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Hc phn
LP TRNH labVIEW C BN
GV: Nguyn Thanh Tn
Lp trnh LabVIEW
LabVIEW
GV: Nguyn Thanh Tn
Lp trnh LabVIEW
labVIEW
GV: Nguyn Thanh Tn
Lp trnh LabVIEW
Lp trnhB1: Phng php pht trin phn mm
B2: Xy dng gii thutB3: Thit k gii thut
B4: Thc thiB5: Kim tra
B6: Bo tr Sa chaB7: To d nGV: Nguyn Thanh Tn
Lp trnh LabVIEW
Ni dung1. Thao tc trn labVIEW 2. K thut g ri (Debug) cho VI
3. Lp trnh theo Modul4. Lp trnh trong VI
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Lp trnh LabVIEW
Phn 1
THAO TC TRN LABVIEW
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Lp trnh LabVIEW
Khi ng LabVIEW
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Lp trnh LabVIEW
Giao din labVIEWThanh menu
Tham khoTo new VI (Blank VI)
M file lu
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Lp trnh LabVIEW
To mt VI mi
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Lp trnh LabVIEW
Thnh phn ca mt VIMt VI trong labVIEW bao gm 3 thnh phn chnh:
Front panel Block Diagram Icon/Connector pane
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Lp trnh LabVIEW
Virtual Instruments (VIs)Giao din chnh Controls = Inputs (Vo) Indicators = Outputs (Ra) Biu khi Cc khi chng trnh ca giao din chnh Cc thnh phn i dy.
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Lp trnh LabVIEW
Icon/Connector paneIcon: biu tng ca mt VI hay mt subVI Connector pane: Bn ch th cc input v output ca mt VI Icon/Connector pane rt cn thit cho mt VI cng nh subVI
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Lp trnh LabVIEW
Giao din lp trnh
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Lp trnh LabVIEW
Giao din lp trnh
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Lp trnh LabVIEW
Thanh cng c
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Lp trnh LabVIEW
Bng cng cLabVIEW t ng chn nhng cng c cn thit. Hin th trn Front panel v trn biu thit lp h thng. S dng cc cng c to v chnh sa giao dn chnh v cc i tng ca biu khi Hin th bng cng c (Tools Palette), chn ViewShow Tools Palette
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Lp trnh LabVIEW
Front panel
1
2
Lp trnh: 1. Thc hin to lnh chng trnh bng cch nhp chut phi (ly cc control v indicator) 2. Chn cc i tng ph hp 3. Ko v th i tng trong khung lnh
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Lp trnh LabVIEW
Front panel
GV: Nguyn Thanh Tn
Lp trnh LabVIEW
Front panel
-To cc lnh trn FP -Thay i nhn, chnh text v nh dng -Thay i c tnh i tngGV: Nguyn Thanh Tn
Lp trnh LabVIEW
Block diagramFunctions (Hm) nm trong Block Diagram Nhm to chng trnh hon chnh
V d: While loop, For LoopWebcam
While room is active door is open LoopGV: Nguyn Thanh Tn
Lp trnh LabVIEW
Block diagram
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Lp trnh LabVIEW
Block diagram1 Cc lin kt vi FP 2 ng lnh: Truyn d liu (Wires) 3 Cc giao im trn mi i tng (Nodes) 4. Dng d liu ca i tng1
3GV: Nguyn Thanh Tn
2
4
Lp trnh LabVIEW
Kiu d liu trn Block Diagram
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Lp trnh LabVIEW
Block diagram
Nhn t hp Ctrl+B xa cc ng dy b t
Chc nng t sp xp cc ng dy gn gng
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Lp trnh LabVIEW
Terminals trong Block DiagramTerminal l i tng tn ti trong Block Diagram ln Front panel Lun tn ti cc ng vo ra truyn thng gia Front panel v Block Diagram Thay i kiu hin th ca cc terminals bng cch nhp phi chut v chn View as Icon
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Lp trnh LabVIEW
Block DiagramFunction NodesPhn t hot ng ch yu ca labVIEW C nn mu vng nht Khng tn ti trong Front Panel
subVI Nodes:subVI l nhng VI c to ra s dng cho nhng VI khc subVI cha Front Panel v Block Diagram ring ca n
VI ExpressL kiu t bit ca subVI Nn mu xanhGV: Nguyn Thanh Tn
Lp trnh LabVIEW
So snhFront panel Cha cc biu tng iu khin v hin th (giao din ngi dng) Truy cp Control Palette (chut phi) Bao gm cc control v Indicator
Block Diagram Chc code chng trnh (lp trnh)
Truy cp Function Palette (chut phi) Bao gm cc terminals, VIs, subVIs, functions, constants, Structures, wires,.Lp trnh LabVIEW
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Tm kim v HelpTm Controls, Functions, Vis s dng nt Search trn Controls v Functions palette Chn Help>>Show Context Help (Ctrl+H) hoc nhp vo biu tng Help: Search the labVIEW Help, NI Example Finder, hin th cc thng tin c bn v i tng labVIEW
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Lp trnh LabVIEW
Phn 2
K THUT G RI (DEBUG)
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Lp trnh LabVIEW
Vn thng gpDy b t (ni dy b sai kiu d liu hoc kiu vo/ra) Mt terminal trn Block Diagram khng c ni dy
Mt subVI b hng hoc do ngi dng sa i cc Connector paneD liu ang s dng mc nh khng chnh xc
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Lp trnh LabVIEW
DEBUG1. Execution Highlighting c s dng xem cc lung d liu chy trn Block Diagram 2. Single Stepping Single-step: chy xuyn qua VI xem kt qu hot ng ca mi VI trn Block Diagram Tm dng thc thi mt subVI chnh sa gi tr cc Control v Indicator
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Lp trnh LabVIEW
DEBUG3. Probes quan st cc gi tr v d liu tc th kim tra li ng ra ca VI v function 4. Gi li gi tr trong cc dy c th quan st d liu trn dy sau khi thc hin xong 5. Breakpoint: VI s tm dng v nt Pause s xut hin mu ti v tr Breakpoint.
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Lp trnh LabVIEW
Phn 3
LP TRNH THEO MODUL
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Lp trnh LabVIEW
subVIsMt VI trong mt VI khc gi l subVI Mt subVI xem nh mt chng trnh con trong ngn ng dng text Gc bn phi ca front panel v block diagram hin th Icon ca subVI Icon xc nh cc VI khi bn t n trn s khi
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Lp trnh LabVIEW
subVIs
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Lp trnh LabVIEW
subVIs
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Lp trnh LabVIEW
subVIs
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Lp trnh LabVIEW
Icon ca subVITa c th to v ty chnh Icon bng cch nhp chut phi vo biu tng gc trn bn phi ca front panel hoc block diagram v chn Edit icon Nhp chut phi vo biu tng gc trn bn phi ca front panel v chn Show connector xc nh cc terminal vo/ra cho subVI
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Lp trnh LabVIEW
s dng mt VI ging nh mt subVI, ta to mt Icon v Connector pane sau thit k Front panel v Blockdiagram
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Lp trnh LabVIEW
To IconIcon: biu tng miu t gi nh ca mt VI Click chut phi ln Icon pane (Panel hoc Diagram) gc ca s v chn Edit icon
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Lp trnh LabVIEW
To Connector PaneConnector pane : bn ch th cc input v output ca mt VI Nhp chut phi vo biu tng gc trn bn phi ca front panel v chn Show connector xc nh cc terminal vo ra
Chn im
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Lp trnh LabVIEW
Tr gip v phn loi u cuiPhn loi u vo/ra:Required Li nu khng kt ni Recommended Cnh bo nu khng kt ni Optional Khng c tc dng nu khng kt ni
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Lp trnh LabVIEW
Tr gip v phn loi u cui
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Lp trnh LabVIEW
S dng subVIs t subVI trn block diagram:Click chn mt VI trn Function palette Di chuyn n VI bn mun s dng nh mt subVI Double click ln n t trong Block diagram
t mt VI ang m trn Block diagram ca VI ang m khcClick vo biu tng ca VI bn mun s dng nh mt subVI Ko biu tng (Icon) cho vo Block diagram ca VI khc
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Lp trnh LabVIEW
To mt subVIChn phn no mun a vo subVI Chn Edit>> Create SubVI
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Lp trnh LabVIEW
Phn 4
LP TRNH TRONG VI
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Lp trnh LabVIEW
DataflowLabVIEW hot ng theo kiu dataflow thc thi cc chng trnh trong VI Block diagram thc thi ph thuc vo lung d liu th Block diagram s KHNG thc thi t tri qua phi Mt nt ch c th thc hin khi d liu c sn ti tt c cc u vo ca n
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Lp trnh LabVIEW
DataflowV d: Nt no thc thi trc?a) Add b) Subtract c) Random Number d) Divide e) Sine
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Lp trnh LabVIEW
Cc kiu d liu trong labVIEWTerminals: thit b u cui trc quan hin th cc kiu d liu Numeries: kiu d liu s hc hin th cc con s hoc bin s (chn Representation thay i kiu d liu numeric)
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Lp trnh LabVIEW
Cc kiu d liu trong labVIEWBoolean: kiu True/False, 1/0, ng/sai Boolean: c th l control (Knob, cng tc,..) hay hin th (led,)
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Lp trnh LabVIEW
Cc kiu d liu trong labVIEWString: mt chui cc hin th hoc khng hin th cc k t ASCII
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Lp trnh LabVIEW
Cc kiu d liu trong labVIEWEnum bao gm mt cp gi tr, mt chui v mt s, cc Enum c th l mt danh sch c xc nh bng cc gi tr Enum lit k cc control, constant hoc indicator
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Lp trnh LabVIEW
Cc kiu d liu trong labVIEWDynamic: kiu d liu ng lu tr cc thng tin c to ra hoc thu thp bi VI Express Nu s dng VI thch hp th labVIEW s t ng chuyn i thnh kiu d liu ng
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Lp trnh LabVIEW
K THUT LP TRNH
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Lp trnh LabVIEW
Th vin trong Block DiagramFunctions\Programming:Structures: cc cu trc vong lp (while, for, case, sequence, formula, feedback,.) Array: cc kiu trch lc, bin i, to mng, Cluster: cu trc gom nhm (bundle, unbundle,) Numeric: s hc (ton hc, random numer, hng s) Boolean: dng logic (cng, bin i s boolean, .) String: text (string constant, bin i chui,) Comparision: cc hm so snh Timing: hm tr thi gian Dialog & user..: button, thng bo, giao din,. File I/O: c, ghi file trong labVIEW.GV: Nguyn Thanh Tn
Lp trnh LabVIEW
Th vin trong Block DiagramMeasurement I/O: DAQmx Data Acquisiton (thu thp d liu): DAQ Assistant Instrument I/O: thu thp d liu qua cc giao tip ni tip, Mathematics: cc hm ton hc, tn hiu (sin,cos...) Signal Processing: tn hiu m phng, dng sng, th, lc, ph,. Express: Input (DAQ, scope, simulate sig), signal analysis, output, Merge signals, Arithmatic & comparision, Timing, Structure,GV: Nguyn Thanh Tn
Lp trnh LabVIEW
Th vin trong Front PanelModern:Numeric: s control, indicator,.. Boolean: logic (SW, button, led,) String & path: text control, text indicator, file path,.. Array, matrix & cluster: mng control, indicator, gom nhm, Graph: th (waveform chart, waveform graph,..)
System / Classic: cc dng control, indicator khc: s hc, boolean, text, button, SW, Express: numeric control, numeric indicator, tex control, text indicator, button& Swithes, led, Graph,..GV: Nguyn Thanh Tn
Lp trnh LabVIEW
While Loop
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Lp trnh LabVIEW
While Loop1. Chn While Loop
2. Khoanh vng m mun t trong vong lp
3. Ko th vo vong lp cc nt v sau i dyGV: Nguyn Thanh Tn
Lp trnh LabVIEW
While LoopBin m (Interation terminal): tr v s ln lp thc hin, bt u t zero-0 iu kin kt thc (conditional terminal): xc nh khi vong lp dng
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Lp trnh LabVIEW
While Loop - TunnelsTunnel truyn d liu vo ra cho vong lp D liu vt ra khi vong lp sau khi n chm dt Khi mt ng d liu ni vo Tunnel ca mt vong lp, vong lp ch thc thi sau khi d liu n Tunnel hoc d liu xut ra Tunnel sau khi kt thc vong lp
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Lp trnh LabVIEW
While Loop Error Checking v Error HandlingDng mt error cluster trong while loop dng vong lp while loop nu xy ra li
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Lp trnh LabVIEW
For Loop
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Lp trnh LabVIEW
For Loop
S ln lp
Bin m
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Lp trnh LabVIEW
For Loops Conditional TerminalConditional Terminal: iu kin dng, vong lp For Loop s dng li khi nhn mt Boolean Conditional hoc mt li xy ra
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Lp trnh LabVIEW
For Loops Conditional TerminalMt khi vung mu c gn vo ging nh while loop iu kin dng nm gc phi ca vong lp
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Lp trnh LabVIEW
For Loops Numeric Conversion
8 bytes
4 bytes
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Lp trnh LabVIEW
V d
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Lp trnh LabVIEW
So snh For Loop/ While Loop
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Lp trnh LabVIEW
nh thi cho mt VI (Timing)Ti sao phi nh thi?Kim sot tn s m ti vong lp thc hin Cung cp ti nguyn cho b vi x l v thi gian hon thnh cc nhim v khc, chng hn nh tc v x l giao din ngi dng
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Lp trnh LabVIEW
Timing Wait FunctionsMt Wait Function trong vong lp cho php VI trng thi Sleep vi thi gian c nh trc Cho php b x l thc hin cc tc v khc trong qu trnh ch i S dng xung ng h ca h iu hnh tnh bng ms
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Lp trnh LabVIEW
Thanh ghi (Shift Registers)Thc hin vic truyn kt qu vo/ra vong lp hoc t vong lp trc sang vong lp sau
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Lp trnh LabVIEW
Shift RegistersClick chut phi vo khung vong lp v chn Add Shift Register Thanh ghi dch bn phi s lu tr kt qu sau mi ln lp Thanh ghi dch bn tri s cung cp d liu cho ln lp tip theo
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Lp trnh LabVIEW
Shift Registers
Output=3
?
Output=5
?
Output=2
?
Output=4
?
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Lp trnh LabVIEW
Stacked Shift RegistersNgn xp thanh ghi lu tr nhiu gi tr t cc ln lp trc v truyn gi tr ny n ln lp k tip Click chut phi vo thanh ghi dch (Shift register) bn tri v chn Add Element
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Lp trnh LabVIEW
Nt phn hiS xut hin t ng trong vong lp For hoc While nu nh bn i dy u ra ca mt subVI, function, hoc group ca subVI v functions ti u vo ca VI tng ng, function, hoc group.
D liu lu tr khi m hon tt mt vong lp, gi d liu ti vong lp k tip v truyn i mi kiu d liu.
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Lp trnh LabVIEW
Nt phn hii dy t u ra ti u vo c t ng to ra mt nt phn hi t mt nt phn hi t bng Functions Structures
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Lp trnh LabVIEW
Cu trc CaseThc hin v hin th ch c mt trng hp hoc mt s con trong mt thi im Ng vo s xc nh cho php trng hp no hay s no c thc thi Cu trc Case cng tng t nh cu trc If..then..else trong cc ngn ng lp trnh dng text
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Lp trnh LabVIEW
Cu trc CaseCase Selector Label: cha tn ca trng hin ti
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Lp trnh LabVIEW
Cu trc Caseu vo lun c sn cho tt c cc trng hp Phi xc nh mi u ra tunnel cho mi trng hp
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Lp trnh LabVIEW
Cu trc Case - DefaultNu ng vo khng ri vo bt k trng hp no th cu trc Case s thc hin chng trnh mc nh (default) Nu khng c dy no a vo th cc kiu d liu s c default theo bng sau:
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Lp trnh LabVIEW
Cu trc Case - BooleanNg vo l Boolean ch to ra hai trng hp: True hoc False
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Lp trnh LabVIEW
Cu trc Case - IntegerTa c mi trng hp (case) cho mi s nguyn Nu s nguyn m khng c mt trng hp c xc nh th t ng chn trng hp mc nh
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Lp trnh LabVIEW
Cu trc Case - StringMi trng hp tng ng vi mt chui nht nh Chui khng xc nh th n s chn trng hp mc nh
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Lp trnh LabVIEW
Cu trc Case - EnumCung cp cho ngi dng mt danh sch cc bng ghi chn
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Lp trnh LabVIEW
Cu trc Case Error Checking v Error HandlingS dng cu trc Case trong VI thc thi chng trnh khi khng c li, v b qua chng trnh khi c li pht sinh
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Lp trnh LabVIEW
Mng (Arrays)Mt mng (Array) c trng bi kch thc v phn t trong nPhn t (Element): d liu nm trong mng Kch thc: chiu di v chiu rng, ct x hng y Mng c th c mt hoc nhiu chiu v c (2^31-1) phn t trn mi chiu
Mng s dng mt tp hp cng kiu d liu v cc php ton mang tnh lp li nhiu ln
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Lp trnh LabVIEW
To mng1. t mt mng trng (array shell) ln front panel 2. Ko hoc th i tng hoc phn t vo array shell
3. V d: mng hng
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Lp trnh LabVIEW
Array Auto IdexingKhi ch Auto index c m ng ra tunnel, mng ng ra nhn tng phn t mi mi vong Mng to ra lun c kch thc (s phn t) bng vi s ln thc thi ca vong lp Click chut phi ln tunnel v chn enable/disable auto- indexing
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Lp trnh LabVIEW
Mng 2DLu tr cc phn t dng li to mng a chiu: click chut phi vo index display v chn Add Dimention
Mng cha khi to
Mng khi to gi tr ban uLp trnh LabVIEW
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To mng 2DC th to 2 vong lp For loop lng vo nhau to mng 2D
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Lp trnh LabVIEW
Array
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Lp trnh LabVIEW
Clusters Kiu gom nhmClusters l tp hp nhiu kiu d liu khc nhau Dng hm Bundle hoc Unbundle t hp hoc phn gii mt Cluster
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Lp trnh LabVIEW
To mt Cluster to mt Cluster control hoc indicator trn front panel:t mt cluster shell trn front panel Ko th cc i tng hoc phn t vo cluster shell nh: numeric, boolean, string, path, refnum, array,
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Lp trnh LabVIEW
Error ClusterLabVIEW s dng error cluster chc thng tin li Mt error cluster cha cc phn t sau:Status: kiu boolean, tr v True nu khng c li v ngc li Code: kiu 32 bits Signed integer xc nh m li Source: kiu string xc nh ni b li
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Lp trnh LabVIEW
So snh vi ArrayGing nhau: c control hoc indicator nhng ng thi khng cha c hai Cluster c kch thc thay i so vi kch thc c nh ca array Cluster cha nhiu kiu d liu khc nhau; con array ch cha mt kiu d liu duy nht
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Lp trnh LabVIEW
MT S K THUT THNG DNG
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Lp trnh LabVIEW
Lp trnh phn on (Sequential Programming)Cc VI bn vit s c thc hin mt cch tun t Khng c bt k s u tin no trong block diagram, v khng bit c ci no trc ci no sau.
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Lp trnh LabVIEW
Lp trnh phn on (Sequential Programming)S dng error clusters u tin qu trnh thc thi
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Lp trnh LabVIEW
Lp trnh phn on (Sequential Programming)S dng cu trc on (Sequence structure) u tin thc thi Mt cu trc vi cc khung (frames), Mi khung s cha mt tc v Khung th 2 khng th thc thi cho n khi mi th trong khung th nht hon thnh
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Lp trnh LabVIEW
Lp trnh phn on (Sequential Programming)Trnh s dng qu nhiu cu trc phn on Bn khng th dng thc thi mt thnh phn trong mt on
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Lp trnh LabVIEW
Lp trnh phn on (Sequential Programming)Cch tt nht vit VI ny l nh km cc hp thoi trong cu trc Case, dy error cluster s c ni ti b chn ca cu trc case(case selectors)
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Lp trnh LabVIEW
Trng thi my (State Machines)Lp trnh trng thi(State Programming) l mu thit k nhm thc hin mt s trng thi hoc mt lu Khi no s dng state machines? Thng c s dng to cc giao din, i vi mi ngi dng khc nhau th s c giao din khc nhau Thng c s dng th nghim qu trnh, trong mi trng thi i din mi phn on ca qu trnh.GV: Nguyn Thanh Tn
Lp trnh LabVIEW
Trng thi my (State Machines)Mt state machine bao gm nhiu trng thi v mt chc nng chuyn tip n cc trng thi k tip Mi trng thi c th dn ti mt hoc trng thi hoc kt thc qu trnh thc thi
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Lp trnh LabVIEW
File I/OFile I/O c v ghi vo mt file Mt phng php in hnh truy xut file:
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Lp trnh LabVIEW
nh dng File (File Formats)LabVIEW c th s dng hoc to ra cc nh dng tp tin sau y: Binary l kiu nh dng cp thp cho tt c cc nh dng khc s dng ASCIIL kiu t bit ca binary file, l nh dng tiu chun cho hu ht tt c chng trnh LVM LabVIEW measurement data file (.lvm) nh dng ca LabVIEW, cha bn tnh v vn bn TDMSMt kiu file Binary c to bi cc sn phm ca NI bao gm 2 file tch ri: Mt binary file v mt binary index fileGV: Nguyn Thanh Tn
Lp trnh LabVIEW
nh dng File (File Formats)Trong phn ny, tp trung vo vic to text (ASCII) files S dng cc Text file trong cc tnh hung sau y : Truy xut cc tp tin t cc ng dng khc Khng quan tm n b nh trng, v tc I/O ca file Khng nn truy xut dng random read hoc write
GV: Nguyn Thanh Tn
Lp trnh LabVIEW
High-level File I/OHigh-level VIs truy xut I/O file phi qua 3 bc chung Khng hiu qu cho vic cu hnh hoc thit k cho cc tnh nng ring ca file
Low-level VIs VI ring bit cho mi bc Nu ghi file trong mt vong lp, s dng hm low-level file I/OGV: Nguyn Thanh Tn
Lp trnh LabVIEW
High-Level File I/OWrite to Spreadsheet File Chuyn i mt mng dng double-precision n text string v ghi string ny n ASCII file Read From Spreadsheet File c cc gi tr s theo dong (line) hoc hng (row) t mt text file cha s v xut ra ngoi mt mng 2D array vi cc phn t dng s doubleprecision Write to/Read from Measurement File Mt dng Express VI cho php ghi (write), c (read) mt LVM hay TDMS fileGV: Nguyn Thanh Tn
Lp trnh LabVIEW
Low-Level File I/O VIs
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Lp trnh LabVIEW
VariableCannot pass data between parallel loops with a wire Variables allow you to circumvent normal dataflow by passing data from one place to another without connecting the two places with a wire
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Lp trnh LabVIEW
VariableVariables are block diagram elements that allow you to access or store data in another location Variables can be of the following types: Local: store data in front panel controls and indicators Global: store data in special repositories that can be accessed from multiple VIs Functional Global: store data in While Loop shift registers Shared: transfers data between various distributed targets connected together over a networkGV: Nguyn Thanh Tn
Lp trnh LabVIEW
Local VariableUse local variables to pass data within a single VI
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Lp trnh LabVIEW
Global VariableUse a global variable or a single process shared variable to share data between multiple VIs Use a global variable to share data among VIs on the same computer, especially if you do not use a project file Use a single process shared variable if you may need to share the variable information among VIs on multiple computers in the future
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Lp trnh LabVIEW
Property NodeProperty Nodes access the properties of an object In some applications, you might want to modify the appearance of front panel objects programmatically in response to certain inputs If a user enters an invalid password, you might want a red LED to start blinking If a data point is above a certain value, you might want to show a red trace instead of a green one Property Nodes allow you to make these modifications programmaticallyGV: Nguyn Thanh Tn
Lp trnh LabVIEW
Property NodeProperty Nodes execute in order from top to bottom If an error occurs on a terminal, the node stops at that terminal, returns an error, and does not execute any further terminals
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Lp trnh LabVIEW
Control ReferencesA control reference is a reference to a front panel object Wire control references to generic Property Nodes Pass control references to subVI
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Lp trnh LabVIEW
Control References
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Lp trnh LabVIEW
Control References
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Lp trnh LabVIEW
Invoke NodesInvoke Nodes access the methods of an object Use the Invoke Node to perform actions, or methods, on an application or VI Unlike the Property Node, a single Invoke Node executes only a single method on an application or VI
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Lp trnh LabVIEW
Invoke NodesTo create an implicitly-linked Invoke Node, rightclick the control and select CreateInvoke Node and choose a method An example of a method that is common to all controls is the Reinitialize to Default method The Waveform Graph has the method Export Image:
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Lp trnh LabVIEW
Events Programming
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Lp trnh LabVIEW
Events ProgrammingExamples of User Interface (Static) Events Pressing a button on the mouse, moves or clicks the mouse Pressing a key on the keyboard Value (Signaling) Property can generate a programmatic event
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Lp trnh LabVIEW
USB 6008
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Lp trnh LabVIEW
THANK YOU!
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Lp trnh LabVIEW