Vibrating rod

[sivafriend]

Dear all,

How do you measure the vibration pattern and amplidute of a vibrating rod ( S - shape). It is 1x1 (cm) and 1.5 m long. The vibration is fixed at 35kHz.

thanking you for your time.
mech.design-eng

 

[GregLocock]

I don't think you mean 35 kHz - 35 Hz sounds more likely. What's wrong with accelerometers?

http://www.globalspec.com/FeaturedProducts/Detail?ExhibitID=1554

for example, if you did get the frequency wrong.

Failing that you could use laser holgraphy.





Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[hacksaw]

which modes of vibration are you considering?

 

[vanstoja]

If you can pull this off and adequately describe the results I expect you will gain worldwide notoriety as the first human to measure vibration modes and amplitudes of an S-shaped beam with an L/d (length to depth) ratio as high as 150. It should be worth an all expenses paid trip to the most prestigious vibration conference held anywhere. I never heard or read of such a feat before and wonder how you could propagate a 35KHz vibration forcing function throughout such a relatively lengthy beam. What are beam end fixity conditions (free, fixed, hinged...etc.)? Is the vibration source input somewhere along the length or at one or both of the ends? What is this thing used for?

 

[sivafriend]

Hello its me again,

Thanks for those giving response. The beam is used for vibrating a thin metal surface. The Transducer fixed on one end is giving the vibration to rod and we don't know whether the 35kHz vibration is passed on to the s-shaped beam. both ends are not fixed and it is only supported on the nodes. ( we believe they are nodes!)

At least, we wanted to know the rms value of the resonance.

thanks.

 

[sreid]

You could probably freeze the motion using a LED or LASER diode strobe synched to the 35 kHz source.

 

[sivafriend]

Thanks you Sreid, for your idea and am going to try this.
Mecheng.

 

[vanstoja]

Pollard,H.F., 1977, "Sound Waves in Solids", Pion Ltd., London, pp.172-174 describes an ultrasonic experimental technique that measures resonant frequency amplitudes in a brass rod with length to diameter ratio of 158.83. Steel balls rolling down an inclined tube impacted the end of the bar for excitation. Your ultrasonic vibrator with a stinger rod presumably provides the needed excitation. "The detector consists of a variable capacitor connected into a resonant circuit which forms part of a crystal-controlled oscillator operating off-resonance as a slope detector...After amplification the output of the detector is fed into a wave analyzer whose response is recorded on a logarithmic level recorder. Frequencies of the resonant modes are located by driving the wave analyzer slowly through the appropriate frequency range while periodic impulses are applied to the specimen. For accuracy,individual resonant frequencies may be manually tuned to the peak response. A stable oscillator is switched into the wave analyzer and adjusted to give maximum response at the same setting of the analyzer. Frequency setting of oscillator is read on a frequency counter... Damping associated with each mode is found by measuring the decay rate at resonance." A figure shows 8 harmonics of the extensional resonance frequecy of the straight rod with the odd harmonics showing higher amplitudes than the even harmonics.
For extensional(longitudinal) modes, impact is on the end of the rod. For torsional and flexural modes, projecting lugs attached to the drive end are impacted while the detector electrode is mounted vertically over a similar lug at the same or other end of the specimen. In the frequency amplitude spectra, extensional and torsional harmonic peaks will be evenly spaced while flexural mode harmonics will be unequally spaced.
How do you know where the nodes in the S-shaped beam are located for support locations if you don't know what modes will be excited? For the shape-shaped beam, I would suspect that a complex combination of extensional, torsional and flexural modes may be excited simultaneously.