Simultaneous Multiple Axis Vibrational Testing

ABSTRACT
The automobile industry uses multiple electrohydraulic (EH, also
known as servohydraulic) shakers to simultaneously shake an
entire vehicle on multiple axes. Similar tests are applied to trucks,
busses and military vehicles. This
method of testing has several
advantages compared with earlier
methods of shaking vehicles in one
axis at a time. For instance, simultaneous
multiaxis testing is more
realistic and saves time compared
to individual single axis testing.
This method of vibrational testing
also is able to identify additional
failure modes.
This article elaborates on these
advantages and on the simultaneous
multiaxis vibration testing of
smaller loads at higher frequencies
using multiple electrodynamic
(ED) shakers. Testing should positively
influence the readiness, performance
and long life of systems,
ranging from submarines and surface
vessels to satellites and aircraft to land vehicles and individual
warfighter armament.
Simultaneous Multiaxis Shaking of Land Vehicles
Figure 1 shows a number of EH shakers simultaneously prepared
to perform a vibration test on a complete vehicle. Similar tests can
be performed on a truck, bus or military land or air vehicle. These
shakers are simulating (often exceeding) future terrain dynamic
inputs to the vehicle as the vehicle traverses and maneuvers upon
that terrain. These inputs cause the vehicle to dynamically
respond in numerous natural modes.
Mechanical Shakers
Approximately five decades ago automotive engineers had found
single-axis-at-a-time vibrational testing was time-consuming,
incapable of revealing certain failure modes and less capable of
evaluating passenger ride-comfort issues.
The earliest shakers were mechanical. Larger mechanical shak-
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Figure 1. Multiple electrohydraulic shakers simultaneously
apply controlled vibratory force in multiple axes to the
wheels of a land vehicle. (Photo provided courtesy of MTS).
ers are still used to provide 5-33 Hertz (Hz) sine sweeps for naval
shipboard hardware. Though inexpensive and long-lived,
mechanical shakers have two major shortcomings. They cannot
simulate the complex multi-frequency vibrations from helicopter
engines and rotors and from reciprocating
engines and drive trains
in trucks, for instance. Nor are
they capable of random all-frequencies-at-the-same-timevibrations,
such as those experienced by
rockets at liftoff, jet aircraft, terrain
inputs to land vehicles and
wave inputs to ships. Random
vibration simultaneously excites all
mechanical resonances, whereas
sine sweeps sequentially excite
those resonances.
Single-Axis Electrodynamic
Shaking
Electrodynamic (ED) shakers
were first developed in the late
1940’s. Similar in principle to
electrodynamic loudspeakers, they
vibrate their test loads in just one axis at a time. ED shakers permitted
single-axis shaking to 500 Hz in the mid-1950’s and to
2,000 Hz in the early 1960’s. The vast majority of ED shakers
have been used for single-axis-at-a-time vibration in sequential
axis testing in order to perform testing on
three axes.
US Military Multiaxis ED Shaking
Three pioneering military test laboratories
use multiple ED shakers simultaneously.
Successes have led to a new Test Method
527 (multiexciter testing, shown in Figure
2) in the “G” revision to the widely used
Military Standard 810.[1]
The US Army Research Laboratory used
Wayne Tustin
Equipment Reliability Institute
Santa Barbara, CA
Figure 2. Multiexciter/
multiaxis – tri-axial
exciter test setup.[1]
multiaxial vibration testing to diagnose a vehicle hardware failure
that was not able to be diagnosed otherwise. Initially, the lab
could not reproduce the failure at any intensity nor any frequen-
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cy of which their shaker was capable. The lab installed two more
ED shakers to make three-axis simultaneous excitation possible,
which helped identify the failure mode.
Aircraft hardware can be tested using an eight-ED shaker, six
degree of freedom (6DoF) system. The east-west (E-W) pair of
shakers can operate together for hardware axis 1 and hardware roll
(axis 6). The north-west (N-W) pair of shakers can operate
together for hardware axis 2 and hardware roll. The four shakers
located underneath can act together for axis 3 (thrust) and in two
pairs for axes 4 (yaw) and 5 (pitch).
Factory-Assembled Multi-Shaker Systems
Testing laboratories may prefer to purchase complete factoryassembled
systems, if available, rather than purchase three individ-
Y
Z
Figure 3. Three ED shakers for simultaneous three-axis
vibration testing of automotive headlamps. (Graphics
courtesy of IMV Corporation)
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ual shakers, then design and fabricate a common foundation.
However, few such systems are built in North America. Within the
US, only two firms build simultaneous multiaxis shaker systems.
Foreign systems are available that offer seismic multiaxis shakers,
specifically long-stroke (about 500 mm) ED shakers, which
are useful to approximately 35 Hz, and shorter stroke shakers
(i.e., 50 mm and useable to 1,000 Hz). A US company imported
a system, shown in Figure 3, which allows automotive headlamps
to be simultaneously vibrated in three axes. A “tent” can be lowered
over the vibrating table for shaking at temperature extremes.
Factors Favoring Simultaneous Testing
The several advantages of simultaneous (compared with sequential)
multiaxis testing are potentially offset by the higher cost of
the system. However, the simultaneous multiaxis
test systems reduce test and setup time and
have fewer tooling components, thereby reducing
potential test machine component failures
and maintenance.
REFERENCES
[1] “Environmental Engineering Considerations and
Laboratory Tests,” MIL-STD-810G, 31 October
2008.