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Principles, characteristics and uses of ultrasonic sensors
Ultrasonic sensors are sensors that convert ultrasonic signals into other energy signals (usually electrical signals). Ultrasound is a mechanical wave with a vibration frequency higher than 20KHz. It has the characteristics of high frequency, short wavelength, small diffraction phenomenon, especially good directivity, which can become rays and propagate directionally. Ultrasound has a great ability to penetrate liquids and solids, especially in solids that are opaque to sunlight. When the ultrasonic wave hits the impurity or the interface, it will produce a significant reflection to form an echo, and it can produce a Doppler effect when it hits a moving object. Ultrasonic sensors are widely used in industry, national defense, biomedicine, etc.
Classification of ultrasonic sensors
Sensor: "Can feel the specified measured piece and convert it into usable according to certain rules
Signal devices or devices are usually composed of sensitive elements and conversion elements." A sensor is a detection device that can sense the information being measured, and can transform the sensed information into electrical signals or other information according to certain rules. Information output in a form is required to meet the requirements of information transmission, processing, storage, display, recording, and control. It is the primary link to realize automatic detection and automatic control.
At present, there is no unified classification method for sensors, but there are three commonly used ones:
1. According to the physical quantity of the sensor, it can be divided into displacement, force, speed, temperature, flow, gas composition and other sensors
2. According to the working principle of the sensor, it can be divided into resistance, capacitance, inductance, voltage, Hall, photoelectric, grating, thermocouple and other sensors.
3. According to the nature of the output signal of the sensor, it can be divided into: switch type sensor whose output is switching value ("1" and "0" or "on" and "off"); output is analog sensor; output is pulse or Code digital sensor.
Here, I mainly introduce a sensor that is widely used in daily life and brings great convenience to human society-ultrasonic sensor and its application in reversing radar.
Basic introduction to ultrasonic sensors
Ultrasonic sensors are sensors developed using the characteristics of ultrasonic waves. To use ultrasound as a detection method, ultrasound must be generated and received. The device that accomplishes this function is an ultrasonic sensor, which is habitually called an ultrasonic transducer, or an ultrasonic probe.
Ultrasonic probes are mainly composed of piezoelectric wafers, which can transmit and receive ultrasonic waves. The core of the ultrasound probe is a piezoelectric chip in its plastic or metal jacket. There can be many kinds of materials that make up the wafer. The main materials of ultrasonic sensors are piezoelectric crystal (electrostriction) and nickel-iron-aluminum alloy (magnetostriction). Electrostrictive materials include lead zirconate titanate (PZT) and so on. The ultrasonic sensor composed of piezoelectric crystal is a reversible sensor. It can convert electrical energy into mechanical oscillation to generate ultrasonic waves. At the same time, when it receives ultrasonic waves, it can also be converted into electrical energy, so it can be divided into transmitters or receivers. Some ultrasonic sensors can be used for both sending and receiving. Ultrasonic sensor is composed of transmitting sensor (or wave transmitter), receiving sensor (or wave receiver), control part and power supply part. The transmitter sensor is composed of a transmitter and a ceramic vibrator transducer with a diameter of about 15mm. The function of the transducer is to convert the electrical vibration energy of the ceramic vibrator into super energy and radiate into the air; while the receiving sensor is transduced by the ceramic vibrator The transducer is composed of an amplifier and an amplifier circuit. The transducer receives the wave to generate mechanical vibration and convert it into electrical energy, which is used as the output of the sensor receiver to detect the transmitted ultrasonic wave. The control part mainly controls the pulse chain frequency, duty cycle, sparse modulation and counting, and detection distance sent by the transmitter.
Working principle of ultrasonic sensor
Ultrasonic sensors are sensors developed using the characteristics of ultrasonic waves. Sound waves are the propagation form of the mechanical vibration state of an object. Ultrasound refers to sound waves with a vibration frequency greater than 20000 Hz. The number of vibrations per second is very high, exceeding the upper limit of human hearing. People call this inaudible sound wave ultrasound.
Ultrasound is a kind of mechanical oscillation in elastic medium, which has two forms: transverse oscillation (transverse wave) and longitudinal oscillation (longitudinal wave). Longitudinal oscillation is mainly used in industrial applications. Ultrasonic waves can propagate in gases, liquids and solids, and their propagation speeds are different. In addition, it also has refraction and reflection phenomena, and attenuation during propagation.
The propagation laws of reflection, refraction, diffraction, and scattering of ultrasonic waves in the medium are not essentially different from the laws of audible sound waves. Compared with audible sound waves, ultrasonic waves have many peculiar characteristics: propagation characteristics-the diffraction power of ultrasonic waves is very poor. It can propagate in a straight line in a uniform medium. The shorter the wavelength of the ultrasonic waves, the more prominent this characteristic. Power characteristics ─ ─ When sound propagates in the air, it pushes the particles in the air to vibrate back and forth to do work on the particles. Under the same intensity, the higher the frequency of the sound wave, the greater the power it has. Due to the high frequency of ultrasonic waves, the power of ultrasonic waves is very large compared with ordinary sound waves. Cavitation ─ ─ When ultrasonic waves propagate in the liquid, small cavities will be created in the liquid due to the violent vibration of the liquid particles. These small cavities expand and close rapidly, causing violent collisions between the liquid particles, resulting in pressures of thousands to tens of thousands of atmospheres. This violent interaction between the particles will cause the temperature of the liquid to rise suddenly, so that two immiscible liquids (such as water and oil) emulsify, accelerate the dissolution of solutes, and accelerate chemical reactions. The various effects caused by the action of ultrasonic waves in the liquid are called ultrasonic cavitation.
Features of ultrasound:
(1) When the ultrasonic wave is propagating, the directionality is strong, and the energy is easy to concentrate;
(2) Ultrasound can propagate in a variety of different media and can travel a long enough distance; (3) The interaction between ultrasound and the sound transmission medium is moderate, and it is easy to carry information about the state of the sound transmission medium (diagnosis or on the sound transmission medium). Have an effect).
In the previous industrial revolutions of human civilization, sensing technology has always played an important role as a pioneer. It is a key technology that runs through various technologies and application fields. It is almost ubiquitous in all fields that people can imagine.
With the advancement of sensor technology, the sensor will develop from a purely judgment function to a learning function, and eventually to a creativity. Looking into the future in the new century, as a new type of very important and useful tool, the ultrasonic sensor will have a lot of room for development in all aspects, and it will develop in the direction of higher positioning and high precision. In meeting the ever-developing social needs, sensors with a new look will play a greater role.
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