Thursday, June 18, 2015

Towards Battery-less Zigbee Devices

The most common way of commercial Zigbee devices is powering them up with batteries. Zigbee technology promises longer of battery lifetime due to very small power consumption. Even some reports say until years of battery lifetime. Low power consumption surely give beneficial in energy saving. Moreover, maintenance procedure may perform much less.  Not only reducing the interruption to the network, the need of resource, let's say the technician for battery replacement, is also reduced.  Imagine how tedious works that need to be done when it comes to maintain lots of nodes within network. In business view, this leads to minimizing the cost.

Picture taken from Morais (2008)

Just for illustration let's make  simple calculation of battery lifetime. It is based on application note released by Meshnetics, one of producer of Zigbee devices. Suppose we have battery with capacity with amount 2000mAh. Meanwhile, active-mode current consumption is 19mA while sleep-mode current consumption is  0.006 mA. With period of duty cycle of 5 seconds and wake up time of 1 second, the battery will last for almost 1.5 half years. One second of wake up time is quite long time, if we take 200ms it should be alive for more than 7 years. Even though it's not the real case, at least we can imagine how Zigbee performs regarding battery lifetime. Real calculation requires more parameters to be taken into account such as the capacity of the battery, type of data and network which influence the duty cycle and current consumption. 

We already have small energy consumption of Zigbee device and we are now in the middle of go green campaign with a lot of types renewable energy. Combining two technologies will be perfect combination in realizing battery-less of Zigbee network. Main issue of renewable energy is the small efficiency, but in this case we do not really need to wait for high efficiency output power because small amount of energy will be sufficient to support Zigbee technology. Apparently, Zigbee alliance also concerns about making Zigbee technology without battery and open the possibility of combination renewable energy specifically in energy scavenging.

We may know common renewable energy technologies such as solar, wind, gas, mechanical, etc in the market. But for now, let's eliminate wind and gas energy.  They're surely incomparable with the size of Zigbee device. Deployment of solar technology, MEMS- mechanical based renewable energy, RF scavenging, toroidal coil, possibly suit with zigbee devices. They are physically small which require less space and can be easily integrated to Zigbee devices. Some research has been done about these studies in order to realize baterry-less zigbee device.

Not only trying out what type energy scavenging that may be utilized, they also encountering problem of too small produced energy. For instance, RF harvesting as one of energy scavenging technologies, produces fairly tiny amount of energy. Integration with Zigbee devices is possible if the energy is stored first. Produced energy may be stored in battery or maybe even a small capacitor bank.  Whenever the devices need to be supplied for "active-mode", they take out energy from the storage of energy. This way, battery-less zigbee device can be realized. Of course, this technique can be applied to another energy scavenging as well.

Another approach in order to tackle the issue of low power production of energy harvester is by combining several technologies to supply energy for Zigbee devices. If one technology is not enough to providing required energy, two or three energy harvester should be good enough to power them up. Implementing more technologies on to Zigbee devices is more make sense rather than being bothered by regular maintenance of replacing battery. Deployment more harvester technologies does not give significant change in size. Some research has demonstrated it and it is still considerably small size in comparison with Zigbee devices.

Finally, with all the efforts towards batery-less wireless sensor network, especially Zigbee technology based, it will not be impossible to see it in the future. It might take some time but it should not be that long. Zigbee alliance is working on standardization of it as well as the researchers try to figure out the suitable technology


References:

Amaro, J. Pedro, et al. "Energy harvesting for zigbee compliant wireless sensor network nodes." IECON 2012-38th Annual Conference on IEEE Industrial Electronics Society. IEEE, 2012.
Meshnetics. ZigbeeNet Software 1.0 - Application Note: Battery Life Estimation. 2008
Morais, Raul, et al. "A ZigBee multi-powered wireless acquisition device for remote sensing applications in precision viticulture." Computers and electronics in agriculture 62.2 (2008): 94-106.
http://www.zigbeeresourceguide.com/