Measurement Testing

Mjr · Site Admin Joined: 13 Jul 2005 Posts: 294

Scenario: You are trying to test a beam (140 lbs) under free-boundary conditions using an electrodynamic shaker (rated 110lbf sine). You connect the shaker (attached to two cinder blocks) to the beam using a long nylon stinger and a force transducer. Problem: when the input is a 10Hz sine wave from the function generator, the output from the force transducer is not "clean" 10 Hz sine -- i.e., there is high frequency content in the signal. But the output of the beam is 10Hz sine wave. You have tried two different force transducers but you still get the same result. What's the problem?

Possible solutions:

1: Look at the voltage going to the shaker amp and the voltage across the shaker. Are they both "clean"? Is the high-frequency noise in the main power line (are you using a line conditioner)?

If the voltages are clean, then in the most probable cause of the noise are:

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

Another possibility: 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.

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A nylon stinger may be a cause for concern for that type of shaker. Rather, use 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 co-linear 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 of the reasons the automotive industry engineers use a spider and an inertial shaker is that the practice makes this sort of misalignment problem uncommon. Free testing with a grounded shaker is slightly tricky; e.g., overnight your suspensions may creep, if they are bungee cords, and that will ruin your shaker.

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

Looking at the situation, the maximum obtainable acceleration on this shaker is:

G = Shaker Force/Total moving weight

Not including the armature, fixture, cinder block and other weights, the obtainable acceleration is:

G = 110/140 = 0.786 G

Add the other weight to 140 lb to obtain your actual G capability.

Depending on what acceleration you want you may be exceeding the shakers capability. What

If it's just a modal test to acquire frequencies and mode shapes. Do this:

For free test, hang 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 -- expect to use 10-100N for that size of part.

Some recommend the force transducer be 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.