Wireless Sensor Networks
formance based on parameters such as
vibration, temperature, proximity, power
quality, and pressure. These nodes are
often composed of a microcontroller,
several sensors, communication modules,
memory for data storage (e.g., EE -
PROM, SDcard), and a power source.
Depending upon the access technology,
the nodes can communicate to external
systems using either Ethernet, Wi-Fi,
ZigBee, Bluetooth, or GPRS/3G. Also
known as the data acquisition (DAQ)
sensor node as well as the gateway, the
sink wirelessly receives the data and
channels it through to the internet with
an internal wired connection such as
Ethernet. This prevents the necessity
for each individual sensor node to store
large amounts of data and provides a
means for network backhaul in order to
process and analyze data.
ENERGY CONSTRAINTS
Industrial WSNs face major resource
constraints with energy, memory, and
processing; most sensor nodes are battery
powered limiting both the processing
power of the sensor as well as the
operational lifetime of the node. There
are many methods for energy harvesting
that are being explored including
energy generations through vibration,
thermal energy, RF energy, and light.
In outdoor environments, photovoltaic
(PV) cells can be installed on the board
for the primary source of energy where
secondary energy storage is accomplished
through a rechargeable battery
bank and/or supercapacitors 5, 6.
On the plant floor, electric motors can
account for more than 90% of energy
output and also happen to be one of
the biggest sources of energy on the
plant floor to be exploited in terms of
temperature and vibration. Thermoelectric
energy can be harvested through
the use of two dissimilar metals where
a temperature gradient produces a
current (Seebeck effect). Thermoelectric
generators (TEG) composed of a
number of n- and p-type semiconductor
pellets use temperature gradients
ranging from a few degrees to hundreds
of degrees to generate energy. These
temperature differentials can be from a
human body or a machine to the ambient
environment. Piezoelectric materials
can convert vibrational and airflow
strain into voltage. Ambient vibration
can also be converted to power by
means of magnetic induction with a
magnet moving with respect to a coil.
Energy harvested through Microelectromechanical
Systems (MEMS) or PV
Table 1: Dimensions of Security
cells go through power conditioning
in order to be stored in the secondary
energy storage, through power
management circuitry and finally to the
load (sensor and radio). While there are
many creative ways to autonomously
generate energy to prevent the costly
maintenance of replacing batteries, the
unpredictability of the power source
adds to challenge of the sensor node’s
reliability. Time sensitive information
could be incorrectly transmitted or lost
without the sensor node’s replenishment
of energy. Moreover, if a disproportionate
amount of energy used up
in transmission, sensors may not have
enough power to detect the environment
thereby degrading the potential
determinism of a system 7.
DETERMINISM AND RELIABILITY
A survey composed by the Inter national
Society of Automation (ISA) and
ON World at the end of 2014 revealed
that data reliability, security, and easy
access to sensor data tend to the be
Figure 3: Depiction of
the various factions
of industrial WSN
end users according
to wireless systems
standards and strategies
2.
the most important features of a WSN
(Figure 2). Interestingly enough, the
concerns for battery life decreased
while the need for IP addressability
increased 2. Two factors mainly contribute
to the reliability of a WSN: mesh
networking and channel hopping. Network
clusters in which every node can
communicate with multiple neighboring
nodes inherently have a higher reliability
through self-healing algorithms than
linear, point-to-point topologies where if
one node is rendered nonfunctional, the
chain for transmission is broken.
Channel hopping is yet another failsafe
for low power and lossy networks
(LLNs) where nodes can use multiple
channels within a given bandwidth in
case there are transmission/reception
challenges in select channels. This is
defined in the media access control
(MAC) layer within the IEEE 802.15.4e
standard--the physical and data link
defined basis for many WSN standards
including WirelessHART, ZigBee, and
ISA100.11a.
12 MW November - December 2017 www.mwee.com
