What is a Piezoelectric Sensor?

Catalog

Ⅰ Introduction

A piezoelectric sensor is a sensor based on the piezoelectric effect. It is a self-generating and electromechanical conversion sensor. Its sensitive components are made of piezoelectric materials. The piezoelectric material generates an electric charge on the surface after being forced. The charge is amplified by the charge amplifier and the measuring circuit and transformed into impedance. Then it becomes a power output proportional to the external force received. Piezoelectric sensors are used to measure force and non-electrical physical quantities that can be transformed into electricity. Its advantages are wide bandwidth, high sensitivity, high signal-to-noise ratio, simple structure, reliable work, and lightweight. The disadvantage is that some piezoelectric materials need moisture-proof measures, and the output DC response is poor. High input impedance circuits or charge amplifiers are needed to overcome this defect.

Ⅱ Piezoelectric effect

The piezoelectric effect can be divided into the positive piezoelectric effect and the inverse piezoelectric effect. The positive piezoelectric effect means that when the 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 external force changes, the polarity of the charge also changes; the amount of charge generated by the crystal 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 application of an alternating electric field to the crystal causes mechanical deformation of the crystal, also known as the electrostrictive effect. Transmitters made with the inverse piezoelectric effect can be used in electroacoustic and ultrasonic engineering. There are five basic forms of force deformation of piezoelectric sensitive components: thickness deformation type, length deformation type, volume deformation type, thickness shear type, and plane shear-type (see figure). Piezoelectric crystals are anisotropic, and not all crystals can produce piezoelectric effects in these five states. For example, quartz crystal does not have the piezoelectric effect of volume deformation but has a good piezoelectric effect of thickness deformation and length deformation.

five basic forms of force deformation of piezoelectric sensitive components

Ⅲ Piezoelectric material

Piezoelectric materials can be divided into a piezoelectric single crystal, piezoelectric polycrystalline, and organic piezoelectric materials. The most commonly used piezoelectric sensors are various types of piezoelectric ceramics and quartz crystals in piezoelectric single crystals. Other piezoelectric single crystals include lithium niobate, lithium tantalate, lithium gallate, and bismuth germanate suitable for high-temperature radiation environments. Piezoelectric ceramics include barium titanate ceramics belonging to the binary system, lead zirconate titanate series ceramics, niobate series ceramics, and lead magnesium niobate ceramics belonging to the ternary system. The advantages of piezoelectric ceramics are convenient firing, easy shaping, humidity resistance, and high-temperature resistance. The disadvantage is that it has pyroelectricity, which will interfere with the measurement of mechanical quantities. Organic piezoelectric materials include more than ten kinds of polymer materials such as polyvinylidene fluoride, polyvinyl fluoride, and nylon. Organic piezoelectric materials can be mass-produced and made into larger areas. It has unique advantages in matching the acoustic resistance of air and is a new type of electro-acoustic material with great development potential. Since the 1960s, crystals with both semiconductor properties and piezoelectric properties, such as zinc sulfide, zinc oxide, and calcium sulfide, have been discovered. The use of this material can be made into a new type of piezoelectric sensor integrating sensitive components and electronic circuits, which is very promising.

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

Positive piezoelectric effect

Certain substances, when they are deformed by applying force in a certain direction, will generate electric charges on a certain surface. When the external force is removed, it will return to the normal uncharged state. This phenomenon is called the positive piezoelectric effect.

Inverse piezoelectric 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 stress will disappear. This phenomenon is called inverse piezoelectric effect, or called the electrostrictive effect.

Ⅳ Main types of piezoelectric sensor

1.Piezoelectric pressure sensor

A piezoelectric pressure sensor is a sensor that uses piezoelectric elements to directly realize force-to-electricity conversion. In tension and compression applications, two or more quartz crystals are usually used as piezoelectric elements. It has a wide measuring range, high linearity and stability, and good dynamic characteristics. When a charge amplifier with a large time constant is used, the quasi-static force can be measured. According to the force measurement status, there are unidirectional, bidirectional, and three-directional sensors, and they are basically the same in structure.

structure of the piezoelectric unidirectional pressure sensor

The figure shows the structure of the piezoelectric unidirectional pressure sensor. The sensor is used to measure the dynamic cutting force of machine tools. The insulating sleeve is used for insulation and positioning. The verticality of the inner and outer bottom of the base to its centerline, the parallelism of the upper and lower bottom surfaces of the upper cover, the wafer, and the electrode, and the surface finish has extremely strict requirements. Otherwise, the lateral sensitivity will increase or the film will be broken prematurely due to stress concentration. In order to improve the insulation resistance, the sensor must be cleaned (including ultrasonic cleaning) many times before assembly, and then assembled in an ultra-clean working environment, and then sealed with electron beams.

There are many types of piezoelectric pressure sensors, but their basic principles and structures are still similar to those of piezoelectric acceleration and force sensors. The outstanding difference is that it must collect and convert the pressure into force through an elastic membrane, box, etc., and then transmit it to the piezoelectric element. In order to ensure static characteristics and stability, quartz crystals are usually used as piezoelectric components.

2.Piezoelectric acceleration sensor

The figure shows the structural diagram of a compression piezoelectric acceleration sensor. The piezoelectric element is generally composed of two piezoelectric sheets. A silver layer is plated on the two surfaces of the piezoelectric sheet, and weld the output lead on the silver layer. Or, clamping a piece of metal between the two piezoelectric sheets, and the lead is welded to the metal sheet, and the other lead of the output end is directly connected to the sensor base.  Place a mass with a larger specific gravity on the piezoelectric sheet, and then use a hard spring, bolt, or nut to preload the mass. The whole assembly is installed in a metal shell with a thick base. In order to isolate any strain of the test piece from being transmitted to the piezoelectric element and avoid false signal output, it is generally necessary to thicken the base or choose a material with greater rigidity.

structural diagram of a compression piezoelectric acceleration sensor

When measuring, the sensor base and the test piece are rigidly fixed together. When the sensor feels vibration, the mass feels the same vibration as the sensor base and is subjected to the force of inertia which is opposite to the direction of acceleration. Why? Because the stiffness of the spring is quite large and the mass is relatively small, it can be considered that the inertia of the mass is small. In this way, the mass has an alternating force proportional to the acceleration acting on the piezoelectric sheet. Due to the piezoelectric effect of the piezoelectric sheet, alternating charges (voltage) are generated on its two surfaces. 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 force, which is proportional to the acceleration of the test piece. The output power is drawn from the output of the sensor, and after input into the preamplifier, the acceleration of the test piece can be measured with an ordinary measuring instrument. If an appropriate integrating circuit is added to the amplifier, the vibration acceleration or displacement of the test piece can be measured.

Ⅴ Applications of piezoelectric sensors

1. Used to reduce vibration and noise

The application research of piezoelectric smart structure in vibration control was carried out earliest, and the research results were also rich, mainly focusing on the vibration control of large aerospace flexible structures. There are usually three methods for the design of control systems, namely, active control, passive control, and active-passive hybrid control. The passive control system is simple in structure, easy to implement and low in cost, but it lacks flexibility in control and has poor adaptability to sudden environmental changes. Compared with passive control, active control uses modern control theory as the main tool and has greater flexibility and strong adaptability to the environment. It is currently a research hotspot in vibration engineering. The combination of passive control and active control to form a hybrid control strategy is an emerging direction of current vibration engineering. Another important application direction of piezoelectric smart structures is active noise control. It is mainly used to control the internal noise of three-dimensional enclosed spaces such as submarines, aircraft, and vehicles. The difference from the active control of wall panel vibration is that the vibration control is to control the mode of the panel, while the noise control is mainly to control the part that produces the sound intensity.

2. Used for structural static deformation control

By controlling the deformation of the piezoelectric smart structure, the geometric shape of the structure can be adjusted, and the accurate shape and position of the structure can be maintained. This has important application value in the control of space stations and other space vehicles and flexible machinery. For example, in a space vehicle, the deformation of the precise structure can be controlled to ensure the normal operation of precision instruments such as space antenna reflectors and telescopes. In a robot, the movement of the flexible manipulator can be controlled by piezoelectric elements to improve the movement accuracy of the manipulator.

3. Used for structural damage monitoring

There are two main ways to apply piezoelectric sensing elements to structural damage monitoring. One is to use piezoelectric sensors to accurately sense the changes in the mechanical properties of the structure, and to predict the damage of the structure through further calculation and analysis; the other way is to predict the damage by analyzing the vibration waves propagating in the structure. These two methods can provide reliable information for structural safety assessment and damage location, thereby providing a new method for long-term, real-time health detection of civil engineering structures.

4.Used for processing technology monitoring

Piezoelectric force, stress, vibration, and acoustic emission sensors have become the ideal choice for condition monitoring in modern automatic control manufacturing because of their unique advantages. For grinding, drilling, and tapping, a novel quartz multi-component force, and torque sensor is designed using the latest telemetry technology. This new type of rotary cutting dynamometer can be directly installed between the shaft and the tool to directly measure the cutting force of the rotation tool, which is of great significance for analyzing and planning the production process and determining the best cutting parameters used in actual production. Acoustic emission occurs in the metal processing process, which contains a wealth of information, and its most important value is to give reliable instructions for the defects and initial failures of the parts. An innovative dual-purpose sensor combines the acoustic emission sensor and the three-component load cell into one. Installing this dual-purpose sensor in the appropriate position of the lathe fixture, we can continuously monitor the cutting force, feed force, and related acoustic emission signals.

5. Used for vehicle weighing

The combination of piezoelectric sensing technology, network technology, and video technology can collect and analyze information on the number of axles, vehicle speed, wheelbase, and vehicle load capacity in driving, thereby playing a major role in intelligent transportation systems. For example, the copolymer piezoelectric shaft sensor researched and developed by the American MSI company can obtain accurate speed signals, trigger signals, and classification information, and long-term feedback traffic information statistics. Brazil, Germany, Japan, and South Korea also have a large number of applications in the weighing function of piezoelectric detection vehicles in driving.

6. Used for piezoelectric sensing cable

People have recently developed a PVF2 piezoelectric material in the form of a coaxial cable with a thicker piezoelectric layer-PVF2 piezoelectric cable. This piezoelectric cable is produced by a continuous process, with a length of several kilometers and a diameter of 1.5 mm. It can convert sound, vibration, impact, pressure, stress, and strain into electrical signals. The method of use is very flexible. It overcomes the shortcomings and limitations of piezoelectric films and piezoelectric ceramics, showing great application prospects. Its main application fields are hydroacoustics, shock sensing, vibration sensing, intrusion alarm and security defense, traffic flow statistics, strain stress measurement, industrial control, and detection, etc.

7. Used for aviation and navigation

The "portable automatic remote inspection system" developed by Sigma Research Company, PARIS for short, is specially designed for in-situ inspection of large-area layered structures or composite structures. The key element is a 200mm times 200mm deformable PVDF piezoelectric film, which contains 1024 transducers. This kind of film is very flexible and can completely fit the curved surface with a radius of curvature of 4. The device connected to it also has a portable controller, data sampler and display device. This receiver has a total signal-to-noise ratio of 100dB and a center frequency of 2.5MHz. Graphite-epoxy composites on airplanes or large structures on ships can be used for convenient testing. For aluminum and steel tests, satisfactory results have been obtained in this regard.

8. Piezoelectric alarm

The glass break alarm device uses the vibration-sensitive characteristics of piezoelectric elements to sense the vibration waves generated when glass is impacted and broken. The sensor converts the vibration wave into a voltage output, and the output voltage is processed by amplifying, filtering, and comparing, then providing it to the alarm system. The minimum output voltage of the sensor is 100 mV, the maximum output voltage is 100 V, and the internal impedance is 15-20 kq. When the glass is broken, it will emit vibrations of several kilohertz to tens of kilohertz. When in use, paste the polymer piezoelectric film sensor on the glass, feel the vibration, and then connect it to the alarm circuit through a cable to transmit the piezoelectric signal to the central alarm system. In order to improve the sensitivity of the alarm, after the signal is amplified, it is filtered by a bandpass filter. It is required that the attenuation of the selected spectrum passband should be small, and the attenuation outside the band should be as large as possible. The wavelength of glass vibration is in the audio and ultrasonic range, which makes the filter the key to the circuit. Only when the sensor output signal is higher than the set threshold, will it output an alarm signal to drive the alarm actuator to work. The glass break alarm can be widely used in cultural relic storage, valuable commodity storage, and other commodity counter storage.

Related Articles:

Introduction to Wireless Sensor Networks

How do Parking Sensors Work?