TDS PVDF SHOCK GAUGES - piezotech.eu€¦ · For shock wave measurement with rising time shorter than time of transit in the sensor (tilt = 0), first short circuit (CVR ~1Ω) current

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


3/11 www.piezotech.eu

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.



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)



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



- 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.



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.



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.



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.



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.


August 2018 www.piezotech.eu

Contact Information

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Phone: +33 4 72 39 87 03

www.piezotech.eu

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TDS PVDF SHOCK GAUGES - piezotech.eu€¦ · For shock wave measurement with rising time shorter than time of transit in the sensor (tilt = 0), first short circuit (CVR ~1Ω) current