Resistance Temperature Detector (RTD) l g? Bi ton ng dngHaiAu2005 | 15:03:29 14-08-09 | Posts: 14121

Bi vit ny s trnh by nguyn l cu to ca RTD v bi ton ng dng RTD trong h thng o nhit dng ch bo v iu khin. 1. Nhit k in tr kim loi RTD RTD hay cn gi l Resistance Thermometer l mt cm bin nhit in tr hay ngn gn hn l nhit k in tr da trn nguyn l in tr kim loi thay i (tng hay gim) khi nhit thay i (tng hay gim). RTDs v thermistors cng da trn nguyn l nhit thay i th in tr thay i nhng khc nhau ch RTDs dng kim loi cn thermistors th dng cht bn dn (nhit tng, in tr gim). Do s khc nhau ny nn ta c th gi tn 2 loi nhit k in tr nh sau: RTD = nhit k in tr kim loi Thermistor = nhit k in tr bn dn Da vo nguyn l ca s thay i in tr, ngi ta chng minh c rng in tr RTD thay i ph thuc vo nhit theo phng trnh sau:

trong : : in tr ( ti nhit T ( ) : in tr ( ti nhit ( ) v l hng s xc nh theo thc nghim. Nu nhit trong khong ngn nht nh, cng thc trn c th c n gin thnh:

Cu to RTD

Hnh 1: RTD

Mch RTD s dng RTD lm dng c o nhit ngi ta thng dng mt mch cu Wheatstone kt ni vi RTD. C mt s cch kt ni nh sau: Kt ni hai dy:

Hnh 2: Kt ni hai dy

Kt ni ba dy:

Hnh 3: Kt ni ba dy

Kt ni bn dy:

Hnh 4: Kt ni bn dy

Da vo mch cu (ly v d mch cu kt ni hai dy) in p ra c tnh theo cng thc:

trong V l in p cung cp cho mch cu. Cc in tr , thng c gi tr bng nhau v thng l mt bin tr iu chnh mch cu cn bng nhit . Trn th trng c mt loi nhit k in tr dng Platinum c in tr nhit 0oC bng 100 Ohms v c t tn l RTD-100 Platinum. Nhit k in tr ny c cng thc tnh in tr nh sau:

Gi tr hng s thng ly bng 0.00385 nhit 0-100 .

v c coi l khng i trong thang

Trong thc t thit k mt h thng o nhit dng RTD th ngi ta thng kt ni mt RTD vi mt b chuyn i tn hiu thnh tn hiu ra dng 4-20 mA dng cho iu khin v ch bo nhit . B chuyn i tn hiu nhit (temperature transmitter) c b ch bo nhit v cung cp tn hiu 4-20 mA cho b iu khin. Tn hiu trong ton b h thng o nhit dng RTD c chuyn i nh sau: Nhit [oC] > in tr [Ohms] > in p [V] > dng [mA] (> nhit [oC]) 2. M phng h nhit dng RTD Bi ton 1: Mt h thng o nhit dng RTD (RTD-100 c s kt ni nh s Hnh 2 (kt ni hai dy). Cc in tr ) c v bin

tr dng cn bng mch cu, oC gi tr ca bin tr . in p cung cp cho mch cu Wheatstone l 5 V v thang nhit o . Hy s dng Simulink m phng h thng o nhit RTD, trong chng trnh Simulink biu din in tr v in p ra . Kt qu m phng:

Hnh 5: Chng trnh m phng RTD v mch cu

Bi ton 2: S dng RTD v mch cu trong Bi ton 1 trn, hy thit k mt mch khuych i dng Op Amp c in p ra 0-5 V v mch chuyn chuyn i in p thnh dng c thang o 4-20 mA. S dng Simulink m phng mch. Mch khuch i:

Hnh 6: S mch khuch i dng Noninverting Op Amp

Kt qu m phng mch khuch i (Rin = 10 123.9 ):

, dng m phng xc nh c RF =

Hnh 7: Chng trnh m phng RTD + Wheatstone Bridge + Amplifier (in p ra 0-5V)

Kt qu m phng mch chuyn i in p sang dng (nu s dng mch chuyn i in p sang dng sau chng ta cn c thm mt mch inverting i cc in p):

Hnh 8: S mch chuyn i in p sang dng dng Op Amp

Thit k s b: chn R1 = 100 Ohms, R2 = 200 kOhms, R3 = 500 Ohms, R4 = 100 Ohms, R5 = 199.5 kOhms th c dng ra trong khong 0-20 mA theo chng trnh m phng sau:

Hnh 9: Chng trnh m phng mch chuyn i in p sang dng dng Op Amp

c dng ra trong khong 4-20 mA th c th dng gii php chuyn i mch khuch i c in p ra trong khong 1-5 V. Bn c t tm hiu thm v thit kt mch in thc t khi c iu kin thc hnh! H thng o nhit dng RTD thng c thm chc nng hin th nhit nn trong mch in cn c thm mch hin th nhit dng LCD. 3. Kt ni RTD Temperature Transmitter vi my tnh dng I/O Interfacing Board 3.1 M phng o nhit bng my tnh Khi s dng RTD Temperature Transmitter c tn hiu ra 1-5 V hoc 4-20 mA th u c th kt ni c vi my tnh c vi iu kin tng ng l I/O interfacing board c cng vo l 1-5 V hoc 4-20 mA tng ng. Nu ch c I/O interfacing board c cng vo dng in p th phi dng b chuyn i dng sang in p, n gin nht l dng in tr 250 Ohms ( cng sut) ni hai u ca RTD Temperature Transmitter. Chng trnh m phng o nhit bng Simulink:

Hnh 10: Chng trnh m phng o nhit bng Simulink

Bn c th thc hnh t lp chng trnh m phng bng Simulink theo hng dn (bng ting Anh) Simulink_Tute_01_01.pdf (137 views) (bn khng cn thit phi bit s dng Simulink khi lm bi thc hnh ny, cn bit cht t v MATLAB x l kt qu trong MATLAB Command Window, bn c th hc Simulink t u theo hng dn ny!). Chng trnh m phng o nhit bng LabVIEW

Hnh 11: Chng trnh m phng o nhit bng LabVIEW

Cc bn xem th chng trnh m phng khi ang chy:

Video clip (*.swf): temp01.swf (99 views) (file nguyn bn). Xem video thu nh:

Bn cng c th chy th chng trnh m phng sau (exe file, khong 180 KB) TempMeasSim01.exe (82 views). Bn cn download LabVIEW Run-Time Engine 8.6 chy file ny. 3.2 Thc hnh o nhit bng my tnh s dng NI PCI-6036E hoc Advantech PCI-1711 Khi s dng bng NI PCI-6036E hoc Advantech PCI-1711 th bc u tin l phi np Device Drivers, sau tt ngun my tnh (cn thn hn th rt in ra), cm card vo cng PCI, khi ng li my tnh v kim tra xem card chy chnh xc cha. Vi PCI-6036E th c th s dng Measurement and Automation Explorer (MAX) kim tra. Khi kim tra c th s dng phng php Loopback Test Technique, ngha l ni knh u ra ca card vi knh u vo, pht tn hiu v kim tra tn hiu vo. Nu card chy tt th tn hiu vo phi bng vi tn hiu ra. Theo phng php ny th khng cn phi c mt ngun no pht tn hiu DC, v cng khng cn phi c mt multimeter no. Tham kho bi S dng MAX kim tra bng PCI-6036E. Cch lp t v kim tra cc card u c m t trong hng dn s dng. Ngoi ra phn lp trnh thu nhn d liu v pht d liu s dng phn mm km theo card cng u c trnh by trong hng dn v khi s dng "ng b" phn cng v phn mm ca cng mt hng thng khng gp kh khn, nu gp kh khn c th lin lc hi Technical Support trc tip hng sn xut (iu kin h tr li thng thng l trong thi gian bo hnh v vn tip tc s dng dch v maintenance ca hng sn xut).

Trong phn ny ti ch trnh by ng dng card vi phn mm MATLAB/Simulink v Data Acquisition Toolbox ca MathWorks (phn mm ca hng th 3) c h tr card PCI-6036E (kh ging PCI-1711) v nhng kh khn gp phi. 3.2.1 S dng MATLAB/Simulink v Data Acquisition Toolboox Ti liu tham kho chnh cho phn ny l Data Acquisition Toolbox 2.14. Bn cng c th c c ti liu ny trong MATLAB/Simulink/Data Acquisition Toolbox ca bn (cho phin bn ci t). Trong phn sau s minh ha vic lp trnh Simulink v MATLAB vi Data Acquisition Toolbox (phin bn MATLAB R2007b) cho mt b cm bin nhit dng RTD (RTD Temperature Transmitter) cung cp tn hiu dng 4-20 mA. Do vy s cn mt b chuyn i dng sang in p (s dng SC-2345 v NI SCC-CO02 Module ca National Instruments). Nu bn cn chuyn i dng sang in p m khng c iu kin mua thit b th c th s dng mt in tr 250 Ohm ni hai u ra ca b cm bin v ni vi knh vo ca NI PCI-6036E l c. Tn hiu chuyn i nh sau: Tn hiu ra ca RTD Temperature Transmitter l dng 4-20 mA, qua b chuyn i dng sang in p tr thnh tn hiu in p 1-5 V, qua PCI-6036E tn hiu c trong my tnh (MATLAB/Simulink hoc LabVIEW) l 1-5 V. Do vy trong chng trnh s thc hin vic nhn d liu v x l tn hiu nh sau: Nhn d liu in p t PCI-6036E (1-5V) uin > hin th nhit bng in p (1-5 V) trong chng trnh > chuyn i in p thnh dng (4-20mA) u - hin th dng (nu o dng u ra ca RTD Temp Transmitter th s o ny phi dng vi s o dng trong chng trnh) > chuyn i dng thnh nhit y (0-100oC). Cng thc chuyn i d liu nh sau:

(theo nh lut Ohm)

(h s 6.25 l nhy (hoc gain) ca RTD Temp Transmitter vi gi thit l trong b cm bin c b tuyn tnh, nu khng c gi thuyt ny bn phi bit y thng tin trong datasheet - v RTD Temp Transmitter). Dng Simulink

Xin hng dn s dng Data Acquisition Toolbox trong Simulink trc. Trong Simulink, Data Acquisition Toolbox c cc (6) khi sau (theo phin bn mi nht, nu bn dng R2007b th ch c bn khi Analog Input, Analog Output, Digital Input v Digital Output): Analog Input Acquire data from multiple analog channels of a data acquisition device. Analog Input (Single Sample) Acquire a single sample from multiple analog channels of data acquisition device. Analog Output Output data to multiple analog channels of a data acquisition device. Analog Output (Single Sample) Output a single sample to multiple analog channels of data acquisition device Digital Input Acquire the latest set of values from multiple digital lines of a data acquisition device. Digital Output Output data to multiple digital lines of a data acquisition device. 1. Nhn d liu Cc bc c bn cho mt chng trnh nhn d liu: Step 1: Open the Data Acquisition Toolbox Block Library Step 2: Create a New Model Step 3: Add the Analog Input Block to the Model Step 4: Add a Scope to the Model Step 5: Specify Block Parameters Step 6: Connect the Blocks Step 7: Run the Simulation Step 8: Look at the Data in the Scope T cc bc trn, ng dng cho chng trnh nhn d liu t RTD Temperature Transmitter chung ta cn thm mt s khi x l d liu. Ti khng trnh by chi tit vic lp chng trnh Simulink m ch trnh by chng trnh mu sau, bn c t lp chng trnh theo cu hnh v cm bin nhit ca mnh. Nu cm bin nhit ca bn c tn hiu dng 4-20 mA v I/O Interfacing Board ca bn tng t PCI-6036E th bn c th dng chng trnh Simulink cho trn, ch cn thay khi Constant v Slider Gain bng khi Analog Input l c. Chng trnh mu nhn d liu bng Simulink/Data Acquisition Toolbox:

Hnh 12: Chng trnh mu nhn d liu bng Simulink/Data Acquisition

Phn quan trng trc khi chy th chng trnh l:

Hnh 13: Thit tr khi Analog Input nhn d liu - Knh AI-10 ca bng PCI-6036E

Thit tr tham s m phng:

Hnh 14: Thit tr tham s m phng

Sau khi thit tr xong bn c th chy th chng trnh v kim tra xem chng trnh nhn d liu ng hay cha. Nu c b ch bo khc th c th so snh s o c trong Simulink v s o c trn b ch bo bn ngoi my tnh. Ch : khi bn dng chng trnh trn, c khi To Workspace lu d liu vo trong Workspace th s gp phi li (Simulink bo li). khc phc li ny th cn cho thm mt khi Rate Transition vo chng trnh nh trong chng trnh mu trn (ti cha/khng r l do ti sao Simulink li bo li ny khi thm khi To Workspace vo chng trnh). Cch khc l khng dng khi To Workspace na m dng khi Scope lu d liu, nh l mun lu c ton b d liu phi tick b Data Limit (5000) trong Scope > Parameters > Data History. Nu cn lu c thi gian th chn Structure with Time. 2. Pht d liu

Cc bc c bn: Step 1: Open the Data Acquisition Toolbox Block Library Step 2: Create a New Model Step 3: Add the Analog Output Block to the Model Step 4: Add a Constant Block/Slider Gain Block to the Model Step 5: Specify Block Parameters Step 6: Connect the Blocks Step 7: Run the Simulation Step 8: Measure the signal pht d liu ng thi vi thu d liu th ch cn lp trnh thm vo chng trnh nhn d liu trn ba khi sau: khi Constant, t gi tr bng 1, khi Slider Gain, dng khi ny c tc dng l khi ang chy m hnh Simulink, thay i gi tr th chng trnh khng ngng, v khi Analog Output:

Hnh 15: Chng trnh mu nhn d liu v pht d liu bng Simulink/Data Acquisition

Thit tr khi Analog Output pht d liu:

Hnh 16: Thit tr khi Analog Output pht d liu - Knh AO-0 ca bng PCI-6036E

Sau khi thit tr xong, bn c th chy th chng trnh, nhn p khi Slider Gain thay i gi tr in p v dng mt multimeter t ch o DC voltage kim tra tn hiu in p pht ra Knh AO-0 ca bng PCI-6036E, khng c sai st, gi tr o c phi bng vi gi tr pht ra nhp vo chng trnh bng khi Slider Gain. ng dng lp trnh iu khin: n y th nh bn thy chng trnh trn chnh l mt chng trnh iu khin bng tay n gin giao tip my tnh vi qu trnh (b cm bin v b chp hnh) dng I/O Interface Card, MATLAB/Simulink v Data Acquisition Toolbox. V nu bn mun lp trnh b iu khin On-Off hay PID bn ch cn thm thut ton cho On-Off controller hay PID controller m khng phi "m hnh ha" h thng ng v tun th cc bc kho st thit k h iu khin theo "l thuyt". Khi khng c thit b thc hnh th mi cn n m hnh ton v thit k h iu khin bng m phng. Bi ng dng lp trnh iu khin cho h nhit ny s c trnh by trong

phn sau. Dng MATLAB 1. Nhn d liu 2. Pht d hiu (to be updated) 3.2.2 S dng LabVIEW (NI PCI-6024E, PCI-6036E hoc cc DAQ cards ca NI th nn s dng LabVIEW (ng b) s tt hn vic s dng phn mm ca hng khc). Trong phn ny hng dn lp trnh iu khin nhit s dng USB-6009.

Hnh 1 Chng trnh LabVIEW iu khin nhit

Vng tn hiu iu khin (control loop): (1) RTD temperature transmitter (4-20mA) -> IV converter (1-5V) -> USB-6009 -> PID Controller -> Control signal (0-5V) -> V-I converter (4-20mA) -> Heater -> Tank (liquid temperature) - RTD temperature transmitter (1) (to be updated) 4. iu khin nhit bng my tnh