Wireless sensor networks are a key element in implementing the Internet of Things in several application fields such as wearable technology, building automation, energy management, smart grid, industrial monitoring and structural monitoring.
Wireless sensors have a great deal of potential in numerous applications where wired read-out is difficult, for example, in harsh conditions, with rotating parts, or where there is a cost implication and complexity of wiring.
The main challenges caused by using disposable batteries as a power source for wireless sensors – maintaining sensors in hard-to-service locations, and scaling a sensor network – can be resolved by Zero Power Sensor Network (ZPSN) Technology providing passive wireless sensors with a long reading distance.
Additionally, reliability of operation and lower installation and maintenance costs result from the technology's wireless and battery-free operation, since the common malfunction of cables and connectors can be avoided, and there is no need to change the batteries.
Revolutionising passive wireless sensing
VTT has developed the intermodulation communication principle in order to wirelessly read out passive sensors at long distances. Intermodulation communication sensors reply to the sensor information at an intermodulation frequency when interrogated by a reader device with two tones. A solid theoretical background and analytical expressions to optimise the sensors have been derived and experimentally verified.
The widespread deployment of passive wireless sensor networks has been impeded by the short operational range provided by the current technology such as RFID-based sensors and SAW sensors. VTT Zero Power Sensors are readable from long distances – reaching up to several tens of metres. This enables new application paradigms for passive wireless sensors in several application fields.
VTT's wireless communication platform for harsh environments
We have developed a wireless communication platform for the monitoring of harsh and inaccessible environments, extending the reach of the emerging industrial internet. The technology has been tested at several industrial sites including those of Talvivaara's bioleaching process, Ekokymppi's biocomposting, and nuclear waste repository monitoring in Posiva's Onkalo.
Tests have proved the technology is robust enough for communication through wet and dry soil, saline sea water, ice and concrete. Potential applications include temperature, humidity and pressure monitoring for mining process control, waste management, industrial measurements through water and oil, and environmental underground and underwater monitoring. Furthermore, the structural monitoring of infrastructure such as tunnels, bridges and roads as well as hydraulic and maritime structures is of great interest.
Solid and liquid media – such as concrete, soil, compost, ore, oil, ice and water – with relatively high electrical conductivity and non-uniform dielectric properties provide a unique challenge for wireless communication systems. Due to their opaqueness to the propagation of high frequency electromagnetic fields, frequencies in the kHz range and magnetic fields are better able to penetrate these media. Existing monitoring solutions comprise mainly wired systems, and existing wireless solutions have severe limitations in deployment in terms of range, power consumption and lifetime. VTT's technology makes it possible to communicate through solid structures and monitor processes in the harshest of environments with low-power long-lifetime sensor nodes. Depending on the application, the sensor nodes can be powered wirelessly or by battery.
VTT developed the through-solid wireless communication platform in collaboration with end user Talvivaara Mining Company Plc. The target application was the monitoring of an open mine mineral extraction process known as 'bioleaching'. The leaching activity was represented in the temperature profile, and measurement reliability was highest in the centre of the ore heap, which was up to 15m high. The ore, with high mineral content, low PH and high conductivity, presented an exceptional challenge for wireless communication. Sensor nodes with ferrite coil antennae were developed to sense temperature deep within the ore pile and relay the information to a loop receiver on the surface of the heap. The system was piloted at Talvivaara's mine located close to Kajaani in central Finland. Communication was demonstrated at depths of more than 10m inside the ore.
It soon emerged that the system developed here was adaptable for communication through all sorts of solid and liquid media opaque to traditional wireless communication technologies, thus enabling a host of other harsh environment monitoring applications. Another targeted application enabled the monitoring of oil and bio-composts with waste management partner Ekokymppi.
An application area of special importance lies in the monitoring of nuclear waste repositories where waste is sealed behind multiple release barriers. Due to the requirement for sealed waste storage, wiring is impractical and could pose a safety risk. The long-term, long-range wireless monitoring of nuclear waste, which has not been possible to date, could provide vital feedback for nuclear waste management operators in a safe and controlled way. VTT is currently performing tests together with Posiva at the Onkalo spent nuclear fuel repository to evaluate the technology.