Modal testing: Difference between revisions
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'''Modal testing''' is a form of vibration testing of an object where [[Fundamental frequency|fundamental frequencies]] of the object under test is determined. |
'''Modal testing''' is a form of vibration testing of an object where [[Fundamental frequency|fundamental frequencies]] of the object under test is determined. |
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There are several ways to do modal testing. The most widely used are Impact Hammer modal testing and Shaker |
There are several ways to do modal testing. The most widely used are Impact Hammer modal testing and Shaker modal testing. |
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Revision as of 11:26, 5 March 2008
Modal testing is a form of vibration testing of an object where fundamental frequencies of the object under test is determined.
There are several ways to do modal testing. The most widely used are Impact Hammer modal testing and Shaker modal testing.
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Impact Hammer Modal Testing
When an object is subjected to the strike, it is excited. It will vibrate according to the bandwidth of the impulse. While it vibrates, the object will behave in such a way that some of the frequencies will not be responded at all (attenuated) and some frequencies will be amplified. Some of the frequencies will be amplified in such a way that the only limiting factor is the energy available for the vibration. This is called resonance.
These frequencies where the object resonates are known as the fundamental frequencies of that particular object or the modal frequencies.
Shaker Modal Testing
Another method is by using an instrument called a shaker. A shaker is a device that excites the structure according to its amplified input signal. Several input signals are available for modal testing, but the sine sweep and random frequency vibration profiles are by far the most commonly used signals.
Shaker armature is attached to the body to be tested by way of piano wire (pulling force) or stinger (Pushing force). When the signal is transmitted through the piano wire or the stinger, the object responds the same way as impact testing, by attenuating some and amplifying certain frequencies. These frequencies are measured as modal frequencies.
When the impact hammer or the shaker stinger is fitted with a device called a load cell, one can measure the excitation signal. The response signal is acquired through an accelerometer fitted to the object under test. Comparing these two signals in the frequency domain (By calculating the transfer function) will reveal how the object moves or vibrates at modal frequencies.
If adequate measurement points are available, mode shapes of the object can be evaluated from the transfer functions.