People can hear the sound due to the vibration of the object, and its frequency is within the range of 20HZ-20KHZ ultrasonic sensor. More than 20KHZ is called ultrasonic wave, and those below 20HZ are called infrasound waves. Commonly used ultrasonic frequencies are dozens of KHZ-tens of MHZ. Ultrasound is a kind of mechanical oscillation in an elastic medium. There are two forms: transverse oscillation (transverse wave) and longitudinal and oscillation (longitudinal wave). Application in industry mainly uses longitudinal oscillation. Ultrasonic waves can propagate in gases, liquids and solids at different propagation speeds. In addition, it also has refraction and reflection phenomena, and has attenuation during propagation. The frequency of ultrasonic waves propagating in air is low, generally tens of KHZ, while in solids and liquids, the frequency can be higher. It decayes faster in air, but propagates in liquids and solids with less attenuation and spreads farther. Using the characteristics of ultrasonic waves, it can be made into various ultrasonic sensors, equipped with different circuits, and made into various ultrasonic measuring instruments and devices, which are widely used in communication, medical appliances and other aspects. The main materials of ultrasonic sensors are piezoelectric crystal (electrostrictive) and nickel iron alloy (magnetostrictive). Electrostrictive materials include lead zirconate titanate (PZT). 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 a transmitter or a receiver. Some ultrasonic sensors can be used for both sending and receiving. Only small ultrasonic sensors are introduced here. There is a slight difference between sending and receiving. It is suitable for propagation in the air. The operating frequency is generally 23-25KHZ and 40-45KHZ. This type of sensor is suitable for ranging, remote ultrasonic sensor control, anti-theft and other purposes. There are T / R-40-60, T / R-40-12, etc. (where T means send, R means receive, 40 means frequency is 40KHZ, 16 and 12 mean outer diameter, in millimeters). There is also a sealed ultrasonic sensor (MA40EI type). Its characteristic is that it has waterproof function (but can not be put into water), it can be used as material level and proximity switch, and its performance is better. There are three basic types of ultrasonic applications, transmission type is used for remote control, anti-theft alarm, automatic door, proximity switch, etc .; separate reflective type is used for distance measurement, liquid level or material level; reflective type is used for material flaw detection, thickness measurement, etc . It is composed of transmitting sensor (or wave transmitter), receiving sensor (or wave receiver), control part and power 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 it into the air; It is composed of an amplifier circuit. The transducer receives the wave to generate mechanical vibration, and converts it into electrical energy, which is used as the output of the sensor receiver to detect the transmitted excess. In actual use, the ceramic vibrator of the transmitting sensor can also be used. Used as a ceramic vibrator for the receiver sensor company. The control part mainly controls the pulse chain frequency, duty cycle and sparse modulation and counting and detection distance sent by the transmitter.
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