Shaker Setup

[SrinivasAluri]

I am trying to test a beam (132 lbs weight) under free boundary conditions using an electrodynamic shaker (rated 110lbf sine). I connected the shaker, which is attached to two cinder blocks, to the beam using a long nylon stinger and a force transducer. The problem I have is that when I input a 10Hz sine wave from my function generator, the output from the Force Transducer is not clean 10 Hz sine, there is high frequency content in the signal. However, the output of the beam is 10Hz sine wave. I tried two different force transducers and it gives me the same result. What could be the likely problem?

 

[MikeyP]

Have you looked at the voltage going to the shaker amp and the voltage across the shaker? Are these both clean? Is the high frequency polution at mains electricity frequency?

If the voltages are clean, then in my experience it will probably be (in order of likelyhood)

1) Damaged or loose force transducer cable
2) Looseness of the stinger or force transducer mounting
3) Damaged force transducer

Another possibility is that the shaker is misaligned slightly with it's attachment point on the beam. This puts a lateral strain on the stinger and hence the shaker voice coil which can give you some harmonic content in the force signal.

M

--
Dr Michael F Platten

 

[GregLocock]

I'd actually rate #4 the most likely.

A nylon stinger is frankly a bit iffy for that sort of shaker, I'd be looking at a 4mm threaded brass one with cross drilling, or even a steel one. It should be at least 6 inches long.

Make sure that the stinger is /absolutely/ colinear with the shaker, you should be able to screw it out of the force gauge into the shaker (and vice versa) with your fingers only.

One reason the automotive people like to use a spider and an inertial shaker is that this sort of misalignment problem is harder to get. Free free testing with a grounded shaker is slightly tricky - for instance, overnight your suspensions may creep, if they are bungee cords, and that will wreck your shaker.








Cheers

Greg Locock

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

 

[SrinivasAluri]

Thanks Dr.Platten and Greg!

Regarding "Free free testing with a grounded shaker is slightly tricky"

Would it help if I suspended my shaker from a modal excitation stand along with the use of steel stinger?

 

[CESSNA1]

SrinivasAluri: Looking at the situation i notice that the maximum obtainable acceleration on this shaker is:

G = Shaker Force/Total moving weight

Not counting the armature, fixture, Cinder block and other weight, the obtainable acceleration is:

G = 110/132 = 0.83 G

Add the other weight to the 132 # to obtain your actual G capability.

Depending on what accleration you want you may be exceeding the shakers capability. What are your test requirements?

Regards
Dave

 

[SrinivasAluri]

It is just a modal test to acquire frequencies and mode shapes.

 

[GregLocock]

For free free test we usually hung the shaker from bungees as well, off the same frame as the test piece.

You will not need anything like the rated output of your shaker for a modal test, I'd expect to use 10-100N for that size of part.





Cheers

Greg Locock

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

 

[blackeye]

I'd recommend the force transducer being at the structure end of the stinger, not the shaker end. Also, if you glue the force transducer to the structure you'll eliminate any undesirable forces due to tightening up.

Like a previous comment, I'd also suspect a damaged force transducer.