Cantilever Beam First Mode of Vibration

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Discussion Overview

The discussion revolves around the identification and differentiation of the first mode of vibration in a cantilever beam, particularly in the context of a COMSOL model and visual observations from videos. Participants explore how to distinguish between normal vibrations and specific vibrational modes.

Discussion Character

  • Exploratory
  • Technical explanation
  • Conceptual clarification

Main Points Raised

  • One participant describes their experience developing a COMSOL model to calculate the first six modes of vibration of a cantilever beam.
  • Another participant questions how to identify the first mode of vibration, noting that normal vibrations and the first mode appear similar.
  • A clarification is provided regarding "normal vibrating beam," defined as a beam vibrating at a frequency other than its mode frequency.
  • A proposed method for identifying normal modes involves varying the excitation frequency and observing increased amplitude at specific frequencies, indicating resonant modes.
  • It is suggested that visual observation of amplitude changes can help determine when the beam is near a normal mode.

Areas of Agreement / Disagreement

Participants express curiosity and seek clarification on the topic, but there is no consensus on the best method for identifying the first mode of vibration versus normal vibrations. Multiple viewpoints and methods are presented without resolution.

Contextual Notes

Participants discuss the concept of damping and its effect on the sharpness of amplitude peaks, but the implications of damping on the identification process remain unresolved.

Oscar6330
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I developed a COMSOL model and calculated the first 6 modes of vibration of cantilever beam. Later out of curiosity, I looked up some videos on youtube to see actual cantilever. I found this video which shows the first mode.



How do you know that its first mode of vibration? More specifically, how do you differentiate between normal vibrating beam and first mode (they look quite the same)?
 
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any help please!
 
Oscar6330 said:
How do you know that its first mode of vibration? More specifically, how do you differentiate between normal vibrating beam and first mode (they look quite the same)?

What do you mean by "normal vibrating beam"?
 
nasu said:
What do you mean by "normal vibrating beam"?


From Normal vibrating beam/ cantilever, I mean a beam which is vibrating at a freq other than its mode freg. For e.g if first mode a beam is at 100 hz and its vibrating at 40Hz.
 
Now I understand. One way to go is the following:
If you vibrate the cantilever and change the excitation frequency continuously, you can tell the normal modes by an increased amplitude. For low damping this may be very sharp.
If you measure amplitude versus frequency, you will see some peaks corresponding to the normal modes (or resonant frequencies).
For the setup in the clip I guess you can tell visually when it is near the normal mode, from the increased amplitude.
 

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