September 14, 2016
When it comes to vibration testing there are two popular types to consider: sine and random. We receive frequent requests for clarification on which test to run specific products on. The answer is, of course, it depends. So here’s a bit of background to help you make a decision.
Sine Vibration Testing
Sine vibration testing applies a single frequency to a test item and selectively excites resonant structures within the device. In a swept sine test a vibration sine tone is ramped up and down through a range of frequencies and for a specified rate and duration.
Sine testing is primarily useful in identifying resonant conditions within a test item. On an electronic module, the circuit boards are often the main resonant structures and typically their fatigue conditions are most important to understand. However, the components themselves mounted on the PCB as well as the electronics housing also have their own unique natural resonant conditions which are usually different than the circuit boards. Using sine vibration to identify resonant conditions on the PCB or on other test item structures is an effective way to understand how mechanical vibration propagates through a device and can help designers stiffen or dampen elements of the product to reduce the probability of fatigue failure.
Swept sine testing is also useful when the product’s application environment includes reciprocating equipment like motors, engines, turbines, or fans. If the application vibration environment has a significant sine characteristic and if the field vibration can be measured then a swept sine test covering the recorded environmental conditions can be run for a specified duration and at an increased amplitude to simulate a lifetime of fatigue stress.
Sine testing is also helpful to simulate worst-case fatigue exposure by programming an electrodynamic vibration input sine tone to line up with the major resonant frequency of the test item. A vibration test that applies a fixed sine dwell at a resonant frequency will usually generate the highest amplitude displacements and accelerations and therefore the most damaging fatigue stress conditions for the product.
Random Vibration Testing
A random vibration test is one comprised of vibration energy at all frequencies over a specified range. The vibration frequency components that make up the input signal for a random test combine in amplitude and phase to create a time waveform that appears on an oscilloscope as random noise. The characteristics of the random waveform are constantly updating and changing within the bounds of the programmed random input.
Random vibration is the most common type of validation and qualification test that is performed for electronic modules. The majority of vibration environments where devices are mounted, especially electronic devices, are characterized as having predominantly random vibration conditions.
The most effective vibration test to run is one based on actual field measurements of the specific environment where a product will be operated. By recording the in-situ vibration environment with an accelerometer or multiple accelerometers a random power spectral density plot can be created and run at the level recorded or increased to accelerate fatigue and shorten test time. On a random vibration power spectral density plot, the acceleration amplitude is scaled on the y-axis in units of G2/Hz and the x-axis units in frequency. If the measured vibration environment has sinusoidal characteristics then the random vibration profile can also be tailored to include sine tones applied over the random vibration background.
For both random and sine vibration tests, durable mounting fixtures are critical to a successful test. Elite’s in-house experts apply their expertise to design and fabricate custom fixtures to accurately simulate installed orientation and transmit interference-free vibratory energy.
If your application and project do not afford the option of measuring field vibration conditions, then the following standards provide recommendations for test conditions and durations. Some of the standards are free for download while others are available for download but for a fee.
STANAG 4370 (AECTP-240, AECTP-400, AECTP-600)
Telecom Equipment Transportation
ETS 300 019-2-0
We’ve been running vibration tests at Elite for a long time so if you need assistance in specifying a vibe test for your product contact me to discuss the particular details of your product and your end-use application. With 8 vibration test systems and large item capability, Elite is among the most capable dynamics testing labs in the world to get your project started quickly.
Watch our tutorial to prepare for your next vibration testing project.