swept sine vibration analysis

[tijutom]

pretty new to vibration analysis...just did my first swept sine test.

am now in a quandary as to which value points to the natural frequency. what i have are 4 peak frequencies with the maximum Gforce, for all 3 axes. now, do i merely take the frequency corresponding to the highest G? (for each axis?)

thanks

 

[MikeyP]

Can you explain what you mean by "3 axes". Do you mean you have data from 3 different response positions plotted against frequency?

M

 

[electricpete]

If it is a complex structure it may have more than one natural frequency.

I believe there can be one for each 'degree of freedom'

 

[zzy]

Even a simple structure like a beam will have multiple modes (natural frequencies). Normal modes are NOT equal to degrees of freedom. Basically, these two quantities are measused in two different domains (or reference space). DOF is in physical domain (i.e., X-Y-Z coordinate) while modes are in what is referred to as modal space in terms of natural frequency, mode shape, and modal damping. In reality, you can only measure a small number of modes in test while your structure has an infinite number of DOFs. Do not come to conclusion that your system have only 3 DOFs because you measured 3 natural frequencies

 

[GregLocock]

tijutom

I strongly suggest that you read B&K's intro to modal testing, URL of which is given in the FAQ.

By axes I think you mean that you have used a triax and are looking at the plots for each direction? As you will have guessed, the three axes are NOT independent - it would not be unusual for a particular frequency to show up as the strongest response on 1,2 or all 3 axes, depending on a whole bunch of things.

Welcome to the world of dynamics. Should keep you in a job for life!



Cheers

Greg Locock

 

[tijutom]

thanks all for the insights..

what i have is a boxed structure consisting of a few sheet-metal parts. i did swept sine vibration for the X,Y,Z axis by placing the accelerometer on the faces in each respective direction.

the problem is that in each axis, the frequencies corresponding to the maximum Gforce encountered are quite different. does this mean that there are 3 different natural frequencies fro the system vis-a-vis the 3 axial directions? or do i just take the frequency corresponding to the largest resonant value, irrespective of direction as the natural frequency of the sytem?

 

[GregLocock]

As the posts above suggest you have an infinite number of natural frequencies to choose from on a real system.

Depending on what you are trying to do with the result it would be common practice to take the highest amplitude or the lowest frequency of the peaks you see, but neither is strictly correct.

Most often when the automotive world refers to 'the' natural frequency we mean the lowest frequency strong mode, but that is laziness and is not correct.

Cheers

Greg Locock