EEG Monitoring SystemThe EEG Porto-Cap™

Team 5

ENGN4017 - Engineering Design

Autumn 1999

Design Process

• Examine project domain– EEG measurement systems

• Meet with intended customer– Dr Richard Clark, Flinders University SA

• Identifying problems with existing skull cap system– Requirements for new system were formulated

• Develop conceptual solutions to problems– Brainstorming

• Project Decomposition– Sub-system groups focussing on specific problems

• Integration– System integration

• Presentation, Final Report

March

June

What is EEG?

• EEG = Electroencephalogram• Measurement of brain wave patterns

in hopes of understanding the mysteries of the mind.

Problem Statement

The reliable measurement, over extended periods, of scalp EEG on

consecutive occasions, during mental functions in everyday environments.

Key Problem Areas

• Lack of repeatability• Different Head Sizes• Long setup times• Difficulty with lengthy experiments• Use of conductive gels• Scalp abrasion• Noise in signal• Lack of portability/environmental constraints

Issues that needed to be addressed in any new system developed by the group:

In finding a solution, we have taken an ambitious approach to solving these problems in a medical research application

Problem Decomposition

P os it ion in gR ep eatab ility

A ccu racyD iffe ren t H ead S izes

M ech an ica lG rou p

S e lec tion o f M ate ria lP o la risa tion

H yg e in eIrrita t ion

E lec trod eG rou p

N o ise R ed u c tionR eso lu tion

P ortab le S ys tem

S ig n a l P rocess in gG rou p

D eta iled D es ig nD ecom p os it ion

Key Requirements

Category Requirement DesignElectrodes positionedaccording to 10/20system

Yes/No Yes/NoFlexible

Position accuracy forrepeatedmeasurements

< 1mm2

deviationRequires 2nd

prototypetesting

Head size Circumferencerange:> 53-60cm

Circumferencerange:> 53-66cm

Setup time < 15 minutes < 15 minutes

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Cap DesignM

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Positioning• Nasion/Ear reference points• Adjustable straps for different head

dimensions (53cm - 66cm)• Electrodes retractable to suit

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Nathan in an early model. Current model.

Demonstration of

Skull Cap Setup . . .

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Key RequirementsCategory Requirement DesignSkull impedance < 5 k Expect < 1k,

2nd prototypetesting req.

I rritation J ury Test:< 2%experienceirritation

Requires 2nd

prototypetesting

Electrodepolarisation

< 1%Deviation

Expected,2nd prototypetesting req.

Cleaning Compliancewith Aust.Standard

Yes

Scalp preparation No No

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Electrode Selection

• Silver/silver chloride selected– Minimal polarisation characteristic

• Dry electrode used (no gel required)– Uses local impedance converting amplifier

• Pellet electrode, 2mm diameter– Small electrode size ensures good hair

penetration and contact with scalp

2mm

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Irritation Issues

• Initial model- helmet supported by electrodes• Prototype - straps support the load

– allows smaller electrodes to be used without increasing irritation

– straps allow for more comfortable fit• No preparation or abrasion of skin surface

required

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Cleaning

• Electrodes detachable for cleaning• Cleaning processes include disinfecting

and sterilisation via autoclave• Use of two sets of electrodes allows

continuous use

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Key Requirements

Category Requirement DesignSignal noise < 1% Expect 0.1%,

2nd prototypetesting req.

Signal resolution .15V 30.5nV

Signal drift Eliminate below0.05Hz

Adjustablehigh-pass filter(.005-.1Hz)

Wireless operation Yes Yes

Sampling rates Variousselectable

8 selectable(0.5, 1, 1.25,2.5, 5, 10kHz)

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ComponentsS

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Amplifiers,Filters,ADC’s,

CPU

CAP

Transmitter,Power Supply,

Batteries

BELT PACK

Receiver

COMPUTERSerial Data and

Control Line

Power Lines Wireless SerialData Line

CAP

Buffer,1000x Amplifier,

Analog Switch

ELECTRODE(x4)

HP/LP Filters,10x Amplifier,Samp. & Hold,

ADC

GROUP(x6)

CPU,Clocks

CENTRAL(x1)

AnalogData Line

Power Lines SerialData Line

DigitalControl Lines

Power Lines

DigitalControl Line

Signal FlowS

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ElectrodeHighpass

Filter

Sample andHold

1000

10ADCCPU

WirelessTransmitter

WirelessReceiver

1Lowpass

Filter

Computer

EEG Signal Flow Control Signal Flow

Conditioning

Conversion

Transm

ission

Signal FlowS

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ElectrodeHighpass

Filter

Sample andHold

1000

10ADCCPU

WirelessTransmitter

WirelessReceiver

1Lowpass

Filter

Computer

EEG Signal Flow Control Signal Flow

Conditioning

Conversion

Transm

ission

Signal FlowS

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ElectrodeHighpass

Filter

Sample andHold

1000

10ADCCPU

WirelessTransmitter

WirelessReceiver

1Lowpass

Filter

Computer

EEG Signal Flow Control Signal Flow

Conditioning

Conversion

Transm

ission

Signal FlowS

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l s

ElectrodeHighpass

Filter

Sample andHold

1000

10ADCCPU

WirelessTransmitter

WirelessReceiver

1Lowpass

Filter

Computer

EEG Signal Flow Control Signal Flow

Conditioning

Conversion

Transm

ission

Other Features

• EMF Shielding– Faraday cage– Cable shielding

• Auto-Zero Function– Remove constant DC offset introduce by

electronic components

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Areas of Further Research

• Gauging depth of electrodes• Improving setup time• Ag/AgCl dry electrodes• Electrode polarisation• Effectiveness of Faraday cage• Noise analysis• Artifacts from physiological rhythms

Manufacturing Materials Cost

• Total - around US$4k– Mechanical - US$100– Electrode - US$2300– Signals - US$1800

• Prototype materials - estimated US$50k

Why you should upgrade to the Porto-Cap™ . . .

Wireless communication – no huge cables to lug around, you are free to walk around the house just like with a cordless phone.

Data recorded from each different electrode is clocked at exactly the same instant – at last, a device which accurately maps which bits of your brain that weren’t working during that embarrassing moment.

No scalp preparation required – the feeling of blunt needles scraping your scalp will only be remembered in your nightmares.

Porto-Cap™ uses dry electrodes only – that gooey conducting paste can now be used as axle grease.

Zero signal drift – the amplifier and filter package has been hand picked so that the only interruptions during a testing period will involve rubble entering your building.

Eight selectable sampling frequencies – if flexible data rates is you, then you won’t die hungry when wearing the Porto-Cap™.

ConclusionThe EEG Porto-Cap™ successfully enables repeatable and reliable measurements of

EEG signals. It achieves this with little discomfort, no paste or gel and in

conventional environments.

Thank you for coming,

we would like to take this

opportunity to answer any

questions you may have.

Team 5

ENGN4017 - Engineering Design

Autumn 1999