Vilnius Gediminas Technical University Naugarduko 41, LT-03227, Vilnius, Lithuania

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1 METHODS OF STRAIN MEASUREMENTS IN CONSTRUCTION MONITORING SYSTEMS Vygaudas Kvedaras Vilius Gedimias Techical Uiversity Naugarduko 41, LT-03227, Vilius, Lithuaia Rokas Kvedaras Vilius Gedimias Techical Uiversity Naugarduko 41, LT-03227, Vilius, Lithuaia Ričardas Masiuliois Vilius Gedimias Techical Uiversity Naugarduko 41, LT-03227, Vilius, Lithuaia ABSTRACT Results of aalysis of recet accidets i public buildigs are preseted. It is show that costructio state idetificatio systems are itegral part of public ad local warig ad security systems. Structure of costructio of state idetificatio system is proposed ad aalysed. It is established that i order to esure wide usage of such systems it is ecessary to solve some scietific problems. Solutio for multipoit measuremet of strai as oe of the most importat parameter for costructio state idetificatio is proposed. It is show that measuremets of strai by usig closed-loop self-balacig system esure ecessary resolutio with high accuracy ad ca be used i multipoit measuremets. Two closed-loop selfbalacig systems usig digitally balaced Wheatstoe bridge ad balacig of currets are preseted ad aalysed. KEYWORDS Stai measuremet, Multi-chael strai measuremet, Digitally Balaced Wheatstoe Bridge, Digital balacig of currets 1. INTRODUCTION Several buildigs costructed of lightweight steel or cocrete structures have collapsed i recet years. Ufortuately i some of these collapses people lives were lost ad every case huge fiacial losses were sustaied. There were several accidets i the Europe recetly i Lithuaia, Russia, Polad ad Germay. Most frequetly such collapses occur durig witer time whe structures are exposed to additioal load because of the sow ad ice which is accumulatig o a roof, strog wids, ad other exposures that overload the structures of the buildig. 343

2 It would be possible to save huma lives if people were otified about a critical coditio of buildig structures at least miutes before collapse. I case it is kow of critical coditios 1 2 hours before the collapse it is eve possible to keep a buildig itself. I this case for example it is possible to clear roof of a buildig from sow reducig critical loads or take other measures to reduce critical load or stregthe the structure to be able to carry icreased loads. It is ot possible to determie the load of a structure visually or by other simple observatio methods. Therefore automatic sigallig systems are to be used. Whe certai parameters of the structure are exceedig defied threshold values, the system must set a alarm iformig about arisig dager. Developmet ad implemetatio of such costat moitorig systems is ot commo yet. Authors are aalyzig possibilities to build ad use such systems. 2. REQUIREMENTS FOR THE MONITORING SYSTEM Basic costructive elemets of a light structures used i various buildigs are usually steel farms. To idetify state of steel costructios automatically some parameter that ca be chaged to electrical value ad the measured should be foud. A set of differet parameters that are measured o the costructio with the mathematical model of a costructio applied will result i the uique state of the costructio which is evaluated for the threshold value. A set of the physical parameters that are to be measured o the costructio may be differet i each case ad each uique costructio. But for the most cases displacemet (deformatio), strai, force ad acceleratio parameters are measured. Nevertheless most of these above-metioed parameters are coverted to the strai measuremets ad therefore measuremet of strai is oe of the most importat issues to be solved for such systems. There is a rigid relatio betwee durability ad strai of a structure elemet i steel structures [1]. O the other had, size of strai of a steel costructio eve i case of exposure of high loads is rather small. For example, if o a steel wire with cross-sectio A = m 2 ad legth l = 10 m, modulus of elasticity E = 6, Pa, force of size F = 1000 N is applied the relative strai will be ε t = F EA = (1) Strai ca be measured usig differet techiques but, for costat moitorig ad applicatios where higher umber of poits eeds to measured, reliable ad comparably low-cost resistive strai gauges are used. Resistive strai gauges trasform strai of the gauge to small chage of its resistace. These small chages are to be measured. I a complex buildig there are may carryig structures (or to say typical poits of those structures) coditio of which idividually or i groups defies state of the buildig ad as a result safety of buildig operatio. Therefore moitorig systems should be multi-chael ad i most cases would have up to several hudreds or eve more chaels. Desigers of a buildig or a structure should apply mathematical model ad obtai such critical poits for measurig parameters of the costructio ad threshold values for these parameters. Durig the costructio respective gauges should be implemeted ito the structure ad coected to the measuremet system. Cocludig the aforemetioed it is clear that moitorig system must a) determie the curret coditio of a buildig (structure, bridge, roof, etc.); b) defie small chages of parameters (strai, vibratio, acceleratio, etc.) of physical quatities i specific poits of a structure; c) trasfer results of a estimatio to the data-processig cetre where results are processed, mathematical model of a structure is applied ad result is carried out [2]. Accordig to these results decisio o the curret coditio of a structure is made (automatically or by huma operator). 3. MULTI-CHANNEL CONSTRUCTION MONITORING SYSTEM Accordig to earlier experiece further coclusios are made: 1. The system should be as simple as possible. I this case the quatity of error sources, amout of 344

3 errors ad therefore the price is relatively low esurig relatively low-cost service of the system. 2. The system should have the feedback cofiguratio esurig that the ifluece of a part of error sources is compesated. 3. The system havig may chaels (up to several hudreds) should be desiged i a block priciple. I this case measuremet uits are placed i a relatively short distace from measuremet poits ad the system ca be easily adapted for differet purposes. It appears that the structure ad problems to be solved for such systems are practically idetical irrespective of measured parameters of a structure, a ed result or a scope of the system ad is a geeral distributed data-acquisitio system. Similar system desig was proposed [3] but it was relyig o the idividual sesors ad ot implemetig groups of sesors. Groupig of sesors esure that the relatively expesive measuremet uit is ot used for oe idividual sesor but rather a sesor group which simplifies the system ad decrease its price. Created geeral block diagram of moitorig system is submitted o a Figure 1. The developed structure is highly agile it is possible to measure ad store various parameters of the object state, if ecessary addig additioal measurig uits. Virtually it is possible to achieve a o-limited umber of measuremet poits. It is possible to costruct moitorig system gradually by addig additioal measuremet uits upo ecessity. It is possible to measure, accumulate data, ad to aalyze other importat parameters of structure by oly chagig sesors, measurig uits ad/or programs. I this system all sigals from measurig uits have digital form ad ca be trasferred through the Iteret or Itraet. It is possible to apply stadard Wireless LAN systems ad overcome oe of the biggest lacks of the systems curretly i use commuicatio lies that are relatively short ad dealig with aalogue sigals. It is established that implemetatio of measuremet lies i buildigs (it is especially applicable i already build-up "workig" buildigs) is quite heavy ad expesive job. At the same time measuremet lies are mouted ear cables of commuicatios, electric supply ad cotrol lies are subject of ifluece of oises. It results to icrease of errors ad reductio of reliability of the system. Sesors Measuremet uit (strai) Service statio of the data ceter Computerized workplace of operator Sesors Sesors Measuremet Measuremet uit uit (strai) Database Measuremet Measuremet Measuremet Sesors Sesors Sesors uit uit uit (strai) Object Sesors Sesors 1 Object Sesors 1 Measuremet Measuremet Cotrol uit uit uit (strai) Measuremet Cotrol uit uit (other) Object 1 Cotrol uit Figure 1. Block diagram of moitorig system Iteret 345

4 Therefore structure of the system that allows reducig of the measuremet ad commuicatio lies is more preferable ad cheaper at the same time. System proposed esures that locatio of separate measurig blocks ca be established i immediate proximity from measured structures ad at the same time legth of coectig cables ca be reduced reducig errors of measuremet. The cetral server ad workplace of trackig ad maagemet i this case ca be placed at ay locatio that is the same or differet from the buildig moitored. Furthermore the cetralized system ca moitor several buildigs or eve all required to moitor buildigs of the city ad build metropolita area moitorig system. Later such system might be itegrated ito cetralized emergecy services system (such as 112, etc.) for fast reactio to ay alarms geerated by the system. Buildig of such systems based o strai measuremet has several difficulties to deal with. Earlier it has bee show; that the size of deformatio is rather small, therefore the chage of strai-gauge resistace is also small: R= RS ε t ; (2) where R strai-gauge resistace; ε t strai of costructio; S sesitivity ratio. For example i case of chage of strai of steel farm is i rage ε t = , strai-gauge resistace is R = 120 Ω ad sesitivity ratio S = 2, the chage of resistace of strai gauge is R = ( ) Ω. It is a very small value ad typically obalaced Wheatstoe bridges are used for such measuremets. It has bee show that applicatio of o-balaced Wheatstoe bridge without feedback is ot coveiet for such type of applicatios where several strai gauges are coected to the same measuremet uit [4, 5]. Nevertheless early all resistive strai gauge measuremet systems i the market are based o o-balaced Wheatstoe bridge applicatio ad therefore are strugglig to expad systems as implemetatio, expasio ad maiteace of such systems are costly. Authors have developed a digitally balaced Wheatstoe bridge that allows implemetatio of the moitorig system proposed Digitally Balaced Wheatstoe Bridge for strai measuremet system The method for measuremet of small resistace chages by usig automatically digitally balaced Wheatstoe bridge has bee developed by authors [4 6]. Method is based o usig of R-2R DAC as balacig elemet. Geeral structure of the method is provided i Figure 2. The resistace of the sesor i this case is: R x = R = R R R4N + 2 R 2 R R 4 IN IN ; (3) where umber of DAC bits, R IN resistace of DAC iput (resistace R-2R matrix elemets), N decimal DAC cotrol code. It is established that proposed method allows avoidig multiple disadvatages that are typical to the classic system. The created method is a closedloop method. Wheatstoe bridged is balaced by chagig resistace of DAC ad checkig whether the balace is exceeded or ot. As code cotrolled resistace DAC of R-2R type is used. MOSFET switches are used for coectio ad discoectio of differet chaels. These switches have resistace of the ope chael less tha 0.05 Ω ad therefore the ifluece of these resistors to the measuremet result is miimal. The mai advatages of this method are: Closed-loop system is regulatig itself ad therefore reducig ifluece of exteral oise to evaluatio results. System is ot highly iflueced by legth ad resistace of the gauge coectig wires ad therefore ca be easily used as multi-chael system by multiplexig gauges usig low resistace switches. The strai measuremet i the system is made i two stages: the status of the bridge after gluig gauges o a costructio to be measured, ad the load is ot applied to the structure. The the status of the loaded costructio is measured. The size of strai is defied as a differece of these two measures. Aalysis ad experimetal research has show that this method ca be used for moitorig tasks with 346

5 Rj1 Rk1 Rj Rk + - U S1 S2 S3 S4 Amp C mp M R1 R2 R DAC R D Figure 2. Fuctioal scheme of the method for evaluatio of small resistace chages by usig automatically digitally balaced Wheatstoe bridge with R-2R DAC. U Bridge excitatio supply uit; Amp Op Amp; C comparator; mp microprocessor; DAC digital-to-aalog coverter; S switch; Rji strai gauge; Rki compesatio gauge; M output code resolutio up to 2 8. The model of the system was checked durig 24 hours ad worked with 10 m legth cables. The measuremet errors are ot established for all this period of time. Nevertheless system with digitally balaced Wheatstoe bridge has several disadvatages: The legth of the wires from the measuremet uit to the gauges is limited by the resistace of the wire ad ifluece of the oises. Resistace rage of usable strai gauges ad maximum resistace chage limits are limited by the iteral resistace of R-2R matrix. Selectio of DAC s with direct access to the R-2R matrix is rather limited. Therefore for the specific applicatios with ostadard strai gauges or i applicatios where comparably loger wires (> 10 m) are ecessary a ew closed-loop system has bee developed by authors Digital Balacig of Currets for Strai measuremet system The ew closed-loop system is based o the balacig two resistive circuits by chagig curret o oe of the circuits. The chage of curret to maitai balace is proportioal to the chage of resistace of strai gauge i the correspodig resistive circuit. Scheme of the method is preseted i Figure 3. Both sides of the scheme are complemeted of equal precise referece resistors R 1 = R 2 = R A ad two strai gauges R j1 ir R j2. Gauges are of the same type but their iitial values may vary by value R s. Because of this iitial differece it is ecessary to balace the circuit as it is doe i Balaced Wheatstoe Bridge method i order to achieve that voltage U 12 is equal to zero. Whe this is achieved, the followig expressio is true: = I R + I R I R I R. (4) 0 1 j1 1 A 2 j2 2 A 347

6 I1 U I 2 12 Rj R1 Figure 3. Scheme of Currets balacig method Currets are digitally balaced usig curret source cotrolled by DAC. For example curret I 2 is I m I chaged by steps I MRS, where MRS = 2 the least step of curret chage, I m maximum value of curret, resolutio of DAC. Curret I 2 after balacig is equal to: I m I 2 = I1+ m, (5) 2 where m decimal umber of I MRS steps. After applyig load to the costructio ad chage of resistace of gauges to R = R + R + R, Rk R2 j1 j2 m S balacig code of resistace chage because of strai will be: I 2 1 Rm + RS mm = ; (6) Im RA+ R j2 where R m chage of gauges resistace because of load. The resistace chage of gauges because of strai is equal to: I1 Rm M = mm m= 2. (7) I R + R m A Based o this method fuctioal diagram of curret balacig system has bee developed by authors ad is preseted i Fig. 4. By usig curret balacig method it is possible to avoid umber of exteral iflueces such as electric oises ad resistace of the coectors ad coectig wires. DACs used for this method does ot require direct access to the R- 2R matrix ad therefore the selectio of low-priced DACs available i the market is much wider. By usig curret balacig method it is possible to use gauges of much wider resistace rage ad also gauges with higher strai resistace chage rates. 4. CONCLUSIONS 1. It is possible to avoid uexpected costructio collapses if the costructios are costatly moitored ad prevetive actios are take. j2 Rj1 Rk1 Rj S1 I1 U Rk S2 S3 R3 I2 DAC S4 M S5 C mp R1 R2 Figure 4. The strai measuremet method with compesatio of currets. S switch; C comparator; I1 curret source; I2 adjustable curret source; DAC digital to aalog coverter, mp microprocessor; U referece voltage; M output code 348

7 2. Most of costructio moitorig systems relyig o direct ad idirect measuremet of strai o multiple poits of costructio. 3. The method for evaluatio of small chages of resistace based o automatically digitally balaced Wheatstoe Resistace Bridge usig DAC R-2R matrix is developed ad ivestigated. It eables to improve evaluatio of small chages of resistace ad to avoid disadvatages observed i classic obalaced Wheatstoe bridge applicatios. The accuracy of the method is ot iflueced by the stability of the bridge excitatio voltage ad the feedback ature of the system reduces the ifluece of exteral iterfereces to evaluatio result. 4. I order to avoid ifluece of the coectig wires ad to improve flexibility of strai measuremet by usig gauges of wider resistace rage a ew strai measuremet closed-loop selfbalacig method with balacig currets has bee developed ad ivestigated. REFERENCES [1] The Pressure, Strai ad Force Hadbook. (1995) Omega Egieerig. Vol p. [2] Doebeli E. O. (1990) Measuremet Systems Applicatio ad Desig. 4 th Ed. McGraw-Hill, 876 p. [3] P. J. Bock, S. Majumdar, W. J. Bock (2007) Iteret-Based Distributed Data Acquisitio System for Fiber-Optic Sesors. IEEE Trasactios o Istrumetatio ad Measuremet. IEEE. Vol. 56, No. 1, p [4] Kvedaras R., Kvedaras V., Martavičius R. (2003) Elektroiė sistema uikalių kostrukcijų būseos pokyčiams įvertiti. Elektroika ir elektrotechika. Kauas: Techologija. No. 6(48), p [5] Kvedaras R. (2004) Fulfillmet of geeral Requiremets for Costructio State Idetificatio Systems. Coferece Archives PTETiS. Vol. 19, No. 1, p [6] Kvedaras R., Kvedaras V. (2008) Deformacijų matavimas ir kostrukcijų moitorigas. Elektroika ir elektrotechika. Kauas: Techologija. No. 1(81), p