For a correct excitation, the following must be ensured:
- Excite the frequency range of interest with high amplitude (above the background noise of the sensor)
- An amplitude uniformly distributed over the frequency range to be examined
- Enough energy must be generated to excite the entire frequency range of interest, but not significantly beyond. The force must not drop more than 20 dB in the excited frequency range to get a sufficient energy input.
- It must be ensured that enough force is induced into the structure to excite the modes of the structures.
A correct excitation should have the following course over the frequency range to be examined:
2. When do I use which Hammer tip for excitation?
The general idea is that resonant frequencies can be more easily identified by applying the same force level over the entire frequency range.
The width of the input force is controlled by the duration of the shock pulse. However, the shorter the duration of the impulse, the wider the frequency range response.
To controll the input force frequency range, you can change the hammer tip in two ways:
- Hammer mass – Reducing the mass of the hammer tip causes the hammer to touch the structure for a shorter time. Because the reduced mass allows the hammer to reverse direction more easily after hitting the structure, thus reducing the contact time.
- Hammer tip stiffness – Increasing the stiffness of the tip also allows the hammer to shorten the time of contact. For example, a rubber tip could be replaced with a metal tip.
A hard tip has a very short pulse and excites a wide range of frequencies. Whereas a soft tip has a long pulse and excites a narrow frequency range. But the hammer tip itself does not completely determine the excited frequency range. The local flexibility of the structure must also be taken into account.
Generally applies: The lighter the hammer and the stiffer the tip, the higher the excited frequency range.
Choice of tips:
3. How do I recognize a double hit excitation?
A double hit excitation can often be detected in the time domain as well as in the frequency domain.
If a second peak occurs in the time domain, this is an obvious evidence for a double hit excitation.
If this is not the case, it is possible that the second peak is just too small or too close to the first peak. In this case, the double hit excitation can be detected in the frequency domain.
It shows one of the following significant forms:
4. What is the effect of a double hit excitation on the frequency spectrum?
The effect of the double excitation can be calculated from the time interval between the two impacts.
The following chart shows how the inverse of the time interval affects the interference in the frequency band: