SIS project n° 1:

Sound detection with a real e-puck

David FitzGeraldDaniel Kroiss

04.06.2013

SIS project: David FitzGeraldSound detection with a real e-puck Daniel Kroiss

Goal:● Program an e-puck in order to find a sound source.

Equipment:● Two e-pucks

● Three batteries

● Bluetooth USB key

Programming language:● C

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Procedure:1. Record the sound

2. Apply the Fast Fourier Transform

3. Calculate the phase and phase differences

4. Determine the sinus of the angle between the e-puck orientation and the sound direction

5. Clean the values and do a normalisation

6. Calculate the angle between the e-puck orientation and the sound direction

7. Set the wheel speeds

SIS project: David FitzGeraldSound detection with a real e-puck Daniel Kroiss

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1. Sound recording:● The sound is sampled with three microphones and stored in an array:

2. Fast Fourier Transform● The fast fourier transform is calculated by the following function:

// 1. Get Sounde_ad_scan_on();while( !e_ad_is_array_filled());

e_ad_scan_off();

// i. Substract mean of vectore_subtract_mean(e_mic_scan[k], FFT_BLOCK_LENGTH, LOG2_BLOCK_LENGTH);

// ii. Copy vector into sigCmpxe_fast_copy(e_mic_scan[k], (int*)sigCmpx, FFT_BLOCK_LENGTH);

// iii. Do FFT. The result is stored in sigCmpx.e_doFFT_asm(sigCmpx);

SIS project: David FitzGeraldSound detection with a real e-puck Daniel Kroiss

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3. Phase:● The phase is determined using the following line of code:

// iv. Phase calculationtheta[k] = atan2(sigCmpx[IND_FFT].imag, sigCmpx[IND_FFT].real);

● It requires the index (IND_FFT) thats corresponds to the sound frequency (880 Hz). We can find it by plotting the magnitude of the signal.

● We chose the index 22, for which the frequency is:

f ≈ 22256/2

⋅33'0002

Hz

SIS project: David FitzGeraldSound detection with a real e-puck Daniel Kroiss

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(33'000 Hz ≈ microphone sampling frequency)

3. Phase difference:● The phase difference is related to the time it takes the sound to go

from one microphone to another:

For example:

● At this point it is possible to useΔφ to make the e-puck turntowards the sound source.

Δ t = Δϕ2⋅π⋅f

Δ ϕ10 = ϕ0 − ϕ1

SIS project: David FitzGeraldSound detection with a real e-puck Daniel Kroiss

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Forward direction

Δφ10

> 0 Δφ10

< 0

● If Δφ10

> 0: turn left

● If Δφ10

< 0: turn right

● Else: go forward or turn around (depending on the sign of Δφ

20 and

Δφ21

)

Works well

Mic 1 Mic 0

Mic 2

Δϕ10 = ϕ0 − ϕ1

SIS project: David FitzGeraldSound detection with a real e-puck Daniel Kroiss

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3. Phase difference:

4. Sinus determination:

θ = arcsin ( v⋅Δ tx )The angle is given by:

where v is the speed of sound.

SIS project: David FitzGeraldSound detection with a real e-puck Daniel Kroiss

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5. Sinus filtering:

● must have a value between -1 and 1

Normalization required

● Plot of a values and cleaned a values

a= v⋅Δ tx

⇒

SIS project: David FitzGeraldSound detection with a real e-puck Daniel Kroiss

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5. Sinus filtering implementation:

// iv. Normalisationfor (k=0;k<3;k++){

// Clean amplified valuesif (a[k] > 1){

a[k] = -0.5;}else if (a[k] < -1){

a[k] = 0.5;}

// Normalisationif (a[k] > 0){

a[k] = a[k]/max_value[k];}else{

a[k] = -a[k]/min_value[k];}// Clean max and min valuesif (a[k] > 1){

a[k] = 1;}else if (a[k] < -1){

a[k] = -1;}

}

SIS project: David FitzGeraldSound detection with a real e-puck Daniel Kroiss

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6. Angle θ:● One angle is not sufficient to determine the direction of the sound.

Mic 2

Mic 0Mic 1

θ20

= θ20

?● We computed the angle θ21

for mic2 & mic1, and θ20 for

mic2 & mic0

SIS project: David FitzGeraldSound detection with a real e-puck Daniel Kroiss

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⇒ θ20 & θ21 → real θ

6. Angle β:● We defined the angle β (comprised between -π and π) based on θ21

and θ20.

Forward direction

β > 0 β < 0

Result when making the sound source turn around the e-puck:

SIS project: David FitzGeraldSound detection with a real e-puck Daniel Kroiss

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7. Wheel speed:

e_set_speed_right

e_set_speed_left

I 1000 1000

II 1000 200

III a 1000 -1000

III b -1000 1000

IV 200 1000 II

III

I

IV

a bStopping condition:

if (e_get_prox(k) > sensitivity level ){

e_set_speed_left(0);e_set_speed_right(0);

}

SIS project: David FitzGeraldSound detection with a real e-puck Daniel Kroiss

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Forward direction

Performance:● The method using β works as well as the method that uses the sign of

Δφ.

● The fluidity was improved by removing the NaN values of φ.

● The time needed to reach the sound source is directly dependent on the wheel speeds.

Problems encountered:● The IR sensors could cause the e-puck to stop randomly due to the

lighting conditions.

● The maximum distance between the e-puck and the sound source up to which sound is detected is pretty small (~70 cm).

● Unreliable Bluetooth connection.

SIS project: David FitzGeraldSound detection with a real e-puck Daniel Kroiss

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Possible improvements:● The performance could be improved by better filtering the signal.

● The accuracy of β could be better with more correct values for the normalization (max_value and min_value).

● Use several consecutive samples before calculating the angle in order to reduce the influence of the noise.

SIS project: David FitzGeraldSound detection with a real e-puck Daniel Kroiss

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Conclusion:● What was important for us was the precision of the angle β.

● The time needed to reach the sound source was secondary.

● Through this project, we were able to see that the method using the FFT works pretty well. However, it would be interesting to compare it with other methods.

//Thank you for your attention !

SIS project: David FitzGeraldSound detection with a real e-puck Daniel Kroiss

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References:● http://ras.papercept.net/conferences/support/word.php

● http://www.labbookpages.co.uk/audio/beamforming/delayCalc.html

● http://moodle.epfl.ch/course/view.php?id=7321

● http://www.e-puck.org/index.php?option=com_content&view=article&id=2&Itemid=8

● http://www.siteduzero.com/informatique/tutoriels/apprenez-a-programmer-en-c

● https://github.com/davidmarek/E-Puck-Algorithms-Library/blob/master/firmware/BTcomDM.c

SIS project: David FitzGeraldSound detection with a real e-puck Daniel Kroiss

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