At a simple level, a radio frequecy identification (RFID) system consists of an RFID tag that emits a radio signal and a device that picks up the signal. The device that receives the analog signal is called an RFID reader, which produces electricity that runs down its cable at a particular rate. When the electricity reaches a piece of metal on the reader's antenna, the antenna radiates the same signal out in space at a certain frequency and wavelength. After generating the outgoing signal, the rfid reader "listens" for a response from the RFID tag, which has its own antenna from which to transmit this response.
The tags referred to here are of the passive type, meaning that they do not have their own power supply and must rely on the signal emitted from the reader in order to transmit data back to the reader. Active RFID tags, on the other hand, have an on-board power supply (e.g., a battery), and they are constantly emitting signals (provided that the battery is not dead, of course). For a more in-depth discussion of the differences between active and passive RFID tags, refer to Passive RFID Tags vs. Active RFID Tags.
An RFID reader can have one antenna or several, and it is not limited to reading data from a tag; it can also write data to the tag. Performance features used to characterize RFID readers are:
Read range: the distance at which all of the tags in use can be read
Read rate: the number of tags that can be read per second
Write range: the distance at which unique identifying information can be encoded on all of the tags in use by the particular application
Write rate: the number of tags that can be encoded per second
Identification range: the distance at which all of the RFID tags in use can be identified
Identification rate: the number of tags that can be identified per second
It's important to note that despite what a reader's specifications indicate, its actual performance depends greatly on the environment in which the RFID system exists, the reader's antenna, and the performance of other components within the system.
Aside from performance, RFID readers can differ based on configuration: they can be handheld, fixed (stationary), or mounted.
Handheld RFID Readers
Handheld readers are typically used to locate items in distribution centers or to read tags that do not go through RFID portals. These wireless, battery-powered readers are convenient and versatile. Some are self-contained computers, while others can be attached to handheld computers made by a variety of manufacturers.
Fixed (Stationary) RFID Readers
Stationary RFID readers can be affixed to walls, doorways or other types of portals, and tables/desktops. As tagged items pass through the portal (or by the table or desk), the reader picks up the tags' data, which are then stored in a database. The database is updated in real-time, making RFID a powerful technology for use in asset tracking and inventory management applications.
Mounted RFID Readers
Mounted readers differ from fixed readers in that they are typically attached to moving objects, such as trucks or forklifts, and they keep track of the movement of items from one location to another. Mounted RFID readers read the tags attached to the items, cases, or pallets that the vehicle is carrying. The locations of the readers themselves can be determined by interfacing the readers with tags embedded in the ground or mounted on walls throughout the facility. Mounted readers can operate in either autonomous mode or interactive mode. The former mode enables the reader to read tags and immediately send their data to the database host, while the latter mode allows the reader to store data for a specified length of time before they are uploaded to the database.
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