How RFID technology is used in location tracking applications

RFID technology is located in the perception layer of the Internet of Things, which is the basis for the development of the Internet of Things and the prerequisite for the realization of the Internet of Things. Compared with RFID tags of other frequencies, UHF tags are more secure and penetrable. With UHF readers, they can better resist interference and have faster read and write speeds. Therefore, in recent years, its development is more rapid and its application is very extensive. Then, RFID technology is used in location tracking applications mainly in the following ways, let's discuss it together.

1. Based on the location of the RFID Reader

The most common way to establish an item's location is to base it on readers. When you have a stationary reader and you know the location of the reader, you can determine the approximate location of the tagged item based on the RFID reader reporting the tag. The item's location will be within the set RFID reader area. For many applications, this approach works, such as in Sports timing applications. RFID readers can also be placed at doorways to detect movement in and out, providing room-level location.

2. Reference position label

Another good common positioning method is to place reference tags in fixed locations where items are Stored, such as on shelves and tables, rooms and doorways. When you use an RFID handheld reader for inventory, in addition to the RFID tag on the item, you also read the reference tag and can determine the location of the item based on the known reference tag location.

3. Find the correct item

Finding the location of a specific item (the location of something) is a very common use case. For example, finding the correct part in a warehouse or finding the correct File in a large archive. A new product in the market that helps in this application is RFID LED tags. To find an item, the handheld reader sends a select command to the item it is looking for. When the reader "finds" the item it's looking for, an LED light on the RFID tag lights up to visually help the user find the item.

Another method is to use "Geiger mode" with a handheld reader. An RFID reader can be set to display a "warming or cooling" indicator when the reader moves closer or further away from an item of interest. An audible indicator beep is usually used more often when an item is close - hence the method is often labeled a "Geiger counter".

4. Marker-based location

The reverse method of RFID reader-based positioning is to place the RFID tag at a fixed location and track the reader location based on the fixed tag location. The benefit of this approach is that RFID tags are cheap. It is feasible to place many tags along known paths, for example, in elevators, trains, conveyors, floor tiles, etc. When a reader walks by and "sees" an RFID tag, that's where you put it on the map.

5. Phased array

Phased arrays are a fairly new approach to locating RFID tags. The technology has been around for decades, mostly in the radar business. A phased array is an antenna consisting of multiple small radiating elements within a single antenna housing, where the phase difference of the elements is controlled to allow the transmit beam to be steered in the desired direction. This allows you to scan the environment from different directions for flagged items.

Overhead bidirectional phased array readers are becoming more and more popular. They allow estimation of item position using a single reader mounted on the ceiling, scanning the left, right, back and front. The overhead reader works well when the items are large and the room is fairly empty. Shelves and furniture in the room can cause reflections and echoes that will affect accuracy.

6. Triangulation

Using two or more horizontal wall-mounted scanning phased array readouts, position can be determined by triangulation. You can also use a phased array antenna placed on a wall to determine the location if you know the angle and distance range to the item. This technique is still rare and relatively expensive.

Range-based triangulation is a more traditional approach where two or more readers find an item and can estimate the item's distance from the reader. Distance data can be used to calculate the location of an item. But estimating distance with RFID is not that simple. Time-of-flight is commonly used in RF, but due to the small distances, the time difference is too small to work accurately. Using RSSI is also tricky because it only works one way. If you get a really high RSSI, you know the item is pretty close. However, if your RSSI is low, the item can be anywhere, far or near.

Determine location using directional antenna-based triangulation and distance-based triangulation

7. Inventory robots

Using inventory robotics to locate items is not yet very common, but it can be a good solution for inventory in large retail stores and warehouses. The robot's position can be accurately tracked by floor plan and rotary encoders, lidar and other technologies. As the robot roams around tirelessly, its antennas can detect tagged items from hundreds of locations along its path. In post-processing, the exact location of the item can be calculated from these data points. This method is best when the item is not moving. Fortunately, bots don't mind staying up all night, allowing inventory rounds to be completed overnight.

8. Phase-based ranging

Phase-based ranging is a technique that uses backscattered phase measurements. In this method you can select a channel and get the reply from the RFID tag. The answer always comes at some stage. As you take the next channel or increase the frequency, more wavelengths will fit in the path from the RFID reader to the RFID tag and back, so the phase will increase. As you go to higher channel numbers, the phase goes up (see image below). The rate of phase change is relative to distance and thus can be used to calculate the distance of an RFID tag. If the phase grows fast, the RFID tag is far away. If the phase increases slowly as the frequency increases, the RFID tags will be closer. However, regional channel regulations pose challenges to this approach.