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Volume 18 - Friction, Lubrication, and Wear Technology Part 8

Tham khảo tài liệu 'volume 18 - friction, lubrication, and wear technology part 8', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | Input assumed tay FFT Hanning window function New Input assumed by FFT Fig. 6 Effect of windowing on time-domain signal Fig. 7 Effect of windowing on spectrum Averaging In many practical situations data collection has to be carried out in a noisy environment which means that the signal to be analyzed is contaminated by unwanted signals from other sources. A commonly used method of attenuating unwanted signals is to limit the bandwidth by conventional filtering. Because the noise is usually broadband this cuts off the extra bandwidth beyond the highest frequency of interest which eliminates much of the noise. Of course this approach is of little use if the noise and the signal occupy the same bandwidth. If the unwanted noise signal has a near-zero mean value then the signal-to-noise ratio S N can be improved by averaging several blocks of data that come from the sensor. Usually between 4 and 64 data blocks are taken. Signal averaging is most effective when a deterministic signal is buried in a random signal. It can be applied to the raw time data or to the resultant spectrum of each block. It is a very effective way to clean up smooth plots. The improvement in S N ratio is equal to the square root of the number of blocks of data used. Random signals cannot be analyzed exactly unless the sampling time stretches from minus to plus infinity unlike deterministic signals where the sampling time can be confined to one cycle because of the repeatability of the cycle. Finite sampling times applied to random signalswill introduce errors. If noise is present in the signal then the quality of the result will depend on the averaging time Tav and the ideal filter bandwidth -J. It has been shown that if the noise in the signal has a Gaussian distribution then the measurement uncertainty is Ref 7 - For example if a random vibration is to be analyzed in 20 Hz bands then the averaging time needed if the measurement uncertainty is to be less than 5 of the mean squared signal will be