Simcenter Testing Solutions What Modal Impact Hammer Tip Should I Use?

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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.

Article contents:
1. Background
2. Time Versus Frequency Domain
3. Tip Adjustments

4. Conclusion


1. Background

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:

  • excite a broad range of frequencies at high amplitude (eg, above the noise floor of the instrumentation)
  • have amplitude evenly distributed versus frequency
A input force spectrum is shown in Figure 1.

Figure 1: An ideal input force should be uniformly distributed vs. frequency

The general idea is that resonant 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.

2. Time Versus 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 as shown in Figure 2:

sine wave.PNG
Figure 2: Sine wave in time domain (left) has a single value in frequency domain (right).

Short, transient pulses in the time domain, on the other hand, have a wide frequency spectrum as shown in Figure 3:

Figure 3: A short duration impulse in time (left) has a broad frequency response (right).

The short duration modal hammer hit is used to excite a broad frequency range.

3. Tip Adjustments

So a short pulse is desired for a wide excitation frequency range, but how is this achieved in practice?  This can be done by selecting the stiffness and/or mass of the hammer carefully.  Mass and stiffness options are shown in Figure 4.

Figure 4: Modal impact hammers come with a variety of tips (rubbers, plastic, metal) with different stiffness values as well as mass extensions (top of picture).

The input force frequency range can be controlled by changing the hammer tip in two ways:

  • Hammer Mass – Decreasing the mass of the hammer tip causes the hammer to contact the structure for a shorter amount of time. The reduced mass allows the hammer to reverse directions more easily after hitting the structure, reducing the time it is in contact.
  • Hammer Stiffness – Increasing the stiffness of the tip allows shortens the duration that the hammer is in contact with the structure. For example, one could replace a rubber tip with a metal tip.

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 as shown in Figure 5.

Figure 5: Tip "too" soft has noise on measurement at higher frequencies. Green: coherence, Red: FRF, Blue: input force spectrum.

Ideally, the hammer should excite fully the frequency range of interest (Figure 6).

User-added image
Figure 6: Uniform force in frequency range of interest - Green: coherence, Red: FRF, Blue: input force spectrum

If the hammer is "too hard", all the modes of the structure can be excited far beyond the frequency range of interest. This may mean that some of the force is "wasted" by exciting frequencies that are not of interest.  It would be better to apply the force only in the frequency range of interest which can be done by using a softer tip or more massive hammer.

The mass of the hammer is also important for assuring that enough force is being input into the structure to excite it. 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.

4. Conclusion

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 :

  • Causes enough energy to excite the full frequency range of interest, but not significantly beyond
  • Ensures that enough force is being input into the structure to excite the modes of interest

Hope this is helpful for your modal test!

For more information, check out the free Modal Testing Seminar.

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KB Article ID# KB000043129_EN_US



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