RFID Tag as a Sensor - A Review on the Innovative Designs and Applications

Abstract

The Radio Frequency Identification (RFID) technology has gained interests in both academia and industry since its invention. In addition to the applications in access control and supply chain, RFID is also a cost-efficient solution for Non-Destructive Testing (NDT) and pervasive monitoring. The battery free RFID tags are used as independent electromagnetic sensors or energy harvesting and data transmission interface of sensor modules for different measurement purposes. This review paper aims to provide a comprehensive overview of the innovative designs and applications of RFID sensor technology with new insights, identify the technical challenges, and outline the future perspectives. With a brief introduction to the fundamentals of RFID measurement, the enabling technologies and recent technical progress are illustrated, followed by an extensive discussion of the novel designs and applications. Then, based on an in-depth analysis, the potential constraints are identified and the envisaged future directions are suggested, including printable/wearable RFID, System-on-Chip (SoC), ultra-low power, etc. The comprehensive discussion of RFID sensor technology will be inspirational and useful for academic and industrial communities in investigating, developing, and applying RFID for various measurement applications.

Full text

MEASUREMENT SCIENCE REVIEW, 16, (2016), No. 6, 305-315
_________________
DOI: 10.1515/msr-2016-0039
305
RFID Tag as a Sensor - A Review on the Innovative Designs and
Applications
Zhaozong Meng1, Zhen Li2
1School of Electrical and Electronic Engineering, University of Manchester, Oxford Road, Manchester M13 9PL, UK
zhaozong.meng@manchester.ac.uk
2School of Materials, University of Manchester, Oxford Road, Manchester M13 9PL, UK
The Radio Frequency Identification (RFID) technology has gained interests in both academia and industry since its invention. In addition
to the applications in access control and supply chain, RFID is also a cost-efficient solution for Non-Destructive Testing (NDT) and
pervasive monitoring. The battery free RFID tags are used as independent electromagnetic sensors or energy harvesting and data
transmission interface of sensor modules for different measurement purposes. This review paper aims to provide a comprehensive
overview of the innovative designs and applications of RFID sensor technology with new insights, identify the technical challenges, and
outline the future perspectives. With a brief introduction to the fundamentals of RFID measurement, the enabling technologies and recent
technical progress are illustrated, followed by an extensive discussion of the novel designs and applications. Then, based on an in-depth
analysis, the potential constraints are identified and the envisaged future directions are suggested, including printable/wearable RFID,
System-on-Chip (SoC), ultra-low power, etc. The comprehensive discussion of RFID sensor technology will be inspirational and useful for
academic and industrial communities in investigating, developing, and applying RFID for various measurement applications.
Keywords: RFID sensor, NDT, pervasive monitoring, energy harvesting, SoC.
1. INTRODUCTION
As a contactless and non-line-of-sight identification and
data transmission technology, RFID has been widely applied
for access control and information tracking in logistics and
industrial processes [1]. It is also considered as an eminent
enabling technology for the realisation of ubiquitous
monitoring in Internet of Things (IoT) [2], [3].
Since the inductive coupling or backscattered radio waves
in RF identification can be used to detect the physical
parameters of tagged objects, RFID technology is also a
potential solution for smart sensing to deal with some
sophisticated problems [4][5]. For instance, it can be used to
discriminate the variation of materials as a transmission
medium of the radio waves. Therefore, RFID tags are also
innovatively used as electromagnetic sensors for different
measurement purposes, such as strain detection [6], [7],
material corrosion analysis [8], [9], crack detection [10],
[11], [12] and food quality evaluation [13], [14].
In addition, RFID tags can also be integrated with
electronic components, such as sensory material, Analogue-
to-Digital Converter (ADC), and Micro-Controller Unit
(MCU) to make an integrated sensor module. The RFID tag
is used as a communication interface for data transmission.
Passive RFID sensors harvest the RF energy from RF
radiation to power the circuit and complete the sensing
function, then save the data in the RFID chip to be accessed
by RFID readers, which is described in [15], [16], and [17].
The passive sensing is of interest for data collection in
remote sensing and RFID Wireless Sensor Network (WSN),
such as health monitoring [18] and indoor localisation [19].
The promising prospect of this inexpensive technology has
fostered studies in many disciplines. The rapid progress of
RFID sensors and the lack of systematic description of this
technology in the literature are the causes of concerns,
which have motivated this investigation to bridge the gap by
providing a comprehensive overview of the novel designs
and applications with new insights.
The structure of this paper is organised as follows: Section
2 introduces the fundamentals of RFID measurement,
Sections 3 and 4 illustrates the enabling techniques and the
state-of-the-art designs and applications, Section 5
summarises the underlying challenges and suggests the
future directions, and finally Section 6 concludes the work.
2. FUNDAMENTALS OF RFID MEASUREMENT
Normally, the RF identification procedure between an
Ultra-High Frequency (UHF) RFID reader and a tag as a
transponder is as depicted in Fig. 1 [20], where CW and
RCS mean continuous wave and radar cross-section
respectively.
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Since RFID sensing is an extended function of RF
identification, it fulfills the measurement function by taking
advantage of variation in inductive coupling and RF
backscattering caused by the objects under measurement.
This section illustrates the methods of utilising RFID tags as
sensors and their fundamentals.
RFID
reader
Received signal
Time
CW CWQuery
Tag
response
Reader transmission
RCS1
RCS21 2
RFID tag
Chip
RCS1
RCS2
Fig.1. Diagram of UHF RFID reading fundamentals.
A. Basics of RFID technology
The RFID operates at a variety of frequencies which are
summarised in Table 1. Both Low Frequency (LF) and High
Frequency (HF) RFID operating in the near field and energy
transfer is through inductive coupling. However, for UHF
and higher like Super High Frequency (SHF) within
microwave frequency range, the communication and energy
transfer is in the far field through backscattering.
Table 1. Frequencies and reading range of RFID techniques.
RFID
Techniques
Operating
Frequencies
Free Space
Reading Range
LF
125-134.3 kHz
< 10.0 cm
HF
13.56 MHz
< 1.0 m
UHF
860-960 MHz
1.0 - 12.0 m
SHF
2.45 - 5.8 GHz
Up to 100.0 m (Active)
For measurement, LF and HF are used more in metal
materials and UHF and higher are used more for dielectric
materials weaker in conductivity, such as concretes and
food. Some RFID sensors are custom-designed, while some
are compliant with certain global regulatory such as
Electronic Product Code (EPC) Class 1 Generation 2 for
UHF RFID [21]. The use of a fully standardised technology
UHF RFID EPC Class 1 Generation 2 can benefit from
plenty of Commercial Off-The-Shelf (COTS) components.
B. RFID tag as an electromagnetic sensor
The fundamentals of LF/HF and UHF RFID tags
measurement are as shown in Fig. 2, and Fig. 3 represents
the equivalent circuit of LF/HF RFID near-field coupling.
Basically, an RFID tag consists of an L-C-R parallel
circuits. When an RFID antenna is put on a measurement
object as a transmission medium of radio waves, its
electrical properties L2 and C2 are changed. Assume the
transformed resistance of the tag is Z’, the resonance
frequency of the tag at the maximum point of the real part of
Z’ can be represented by:
22
02
1
LC
f


(1)
R2 in the circuits decides the bandwidth of resonance.
When there is variation in C2 and L2 caused by the measured
object, f0 shifts.
In addition, Q-factor of the tag which is assumed to be a
simple parallel resonant tank is defined by:
2
2
2L
C
RQ 
(2)
Therefore, by determining the f0 and Q, and analysis of the
variation in L2, C2, and R2, the electrical properties of
measured objects can be obtained. This basic theory is
explored in different ways for measurement purposes. For
example, a material sensitive to moisture can be located on
the substrate of a RFID tag to affect the electrical properties
C2 and L2. The moisture content can be obtained with a
calibration procedure. UHF measurement based on RF
backscattering RFID are similar in fundamentals.
Electromagnetic field
RFID reader RFID chip
Reader
coil Tag
coil
a) LF/HF RFID
RFID
reader
antenna
RFID
tag
RFID
tag
Power & Data
Data
uplink
downlink
b) UHF RFID
Fig.2. LF/HF and UHF RFID tag sensing.
RFID
reader
I1 R1
L1 L2
I2
C2 R2
U1 U2
Z
Reader coil Tag coil
Inductive coupling
Z’
Fig.3. Equivalent circuit of LF/HF RFID near-field coupling.
C. RFID tag as an energy harvesting and data transmission
medium
In addition to taking advantage of resonance frequency of
the RFID antenna for measurement, the capability of RF
energy harvesting for data transmission is another promising
technique, which allows the integrated sensor module to be
able to perform passive, batteryless and remote sensing.
To achieve the functions, an RFID antenna with matching
network is connected to a RF-to-DC rectifier, which
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generates DC voltage from RF signal. A charge pump with a
voltage supervisor controls the charging and discharging of
a storage capacitor. When the storage capacitor is charged,
the voltage supervisor starts to discharge and power the
MCU which collects sensor data and writes to particular
data banks of the RFID chip that are readable by RFID
readers. A reference circuits design is given in Fig. 4.
`Charge pump &
voltage supervisor
RF-DC
rectifier
Voltage
regulator
Lower power
MCU
RFID
module
Sensor 1 Sensor 2 Sensor n
I2C ADC/I2C
...
Matching
network Storage
capacitor
RF energy
harvesting
Sensor
module
DC2DC1
Fig.4. A reference design of UHF RFID sensor.
The passive RFID tag is a promising candidate to integrate
with an ultra-low power sensor to constitute a batteryless
RFID sensor. Some commercial RF-to-DC converters such
as Powercast P2110 [22], I2C UHF RFID Gen2 ICs such as
Impinj Monza X-2K, ultra-low power MCUs such as TI
MSP430 serials are key enabling technologies [15]. The
integration of sensor module and RFID module has allowed
the sensors to transmit data conveniently in a wireless way,
and is therefore widely investigated and applied [23].
Generally, RFID sensors are based on the two fundamental
methods described above and their further extensions, and
numerous novel designs and applications are conducted
accordingly.
D. RFID sensing technology classification
RFID sensors achieve measurement tasks in different
ways. According to operating fundamentals, the related
methods of RFID measurement can be classified into four
categories: RFID electromagnetic sensor, RFID tag
integrated sensor, RFID tag array, and RFID sensor
networks. The former two kinds are commonly used, and the
latter two are the functional extensions of the former. The
different kinds and their fundamentals of measurement are
given in Table 2.
Table 2. Categories of RFID sensor measurement.
Categories
Fundamental of Measurement
RFID
electromagnetic
sensor
Normal passive RFID tag or chipless RFID, for
which measurement is based on the analysis of
its spectral or phase characteristics
RFID tag
integrated sensor
Integrated with a sensor module, RFID is used
for energy harvesting and data transmission
RFID tag array
RFID array for expanding measurement space or
for localisation and tracking
RFID sensor
networks
Batteryless and low-cost solution for wireless
sensor networks monitoring
A plethora of studies on measurement of strain, crack,
corrosion, temperature, moisture, gas, blood glucose, etc.,
localisation, and RFID WSN monitoring fall in the four
categories. The prosperity of different kinds of RFID
sensing technology is also the result of continuous progress
of enabling techniques to be discussed in Section 3.
3. ENABLING TECHNIQUES OF RFID SENSORS
According to the fundamentals of RFID measurement
described in Section 2, the advantages of RFID sensing are
introduced by techniques of multi-disciplines, such as RF
and antenna, RF identification components, energy
harvesting, and integration of ADC and low power digital
components. As one of the research focuses which
are promising in practical applications, RFID technology
has attracted many research efforts. This section illustrates
the major enabling technologies that offer the RFID sensors
with the addressed advantages.
A. Innovation in RFID antenna design
Antenna is a critical component of RFID tags. Its
performance can be improved by introduction of new
structures and materials. For example, in terms of shape of
antenna, a novel 3-D cubic antenna for wireless sensor
networks and RFIDs for environmental sensing is reported
in [24], and this cubic design has achieved nearly isotropic
pattern. In [25], an ultra-low-cost RFID tag is presented,
which comprises a soft magnetic ribbon that is biased by an
adjacent permanent magnet. The tag is read by measuring
changes in the ribbon’s magnetisation while applying
magnetic AC and DC fields. With respect to material of
antennas, a printable RFID antenna with low temperature
processing of graphene ink is introduced in [26], and its
feasibility has been demonstrated for low-cost printable RF
applications. A highly conductive textile-based elastic RFID
tag antenna is fabricated in [27] to introduce elasticity,
flexibility, and mechanical strength. In addition, a flexible
magnetic composite material is introduced for wearable
RFID antenna [28], which potentially has enabled the
significant miniaturisation of RFID antennas in UHF
frequency band.
These novel designs of RFID antenna have either
improved the performance of RFID tags or extended their
scope of applications.
B. Chipless RFID
Rather than to embed silicon integrated circuits for ID
extraction, chipless RFID uses an electromagnetic signature
for data encoding to reduce the cost of RFID tags [29]. The
challenges facing the design of a chipless RFID transponder
is how to perform data encoding without the presence of
a chip. To this end, two general types of RFID transponders
can be identified: Time-Domain Reflectometry (TDR)-based
and spectral signature-based. The former transponder is
interrogated by sending a signal with the reader in the form
of a pulse and listening to the echo of the pulse sent by the
tag, while the latter decodes spectrum features of RFID tags
encoded using resonant structures. A Ultra-Wideband
(UWB) impulse radar based reader is presented in [30],
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which interrogates a chipless tag with a UWB pulse and
analyses received backscatter in the time domain to obtain
the IDs. A chipless RFID transponder based on phase
encoded backscatter is proposed in [31], which comprises 3
microstrip patch antennas loaded with open circuited high
impedance stubs. The antennas re-radiate backscattered
signals with distinct phase characteristics that are encoded
as hexadecimal bits for the proposed chipless RFID tag. A
typical study on spectral signature-based RFID is to apply
exciting RF to recognise metallic strip letters on dielectric
substrate [32]. Results demonstrate the possibility to
uniquely identify the alphabet with high certainty by
observing the resonance peaks.
From the fundamentals and example applications
described above, both TDR-based and spectral signature-
based chipless RFIDs are good candidates for RFID
electromagnetic sensors.
C. RF energy harvesting and hybrid power supply
RF energy harvester or RF-to-DC converter is a critical
component in RFID tags, the function of which is to convert
RF signal into DC voltage that can be used to power low
voltage electronic devices. In order to fix the efficiency drop
problem of Dickson’s circuits due to turn-on voltage of
diodes, the Metal–Oxide–Semiconductor Field-Effect
Transistor (MOSFET) diode based Complementary Metal-
Oxide-Semiconductor (CMOS) RF-to-DC converter is
investigated in order to generate stable DC power with
an appropriate voltage level [33]. In addition, some studies
introduce novel hybrid powering solutions, such as semi-
passive solar-powered temperature sensor based on a time-
coded UWB RFID tag, which is built up with simple COTS
components [34]. In order to extend the reading range,
optional battery is also included for some designs.
Both energy harvesting and hybrid power supply methods
are effective approaches to extend the batteryless sensing
advantage of RFID based sensing devices.
D. SoC RFID sensor
In addition to energy efficiency, miniaturisation is also a
key parameter of RFID based applications. As shown in Fig
5, integration of RFID technique with other modules on an
integrated chip as a SoC device is a prospective direction
and heuristic studies have already started. Investigation of
integrating sensing materials on 0.18𝜇𝑚 CMOS becomes
common in laboratory studies [36], [37]. Highly integrated
ICs with RF harvesting, DC regulator, ADC, and MCU are
the future of RFID sensors and other wireless sensor nodes.
Fig.5. Implantable SoC RFID for glucose monitoring, [35].
E. Printable/wearable RFID
Printable and wearable RFID that is compliant with
available technologies and the safety standards is highly
demanded. Printable RFID is a solution to produce low-cost
and environment friendly RFID tags for identification and
sensing applications. One promising solution is to use
inkjet-printed RFID circuits on flexible and paper substrates
to provide RFID sensing functionalities, which is as shown
in Fig. 6 [2]. This method is more economical and
environment friendly compared to traditional methods.
Development of wearable RFID tags involving bio-
monitoring of human still remains challenging as the
interaction of antenna with the human body results in
efficiency and sensitivity degradation of RFID sensing. The
effects of human body on a wearable UHF RFID is
examined in [38] focusing on variation of antenna-body
separation distance on the antenna properties. The measured
three-dimensional radiation pattern is useful to peer
investigations. A tag geometry combining folded conductors
and tuning slots with numerical analysis and experiments is
presented in [39], which has achieved a design applicable to
any part of human body. Evidently, the printable and
wearable RFID techniques have also expanded the
application scope of conventional RFID tags.
The techniques either for improving the performance of
RFID tag, or for integration of RFID with other disciplines
have finally contributed to the RFID sensor technology. The
continuous progress of enabling techniques in the above
aspects has extended the functions of different RFID sensing
technologies for measurement purpose and fostered some
new applications in a variety of fields.
Fig.6. Inkjet-printed RFID sensor on paper substrate, [2].
The techniques either for improving the performance of
RFID tag, or for integration of RFID with other disciplines
have finally contributed to the RFID sensor technology. The
continuous progress of enabling techniques in the above
aspects has extended the functions of different RFID sensing
technologies for measurement purpose and fostered some
new applications in a variety of fields.
4. INNOVATIVE DESIGNS AND APPLICATIONS OF RFID
SENSORS - A REVIEW OF THE STATE OF THE ART
With the continuous progress of enabling techniques,
many novel designs and innovative applications of RFID
sensor technology are reported in the literature. This section
gives a brief introduction to the application areas, and
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classifies the reported designs and applications with the
illustrations of some typical examples.
Batteryless, miniaturised, remote sensing, low-cost, easy
fabrication and integration are key advantages identified that
make RFID sensors accepted in a wide range of
applications. From the perspective of application fields, the
categorisation of RFID sensors and the variables to be
measured in order to fulfil the functions are given in Fig. 7.
Environment monitoring
Structural health monitoring
Food quality and safety
Positioning & tracking
Health & bio-monitoring
Physical
variables
Human activity recognition
Composition analysis
RFID sensor network
Functions
Motion
Corrosion
Strain
Crack
Humidity
Pressure
Temperature
Gas
...
RFID
sensor
Brightness
Fig.7. Applications of RFID sensors.
A. Structural health monitoring (SHM)
Monitoring mechanical conditions of critical structures so
as to prevent catastrophic failure is a very significant task.
RFID sensors that can be used for passive, non-destructive
and remote evaluation are widely applied. Commonly, the
structure under test is examined by measurement of
deformation, crack, corrosion, and strain. For high
conductivity materials like metals and carbon fibre
composites, the penetration of electromagnetic signals is
limited. Thus, only the surface damage can be detected [12].
With respect to crack detection, a Surface Crack Antenna
Reflectometric Sensing (SCARS) is introduced in [10],
which implements chipless RFID for pervasive and wireless
detection to identify the length and orientation of surface
crack in structural materials. A crack width RFID sensor
based on high sensitivity phase detection of passive RFID is
presented in [11]. The described sensor can detect sub-
millimetre deformation occurring on the object. In addition,
an RFID antenna for detection of surface crack in civil
infrastructure is introduced in [40], which also proposes a
solution to improve spatial coverage with 2-D grid of tags.
In terms of corrosion detection, LF RFID is applied to
characterise steel corrosion in [8], and experiments
demonstrate the sensitivity of this RFID Wireless Power
Transfer (WPT) solution. An RFID based sensor for
corrosion monitoring of reinforced concrete structures is
presented in [9], which can perform linear polarisation, open
circuit potential and temperature measurements. Test results
can show the initiation and progression of corrosion with
obtained linear polarisation resistance measurement. In
addition, a batteryless RFID based embedded sensor is
proposed for long-term structural health monitoring in [41]
to monitor the corrosion, temperature, and humidity level in
reinforced concrete structure.
RFID sensor strain detection is also widely investigated.
Intel Wireless Identification and Sensing Platform(WISP) is
modified to interface with a foil resistance strain gauge for
uniaxial tension tests of carbon fibre composites [6]. The
results show excellent agreement with the prediction of
NASTRAN finite element model. A Breakage-Triggered
(BT) strain sensor integrated with an RFID tag for wireless
communication is designed to perform non-contact scanning
of structural deformation condition [7]. This system can
rapidly identify the spot where the stain has surpassed the
threshold pre-set by engineers and decision makers. A
flexible and stretchable inductor-capacitor (LC) resonator
based chipless RFID tag fabricated by stamping with silver
nano ink for strain sensing is presented in [42]. A single tag
and multiple tags identifications are achieved by changes in
resonance frequency, and results agree well with theoretical
calculation. A dual-interrogation-mode method combining
chipped and chipless RFID to detect embroidered RFID
strain sensor is described in [43]. This work validates a dual
interrogation mode and proves that chipped RFID sensor
tags can be detected accurately utilising backscattering RCS
measurements.
According to the above studies, passive RFID is widely
applied for non-destructive testing, and the principle of
measurement is to identify the shift of resonance frequency
due to variation of structural conditions. There is also
investigation that uses 2D array to extend spatial coverage.
The application of battery free RFID sensor has alleviated
the constraints caused by various measurement conditions,
which has been provided a promising and inexpensive
remote sensing solution for structural health monitoring.
B. Environment monitoring
RFID sensors are also applied for environment monitoring
mainly on measurement of gas, humidity, and temperature.
Gas detection is a typical case of environment monitoring
applications. A batteryless UHF RFID tags coated by
different sensitive layers for volatile compounds detection
and ambient sensing is described in [44]. Results are
encouraging while packing four RFID sensors into
a compact array still remains an open issue due to an
increase in cross sensitivity of each tag. A single-wall
Carbon Nanotube (CNT) buckypaper based RFID tag as
a low-cost and maintenance free sensor solution for NH3
measurement is investigated and verified by experiments in
[45]. An inkjet printing method to deposit single-walled
CNT film on a fully printed UHF RFID module on paper to
form a wireless gas sensor node for toxic gas detection is
introduced in [46]. This work demonstrates the feasibility of
inkjet-printed CNT for RFID enabled sensor nodes.
For humidity measurement, [47] presents a prototype
passive wireless UHF RFID sensor for humidity monitoring
in a built environment through far-field backscatter
coupling. This sensor is implemented by incorporating
a humidity sensitive polyimide film onto top surface of
RFID tags. Some proactive investigations attempt to
integrate sensor elements to RFID chips to build SoC
modules. A UHF RFID tag with ultra-low power, small size,
high resolution temperature sensor adopting a double
Voltage Controlled Oscillator (VCO) is fabricated using the
SMIC CMOS 0.18𝜇𝑚 EEPROM 2P4M process [48]. These
properties allow the use of the RFID tag as a batteryless
sensor for a long-range wireless temperature monitoring. In
[49], a capacitance humidity sensor with a co-integrated
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energy efficient Capacitance-to-Digital Converter (CDC)
which is implemented on a 0.16𝜇𝑚 CMOS technology is
presented. Measurement results prove that it achieves
a resolution of 12.5 bits with a measurement time of 0.8ms,
while drawing only 8.6𝜇𝐴 from 1.2V power supply which
outperforms the state-of-the-art in capacitive-sensor
interfaces in terms of energy efficiency. An RFID based
capacitive humidity sensor tag fabricated by 0.18𝜇𝑚 CMOS
process with a top metal layer deposited to form a
interdigitated electrodes filled with polyimide as humidity
sensing layer is presented in [50]. With 0.5V power supply,
results show excellent linearity, hysteresis and stability. In
addition, printed LC resonator structures are also
investigated for humidity detection. A two planar LC
resonators RFID humidity sensor tag operating wirelessly
through inductive coupling for short-range item tracking and
humidity monitoring is presented in [51]. The RFID
humidity sensor provides excellent sensitivity and
reasonable response time to humidity. Applying the
fundamentals of RFID tag resonance frequency shift
measurement, an inductor coil and an interdigitated
capacitor are developed by screen printing and inkjet
printing to form an LC resonator for humidity detection in
[52].
With respect to RFID temperature sensor, there are plenty
of investigations reported in the literature. An investigation
of RFID sensor application for soil solarisation purposes is
presented in [53]. RFID temperature sensors are evaluated
and results demonstrate that it is an easy-to-use and cheap
tool to support the decision-making process during long-
term treatment like solarisation. A 0.18𝜇𝑚 CMOS SoC
passive RFID tag with an embedded temperature sensor for
UHF EPC Gen2 is introduced in [54]. The proposed gain-
compensation technique and low-power time-readout
scheme decrease sensing error and power consumption.
From the above studies, it is found that integration of
RFID tag with various sensing techniques is effective and
promising way for environment monitoring applications.
C. Food quality and safety
Food quality and safety is an area electromagnetic sensors
are preferred since they can test the internal of food and
satisfy the hygienic requirements. The inexpensive RFID
sensor solution is a competitive alternative for food quality
assessment. Examples of applications include monitoring of
freshness of fruit, milk, fish, and bacterial growth, etc. [14].
An approach to modify RFID tag with chemically
sensitive conductive composites is introduced in [55] to
detect different biogenic amines associated with food
spoilage. The RFID response is then dependent on amine
concentration, tag initial resistance, and type of biogenic
amine. UHF RFID is applied to detect contamination and
meat quality in [13]. The contamination of food can be
obtained, provided the variation of food permittivity over
time due to aging is known. An RFID sensor system for
vegetable freshness monitoring is proposed in [56], which is
designed by integrating an oxygen and carbon dioxide
concentration sensor with a RFID tag for wireless data
transmission. In a similar way, an HF RFID based sensor for
monitoring of freshness of packaged vegetables based on
measurement of temperature and humidity is presented in
[57], which has extended the reading distance to 30cm.
Differently, a multi-sensor RFID tag uses both HF and
UHF in [58], which can check quality of food and also
monitor the distribution of food with the two frequency
bands. An ultra-low power CMOS temperature sensor is
proposed targeting at RFID food monitoring applications by
employing serially connected sub-threshold MOS as sensing
element in [37], which reaches 119nW power consumption
at room temperature. Validation study on milk freshness is
conducted, and model equations are obtained based on
experimental studies. In addition to individual
measurements, the RFID sensor is also integrated with WSN
technology for food quality monitoring and control in a
wireless network with databases [59], [60].
Since food materials are usually not bulk materials of
regular shapes, RFID is seldom used as an electromagnetic
sensor but more for data transmission of integrated sensors
for PH, temperate, and moisture content, etc.
D. Positioning and tracking
Due to the strengths in non-line-of-sight, object
positioning and tracking based on RFID technology is
another widely accepted use case, including the localisation
and tracking of vehicles, equipment, robots, and human, etc.
An RFID positioning approach for connecting vehicles as
an alternative when GPS is not available is proposed in [61].
In this approach, RFID tags are installed on the road surface
and a tag reader is on-board in vehicles. Low-cost and
reasonable accuracy are the strengths of this solution. A
Real-Time Location System (RTLS) for hospital equipment
tracking with RFID technique is presented in [62]. The
system utilises passive RFID tags mounted on flooring
plates and several peripherals for sensor data interpretation
and achieves the desired accuracy.
A partially observed feedback controller for a wheeled
mobile robot is presented in [63], where the feedback signal
is in the form of noisy Received Signal Strength Indicator
(RSSI) emitted from RFID tags. The proposed controller
requires neither an accurate mapping between the line-of-
sight distance and the RSS measurement, nor the
linearisation of the robot model. In [64], differential
evolution approach is applied in an RFID sensor deployment
for mobile robot localisation. An RFID based system
performs both real-time monitoring of body temperature and
location of the body is demonstrated in [65], where the
RFID chip’s integrated sensor is used for temperature
detection, and reference tags and a multiple-antenna time
division multiplexing system are used for localisation with a
K-Nearest Neighbours (KNN) algorithm.
For RFID positioning and tracking, there may be
uncertainties, gap, and errors due to the speed of objects or
the arrangement of RFID tag array. The calibration methods
and algorithm to eliminate these errors are critical to the
accuracy of measurement. Least Mean Square (LSM),
Interactive Multiple Model (IMM)-based global fusion,
Kalman Filter algorithm, hierarchical fusion algorithm,
Heron-bilateration location estimation, etc. are investigated
[19], [66], [67]. In order to solve the multi-path
phenomenon of RSSI based RFID indoor localisation,
algorithms SA-LANDMARC and COCKTAIL are proposed
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to improve accuracy and scalability for RFID WSN
localisation in [68], and the accuracy can reach 0.7m and
0.45m respectively.
E. Health monitoring and bio-monitoring
Newly invented digital health devices have emerged
frequently in recent years. Applications of RFID for health
monitoring and bio-monitoring have also begun to happen.
One direction is to take advantage of the strength of RFID
sensor in remote sensing for health parameter monitoring. A
set of frameworks based patient life cycle and time-and-
motion perspectives is introduced in [69] for making use of
RFID information system to offer benefit for patient care
and hospital operations. An investigation combining RFID
real-time remote monitoring of body temperature and the
location is introduced in [65], which is considered to be of
interest in disaster relief. A long range UHF RFID sensor
designed using 0.35𝜇𝑚 CMOS standard process is presented
in [70], which allows an ID and a temperature reading range
of 2m from a 2W effective radiated power reader. Test
results demonstrate potential of the sensor as a batteryless
solution for wireless human body temperature monitoring.
Another direction is to innovatively make use of the
electromagnetic field of RFID tag for measurement. A
wireless powered implantable electromagnetic sensor tag for
continuous blood glucose monitoring is presented in [35],
which is remotely powered by 13.5MHz RFID for sensing
and communication. This system can produce reliable and
accurate measurement of glucose, which is of interest by
clinical and patient care. An epidermal passive strain sensor
using UHF RFID tags is presented in [71], which intends to
detect eyebrow or neck skin stretch to offer the possibility of
allowing paraplegic patients to control wheelchairs. In [72],
an adhesive RFID sweat sensor bandage is reported, which
can be made completely intimate with human skin for
chronological monitoring of biomarkers in sweat.
There are many innovative applications for RFID health
monitoring and bio-monitoring, and the studies in this field
will go broader and deeper in the future.
F. Human activity recognition
Another potential application is human motion detection
which is of interest for hospital patient care and elderly care.
A single passive body-worn RFID sensor attached over
clothing for recognising activities, such as walking and
transfers out of beds or chairs in the context of ambulatory
monitoring is reported [73]. A prototype sensor-enabled
RFID system consists of RFID tags paired with proximity
and movement sensors for arm activity monitoring is
presented in [74]. Test results demonstrate the reliability and
validity in individuals with unimpaired movements. A new
tag geometry combining folded conductors and tuning slots
which also includes a passive motion detector is introduced
in [39]. The measured performance indicates a possible
application of these body-worn tags for continuous tracking
of human movements. A study on wearable RFID based
system for real-time activity recognition is conducted in
[75], where recognition is realised by exploiting RFID radio
patterns towards easy-to-use solution and high detection
coverage. In addition to custom-designed systems, there are
specific RFID based platforms suitable for human activity
detection, such as iGlove, iBracelet, and WISP [76].
Compared to visual based activity recognition, strengths of
RFID based system lie in low-cost and privacy protection.
G. Composition sensing and analysis
Although not widely investigated, it is an innovative
solution to apply RFID sensors for material composition
analysis. The conventional passive HF RFID tag is applied
for chemical sensing in [77] by coating the RFID tag with
chemically sensitive films to form a chemical sensor. By
using multivariate statistical analysis tools, the sensor is
capable for position independent analyte quantification.
Chipless UWB RFID is applied for non-destructive wireless
concrete quality measurement in [78]. The measurement is
based on detection of the delay between two scattering
modes induced by permittivity changes in the concrete, and
composition of concrete can be remotely detected and
classified. Since electromagnetic wave of RFID is sensitive
to materials of different permittivity, composition analysis
with RFID is feasible. However, a systematic calibration of
the measurement might be a challenging procedure.
H. RFID sensor network
There have been numerous efforts applying RFID to form
a sensor network. By integrating the RFID tags as sensor
nodes, an RFID sensor network can be built to perform
various measurement tasks. The architecture and technical
issues of integrating RFID into WSN is analysed in [79],
and three forms of new architectures are proposed with
discussions of the feasibility and technical challenges.
An RFID sensor network is introduced in [68] to solve the
environment factors and multi-path problem in RSSI based
localisation. The system consists of a few nodes, each of
which acts as both transmitter and receiver. The final
location of target is estimated by using the RSSI relationship
between the target tag and candidate reference tags. The
system design of a UWB-RFID network for tag localisation
in IoT applications is presented in [80], and it is found that
the architectural choice is strictly application dependent, and
must account for costs, complexity, energy efficiency,
backward compatibility and performance. It is evident that
the architecture design is significant to the success of an
RFID sensor network application.
From the above discussion, we can find that RFID sensor
technology becomes more and more prosperous. It has
penetrated into numerous disciplines and integrated with a
variety of new emerging research directions and application
fields. The innovative applications of RFID sensors have
significantly facilitated and sometimes revolutionised the
measurement and monitoring solutions.
5. DISCUSSION
From the applications illustrated in Section 4, RFID tag is
already considered as a very useful electromagnetic sensing
solution. It can be easily integrated with sensing materials
and electronic systems for applications using conventional
RFID readers and antennas. Using RFID tag as either an
electromagnetic sensor or a medium for power and sensor
data transmission, they both can take advantage of the RFID
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tags in low-cost, batteryless, miniaturised remote sensing,
and easy fabrication.
The application of RFID sensors is not limited to obtaining
measurement results through electromagnetic resonance or
integrating with sensing components for wireless data
transmission. Investigations are bidirectional: (1) from
micro-view, innovative designs have invaded to CMOS
integrated circuits design to integrate sensitive material and
RFID chips; (2) From macro-view, RFID sensors are used to
form 2D arrays for different measurement purposes. It is
also attempted to integrate with WSN towards pervasive
systems for long-term and large-space monitoring.
A. Technical challenges
Together with the progress of RFID sensing methods and
like any newly developed technologies, some intrinsic
challenges that hinder its further developments and
applications exist, which are summarised as follows:
1. Calibration for accurate measurement
As is the case with any electromagnetic sensors, the RFID
sensor is sensitive to ambient environment and object under
test, such as temperature, ambient metal and electromagnetic
radiation, and size and shape of object under test, which
interact with reflection, refraction and scattering of radio
waves. Suffering from the environmental affects is common
to resonance based electromagnetic sensors. Therefore, the
calibration is an essential but sophisticated step to achieve
desired accuracy in measurement.
2. Sparsity and noise of RFID sensing
RFID sensors can perform real-time measurement for
static and low speed variations. Data stream from RFID
sensors is characterised by sparsity and noise due to the
inherent attributes of RFID reading, which makes streaming
based high-speed continuous measurement a challenging
task.
3. Electromagnetic coupling in RFID sensor array
RFID array is a way to extend the spatial dimension of
RFID sensor measurement, and it is a low-cost solution for
2D electromagnetic measurement. However, the
electromagnetic coupling between tags results in cross
sensitivity, which harms the accuracy of measurement.
4. Security for some use cases
For many sensor occasions such as healthcare assisted
diagnosis, and smart home, the parameters measured might
be sensitive and confidential for users. Thus, the data
protection for these use scenarios is a critical issue for the
systems. Since RFID sensors work in distance, security of
data is an issue to be considered for some applications.
Corresponding solutions to overcome the technical
challenges are being investigated and the open issues are
major concerns of future studies in the related areas.
B. Future perspectives
RFID sensing will stay as a hot research topic in the
envisaged future. It will be integrated with new emerging
technologies in material science, integrated circuits, and
computer science and applied in some new fields. Some
commercial components such as integrated RF-to-DC
converter, programmable RFID chips, integrated SoC RFID
reader chip become common, which will also promote the
innovative designs and applications. The major studies of
RFID sensor in the future might fall in the following topics:
1. Performance improvement of conventional RFID
Investigations of new RFID tags to improve the
performance of RFID measurement for some special use
scenarios or for common uses is a promising direction, such
as low-cost tags, long distance reading solutions, data
security and trust for sensitive applications, etc. For
measurement purpose, integration of RFID chips with
sensitive materials is a promising solution for passive and
wireless measurement.
2. SoC and energy efficient RFID sensors
Integration and miniaturisation are essential parameters to
electronic systems including sensors, and SoC is a very
promising solution. The integrated SoC RFID circuits with
ADC and sensitive materials are of interest for many
applications. This is also an effective way to reduce energy
consumption of the system. The miniature, ultra-low power,
passive, and non-contact sensor solution can be extended to
many different fields of measurement.
3. Wearable and bio-monitoring RFID sensor
Some flexible and foldable materials for fabrication of
RFID antenna and substrates are enabling techniques for the
wearable RFID tags. Since RFID is light-weight and small,
it becomes a promising solution to stick on or embed in
human body for the measurement of biomedical parameters
for health monitoring and elderly care.
4. WSN, body area network, and IoT applications
RFID is considered a key building block for future IoT
world, where sensors are connected for pervasive
monitoring and control. RFID sensor based WSN is
a promising technical approach for collaborative
optimisation in some light weight monitoring systems. Body
area network is a typical use case. The versatile, low-cost,
and miniature RFID sensors will be of interest and play an
important role in the IoT ubiquitous sensing environments.
Investigations of new materials for RFID tag, SoC systems
and their applications, and the integration with new
emerging technologies and computing paradigms are
research focuses and key promising directions of RFID
sensor technologies in the future.
6. CONCLUSION
The rapid progress of batteryless RFID sensors has created
tremendous opportunities for wireless measurement in a
variety of areas. Applications that utilise an RFID tag as a
sensor become common and some novel applications for
different use cases appear overwhelmingly. Focusing on this
prevalent research direction, this review paper illustrates the
fundamental of RFID sensors and the enabling techniques,
classifies the novel applications with typical examples,
discusses the technical challenges, and outlines the future
perspectives of this particular research area. This review is a
timely supplement to the literature regarding RFID sensors.
Based on the reported designs and applications, we can
envisage that the integration of RFID technology with many
fields of science and engineering, and their practical
applications will continue to expand deeply and broadly.
Especially due to the strengths of low-cost, easy-to-use, and
convenient integration, RFID sensors will play a very
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important role in pervasive IoT applications in the future.
The comprehensive overview of the innovative studies and
in-depth analysis reported in this work will be of interest for
academic and industrial communities in investigating,
developing, and applying RFID for measurement purposes.
ACKNOWLEDGMENT
The authors would like thank all staff and students in the
microwave sensor laboratory of University of Manchester.
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Received June 23, 2016.
Accepted November 28, 2016.
Unauthenticated
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