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TDS PVDF SHOCK GAUGES
Piezoelectric Sensors
Electroactive Polymers for Organic Electronics
www.piezotech.eu
Table of Contents
I. PVDF shock sensors .......................................................................................................... 3
II. Standard PVDF sensor specifications ............................................................................ 4
III. Piezoelectric Polyvinylidene Fluoride (PVDF) films .................................................... 5
III.1 Piezoelectric and Pyroelectric effects ................................................................ 5
III.2 Piezoelectric Films .................................................................................................. 5
III.3 Properties of PVDF Piezoelectric films ................................................................. 5
III.4 Examples of Applications ..................................................................................... 5
IV. Piezoelectric Sensors .................................................................................................... 6
IV.1 Principle ................................................................................................................... 6
IV.2 Equivalent Circuit ................................................................................................... 6
IV.3 Calibration Curve .................................................................................................. 7
IV.4 Measurement Methods ........................................................................................ 7
IV.4.1 Current mode: RC < T (rising time) ................................................................... 7
IV.4.2 Voltage mode: RC >> T ..................................................................................... 9
IV.4.3 Charge Mode..................................................................................................... 9
V. Handling ............................................................................................................................ 9
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I. PVDF shock sensors
Polyvinylidene fluoride sensors (PVDF) are the sensors of choice for a wide range
of measurement applications because they have unique characteristics:
Rapid response (Nanosecond)
Large stress range (kPa to GPa)
Large signal to noise ratio
Sensitivity (4 μC/cm2 for 10 GPa)
Very thin (25 μm)
Simple circuitry
Piezotech® is the sole source for PVDF sensors suitable for shock physics research. The
important features available only with Piezotech® sensors include:
Reproducibility: The sensors are all made from a high quality biaxially stretched
polymer material, poled by a patented process to provide a stable and
consistent polarization. Evaporated gold over chromium electrodes allow
precise active area measurement.
Calibration: These are the only PVDF sensors that are supported by extensive
shock calibration data. These data range from a few kPa to over 25 GPa.
Technical support: The Piezotech staff has significant experience in all aspects
of PVDF fabrication and use. We stand ready to support your experimental
efforts. We provide the guidance you need to apply the transducer to your
experimentation.
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II. Standard PVDF sensor specifications
Description Model
Basic PVDF Sensor with parallel lead S_25
S_25 with connector mounted S_25C
S_25C with laminated polyester protection on both sides* S_25CP
S_25CP electrically shielded S_25CPB
BNC Cable (1 m)
*The polyester thickness is 55 μm for each side
Sensing areas available:
1x1 mm
5x5 mm
Material: Biaxially stretched 25 μm PVDF
Poling – Calibration:
Crossed lead strip sensing area.
Remnant polarization = 9.2 ± μC/cm2, by the patented
method
Leads: Evapored (1500 Ǻ of Au over 100 Ǻ of Cr)
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III. Piezoelectric Polyvinylidene Fluoride (PVDF) films
III.1 Piezoelectric and Pyroelectric effects
When certain materials are subjected to mechanical stress, electrical charges
proportional to the stress appear on their surface. When an electric potential
difference is applied to these materials, mechanical deformation occurs. This effect is
known as piezoelectricity. When the temperature of the material is changed, an
electric potential appears between the terminals, this is called the pyroelectric effect.
III.2 Piezoelectric Films
Piezoelectricity can be obtained by orienting the molecular dipoles of polar
polymers such as PVDF in the same direction by subjecting films to an intense electric
filed, this is the polarization. The polarized electrets are thermodynamically stable up
to about 90 °C.
PVDF is particularly suitable for the manufacture of such polarized films because of its
molecular structure (polar material), its purity – which makes it possible to produce thin
and regular films – and its ability to solidify in the crystalline form for polarization.
III.3 Properties of PVDF Piezoelectric films
Flexibility (possibility of application on curved surfaces)
High mechanical strength
Dimensional stability
High and stable piezoelectric coefficients over time up to approximately 90 °C
Characteristic chemical inertness of PVDF
Continuous polarization for great lengths spooled onto drums
III.4 Examples of Applications
Pressure pick-ups
- Distribution of pressure on surface
- Localization of impacts
- Accelerometers
- Keyboards
Robotics
- Artificial sensitive skin
- Pressure sensors
Acoustic components
- Microphones
- Ultrasonic detectors
- Hydrophones
- Sonars
Optical devices
- Laser diameter measurement
- Variable mirrors
Electrical components
- Switches
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- Miniature electric fan
Security devices
- Intruder alarms
- IER alarms
- Vibration sensors
Medical instrumentation
- Catheter
- Pedobarography
- Osteogenesis
- Lithotrophy
- Medical echography
- Blood pressure detector
IV. Piezoelectric Sensors
IV.1 Principle
Piezoelectric material delivers electrical charges under mechanical stress. Pressure
can be related to measured charges according to calibration data.
IV.2 Equivalent Circuit
As other piezoelectric components, equivalent circuit can be simplified and be
presented as a current (charge) generator.
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IV.3 Calibration Curve
At low pressures (< 1000 bars), electrical charge delivered is linearly proportional to
applied stress. The sensors sensibility is constant and equal to 15.7 pC/N.
For higher pressure ranges, though sensibility remains high, it cannot be considered as
constant anymore. Calibration data will then show the pressure reached. Maximum
pressure that can be measured is about 30 GPa.
IV.4 Measurement Methods
IV.4.1 Current mode: RC < T (rising time)
The gauge is directly connected to a low value current viewing resistor (CVR).
Current is measured on two channels (i(t)=V(t)/R). Data are transferred to a computer
to perform software aided mathematical operation.
Offset is reduced and both channels are first mixed together to get a noise free
current.
Current is then integrated versus time and divided by the sensor area to get the density
of charge that has been delivered.
Pressure is computed according to the calibration curve that relates density of charge
to pressure.
This method is highly recommended for fast phenomenon measurement.
Voltage is measured. Its value is proportional to the current passing through the
resistance which is the derivative of the generated charges.
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𝑄(𝑡) = ∫ 𝐼(𝑡). 𝑑𝑡𝑡
𝑂
𝐼(𝑡) = 𝑉(𝑡)𝑅⁄
𝑄(𝑡) =1
𝑅∫ 𝑉(𝑡). 𝑑𝑡
𝑡
𝑂
Numerical integration of this signal will give electrical charges delivered by the sensors.
Digital data acquisition devices with high sampling rates are particularly adapted to
the signal processing. If possible, it is recommended to measure the signal on two
channels with different sensitivities.
Relatively to the calibration data, the pressure can then be computed.
For several applications, a 50 Ω cable can be used and connected to the 50 Ω input
of the acquisition device. For short rising time, a low resistance CVR should be used (1
Ω or even less) in order to reduce RC time constant. CVR is to be mounted as close to
the sensor's connector as possible. Measurement components have to be carefully
chosen. Impedances have to be adapted and the inductances minimized. The level
of current that is reached is proportional to the gauge area, to the rising time of the
pressure and to the pressure level.
For shock wave measurement with rising time shorter than time of transit in the sensor
(tilt = 0), first short circuit (CVR ~1Ω) current peak can be estimated at 0.75
A/mm²/GPa.
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Sensor delivers a derivate signal. This can be an advantage when chronometric
information is needed.
IV.4.2 Voltage mode: RC >> T
The gauge is connected to an external capacitance. Measured voltage is directly
proportional to charges delivered by the sensor.
𝑄(𝑡) = 𝐶. 𝑉(𝑡)
High impedance acquisition devices can be used. Total capacitor has to be
considered (source capacitor, cable capacitor, device capacitor and optional
additional capacitance).
Measurement is only valid as long as t << R.C. This method doesn‟t require specific
instrumentation but capacitor value C has to be precisely determined. As electrical
impedances are not adapted, it can only be used for slow signals. For fast signals,
current mode should be used.
IV.4.3 Charge Mode
The gauge is directly connected to a charge amplifier. Its output voltage is
proportional to charges delivered by the sensor. Charge amplifier characteristics
define the high and low cut frequencies. This measurement mode is simple and can
be easily used for dynamic low pressures. The accuracy and the minimum pressure
that can be measured are given by the signal/noise ratio of the amplifier.
V. Handling
The active area has to be perfectly clean. Any area exposed to air which has not just
been cleaned is to be considered as dirty and needs to be cleaned. Any contact with
fingers should be avoided. Therefore, clips must be used to handle the gauge from the
moment it is removed from its protecting sleeve until it is mounted. Eventual dust will
be removed with a soft clean cloth. To clean gauges, only use pure ethanol.
In many cases, gauge has to be glued. Materials in contact with the gauges must be
neither conductive nor polar. If this is not the case, a Teflon protected gauge has to
be used. Polar materials have to be eclectically shielded and connected to ground.
The active area of the gauge has to be perfectly plane. Its existing deformation is
normal, but it has to be removed by pressure during the mounting of the gauge.
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Assembly can be made using epoxy or cyano-acrylate glues, under high pressure to
reduce the thickness of glue. These glues are non polar, have a mechanical
impedance close to PVDF and do not damage the gauge. If you do not use a pre-
connected gauge, great care should be taken during electrical wiring in order to
minimize resistance and inductance. Low temperature weld (< 85 °C) or clincher
connectors can be used.
Positive lead is marked with a “+” sign and sensor upper corner is cut on this lead side.
Positive signal is measured on this lead when sensor is under compression.
Please contact us for more information.
Electroactive Polymers for Organic Electronics
August 2018 www.piezotech.eu
Contact Information
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Piezotech
Arkema-CRRA
Rue Henri Moissan
69496 Pierre-Benite Cedex
FRANCE
Info.piezotech@arkema.com
Phone: +33 4 72 39 87 03
www.piezotech.eu
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