RFID-powered Sensors Can Play a Big Role in the Internet of Things

While energy-harvesting and battery-powered devices can be the right choice for certain applications, passive RFID sensor tags offer a number of advantages.
By Mikel Choperena
Oct 14, 2013

The quantity of devices connected to the Internet exceeds the population of people on Earth. That's right—there are more devices tapping into the Internet than people on Earth to use them. Different sources predict that by 2020, wireless sensors and other types of wireless network nodes (such as actuators) will account for the majority (60 percent) of the total installed base of Internet of Things (IoT) devices.

Autonomous sensors will play a large part in those predictions. These sensors autonomously execute their functions in the environment in which they have been deployed. They are wireless, and their most distinctive characteristic is that they are self-powered while still being capable of monitoring the environment and transmitting data. Such devices range from simple detectors that trigger an alarm signal if the sensor passes a measurement threshold, to monitoring systems that collect data regarding different products or processes.

Autonomous sensors can potentially be deployed everywhere, though there are usually constraints on the power supply of such devices, so the communication technology must be carefully selected. Most of these sensors employ low-power-consumption wireless protocols, such as ZigBee or Bluetooth Low Energy (BLE). ZigBee is a protocol specifically developed for mesh communication—each sensing device can transmit data either directly to the acquisition module, or to another nearly ZigBee-based sensing device, which in turn will transmit both devices' data to the acquisition module. When they are deployed in urban areas, it is also typical to implement Wi-Fi-based devices, taking advantage of the Wi-Fi coverage associated with these places. A new, interesting trend with sensor devices connected to the IoT involves using smartphones to serve as a bridge between the sensors and the cloud. A force still to be reckoned with is that many citizens are eager to help their communities by installing smartphone applications that provide such a bridge to data, as long the information is to be used for something they believe in.

Power Supply of Autonomous Sensors
The power supply of these autonomous devices is the key. Even if research is continuously decreasing the power requirements of wireless sensors, the energy levels required are still too large a burden for the energy-harvesting technologies at hand. Power requirements from different communication protocols generally range from tens of milliamps to hundreds of milliamps. However, in most applications, there is no need to have the device continuously transmitting data, so the average power consumption is lower. The table below shows some general data comparing power requirements of the three wireless protocols widely used: Wi-Fi, ZigBee and Bluetooth Low Energy.

Please note that this comparison of power requirements is intended for the purpose of analyzing the impact on the power supply of autonomous sensor devices. There are, of course, multiple other variables to compare, such as communication range (Wi-Fi and ZigBee are much longer than Bluetooth), data rate (again, Wi-Fi and ZigBee are significantly better) and others.

In addition to wireless data transmission power consumption, we need to add the power consumption of the sensor itself. Fortunately there are many ultra-low-power sensors currently available on the market. For estimation purposes, we will agree on a sensor requiring some 50 microamperes (μA) to perform 10 measurements daily. Note that the total power consumption will range from hundreds of microamps to several dozen milliamps for a small amount of messages per day.

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