up in the official Arduino language reference (see Bibliography).
6. Plan out an algorithm for how your robot should react to the microphone readings. Your goal is to make the robot drive toward the source of a sound. Here are a few things to consider (this is not an exhaustive list):
   1. The robot's motors are pretty loud. Should you take readings while the robot is moving, or "stop and listen" so the motors' noise does not affect the readings? Can you calibrate the microphones to filter out (ignore) noise from the motors?
   2. There is probably some level of background noise in your environment. Do you want your robot to react to every single sound? Should you set a threshold so it only reacts if sound amplitude exceeds a certain volume? Do you want it to react to a person's voice, or something like a person clapping their hands?
   3. If a sound comes from in front of the robot, the two microphone readings will not be exactly the same. If the microphone readings are close, but not exactly equal to each other, you may want the robot to drive straight instead of turning.
   4. You can leave your robot plugged into the computer and use the serial print command to print out the values of different variables. This is useful for debugging and calibrating the microphones (e.g., measuring the amplitude produced by different sounds at different distances from the robot).
7. Convert your algorithm to Arduino code. Make sure you document your code with comments so you understand what it does if you come back to it later.
8. Upload and test your code. At first, it may be helpful to leave the robot's motors turned off (using the switch on the breadboard) and leave the Arduino connected to the computer with a USB cable. This will let you use the serial monitor to debug and observe the measured sound values. When you are ready, you can put the robot on the floor and let it drive around. You can leave the USB cable connected during testing, but it will pull on the Arduino and affect the robot's motion, so you will want to disconnect it eventually.
9. Your robot may not work perfectly on the first try, and that is OK! Iteration is an important part of the engineering design process. Make observations about your robot's behavior. For example, does it ever turn in the wrong direction? Does it drive forward when it is not supposed to? Are there parameters you can tweak in your code to change these behaviors? What about changing the physical location of the microphones?
10. Continue iterating until you can get your robot to reliably drive toward a sound source. See the Variations section for other ways to add to or improve your robot.

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Global Goals

The United Nations Sustainable Development Goals (UNSDGs) are a blueprint to achieve a better and more sustainable future for all.

This project explores topics key to Sustainable Cities and Communities: Make cities inclusive, safe, resilient and sustainable.

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