data acquisition
Recently, a major aerospace corporation
discovered a transient pulse with almost
infinite amplitude and frequency content
was fed into their amplifier while the
control system was inactive. After
investigation, the root cause of the failure
was found to be a frayed three-inch cable.
The cable, which patched the drive signal
from a single control system to several
shakers, had been situated in a cabinet
located in an area with a lot of human
traffic. The simple act of someone leaning
on the cabinet or grazing the cable while
the shaker’s amplifier was ‘on’ caused a
relatively large shock pulse to resonate
through the US$40,000,000 structure
sitting atop the shaker table. The
magnitude of this pulse? Unknown.
This scenario demonstrates the
profound importance of a monitor system
which has evolved to do much more than
simply clamp a signal whenever a value
is reached.
m+p Coda (Continuous online data
acquisition) is an advanced system which
actively monitors tens or even hundreds of
channels, including amplifier current and
voltage, strain, stress, and pressure while
recording time domain data for postanomaly
analysis should any signal reach
predefined levels. In the case of the frayed
patch cable, the transient pulse would have
immediately put the m+p Coda system into
transient capture mode. The acceleration of
the table, the amplifier current and voltage,
and even the drive signal itself would be
recorded. With this data, a review board
determining the anomaly’s impact on the
test article could accurately asses the risk
said anomaly imposed on the system and
determine a path forward.
The protection offered by advanced
monitoring systems is not limited to the
context of amplifiers, shakers and
vibration control systems. Pressurized
tanks, battery voltages, critical structures
and more can all be monitored and
equipped with automated shutdown
circuitry for safety. It is not only overtest
protection anymore, it is simply protection.
AEROSPACETESTINGINTERNATIONAL.COM // SHOWCASE 2020 65
allows dynamicists to process data while
test technicians prepare for the next test.
m+p international is at the forefront of
these developments and provide
comprehensive dynamic testing packages
that enable even the most advanced tests to
be run safely and efficiently. To understand
the complete system presented here, it is
important to first examine each
component and its individual contributions
before viewing the system as a whole.
VIBRATION OVERTEST
PROTECTION SYSTEM
Vibration overtest protection systems
(monitors) are simple devices - an
accelerometer or force transducer is fed
into a basic limiting system wherein limits
are entered in either peak or root mean
square. Should one of these limits be
reached, the system compresses the drive
signal to ensure a soft shutdown of the
shaker system. Alternatively, a relay-driven
enable loop opens, signaling the vibration
controller to end the test. The importance
of a monitor system cannot be understated,
as it can prevent unwanted physical
movement of the shaker table even if all
other systems fail.
But what about overtest situations that
do not involve control system? There are
many circumstances in standard Failure
Modes and Effects Analysis (FMEA) of
vibration systems that are not caused by
a controller-gone-rogue or miscalculated
sensitivity setting. These many and
varied possibilities call for a more robust
solution than a standard monitoring
system can offer.
Spacecraft require dynamic testing to
guarantee that their sensitive
components are assembled properly
and will withstand rather rigorous
journeys to their final destination among
the stars. Be it Low Earth Orbit (LEO)
satellites for critical scientific missions,
Geosynchronous (GEO) satellites for
telecommunications, or more experimental
deep space spacecraft, each has a
specific set of dynamic testing to undergo
in order to reduce risk and to ensure
mission success.
THE EVOLUTION
OF VIBRATION CONTROL
In the early days of dynamic testing, a
shaker and a simple signal generator with
variable gain were used to excite a
structure with discrete frequencies or
broadband random signals. The results of
these manual sine burst or sinusoidal
sweep methods were inconsistent at best
and dangerous if not used properly. As a
result, the primitive signal generators were
soon replaced by significantly more reliable
closed-loop vibration control systems.
Modern test systems are now arguably
in the third generation and require more
than just a control system and a shaker.
Many major manufacturers in the
aerospace industry now use a complete
dynamic testing package including
independent vibration overtest protection
systems, advanced safety interlocks and
large channel-count data acquisition
systems. Some laboratories have even
adopted built-in analysis software as part
of their vibration control system, which
1 // Positioning the James
Webb Space Telescope
onto the shaker table
(Photo: NASA)
2 // m+p international’s
high-channel count
systems used for largescale
spacecraft testing
(Photo: Johns Hopkins
University APL)
3 // Monitoring
large structures
from workstations
2 3
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