tailieunhanh - Harris' Shock and Vibration Handbook Part 7

Tham khảo tài liệu 'harris' shock and vibration handbook part 7', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | CHAPTER FOURTEEN C FIGURE Use of a fixed low-pass filter to prevent aliasing when tracking with an FFT analyzer employing zoom to analyze in a lower-frequency band. For illustration purposes the sampling frequency at maximum shaft speed has been made four times greater than that appropriate to the analog LP filter. The shaft speed range could be made proportionally greater by increasing this factor. A Situation at maximum shaft speed. All harmonics of interest must be contained in the display range. B Situation at one-fourth maximum shaft speed. The analog filter characteristics overlap but are well separated from the display range. C Situation at three-sixteenths maximum shaft speed. The aliasing range almost intrudes on the display range. located in line no. 240 the fundamental must be in line no. 8 there must be eight periods of the fundamental component along the data record. Where the data record contains 1024 samples . N 1024 the sampling frequency must then be 128 times the shaft speed thus a frequency multiplier with a multiplication factor of 128 should be used in this specific case. For FFT analyzers with zoom a simpler approach can be used as illustrated in Fig. . An analog low-pass filter is applied to the signal with a cutoff frequency corresponding to the highest required harmonic at maximum shaft speed. However a frequency multiplying factor is chosen so as to make the sampling frequency say 10 or 20 times this cutoff frequency instead of the normal . The spectrum then VIBRATION ANALYZERS AND THEIR USE is obtained by zooming in a range corresponding to the highest required harmonic. As shown in Fig. the shaft speed and thus the sampling frequency can then be varied over a wide range without aliasing components affecting the measurement results. A somewhat similar procedure is used in conjunction with the digital resampling technique mentioned above. By using four times oversampling a maximum speed range of 1