Common detection methods and principle analysis of ultrasonic sensors

In industry, the typical applications of ultrasonic are non-destructive testing of metals and ultrasonic thickness measurement. In the past, many technologies were hindered because they were unable to detect the inside of the object's tissues. The emergence of ultrasonic sensing technology has changed this situation. Of course, more ultrasonic sensors are fixedly installed on different devices to quietly detect signals that people need. In the future application of ultrasonic sensors, ultrasound will be combined with information technology and new material technology, and more intelligent and highly sensitive ultrasonic distance measuring sensors will appear.

1. According to the characteristics of the detected object's volume, material, and whether it is movable

The detection methods adopted by ultrasonic sensors are different. There are four common detection methods as follows:

1. Through type: the transmitter and receiver are located on both sides, when the detecting object passes between them, the detection is performed according to the attenuation (or occlusion) of the ultrasonic wave.

2. Limited distance type: The transmitter and receiver are located on the same side, and when the detected object passes within the limited distance, the detection is performed based on the reflected ultrasonic waves.

3. Limited range type: the transmitter and receiver of ultrasonic distance sensors are located at the center of the limited range, the reflector is located at the edge of the limited range, and the attenuation value of the reflected wave without being blocked by the detected object is used as the reference value. When the detected object passes through the limited range, the detection is performed according to the attenuation of the reflected wave (compare the attenuation value with the reference value).

4. Retro-reflective type: the transmitter and receiver are located on the same side, and the detection object (flat object) is used as the reflection surface, and the detection is performed according to the attenuation of the reflected wave.

Second, the test is good or bad

There is nothing reflected when the ultrasonic sensor is directly tested with a multimeter. If you want to test the quality of the ultrasonic sensor, you can build an audio oscillator circuit. When C1 is 390OμF, an audio signal of about 1.9kHz can be generated between the pins of the inverter. Connecting the ultrasonic sensor to be detected (transmitting and receiving) between the foot and the foot; if the sensor can emit audio sounds, it can basically be determined that it is better than the ultrasonic sensor.

Note: When C1=3900μF, it is about 1.9kHZ; when C1=0.O1μF, it is about 0.76kHZ.

Three, liquid level test

The basic principle of ultrasonic measurement of liquid level is the ultrasonic pulse signal sent by the ultrasonic probe propagates in the gas, and is reflected after encountering the interface of air and liquid. After receiving the echo signal, calculating the propagation time of the ultrasonic wave. Convert the distance or liquid level height. The ultrasonic measurement method has many advantages that other methods cannot compare:

(1) There are no mechanical transmission parts, and there is no contact with the measured liquid. It is a non-contact measurement, it is not afraid of electromagnetic interference and strong corrosive liquids such as acid and alkali, so it has stable performance, high reliability and long life;

(2) Its short response time can easily realize real-time measurement without delay.

The working frequency of the ultrasonic sensor used in the system is about 40kHz. Ultrasonic pulses are sent out by the transmitter sensor, transmitted to the liquid surface and then returned to the receiving sensor. The required time for the ultrasonic pulse from transmission to reception is measured. According to the speed of sound in the medium, the distance from the ultrasonic sensor to the liquid surface can be obtained To determine the liquid level. Taking into account the influence of ambient temperature on the propagation velocity of ultrasonic waves, the propagation velocity is corrected by the method of temperature compensation to improve the measurement accuracy. The calculation formula is:

V=331.5+0.607T (1)

Where: V is the propagation speed of ultrasonic waves in the air; T is the ambient temperature.

S=V ×t/2=V×(t1－t0)/2 (2)

In the formula: S is the measuring distance; t is the time difference between transmitting the ultrasonic pulse and receiving its echo; t1 is the receiving time of the ultrasonic echo; t0 is the time of transmitting the ultrasonic pulse. The capture function of MCU can easily measure t0 and t1. According to the above formula, the measured distance S can be obtained by software programming. Because the MCU of this system selects a mixed-signal processor with SOC characteristics and integrates a temperature sensor inside, the software can be used to easily realize temperature compensation for the ultrasonic level sensor.