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Sensors and Actuators A 190 (2013) 197– 202

Contents lists available at SciVerse ScienceDirect

Sensors and Actuators A: Physical

jo u rn al hom epage: www.elsev ier .com/ locate /sna

assive wireless displacement sensor based on RFID technology

ario J. Cazecaa, Joey Meada, Julie Chenb, Ramaswamy Nagarajana,∗

Department of Plastics Engineering, University of Massachusetts, Lowell, MA 01854, United StatesDepartment of Mechanical Engineering, University of Massachusetts, Lowell, MA 01854, United States

r t i c l e i n f o

rticle history:eceived 13 June 2012eceived in revised form 28 October 2012ccepted 5 November 2012vailable online 16 November 2012

a b s t r a c t

This research demonstrates feasibility of using off-the-shelf radio frequency identification (RFID) tags tobuild a low cost passive wireless displacement sensor, suitable for applications such as crack detectionin buildings and bridges. The sensor was constructed by splitting a RFID tag into two component partsconsisting of the tag antenna and the chip-loop, which can be displaced by a distance represented by‘d’. The displacement d could represent the width of a structural crack that may be found in a building

eywords:FIDisplacement sensorireless sensor

assive tagtructural health

or bridge. A loop antenna attached to a RFID reader can be placed at some distance ‘D’ in proximity tothe sensor to measure the effect of the displacement (d) on the power transmitted by the RFID chip backto the reader. By incrementally increasing the power transmitted by the RFID reader to the sensor (Pt)when interrogating the RFID chip, calibration curves of the displacement d versus transmitted power Pt

for different values of D were obtained. The use of the calibration curves along with known values of thetransmitted power Pt allows for the determination of the displacement ‘d’.

© 2012 Elsevier B.V. All rights reserved.

. Introduction

Radio frequency identification (RFID) systems are well knownor being reliable and versatile to track and identify objects atistances without the line of sight. RFID technology allows for iden-ification of many tags simultaneously in a very fast and efficient

anner. In applications such as supply chain management, wherepeed is of paramount importance, this technology has alreadyeen implemented for inventory monitoring especially in largeetail stores and warehouses with thousands of different typesf products [1,2]. RFID technology has also found large applica-ions in the area of sensors owing to their wireless and far field

easurements capabilities. More specifically, RFID combined withensors can be used in surveillance [3], monitoring and measure-ents of environmental factors such as moisture [4], humidity [5,6]

nd temperature [7,8], in capacitive and resistive applications [9],as sensing [10], and displacement sensors for monitoring healthf structural parameters [11–13] among many other applications.he use of RFID tags to build wireless displacement sensors are ofundamental importance for monitoring structural health of build-ngs and bridges. Wireless displacement sensors can help prevent

nnecessary and expensive structural repairs by constantly moni-oring the condition of structures and transmitting the information

∗ Corresponding author. Tel.: +1 978 934 3454; fax: +1 978 934 3089.E-mail address: Ramaswamy Nagarajan@uml.edu (R. Nagarajan).

924-4247/$ – see front matter © 2012 Elsevier B.V. All rights reserved.ttp://dx.doi.org/10.1016/j.sna.2012.11.007

collected to monitoring stations for appropriate decision-makingand maintenance.

This paper addresses the fabrication and testing of a reliable andinexpensive passive wireless displacement sensor prototype basedon RFID technology for structural health monitoring applications.A conventional RFID tag operating at 920 MHz was used to buildthe passive wireless displacement sensor prototype. Two sensorswhich differ from each other by the relative position of the RFID chipwith respect to the antenna were assembled and tested. Measure-ments performed using the proposed sensors show their feasibilityand versatility to wirelessly measuring displacement. The sensorscan be made reliable and low cost by using commercially available(off-the-shelf) components. Furthermore, a RFID displacement sen-sor system using the principle described in this manuscript can befabricated with several different tags dispersed in a general area ofinterest on the structure, to facilitate simultaneous remote moni-toring of a large area.

2. Methods

An ultra high frequency (UHF) Class 1 Generation 2 model BeltRFID tag from UPM using the NXP UCODE G2iL chip from NXP oper-ating at frequencies of 860–960 MHz for far field application, (seeFig. 1), was used to perform the experiments described in this work.

The tag dimensions were 70 mm × 14 mm, and was made on an alu-minum film deposited on a thin flexible polyester (PET) substrate.It was used in experiments geared towards testing the concept ofthe wireless displacement sensor.

198 M.J. Cazeca et al. / Sensors and Actuators A 190 (2013) 197– 202

F he pasc

2s

bwmaahica

P

wotura

P

w

�

wr

mc

tf‘tttpocpftpgc

ig. 1. Image of a UPM Belt UHF tag similar to the one used in the fabrication of thip-loop, and the displacement direction d are shown in the RFID tag.

.1. Description of the proposed passive wireless displacementensor

In a passive displacement sensor the RFID reader will alternateetween providing a continuous wave transmitted to the tag toirelessly powers the chip, and receiving the modulated RF com-ands from the tag. The sensor has been fabricated by separating

RFID tag into two component parts, namely chip-loop and the tagntenna, see Fig. 1. In principle, the displacement sensor works byaving the tag antenna fixed, while allowing the chip-loop to move

n relation to the tag antenna in a linear path, while the RFID readerontinuously interrogates the chip. The power received by the tagntenna is given by the equation

T = PtGRGT

∣∣�t�T

∣∣2(

�

4�D

)2

(1)

here Pt is the power transmitted by reader [11], GR is the gainf the transmitting reader antenna, GT is gain associated with theag, |�t�T|2 is the polarization loss factor with �t and �T being thenity vector of the electric field of the power transmitted by theeader and tag respectively, D is distance between the transmittingntenna and the tag, and � is the set wavelength at the reader.

The amount of power received by the RFID chip Pc is given by

c =(

1 −∣∣�T

∣∣2)

PT . (2)

here � T the power reflection coefficient of the tag and is given by

T = Zc − Z∗a

Zc + Za, (3)

ith Zc and Za being the impedance of the RFID chip and antenna,espectively.

Therefore, the power transferred from the tag to chip is maxi-um when |� T|2 = 0, due to perfect match between Zc and Z∗

a theonjugate of the antenna impedance.

The proposed displacement sensor will work on the principlehat for a fixed transmitted power, the amount of RF energy trans-erred to the RFID chip by the tag antenna depends on the distanced’ between chip-loop and the tag antenna, see Fig. 1. The first stepo build the displacement sensor is to generate a calibration curvehat can be used to determine the values of displacement d, whenhe interrogating power of the RFID reader is known. Initially theower at the RFID reader is adjusted to a value Po; the initial thresh-ld power; which is the lowest value of interrogating power thatan establish communication with the RFID chip. At Po the dis-lacement d is set to zero, and any displacement of the chip-looprom zero will interrupt the established communication between

he RFID chip and the reader. This procedure establishes the firstoint (Po,0) of the calibration curve. In the next step, the interro-ating power is incrementally increased once again establishingommunication between the chip and the reader, the chip-loop is

sive wireless displacement sensor presented in this manuscript. The tag antenna,

then slowly displaced from the zero position until the communi-cation between the RFID chip and the RFID reader is interrupted,this is the second point of the calibration curve (P1,d1). This pro-cedure is repeated until there are a sufficient number of points toconstruct a calibration curve for the displacement sensor, see Fig. 2.A calibration curve was generated for distance D between the loopand the tag antenna.

2.2. Experimental setup

To test the proposed wireless displacement sensor the experi-mental set-up shown in Fig. 3 was utilized. Two insulating blocks,denoted by ‘A’ measuring 10 cm × 15 cm × 0.5 cm each, were placedparallel to each other with a 2.2 cm gap between them. The twoblocks, A, were then glued to a 30 cm × 30 cm × 0.5 cm insulatingblock. Insulating material is necessary when working with RF toavoid increasing the impedance mismatch between the tag antennaand the RFID chip. This results in an increase of the reflection coef-ficient resulting in decrease of the power available to the chip. Themismatch occurs when a conducting material is placed in proxim-ity to the chip. The tag antenna was symmetrically fixed on top ofthe two blocks, A, with the straight part of the tag antenna beingcentered over the gap between the two blocks. The chip-loop wasglued onto one of the extremities of an insulating block, B, mea-suring 2 cm × 20 cm × 0.5 cm, and placed into the gap between thetwo blocks, A, see Fig. 3. An insulating extension was used to con-nect block B to a translational stage, with maximum displacementof 2.0 cm and a least count (minimum displacement possible) of 10microns. The translational stage allows one to adjust the distanced between the chip-loop and the tag antenna. To interrogate theRFID chip, an 85 mm × 85 mm loop antenna made of 2 mm diame-ter copper wire was used together with an off-the-shelf RFID reader(model Mercury5e from ThingMagic). The distance D in the mea-surements was from the center of the loop antenna to the centerof the straight line in the tag and was in the range of 25–45 cm,in increments of 5 cm. All measurements were performed with theheight between the loop antenna and the RFID being exactly 25 cm.The loop antenna was set at an angle of 45 degree with respect tothe tag antenna as shown in Fig. 3.

3. Results and discussions

The amount of RF power transferred from the tag antenna tothe RFID chip depends on the match between Zc and Za, the chipand antenna impedances, respectively. The transferred power is atits maximum when Zc = Za

*. Since the proposed wireless displace-

ment sensor was constructed by separating the chip-loop part fromthe rest of the RFID tag, a mismatch between Zc and Za

* increasedreflection, thus decreasing the reading distance of the RFID tag bythe reader for any given value of transmitted power Pt. The results

M.J. Cazeca et al. / Sensors and Actuators A 190 (2013) 197– 202 199

Fig. 2. Flowchart of the RFID displacement sensor operation.

Fig. 3. Schematics of experimental setup.

200 M.J. Cazeca et al. / Sensors and Actuators A 190 (2013) 197– 202

Directio n of

displac ement

RFID chip

Chip-loop

d

Tag ante nna

Direction of

displac ement

RFID chip

Chip loop

d

Tag ante nna

close

pti

3

psv

fw

dm

(A)

Fig. 4. Schematics of sensors A and B, with the RFID chip

resented in this section were obtained by placing the RFID chip inwo different positions with relation to the tag antenna as shownn Fig. 4.

.1. Sensor A (RFID chip close to the tag antenna)

Using the configuration shown in Fig. 4A, the experiment waserformed by fixing a value of D, and varying the distance d. Fig. 5hows the results of the measurements performed for differentalues of D ranging from 25 to 45 cm in steps of 5 cm.

Each of five sets of points obtained from measurements at dif-erent values of D were fitted with second order polynomial curveshich are given below with their respective R2 values.

Curve 1: d(Pt) = 0.090 Pt2 – 1.687 Pt + 7.417, with R2 = 0.9992, for D = 25 cm

Curve 2: d(Pt) = 0.081 Pt2 – 1.639 Pt + 7.650, with R2 = 0.9995, for D = 30 cm

Curve 3: d(Pt) = 0.079 Pt2 – 1.940 Pt + 11.192, with R2 = 0.9994, for D = 35 cm

Curve 4: d(Pt) = 0.086 Pt2 – 2.834 Pt + 23.134, with R2 = 0.9993, for D = 40 cm2 2

Curve 5: d(Pt) = 0.105 Pt – 3.909 Pt + 36.348, with R = 0.9978, for D = 45 cm

Any of the 5 curves shown in Fig. 5 can be used to build aisplacement sensor, or using the 5 curves together a displace-ent sensor system composed of 5 displacement sensors can be

0

2

4

6

8

10

12

14

16

18

12 14 16 18 20

Transmitted p

Dis

pla

cem

en

t, d

(m

m)

1: D = 25cm

2: D = 30cm

3: D = 35cm

4: D = 40cm

5: D = 45cm

Fig. 5. Set of second order polynomial curves obtained by translating the

(B)

r to and further away from the tag antenna, respectively.

constructed. The displacement sensor system can be constructedby keeping the loop antenna at a chosen (fixed) position to simulta-neously monitor 5 or more displacement sensors placed at differentdistances D from the loop antenna. The displacement sensor sys-tem has the advantage of covering a large area on the structurebeing monitored. In this sensor, knowing the initial power Po ford = 0 is crucial for one to correlate measured values of transmit-ted power Pt from the reader to displacement d as described in theflowchart of the proposed sensor, see Fig. 2. Comparing the valuesof Po obtained from the theoretical curves 1 to 5 with the measuredexperimental values of Po for each of the distances D (from 25 to45 cm), the errors were found to be 4.86, 2.78, 3.21, 1.35 and 5.28percent, respectively.

3.2. Sensor B (RFID chip far from the tag antenna)

Performing the same type of experiment as described in Section3.1, but with the RFID far from the tag antenna as shown in Fig. 4B,

yielded the results shown in Fig. 6.

Linear fitting was used on the measured results, and the equa-tions for linear fitting with their respective values of R2 obtainedfor different values of D are given below.

22 24 26 28 30

ower, P t (dBm)

1

2

3

4

5

chip-loop with relation to the tag antenna for different values of D.

M.J. Cazeca et al. / Sensors and Actuators A 190 (2013) 197– 202 201

0

2

4

6

8

10

12

12 14 16 18 20 22 24 26 28 30

Transmitted power, P t (dBm)

Dis

pla

cem

en

t, d

(m

m)

1: D = 25cm

1

2

3

4

5

2: D = 30cm

3: D = 35cm

4: D = 40cm

5: D = 45cm

anslat

ev2SPr

3

w

Fig. 6. Set of points and their respective linear fitting curves, obtained by tr

Line 1: d(Pt) = 0.631 Pt – 8.817, with R2 = 0.9922, for D = 25 cmLine 2: d(Pt) = 0.586 Pt – 8.807, with R2 = 0.9912, for D = 30 cmLine 3: d(Pt) = 0.513 Pt – 8.402, with R2 = 0.9887, for D = 35 cmLine 4: d(Pt) = 0.442 Pt – 8.278, with R2 = 0.9869, for D = 40 cmLine 5: d(Pt) = 0.539 Pt – 11.479, with R2 = 0.9907, for D = 45 cm

Calculating the theoretical values of Po obtained from the lin-ar equation of sensor B and comparing them with the measuredalues of Po, the errors were found to be 1.32, 1.58, 0.51, 0.36 and.24 percent for values of D ranging from 25 to 45 cm, respectively.ensor B has a linear behavior with a smaller percent deviation ino when compared with sensor A; therefore, for low displacementegimes more precise results can be expected when using sensor B.

.3. Comparison between sensors A and B

Power irradiated away from the tag antenna is a function of 1/d2,here d is the measured displacement of the chip-loop. Sensor A

0

2

4

6

8

10

12

14

16

18

12 14 16 18 20

Transmitted po

Dis

pla

ce

me

nt,

d (

mm

)

D = 25 cm se

Fig. 7. Comparison of fitted curves fo

ing the chip-loop with relation to the tag antenna for different values of D.

was built with the chip placed very close to the linear part of the tagantenna, and in sensor B the chip was placed approximately 20 mmaway from the linear part of the tag antenna (when d was zero). Theposition of the chip with relation to the tag antenna may accountfor the linear and second order polynomial behavior of the calibra-tion curves. Therefore, the power transfer from the tag antenna tothe chip-loop is more efficient for sensor A than for sensor B duethe closer proximity of the RFID chip to the tag antenna in sensorA. The plot of d versus transmitted power measured at differentvalues of D, shows that sensor A can be described by a second orderpolynomial curve as confirmed by the values of R2 being very closeto 1. Sensor B has a linear behavior with values of d changing lessabruptly when compared to sensor A. Fig. 7 shows measurements

performed for D = 25 cm for sensors A and B. Sensor A can be used tomeasure larger displacement d for transmitted power ranging from14 to 23 dBm when compared to sensor B. For transmitted powerof 23 dBm the measured displacements were 15.96 and 5.37 mm

22 24 26 28 30

wer, Pt (dBm)

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sensor B

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02 M.J. Cazeca et al. / Sensors and

or sensors A and B, respectively. Sensor A can measure displace-ents approximately 5 times larger than sensor B for the same

alue of interrogating power, and it would be useful for monitor-ng structures with large and rapidly varying displacements, suchs expansion of a metallic structure in bridges. Sensor B would per-orm better when monitoring structures with small cracks or slowhanging displacements.

The data presented here has been obtained primarily from unidi-ectional displacement. But in practice if this wireless displacementensor is deployed to monitor displacement of civil infrastructurehe displacement may not be entirely unidirectional. If the dis-lacement occurs in directions orthogonal to the one described

n this manuscript the sensitivity of the system may be dimin-shed. This is one of the limitations of this technology. Ensuring theisplacement to be truly unidirectional while minimizing vibra-ions/displacements in other directions can be a challenge.

. Conclusions

The change of RF power observed by displacement of a chip-oop with relation to the tag antenna has been successfully used toabricate and demonstrate an RFID based displacement sensor. Inhe experimental configuration, for any value of D presented in theaper, the RFID reader cannot interrogate the RFID chip withouthe tag antenna being present as shown in the experimental setup,ven when the transmitted power is set to maximum (30 dBm-aximum power generated by the reader). Therefore, all effects

bserved are due to the presence of the tag antenna being in prox-mity to the chip-loop. The proposed wireless displacement sensorased on RFID technology is inexpensive to build and simple toeploy. When compared with sensor A, sensor B that has the RFIDhip at a greater distance from the tag antenna, shows a higheregree of linearity. As a wireless detection system, it is possible toeploy several sensors for structural health monitoring using a sin-le loop antenna to monitor all deployed sensors simultaneously.ith tag readers that are more advanced and powerful than those

sed in this work, it is conceivable that several antennae can be usedo simultaneously interrogate multiple sensors. Therefore one RFIDeader can be used to cover large areas of a structure for continu-us structure health monitoring. In summary, the strategy outlinedere opens numerous possibilities for the development of a newlass of low cost wireless displacement sensors.

cknowledgements

The authors gratefully acknowledge the ARMY RESEARCH LAB-RATORY under grant number W911NF-07-2-0081 for the supportf this research. The authors thank Mr. Mac Chinwala of NXP semi-onductors for his valuable suggestions and for providing us theXP UCODE G2iL Chip. The authors thank Dianne Cazeca for helpingith manuscript preparation.

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Biographies

Mario J. Cazeca received his Ph.D. in applied physics from University of Mas-sachusetts Lowell. He worked for several years in industry with polymeric thinfilms for nonlinear optics applications, conductive polymer for organic light emittingdiode (OLED), thin films for bottom antireflection coating (BARC), and high temper-ature superconductors. Since 2009, he has been working as Research Associate atthe Department of Plastics Engineering at University of Massachusetts Lowell, onthe development of sensors for detection of cracks on ceramic body armor and civilstructures.

Joey Mead is a professor in the Plastics Engineering Department at University of Mas-sachusetts Lowell and Deputy Director of NSF Nanoscale Science and EngineeringCenter for High-rate Nanomanufacturing. She received her B.S., M.S., and Ph.D. fromthe Massachusetts Institute of Technology. Her interest are in areas of elastomers,thermoplastic elastomers, nanomanufacturing, electrospinning, mechanical behav-ior of elastomers and polymers, interfacial properties of fiber reinforced composites,structure-property relationships, recycling of rubber and elastomeric barrier mate-rials.

Julie Chen is the Vice-Provost for research, a professor and Co-Director of theAdvanced Composite Materials and Textiles Laboratory at University of Mas-sachusetts Lowell. She received her B.S., M.S., and Ph.D. from the MassachusettsInstitute of Technology. She is considered one of the region’s leading experts onnanotechnology. Her interests are in mechanical behavior and deformation of fiberstructures, fiber assemblies and composite materials. Additionally, her work focuseson experimental investigation and analytical modeling of processing such as form-ing, stamping, energy absorption, fatigue, and failure behavior of composites as theyrelate to the fiber architecture and manufacturing defects.

Ramaswamy Nagarajan is an associate professor in the Department of PlasticsEngineering at University of Massachusetts Lowell. He received his Ph.D. in Poly-mer Science from the University of Massachusetts Lowell. His research interestsinclude developing ‘Greener’ routes to advanced functional materials (electronic,

photo-responsive polymers) and therapeutic materials, biocatalytic polymeriza-tion, polymers from renewable resources, mechanistic aspects of enzyme catalysis,developing novel eco-friendly biomimetic catalysts, RF-based sensors for wirelessstructural health monitoring and sensors for detecting explosives in air and toxicmetals in soil and water.