Artificial legs of the future may be controlied by the same messages from the brain­tapped at various thigh muscles (far lefi)-that control normal limbs.

When ~he ~:t:3 At"noi:COil maiherfaiiiieianNorbert Wiener Introduced the concept ofcybernetics In 1947, his goai was to expandthe bluiuyi\;Ci; '-iUrlcept of "homeo­stasis"-the equilibrium between variousfunctions In the human body-to cover theentire fls!d of control arid communicationtheory, whether In machines or animals.Tha term was derived from the Greek wordluI' !~htfiftiSnicln/~ and tne concept grewfrom observations by bioloqists that con­trol of voluntary movement In animalsshowed striking similarities to the behaviorof servomachanlsms.

Biologists Quickly adopted the servoprincipia to explain how, for example, hor­mones secreted by various glands regulategrowth; how the kidney-acting as anInfinite-gain servo-regulates bloodpressure indirectly by regulating bioodvolume; and howmuscle systemsfunction.As most engineers know, cybernetics hasalso contributed to robotics and artificialintelligence; but they :'n~y net ~nc':: th::tbiomedical engineers, using cyberneticprinciples to Improve the functions of ar­tlficlailimbs, areclose at last to fulfilling adream described by Wiener in 1951: thebuilding of artificial limbs that can doalmost all natural functions because theyare controlled by bioelectric messagesfrom the arnoutee's brain-the same tvoesof messages the brain would have sent tothe muscles in a natura! Hrr.::;.

The messages from the brain, which canbe picked up by tiny electrodes placed onthe muscles of the stump of the amputatedlimb, would be relayed to a micro­processor- or eiectromechanlcally-basedcontroller in the ait:ficial limb. The con­troller would be programmed to recognizesuch messages-caHad myoelectric sig­nals (myo refers to muscle)-and convertthem into corresponding motions of theknee, elbow, wrist, or fingers. The iTiOt!onsare Implementea tnrough torquing motorsor pneumatic or hydraulic pistons.

several types of the cybernetic limbsnow in university laboratories will be testedclinically In about two years. They ared~s~gned fo!' the severest cases-amputa­tions above the knee or elbow-and will beeasier to use and maintain as weUas morenatural than current devices,says GordonD. Moskowitz, director of the Moss-Temple­Drexel H6habmtation Engineering centerIn PiiiiaJ"ipilia

Cybernetic t .' .:,<'\are needed, Mr.Mosko­witz points out, oecause present limbs-

operated by cables or hydraulics and ac­tivated by brute mechanical forces In adja­cent mu~!es or Inadequate myoelectriccontrol systems-require long training andgreat concentration to use and are limitedin function.

Although VGi;OUS tecnnlcat pfuuitnTISdeiayed the development of cyberneticlimbs, the main reasons for the long delayificiude the reiaiiveiy small market for themand the lack of coofdiFiaieu effort by thehealth-care systems that serve the averageamputee: according to Mr. Moskowitz andRobert 'Iv. Mann, Whitaker Professor ofBiomedical Engineering at the Massachu­setts institute of Technology In Cambridge.Funding has tneretorebaanilmlted andih~

devices have been developed In just ahandful of university centers. Besides thework done in Philadelphia and at MIT,cybernetic limb oroararns ara run hv {lllli.

ley-S.Childress at No'rthwestem-UnlversltY,Evanston, III.; Richard B. Stein at theI I "luAPeI." ,..1 AlhA 1_ r" ....__~_. :;';;-;..-......._...._._.~I _. ,...,.~.~ "' "'-'U"QUQ, ...--K"IuglL IYI.

SCott at the University of New Brunswick,Canada; and S.C. Jacobsen at the Univer­sity of Utah, Salt Lake City.

On the technical side, two developmentsIn recent years have made the cyberneticlimb feasible: (1)techniques for quickly pro­cessing the Inherently noisy myoelectricsianals to obtain adeauate sianal-to-nolseratlos-and relatively dlstortlon4ree signals;and (2) methods for automaticallydiscriminating between different mvo­electric patterns and matching them- totheir corresponding motions (knee or_ .... _ ••• £1_••1___ _ __..... :___ _11__••• __

alUUw t!~~~~:::i UI aAlall~IUII:t, II:'IUUYV UI

wrist torques, and so on).The muscle signals picked up by an elec­

trode are noisy because the measuredvoltagas aiGthe vectorsumof signals ema­nating from thousands of muscle fiberswith different time constants. The spacingof electrodes Introduces addltiona: noiseand cii~iofiivn caused by fc'iI"llvc ulfter­ences in-the time of arrival of signals at themeasuring points.

To optimize slgnal-ta-nolse ratios, engi­neers In some laboratories use statisticalmethods from modern control theory. Forexample, an approach called the "optimalestimator" works as follows: A micro­processor continually computes the bestposslbie estimate oi tne signal by combin­ing the nc;~y s;gnal with one representing amodel of the tissue and electrode actionsthat cause the noise. iha noise model isImplemented with a white-noise signal

passed through a shaping tiiter. A simplerapproach, perhaps just as adequate, pro­cess= ih{; signal with a bancpaee filterthat removes high-frequency noise, fol­lowed by logarithmic amplification to pro­teet against overrangfng and maintain thesignal despite poor electrode contact.Finally, envelope detection yields a dcenllivsllAnt nf tho QI"",o.

-'Most-oi the·~y~~t~ic-pattern classlfl­eiS hawQ so far t~n des~unEd for art~f~c:Ja~

arms. The first such arms-the so-calledTemp!e, Boston, and Utah arms {as weH asthe "SwedIsh niSF10 t j - 08veioped controlslana's from eorreletlons ~tween. 810"818from different muscles In the shoulder andthe desired elbow torques. Basically, how­evai, they used only the total energy in themvoelectric ~innAI frnm a alven muscle-on ths -premlsethat-this gave a reasonatse~~.lmQtA nf t~- ~-.- ""f 1""'1 'Q~lo l'.nntrAt'..-

tl~~~~hlle ig~;i;~a;u~'h··d~t;iied-sig~;.tures of the signal as the patterns from dif­ferent muscles. The most advanced suchdevice-the Utah arm designed by Dr.Jacobsen-is based on an experimentallyprovsdmodet that correlates sIgnalsrepresenting shoulder and collarbonetorques with elbow and wrist torques.

The state of the art In cybernetic legs Isa device designed by Woodruff Flowers, aprofessor of -biomedical engineering atMIT, which has a fixed walking rate oversmooth surfaces only; in climbing steps,the amputee drags the prosthesis to thestep just reached by the normal leg. Thetorques at the knee are damped by is fili~fu­

pioceSSoi-i6gulatad eddy-currant brake.Byadjusting slide potentiometers, the usercan tell the microprocessor to commandvaried damping profiles as needed forstanding stili, sitting, walking, or swingingdown stairs, for example.

Amore advanced leg Is being developedat the Moss-ierilp;e-Orexal canter and willbe tested clinicaiiy ill two years, says Mr.Moskowitz. The device Is based on adesign by Donald R. Myers, a former doc­toral student now with the National Bureauof Standards in Washington, D.C. [seefigure].

The "Philadelphia leg" works on tne prin­ciple that when an amputee Imagines themissing 11mb making certain knee actions,the myoelectric signals in the stump will besimilar, on average, to those measured innormal subjects. (Mr. Moskowitz and Mr.Myers say they have confirmed that princi­ple In experiments.) During a trainingperiod, data are collected with which pat­tern Classifiers!n the art!!k~; ;~ wi;; ;&tQrrecognize the imagined kiieii actlcns,

CONTROLFORCE

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MUSCLESITES FOR ELECTRODES

24 IEEE spectrum APRIL 1982

Onedeviceoutof200... Oneboard outof 10... Twosystems outof 5

Howarespectable 0.5% devicefailure rale can turn intoa,~nft/. ,,- I........ "-., --. .~!I~ If.... , _----~ I ~ _~~~ _~_

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ponents at static safeguardedwork areas.

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-iEEE spectrum APRIL 1982 Circle No.7 2S