When running a vibration test on a large structure, the engineer must account for the response of all components. Test levels that are suitable for one may excite the resonance of another. In such instances, the engineer may need to control the response in one or several areas to avoid damage. Notching is a technique engineers can use to limit vibration at specific frequencies during testing.
Notching is particularly useful for test items with various bending motions, such as an aircraft with wings, or critical structures that cannot incur damage. It allows engineers to avoid over-testing at major resonant frequencies while achieving the proper test levels.
How a Notch Works
A notch is a minimum spectral value at a natural frequency. Its purpose is to prevent unnecessary excitation of large resonances that may occur during a vibration test. A notch does not remove frequencies from the spectrum but limits the peak vibrations in the notched area to prevent any resonances from exceeding safe limits.
A vibration test runs on a closed-loop system, allowing the controller to notch at specific frequencies. The controller sends a drive signal to the power amplifier driving the shaker’s armature. It then compares the measured response on the shaker table or test item with the desired demand reference and adjusts the drive signal if necessary.
If a response channel includes a notch limit at a specific frequency, the system will identify if measurements exceed that value. If so, the system reduces (or notches) the drive until the response is at the correct level. There is no adjustment to the drive signal at frequencies where the signal does not exceed the limit. Engineers can also set an overall notch limit for a channel regardless of frequency.
NAVMAT test on a lawnmower blade with notching at 65 to 95Hz for Channel 2.
Each measurement channel has individual limits the engineer can adjust. The limits they select may derive from a field recording or previous testing. In the VibrationVIEW software, the limit can be a decibel level relative to the demand profile or a user-defined value.
Notching: Sine & Random Tests
Notching is the deliberate reduction of a portion of a test spectrum. It can be added to a sine sweep or random test profile and limits measurements such as acceleration, velocity, and displacement.
A sine sweep test outputs a single-frequency sine tone at a defined amplitude and duration. Its main purpose is to identify resonance frequencies and then expose the test item to a single resonance until failure. As a sine test runs a fixed-frequency tone, it can excite resonances to high acceleration (G) levels. Notching avoids damage incurred by resonance.
A random test excites all frequencies in a defined spectrum that might occur at any given time. By exciting all resonances, engineers can determine the interaction between them. Unlike a sine test, which is often controlled to a peak G level, a random test is controlled to a PSD level.
As a random test is more realistic than a sine test, it is the more common selection for validating complete structures. In this case, notching avoids damage incurred by coupled behavior.
WATCH: VibrationVIEW Random Notch Quick Tip Video
A force transducer mounted to a shaker system.
An alternate unit, such as force, is also valid if the channel is configured for a force transducer. Force limiting is often implemented when the mechanical impendence of the shaker may result in over-testing.
Notch at the Response or Control?
A test engineer can set limits on a response channel to account for resonance peaks that may appear during testing. Using notching, the engineer can avoid damaging resonances while testing at others.
Some advocate that notching is unnecessary because the drive compensates for resonance. Although the drive signal compensates for product resonances, it does so across the frequency spectrum. Therefore, it is wise to use notching in appropriate locations to avoid testing around unwanted resonances.
However, as far as control channels are concerned, there is no need to use notching because the drive signal accomplishes the same goal as the notch on the control channel.