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Blog Impact, Vibration and Ultrasound Sensing with PVDF Piezo Sensors
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  • Author Author: shabaz
  • Date Created: 13 Apr 2014 10:57 PM Date Created
  • Views 9397 views
  • Likes 10 likes
  • Comments 54 comments
  • ultrasound
  • ioe
  • piezoelectric
  • sensors
  • piezo-electric
  • piezo
  • iot
  • vibration
  • pvdf
  • sensor
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Impact, Vibration and Ultrasound Sensing with PVDF Piezo Sensors

shabaz
shabaz
13 Apr 2014

This post is about an interesting, low-cost sensor that doesn’t need much processing to use, and has some unique characteristics – a PVDF (polyvinylidene difluoride) Piezoelectric sensor. The sensors looks like a small strip of plastic, and can be used for detecting movement or vibrations even into ultrasound. Such devices can help sense in many practical, real-world scenarios. They are extremely sensitive, low cost and easy to use.

Some simple practical experiments with these sensors are described, finally looking at detecting ultrasound.

image

 

Introduction

Sensors will play a big role in IoE helping to identify what is happening in the real world. I was keen to try out PVDF sensors because I think they could have a lot of applications. For example, they could be used to identify when home appliances are in use (due to vibrations), such as a washing machine, or as movement detectors (sensing people walking on a floor) or glass breaking.

There is a general overview on Wikipedia, but in summary PVDF (also known as PVF2) is a plastic that, when suitably manufactured (it is stretched, heated and an electric field applied during manufacture), becomes a piezoelectric material, meaning that it can generate a charge when some force is applied to it. It appears that stretching orientates the molecules into a common direction, and the electric field (at a temperature close to melting point) further aligns the molecules from an electrical orientation standpoint.

The PVDF sensor available from Farnell / Newark is a small sheet of the material with a layer of metal on each side and two crimped contacts (PVDF has a low melting point so be quick with the soldering iron). It is about the size of a small stamp.

There is a wealth of information for PVDF sensors here (in particular the Piezo Technical Manual on that page). It shows that PVDF has many applications including switches, impact, vibration and ultrasound.

 

Initial experiments

According to what I’d read, the voltage output from such sensors was quite high. The first step was to connect it up to the oscilloscope and see if this was the case, and if anything was measurable. It turns out the answer was Yes!

Here is the output when the sensor was lightly flicked – the amplitude was 6V.

image

This level of output could be interfaced to a microcontroller with no amplification needed, but some protection would be needed (either clamp with diodes, or (as shown in the technical document) limit the current with a series resistor).

This was the output when gently blowing on the sensor from a few centimetres:

image

Again, this is quite a high voltage. It would not need much amplification to allow connection to a microcontroller.

 

Maximising Sensor Output

PVDF sensors generate a charge when mechanically stressed. It turns out that to get the maximum output from the sensor, the stress needs to be in a direction that depends on which axis of the sensor the film was stretched during manufacture. For the Farnell sensor the stretched direction is along its longest direction. The most convenient way to use the sensor for general vibration or impulse detection is to perform tensile stress in that direction. A sideways force can do that. In other words, if it is bent then you’ll get a high voltage out.

image

The reason that bending the PVDF sensor generates such a stress is that the sensor is being stretched on one side when flexed. Both sides need to be stretched for maximum output, and that can be achieved by having the PVDF film on some thickness of material, to move the center onto one side. The particular sensor used already has the film off-center as far as I can tell (it has a protective layer with a certain width on one side, but not the other).

 

Example Application

As a quick experiment it was decided to try to create an example application. Here the sensor was mounted with a springy metal end, to try and detect paper labels.

image

 

When the strip of labels is dragged across, a signal is clearly observed on the oscilloscope. However some processing would be needed, because even dragging across plain paper generates a signal. In fact, even a tap on the table would generate a signal. Clearly it is extremely sensitive.

 

Amplifying the Signal

The fact that it was so sensitive got me interested in seeing how the sensor could be used with yet more subtle disturbances.

image

The circuit shown above was constructed. The op amp does have input protection up to some limit, but to protect against any accidental knocks to the sensor it would be good to use a series resistor (a few hundred ohms) to the input of the op amp, and diode clamps (two low leakage diodes like BAS116) connected from the input to +5V and from the input to -5V (both in the reverse biased direction).

image

 

The gain was set to 100. The amplifier functions from DC to several hundred kilohertz, so it needs a bit of care during construction. The circuit functioned, but, as expected, the high input impedance makes it very sensitive to pick up 50/60Hz mains hum too. Any mains cable placed close to the sensor will cause a signal to get picked up.

 

The oscilloscope trace below shows the result from placing the circuit in a biscuit tin (not fully enclosed because I didn’t make a hole for the cable). There is some source being picked up (40mV p-p output from the circuit) at about 1.3kHz – I couldn’t identify it (it may be a nearby equipment fan which I couldn’t switch off). At this level of sensitivity sounds are picked up by the PVDF sensor.

image

 

A high-pass filter or alternatively a notch filter could be used to reduce the effect. However, no high pass filter was used so I could see low frequency vibrations too. It was extremely sensitive. If placed on the floor so that the sensor was gently pressing on the desk (as shown in the photo above), a light touch of the desk would result in a signal. On the floor, it could detect anyone walking nearby.

For a real application, as mentioned, some filtering would be needed to expose just the frequencies of interest. The sensor easily picks up mains hum (shielded versions are available). The filtering could be analog, or alternatively it could be digital once the signal has converted using an ADC.

 

Experimenting with Ultrasound sensing

One application for PVDF sensors is ultrasound detection. PVDF can also be used for ultrasound generation but that was not explored here. To generate ultrasound I used a low cost ‘ping’ module and positioned it facing the PVDF sensor.

image

 

The output from the module was tapped (from the output) and observed on the oscilloscope. For the PVDF sensor, the same op amp circuit as before was used. It was positioned so that the end of the PVDF sensor was not touching anything. The traces here show the captured result. For each brief 40kHz burst from the ping module, a clear signal was picked up (the signal is on top of 50Hz mains hum here; I performed this test outside of the biscuit tin. A 50/60Hz notch circuit could filter it out if ultrasound was the only signal of interest).

image

The image below shows a zoomed-in view of the signals:

image

It is possible to see that the ping module sends a burst of 8 pulses to the ultrasound transmitter. Resonance causes the generated signal to rise in amplitude; although the distance was not accurately measured the transmitter and the sensor were positioned quite close (a few centimeters) from each other and that agrees with the time difference between the signals (the trace is marked from the end of the burst and the peak of the received signal in the oscilloscope capture above).

 

According to the document mentioned earlier, ultrasound sensing works best when the PVDF film is bent into a curved shape. So, the sensor was flexed slightly and the test was reattempted.

image

This time, the output from the circuit was far greater; almost 1V p-p from a distance of about 2cm. It was still easily detectable from 30 cm away. I didn’t try further.

 

Summary

The above only touches on some aspects of PVDF sensors, but there are many interesting use-cases that could be explored. It can be seen that very little interfacing circuitry is needed to make use of these sensors!

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Top Comments

  • koudelad
    koudelad over 7 years ago +4
    For the past few days I was trying to figure out, what sensors are used in the babies breathing monitors (to quickly react in case of a Sudden infant death syndrome ). They seem to use a simple piezoelectric…
  • Instructorman
    Instructorman over 11 years ago +2
    Thanks for posting on this very interesting sensor technology Shabaz. I have some of these sensors equipped with small masses on the far end (away from the connection points). It is easy to get >60 V output…
  • mcb1
    mcb1 over 11 years ago in reply to Instructorman +2
    At work we use Vaisala Ultrasonic Wind gauges. They have quite detailed information about how they work. (which I can't find online) Despite the use of Tx and Rx at all 3 heads (total of 96 measurements…
  • balearicdynamics
    balearicdynamics over 6 years ago

    Hey Shabaz! Thank for this new piece of experimental knowledge. image

     

    This post is very helpful, I have some of these around and for now, as I have no time to make tests I have just bookmarked the article. Very clear and super useful.

     

    Enrico

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  • dougw
    dougw over 6 years ago in reply to nuzhat

    In order to measure vibration a PVDF sensor needs to be glued rigidly to the part that is bending. If the part is not deforming or bending, the sensor might not work well as it only responds to deformation. If you let the sensor flop around, it will bend due to its own mass, but the vibration modes will not be the same as the test subject. For vibration it is all about setting things up so that the sensor deforms at the same frequency that the test subject is vibrating. We cannot provide much more help without knowing the materials, the geometry, the frequencies and the application.

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  • nuzhat
    nuzhat over 6 years ago in reply to dougw

    Basically we give vibration and record the response in the form of voltage through oscilloscope, but the impedance of the probe used it 1M ohm and as the sensor is tailor made at our lab the impedance is high and m not able to measure it and m not able to bifercate the difference between noise and sensor response in the output. These sensors when I test for eg: if 1g acceleration is given output response is obtained, again if same 1g acceleration is given on same material I get a different output. So when check its being mentioned that there should be impedance matching done inorder to use any sensor with oscilloscope. So I required a clarification on that sir

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  • dougw
    dougw over 6 years ago in reply to nuzhat

    PVDF is a non-conductive homopolymer. PVDF sensors have PVDF sandwiched between 2 metallic conductors. It is possible to get Kynar in a conductive formulation which would make the sensors much lower in impedance, but I'm not sure why you might want this as the response would be dramatically degraded.

    https://www.extremematerials-arkema.com/en/product-families/kynar-pvdf-family/physical-and-mechanical-properties/

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  • dougw
    dougw over 6 years ago in reply to nuzhat

    It would be easier to suggest a good solution if you explained what you need to measure.

    Kynar is a trade name for some types of PVDF. It melts at 155C and can be used up to 150C. You can make senors or actuators with it.

    These materials do not generate charge directly from force - the charge they generate is proportional to strain (deformation) which is produced indirectly by force.

    If you want the response to be consistent the strain must be consistent and the strain versus time must be identical.

    Very small differences in the strain versus time characteristic can result in large differences in output.

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