Environmental sensors and networks of sensors

Environmental sensors and networks of sensors

Article Type: Viewpoint From: Sensor Review, Volume 28, Issue 4

Keywords: Environmental management, Sensors, Wireless, Control networks

Environmental sensors and networks of sensors are transforming everyday life by scrutinizing our environment and sometimes feeding into control systems that then adjust our environment to improve our processes and lives.

Individual environmental sensors have tended to obtain data (and sometimes information) about their environment and then to transform that data into electrical signals to feed higher level systems around the individual sensors. The development of such environmental sensors has been driven by needs to reduce size and cost while increasing performance and nano-systems and micro-systems (such as micro-electro-mechanical system – MEMS) could revolutionize the environmental sensor markets by providing small, cheap and reliable devices at little cost.

Until recently, environmental sensors tended to be simple, unintelligent, connected directly into control systems, and static (or at best moved from place to place by separate transportation systems)… but all that is changing.

As this journal has said, “it would be hard not to notice that we are now living in an increasing wireless society.” Wireless networks are becoming more and more common and some smaller environmental sensors are becoming mobile so that networks of sensors can work in mobile teams (or swarms). They can deploy and locate themselves to efficiently sample (and sometimes then control) their environment. At first, these sensors tended to be remotely interrogated using devices such as hand-held communicators but now systems are becoming automatic. Environmental sensors have also become “Smart sensors” that can pre-process their own data to improve quality and reduce amounts of communicated data. These sensors become really smart when the integral processing results in an adaptive sensing system that can react to environmental conditions and still provide useful measurements in harsh conditions.

Our future may be set to change through a combination of: smart mobile environmental sensors with enough energy to change themselves within their environment (for example, to move themselves around); effective wireless communication; automatic ranging; remote calibration; advances in microprocessors; new algorithms; and reduced costs in some key areas.

Increasing processing power within individual environmental sensors is improving the performance of sensor arrays and allowing for the more accurate sensing of some phenomena that have traditionally required a large amount of off-line signal processing, for example arrays of mass flow sensors, image processing, sensor integration from many different sensors, and gas sensor arrays.

As the environmental information from numbers of sensor arrays becomes larger (and therefore more complicated), then this leads to a need for systems to model and then convey the information in a simple way (and sometimes in real time) to human beings. Such systems should then also be able to simulate and then predict future scenarios. Large amounts of numerical data can already be handled by some programs such as Octopus, SpyGlass, SenSor and MonSense and the Open Geospatial Consortium is specifying standards for interfaces and metadata encoding to enable (and encourage) the real-time integration of sensor networks and then the display of the environmental information via a standard web browser. That said, there may be a need for a meta-level above them.

Many companies are predicting major growth in environmental sensor applications and technologies, for example: gas, chemical, optical, environmental nano-sensors, and biosensors. Examples of expanding markets are in security (where particularly in the USA, this area is still receiving massive funding) and for chemical sensors, with markets for optical sensors, biosensors, indoor air quality sensors, ozone monitors, explosive detectors and chemical and biological detectors potentially growing the fastest. Electronic “noses” and “tongues” may have the potential to generate multi-million dollar sales.

These electronic sensor systems and their components are sometimes exposed to harsh environmental conditions and some new sensor systems are suggesting that new environmental sensors could be especially robust in harsh conditions, for example some MEMS-sensors appear to be able to withstand very high humidity, pressure and temperature; these sensors-on-a-chip are potentially small and low cost.

The size of such sensor systems can be envisaged by considering a couple of examples. This journal reported recently that Harold Craighead and colleagues at Cornell University are studying a highly-sensitive biological agent detector that uses a “nanoelectromechanical” oscillating silicon cantilever which is just 4 μm long and 500 nm wide. Meanwhile, Harvard University is using an array of silicon nanowire-based field effect transistors to detect influenza viruses; they are now developing sensor arrays that can sense up to 100 different viruses simultaneously.

The deployment of such small environmental sensors may become a critical issue as it affects costs and the ability for the sensors to detect events. This can be seen as having two parts to the problem: placement and dispatching. The first considers the number of sensors required to achieve area coverage and the second considers how to decide when sensors should be moved or deployed. These are not trivial problems in environmental engineering and research is being directed towards both centralized solutions and distributed solutions using smart sensors.

Although originally motivated (and funded) for military applications such as battlefield surveillance, wireless sensor networks are now being used to monitor environmental conditions such as temperature, sound, habitat monitoring, vibration, fire detection, pressure, motion and pollutants. An environmental sensor network is often an ad hoc network where each sensor supports a multi-hop routing algorithm so that several nodes may forward data packets. Wireless sensor networks are an active research area in computer science, systems engineering and telecommunications.

With all these enhancements, environmental sensors are becoming small computer systems with basic interfaces, some limited computational power, memory, sensing ability and communications equipment wrapped up with a power source (often a battery). Energy from the power source is the scarcest resource and that determines the lifetime of an environmental sensor. One of the most energy-expensive operations for an environmental sensor can be data transmission so that research may need to focus on energy aware algorithms and data transmission may need to use multiple hops from node to node to reduce the polynomial growth in energy-cost with respect to transmission distance. The mathematical models and techniques may need to be more abstract and general than those currently used for other protocols.

In wireless networks then environmental sensors could be deployed in large numbers and they could become throw-away (disposable) “green” devices that degrade.

Systems for wireless environmental sensor networks can be seen as less complicated than general-purpose systems because of the specific requirements of the environmental applications, for example, environmental sensors are often not interactive so that systems do not need to include extras such as user interfaces or extra memory. That said, the complexity can be in the environmental system as a whole, as the environmental networks of the future may consist of large numbers of sensors so that distributed algorithms may become more and more important.

So, environmental sensors are going to need an ability to self-configure and reconfigure, some will need to become mobile and to operate autonomously (and with some intelligence) in some uncontrolled hostile environments. Many will need self-fault diagnosis.

Where then should we be concentrating research into environmental sensors in the future?

Well, there are some interference problems that can become critical for wireless communications and they can also be limited by bandwidth (especially as more and more devices compete for time in the same area of the radio spectrum)… but that is being explored by radio and telecommunications engineers. Environmental sensor systems need the ability to cope with technology or communication failures and large-scale deployments and large amounts of data need new computer science algorithms… but the computer scientists are investigating this.

It is the size and cost constraints that result in corresponding constraints on resources such as energy, memory, computational speed and bandwidth. These limitations really push research into environmental sensors towards distributed, energy-efficient sensors. It is that potential need for smaller and more energy efficient sensors that can operate autonomously in harsh conditions that will drive research towards more robust and fault tolerant sensors and systems that can compensate for variables such as temperature (and other cross-sensitive factors). That is probably where Government needs to concentrate R&D effort.

David SandersSystems & Knowledge Engineering, Faculty of Technology, University of Portsmouth, UK