Low freq shaker motion subtract from unit motion
Low freq shaker motion subtract from unit motion
(OP)
I've been told that at low frequencies, <3ish Hz, that the motion of the whole airbag supported shaker subtracts from the motion of the unit under test, but I'm having a hard time wrapping my head around it.
For example, when we ran a dwell at 2 Hz the whole shaker moved, about 1/2", roughly out of phase of the slip plate. This makes sense because the frequency is close to the natural frequency of the table which is supported by airbags. I've been told the 1/2" of shaker motion should subtract from the motion that the unit sees, but that doesn't make sense to me.
Assuming it's under control, the control accel still sees the required acceleration. This acceleration translates into the required displacement and therefore, it seems to me that the shaker motion does not subtract from the unit displacement.
Am I missing something?
For example, when we ran a dwell at 2 Hz the whole shaker moved, about 1/2", roughly out of phase of the slip plate. This makes sense because the frequency is close to the natural frequency of the table which is supported by airbags. I've been told the 1/2" of shaker motion should subtract from the motion that the unit sees, but that doesn't make sense to me.
Assuming it's under control, the control accel still sees the required acceleration. This acceleration translates into the required displacement and therefore, it seems to me that the shaker motion does not subtract from the unit displacement.
Am I missing something?





RE: Low freq shaker motion subtract from unit motion
To the extent that the shaker base moves, the UUT is not moving, and not being tested.
Similarly, I have been in a commercial test lab where the UUT was pretty much stationary, and all the shaker's activity went into flexing the spindly fixture between the UUT and the shaker. A totally bogus test, producing totally worthless data. The poor schmuck customer didn't have a witness present, so probably thought the UUT design was good enough.
Mike Halloran
Pembroke Pines, FL, USA
RE: Low freq shaker motion subtract from unit motion
Incidentally we often hang small shakers from vehicle bodies via their stingers, using their inertia to react the driver force. Works very well.
Cheers
Greg Locock
New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?
RE: Low freq shaker motion subtract from unit motion
Tobalcane
"If you avoid failure, you also avoid success."
RE: Low freq shaker motion subtract from unit motion
Tobalcane
"If you avoid failure, you also avoid success."
RE: Low freq shaker motion subtract from unit motion
Greg - "But if the accel is on the UUT there is no problem."
I think this is where I'm getting confused. Our control accel is on the unit. It seems to me that the displacement relative to the floor is not important and what is important is what the control accel sees.
Like Twoball's example, the unit should be seeing 2Hz and 5g's.
RE: Low freq shaker motion subtract from unit motion
I'm just not used to having accelerometers.
Mike Halloran
Pembroke Pines, FL, USA
RE: Low freq shaker motion subtract from unit motion
HOWEVER, one should also be aware of any potential influences caused by the table itself. We had a UUT that experienced an unpredicted resonance around 35 Hz on a slip-table shaker. After months of this and that, we determined that the resonance was actually a shaker table mode excited by our UUT. Turns out the slip-table rides on a film of oil, and if the UUT is sufficient top heavy or tall, it can cause the granite slab to rock on the oil film, resulting in a rocking mode resonance. We wound up changing vendors to get a hydrostatic table shaker, which eliminated the problem.
So, if your shaker setup or fixturing is dubious, they may introduce modes that aren't actually coming from the UUT.
TTFN
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RE: Low freq shaker motion subtract from unit motion
Tobalcane
"If you avoid failure, you also avoid success."
RE: Low freq shaker motion subtract from unit motion
Cheers
Greg Locock
New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?
RE: Low freq shaker motion subtract from unit motion
Tobalcane
"If you avoid failure, you also avoid success."
RE: Low freq shaker motion subtract from unit motion
Seems to me, part of the discussion here is an issue of terminology. When some people refer to "what the UUT sees," they're referring to the input stimulus, ala MIL-STD-167, or the like, not the end result (what the UUT does when it "sees" a particular stimulus.
TTFN
FAQ731-376: Eng-Tips.com Forum Policies
RE: Low freq shaker motion subtract from unit motion
We've done it both ways here, with the vast majority having the control accel(s) right next to the unit on the fixture. For the sake of this discussion, I just said "on the unit" for simplicity.