Ultrasonic sensors are sensors that convert ultrasonic signals into other energy signals, usually electrical signals. Ultrasonic waves are mechanical waves that oscillate at frequencies higher than 20KHz. It has the characteristics of high frequency, short wavelength, small diffraction phenomenon, especially good directionality, and can become a ray and propagate directionally. Ultrasonic waves have a great ability to penetrate liquids and solids, especially in solids where sunlight is opaque. When the ultrasonic wave encounters impurities or interfaces, it will be reflected obviously, and the reflection will become an echo, and the Doppler effect can occur when it touches an active object. Ultrasonic sensors are widely used in industry, defense, biomedicine, etc.
Commonly used ultrasonic sensors consist of piezoelectric wafers that can either emit or receive ultrasonic waves. The low-power ultrasonic probe is more effective for detection. It has many different configurations, which can be divided into straight probes (longitudinal wave), oblique probes (shear waves), surface wave probes (surface waves), Lamb wave probes (Lamb waves), dual probes (one probe emitted, one probe accepted), etc.
The ultrasonic interval sensor adopts the principle of ultrasonic echo ranging, uses accurate time difference measurement skills to detect the interval between the sensor and the guideline, and selects a small point of view and a small blind area ultrasonic sensor, which has the advantages of accurate measurement, no touch, waterproof, anti-corrosion, and low cost.
The principle of ultrasonic ranging is to emit ultrasonic waves in a certain direction through the ultrasonic transmitter, start timing at the same time of transmission, return immediately when the ultrasonic waves hit obstacles when propagating in the air, and stop the timing immediately when the ultrasonic receiver receives the reflected waves.
The propagation speed of ultrasonic waves in the air is v, and according to the time difference △t recorded by the timer to measure the emission and acceptance echoes, the interval S between the emission point and the obstacle can be calculated, that is: (1) S = v· △t /2 This is the so-called time difference ranging method.
Since ultrasound is also a sound wave, and its sound velocity C is related to temperature, Table 1 lists several sound velocities at different temperatures. In use, if the temperature does not change much, it can be assumed that the speed of sound does not change at all. The propagation velocity of ultrasonic waves at room temperature is 334 m/s, but its propagation velocity V is easily affected by the temperature, humidity, pressure and other factors in the air, among which the temperature is greatly affected, such as the sound velocity increases by about 0 for every 1 °C increase in temperature. 6 m/s.
If the ranging accuracy is required to be very high, it should be corrected by the method of temperature compensation (the system is selected by the method of temperature compensation). When the ambient temperature T is known, the formula for calculating the ultrasonic propagation velocity V is: (2) V = 331.45 + 0.607T.
To measure the time of ultrasonic round-trip, the interval can be found. This is how an ultrasonic rangefinder works.
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