{"id":15,"date":"2007-02-22T10:01:11","date_gmt":"2007-02-22T10:01:11","guid":{"rendered":"http:\/\/new.flugmodel.is\/?page_id=15"},"modified":"2013-06-23T22:33:38","modified_gmt":"2013-06-23T22:33:38","slug":"pcm-og-ppm","status":"publish","type":"page","link":"https:\/\/www.flugmodel.is\/?page_id=15","title":{"rendered":"PCM og PPM"},"content":{"rendered":"

H\u00f6fundur: Dave Edwards af s\u00ed\u00f0u hans Dave’s RC Flight : http:\/\/davesrcflight\/mysite.wanadoo-members.co.uk<\/a>
\n Þ\u00fdtt me\u00f0 leyfi h\u00f6fundar: Gu\u00f0j\u00f3n \u00d3lafsson <\/p>\n

\u00cd \u00feessari grein ver\u00f0a fyrirb\u00e6rin PPM (enska Pulse Position Modulation) og PCM (e. Pulse Code Modulation) \u00fatsk\u00fdr\u00f0, sagt fr\u00e1 hvernig \u00feau virka og gefnar hugmyndir um kosti og galla \u00feeirra. Einnig er l\u00edtillega tala\u00f0 um PCM \u00f6ryggi (e. fail-safe) og hvernig \u00fea\u00f0 virkar.<\/p>\n

Verki\u00f0 sem fjarst\u00fdrigr\u00e6jurnar vinna er a\u00f0 sko\u00f0a stanslaust \u00ed hva\u00f0a st\u00f6\u00f0u st\u00fdripinnarnir eru og yfirf\u00e6ra \u00fe\u00e1 st\u00f6\u00f0u \u00e1 st\u00fdrisarmana \u00e1 serv\u00f3unum. Þa\u00f0 er vanalegt a\u00f0 hafa fr\u00e1 tveim upp \u00ed \u00e1tta r\u00e1sir \u00ed notkun \u00ed einu, \u00feannig a\u00f0 t\u00e6ki\u00f0 \u00fearf a\u00f0 samstilla st\u00f6\u00f0u margra pinna og takka vi\u00f0 marga serv\u00f3arma. <\/p>\n

N\u00e6stum \u00fev\u00ed \u00f6ll fjarst\u00fdrit\u00e6ki sem notu\u00f0 eru fyrir flugv\u00e9lar nota t\u00ed\u00f0nim\u00f3tun (FM e. Frequency Modulation) og me\u00f0 henni er sett upp ein st\u00f6k \u201e\u00fatvarpstenging\u201c \u00e1 milli fjarst\u00fdringarinnar (TX) og m\u00f3ttakarans (RX). \u00d6nnur ger\u00f0 m\u00f3tunar, PPM e\u00f0a PCM er s\u00ed\u00f0an notu\u00f0 til a\u00f0 bera \u201est\u00fdrir\u00e1sirnar\u201c \u00e1 \u00fatvarpstengingunni, ein r\u00e1s fyrir hvert serv\u00f3. PPM e\u00f0a PCM er nota\u00f0 til a\u00f0 sam\u00fe\u00e6tta (multiplex) st\u00fdrir\u00e1sirnar \u00ed eina \u00fatvarpsr\u00e1s. Hver framlei\u00f0andi fjarst\u00fdringa notar s\u00edna eigin a\u00f0fer\u00f0 vi\u00f0 a\u00f0 b\u00faa til PCM og l\u00fdsingin h\u00e9r \u00e1 eftir gefur grunn hugmynd a\u00f0 \u00fev\u00ed hvernig \u00feetta virkar.<\/p>\n

Þa\u00f0, hvernig st\u00fdrir\u00e1sir eru sam\u00fe\u00e6ttar, er nokkurn vegin eins \u00ed b\u00e6\u00f0i PCM og PPM. Sta\u00f0setning pinnans fyrir r\u00e1s 1 \u00ed fjarst\u00fdringunni er lesin og h\u00fan send til m\u00f3ttakarans. Strax \u00e1 eftir er sta\u00f0setning pinna 2 lesin og \u00fe\u00e6r uppl\u00fdsingar sendar. Þetta er endurteki\u00f0 me\u00f0 allar r\u00e1sirnar, hverja \u00e1 eftir annarri. Þetta gerist nokku\u00f0 \u00f6rt, svo a\u00f0 svo vir\u00f0ist sem serv\u00f3in viti strax ef einn pinni e\u00f0a fleiri hafa veri\u00f0 hreyf\u00f0ir, en \u00ed raun eru \u00feessar uppl\u00fdsingar ekki sendar nema 25 til 50 sinnum \u00e1 sek\u00fandu (eftir framlei\u00f0anda og g\u00e6\u00f0um t\u00e6kja). <\/p>\n\n\n\n
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A\u00f0fer\u00f0in til a\u00f0 senda uppl\u00fdsingar um sta\u00f0setningu pinnans er me\u00f0 \u00fev\u00ed a\u00f0 breyta lengd p\u00falsins. P\u00fals me\u00f0 lengdina 1,5ms (millisek\u00fanda) mi\u00f0jar serv\u00f3i\u00f0. Ef p\u00falsinn er styttur \u00ed 1,0ms, \u00fe\u00e1 fer serv\u00f3i\u00f0 alla lei\u00f0 \u00ed a\u00f0ra \u00e1ttina en ef hann er lengdur \u00ed 2,0ms, \u00fe\u00e1 fer \u00fea\u00f0 alla lei\u00f0 \u00ed hina \u00e1ttina. A\u00f0rar sta\u00f0setningar falla \u00fe\u00e1 \u00e1 \u00feetta bil. Einhver munur er \u00e1 bilum \u00e1 milli framlei\u00f0anda serv\u00f3a, en hann er svo l\u00edtill a\u00f0 \u00fea\u00f0 skipti ekki m\u00e1li. <\/p>\n

Sko\u00f0um \u00feetta aftur. Sendirinn les sta\u00f0setningu pinna 1 og p\u00fals af r\u00e9ttri lengd er b\u00fainn til, sem segir serv\u00f3inu hvert \u00fea\u00f0 \u00e1 a\u00f0 fara. Strax \u00e1 eftir er sta\u00f0setning pinna 2 lesin og annar p\u00fals af vi\u00f0eigandi lengd b\u00fainn til og svona gengur \u00fea\u00f0 fyrir allar r\u00e1sir st\u00fdringarinnar.<\/p>\n

N\u00fa komum vi\u00f0 a\u00f0 muninum \u00e1 PPM og PCM. \u00cd PPM er hverjum p\u00fals bara skeytt aftan vi\u00f0 \u00feann n\u00e6sta \u00e1 undan. Þetta \u00fe\u00fd\u00f0ir a\u00f0 \u00ed tveggja r\u00e1sa st\u00fdringu eru tveir p\u00falsar af vi\u00f0eigandi lengd sendir til m\u00f3ttakarans, s\u00ed\u00f0an kemur langt bil og svo aftur tveir p\u00falsar. \u00cd 10 r\u00e1sa fjarst\u00fdringu eru sendir 10 p\u00falsar og s\u00ed\u00f0an stutt bil og svo 10 p\u00falsar aftur. Þessi r\u00f6\u00f0 p\u00falsa og bila , e\u00f0a \u201eramma\u201c er send m\u00f6rgum sinnum \u00e1 sek\u00fandu. M\u00f3ttakarinn ver\u00f0ur a\u00f0 finna \u00fat hva\u00f0a p\u00fals fer \u00e1 hva\u00f0a serv\u00f3 og \u00fea\u00f0 getur hann gert me\u00f0 \u00fev\u00ed a\u00f0 b\u00ed\u00f0a eftir bilinu. Fyrsti p\u00falsinn eftir bili\u00f0 er fyrir serv\u00f3 1, n\u00e6sti p\u00fals fyrir serv\u00f3 2 o.s.frv. Þess vegna er \u00feetta kalla\u00f0 \u00e1 ensku Pulse Position Modulation, e\u00f0a m\u00f3tum me\u00f0 sta\u00f0setningu p\u00falsa. Lengd p\u00falsins segir serv\u00f3inu \u00ed hva\u00f0a st\u00f6\u00f0u \u00fea\u00f0 \u00e1 a\u00f0 fara. <\/p>\n\n\n\n
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\u00cd PCM st\u00fdringu gerist anna\u00f0. Þegar fjarst\u00fdringin les sta\u00f0setningu pinnanna, \u00fe\u00e1 b\u00fdr h\u00fan ekki til p\u00falsa af mismunandi lengd fyrir serv\u00f3in, heldur b\u00fdr h\u00fan oftast til 10 bita k\u00f3\u00f0a (fer eftir framlei\u00f0endum og g\u00e6\u00f0um t\u00e6kja), \u00fe.e. 10 bita b\u00e6ti uppl\u00fdsinga \u00e1 forminu 1 og 0. Hver pinni er lesinn \u00ed r\u00f6\u00f0 og vi\u00f0eigandi 10 bita k\u00f3\u00f0i fyrir pinnana b\u00fainn til. S\u00ed\u00f0an eru \u00feessir k\u00f3\u00f0ar settir saman \u00ed ramma og ramminn sendur til m\u00f3ttakarans m\u00f6rgum sinnum \u00e1 sek\u00fandu. M\u00f3ttakarinn \u00fearf n\u00fa a\u00f0 \u00e1kve\u00f0a hva\u00f0a 10 bita k\u00f3\u00f0i er fyrir serv\u00f3 1, lesa hann og b\u00faa til p\u00fals af vi\u00f0eigandi lengd fyrir serv\u00f3i\u00f0. Hann gerir \u00feetta fyrir hverja einustu r\u00e1s. <\/p>\n

Hvers vegna gera \u00feetta svona fl\u00f3ki\u00f0? Vi\u00f0 fyrstu s\u00fdn vir\u00f0ist sem vi\u00f0 h\u00f6fum gert \u00feetta bara verra, vegna \u00feess a\u00f0 me\u00f0 10 bitum eru bara 1024 m\u00f6guleikar og \u00fear me\u00f0 bara 1024 m\u00f6gulegar sta\u00f0setningar fyrir serv\u00f3i\u00f0. Me\u00f0 PPM eru fr\u00e6\u00f0ilega til \u00f3endanlega margar sta\u00f0setningar fyrir serv\u00f3i\u00f0. Veiki hlekkurinn \u00e1 milli pinnanna \u00e1 fjarst\u00fdringunni og serv\u00f3anna \u00ed flugv\u00e9linni er \u00fatvarpsbylgjan. Me\u00f0 PPM er \u00fatvarpsbylgjan hli\u00f0r\u00e6n (e. analogue), en me\u00f0 PCM er h\u00fan stafr\u00e6n (e. digital). Eins og \u00ed flestum tilfellum \u00fear sem stafr\u00e6n g\u00f6gn eru notu\u00f0, \u00fe\u00e1 aukast g\u00e6\u00f0in. Me\u00f0 PPM, \u00fe\u00e1 \u00fearf m\u00f3ttakarinn a\u00f0 m\u00e6la lengd p\u00falsanna, \u00fe.e. hven\u00e6r p\u00fals byrjar og hven\u00e6r hann endar. Ef merki\u00f0 fr\u00e1 fjarst\u00fdringunni er gott og sterkt, \u00fe\u00e1 gerir hann \u00feetta n\u00e1kv\u00e6mlega og \u00e1n mikilla vandkv\u00e6\u00f0a, en ef merki\u00f0 er veikt e\u00f0a \u00fea\u00f0 koma inn truflanir, \u00fe\u00e1 getur hann \u00fea\u00f0 ekki. Vi\u00f0 h\u00f6fum allir s\u00e9\u00f0 titring \u00ed serv\u00f3um sem koma vegna \u00feess a\u00f0 m\u00f3ttakarinn getur ekki reikna\u00f0 \u00fat upphaf og endi p\u00falsanna n\u00f3gu vel. Me\u00f0 PCM, \u00fearf lesarinn \u00ed m\u00f3ttakaranum ekki a\u00f0 finna neitt nema r\u00f6\u00f0 1 og 0, \u00fe.e. P\u00fals e\u00f0a ekki og hann veit n\u00e1kv\u00e6mlega (e\u00f0a svo til) hven\u00e6r hann \u00e1 a\u00f0 leita \u00feeirra. Þegar hann er ekki a\u00f0 leita a\u00f0 p\u00fals, \u00fe\u00e1 skiptir ekki m\u00e1li hva\u00f0 er a\u00f0 gerast me\u00f0 merki\u00f0 fr\u00e1 sendinum. Me\u00f0 PCM getur m\u00f3ttakarinn lesi\u00f0 \u00far uppl\u00fdsingunum me\u00f0 mikilli n\u00e1kv\u00e6mni \u00fe\u00f3 verulegar truflanir s\u00e9u \u00ed umhverfinu. Lesarinn \u00fearf \u00fe\u00f3 a\u00f0 geta s\u00e9\u00f0 hven\u00e6r uppl\u00fdsingarnar sem hann f\u00e6r eru gildar e\u00f0a ekki. Þetta er gert me\u00f0 svok\u00f6llu\u00f0um rammavart\u00f6lum (e. frame check sums) Einf\u00f6ld a\u00f0fer\u00f0 til a\u00f0 gera \u00feetta er a\u00f0 sendirinn telur heildarfj\u00f6lda t\u00f6lunnar 1 \u00ed hverjum ramma og sendir \u00feessa t\u00f6lu til m\u00f3ttakarans, sem aftur telur fj\u00f6lda t\u00f6lunnar 1. Ef t\u00f6lurnar tv\u00e6r eru \u00fe\u00e6r s\u00f6mu, \u00fe\u00e1 er ekki l\u00edklegt a\u00f0 um villu s\u00e9 a\u00f0 r\u00e6\u00f0a. Þa\u00f0 eru til betri a\u00f0fer\u00f0ir til a\u00f0 gera \u00feetta sem gefa n\u00e1kv\u00e6mari ni\u00f0urst\u00f6\u00f0ur, en hugmyndin er s\u00fa sama. M\u00f3ttakarinn hunsar einstaka ramma me\u00f0 ranga vart\u00f6lu og notar bara aftur s\u00ed\u00f0asta heila ramma sem hann f\u00e9kk.<\/p>\n\n\n\n\n
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Futaba 6EXA \u2013 frr\u00e1b\u00e6r st\u00fdring fyrir byrjendur<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n

Þannig gefur PCM mun \u00f6ruggari tengingu \u00e1 milli fjarst\u00fdringar og m\u00f3ttakara og \u00fatilokar allar litlu truflanirnar sem vi\u00f0 getum fengi\u00f0 me\u00f0 PPM. Þa\u00f0 er aftur \u00e1 m\u00f3ti sm\u00e1 vandam\u00e1l \u2013 PCM gefur fullkomna stj\u00f3rn \u00fe\u00f3 a\u00f0 m\u00f3ttakarinn s\u00e9 a\u00f0 f\u00e1 frekar dauft merki fr\u00e1 sendinum, en \u00fea\u00f0 eru takm\u00f6rk fyrir \u00feessu og handan \u00feeirra hefur ma\u00f0ur enga stj\u00f3rn. Stj\u00f3rnin er fullkomin e\u00f0a engin og \u00fea\u00f0 koma engir kippir e\u00f0a hopp sem vara mann vi\u00f0. Fjarl\u00e6g\u00f0in \u00e6tti a\u00f0 vera jafn mikil og \u00feegar ma\u00f0ur notar PPM, en \u00feegar fari\u00f0 er \u00fat fyrir hana, \u00fe\u00e1 er stj\u00f3rnin horfin \u00e1n fyrirvara! Til a\u00f0 gir\u00f0a fyrir \u00feennan galla var \u00f6ryggi (e. fail-safe) b\u00fai\u00f0 til og er selt me\u00f0 sem aukab\u00fana\u00f0ur, en \u00e9g hef ekki tr\u00fa \u00e1 a\u00f0 sl\u00edkt \u00f6ryggi virki \u00e1 neina fjarst\u00fdr\u00f0a flugv\u00e9l. Ef \u00fe\u00fa tapar sendingu \u00e1 milli fjarst\u00fdringar og m\u00f3ttakara, \u00fe\u00e1 er engin lei\u00f0 til a\u00f0 flugv\u00e9lin lendi sj\u00e1lf \u00f6rugglega. Ekkert flugm\u00f3del er \u00f6ruggt fyrr en \u00fea\u00f0 er hreyfingarlaust \u00e1 j\u00f6r\u00f0u ni\u00f0ri. A\u00f0 \u00feessu s\u00f6g\u00f0u, \u00fe\u00e1 tel \u00e9g samt a\u00f0 \u00fea\u00f0 s\u00e9 gott a\u00f0 hafa PCM \u00f6ryggi, en nafni\u00f0 \u00e1 \u00fev\u00ed hlj\u00f3mar betur en \u00fea\u00f0 virkar \u2013 ekki b\u00faast vi\u00f0 of miklu af \u00fev\u00ed. <\/p>\n

(Þa\u00f0 m\u00e1 taka fram h\u00e9r a\u00f0 \u00fe\u00fd\u00f0andinn hefur alltaf liti\u00f0 svo \u00e1 a\u00f0 \u00f6ryggi e\u00f0a \u201efail-safe\u201c s\u00e9 ekki hanna\u00f0 til a\u00f0 auka \u00f6ryggi m\u00f3delsins e\u00f0a bjarga \u00fev\u00ed fr\u00e1 skemmdum, heldur til a\u00f0 koma \u00ed veg fyrir e\u00f0a l\u00e1gmarka eins og kostur er \u00fe\u00e6r skemmdir og\/e\u00f0a mei\u00f0sl sem stj\u00f3rnlaust m\u00f3del getur valdi\u00f0.)<\/p>\n

PCM \u00d6ryggi.<\/h3>\n

Þeir sem hafa PCM st\u00fdringu geta stillt inn fyrirfram \u00e1kve\u00f0nar sta\u00f0setningar fyrir hvert serv\u00f3 sem \u00feau eiga a\u00f0 fara \u00ed ef sambandi\u00f0 \u00e1 milli fjarst\u00fdringar og m\u00f3ttakara fellur ni\u00f0ur vegna \u00feess a\u00f0 merki\u00f0 er of dauft e\u00f0a vegna truflana. Þetta er venjulega gert \u00ed tveim hlutum:<\/p>\n

\u00cd fyrsta lagi er SERV\u00d3ST\u00d6ÐVUN (e. servo hold), sem fer \u00ed gang um lei\u00f0 og merki\u00f0 fer a\u00f0 hverfa og gildir \u00ed allt a\u00f0 eina sek\u00fandu (\u00e1 sumum t\u00e6kjum er \u00feetta stillanlegt) Þetta heldur s\u00ed\u00f0ustu st\u00f6\u00f0u \u00e1 \u00f6llum serv\u00f3um og \u00fatilokar a\u00f0 mestu allar hreyfingar vegna truflana sem \u00ed flestum tilfellum eru stuttar.<\/p>\n

N\u00e6sta \u00d6RYGGI kemur inn \u00feegar merki\u00f0 hefur veri\u00f0 veikt e\u00f0a fjarverandi lengur en t\u00edminn sem gefinn er fyrir SERV\u00d3ST\u00d6ÐVUN og setur \u00f6ll serv\u00f3 \u00ed fyrirfram \u00e1kve\u00f0nar st\u00f6\u00f0ur sem notandinn hefur stillt inn me\u00f0 fjarst\u00fdringunni. Ef engar st\u00f6\u00f0ur hafa veri\u00f0 stilltar, \u00fe\u00e1 er venjulega sj\u00e1lfgefi\u00f0 a\u00f0 serv\u00f3 halda \u00feeim st\u00f6\u00f0um sem \u00feau eru \u00ed, \u00fe.e. halda serv\u00f3st\u00f6\u00f0vun \u00e1fram. Ef inngj\u00f6fin er undanskilin, \u00fe\u00e1 er \u00feetta l\u00edklega besta a\u00f0ger\u00f0in, en hver og einn ver\u00f0ur a\u00f0 \u00edhuga vandlega hvernig \u00feetta er nota\u00f0.<\/p>\n\n\n\n\n
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Futaba 9C ser\u00edan \u2013 magna\u00f0ur sendir me\u00f0 \u00f3t\u00e6mandi m\u00f6guleika<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n

\u00c1 me\u00f0an \u00e1 SERV\u00d3ST\u00d6ÐVUN og \u00d6RYGGI stendur er m\u00f3ttakarinn stanslaust a\u00f0 leita a\u00f0 merki og um lei\u00f0 og hann f\u00e6r gildan gagnaramma \u00fe\u00e1 eru serv\u00f3in aftur undir stj\u00f3rn fjarst\u00fdringarinnar. Þannig a\u00f0 ef stuttar truflanir hindra merki\u00f0 \u00e1 milli fjarst\u00fdringar og m\u00f3ttakara t.d. ef elding sl\u00e6r ni\u00f0ur \u00ed fjarska, \u00fe\u00e1 virkar SERV\u00d3ST\u00d6ÐVUN ekki nema \u00ed brot \u00far sek\u00fandu.<\/p>\n

Uppl\u00fdsingarnar fyrir \u00f6ryggis-sta\u00f0setningar serv\u00f3anna eru stanslaust sendar til m\u00f3ttakarans \u00e1 milli ramma \u00feegar engar a\u00f0rar sendingar eru \u00ed gangi, en vegna \u00feess a\u00f0 sendingar eru venjulega \u00f6rar og miki\u00f0 a\u00f0 gera, \u00fe\u00e1 geta li\u00f0i\u00f0 nokkrar m\u00edn\u00fatur fr\u00e1 \u00fev\u00ed kveikt er, a\u00f0 allar \u00f6ryggissta\u00f0setningar eru komnar til m\u00f3ttakarans. Uppl\u00fdsingar um SERV\u00d3ST\u00d6ÐVUN eru aftur \u00e1 m\u00f3ti \u00ed m\u00f3ttakaranum fr\u00e1 upphafi. Þessi t\u00f6f er venjulega ekki vandam\u00e1l ef t\u00edminn fr\u00e1 \u00fev\u00ed ma\u00f0ur kveikir \u00e1 og \u00feanga\u00f0 til ma\u00f0ur tekur \u00e1 loft er tekinn me\u00f0 \u00ed reikninginn. \u00cd Futaba FF9 sendinum er snj\u00f6ll lausn sem virkar \u00feannig a\u00f0 ef ma\u00f0ur kveikir \u00e1 m\u00f3takaranum fyrst og s\u00ed\u00f0an \u00e1 sendinum, \u00fe\u00e1 sendir hann fyrst af \u00f6llu \u00f6ryggisstillingarnar fyrstu 10 sek\u00fandurnar. S\u00ed\u00f0an endurstillir hann \u00fe\u00e6r einu sinni \u00e1 hverri m\u00edn\u00fatu. PCM t\u00e1knmyndin \u00ed efra horninu vinstra megin blikkar og \u00e9g tel \u00fea\u00f0 vera merki um a\u00f0 \u00f6ryggisramminn hafi veri\u00f0 sendur, \u00fe\u00f3 \u00e9g hafi hvergi fengi\u00f0 \u00fea\u00f0 sta\u00f0fest. A\u00f0rar st\u00fdringar hafa hugsanlega svipa\u00f0a eiginleika.<\/p>\n

Hver framlei\u00f0andi hefur sj\u00e1lfgefnar stillingar fyrir \u00f6ryggi sinna t\u00e6kja og \u00fea\u00f0 er mikilv\u00e6gt a\u00f0 notendur skilji n\u00e1kv\u00e6mlega hva\u00f0 muni gerast \u00ed m\u00f3delum \u00feeirra \u00feegar \u00fea\u00f0 fer \u00ed gang. <\/p>\n\n\n\n\n
<\/a><\/td>\n<\/tr>\n
Langt \u00e1 undan sinni framt\u00ed\u00f0: Futaba 14-MZ:<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n

Kostir PCM.<\/h3>\n