Module 4.2 – Multi-Mode Testing Random on Random

Back to: VibrationVIEW Syllabus

Module 4.2 is a variation of a specification used to test hull/sponson vibration. The test includes a broadband random spectrum that is entered in the first breakpoint table. This spectrum is the source of the base random noise apparent throughout the test.

Then, three sweeping narrow bands of random energy are inserted into the test. The three bands start below the broadband random spectrum, slowly increase in amplitude, and then rise out of the broadband spectrum. In addition to specifying the breakpoint tables, the test also requires that the control system frequency resolution equal 1 Hz. This will be configured during the test setup.

Procedure

Use the Hardware, Inputs, and System Limits configuration from Module 1.1.

1. Create an Advanced Random on Random Test

a. Select New Test > Random on Random.

b. Select Table and create a breakpoint table as displayed below:

Frequency (Hz)	PSD (G2/Hz)
10	0.00054
60	0.00650
100	0.00800
150	0.01100
200	0.00640
275	0.00400
325	0.00600
375	0.00175
425	0.00160
480	0.00250
530	0.00140
575	0.00075
625	0.00053
700	0.00425
750	0.00150
800	0.00200
850	0.00091
900	0.00048
950	0.00035
1000	0.00040

c. Select Schedule and set the Run for time to 10 minutes.

d. Select Parameters and set the following parameters:

• General Parameters:
  • Sample Rate: 32768.0
  • Lines: 13000

The test specification calls for a frequency resolution equal to 1 Hz; with these parameters, the frequency resolution will change to 1 Hz.

e. Select Limits and enter the following limits:

• Spectrum Limits:
  • Plus Abort (+): 15 dB
  • Minus Abort (-): 15 dB
  • Plus Tol (+): 9 dB
  • Minus Tol (-): 9 dB
• RMS Limits
  • Plus Abort (+): 12 dB
  • Minus Abort (-): 12 dB
  • Plus Tol (+): 12 dB
  • Minus Tol (-): 12 dB
• Startup Drive Limits:
  • Max System Gain: 1 V/G
• Running Drive Limits:
  • Max System Gain: 1 V/G
  • Max Output: 3 VRMS

f. Select R-o-R and create Band 1, Band 2, and Band 3 as follows:

• Band 1
  • Sweep up from 30 Hz
  • Bandwidth: 4 Hz
  • Sweep rate: 1 min/sweep linear
• Band 2
  • Sweep up from 60 Hz
  • Bandwidth: 8 Hz
  • Sweep rate: 1 min/sweep linear
• Band 3
  • Sweep up from 150 Hz
  • Bandwidth: 20 Hz
  • Sweep rate: 1 min/sweep linear

g. Select OK and save the test as Name_MultiMode_RoR.

2. Create the Graphs

a. Select New Graph.
b. Graph 1 configuration

• Graph Type: Acceleration vs. Freq.
• Graph Traces:
  • Control Loop Traces: Demand, Control, Tolerance, Abort
  • Input Channels: Ch2, Ch3, Ch4

c. Graph 2 configuration

• Graph Type: Acceleration vs. RMS History
• Graph Traces:
  • Control Loop Traces:
  • Input Channels: Ch1, Ch2, Ch3, Ch4

d. Graph 3 configuration

• Graph Type: RoR Frequency
• Graph Traces:
  • Random Bands: Band 1, Band 2, Band 3

e. Graph 4 configuration

• Graph Type: RoR Amplitude
• Graph Traces:
  • Control Loop Traces: Demand, Control
  • Random Bands: Band 1, Band 2, Band 3

f. Select OK.

3. Run the Test

a. Allow the test to run for at least 10 minutes.
b. Observe the changes in the broadband random spectrum as the individual narrowband spectrums sweep.

4. Evaluate the Test

a. As the sweep passes, note the effects of the narrowband spectrum on the Ch2, Ch3, and Ch4 resonances. As the narrowband spectrum passes, channels 2, 3, and 4 begin to accelerate and spike much higher than the current resonance, causing an increased amount of damage.
b. When the bands continuously change the sweep rate, the amount of distortion increases. If you watch closely, you can see the channels lag because of the distortion added to the test.