Product
Features
Advantages of
Hydraulic Excitation for
Measuring the Frequency Response of Structures
The FFT analyzer has rapidly gained popularity for making frequency
response measurements on mechanical systems. One reason for this popularity is the ability
of these analyzers to quickly and easily measure frequency response functions using
impulsive excitation generated by an impact hammer. For lightly damped, linear structures,
this low energy excitation technique provides good results, which accurately predict the
dynamic behavior of the structure under higher level inputs.
However, many real-life structures exhibit a considerable degree of non-linearity and/or
high damping. For example, bolted or riveted joints produce frictional damping and
"gap" discontinuances; elastomeric isolation mounts are often highly non-linear.
In such cases, the frequency response function can change dramatically depending on the
amplitude of the input, and so a useful measurement requires higher energy excitation
techniques which approximate the dynamic levels actually observed in the operating
environment.
These techniques involve
the use of an exciter which is able to reproduce a desired force waveform, usually either
random or sinusoidal nature.
A properly designed hydraulic exciter
system provides an excellent general purpose tool for high energy structural excitation.
Some of the key features which may be important for a successful frequency response are:
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High Force Capability Hydraulic
exciters are available with peak force ratings from 1,000 lbs. to 20,000 lbs. or more.
High force is essential for testing smaller structures which are highly damped, and for
testing non-linear structures at a variety of input levels. It allows the use of broadband
random excitation for faster results than would be possible using swept-sine testing with
a lower force exciter. |
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Ease
of Fixturing A hydraulic exciter provides an
extremely compact point source of force which can normally be applied directly to the
structure under test. This eliminates the need for complicated fixturing and the attendant
distortion and resonance problems. |
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High Frequency Response With hydraulic excitation there is no loss of performance at low
frequency, and with a suitable design, excellent response can be achieved up to 1,000 Hz.
This frequency range covers the vast majority of all structural analysis applications. |
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Preload
Capability Many structures require an
accurately controlled static "preload" force in order to take up the slack in
bearings, gears, or joints. Hydraulics provide a high preload capability, which is easily
adjusted for studying the effects of variations in static loading. |
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Independent Static and Dynamic Control This feature allows independent control of separate static and
dynamic feedback variables. For example, in testing compliant or freely-suspended
structures, it is often necessary to control the dynamic input force while maintaining
static displacement control. This helps to keep the exciter from drifting to the end of
its stroke. |
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Automatic Gain Compensation This feature maintains a constant level of force (or some other
dynamic feedback variable) during a sine sweep, in order to automatically compensate for
variation in the dynamic stiffness of the structure being tested. |
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Long
Stroke Hydraulics can provide up to several
inches of stroke for testing vehicle suspensions, exciting total vehicle or similar
applications. |
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Variety of Exciter Models In addition to the full range of linear and torsional exciters,
inertial mass exciters are available in linear and torsional models for applications in
which backup fixturing is difficult, such as exciting buildings, turbine rotors,
automotive drivelines, or shipborne structures. |
Other
Features
| The Xcite Torsional Modal
Excitation System allows the structural dynamist to apply pure moments in structures for
the evaluation of torsional modal participation without the addition of side loads. (A
problem which always accompanies the use of linear exciters and moment arms.) These
torsional systems have been used in the development of diesel engine torsional dampers,
anti-lock braking systems, automotive axles, half axles and turbine rotors. |
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| The Xcite 1100 Series of Modal
Exciters have found applications in component testing in addition to total vehicle or
structure excitation. As shown in the photo, the Xcite 1100 is being used on a bench to
simulate the wheel/ road induced forcing functions into a prototype electric steering
assembly to simulate noise and vibration levels acquired on the test track. By allowing a
bench simulation, the exciter system allows the engineers to evaluate fixes at a fraction
of the time and cost of repetitive test track runs. |
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| The Xcite 1100 Field Test
System provides the mobility required by field test engineers who can efficiently air
freight the hydraulic modal system to remote locations or move it across town to another
manufacturing area. The Xcite 1001P Field Test Hydraulic Power Supply operates on 110/220
VAC single-phase power so the excitation system can be used in any country or on
shipboard. This application shows the modal system being used to evaluate the dynamic
forcing functions of a farm tractor caused by PTO side loads. |
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| The compact size and
versatility of control options when using the Xcite 1200 Modal Excitation System allows
NVH engineers to simulate the operational static torque loading on engine mounts.
Simultaneously, the engineers apply road acquired or synthesized dynamic loading profiles
to evaluate the vibration isolation of various mounting configurations without needing to
move the vehicle to a road simulator. This application shows two Xcite systems holding
opposing engine mounts under tension and compression loading while dynamically driving the
engine mounts with opposing phase signals generated by an LMS system. |
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