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Multi-Axis Vibration
Reduces Test Time
by Clyde Harman, Team Corporation, and Michael B. Pickel, Spectrum
Technologies
Reproducing real-world vibration environments for testing may introduce a
higher level of product reliability.
Widely accepted as a method to improve product quality, vibration testing is used
to qualify products for production. Either alone or combined with an
environmental chamber, vibration testing is a critical step in the successful
development of new products.
Historically, single-axis, single-shaker testing is the method of choice.
Vibration tests are conducted by sequentially applying single-axis vibration to
a test article along the product’s X, Y, and Z axes. These tests are performed
using a linear shaker and rotating the test product to the next axis after each
test.
However, complex, multi-axis motions are characteristic of field environments.
The more a vibration test can replicate the field environment, the more
realistically it can induce a vibration stress loading equivalent to that
experienced by the product in the field. Testing all three axes simultaneously
can reduce traditional test times by two-thirds and more closely duplicate
real-world vibration environments.
Standards and recommended practices attempt to contain the spectral peaks of
specific environments. However, published standards requiring multi-axis testing
are almost nonexistent.
One exception is IEEE 344, a nuclear-power industry standard requiring bi-axial
testing. Specifications such as MIL-STD-810-F allow for multi-axis testing but
do not require it. As a result, published standards and testing requirements
have not pushed for the replacement of single-axis testing by multi-axis
testing.
With rigorous environmental testing, design problems can be identified and
corrected, life span anticipated, and the where and when of product degradation
predicted. Although it is critical that a product’s performance meets customer
expectations, there is constant pressure to bring products to market faster. For
a number of commercial reasons, shortening the product time-to-market cycle is
extremely important.
With the preponderance of higher-frequency vibration testing being done with
single-axis shaker systems, product qualification testing can be a bottleneck.
Instead of running these tests single-axis sequentially, running them using
simultaneous multi-axis excitation improves test realism, increases throughput,
and decreases risk.
Realism is important for several reasons. First, multi-axis testing excites all
modes simultaneously with a more realistic stress loading. However, sequential
single-axial excitation may not excite all the critical modes of the test object
concurrently and, as a result, may fail to detect defective design.1 Second,
test objects may pass uniaxial testing but fail under actual operating
conditions.2 Third, fatigue damage is increased by a factor of two with
three-axis excitation.3
In the automotive industry, the lack of published standards has not prevented
the use of multi-axis testing procedures. The continued search for improved
product quality and reduced warranty has led to the development of a number of
multi-exciter, multi-axis test systems.
With the exception of some small-force-rated dual-axis systems, these generally
have been servo-hydraulic systems. The result is that the maximum excitation
frequency of even the most sophisticated systems is limited to several hundred
Hertz.
Product qualification specifications from the major car manufacturers regularly
require test profiles that extend to 1,000 Hz. Until now, that requirement has
eliminated the multi-axis systems and forced product qualification tests to be
done single-axis sequentially using electrodynamic shakers. A high-force, 3-DoF
electrodynamic shaker system now exists that meets that requirement and allows
significant reductions in testing time and cost.
Automotive Multi-Axis Testing
Automotive qualification tests seek to duplicate the stress a component sees in
a required number of passenger-car miles. To ensure that a minimum standard of
design has been met, multiple quantities of the component being qualified are
tested.
Duplication of the stress accumulated in the specified number of passenger-car
miles requires running the product sample for a large number of hours. The tests
typically are done using either field data replication or a random-vibration
profile based on the analysis of field data usually with some form of time
compression.
 The length of the tests can be anywhere from several hours to several hundred
hours per axis. Multiplied by the number of samples, the length of time required
to qualify a product is significant.
For instance, testing single-axis sequentially, a qualification test 30 hours
long will take 90 hours plus setup time between axes to complete. Assuming only
one product will fit on the shaker at a time, a 30-hour qualification test with
10 samples and 30 minutes of setup time per axis will require 915 hours of test
lab time to complete. If the same tests were run three-axis simultaneously, the
testing time would be reduced by 60 hours per sample, and the setup time would
be lessened by one hour per sample or a total reduction of 305 hours of test lab
time to complete.
Another example of increasing throughput using multi-axis testing is in a
production situation requiring 100% testing of all products prior to shipment.
For illustrative purposes, assume the product requires a 30-minutes-per-axis
vibration test and each axis change takes 15 minutes.
Figure 1 illustrates the time saved for a production run of 500 units. Three
conditions are illustrated as:
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Figure 1. Productivity Improvement |
• Single-axis shaker doing sequential X-, Y-, and Z-axis testing, one setup per
axis
• Tri-axial shaker doing sequential X-,Y-, and Z-axis testing, one setup
• Tri-axial shaker simultaneous X-,Y-, and Z-axis testing, one setup
Time savings of 25% and 75% are demonstrated for the second and third test
options, respectively. Lower cost results from the fewer numbers of setups
required, and risk is decreased because the potential for errors inherent in
each of those setups is less.
Testing a Lighting Assembly
Spectrum Technologies is an independent test laboratory that provides vibration
and environmental testing services to the automotive manufacturing community.
The company’s focus is on solving warranty problems and offering product
qualification testing services.
Spectrum has been using a Team CUBE to provide multi-axis testing services for
some time. However, the frequency limitation of the CUBE has prevented Spectrum
from running higher-frequency automotive qualification tests simultaneously
rather than sequentially.
Until recently, these tests were run using single-axis shaker systems. At the
end of 2005, Spectrum took delivery of a Team TE3-9.8 Tensor, a 3-DoF
electrodynamic shaker system with 2,200 pounds of force in each axis. The lowest
force rated of the three models available, the TE3-9.8, has a frequency
bandwidth of 5 to 2,000 Hz.
A major automobile manufacturer has a product qualification test that requires a
2.9-grms random vibration profile from 5 to 1,000 Hz to be run for 30 hours in
each axis. The durability test is used for qualifying, among other things, light
bulbs and lighting assemblies.
A tier-one OEM providing the vehicle manufacturer with lighting assemblies
contracted Spectrum Technologies to run product qualification tests on headlamp
assemblies. As part of the test requirement, the lighting assembly must be
mounted in a sheet-metal fixture; in the case of the test detailed here, a small
portion of the fender assembly.
A contract requirement was to reduce test time while still qualifying the parts
for production. Spectrum Technologies succeeded in reducing testing time by more
than 66% by using simultaneous excitation and eliminating two 30-minute setup
times per pair of headlamp assemblies tested. At the end of the tests,
comparisons of product wear on assemblies tested using the Tensor 3-DoF system
and using the traditional single-axis sequential method were identical. Put
another way, the product tested for 30 hours three-axes simultaneously showed
the same product wear as samples tested sequentially for 90 hours one axis at a
time.
The tests were conducted using a Vibration Research 8500-12 Vibration Test
Controller. It runs three different and independent random profiles
simultaneously, one per axis, while using up to four channels of control on each
axis.
The VR 8500 offers a control option called Multi-Channel Extremal. The
controller uses the highest accelerometer reading from more than one input
channel to control the test. Three control accelerometers were mounted on the
qualification product and one on the shake table in each axis of excitation for
a total of 12 channels of control.
By using the highest accelerometer input and changing the drive signal to
control the test in that axis, Spectrum Technologies prevents the product from
excessive accelerations. Because the fixturing is sheet metal, it is prone to
multiple resonances. Without this control, local accelerations from 70 to 100
times the desired test levels could be experienced. The goal is to qualify the
product at the appropriate test levels, not break it.
The acceleration vs. frequency points defining each of the profiles are shown
in Table 1. The simultaneous test results are shown in Figure 2a, b, and
c. Each
axis has a different grms level and power spectral density (PSD) profile.
Table 1. Acceleration Profile Frequency Break Points |
Figure 2a. Loop 1 |
Figure 2b. Loop 2 |
Figure 2c. Loop 3
Figure 2. Acceleration Profiles |
Summary
As production cycles shorten, time to market becomes more critical. Product
qualification testing, either in batch lots before the start of production or as
part of the production process itself, can be a choke point in the production
process.
The test requirements often demand frequency bandwidths beyond the capability of
servo-hydraulic shaker systems. Traditional testing methods have relied on
single-axis electrodynamic shakers to meet the frequency requirements. Until
recently, cost-effective, multi-axis electrodynamic shakers and control systems
were not available. With the availability of these systems and controls, product
qualification managers now can look at more accurately reproducing real-world
vibration environments and decreasing product qualification test times.
References
1. Berman, M.B., “Inadequacies in Uniaxial Stress Screen Vibration Testing,”
Journal of the IEST, Vol. 44, Fall 2001:20-23.
2. Freeman, M.T., “3-Axis Vibration Test System Simulates Real World,” Test
Engineering and Management, Dec/Jan 1990-91:10-14.
3. Himelblau, H. and Hine, M.J., “Effects of Triaxial and Uniaxial Random
Excitation on the Vibration Response and Fatigue Damage of Typical Spacecraft
Hardware,” Proceedings of the 66th Shock and Vibration Symposium, SAVIAC 1995.
About the Authors
Clyde Harman is the director of sales and marketing at Team Corporation. He has
more than 28 years experience as an engineer doing design, field service, and
sales. Mr. Harman has published several articles on multi-axis testing and holds
a bachelor’s of engineering degree from the University of Michigan. Team
Corporation, 11591 Watertank Rd., Burlington, WA 98233, 360-757-8601, e-mail:
Clyde.Harman@teamcorporation.com
Michael B. Pickel is president of Spectrum Technologies. He received a degree
from Lawrence Technological University and has worked for 21 years in the
automotive industry, before opening Spectrum Technologies 15 years ago. Spectrum
Technologies, 12245 Wormer Ave., Redford, MI 48239, 313-387-3000, e-mail:
mbpickel@spectrum-technologies.com |
