Sensor Sense: RFID Safety Sensors

Jan. 12, 2010
RFID safety sensors offer machine safety with built-in integrity checks.

Guards on potentially hazardous equipment are designed to keep personnel out of harm’s way or to shut down the machinery if access is needed. Mechanical safety interlocks and magnetic safety switches are still widely used to determine the status of access doors and gates. Although radio-frequency identification (RFID) has been around for years, it has only recently found use in safety systems for new machine designs.

A typical RFID safety system uses three components: a tag, a read head, and a controller. The tag attaches to a movable gate or sliding fixture while the read head mounts so it can read the tag in the safe position. The controller determines the safe integrity of the entire system.

The tags, also known as transponders, contain no battery or other power source. The read head supplies power inductively to the tag to run a low-frequency (125-kHz) RF circuit that transmits a 32-bit number to the head. Thirty-two bits create almost 4.3 billion possible combinations, making each tag code unique. A teach procedure links the tag information to a specific read head upon system setup. Once the tag number is logged into a read head, no other tag can trigger that read head’s output.

The read head contains a simple ring antenna to communicate with the tag at ranges up to 15 mm. Tags can enter the read field-of-coverage area from any direction. This differs from older magnetic safety systems where independent contacts switch when targets move in from the side.

The controller is the brain of the safety system. An internal multiplexer lets a controller simultaneously monitor four read heads mounted up to 30 m from the controller without signal degradation. The multiplexer lets the heads run in close proximity without mutual interference. The controller also performs safety evaluations. When the tag moves into position over the read head, the data from the tag is read and evaluated by two different microprocessors. If both microprocessors read identical data, the controller signals the safe state and the machine is ready to run.

Pepperl+Fuchs ( supplied information for this column.

Edited by Robert Repas

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