Piezoelectric Element Complete Reference

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PIEZOELECTRIC ELEMENT

1.1 INTRODUCTION

Generating power locally from the industrial machines  with in the industries itself can solve many of the problems. Piezoelectric material can be used to harvest energy from such vibration, and use the harvested energy as a power supply for the sensors. The unique physical properties of piezoelectric material is that when a piezoelectric material is subjected to stress/strain it produces electrical charge on its surface, and vice versa, i.e. when charge flows through such material, the material goes through physical deformation which is shown in Figure 2.1.stress and strain on the piezoelectric material, the charge produced moves in one direction under positive stress and in the opposite direction under negative stress. This indicates that the power produced at the output of the piezoelectric material is in the form of alternating positive and negative cycles i.e., ac output is produced.

1.2 PRINCIPLE OF OPERATION

The piezoelectric effect is stated as follows :

“Whenever a pair of opposite faces of a piezoelectric crystal is subjected to stress or strain , an equivalent amount of voltage drop is produced across the other pair of opposite faces of the crystal. The amount of voltage obtained depends upon the intensity of the stress or strain applied over the crystal”.

1.3 PIEZOELECTRIC EFFECT BASICS

A piezoelectric substance is one that produces an electric charge when a mechanical stress is applied (the substance is squeezed or stretched). Conversely, a mechanical deformation (the substance shrinks or expands) is produced when an electric field is applied. This effect is found in crystals that have no center of symmetry. To explain this, we have to look at individual molecules that make up the crystal.

Each molecule has a polarization, one end is more negatively charged and the other end is positively charged, and is called a dipole. This is a result of the atoms that make up the molecule and the way the molecules are shaped. The polar axis is an imaginary line that runs through the center of both charges on the molecule. In a monocrystal the polar axes of all of the dipoles lie in one direction.

The crystal is said to be symmetrical because if we were to cut the crystal at any point, the resultant polar axes of the two pieces would lie in the same direction as the original. In a polycrystal, there are different regions within the material that have a different polar axis. It is asymmetrical because there is no point at which the crystal could be cut that would leave the two remaining pieces with the same resultant polar axis. Figure 2.3 illustrates this concept.

In order to produce the piezoelectric effect, the polycrystal is heated under the application of a strong electric field. The heat allows the molecules to move more freely and the electric field forces all of the dipoles in the crystal to line up and face in nearly the same direction 

The piezoelectric effect can now be observed in the crystal. Figure 2.5 illustrates the piezoelectric effect. Figure 2.5(a)shows the piezoelectric material without a stress or charge. If the material is compressed, then a voltage of the same polarity as the poling voltage will appear between the electrodes (b). If stretched, a voltage of opposite polarity will appear (c). Conversely, if a voltage is applied the material will deform. A voltage with the opposite polarity as the poling voltage will cause the material to expand, (d), and a voltage with the same polarity will cause the material to compress (e). If an AC signal is applied then the material will vibrate at the same frequency as the signal (f).

1.4 EQUIVALENT CIRCUIT OF A PIEZO FILM

             We are now ready to draw out an electrical equivalent of the piezo film element. There are two equally valid “models” – one is a voltage source in series with a capacitance, the other a chargegenerator in parallel with a capacitance – but the latter is uncommon in electrical circuit analysis and we will concentrate on the voltage source (see Figure 2.6). The dashed line represents the “contents” of the piezo film component. The voltage source Vs is the piezoelectric generator itself, and this source is directly proportional to the applied stimulus (pressure, strain, etc).

1.4.1 ADDING IN A RESISTIVE LOAD

                  Now we can add in the effect of connecting up to the oscilloscope.The oscilloscope and its probe are modeled simply as a pure resistance, although in reality there will be a very small capacitance associated with the probe and the cable (usually in the region of 30 to 50 pF). This can be neglected if the film capacitance is significantly higher in value. The voltage measured across the load resistor RL will not necessarily be the same voltage developed by the “perfect” source (Vs). To see why, it is helpful to redraw this circuit in another way (Figure 2.7).

1.4.2 POTENTIAL DIVIDER

                   With the circuit shown in Figure 2.7 redrawn as in Figure 2.8, it is easier to see why the full source voltage does not always appear across the resistive load. A potential divider is formed by the series connection of the capacitance and the resistance. Since the capacitance has an impedance which varies with frequency, the share of the full source voltage which appears across RL also varies with frequency.

1.5 PROPERTIES OF PIEZOELECTRIC FILM

                      Piezo film is a flexible, lightweight, tough engineering plastic available in a wide variety of thicknesses and large areas. Its properties as a transducer include:

Ø Wide frequency range – 0.001 Hz to 109 Hz.

Ø Vast dynamic range (10-8 to 106 psi or μ torr to Mbar).

Ø Low acoustic impedance—close match to water, human tissue and adhesive systems.

Ø High elastic compliance.

Ø High voltage output—10 times higher than piezo ceramics for the same force input.

Ø High dielectric strength—withstanding strong fields (75V/μm) where most piezo ceramics depolarize.

Ø High mechanical strength and impact resistance (109—1010 Pascal modulus).

Ø High stability—resisting moisture (<0.02% moisture absorption), most chemicals, oxidants, and intense ultraviolet and nuclear radiation.

Ø Can be fabricated into unusual designs.

Ø Can be glued with commercial adhesives.

1.6 MODES OF VIBRATIONS

The piezoelectric crystal bends in different ways at different frequencies. This bending is called the vibration mode. The crystal can be made into various shapes to achieve different vibration modes. To realize small, cost effective, and high performance products, several modes have been developed to operate over several frequency ranges. These modes allow us to make products working in the low kHz range up to the MHz range. Table 2.1 shows the vibration modes and the frequencies over which they can work. An important group of piezoelectric materials are ceramics. Using these various vibration modes and ceramics many useful products, such as ceramic resonators, ceramic bandpass filters, ceramic discriminators, ceramic traps, SAW filters, and buzzers are made.

1.9 SPECIFICATIONS

Operating voltage                     : 30Vp-p max

Resonant frequency                  : 4.5KHz ± 0.5

Resonant impedence                 : 500Ω

Electrostatic capacity               : 16,000 ±30%

Operating temperature             : -20 to +70°C

Storage temperature                : -30 to +80°C

Diameter                                   : 27.0mm

Depth                                       : 0.51mm

Weight                                      : 2.0 grams

Material                                    : brass

Termination                              : wire leads

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