As part of Stimulant’s focus on smart spaces, we are always evaluating products and technologies that help us determine the position of people and objects in space. One exciting technology is RFID, which enables tracking of small “tags” which can be embedded into badges or other objects given to visitors to a space. The RFID system we’re trying out lately is the new xArray reader from Impinj.
The Impinj RFID Reader is used for spatial tracking of assets and people. It’s about 18″ square, weighs about 18 pounds, and is meant to be mounted 10-15 feet above the area to be tracked. The unit contains 52 directional tracking antennae that radiate RF frequencies in a conical pattern around them. One can think of this as a cone shaped projection from the reader towards the floor. All of the assets that fall in the line of sight of this cone can be tracked when tagged with passive RFID tags.
Passive RFID tags integrated into various form factors.
Whenever the Impinj Reader pings a tag, the tag uses this radiation to charge up a tiny capacitor on it’s surface. Once the capacitor is fully charged, a microchip on the tag’s surface uses the energy stored in this capacitor to send a signal back to the receiver which contains information such as its ID, its X and Y co-ordinates with respect to the receiver, the read count (which is a measure of confidence of the coordinate reading), and the timestamp of the signal.
Along with the hardware, there are .NET and Java SDKs that allow integrators to write software to configure the sensors, gather data from them, and interpret that data as needed.
One of the unique features of the Impinj Reader is that it provides pretty precise tracking using passive RFID tags, which are much cheaper than active tags. This can be of great advantage when tracking assets or people across a space, while keeping the costs low. However, passive RFID tracking also has certain drawbacks. First, it always has to be in the line of sight of the reader. If it is blocked by even a thin obstacle, the signal from the tag won’t be strong enough to make it to the receiver. Further, metal, liquids, human body create lot of interference for the tag to pick up the weak radiation from the reader.
A single reader provides up to 1-1.5 meters of accuracy with passive Monza 5 tags. The accuracy can be slightly improved by using multiple readers together and writing software to use a weighted averaging algorithm to calculate the average of positions reported by all the readers in question. This method can only increase the accuracy of tracking to some extent. To further improve the accuracy one can use Monza 6 tags. These provide a tracking resolution of about 1.5 to 2 feet. However, they are relatively new at the moment and are slightly expensive and scarce in production.
Another drawback of the Impinj Reader is that it can only scan a space once per second. This hardware limitation is specifically because the system pings frequencies separated in time across the 52 directional antennae. So, it can do so only once per second.
Despite some limitations, the xArray system is a great piece of technology for tracking objects and people in space. We’ve prototyped some interactive visualizations of the data coming from the sensor and are hoping to use it in real deployments. Reach out to us if you’d like to learn more about what we’re working on and how we can apply it to your installation project.
