Module 4.1 – Multi-Mode Testing Sine-on-Random
March 29, 2018
Background
Using VibrationVIEW
Laboratory Exercises
Reference
Back to: VibrationVIEW Syllabus
Module 4.1 follows the MIL-STD-810 specification to simulate helicopter vibration exposure for the U.S. Army Ch-47M Chinook Helicopter. The test settings depend on the type of helicopter and the location of the material being tested.
Essentially, the test generates a broadband random spectrum, and sine tones replicate the rotor blade vibrations and harmonic frequencies. These values vary based on the helicopter model, the rotational speeds of the main and tail rotors, and the number of blades on the main and tail rotors. In this case, the helicopter has two main rotors.
Procedure
For Demo Mode only.
Use the Hardware, Inputs, and System Limits configuration from Module 1.1.
1. Create an Advanced Sine-on-Random Test
a. Select New Test > Sine on Random.
b. Select Table and enter the following profile:
| Frequency (Hz) | Amplitude (G2/Hz) |
| 3 | 0.00001 |
| 10 | 0.001 |
| 100 | 0.01 |
| 300 | 0.01 |
| 500 | 0.001 |
c. Select Schedule and set the Run for time to 10 minutes.
d. Select Parameters and enter the following parameters:
General Parameters
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- Sample Rate: 16384.0
- Lines: 6500
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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
| (+) | (-) | |
| Tol | 9 dB | 9 dB |
| Channel Tol | 3 dB | 3 dB |
| Abort | 15 dB | 15 dB |
Startup Drive Limits
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- Max System Gain: 1 V/G
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Running Drive Limits
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- Max System Gain: 1 V/G
- Max Output: 3 VRMS
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f. Select S-o-R.
g. Using the following information, calculate the four sine tones for the Ch-47D and enter the values as Hold at frequencies.
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- f1 = 1P
- f2 = n * 1P
- f3 = 2 * n * 1P
- f4 = 3 * n * 1P
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| Helicopter | Rotation Speed, 1P (Hz) | Number of Blades, n |
| AH-1 | 5.40 | 2 |
| AH-6J | 7.95 | 5 |
| AH-6M | 7.92 | 6 |
| CH-47D | 3.75 | 3 |
| MH-6H | 7.80 | 5 |
| OH-6A | 8.10 | 4 |
| UH-1 | 5.40 | 2 |
h. Using the following information, calculate the Gpeak for each frequency and enter the values.
| Source Frequency (fn) Range (Hz) | Peak Acceleration at fn (Gpeak) |
| 3 Hz to 10 Hz | 0.70 / (10.70 – fn) |
| > 10 Hz to 25 Hz | 0.10 * fn |
| > 25 Hz to 40 Hz | 2.50 |
| > 40 Hz to 50 Hz | 6.50 – (0.10 * fn) |
| > 50 Hz to 500 Hz | 1.50 |
i. Select OK and save the test as StudentName_MultiMode_SoR.
2. Create the Graphs
Graph 1 Configuration
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- Graph Type: Acceleration vs. Freq.
- Control Loop Traces: Demand, Control, Tolerance, Abort
Graph 2 Configuration
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- Graph Type: Output Drive Waveform
Graph 3 Configuration
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- Graph Type: Sine Tone Frequency
- Sine Tones: Tone 1, Tone 2, Tone 3, Tone 4
Graph 4 Configuration
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- Graph Type: Sine Tone Amplitude
- Control Loop Traces: Demand, Control
- Sine Tones: Tone 1, Tone 2, Tone 3, Tone 4
3. Run the Test
a. Run the test for at least 10 minutes.
b. Listen for the effects of the low-frequency sine tones on the broadband random spectrum.
4. Change the Sine Tones
a. Change the sine tones to reflect the properties of a different helicopter and run the test again.
Questions
- What effect do the low frequency sine tones have on the tone of the broadband random spectrum?
- When listening to a different helicopter configuration what changes were noticed?
- Does increasing the number of blades increase or decrease the Peak acceleration for a specific sine tone?
- What is the G RMS level of the entire test? Note, this is not the overall level of the broadband spectrum, but a combination of it and each sweeping tone.
Answers in syllabus PDF.