Controles Electricos y Electronicos en Hidraulica Industrial

8/8/2019 Controles Electricos y Electronicos en Hidraulica Industrial

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Ele clrical contro ls are thc dom ina nt m ct hod [or co n-Iro ll in g fl u id powcr systems. The solenoid-oper at eddi rectional control val ve is t hc basic bu ild ing blockfor elec lr ic al co ntro l cir cu its ,Ju st as the designer had .10 lea rn ISO graphicsyrnbols to co rnmunic a te ab ou t-R -u id po wer- circuits ,he rn ust also lea rn standard electrical syrnbolcgy.Thi s cha pter de als pr irn arily w ith electrical controlcircuits, not power circuits .Switches and other electrical co ntrol sSw i ch cs a re lhe ba si c clcviccs 'ro r gC llcr:11 i ng COII -tr ol si gna ls. fi gu re s 21.1 t lu ou g h 21.10 illu st rutea nd bricfly describe (1)(; sy m hn lx nI' lIll sl couunonlyus ed types 01 'elec lri c sw itch cs , contac ts, re lays, andmotors, - r ./v_/:/5)

RATINGS AND SPECIFICATIONS

Electrical power circuitsEleclrical po wer is primarily use d lo drive fl ui dpower sy sterns. A lterating cu rre nt (A C) power cir-cu it s are ra ted at 11 0 -1 2 0 V (volts), 220-240 V , 44 0V , 88 0 V , and 2300 V . The y rna y be ei the r single- orth r ee -phase sys le rns . Sr nall motors , up to 1 hp , rn aybe st art cd "across th e linc, " i,c., with a sim ple pu sh-but ton cont ro l.I n these fractiona l horsepower rnotors, the inr ushcu rre nt duri ng start- up is not large enough to causesignifica nt prob lern s. However, m otors large r th an 1hp rnu st hav c a motor st arter bu ilt in to th eir co nt rolci rcuit.M ol ar starte rs prote ct ag ainst excessive inrush

cu rrenl du ring star t-up . Thcy m ay also pr ov id e"overload" protection when thc electric motor is run-ning lo pre ven t o verhe atin g of the motor w indingsand dam age Irom excess ive loads on thc motor shaft.Motor star ters m ust be ma lch ed lo the elec tric motoron the basis of vo ltage ra ting, phase, a nd horsepowerrating. Fo r the highcr-horse pow cr motors, th e starlercosts a lm ost as much as thc motor. ThI lS , t he motorst a r t cr is a significanl cost [actor in thc dcsign of thcsystcm and can not be ovcr lookcd.

CHAPTER 21

ELECTRleAL/ELECTRONICCONTROLS

Electrical control circuitsDirect-cu rrent (DC) elec tr ical conl ro l cir cu its arera ted at 6 , 12, 24 , or 36 volt s; th e Ii rst three are themost po pu lar.A lternating- cu rr ent ratin gs for co nl ro l circuitsare 6, J 2, 24 t, and 110 vol ts . Contr ol circuit s ca noperate al any voltage e v i suitable for t he .system.However , op erating con trol systerns at hi g h v olta ge sis inefficicn t a nd introduces unnecessary haz ards forpcrsonncl.

Switchcs HC r.i t cd on th e basis of syst cr n voltugcan d t hc m a x iuunu currcnt lhe)' can h .u id lc sn fc ly . Atypi c: d r:ll in). '. fm:1 push -bu t to n sw itch dcsiuncd [or1 '/, hp du tv iS. J():lI llllL 'res (/\): II It1 120V ,or IO:llld250 V . Somc switch ratings include data on De asw el l as AC cu r re nt and voltage. O thers specify fu r-th er th al the ra ting given ap pl ie s lo a non-inductiv eloa d. Induct ive loads can ea use hig h currenl su rges inth e circuir whieh t he switch rnay not be abl e tohan dlc.W hen rel ays are used, it is nec es sary lo specif y coi Iand co ntact rating s. fOI exa rnple, a des igner m ig htuse a relay with a 6- V DC opcrating coil and eo ntac tsca pable of handlin g a lO -A , 24 0- V loa d. O r he rn aywant a relay lo opera te on le sa rn e vo lt age as th eco ntact cir cu it: in hi s case th cdc signcr co uld spccify110 V Ior th e coil , an d 20 A and 110 V for thecontacts. Spccifications for a relay ca n bec om e ascomp lex as th os e Ior a directio na l co nlrol va lve. Thedesigner must in elude the vo lt age rat in gs , the con-ta cl paucrn, and a ny spccial co nl ro l Iu nctions , suc has tirne de lay , latehing , or ste pping.Application example 1One of t he sim ples t conl ro l ap pli ca tions in flu idpow cr involvcs t he actuat ion of a doubl c-ac ting cy lin-de l' th rough a cycle: start , ex tend ihrough a givenstr oke, reverse, and stop after full relra clion. F ig u re2 1 . 1 1 (a) illustr a tes l he hydr aulic c ir cu it Io r th isa ppl ica lon usi ng IS O gra ph ic sym bols. The se-tl IC IICC diagr:llll, (rc lc r a lso to Chu ptcr 22 ), cl ct cr-mincsthc or dcr of cvcnts, Figure 21 .1/ (h ); th e clcc-tr ic n l cO III rol circu it , Figure 1..1 .11 (r ) shows how lhe

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Fig. 21.1. Basic electrcel pushbutton switches

\ I t, I)'JJ - '> r) r. 1).t":) e f) " . f !~ I) r- 1_, '"1

, ,'-

Fig. 21.2. Basic electricallimit switches

Fig. 21.3. Basic pressure switches

Fig. 21.4. Basic temperature switches

Fg. 21.5. Floal switcnes

224

NO-SPST Normally open - single pote,single thrQw-< o---NC-SPST Normally closed - single pole,single throw

NO/NC-DPST Oouble contacts normal ly open!norrnaly closed - double poie , s ingle throw---D o---NO/NC-DPDT Two pairs normally open and twopairs normally closed - double pole,double throw--o o---

--~-- NO Nomwlly open --_ .._------~~. .NO Normally open, held closed

NC Normally closed

NC Normally closed, held open

NC!NO Double pole, normally closed/normally open

~o-o , - NO Normally open~ NC Normally closed

~ ~ NO Normally open

~ NC Normallyclosed

" - - - 9 - ' NO Norurulty opcu.__- - - o ~-6-0 '- ' NC Normallyclosed


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'-~ NO Normally open

~ NC Normally closed

NO Normally open (timed closedwhen energized)

NC Normally closed (timed openwhen energized)I----~--------__-_ ..-...

NO Normally open [timad opcnwhen deenergized)

NC Norrnally closed (timed closecwhen deenergized)

~o- 6 .- - NO Norrnallv opon1-------___-__-_........------.--.---~ .-- NC Nonnnyd(l~;(~d

-o-- Control relay coilI------------,-!--'----------~ ~. -NO Normally open relay contacts - .1-------------_ .._--- .----~ NC Normally closed relay contacts1------------ ..-----------..,--o-A.ro- Solenoid1------,----_ ..--_._---_._-----1~ Fuse--n- Pilotlight.- - ::= :'0 3phase'-.Q Q .QQ .QQ .r Field winding----.--.-------.------------1---0- Molor armature

Fg. 21.6. Foot actuated switches

Fig. 21.7. Time de/ay contacts

Fig. 21.8. F/ow switches

Fg. 21.9. Co/s andcontact symbols

Fig. 21.10. E/ectricmolorsymbo/s

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co ntrol Iuncrions ar e acco rnpl ishc d.A nor m ally opc n SPST push buuon initia tcs IhL:cyclc. W hcn t hc opc rator dcprcss es rh c cvc!c st artpush buuon , il cncrg izcs co n tro l r clay , eR-I. '(h ecoil clos es onc p.iir u f cont u cts ("1


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Application example 4A Iu rth er m od if icati on of th e basic cy li nde r cir cu itu ses the retracting ey linder lo l~ igger the ex tendingeylind er al so rne prese t poi n t a long th e ret ractionstr oke . The fluid po wer eir eu it is show n in F igur e21.14(0 ), the eleelr iea l ladde r diagra m in F igu re21.14(c ). ,/~0Dle{l.vThe scqucnce of e~rls , as show n in th e seq uenc ediagrarn , Fig u re 21 .,1 4(b) is as fo ll ow s: Cylin der. 1 '[eXlelld~ . W hen it 6 'a~he .s it s end of sl.r oke pos iti on,thc cylindcr rod trrps limit sw itch 2-L,) lo rever so thc 1stroke di rection of C ylin der 1 , w hich beg ins to re- /tr ac t. A l som e poin l al ong its re tu rn stroke, Cylind er r ; tl ac lua les limit swi tch 5-LS which initiatcs thc ex -te nsi on str oke of C ylin der 2. Cylinder l continues lo < )retrae r, trip s lirn it sw itch I-LS and stops al th e end ofit s str oke. C ylind er 2 ex tends until it rcaches th e end Gof its str oke , when il ac tuates limit sw it ch 3-LS an d 1 - -T~~",-o-~"";"=~~-"""'--.--------Ireversos it s dire cti on . W hen Cylind er 2 is fu lly re- I- 2.LS Itractcd, it actuales lim it sw itch 4-LS whi ch stops th e J CR3cy cle . 1~--~ ....-=-=-=========~~~Apptication example 5 j "A cy linder must reci p roc ate a preset number of ,f.,cy cles . The f lu id pow er cir cu it is shown in F igu re21.15(0 ); th e scq ucncc di ag r.u n in f-igu re 21 .15(b); /the elcctrical ladder d ia g r a m in F igur e 21.1 5 (c).

alternately. Figure 21 .12(0) show s t he flu id pow erdiagr arn ; th e se qu enc ed ia gr am , in clu d ing elec tri ca lcon trol designations , is show n in F igure 21. 12( b);th e eleclric a l di agram is in F igur e 21.12 (c) ..The cycl e is s tarted by de pr ess ing a push bu tton , aswas th e ca se in Exa m ple l. Lim it sw it ch, I-LS, isnow of the DPST type.Carnm ing I-LS op ens the ho ld ing cir cu it.Io r th eIi rst relay el? -1 a nd s imul la nco usl y clos cs [h e hold-in g ci rcu ir Ior th e scc ond rc lay , CR-J. Lim it sw it chI-LSenerg ize s re lay CR-J.Thc cir cu it di agra m in f ig u re 21 .12') show s thatthc elcclrical conl ro l circuir is esscntially th al o fExarn pl e 1 , w ith extra path s added to cont ro l cy li n-der 2 . The two b r a nches of th e circu ita r c in terl oc ke dthr ough lim it sw itc h LS -l.Application example 3The hydr aul ic circuir , F igu re 21 .13(0 ) m ust pro- 'v id e the follow in g work sequence: Cylinder 1 ex -tends, conl ac ts the workp iece, dwells un til a presetpressure is rcachcd, reversos. a nd rct racts. W he nCylindcr 1 reversos. Cylindcr 2 bcgi ns lo cx tcnd ,contacts thc workpiece; dwell s unt il a prcsct pres su reis r ca chcd, then rcvcrscs ami rct ra c ts. Thc scqucnccdiagram Ior t hc opcr a t io n is s how n in F igu re21.13 (b); th e el ec tr iea l laddcr dia gr a m is shown inF igu re 21 .13( c). .

'~I(1r~!,'.'

1 2Fig. 21.13. Syslem tor aperaling Iwo eylin


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(a)

S " ' , I Cycle 0;, oper ateo .' . stan .

(b)

CR-11 1- 1 - - -Cycle start~O

CR-,2-LS . - . . . - - - -

CR-2 4-LS 5-LS)--~_C~R..;,-4_ -_-C-iR~~ '\:7I \i'R-3

CR-1 SOL-11 50L:-2 -R-21 /

SOL-3R-3SOL-4R-4

(e)lu]. 21 I~J. .sy:;/I~111 t. PI )!! '' '''''!.! (l\'() t:yilllt/:I:; vvttl t 1 I / f / 1 1 f l ' h . timit switcnes: (l) IlyuI uutic cit cint; (b) SU,/lJUIICt! d1J!J1 ;111I; le)eJeclricaJ circuito

1-+1-+- ---_._-.~-._----

PS-,, - - - - ~ - ' . - . , ; : . . . - r : r ~ - A:'_ .~v.'-1I.J(a)

1imer-Oforreset

Cour iter'Oltorres e t

O1iming:. . Timed _outO0= open '= closed

,1imig TR lrelay1imer M- rnotor .1imer Tclutch 'Relay, CR-2CR-3------CounlerclutchCounlcreoilSolenoid 1Cylinder 1 - r- Retrael

StartCyclestart

~ operated

Reverso

CR-3 SOLo'

I ~~:~,PS-1. ', l-LS. ,_PS-2. out -Counter eounts TR-I openoullime, starts T-' reseton press, dwell

(e)lid; ;,> /.15. SV:;[CUfl 1,)1 /(.';I)f(JCiltUI!) .1 c/o u iJ /u !GIIIIY cvtu uto,with tnnu iJlJ(l oi essuro swucues: (iJ) IIY(/iJlJlic circuit, (b) se-auence diagram: fe) eleelrieal circuit.

228

-, (b) _:-"

1-LS Counterclutch

~::.:..,---'. - . _ : _ - - _ . _ - -------- -------------------'----.:._-----~;::

CR-2PS--1- .ot--'---~>-""o---jJ-+-----I

Cyl-lCR-2


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Thc cy cle is as follows: I he cy l inder ex tcnd s a nelcoutacts th e workpiecc, Whcn a prcset pres su re ascontrolled by p ressure sw itc h , PS -1 , is reached, th ecylindcrreverses and retracts. A t sornepo int duringth e re tra ct str oke, the cylind er actu ale s lim it sw itchI-LS, ca usin g the cylinder to re ver so an d ex tendag ain until it co nta cts the workpicce, again actuatin gpr css u rc sw itch PS -l . The cyclc is rcpcatcd a pr esetnurnber of tim es, as controllcd by ac ouutcr. 'The co unter is a n clectrical co nt ro l whiclr; for ea chpu lse it rcccivcs, su bt racts onc digit Irom a num bcrpr ese t int o it. W hen the balance in Ih e countcrrcaches zero, th e co unt er perf orr ns a 'co ntro l f unc tio n- for ex arnplc, it m ay opcn a sw itch or send a ne lect r ic a l pu lse, Thes e type count ers are com rn on lyu sc d to cont ro l nurnbe rs o f cyc lcs o r cv cn ts. Thcoperalor rncr cl y dials in to th c co u nt cr thc numbcr o fcvcn ts dc sircd bcforc slarl i~ 'lg th c sysicm.

Th is sysrcm isnlso cq uippcr! '\Vil h :1 t imcr, Fkc-tr ic u l t imcrs ar e mcchan ism s drivcu by xynchr ouousmotors. Thcsc rn o tors run at consta nt spccd, usuallybased on 60-Hz, AC current in th e United States andCanada - (the current would be 50-eycle in Eu-'rope) . Su ita blegea r tra ins prov id e the des ired time '.,1. The.co n tr o l or data pr ocessing functio n of m icr o-del ay . A t th e end of a tim ed peri od , the tim er in itia tes : elec tr oni c tec hn olog y and ',' . .' -som e co nt rol ac tion such as ope ning or closing a' 2 . The pow er tra nsm iss ion and co nt ro l function ofsw itch , Pneurnatic an d electronic tirners are also Ilu id powcr .available .In t his exarnple , the tirner shu ts dow n the ent irecircui r aft er a p r es e tl cngth of tim e by co nl roll ing lh esolenoi d operator on the directi onal co ntro l valve .Application exarnple 6An in tcrcs ting co nt rol np plic.ui on is shown in Fig-u re 21. 16(a) in which a cylinde r re ciproca te s, ea chsucccssivc sI rok e bcin g longc r I ha n I he prcccd ingone . Thi s.stcp pcd co nt rol is ac hi cv cd w i t h a numbcrof lim it sw ilches app ropr iarcly spac cd long the cyl--- indcr stro kc. Thc clcc trical lad dcr diugra m is s hownin Figu re 21.16(c) . W hen a cycle is started , thecylindcr cx tcnrlx . un il il ClllIl;ICts 1)Ic sccond limitsw itch , 2-LS, Thc cy lindcr re versos and retracls untili t co nt ac ts I-LS. Lim it sw ilch ' -LS rev erses th ecy linde r whic h ex tend s aga in until i t contacts 3-LS,and so on.The seque nce d iag rarn , figure 21. 16(b ) di spl ay s hc sw itching scquence Ior t his circuit. Two addi-1ionn 1 cout rol Iu n ct io ns a re 1101:\ppa rC1l1 [rom th copcr a ing scqucnce. First , it is possiblc lo reverso hccylindcr and rctur n it to it s st.ut posi t ion :\1 a ny poi nlin t hc cyclc. Second, a ny imc t hc rc t r.ict Iu u ct ionhas bccn dcp rcs sc d, hc cy lc stu rt bu t lon mu st beopcratcd lo in itia te a new cyclc.The nee d for these additional co nt rol fu nc tionsbr ings up an im po rl ant poin t. D es igners of co nl ro lcircuits mu st hav e the necessary sk ill and experien ceto pe rcei ve the nee d Ior th ese little "extras," Su chextr as ar e [r eq uent ly r eq u ir ed to co nve rt a th e-oretical ci rcu it derived from a se quence diagr arn in loa pra ctical sy stem . The diagrum alon e ca nn ol en-corn pas s and display all the req u ir cm ent s such as th e

229

ncc d Ior intcr lock s bc twecn parts of a cir cu it,cm crgc ncy shuu lowns, c te . The se qucncc diag ra mrn ay indica te where thes e functio n s ou ght to be in -c1u de d, bu t it is up to th e des igner lo re cog ni ze th enee d for thcm .' ,MICRO ELECTRONICSThe elcc lr on ics tcc hn ology was stim u latcd not

onl y by aer os pac e .and defense system s an d asso-cia ted rcscarch and de ve lopr nent ef forts, bu t alsothrough spcc tu cu lar dc vcl opm cnts in t hc COIllIllU-nications industr y , televisin, consurncr-oricntcd ac-tiv it ie s.The resu lt w as a series of ad vance s in elec tr onic s:Iro rn vacuurn tube icc hnology , lo disc rete elern entso lid-stu tc dcv iecs, lo in leg raled ci rcu i t r y bas cd onso li d -s uu c dcv iccs, to m ucr o-in tcg r.u cd circu i ts , amiIina lly lo micro-intcg rut cd cir cu ii s. To progrcss [romlile solid-sl:llc C!)lIiV:i1CIII of lile c lcctromcchn nicn li.:I;IY lo Ilit' Illin(ll'l!)lTSSOr 1001\bu l OIlC dCC:lllc,To pu t hcsc dcvclopmcnts into pr opcr pcrspcct ivc,we m ust di stingu ish bc l w ccn:

Indu str ial cont rol s, to which Ilu id power is cl ose lyrela tcd , hav e barel y bcgun lo adop t new clectronicdeve lopmcn ts . The use of sta te-of-the-art electronics 'by aerospace-defensc. t he co rnputer industry, thcco mmunicat ions industry, and proces s conl rol is Ia rah ca d 0 1 ' currcnl pruc iccs in ba sic m anu factu ring ,hard goods, and m etal wo r king industries. Most con_otrol in stal l.u ions use old , siand by, clcctrom cc huu ic alrclays.K ee p in m ind rhatclcctr ohydraul ic systcms areon ly (J/I(' part of a Ilu id pow cr contro l systcm ; o t hc rconl ro ls i n c lu d c: man ual , m ec h a n i ca l, e a m -np cru tcrl, lcv crs, pllsh but lons, trcn dl cs, n nd ai r-I(;~'oil as wcll as oil-to-uir pilots.Prograrnrnable controllersAcco rding to N EM A Standa rd 1- 28-1976, a pro-gr ammable conlro ller is a "digitu lly operu tcd elec-lronie appa ra t us w h ich uses a (progrn mruab le)11ICll)(1ry rol' t hc int c rna l st orn g c 0 1 ' inst ruct io us rol'im plcmcn t ing spccific fllnelions such as log ic , se-qucnc ing. tilnillg, cou nt ing. a11(1ur it hmcr ic lo C01l-t rol , t hroun h digil:\1 or an:!lllg illJllll/ollI 1 ' 1 I t uiud-u les, v.uious t ypcs 0 1 ' machincs or proccsscs."A dc signer who is a lso experienccd in the Il u idpower lee hn ol ogy shou ld ex pan d the clefin ition ofprogrammable controllers. (PC s), lo inclucle (luid -rnechanica l and eleclro-mechani cal sequencing de-. v ice s suc h as drurn-switch progr anuners, fluidic se-qu enccs, et c.Sorne definitionsFigu re 21, 17 illustra tes a block di ag rarn of a

1_ "_ j t~) h ' n , ' - . [ (':ll';,' _"~,~,;_,1-.('"1 - J . : .. l ..1 J J kA -c, le .. c . . ' '.f'- C : . J ( !


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,-~..'0,"- .,

~ I~;~~ Lt~1 IP ;~ I'.fTIF~~-, ,r--' 1 Path o{ cylinder

~ ~ ~ = = = = J./'L - " _ _ - " ~ ' - - _ ~ .I,- -_ ... J3,L ,, .r- ----------,----J4/ .L .. -,

/'.. --- ---...J5(a)

..~ operated o ~ re/eased (b)Cyc/estart.-L

~--------~-L~------~_o o'C~'J. 1-1---'

Reverse SL.$.CR.lll-..,.--.c;~=m:.-..--ICA-2,......,-__;

,...

.: 1-t~."_.A" . 'J. .. . t - - - - - - -, - ,- , - . ~ . ~. .- , " ' ,~..::;r_------..,,--~-------------{- . '. J . . " " " " " . ' C :: - " . C-:,/~,-. --o-"'.~,.....,-----'-::~-'1

yt--:--~C~R~.5~. _.. _+------o.3~.L.So/ o/ /1 1 - CR.71- _1 > . + . o4~.LSo/ < 1 . R.S-!lbn' Oig. 21.16. System for recip-rocating a double acting cylinder (ll +- ---'C:.,cR..:..c.:.7- --=C.:..cR'-'.S~I---4

successively longer .strr;kes: (%0hydrauflc circuit; (b) sequence oie:gram: (e) etoctricet circuito

:"; . .,..CH.:JI~.: : ,- - - CR.211CR.31~ CR.21

CR.l/1 J CR.411'CR-511~ CR.211

CR-lll' CR.6CR.71~ CR.2j

- 1 CR5R' 1-1,;.'._ --,. _r .&...- -o.2'-..LS.,- O CR6CR4CR6

CR~9

CR.l, - I ' _'CR9 - j ' _ ' J .CR.S11- --'_-~r9'1-----"

t-__..,------..,,---..,~C~R..:..~3 r- ~C~R.:..c~_'_4+r--~------S-O-LC-l~CR-6R-5

CR9(e)

230


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rnic roproce sso r- base d pr ogra m m able con tr o ller .N o te that m any of th es e Iunctions are also u se d in. rnic rocornputers. .P rogramm ing device m ay be a built-in se ction of therc or a sep arale , sel f-c on ta in ed accessory .CPU (C entral Processing Unit) is an in leg ra tedcircuit (ICjon a ch ip w hic h includes t hc n ec es sa rycircuitry , logic Iunctions, ct c. , lo prov id c hc fu nc-tions requ ired of the PC. .Pow er su pp ly is the e lec tr ica l p ower so u rce fo r there. It is a regula ted pow er supp ly w hie h prov id esclcctricity lo th e rc a l rcq uircd vo ltag c an d currenlle vel s.Memo ry is another in tegratcd circui r whi ch may beon th e samc ch ip as th c CPU or 011 an indi vidual ch ipdepending on PC requir c m e n ts. Som e m icr o-el eclr on ic 'sy s terns are tot a lly co nt ai ned inthe ci r- .cu itry en .a single ch ip . O th ers are "board 'lev el"systcms in which ind iv id ual functional chips aremoun ted ona board or ca r d a nd int er co nnect ed byco nduo i ngpa t hs deposited on (he board orca rd . ':The memory is the pa rt .of the ptograr nm ablecont rolle r wher e instruct ions (t he program ) arestorcd. Da taT rom inpu ts or lo oul pu ts IlIa y ,a lso bestor ed in appropria te addresses in.t hc m ern ory . O neof t hc Iunct ions w hic h t he CI'U proccsscs am i con-trols is the rout ing, storin g, an o re tr icv al of dal a in 'the m ern ory .Input/output (l/O) modules are the in teg rated cir-cu its d esigned lo hand le i npu l o r ou tpu l da ta (in thefo rrn o f an c lectricalvo ltage) from exle rna l inpu t o rou tpu t dev ic es .In put devices Ior flu id power syslem s lyp ica lly m ayinc lude pu sh-bul lons, lim i t sw it ches, selectorsw itches, re lay s, punch ca rds rea der s, se nso rs , lrans-ducer s, o r any device w h ic h ca n supp ly an e lectricalvo ltage signal to the r c inpu t ;il~d til, ln 'pu t dev icesar e hard- w ired lo thc term ina l strip on t hc re.Output devices typ ic ally might be rclay coils , val veso leno ids, m olar s la rte r co il s, cte. O ulpu t dev ices area lso hard w ired lo the re. The CPU prograrn in-s tructions from mernory and fee dback signal s on thes ta tu s o f the l/O dev ices lo co n lr ol t he operatingscq llencc of t he output dcviccs.P rogrammable co n lr oll ers o f th e type des cribedabovc rangc from ver y s imple sys tc rns, Figure 21,18lo h ig hly so ph istica ted di str ibu ted syste rn s w hich arehardly d istingu ishable fr orn co rnpu ter co n tro l in sta l-la t ions .The principa l diffcrencc belween.relay~Iagic an dprogrammable cont rol le rs is programmability andcost . lb ch ange a relay log ic systern , com ponen lsmay hav e lo be added or rem oved an d the system hardre w ir cd . 10 makc a corrc spond ing chango in a PCrequires only reprogramming, or al worst, addingadditional modu lc s, if incrcnscd capn ci t y is rcquircd.

lmplcmcnting PC controls Ior Ilu id p ow cr a p plica-t ions m ust m cct sorne lirn ilations a n d sa tisfy ccrt a i n

Fig, 21.17, BlOCk diagram of a microprocessor-based pro-, qremmsbteconroier.

Fig, 21, 18, Iflus Ira tion ot simple programmable comrouer.requi rerncnts. Typically, the designer m ust p ropcrl ym atch th e char acteristics o f l/O dev ices , tirn ing re-quirernents, program fau lts, defec tive rnem ory , irra-tional commands, fau lty logic , etc. a ll o fw h ic h make. p ro grarnm able con tro lle rs m ore d ifficu lt to u se thanm igh t be construcd by an inexperienced designer.Such dctailcd d iscu ss ion is beyond the scopc ofthisbook : H ow ev er , the con cerned reader shou ld s lu dytheabundan t availableliterature which des cri bes ihej-c t cchnology .

' ,_ . , .. . '. 0,

Microprocessors and microcomputersWhy should the fluid power lech nology prep are lointcrFacc wit h control computcrs? Beca use ihn t ist he d ireclion indu slria l con lr o ls a re following. Flu idpowcr is not a lcnd cchnolog .y. W hen the grow th of agivcn indusuy is spur rcd by mu rkcl dcma nds ort cchnologica l innova t ions WI;lhi 'll t hat indu stry , t hcr c

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are changes in production tcchn iqucs, m ac hin cr y,and p roc ess es. F lu id pow er m ust accomrnod ate thes echange ~. ;M os t arcas of ind ustrial act iv ity incviiably .ad optcom pu te r contr o l. The m icroproc ess or has m ade th ispos sib le bccause of the ca pabi lit y in corp ora tcd iu tolow -c os t co nt ro l corn ponents. .A tthe tirn eof th is w rit ing , th e adap tai ion of.e lcc -troni c and rnicro-elcctr onic cont ro l is proc ccdi ng .rn uch m ore rapidly than hadbeen anticip ated in thela t e 1970s. The indu stry will-nccd o t ra inhybriden gineers/ tec hn icia 'ns w ho und crstand m icroelect ronis and Ilu id pow er arid hav eihe kno w ledgeand skill to comb ine them ." 'Som e cornponent p roblern s m al ha ve to be re-so lved before the hybrid tec hn olog y can overco rn e

cxisting co nstru ints .S ig nal lev el ~ Bec ause solenoidvalves cu rrentlyu se d in Ilu id pow cr system s operate at pow er levclssubs tant ia ll y above the norm al sign alleve l gene ratedby m icropr ocesso rs, s ign ai am pli fica tion is ne ce s-sary . The am pl ifi catio n stage co u ld be :' par! o f the m ic roc om pu ter packag e a separa le , .t hird lev e! between t hc rnicro-,co rnputer an d th e llu ic t pow er valve , or pa rt of the flu id po wer va lve package , for exa rn -p lc, by addi ng i t i o hcsolcn oid opc rator. Bcc ause them ajori ty of so leno id valves are di g ital dev ices , th eyw ou ld be m ost com patible w ith m icrocornpu tcr ou t-pu l signals .E lec tr ohydraul ic servovalves are analog de vicesand therefor e harder to in terf ace t han solenoid

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