It’s reasonably common now to see wireless sensors and measurement devices in industrial applications. These small devices can drastically reduce wiring because they can be placed virtually anywhere to measure anything. Powering such small devices is also relatively simple. Or is it? As with consumer devices, battery power seems the most obvious solution. Batteries are cheap and readily available. Some battery types can source low-power devices for years, with lives of over 10 years claimed in some cases. But are these batteries really the answer and do they really last 10 years?
A battery, in a laboratory environment, being tested on a device at a constant ambient temperature may indeed last 10 years, but out on the factory floor it is a different scenario altogether! There is dust, vibration, electrical noise and heat, all of which can drastically shorten a battery’s health and life. This may not be an issue if a factory has only a handful of wireless devices that are easily accessible. But if there are thousands of wireless sensors it could take hundreds of man hours to check and replace all the batteries. These are hours which most companies cannot afford and could easily be spent elsewhere. In order to combat this, some devices are capable of reading battery health and transmitting a reading, together with other data, to a central source. This can be useful, although transmitting this extra data puts further strain on the battery.
There are other options available, all of which reduce strain on the battery, or eliminate it altogether. Low-power wireless technologies, such as Bluetooth Low Energy or ZigBee, reduce the power required to transmit data in the first place. Some technologies also offer the option to mesh network devices in order to use a lower power network. This has issues in itself in that latency times can increase if the mesh network is sufficiently big. This isn’t an issue if data readings are only required every minute or so. It can become a big problem, however, as the time intervals decrease.
Of course, a significant way to reduce issues related to battery use is to remove the battery altogether. This can be done by energy harvesting, which can be achieved in a number of ways and works very well for low power devices such as sensors. Solar is definitely the most common harvesting solution. Solar panels are more efficient than ever and cost is constantly falling. Solar powered sensors can be implemented almost anywhere and with a long-haul radio solution can be put in the most remote of places.
Thermal and kinetic energy recovery are probably the next two most common energy harvesting methods. Both work well in factory environments, providing power to sensors in fairly extreme environments, which would make a battery suffer. Perhaps the most major issue with all these solutions is cost. Although energy harvesting is great in principal, it is still more expensive than conventional batteries. There is also the issue of cold starting. How do you cold start a thermal energy harvesting sensor without causing potential damage to the system it is measuring? To most the answer would be simple, implement a battery—but that, of course, voids the reason for installing energy harvesting in the first place.
Wireless is very suitable for certain industrial applications, but until the issue of power can be addressed, along with issues related to security and perceived reliability, it will still be seen as an add-on as opposed to a complete solution. Advances in technology and economies of scale are key to unlocking the potential of battery-free wireless systems. These solutions remain some way off, but are definitely on the horizon.
Tom Moore, is a discrete automation analyst with IHS.
