What is Piezoelectric Transducer?

A Piezoelectric transducer is an electrical transducer that can transform any kind of physical quantity into a measurement-capable electrical signal. A piezoelectric transducer is an electrical device that transforms physical quantities into electrical signals by utilizing the characteristics of piezoelectric materials.

Piezoelectric materials display the piezoelectricity property, which states that the generation of an electric voltage is proportional to the application of any kind of mechanical stress or strain. By measuring the produced electric voltage with voltage measuring equipment, the amount of stress or strain placed on the material can be determined.

Types of Piezoelectric Materials

Some of the types of piezoelectric materials are:

Naturally Available Ones: Minerals from the Tourmaline group, quartz, Rochelle salt, topaz, and some organic materials like silk, wood, enamel, bone, hair, rubber, and dentin are examples. Barium titanate, Lead titanate, Lead zirconate titanate (PZT), Potassium niobate, Lithium niobate, Lithium tantalate, and other lead-free piezoelectric ceramics are examples of artificially produced piezoelectric materials.

Piezoelectric transducers cannot be made from all types of piezoelectric materials. The piezoelectric materials must fulfill a number of conditions in order to be used as transducers. The materials used for measurement purposes should be flexible enough to be manufactured into a variety of shapes without affecting their properties, have frequency stability, high output values, and be insensitive to extreme temperature and humidity conditions.

Unfortunately, no piezoelectric material possesses all of these characteristics.Quartz is a naturally occurring, highly stable crystal with low output levels. Quartz is useful for measuring parameters that change gradually. The highest output values are produced by Rochelle salt, but it is sensitive to environmental factors and cannot be used above 1150F.

Piezoelectric Transducer Working

The piezoelectricity principle is used by the Piezoelectric transducer. A thin layer of conductive material, usually silver, is applied to the faces of the piezoelectric material, which is typically quartz. Ions in the material move away from one conducting surface and toward the other when stress is applied. Charge is created as a result of this. The stress calibration process uses this charge. The direction of the applied stress determines the polarity of the generated charge. Compressive stress and tensile stress are two different types of stress that can be applied, as shown below.

Piezoelectric Transducer Formula

The amount of voltage produced is also influenced by the crystal’s orientation. A transducer’s crystals can be arranged either longitudinally or transversely.

Longitudinal and Transverse Effect

In the longitudinal effect, the charge generated is given by

Q = F * d

Where F is the applied force,d is the piezoelectric coefficient of the crystal.

Piezoelectric coefficient d of quartz crystal is around 2.3 * 10^-12 C/N.

In the transverse effect, the charge generated is given by

Q = F * d * (b/a)

The charge produced by the transverse arrangement will be greater than the charge produced by the longitudinal arrangement when the ratio b/a is greater than 1.

Piezoelectric Transducer Circuit

The working of a basic piezoelectric transducer can be explained by the below figure.

Here, a silver-coated quartz crystal is used as a sensor to produce a voltage when under stress. To measure the produced charge without dissipation, a charge amplifier is employed. R1 has a very high resistance and can only draw very little current. The calibration is also impacted by the capacitance of the lead wire connecting the piezoelectric sensor and transducer. Consequently, the charge amplifier is typically positioned very close to the sensor. Therefore, when mechanical stress is applied to a piezoelectric transducer, a proportional electric voltage is generated. This voltage is amplified by a charge amplifier and used to calibrate the applied stress.

Piezoelectric Ultrasonic Transducer

The reverse piezoelectric effect serves as the basis for how the ultrasonic piezoelectric transducer operates. In this effect, a piezoelectric material experiences physical deformations proportional to the applied charge when electricity is applied. The ultrasonic transducer’s circuit is shown below.

In this instance, the primary L3 of the transformer is connected to the quartz crystal, which is positioned between two metal plates A and B. The electronic oscillator and the transformer’s primary are inductively coupled. The secondary coils of the transformer, L1 and L2, are connected to the electronic oscillator. The oscillator generates high-frequency alternating voltage pulses with a frequency of  f=1÷(2π√L1C1) when the battery is turned ON. As a result, L3 experiences an e.m.f. that is then transmitted through plates A and B to the quartz crystal. The crystal begins contracting and expanding alternately as a result of the opposing piezoelectric effect, producing mechanical vibrations.

When the electronic oscillator’s frequency and the natural frequency of quartz are equal, resonance occurs. Quartz now emits longitudinal ultrasonic waves with a significant amplitude.

Piezoelectric Transducer Applications

• Used by automotive industries to measure detonations in engines.
• Piezoelectric materials are primarily used for measuring surface roughness, in accelerometers, and as a vibration pickup because they cannot measure static values.
• In strain gauges to measure force, stress, vibrations etc…
• They are used in seismographs to measure vibrations in rockets.
• These are used in ultrasonic imaging in medical applications.

Advantages and Limitations of Piezoelectric Transducers

The advantages and limitations of piezoelectric transducers include the following.

Advantages

• These transducers are a good option for many applications due to their high-frequency response.
• These are active transducers, making them self-generating since they don’t need outside power to function.

Limitations

• They are useless for measuring static parameters because they can only measure changing pressure.
• The transducer’s behavior can be impacted by temperature and the environment.