Types of Acoustic Emission Instruments

1 There are many ways to classify acoustic emission instruments. According to the number of channels for collecting signals, they can be divided into single-channel and multi-channel.
Single-channel acoustic emission instruments have only one channel and are mainly used for detection and monitoring of single points or small areas. Generally, they cannot locate the position of the signal source. Acoustic emission instruments with 2 channels or more are called multi-channel acoustic emission instruments. The former can complete the detection and monitoring of larger areas and locate the acoustic emission signal source in linear areas; in addition to the above functions, 3-channel acoustic emission instruments can also locate the acoustic emission signal source in triangular areas and linear areas; more than 3 can complete the detection and monitoring of signal sources in larger areas and locate the acoustic emission signal source in the above areas using multiple algorithms. Instrument manufacturers are generally equipped with analysis and display software based on the above positioning algorithms. For equipment with specific shapes such as spherical and cylindrical shapes, some manufacturers have developed more intuitive positioning analysis software.
Whether to buy single-channel or multi-channel, or how many channels, needs to be selected according to specific needs. Factors to consider include: the material of the equipment to be tested (affecting frequency, sound velocity and attenuation), the maximum geometric size of the equipment to be tested, the geometric shape, the required positioning type, etc. The detection applications with large geometric size, large material attenuation and special shape require more channels, and vice versa. Generally, in steel, the propagation distance that the acoustic emission signal can be received should be within 5m. If it exceeds this length, the number of channels should be increased. For narrow and long equipment, such as long tube trailers, the positioning type needs to be considered. Generally, the number of channels required can be reduced when line positioning is used.
In addition, when purchasing, you can consider 1 to 2 more channels than the required number of channels as spare channels.

2 According to the technology used in the acoustic emission signal, it can be divided into analog acoustic emission instruments based on analog circuits and digital circuit acoustic emission instruments based on digital circuits.
Analog acoustic emission instruments have a long history of use, relatively mature technology, relatively stable and reliable instruments, simple operating software, and are easy to learn. However, this type of instrument has disadvantages such as large temperature drift, susceptibility to interference, high power consumption, large weight, and large volume. Digital instruments generally use technologies such as CPLD, FPGA, and DSP. Due to the small number of analog circuits, their overall volume is small, weight is light, power consumption is low, and temperature drift is small. However, since many functions are superimposed on the processing software of the digital acoustic emission instrument, it is powerful; but it is relatively difficult to learn, which is a lot of trouble for engineering detection. In addition, this type of equipment has a relatively short history of use, and more attention should be paid to its reliability and stability. Due to the use of many new technologies, it is easy to cause problems if it is not handled properly.
It should be noted that there is no truly fully digital acoustic emission instrument. Because the signals in nature are continuous (in macroscopic terms), even if the most ideal situation is to directly convert the signal from the sensor to analog-to-digital (ADC), analog circuits are still needed (ADC itself should be classified as an analog circuit); and generally, a preamplifier and signal conditioning circuit are added after the sensor, so it is impossible to be a fully digital instrument. In general, a fully digital acoustic emission instrument means that
in ADC and signal conditioning circuit, there are no other analog circuits that affect the parameters and waveforms!

3 According to the signal processing method, acoustic emission instruments can be divided into parametric acoustic emission instruments, direct wave acoustic emission instruments and parameter-wave acoustic emission instruments.

3.1 Parametric acoustic emission instrument is the most widely used and longest-used acoustic emission instrument at present. Its analysis method is also recognized by most people in the industry. The analysis method is relatively mature and simple to operate. Most multi-channel acoustic emission instruments are of this type. Parametric acoustic emission instrument refers to an instrument that directly processes the acoustic emission signal into parameter data of a certain significance with the help of analog or digital circuits in the circuit of the signal processing channel, and then sends it to the computer for display, analysis, processing and storage. This instrument generally needs to set the threshold (threshold), HDT (impact definition time), and some need to set the gain, PDT, HLT, etc. Due to the small amount of parameter data, the real-time response capability of the acoustic emission signal is strong, the data flow is small, and the requirements for the computer's display, analysis, data storage and other capabilities are low. Generally, computers can meet the requirements, regardless of PCI bus, ISA bus, etc. At the same time, the probability of parametric instruments losing signals is relatively low. Since the circuit directly generates parameter data and cannot transmit waveforms, more effective information is lost. However, for general detection engineering, parameter acquisition can meet most applications, while parametric instruments have certain limitations if acoustic emission signal analysis and application research are carried out.

3.2 Direct wave form acoustic emission instrument refers to the instrument that performs A/D conversion on the acoustic emission signal, and then directly transmits this waveform data to the computer, which then performs parameter extraction, display, analysis, data storage and other work. Under the premise of meeting real-time requirements, this is undoubtedly the most ideal instrument, because in this way, a general acquisition card can meet the needs and the price will be very low. However, under the current electronic design technology and computer technology, direct waveform acquisition and analysis of a few channels can meet the real-time requirements; but when the number of channels is large, the conversion frequency of the ADC is high, and the analysis function is complex, real-time performance is difficult to guarantee, and there is bound to be the possibility of losing some signals, so it needs to be taken seriously when choosing. In order to achieve high data transmission speed, fast analysis and calculation time, direct wave form acoustic emission instruments generally require very high computer configuration.

Taking the 32-bit, 33MPS PCI bus supported by most computers as an example, the maximum theoretical data transmission speed is 132MB/S (bytes/second). In fact, it is affected by the design level of the computer system, the non-real-time operating system used by the computer operating system, the design level of the acquisition software driver, and the delay introduced by other system software. Generally, 100MB/S is almost the highest acquisition speed that can be seen. In addition to data acquisition, the computer system also performs operations such as parameter data extraction, data analysis, storage, parameter display, and waveform display. Considering the mutual influence of switching between multiple acquisition cards in the system, the overall speed of the entire system is generally 60MB/S, which is quite good. We assume that the acquisition speed of each signal channel is 5MS/S (also recorded as MSPS--Mega-Samples Per Seconds, millions of samples per second), considering that the number of bits of the ADC is more than 12 bits, the data flow speed of each channel is 10MB/S (assuming no real-time compression processing). Obviously, the entire system can only guarantee real-time acquisition of 6 signal channels at an acquisition speed of 5MS/S. The above estimate assumes that the number of acoustic emission impacts per unit time is small, so that the computer will not take too long to write to the hard disk and affect the acquisition speed. Therefore, for applications with a large number of channels, high real-time requirements, and no need for waveform data, it is not appropriate to choose a direct wave acoustic emission instrument.

3.3 Parameter-wave form acoustic emission instrument combines the characteristics of parameter type and direct wave form acoustic emission instrument. While ensuring the real-time performance of parameter data, it transmits a certain number of waveforms when the system allows. This instrument first directly generates part or all of the parameter data by setting a threshold in the processing circuit of each signal channel. At the same time, it retains part or all of the waveform data, which is generally a certain length of waveform data after passing the threshold. Since it takes into account the real-time requirements of parameter data and the waveform only takes a certain length, this is a good compromise between real-time performance and full waveform. It is generally believed that as long as it is properly designed, this type of instrument can meet most application requirements. Its disadvantage is that due to the consideration of waveform and parameter data, the circuit is slightly more complicated and the cost will increase; due to the consideration of parameter data, when the waveform data has a large number of acoustic emission impacts per unit time, there is still a problem of losing part of the waveform data; due to the preservation of some of the threshold waveforms, some meaningful details may be lost. This type of acoustic emission instrument has flexible requirements for computers. If the requirements for waveform data are not very strict, the configuration requirements for the computer system can be reduced.