What is ultrasonic ranging
Due to the strong directivity of ultrasonic waves, slow energy consumption, and the long distance of propagation in the medium, ultrasonic waves are often used for distance measurement, such as rangefinders and level measuring instruments. The use of ultrasonic testing is often relatively fast, convenient, simple calculation, easy to achieve real-time control, and can meet the industrial practical requirements in terms of measurement accuracy, so it has also been widely used in the development of mobile robots.
In order for the mobile robot to walk automatically while avoiding obstacles, it must be equipped with a distance measuring system so that it can obtain distance information (distance and direction) from obstacles in time. The three-direction (front, left, and right) ultrasonic ranging system introduced in this article is to provide the robot with a range of motion distance information to understand the environment in front of it, left, and right.
Principle of Ultrasonic Ranging
1. Ultrasonic generator
In order to study and use ultrasound, many ultrasound generators have been designed and made. 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. The electrical methods include piezoelectric type, magnetostrictive type and electric type, etc .; the mechanical methods include Galton whistle, liquid whistle and air flow whistle. The frequency, power and sound wave characteristics of the ultrasonic waves generated by them are different, so their uses are also different. At present, the piezoelectric ultrasonic generator is more commonly used.
2. The principle of piezoelectric ultrasonic generator
The piezoelectric ultrasonic generator actually uses the resonance of the piezoelectric crystal 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 poles and its frequency is equal to the natural oscillation frequency of the piezoelectric wafer, the piezoelectric wafer will resonate and drive the resonance plate to vibrate, which generates ultrasonic waves. Conversely, if no voltage is applied between the two electrodes, when the resonance plate receives ultrasonic waves, it will press the piezoelectric wafer to vibrate and convert mechanical energy into electrical signals. At this time, it becomes an ultrasonic receiver.
3. The principle of ultrasonic ranging
The ultrasonic transmitter emits ultrasonic waves in a certain direction, and starts timing at the same time as the transmission time. The ultrasonic waves propagate in the air. When an obstacle is encountered on the way, it returns immediately. When the ultrasonic receiver receives the reflected wave, it immediately stops timing. The propagation speed of ultrasonic waves in the air is 340m / s. According to the time t recorded by the timer, the distance (s) from the launch point to 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 propagation speed of ultrasonic waves in the air to be known. Measure the time when the sound wave meets the obstacle and reflects back after transmission, and calculate the actual distance from the transmission point to the obstacle according to the time difference between transmission and reception. This shows that the principle of ultrasonic ranging is the same as that of radar.
The formula for ranging is expressed as: L = C & TImes; T
In the formula, L is the measured distance length; C is the propagation speed of the ultrasonic wave in the air; T is the time difference of the measured distance propagation (T is half the value of the time 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 often only reach the order of centimeters.
Due to the advantages of easy directional emission of ultrasonic waves, good directivity, easy control of strength, and no direct contact with the measured object, it is an ideal method for liquid height measurement. In precision liquid level measurement, it is necessary to achieve measurement accuracy of millimeter level, but at present, domestic integrated circuits for ultrasonic distance measurement only have measurement accuracy of centimeter level. By analyzing the causes of ultrasonic ranging errors, increasing the measurement time difference to the microsecond level, and using the LM92 temperature sensor to compensate for the sound wave propagation speed, our designed high-precision ultrasonic rangefinder can achieve millimeter-level measurement accuracy.
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