Product Testing Services

NOTE:  For an explanation of sine wave vibration and control methods, please refer to the fixed frequency sine vibration portion of this series.

Swept sine is the logical extension of fixed-frequency sinusoidal vibration.  It is commonly used in product testing to expose a product to a full range of vibrational excitation, but only one frequency at a time.  This frequency range is generally selected to cover those frequencies that the product might see in actual use, but the intensity of the vibration may be increased over real world expectations.  This is to help ensure appropriate design margin, as the testing will be very short in duration compared to the projected life of the product.

One very specific use of swept sine is to determine resonant frequencies.  All products have naturally occurring resonant frequencies.  At these frequencies, the applied vibration will be magnified by the natural structure of the device.  To parametrically evaluate resonance, a dimensionless number, transmissibility, which is the ratio of the acceleration experienced by the product to the acceleration input to the system, is used.  Accelerometers mounted to the vibration table (or fixture) and to the product itself are used to record this data.  Any frequency where the transmissibility exceeds a predetermined cutoff (typically 2.00) is considered resonant.  As this may include a large number of frequencies within the range, this may be simplified to include only the local peaks ignoring those frequencies surrounding the peak.  Making this simplification does incur some risk, but this risk is usually outweighed by practical testing requirements (time, money, resources, etc.).

Electrodynamic Vibration

Once resonant frequencies are determined, a common test to examine the worst case performance of the device under test is to dwell at those frequencies for a set period of time.  The term dwell refers to running a fixed frequency sine test at the designated frequency.  The control parameter for this fixed-frequency test must be determined based on the projected usage environment for the device.  The overall test concept is that if the device will experience greater than normal acceleration (e.g. resonance) at these frequencies, then it is prudent to find out if continual vibration at these frequencies will cause a device failure.  This dwell time varies widely but 5 minutes is a common duration. 

There are many test parameters associated with swept sine.  Most basically is that the vibration must be controlled through acceleration, displacement, or velocity just like a fixed-frequency test.  Unlike the fixed-frequency vibration, we have the added option of controlling part of the range with one parameter, while other parts of the range are controlled by completely different parameters with different magnitudes.  As an example, we might run a swept sine from 20 to 200 Hz with 20 to 50 Hz controlled with a 0.060" displacement, 50-150 Hz controlled with 3 g's acceleration, while 150-200 Hz is controlled with 6 g's of acceleration. 

What about the sweep itself?  Although we know the minimum and maximum frequencies, at what frequency do we start?  This value can be any frequency within the range of the test.  Which direction do we go from that starting frequency (increasing or decreasing)?  Do our sweeps only go one direction, or both (increasing, decreasing, or bi-directional)?  How do we change frequencies (logarithmically or linearly) and what is the rate of frequency change?  How long will we run the test, or alternately, how many sweeps (unidirectional) will we need?  The variations are many.  Which is appropriate will depend on the goals and constraints of the testing under consideration.

Test Specification Template (Swept Sine Vibration)