common method of HIFU for sound field testing (2)

Due to the complexity of the HIFU sound field, there has not been a completely ideal method to fully detect the HIFU piezo crystal sound field. The improved radiation balance system and hydrophone have been successfully applied to the measurement of the HIFU field with a sound intensity of 5 kW / cm 2 and a sound power of 500 W. It is recommended by China GB/T 19890-2005 "acoustic high-intensity focused ultrasonic sound power and sound field characteristics measurement" national standard. The standard has passed the IEC exchange and has been widely circulated abroad. In February 2006, it was quoted by the National Physical Laboratory (NPL) technical report and submitted to IEC/TC87 for forwarding to all countries in the world. This shows that China's HIFU sound field measurement .And the standardization research has reached the forefront of the world. High-intensity focused ultrasound (H IFU) sound field sound pressure is generally more than 20MPa, negative pressure may exceed - 10 MPa, accompanied by strong nonlinear effects, cavitation and sound flow, which greatly affect the accuracy of the measuring device. 

whether it is radiation method, hydrophone measurement or fiber measurement, must consider the bearing device's ability to withstand the HIFU sound field and the uncertainty of measurement. Moreover, the temperature rise caused by the HIFU sound field in a moment will also have a great influence on the sensitivity of the measuring device such as the sensor, causing measurement deviation, and it is likely to generate an interaction of temperature and sound pressure. If fiber optics is used, temperature and sound pressure will cause changes in the shape and refractive index of the fiber. Therefore, how to effectively separate the interaction between temperature and sound pressure is also a major issue in HIFU sound field detection. In HIFU sound field measurement, cavitation is also one of the factors limiting its measurement accuracy. When the sound pressure exceeds the cavitation threshold, the extremely high pressure, high-speed jet and shock wave generated by the cavitation bubble will be generated on the measuring device. The effect is even damaged. Degassing water alleviates the influence of cavitation on the measuring device to a certain extent, but when the power is increased to a certain extent, a large amount of cavitation bubbles will be formed in the measuring medium (degassed water), which will result in a larger measuring device and the result. Impact. In short, the detection of ultrasonic high focusing piezo is still one of the bottlenecks restricting the development of HIFU technology. The safe and effective measurement depends on the research progress of cavitation and nonlinear propagation, and also depends on optical fiber, sensor technology and its materials.

Piezoelectric ceramic sensor for pickup

The pickup is an important part of the record player, which in turn is the heart of the pickup. The function of this piezoelectric sensor is to convert the recording sound vibration signal into an electrical signal output to achieve the purpose of the record . Since the piezoelectric ceramic sensor has easier manufacturing,it is lower cost, higher sensitivity and higher sensitivity than other types of piezoelectric sensors. In the playback circuit, the advantages of preamplification are not required, so in recent years, piezoelectric ceramic sensors have been used in the pickups  First, the structure and working principle of the pickup. With the development of audio equipment, the pickup has been developed from the past mono pickup to the two-channel (stereo) pickup stage. The two-channel pickup is composed of a casing, a stylus, a piezoelectric sensor, a rubber fixing member, a damper insert and a bracket. When the record player plays the tip of the pickup moves along the record sound groove to generate a synthetic mechanical vibration. The splicing part breaks the vibration into two mutually perpendicular vibrations, and then respectively transmits the ends of the two piezoelectric ceramic sensors, which cause the piezoelectric ceramic sensor to generate bending vibration, which is converted and restored to the left and right channels by the positive piezoelectric effect. Audio signal. Mono pickups and two-channel pickups are similar in construction and operation. The main difference between the two is that the former has a piezoceramic sensor and the latter has two piezoceramic sensors.

Second, the piezoelectric sensor

1 The piezoelectric ceramic sensor for the pickup is made up of two high focus piezoelectric ceramics sheets with opposite polarization directions. This structure is called a double diaphragm type piezoelectric ceramic sensor. When the tip of the pickup moves along the record sound groove, a slight force of 1 to 5 9 is obtained from the sound groove, which causes one piece of the ceramic of the double diaphragm type piezoelectric ceramic sensor to be compressed, and the other piece of the ceramic piece is extended, thereby generating the bending stress produces a corresponding perpendicular to stress electric field between the electrodes of the outer surface of the piezoelectric ceramic sensor. The output voltage of a typical dual diaphragm type piezoelectric ceramic sensor is about 1 SV .In order to obtain high-voltage electrical sensitivity and wide frequency response, the piezoelectric ceramic material should have a large piezoelectric voltage constant of 9 3 , a high transverse electromechanical coupling coefficient R 3 1 , and a large dielectric. Constant expansion, low mechanical Q. The value of the soft piezoelectric ceramic material for the working principle of piezoelectric ceramics, for the sake of simplicity, we first discuss the monolithic tiles. A single piezoelectric ceramic sheet applies a compressive force or a tensile force F between two end faces perpendicular to the surface of the two electrodes. Due to the positive piezoelectric effect, a charge Q proportional to the force F is generated on the electrode, and the relationship is d is the thickness of a single piece. , the absolute absolute dielectric constant of piezoelectric ceramics. In relation to its electrostatic capacitance C, b is the width of a single piece. The voltage V between the electrodes is such that the mechanical impedance of the monolithic piezoelectric ceramic sensor is too large, and the stylus cannot correctly perform the sound groove tracking. . If it is changed into a strip, and the two pieces are bonded into a double diaphragm type sensor, and one end is fixed, the other end is forced to make a bending vibration. The bending vibration of the thin long double diaphragm type piezoelectric ceramic sensor is higher than that of the monolithic piezoelectric ceramic sensor. Similarly, a dual diaphragm type piezoelectric ceramic sensor can also obtain a voltage output proportional to an external force.

3. Piezoelectric ceramics for domestic record players
The sensor made 206 type record player uses the double diaphragm type piezoelectric sensor. The authors used a niobic acid lead ternary piezoelectric ceramic material to make a piezoelectric ceramic sensor that meets the requirements of the pickup. The chemical formula of the ternary piezoelectric ceramics is to improve the properties of the material by replacing a small portion of Pb. The main indications are shown in Table . A double diaphragm type piezoelectric ceramic sensor made of this material.

Fourth, the application of rare earth high temperature thermal power.Since rare earth oxides are resistant to high temperatures and are not easily decomposed, they are stable even at high temperatures, and their specific resistance is small, so that the temperature range is wide. There is no piezoelectric and polarization effects, etc. They also have the advantages of other materials high temperature thermistors: The temperature coefficient of the resistor is large and can directly indicate the temperature. The output signal is strong, and the control circuit is simple without the need for an amplifying circuit. Zero offset compensation and compensation wires are not required for long distance measurement and drawing. Therefore, the rare earth high-temperature thermistor is one of the sensors with large application and wide coverage, and can be applied in the following aspects.

High temperature detection system for various aircraft in aerospace technology

Prevent the pollution of the environment caused by the exhaust of the car, and be used for the detection and extraction of the exhaust gas temperature.