Sensor Sense: Optical data transmission

Sept. 13, 2007
Optical data transmission (ODT) offers a means of sending data between moving objects without some of the drawbacks associated with radio-frequency and wired-communications methods.

Edited by Robert Repas

ODT essentially replaces the radio waves or electrical wires with a beam of light.

The sensors in ODT devices are called couplers. They use pulse-modulated infrared light to transfer digital signals. The use of light as the transfer method means that ODTs do not incur physical wear like drag cables and slip rings nor require recalibration like radio systems. They operate without problems in electrically noisy areas and in the presence of metal wave guides, both situations where radio signals have high failure rates.

Data travels over the light beam using a modulation method known as frequency-shift keying or FSK. FSK rapidly switches the IR light between two different modulating frequencies. One frequency represents a logic 1 value while the other frequency represents a logic 0. This resembles the way modems use two different audio tones to send data over telephone lines. The signal from the infrared sensor in the receiver feeds narrow-band filters tuned to the two frequencies. The filters minimize any adverse effect from ambient light that could interfere with the signal.

ODT technology works with any digital signal such as parallel I/O, serial RS-232 and RS-422, and many bus protocols. The most common application consists of one coupler mounted in a stationary position while another coupler travels with the equipment along a path towards or away from the first coupler. A good example of that type of system is an overhead crane where the operator on the ground uses it to send control information to the moving carriage of the crane.

ODT devices can support transmission distances up to 700 ft or more under good conditions. Most have alignment aids to help expedite setup over long distances. For example, LEDs indicate signal strength on some units.

One drawback is that the couplers require a line-of-sight installation. Each coupler has to "see" the other to send data, which means the link fails if the device goes around a corner or if an object or grime blocks the path of the IR light beam. Pepperl+Fuchs (www.am.pepperl-fuchs.com) supplied information for this column.

Edited by Robert Repas

 

Optical data couplers use a beam of infrared light to transmit digital information between moving objects without the problems associated with wired or wireless systems.

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