Mechanism of ultrasonic range finder

As the medium for transmitting information, ultrasonic is more and more important in various fields such as flaw detection, ranging, and speed measurement due to its direct and reflective properties and its characteristics that are not easily affected by external factors such as light and electromagnetic waves. Ultrasonic wave is a sound wave with the frequency higher than 20KHz. It has good directionality, strong penetrating power, easy to obtain concentrated sound energy, which has a long distance in the water. ultrasonic transducers can be used for distance measurement, speed measurement, cleaning, welding, crushing, sterilization. disinfection, etc. There are many applications in the medicine, military, industry, and agriculture. Ultrasound is named for its lower frequency limit, which is approximately equal to the upper limit of human hearing.

In the ultrasonic detection circuit, the output pulse is a series of square waves at the transmitting end. The width of this series of square waves is the time interval between transmitting ultrasonic waves and receiving ultrasonic waves. Obviously, the larger the distance between the measuring objects, the larger the pulse width, and the output pulse. The number is proportional to the measurement distance. Ultrasonic ranging transducer generally has the following methods:

(1) taking the average voltage of the output pulse, the voltage (the amplitude of the voltage is basically fixed) is proportional to the distance, and the measuring voltage can be measured;

(2) Measuring the width of the output pulse, that is, the time interval t between the transmitting ultrasonic wave and the receiving ultrasonic wave. Therefore, the measuring distance is S = 1/2 vt.

Ultrasonic transmitters transmit ultrasonic waves in a certain direction, and start timing at the same time is the transmission time. When the ultrasonic waves propagate in the air, they will immediately return to the obstacles, and the ultrasonic receivers will stop timing immediately upon receiving the reflected waves. The propagation speed of the ultrasonic wave in the air is V, and according to the time difference measured by the timer to measure the transmitted and received echoes, the distance S of the emission point from the obstacle can be calculated, that is: S = V·Δt / 2 ,This is the time difference ranging method. Since ultrasonic distance sensors are also a kind of sound waves, their speed of sound is related to temperature, and the speed of sound at several different temperatures is listed. In the use, if the temperature does not change much, the speed of sound can be considered to be substantially constant. The propagation speed of ultrasonic wave at normal temperature is 334 m / s, but its propagation velocity V is easily affected by factors such as temperature, humidity and pressure in the air, which are greatly affected by temperature. For every 1°C increases in the temperature, the speed of sound increases 0. 6 m / s. If the accuracy of the ranging is very high, it should be corrected by the method of temperature compensation. When the ambient temperature T is known, the formula for calculating the ultrasonic propagation velocity V is:V = 331.45 + 0.607T

After the sound speed is determined, the distance can be obtained by measuring the time of the ultrasonic round trip. This is the mechanism of the ultrasonic range finder.

1. Use IO port TRIG to trigger ranging, giving a minimum of 10us high level signal (one pulse width is 10us / one high power ,The flat duration is 10us.

2. The module automatically sends 840khz square waves to automatically detect whether there is a signal return.

3. There is a signal return, and a high level is output through the IO port ECHO. The high level duration is the time from the transmission to the return of the ultrasonic wave. Test distance = (high time * sound speed / 2), VCC GND is the power supply, TRIG is the control end (input), and ECHO is the return end (output). TRIG gives a high level pulse, and SR40 starts to send ultrasonic waves. When receiving the reflect wave, ECHO outputs a valid signal. The distance can be converted by measuring the time difference from the start of the TRIG trigger to the receipt of the ECHO.

HCSR04 ultrasonic ranging module, VCC provides 5V power supply, GND is ground, TRIG trigger control signal input, ECHO echo signal output and other four interface terminals. A pulse trigger signal of 10us or more is provided, and the module will internally emit 40khz cycle levels and detect echoes. Once the echo signal is detected, the output echo signal is detected. The pulse width of the echo signal is proportional to the measured distance, whereby the distance can be calculated by transmitting the signal to the receiving echo signal .