Piezoelectric sensor and its application

When a crystal undergoes strain, it displaces a small amount of charge. In other words, when the distance between atoms in the crystal lattice changes some electrons are forced out or drawn in. This also changes the capacitance of the crystal. This is known as piezoelectric effect.

A piezoelectric substance is one that produces an electric charge when a mechanical stress is applied.On the application of electric field substance squeezed or stretched. This effect is formed in crystal that have no center of symmetry. To clarify it we should study each internal molecular activity. Each molecule has polarization, One end is positively charged and another end is negatively charged, and is called Dipole. This is the result of the atoms that make up the molecule and the way molecule is shaped. The polar axis is imaginary line that runs through the center of both charges on the molecule. In mono crystal the polar axes of all the dipole lie in one direction. In poly crystal, there are different regions within the material that have different polar axis. This is asymmetrical. Below figure suggest you to analyze molecular nature…

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

The below figure illustrate the piezoelectric effect.

 Fig 3

(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 is 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 frequency.

The piezoelectric crystal bends in different ways at different frequencies. This bending is called Vibration Mode. The crystal can be made into various shapes to achieve different vibration modes. An important group of piezoelectric materials are ceramics. Murata utilizes various vibration modes and ceramics to make many useful products, such as ceramic resonators, Ceramic band pass filters, Ceramic discriminators, Ceramic traps, SAW filters and buzzers.

The below figure shows the vibration modes and the frequencies over which they can work.