Piezoelectric Film Transducers



 

 "Imagination is more important than knowledge."

 Albert Einstein

Table of Contents

Executive summary
Educational Objectives
Background Information
Procedures
Questions
Performance Test Questions


Executive summary

Transducers convert one form of energy into another.  They have a range of uses, particularly as sensors.  In the last 20 years Polyvinylidene Fluoride, or (PVDF), Film along with the piezo electric effect has been used in thousands of sensing applications.  These applications range from infrared sensors, stress gauges, and vibration detectors.  There is still a myriad of possibilities to which PVDF Film can be applied.  The purpose of this lab is to familiarize you with PVDF Film, and to give you an idea of its possibilities.

Educational Objectives

After completing this lab a student should be able to:

1. Understand the basic properties of PVDF Film.

2. Familiarize themselves with some applications of the film.

3. Integrate LabVIEW by controlling the DMM and Power Supply.


Background Information

The piezoelectric effect was discovered in the late 1800s by the Curie brothers. It was discovered that when certain ceramic and crystalline structure underwent some kind of deformation they emitted an electrical pulse. Conversely, when an electrical current was run through the material it would vibrate. The first application of this technology was for SONAR. In 1969 PVDF was first created. It was found that the piezoelectric effect was higher than that found in materials used previously.

PVDF Film, or piezo film, has interesting characteristics which make it highly useful as a transducer. The film is highly sensitive to infrared energy, making it a great sensor for quick changes in temperature. When deformed it emits a high voltage spike which can be easily measured by an oscilloscope. Additionally, the voltage output is anagalous to the frequency and degree of deformation, making the material good for detecting high-pitched sound. This leads to a wide variety of industrial applications such as a strain gauge.

Figure 1 - Simple piezo film sensors.



Think of piezo film as an "electrical sponge". If the sponge is full of water and you squeeze it, most of the water flows out, as is the case with electrical current from piezo film.
Once you have allowed all the water to drain out you must relax the sponge for it to soak up more water. In the same respect, the film must be allowed to "relax" before a charge will return to it.
Once the charge returns the "electrical sponge" can be squeezed again to get a current.

In addition to giving off a current, the film has the opposite effect when receiving a current. This characteristic allows it to be used as a speaker element, a fan, or even as a component in an accelerometer.

Figure 2 - A piezo buzzer, commonly used in every day products because of its size, weight and durability. It is also inexpensive and requires little power, which makes it an ideal electrical device.



In this lab you will be examining the film's many facets. Additionally, you will create a strain gauge from the film, calibrate it, and integrate LabView to receive its input.

Procedures

Part A - Basic Piezo Film Qualities

Connect the leads of the piezo film sensor to a BNC cable which is connected to the oscilloscope, as shown below in Figure 3.

By connecting the piezo film to the oscilloscope, we are able to see the electrical output of the film. Flexing the film will create noticable waves, and an exact voltage can be determined by adjusting the oscilloscope.

Now that you have an idea of what the output looks like on the oscilloscope, try different ways of bending, stretching, and squeezing the film.

Note the qualities of the film. What seems to be the best method for producing the greatest voltage?


Figure 3 - Schematic for set-up of Part A.



Part B - Temperature Response

As was previously mentioned, piezo film is highly sensitive to heat. In this part, you will see the effect that heat has on the voltage output. First, turn on the hotplate. While the hotplate is warming up, connect the piezo film to the digital multimeter, as shown below in Figure 4.

After the hotplate warms up, hold the piezo film over the hotplate briefly, but be careful not to let it get too close.

Warning - Use common sense, hotplates get hot! Don't let the piezo film touch the hotplate or the film will melt and be destroyed
Also, make sure to turn off the hotplate when you are finished!


Watch the multimeter to see how a voltage is produced when you wave the film over the hotplate. Now try holding the film over the hotplate. What happens? What do you think the reason for this is? (Hint: think about the "electrical sponge")

Figure 4 - Schematic for set-up of Part B.



Part C - Sound Qualities

In this part of the lab you will be using a piezo buzzer to test its sound qualities.

1. Turn the DC Power Supply on and set Output 1 to a voltage in the range of the specification on the buzzer. Disable the output. Now connect the buzzer to Output 1, making sure that the positive alligator clip is attached to the lead on the buzzer marked as positive. Turn the Output 1 on, and listen to the sound created. Try using a different voltage, does it affect the sound?

Figure 5 - Schematic for set-up of Part C1.



2. Turn off the Power Supply, and connect the piezo buzzer to the function generator as shown in Figure 6. Set it to the same voltage used in step one. Now set the frequency to 1.7 KHz. Try panning the frequency up to 11.8 KHz. Notice the change in pitch and volume.

Figure 6 - Schematic for set-up of Part C2.



3. In this step, the same qualities will be examined, but in reverse. Instead of the piezo film being used as a speaker, you will use it as a microphone. To do this, a styrofoam cup will aid us in the absorbtion of the sound waves, which will then be picked up by the second piezo sensor, and finally converted into electrical form.

First, tape the piezo sensor to the back of a stryofoam cup. Then connect the leads of the sensor to the oscilloscope, as was done previously in figure 3. Now you should have a working microphone. Try speaking into the cup, and notice the voltage on the oscilloscope. The frequency of the output corresponds to the pitch of your voice.



Part D - Integration with LabVIEW

In this final part of the lab you will be creating a strain indicator. You will need the film sensor, a ruler, and tape. Also, you will be using LabVIEW as a way of interpreting the signal.

First open the LabVIEW document called piezo.vi, it should be located on the desktop. The front panel looks like this:

Control Panel

Figure 7 - Front panel view of piezo virtual instrument.



Figure 8 - Setup of Part D.



Tape the film sensor to the ruler as pictured above in Figure 8. Then hook the leads up to the DMM as was done in Part B, in Figure 4. Bend the ruler several times. Get an idea of what kind of voltage spike to expect. The VI has been calibrated to .5 mV, however, if your estimate is much greater or less than that adjust the sensitivity on the front panel.

This is an example of a real life application for piezo film. Imagine scaling the ruler and film to the size of an iron construction girder. If a building with many such girders were to experience structural damage or a severe loss in structural integrity the alarm would sound. This would signal the need for immediate evacuation of the building.

Questions

1. What does PVDF stand for?.

2. Explain the piezoelectric effect and list 3 conditions that cause piezo film to produce electricity.

3. List five applications of piezo film.

4. When a force is applied to a piece of piezo film causing it to flex, why does the voltage stop quickly even when the force is still present?

5. Based on what you saw with the oscilloscope, sketch the voltage as a function of time of what you would expect the output to be if a piece of piezo film is placed in the path of a rotating fan blade.

6. Explain the importance of the material in engineering.

Performance Test Skills and Questions