In this measurement column, we address frequently asked questions received by our customer support center and provide answers. This time, we will continue to cover topics related to hammering measurements using impulse hammers.
Impulse hammers come with several types of tip attachments. Generally, a hard tip is used for measuring high frequency bands, while a soft tip is used for measuring low frequencies. This time, we will introduce the time-domain waveform and power spectrum when striking with each tip and explain the necessity of using different tips.
Time-domain waveform and power spectrum of impact force from an impulse hammer.
Figures 1 and 2 show the time-domain waveform and power spectrum (example) of the impact force produced by an impulse hammer. The yellow line represents the case using a medium tip (plastic), and the red line represents the case using a super-soft tip (red). These waveforms were measured at a frequency range of 20 kHz with 8192 sample points, and the X-axis is magnified for display.
The maximum impact force is approximately 96 N for the medium tip (plastic) and approximately 29 N for the super soft tip (red), indicating that a greater force can be applied when using a harder tip. From the power spectrum, it can be seen that the frequency components extend up to approximately 1100 Hz for the medium tip (plastic) and up to approximately 290 Hz for the super soft tip (red).
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Figure 1. Time-domain waveform of impact force from an impulse hammer <Yellow: Medium tip (plastic), Red: Super soft tip (red)>
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Figure 2 Power spectrum of impact force from an impulse hammer <Yellow: Medium tip (plastic), Red: Super soft tip (red)>
Time-domain waveform and frequency band of impact force
When measuring frequency response functions and natural frequencies by hammering, select the impulse hammer tip according to the frequency band to be measured. Even with the same tip, the power spectrum of the impact force will change depending on the material of the object, so actually strike the object and check the power spectrum of the impact force, the coherence function, and the frequency response function. If the power spectrum of the impact force does not extend to the frequency band you want to measure, or if the coherence function is low, try changing to a harder tip.
Conversely, if the frequency response extends to unnecessarily high frequencies, the signal-to-noise ratio (S/N) of the measurement band will deteriorate due to the influence of high-frequency components, and it will also become difficult to adjust the trigger detection level for the reasons described below. Therefore, a soft tip is used.
In an FFT analyzer, lowering the frequency range applies a low-pass filter that cuts out frequency components higher than the frequency range contained in the time-domain waveform. Therefore, when measuring a time-domain waveform containing high-frequency components at a lower frequency range, the apparent height of the time-domain waveform decreases.
Figures 3 and 4 show the time-domain waveforms of impact force when using a medium tip (plastic) and a super-soft tip (red), with a low-pass filter applied.
The maximum values of the time-domain waveform of the medium tip (plastic) (Figure 3) were 96.1 N for the original waveform, 95.6 N at 2000 Hz, 64.9 N at 500 Hz, and 29.1 N at 200 Hz. It can be seen that the amplitude decreases when a low-pass filter is applied (high-frequency components are cut). Therefore, the signal-to-noise ratio of the measurement results will be poor when measurements are performed with such impact force. Also, because the amplitude decreases, the trigger detection level needs to be readjusted each time the frequency range is changed.
The maximum values of the time-domain waveform (Figure 4) for the super soft chip (red) were 28.8 N for the original waveform, 28.8 N for 2000 Hz, 29.2 N for 500 Hz, and 26.0 N for 200 Hz. Since the original waveform only has frequency components up to approximately 290 Hz, the amplitude does not change even when a low-pass filter is applied. Therefore, there is no need to readjust the trigger detection level when changing the frequency range.
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Figure 3. Impact force time-domain waveform when using a medium tip (plastic).
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Figure 4. Impact force time-domain waveform when using the super soft tip (red).
summary
In this article, we presented the time-domain waveform and power spectrum of the impact force produced by an impulse hammer when using two different types of tips with varying hardness (medium tip and super soft tip), as well as the time-domain waveform when a low-pass filter is applied to them.
When measuring in the low frequency range, using a rigid tip can result in a poor signal-to-noise ratio (S/N) and make it difficult to adjust the trigger detection level. Therefore, it is important to select a tip that matches the frequency range being measured.
(Excerpt from the email newsletter issued on August 27, 2015)
