A new technology for monitoring tool breakage using acoustic emission sensing-EEWORLD

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New Acoustic Emission Sensor Monitoring Technology for Tool Breakage

　　With the increasing use of FMS and CIMS, mechanical processing is rapidly developing towards automation and unmanned operation. Tool status monitoring during machining has become an important factor restricting the development of cutting processing. The acoustic emission (AE) method is one of the most promising monitoring methods developed in recent years. Since it monitors the high-frequency elastic stress wave signal generated by tool wear and breakage, it avoids the low-frequency area where vibration and audio noise pollution are serious during machining. It has high sensitivity and strong anti-interference ability within the frequency range of interest. Therefore, it is widely used in tool cutting status monitoring〔1〕〔2〕. At present, when using acoustic emission to monitor tool cutting status at home and abroad, most of them use contact-type acoustic emission sensors, which have a simple structure and low cost. The disadvantage is that it is easy to misjudge due to actual environmental noise interference; because the acoustic emission signal has a large loss when propagating at the contact interface, it is generally required to install the sensor on the workbench or tool bar, which brings about the problem of signal transmission and sensor installation difficulties.
　　In response to the above problems, the author proposed a new type of non-contact acoustic emission sensor. This sensor uses fluid (liquid or air) to propagate acoustic emission waves and integrates a wide-band, high-gain preamplifier. It has the advantages of strong anti-interference and driving capabilities, high sensitivity, and easy installation and use. Another feature of the sensor is that the preamplifier uses a logarithmic amplifier, which greatly broadens the dynamic range of the sensor's input signal.
      2 Structure of the new non-contact acoustic emission sensor
　　In response to the shortcomings of traditional acoustic emission methods, based on in-depth research on the mechanism of acoustic emission of friction and wear between metal materials, the propagation and attenuation laws of acoustic emission waves〔3〕, a new type of non-surface contact acoustic emission sensor that uses liquid media to transmit tool cutting status information has been developed. Its structure is shown in Figure 1. When used, it is installed on the nozzle of the original lubrication and cooling pipe of the machine tool. When the tool is damaged, the AE signal is transmitted to the sensor through the lubrication coolant in the opposite direction of the flow of the lubrication coolant, and then enters the monitor for analysis and processing. For AE signals, these liquids are good media. The propagation frequency range is large, from a few hertz to tens of megahertz, and because only longitudinal waves can be propagated in liquids, the interference loss and signal distortion during signal propagation are small, and under certain boundary conditions, the signal can be guaranteed to have a small attenuation as the propagation distance changes. This provides a guarantee for us to use fluid as the propagation of acoustic emission signals. Figure 2 shows a schematic diagram of the installation and use of the sensor on a machining center machine tool. This sensor has high sensitivity and strong anti-interference ability because the signal transmission path is short and there are no other contact surfaces in the transmission path.
      Considering that no cooling lubricant is used when machining cast iron materials or high-speed cutting, the author has also developed an air-conducted acoustic emission sensor, the structure of which is shown in Figure 3. Its usage scheme is exactly the same as that of the liquid acoustic emission sensor, and it is also installed on the nozzle of the lubricating cooling pipe. This sensor uses free air as the medium for propagating AE signals. Due to the diffusion of the acoustic wave front and the low air density, the attenuation during AE propagation is large. Therefore, the sensor is required to have high sensitivity and anti-interference ability. When designing the sensor, the above shortcomings are overcome by optimizing the structure and selecting sensitive element materials with high sensitivity and high signal-to-noise ratio. Experimental verification〔4〕 shows that this sensor has achieved good results.
      In order to adapt to the changing working conditions, the author has also developed a liquid-gas combined conduction acoustic emission sensor that integrates the above two sensors into one. On one sensor, liquid and air can be used to conduct acoustic emission signals. At the same time, when there is no cooling lubricant, it is equivalent to an air-conducted acoustic emission sensor; when there is cooling lubricant, two signals can be used to monitor the tool status at the same time to improve the "redundancy" of monitoring information. Its usage method is the same as the above two sensors.
      3 Acoustic emission signal processing method for tool breakage
　　As shown in Figure 4, the acoustic emission signal of tool breakage generally has a frequency greater than 100kHz and a duration of about 200 microseconds to 1 millisecond, which is 4 to 5 times larger than the normal cutting signal. Therefore, a high-speed signal collector is required to extract the original AE signal. If the envelope signal is collected, the sampling frequency can be reduced, and a low-cost acquisition board can be used. Figure 5 shows the block diagram of AE signal processing for tool breakage. The
　　AE signal from the sensor is amplified, high-pass filtered, and the envelope is taken for threshold comparison. The dual threshold discrimination method is used here, that is, a discrimination method that compares both amplitude and pulse width. First, the envelope of the acoustic emission signal is compared in amplitude. When the envelope signal exceeds a certain set threshold, the pulse width comparison is started. When the pulse width exceeds another set threshold, it is considered that the tool is broken and the machine is stopped and an alarm is sounded. When the envelope amplitude signal of the acoustic emission is lower than the set threshold, the pulse width comparator does not work. When the envelope amplitude signal of the acoustic emission exceeds the set threshold, but the pulse width does not exceed the set threshold, there is still no abnormal state alarm, and it is considered to be a noise interference signal. Practical application proves that this method is very suitable for monitoring the acoustic emission pulse signal when the tool is broken. The amplitude of the AE signal changes with the change of processing conditions during the cutting process, and the use of a fixed threshold is prone to misjudgment. For this reason, the author uses a floating threshold for AE amplitude judgment, that is,
　　L(n)=λ.max｜xi(n)｜ (1)
      where L(n) is the acoustic emission amplitude threshold;
　　n is the number of floating times; λ is the coefficient;
　　xi is the sampling sequence value of the acoustic emission envelope signal (i is the sampling sequence number).
　　After entering the monitoring state, sampling is first performed to obtain the working environment noise signal data xi(0). According to formula (1), the maximum value of the sampling sequence is multiplied by the coefficient λ as the threshold. After that, automatic regular sampling is performed to obtain the working environment noise signal data xi(n), and the new threshold is determined again according to formula (1), so that the threshold changes with the working conditions. The coefficient λ is determined by the working conditions. For example, under light load conditions, λ=1.50; under medium load conditions, λ=1.30; under heavy load conditions, λ=1.10.
　　Due to the complexity of the processing conditions, the dynamic range of the acoustic emission signal is relatively large, generally from the microvolt level to the millivolt level. Therefore, the acoustic emission sensor requires a preamplifier so that the signal can be effectively transmitted to facilitate post-processing. The amplification factor of the traditional acoustic emission sensor is fixed, and generally in order to obtain sufficient signal strength, the amplification factor of the small signal is first satisfied. In this way, a slightly larger signal will lead to saturation. To address this problem, a logarithmic preamplifier acoustic emission sensor was developed. According to the logarithmic amplification function of formula (2), the dynamic range of the acoustic emission input signal can be compressed and the monitoring range of the acoustic emission sensor can be widened.
　　Uout = K1lg (K2.Uin) (2)
      In the formula, Uout is the acoustic emission logarithmic output signal;
　　K1, K2 are constants (generally called logarithmic slope);
　　Uin is the acoustic emission input signal.
      4 Monitoring system
　　The main control module is responsible for the main control task of the monitoring system and manages the display of the keyboard and LCD screen through the I/O template; the acoustic emission conditioning template completes the aforementioned acoustic emission signal monitoring and processing tasks and alarms when an abnormal state occurs, that is, applying for an interrupt to the main control template; the A/D and D/A templates complete the sampling of the acoustic emission envelope signal in the acoustic emission conditioning template and send the acoustic emission threshold from the main control template to the acoustic emission conditioning template for comparison; the parallel communication template completes the communication task between the monitoring system and the machine tool control system, that is, the machining center system transmits various working parameters and machine tool operation status parameters to the monitoring system, and the monitoring system transmits the tool cutting status signal to the machining center system.
　　In addition to manual monitoring, this system also has the function of automatic monitoring by using communication functions and a certain range of automatic floating thresholds. The system has the advantages of modularity, easy maintenance, reliable use, and strong anti-interference ability. The
　　above tool breakage monitoring system has been proven to be stable and reliable after many experiments〔3〕, and the success rate of tool breakage identification has reached 98%. It is currently being used in the Changchun FMS Experimental Center.
      5 Conclusion
　　The new AE sensor developed by the author uses the original lubricating coolant of the machine tool as the medium for transmitting AE signals. It can be installed at the end of the lubricating coolant pipeline. Therefore, it is not only easy to install, but also highly sensitive. It can automatically identify the breakage of drills with a diameter of more than Φ0.5mm, with a success rate of 98%. It is an ideal sensor for real-time monitoring of tool breakage in automated processing such as FMS and CIMS.

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