design review electronic stethoscope array

1 ECE445 – Design Review Fatima Zohra Hasnaoui Robin Guigny

ECE445 Senior Design Laboratory Spring 2016

DESIGN REVIEW

Electronic Stethoscope Array

Fatima Zohra Hasnaoui

Robin Guigny

TA : Cara Yang


Table of content 1 Introduction .................................................................................................................................................... 3

1.1 Statement of Purpose ............................................................................................................................ 3

1.2 Objectives .............................................................................................................................................. 3

1.2.1 Goals & Benefits ............................................................................................................................ 3

1.2.2 Functions & Features .................................................................................................................... 3

2 Design ............................................................................................................................................................. 4

2.1 Block Diagram ........................................................................................................................................ 4

2.2 Block descriptions .................................................................................................................................. 4

2.2.1 Electret microphone ..................................................................................................................... 4

2.2.2 Pre-amplifier ................................................................................................................................. 4

2.2.3 Amplifier ....................................................................................................................................... 5

2.2.4 Butterworth band-pass filter ........................................................................................................ 5

2.2.5 Headphone amplifier .................................................................................................................... 5

2.2.6 Bluetooth module ......................................................................................................................... 5

2.2.7 Power supply unit ......................................................................................................................... 5

2.3 Software flowchart ................................................................................................................................ 6

2.4 Simulations & calculations ..................................................................................................................... 7

2.4.1 Signal-to-noise ratio ...................................................................................................................... 7

2.4.2 Analog filter .................................................................................................................................. 8

3 Schematics .................................................................................................................................................... 13

3.1 Microphone & preamplifier ................................................................................................................. 14

3.2 Amplifier schematic ............................................................................................................................. 14

3.3 Headphone to jack schematic ............................................................................................................. 15

3.4 Bluetooth module schematics ............................................................................................................. 15

3.5 Butterworth filter schematic (as shown above) .................................................................................. 16

4 Requirements & Verifications ...................................................................................................................... 16

5 Safety & Ethics .............................................................................................................................................. 17

5.1 Safety ................................................................................................................................................... 17

5.2 Ethics ................................................................................................................................................... 18

6 Costs & schedule .......................................................................................................................................... 18

6.1 Costs .................................................................................................................................................... 18

6.2 Schedule .............................................................................................................................................. 19

7 References .................................................................................................................................................... 20


1 Introduction

1.1 Statement of Purpose One issue that doctors face today is that the traditional stethoscope only provides a

qualitative diagnosis of lung activity. When getting a physical exam done, the doctor holds the

stethoscope to the patient's chest and back and asks him/her to take deep breaths and listens

for any abnormal sounds.

Our plan is to create a self-contained electronic stethoscope with multiple inputs that will

amplify and record these signals using a microphone chip in order to assess the entire lung.

The microphone chip will be attached in between the chest piece and the tubing, capturing

the vibrations that the diaphragm produces. We plan to process these signals using computer

software.

The final data will be put on a display for the doctor to read. This will allow us to analyze both

the amplitudes and frequency bands that correlate to healthy and unhealthy lung activity.

Current electronic stethoscopes only contain a single sensor that has the capability to amplify

lung activity and reduce outside noise. Our idea is to both capture and analyze the signal

across the lung and provide details about the symptoms.

1.2 Objectives

1.2.1 Goals & Benefits

Improve diagnosis of lung-related illnesses

Capturing and processing the lung signal

Have a quantitative assessment of lung activity

Easy-to-read output for doctor

Have a better reliability than doctors’ diagnosis

1.2.2 Functions & Features

Analyze lung activity in both frequency and time domain

Recordings allow for further analysis later on

Software should be able to compare patient’s data to benchmark and declare

whether the lungs are healthy


2 Design

2.1 Block Diagram

Figure 1 Block diagram

2.2 Block descriptions

2.2.1 Electret microphone

Electret condenser microphone CUI Inc. CMA-4544PF-W :

The use of an electret condenser microphone will allow the conversion of acoustical energy into electrical energy. The microphone we chose is omnidirectional, and a good sensitivity and signal to noise ratio Datasheet : : http://www.cui.com/product/resource/cma-4544pf-w.pdf

2.2.2 Pre-amplifier

Pre-amplifier MAX4466EXK-T

This preamplifier, preassembled with the electret microphone, has a really good power supply noise

rejection. It provides an amplification of 100.

Datasheet: https://www.adafruit.com/datasheets/MAX4465-MAX4469.pdf

The schematics for the microphone and preamplifier can be found in the figure below.

http://www.cui.com/product/resource/cma-4544pf-w.pdf

https://www.adafruit.com/datasheets/MAX4465-MAX4469.pdf


2.2.3 Amplifier

The signals we are interested in aren’t loud. Looking into medical data, we decided to only consider

useful the sound with intensity below 40 decibels.

The microphone’s sensitivity of -44dB leads to a transfer factor of 6.3mV/Pa. A sound with an

intensity of 40 decibels corresponds to a sound pressure of 0.002Pa. In order to obtain a signal with

an amplitude of about 1V, we will then need to amplify the signal coming out of the preamp by a

factor of 1000. The schematics for the amplifier chain are provided below.

2.2.4 Butterworth band-pass filter

We will use a band-pass Butterworth filter that filters unwanted frequencies and provides us only

those of lung sound, as to say between 50Hz and 2500Hz. The advantages of a Butterworth filter rely

on its linearity; indeed the waveform distortion is lower than for classical filters and its gain in the

band pass is constant.

2.2.5 Headphone amplifier

For the headphone amplifier, we followed a TI analog input amplifier design using a OPA amp. Details

for the choice of the design can be found in the references section of this paper.

Moreover, the headphone needs a analog input.

The schematics for the HP amplifier to 3.5mm jack can be found on the figure below.

2.2.6 Bluetooth module

We will be using an 8 channel Bluetooth GPIO Module from Numato Lab to transfer the signal from

the filter to the computer. This module is compatible with most computers’ operating systems

(Windows, Linux, Mac) and can be programmed easily through C, Java, Python…

2.2.7 Power supply unit

We will be using a Lithium Polymer battery to power our device. Those high density batteries are

well-adapted for any portable device such as ours, and are not expensive.

Details about the battery are in section 2.4.1.

Datasheet : https://www.adafruit.com/datasheets/LiIon2000mAh37V.pdf

As for the regulators, we’ll be using NCP4626 3.3V and 4.5V linear regulators.

https://www.adafruit.com/datasheets/LiIon2000mAh37V.pdf


2.3 Software flowchart

The data we gathered from our research on lung diseases allowed us to categorize different

symptoms we can detect. During the processing, the software will analyze the sound signal and

should be able to tell if any of the known symptoms has been detected or not.

Two main sounds are characteristics of an existing anomaly in lung activity that are wheezes and

crackles. According to their signal length, their analog waveform and the frequencies for which their

amplitude is maximum, we can identify them and that will be the aim of our software.

The Matlab simulation plot above allow us to compare 4 different plot; one healthy lung

sound signal and three unhealthy lung sound signals. The waveform and cycle duration are

different.

Figure 2 Sick and healthy lung sounds


The software flowchart is given below.

Figure 3 Software flowchart

2.4 Simulations & calculations

2.4.1 Signal-to-noise ratio

In order to find the signal-to-noise ratio value we added random noise to our original lung sound

signal. For that, we used the Matlab function randn.m that generates and random white Gaussian

noise to our signal.

The Matlab code we used is :


Where « murmurevesic.wav » is our orignal lung sound.

The result we find is :

>> snr(y,noisy-y)

ans =

47.5859

For the calculations, we’ll use a 48dB SNR value.

2.4.2 Analog filter

2.4.3.1 Order determination

A band-pass Butterworth filter is described with a template as shown in figure 1 below where a1 is

maximum accepted attenuation in the passing band, and a2 the minimum attenuation in the cut off

band, which is here the signal-to-noise ratio we found above.


Figure 4 Filter template

Here, a1=3dB from f2=50Hz to f3=2500Hz, and a2=48dB from f1 to f4.

In our case, we choose f4 close to 2500Hz to have a selective filter, but not too close to have a

reasonable order. Hence we state f4=3500 Hz. Moreover, the relationship f1f4=f2f3 gives us

f1=35,7 Hz.

We standardize the template by introducing the variable x=f/f0 where f0 is the center frequency

√ √ .

Next step is finding our filter order. For that, we need to consider the equivalent low-pass filter.

Let’s consider, as an example, a second order band-pass filter which transfer function is

( )

where s is the Laplace standardized variable s=p/w0 and 2m=Δx.

( )

(

)

(

)

and

(

).

Then T(S) is a low-pass filter with the Laplace variable S. We then can look for the low-pass template

and deduce that the band-pass.


In our case, Δx=

=6.9 .The variable change

|

| gives X2=X3=1 and X4=1.41

The value of X4 imposes the filter’s order. Since X4=1.41, the minimum order is 2.

For a low-pass order 2 Butterworth filter, the transfer function is ( )

( )

With the variable change we found above, we find the corresponding band-pass filter transfer

function

( )

(

(

))

( ) ( )

With s=jx.

The corresponding band-pass filter is thus a fourth order filter. Let’s plot it on Matlab :

Figure 5 Bode gain diagram

2.4.3.2Filter design

The circuit is realized with two Sallen-Key circuits (one low-pass and one high-pass Sallen-Key filters)

We can rewrite the transfer function we found above as:

( )

( )

=T1(s)T2(s)


A low pass filter has a circuit as given in figure 3, and a high pass filter has a circuit as given in figure

4.

Let’s start and find the value of each component.

Figure 6-Low pass Sallen-key circuit

T1(s)=

( )

So

√ and

(

)

There are two equations but four unknowns. The design procedure fixes one resistor as ratio of the

other resistor and one capacitor as a ratio of the other capacitor.

R1=mR2 =mR and C1=nC2=nC

And

√ ,

√

. Thus, every (R,C) value comes with a (m,n) value to set the

wanted filter.


Let’s set R=10MΩ and C=1nF. Then we find m= 688 and n=14.5

Then we have :

R1=10MΩ

R2=6,88GΩ

C1=1nF

C2=14.5nF

Figure 7-High-pass Sallen-Key circuit

The same reasoning can be used to find the components of the high-pass Sallen-Key.

T2(s)= (( ) )

( ) ( )


Here

√

and

And with R1=mR2 =mR and C1=nC2=nC , we have

√ and

( )

√ √

We use R1=100 Ω and C1=1mF

We find n=4.8x105 and m=2x10-4

Then

R1=100 Ω

R2=48MΩ

C1=1mF

C2=20nF

Finally, the final schematic is given below :

Figure 8 Final analog filter

3 Schematics


3.1 Microphone & preamplifier

Figure 9 Mic&Preamp schematics

3.2 Amplifier schematic

Figure 10 Amplifier chain


3.3 Headphone to jack schematic

Figure 11 Headphone to jack schematics

3.4 Bluetooth module schematics

Figure 12 Bluetooth module schematics


3.5 Butterworth filter schematic (as shown above)

Figure 13 Final analog filter

4 Requirements & Verifications

Table 1 Requirements and verifications

Requirements Verifications Points

Power supply unit : - Li-ion Battery : 1) Supply +3.7V+/-185mV power at 0,2 m 2) Lifetime of the device should be min. 9.8 hrs -Linear regulators LM2937 : 1) Voltage must be regulated to 3.3 V at 500mA draw

- Li-ion Battery:

1) Using a multimeter can allow us verify voltage and current outputs, and an oscilloscope can be used to check if the voltage signal is steady.

2) Measure the lifetime during both active usage and standby and make sure the values found are close to the one calculated.

- Linear regulators 1) With a multimeter and an oscilloscope

connected across the linear regulator and make sure the values are close or equal to those specified in the requirement.

15/100 5/100

Microphone and pre-amp : 1) Capture frequency range :

50Hz – 2.5kHz 2) Signal to Noise ratio :

48 +/-10dB

1) For the high frequencies, use known frequencies sounds coming out from a speaker and make sure the microphone can capture them. For the low frequencies, use sounds coming from a big speaker (that can produce low frenquency sounds) and do the same.

2) - Record 2 different signals : a noise signal

and a breathing signal

20/100


- Read them on Matlab - Compute for each the power spectral density (PSD) of the signal - Calculate PSD(breathing)/PSD(noise) - Make sure the previous value is between 38dB and 58dB

Butterworth filter

1. Cut off frequencies : between 50Hz and 2500 Hz

2. Attenuation max of 3db between 50Hz and 2500Hz and attenuation min of 48dB below 35.7Hz and above 3500Hz

1. - Generate a 5V sinusoidal signal - Attach oscilloscope to the circuit - Make sure the signal is attenuated below 50Hz

and above 2500Hz 2. - Use the previous oscilloscope

- Switch XY mode

- Measure

√ when between 50Hz and

2500Hz and compute 20log of the value found, make sure it is 3dB

- Measure

√ when above 3500Hz and below

35.7Hz, compute 20log of the value found and make sure it is 48dB

20/100

Headphone amplifier Analog signal input

5/100

Bluetooth module Rate speed : >100Mbps

- Send an analog signal with a size of 125 Mo through the Bluetooth module and start a timer.

- Once the whole signal has been fully received by the computer, stop the clock.

- The time it took to transfer the signal has to be <10s

15/100

Software Latency < 2s +/- 1s

- Run the software for a lung sound record - Measure time and make sure it stays between

1s and 3s

15/100

5 Safety & Ethics

5.1 Safety Our project is to design a medical disposal, so it has to be reliable enough to be used by doctors.

To make it the most reliable, it should be used carefully.

First, it is not designed to be waterproof or water-resistant so it has to be kept away from water.


Moreover, the software has to be used properly in order to avoid any misdiagnosis and provide a

reliable conclusion to the patient condition.

Furthermore, the stethoscope membrane has to be cleaned/changed from a patient to another in

order to ensure hygienic auscultation.

Anyone using the disposal has to be properly trained with electrical safety before working with it or

maintaining it.

5.2 Ethics The main issue that our device will possibly face is the chance of misdiagnosis. In that regard, the

data provided by the electronic stethoscope shall be seen as a way to complete and help the doctors

in their diagnosis, but not in any case as a way to replace it.

We intend to follow the IEEE Code of Ethics during all the attainment of our project. Yet, we want

to highlights some particular points.

Indeed, we want (5) to improve the understanding of technology ; its appropriate application, and

potential consequences . In fact, we are still student and improving everyday our knowledge, and

this project can only help us achieve this goal.

(6) to maintain and improve our technical competence and to undertake technological tasks for

others only if qualified by training or experience, or after full disclosure of pertinent limitations.

(7) to seek, accept, and offer honest criticism of technical work, to acknowledge and correct errors,

and to credit properly the contributions of others ;

We are students, but also human, so can be possibly making errors to some point, and we need to

correct any of it in order to ensure other people’s safety.

6 Costs & schedule

6.1 Costs

4.1.1 Labor cost:

Table 2 Labor cost

Name Hourly rate Total hours invested Total=hourly rate*total hours invested*2.5

Robin $28 230 $16,100

Fatima $28 230 $16,100


4.1.2 Parts cost: Table 3 Parts costs

Item Unit cost Quantity Total cost

Electret Microphone & PreAmplifier $6.95 1 $6.95

Li Polymer Battery pack $35 1 $35

8 Channel Bluetooth GPIO Module $34.95 1 $34.95

Regulators, resistors, capacitors Around $10 - $10

Grand total

86.9$

4.1.3 Totals: Table 4 Total costs

Scenario Labor Parts Total

Paid engineering $32200 $86.9 $32286.9

Volunteer engineering $0 $86.9 $86.9

6.2 Schedule The following schedule sums up what we both hope to achieve. Having similar qualifications, we plan

to start executing each step of this schedule together and eventually split the work once we start.

Table 5 Schedule

Week Task

02/08/2016 Prepare project proposal + library research

02/15/2016 Library research (lung activity, illness data, existing hardware)

02/22/2016 Prepare design review

02/29/2016 Ordering parts, testing them, improve design

03/07/2016 Start programming the software and assembling the parts

03/14/2016 Software programming and parts assembling

03/21/2016 Software programming and parts assembling

03/28/2016 Testing

04/04/2016 Testing

04/11/2016 Fix possible issues

04/18/2016 Fix possible issues

04/25/2016 Prepare presentation & final paper

05/02/2016 Finalize presentation and final paper


7 References

[1] TI’s Linear and switching voltage regulator fundamental part 1 :

http://www.ti.com/lit/an/snva558/snva558.pdf

[2] Electret microphone+preamp choice : https://www.adafruit.com/products/1063

[3] http://miv.u-strasbg.fr/collet/ftp/PhD/PhD_LeCam.pdf

[4] IEEE Code of Ethics : http://www.ieee.org/about/corporate/governance/p7-8.html

[5] Headphone amp design : http://www.ti.com/lit/ug/tiduaw1/tiduaw1.pdf

[6] Numato’s 8 channel blutooth GPIO module user guide :

http://community.numato.com/api/productdata/assets/downloads/bluetooth/bluetoothgpio/gpio8/

8ChannelBluetoothGpioModuleV9.pdf

[7] Sellen-Key circuits http://www.d.umn.edu/htang/ece3235_doc_F10/Active_Filter_Summary.pdf

[8] Butterworth filters : http://siversima.com/rf-calculator/bandpass-filter-designer/

[9] Abnormal lung sounds detection:

http://www.ijarcce.com/upload/2014/june/IJARCCE4H%20s%20parveen%20khan%20Artificial%20El

ectronic%20Ear.pdf

[10] http://www.jomb.org/uploadfile/2013/0412/20130412053338852.pdf

[11] http://www.bmjopen.bmj.com/content/2/1/e000506.full.pdf+html?sid=b70ced9d-a64e-463a-

8c44-cc9d09fbabdf

http://www.ti.com/lit/an/snva558/snva558.pdf

https://www.adafruit.com/products/1063

http://miv.u-strasbg.fr/collet/ftp/PhD/PhD_LeCam.pdf

http://www.ieee.org/about/corporate/governance/p7-8.html

http://www.ti.com/lit/ug/tiduaw1/tiduaw1.pdf

http://community.numato.com/api/productdata/assets/downloads/bluetooth/bluetoothgpio/gpio8/8ChannelBluetoothGpioModuleV9.pdf

http://community.numato.com/api/productdata/assets/downloads/bluetooth/bluetoothgpio/gpio8/8ChannelBluetoothGpioModuleV9.pdf

http://siversima.com/rf-calculator/bandpass-filter-designer/

http://www.ijarcce.com/upload/2014/june/IJARCCE4H%20s%20parveen%20khan%20Artificial%20Electronic%20Ear.pdf

http://www.ijarcce.com/upload/2014/june/IJARCCE4H%20s%20parveen%20khan%20Artificial%20Electronic%20Ear.pdf

http://www.jomb.org/uploadfile/2013/0412/20130412053338852.pdf

design review electronic stethoscope array

Documents