Let the frequency fit the RFID application

Is a low-frequency, high-frequency or ultra-high-frequency system right for you?

By Tom Stevic, contributing editor

Radio-frequency identification (RFID) technology is used in a wide variety of applications, from the chips used to identify a lost pet, key cards and security packaging for consumer goods to manifests of products carried in the transportation industry. In an industrial context, RFID tags are most often used to identify specific devices and to store manufacturing data. The tags can be divided into two classes. Passive tags use the electromagnetic energy generated by an RFID systems antenna. Active tags use a local power source, most often a battery, to allow the tag to respond to the radio waves. Passive tags are limited in transmission distance because the amount of energy generated by the antenna’s radio signal must be restricted, so it does not interfere with other electronics and does not produce harmful radiation. Active tags can either periodically broadcast a signal or can wait for a signal to be detected before responding.

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The three top frequencies used in industrial RFID system are low-frequency (LF, 125 kHz), high-frequency (HF, 13.56 MHz) and ultra-high-frequency (UHF, 865–928 MHz). The LF and HF systems have a smaller operational range in the 10–150 mm range. UHF system distances are measured in meters. The size of the tag and the size of the antenna greatly affect the operational distance.

The LF systems are used when a limited amount of read-only information is required. They perform well in a variety of environments, such as machine tool coolant, dirt, oil and water. Computer numerical control (CNC) machines with automatic tool changers can verify the correct tool has been selected by reading a tag embedded in the tool itself. Containers, carriers or pallets on a conveyor simply need a unique identifier. Some tags are available with write-once or factory-written codes that prevent modification.

The HF systems are available with larger memory capabilities—up to 128 kB. They can be used as read-only systems, or, more often, they are utilized in a read-write mode. A data set, such as a recipe, build instructions or an order to be filled at a warehouse is stored on the data tag at the beginning of its journey. In read-write mode, production data is updated as the product moves through the manufacturing process. The data tag can have component lot numbers, results of in-process testing, torque values and production timestamps. At the end of the production cycle, the data on the tag is read and the information stored for future use or analyzed to detect processes going out of specification.

The UHF systems share many of the traits and potential uses that the HF system provides; however, the higher frequency has two advantages—faster data transfer speed and longer distance between the antenna and the tag.

A central computer can be programmed to provide the same information, but the RFID systems remove the nonlinear time factor and create data packets that travel with the container or product. The RFID system will help to keep all of the information in sync, especially if products are sometimes removed from the process flow for testing or repair.

One-dimensional (1D) and two-dimensional (2D) barcodes can be used as identifiers for a particular product. An RFID system has the advantage of operating in dirty environments and not requiring a line of sight to the systems antenna. RFID tags do require some forethought in how they will be mounted. Each manufacturer has specific mounting instructions for each of the tags they offer.

Mounting style and the backing material used to mount the tag also have a large effect on system performance.

RFID data tags are available in a large variety of physical sizes, memory size, mounting styles and environmental resistance. Writable data tags support a limited number of write operations. Technical data should include the maximum number of write operations the manufacturer guarantees.

The RFID antenna must be connected to an amplifier of some sort to make the data useful. Amplifiers come in a variety of form factors, including specialty PLC cards, amplifier cards that plug into a computer backplane and stand-alone devices. The stand-alone amplifiers, though more costly, will provide more connectivity options. Serial communications and all popular fieldbus connection options are available. A stand-alone amplifier does not tie a designer to a specific brand of controls or to a custom-written PC program.

A number of standards from the International Organization for Standardization (ISO) and EPCglobal exist that reference RFID use and data handling. Manufacturers that list compliance to these standards may or may not produce products that are interchangeable. There is no guarantee that tags from one company will work with antennas from a different company. Some manufacturers even go so far as to discourage using some tag-antenna combinations from the same product line.

Careful definition of system requirements and testing the application are the first steps in designing a successful project. The distance between the antenna and the tag and the speed at which the tag is moving relative to the antenna are major considerations. Mounting style and the backing material used to mount the tag also have a large effect on system performance.

As always, consult the manufacturer for correct use. And determine the ease of securing technical support, if required.

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