Ultrasonic Distance Measurement Transducer

The frequency of the ultrasonic transducer sensor that people can hear is 20Hz~2KHz, that is, audible sound waves. The sound beyond this frequency range, the ultrasound below 20Hz is called low-frequency sound wave, and the ultrasound above 20KHz is called ultrasound (ultrasound), the frequency of general speaking .The range is 10Hz-8KHz. ultrasound has good directionality, strong penetrating ability, easy to obtain concentrated sound energy, and it has a long distance in water. Ultrasound is named because its lower frequency limit is approximately equal to the upper limit of human hearing.

Ultrasonic frequency distribution

Ultrasonic waves can propagate in gases, liquids and solids at different speeds. The waveform of ultrasonic waves propagating in a medium depends on what kind of force the medium can withstand and how to excite ultrasonic waves on the medium.

Usually there are three types:

(1) Longitudinal wave type

When the particle vibration direction in the medium is consistent with the propagation direction of the ultrasonic wave, the ultrasonic wave is in the longitudinal wave mode. Any solid medium can produce longitudinal waves when its volume changes alternately. The application in industry mainly adopts longitudinal oscillation.

(2) Shear wave type

When the vibration direction of the particle in the medium is perpendicular to the propagation direction of the ultrasonic wave, the ultrasonic wave is a transverse wave. Since the solid medium can bear shear deformation in addition to volume deformation, shear waves can be generated when the shear force alternately acts on the solid medium. Shear waves can only propagate in the solid media.

(3) Surface wave mode

It is a wave with dual properties of longitudinal wave and transverse wave propagating along the solid surface. The surface wave can be regarded as a synthesis of longitudinal waves parallel to the surface and transverse waves perpendicular to the surface. The trajectory of the vibrating particle is an ellipse. The amplitude is the strongest at the depth of 1/4 wavelength from the surface, and it attenuates quickly as the depth increasing. In fact, the amplitude of particle vibration is already very weak when it is more than one wavelength away from the surface. In addition, ultrasonic waves also have refraction and reflection phenomena, and they are attenuated during propagation. Ultrasound propagates in the air, and its frequency is low, generally tens of KHz, while in is solid and liquid, the frequency can be higher. It attenuates faster in the air, but propagates in liquids and solids with less attenuation and farther propagation.

Utilizing the characteristics of ultrasound range sensor, it can be made into various ultrasonic sensors, coupled with different circuits, and made into various ultrasonic measuring instruments and devices, which can be used for distance measurement, speed measurement, cleaning, welding, gravel, sterilization, etc., and in communication , medical, home appliances, military, industry, agriculture and other aspects are widely used. There are many methods that can generate ultrasonic waves, commonly used are piezoelectric effect method, magnetostrictive effect method, electrostatic effect method and electromagnetic effect method. When a short voltage pulse is applied to the two poles of the piezoelectric wafer, due to the inverse piezoelectric effect, the piezoceramics wafer will undergo elastic deformation and produce elastic oscillation. The oscillation frequency is related to the thickness of the wafer and the speed of sound. proper selection of the thickness of the wafer can obtain elastic waves in the ultrasonic frequency range, that is, ultrasonic waves. What is emitted in this way is an ultrasonic wave packet, usually called a pulse wave.

Ultrasonic ranging transducer

Ultrasonic distance measuring sensor is mainly used in car reversing radar, robot automatic obstacle avoidance walking, construction site and some industrial sites such as liquid level, well depth, pipeline length and other occasions. There are currently two commonly used ultrasonic ranging transducer, One is an ultrasonic ranging transducer based on a single-chip or embedded device, and the other is an ultrasonic ranging system based on a CPLD (Complex Programmable Logic Device). Several schemes were used in the experiment,by using embedded device programming to generate a square wave with a frequency of 40KHz transducer which was amplified by the transmitting drive circuit to make the transmitting end of the ultrasonic sensor oscillate and emit ultrasonic waves. The ultrasonic wave is reflected back by the transmitter, received by the sensor receiving end, and then amplified and shaped by the receiving circuit. The ultrasonic ranging transducer with embedded micro-core records the time of ultrasonic emission and the time of reflected wave through embedded equipment. When the reflected wave of the ultrasonic wave is received, a jumping occurs at the output end of the receiving circuit. By counting the timer and calculating the time difference, the corresponding distance can be calculated.

Principle of ultrasonic ranging transducer

The principle of ultrasonic distance measurement  transducer 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 in order to calculate the actual distance from the emission point to the obstacle based on the time difference between emission and reception. First of all, the ultrasonic transmitter transducer 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 it 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 C=340m/s. According to the time T seconds recorded by the timer, the distance L between the emission point and the obstacle can be calculated, namely: L= C×T /2. This is the so-called time difference ranging method. Since ultrasound is also a kind of sound wave, its sound velocity C is related to temperature. Table 1 lists the sound velocity at several different temperatures. In use, if the temperature does not change much, it can be considered that the speed of sound is basically constant. If the distance measurement accuracy is very high, it should be corrected by temperature compensation.

The relationship between ultrasonic wave speed and temperature

Due to the advantages of easy directional emission of ultrasonic waves, good directionality, easy control of intensity, and no need for direct contact with the measured object, it is an ideal choice for measuring the reversing distance. Ultrasound propagates in a straight line. The higher the frequency, the weaker the diffraction ability, but the stronger the reflection ability. Therefore, ultrasonic sensors mould can be made by using this property of ultrasonic waves. In addition, ultrasonic waves travel slowly in air, which makes the use of ultrasonic sensors simple. Ultrasonic sensors are sensors developed using the characteristics of ultrasonic waves. Ultrasound is a mechanical wave with a vibration frequency higher than that of sound waves, which is generated by the vibration of the transducer chip under the excitation of voltage. It has high frequency, short wavelength, small diffraction phenomenon, especially good directionality, and can be oriented as rays dissemination characteristics. Ultrasound has a great penetrating ability to liquids and solids, especially in solids that are opaque to sunlight, it can penetrate to a depth of tens of meters. Ultrasonic waves will produce significant reflections when they encounter impurities or interfaces to form echoes, and when they encounter moving objects, they will produce Doppler effects. Therefore, ultrasonic detection is widely used in industry, national defense, biomedicine, etc. ultrasound is used as a detection method, and ultrasonic waves must be generated and received. The device that accomplishes this function is an ultrasonic sensor, which is customarily called an ultrasonic transducer, or an ultrasonic probe.

The ultrasonic sensor is mainly composed of a bimorph vibrator, a conical resonant plate and electrodes. When a certain voltage is applied between the two electrodes, the piezoelectric ceramics wafer will be compressed to produce mechanical deformation, and the piezoelectric wafer will return to its original shape after the voltage is removed. If a voltage is applied between the two poles at a certain frequency, the piezoelectric ceramics will also vibrate at a certain frequency. It is tested that the natural frequency of this type of piezoelectric transducer is 38.4 KHz, and a square wave pulse signal with a frequency of 40 KHz transducer is applied to the two poles. At this time, the piezoelectric chip resonates and emits ultrasonic waves. In the same way, an ultrasonic sensor without an external pulse signal will also resonate when the resonance plate receives ultrasonic waves, and an electrical signal will be generated between the two poles. The ultrasonic probe is mainly composed of piezoelectric wafers, which can both transmit and receive ultrasonic waves. Low-power ultrasonic transducers are mostly used for detection. It has many different structures, which can be divided into straight probe (longitudinal wave), oblique probe (shear wave), surface wave probe (surface wave), Lamb wave probe (Lamb wave), double probe (one probe reflection, one probe reception) wait.