Image Processing Reference
In-Depth Information
channel). WISA is suitable for discrete manufacturing if the single-hop requirement is met (the star
topology). Another option is the IEEE ../ZigBee [] technology. ZigBee supports multihop
transmission mode of operation which requires the intermediate nodes (typically router nodes) to
be mains powered. Unlike TDMA-based WISA, there is no allocation of timeslots to messages giving
rise to contention for channel access, increased latency, and power consumption. It is suitable for
applications with a relatively slow process dynamics such as process control, asset monitoring, etc.
The two most notable standardization initiatives aiming at wireless sensor networks for indus-
trial applications are WirelessHART [] and ISA []. WirelessHART, which extends the HART
standard into the wireless domain, is the only standard available to cover wireless sensor net-
works for industrial applications. ISA, still in the standardization process, aims at supporting
different fieldbus formats, including HART, and applications ranging from safety-critical control to
monitoring.
In practical implementations of wireless sensor networks in industrial applications, to leverage
low cost, small size, and low power consumptions, standard Bluetooth (IEEE ..) . GHz radio
transceivers [,] may be used as the sensor/actuators communication hardware. To meet the
requirements for high reliability, low and predictable delay of data transfer, and support for high
number of sensor/actuators, custom-optimized communication protocols may be required as the
commercially available solutions such as Bluetooth (IEEE ..), IEEE .. [], and IEEE
. [-] variants may not fulfill all the requirements. A representative example of this kind of
systems is a wireless sensor/actuator network developed by ABB and deployed in a manufacturing
environment []. [].The system, known as WISA (wireless sensor/actuator) has been implemented in
a manufacturing cell to network proximity switches, which are some of the most widely used posi-
tion sensors in automated factories to control positions of a variety of equipment, including robotic
arms, for instance. he sensor/actuators communication hardware is based on a standard Bluetooth
. GHz radio transceiver and low power electronics that handle the wireless communication link.
The sensors communicate with a wireless base station via antennas mounted in the cell. For the
base station, a specialized RF front end was developed to provide collision-free air access by allo-
cating a fixed TDMA time slot to each sensor/actuator. Frequency hopping (FH) was employed to
counter both frequency-selective fading and interference effects, and operates in combination with
automatic retransmission requests. he parameters of this TDMA/FH scheme were chosen to satisfy
the requirements of up to sensor/actuators per base station. Each wireless node has a response or
cycletimeofms,tomakefulluseoftheavailableradiobandofMHzwidth.heFHsequencesare
cell-specific and were chosen to have low cross-correlations to permit parallel operation of many cells
onthesamefactoryloorwithlowself-interference.hebasestationcanhandleuptowireless
sensor/actuators and is connected to the control system via a (wireline) fieldbus. To increase capacity,
a number of base stations can operate in the same area. WISA provides wireless power supply to the
sensors, based on magnetic coupling.
1.5 Networked Embedded Systems in Building Automation
Another fast growing application area for networked embedded systems is building automation [].
Building automation systems aim at the control of the internal environment, as well as the imme-
diate external environment of a building, or building complex. At present, the focus of research
and technology development is on commercial type of buildings (office building, exhibition cen-
tre, shopping complex, etc.). In future, this will also include industrial type of buildings, which pose
substantial challenges to the development of effective monitoring and control solutions. Some of the
main services to be offered by the building automation systems typically include climate control to
include heating, ventilation, air conditioning; visual comfort to cover artificial lighting, control of day
light; safety services such as fire alarm, and emergency sound system; security protection; control of
