Table of Contents
Advertisement
10
Modal Exciters Types 4825 & 4826
Pre-load sufficiency should always be verified before acquiring Frequency Response
Functions (FRFs) by use of the Coherence function of the driving point FRF. Signifi-
cant drop-offs will be seen at and above frequencies of the first bending mode of the
tension wire. For most applications, frequencies at which coherence values fall be-
low approximately 0.7 to 0.8 will indicate maximum useful frequency range for that
particular setup.
Note that at structural resonance frequencies, Coherence values will normally al-
ways drop to lower values. This, however, is typically a result of the so-called "force
drop-offs" phenomenon and is hence to be expected regardless of the above discus-
sion.
Push/Pull Stingers
Traditional push/pull stingers are normally connected to Modal Exciters Types 4825
and 4826 using one of the UA 1612 Threaded Adaptors that come as a standard
accessory with the exciter. One end of this adaptor screws into the female M6
threaded end of the exciter rod while the other end accepts a 10–32 UNF threaded
stud.
Highest force measurement accuracy is achieved by ensuring that Modal Exciters
Types 4825 and 4826 are setup to be perfectly co-linear with the measurement axis
of the input transducer. In this way, the interconnecting push/pull stinger will trans-
fer the least possible torque to the input transducer and ensure that exciter and test
object are effectively rotationally decoupled.
In theory, setting up for the longest possible stinger is recommended as this pro-
vides for the lowest bending stiffness of the push/pull stinger. However, in practical
applications, it is often necessary to shorten the length of the push/pull stinger to
avoid bending of the stinger at higher excitation frequencies or during excitation at
high force levels. Likewise, theory calls for use of the thinnest possible stinger. Prac-
tical test applications will, however, decide the actual diameter of the stinger that
can transfer sufficient force in the desired frequency range.
Brüel & Kjær offers two types of push/pull stingers – fixed length types of various
diameters and variable length types also of various diameters. While the latter offer
maximum flexibility and ease of use during setup, the former offer the possibility of
excitation with the highest force levels, and may, even with smaller modal exciters
such as Modal Exciters Types 4825 and 4826, be advantageous during long test ses-
sions at highest force rating. Brüel & Kjær fixed length types of push/pull stingers are
fitted with 10–32 UNF threaded ends, allowing for direct attachement to an input
transducer with a 10–32 UNF threaded hole at the one end, and for direct attach-
ment to the M6 to 10–32 UNF adaptor at the other end. Brüel & Kjær variable length
types of push/pull stingers are fitted with one 10–32 threaded end and one "free"
end that allows for insertion into a collet chuck.
Advertisement
Table of Contents
