wilamowski-b-m-irwin-j-d-industrial-communication-systems-2011

6-10 Industrial Communication Systems
Moreover, radio modulation techniques can be applied to reduce the interference and improve wireless
communication reliability in an industrial facility. In this respect, to reduce the interference in IWSNs,
spread spectrum radio modulation techniques can be applicable because of their multiple access, antimultipath
fading, and antijamming capabilities. The two main spread spectrum techniques employed
are direct sequence spread spectrum (DSSS) and frequency hopping spread spectrum (FHSS). These have
different physical mechanisms and thus react differently in industrial settings. The choice between radio
techniques is dependent on application requirements and the industrial environment characteristics.
In IWSNs, interference to the mission-critical data can have costly consequences in terms of money,
manpower, and even lives of employees and public [28]. Therefore, the coexistence of WSN with other
systems operating in the same band should be examined. The coexistence should be considered in both
directions, “from system” and “to system” points of views, i.e., how the system reduces its effects to other
systems or how it can reduce the effects from other systems and interference sources to itself. An example
about coexistence impact analysis between IEEE 802.15.4 and 802.11b is presented in [4,17,19,36].
6.5.1.3 Energy-Harvesting Techniques
In IWSNs, the use of batteries as power source for the sensor nodes can be troublesome due to their
limited lifetime, making periodic replacements unavoidable [5]. In this respect, energy-harvesting (also
referred to as energy scavenging) techniques, which extract energy from the environment where the
sensor itself resides, offer another important way to prolong the lifetime of sensor devices.
Systems able to perpetually power sensors based on simple COTS photovoltaic cells coupled with
rechargeable batteries and supercapacitors have already been demonstrated [21]. In [33], the state of
the art in more unconventional techniques for energy harvesting is surveyed. Technologies to generate
energy from background radio signals, thermoelectric conversion, vibrational excitation, and the
human body are investigated. As far as collecting energy from background radio signals is concerned,
unfortunately, an electric field of 1.V/m yields only 0.26.W/cm 2 , as opposed to 100.W/cm 2 produced by
a crystalline silicon solar cell exposed to bright sunlight [2]. Electric fields of intensity of a few volts per
meter are only encountered close to strong transmitters.
Another practice, which consists in broadcasting RF energy deliberately to power electronic devices,
is severely limited by legal limits set due to health and safety concerns. Recently, it has been also demonstrated
that wireless power transfer using resonant magnetic coupling between two copper coils is
possible [23]. In this experiment, it was shown that a 60.W light bulb was lighted at an efficiency of
40%, with the distance between the transmitter and receiver being 2.m. The efficiencies demonstrated
were almost a million times larger than nonresonant magnetic induction. Wireless energy transfer via
magnetic coupling is important, since biological organisms only weakly interact with magnetic fields.
While thermoelectric conversion may not be suitable for wireless devices, harvesting energy from vibrations
in the surrounding environment provides another useful source of energy. Vibrational magnetic
power generators based on moving magnets or coils could yield power that range from tens of microwatts
when based on micro-electromechanical system (MEMS) technologies to over a milliwatt for larger devices.
Other vibrational microgenerators are based on charged capacitors with moving plates and, depending on
their excitation and power conditioning, yield power in the order of 10.μW. In [33], it is also reported that
recent analysis [31] suggested that 1.cm 3 vibrational microgenerators can be expected to yield up to 800.W/cm 3
from machine-induced stimuli, which is orders of magnitude higher than what is provided by currently
available microgenerators. Hence, this is a promising area of research for small battery-powered devices.
Other energy-scavenging approaches employ piezoelectric materials. In [5] and [33], it is reported
that these materials can generate power between 100 and 330.μW/cm 3 . Please note that while energyharvesting
techniques provide an additional source of energy and help prolong the lifetime of sensor
devices, they yield power that is several orders of magnitude lower as compared to the power consumption
of state-of-the-art industrial multimedia devices. Hence, they may currently be suitable
only for very-low duty cycle devices. An overview of different energy-harvesting techniques is presented
in Table 6.4 [5,2,25,33].
© 2011 by Taylor and Francis Group, LLC