Piezoelectric sensor

1 Overview of Piezoelectric Effects

The piezoelectric effect can be divided into positive piezoelectric effect and inverse piezoelectric effect. The positive piezoelectric effect means that when the crystal is subjected to an external force in a certain fixed direction, an internal electrode is generated, and at the same time, opposite charges are generated on two surfaces; when the external force is removed, the crystal is restored to an uncharged state. State; When the direction of the external force changes, the polarity of the charge also changes; the amount of charge generated by the force of the crystal is proportional to the magnitude of the external force. Piezoelectric sensors are mostly made using positive piezoelectric effects. The inverse piezoelectric effect refers to the phenomenon that the crystal is mechanically deformed by applying an alternating electric field to the crystal. It is also called the electrostrictive effect. Transducers manufactured with inverse piezoelectric effects can be used for electro-acoustic and ultrasonic engineering. Piezoelectric elements are deformed in the following five basic forms: 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 have no volumetric deformation piezoelectric effect, but have good thickness deformation and length deformation piezoelectric effect.

Piezoelectric material

It can be divided into piezoelectric single crystals, piezoelectric polycrystals and organic piezoelectric materials. Piezoelectric sensors are most commonly used in piezoelectric ceramics and piezoelectric crystals in various types of piezoelectric crystals. Other piezoelectric single crystals include lithium niobate and lithium niobate, lithium niobate, and barium niobate which are suitable for high-temperature radiation environments. Piezoelectric ceramics include barium titanate ceramics, lead zirconate titanate ceramics, niobate series ceramics and lead magnesium niobate ceramics belonging to the binary system. The advantages of piezoelectric ceramics are easy firing, easy molding, moisture resistance, and high temperature resistance. The disadvantage is pyroelectricity, which can interfere with mechanical measurements. There are more than ten kinds of polymer materials such as polyvinyl difluoride, polyvinyl fluoride and nylon in organic piezoelectric materials. The organic piezoelectric material can be mass-produced and made into a large area. It has unique advantages in matching with the acoustic resistance of air, and it is a new electro-acoustic material with great development potential. Since the 1960s, crystals with both semiconductor and piezoelectric properties have been discovered, such as zinc sulfide, zinc oxide, and calcium sulfide. This kind of material can be used to make a new type of piezoelectric sensor that integrates a sensitive element and an electronic circuit, and has a promising future.

Piezoelectric sensors can be roughly divided into four types, namely: Piezoelectric force sensors, piezoelectric pressure sensors, piezoelectric acceleration sensors, and polymer material pressure sensors.

Piezoelectric effect

Some substances, when they are forced along a certain direction to deform it, will generate a charge on a certain surface, and when the external force is removed, it will return to the uncharged state. This phenomenon is called the piezoelectric effect.

Reverse effect

If an electric field is applied in the polarization direction of these substances, these substances will produce mechanical deformation or mechanical stress in a certain direction. When the external electric field is removed, these deformations or stresses will also disappear. This phenomenon is called inverse piezoelectric effect. , or electrostrictive effect.

Piezoelectric material

Sensitive functional materials that clearly exhibit piezoelectric effects are called piezoelectric materials.

Piezoelectric single crystals, such as quartz, potassium sodium tartrate, etc.;

Polycrystalline piezoelectric ceramics, such as barium titanate, lead zirconate titanate, lead magnesium niobate, etc., are also known as piezoceramics. In addition, polyvinylidene fluoride (PVDF) has been widely used as a novel high-molecular property sensing material.

2 main parameters

(1) The piezoelectric constant is a parameter that measures the strength of a piezoelectric material. It is directly related to the sensitivity of the piezoelectric output.

(2) The elastic constant and stiffness of the piezoelectric material determine the natural frequency and dynamic characteristics of the piezoelectric device.

(3) For a piezoelectric element of a certain shape and size, its intrinsic capacitance is related to the dielectric constant, and the intrinsic capacitance affects the lower limit of the frequency of the piezoelectric sensor.

(4) In the piezoelectric effect, the mechanical coupling coefficient is equal to the square root of the ratio of the output energy (such as electrical energy) to the input energy (such as mechanical energy); it is an important parameter to measure the electromechanical energy conversion efficiency of the piezoelectric material.

(5) The insulation resistance of the piezoelectric material will reduce the charge leakage, thereby improving the low frequency characteristics of the piezoelectric sensor.

(6) The temperature at which the piezoelectric material begins to lose piezoelectricity is called the Curie point temperature.

3 Piezoelectric ceramics

Piezoelectric crystal and piezoelectric ceramic comparison:

The same point: all piezoelectric materials with piezoelectric effect.

Different points: The advantage of quartz is that its dielectric and piezoelectric constants have good temperature stability and are suitable for sensors with a wide range of operating temperatures. After the polarization of the piezoelectric ceramics, when the deformation by the external force, due to the repositioning of the electrode moment and generate charges, the piezoelectric coefficient of piezoelectric ceramics is dozens or even several hundred times that of quartz, but the stability is not as good as quartz, ranking The point is also low. [2]

4 equivalent circuit

1. The equivalent principle of the capacitive effect

1) Piezoelectric sensor structure

Metal vapor deposition is performed on the two working surfaces of the piezoelectric wafer to form a metal film and two electrodes are formed.

2) Equivalent capacitance

When the piezoelectric sensor is subjected to an external force along its sensitive axis, opposite polarity charges are generated at both electrodes, so that it corresponds to a charge source (electrostatic generator). Since the piezoelectric crystal is an insulator, when its two-pole surface accumulates charge, it is also equivalent to a capacitor whose capacitance generates a longitudinal piezoelectric effect along the x-axis and generates a lateral piezoelectric effect along the y-axis. The tangential piezoelectric effect is generated by applying force along two opposite planes.

3) Equivalent voltage

When the piezoelectric crystal is acted by an external force, the two surfaces generate an equal amount of positive and negative charges Q, and the open circuit voltage can be found (when the load resistance is infinite). The piezoelectric sensor can be equivalent to a charge source and can also be equivalent to The equivalent circuit of a capacitor can be considered as a parallel connection between the two; it can also be considered as a voltage source connected in series with a capacitor. Where Ra is the leakage resistance of the piezoelectric element.

5-series parallel

Monolithic piezoelectric wafers are difficult to generate enough surface charge. Piezoelectric sensors often use two or more piezoelectric wafers in combination. Because piezoelectric crystals are polar, two piezoelectric crystal sensors have two connections: series and parallel.

6 Application piezoelectric force transducer

Piezoelectric force sensors use piezoelectric elements to directly perform force-electricity conversion. In the case of tension and pressure, two or more quartz crystals are often used as piezoelectric elements. Its large rigidity, wide measurement range, high linearity and stability, and good dynamic characteristics. When using a large time constant charge amplifier, quasi-static forces can be measured. According to the state of force measurement, there are one-way, two-way and three-way sensors, which are basically the same in structure.

The sensor is used to measure the dynamic cutting force of the machine tool. Insulation sleeves are used for insulation and positioning. The inside and outside bottom of the pedestal have very strict requirements on the verticality of the centerline, the upper cover and the wafer, the parallelism of the upper and lower sides of the electrode, and the surface finish, otherwise the horizontal sensitivity may increase or the film may be premature due to stress concentration. broken. In order to improve the insulation resistance, the sensor must be cleaned (including ultrasonic cleaning) several times before assembly, and then assembled in a clean working environment. After sealing, electron beam sealing is used.

Piezoelectric pressure sensors have many types of structures, but their basic principles and structures are similar to those of piezoelectric acceleration and force sensors. The outstanding difference is that it must collect, convert, and transfer the pressure to the piezoelectric element through elastic membranes, boxes, etc. To ensure the static characteristics and its stability, quartz crystals are often used as piezoelectric elements.

Piezoelectric acceleration sensor

Piezoelectric elements generally consist of two piezoelectric sheets. A silver layer is plated on both surfaces of the piezoelectric sheet, and an output lead is welded on the silver layer, or a piece of metal is sandwiched between the two piezoelectric sheets. The lead is welded to the metal sheet, and the other lead of the output end is directly connected to The sensor base is connected. Place a mass with a larger specific gravity on the piezo-electric chip and then preload the mass with a hard spring or bolt or nut. The entire assembly is housed in a thick metal base shell, in order to isolate any strain of the test specimen from being transmitted to the piezoelectric element to avoid false output. Therefore, it is usually necessary to thicken the base or use a material with higher rigidity to manufacture the specimen. .

During measurement, the sensor base and the test piece are rigidly fixed together. When the sensor senses vibration, because the stiffness of the spring is quite large and the mass of the mass is relatively small, the inertia of the mass can be considered to be small, so the mass feels the same vibration as the sensor base, and is subject to the acceleration direction. The opposite inertial force effect. In this way, the mass has an alternating force that is proportional to the acceleration acting on the piezoelectric sheet. Since the piezoelectric sheet has a piezoelectric effect, alternating charges (voltages) are generated on both surfaces thereof. When the vibration frequency is much lower than the natural frequency of the sensor, the output charge (voltage) of the sensor is proportional to the acting force. , which is proportional to the acceleration of the test piece. The output power is drawn from the sensor output. After inputting to the preamplifier, the acceleration of the test piece can be measured with an ordinary measuring device. If an appropriate integrator circuit is added to the amplifier, the vibration acceleration or displacement of the test piece can be measured. .