“Surface acoustic wave technology is an emerging science and technology developed in the late 1960s. It is a marginal subject combining acoustics and electronics. Since the propagation speed of SAW is 100,000 times slower than that of electromagnetic waves, and it is easy to sample and process on its propagation path, using SAW to simulate various functions of electronics can realize ultra-miniaturization of Electronic devices. and multifunctional.
Surface acoustic wave technology is an emerging science and technology developed in the late 1960s. It is a marginal subject combining acoustics and electronics. Since the propagation speed of SAW is 100,000 times slower than that of electromagnetic waves, and it is easy to sample and process on its propagation path, using SAW to simulate various functions of electronics can realize ultra-miniaturization of electronic devices. and multifunctional.
surface acoustic wave technology
A surface acoustic wave is an elastic wave that propagates along the surface of an object. Such waves have been studied more than ninety years ago. In 1885, Rayleigh theoretically elucidated the properties of elastic waves on isotropic solid surfaces based on the study of seismic waves. However, due to the limitation of the scientific and technological level at that time, this elastic surface wave has not been practically applied. Until the 1960s, due to the development of semiconductor planar technology and laser technology, a large number of artificial piezoelectric materials appeared, which provided the necessary material and technical basis for the development of surface acoustic wave technology.
In 1949, a single crystal of LiNbO3 was discovered by Bell Telephone Laboratories in the United States. In 1964, he published a patent on a planar structure transducer for exciting surface acoustic waves. In particular, it should be noted that in 1965, R.M.white and FWvoltmer published in the Journal of Applied Physics entitled “A Novel Surface Wave Acoustic-Electrical Transducer D Interdigital Transducer “The paper, which made a key breakthrough in surface acoustic wave technology.
1. Structure and principle of surface acoustic wave device
The surface acoustic wave device is made of two acoustic-electrical transducers – interdigital transducers on a piezoelectric substrate. The so-called interdigital transducer is to form a metal pattern shaped like the fingers of two hands on the surface of the piezoelectric substrate, and its function is to realize acoustic-electrical conversion.
The working principle of the surface acoustic wave device is that the transducer on the left end of the substrate (input transducer) converts the incoming electrical signal into an acoustic signal through the inverse piezoelectric effect. The transducer on the right side of the sheet (the output transducer) converts the acoustic signal to an electrical output. The function of the entire surface acoustic wave device is accomplished by performing various processing on the acoustic signal propagating on the piezoelectric substrate and utilizing the standby properties of the acoustic-electric transducer.
2. Characteristics of surface acoustic wave technology
First, surface acoustic waves have extremely low propagation speeds and extremely short wavelengths, each of which is one hundred thousand times smaller than the wavelength of the propagation speed of the corresponding electromagnetic waves. In VHF and UHF ropes, the size of the electromagnetic wave device is comparable to the wavelength. In the same way, as the acoustic simulation surface acoustic wave device of the electromagnetic device, its size is also compared with the acoustic wavelength of the signal. Therefore, in the same frequency band, the size of the surface acoustic wave device is much smaller than that of the corresponding electromagnetic wave device, and the weight is also greatly reduced.
For example, the delay that can be obtained with a one-kilometer-long microwave transmission line only requires a transmission path of 1. A surface acoustic wave delay line of m can be completed. This shows that surface acoustic wave technology can realize the ultra-miniaturization of electronic devices.
Second, since the surface acoustic wave system propagates along the solid surface and the propagation speed is extremely slow, the time-varying signal can be completely presented on the surface of the crystal substrate at a given instant. It is then easy to sample and transform the signal as it travels between the input and output of the device. This gives the SAW device great flexibility, enabling it to perform various functions in a very simple manner that are difficult or too burdensome to perform with other technologies.
Such as compression and stretching of pulse signals, coding and decoding, and signal correlation and convolution. A practical example is a one-inch-long SAW convolver reported in 1976, which has the function of convolving two arbitrary analog signals, and its adaptable bandwidth can reach 100MHz, and the time-bandwidth product can reach 100MHz. Ten thousand. Such a convolver can replace a digital convolver made up of several Fast Fourier Transform (FFT) chains, ie effectively a dedicated convolution computer.
In addition, in many cases, the performance of SAW devices far exceeds the level achieved by the best electromagnetic wave devices. For example, SAW can be used to make a pulse compression filter with a time-bandwidth product greater than 5,000, a resonant cavity with a Q value of more than 50,000 in the UHF frequency band, and an out-of-band suppression of 70dB and a frequency of 1 low Hz. bandpass filter.
Third, since the surface acoustic wave device is fabricated on a single crystal material with a semiconductor planar process, it has good consistency and repeatability, and is easy to mass produce, and when some single crystal materials or composite materials are used, SAW devices have extremely high temperature stability.
Fourth, the surface acoustic wave device has strong radiation resistance and a large dynamic range, up to 100dB. This is because it uses elastic waves on the crystal surface and does not involve the migration of electrons.
surface acoustic wave sensor
A surface acoustic wave (SAW) sensor is a new type of micro-acoustic sensor developed in recent years. It uses a surface acoustic wave device as a sensing element to pass the measured information through the Changes in wave speed or frequency are reflected and converted into electrical signals output by the sensor.
SAW sensors can accurately measure physical, chemical and other information (such as temperature, stress, gas density). Due to its small size, SAW devices are hailed as creating a new era of wireless and small sensors; at the same time, they have strong compatibility with integrated circuits and have been widely used in analog and digital communication and sensing fields.
The surface acoustic wave sensor can concentrate the signal on the surface of the substrate, has a high working frequency, has extremely high information sensitivity accuracy, can quickly convert the detected information into electrical signal output, and has the characteristics of real-time information detection; The wave sensor also has the advantages of miniaturization, integration, passive, low cost, low power consumption, direct frequency signal output and so on.
At present, various types of surface acoustic wave pressure sensors, surface acoustic wave temperature sensors, surface acoustic wave biological gene sensors, surface acoustic wave chemical gas sensors and smart sensors have been formed in China.
SAW is an elastic wave that propagates on the shallow surface of a solid, and has multiple modes. Rayleigh wave is currently the most widely used surface acoustic wave. Different types of surface acoustic waves have different characteristics, and sensors made of them can be applied to detection in different situations.
1. Structural type of surface acoustic wave sensor
The two basic configurations of surface acoustic wave sensors are delay line and resonator. Figure 1 shows the sensor structure categories of delay line and resonator. The delay line and resonant surface acoustic wave sensors are both composed of piezoelectric substrates, interdigital transducers and emission grids.
The delay line type surface acoustic wave sensor receives the sinusoidal excitation signal through the antenna, and transmits it to the interdigital transducer (IDT). The surface acoustic wave propagates on the piezoelectric substrate and reaches the reflection grid after a delay for a period of time. The reflection grid reflects part of the sound wave back, and the reflected sound wave is converted into a sinusoidal excitation signal through the IDT, thereby realizing electro-acoustic conversion.
The resonant surface acoustic wave sensor places the IDT between two total reflection gratings. When the frequency of the excited surface acoustic wave is equal to the frequency of the resonator, the surface acoustic wave forms a standing wave between the reflection grids, and the energy reflected by the reflection grid reaches the maximum. The external excitation signal is loaded on the input IDT, and the IDT converts the electrical signal into a surface acoustic wave. The surface acoustic wave propagates along the surface of the piezoelectric crystal to both sides, and is reflected and superimposed by the reflection grids on both sides.
2. The working mode of the surface acoustic wave sensor
SAW devices generally use piezoelectric crystals (such as quartz crystals, etc.) as a medium, and then generate acoustic waves by applying a positive voltage, propagate through the substrate, and then convert them into electrical signals for output. The piezoelectric effect plays a leading role in the SAW sensor, and its design needs to consider many factors: such as relative size, sensitivity, efficiency and so on. Generally, the signal frequency of wireless passive SAW sensors ranges from 40 MHz to several GHz. Figure 2 shows the common structure of surface acoustic wave sensor, the main parts include piezoelectric substrate, antenna, sensitive film, IDT and so on. The sensitive layer of the sensor changes the frequency by changing the speed of the surface acoustic wave.
3. Working principle of surface acoustic wave sensor
Wireless passive SAW system package: transmitter, receiver, SAW device, communication channel. The transmitter and receiver are combined into a single module of a transceiver or reader. Figure 3 shows a surface acoustic wave system and its interrelated basic components. The reader delivers power to the SAW device, which can be a continuous wave, pulsed or chirped input from the transceiver. Generally, the power obtained by the surface acoustic wave device has a certain limit to reduce the maximum transmit power, so as to obtain the chirp of the same average power. Depending on the isotropic radiator, the received signal can generally be transmitted by a highly efficient radiated power antenna.
Applications of SAW Sensors
1. Application of surface acoustic wave sensor in smart substation
In order to overcome the shortcomings of complex temperature detection environment, non-contact, low precision and high cost of smart substations, Zhang Peng et al. from China Institute of Technology developed a passive wireless surface acoustic wave smart temperature sensor that can be applied in smart substations. The detection mechanism of the temperature sensor and the sensor transceiver system are studied. At the same time, a smart substation temperature detection system is constructed based on the developed passive wireless surface acoustic wave sensor. The experimental results show that the passive wireless surface acoustic wave temperature sensor can completely solve the problems of inconvenient installation, strong electromagnetic interference, high working environment temperature and signal transmission of power equipment such as cable joints, switch cabinets, and isolation switches.
2. Application of surface acoustic wave sensor in power equipment
Because the power equipment works under high voltage, heavy load and long-term power-off state, the requirements for temperature measurement devices are naturally higher. There is a strong electric field around the operating medium and high voltage power equipment, and its temperature detection sensor must have a passive or self-energy function to ensure the safety of the power equipment. In addition, a certain safety distance is required to be maintained between electrical equipment, so the volume of the detection device should be as small as possible. All types of power equipment are suitable for installation, and the equipment maintenance cycle should be as long as possible to ensure long-term uninterrupted operation of power equipment. The researchers investigated the possibility of RF energy harvesting technology in monitoring temperature changes in power systems, and also developed a surface acoustic wave temperature sensor based on RF energy power. The system is mainly composed of a dual-channel reader and many sensor nodes. The sensor nodes obtain energy from the energy delivered by the reader, and the transmitted RF energy is used as wake-up information to turn on the sensor to avoid data conflict. According to the authors’ analysis, RF energy harvesting technology is a surface acoustic wave sensor technology that is well suited for power equipment.
3. Application of surface acoustic wave sensor in train
The fast running speed of the train leads to the increase of traction power, which increases the frictional impact between the wheel and the rail, the vibration amplitude of the axle and the dynamic effect. With the wear of the train axles, the axles will increase the heat generation and the vibration amplitude, thereby accelerating the expansion of the axle defects and affecting the normal operation of the train. Generally, the operating status of the train axle is directly reflected by monitoring the temperature and vibration of the axle. The surface acoustic wave temperature sensor is a detection device that can reflect the state of the train axle. Generally, the surface acoustic wave temperature sensor detection system is mainly composed of three parts: the surface acoustic wave temperature sensor chip, the signal reader and wireless relay, and the background monitoring system. Since the surface acoustic wave temperature sensing chip is passive wireless, additional power supply is required. The surface acoustic wave temperature sensor can be installed on the axle of the train where temperature measurement is required to accurately track the temperature change of the hot spot. The advantages of the surface acoustic wave temperature sensor applied to trains are mainly manifested in: the temperature measurement chip can communicate wirelessly through the antenna and the signal reader, each signal reader corresponds to multiple detection points, plug and play, easy to expand the scale and system upgrade; the signal reader processes the temperature signal into a digital signal and transmits it to the background monitoring system through optical fiber, so as to realize long-distance non-relay transmission; the background monitor adopts time division multiplexing or frequency division multiplexing and other methods to control the ― 100 signal readers, and each signal reader can correspond to multiple acoustic surface temperature sensors at the same time.
4. Surface acoustic wave sensor and its application in humidity detection
Humidity detection plays an increasingly important role in storage, grain and food mildew prevention, greenhouse planting, environmental monitoring, instrumentation, transportation, meteorology, and military. Since in a conventional environment, humidity is a parameter that is difficult to measure accurately.Therefore, humidity measurement needs to have high sensitivity, fast response speed
high performance. The research group of Zhejiang University conducted an in-depth analysis of the perturbation theoretical model of the surface acoustic wave sensor and its response mechanisms such as mass loading effect, acousto-electrical accident effect, etc., which fundamentally provided the structural design of the surface acoustic wave sensor and the selection of humidity-sensitive materials. theoretical basis and reference. At the same time, a high-frequency surface acoustic wave single-ended resonator was prepared by using a precision lithography process as the basic transducer element of the humidity sensor, and a high-performance surface acoustic wave high-frequency oscillation circuit and a complete set of detection systems were developed. The new structure of the interdigital electrode series surface acoustic wave sensor provides a new idea for the design of high frequency surface acoustic wave sensor and satisfies its application in humidity detection.
5. Application of surface acoustic wave sensor in complex and changeable environment
For a long time, traditional temperature sensors have many insurmountable defects, which cannot meet the actual and changeable measurement needs. The research group of Zhejiang University designed and fabricated a single-port resonant surface acoustic wave temperature based on four different piezoelectric sensitive materials: YZ-cut lithium niobate (LiNbO3), 128°YX-cut LiNbO3, ST-cut quartz and YX-cut quartz. sensor. The research results show that the LiNbO3 surface acoustic wave temperature sensor has a larger frequency temperature coefficient than the quartz sensor; in the range of 0-80 ℃, the YZ-cut LiNbO3, 128°YX-cut LiNbO3 and YX-cut quartz temperature sensors have higher frequency than the ST-cut quartz temperature sensor. Linear temperature-frequency characteristics; the quartz surface acoustic wave temperature sensor has a larger quality factor and a stronger echo signal than the LiNbO3 sensor; under the same test conditions, when the wireless transmission distance is less than 10 cm, the YZ cut LiNbO3 temperature sensor The measurement accuracy is higher; when the distance exceeds 10 cm, the YX cut quartz sensor has higher measurement accuracy. The research results have general significance for the design and fabrication of single-port resonant surface acoustic wave temperature sensors, and provide important guidance for the fabrication of surface acoustic wave sensors in complex and variable environments.
In order to adapt to the future changeable environment, fast and intelligent life mode, the future surface acoustic wave sensor should develop in the direction of miniaturization, flexibility, intelligence, high precision and high reliability. Specific researches such as: 1) Development and preparation of new device sensitive materials to improve the performance and reliability of surface acoustic wave sensors; 2) Strengthen the theoretical design of surface acoustic wave sensors to provide intelligent and miniaturized surface acoustic wave sensors. Strong theoretical guidance; 3) Develop the integration process of the surface acoustic wave sensor, so that the surface acoustic wave sensor can be compatible with a variety of equipment.