Ultra-low-power ZigBee-based wireless mesh networks, powered by a variety of energy-harvesting technologies, make it possible to create the first truly wireless and battery-less sense and control networks for regulating energy consumption in domestic and industrial uses.
Homes, offices and factories waste energy on lighting, heating and air conditioning. With the help of light- and temperature-sensor network technologies, energy conservation processes such as turning off lights and adjusting temperature now can be automated, reducing costs.
Until now, wires and cables for power and connectivity have limited the widespread adoption of sensor networks by making them difficult and expensive to install and maintain.
Battery-powered wireless networks can simplify installation, but their high power consumption and the corresponding need for regular battery replacement renders this option costly to maintain. Nobody wants to replace hundreds or thousands of window sensor batteries in a large building on a regular basis.
The promise of wireless sensor networks can only be fully realized when the wiring for both data communications and power supply is eliminated.
The no-batteries movement
Besides reducing cost and maintenance requirements, doing away with batteries eliminates the waste and headaches associated with disposing of and recycling batteries. This environmentally friendly approach also extends the reach of sensor technology to distant and inhospitable climates and regions.
Consider, for example, a network of sensors for leak detection on a remote oil pipeline. Accommodating thousands of standard wired sensors spread out over hundreds of miles of pipeline is financially challenging, even with the enormous environmental risk associated with oil leaks.
Alternatively, a wireless sensor network equipped with energy harvesters is fairly inexpensive and straightforward to install. No batteries, no power lines, no network cables.
Two of the most significant requirements for widespread adoption of wireless sense and control products are ease of use and reliability. Battery-free technology helps address these requirements for the units themselves, but the reliability of the communications chain that interconnects the sensors is just as critical.
One way to enhance network reliability is to use mesh technology so each wireless sensor and actuator device, utilizing integrated mesh software, can act as a repeater for other wireless devices in the network. The sensor nodes form a mesh by sending messages through intermediate nodes from source to destination.
This approach enables the network to span larger distances (even when individual nodes can see only a local area of the network), allowing wireless coverage throughout a facility without the need for dedicated base stations or routers.
To streamline installation and day-to-day operation, it is also desirable to have a self-organizing network that is self-forming and self-healing. Once a sensor node is powered, it should automatically link to the wireless network, meaning technicians installing the network don’t require sophisticated training. The self-healing nature of the technology should also make it easier to accommodate environmental changes, enabling nodes to automatically find alternative communication routes when the signal quality decreases or building infrastructure changes.
How does it work
Ultra-low-power IEEE 802.15.4 and ZigBee-based wireless technologies, operating in the 2.4GHz radio spectrum, can be powered by a variety of environmental sources, such as light, motion and vibration.
But you need on-board power-management circuits and software to optimize use of the energy. Because wireless sensors can commonly draw significant current at peak usage, system designers need to be able to manage the network’s power consumption in several ways:
- Peak current control – When examining the power consumption of electronic circuits, it becomes apparent that current curves resemble a mountain range with peaks and valleys. When certain functional blocks become active, they draw peak current. When two functional blocks switch on simultaneously, the peak amplitude doubles. The secret to reducing peak power lies in carefully managing the turn-on and turn-off time for key functions so double peaks can be avoided.
- Low-power mesh routing, or multi-hop networking – One of the most dramatic differences between wireless sensor communications technology and other wireless technologies is the ability of sensor nodes to forward messages from other nodes located further down the communications chain. The breakthrough lies in synchronizing the sleep/wake-up cycles of nodes. Nodes wake up when they expect a message from a neighboring node. This enables the routing nodes to operate in a nearly powerless sleeping state most of the time, thereby achieving ultra-low-power operation.
- Controlling sleep current – Even when wireless chips are in sleep mode, not transmitting, they still require power. By optimizing for low-leakage current, it is possible to greatly limit the amount of power required for the network to operate.
- Graceful power failure – During normal operation, the devices monitor the state of the power circuits. As they encounter declining power levels, they raise different levels of alarms, ranging from early warning to near-death. The alarms are communicated to other parts of the system, thereby enabling the system to be placed in a state consistent with the alarm condition.
By integrating these technologies into radio chip and module designs, it is now possible to develop wireless network solutions for a variety of real-world applications where extreme low power is an absolute requirement.
These wireless sensor networks that require very little power, combined with a energy-harvesting solutions, will enable us to better control our lives, homes and environment, creating a truly connected world that will a provide comfortable, safer and cleaner environment for future generations.
Links is CEO of GreenPeak. He can be reached at [email protected].
