ZigID: Improving visibility in industrial environments by combining WSN and RFID

Abstract

The objective of this work is to provide decision-making processes with an updated/real picture of the mobile resources in
industrial environments through a constant feedback of information. The combination of identification technologies and wireless
sensor networks (WSN) is proposed as a key development to guarantee an accurate and timely supply of online information regarding
the localization and tracking of the mobile wireless devices. This approach uses a cooperative and distributed localization
system, called ZigID, which is a WSN based on a Zigbee network with radio frequency identification (RFID) active tags as end
nodes. The WSN can recover not only the ID information stored at the tags attached to mobile resources, but also any other
useful data captured by specific sensors for acceleration, temperature, humidity and fuel status. This paper also shows the
development of ZigID, including devices and information flows, as well as its implementation in ground handling operations
at the Ciudad Real Central Airport, Spain.

Key wordsRadio frequency identification (RFID)–Tracking–Wireless sensor networks (WSNs)

Full text

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ZigID: Improving visibility in industrial environments
by combining WSN and RFID

Pablo GARCÍA ANSOLA†1, Andrés GARCÍA1, Javier de las MORENAS1,
Javier GARCÍA ESCRIBANO1, Francisco Javier OTAMENDI2
(1Autolog Labs, School of Industrial Engineering, University of Castilla-La Mancha, Ciudad Real 13071, Spain)
(2Applied Economic Department, University Rey Juan Carlos, Madrid 28032, Spain)
†E-mail: pablo.garcia@uclm.es
Received Feb. 7, 2011; Revision accepted Aug. 19, 2011; Crosschecked Sept. 29, 2011

Abstract: The objective of this work is to provide decision-making processes with an updated/real picture of the mobile re-
sources in industrial environments through a constant feedback of information. The combination of identification technologies and
wireless sensor networks (WSN) is proposed as a key development to guarantee an accurate and timely supply of online infor-
mation regarding the localization and tracking of the mobile wireless devices. This approach uses a cooperative and distributed
localization system, called ZigID, which is a WSN based on a Zigbee network with radio frequency identification (RFID) active
tags as end nodes. The WSN can recover not only the ID information stored at the tags attached to mobile resources, but also any
other useful data captured by specific sensors for acceleration, temperature, humidity and fuel status. This paper also shows the
development of ZigID, including devices and information flows, as well as its implementation in ground handling operations at the
Ciudad Real Central Airport, Spain.

Key words: Radio frequency identification (RFID), Tracking, Wireless sensor networks (WSNs)
doi:10.1631/jzus.A1100024 Document code: A CLC number: TP393


1 Introduction

New management systems are steadily in-
creasing their complexity due to pressing require-
ments for productivity and flexibility. This com-
plexity becomes especially relevant when dealing
with large industrial plants, airports or logistic cen-
tres where the usual control is too rigid and hard to
manage, maintain and update (Thorne et al., 2007).
For a modern system, however, finding an adequate
response to a broad range of possible new situations
must be business-as-usual; and this should apply even
while the system is being modified or updated (Garcia
et al., 2003). Therefore, flexible control becomes
standard practice as it allows a division of the prob-
lem into smaller and more manageable ones. In this
case, the system becomes distributed in its control,
which makes a negotiated and cooperative process
necessary to take decisions that affect more than one
of these parts (Durfee, 1996). This situation requires
an improvement in the connection between the real
environment and the information systems (Sandholm,
1999). Enterprise resource plannings (ERPs) or other
information/management systems have given place to
new architectures, which need constant feedback of
information from the real environment. This infor-
mation is managed through artificial intelligence
techniques such as Bayesian networks (Chalkiadakis
and Boutilier, 2003), multiagent systems (Wooldridge
et al., 2002), neural networks and fuzzy systems
(Kosko and Burgess, 1998), making expert systems
adapted to the environment (Russell and Norvig,
2010).
To obtain information from the environment, the
systems need transparent identification technologies
Journal of Zhejiang University-SCIENCE A (Applied Physics & Engineering)
ISSN 1673-565X (Print); ISSN 1862-1775 (Online)
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© Zhejiang University and Springer-Verlag Berlin Heidelberg 2011

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and quick communication protocols (Mao and Wu,
2007). The frequency identification (RFID) technol-
ogy emerges as especially suitable for capturing in-
formation from the physical world, offering rapid
responses and continuous tracking, and has become a
standard for communication protocols, hardware
implementations and information systems due to the
electronic product code (EPC)-Global initiative (GS1,
2010). One of the problems of RFID is that, due to
battery limitations, the reader is usually set so that it
can only communicate for a distance of about 8–12 m
with standard active (with batteries) tags—and this
distance is necessarily smaller for more typical case
of passive tags (not powered)—while there is no
defined network topology to coordinate several
readers.
This inconvenience can be directly minimized
using mobile WSN (Xia et al., 2007; Sahinoglu et al.,
2008). The current WSNs abstract the mobility, to-
pology, scalability and communication between the
information system and the mobile nodes in the net-
work (Gezici, 2008). Besides routing information,
which can be added to the submitted information
package, more information can also be attached to
this package (Akkaya and Younis, 2005). In this way,
product/resource ID is also added to the information
about the environment, captured by the sensors. All
this information, together with the routing path, con-
stitutes a complete information package in the WSN.
Different researchers and companies have developed
real industrial applications of WSN, with satisfactory
results both for indoor (Ferrari et al., 2007; Morelli et
al., 2007; Rovňáková and Kocur, 2010) and outdoor
(Sung et al., 2007) applications. For example, Ansari
et al. (2009) focuses on specific technological de-
velopments to optimise battery life by suppressing
the power consumption during idle periods, which
could be an improvement to the system proposed in
our paper. It has been conceived as a broader
development.
However, most of the energy consumed by de-
vices is used in the processes of activation, routing
and sending of data via ZigBee. To reduce consump-
tion and according to the regulations of the ZigBee
protocol, the use of beacons is an available option.
This is the way many authors try to reduce the duty
cycle of nodes. Nevertheless, beaconing methods can
not always be adapted to the requirements of RFID
applications. Another way to reduce consumption is
to use the time division beacon scheduling (TDBS)
(Koubâa et al., 2008). Using this method, activation
and de-activation beacons are sent, by final nodes,
just when they need to start or finish the communica-
tion with their parents. Other works such as that pre-
sented in (Medagliani et al., 2010) also propose a
combination of the technologies ZigBee and RFID.
Still, in that case both networks overlap as RF is used
to activate/de-activate ZigBee modules by using the
specific characteristics of the protocols commonly
used in RFID and originally designed to turn off the
tags in the field of a reader when read (and to turn
them on again to restart a read cycle). Therefore,
RFID is not really used there to identify passing ob-
jects but to take advantage of some of its specific
characteristics.
In (Sánchez et al., 2009), active RFID technol-
ogy with sensors is used to monitor the surrounding
elements. The objective in these cases is for all the
elements to be capable of transmitting not only their
identity, but also information regarding their physical
condition. These elements are already known as
“Smart Objects”, where they are located into is
known as “Smart Environments”. The integration of
RFID and WSN is apparent and, in the present paper,
it can be named as wireless sensors and RFID for
smart environments (WISSE) by Sánchez et al.
(2009). However, the objective of the present work is
not the development of a new protocol for a global
use of active tags but rather to find the solution to a
recurring problem through the fusion of RFID and
WSN in one single platform. With a similar objective,
in (Escribano et al., 2010) a device is designed which
is capable of operating multiple communications
modules simultaneously. In the designed system, both
RFID and WSN are used in a way in which the in-
formation is collected through readers from active
tags, and these readers are mounted on network nodes
of a large scale personal area network (PAN).
Therefore, this paper approaches the combina-
tion of RFID to identify objects and WSN for trans-
mission of the collected data, which drives the de-
velopment of a cooperative and distributed architec-
ture, called ZigID. This paper aims at the develop-
ment of a complete system for a specific application
that takes advantage, in an inclusive way, of different
techniques found in the related literature.

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2 ZigID architecture

This research proposes the combination of RFID
and WSN technologies in a framework, called ZigID,
to localize and track mobile resources at industrial
environments. The feasible applications range from
airports to factories or logistic centres, in which ve-
hicles, products, pallets, machinery or employees
comprise the activities. The proposed system is based
on the development of a Zigbee wireless communi-
cation network of RFID readers that is capable of
covering medium/long distances within specific
zones over the whole industrial area (Zhao and Gui-
bas, 2004).
2.1 Wireless sensor network with active RFID
As a communication network, Zigbee is selected
for its characteristics of range, auto routing capability,
low power consumption and a sufficient bandwidth.
Zigbee is a specification of a set of high-level proto-
cols of wireless communication for use with
low-power digital radios based on IEEE 802.15.4
wireless personal area network (WPAN). Its main
features are low power consumption, mesh network
topology and ease of integration (nodes can be
manufactured with very little electronics). Zigbee
networks can adopt three alternative topologies: star,
tree and mesh. For the current situation, the mesh
topology is particularly well suited. It allows com-
munications to be maintained even in the case of a
Zigbee node breakdown by means of dynamically
recalculating new routes, while covering medium/
long distances. By using WSN based on Zigbee with
RFID reader nodes, the ID information can be ob-
tained over large distances. The communication ar-
chitecture of a general airport is shown in Fig. 1.












port application
To summarise, the benefits of this combination of
technologies are: (1) Better and more transparent flow
of information. (2) The RFID identifies elements us-
ing EPC, which is a standard in all levels of the busi-
ness model. (3) The WSN allows setting up networks
to cover large areas in an easy and dynamic way, while
RFID focuses on the identifications of items in spe-
cific zones. (4) The cost of the RFID End Nodes (10–
12 USD) is low compared to other existing devices.
(5) The battery consumption of the RFID End Nodes
is really low compared with other electronic devices,
such as mobile phones and radio devices. (6) Network
re-routing is also possible in case of breakdown of one
node in the WSN by using mesh topology.
2.2 ZigID definition
To take advantage of the technologies presented
above as well as those resulting from their combina-
tion, a new architecture has been developed with the
name of ZigID. ZigID is divided into three levels to
facilitate the distribution of tasks and foster the co-
operation among devices. The first of these levels is
the End Node level, whose features are: small and
unobtrusive size, low consumption (enabling battery
supply), possibility of integrated sensors, short range
coverage (8–10 m) and identification capabilities.
The second level is the Router-Reader Level and its
objective is to facilitate the communication and
routing of the information among End Nodes and the
information systems. The third level is the coordina-
tion level, which collects the distributed information
across the network and provides an interface with the
existing information systems (Fig. 2).















Fig. 2 ZigID architecture
Sensors, variables, ID
RFID
RFID
Filtering & capturing
Zigbee
Zigbee
Coordination
Interface
Information systems
End Node
Router
Coordinator
<12 m
<400 m
USB or Ethernet
Fig. 1 Communication architecture for a general air-

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3 ZigID devices
3.1 End node level
Hardware devices at this level are attached to the
resources to be tracked. Their function is to commu-
nicate the identification and sensors information to
the WSN through an active RF communication. We
call it RFID+, as it includes identification and more
information than that captured by sensors. At this
level, there are some limitations such as battery life,
computation capacity and communication bandwidth.
The proposed devices are based on active tags
with sensors; which means that they must be pow-
ered by batteries. These devices are active tags with
sensors that incorporate a microcontroller and a
transceiver (Fig. 3). The microcontroller is a
PIC16F819 by Microchip with low power con-
sumption (7 μA) as it integrates nanoWatt Tech-
nology (Microchip, 2004). This microcontroller is in
charge of managing the operations of the end node,
which include communications with routers and data
acquisition from sensors.















The transceiver is an nRF905 by Nordic (2008),
which is an RF transmitter/receiver that operates in
the ultra high frequency (UHF) band to manage the
communication between nodes and routers. It is con-
figurable to work at 433, 868 and 915 MHz. It also
has different transmission power modes from −10 to
10 dBm, providing a good relationship between range
and power consumption. It is commanded through a
Synchronous Serial Port, SPI. Other elements of the
node are the antenna, sensors and batteries. The an-
tenna is a dipole printed on the board. The node can
operate different sensors to measure temperature,
acceleration (shocks), fuel or other chemical elements
among others.
Two conventional rechargeable AAA batteries
have been used to power the prototype. A Cr2032 Li
Button Coin cell was first tested but it did not provide
enough autonomy for the required communication
ranges in the airport application. After some tests,
autonomy has been estimated at around 2845 h with
the AAA batteries, for a duty cycle where there is an
RF communication every 2 s with a duration of ap-
proximately 0.1 s in each transfer. This measurement
has been undertaken at 10 dBm, which is the highest
power the device can manage, but that is not likely to
be required in most applications. In most cases, End
Nodes are attached to vehicles, so it is possible to
power them using the vehicles battery, maximizing
the lifetime.
3.2 Router level
The second level is in charge of routing the in-
formation to the coordinator. This level uses two
communication protocols. The first one is used for RF
communication, which receives information from the
End Nodes. The second one is used for Zigbee con-
nection (network of readers), which routes the in-
formation through the wireless network until it
reaches the coordinator.
A significant asset of Zigbee for this application
(or Zigbee-like networks based on IEEE 802.15.4, as
top-most layers are not necessarily required) is its
characteristic of dynamic auto-routing between
nodes, which provides excellent reliability in the flow
of information to the coordinator. This feature is very
useful in case of break-down/disconnection of nodes
and allows adding new devices to the WSN in a dy-
namic way. Another function of this device is the
filtering of unnecessary information, such as redun-
dant identifications or not well-defined information,
to avoid jamming.
The elements that constitute the router (Fig. 4)
are as follows: an nRF905, in charge of RF commu-
nication, and a JN5139 by NXP Semiconductors
(2011). The JN5139 is a module that enables the im-
plementation of Zigbee networks and that is available
in two versions: normal and high power. The high
Fig. 3 End node prototype

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power module JN5139M02 has been used for this
development as it can provide an output power of
19 dBm, which can make a possible theoretical range
of up to 4 km as specified in the IC data-sheet (NXP
Semiconductors, 2011); although its actual perform-
ance for a reliable enough connection is approxi-
mately 400 m in a real environment. JN5139 also
includes microcontroller features to control serial
communications and input/output pins. The JN5139
manages the Zigbee network and the RFID commu-
nications, as well as the buttons and light emitting
diodes (LEDs) in the device (Fig. 4).
The routers composing the second level modules
are outdoor modules to be placed at fixed locations.
The best option is to plug them into the power grid. In
isolated places, the use of high power rechargeable
batteries with alternative power sources, such as
photocells, would provide them with additional
autonomy. Specific battery management functions
have also been implemented to increase battery life
(Section 3.4).



























3.3 Coordinator level
The coordinator level connects the WSN with
the existing information systems. The information
captured by the WSN is properly formatted in XML
or submitted in a query to a database. The coordinator
submits an XML file to an EPC repository developed
by Floerkemeier et al. (2007), which is accessible to
the rest of departments in the company. There is only
one coordinator device in the WSN, which has been
called “PAN coordinator”. Firstly, it selects the fre-
quency channel to be used by the network (usually the
one with the least detected activity). It then starts the
network and allows child nodes to connect to it. It can
also provide message routing, security management
and other services, which will be detailed in the data
model definition.
The prototype shown in Fig. 5 has been devel-
oped to allow direct universal serial bus (USB)
communication with a personal computer; future
developments will also include Ethernet communica-
tion and power over Ethernet (PoE).

























Fig. 5 Prototype of the Coordinator Node
Fig. 4 Prototype of router node