Measuring fiber optic wire vibration during manufacturing

[VibeGuy]

What would be the best method to measure the vibration of a .007" fibre optic wire? The manufacturing process is likely causing the wire to vibrate and I want to measure the frequency that it is vibrating at, the phyical displacement of the vibration etc. Laser or strobe is about all I can come up with since it must be done without contact to the wire. I'll need to gather as much information on the wire as I can before I can decide how to proceed with determining the source and stopping the vibration. Let me know your ideas!

Thanks!

 

[butelja]

In addition to the laser you already mentioned, would it be possible to sense the vibrations via monitoring of the fiber tension?

 

[VibeGuy]

Hmmm.. measuring the tension may be even harder, how would it (tension) relate to vibration frequency etc.?

 

[butelja]

I was assuming that there was probably some sort of web tension control system in place that may have a tension signal that could be monitored. If not, then adding it would probably be more trouble than a laser doppler vibrometer.

To answer your 2nd question, higher tension will cause higher frequency of vibration. An example of this is how a guitar or piano is tuned. The fundamental frequencies of a vibrating string is given by:

fn = Kn/(2*Pi)*sqrt((T*g)/(w*L^2)), where

fn = Natural frequency (hz)
Kn = Pi, 2*Pi, 3*Pi, etc.
T = String tension (lbf)
g = 386 lbm*in/lbf/s^2
w = Weight of string (lbm/in)
L = Length of strin (in)

This is assuming small displacements and zero bending stiffness of the fiber. For large displacements, the tension will fluctuate with displacement and cause a nonlinear stiffness increase.

 

[butelja]

Another thought. If the tension and theoretical frequency are known, could an acoustic sensor with appropriate signal processing monitor for vibration?

 

[VibeGuy]

Thanks for the information. Our research has shown this is a very new diagnostic field (fiber optic) and we have some been getting some good feedback from people like yourself. I'll post again if and when we figure out what the final results/test equipment will be.

Thanks again!

 

[VibeGuy]

One more question... what does Kn equal (stand for) in this formula?

fn = Kn/(2*Pi)*sqrt((T*g)/(w*L^2)), where

fn = Natural frequency (hz)
Kn = Pi, 2*Pi, 3*Pi, etc.
T = String tension (lbf)
g = 386 lbm*in/lbf/s^2
w = Weight of string (lbm/in)
L = Length of strin (in)

 

[butelja]

Kn is a constant. K1 is Pi, K2 is 2*Pi, K3 is 3*Pi, etc. These correspond to the fundamental frequencies f1, f2, f3, etc. In theory, there are infinitely many modes and frequencies. In practice, only the first few modes are likely to have any significant amplitude.

 

[byron]

Greetings,

Is it totally impossible to send a laser signal down the length of the fiber? Is the free end of the fiber accessible (even there's a mile of fiber between it and the vibration source)? Maybe there is a sharp enough impedance change at the point of extrusion to bounce a laser off of it. I ask because I have a friend at MIT who's doing part of his PhD work on phase smearing due to acoustical waves inside installed fiber optics. He would know.

Barring that, maybe a good question to ask would be: where exactly does the vibration input matter? If the problem is occurring at the point of extrusion and goofing up the cross-sectional integrity, then maybe measuring equipment displacement would be a good proxy for displacement in the fiber.

Also, have you considered modeling any parts of the system? Depending on where the vibration input is occurring, maybe a bit of finite element work on the equipment design would give you some direction on what to look for.

Byron J. Byron Davis
Vibro-Acoustic Consultants
byron@va-consult.com

 

[dgallagher]

I believe Dr. Bob Jones of SKF has done vibration analysis on Fiber optic wire - he may be contacted at Bob.M.Jones@skf.com