Reference no: EM132543627
48601 Mechanical Vibration and Measurement - University of Technology Sydney
Multi DoF Vibration Analysis
The 8-level tower described in Project 2 is instrumented with two accelerometers on different levels to measure the local horizontal acceleration of the tower. Only vibration in one vertical plane is studied (the plane that has the lowest stiffness), and transverse vibration and torsional vibration are ignored.
Procedure:
1) Measurement of Modal Frequencies
An instrumented hammer is used to input a horizontal force pulse into the tower. The piezoelectric force transducer mounted on the hammer measures the impact force and also triggers the data acquisition software. The accelerometer on level 8 is used to capture the acceleration response of the tower.
The software calculates the FFT of the acceleration response and the FFT of the force input, and then divides the response FFT by the input FFT to calculate the frequency response (FR). A curve fitting function is applied to the peaks of the FR to improve frequency resolution. Identify and record the first 5 modal frequencies. Use an average of 5 impacts to help remove random components in the frequency response plot.
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Mode
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1
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2
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3
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4
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5
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Freq. from FR plot (Hz)
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Freq. from curve fitting (Hz)
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2) Measurement of Mode Shape for 2nd Mode
A shaker is attached to the tower at level . The shaker is driven by a function generator and power amplifier, with the function generator set to the frequency of the 3rd mode. One accelerometer is used as a reference accelerometer to record the acceleration where the shaker is attached. The other accelerometer is a "roaming" accelerometer that is used to measure the local acceleration on the levels listed in the table below. An oscilloscope or computer is used to display the signals coming from the two accelerometers and measure the rms amplitudes.
Note: when re-attaching the roaming accelerometer to the tower, make sure its sensitive axis is horizontal and orientated the same way as the reference accelerometer.
Record the acceleration magnitudes and phase (relative to the reference acceleration) at the levels listed in the table below.
Using your hand, feel the vibration displacement along the length of the tower, noting the nodal lines (zero displacement amplitude) of the mode shape. Note between which masses these nodal lines occur.
PLEASE TAKE CARE NOT TO DROP THE ACCELEROMETERS AS THE SHOCK WILL DESTROY
THEM! Use synthetic beeswax to attach them to the structure and use cable ties to prevent them from falling off. When roaming the second accelerometer, keep your hands close to it.
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Level
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Local Acceleration (mVrms)
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Ref Accel. (mVrms)
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Phase
+1=In
-1=Out
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Local acceleration relative to ref
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8
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7
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6
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5
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4
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3
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2
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1
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0
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Items for consideration:
a) The raw frequency response data (real & imaginary components) obtained in part 1 should be saved to your USB drive. Use these data to calculate the magnitude spectra and plot these against frequency using linear scales for each axis. Then convert magnitude to dB (using 1 for the dB reference) and again plot against frequency, again using linear scales.
b) Use the "quadrature peak picking" technique to plot the mode shapes for each of the 5 natural frequencies.
c) Note how the shaker is supported and connected to the tower in part 2. Why is it done this way?
d) Using the data obtained in part 2 plot the mode shape (taking phase into account) and compare this with that obtained via quadrature peak picking.
e) Sketch the experimental arrangement in the space below.
f) Refer to UTSOnline for further information relating to the completion of Project 2.
Attachment:- Mechanical Vibration and Measurement.rar
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