This set of posts will guide you through the perils of developing a complete solution with battery free RFID sensor tags.
Following our introduction into wireless battery free sensor tags I would like to go a little bit deeper on each of the keys we mentioned. This is a general post introducing the technology and highlighting the consequences of introducing a sensor in an RFID tag. You will receive more information about how to configure a commercial RFID reader and develop a SW program that is well suited for battery free sensor solutions, more about developing battery free sensor tags and more about housings to be used on different blog posts.
Let’s start with a basic understanding of how RFID technology works. The reader is the power source for the battery free tag. The reader sends both energy and signal through the reader antenna. The energy is then harvested by the tag antenna, converted into DC and used to switch on a circuitry specially designed to send a unique ID number back to the reader. Please note that in passive UHF RFID (EPC C1G2 – ISO 18000 6c standards) the tag does not transmit data, it just backscatters the information. The RFID tag uses a module (generally referred to as “load modulator”) that acts as a mirror with RF waves: when the waves are reflected they get back to the reader; when they are not reflected, the reader receives no signal back. These are read by the reader as zeros and ones and the full string will be duly converted into meaningful data by the reader.
This imaginative way of communication allows the system to have very low power consumption – there is no need to transmit data, which is very power demanding – thus making passive RFID a reality: RF energy harvested and a unique ID communicated back. Most of the commercially available solutions can communicate to up to 8-10 meters away when set up properly. On top of that, most of the readers come with a handy piece of Software you can easily use to detect RFID tags. You only have to click on a button that says “Power”, “Read EPC” or a similar text and the Software handles it for you.
Once you want to implement a sensor on the tag, things change a little bit. There are a couple of things to address once you do that:
New power supply required. As you can suspect, introducing a sensor on your equation increases the energy consumption of the tag. This is bad news for the communication range of your system but there are some more subtle and deadly requirements associated to this:
You need to harvest more energy. As discussed previously, your new RFID sensor tag has a higher power consumption. Since the output power of the readers is limited by country regulations, you need to be smarter at every point of the system. This includes both the tag and the reader. Developing a good reader Software greatly increases the performance of the whole system (increasing the RF transmission duty cycle – same output power but for longer periods of time – improving the rate at which you access sensor data, etc.). Remember that default RFID Software is not thought for your RFID sensor tags but just for standard ID tags.
You need a supply voltage on your tag. In order to implement an external sensor to your RFID sensor tag you need to have an RFID IC specifically designed to supply an external device. Most commercially available sensors require supply voltages around 1.8V – 2.5V. While 1V internal supply is enough for the RFID chip to provide the reader with a unique ID, your selected chip will need to provide higher voltages to an external device. Bear in mind that for a given chip, the distance at which you can have a 1.8V supply will be shorter than the distance at which you have 1V (the range for retrieving the unique ID is longer than the range for retrieving sensor data). Make sure the RFID IC you select has the correct supply for the sensors (or other devices) you are going to use on your tag.
You also need to prepare your new RFID sensor tag design for peak power consumptions. There are lots of low power sensors available in the market at the moment, and the industry is constantly producing lower and lower power consumption devices. That’s good news, right? We are seeing datasheets of sensors with typical power consumptions of just a few microamps, which is great for your new battery free sensor system. Now the bad news: most of the sensors require an initial peak current that can be 10, 100 and even 500 times higher than the typical one. You will need to make sure your RFID sensor tag is able to handle those kinds of currents so that your system performs correctly!
Communication with an external device needs to be established. Now that you have a sensor you can switch on, you also want to be able to control it and obtain the correct data from it to be sent back to the reader. Again, a couple of things you need to take into account:
Physical connection with the sensor. In order to be able to connect a sensor with your battery free RFID tag, you need to develop the proper communication protocol. There are a couple of widely used protocols: I2C and SPI. Make sure you have the correct logic in place to connect with your selected sensor.
Ability to send the proper commands from the reader. It may seem like a very straightforward point, but it can be a complex one once you dig deeper. Note that you are using an RFID IC as a bridge between the sensor and the reader. Collecting the information from the sensor is pretty simple: just ask for the specific user memory address where the RFID sensor tag has stored the sensor information. However, think about some other commands you will have to use. For example, many sensors require an initial configuration such as setting the power mode or similar commands. You will need to be able to include these kind of signals in your reader software!
These are the main differences between a standard RFID tag and an RFID sensor tag solution. There is one last big component you need to take into account (this one for both standard and sensor tags): tag antenna. It is a very tricky and complicated topic but we will try to share with you our experience on this field.