Notching Demonstrations

Vibration Research designed the following experiments to illustrate the usefulness of notching in random vibration testing. The engineer used a shaker to vibrate a lawnmower blade and placed accelerometers on the shaker head and the end of the blade (Figure 1). They ran four tests on the system.

The engineer ran a modified NAVMAT profile that ramped up from -20dB to full level and varied the control of each test:

• Test 1: Control on the shaker head without notching
• Test 2: Control on the shaker head with notching
• Test 3: Control on the end of the mower blade without notching

Finally, they ran a test on the shaker head alone to view the effect of notching on a control channel.

Figure 1. Lawnmower blade mounted on the shaker.

Test 1

The engineer ran the test with no notching on Channel 2, which they mounted at the end of the lawnmower blade. The control was on the shaker head. Figure 2 displays the NAVMAT test on a lawnmower blade with no notching. Notice the large resonances on the mower blade (Ch2) in the 70 to 80Hz range and the 300 to 400Hz range.

Figure 2. NAVMAT test on a lawnmower blade with no notching.

Test 2

Next, the engineer re-ran the test with a notch at the 65 to 95Hz range for Channel 2. They set the amplitude level at 65Hz to 0.0025g²/Hz and 95Hz to 0.0055g²/Hz. In doing so, the notch limited the large resonances (2.98g²/Hz) in Test 1 (Figure 3).

Figure 3. NAVMAT test on a lawnmower blade with notching at 65 to 95Hz for Channel 2.

Test 3

The engineer ran the test without notching but set the control to Channel 2. The drive compensated to keep the lawnmower blade’s large resonances to a minimum, i.e. within the tolerance and abort lines (Figure 4). To do so, the drive signal dipped at the 65 to 95Hz range, similar to when it was notched (Figure 3). It also dipped in the 300 to 500Hz range.

The lawnmower blade’s large resonances were reduced across the PSD spectrum (Ch2) because of the low-level drive signals at those resonances.

Figure 4. NAVMAT test on a lawnmower blade with control on the end of the lawnmower blade (Ch2) with no notching.

Test 4

In the final test, the engineer ran the NAVMAT profile on the shaker head alone. They determined that the shaker head had a resonance at approximately 1,700Hz (Figure 5). They then added a notch (boost) at the shaker head’s resonance (1,690 to 1,715Hz). See Figures 6 and 7. The drive signal without the boost (Figure 5) produces the same outcome as the boost (Figure 6).

Figure 5. NAVMAT test on the shaker head. There is a resonance at approx. 1,700Hz.

Figure 6. NAVMAT test on the shaker head with a boost (notch) at 1,690 to 1,715Hz.

Figure 7. Zoomed-in view of NAVMAT test on shaker head with a boost (notch) at 1,690 to 1,715 Hz.

Benefits of Notching

When discussing notching, it is appropriate to separate the discussion into two subtopics: the benefits of notching for response channels, and the benefits of notching for control channels.

Response Channels

For a response channel, the test engineer can set boundaries for large resonances that may appear in the test of a specific product. With notching, the test engineer can avoid certain resonances while still testing others. In Figure 3, the low-frequency resonance (70 to 80Hz) was eliminated, but the higher frequency (300 to 400Hz) was tested.

Some advocate that notching is unnecessary because the drive compensates for the resonances in a product. Controlling on the end of the lawnmower blade rather than the shaker head seems to reduce the drive input and control the resonances on the lawnmower blade, but it does so across the entire frequency spectrum (Figure 4). This outcome is not desirable. Reducing the resonances in this manner affects the entire frequency band rather than the narrow frequency band of the one or two resonances of interest.

Control Channels

As displayed in Test 4, there is no need to use notching for control response resonances. The drive compensated for the notch across the 1,700Hz resonance of the shaker head (Figure 7) in the same way when no notch was in place (Figure 6).

Conclusion

Notching is helpful when dealing with unwanted resonances on response channels. Test engineers should use a notch on a response channel to avoid a particular resonance when testing other resonances. Although the drive compensates for resonances in a product, it does so across the frequency spectrum. Therefore, it is wise to use notching in appropriate locations to avoid testing around unwanted resonances and still be able to test others.

However, as far as control channels are concerned, there is no need to use notching because the drive signal accomplishes the same goal as the notch on the control channel.