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The hole-through design of Modal Exciters Types 4825 and 4826 means that the
desired length of a variable stinger can be easily achieved by pushing the stinger
into the modal exciter, acommodating the precise length between modal exciter and
test object. Once the desired length has been set, the chuck nut is turned clockwise
with a spanner.
Practical Measurement Considerations
Distributing and Minimising Dynamic Force Levels
There are several reasons why exciting the structure under test with the smallest
possible force level, at more than one input location, is desirable. Firstly, excessive
force levels may drive the structure into non-linear behaviour. Secondly, higher force
levels require larger exciters, which, inherently, have more massive armatures lead-
ing to increased force drop-offs at resonance frequencies. Thirdly, large and complex
structures that exhibit local modes generally require force inputs at several distrib-
uted locations in order to excite all modes sufficiently well. Finally, "repeated roots",
i.e., two modes at the same frequency, can occur in certain, mostly bisymmetrical,
structures. The only way to obtain a valid modal model in such a situation is by
employing a modal test setup with multiple inputs, that is, a Multiple Input Single
Output (MISO) or Multiple Input Multiple Output MIMO test strategy
Minimising Exciter/Test Structure Interaction
Effective mechanical de-coupling of the structure under test and the modal excit-
er(s) is required to minimise the mechanical impedance "disturbance" caused by the
exciter(s). At resonance frequencies, where modal parameters are subsequently be-
ing extracted, force drop-offs are inevitable
mode shape displacement determination.
Force drop-offs are caused by the structure becoming highly compliant at resonance.
The exciter may then use all the available energy to accelerate its own mechanical
components, leaving no force with which to drive the structure under test. Very
little force is therefore being put into the structure and the signal level of the force
may drop towards the normal noise level in the instrumentation.
The lighter the mass of the moving element in the exciter, the smaller this problem
will be. Modal Exciters Types 4825 and 4826 have been fitted with low weight, preci-
sion machined magnesium armatures that help to ensure minimial problems with
force drop-offs.
1 Note that any multiple input test strategy will require a dedicated MIMO Frequency Response
Function estimator. The FFT analyzer used as part of the test setup will accordingly need this special
FRF estimator in order to work. By using the optional Modal Test Consultant™ Type 7753,
Brüel & Kjær's PULSE™ Multi-analyzer is capable of providing MIMO Frequency Response Function
estimations.
2 In addition to the weight of the armature, other parameters will influence the degree of force drop-
off experienced. In general, lightly damped structures will be more susceptible to force drop-offs than
heavily damped structures. Also, driving the exciter with a power amplifier, such as matching Power
Amplifier Types 2720 or 2721, in current feedback mode will significantly minimise force drop-offs.
2
, thereby decreasing the accuracy of the
CHAPTER 2
11
Operation
1
.
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