What is a piezoelectric sensor?

1. Piezoelectric effect

Positive piezoelectric effect ( parallel piezoelectric effect ) : When a certain dielectric is deformed by a force applied in a certain direction , polarization occurs inside it and electric charges are generated on its certain surface . When the external force is removed, it returns to a neutral state. When the direction of the force changes, the polarity of the charge also changes.

Inverse piezoelectric effect ( electrostrictive effect ): When an electric field is applied in the polarization direction of a dielectric, the dielectric will produce mechanical deformation or mechanical pressure in a certain direction . When the external electric field is removed, these deformations or stresses disappear.

A sensor based on the piezoelectric effect. It is a self-generating and electromechanical conversion sensor. Its sensitive element is made of piezoelectric material. When the piezoelectric material is subjected to force, a charge is generated on the surface. After the charge is amplified and the impedance is transformed by the charge amplifier and the measuring circuit, it becomes an electrical output proportional to the external force. Piezoelectric sensors are used to measure force and non-electrical physical quantities that can be converted into force, such as pressure, acceleration, etc. (see piezoelectric pressure sensor, accelerometer). Its advantages are wide frequency band, high sensitivity, high signal-to-noise ratio, simple structure, reliable operation and light weight. The disadvantage is that some piezoelectric materials require moisture-proof measures, and the output DC response is poor, and a high input impedance circuit or charge amplifier is needed to overcome this defect. The emergence of supporting instruments and low-noise, low-capacitance, and high-insulation resistance cables makes the use of piezoelectric sensors more convenient. It is widely used in technical fields such as engineering mechanics, biomedicine, and electroacoustics.

Piezoelectric effect Piezoelectric effect can be divided into positive piezoelectric effect and inverse piezoelectric effect. The positive piezoelectric effect means that when a crystal is subjected to an external force in a fixed direction, electric polarization occurs inside the crystal, and charges of opposite signs are generated on two surfaces at the same time; when the external force is removed, the crystal returns to an uncharged state; when the direction of the external force changes, the polarity of the charge also changes; the amount of charge generated by the crystal under force is proportional to the magnitude of the external force. Most piezoelectric sensors are made using the positive piezoelectric effect. The inverse piezoelectric effect refers to the phenomenon that the mechanical deformation of the crystal is caused by applying an alternating electric field to the crystal, also known as the electrostrictive effect. Transmitters made using the inverse piezoelectric effect can be used in electroacoustic and ultrasonic engineering. There are five basic forms of force deformation of piezoelectric sensitive elements: thickness deformation, length deformation, volume deformation, thickness shear, and plane shear (see figure). Piezoelectric crystals are anisotropic, and not all crystals can produce piezoelectric effects in these five states. For example, quartz crystals do not have volume deformation piezoelectric effects, but have good thickness deformation and length deformation piezoelectric effects.

Piezoelectric materials can be divided into piezoelectric single crystals, piezoelectric polycrystals and organic piezoelectric materials. The most commonly used piezoelectric sensors are various types of piezoelectric ceramics belonging to piezoelectric polycrystals and quartz crystals in piezoelectric single crystals. Other piezoelectric single crystals include lithium niobate, lithium tantalate, lithium gallate, bismuth germanate, etc., which are suitable for high-temperature radiation environments. Piezoelectric ceramics include barium titanate ceramics, lead zirconate titanate series ceramics, niobate series ceramics, and lead magnesium niobate ceramics belonging to the ternary system. The advantages of piezoelectric ceramics are easy firing, easy molding, moisture resistance, and high temperature resistance. The disadvantage is that they have pyroelectricity, which will interfere with the measurement of mechanical quantities. Organic piezoelectric materials include more than ten polymer materials such as polyvinylidene fluoride, polyvinyl fluoride, and nylon. Organic piezoelectric materials can be mass-produced and made into large areas. They have unique advantages in matching the acoustic impedance of air and are new electroacoustic materials with great development potential. Since the 1960s, crystals with both semiconductor properties and piezoelectric properties have been discovered, such as zinc sulfide, zinc oxide, and calcium sulfide. This material can be used to make a new type of piezoelectric sensor that integrates sensitive components and electronic circuits, which has great development prospects.