As a key part in rotating machinery and equipment, bearings are an important monitoring object in condition monitoring. It is an important part of predictive maintenance to always ensure its stable and reliable operation. At present, the main form of bearing failure is the cracking of the surface material of the raceway or rolling element due to contact fatigue, which forms surface defects. In the early stage of the development of bearing fatigue damage, that is, before the material fatigue peels off to form a pit, the vibration signal of the bearing is very weak, mixed with the vibration signals from other components of mechanical equipment, coupled with noise interference, it is often difficult to clearly identify the fault . The use of acoustic emission technology will make it possible to detect the crack growth caused by fatigue damage, which has great advantages in the early prediction and diagnosis of rolling bearing failures.
Acquiring acoustic emission signals with full waveform
Acoustic emission is an important method for dynamic monitoring of material or structural conditions. Acoustic emission gradually developed from the early parameter acquisition to the current full waveform acquisition. The acoustic emission signal acquisition system consists of an acoustic emission sensor, a preamplifier, and a high-speed data acquisition system. Because the frequency spectrum of the acoustic emission signal is relatively wide, from thousands of Hz to several megahertz, in addition to data collection, filtering, parameter feature extraction and other processing are also required, so there are higher requirements for data collection equipment.
At present, the special acoustic emission instrument with full waveform acquisition function is expensive, which is not conducive to the work in the initial stage of acoustic emission research. Using the high-speed and high-precision sampling capability of ADLINK PCI-9846 high-speed digitizer as the hardware foundation, the acoustic emission test system developed by LabVIEW can also complete the functions of acoustic emission signal acquisition, storage, and ringing feature parameter extraction in real time. It provides a practical solution for the acoustic emission research in the laboratory.
The data acquisition system is the core of the acoustic emission testing hardware. Because the frequency of the acoustic emission signal can reach several megahertz, in order to accurately acquire the waveform of the acoustic emission, the sampling rate of the collector is often required to reach 20MS / s or even higher. At the same time, because the acoustic emission signal is weak, the original acoustic emission signal is generally in the range of μV, even after pre-amplification, it is only a few mV, and the amplitude of the signal waveform varies widely, so the sampling accuracy and dynamic range of the acquisition device also have Higher requirements.
Acrylic emission signals are collected using the high-speed digitizer PCI-9846 of ADLINK. The main parameters are compared with the main performance of the currently widely used PCI-2 acquisition card of the American Physical Acoustics Company (PAC). Some of the proprietary functions are lacking, and they can fully meet the requirements of acoustic emission full waveform acquisition.
Acoustic emission signal acquisition
ADLINK provides LabVIEW driver for PCI-9846. After installing the DAQPilot driver, you can find the DAQPilot toolkit in the function library of LabVIEW, which provides control functions for data acquisition. Its functions and calling methods correspond to NI DAQmax and are very convenient to use. For example, the four functions in the following figure: PLT Create Virtual Channel, PLT TIming, PLT Read, and PLT Clear Task constitute a complete continuous data acquisition function. In addition, DAQPilot also provides LabVIEW sample programs for various functions such as analog and digital signal input and output, so that developers can quickly get started.
The signal acquisition module in the acoustic emission test system developed in this paper is based on the acquisition program in Figure 1. After the user sets the input channel, preamp gain, filtering and other parameters, continuous data acquisition is started. After the original signal is restored by magnification, digital filtering is performed to remove noise and unnecessary frequency bands. The filtered continuous signal can be saved in binary or text form. Figure 2 is the acoustic emission signal collected on the bearing in operation, the sampling parameter is 2MS / s, the filter is fourth-order Butterworth low-pass filtering, and the cut-off frequency is 50KHz.
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