Underwater Pressure Sensor1

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6/17/2013

Presented by:

AYSWARYA B(551)ANSHAD ASHRAF(510)S7 ECE

A FLEXIBLE UNDERWATER PRESSURE

SENSOR ARRAY USING A CONDUCTIVE

ELASTOMER STRAIN GAUGE

Guided by:

Mrs.Mridula Reghunathan



INTRODUCTION

A pressure sensor measures pressure, typically of gasses or liquids.

Pressure is an expression of the force required to stop a fluid from

expanding, and is usually stated in terms of force per unit area.

A pressure sensor usually acts as a transducer; it generates a signal as a

function of the pressure imposed.

Pressure sensors can alternatively be called pressure transducers, pressure

transmitters, pressure senders, pressure indicators and piezometers,

manometers, among other names.

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Pressure Sensors

• Not easy to measure pressure directly from its action on theproperties of a particular material.

• Low sensitivity and poor performance.

• Only advantage - very low cost.

• The great majority of pressure sensors are “composite sensors”



Strain gauge type

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• Sensitivity of the sensor or Gauge factor,

• For metal wire gauges (constantan), G ~ 2

• silicon semiconductor gauges higher sensitivity, G ~ 200

l/l

R /R G



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A Flexible Underwater Pressure Sensor Array – Why ???

The Automated Underwater Vehicles (AUVs) uses sonar and vision-based

systems.

Sonar suffers from multipath propagation issues in the cluttered seabed

environment.

Vision based systems are limited by turbidity of the sea water.

Both systems are forms of active sensing

But less energy efficient



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A flexible sensor array

transduces underwater pressure variations produced by moving

objects and surface waves.

Exhibit a 0.0014 fractional resistance change per 100 Pa.

achieving a high 1.5 Pa pressure resolution.Measurement has a repeatability of 22% of the peak amplitude

of the pressure waveform, due to creep.

Consists of a strain-concentrating polydimethylsiloxane(PDMS) diaphragm and a resistive strain gauge made of a

conductive carbon-black-PDMS composite.



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Comparison between silicon pressure sensors based on

MEMS and an elastomer material set

First one is limited in their spatial resolution, but sensor array using a

conductive elastomer have a high 1.5-Pa pressure resolution.

First one does not exhibit the flexibility or chemical robustness required

for use as an artificial lateral line and must be protected from the water

with a parylene coating. The other have all these properties.



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A Flexible Underwater Pressure Sensor Array using a

Conductive Elastomer Strain Gauge – What ???

PDMS is used for the substrate and the diaphragm.

A conductive elastomer composite made of PDMS doped with carbon

black nanoparticles is used as the resistive strain gauge.

Consists of individual sensors arranged in a 1-D strip.

The array’s flexibility allows it to be mounted along the curved hull of

an AUV.



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A Flexible Underwater Pressure Sensor Array – Use ???

It can transduce tiny underwater pressure variations on the order of 10

Pa produced by relative motion between objects and the water surrounding

the AUV.

Information about the fluid flow provided by the array enables passive

detection of underwater obstructions or targets.

Can be used to enhance the AUV’s propulsive efficiency and

maneuverability.

Exhibit a high pressure resolution, flexibility and underwater operating

capability.



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A Flexible Underwater Pressure Sensor Array-

Design???

It Consists of four pressure sensing cells. Each cell in the array

comprises four main components.

PDMS Substrate

Strain-Concentrating PDMS Diaphragm.

Pressure Equilibration Channel

Carbon Black PDMS Resistive Strain Gauge



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1. PDMS Substrate

flexible substrate for the pressure sensor array

it is widely used in MEMS and provides electrical insulation.

While watervapour diffuses rapidly through PDMS, liquid water can

not diffuse deeply into PDMS, although surface properties are

affected.

i.e. PDMS substrate provide flexibility and water-proofing



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2. Strain-Concentrating PDMS Diaphragm

transduces a pressure difference between the environment and sensor’s

internal cavity into a strain.

PDMS diaphragms will have a linear stress-strain response to dynamic

(ac) signals.

Linearity is improved by keeping the deflection smaller than the

membrane thickness.



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3. Pressure Equilibration Channel

It has 2 by 1 mm cross-sectional area.

connects all four pressure sensor cavities to an external pressure

reference.

Allow the array to operate arbitrary water depth.



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4. Carbon Black PDMS Resistive Strain Gauge

The strain gauge material is a composite made of an elastomer doped

with conductive filter particles.

A contact resistance R contact exists between the metal contact and the

strain gauge resistor of interest R gauge. Additionally, metal wiring is

stiff and can not be routed onto the PDMS diaphragm. This creates an

additional series resistance R tap which reduces sensitivity.



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The four-point probe, alternatively known as a Kelvin structure, solves

the contact resistance problem by measuring R gauge alone, without

being affected by series resistances R contact or R tap.

A constant current Itest is passed through the two current terminals

while the voltage Vtest is measured across the two terminals. Since, no

current passes through the voltage terminals, no voltage appears across

the R contact and R tap on the voltage terminal branches.



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The four-point probe, alternatively known as a Kelvin structure, is

shown.



FABRICATION

PDMS is cast into Al moulds to form cavity, channel structure and the two

sheets.

Carbon black-PDMS composite-Ratio 1:6.

screen patterning technique-pattern the strain gauges and carbon black

PDMS paste is spread over the screen Glass slide is swept over the screen.

entire device is flash cured on a hot plate at 120C whilw filler particles

are still uniformly dispersed.

Finally additional pdms is used to seal the remaining exposed regions of

starin gauges and wires.

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A Flexible Underwater Pressure Sensor Array using a

Conductive Elastomer Strain Gauge –

Characterization It is dynamically characterized by applying small-amplitude pressure

signals at several dc bias pressures up to 1 kPa.

The sensor output voltages, proportional to the strain gauge resistances

R gauge .

An estimate of power consumption of a single sensor is given by the

power dissipated in R gauge.

Pgauge = I²src * R gauge



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An estimate of power consumption of a entire array is,

Parray = I²src * R total

Here, Pgauge ≈ 10 µW

Parray ≈ 162 µW

If, R gauge ≈ 3.2 to 32 kΩ

R array ≈ 448 kΩ

This shows that much power is lost due to contact resistances and current

branch series resistances.



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Carbon Black Particles sinter together inside the elastomer matrix to

form conductive chains which provide macroscopic conductivity to the

carbon black PDMS composite.

A large tensile strain applied to the strain gauge creates fractures in the

chain, resulting in an increase in strain gauge's resistance.

Upon removal of the strain, the decrease in the resistance back to its

initial value is slower because the fractured chains must rely on their

structural stresses and Vander Waals forces to reconnect.



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UNDERWATER TESTING…How it works??

• The motion in the surface of water due to some object, leads to deflection

in diaphragm outward if a positive bias pressure is applied to the cavity,

allowing the strain gauges to operate in the tensile strain regime.

• In this pressure biasing scheme, external pressure acts to reduce the

outward deflection of the diaphragm, reducing the strain on the strain

gauge. Thus, higher external pressure corresponds to lower strain gauge

resistance and a reduced output voltage.



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• As diagrammed below, the wave machine is used to generate sinusoidal

surface waves which propagate across the pressure sensor array.

• The center-to-peak amplitude A of the surface waves was measured to be 4

mm. Given plot shows array output in response to the surface waves. The

period of the surface waves is 0.67 s, determined using the time elapsed

during eight periods. This corresponds to a frequency f of 1.5 Hz. Based on

the time offset ∆t = 0.2 s between the waveforms of the sensors 1 & 4, the

velocity v of the waves is determined to be



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Where D = Spacing between the centers of sensors 1 & 4.

Thecomplete expression for the pressure P that the sensor array will experience isgiven by

Where Ps = Static component of average underwater depth pressure.Pd = Dynamic component of Pressure which decay

exponentially with water depth since there is less flow farther away from thesurface.

ρ = Density of water; g = acceleration due to gravity; k =wave number; y = depth of sensor; w = angular freq.



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CONCLUSION

• In this pressure sensor array, small changes in strains results in more

reversible linear resistance changes due to bending of the carbon chains in

diaphragms(motion in underwater).

• high pressure sensitivity over a small range that is the basic requirement

for artificial lateral line application.

• This pressure sensor array utilizes the flexibility, chemical robustness

and waterproofing along with resolution required for use as an artificial

lateral line without requiring the cost and processing complexity of silicon.



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REFERENCE

• F. M. Yaul, V. Bulovic, J. H. Lang, ‘ A Flexible underwater pressure

sensor Array using conductive elastomer strain gauge’.

• F. M. Yaul, ‘ A Flexible underwater pressure sensor Array for

artificial lateral line applications’.

• D. W. Lee and Y. S. Choi, ‘ A Novel pressur sensor with a PDMS

diaphragm’.



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THANK YOU

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Underwater Pressure Sensor1