Ultrasound

Sound Wave is a kind of mechanical wave which can propagate in gas, liquid and solid state. Sound waves are generally divided into infrasonic, acoustic and ultrasonic waves by frequency.

Sound waves range in frequency from 16 Hz to 20 khz, which is a mechanical wave that can be heard by the human ear; infrasound is a mechanical wave whose frequency is lower than 16 Hz, while ultrasonic wave is a mechanical wave whose frequency is higher than 20 khz.

The ultrasonic wave is characterized by high frequency, short wavelength and small diffraction. Its most obvious characteristics are good specificity, and in the liquid, solid loss is not large, through really smart, encounter the interface between the media will produce more obvious reflection and refraction, therefore, it is widely used in industrial detection.

The rapid spread of ultrasound: ultrasonic wave generally have p-wave, s-wave and surface wave, the rapid spread of these people, the key lies in the medium elastic constant and medium density. Only longitudinal waves can be propagated in gas and liquid. The sound velocity is 344 m/s in gas and 900-1900 m/s in liquid. In the solid state, the velocities of longitudinal, shear and surface waves are related. It is generally considered that the velocity of S-wave is half of that of p-wave and that of surface wave is about 90% of that of s-wave.

As the ultrasonic wave propagates in the medium, the distance between the propagations increases and the energy is slowly lost. The key of energy loss lies in the diffusion, scattering and absorption of ultrasonic wave.

The ultrasonic wave can be produced and received by the ultrasonic wave as a detection method. The device that performs this function is the ultrasonic generator.

The core performance of the ultrasonic generator includes;

(1) operating frequency.

The operating frequency is the resonant frequency of the piezoelectric chip. When the frequency of the AC voltage and the resonant frequency of the chip are equal, the output energy is the highest and the sensitivity is the highest.

(2) working temperature.

Because the Curie point of piezoelectric materials is usually very high, especially when the diagnostic ultrasound probe uses less power, it operates at a relatively low temperature and can work for a long time without becoming ineffective.

(3) sensitivity.

It all depends on making the chip itself. The electromechanical coupling coefficient is large, the reaction speed is fast, on the contrary, the sensitivity is low.

Basic principle of operation

Ultrasonic generator in accordance with its basic principles of work, generally divided into piezoelectric, magnetostrictive, electromagnetic and so on, piezoelectric is extremely common.

Piezoelectric ultrasonic generator

Piezoelectric ultrasonic generator is based on the principle of piezoelectric effect of piezoelectric materials to work. The common materials of sensitive elements include piezoelectric crystals and piezoelectric ceramics.

Based on the difference between positive and piezoelectricity ultrasonic generators, piezoelectric ultrasonic generators are divided into two types: generators (emission probe) and receivers (receiving probe) , according to the different structure and wave type, it can be divided into straight probe, surface wave probe, Lamb wave probe, variable angle probe, double crystal probe, focus probe, water immersion probe, spray probe and special probe.

The piezoelectric ultrasonic generator uses the principle of piezoelectricity to convert high-frequency electrical vibration into high-frequency mechanical vibration to produce ultrasonic waves. Resonance occurs when the frequency of the applied alternating voltage is equal to the natural frequency of the piezoelectric material, which produces the most intense ultrasonic waves. Piezoelectric ultrasonic generator can form tens of kilohertz to tens of megahertz of high-frequency ultrasound, its sound intensity can reach tens of watts per square centimeter.

The typical piezoelectric ultrasonic sensor structure is mainly composed of piezoelectric chip, absorption block (damping block) , protective film and so on. Piezoelectric wafers are usually circular, and the ultrasonic frequency is inversely proportional to the thickness of the wafers. The two sides of the piezoelectric chip are coated with silver layer, which acts as a conductive electrode plate, the bottom surface is grounded, and the top surface is connected to the lead wire. In order to prevent the damage caused by the contact between the sensor and the measured part, a protective film is bonded under the piezoelectric chip. The function of the absorber is to reduce the mechanical quality of the piezoelectric chip and absorb the energy of the ultrasonic wave.

Magnetostrictive ultrasonic sensor

Ferromagnetic materials in the alternating magnetic field along the direction of the formation of expansion of the phenomenon, known as magnetostriction. The strength of the magnetostriction, or the degree to which the material elongates and contracts, varies from ferromagnetic material to ferromagnetic material. Nickel has the greatest magnetostriction, and if a certain DC magnetic field is applied at the beginning, and alternating current is applied, it can hold to the best range of characteristics. Magnetostrictive sensor materials in addition to nickel, but also iron drill vanadium alloy and containing zinc, nickel ferrite. They have a narrow range of operating capabilities, which range from a few tens of thousands of hertz to 100,000 watts of power, several kilowatts of sound per square millimeter, and high temperatures.

Magnetostrictive ultrasonic generator is to put the ferromagnetic material in the alternating magnetic field, let it form the mechanical size of the alternating changes that mechanical vibration, and then produce ultrasonic. It is made by stacking several nickel sheets 0.1-0.4 mm thick to insulate between the sheets to reduce eddy current damage, and its structural shapes are rectangular, window-shaped, etc. .

The principle of magnetostrictive ultrasonic receiver is that when the ultrasonic wave acts on the magnetostrictive material, the material will expand, which may lead to the change of its internal magnetic field. Based on electromagnetic induction, the magnetostrictive material is wound in a coil to obtain electromotive force. This potential is sent to the measuring circuit and is eventually recorded or displayed.

Type selection requirements

In the selection and installation of ultrasonic sensors will have to establish some preconditions, otherwise it will be directly related to the results of the sensor measurement.

Scope and size of inspection

The size of the object to be inspected seriously affects the detection range of the ultrasonic sensor. The sensor must detect a certain level of sound in order to carry out output. Large parts reflect most of the sound to the ultrasonic sensor, so that the sensor can detect the part at its farthest sensing distance. Small components can reflect only a small portion of the sound, which may result in a significant reduction in the sensing range.

Examine the characteristics of an object

The ideal object examined by ultrasonic sensor should be large, flat, high density and perpendicular to the front of the converter. The most difficult objects to examine are those that are small, made of sound-absorbing material, and are angled to form an angle with the converter.

If the liquid level is static and perpendicular to the surface of the sensor, it is easy to check the liquid state. If the liquid level fluctuates greatly, the response time of the sensor can be prolonged, and then take the average of the fluctuation changes to obtain more consistent readings. However, ultrasonic sensors can not accurately detect a liquid with a frothy surface because the froth can deflect the direction of the sound.

In this case, the reverse ultrasonic mode of the ultrasonic sensor can be used to verify the irregular shape of the pattern of objects. In reverse mode, the ultrasonic sensor will examine a flat background, such as a wall. Anything that passes between the sensor and the wall blocks the sound waves. The sensor can then identify an object's presence by examining the effect.

Whether there is vibration around

Whether it is the vibration of the sensor itself or the vibration of a nearby machine, will seriously affect the accuracy of distance measurement. Guide rails are sometimes used to eliminate or reduce component vibration.

False positives caused by environment

Objects around them may reflect sound waves. In order to inspect the object reasonably, it is necessary to reduce or eliminate the influence of the nearby sound reflection surface. In order to prevent misdetection of objects nearby, many ultrasonic sensors are equipped with LED indicator lights to guide the installation of workers, to ensure the reasonable installation of sensors and reduce the risk of misdetection.

Ultrasonic sensors to be examined non-contact non-destructive detection of objects, it is transparent or non-ferrous objects, metal or non-metallic objects, solid, liquid, powder materials can be examined. Its inspection performance is basically not affected by the use of the environment, including dust environment and rainy days.