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01 June 2003

Avoid fiascoes with fiber optics

By Derek Montgomery

New technologies, such as mechanical, hydraulic, acoustic, and pneumatic systems, have always attempted to eliminate electricity from control systems in hazardous industrial environments. Today you're more likely to run into commercially viable optical systems.

Such technologies allow optical sensing in almost any industrial process—temperature, pressure, displacement, strain, liquid level, and electric field. Fiber optics are also durable and chemically inert. They fair well in atmospheres where metal electrodes are subject to corrosion, such as marine environments.

Fiber-optic control components offer advantages over electrical technology, and manufacturers often consider them first in situations where electrical devices suffer from extraordinary environments. Optical signals are unaffected by electric fields and radio interference, and you can route fibers without concern for antenna effects. You'll find manufacturers applying fiber optics mostly in the presence of high electric fields—electric power distribution stations, industrial microwave dryers, or locations near radio communication towers.

Because fiber-optic signals are not affected by electromagnetic interference, you don't need the sensor transmitter in the immediate vicinity of the measuring probe. Relatively long fiber runs can usually reach an electrically safe operating location where you don't need explosion-proof enclosures. You could put the opto-electronic converters in convenient intermediate locations and network them together on a standard data bus loop, connecting them either electronically or by data communication fibers.

In a typical fiber-optic control system, switches and sensors appear in the hazardous environment. They use only light transmitted by the optical fibers to convey control information. The opto-electronic conversion modules appear in a nonhazardous environment—connecting to a conventional electronic control system through a standard electrical interface. These modules are comprised of a light source, a photodetector, and processing electronics to interpret the optical signals and generate electrical outputs in accordance with protocols.

Ubiquitous acceptance in the commercial marketplace hasn't happened yet, however. And the main complaints remain—it's too expensive, and glass fiber is too difficult to install. Glass fibers tend to be fragile and you need to terminate, polish, and splice them using specialized equipment.

Plastic optical fiber might change all that. It looks and feels like fishing line, and you can trim it and terminate it with a simple tool in seconds. You can find plastic optical fibers with core sizes up to 1 millimeter in diameter, which dwarfs the glass varieties. They don't transmit light pulses as well as glass fibers, but the plastic is much tougher, which makes it more suitable for most industrial environments, without resorting to expensive cable protection. The plastic is not as transparent as glass, but the large core diameter means that much more light launches over relatively short distances, and the plastic fibers can deliver more illumination than their tiny glass counterparts.

Photon Control's fiber-optic temperature sensor has a special fluorescent material (phosphor) coating the tip of the fiber probe, which is in thermal contact with the measuring environment. Light-emitting diodes generate light that transmits down the fiber to the probe tip, which charges the phosphor. The phosphor then discharges the energy by emitting light, some of which the optical fiber transmits back to the photodetector. The signal generated by the photodetector processes electronically. You can determine the temperature of the phosphor by measuring the persistence of the fluorescent afterglow light.

Fiber optics have also become the standard interconnect medium for most data communication networks. Industrial plants are now using fiber optics for data communication to connect their control systems to their computer networks, so they can monitor industrial processes online. This fiber has a larger core than its long-distance counterpart, which makes it less expensive to terminate at the expense of operating distance.

The telecommunication-grade fiber has a microscopic core, which allows light pulses to travel great distances without distortion. The data communication–grade fiber has a somewhat larger core, but the light pulses rattle more inside its core, which distorts the communication signals and limits their operating range to less than a mile. The plastic-coated glass fiber has an even larger core and is commonly used in instrument illumination applications.

Fiber optics might not replace electricity for most industrial control applications any time soon, but the cost of this technology is competitive with conventional explosion-proof electrical systems. Guaranteed safety is perhaps an unrealistic goal, but optical fibers can play their part in reducing catastrophic risk one more notch. IT

Derek Montgomery is vice president of operations at Photon Control in Burnaby, British Columbia, Canada.

A comparison of optical fibers.