AE sensors are the most important instrument about measuring the AE signal. Sensor must be suitable for the purpose should be used. According to the recent development of electronic circuit technology, advanced signal processing is possible. But before it can accurately signal from the AE sensor to detect objects should be. AE measurement field, the evaluation of the materials, health care, structural diagnosis can range from the field. The size of objects in the ceramic specimens from bridges and concrete structures are different kinds. Of course, everything the same as AE sensors is impossible to measure.
AE sensor for each measurement should be selected. In our standard AE sensor is introduced in the catalog. But in fact, designed to suit the individual requirements for specific AE sensors are often fabricated. This article discusses the AE measurements before choosing the most appropriate sensor for your information the principle of priority AE sensor, and to consider the basic structure describes the attributes and, finally, introduce a special example of using AE sensor.

▶ Principle

AE sensor detects the device, except for special cases, PZT has been used.

Other materials include Niobate or Lithium Niobate. However, the sensitivity is quite low compared with PZT due to be used for special purposes, such as high-temperature environments. PZT piezoelectric material, such as the characteristics of force has caused the charge. AE waves propagated through the metal surface passed to the PZT inside AE sensor, Deformation of PZT is converted into electrical signals by.

▶ Structure

AE sensor's basic architecture is shown in Figure 1. AE sensor detection device (PZT) using the high sensitivity of the mechanical resonance is obtained. AE sensors are introduced into the interior through the receiving plate of PZT AE waves are reflected back from the bottom, go to the top again, coming back to the bottom. This is reflected by the PZT to resonate, but the actual use AE sensors attached to the fixed, because if the resonant frequency of PZT is a fixed resonance. The following is the way can save.
    l : length, c : velocity of sound

Resonance model

Therefore, in order to obtain a low resonant frequency to increase the length of the PZT, but too high, the capacitance of the PZT is underpowered due to the capacitance of the cable is limited because of the loss too great. Is generally lower level of 60 kHz. Lower the resonant characteristics of the AE sensor if necessary, the structure of piezoelectric acceleration sensor is used.
AE sensors can be spread directly reach. However, large structures, board structures, pressure vessels, for example, in many cases reaching as surface waves.
Until now, the description of the structure of resonant AE sensor, and wideband AE sensor on the PZT paste buffer in a section of PZT AE waves are not reflected in cushioning material to absorb and suppress the resonant structure is.

▶ Noise suppression

AE is a fine target signal. Therefore, it is important how to reduce noise. Effective to reduce the noise below the sensor is introduced.

① Differential type
From outside noise caused by induction is effective in an environment, Differential type preamplifier is required.

② Insulation type
Especially in the field using a long distance from the instrument sensors, sensors and instruments also can not ignore the potential difference between.
If you use multiple AE sensors Ground loop is formed.
Therefore, AE sensors of insulation type are needed. Because sent to low-impedance it is effective as a noise countermeasure.

③ Internal preamplifier
It is more effective noise countermeasure, due to low impedance signal is amplified without loss of cable outage capacity.

▶ Environmental-resistant Characteristics

① Temperature
Typically, the detection of AE sensor device (PZT) is bonded to the case.
Therefore, the temperature limit is -20 to 80℃ according to the adhesive.
But that does not use glue to design structures at liquid nitrogen temperature (-197℃) could respond to the 200℃

② Waterproof, Oil-resist
In and outdoor plant needs waterproof, and the level of protection is also important.
The oil can be transferred to the environment at the same time the sensor body structure, and cables also; oil-resist materials should be selected.

▶ Frequency Characteristics

Typically, the sensitivity of AE sensors Frequency characteristic is measured in a pure sectarian or surface wave with the size of a half space as attached to the iron block.
However, the actual measurement target is the finite size, reflected AE waves from the sectional complex mixes; the wave is often not equal attached wave frequency data in AE sensor.
If a thin plate, changes resonant state of the sensor internal oscillator and Frequency change. (Resonant frequency increases) And depending on the material of the test subjects were affected.
For example, FRP resin, in such measure AE waves from the target material AE sensor when they enter the difference in acoustic impedance is reflected up the sensitivity is decreased

▶ Attached method

AE sensor attached to the side that is difficult because detect plane is attached plane.
Put glue on the attached plane, sensor fixed as tape or hot melt or adhesive, also using the magnetic holder and adhesive fixed holder.
Attach a difficult field, use made to sensor shape change design; flange type and screw attach type.

▶ Mounting dice AE sensor

Developed AE sensor for Formed Dice type, having a magnetic between 2pieces plate absorbed into magnetism to V shape formed Dice. Due to formed Dice use the cooling water tank, AE sensor is used waterproof-insulation type

▶ Civil type AE sensor

Inside, resonator type with PZT oscillator, resonant frequency is 60 kHz. Built-in preamp of 20~40dB amplification and consist of the pressure-proof type structure considering installed in the ground during long-term.

▶ Subminiature AE sensor

Ø5.5-h10 small size and in case the built-in amplifier head, since the amplified signal output is beneficial in noise. This sensor has been developed for ceramic material test, small type, high-Sensitivity, and low-noise.
This is less constrained because of small. And it is easy to use. R-CAST is used in various fields due to Wide coverage.

Subminiature AE sensor(R-CAST)

▶ Air conditioner rotary compressor inspecting AE sensor

AE sensor and magnetic holder is the integral, AE sensor is bonded to the compressor surface by spring. Detection plane of AE sensor and surface of magnetic holder consist of curve to suitable compressor shape. For less noise, using the insulation type and built-in vibration sensor for detect compressor rotation.

It has been diagnosed of algorithm quality as well as poor-content using the diagnosis. This system operate at really production line, contribute reliability improve of room air conditioner.

rotary compressor inspecting AE sensor

AE can be applied because of wide application range but in special case, it needs detailed optimization, after consultation is better to choose regarding AE sensor selection.