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Technology

Operating principle

When introduced to the radio frequency created by the reader/controller – the tag will charge its internal capacitor wirelessly and communicate directly with the reader using the RF backscatter principle. Unlike inductive field modulation in HF frequencies (13.56 MHz ISO 15693) – the tag rebroadcasts its communication in the ~900-920 MHz frequency band (FCC) which allows for significant increases in read/write distances compared to HF.

EPC (Electronic Product Code) Generation 2 standards allow for the communication between a number of tags (up to thousands) and a single antenna. Due to the re-transmission of the signal, many tags can be read at once at a distance up to 10 m. The tag is first identified with its EPC, but many databases can be accessed.

Limitations

  • The tag must be within the transmission field to charge the capacitor and re-transmit.
  • Because of the increased field size and the nature of the 900...920 MHz band, ambient temperature and RF blocking / reflecting materials can affect the system.
  • RF blocking materials include water, eater-based chemicals and metals.
  • If not properly shielded or if the antenna field is not properly tuned, stray emissions or reflections from undesired tags may cause those tags to be read by mistake.
  • Specialized pre-tuned tags are required for applications where RF blocking substrates are installed.

Common standards overview

EPC Global -- EPC Class 1 Generation 2 standards are the most common in industrial production.

  • Unlimited read / write capabilities on a passive tag.
  • Interrogator first protocol -- antenna queries the tag EPC memory bank to initiate communication.
  • Other classes of tags exist for specific applications (Classes 0-5 EPC Global).
  • ifm UHF equipment can be used with any tags that follow EPC Global standards.

ISO standards -- ISO and WTO accepted the EPC Global standards for Class 1 Gen 2 tags and this became the ISO 1800-6C air interface protocol.

  • ifm UHF equipment can be used with any tags that follow this standard.

Tags

A UHF tag contains four main memory banks for manipulation and storage of information (under the UHF Gen2 standard) and they are summarized below.

EPC (Electronic Product Code) TID (Tag Identifier) User Memory RFU (Reserve for Future Use)
Identifying information for the tag to initiate communication Locked unique identifier for chipset Full read / write capability Full read / write capability
Full read / write capability with redundancy check for data integrity Non-repeatable, cannot be rewritten 512 bits to 8 kbits typical Storage for passwords, kill, lock and unlock passwords
96 or 128 bits typical, expandable to 496 bits 96 bits typical Used for additional information outside of hte EPC memory May not be included in all chipsets
Present on all chipsets Present on all chipsets May not be included in all chipsets  

Typical memory structure of UHF RFID tags

Anti-collision
Since UHF antennas read many tags at once, it is possible for two tags to broadcast their EPC memory bank at the same time. Gen 2 standards provide an algorithm tags report EPC information in sequence so no tags are missed.

Tag types

  • Standard tags must be mounted to non-metalic, non-hydrous materials and are tuned to maximum range.
  • Pre-tuned tags have a metal film backing and are tuned to operate best when mounted directly to metal.
  • Metal-embeddable tags can be completely surrounded by metal (except for the read face) and are typically friction fit into an application.
  • High temperature tags withstand environmental temperature up to 450 °C, but read / write operations must occur in the -40...85 °C range. Most high temperature tags are also pre-tuned for metal mounting.
  • Labels offer a low cost option for tracking of materials and components. Some are pre-tuned for metal mounting.
  • Specialized tags such as databolts, cards, bracelets and fobs are available for targeted applications.

Tag reading and writing
UHF RFID communications relies on the tag's ability to effectively receive and retransmit radio frequency signals produced.

  • The read / write range is a function of tag and antenna, not either one alone.
  • Typically, the larger the tag, the larger the read range with a given antenna.
  • Larger antennas and / or larger tags may increase the chance for interference or stray reflections requiring additional shielding.
  • Note:  Thoroughly test your installation with the selected tag / antenna combination in the application.

Antennas

Linear polarization is achieved by broadcasting electromagnetic waves on a single plane. By focusing communication on a single plane, more power can be effectively used to achieve distance and still adhere by the FCC standards for the UHF spectrum (increase in about 3dB on a single plane). While this does not seem like much, it can allow for significantly longer read distances, with some major drawbacks:

  • There must be uninterrupted RF visibility to the target tag
  • The tag must be on the same plane as the emitter
  • The tag must be perpendicular to the antenna

While there are some specific applications where this antenna polarization is beneficial, when investigating an asset tracking program, circular polarization should be used.

Circular polarization utilizes RF transmission in two planes, which results in a rotating electromagnetic wave, completing a whole revolution throughout a single wavelength emitted. Simply, this results in a corkscrew-like emission of the RF signal. By broadcasting on two planes, the effective emission loss is about 3dB, resulting in a shortened read range than its linear counterpart. However, when discussing asset tracking with UHF, the benefits of the circular polarization far outweigh the loss in read range.

Because of the corkscrew-like nature of the field – the tags orientation can be effectively omni-directional, and the tags no longer need to be on the same plane as the reader. This is significant in discussing the tracking of assets when location, orientation/plane, and distance are never guaranteed. Circular antenna polarization is what allows for tracking portals and UHF mobile automation tracking for intralogistics applications.

Speed of data transfer

The two main factors influencing the speed of data transfer are local storage / memory and the amount and type of data read / written.

Local storage / memory

  • Limiting factor is data transmission time to a higher level system.
  • Communication cycle over Ethernet can take 1000+ msec to complete, but includes a large amount of data for many tags.
  • Local storage on readers capture and store large batches of information communicated on each cycle.
  • Typical applications include inventory and perimeter control in logistics.

Amount and type of data

  • Reading EPC memory is a native process in EPC Gen 2 and provides the fastest speed of transfer.
  • Reading TID or User memory requires an additional communication cycle, and therefore increases transfer time.
  • Interacting with RFU and commands as well as writing data to any memory bank requires many communication cycles, increasing interaction time.