IEEE 1451 SMART SENSOR - Engenharia Eletrica

Anais do XIX Congresso Brasileiro de Automática, CBA 2012.
IEEE 1451 SMART SENSOR: LOW COST, LOW POWER WIRELESS CASE STUDY
LEANDRO PRYTULA, IVAN MÜLLER, VALNER BRUSSAMARELLO, CARLOS E. PEREIRA
Department of Electrical Engineering, Federal University of Rio Grande do Sul
Porto Alegre, RS, Brazil
E-mails: {leandro.prytula, ivan.muller}@ufrgs.br,
[email protected], [email protected]
SEBASTIAN Y. C. CATUNDA
Department of Computing Engineering and Automation, Federal University of Rio Grande do Norte
Natal, RN, Brazil
E-mails: [email protected]
Abstract
 The combination of transducers and microprocessors facilitate the insertion of a device called smart sensor in a digital communication network and decrease the efforts of proper configuration. However, the diversity of existing protocols or the
creation of independent solutions by manufacturers hinders the interoperability of these transducers, making them less flexible
and more expensive. Within this context the IEEE 1451 was created, which standardizes the hardware and communication
method for a smart transducer. This paper presents the development of a smart sensor based on the IEEE 1451. The developed
device is composed of two parts: the sensor nodes and network node. Communication between these two parts is accomplished
through a wireless interface using the IEEE 802.15.4 protocol. The main features of the developed smart sensor are its ability of
self-configuration, self-calibration and self-identification when connected to the network. A case study validated the results using an evaluation platform consisting of sensor devices and a base station. The feasibility and practical issues about the standard
are also discussed.
Keywords
 Smart sensors; IEEE 1451; Wireless sensor networks; Self calibration systems.
Resumo
 A combinação de transdutores e microprocessadores facilitam a inserção de um dispositivo denominado sensor
inteligente em uma rede de comunicação digital e diminui os esforços de configuração. No entanto, a diversidade de protocolos
existentes ou a criação de soluções independentes de fabricantes dificultam a interoperabilidade desses transdutores, tornando-os
menos flexíveis e mais caros. Dentro deste contexto, o IEEE 1451 foi criado para padronizar o hardware e o método de
comunicação para um transdutor inteligente. Este trabalho apresenta o desenvolvimento de um sensor inteligente baseado no
IEEE 1451. O dispositivo desenvolvido é composto de duas partes: os nós sensores e nó de rede. A comunicação entre estas duas
partes é realizada através de uma interface sem fio IEEE 802.15.4. As principais características do sensor desenvolvido
inteligente são a sua capacidade de auto-configuração, auto-calibração e auto-identificação quando conectado à rede. Um estudo
de caso valida os resultados, utilizando uma plataforma de avaliação consistindo de dispositivos sensores e uma estação base. A
viabilidade e uso prático do sensor desenvolvido também são discutidos.
Palavras-chave
 Sensores inteligentes; IEEE 1451; Redes de sensores sem fio; Sistemas auto-calibráveis.
1
Introduction
Wireless sensor networks (WSN) have been the
object of study for some time, but the development of
these networks has become more common a few
years ago due to advances in the areas of
microprocessors,
sensing
materials,
microelectromechanical
systems
and
wireless
communications. Today, it is usual to have cheap
radio transducers incorporated with microcontroller
units (MCU) which facilitates the rapid development
of wireless devices. The replacement of traditional
wired sensor networks for wireless ones has many
advantages and allows the deployment in several
different areas. Among these, we can mention
environmental applications, in which WSN are used
for monitoring environmental conditions. These
networks can be used to the rapid detection of, for
instance, points of fire in a forest, floods, monitoring
animal’s movement or other environmental factors,
such as detection of air pollution levels or pesticides
in water. In human health related applications,
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wireless sensor networks can be used for medical
screening, monitoring patient’s movements, body
functions (heart rate, blood pressure) and medicine
administration. Other applicable areas include traffic
control, modeling and monitoring structures, quality
control of products, etc. (WICZER, 2001), (MEKID,
2006), (BUZDUGAN, 2008). But the main idea of
wireless sensor networks is beyond of current
applications: taking advantage of low power
consumption, small size, inexpensiveness, easy
replacement and maintenance devices that can be
used on a large scale deployment that will enable
applications previously only imagined in science
fiction. Intelligent devices would be so small that
they could remain suspended in the air,
communicating for hours, days or even years. They
could identify themselves and track virtually any kind
of process or physical object in their scope. The
information provided by these sensors could be used
independently or, combining gathered data, and they
can act in a complex system where they could
perform self-calibration of sensors. Self-identification
and self-calibration of sensors is facilitated if the
sensors share the same network communication
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protocol (KIYONO, 2004). But with the wide variety
of protocols and manufacturers this task has become
very complicated. Because of this drawback, IEEE in
partnership with NIST created the IEEE 1451
standard to facilitate these operations (IEEE, 2007),
(EUGENE, 2008).
The development of this work takes place over
the three points presented in the previous paragraph:
the self-identification, self-calibration of sensors and
wireless communication using the IEEE 1451 family
of standards. The main objective is the development
of a wireless smart sensor based on the IEEE 1451,
more specifically in the 1451.0 and 1451.5 variations
of the standard that relate respectively the common
definitions to all points and wireless physical
interface. The developed system is based on an IEEE
802.15.4 protocol variant and is capable to selfcalibrate. To reach the objective, specific
implementations are needed, namely:
•
Development of Transducer Electronic Data
sheets (TEDS) from IEEE 1451.0 with emphasis
on TEDS calibration;
•
Development of IEEE 1451.5 TEDS that
describes the physical interface;
•
Inclusion of the TEDS in the chosen hardware
and software platform;
•
Development of algorithms for the sensor based
on the commands listed in the IEEE 1451
standard.
Previous related works are presented in the Section two and Section three deals with a brief overview about the standard. The development of an
IEEE 1451 smart sensor is explained in Section four
and a case study about it is explored in Section five.
2 Related works
The IEEE 1451 had its first results published in
1997. Thereafter several other studies have been
published and others were reviewed in order to make
the standard more flexible and comprehensible. The
IEEE 1451 addressed to extended applications such
as described in this work, with wireless applications
(IEEE 1451.5) firstly published in 2007.
During this period several IEEE 1451 based
works have been published. Among them we can cite
the one presented in (MANUEL, 2003), which deals
with the implementation of a sensing module based
on IEEE1451.2 in which development is still based
on wired networks. The proposed system does not
perform self-calibration. The work presented in
(ROSSI, 2009) deals with the implementation of a
dynamic and interactive web application for the
control and monitoring of networked smart
transducers in accordance with IEEE 1451. Again,
the used sensors are connected by wires and the
system is not capable to perform self-calibration. The
work mentioned in (VIEGAS, 2008) deals with
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IEEE1451.5 and IEEE1451.0 through wireless
sensors connected to a computer that plays the role of
network node but does not implements a sensor
capable to perform self-calibration.
Concluding, several works were developed in
accordance with IEEE 1451 self-identification
feature but only few ones cope with self-calibration.
Also, the increase of wireless sensors networks usage
permits the easy development of wireless smart
sensors and the IEEE1451 is an alternative to unify
the sensors protocols. The motivation for the
development of this work is precisely based on these
facts.
3 IEEE 1451 standard
The main purpose on the integration of
microprocessors, transducers and communication
transceivers is to add intelligence to devices, making
possible to insert them in a digital communication
network. However, with the wide variety of field
networks currently available, each manufacturer of
microprocessor-based sensors tend to choose a
network protocol and work around it, or create a new
protocol for their products. This diversity of field
networks and protocols leads to inflexibility and
generally more expensive solutions. The 1451 family
of standards aims to solve this problem by proposing
a set of standardized hardware and software
interfaces that act as "plugs", allowing different
transducers to be connected to the same network
(VIEGAS, 2008). It should be noted that the 1451
family is not a network protocol, but standardization.
The IEEE 1451 standard is composed of several
working groups, each one responsible for the
development of a specific feature, necessary to the
general purpose of the standard. Currently six of
these groups have already made available their work
and two are still working on the development. The
common terms used in the IEEE 1451 are (IEEE,
2007), (EUGENE, 2008):
•
TII (Transducer Independent Interface) is the
physical communication and information transfer
protocol between the Transducer Interface
Module (TIM) and the Network Capable
Application Processor (NCAP).
•
API (Application Program Interface) are sets of
routines for the application programs that allow
the usage of services.
•
TIM (Transducer Interface Module) contains the
interface, signal conditioning, analog to digital
converters and in many cases the transducer
itself. The composition of the TIM can be
anything from a single sensor or actuator to a
unit containing many transducers.
•
NCAP (Network Capable Application Processor)
is a set of hardware and software that acts like a
gateway between the TIM and the network or
host application.
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•
STIM (Smart Transducer Interface Module) is a
TIM when implemented in accordance with the
IEEE 1451.2. The STIM is a transducer that
communicates with the NCAP-based interface in
a similar way to a Serial Peripheral Interface
(SPI) with additional lines for hardware flow
control and time, resulting in a total of 10 lines
for the interface.
•
TBIM (Transducer Bus Interface Modules) is a
TIM when implemented in a distributed array of
transducers defined in IEEE 1451.3.
•
WTIM (Wireless Transducer Interface Module)
is a TIM when implemented according to the
IEEE 1451.5. The communication is established
using approved standards, like IEEE 802.11,
IEEE 802.15.4, Bluetooth, and ZigBee.
•
TEDS (Transducer Electronic Data Sheet) are
files stored in the TIM and contain information
related to the manufacturer such as name, serial
number, sensor type, calibration data, etc. The
TEDS allow automatic identification for smart
sensors or actuators.
in the structures of communication between the
sensor node and the network node.
ii. Transducer Electronic Data sheets - TEDS: the
TEDS was written intended for self-configuration and
self-identification in a sensor network. IEEE 1451.0
and IEEE 1451.5 were used to accomplish the writing of the TEDS. Five TEDS structures were created:
a. Meta TEDS, which contains information common to the channels of transducers, and general information of the sensor node;
b. Transducer channel TEDS, which describes the
transducer that is connected to the channel of the
sensor node;
c. Username transducer, that is the information relating to the identification of the sensor;
d. Calibration TEDS, an information required to
correct the reading of the channel sensor node;
e. Physical layer TEDS, which describes the
physical interface between the network node and sensor node.
4.1 IEEE 1451 structure in the firmware
4 Development of a wireless smart sensor
The hardware selection for the development of an
IEEE 1451 smart sensor is based on the need for a
wireless interface between the network node and the
sensor node. Concerning platforms that include MCU
and radio in the same chip, there are several options,
among them, the Freescale’s MC1322X. This
platform includes an ARM 7 MCU and an IEEE
802.15.4 radio, besides standard peripherals such as a
12 bit ADC with multiple inputs. These features are
quite adequate to the development of a low cost, low
power IEEE 1451 smart sensor.
The network node and a sensor node were both
implemented based on the Freescale’s MC1322X. In
order to exercise the applicability of the platform,
two integrated-circuit temperature sensors (LM35)
were connected to the sensor node. One of these
sensors was used as reference, representing the gold
standard and the other as a regular temperature
measurement point. The last temperature sensor was
connected to an electronic voltage divider, in order to
emulate deviations on the sensor from the original
calibration curve. Thus, the auto-calibration software
should adjust offset and sensibility coefficients in
order to compensate the induced errors. One should
observe, however, that the sensor is still working, i.e.
the output is a linear function of the temperature. In
fact, it should be emphasized that a calibration is a
much more complex process than just a set of
coefficients (VIM, 2012).
The TEDS are written in the MCU’s firmware to
describe the sensor as a whole and identify it to the
network. The description is adaptable to any type of
transducer. Some of the written TEDS structures of
this work are mandatory and others optional. Figure 1
shows the structure of the developed smart sensor
and the role of the TEDS sensor can be seen. The
TEDS are composed of three types of fields, namely:
•
Field length, made up of four octets, representing
the total number of octets contained in the data
block plus two checksum octets;
•
Block Data Field, with a variable number of
octets. This is the field that contains the TEDS
information, being composed of several
structures type, length, value (TLV). The type is
composed by an octet that identifies the TLV
structure. The length specifies the number of
octets of the value, which in turn represents the
actual data;
•
Checksum field, two octets. This field is formed
by a complement of the sum of all preceding
octets.
The suitability of the sensor to the IEEE 1451
standard involves two basic tasks:
i. Firmware development: the firmware was
developed according to the IEEE 1451.0 standard,
since it describes the commands and messages used
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Figure 1. IEEE 1451 based smart sensor structure.
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In order to verify the applicability of these
definitions, a calibration procedure is done. In the
linear conversion method, the TLV has octets that
represent the slope and the coefficient of the linear
calibration curve. All of the TLV's TEDS are
initialized with the exception of fixed TLV, stored in
the MCU’s flash memory during the process of selfcalibration.
The communication between the sensor node and
network node is based on three pillars:
communication protocol, message structure and
command standard. The communication protocol
used is a variant of IEEE 802.15.4. The commands
and message structure are defined and encapsulated
in IEEE1451.0 structure. The messages are
exchanged between sensor node and network node,
governed by three types of structure: command
message structure, response message structure and
message structure initiated by TIM (sensor node).
Each field of the message structure consists of an
octet and the size of the structure is variable
according to the message information. The total
message size is limited by the communication
protocol.
4.3 Sensor operation
As the smart sensor consists of two controllers,
the operation thereof is made as follows:
•
The sensor node acquires data from the
temperature transducers in free running mode. It
keeps reading the temperature sensors outputs
until solicited by network node. Different actions
can be requested by the network node such as the
self-calibration routine and TEDS information.
The basic operation flowchart can be seen in
Figure 2.
•
The network node request information from
TEDS once it is energized and in every second,
showing acquired data in a LCD display. The
reading of the sensors is available to the network
node in the format described in the TEDS
transducer channel definitions relating to
sampling. Also, the equation contained in the
TEDS calibration is applied and finally the
temperature in converted to Kelvin scale.
The network node basic fluxogram is depicted in
Figure 3.
4.2 Self calibration
The self-calibration routine employed in the
design of the smart sensor is based on linear
regression using the least squares approach.
The sensor output is supposed to be a linear
function of the temperature:
(1)
y = ( aT + b )
Where T is the temperature, a the slope (sensibility) and b, the offset coefficient.
The reference sensor and the measurement sensor
to be calibrated are inserted into a temperature
controlled chamber in which 20 samples for each
sensor are collected. The reference sensor has a
response defined by the manufacturer and it is used to
provide ambient temperature y for the linear
regression method:
n
n
n
a=
n∑ i =1Ti yi − (∑ i =1Ti )(∑ i =1 yi )
b=
n∑ i =1Ti − a ∑ i =1 Ti
n
n
2
n ∑ i =1 Ti − (∑ i =1 Ti ) 2
n
(2)
n
n
Where Ti are the temperature measurements and
n the number of samples.
The slope and the offset coefficients are
computed and stored in the calibration TEDS sensor
to be calibrated.
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Figure 2. Basic fluxogram of the sensor node.
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evaluated. As two LM35 sensors are used, they
provide very similar output voltages under the same
operating temperature. To generate a difference
between the output signals for calibration test
purpose, the measurement sensor output is connected
to a resistive divider to provide a different output
voltage. Figure 4 shows the voltage versus
temperature obtained from the sensors. The routine is
based on the content discussed in Section four. The
calibration procedure takes place in five minutes. The
total amount of 128 kB MCU Flash memory was
occupied by wireless routines and IEEE 1451
standard implementation in a 23% - 67% proportion.
At the end of the tests, the proper functioning of the
sensor is verified and its feasibility confirmed.
Figure 3. Basic fluxogram of network node.
5 Case study
5.1 Experimental setup
In order to perform tests with the developed smart
sensor, the MCU’s 12 bit ADC operating at 3.3 V
reference was used. National’s LM35 temperature
sensors output values were used to provide the
operating limits of the sensor. The 275 K and 420 K
lower and higher limits are described respectively in
the TLV's lower and higher limits of operation.
In order to have a simple and cost effective
solution for wireless networking, the Simple Media
Access Control (SMAC), which is incorporated into
Freescale’s BeeKit framework, was employed. This
proprietary protocol is based on the 802.15.4 PHY
and supports point-to-point and star networks. Figure
4 shows the central network node, in which it is
possible to notice the acquired data displayed on the
LCD module.
Figure 4. Freescale’s development kit, used to evaluate the smart
sensor.
5.2 Evaluation
With the smart sensor already operating under the
IEEE 1451 standard, the auto-calibration routine is
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Figure 5. Calibration curves of temperature sensors
6 Conclusions
In this paper we presented the development of a
wireless smart sensor in accordance with IEEE
1451.5 in which a self-calibration procedure is
performed. The IEEE 1451 covers a wide range of
sensor network configurations and is still being
expanded to become more comprehensive and
conform to widely distributed networks. The great
advantage an IEEE 1451 based sensor is the
information carried in its memory, the TEDS, which
allow self-identification and self-configuration. This
eases the insertion of nodes in a network, a great
feature for wireless networks. The possibility of
changing some of the TEDS information during
operation of the sensor is also very useful and it was
this feature that allowed the creation of a selfcalibration routine.
Within the development of this work, the
inclusion of IEEE 1451 in a smart sensor revealed
not to be a very complex task but very time
consuming. That is because of a large amount of
information needed for the TEDS and also the
number of commands that must be inserted into the
sensor. The employed hardware included a wireless
communication interface and enough memory to
allocate the TEDS and the routine operation of the
sensor. In short, it is possible to afirm that IEEE 1451
family of standards is a standard not yet widely used
due to their recent time of publication, but with a
great potential for application.
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The main objective of this work was to explore
the standard and for this, we developed a device for
research purposes. For a practical device, we propose
the use of only one sensor in the STIM and the usage
of a high precision sensor inside the calibration
chamber. During the calibration process, data are
transferred for the TEDS of the STIM through the
wireless channel.
6 Acknowledgment
We would like to express our gratitude to CNPq
and Capes, our governmental commissions for post
graduation and research on their support for this
work.
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