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Predictive maintenance: starting with data acquisition

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Wu Xing, Liu Xiaoqin, Liu Chang,
Faculty of Mechanical and Electrical Engineering
of the Kunming University of Science and Technology

Bearings are key parts of rotary machines and important monitored objects in machine status monitoring. Acoustic emission signal acquisition systems based on high-speed data acquisition digitizers are significant for predictive maintenance of roller bearings and diagnosis of roller bearing faults.

Excellent manufacturing aims to meet market needs with high-quality, highly cost-effective products. Operating in an environment of just-in-time system-based production, modern manufacturers are undoubtedly under great pressure. The most important key for manufacturers is to always guarantee the best performance for their equipment, as equipment faults will result in numerous types of damage.

As key parts of rotary machines, bearings are important monitored objects in machine status monitoring. The object of their predictive maintenance is to guarantee their contuned stable and reliable operation. At present, the main bearing faults are surface defects caused by fractures of the surface materials of the raceways or rolling bodies of the bearings as a result of contact fatigue. In the early stage of bearing fatigue damage development, or before the formation of pits as a result of material peeling because of fatigue, the vibration signals of bearings are very weak, they get mixed with the vibration signals of other parts of machines, and are subject to noise interference, which makes it difficult to identify faults easily. Fortunately, acoustic emission technology may detect crack expansion caused by fatigue damage, thus showing a great advantage for the early prediction and diagnosis of rolling bearing faults.

Full waveform acquisition of acoustic emission signals

Acoustic emission is an important method in the dynamic monitoring of material or structure statuses. It has developed gradually from early parameter acquisition to the present full waveform acquisition. An acoustic emission signal acquisition system consists of an acoustic emission sensor, a preamplifier and a high-speed data acquisition system. Acoustic emission signals have broad frequency spectrums, from thousands to trillions of Hz, and need to undergo filtering and parameter characteristic extraction in the process of data acquisition, so there are strict requirements for the data acquisition equipment used.

At present, special acoustic emission instruments with full waveform acquisition functionality are expensive, which is disadvantageous for work in the early stage of acoustic emission studies. The acoustic emission test system taking the high-speed and high-accuracy sampling capabilities of the PCI-9846 high-speed digitizers produced by ADLINK Technology Inc. as the hardware foundation and developed based on LabVIEW can realize such functions as acoustic emission signal acquisition and saving and ring-down characteristic parameter extraction, providing a feasible solution for acoustic emission studies done in laboratories.

Data acquisition systems are the core of acoustic emission test hardware. The frequency of an acoustic emission may be up to trillions of Hz, so it is necessary to have a sampling rate of a data acquisition digitizer up to 20MS/s or higher for accurate sampling of the acoustic emission waveforms. Furthermore, acoustic emission signals are very weak. Original acoustic emission signals are usually at the μV level or at a level of several mV even after pre-amplification and have large waveform amplitude ranges, so requirements for the sampling accuracies and dynamic ranges of the sampling equipment are very high.

If PCI-9846 high-speed digitizers produced by ADLINK Technology Inc. are used for sampling acoustic emission signals, the results of the comparison between their main parameters and the main performance of the PCI-2 data acquisition digitizers produced by PAC and are now extensively deployed, are given in Table 1. According to the table, PCI-9846 high-speed digitizers do not have some special functions, but they can fully meet requirements for the full waveform acquisition of acoustic emission signals.

Dynamic range 85 dB 80 dB
Resolution 18 bit 16 bit
Highest sampling rate 40 MHz 40 MHz
Number of channels 2 4
DMA high-speed data transmission Available Available
Onboard memory 512 MB
FPGA high-speed signal processing Available
Interface PCI PCI

Table 1 Comparison of the Sampling Parameters of ADLINK PCI-9846 High-speed Digitizers and PAC PCI-2 Data Acquisition Digitizers

Realization of the acquisition of acoustic emission signals

ADLINK Technology Inc. provides each PCI-9846 high-speed digitizer with a LabVIEW drive. After a DAQPilot driver is installed, a DAQPilot tool kit can be found in the LabVIEW database. The DAQPilot tool kit provides control functions for data acquisition. The control functions and call methods correspond to the DAQmax of NI and can therefore be conveniently used. For example, the four functions in the figure below (including PLT Create Virtual Channel, PLT Timing, PLT Read, and PLT Clear Task) constitute a complete function of continuous data acquisition. Furthermore, the DAQPilot tool kit also provides LabVIEW example programs with such functions as input and output of analog signals and digital signals to enable developers to quickly acquire the necessary skills.

Fig. 1 Continuous data Acquisition Procedures of a PCI-9846 High-speed Digitizer

The signal acquisition module in an acoustic emission test system developed in this paper is based on the sampling procedures in Fig. 1. After setting the input channel, the pre-amplifier gain, the filter and other parameters, the user can start the process of continuous data acquisition. After the original signal is reduced according to the magnification factor and put through digital filtering processing, the noise and unnecessary bands are removed from it. The filtered continuous signal can be saved in binary or text form. Fig. 2 shows the acoustic emission signal acquired from a bearing during operation. The sampling parameter is 2MS/s; the filter is a Butterworth four-order, low-pass filter; the cut-off frequency is 50KHz.

Fig. 2 Continuously-acquired Acoustic Emission Signal

Parameter analysis of an acoustic emission signal

After filtering, the continuously-acquired signal will be sent to an amplitude detector. According to the set amplitude threshold and time length, the waveform data of the paroxysmal acoustic emission signal is intercepted from the continuously-acquired signal (Fig. 3 shows the waveform of the paroxysmal acoustic emission signal generated in the operation of a bearing with a rolling body surface defect). After that, for each waveform datum, characteristic parameters like ring-down count, amplitude, frequency, energy, rise time and duration are calculated and saved into an acoustic emission characteristic parameter table for direct viewing and further system analysis. The acoustic emission waveform data detected may be saved in text form. Functions of waveform data saving and continuous data saving are independent. The two kinds of data may be saved at different times or concurrently.

PCI-9846 high-speed digitizers provide abundant triggering sampling modes, including edge, window, reference and other analog triggering modes, and are suitable for the hardware triggering sampling of acoustic emission waveform data, thus saving resources for the software extraction algorithm and reducing the calculation load. It has been discovered in an experimental setting that acoustic emission signals are easily interfered with by such factors as electromagnetic energy in the environment and need to be filtered before waveform detection. However, PCI-9846 high-speed digitizers do not have programmable filtering functionality. In future system improvements, a filter module may be added to the front end of each PCI-9846 high-speed digitizer so that the hardware triggering capabilities of the PCI-9846 high-speed digitizer can be fully utilized and the necessary system performance can be provided.

Fig. 3 Waveform of a Paroxysmal Acoustic Emission Signal of a Bearing

Effectiveness checking

In order to check the effectiveness of the functionality of the acoustic emission test system, a comparison test was done on a QPZZ-II rotary machine vibration and fault simulation experiment table for the acoustic emission and vibration signals of a bearing with a rolling body fault. The bearing is a model NU205, with a pitch diameter of 39mm, a rolling body diameter of 7.5mm, 12 rolling bodies and a contact angle of 0°. Tiny gaps were made on the surfaces of the rolling bodies by means of linear cutting to simulate the defects caused by peeling of the surface materials as a result of fatigue. The speed of rotation was 570 r/min, with a specific rotational frequency of 9.5 Hz. According to the calculation result, the rolling body passing frequency was 18.3 Hz.

Fig. 4 Comparison of the Acoustic Emission and Vibration Signals of a Defective Bearing

A sensor is installed on the vertical position on the bearing seat corresponding to the axis. The vibration test system consists of an acceleration sensor and a NI USB9234 dynamic signal acquisition device. The vibration data were imported into an MATLAB environment for analysis. While the speed of rotation is unchanged, an acoustic emission sensor was installed on the installation position of the acceleration sensor. The acoustic emission test system developed in this paper was used for data acquisition, and the saved data were imported into an MATLAB environment for analysis. Fig. 4 shows the results of the comparison between the acoustic emission signal and the vibration acceleration signal. According to the figure, the signal-to-noise ratio of the acoustic emission signal generated by the roller surface defect is much higher than that of the vibration acceleration signal. Fig. 5 is an envelope spectrum plot of the two signals after narrow-band filtering. According to the figure, the acoustic emission signal has a simpler envelope spectrum line structure, making it easier to discover the characteristic frequency of the defect.

Fig. 5 Comparison of the Envelope Spectrum Plots of the Acoustic Emission Signal and Vibration Acceleration Signal of a Defective Bearing

The acoustic emission test system integrates the outstanding cost performance of PCI-9846 high-speed digitizers and the advantages of the simplicity and ease-of-use of LabVIEW and has the advantages of low cost, convenient development and great expansion potential. It was used for testing the acoustic emission signal of the operation of a bearing with a rolling body surface defect. The results of the test were compared with the results of a conventional vibration test. According to the comparison result, the acoustic emission signal had a higher signal-to-noise ratio. It is easy to see that the acoustic emission method has great potential for the early prediction and diagnosis of rolling bearing faults.

*LabVIEW™ is a registered trademark of National Instruments.

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