Rule-of-thumb modal test
Rule-of-thumb modal test
(OP)
Hi pals, any of you has previous experiences in performing impulse modal testing WITHOUT using an instrumented hammer, interpreting the modes directly from the accelerometers signals? I guess this is a kind of 'extreme sport' in modal testing, but that's it...
Thanks, bye
Spirit
Thanks, bye
Spirit
'Ability is 10% inspiration and 90% perspiration.'





RE: Rule-of-thumb modal test
A second approach to input the force without measuring it is to deflect the structure with a string and then cut the string.
I'm not sure what the question is, can you reword it?
RE: Rule-of-thumb modal test
There are ways of getting better estimates of the modal parameters (although not the modal mass) using "operational modal analysis" but I think I'm right in saying that these only work with statistically stationary signanls, eg. broadband random exitation.
M
RE: Rule-of-thumb modal test
Also people use to do this a lot with acoustics - clapping or bursting a balloon to get the first few room modes.
So, yes you can do it, if you've got the time to waste. The rest of us use a 2 channel analyser.
Cheers
Greg Locock
RE: Rule-of-thumb modal test
a) striking perpendicular to the neautral axis for a particular direction will give the the fundamental tangential and axial modes. Striking the blade at various points along the tip and only measuring response can show the node point when you strike there for the first torsional mode. This ferrets out the second tangential mode which is sometimes fairly close to the first torsional. With a two-channel analyzer and an instrumented hammer, frequency response function amplitudes can be measured without doing a full modal - to save time. I imagine others could do the same and save time for other types of structures. Log decrement values as one respondant suggested are almost always meaningless from light rap tests. We don't even document them when using transfer functions from 2-channel analyzers.
RE: Rule-of-thumb modal test
Most of my work is machinery diagnostics in the field where the budget is extremely limited. The main goal is to identify the source of the problem as quickly as possible and correct it. "Modal" analysis is usually out of the question (often we can't even shut the machine down!). We have to use many different techniques to make a determination of the presence or absence of a resonant condition. Most people involved in the use of true modal analysis would consider what I do to be incredably inaccurate, but it is hard to argue with results that solve problems.
What is your application?
Skip Hartman
http://www.machinerywatch.com
RE: Rule-of-thumb modal test
Cheers
Spirit
'Ability is 10% inspiration and 90% perspiration.'