Bài 1

Bi 1. KHI NIM CHUNG V O LNGV tr ca o lngGii thiuo lng l s so snh i lng cha bit (i lng o) vi i lng c chun ha (i lng mu hoc i lng chun). Nh vy cng vic o lng l ni thit b o vo h thng c kho st kt qu o cc i lng cn thit trn thit b o.Trong thc t kh xc nh tr s thc cc i lng o. V vy tr s c o cho bi thit b o c gi l tr s tin cy c. Bt k i lng o no cng b nh hng nhiu thng s. Do kt qu o t khi phn nh ng tr s tin cy c. Cho nn c nhiu h s nh hng trong o lng lin quan n thit b o. Ngoi ra c nhng h s khc lin quan n con ngi s dng thit b o. Nh vy chnh xc ca thit b o c din t bi hnh thc sai s.Mc tiu: - Trnh by c cc sai s trong k thut o lng, nguyn nhn v bin php phng trnh gim sai s trong o lng. - C thc trch nhim v bo qun thit b dng cNi dung chnh: 1. o lng- Mc tiu:Trnh by c khi nim, cc tiu chun qui nh trong o lng.- o lng in t: l o lng m trong i lng cn o c chuyn i sang dng tn hiu in mang thng tin o v tn hiu in c x l v o lng bng cc dng c v mch in t.- o lng l mt qu trnh nh gi nh lng i tng cn o c kt qu bng s so vi n v. Vd: U= 380v, U in p, 380 con s, V n v o.Vi nh ngha trn th o lng l qu trnh thc hin ba thao tc chnh: - Bin i tn hiu v tin tc. - So snh vi n v o hoc so snh vi mu trong qu trnh o lng. - Chuyn n v, m ho c kt qu bng s so vi n v. Cn c vo vic thc hin cc thao tc ny ta c cc phng php v h thng o lng khc nhau. 1.1. chnh xc v mc chnh xc - chnh xc l tiu chun quan trng nht ca thit b o. Bt k mt php o no u c sai lch so vi i lng ng

chnh xc tng i: V d: in p hai u in tr c tr s tin cy c l 50V. Dng vn k o c 49V. Nh vy: chnh xc tng i:

Mc chnh xc l chc chn ca thit b vi gi tr ca i lng ng ra khi ta a mt i lng u vo.

Trong : Xn - gi tr o ln th n

- gi tr trung bnhV d: Cho bng 2.1c gi tr nhn c 10 ln o, tnh s chnh xc ca ln o th 6.S ln oGi tr o c Xn

198

2101

3102

497

5101

6100

7103

898

9106

1099

Bng 2.1Gi tr trung bnh ca 10 ln o c tnh nh sau:

chnh xc ca mt php o v mc chnh xc ph thuc vo rt nhiu yu t nh cht lng ca thit b o, ngi s dng cc thit b v yu t mi trng. Cp chnh xc ca dng c o l c trng tng qut ca n, c quy nh bi cc tiu chun quc gia v quc t v cc gii hn ca sai s o c bn v th yu, cng nh v cc thng s khc c nh hng n chnh xc ca cc dng c o. nh gi chnh xc ca ng h o in, ngi ta dng khi nim cp chnh xc ca dng c o. Cp chnh xc c th k hiu bng ch hoc s theo cc quy nh xc nh. Cp chnh xc c biu din bi biu thc 2.3

Dng c o in c 8 cp chnh xc sau: 0,05, 0,1, 0,2, 0,5, 1,0, 1,5, 2,5 v 5. Cp chnh xc c ghi trn mt ca ng h o. Bit cp chnh xc ta c th tnh c sai s tuyt i ln nht cho php ca php o:

V d: Mt vn-k c ghi cp chnh xc l 1, ngha l gii hn sai s ca n cho tm do l 1%.V d: Mt miliampe k c thang ln nht Amax = 100mA, cp chnh xc l 2,5. Sai s tuyt i ln nht cho php s l:

Vt qu gi tr 2,5mA ny ng h s khng cn t cp chnh xc 2,5 na. V d: Mt vn k c cp chnh xc 1,5 khi dng thang o 50V mc sai s cho php ln nht l: Xmax = 1,5. 50 / 100 = 0,75V Nhng nu dng thang o 100V th sai s tuyt i ln nht cho php li l Xmax = 1,5. 100 / 100 = 1,5V 1.2. Cc tiu chun Khi s dng thit b o lng, chng ta mong mun thit b c chun ha (calibzate) khi c xut xng ngha l c chun ha vi thit b o lng chun (standard). Vic chun ha thit b o lng c xc nh theo bn cp nh sau: Cp 1: Chun quc t (International standard) - cc thit b o lng cp chun quc t c thc hin nh chun ti Trung tm o lng quc t t ti Paris (Php), cc thit b o lng chun ha cp 1 ny theo nh k c nh gi v kim tra li theo tr s o tuyt i ca cc n v c bn vt l c hi ngh quc t v o lng gii thiu v chp nhn. Cp 2: Chun quc gia - cc thit b o lng ti cc Vin nh chun quc gia cc quc gia khc nhau trn th gii c chun ha theo chun quc t v chng cng c chun ha ti cc vin nh chun quc gia. Cp 3: Chun khu vc - trong mt quc gia c th c nhiu trung tm nh chun cho tng khu vc (standard zone center). Cc thit b o lng ti cc trung tm ny ng nhin phi mang chun quc gia (National standard). Nhng thit b o lng c nh chun ti cc trung tm nh chun ny s mang chun khu vc (zone standard). Cp 4: Chun phng th nghim - trong tng khu vc s c nhng phng th nghim c cng nhn chun ha cc thit b c dng trong sn xut cng nghip. Nh vy cc thit b c chun ha ti cc phng th nghim ny s c chun ha ca phng th nghim. Do cc thit b o lng khi c sn xut ra c chun ha ti cp no th s mang cht lng tiu chun o lng ca cp .Cn cc thit b o lng ti cc trung tm o lng, vin nh chun quc gia phi c chun ha v mang tiu chun cp cao hn. V d phng th nghim phi trang b cc thit b o lng c tiu chun ca chun vng hoc chun quc gia, cn cc thit b o lng ti vin nh chun quc gia th phi c chun quc t. Ngoi ra theo nh k c t ra phi c kim tra v chun ha li cc thit b o lng.1.3 K thut o Php o cn phi c thc hin mt cch cn thn v s th hin cc s liu o phi ph hp sau khi c tnh ton n cc gii hn v nhy, chnh xc v kh nng ca thit b o. i khi s o c th ng nhng nu th hin kt qu sai, ngi ta c th hiu mch ang tt l c sai hng v ngc li. Hn na, vic s dng thit b o sai c th to ra cc nguy him cho s an ton ca ngi o v thit b o. Cc k thut o sau y cn phi tun theo khi o th hay thc hin cc php o trong vic chn on h hng, sa cha v bo dng cc thit b in t. 1.3.1. Ni thit b n ngun in li, tt hn ht l thng qua u ni ba chn, v thc hin bt ngun cho h thng theo trnh t sau: Cc im quan trng c chuyn mch ON u tin, tip theo l ng [ON] ngun cung cp, sau ng [ON] thit b o, v cui cng ng ngun cung cp cho mch cn o th. Khi tt (chuyn mch sang OFF), th trnh t l ngc li, th trnh t phi c thc hin ngc li: trc tin tt ngun cung cp cho mch cn o, tip theo l tt thit b o, sau tt ngun cung cp v cui cng l ngt in li. iu ny s bo v thit b o v thit b cn o khi cc xung qu . Khng hn hay tho mi hn linh kin khi ngun cung cp ang bt. 1.3.2. Bt k lc no cng phi tt thit b o cn nu thit b o c chuyn mch sang ng [ON] ngay sau th cn phi c khong thi gian ng k cho php cc t x in. 1.3.3. Cc thit b o th cn phi c ni t mt cch hiu qu gim thiu cc bin thin ca nhiu. 1.3.4. Chn thang o ph hp theo tham s cn o, tu theo gi tr o yu cu. Nu khng bit gi tr o yu cu, th hy chn thang o cao nht v sau gim dn thang o cho ph hp, trnh cho thit b o b qu ti v b h hng. Thang o c chn cui cng s cho kt qu o gn vi lch ln nht c th c i vi php o in p v dng in, v gn mc trung bnh i vi php o in tr, c chnh xc ti u i vi h thng o. 1.3.5. Khi gi tr o bng 0, th ng h o cn phi ch th bng 0, nu khng th cn phi c chnh v 0 cho ph hp.1.3.6. Khng s dng cc u que o nhn c kch thc ln v chng c th gy ngn mch. Cc u que o cn phi nhn nht nu c th c. 1.3.7. iu quan trng ca vic ni cc im o th: cc hng ch to thit b thng quy nh cc im o th ti cc v tr thun tin trn bng mch in. in tr, mc in p DC, mc in p tn hiu v cc dng sng ca tn hiu s c quy nh cho mi im o th. (im o th thng l cc lp ng trn bng mch in). Cc im o th s c m tt nht trnh nguy him qu ti cho mch cn o. Cc im o th c thit k bi cc nh chuyn mn c kinh nghim, khi cn kho st thit b, khng c b qua cc im o th nh vy trong qu trnh sa cha. 1.3.8. Thng thng cc u que o mang du dng v m i vi cc php o in p v dng in trong mch. Ngun pin bn trong ng h o s c cc tnh ngc li, tc l u que o m ca ngun pin trong ng h o s c ni u que c nh du dng (que o mu en) v ngc li th hin hnh 2.1. Thc t ny cn phi nh khi o th cc diode, cc t in phn, cc transistor v cc vi mch.

Hnh 2.11.3.9. Nu cc im o th l khng cho trc, hoc nu cc php o l c thc hin ti cc im khc nhau, th cn phi ch cc im nh sau: a) Khi o cc in p DC, php o cn phi c thc hin ngay ti cc linh kin thc t, v i vi vi mch o trc tip trn cc chn. b) S dng u kp o th IC thc hin cc php o trn cc chn ca IC. c) Khi cn o tn hiu trn mch in trong bng mch, nn kp u o trn chn ca cu kin in t c ni vi ng mch in. d) Khi thc hin cc php o trn bng mch, cn phi m bo rng cc IC khng b in tch tnh o thit b o. e) Khi kim tra h mch, hy tho mt u ca cu kin in t ri thc hin php o. Nu cu kin khng c tho mt u, th cc cu kin khc mc song song vi cu kin nghi ng s ch th khng ng tin cy. C th kim tra cu kin nghi ng bng cu o. Khi tho mi hn ra khi bng mch in l kh khn th c th ct ng mch in lin quan, do d dng hn li vt ct hn so vi vic tho mi hn cu kin o ri hn li, nhng khi hn li vt ct, cn phng mi hn b nt khng xy ra. f) Vic tho v hn IC l mt qu trnh kh phc tp cn phi ht sc cn thn. Cn phi tho mi hn cho IC o th ch khi xc minh chc chn cc php o trn bng mch cho thy IC thc s hng. 1.3.10. Cn phi tun theo cc luu v an ton m bo an ton cho ngi o, thit b o. 1.3.11. Cn phi tun theo cc ch dn t hng dn s dng thit b o th, cng nh trnh t o th. 1.3.12. Cn phi nghin cu k cch vn hnh thit b o thc hin php o v cn phi tun theo tt c cc im lu c cp.1.4 Dch s liuKhi thc hin php o, iu quan trng l s liu nhn c c ng vi gi tr ca linh kin cn o t nhn ra nguyn nhn ca s khc bit gia kt qu o c v kt qu d kin. Nu kt qu thu c, khc vi d kin th cng c th l dng c o b hng, hay b phn c s liu b hng hoc km, s hiu bit v cc thng s o cha y , 2. Sai s trong o lng:- Mc tiu: Xc nh c cc nguyn nhn gy ra sai s v nhng nh hng cu n trong o lng.L chnh lch gia kt qu o v gi tr thc ca i lng o. N ph thuc vo nhiu yu t nh: thit b o, phng thc o, ngi o - Nguyn nhn gy sai sKhng c php o no l khng c sai s. vn l khi o phi chn ng phng php thch hp, cng nh cn cn thn, thnh tho khi thao tc , hn ch sai s cc kt qu o sao cho n mc t nht.Cc nguyn nhn gy ra sai s th c nhiu, ngi ta phn loi nguyn nhn gy ra sai s l o cc yu t khch quan v ch quan gy nn. Cc nguyn nhn khch quan v d: dng c o lng khng hon ho, i lng o c b can nhiu nn khng hon ton c n nhNguyn Nhn ch quan, v d: o thiu thnh tho trong thao tc, phng php tin hnh o khng hp l V c cc nguyn nhn v ta cng khng th tuyt i loi tr hon ton c nh vy nn kt qu ca php o no cng ch cho gi tr gn ng. Ngoi vic c gng hn ch sai s o n mc thp nht, ta cn cn nh gi c xem kt qu o c sai s n mc no.- Phn loi sai sMi thit b o c th cho chnh xc cao, nhng c th c cc sai s o cc hn ch ca thit b o, do cc nh hng ca mi trng, v cc sai s o ngi o khi thu nhn cc s liu o. Cc loi sai s c ba dng: Sai s ch quan (Sai s th), sai s h thng, sai s ngu nhin. 2.1 Sai s ch quan (Cc sai s th): c th quy cho gii hn ca cc thit b o hoc l cc sai s o ngi o. Gii hn ca thit b o: V d nh nh hng qu ti gy ra bi mt voltmeter c nhy km. Voltmeter nh vy s r dng ng k t mch cn o v v vy s t lm gim mc in p chnh xc. 2.2 Sai s h thng: Sai lch c cng dng, khng thay i c gi l sai s h thng. V d: Gi s dng thc 20m o mt on thng no , nhng chiu di tht ca thc lc li l 20,001m. Nh vy trong kt qu mt ln ko thc c cha 1mm, sai s ny c gi l sai s h thng. C hai loi sai s: Sai s ca thit b o v sai s do mi trng o. Sai s ca thit b o: l do ma st cc b phn chuyn ng ca h thng o hay do ng sut ca l xo gn trong c cu o l khng ng u. V d, kim ch th c th khng dng mc 0 khi khng c dng chy qua ng h. Cc sai s khc l o chun sai, hoc do ao ng ca ngun cung cp, do ni t khng ng, v ngoi ra cn do s gi ho ca linh kin. Cng l loi sai s tng t sai s c, nhng khng phi do mt, m do s hin th ca cc thit b o k thut s. Cc gi tr m chng c th cho hin th trn mn hnh ch l cc gi tr gin on (v d: card chuyn t analog tn hiu tng t sang digital tn hiu s, nu l loi 8 bits th ch c th hin th c 28=256 mc khc nhau), nu kt qu o khng trng vi cc mc th s c lm trn. Ngoi ra, khi i lng cn o c s dao ng ln hn khong cch gia hai mc tn hiu s cnh nhau, ta cn thy cc con s hin th thay i lin tc, vic chn gi tr no l ty ngi s dng. Sai s do mi trng o: l sai s do cc iu kin bn ngoi nh hng n thit b o trong khi thc hin php o. S bin thin v nhit , m, p sut, t trng, c th gy ra cc thay i v dn in, r, cch in, in cm v in dung. Bin thin v t tnh c th o thay i m men quay (tc lch). Cc thit b o tt s cho cc php o chnh xc khi vic che chn cc dng c n mc ti a, s dng cc mn chn t trng, v. v... Cc nh hng ca mi trng o cng c th gy ra dch chuyn nh kt qu, do thay i nh v dng in.2.3 Sai s ngu nhin: Gi s thc c vch chia nh nht n 1mm, th sai s dc thc phn c lng nh hn mm l sai s ngu nhin.Sai s ngu nhin l nhng sai s m tr s v c im nh hng ca n n mi kt qu o c khng r rng, khi th xut hin th ny, khi th xut hin th kia, ta khng th bit trc tr s v du ca n.V vy sai s ngu nhin xut hin ngoi mun ch quan ca con ngi, ch yu do iu kin bn ngoi, ta kh khc phc m ch c th tm cch hn ch nh hng ca n. Sai s ngu nhin c cc c tnh sau. Sai s ngu nhin c tr s v du xut hin khng theo quy lut, nhng trong cng mt iu kin o nht nh, sai s ngu nhin s xut hin theo nhng quy lut. c tnh gii hn: Trong nhng iu kin o c c th, tr tuyt i ca sai s ngu nhin khng vt qu mt gii hn nht nh. c tnh tp trung: Sai s ngu nhin c tr tuyt i cng nh, th c kh nng xut hin cng nhiu. c tnh i xng: Sai s ngu nhin dng v m vi tr s tuyt i b c s ln xut hin gn bng nhau. c tnh b tr: Khi s ln o tin ti v cng, th s trung bnh cng ca cc sai s o c ngu nhin ca cng mt i lng s tin ti khng. Tc l:

- Ngoi cc sai s trn nh gi sai s ca dng c khi o mt i lng no ngi ta cn phn loi.- Sai s tuyt i:l hiu gia gi tr i lng o Yn v gi tr thc Xn e = Yn - Xn

- Sai s tng i (tnh theo %): Trong : er - sai s tng i, Yn - gi tr i lng o; Xn - gi tr thc (tr s o c)

- chnh xc tnh theo %: a = 100% er = (A100%) (2.6)3. Th saiTh sai th hin trng thi trong ch c mt im xc nh ng thng t mt n thang o v im ny chnh l u kim hay u nhn ca thc o (ph thuc vo im nhn). S khc nhau trong vic c khng do dng c gy nn m do v tr ca mt so vi mi nhn ca kim o. Cc nhm ln nh vy c th do nh gi sai khi kim nm gia hai vch chia. Nhim v ca ngi quan st khi thc hin php o: Chun b trc khi o: phi nm c phng php o, am hiu v thit b o c s dng, kim tra iu kin o, phn on v khong o chn thit b ph hp, chn dng c o ph hp vi sai s yu cu v ph hp vi mi trng xung quanh. Trong khi o: Phi bit iu khin qu trnh o c kt qu mong mun. Sau khi o: nm chc cc phng php gia cng kt qu o gia cng kt qu o. Xem xt kt qu o t yu cu hay cha, c cn phi o li hay phi o nhiu ln theo phng php o lng thng k.- Khng c thang o no c cc vch cho mi gi tr ( v d: Thc k ch chia vch n mm, do cc di khng phi s nguyn ln mm th ngi o phi nhn nh v phn l l bao nhiu phn trm ca 1mm). Sai s loi ny rt ph bin v do tnh ch quan ca ngi c.- Khi dng ng h kim, kim ca ng h khng nm trong mt phng cha cc vch chia . Khi v tr t mt khng ng s lm tng sai s c. V tr ng l v tr m mt phng do con ngi ca mt v kim ca ng h to thnh mt mt phng vung gc vi mt chia . Do vy, i khi ngi ta phi c gng phn x trn mt chia , v ch cn chn v tr ca mt sao cho nh ca kim b khut sau chnh kim .

Cc b phn chnh ca my o1.5.1. S khi ca dng c oS cu trc chung ca dng c o Mt dng c o c bn c ba b phn chnh l chuyn i s cp, mch o v c cu ch th.

Hnh. Cu trc c bn ca dng c oTa ln lt xt cc khu chc nng ny: Chuyn i s cp:lm nhim v bin i cc i lng o thnh tn hiu in. y l khu quan trng nht ca thit b o. Mch o: l khu thu thp gia cng thng tin o sau chuyn i s cp lm nhim v tnh ton v thc hin cc php tnh trn s mchC cu ch th: l khu cui cng ca dng c th hin kt qu o di dng con s so vi n v. C ba cch th hin kt qu o l: ch th bng kim, ch th bng thit b t ghi, ch th di dng con sS cu trc ca dng c o bin i thng

Hnh S cu trc chung ca dng c bin i thng

Vic bin i thng tin o ch xy ra trn mt ng thng, tc l khng c khu phn hi. Theo s trn th i lng o X c a qua cc khu chuyn i C1 (s cp), C2,, Cn bin thnh i lng Yn tin cho vic quan st v ch th. Cc i lng Y1, Y2, Yn l cc i lng trung gian.S cu trc ca dng c o kiu so snh

Hnh S cu trc chung ca dng c o kiu so snhDng c o ny c mch phn hi vi cc b chuyn i ngc (CN) to ra tn hiu Xk so snh vi tn hiu o X. Mch o l mt mch vng khp kn. Sau b so snh ta c tn hiu X = X - Xk.Khi X = 0 ta c dng c so snh cn bng. Khc i ta c dng c so snh khng cn bng.1.5.2. Cc c tnh c bn ca dng c o Dng c o c nhiu loi ty theo chc nng ca chng nhng bao gi cng c nhng c tnh c bn nh sau: Sai s ca dng c o: Nguyn nhn gy ra sai s ca dng c o th c nhiu loi. C th l nhng nguyn nhn do chnh phng php o gy ra hoc 1 nguyn nhn no y c tnh quy lut hoc cng c th l do cc yu t bin ng ngu nhin gy ra. Trn c s ngi ta phn bit hai loi sai s l sai s h thng v sai s ngu nhin. Sai s h thng: cn gi l sai s c bn, l sai s m gi tr ca n lun lun khng i hay thay i c quy lut. Sai s ny v nguyn tc c th loi tr c. Sai s ngu nhin: l sai s m gi tr ca n thay i rt ngu nhin do cc bin ng ca mi trng bn ngoi (nhit , p sut, m). Sai s ny cn gi l sai s ph nhy: nhy ca dng c o c tnh bng vi X l i lng vo v Y l i lng ra. Nu F(X) khng i th quan h vo ra ca dng c o l tuyn tnh. Lc thang o s c khc u. Nu mt dng c gm nhiu khu bin i, mi khu c nhy ring th nhy ca ton dng c l: in tr ca dng c o v cng sut tiu th o in tr vo: l in tr u vo ca dng c. in tr vo ca dng c o phi ph hp vi in tr u ra ca khu trc ca chuyn i s cp.o in tr ra: Xc nh cng sut c th truyn ti cho khu tip theo. in tr ra cng nh th cng sut cng ln. Tc ng nhanh tc ng nhanh ca dng c o chnh l thi gian xc lp kt qu o trn ch th. i vi dng c tng t thi gian ny khong 4s. Cn dng c s c th o c hng nghn im o trong 1 giy tin cy tin cy ca dng c o ph thuc nhiu yu t nh: tin cy ca cc linh kin ca cc dng c o, kt cu ca dng c o khng qu phc tp, iu kin lm vic ca dng c o c ph hp vi tiu chun hay khng. Ni chung tin cy ca dng c o c xc nh bi thi gian lm vic tin cy trong iu kin cho php c ph hp vi thi gian quy nh hay khng.

4. CU HI N TP 1. n v o l g? Th no l n v tiu chun? c my n v tiu chun.2. K thut o l g?3. Sai s o l g? Phn bit cc loi sai s o4. Cp chnh xc ca dng c o l g? Phn bit sai s ca php o v cp chnh xc ca dng c o khc nhau ch no?