Vibrationdata
Unit 2a
1. Calculate the RMS value for a constant amplitude signal x(t) = A.
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2. Calculate the standard deviation for the signal in problem 1.
STD DEV^2 = RMS^2 – MEAN^2
The mean and the RMS are both equal to A. Thus the standard deviation is zero
3. A signal consists of two components:
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what is the standard deviation?
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Mean = A by inspection
STD DEV^2 = RMS^2 – MEAN^2
STD DEV^2 = [pic]
STD DEV^2 = [pic]
STD DEV^2 = [pic]
STD DEV = [pic]
STD DEV = [pic]
STD DEV = [pic]
4. Plot the data file sine.txt. This is measured data taken on a floor adjacent to a wafer polishing machine in a semiconductor facility. It is a velocity time history with amplitude in units of (in/sec). It is not a pure sine signal, but measured data never really is. What is the dominant frequency?
48.3 Hz
5. The program maxfind.exe generates descriptive statistics for a time history with two columns: time(sec) and amplitude(units). Copy this file to same directory as the sine.txt file. Then type:
maxfind sine.txt
What is the average, standard deviation, RMS, and kurtosis values of this signal?
Note that the maxfind.exe program assumes amplitude units of G, but this is irrelevant. Substitute in/sec for G in your notes.
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6. Pogo is a type of sinusoidal vibration that occurs in certain rocket vehicles due to combustion instability. Read tutorial pogo.pdf to learn more about this effect. This tutorial is not concerned with descriptive statistics per se, but it gives a real-world example of sinusoidal vibration.
Unit 2b
1. File trailer.txt is measured acceleration data from a semi-trailer tested at an automotive proving ground oval. The data is not purely sinusoidal, but is reasonably so. First, determine the frequency in Hertz. Then use equation (5) to estimate the peak displacement. Remember to convert G to in/sec2, as part of the calculation. You may use a hand calculation or Excel.
Fn = 18.1 Hz
Amplitude = 0.49 G, but peak value is 1.2 G
2. Check your calculation from problem 1 using program sine.exe. Use the pure sine option in this program.
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(again peak accel is 1.2 G).
3. Use sine.exe for this problem. A shaker has a displacement limit of 1 inch peak-to-peak, or 0.5 inch zero-to-peak. What is the maximum velocity at a frequency of 10 Hz?
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