Basic HTML Version
www.geotechnicalnews.com
Geotechnical News • December 2014
37
GEOTECHNICAL INSTRUMENTATION NEWS
to extend, adapt, move or redeploy
them during the monitoring period?
Network topology
If you only need one, two or a very
small number of sensor points, and
they are not clustered, then a simple
point to point system may be quite suf-
ficient. However these days with even
a relatively few sensors, a network
based solution will be equally cost
effective in comparison. Networks are
invariably more flexible and allow for
adaptation and extension through the
life of the monitoring project.
Both hub & spoke and mesh archi-
tectures provide solutions that readily
support multiple sensors; however
there are significant differences in con-
figuration, flexibility, robustness and
power requirements across the instal-
lation, depending on the architecture
and the specific product selected.
Important considerations in the choice
of architecture and supplier do require
careful teasing out, as parts of the
industry are still very immature. How
easy is it to configure the network?
Is configuration required on a sensor
by sensor basis? This can particularly
be an issue with hub and spoke type
networks where the controller and
relay nodes may require configuring as
nodes are added/removed.
In a multi-hop network, such as a
mesh network, how many ‘hops’ can
be supported? If this is small, then this
could considerably limit the area over
which the sensors can be installed.
Power
What power is required, and how is
this different for different parts of the
system? Typically sensor nodes should
be battery powered and give a long
operational life of 5-15 years. The life
however will depend on the type and
make of sensor, the frequency with
which readings are taken, the size
of the battery cell, but it can also be
influenced by where the node sits in a
network. In some mesh networks for
example, where there are many nodes,
those nearer the gateway may use up
their batteries slightly faster.
In hub & spoke systems the relay
and control nodes will need external
power, as they need to be on all the
time. In a mesh network, typically the
only item to require power will be the
gateway. Is it possible to use energy
harvesting to provide power where
needed, for example with a solar
panel? This will depend on the system
supplier, and the type of backhaul
used. In some implementations the
power requirement for the gateway
and backhaul is such that solar power
is not practicable. If not, is a suitable
source of mains power available in the
locations required?
Data robustness
How reliable is the data transmission?
Does the system retransmit ‘lost’ data
readings? Are data readings buffered
on the nodes? If so, how many read-
ings can be stored? If a communica-
tion link is lost temporarily, does the
system retransmit them when com-
munication link is re-established? This
applies to individual sensor nodes,
but also to relay/controller/ gateway
nodes.
Sensor stability
It may seem self evident, but of course
the quality of the data is paramount,
and needs to be fit for purpose. This
applies particular to systems with
integrated sensors, as well as those
connected to external sensors. Is the
resolution of the data sufficient for
purpose? How stable is the data over
time and temperature? Is the data
liable to noise, spikes or anomalous
readings?
Installation
How easy is it to install the system? Is
a lot of configuration required, either
before, during or after the installa-
tion? Is an intervention required on
the nodes themselves? Is it possible to
determine network wireless perfor-
mance at the time of installation, so
that the installer can be confident of
system operation before leaving site?
Can a contractor, surveyor or any
reasonably trained individual install
without significant help or support?
Wireless range
Range capability can vary consider-
ably not just with type of system but
placement and height of the wireless
nodes. Key factors depend on the site,
where the gateway can be located,
how far the sensor deployment needs
to extend and what obstructions may
exist.
What is the range of each wireless
node? How is this affected by local
environmental factors, such as height,
obstructions and vegetation? Does the
system need repeaters to get around
obstructions and do those repeaters
need to be powered on all the time?
What sort of obstructions can the
wireless signal pass through? Note
that generally speaking the higher the
position of the antenna, the better the
wireless range that can be achieved.
Frequency bands
Generally speaking wireless sensor
systems (not the backhaul) operate
on the internationally agreed Indus-
trial, Scientific and Medical (ISM)
radio bands, typically in the 2.4GHz
or 900MHz bands. These should be
license exempt, but it is important to
check this against the individual coun-
try where they are to be installed, and
what local restrictions there may be on
wireless power or indeed the sort of
application to be used.
Data backhaul
Data backhaul will depend very much
upon the facilities available where
the system is to be installed. In most
parts of Western Europe, GSM is of
good quality and available, although
it should be checked in more remote
locations. At its simplest, data can be
stored at the gateway and collected
manually, but this is clearly less desir-
able. For very remote locations where
GSM is not available then Satellite
may be a good alternative.
For confined and underground loca-
tions, then the only viable solution
may be to use a wired communica-