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By Monty). Smith, PhD and David C. Slater, PhD
S
cientists looking into deep space
through the lens of time continuously strive for more accurate
instrumentation to enhance their vision
of the universe. Random noi se, either
electronic or structural, impairs the ability of these instruments to gather accurate readings, a problem that has been
previously addressed through linear filtering technology. An internally funded
Southwest Research Institute project h as
shown, however, that even greater accuracy is possible through the use of nonlinear filters.
Nonlinear filter technology, recently
developed at SwRI for improved telescope
pointing, helps resolve problems associated w ith power sensitivity and aligrunent
errors in instruments used to study asteroids, comets, p lanets and other space
objects. This technology also h as applications for missile guidance, aircraft flight
control systems, industrial processes and
improved machine tool accuracy. For
example, this technology might have
improved the Gulf War performance of the
U.S.-launched Patriot missile by keeping it
on an ultra-high precision trajectory.
Studies performed after the 1991 Gulf War
have shown that Patriot rrtissiles destroyed
w ith high confidence only 25 percent of
the enemy's Scud missile warheads.
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Electronic and structural noise, inadvertently introduced into all telescopic
hardware, adversely affects instrument
precision, alignment and pointing.
Structural noise includes vibration of fle xible sp ace structures caused by fe-alignment maneuvers and contraction and
expansion of material from exposure to
extreme temperature ch anges. Electronic
noise can originate from transformers,
power supplies and instrumentation.
Devices relying on di gital information for
processing and sending corrective signals
to instrument hard ware can be installed
to correct or attenu ate deviations caused
by noise. Their digital logic is derived
from linear systems theory because of
computa tional restrictions imposed by
existing computer platforms.
Motivated by the progression of
core processor technology, SwRI
researchers proposed the new, nonlinear
filtering approach with applications
directed toward the improvement of
astronomical h ardware. Additi onal
numerical simulation based on earlier
theoretical studies gave SwRI investigators confidence that instrument pointing
accu racy could be improved by an
order of magnitude.
Dr. Monty Smith is a research engineer
in the Engine and Vehicle Research
Division. His responsibilities include
advanced developmental controls
research and modeling for the aerospace, automotive, electronic and manufacturing industries. Areas of expertise
also include hardware development and
implementation for new and enabling
technologies in the control, identification
and signal processing arenas using the
latest in processor technology.
Teclmology Today. Fall 2001
The gimbal angular position error and motor command voltage signals were recorded using the low power, conventional, and maximum
accuracy filters under similar noise environments. The additional freedom in nonlinear filter design accounts for predetermined power constraints or results in improved instrument precision.
Background
Lin ear filters to control the effeds of
noise in instruments used to survey the
heavens were developed long before the
advent of the digital computer. Some of
the earliest analytical contributions
carne from Bell Laboratories for the
design of electroni c amplifier circuits
during the 19305. These filters also estimate p roblems caused by random noise
and reconstruct the signal. Engineers
working to resolve disturbance attenuation issues often im plement linear filters, typically Kolmogorov Weiner and
Kalman filters, on computers using
closed form solutions, or solutions based
upon a fixed number of computations
between sampling intervals. With recent
technological advances in digital signal
p rocessing (DSP), instrumentation has
become readily available that enables
engineers to implement nonlinear, digital fiJtering strategies with performance
that was previously unachievable.
Experimental platform
To test the hardware, SwRI
researchers developed nonlinear filtering algorithms with a gimbaled, laserpointing experiment th at emulates a
telescope. Essentially, these nonlinear filtering strategies give the instrument
designer two options beyond the conventional approach based upon the following criteria: low power (with
stability preserved ) and maximum accuracy (using all available power).
SwRI staff compared variations of
low power, maximum accuracy nonlinear filters to the conventional filtering
methodologies being used today.
Researchers chose a Texas Instruments
evaluation modu le based on DSP technologies fo r the final platform for successful im plementation of these
nonlinear filtering strategies.
injected random electronic
noise into the hard ware to
induce unwanted excitation.
They used three filters - conventional, low power and
maximum accuracy - under
the same noise environment
for comparative studies.
Researchers implemented
all three filteri ng strategies
under the influence of artificially generated external
noise. The latter two
approaches involved the
implementation of nonlinear
fi ltering tecJmjques developed
at SwRI.
The ex perimental stage of
this program readily proved
the effectiveness of using nonlinear filtering to redu ce the
effects of exogenous disturbances. Using the maximum
accuracy filter, SwRI engineers
observed from the data that a
20 perce nt decrease in error
over the conventional
approach could be achieved
under the same
disturbance/ noise environment. In ad di tion, with stability as the primary objective
and perform ance the low priority, the implementation of
the low power filter decreased
energy requiremen ts by 74
percent.
...,;~
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20
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Co n trol Signa l, p = 2
40
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Tim e , sees
30
Conventional Filter
Angu lar Position, p
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10
20
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Con tro l S igna l, p = 10
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10
20
Time, sees
30
40
Low Power Filter
Ang ular Pos iriOD, p
~
= 1. 1
~ 0.2
...,;0
Conclusion
.
:E 0
~
0
SwRI is developing an
extension to this nonlinea r
filtering ap p roach. Other
organiza ti ons have expressed
interest in this technology,
and communication is under
way for furtheri ng the develop ment fo r their specific
applications. +:+
Experimental results
Comments about this article?
To test the capabilities of nonlinear
filtering for noise rejection, SwRI staff
Ang ular Pas ition, p = 2
~ 0.2
-0.2
0
10
Co nt ro ~
20
S ignal, p
30
40
= 1.1
1
~
~
0
-1
-2
0
10
20
TIme, s ees
30
40
Contact Sm ith at (210) 522-3208
or [email protected].
Tech nology Today . Fall 2001
Maximum Accu racy Filter
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