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8
Modal Exciters Types 4825 & 4826
Connecting Structure, Force Transducer and Modal Exciter
Setting up Modal Exciters Types 4825 and 4826 for a traditional frequency response
function (FRF) based modal test, requires, apart from the modal exciter with a
matching power amplifier, an input transducer
normally called a stinger.
Any force transducer/impedance head is, to a certain degree, sensitive to unwanted
environmental effects as well as to bending moments and transverse movements.
The purpose of the stinger, i.e., the component that connects the modal exciter to
the force transducer/structure, is to transmit axial forces, as well as to isolate and
decouple the force transducer/impedance head from side loads and bending mo-
ments. For optimum test results, use a stinger in every setup.
Two types of stinger technology exist – tension (piano) wire-based stingers and
push/pull stingers. Modal Exciters Types 4825 and 4826 accommodates both types of
stingers.
Tension Wire
A thin metal (piano) wire will have zero compression and negligible bending stiff-
ness. Therefore, in a tension wire test setup, there is a constant tension in the wire,
upon which the oscillatory force is superimposed. A piezoelectric-based force trans-
ducer/impedance head is by definition an active transducer that requires a dynamic
force input in order to give an output. Hence, the constant tension provides no
output from the input transducer and only the oscillatory part of the signal will be
measured.
Accomodating a tension wire
by using a "hole-through" design of exciter and trunnion, whereby the modal exciter
can be "slid" along the wire. A chuck assembly that clamps the exciter to the wire at
the desired position replaces the traditional exciter table.
In connection with Lateral Modal Exciter Stands UA 1607 or UA 1608 (optional), the
tension wire concept becomes especially simple and elegant. The tension wire goes
through the exciter, then goes around the pulley and connects to the spring fastened
to the base of the exciter stand. The turnbuckle, positioned between the spring coil
and the base, achieves adjustable pretensioning.
In a practical setup, connect the input transducer to be used in the test at the
chosen input location on the structure. Then connect the tension wire to the input
transducer, and finally route the tension wire to the appropriate point or "ground"
connection.
1 The input transducer is either a force transducer or an impedance head, i.e., a transducer with a force
transducer and an accelerometer built to form a combined transducer. Note that use of Brüel & Kjær's
Impedance Head Type 8001 requires that the structure under test has a dynamic mass of less than
1.5 kg for force measurement errors less than 10%.
2 Brüel & Kjær recommends use of UA 1600 piano wire set complete with a 5 m long tension wire, 10–32
UNF threaded hole to 0.75 mm wire adaptor, fastening screw and collet chuck assembly (chuck nut
with matching collet insert)
1
and an interconnecting component,
2
with Modal Exciters Types 4825 and 4826 is achieved
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