0-60 Meurement nd Intrumenttin Lecture Nte: Strin Meurement eview f Stre nd Strin Figure : Structure under tenin Frm Fig., xil tre σ, xil trin, trnvere trin t, Pin' rti ν, nd Yung mdulu E re σ F A, dl dd, t L D t dd / D υ 0., dl / L σ E, where A i the cr-ectinl re. Electricl-eitnce Strin Gge -D trin gge ette gge Figure : Different type f trin gge
Widely ued fr trin meurement eitnce f gge mteril chnge with defrmtin Ue t meure lcl trin in lterl directin f the gge (ee Figure ) Cn meure multi-directinl trin uing rette gge (ee Figure ) Principle f Opertin Electricl reitnce f ny gge wire i determined frm ρl, A where L i the gge length, A i cr-ectinl re f the wire nd ρ i the reitivity in Ω- m. Electricl reitnce f the wire chnge with trin, i.e. trin increed, L decreed, A decreed nd ρ increed. Therefre with the trin increed, the reitnce increed. Fr the trin gge, the electricl wire re deigned uch tht their reitnce linerly incree with trin, prvided cntnt temperture. eltinhip f nd Frm ρl where A cd, c (qure) nd c π/ (circulr). The chnge f i then A d r ρl d A L ρ dρ + dl A A L ρ dρ + dl A A ρl da A ρl dd A D d dρ dl dd + ρ L D Divide the whle equtin with dl/l d / dl / L d / dρ / ρ + + υ If ρ i cntnt, i.e. reitivity de nt chnge with trin, thu d / + υ Gge Fctr (GF)
emrk: GF typiclly vrie frm.6 t. High GF (high enitivity) i deirble becue lrge chnge in reitnce cn be prduced fr given trin. i nrmlly 0 Ω r 50 Ω. Axil trin i in the rnge f 0-6 t 0 -, reulting in d 0.000-0. Ω. Nte tht hw mll d i, therefre we cnnt imply tick n hm meter cr the trin gge. Whettne bridge re cmmnly circuit ued t mplify uch lw reitnce vlue in trin meurement. Whettne Bridge Figure : Whettne bridge Frm the circuit in Fig., the utput vltge i ( + )( + ) If ll fur reitr re identicl r if, the bridge i blnced r 0. ) Qurter Bridge Circuit (ne ctive gge) Figure : Qurter bridge-ne ctive gge
Figure hw the qurter bridge circuit where the trin gge i et t in the bridge, r i n ctive gge. Befre the meurement, the bridge i initilly blnced, i.e. i 0 r. If 0 Ω, then the trin cue chnge in + d. The utput vltge per input vltge i therefre ( + d) ( + + d )( + ) ( + ) d Since d / GF nd, hence GF Or the enitivity (in unit f /ppm) f ne ctive gge (qurter bridge) i GF Figure 5 hw ne ctive gge intlltin t meure the xil trin n br. Figure 5: Qurter bridge intlltin Exmple: Fr the etup in Fig. 5, if under tenin the utput vltge i. m. Given 0 nd GF.8, determine the meured trin. Frm the enitivity reltin, we btin GF. 0 0.8 0.0009, r 9 ppm
) Hlf Bridge Circuit (tw ctive gge) Figure 6: Hlf bridge intlltin We cn ue tw ctive gge t nd hwn in Figure 6. Initilly blnce the gge, hence i. When lded, we cn get the utput vltge per input vltge ( + ) d i d + where d d GF. Therefre the enitivity, fr thi ce, i i i GF In generl, if we ue ll fur reitr ctive gge the utput vltge per input vltge i ( + ) d i d i d + d i Nte tht: d nd d bth cntribute t pitive utput vltge where d nd d cntribute t negtive vltge trin i increed. Accrding t pitive nd negtive ign in bridge circuit, it i imprtnt where yu plce trin gge in the circuit. Fr the previu exmple, if we put the firt gge t, it de nt mke ene t put the ecnd gge t nd, becue reitnce chnge wuld cncel nd utput vltge wuld be zer.
Figure 7: Strin meurement f bem Fr little exercie, if we hve tw ctive gge, ne i in tenin nd the ther i in cmprein hwn in Fig. 7, where cn we plce thee tw ctive gge in the Whettne bridge? Nte tht the reitnce chnge due t cmprein i minu f tht in tenin. (Anwer: nd, nd, nd, r nd, ele reult in zer utput vltge.) ) Temperture Cmpentin Figure 8: Temperture cmpentin T cncel the reitnce chnge f the trin gge due t therml effect, we cn etup the gge hwn in Fig. 8. With n ctive gge, we cn ue r t cmpente fr the temperture. ) Full Bridge Circuit (fur ctive gge) When ll fur reitr n the bridge re replced by the trin gge, we cn mke creful chice fllw: " nd mut hve pitive trin (tenin) while nd hve negtive trin (cmprein)". In thi ce, we get the enitivity f full bridge fur time f tht fr the qurter bridge nd tw time f tht fr the hlf bridge.
5) Bridge Cntnt Bridge cntnt K i defined the rti f the utput frm n ctive bridge t the utput f the bridge with primry (ne ctive) gge. 0 Fr ne ctive gge (K), the enitivity i GF, fr tw ctive gge r hlf 0 bridge (K), 0 GF, nd fr fur ctive gge r full bridge (K), GF. In 0 K generl, GF, where K i the bridge cntnt. Nte tht bth gge fctr GF nd bridge cntnt K determine verll enitivity f the trin gge ytem.