Better economics for industrial fiber optics

Nov. 20, 2012
• Recent developments in plastic optical cable have brought down prices. • Optical fiber can be an economical way of warding off problems caused by electromagnetic interference.

Researchers at NASA’s Langley Research Center got some alarming results when they looked at how electromagnetic interference could potentially affect flight navigation and communication systems. Interference from some ultrawide-band signals, for example, could suddenly silence aircraft radios with no warning. EMI could also lead radio-navigation systems to give erroneous readings. Distance-measuring equipment could be influenced by EMI as well, in some cases reading out distances that were off by a full nautical mile. Perhaps most worrying, sufficiently bad EMI could knock airplane images completely off displays of airtraffic collision-avoidance systems.

Such are the difficulties EMI can cause not just in aircraft, but in industrial equipment of all kinds. One often encounters industrial applications where hundreds of amps switch within a microsecond. Examples include motor drives, IGBT power circuits, SCR trigger circuits, and power contactors. Such current switching radiates large magnetic and electric fields, creating electrically noisy conditions. Fortunately, optical fiber links can remedy such problems. For example, polymer optical fiber (POF) is steadily replacing copper cabling in many industrial RS-485 and Fast Ethernet communication schemes. Unlike copper cables that can act as an antenna, glass and plastic fibers are dielectric materials and, thus, immune to the stray EM fields common in motors drives, ac/dc power inverters, and power-distribution systems. These fibers can even sit in a duct alongside high-voltage power cables without concern for cross talk.

Optical fiber also completely eliminates ground loops and their potential noise and safety issues. Fiber is a candidate for monitor and control functions in high-voltage applications and excels at connecting control triggers to high-current/voltage switching circuits through an isolation barrier.

Nevertheless, many design engineers have been reluctant to build fiber optics into data and control networks. Reasons include a perceived cost disadvantage, concerns about ease of use and installation, or simply a greater familiarity with copper and its infrastructure. But recent developments in fiber-optic transmitters and receivers now make fiber an economical alternative to copper links. Moreover, the new devices can be easier to use and install than copper lines.

Galvanic isolation

Designers must sometimes go to great lengths to galvanically isolate copper wiring to prevent dc-current flow. Typical measures include differential line receivers; RF, magnetic, capacitive or optical coupling; and transformers. However, these precautions can prove inadequate in the presence of extremely large switching currents and voltages as arise from variable-frequency motor drives, utility-scale wind turbines, or beefy dc-to-ac inverters.

Unlike copper wiring, optical fibers need neither rigorous grounding rules to avoid ground-loop interference nor termination resistors to avoid reflections. Properly used, optical transceivers and fiber cables can prevent lightning strikes from catastrophically damaging equipment and can safely isolate outdoor and tower-mounted electronics.

IEC 664-1:1992 is the international insulation standard for low-voltage equipment. It dictates that even in the worst possible environment (i.e., outdoors) the minimum standard distance for a working voltage of 10 kV is 45 cm, a little less than 18 in. A plastic optical fiber this long is considered an ultrashort link. The average installation length of plastic fiber is 10 m. At this length, the possible working voltage exceeds the standard by 20 times, an illustration of why the galvanic isolation properties of optical fibers work well for harsh industrial environments.

Industrial communication links must also reject common- mode noise and be designed to avoid ground loops. One common means of realizing these goals is to use an optocoupler. It can place up to 15 mm of galvanic insulation between the data source (μP, UART, etc.) and the copper transceivers driving twisted-pair cable. (Galvanic insulation distance is the maximum distance between the input and the output based on the maximum physical dimensions and limits on package stability.)

This insulation can prevent common-mode noise from propagating into sensitive receiver decision circuits, which can cause errors in the data transmission. However, the transmitter and receiver in couplers are in close proximity, so close that stray capacitance across the isolation barrier can be an issue. So optical coupling is effective only to the extent that this capacitance is minimized. Typical parasitic capacitance of optocouplers is 0.4 pF. POF links have a parasitic capacitance that is directly proportional to link length — about 1 pF/m, i.e., essentially zero. And a fiber link with a standard length of several meters produces an isolation barrier measured in meters, not millimeters. All in all, POF eliminates any pathway for common-mode noise.

The benefits of fiber links don’t always come at a substantial cost penalty. To illustrate this point, consider two bills of materials (BOMs) for equivalent industrial link designs, one for a 10-m RS-485 copper link, the other for a 10-m plastic optical fiber (POF) link. As the requirements for shielded/certified cables rise, so do the associated costs. In contrast, POF costs remain the same regardless of the application environment.

One reason the cost of POF links compares more favorably with copper in recent years is better integration. It is now possible to find fiber transmitter and receiver packages that include driver ICs. For example, the AFBR- 1624Z transmitter integrates a 650-nm LED source with optics and a driver IC for use with 1-mm POF. Similarly, the AFBR-2624Z receiver consists of an IC with an integrated photodiode to produce a logic-compatible output. Designers can connect TTL, LVTTL, PECL, LVDS logic signals to the inputs and need little or no knowledge of optical drive circuits.

Clearly, the copper cable itself dominates the cost of a copper link. Since the beginning of 2009 when copper dipped to $2,000/metric ton, the price of copper has risen to over $10,000/metric ton and is now sitting at about $8,400/metric ton. Thus, copper prices can be volatile. In contrast, the cost of plastic in POC is quite stable and may potentially drop as sales volumes rise. Such price dynamics favor the economics of plastic fiber. This is especially true for conservative or high-reliability designs. Here, most engineers will choose a wellshielded, high-quality cable to help prevent noise ingress and egress. Fiber easily competes on cost in such cases. The cost of the copper cable drops in applications that can get by with less shielding (i.e., CAT5) or shorter links, on the order of 10 m. For longer cables, however, POF runs about $0.20/m compared $1/m for copper without connectors.

Terminating fibers

Typically, it takes more time to terminate glass fibers than to connect twisted-pair wire because these fibers require end polishing and epoxy. The relatively large 1-mm polymer core in POF, however, makes this kind of fiber much easier to handle. Today vendors offer connectorless and crimpless style connectors that let POF cables terminate more easily than shielded twistedpair cables.

Connectorless transmitters and receivers have 650-nm fiber-optic components that work with unconnectorized POF that needs no end polishing. The user simply cuts the fiber to the desired length, inserts it in the active component ports, and tightens a built-in locking mechanism. Another design uses a simple, snap-together concept that eliminates the need for crimping, thereby greatly reducing labor and tool costs as well as potential yield loss caused by installation errors.

Optical fiber is also a way of future-proofing applications for the inevitable rise of communication data rates. For example, fiber links can transmit 125 Mbytes/sec Fast Ethernet (100BASE-FX) signals up to 2 km, while equivalent copper links (100BASE-TX) are limited to 100 m. Moreover, optical fibers are thinner and weigh less than equivalent lengths of copper cable. So a given installation space can hold more optical than copper links and the optical links will be easier to handle.

Avago Technologies Ltd.

© 2012 Penton Media, Inc.

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