Analysis of ultrasonic sensors and ultrasonic ranging principles
The sound wave frequency that the human ear can hear is 20HZ ~ 20KHz. When the vibration frequency of sound waves is greater than 20KHz or less than 20Hz, we cannot hear it. Therefore, we call sound waves with frequencies higher than 20KHz "ultrasound". Because of its good directionality and strong penetrating ability, it is easy to obtain relatively concentrated sound energy and can propagate far in water. It can be used for distance measurement, speed measurement, cleaning, welding, gravel, sterilization and disinfection, etc. It has many applications in medicine, military, industry, and agriculture. Such as ultrasonic cleaning machine, ultrasonic humidifier, medical examination B-ultrasound, color ultrasound, ultrasonic flaw detector, etc.
Sound is produced by vibration, and the device that can generate ultrasonic waves is an ultrasonic sensor, which is customarily called an ultrasonic transducer, or ultrasonic probe. The ultrasonic probe is mainly composed of a piezoelectric chip, which can both emit and receive ultrasonic waves. Wafers can be made of many materials. The size of the chip, such as diameter and thickness, also varies, so the performance of each probe is different, and its performance must be understood in advance before use.
Commonly used is the piezoelectric ultrasonic generator, which uses the resonance of piezoelectric crystals to work.Ultrasonic sensor probeThere are two piezoelectric wafers and a resonance plate inside. When a pulse signal is applied to its two poles and its frequency is equal to the natural oscillation frequency of the piezoelectric crystal, the piezoelectric crystal will resonate and drive the resonance plate to vibrate, thus generating ultrasonic waves. On the contrary, if no voltage is applied between the two electrodes, when the resonance plate receives ultrasonic waves, it will press the piezoelectric chip to vibrate and convert the mechanical energy into electrical signals. At this time, it becomes an ultrasonic receiver. Ultrasonic sensors use the principle of piezoelectric effect to convert electrical energy and ultrasonic waves into each other. That is, when transmitting ultrasonic waves, they convert electrical energy into ultrasonic waves and send them out; when receiving, they convert ultrasonic vibrations into electrical signals.
Ultrasonic ranging principle:
The most commonly used method of ultrasonic distance measurement is the echo detection method. As shown in the figure below, the ultrasonic transmitter emits ultrasonic waves in a certain direction. At the same time as the emission time, the counter starts timing. The ultrasonic waves propagate in the air and immediately stop when they encounter obstacles on the way. Reflected back, the ultrasonic receiver stops timing immediately after receiving the reflected ultrasonic wave. The propagation speed of ultrasonic waves in the air is 340m/s. According to the time t recorded by the timer, the distance s between the emission point and the obstacle surface can be calculated, that is: s=340t/2
Ultrasonic transmitter circuit: The 555 timer generates a 40KHZ pulse signal and adds it to the pin of the ultrasonic probe, causing the internal piezoelectric chip to resonate and emit ultrasonic waves outward.
Ultrasonic receiving circuit: Since the electrical signal generated by the ultrasonic receiving probe is very weak, it needs to be amplified. As shown in the figure below, the amplifying circuit is composed of a transistor and an operational amplifier LM324. After amplifying the received signal, it drives the relay.
Review Editor: Huang Fei
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