Acoustic Triangulation Device

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Acoustic Triangulation Device Group 13 Senior Design Group 13 - Ben Noble, Johnathan Sanders, Jeremy Hopfinger

description

Group 13 Senior Design. Acoustic Triangulation Device. Project Description. The Acoustic Triangulation Device (ATD) is an electronic system designed to detect the location of a sonic event, specifically gunshots or explosions, and relay that location in real time to the user. - PowerPoint PPT Presentation

Transcript of Acoustic Triangulation Device

Page 1: Acoustic Triangulation Device

Acoustic Triangulation Device

Group 13 Senior Design

Group 13 - Ben Noble, Johnathan Sanders, Jeremy Hopfinger

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Group 13 - Ben Noble, Johnathan Sanders, Jeremy Hopfinger

Project Description The Acoustic Triangulation Device (ATD) is an

electronic system designed to detect the location of a sonic event, specifically gunshots or explosions, and relay that location in real time to the user.

Its main subsystems include but are not limited to a microphone array, GPS locator, and real time triangulation software. These systems will work together to determine the origin of any sonic event within a predetermined frequency range.

As will be shown, the philosophy of use is broad and the device will prove useful in a variety of applications.

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Philosophy of Use

VIP Protection (Speeches etc.) Inner city law enforcement Military use Civilian Property / National Park

Regulation

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Requirements and Specifications

Requirement SpecificationsHigh Accuracy Within 5 Meters of true target positionLong Range Locates a target within 400 metersLow Cost Costs less than $500Portability Under 10lbs and less than 1 cubic meterEase of Use Less than 5 minute setup time for

inexperienced usersHighly Discriminate Able to distinguish categorized sounds with

90% accuracy

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Problem Solving Methods

There are many algorithms for locating the origin of wave based events.

The majority of these techniques require the event to occur within the area of a microphone array.

E.g. Mic BMic A

Mic C

Source

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Technique: Multilateration Many military applications use a technique called

Hyperbolic Multilateration. In this technique the time difference of arrival

(TDOA) of an acoustic event between two microphones limits the possibility of a source location to one half of a hyperbola.

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Multilateration A second and third TDOA are obtained from a second and

third microphone. These TDOAs produce respective second and third

hyperbolae. The intersection of the hyperbolae is the location of the

source. In order to have an accurate measurement for the speed of

sound which we will subsequently refer to as C, we will need the temperature T.

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Multilateration Due to the fact that there is a constant distance

differential along the hyberbolae this method can easily be derived from the distance formula.

Here A, B, and C reference microphones A, B and C respectively.

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Multilateration Unfortunately since we do not know the exact time the

sonic event initially occurs we cannot know the time it takes the sound wave to travel to the closest microphone (in this case mic A).

We do however have the TDOA between A and all subsequent microphones, therefore we can translate the source location to A.

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Multilateration Also if we choose an origin for the system at microphone A

then we can simplify further. This means that the locations for microphone B and C are

relative locations with respect to microphone A. There are then two equations and two unknowns which are

the x-coordinate and y-coordinate for the sound source.

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Multilateration This can be easily expanded to the 3 dimensional

case using a 3rd dimensional hyperboloid. All this takes is one additional mic and a third

equation.

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Problems Accuracy improvement in Multilateration involves a technique

known as Nonlinear Least-Squares Stochastic Gradient Regression.

This technique involves more microphones and is an advanced way of averaging the perceived source location from each mic.

Needless to say, this is difficult, time consuming, and increases the price of the unit due to additional equipment.

Our initial design required eight microphones for accurate results.

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Problems Suppose we have an acoustic event located 400 meters

away from the unit oriented directly in front of mic A with microphones spaced 1 meter apart (as per specifications).

This yields the equations:

where A and B are the TDOA’s.

Solving this yields B=0.00206025 seconds, A=0.00206025 seconds (intuitively both TDOA’s are the same for this configuration)

400 mMic B

Mic C

Mic A

Source

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Problems Now suppose we have an acoustic event located 405

meters away from the unit oriented directly in front of mic A. with microphones spaced 1 meter apart.

This yields the equations:

where A and B are the TDOA’s.

Solving this yields B=0.00206026 seconds, A=0.00206026 seconds

405 mMic B

Mic C

Mic A

Source

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Problems Problem: A 5 meter difference in position yielded only

a .00000001 second TDOA differential. That is 10 nanosecond difference.

In order to be accurate to within 5 meters we must sample nearly 100 million times a second for each of 8 microphones.

This would require a 800 MHz sample rate. This along with the clock time for each instruction

(assignment statements, conditional operators etc.) needed to process each sample, would require a very expensive processor.

A microcontoller with this processor would push the project out of budget, as would a PC sound card with 8 inputs.

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Technique: Triangulation For 2-dimensional triangulation we

need 2 arrays of 3 microphones each, oriented in an equilateral triangle.

Each array will give us an angle which tells us the direction of the source.

Using these two directions and the known distance between the two arrays we can determine the sound source’s location.

If the microphones are close enough together and the sound source is sufficiently far away we can assume the sound wave approaches as a straight line perpendicular to the line originating from the source.

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Triangulation We can find the distance Δx1

using the distance formula.

C is the speed of sound and tB – tA is the difference between the time of first detection and the time of second detection.

Knowing the distance Δx and the side of the array S we can find the angle θ1 using trigonometry.

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Triangulation We can find the angle α1 based on its

relationship with θ1

These equations will work regardless of the orientation of the array, therefore the times tA and tB will always be the time that the first and second microphones detect a sonic event, respectively.

Based on these equations the sound source could be from either side of the line which connects the first and second microphones. The third microphone tells us that the source came from the opposite side that it is located.

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Triangulation The relationship

between the β angles and the α angles are determined by knowing the orientation of each array with respect to the line L.

This information will be determined using a compass.

In this case:

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Triangulation Then using the Law of

Sines we can determine the distance from the first array to the sound source.

The coordinates for the sound source are found by adding each portion of the distance (vertical and horizontal) to the coordinates of the GPS for the first array.

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Triangulation Combining these

equations we get α for each array.

We can also combine the previous equations to find D.

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Triangulation Notice that there are two

equations which can tell us the value of Δy1.

If these two equations are not equal then there is some amount of error in the system.

Setting the two equations equal allows us to solve for an expected angle and compare that angle with the one found in the previous equations.

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Triangulation The same equations apply

to the 3D case except that the speed of sound is broken into two parts.

These two equations can be solved simultaneously and will give us a horizontal angle and a vertical angle.

The same method can then be used as before to determine the exact location of the sound source.

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Solutions Lets return to the same event located 400 meters away from

the unit oriented directly in front of mic A of the first array with mics spaced 1 m apart and 10 m between arrays.

This yields the equations:

where t1 and t2 are the TDOA’s.

Solving this yields t1=0.00248763 seconds, t2=0.00252485 seconds

400 mMic B

Mic C

Mic A

Source

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Solutions Now lets look at the event located 405 meters away from the

unit oriented directly in front of mic A of the first array with mics spaced 1 m apart and 10 m between arrays.

This yields the equations:

where t1 and t2 are the TDOA’s.

Solving this yields t1=0.00248810 seconds, t2=0.00252485 seconds. The second time being the same due to the angle not changing.

405 mMic B

Mic C

Mic A

Source

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Solutions This time a 5 meter difference in position yielded

only a .00000047 second TDOA differential. That is 470 nanosecond difference.

This is a significant decrease in necessary sample rate. The sample rate required for this worst case scenario which used only 6 microphones is 12MHz.

This allowed us to use a significantly cheaper microcontroller.

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Code Sample This short piece of code

shows the pre-scale factor for an ADC.

At maximum speed an ADC loses some ability to give an accurate analogue voltage reading.

After some trial and error we determined that a pre-scale factor of 16 was a good average between accuracy and speed.

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Signal Analysis Wavelet Vs. Fourier

Transforms Time Domain Signal: The Fourier

transform shows the frequency spectrum of the time domain signal.

Fourier Transform:Frequency Domain Signal

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Signal Analysis Problem: Notice that the Fourier transform graph tells us

nothing about the times at which a frequency occurs. Example: Observe the following two signals. (Note: Both

the signals contain only the frequencies 5, 10, 20, and 50 Hz)

Stationary Non-Stationary

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Signal Analysis Problem: The Fourier

transform of both functions provides the same frequency response graph.

Both signals contain the same frequencies at different times. Note that the Fourier graph cannot provide any time information at all… whatsoever.

This means Fourier analysis is useless for distinguishing between non-stationary audio signals. (Most audio signals)

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Signal Analysis How can we solve this problem? One approach is the Short Time

Fourier Transform (STFT).

In this method the wave to be analyzed is broken up into time segments via a window function

The window function lets us correlate different frequencies to different times and as the STFT is just a Fourier transform in two variables, we can again graph the result.

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Signal Analysis Notice now that for each frequency

there exists a band in time, with a certain amplitude (amount of that frequency, in that time band)

Notice also that the width in frequency is small but non-zero.

This indicates that the frequency resolution is not perfect. We should have impulse responses at 5, 10, 20, and 50 Hz.

Instead there are bands around these frequencies.

This is known as frequency resolution and the wider the band, the poorer the resolution.

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Signal Analysis Recall, the Heisenberg uncertainty principal. The same principal can be applied to the time and

frequency information of a signal.Poor Frequency Resolution

Poor Time Resolution

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Signal Analysis To solve the problem of resolution, we will use wavelets in

our signal analysis.

Note: This is a STFT (a FT windowed in time) but windowed in frequency as well.

The window function (psi in the equation) is now a frequency/time window and is known as the mother wavelet.

Note that Psi is varied in time and scaled in frequency.

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Signal Analysis Examples of suitable mother wavelets:

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Daubechies 4th Mother Wavelet

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Signal Analysis Gun shot comparison:

By using the wavelet transforms of prerecorded gunshots and comparing the frequency time wavelet coefficients to an event currently occurring, we can distinguish between gunshot types with a high degree of accuracy.

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Components Microphones Knowles MD9745APZ-F Omni directional Flat frequency response Operates at 5V Highly Sensitive Cheap Very small Came with 100x preamp

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Components Development Board Arduino Mega Microcontroller

ATmega1280 16 Analog Inputs 16 Mhz clock speed 128kb Flash memory Easy to program

using C++

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Components GPS EM 408 Extremely high

sensitivity : -159dBm 5m Positional

Accuracy Cold Start : 42s 75mA at 3.3V 20 gram weight Outputs NMEA 0183

binary protocol

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Components

Compass HMC-6352 2.7 to 5.2V supply

range Simple I2C

interface 1 to 20Hz

selectable update rate

0.5 degree heading resolution

1 degree repeatability

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Components

Temperature Sensor

DS18B20 Digital Temperature Sensor

± 0.5º C accuracy from -10º C to +85 º C

Converts temperature to 12-bit digital word in a max of 750ms

Operates at 3.3 V

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Design Summary

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Design Summary

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Administrative Content

Part Price

Compass - HMC 6352 59.95

GPS - EM 408 64.95

8 Mics/Preamps - MD9745APZ-F 31.80

Arduino Mega 64.95

Temperature Sensor - DS18B20 4.25

PCB 100.00

Total (Basic) 325.90

Budget

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Administrative Content

For the duration of the project our group worked together to accomplish all goals.

Each member however had a specific area of expertise.

Work Distribution

Johnathan Sanders Ben Noble Jeremy Hopfinger

Multilateration,Software – UI, Coordinate Translation, Wavelet

TriangulationSoftware – GPS, Google Earth, Microcontroller

PCB Design, Component Integration, Array Fabrication

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Special Thanks

Robi Polikar – Rowan UniversityThe Wavelet Tutorial Special thanks to Robi Polikar for

allowing us to use pictures from his website at

http://users.rowan.edu/~polikar/WAVELETS/WTtutorial.html

Special thanks to Louis Scheiferedecker for allowing us to use several of his firearms for testing.