Art Illustration
What is a Vibration Test Fixture? Looking at its appearance, vibration test fixture is needed for mounting our Unit Under Test (UUT) to the Vibration Testing machine. Is that it? Unfortunately, not that so straight forward.
For some engineer, vibration test fixture may be considered just like an adapter. As long as the UUT can be mounted to vibration machine platform, then it is good enough. But for test engineer with experience, designing a vibration test fixture may be quite challenging as well.
The main reason for us to fabricate a vibration test fixture is that we are not able to mount our UUT directly to the vibration table or armature due to the following reasons:
The UUT mounting hole pattern doesn't match the vibration table hole pattern
The UUT size is much bigger than the size of armature
The UUT need to be mounted in certain orientation (example: car audio system, aircraft avionic, Unmanned Aerial Vehicle camera, etc)
Fixture
The main objective of designing vibration test fixture is that the fixture must be able to transmit the vibration load from vibration test machine to the UUT without any distortion.
There are two key important factors need to be considered for designing a good vibration test fixture:
Vibration test fixture must be designed to be as light as possible in order not to significantly reduce the testing capacity of the vibration test machine
Vibration test fixture must be designed to be as stiff as possible to allow the resonance to lie far beyond the upper limit of the test frequency range
Having resonance far beyond upper limit of our test frequency range may allow our test frequency range to lie within relatively flat region of the fixture frequency response curve. This will provide more warranty that our UUT will not be over tested. Some of test standards even also mention some specific requirements related to resonance evaluation of the vibration test fixture.
Resonance
Example: MIL-STD-810G Method 514.6 Clause 4.2.2.4.a. Test Fixture Resonance. Prior to test, a test fixture survey should be conducted to ensure that the structural characteristics of the test fixture do not introduce uncontrollable resonances into the test setup.
From the material perspective, there are 2 types of materials used for vibration test fixture:
Aluminium
Magnesium
Steel is not commonly used for vibration test fixture due to its weight and light damping value. The most common material used is aluminium, since it is much lighter and has better damping characteristic than steel. If we have enough budget, then we can also try to use magnesium.
Is there any tool that can help us to design vibration test fixture? The answer is YES, if you have capability to perform Finite Element Model (FEM) simulation, then this will be very useful for predicting the dynamic characteristic of our vibration test fixture. Of course, the more complex design you have, it will need more modelling skill on your simulation.
Finite Element Model Simulation
