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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
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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”
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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.
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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’.