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MCH KHUCH I STRAIN GAUGE DNG VI MCH CHUYN DNG 1B31AN V Minh Tr1 1B mn T ng ha, Khoa Cng ngh, Trng i hc Cn Th

Thng tin chung: Ngy nhn: 31/10/2013 Ngy chp nhn: 28/04/2014 Title: Design of a strain gauge amplifier using integrated circuit 1B31AN

T kha: Cm bin bin dng, khuch i strain gauge, cm bin trng lng Keywords: Strain gauge measurement, strain gauge signal conditioning, amplifier

ABSTRACT Output signals directly from strain gauges have small amplitude expressed in microstrains; therefore, amplifying and processing these signals has always been considered in precise measurement. The aim of this paper is to survey and implement a special integrated circuit designed for strain gauge amplification, namely 1B31AN (Analog Devices), in order to support research and teaching activities at Measurement and Sensors Laboratory, Department of Automation Technology, College of Engineering Technology, Can Tho University. The study results show that this integrated circuit is highly functionable, being suitable for training on the principles and configuration, amplification, and signal processing of strain gauges. This integrated circuit is compatible with strain-gauge configurations of quarter-bridge, half-bridge, and full-bridge circuits. Besides, the amplitude and quality of the output signal from the amplifier can be adjusted depending on the requirements of low pass filtering and gain adjustment.

TM TT Tn hiu ng ra trc tip t cc cm bin bin dng strain gauge c bin rt nh tnh bng micro strain, v vy vic khuch i v x l tn hiu o t cc train gauge lun l vn c quan tm trong o lng chnh xc. ti ny nhm kho st v ng dng mch khuch i tn hiu cho cm bin bin dng dng vi mch 1B31AN ca hng Analog Device phc v cho nghin cu v ging dy phng th nghim o lng v cm bin, b mn T ng ha, Khoa Cng ngh, Trng i hc Cn Th. Kt qu cho thy 1B31AN c thit k c tnh m un ha rt cao, rt ph hp dng cho ging dy v nguyn l v kt ni strain gauge, khuch i, v x l tn hiu o. Vi mch ny c thit k tng thch vi cc loi cu hnh cu nh n, hai na, hay ton cu. Bn cnh , bin v cht lng ca tn hiu ng ra c th thay i c ty theo yu cu da vo tnh nng lc h thng v iu chnh h s khuch i.

1 GII THIU Strain gauge l nhng phn t cm bin c

ch to bng bn dn hay kim loi m in tr ca n thay i khi c tc ng ca ngoi lc. Cu to ca strain gauge l mt on dy dn kim loi c dn trn mt mng mng thng l plastic theo hnh zigzag. Khi b ko hay nn theo chiu tch cc

th in tr ca strain gauge thay i. Strain gauge c s dng nhiu trong ch to loadcell, cm bin o moment, p sut, lu lng nh l nhng phn t s cp.

S thay i in tr ca strain gauge l rt nh v cc bin dng ch tnh bng micro-strain. Yu

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cu t ra khi thit k mch khuch i tn hiu cho strain gauge l phi m bo c cc yu cu:

iu chnh h s khuch i theo yu cu Cho php loi b cc thnh phn nhiu H tr cc loi cu hnh mch cu Thit k mch khuch i c th s dng cc

linh kin in t thng dng nh BJT, OPAMP v kt hp vi cc linh kin th ng R, L, C. Tuy nhin chnh xc khng cao do cht lng linh kin v kh nng kh nhiu khi khuch i. V th vic s dng cc vi mch chuyn dng thit k mch khuch i cho strain gauge l xu hng mi hin nay. Texas Instrument pht trin vi mch INA125 c th dng khuch i strain gauge. Tuy nhin vi mch ny cn nhiu hn ch v kh nng cu hnh cu v chng nhiu.

ti ny nhm mc ch kho st chc nng v thit k mch khuch i tn hiu cho strain gauge dng vi mch 1B31AN phc v cho hc tp v ging dy mn o lng v Cm bin B mn T ng ha.

2 PHNG PHP THC HIN 2.1 Tng quan Nh gii thiu strain gauge c dng nh

nhng phn t s cp ch to load cell, cm bin o moment, p sut, lu lng,... v hu ht cc cm bin ny u s dng strain gauge dng cu Wheatstone.

Cu Wheatstone l mt mch in c s dng o mt in tr cha bit bng cch cn bng hai nhnh ca mt mch cu, trong c mt nhnh cha cc thnh phn cha bit. Hot ng ca n tng t nh mt cu phn th. N c pht minh bi Samuel Hunter Christie vo nm 1833 v c ci thin v ph bin bi Charles Wheatstone vo nm 1843. Mt trong nhng mc ch s dng ban u ca cu Wheatstone l dng vo vic phn tch v so snh (Ekelof, 2001).

Hnh 1: Mch cn bn ca cu Wheatstone

Mt cch tng qut, cu Wheatstone c dng nh Hnh 1, cc thnh phn tr khng R1, R2, R3, R4 l cc in tr thun. Cu c kch bng ngun in p VEX, ng ra VO l in th chnh lch gia hai nt a v b. in th VO c tnh:

3 23 4 1 2

R RV VEXO R R R R

= -+ +

(1) R1, R2, R3, R4 c chn sao cho khi trng

thi cn bng th VO c gi tr 0V. Thng thng khi thit k ngi ta thng

chn cc gi tr in tr sao cho trng thi cn bng R2 = R3, R1 = R4, hoc R1 = R2 = R3 = R4.

Trong cc mch cm bin, cu Wheatstone c s dng bng cch thay th cc in tr R1, R2, R3, R4 trn Hnh 1 bng cc strain gauge, ty trng hp m s strain gauge c mc vo cu c th l 1, 2 hoc 4. Do t dng mch cu Wheatstone tng qut ta c cc dng bin th sau:

a. Quarter-Bridge

Hnh 2: Quarter-Bridge dng mch cu ny, ch c mt strain gauge

c s dng, s nguyn l nh Hnh 2. Do R1 = R2 = R3 = RG = R, vi RG l in tr ca strain gauge khi cha b bin dng. Khi c tc ng vo vt chng, in tr ca strain gauge s thay i mt lng R. Khi , VO c tnh:

1 .2 2R

V VEXO R R (2)

nh hng ca nhit Thc t, yu t nhit bn ngoi nh hng

trc tip ln in tr ca strain gauge lm gim chnh xc ca kt qu o. Trong trng hp ny hn ch nh hng ca nhit ngi ta s dng thm mt dummy gauge dn vo vt chng nhng trng thi khng hot ng.

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Hnh 3: S dng dummy gauge trnh nh

hng ca nhit

Nh trn Hnh 3 lc ny strain gauge A l strain gauge c dn ln vt chu bin dng, cn strain gauge D khng c dn trc tip ln vt chu bin dng m dn ln vt khng chu bin dng gn . Lc ny c 2 strain gauge cng chu tc ng ca cng mt iu kin nhit nn nh hng ca nhit c b qua. Tng ng cc strain gauge A v D trn Hnh 3 v RG v R1 trn Hnh 2 vi nhau th cng thc tnh in th VO vn c tnh nh cng thc (2).

nh hng ca in tr dy dn

Hnh 4: nh hng ca in tr dy dn mch o dng Quarter-

Bridge

Do mch o v vt chu bin dng t xa nhau

mt khong cch nht nh nn cn c dy ni gia strain gauge v mch o. Chnh cc on dy dn ny lm xut hin thm thnh phn in tr RL nh trn Hnh 4a.

Thnh phn in tr ny lm cho cu b mt cn bng ngay c khi khng c tc ng lm strain gauge thay i in tr. Mt khc, in tr ca dy dn cng chu tc ng ca nhit bn ngoi nn khi nhit thay i RL cng thay i theo lm

cho cu b mt n nh hn. hn ch nh hng ca in tr dy dn v

nhit ngi ta ci tin bng cch thay i cch u dy dn nh Hnh 4b. Lc ny c 2 on Aa v aB u c b in tr dy dn v cng chu tc ng ca nhit nn cu s cn bng khi khng c tc ng. on dy dn c in tr RL2 mang dng in rt nh v mch o nn in th ri trn dy dn ny rt nh v c th b qua.

b. Half-Bridge

Hnh 5: Half-Bridge

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S nguyn l v ng dng ca mch o dng Half-Bridge c gii thiu Hnh 5. Mch o Half-Bridge s dng 2 strain gauge trng thi tch cc nhng trong c 1 strain gauge chu nn v 1 strain gauge chu ko, nhy tng gp i so vi dng Half-Bridge. Mch thit k dng ny loi b nh hng ca nhit . in p lch VO c tnh theo cng thc (3):

1 .0 2 2R R

V VEXR (3)

c. Full-Bridge

Mch Full-Bridge s dng 4 strain gauge, trong c 2 strain gauge chu nn v 2 strain gauge chu ko. S mch Full-Bridge nh Hnh 6.

Hnh 6: Full-Bridge

in th Vo c tnh theo cng thc (4): .0 2 2

R R R RV VEX

R R

(4) Nh im qua trn, cu Wheatstone

hot ng cn phi cp mt in p nui VEX (excitation voltage). Ngun in p ny thng c gi tr trong khong 3V 15V. Nu in p ny ln th in dng in chy qua cc strain gauge trong cc nhnh cng ln v th s lm nh hng n in tr ca strain gauge do nh hng ca nhit bn thn cc strain gauge sinh ra (self-heating).

in p kch thch VEX cng nh hng n in p lch Vo ca cu (th hin trong cc cng thc (1), (2), (3)), nu VEX khng c gi n nh th VO cng s khng n nh v lm nh hng n kt qu o. V th mch ngun nui cho cm bin c thit k thm chc nng remote sensing. Chc nng ny cho php mch kim tra in p VEX trn mch cu bng ng hi tip in th dng 2 dy dn sense high v sense low ti im ni dy cp in th kch thch cho cm bin. ng hi tip in th ny a v b ngun so

snh v b thm lng in p ri trn dy dn. V th nhng cm bin loi ny tr nn c 6 dy (Hnh 7).

Hnh 7: S nguyn l ca cu 6 dy 2.2 Ni dung 2.2.1 IC khuch i tn hiu 1B31AN a. Chc nng IC c thit k chuyn dng khuch i

tn hiu cho cc cm bin, mch o c dng cu v na cu.

in th kch thch cho cm bin thay i c v chc nng remote sensing h tr cho cc cm bin 6 dy, 4 dy, na cu.

B lc tn hiu (10 Hz 20 kHz), cho php ty chnh.

H s khuch i thay i c trong khong 2 mV/V 5.000 mV/V.

Hnh 8: S khi chc nng ca IC 1B31AN

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b. Kt ni vi cm bin IC 1B31 h tr cc loi cm bin loi 6 dy, 4

dy v mch o dng na cu c tr khng 120 hoc ln hn (Analog Devices, 1996).

a. Cm bin 6 dy b. o na cu Hnh 9: S nguyn l kt ni vi cm bin v strain gauge

c. iu chnh in p cp ngun cho cm bin VEXC

in p cp ngun cho cm bin (in p kch) VEXC c t trc 10 V khi ni chn 19 (REF OUT) vi chn 20 (REF IN). tng in p VEXC cn mc thm in tr REXT t chn 21 (EXC ADJ) ti chn 26 (HALF-BRIDGE COMPL) nh Hnh 10a.

Gi tr in tr REXT c tnh theo cng

thc (5), (6):

OUTREFEXC

OUTREFT VV

VkR

_

_10 , (5)

VREF_OUT = 6,8V

T

TEXT Rk

RkR 20

20 (6)

a. VEXC 10 V 15 V b. VEXC 4 V 10 V

Hnh 10: in th cp ngun cm bin VEXC gim in th VEXC th mc mt bin tr 20

k gia chn 19 (REF OUT) v chn 20 (REF IN) v mt t 4.7 F t chn 20 ti chn 16 (COMMON) nh Hnh 10b, lc ny VEXC thay i c trong khong 4 V 10 V.

d. iu chnh h s khuch i H s khuch i G c tnh theo cng thc:

802 kGRG

(7)

Trong RG l in tr ni gia chn 3 v chn 4. Da vo cng thc (7) nu mun thay i h s khuch i th ch cn thay i gi tr in tr RG, v th nn s dng RG l mt bin tr d dng iu chnh khi s dng.

e. iu chnh tn s ct ca b lc h thng (Low pass filter)

Tn s ct ca b lc h thng c thit k mc nh 1 kHz. thay i tn s ct thp hn 1 kHz cn mc thm 2 t in gia chn 12 vi chn 16 v chn 13 vi chn 14 nh Hnh 11a. Gi tr ca cc t in c tnh theo cng thc:

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10.015 11kHz

C FSEL fC

(8)

10.0022 12kHz

C FSEL fC

(9)

Vi fc l tn s mong mun (fc

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Hnh 13: Khi mch kim tra in p dng Voltmeter

Xoay SW1 thay i cc v tr kt ni ca chn (+) v chn (-) ca Voltmeter vi cc im cn o in p (Hnh 13).

V tr 1: o in p -VS V tr 2: o in p +VS

V tr 3: khng dng V tr 4: khng dng V tr 5: o VEXC ca knh 1 V tr 6: o VEXC ca knh 2

Hnh 14: Chn kiu kt ni cm bin

Load cell kt ni vi mch khuch i thng qua u ni 16S-1. Ty theo loi cm bin m xoay SW2 chn cu hnh kt ni cho thch hp (Hnh 14).

V tr 1: kt ni vi cm bin 6 dy

V tr 2: kt ni vi cm bin 4 dy V tr 3: ni tt ng vo INPUT+ v

INPUT- ca IC iu chnh input offset Vi tr 4: kt ni vi mch na cu

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Hnh 15: iu chnh tn s ct ca b lc h thng

Tn s ct ca mch lc h thng c th thay i trong khong t 10 Hz n 1 kHz bng cch kt hp cc t hay in tr bn ngoi. Tn s ct ca mch lc h thng c thay i bi SW3 (Hnh 15), khi xoay SW3 th cc chn chc nng ca mch lc c thay i v tr ghp ni vi cc gi tr t in khc nhau, t tn s ct c thay i.

V tr 1: tn s ct 10 Hz V tr 2: tn s ct 90 Hz V tr 3: tn s ct 180 Hz V tr 4: tn s ct 500 Hz V tr 5: tn s ct 1000 Hz

Hnh 16: iu chnh h s khch i

li ca mch khuch i c iu chnh thng qua SW4 v mt bin tr tinh chnh (Hnh 16). iu chnh SW4 s lm thay i cc gi tr in tr c gi tr c nh (10 - 280 ) v cc in tr ny c mc ni tip vi bin tr tinh chnh c gi tr thay i t 0 - 100 .

Nh vy i vi mch khuch i ny li c th iu chnh trong khong:

802 800210 0k

GMAX (13)

802 212280 100k

GMIN (14)

3 KT QU V NH GI kim tra hot ng v nh gi cht

lng ca thit b ta tin hnh mt s th nghim kim tra:

3.1 Th nghim 1: Kho st p ng tn s ca mch khuch i

Ni cc dy ca loadcell vo ng vo ca mch khuch i

Xoay nm chnh t khuch i mong mun

iu chnh VEXC sao cho gi tr bng 10 V S dng Card NI myDAQ xem tn hiu

ng ra Thay i nm chnh tn s ct ca b lc v lu

li kt qu. Kt qu th nghim cho thy khi iu chnh

nm chnh tn s ct ca mch lc gim dn th cc thnh phn tn s nhiu b trit tiu dn v bin sng nhiu cng gim dn. C th:

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Khi fc = 1000 Hz, trong sng nhiu c cc thnh phn:

Chu k 20 ms: tn s 50 Hz, bin khong 100 mV

Chu k 2.5 ms: tn s 400 Hz

Chu k 2 ms: tn s 500 Hz Chu k 1 ms : tn s 1000 Hz

Khi fc = 10 Hz, tn hiu ng ra ch cn li thnh phn nhiu c tn s 50 Hz nhng bin suy gim cn rt nh (khong 5 mV).

a. fc = 1 kHz b. fc = 500 Hz

c. fc = 180 Hz d. fc = 90Hz

e. fc = 10 Hz

Hnh 17: Tn hiu ng ra quan st trn oscilloscope (myDAQ) 3.2 Th nghim 2: Khuch i load cell u dy load cell vo mch khuch i

Ln lt t cc ti bit trc trng lng ln load cell v ghi li gi tr c c trn MyDAQ. Kt qu thu c nh Bng 1.

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S dng MATLAB v li cc gi tr trn v tm dng hm m t mi quan h gia khi lng v in th (Hnh 18).

T kt qu tnh ton ta c phng trnh biu din quan h v-m nh sau: v (mV) = 3,885 m + 8,022 (15)

Hnh 18: th mi quan h gia in th v khi lng

Bng 1: Cc cp gi tr khi lng, in th thu

c t th nghim Khi lng m (gram) in th v (milivolt)

0 0 180 705 280 1108 360 1408 385 1480 540 2110 560 2190 665 2565 720 2805 945 3660

1000 3910 1280 5010 1665 6480 1945 7585 2445 9520 2665 10380 3000 11610

4 KT LUN V XUT Bng cch s dng IC chuyn dng 1B31AN

thit k mch khuch i tn hiu cho cm bin vi cc thng s k thut theo ngh ca nh sn

xut gim bt s phc tp trong tnh ton v thit k phn cng, ng thi tin cy v chnh xc cng cao hn so vi mch thit k bng nhng linh kin ri. Kt qu ca cc th nghim cho thy mch sau khi thit k ch to p ng c nhng yu cu t ra cho mt bi thc hnh v bn cht v k thut x l tn hiu t mt strain gauge. Thit b c trin khai ging dy mt cch rt hiu qu cho thc hnh hc phn Cm bin chuyn nng ti phng th nghim o lng v Cm bin, thuc B mn T ng ha, Khoa Cng ngh.

TI LIU THAM KHO 1. Ekelof, S., 2001. The genesis of the

Wheatstone bridge. Engineering Science and Education Journal, ISSN: 0963-7346.

2. John P. Bentley, 2005. Principles of Meaurement Systems. NXB Pearson Prentic Hall.

3. Analog Devices, 1996. Wide Bandwidth Strain Gauge Signal Conditioner - 1B31. USA.

4. National Instrument, 2013. Measuring strain with strain gauge. http://www.ni.com/white-paper/3642/en/ truy cp ngy 10/10/2013.