Say I hit the free end of cantilever beam hinged at other end with a hammer and find the frf of middle point on the beam.
In the second case I put the shaker at the free end of the same hinged cantilever beam and calculate the frf at the middle point on the beam.
So what differences I may find between the two frfs?
I suppose that both will be same only difference I feel is the impluse excitation by hammer will consists of all the frequencies while the shaker test will contain only the frequencies that the shaker is configured for.
Please correct me If I am wrong.
Hi Suman,
Your opinion is right. The hammer impact test will give you a much richer frequency content and can be useful for identifying the higher modes. The shaker test is a useful test when you know something in advance about your structure and you only want to select a certain frequency range of interest. Regards. Luiza
Two more points:
1. Shaker gives better control over frequency, but might cause additional stiffening effect.
2. Impact excitation is very expedient as compared to the shaker test.
Impact test is a type for free response; however, in case of the shaper the forced response is obtained.
In case of Impact test, u ve to check spectrum of hammer. Sometimes hammer gives nonlinear spectrum hence affecting FRF of sensor. in this case sensor FRF is not reliable. Where as shaker input spectrum will be linear hence FRFs are reliable. So shaker test is much reliable. But basic difference depends on your requirements and further processing requirements.
I'd say the shaker allows for more control and speed: more control since you define the exciting force (type, amplitude, phase, freq. range, etc.) and speed in the sense that you usually can perform hundreds of samples in a few seconds (obtaining a stable average). while you are usually limited to a few samples for the hammer. The fact that you can control better the applied force means that you are not so prone to miss some modes due to "holes" in the impact spectrum. Of course that you cannot overcome the hardware limits of the shaker, but that depends on the shaker model you are using.
On the other hand, the hammer advantages relate to the fact that you don't need to attach any mass or stiffness to the structure (although, in some limited cases, that can also be accomplished with a hand shaker); also, the free space for the hammer and the shaker has different geometries.
The setup time is always several order of magnitude longer then the test time, for both excitation methods. Of course the windowing is also different for each method.
What you have done is a good one. Shaker gives continuous force on the structure and the frf can be obtained by the placement of the accelerometer or any other sensor.
But Impact hammer is mainly used for intermittent force supply and it will be helpful for finding many no. of natural frequencies of the structure and modal analysis.
It was already mentioned that with shaker you can apply energy to the structure for a long time so you can get more samples. Sometimes you need big number of samples to estimate reliable modal parameters. Bu there are also cases that the the record time must be long due to the frequency (low frequencies). Application of the shaker could be crucial in such a case, while with impact testing the response time is limited. Moreover, shakers are often used for modal testing of big structures not only because the low frequency. You use multiple synchronized shakers and thanks to that approach you apply the energy to the structure in uniform way. Of course you cover, already mentioned, long time excitation of low frequencies. Having only hammer impact you can have problems to excite whole structure uniformly. You could use multiple hammers but synchronization can be the problem.
Your understanding is approximately correct. This can be explained with the help of fundamental theory of vibration.
a) When a multi-dof system is excited by the constant frequency source, ideally all the points in the system will vibrate with the excitation frequency.
b) Whereas the response of a multi-dof system to the initial excitation (condition) is the combination of natural modes and will consists of all the natural frequencies.
The system will vibrate in a particular mode with a single frequency only when all the points in the system are given the initial displacements which are corresponding to that mode. This a most desirable condition during experimental modal testing, but practically it is impossible for a continuous system or a system with more dof.
c) It seems that the shaker test will give more reliable FRF than impact hammer test because the excitation is well defined and controlled, but depends on the reliability of the shaker. This in agreement with opinions of Ribeiro and Damian.
My understanding is: in the following three cases, you have to use shaker testing rather than hammer testing to get a successful analysis:
1. Heavily damped structures, or heavy nonlinear structures (whether it is varying stiffness or damping caused).
2. where dynamic loading and/or its orientation has special meaning for complex structure, such as robot, or floating-raft structural design. this is also why people likes to use the term of 'dynamic-stiffness' rather than FRF here for the derivations.
3. whereever a constant excitation energy is needed (hammer testing is a transient impulsive) to fully excite the structure for a better parameter estimations. someone will argue the tap-testing is easier and give a similar results. but they don't notice the differences on the details, such as the hidden modes, multi-eigen etc.
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