Piezoelectric Transducer Working Principle

Hi friends,
In this article, I am discussing the piezoelectric effect, piezoelectric transducer working principle, its working, and applications. You will find this article informative and interesting, I hope so.
The Piezoelectric transducer is an active transducer. It is generally used to convert mechanical stress or pressure into an electrical signal. These physical quantities cannot be measured directly. Therefore, with the help of piezoelectric transducer, these are converted into an electrical signal first after that used for other purposes like measurement or process control.

Piezoelectric Transducer Working Principle

A quartz crystal exhibits a very important property known as the piezoelectric effect.
When some mechanical pressure is applied across faces of a quartz crystal, a voltage proportional to the applied mechanical pressure appears across the crystal. Conversely, when a voltage is applied across the crystal surfaces, the crystal is distorted by an amount proportional to the applied voltage.

piezoelectric transducer working principle, piezoelectric sensor working principle

This phenomenon is known as the piezoelectric effect and the material that exhibits this property is known as a piezoelectric material.
All piezoelectric transducers work on the piezoelectric effect. In a piezoelectric transducer, a piezoelectric material is used as a sensing element which transforms input mechanical quantity into a proportional electrical signal. This is the basic piezoelectric transducer working principle.
Besides quartz, the other substances that exhibit the piezoelectric effect are Rochelle salt and tourmaline.
Rochelle salt exhibits the greatest piezoelectric effect, but its applications are limited to manufacture of microphones, headsets, and loudspeakers. It is because Rochelle salt is mechanically weakest and strongly affected by moisture and heat. Tourmaline is most rugged but shows least piezoelectric effect.
Quartz is a compromise between the piezoelectric effect of Rochelle salt and the mechanical strength of tourmaline. It is inexpensive and readily available in nature.

Piezoelectric Transducer Working

In a piezoelectric transducer, a piezoelectric material like quartz crystal is used as a sensing element. When a dynamic force or dynamic pressure is applied to a piezoelectric transducer a charge generates on the surface of the crystal. This charge appears as a potential difference across the electrodes fitted on opposite sides of the crystal.

  • The charge so generated is very small in magnitude. Therefore it has to amplify with the help of a charge amplifier to get a sufficient output. The output instrument is calibrated in terms of input measuring quantity.
  • If we apply a static force or static pressure, there will be no output voltage. Therefore, input measuring quantity should always be dynamic.
  • The magnitude of the output voltage is directly proportional to the applied force.
piezoelectric working principle, piezo electric transducer working
  • The polarity of the generated voltage depends upon the direction of the applied force. Therefore, the polarity of generated voltage for tensile force and compressive force will be opposite in polarity on the same piece of piezoelectric material.

Charge Sensitivity of Crystal

Charge induced on a crystal is proportional to the applied force,
i.e.    Q α F
or      Q = dF
or      d = Q/F, is the charge sensitivity. Its unit is C/N.
The charge sensitivity, d is direction dependent. 
If d11 is the charge sensitivity in direction 1,
d22 is the charge sensitivity indirection 2,
dnn is the charge sensitivity indirection n,
Then the overall charge sensitivity of the crystal, g = √(d112 + d222 + ……. + dnn2)

Output Voltage Equation

Output voltage, E = gPt
where, P = applied pressure, t = thickness of the crystal,
g = voltage sensitivity of the crystal, its unit is Vm/N
g = d/ɛ = d/ɛo ɛr

Advantages of Piezoelectric Transducer

  • Piezoelectric crystal transducers have a high-frequency response.
  • They have a high transient response.
  • They are very rugged.

Disadvantages of Piezoelectric Transducer

  • The output obtained from the piezoelectric transducers is very low.
  • Temperature and humidity may affect the output in some cases.
  • They have high impedance.
  • They cannot measure static pressure or force. If a static pressure or force is applied to a piezoelectric transducer, the output will be zero.

Applications of Piezoelectric Transducer

  • They are very useful for dynamic measurement.
  • They are very useful to study a high-speed phenomenon like explosions and blasts.
  • They are very useful for stress, force, and pressure measurements.

Applications of Piezoelectric Effect

Crystal Oscillators: In crystal oscillators, the usual electrical resonant circuit is replaced by a mechanically vibrating crystal. The crystal (usually quartz) has a high degree of stability in holding constant at whatever frequency the crystal is cut to operate. The crystal oscillators are, therefore, used whenever great stability is needed, for example, communication transmitters and receivers, digital clocks, etc.
Impact printer head: Dot matrix impact printers driven by multilayer piezoelectric ceramic actuators have been successfully produced on a large commercial scale. The printing pin element consists of a piezo-actuator, a stroke amplifier operated on the lever principle and a printing wire.
When a pulse with a peak voltage of 150 V is applied to a piezoelectric actuator, the printing wire moves by about 40 µm, making the tip of the wire to hit the paper through ink ribbon.
Medical ultrasound applications:  A piezoelectric material can be used for both active and passive transducer applications. In the passive mode the transducer act as a sound receiver i.e. there is the conversion of sound energy into an electrical signal. The converse piezoelectric effect permits a transducer to act as an active sound transmitter.
In the pulse-echo mode, the transducer is used to perform both the active and passive functions at the same time. A sound wave is propagated into the medium and a faint echo is received back after a small time gap due to the acoustic impedance mismatch between interface materials. This principle is used in transducers for ultrasonic medical imaging applications.
Thanks for reading about “piezoelectric transducer working principle”.

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