FEATURE POWER ELECTRONICS
PLACING WIRELESS SENSORS ANYWHERE
Jonathan Simon, systems engineering director, Lance Doherty and Thomas Watteyne, both systems engineers and Ross Yu, product marketing manager all at Dust Networks Products, Power by Linear Group, Analog Devices Inc. discuss how, by placing wireless sensors anywhere can deliver low power and reliable wireless sensor networks over long distances
O
ne of the visions of the Internet of Things (IoT) is to be able to measure
and instrument things that have never before been measured. Whether monitoring aging infrastructure, such as bridges, tunnels or power transmission lines, or providing real-time parking and traffic information, these applications call for wireless sensor networks (WSNs) to deliver wire-like performance and yet be practical to deploy. These WSNs need to be able to scale to large numbers of wireless nodes, or motes, and in many cases, cover long distances.
KEYS TO WIDE SPREAD USAGE In order for WSNs to be deployed on a wide-scale basis, they must be practical to deploy and run reliably for many years, often over a decade. To enable this, WSNs must meet a number of key requirements: Place a sensor anywhere – Measurement points need to be placed where optimal for sensing, but not necessarily optimal for communication. As a result, sensor nodes are often placed in locations that do not necessarily have convenient access to communications or power infrastructure, and often are in challenging RF environments (e.g. close to ground, in tunnels, under cars or deep within machinery). Low maintenance – The network must be largely self-maintaining, and any physical maintenance (e.g. battery replacement) must not incur additional ‘truck rolls’ or technician visits. For example, in smart parking applications, battery-powered sensors embedded in the street surface are permitted only if they can be replaced at the same interval as regular road repair, which occurs no more frequently than every 5-7 years. In other applications, WSNs are deployed for more than a decade. Communications reliability – Must be able to reliably communicate with the sensors despite the fact that they may be located in a rough RF environment.
16 FEBRUARY 2018 | ELECTRONICS
Scalability - A network needs to be suitable to a variety of similar yet unique deployments which cover a fairly wide range of network sizes (both number of motes and area coverage), depth (i.e. number of radio hops a node can be from a data egress point), data traffic level, etc.
BUILDING A PREDICTABLE NETWORK ON AN UNPREDICTABLE MEDIUM Low power is difficult without making tradeoffs. There are many approaches in wireless sensor networking that target low-power operation. Some wireless networks, such as ZigBee, achieve low power only on the sensing devices at the edge of the network but require line power for any routing nodes. Other networks introduce a basic form of duty cycling, called ‘beaconing’, in which the entire network shuts down to a low-
Figure 1:
Path and frequency diversity - If
communication fails on the 'green' arrow, node D retries on the 'purple' arrow using another channel
Figure 2:
A deep hop network – the motes in gray are within range of mote 50
power sleep mode for extended periods of time, but sacrifice network availability and overall network capacity. However, for the types of applications talked about for the IoT, wireless sensor networks must be able to accommodate much larger networks and publish at regular data intervals. The challenge, therefore, is to provide low power without sacrificing reliability or network availability. RF Is unpredictable – Radio (RF) is an unpredictable communications medium. Unlike wired communications where the communications signal is shielded from the outside world by cabling, RF propagates in the open air and interacts with the surrounding environment. There is the possibility of other RF transmission sources to cause active interference. Much more common is the effect of multi-path fading, where the RF message may be attenuated by its own signal reflected off of surrounding surfaces and arriving out of phase. Mobile phone users experience multi- path fading every day when their phone has poor signal strength in one spot, but can improve it by moving a few centimeters. Furthermore, the effects of multi-path change over time, as nearby reflective surfaces (e.g. people, cars, doors) typically move. The net result is that any one RF channel will experience significant variation in signal quality over time. However, since multi-path fading affects each RF channel differently, using channel hopping for frequency diversity minimises the negative affects of multi- path fading. The challenge for WSNs, then is the ability to use frequency hopping over large networks with multiple hops.
TIME SYNCHRONISED CHANNEL HOPPING MESH NETWORKS Low-power, reliable wireless sensor networks are a reality with Time Synchronised Channel Hopping (TSCH) mesh networks, pioneered by Analog devices’ Power by Linear, Dust Networks
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