Force Limited Vibration Testing
The importance of vibration testing in qualifying and accepting products cannot be overstated. Especially in aerospace, it is important to implement techniques to prevent over-testing that might cause damage to expensive structures and the shaker. Over-testing in sine and random tests is mainly caused by the differences between the mechanical impedance of the shaker and the specimen fixture and the usual frequency excitation control using accelerometers. These result in artificially high shaker forces and reaction forces at the resonances of the test item. Thus, a load occurs during testing which does not exist in the real-life environment. These high forces can damage expensive payloads.
Force Limited Vibration Testing Prevents Over-Testing
During real flight, the acceleration at the resonances of the payload is negligible. When testing on the shaker table, however, the reaction forces occurring between the test item and the table are disproportionately higher because the shaker table has a much higher mechanical impedance than the payload.
In terms of preventing a load on the specimen during testing that is much higher than in the real application, Force Limited Vibration Testing (FLVT) is the most effective method. With our m+p VibRunner measurement hardware and m+p VibControl vibration control software, we offer a FLVT solution that is successfully used in laboratories worldwide. The outputs from triax force transducers, installed at the fixing points of the unit under test, are used to limit the vibration excitation forces at resonances of the test item to those predicted for the actual flight. This type of vibration control is often referred to as "notching".
m+p international Solution
The m+p VibRunner measurement hardware has been specifically developed for noise and vibration engineering applications. For the high-channel counts required for Force Limited Vibration Testing, the self-contained, low-profile units are combined and usually mounted in a rack. Data acquisition is precisely synchronized across all units and channels. All communications are via Gigabit Ethernet so that the individual m+p VibRunner units can be located close to the measuring points to minimize transducer cabling. The m+p VibRunner hardware enables both frequency and time domain data to be displayed simultaneously in real time; even while controlling tests with over 500 channels, any of which can control or “notch” to the test spectrum.
The m+p VibControl vibration control software supports all excitation types relevant for vibration testing. Force Limited Vibration Testing is available for sine and random tests. Features such as time domain data recording, support for multiple monitors and remote workstations make work more efficient, particularly when working with many transducers. All input channels can be defined as control, watchdog and/or measurement channels. Test safety has top priority: Alarm and abort limits are defined for each channel; m+p VibControl detects an open channel and aborts the test immediately.
Limiting the Excitation
In Force Limited Vibration Testing, the excitation is limited in a defined frequency range (bandwidth) so that the defined level is not exceeded. As long as a notch channel does not exceed the defined level, m+p VibControl controls the test according to the reference spectrum. The notch channel with the highest level exceedance is used for control only if the test item is overloaded at the defined measuring point.
If you want to know more about Force Limited Vibration Testing and how you can reliably protect your test item and shaker from unnecessary testing, read the 3-page brochure.
Together with the Swiss company Kistler, world market leader in the manufacture of sensors and other products, we recently organized a practical webinar on this exciting topic. You can find valuable information and tips in the recording of the webinar Practical Considerations for Force Limited Vibration Testing.