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Wireless Sensor Networks

Published: 01st Jun 2008

Wireless sensor network technologies provide a bridge between information systems and the physical world. While cost effective sensors and actuators have been available for decades connecting, installing and commissioning these sensors and actuators has been expensive due to the cost of wiring.

Computing power, memory and sensors are all showing the exponential decrease in cost, size and power associated with Moore’s Law. The costs involved in installing and commissioning sensors are by comparison increasing.

With the introduction of reliable wireless sensor network technology the costs of installing and commissioning such devices can be up to 90% less than a wired alternative.

The use of sensors is very widespread and therefore opportunities have been identified in many areas of industry and commerce. The problem facing anyone looking into these technologies today is trying to decide what represents technology and products actually in use rather than the claims of vendors or the forecasts of market researchers.

What this article sets out to do is illustrate real applications in specific sectors and describe some of the benefits that are already being achieved.

What is a wireless sensor network?

A wireless sensor network (WSN) is a wireless network consisting of distributed autonomous devices using sensors to cooperatively monitor physical or environmental conditions, such as temperature, sound, vibration, pressure, motion or pollutants, at different locations.

In addition to one or more sensors, each node in a sensor network is typically equipped with a radio transceiver or other wireless communications device, a small microcontroller, and an energy source, usually a battery.

A sensor network normally constitutes a wireless ad-hoc (mesh) network, meaning that each sensor supports a multi-hop routing algorithm (data may travel along various paths through the network to its desired destination).The applications for WSNs are many and varied. They are used in commercial and industrial applications to monitor data that would be difficult or expensive to monitor using wired sensors. They could be deployed in wilderness areas, where they would remain for many years (monitoring some environmental variables) without the need to recharge/replace their power supplies. They could form a perimeter about a property and monitor the progression of intruders (passing information from one node to the next).

Limitations of wired and existing wireless solutions

Studies show that up to 90% of actionable process and environmental data remains uncollected. Wired monitoring systems are expensive and unrealistic in challenging physical environments, and manual monitoring has proven simply to be cost-prohibitive.

There a number of traditional wireless solutions which fall in to the category of point-to-point or point-to-multipoint. The reliability of these networks is set by the quality of the radio link between the central access point and each endpoint. In industrial settings it can be hard to find a location for an access point that provides dependable communications with each endpoint. Moving an access point to improve communications with one endpoint will often degrade communications with other endpoints.

Benefits of WSN

Wireless mesh sensors networks communicate with each other through routers and a gateway to form a mesh network. Router devices relay signals for each other, so if a given sensor is out of range of the gateway, other members of the network carry data. Since interference patterns can change, the network will automatically adapt to keep all members communicating. This self-healing capability keeps data flowing with up to 100% reliability.

WSNs generally use spread spectrum communications because it delivers the very highest level of data transmission reliability. Frequency hopping, spread spectrum eliminates the effects of interference by 'spreading' the transmission over a range of frequencies and randomly 'hopping' from one frequency to another. If a transmission is blocked on one channel it moves to a clear channel and retries.

A self-configuring, self-healing network offers tremendous advantages, particularly in companies with changing requirements, the need to relocate sensors at will or that have an environment with moveable production equipment. In the event a node becomes overloaded or unavailable, a mesh network will automatically reroute the traffic to its destination without outside intervention. The goal should be to spend more time collecting information, not setting up and managing the network.

Many monitoring products are single application systems that specialise in one particular measurement, such as food storage temperature or humidity monitoring. The reality is that most companies have multiple monitoring requirements. Investing in multiple systems can be costly and make it difficult to get a comprehensive “dashboard view” of the operation. A flexible wireless mesh sensor network supports multiple applications on the same infrastructure and provides more valuable, comprehensive results.

Scaled architecture enables a small network to expand without changes to the underlying infrastructure. Given the wide range of data that can be collected, this is a smart investment in anticipation of future requirements. A scalable architecture that allows additional sensors to be added easily, without significant advance planning or technical reconfiguration, is most valuable and should be able to increase in scale to thousands of points as the technology develops. Today, networks of hundreds of monitored points are becoming common.

An effective wireless sensor network solution must offer extensive facilities for connecting to external systems and third-party software packages already in use. Using industry standard protocols such as Modbus and OPC and Web 2.0 connectivity through XML, SOAP, and ODBC ensures seamless and flexible connectivity with all SCADA systems and business applications.

Collecting the data solves only part of the problem; making sense of it is where the real payoff happens. Ideally, data should flow through the network gateway into a console such as OnCall that can display realtime information, historical data and facilitate certain administrative tasks.

Water industry

One of the major areas of applications for sensor networks is the monitoring and management of scarce water resources.

One application using solar powered sensor nodes measures environmental variables such as temperature, soil moisture, water quality, humidity and solar energy levels. Charged by miniature solar panels, these sensor nodes can exchange data to deliver it back to a central database for analysis.

This network is used to advise local sugar cane farmers of the point at which water becomes too saline to use for irrigation, thus saving water, time, money and crops.

Environmental monitoring

In a recent project a team of researchers have used a network of wireless sensors to monitor various features of glaciers in order to understand how the world’s ice masses will respond to the threat of global warming.

Each sensor node monitors parameters including temperature and pressure. One advantage of a wireless solution is overcoming the problem of damage to cables caused by movement of the ice which occurs with traditional wired approaches.

Pharmaceuticals

Quality control and compliance with FDA and European mandates is imperative to pharmaceutical manufacturers. A wireless sensor network allows companies to monitor ambient temperature in their manufacturing and storage facilities. Benefits include improved process and quality control, reduction in manual data collection and accelerated manufacturing.

A recent installation by a large pharmaceutical manufacturer uses a wireless temperature monitoring solution to record storage temperatures of finished product which were previously monitored by staff using hand held data loggers. Manual collection of data is often subject to errors in collecting data or a break down in the process of regular data collection.

Food and beverage

Food and beverage manufacturers can use a wireless sensor network to ensure hygienic production, storage and distribution of their products. By monitoring the state of refrigerated or frozen foods, production and storage conditions (temperature and humidity), equipment and processes (electricity), food manufacturers can improve and ensure quality, reduce spoilage and food poisoning, and prove conformity/traceability for regulatory compliance and identification of problems. A recent application in a bakery used a wireless network to monitor gas consumption in ovens. In the first year (2005/06) savings averaged 20,000 kWh of gas per week. Savings in 2006/07 were around £75K.

Computer data centres

The power demands of newer high density computing equipment and the need to keep them cool are growing exponentially. This is causing major problems in terms of power supply capacity and environmental concerns. There is growing awareness of the need to improve monitoring in data centres to help identify hot spots where cooling is most critical. By directing air flow more effectively better use can be made of the existing cooling capacity. One key difficulty has been the lack of enough flexible temperature monitoring points in the facility to check performance in detail and raise alerts when hot spots arise. Traditional wired approaches can be too expensive and inflexible.

To solve this problem a wireless sensor mesh network can be used which is easy and quick to install with minimal disruption and allows monitoring points to be moved around and added without having to re-commission the whole network. Data collected by the sensors is fed back to a data analysis application which can be configured to raise alarms when limits are exceeded as well as logging the temperature data for further analysis. Alternatively, data from the wireless system can easily be interfaced with existing infrastructure monitoring systems if required.

In a recent data centre installation for an oil company in the Middle East the sensing network quickly highlighted a potential overheating situation which enabled advance remedial action to be taken avoiding the cost of equipment repair and the impact on computing availability.

Heavy industry

There are few places more inhospitable to a sensor than a steel plant. Extreme temperatures and harsh chemical, metallurgical, and mechanical processes make any monitoring a challenge. Yet process monitoring is critical to a steel plant’s profitably. Tough competition in the world steel market holds prices down. Any unexpected maintenance shutdowns reflect directly on the bottom line. At a point in steel making, electrical current often arcs to the sides of the furnace. This super heats the furnace wall, sometimes overloading its temperature control mechanism.

For one major steel manufacturer, it was critical to find an accurate and cost effective way to monitor and record temperature fluctuations within the water jackets of their giant furnaces. With 24/7 production, there was almost no downtime to install or replace the sensors. They had to be placed or replaced quickly during a brief cool period in the melting cycle. Even during these 'cool' periods the temperature at the sensor site measures over 125°F! The need for a quick installation, the high heat, and difficult positioning made installing wired sensors impossible. Plus, molten steel splashing around would vaporise any wires it touched!

Simple traditional wireless monitoring techniques were out, too. Strong magnetic and electrical fields, plus the large masses of steel would interrupt their transmissions, causing communication outages. In an application where a minute’s delay can mean disaster, this was not acceptable. Sensor nodes were placed between the inner and outer walls of the furnace. They were able to withstand the heat from melting steel, wide temperature swings, powerful magnetic fields, water spray, and vibration.

Mesh networking routes data around any temporary trouble spots that might occur. Frequency hopping lets the nodes search out the best channel for clear communication. This insures a constant and accurate flow of information to the plant’s maintenance and management teams. The company depends on this critical information to handle preventive maintenance more efficiently, eliminating the unscheduled downtime that can turn a profitable steel plant into a loser.

Building automation

A wireless system deployed at an industrial facility in Sweden reduced the facility’s energy costs by 37% over the course of one year.

The entire system was installed in an industrial building with a total heated area of 3,600 square meters holding one oil-fired boiler and nine air handling units with air heaters and recycling air control. Using real-time temperature and energy usage data, the wireless sensor network-based system optimised energy management – achieving a dramatic improvement in efficiency that translated to significant bottom-line gains. For a total installation cost of $45,000, the energy management system returned annual energy savings in the amount of $34,974 – nearly paying for itself over the course of a single year.

The energy management system features multiple wireless sensor and control devices that are connected into a self-healing multi-hop mesh network, capable of rerouting a signal if line-of-sight is blocked. Every ten minutes, all data collected by the modules are sent to the web server, which in turn channels the values over the internet to a SQL database. Operators connect to the database to read and change values, allowing efficient management of the facility’s energy usage. Each time the web server connects to the database, changed values are read and sent back to the local controllers. In the industrial energy management facility, a wireless module uses this information – including data referencing both outside temperature and inside temperature – to precisely control the radiator temperature.

Health care premises

In a recent installation for the pharmacy operations of an NHS provincial hospital the wireless system monitors ambient temperature and refrigerator temperatures in production, packing and storage areas. The wireless system is deployed in two physically dispersed areas.

The pharmacy had a problem in that an important storage building was in a remote location over 350 metres away on the other side of the hospital site. In addition, the main hospital buildings rise up on a hill in between the two locations making any ‘line of sight’ wireless solution impossible to implement. Due to cost and ongoing hospital improvement works a wired system was not an option. In this application a multi-hop wireless solution was the only viable solution.

By mounting mesh repeater nodes in weatherproof housings on the apex of two intervening buildings the remote store temperature data is relayed back to the main pharmacy system. The monitoring software provides fully compliant and easy-to-manage system and alarm status information.

Waste processing

With recycling and efficient management of compostable waste high on the UK agenda the technology being used to meet these challenges is evolving quickly.

One company is processing the waste through large rotating drums to achieve the desired results. To ensure proper sterilisation of the finished compost the temperature of the drum contents in the final stage must be monitored continuously and an auditable data set maintained. A number of methods including Wi-Fi wireless had been tried but were unreliable and performed poorly.

Following a very successful site trial using industrial mesh networking devices the company purchased a 6-point system to cover both the rotating sterilising drums and the plant ambient conditions. With fast and easy installation, faultless data capture performance and reliability, IP66 housings and easy integration into the plant Wonderware monitoring system, the system exactly meets this company’s challenging requirements. A second system at the company’s newest plant is now being commissioned.

Conclusion

As the demand for monitoring of critical facilities continues to increase, the use of wireless sensor networks is growing due to their ease of installation, flexibility and ability to solve challenging problems allowing customers to quickly and more economically comply with legislation and regulatory requirements.

Published: 01st Jun 2008 in AWE International

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