Basic Principles of Ultrasonic Range Finder
1. Ultrasonic generator
In order to study and utilize ultrasonic waves, many ultrasonic generators have been designed and manufactured. Generally speaking, ultrasonic generators can be divided into two categories: one is to generate ultrasonic waves by electrical means, and the other is to generate ultrasonic waves by mechanical means. Electrical methods include piezoelectric, magnetostrictive, and electric types; mechanical methods include garrison flutes, liquid whistles, and airflow whistles. The ultrasonic waves they generate vary in frequency, power, and sonic characteristics, and thus have different uses. The most commonly used is the piezoelectric ultrasonic generator.
2. Principle of piezoelectric ultrasonic generator
Piezoelectric ultrasonic generators actually use the resonance of piezoelectric crystals to work. The internal structure of the ultrasonic generator is shown in Figure 1. It has two piezoelectric wafers and a resonance plate. When a pulse signal is applied to its two poles, the frequency of which is equal to the natural oscillation frequency of the piezoelectric chip, the piezoelectric chip will resonate, and drive the resonance plate to vibrate, thus generating ultrasonic waves. Conversely, if no voltage is applied between the two electrodes, when the resonant plate receives ultrasonic waves, it will press the piezoelectric chip to vibrate, converting mechanical energy into electrical signals, and then it becomes an ultrasonic receiver.
3. Basic principle of ultrasonic range finder
The ultrasonic transmitter emits ultrasonic waves in a certain direction, and starts timing at the same time as the emission time. The ultrasonic waves propagate in the air, and return immediately when encountering obstacles on the way, and the ultrasonic receiver stops timing immediately after receiving the reflected waves. 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 can be calculated, namely: s=340t/2. This is the so-called time difference ranging method.
The principle of ultrasonic distance measurement is to use the known propagation speed of ultrasonic waves in the air to measure the time it takes for the sound waves to encounter obstacles and reflect back after they are emitted, and to calculate the actual distance from the emission point to the obstacle based on the time difference between emission and reception. It can be seen that the principle of ultrasonic ranging is the same as that of radar.
The formula for ranging is expressed as: L=C×T
In the formula, L is the measured distance length; C is the propagation speed of ultrasonic waves in the air; T is the time difference of the measured distance propagation (T is half of the time value from transmission to reception).
Ultrasonic ranging is mainly used for distance measurement of reversing reminders, construction sites, industrial sites, etc. Although the current ranging range can reach 100 meters, the measurement accuracy can only reach the order of centimeters.
Due to the advantages of easy directional emission of ultrasonic waves, good directionality, easy control of intensity, and no direct contact with the measured object, it is an ideal means for liquid height measurement. In precise liquid level measurement, it is necessary to achieve millimeter-level measurement accuracy, but at present, domestic ultrasonic ranging ASICs only have centimeter-level measurement accuracy. By analyzing the causes of ultrasonic ranging errors, improving the measurement time difference to the microsecond level, and using the LM92 temperature sensor to compensate for the sound wave propagation velocity, the high-precision ultrasonic rangefinder we designed can achieve millimeter-level measurement accuracy.
