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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. 10 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. ...,;~ :5 . 0 ~ 0 -0 .2 0 10 20 30 Co n trol Signa l, p = 2 40 0 10 40 1 ~ 0 ~ -1 -2 20 Tim e , sees 30 Conventional Filter Angu lar Position, p = 10 ~ 0.2 r---~~-r------~~~~~~~----~ ,;:~ 0 . ~ o -0 .2 L-----'-----,L-- ---.Lo 10 20 30 Con tro l S igna l, p = 10 ---,J 40 ..................... ·. ··r ...........................................................j .... ~ 0 f=~~~~~-.-.~. . ,. . +.l~. .~. .~. .~. .~. .~. .~. ·4· i~·· ~~~1 ~ -2 ......, ........................ ...+......... ............+............... -1 o I ! 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 11