RFID battery free sensors are a very good alternative to natural source energy harvesting solutions for those applications in need of a reliable wireless energy source.
As you already know, this blog is about the challenge of implementing autonomous sensing and connecting it to the IoT. We will start with a very specific topic: using passive battery free sensors as a source of data in a short communication range – generally referred to as Personal Area Network (PAN).
There is a lot of controversy around the number of devices that will be connected to the IoT but there is no doubt that the potential is enormous. Millions of devices linked together to provide us with continuous meaningful real-time data. Sensors are amongst the most important devices to be implemented: strain gages embedded in bridges or tunnels will alert maintenance staff of dangerous deformations, position sensors will inform about parking places available in your neighborhood directly to your mobile or car display, etc. However, what is preventing a wider and faster implementation of sensor networks?
For a wireless sensor network (WSN) to be effectively deployed, you want sensors located virtually everywhere. Some of these locations are easily accessible and are close to power supply the sensor nodes can be connected to. Unluckily, that’s not the standard and power supply is the biggest issue when talking about WSN. Battery assisted solutions are currently being implemented in those applications where the battery change is not an issue. However, in the long run, battery maintenance is not cost effective for millions of devices.
Energy harvesting from natural sources is the optimum solution. However, the efficiency of devices commercially available is still a bit limited. On top of that, natural energy sources are generally not reliable (i.e.: photovoltaics depend on a sunny day). See a brief article here.
As an alternative solution, RFID battery free sensors are dependent on a reliable power source: an RFID reader. These devices can be fixed or handheld (the size of a PDA) and can power up and communicate with battery free sensors located in their read range. Generally limited to around 2 meters (PAN) if using UHF and some centimeters when using HF technology, the information collected by the reader can then be shared via Wi-Fi, Ethernet, Bluetooth or similar means.
So how do you actually implement RFID battery free sensors solutions? There are multiple variables you will have to take into account when implementing an RFID system. On top of the typical standard RFID implementation consideration, there are three generic keys to implementing battery free sensor tags:
Power Source – The power source in UHF RFID systems is a UHF RFID reader. On top of typical considerations, the output power is a key parameter when dealing with sensor tags. You are trying to power up a sensor with these systems, not just an RFID tag that will give you a unique ID number. The energy harvested by the sensor tag needs to be enough to power up the sensor, make the required measurements, transmit it to the tag and allow the tag to backscatter the information (unique ID plus sensor data) back to the reader. Generally speaking 2W ERP and 4W EIRP are the maximum output power levels to be used in commercial readers. Check your local regulation to design your solution for the best performance possible.
Battery free UHF RFID works under the EPC C1G2 and ISO 18000-6C standards. These standards are thought for ID purposes plus some extra information that can be stored in the user memory of the RFID tags. To be able to work with battery free sensors, you will have to ‘tweak’ the software of the reader so that you do not ask just for the ID. You actually want to send the commands to switch on the sensor, select a proper sensor mode in each case, ask for a measurement, etc. All of this can be performed with standard commands but, in general, my recommendation is to look for UHF RFID readers which APIs are as open as possible. This will allow you to create the best software for your needs, be it a faster data communication (meaning more readings per second), a longer communication range or any other application objective.
Energy Harvesting, Power Management and Low Power Sensors – This is most likely the biggest technical challenge of the three: design of an RFID tag that is able to communicate with and supply power to the sensor included in the tag. You will have to look for characteristics such as sensibility or communication range to know what performance to expect. However, we always encourage testing the products to assess the conformance of the functionalities with the data shown in the datasheet. In general, bigger tags will have a better communication range – due to a bigger antenna to collect more RF energy.
You will also most of the times want to make sure you select a product that is compatible with commercially available readers. Lots of applications are dependent on RFID infrastructure that is already deployed so you want your new tags to be compatible with that – expensive, by the way – infrastructure. Even for those applications where there are no limitations, tying yourself to a single provider is probably not a good idea in the long run.
Then there is the key characteristic of battery free sensors: the sensor itself. Even though an ambient temperature sensor is the typical sensor included in a sensor tag, the actual market demands are very varied: from accelerometers to contact temperature sensors (such as thermistors or thermocouples) to ambient light to switches. These are the real challenge to turn wireless and battery free.
Housing – Many times not given the importance it has, the housing has a tremendous impact in the implementation of a solution. You will need to think about how the tags are going to be fixed to the item you want to monitor (screws, welding, adhesive, rivets, etc.), special housing shapes required, materials required, IP protection, specific certifications (i.e. ATEX), etc.
Moreover, now that you are using sensors, you need to take into account that some of them need to be in contact with the item to be monitored. This means that you have a sensor out of the housing. There is also the need to agree on how that sensor is going to be connected to the desired item and how long the wire out of the encapsulated tag needs to be.
As a simple and short example, I thought a brief video of a wireless battery free strain gage would help you understand how the system works. You can access the video clicking here.
As you can see, a battery free sensor tag is powered by a commercial UHF RFID reader – the MC9090, a Motorola handheld reader. We have developed similar solutions with the Sirit IN610, the ThingMagic M6, the Alien ALR9900, the Impinj R420 or the Motorola FX9500. As discussed above, the performances vary a little bit from one system to another but they all work well.
Now tell us: what are your current challenges on battery free sensor solutions? Reader SW? Sensor tags? Housing?