Ultrasonic sensor application
Ultrasonic sensor is a kind of sensor developed by using the characteristics of ultrasonic wave. The ultrasonic sensor has the advantages of low cost, easy installation and maintenance, small size, non-contact measurement, and is not easy to be affected by electromagnetic, smoke, light, color of the measured object, and can be realized in the dark, dust, smoke, electromagnetic interference and toxic environment. Therefore, it is widely used in industrial field. Based on the working principle of ultrasonic sensor, the application status of ultrasonic in ranging, automatic weld tracking, non-destructive testing of parts, flow measurement and liquid concentration detection is summarized in this paper.
1. Working principle of ultrasonic sensor
Ultrasonic positioning technology is used by bats and other creatures without visual ability as a means of survival to defend against natural enemies and capture prey. These organisms can emit ultrasonic waves (mechanical waves above 20kHz) that people cannot hear. With the help of air medium, they can propagate according to the time interval and strength of the echoes reflected by prey or obstacles. Judge the nature of prey or the location of obstacles . According to the principle of bionics, a series of practical ultrasonic sensors such as ultrasonic ranging and non-destructive damage detection have been developed.
Ultrasonic sensor is a kind of reversible transducer, which uses the piezoelectric effect and electrostriction effect of crystal to convert mechanical energy and electric energy to each other, realizing the measurement of various parameters. Ultrasonic generators can be divided into two categories: one is the electrical way to generate ultrasonic waves; One is to produce ultrasonic waves mechanically. Electrical class includes piezoelectric type, magnetic extension type and electric type; The mechanical category includes the Gaal flute, liquid whistle and air - swirling flute. They produce different ultrasonic frequencies, powers and acoustic characteristics, and therefore different uses. Currently commonly used is the piezoelectric ultrasonic generator, it is the use of piezoelectric crystal resonance to work, the sensor has two piezoelectric wafers and a resonance plate, when the poles of the pulse signal, and the frequency is equal to the natural oscillation frequency of the piezoelectric wafers, piezoelectric wafers will resonance, and drive resonance plate vibration to produce ultrasonic. On the other hand, if there is no voltage applied between the two electrodes, when the resonant plate receives the ultrasonic wave, it will force the piezoelectric wafer to vibrate, converting mechanical energy into electrical signals, at which point it becomes the ultrasonic receiver. According to the time difference between the echo and the transmitted wave or the intensity of the echo, the ultrasonic sensor can obtain the distance or attribute of the measured object.
2. Application of ultrasonic sensor
2.1 Ultrasonic Ranging
The basic principle of ultrasonic ranging is that the ultrasonic transmitting sensor sends out sound waves, and the sound waves meet the measured object and return to the ultrasonic receiving sensor. The measured distance can be calculated according to the transmission time t of the sound waves:
In Formula (1), c is the propagation velocity of sound wave in medium (m/s).
Ultrasonic ranging principle is simple, data processing speed, convenient installation and maintenance and low cost advantages, in the liquid level measurement, robot obstacle avoidance and accurate ranging positioning has been widely used. Li Minzhe et al.  designed a distributed ultrasonic liquid level measurement system based on wireless transmission, which can measure the liquid level of multiple liquid storage tanks at the same time. Temperature measurement circuit is adopted in the system to compensate the temperature and improve the measurement accuracy. Liu Yuqin et al.  adopted two ultrasonic sensors to detect the obstacle information around the robot in the walking process and realized the obstacle avoidance function of the mobile robot. However, they could only obtain the distance information of the target, but could not accurately obtain the boundary information of the target. The ultrasonic transmitter is installed at several known locations in the room, and the receiver is installed on the mobile robot, which is combined with an extended Karman filter to realize the accurate positioning of the indoor mobile robot. In order to overcome the blind spot of ultrasonic sensor, a sensing system based on ultrasonic sensor and infrared sensor is developed. The infrared sensor compensates the blind spot of ultrasonic sensor and improves the sensing range. It has been used well in obstacle avoidance and navigation of mobile robot. We design a kind of ultrasonic ranging system which takes microcontroller as the main control chip, CPLD is responsible for the generation, sending and receiving of ultrasonic wave and the high-speed counting of the ultrasonic propagation time, and uses the digital temperature sensor for temperature compensation, which reduces the ultrasonic timing error and sound velocity error, and realizes the high precision distance measurement.
2.2 Ultrasonic automatic weld tracking
Ultrasonic sensors have been used in welding seam tracking in recent years because of their advantages such as being free from arc light and strong electromagnetic interference, sensitive to the fluctuation and change of detection object surface, high cost performance and penetrating dust. Tian Songya et al.  designed a corrugated plate polyline weld tracking system based on ultrasonic sensor. They used the ultrasonic sensor to track a corrugated cycle of the corrugated plate. Wang Yingge et al.  used the least square fitting method to calibrate the ultrasonic sensor, and achieved good results in the automatic welding tracking of right-angle welds. Zhang Chenshu et al. [9-10] summarized ultrasonic tracking in welding seams and proposed measures to improve ultrasonic detection accuracy in welding environment, such as information fusion technology and information processing technology.
2.3 Ultrasonic flow measurement
The basic principle of ultrasonic flow detection is based on the fact that the velocity of ultrasonic propagation in the fluid is equal to the vector sum of fluid velocity and ultrasonic velocity. Its specific implementation methods include propagation velocity difference method (including difference frequency method, time difference method and phase difference), beam migration method, Doppler method, correlation method, spatial filtering method and noise method, etc. . Yao Binbin et al.  developed a flowmeter based on ultrasonic time difference method, adopted a high-precision TDC-GP2 time digital converter, and the flow measurement accuracy reached 5%. GUI Yongfang et al.  designed an ultrasonic gas flow measurement system based on the cross-correlation theory. DSP was adopted as the controller to collect the upstream and downstream flow signals for cross-correlation calculation, calculate the transit time corresponding to the peak value of the cross-correlation function, and measure the flow indirectly. When the flow velocity is greater than 0.16m/s, the measurement error is less than 3%.
2.4 Ultrasonic liquid concentration detection
The principle of ultrasonic liquid concentration detection is based on the functional relationship between ultrasonic propagation velocity in liquid and liquid concentration and temperature. According to the principle of acoustics, the velocity of ultrasonic propagation in liquid is a function of the elastic modulus and density of liquid. The velocity of ultrasonic wave varies with the elastic modulus or density of liquid, and is also a function of the mass concentration and temperature of solution. So as long as the ultrasonic conduction velocity is measured at different temperatures, the mass concentration of the liquid can be obtained. Huang Jia et al. [14-15] designed a yeast concentration detection system based on ultrasonic sensors, studied the corresponding relationship between yeast cell concentration, ultrasonic propagation time and temperature in the production of Tablet wine, discussed various factors influencing the measurement and proposed solutions, and realized the online measurement of yeast concentration in beer production.
2.5 Nondestructive inspection of ultrasonic parts
Ultrasonic inspection can not only detect defects on the surface of materials, but also detect defects several meters deep inside. Compared with X-ray inspection, ultrasonic inspection has the advantages of high sensitivity, short cycle and harmless to human body. Its disadvantage is that the surface of the workpiece is smooth, experienced detection personnel can distinguish the type of defect, the defect is not intuitive. Therefore, ultrasonic flaw detection is suitable for parts with large thickness. Li Kaijun  proposed an ultrasonic nondestructive testing method for high pressure seamless steel pipes, which can carry out automatic continuous and spot detection for straight pipes, and stored various steel pipe detection processes and standards, realizing automatic ultrasonic nondestructive testing for seamless steel pipes. Yang Lijian et al.  designed an ultrasonic Lamb wave nondestructive testing system, which achieved a high sensitivity for nondestructive testing of sheet metal. Peng Guangjun et al.  studied the rapid calculation technology in nondestructive testing, providing a reference for the reliability of nondestructive testing technology, reliability evaluation methods and influencing factors.
3. Disadvantages and solutions of ultrasonic sensor
1) Measuring blind areas. The ultrasonic measurement blind spot is caused by two factors: the first is that after the ultrasonic sensor transmits the signal, there is residual vibration of the transducer. If the transmitting signal is immediately opened after the receiving circuit, the residual vibration signal will cause misjudgment. Generally, after starting the transmitting signal, it is necessary to delay for a period of time before opening the receiving circuit. During this period, the propagation distance of ultrasonic wave cannot be detected, so there will be a measurement blind area. The strength of ultrasonic residual vibration signal is related to the performance of the transducer and the strength of the transmitted signal. By improving the performance of the transducer and reducing the power of the transmitted signal, the residual vibration and the blind area can be reduced. Second, the ultrasonic sensor of the integrated probe needs the controller to control the transmission and reception through the switching circuit, and the time interval of switching will also produce a blind area. A higher master frequency controller and faster switching circuit can be used to reduce such blind spots.
In addition to improving the performance of controllers and sensors, Ding Lijun et al.  combined ultrasonic sensors with infrared sensors to realize complementary advantages and overcome the problem of ultrasonic sensors measuring blind areas by taking advantage of the infrared sensors, which have no blind areas, high measurement accuracy, strong directivity, but are greatly affected by the environment and have a relatively close detection distance.
2) Ultrasonic propagation speed is affected by environmental changes. The propagation velocity of ultrasonic wave in the medium is affected by environmental changes (such as temperature, pressure, humidity, etc.), among which the influence of temperature is the most obvious. Generally, every 1℃ increase in temperature will increase the sound velocity by about 0.6 m/s. Equations (2) and (3) respectively show the functional relationship between ultrasonic wave propagation velocity and environmental parameters in air and seawater .
Type of T as the ambient temperature (in degrees Celsius ℃), S (calculated on per mil) for water salinity, P for 牗 water static pressure units for Pa dozen.
In order to improve the accuracy of ultrasonic measurement, sound velocity must be corrected. The main methods of sound velocity correction are temperature compensation method and setting corrector method. Temperature compensation method uses temperature sensor to detect the ambient temperature, and then calculates the corresponding sound velocity. Setting corrector method is in the process of distance measurement using two channels: one channel through the known distance of the preset reference to determine the acoustic velocity in the current environment; The other channel takes the measured sound velocity as the standard and measures it in a normal way to avoid the influence of environmental changes on sound velocity . Mo Deju et al.  used the method of setting corrector to compensate the sound velocity and measure the liquid level, achieving a higher precision measurement than the temperature compensation method.
3) The volume of obstacles should not be too small. According to the ultrasonic propagation theory, when the size of the obstacle is less than half of the length of the ultrasonic wave, the ultrasonic wave will be diffracted. Only when the obstacle size is larger than 1/2 of the wave length can it be reflected. If the ultrasonic sensor is adopted, the transmitting frequency is 40kHz, and the corresponding half wavelength is 0.025mm, so the obstacle with side length of 0.025mm can be measured at least theoretically. The transmitting frequency of ultrasonic wave can be improved to realize the ranging of smaller obstacles.
4) Measuring distance is limited. The amplitude of ultrasonic echo signal attenuates with the increase of the propagation distance, which makes the echo signal received by the receiver decrease greatly with the increase of the measurement distance, resulting in the limited measurement distance. However, increasing the transmitting power of the transmitter will increase the residual vibration of the transducer and increase the measurement blind area. The AGV is installed in the circuit to make the circuit magnification increase with the increase of the measurement distance according to the corresponding law, which can reduce the influence on the measurement blind area. However, due to the limitation of ultrasonic transmitting power and loss, the maximum measuring distance of ultrasonic sensors in China is generally less than 15m.
4. Development trend of ultrasonic sensor
As a typical non-contact detection technology, ultrasonic sensor has the advantages of small size, low cost, and no interference from electromagnetic, light, smoke, etc., so it has broad prospects for development. Comprehensive analysis of ultrasonic sensor in ranging and other aspects of the application of the current situation, the existence of questions and solutions, the development trend of ultrasonic sensor to do the following points of prospect:
1) Integrated, high precision, the future ultrasonic sensor will be built-in temperature compensation circuit, when the temperature of the external environment changes, the temperature compensation circuit automatically proofread, improve the accuracy of measurement.
2) Improve anti-interference, the new type of ultrasonic sensor induction head should have stronger self-protection ability, can resist material damage, adapt to the messy environment. Make the ultrasonic sensor can adapt to the harsh environment measurement.
3) Intelligent, digital, new ultrasonic sensor should be easy to adjust, adapt to different measuring distance, output signal has a variety of types, make the application more flexible.
4) A variety of sensor fusion technology, with the industrial field sensor detection precision and reliability requirements more and more high, a variety of sensors (such as laser ranging, infrared, etc.) and ultrasonic sensor redundancy combination, give full play to their advantages, improve the overall performance of the sensor, will also become a development trend of ultrasonic sensors.
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