What modal impact hammer tip should I use?
Getting high quality frequency response function (FRF) measurements is key to identifying the resonant frequencies of a structure. Using the appropriate hammer tip is a big part of getting a quality FRF measurement.
During a modal impact test, a Frequency Response Function (FRF) is calculated to determine the natural frequencies of the structure under test. A FRF is a measure of the systems output in response (usually acceleration, velocity or displacement) to a known input (usually force).
To calculate an experimental FRF function, both the input and output response signals are measured using sensors, like load cells and accelerometers.
For a good measurement the input force must:
The general idea is that resonance frequencies can be easily identified by applying the same force level across the entire frequency range. Frequency peaks in response to the force correspond to resonant frequencies.
Time vs Frequency domain
The width of the input force is controlled by the length of time of the impact pulse. The shorter the impulse duration, the broader the frequency domain response becomes.
In general, there is an inverse relationship between the time and frequency domain of a signal. Signals with short durations is time, have a broad response in frequency and vice versa.
For example, a sine wave, which is continuous in the time domain has a narrow frequency spectrum.
Short, transient pulses in the time domain, on the other hand, have a wide frequency spectrum.
So a short pulse is desired for a wide excitation frequency range, but how is this achieved in practice?
The input force frequency range can be controlled by changing the hammer tip in two ways:
The desired result is a clean FRF over the full frequency range of interest and a relatively even input spectrum throughout that same frequency range.
If the FRF becomes noisy at higher frequencies and the input spectrum drops off significantly, this is an indication that our hammer tip may be too soft.
If all the modes of the structure are excited, far beyond the frequency range of interest, there may be noise on the FRF at lower frequencies which indicates that the hammer tip may be too hard. This would be caused by creating “out-of-band” overloads.
The mass of the hammer is also important for assuring that enough force is being input into the structure to excite it. So a heavier hammer tip may be needed to ensure that the force levels are high. For example, hitting a 50 ton boat with a one pound hammer will not excite the modes of the boat.
Like Goldie Locks looking for a bowl of porridge or a place to sleep, we are looking for hammer tip that is ‘just right’.
The correct tip will cause enough energy to excite the full frequency range of interest, but not significantly beyond.
The correct tip will also ensure that enough force is being input into the structure to excite the modes of the structures.
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