Sine Vibration Testing may be considered as the most basic vibration testing method used. This type of vibration testing requires less complex parameters compare to other type of vibration testing. Knowing the nature of sinusoidal signal, makes this type of vibration testing easier to understand and to get familiar with. Though, easier to understand does not make it less important compare to other type of vibration testing.
Sine vibration testing may still be at the top rank when we are thinking of dynamic characteristic measurement. Basically.
Below are basic parameters required for running sine vibration testing:
Vibration Amplitude
Frequency
Duration
Sweep Rate
There are two types of sine vibration test:
1. Sine Dwell Test
When we are running sine dwell test, then we just need combination of vibration amplitude and frequency for each test, plus time duration. The vibration amplitude normally can be defined in form of acceleration, velocity, and/or displacement. Displacement is normally used at low frequency region, while acceleration is used at high frequency.
2. Sine Sweep Test.
During sine sweeping, the vibration controller will show us a sine tracking curve, allowing us to compare the actual vibration amplitude with the target curve. Many of test standards require the actual vibration amplitude to be within +-3db tolerance from the target curve. Most of vibration controller manufacturer also set +-6db as abort limit, which will make the test aborted automatically when the actual vibration amplitude exceed this abort limit.
The tracking curve is calculated after a tracking filter has been applied to the feedback signal. This tracking is similar like an "adaptive bandpass filter", to allow the software to remove the effect of harmonic signal from the signal, in order to obtain more accurate amplitude.
Commonly, sine vibration testing is used for below purpose:
1. Resonance Search
Resonance search is needed when we want to perform the dynamic characteristic measurement of our product or structure. There are other similar terms used for dynamic characteristic measurements. Those include modal testing, FRF testing, transmissibility testing, etc. Resonance search is normally performed in low vibration amplitude level, in order to minimize the fatigue damage caused by this test procedure. In some cases, engineers only perform resonance search since it is required by test standard or test specification. Normally, the test standard only require the engineers to ensure there are no resonance pear within a specific test frequency range, or peaks are allowed as long as the peak amplitude less than a specific value. But some engineers use the result of resonance search for further modal analysis in order to get more information related to dynamic characteristic, such as damping ratio and actual mode shape. By knowing these information, engineer will be able to use it for improving their structural design. They can use it also for validating their Finite Element simulation model.
2. Durability/Endurance Test
Sine vibration testing may not be the most common type of vibration test used for environmental test. Transportation Test Specification will use Random Vibration Testing due to the nature of its random signal. In aerospace industry, many engineers already start to use Sine on Random Vibration Testing to simulate environmental condition due to turbulence and engine vibration. But it does not mean there are no such condition that can be represented by sine vibration testing. Many automotive manufacturers are stil using sine vibration testing to simulate environmetal condition due to engine vibration. Sine Sweep Vibration Test can also be used to check the product or structure vibration response from idle to max engine rpm.
Other than sine sweep, some engineers use Sine Dwell Test to run durability test at constant frequency. Commonly they will run dwell test at one or more product or structure resonance frequencies. The purpose of having this test is to understand the worst possible scenario can happen to the product or structure, which most probably happens due to one or more of its resonance modes. Sine dwell test may show the weakest and most vulnerable area at the product or structure. Some engineers may even combine sine dwell test with high speed camera, in order to see the vibration mode at each resonance frequency, especially at low frequency where the displacement is significant enough to be measured by a high speed camera.
Nowadays, most of vibration controller manufacturers already developed a special test module to combine and perform resonance search and sine dwell in more automated way. This special test module will automatically search and list up resonance peaks for us according to certain resonance criteria (transmissibility value and/or Q factor). Once the resonance peaks are listed up, we can assign the vibration controller to run sine dwell at a selected resonance frequencies with time duration we prefer.
During sine dwell test, due to fatigue load, crack failure may start to initiate. When the crack starts to propagate, the overall stiffnest of product or structure will decrease, causing the resonance frequency to shift into lower frequency. Other than allowing us to combine resonance search with sine dwell test, this special test module may allow us to perform sine dwell test, track, and follow the product or structure resonance frequency at the same time. When the resonance frequency starts to go lower, the vibration controller will adjust the test frequency to follow the actual product or structure resonance.
