Data Acquisition: No Limits
How digital
wireless technology using the IEEE802.15.4 ZigBee standard is helping datalogging
applications break free
Mark Lee
The consumer
electronics market has seen an explosion of gadgets enabled by wireless
technology over the past 5 years, with the wide uptake of devices such as
wireless broadband routers, Bluetooth headsets, DECT phones and audio-visual
senders. The commercial market has been far more cautious in embracing these
technologies when designing environmental monitoring and process control
systems, with concerns over reliability, data security and potential cost
outweighing the obvious convenience benefits.
With an
increasing demand for easy, flexible environmental monitoring systems in
sensitive sites such as museums, healthcare manufacturers and heritage listed
buildings, the massive advantages offered by new wireless technology are
becoming impossible to ignore when specifying data acquisiton systems.
Cables?
Local and
high speed data acquisition has historically been a case of connecting sensors to
a data logging device to a PC or PLC, via cable. Cables typically offer a
guaranteed, reliable point-to-point connection, several decades of reliable
service, and high bandwidth capacity resulting in large amounts of dataflow at
high speed.
Long cable
runs to remote sensors around a site however, suffer from two main problems.
The first is disruption to the building fabric and operation if new cable
routes have to be laid to install the system, and
subsequent further disruptive work if the system needs to be modified, or sensors
added or moved.
The second major
problem is one of maintaining signal integrity over long cable runs. All cables
are subject to interference from nearby strong magnetic fields and signal decay
(where the measured signal at the sensor end fades along the length of the
cable, changing the measurement). Active repeaters, amplifiers and filters are common
processes used to try to boost signal-to-noise ratio (SNR), often with mixed
success.
Wireless!
Wireless
data acquisition makes installation as easy as placing sensors in the correct
locations, then adding or moving them if the system needs to be modified. It
also overcomes the problem of maintaining signal integrity by sampling and
converting the analog measurement signals into digital data at the monitoring
point ready for radio transmission. Wireless signals are of course just as
prone to interference and decay as wired signals, but with digital
transmission, the carried data includes its own error checking and correction,
and higher level protocols will manage the checking and re-sending of data
should an interference to transmission occur.
In addition, modern digital coding techniques allow much more
information to be carried in the same bandwidth than would be the case with an
analog signal.
Technology
The latest
wireless environmental monitoring systems in
The ZigBee
protocol was designed to be simpler, cheaper and more power efficient than
other 2.4GHz technologies, with devices requiring significantly less
programming code than Bluetooth and wireless LAN devices. This makes ZigBee
radio nodes up to five times cheaper than similar Bluetooth devices – enabling
great value for data acquisition systems requiring many wireless nodes.
Security
The ZigBee
protocol was designed with built-in security as a top priority, unlike 802.11
wireless LANs or Bluetooth which suffer from several security weaknesses. While
the 802.15.4 standard specifies the use of AES (Advanced Encryption Standard)
encryption using up to 128-bit shared keys for secure transport, the ZigBee
protocol defines the way the keys are established, and how nodes recognise and
react to each other throughout the mesh network (authentication). The master
network controller keeps a list of authenticated devices (those that have
specifically joined the network in a process known as association), and only
responds to these devices, blocking messages from foreign nodes.
The master
also supervises the distribution of encryption keys to new nodes across already
secure parts of the network. ZigBee network nodes can choose from three
different types of key: master, network and link. The network key protects the
entire network from outside intrusions, and the link key forms the basis of
security between two devices. If the link key is available, it is always used.
Nodes can use the less secure network key if required to conserve memory, but
as this key is used across several devices it can be susceptible to insider
attacks. End-to-end security is maintained from sender to receiver by
encrypting the data once. Intermediate nodes do not decrypt and re-encrypt the
packets, but only relay them on to their destination. Finally the network
master also maintains a ‘freshness’ list, which assigns a unique counter id
when each key is established. If the master detects messages it has already
received (with ‘stale’ or out-of-synch freshness counters) it suspects a hacker
is replaying or falsifying messages and chooses to ignore the intrusion.
Interference Robustness
The ZigBee network copes with potential interference from other 2.4GHz devices by scanning the radio environment for traffic, and selecting appropriate channels away from (or slotted between) competing signals. 802.11 devices have eleven 22MHz spread spectrum channels defined at a 25MHz spacing between 2.401GHz and 2.473GHz. ZigBee devices however have 26 narrower (3MHz) channels at 5MHz spacing defined in the same frequency range, meaning lower bandwidth channels, but far greater scope to find an unused portion of the spectrum to communicate in.
Networking
ZigBee
devices form a mesh network, allowing devices to relay messages from a more
distant node to the network’s central base station. This significantly extends
the range of the system from a basic point-to-point range of 80m, to a maximum
network range of 1.2km, and allows messages to find different routes through
the system when certain nodes become unavailable for any reason. This mesh packet
network concept is so powerful and robust it forms the basis of the entire internet,
a firm foundation for a reliable wireless data acquisition system.
Figure x – The wireless
‘mesh’ network allows the system’s range to be extended by repeating signals
Considering the Operating
Environment
When new
wireless data acquisition systems are planned, an initial site survey should be
carried out, which takes into account how the wireless signal will propagate
from the wireless nodes to the base station. Factors such as whether the
wireless signal has to travel through walls, glass, doors, or openings must be
taken into account, all of which effectively attenuate the wireless signal and
reduce the range. Attenuations encountered can be added to produce a power
budget and determine whether the site for a particular node is viable, or
whether the site requires enhanced antennas or cabling.
|
Items
to losses to be added |
dB |
|
Cubicles |
3 to 5 |
|
Marble |
5 |
|
Window, Brick Wall |
2 |
|
Clear Glass Window |
2 |
|
Glass wall with metal frame |
6 |
|
Metal Screened Clear Glass Window |
6 |
|
Wired-Glass Window |
8 |
|
Brick Wall next to a Metal Door |
3 |
|
Plasterboard wall |
3 |
|
Cinder block wall |
4 |
|
Dry Wall |
4 |
|
Sheetrock/Wood Frame Wall |
5 |
|
Sheetrock/Metal Framed Wall |
6 |
|
Office Wall |
6 |
|
Brick Wall |
2 to 8 |
|
Concrete Wall |
10 to 15 |
|
Wooden Door |
3 |
|
Metal Door in Office Wall |
6 |
|
Metal door in brick wall |
12 to 13 |
Table 1 – Attenuation
losses for common indoor obstacles
Data acquisition
nodes based on ZigBee have a standard range of 80m in free air with -1dBm at
the transmitter, -94dBm receiver sensitivity, and a standard 2.2dBi antenna (as
found on wireless routers). The range can be extended with a higher gain
antenna, every 6dB of gain introduced into the system giving an effective
doubling of the range in free air. The wireless mesh can be extended into potentially
radio 'dead' locations, such as underground tunnels and store rooms using
custom designed combinations of antennae and cabling.
Information Flow
In the
latest wireless data acquisition products, nodes with internal sensors capable
of measuring temperature, humidity, light level, sound and vibration and dew
point are available. Nodes also have additional analogue inputs which accept
industry-standard 0-5V and 4-20mA signals suitable for connecting external
sensors, and digital inputs suitable for measuring contact closure, counting
and totalising events.
The sampled
data is fed from the nodes back to a base station, which coordinates the acquisition
from all nodes at set time intervals, gathers the measurements and feeds them
back to a database or PC-based application via standard ethernet. If the base
station is required at a location with no local network available, GPRS based
wireless modems can be used and the signals transmitted across mobile phone
networks, allowing for a truly remote monitoring system.
Network Robustness
If
communication is lost from node to base station the node can buffer data until
the line is restored, and similarly data buffering is available at the base
station if the network is interrupted for any period of time. Once the data has
been fed into a PC-based enterprise system backed up with a database, every
measurement, system configuration change, alarm activated and acknowledged is
logged and traceable, providing a unique and compliant data acquisition system.
Hard To Ignore
With
customers’ demands for easy, unobtrusive, flexible data acquisition driving the
development of high specification wireless data acquisition and monitoring
systems, it seems that wireless devices are no longer just ‘gadgets’ for the
consumer electronics market – they have a powerful role to play in demanding
data acquisition applications.
Mark Lee is a Technical
Specialist for Neo Vista Systems Integrators Pty Ltd.(NVSI) of Sydney, Australia and Auckland, New Zealand
(www.nvsi.com.au).
Mark studied wireless digital
communications systems at
NVSI are the developers and
distributors of the Enviropoint wireless monitoring system powered by Accsense.
For more information please call
(02) 9809 7899 or e-mail info@nvsi.com.au