Design of four-channel acoustic emission signal acquisition system based on STCl2C5410AD single chip microcomputer

0 Introduction
As a new type of dynamic monitoring technology, acoustic emission technology occupies an important position in non-destructive testing technology. Non-destructive testing technology is a common and effective method in fault diagnosis. Therefore, acoustic emission technology has broad application prospects in online detection of fault diagnosis. Especially in large pressure vessels that are performing production tasks. Due to the need for long-term continuous work without stopping production, it is easy to cause fatigue damage to pressure vessels, posing a serious threat to safe production. Acoustic emission detection can dynamically monitor large pressure vessels or storage tanks without interrupting production, and can quickly capture defect locations, thereby effectively avoiding major accidents.

1 Characteristics and collection principle of acoustic emission signals
Acoustic emission (AE) technology is a non-destructive testing and diagnostic technology that can be used to evaluate the damage of materials or components. The so-called acoustic emission refers to a phenomenon in which a local source of a material quickly releases energy and generates transient elastic waves under the action of external or internal forces. The elastic waves released by the material reflect some physical properties of the material during the destruction process. The elastic waves generated by the acoustic emission source in the material can be converted into electrical signals by coupling the piezoelectric ceramic probe on the surface of the material. These electrical signals are then amplified and sampled by electronic equipment, and the collected data are then transmitted to the host computer for storage and analysis.
The elastic waves generated by the acoustic emission signals generated inside the material will be affected by the combined effects of reflection, diffraction, and wave conversion during the propagation process, thereby causing the attenuation of the elastic wave energy. In this way, the intensity of the acoustic emission signal will weaken according to the increase in the distance between the position of the sensor and the acoustic emission source. Therefore, when the acoustic emission signal is propagated from the material deformation or crack formation point to the sensor receiving point, the detected acoustic emission signal will become very weak.
The most important thing in acoustic emission detection technology is to locate the acoustic emission source, and the way and method of locating the acoustic emission source is determined by the number of channels of the acoustic emission signal acquisition system. One-dimensional line positioning requires two acoustic emission signal channels, while two-dimensional plane positioning requires at least three acoustic emission signal channels. Therefore, four-channel acoustic emission signal acquisition is the most widely used acoustic emission signal acquisition system, and on this basis, it can be expanded to an acoustic emission signal acquisition system with more channels.

2 Circuit design of acoustic emission signal acquisition system
2.1 Composition of acoustic emission signal acquisition system
The acoustic emission data acquisition system is mainly composed of sensors, preamplifiers, data acquisition, data communication, signal processing and other modules. The multi-channel acoustic emission signal acquisition system usually consists of four independent signal acquisition channels. This paper will discuss the design of four independent signal acquisition systems in detail. Figure 1 shows a block diagram of the composition of a multi-channel acoustic emission signal acquisition system.

2.2 Amplification and filtering circuit of acoustic emission signal
Since the acoustic emission signal is very weak, an amplifier with high input impedance must be selected; and the charge amplifier has high input impedance and good linearity. Moreover, because the acoustic emission probe is a piezoelectric ceramic with capacitive characteristics, the selection of an ordinary amplifier will cause the static charge during operation to flow back into the sensor plate, making the sensor unable to output the detected signal normally. Therefore, in this system, the charge amplifier LF356 is selected as the first stage amplifier circuit. Figure 2 shows the charge amplifier circuit of this system.

In the charge amplifier circuit, the resistance value of the resistor and the capacitance value of the capacitor must meet the signal frequency requirements of the sensor output:

This design adopts two-stage amplification, and the amplification factor A of the first-stage charge amplifier is designed to be 20, that is:

Where: Ct is the equivalent capacitance of the transducer, the capacitance of the feedback capacitor Cf is selected to be 1 nF, and the equivalent capacitance of the transducer is 20 nF. In this way, Cf is selected to be 1 nF, and the resistance value of Rf is selected to be 10 MΩ. Then, the resistance and capacitance are substituted into the relationship to obtain:

The frequency of the transducer output signal is greater than 60 Hz, that is: f>fx, which is greater than the cutoff frequency of the charge amplifier and can meet the design requirements. [page]

2.3 Synchronous sampling circuit
In acoustic emission detection, one of the basic conditions for determining whether an acoustic emission signal is generated is the threshold voltage of the acoustic emission signal. When the signal is higher than the threshold voltage of the acoustic emission signal, an acoustic emission event is considered to have occurred. In this system, a voltage comparator is set. When a strong acoustic emission signal is detected, the reference voltage of the voltage comparator is the threshold voltage in acoustic emission detection. The four independent acoustic emission signals in the system can enter the voltage comparator LM239 after amplification and filtering. LM239 has four independent voltage comparator modules. When the signal enters each voltage comparator module, it can be compared with the threshold voltage. When the signal is higher than the threshold voltage, it is determined that the acoustic emission signal has arrived, and the output of the voltage comparator is high.
Since the propagation speed of sound waves in steel plates is relatively fast (the typical propagation speed is 3000 m/s), if the instruction time of multiple channels controlled by a central processing unit is not fast enough, data acquisition will be asynchronous. This paper adopts the design scheme of "preferential arrival and simultaneous triggering of four-way AD synchronous acquisition".
Among the four sensors, the channel that receives the acoustic emission signal first is also the channel that makes the output of the voltage comparator of the channel high level first, so the high level signal of this channel can be sent to the external interrupt of the four single-chip microcomputers at the same time, and then the on-chip AD of the four single-chip microcomputers is started to collect the acoustic emission signals of the four channels at the same time. In this way, the problems of asynchronous, missed or incomplete collection caused by the delay of instruction execution time and other internal factors in the program are fundamentally avoided. The synchronous sampling circuit is shown in Figure 3.

2.4 Data Communication
The signal acquisition communication part of this system is divided into two parts: the data exchange system composed of multiple SPI microcontrollers in the lower computer and the communication between the master microcontroller and the upper computer.
The STCl2C5410AD microcontroller has its own SPI communication interface and a built-in 8-bit shift register, which can ensure the accuracy of data transmission, simplify the circuit design, and improve the reliability of the design.
The four microcontrollers in the four signal channels set in this design are all slave microcontrollers. They are controlled by the master microcontroller and can select the slave microcontroller that needs to communicate through the slave selection line. The four slave microcontrollers can share the host input/slave output data line MISO, the host output/slave input data line MOSI, and the clock SPICLK bus. At the same time, they can save the collected data in the registers of their respective slave microcontrollers, so that the master control can use the SPI bus to retrieve the data from each slave microcontroller, thus forming a data exchange system by the SPI bus.
Communication with the host computer can be achieved through serial communication, and the computer and the microcontroller are connected through serial communication design, and then the collected data is transmitted to the computer in channel order.
Finally, the host computer sends a command, and the main control microcontroller transmits the collected data to the computer in channel order.

3 Experiment and simulation The acoustic
emission signal can be simulated by the signal of the broken pencil lead with a hardness of HB. The diameter of the pencil is 0.5 mm, the length of its lead is about 2.5 mm, and the angle with the surface of the steel plate is about 30°.
The acoustic emission signal comes from the defect itself. Defects of the same size and nature will have different acoustic emission signals due to different positions and stress states. In the experiment, the pencil lead was broken at the same point many times while the sensor position was fixed. The state of the signal was observed, and finally the situation where the acoustic emission simulation signal appeared more frequently was selected as the state of the acoustic emission simulation signal. This paper uses the STC12C5410AD microcontroller, which is an AD with 8 10 bits and 100 ksps, which fully meets the sampling theorem, and has an SPI bus inside the chip, so it can be used to test the multi-channel acoustic emission signal acquisition system.
The experiment uses piezoelectric ceramics as acoustic emission signal sensors, so it has the characteristics of simple structure, light weight, and high sensitivity. Piezoelectric ceramics are sensitive to external forces and can convert extremely weak mechanical
vibrations into electrical signals.
According to the characteristics of the acoustic emission signal, the substrate material selected in this experiment is brass, the resonant frequency is 20 kHz, and the piezoelectric ceramic piece with a capacitance of 20 nF is used as the transducer of the acoustic emission simulation signal, which can meet the reception requirements of the pencil lead break signal.
This experiment selected two-channel data acquisition, and two sensors were distributed at intervals on a steel plate with a thickness of 10 mm. Then the pencil lead was broken, and channel one and channel two were selected on the host computer respectively, and then the data of the two channels were uploaded to the host computer respectively, and the collected data was expressed as a waveform using MATLAB. In this way, it can be seen that the amplitude of the acoustic emission simulation signal decays with time, which proves the energy attenuation of the elastic wave during the propagation process. At the same time, the interference phenomenon of the elastic wave also appeared at the position of sensor 2, which is caused by the longitudinal wave and transverse wave of the elastic wave reaching transducer 2 successively. Figure 4 shows the waveform simulation diagram of signal acquisition.

4 Conclusion
Based on the characteristics of acoustic emission, this paper proposes a design method for a multi-channel data signal acquisition system using the sound source localization method of acoustic emission. A large number of experiments have proved that this design has the characteristics of fast data transmission and accurate positioning, and can meet the requirements of sound source localization.