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Transcript of Piezoelectric Transducers
A piezoelectric impulse hammer consists of a head assembly containing a force sensor and a handle. Interchangeable head extenders and several different impact tips (steel, aluminum, hard and soft plastic) are available.
The piezoelectric effect describes the relation between a mechanical stress and electrical voltage in solids.
An applied mechanical stress will generate a voltage, and conversely an applied voltage will change the shape of the solid by an amount 4% maximum.
Piezoelectric high frequency transducers generate, receive, or generate and receive ultrasonic signals that can be used to measure distances in air, water, or other fluid media, to determine flow rates or for other applications.
When an electrical signal is applied to a thin wafer element, it vibrates with the wavelength twice of its thickness. So that is why Piezoelectric crystals are cut to a thickness that is 1/2 of the desired wavelength.
The piezoelectric effect occurs only in non-conductive materials. They are divided into two main groups: crystals and ceramics.
The most important one is Quartz, and there are also synthetic ones such as Lithium Sulphate and ferro-electric ceramics such as Barium Titanate.
The distortion due to the mechanical forces in the asymmetrical lattice of material causes a reorientation of electrical charges within the material, resulting in a relative displacement of positive and negative charges. The charge displacement induces surface charges on the material of opposite polarity between the two sides. By implanting the electrodes into the surface of the material, these charges can be measured as an output voltage, defined by:
Polarization of Piezoelectric Material
Subject a piezoelectric material to a large voltage near Curie Temperature (above which the material loses its piezoelectric property), then the dipoles align.
The piezo effect is used in sensors, and the actutator employs reverse piezo effect.
Piezoelectric and Reverse Piezo Effect
As the voltage is applied across piezoelectric material, it transmits waves.
When the receiver receives an ultrasonic wave, alignment of dipoles occur and we get electrical signals in form of voltage.
Piezoelectric Pressure Sensor
Piezoelectric pressure transducer take advantage of the electrical properties of naturally occurring crystals such as quartz. These crystals generate an electrical charge when they are stressed. Piezoelectric pressure sensors do not require an external excitation source and are very rugged.
Charge Amplifier for Pressure Sensor
The amount of charge and in turn voltage generated in the crystal due to application of pressure is very less, that is why a high impedance is connected. Charge amplifier is applied whose primary function is to amplify charge/voltage, so that signal can be recorded with a low impedance voltage circuit.
Muhammad Sohaib Akram (149 - 2011)
Muneeb Ul Qayyum
(154 - 2011)
Department of Mechanical Engineering
College of Electrical and Mechanical Engineering, NUST