JSSSJournal of Sensors and Sensor SystemsJSSSJ. Sens. Sens. Syst.2194-878XCopernicus PublicationsGöttingen, Germany10.5194/jsss-6-19-2017A wireless communication system for energy and environmental monitoringKrushDmytrodmytro.krush@hsu-hh.deCamminChristophHeynickeRalfSchollGerdKaercherBerndElectrical Measurement Engineering, Helmut Schmidt University,
University of the Federal Armed Forces Hamburg, Hamburg, GermanyFesto AG, CR-MC, Ruiter Strasse 82, 73734 Esslingen, GermanyDmytro Krush (dmytro.krush@hsu-hh.de)10January201761192612August201627September201624November2016This work is licensed under a Creative Commons Attribution 3.0 Unported License. To view a copy of this license, visit http://creativecommons.org/licenses/by/3.0/This article is available from https://jsss.copernicus.org/articles/6/19/2017/jsss-6-19-2017.htmlThe full text article is available as a PDF file from https://jsss.copernicus.org/articles/6/19/2017/jsss-6-19-2017.pdf
Custom-fit communication systems are key elements in modern cyber-physical
sensor systems. Therefore a wireless communication system (WCS) for
sensor/actuator
communication has been developed to facilitate energy and
environmental monitoring on the shop floor of industrial production sites.
Initially, the distinct demands and requirements are described. As the WCS
has been designed for new installations as well as for retrofitting already
installed facilities, the WCS has to be able to coexist with other wireless
communication systems already allocated in the same frequency band. The WCS
handles measurement data from both, energy-autarkic sensors and fast
line-powered sensors. Mobile users in the field equipped with mobile devices
are served by the system, too. A modular hardware concept has been chosen for
easy system modification or for the integration of new wireless standards.
Finally, measured results for the coexistence capability are presented.
Introduction
An ongoing tendency indicating increasing energy costs can be observed.
Therefore energy efficiency of industrial manufacturing processes is becoming
more important. Moreover, there is an additional demand by industrial
manufacturers to assign carbon dioxide emissions to specific products and
individual process steps.
The publicly funded research project ESIMA
aims at measuring energy flows and at analyzing energy consumption in
different production steps during the manufacturing process. The acronym
“ESIMA” stands for “Optimierte Ressourceneffizienz
in der Produktion durch Energieautarke
Sensorik und Interaktion mit mobilen Anwendern” (English: “Optimized
resource efficiency in the production process by energy-autarkic sensors and
interaction with mobile users”) . The ESIMA consortium consists
of the following partners: FESTO AG (project coordinator) as manufacturer
of pneumatic and electronic components, Varta Microbattery GmbH as
manufacturer of storage battery systems, EnOcean GmbH as manufacturer of
energy-autarkic sensors, the chair of Electrical Measurement Engineering at
Helmut Schmidt University, University of the Federal Armed Forces Hamburg,
for the development of a robust wireless communication system, Hahn-Schickard
as a service provider for sensor technology, c4c Engineering GmbH for
software engineering, the Institute of Machine Tools and Production
Technology at the TU Braunschweig for concept studies with respect to energy
and resource efficiency, and Daimler AG as operator of the industrial target
application.
One aspect in ESIMA is focused on interaction with mobile users. In the
context of Industrie 4.0 a consistent information concept is realized, driven
by the demands of the facility operators. Energy flows through the electrical
and pneumatic supply lines of the production machinery are measured to
extract and to visualize significant key figures. These key figures are used
to reveal energy and cost savings. A role model has also been developed,
including individual user groups like machine operators, maintainers, and
management personal, which are provided with user-specific, especially
reprocessed, and visually optimized data sets. Additionally, combined user
groups, e.g., sitting in “shop-floor meetings”, are taken into account.
Besides electrical and pneumatic quantities, environmental quantities of the
production sites like temperature and illumination intensity are measured
with highly miniaturized embedded machine and application-specific sensor
systems. In order to enable an easy and flexible integration into the
production environment or to enhance existing production facilities, a
wireless communication architecture is suggested. The wireless approach also
has the advantage of providing measurement data and analysis results to
mobile users and getting data from measurement points, where standard cabling
is too expensive or too awkward to install.
System requirements
Sensor setup, configuration, and diagnosis as well as the accessibility and
the processing of the sensor data are the key matters of concern in the ESIMA
project. Mobility and flexibility requirements make a wireless communication
system (WCS) necessary.
System requirements are derived from the needs of the individual user groups.
In standard mode a measurement rate of 1 Hz has been chosen, with the option
of increasing the sample rate to 10 Hz in ESIMA-streaming mode. For
quantities like machine or room temperature with large time constants, the
sample rate can be appropriately decreased. This ability is especially
important for sensors, which have no energy harvesters, but which have to
operate battery-powered over many months . Thus,
energy-autarkic sensors with long sleep times can also be integrated into the
WCS.
The prospective number of sensors for the energy and environmental monitoring
in ESIMA is on the order of 10 sensors per manufacturing cell with a
footprint of 10 m by 10 m. More sensors and also actuators are intended to
be installed for enhanced functionalities, e.g., process monitoring. The
following Table is extracted from the guideline itemizing
typical system requirements in the various fields of automation and has also
been taken as a guideline in ESIMA.
Requirements for different wireless automation
domains, extracted from .
WLAN communication systems based on the standards according to IEEE 802.11
a/b/g/n exhibit RF bandwidths of at least 20 MHz and gross data rates
usually greater than 11 MBit s-1. Thus they are
often used for applications where high data throughputs are required, e.g.,
intra-logistic solutions. In comparison with Bluetooth or WirelessHART
devices, which comply with the IEEE 802.15.1 and IEEE 802.15.4 standards,
respectively, energy consumption is much higher, so that these devices cannot
be used for energy-autonomous applications
. Another drawback is
that latency times for data transmission and jitter in latency times are not
defined, so that this communication medium cannot be used for time-critical
applications.
Bluetooth was primarily designed for robust short-range
applications offering sufficient data rates for many industrial applications
and low energy consumption, but Bluetooth piconets are limited to seven
active devices, also limiting the application spectrum
. During the last few years WirelessHART has become
the de facto standard for process automation applications, but cannot be used
for applications, where cycle times on the order of several milliseconds and
below must be realized . Thus the
ESIMA network is the first system that can be used across the different
application domains. WirelessHART and the ESIMA communication network also
significantly differ from classical office communication standards as they
offer all tools and resources, which are required for professional industrial
sensor/actuator communication, i.e., appropriate engineering and development
tools as well as tools for network setup, security management or network
monitoring, and diagnosis or data acquisition. Thus WirelessHART and also
ESIMA (yet to a smaller extent) offer substantially more than a bare data
link like WLAN or Bluetooth and represent the connection to the cyber world
for modern cyber-physical systems.
As many production environments are already equipped with wireless systems,
in particular WLAN , and these systems are also utilized for
production-critical data transmission such as, e.g., intra-production site
logistics, the established performance of these systems is protected by
prohibiting any possible degradation. Thus, an additional WCS has to ensure a
continuous coexistence with the existing wireless systems.
Wireless communication technology
An industrial environment is typically characterized by many metallic
structures causing diffraction, multiple reflections,
and shadowing, where there is often no line-of-sight
between the transmitting and receiving antennas. Every radio channel can be
characterized by its time-variant and frequency-selective behavior, where the
coherence bandwidth is a characteristic quantity of a radio channel
describing the frequency behavior . Many measurements
of radio channels in industrial environments were performed and analyzed,
resulting in the coherence bandwidth being on the order of a few MHz in the
2.4 GHz industrial, scientific and medical (ISM) band
. If the transmission bandwidth of a wireless
system is significantly smaller than the coherence bandwidth of the radio
channel, the radio channel can be treated as frequency-flat and the
communication system is denoted as narrow-band . This
has the advantage that there is no need for complex radio channel
equalization, as is the case for wide-band communication
. Accordingly, this
reduction of the receiver's complexity is beneficial for low-power operation.
More importantly, it has been shown that in complex industrial environments
low-latency times in conjunction with a high reliability can be ensured using
narrow-band wireless communication systems
. The implemented WCS is
derived from the IEEE 802.15.1 standard as an open standard and
utilizes the 2.4 GHz ISM band, as it is available in most countries. Another
advantage is that there is a large availability of highly integrated
commercial transceiver chips that can be tailored to the specific customer
needs .
Gaussian frequency-shift keying (GFSK) is used as a modulation technique at a
bit rate of 1 Mbit s-1. In accordance with the IEEE 802.15.1 standard
, up to 81 channels are utilized with a channel spacing of
1 MHz. A so-called “blacklisting” is implemented to avoid frequencies
already occupied by other wireless systems. The network has a star-shaped
topology. The gateway provides the interface of the WCS to the wired LAN with
the connected backend system or the database. Moreover, sensor nodes
facilitate low-energy communication and a predefined latency time can be
assured also for energy-autarkic sensors .
The WCS provides bidirectional communication offering the possibility of
reconfiguring sensors and returning acknowledge messages for correctly
received packets to ensure a high safety level. As an additional advantage,
wireless actuators can also be addressed.
Protocol structure
A combined time and frequency division multiple access scheme has been
chosen. The sensors/actuators are assigned to one of up to four uplink tracks
(ULs) for the uplink communication from the sensors/actuators to the gateway.
The frequency spacing between the DL track and the UL tracks is at least
22 MHz. This frequency spacing allows one to operate a WLAN system between
the DL track and the UL tracks. Additionally, crosstalk between the DL track
and the UL tracks is sufficiently avoided. Measurements have shown that WLAN
systems operated at frequencies close to the WCS do not degrade the WCS's
performance, even if the power level of the WLAN is up to 20 dB higher than
the power level of the WCS .
Each UL track carries the communication load of up to 15 nodes, so that in
total up to 60 sensor/actuator nodes can be served. There is only one
downlink track (DL) for transmissions from the gateway to the
sensor/actuator nodes. Data transmission is asymmetric due to the fact that
in total more data have to be transmitted by the sensor/actuator nodes than
data have to be received from the gateway.
The communication protocol is organized into time frames, where each frame
has a duration of 3.328 ms, so that in the case of disturbances two
retransmits can be carried out within a time frame of 10 ms. The structure
of the frames is shown in Fig. . Each frame consists of 16
time slots, each lasting 208 µs. Within the 16th time slot at the
end of each frame a frequency change is carried out. A frequency-hopping
scheme is used to circumvent fading effects of the radio channel. A second
advantage is the probability of interferences with other wireless systems
being further reduced. Different frequency-hopping algorithms can be chosen,
depending on the application, prospective environment, and specific
requirements .
Downlink frame with 16 slots and a downlink packet.
Packet structure
Each DL packet comprises a 16 bit preamble, a 24 bit sync word, and
additional 24 bit for protocol management. The payload consists of 112 bit.
A cyclic redundancy check (CRC) with a length of 32 bit is used for error
detection, so the probability that a corrupted message will be falsely
handled as an allowed message is below 10-9. The
structure of the DL packet is presented in Fig. . The
structure of a UL packet is shown in Fig. . The UL packet
contains an additional guard interval of 8 µs, ensuring that
multiple echoes caused by the radio channel will have decayed and that the
gateway's receivers can cope with the fast packet handling, even if the
sensor/actuator nodes are imperfectly synchronized. As 2 bytes for the
protocol management are sufficient, the total duration of a UL packet is
208 µs too.
Downlink packet.
Additional features
Beyond the scope of energy monitoring, the capabilities described below were
also implemented to cover additional prospective applications.
Time stamping
Due to the relatively low sample rate of typically 1 s and the fast
processing time of the data wirelessly sent by the sensor/actuator nodes, it
is sufficient that the individual information messages are time-stamped when
they arrive at the interface of the backend database. This omits the need for
data synchronization of the sensor/actuator nodes, resulting in less
computational effort and hence a reduced energy consumption. Also, data size
and thus channel occupancy can be kept small.
Uplink packet.
Streaming mode
To allow a higher time resolution of sensor data, a streaming mode has been
implemented, which has to be initiated by the user via the backend system.
After the streaming mode has been triggered, a control command will be sent
to the gateway by the backend system. When the sensors receive the control
signal in the downlink message, they increase their wireless transmission
rate. Supplementary to Sect. , the transmission rate
can then be increased beyond 10 up to 100 Hz. The net-data rate equals
11.2 kbit per sensor/actuator node in the uplink. Assuming measurement
values with 8 bit resolution, sample rates of up to 1.4 kHz for each node
can be realized if multiple measurement values are transmitted in one packet.
Hardware architecture
In what follows the hardware components of the gateway and the sensor nodes are described.
Hardware of the gateway
The gateway is realized energy-efficiently and compactly on the basis of a
Xilinx Zynq system-on-chip (SoC), combining a field-programmable gate array
(FPGA) with an ARM dual-core microcontroller . The structure of
the hardware architecture is shown in Fig. .
Hardware architecture of the gateway.
VHDL state machines were implemented in the logic section of the SoC to
control the radio transceivers. A dual-port RAM is employed as a buffer for
data communication between the ARM microcontroller and the VHDL structures.
Data and protocol handling is organized with the help of the ARM
microcontroller, which also transfers data between a LAN interface and the
VHDL structures.
Photo of the demonstrator gateway.
Figure shows a photo of a gateway demonstrator. That
gateway is equipped with five radio transceiver modules, where one is based
on a Texas Instruments/Chipcon CC2400 transceiver chip to
transmit DL packets due to the ability for fast frequency change and their
spectral performance in transmit mode. Four radio transceiver Nordic NRF
2401A transceiver chips have been chosen in the uplink part
of the gateway due to their selectivity in receive mode and their ability for
fast frequency change, too. It is possible to modify the gateway easily owing
to its modular design. As an example, the radio transceivers above can be
replaced, e.g., by EnOcean transceivers . A photo of the
gateway inside a housing is shown in Fig. . Four antennas
for the UL tracks are integrated into the box on a common ground plane. The
transmit signal for the DL signal is transferred to an external antenna. The
housing was 3-D-printed for compactness.
Photo of the gateway with internal antennas
inside a housing.
Hardware of the sensors/actuators
The generic sensor/actuator architecture is shown in Fig. .
To ensure a quick and comfortable exchange of components, specific connectors
were standardized and employed. The internal communication between the main
controller and the transceiver is realized via SPI. Radio transceiver modules
based on the Texas Instruments CC2541 SoC are used as they
already include a microcontroller to handle the radio control and media
access. Near-field communication (NFC) is used for sensor/actuator
configuration prior to installation and for additional service
functionalities. Energy management software is implemented in the main
controller, allowing one to reduce the communication load or even to
interrupt communication in a defined way if energy is low.
Generic hardware architecture of the sensors/actuators.
All sensors are equipped with a rechargeable battery. Even the line-powered
sensors are battery buffered to facilitate condition monitoring of the power
supply line itself. Some
sensor types are equipped with an energy harvester or with electronic modules
for wireless charging. In the following Table , the sensor
devices, developed by the ESIMA partners up to now, are listed.
Two different sensor designs for compressed air were implemented for
different applications: the MS6-type sensors are intended for fixed
installation. They contain a turbine-based generator unit driven by
compressed air to charge the internal battery. The measuring range is
0–10 bar pressure. By using one of three different internal bypasses, the
measuring range for compressed air flow rate can be chosen between 2–200,
10–1000, or 50–5000 L min-1. The Ad-Hoc
type sensor is intended for
flexible temporary mobile use. It has measuring ranges of 0–10 bar pressure
and 2–200 L min-1 flow rate. To keep the housing lightweight and
compact, the internal generator unit was omitted. The internal battery lasts
for 10 h continuous operation with a measuring and data transmission rate of
1 Hz each.
The E-Meter measures voltages, currents, phase angles (cos(ϕ)), and
frequencies such that all relevant parameters for a three-phase power network
as active, reactive, and apparent power can be calculated and transmitted. By
selecting different current clamps, measurements can be taken over a wide
range.
The EIS (Environmental Information System) environmental sensor is intended
to measure quantities related to environmental conditions around and within
the machines and for working safety and comfort. Measured quantities are
temperature, humidity, pressure, flow, and CO2 content of the
surrounding air and luminosity. Trends and forecasts for the quantities
related to the surrounding air can be derived by measuring the air flow rate
horizontally in two dimensions at several installed EIS sensors. Estimations
can also be derived for, e.g., the heating control system of the production
hall. Another example is the measurement of the luminosity to determine
whether there is enough light in the various working areas.
The coexistence behavior of the WCS is not only based on blacklisting, but
also on frequency-hopping schemes especially developed for operation in
industrial environments. In the next paragraph, characteristics of
blacklisting algorithms are presented and in the second part a measurement of
a coexistence test is presented.
Blacklisting
Blacklisting denotes a frequency or channel separation technique to manually
or automatically separate different wireless systems operations in the same
frequency band. It can be applied in general, but some limitations have to be
taken into account: from a regulatory perspective there has to be a minimum
number of hopping frequencies for such frequency-hopping devices that are
used in the WCS. According to EN 300 328 at least 15 frequency channels have
to be employed by the devices in this case .
WLAN channels in the 2.4 GHz band usually show a bandwidth of 20 MHz (e.g.,
802.11n) or 22 MHz (e.g., 802.11b) . Thus, if three WLAN
channels are allocated, there remain 15 or 21 frequency channels for the
operation of the WCS, respectively. As described in
Sect. , one fully employed WCS with 60
sensor/actuator nodes in four UL tracks needs in total five 1 MHz wide
channels at a time for no loss of performance or reduction of sensor nodes.
The frequency-hopping schemes are designed to operate at the maximum three
fully employed WCSs at one place with minimal overlapping of the frequency
channels, so the WCSs do not have to be synchronized with each other.
Therefore it is possible to operate three fully employed WCSs in parallel
with three WLAN systems, if sufficient filtering and spectral masks are
preconditioned. Therefore no constraints regarding the number of nodes or the
timing behavior exist.
Coexistence test
A coexistence scenario, which can often be found in the target applications,
is that three WLAN systems are already operated in the 2.4 GHz ISM band.
Typically, channels 1, 7, and 13 are chosen, occupying the frequency bands
between 2402 and 2422, 2432 and 2452, and 2462 and 2482 MHz
. These bands are cancelled (“blacklisted”) in the
frequency list for possible data transmissions. The test setup for this
scenario is presented in Fig. . The WLAN access points are
connected via 30 dB attenuators to a 16-port power combiner/splitter. The
downlink channel is attenuated by 20 dB. For the sensor nodes 10 dB
attenuators are used. The isolation between the ports is 30 dB; the total
insertion loss is 12.5 dB, typically . The power levels at the
different receivers given in Table are calculated by
typically applying 3.5 dB additional loss per connection due to cables,
adapters, and connectors. The attenuations are chosen to have power levels
comparable with the target application.
Receive power levels in dBm.
Power (dBm) Transmitter Gateway, DLSensorWLANOutput power (dBm) 00+10ReceiverGateway, UL-66-56-56Sensor-76-66-66WLAN-96-86-111RSA-66-56-66
Test setup for coexistence investigations.
With the help of a Tektronix real-time spectrum analyzer ,
signals can be monitored precisely. A typical spectrogram is shown in
Fig. . The long narrow tracks between the WLAN packets
indicate the downlink packages, whereas the short packages represent uplink
data. The lengthy transmissions in Fig. of the WCS
demonstrate that the downlink packets are sent for 3.328 ms without gaps.
For this setup five sensors have been used. Three sensors transmit in every
second frame, whereas the remaining two sensors transmit in the alternating
frames, respectively. The frequency spacing between the UL tracks varies in
time according to the hopping algorithm. No interference between the WCS and
the WLAN systems could have been measured in this laboratory test setup. An
additional detailed study on the coexistence behavior of the WCS is presented
in .
Spectrogram of the test setup.
Conclusions
A new wireless communication system (WCS) to facilitate energy monitoring for
resource-efficient production was presented, enabling end users to precisely
monitor energy flows and to develop energy-saving strategies in the field.
The WCS provides customized energy-autarkic sensors as well as continuously
powered sensors, offering higher data rates. Utilizing a blacklisting and
application-specific frequency-hopping algorithms, the WCS is able to
efficiently use the available spectrum together with the other wireless
systems around. A defined maximum latency time below 10 ms can be achieved
at a reliability level comparable with wired sensor/actuator networks. Thus,
the requirements of energy and environmental monitoring in industrial
applications are entirely fulfilled.
Acknowledgements
This work was done in research project ESIMA, which is funded by the Federal
Ministry for Education and Research (BMBF) and managed by the VDI/VDE-IT. The
authors would like to thank the project partners for their support and
cooperation.Edited by: J. Auge Reviewed by: two anonymous
referees
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