The phenomenon of deformation or fracture caused by the force of the structure, and the release of strain energy in the form of elastic waves is called acoustic emission (AE).
The structural changes occurring inside the material can be inferred by detecting and analyzing the acoustic emission signal. Due to the real-time, dynamic and sensitive characteristics of AE detection, acoustic emission has developed into an important means for non-destructive testing and online safety monitoring of equipment. It has been widely used in many fields, such as online detection of large storage tanks, pressure pipeline leak detection, Valve leak detection, etc. Practice has shown that acoustic emission technology has a good application effect for such stationary objects, but for the condition monitoring of gear devices, on the one hand, the gear operation is often accompanied by strong vibration and noise, and the acoustic emission signal itself It is a weak signal, which is easy to be immersed in strong background noise. On the other hand, the acoustic emission sensor is generally placed on a non-moving part such as a box. The acoustic emission signal generated by the gear passes through the shaft, the bearing and the box. In order to reach the sensor, it will inevitably lead to severe attenuation, which is also an important reason for restricting the application of acoustic emission technology in the field of gear state monitoring. Therefore, it is of great significance to study the detection of weak acoustic emission signals in strong background noise.
In recent years, the use of chaotic properties to detect weak useful signals in noise has become one of the research hotspots of chaotic engineering.
As a typical chaotic system, the application of Duffing oscillator in weak ultrasonic signal detection includes two methods:
(1) The intermittent chaos using the Duffing oscillator is immune to noise to detect the signal, that is, the chaotic weak signal detection method. This method is simple to implement, low in cost, and has application value. (2) The noise is excited by the Duffing oscillator to detect the signal. Since the noise is random and difficult to control, using the noise to excite the Duff2ing oscillator is very difficult in practical operation.
Based on the analysis of chaotic weak signal detection method and the gear crack experimental data, the application of this method in gear crack detection is studied, and the results are compared with the spectrum analysis method. It can be seen by comparison. Its detection performance is greatly improved.
1 Chaotic weak signal detection method Intermittent chaos (also known as burst chaos) is a special dynamic form of nonlinear systems that appear in order and disorder in time and space. In some time and space segments, the movement is very close to the regular periodic motion; and between the regular motion paragraphs, there is a seemingly random jump. In this paper, we mainly study the acoustic signal detection of gear cracks due to the intermittent chaos caused by the small frequency difference of Duffing oscillator.
The Duffing oscillator with externally applied power is shown as the timing diagram of the Duffing oscillator in the above system. It can be seen that the appearance of the period and chaos is clear. As a key point of this method, it is precisely through the alternate appearance of such distinct periods and chaos in the Duffing oscillator timing diagram to determine whether the desired acoustic emission signal is included in the signal to be detected.
2 Experimental analysis The platform used in the experiment is mainly composed of an electric motor, a single-stage gear box, a coupling, and an oil pump, as shown in 1. The prime mover is an electric motor with a rated speed of 2830r/min. The load can be changed by adjusting the outlet pressure of the oil pump. The gear box is driven by a pair of closed spur gears. The number of teeth of the large gear is z1=30, the number of teeth of the small gear is z2=180, the modulus is 1mm, the tooth width of the large gear is b1=20mm, and the tooth width of the small gear is b2=22mm.
Both the large and small gear materials are 45, which are treated by high frequency quenching.
In order to simulate the crack failure, a 2 mm crack was prefabricated by a wire cutting method at the root of the large gear. In order to rapidly expand the gear crack, the tooth was milled in the width direction, leaving only a 6 mm tooth width.
The acoustic emission detection instrument is mainly composed of a sensor, a preamplifier, a data acquisition card and an industrial computer. The sensor uses the SR150 acoustic emission sensor of Beijing Shenghua Xingye Technology Co., Ltd., with a bandwidth of 50-400 kHz; the preamplifier gain is 40 dB, and the bandwidth is The bandwidth of the sensor is the same; the main amplifier is the Shenghua SA4.
The acquisition system is the Topview2000 high-speed data acquisition card produced by Sichuan Tuopu Measurement and Control Technology Co., Ltd.
The sensor is arranged on the gear box case, and the acoustic emission signal is received by the sensor, then passed through the preamplifier, then sent to the main amplifier for further amplification and conditioning, and finally enters the computer acquisition system.
The sampling frequency is 500 kHz during the experiment, the data acquisition time is 1 s, and 2 is the detected acoustic emission signal of the gear crack.
The acoustic emission signal of metal crack propagation has a main frequency range of 100 to 550 kHz, and most of the energy of the AE crack propagation signal is accumulated in this frequency band.
The peak frequencies of different materials are different. According to the actual measurement, the peak frequency of the acoustic emission signal generated by the gear crack in this experiment is around 150 kHz, which approximates a narrowband signal. For the Duffing detection system, the damping coefficient δ=0.4, ω0=150 kHz. At this time, the critical threshold fc=0.672 of the phase transition from the chaotic state to the large-scale periodic state, so fr=0.671. Using the spectrum analysis method to collect the above experimental platform The acoustic emission signal (the gear has been cracked) has been tested and analyzed, and the result is shown in 3. It can be seen from 3 that the spectrum analysis method is used for detection. Because the noise signal is strong, the amplitude of the desired signal in the frequency domain is too small compared with the amplitude of the noise signal, and it is almost impossible to observe, resulting in the desired signal not being correctly Perform test analysis.
The chaotic weak signal detection method is used for detection and analysis. For the convenience of comparison, the acoustic emission signals of the gears with pre-crack and normal without crack are measured. The results are shown in 4 and 5.
4 The acoustic emission signal on the box produces intermittent chaotic state (when there is a crack fault). The chaotic state generated by the acoustic emission signal on the box of Figure 5 (when there is no crack failure) can be seen from 4, the signal is added to the detection system as an external excitation. Because the frequency of the acoustic signal of the gear crack is close to the frequency of the internal driving signal, although the amplitude of the acoustic signal is small and the noise signal is strong, obvious intermittent chaos can be observed from the timing diagram of the system. Since the acoustic signal of the gear crack is not a single-frequency signal, the observed intermittent chaos is not a regular intermittent chaos, but it is sufficient to illustrate the problem. It is the detection result when the gear is free of cracks, and in this paper, the detection result of noise. It can be seen from the figure that although the noise is very strong, since the frequency is different from the frequency of the internal driving signal of the detecting system, it does not cause intermittent chaos.
3 Ends the sensitivity of the chaotic system to small signals and immunity to noise, making it very promising in signal detection. Based on this, the principle of intermittent chaos in Duffing equation is analyzed in detail, and the method of detecting weak ultrasonic signal in strong noise by using the principle of intermittent chaos is studied. Combining the method with the experimental data and comparing with the spectrum analysis method: the chaotic weak signal detection method is simple, easy to implement, and has good application value; compared with the spectrum analysis method, it has strong noise immunity and Anti-interference ability.
Therefore, the chaotic weak signal detection method is an effective detection method for gear crack detection.
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