Fiber-Optics - Finding a New Home in the Factory

By Michael Messenger, Field Applications Engineer, Future Electronics (UK)

READ THIS TO LEARN ABOUT The requirements of industrial data links Designing-in industrial-grade fiber-optics

The coming together of two trends is leading developers of industrial systems and network installations to consider fiber-optic data links as an alternative to copper. Firstly, there is growing demand for high data rates over transmission distances ranging from a few centimeters to hundreds of meters. Secondly, fiber-optic cable and transmission equipment has fallen in price and become easier than ever to implement.

Systems integrators need only a little guidance to design fiber communication links to achieve a cost effective solution meeting performance, safety and scalability targets.

 

Communicating in the Factory: Special Requirements

Any industrial application, for example performing automation or process control, must be able to transfer data reliably, predictably and in real-time. Communications may be point-to-point or, alternatively, they may be networked. Various industrial communications standards now exist to support networked communications between connected devices. These include established standards such as Fieldbus, Profibus and Sercos, which are popular in applications requiring low data rates from 2Mbaud to 16Mbaud.

But demand from system developers has moved on since these standards were defined. The trend is towards higher data rates and an open architecture. For example, the recent introduction of a standard based on Internet Protocol, with additional features for real-time data transfer, now allows low-level, low data-rate communications between controllers, sensors and actuators to be integrated into an enterprise network. This will enable higher levels of integration throughout the network, allowing industrial processes to be initiated, monitored, adjusted and managed from a remote desktop away from the factory floor.

Industrial applications in the past tended to operate at data rates up to approximately 50Mbit/s. With this move to integrate factory and enterprise networks, much higher data rates extending up to 125Mbit/s are now commonly required, providing compatibility with the Fast Ethernet standard. Fiber is easily able to support these higher data rates over longer distances.

 

 

Copper-replacement Technology

Next-generation industrial networks, then, must be capable of supporting higher requirements going forward. When designing a new network, a fiber-optic infrastructure provides a number of advantages over conventional copper transmission lines. For instance, fiber infrastructures create no electromagnetic interference and are also immune to EMI generated externally. This makes fiber-optic links highly suited to use in electrically noisy environments, such as power distribution and switching equipment. This immunity also allows fiber-optic cabling to be carried along the same infrastructure as power lines and close to equipment carrying high currents such as power supplies and heavyduty electrical motors.

Apart from cost considerations, certain applications demand an alternative to copper for safety reasons. In particular, applications that have high electrical fire risks or that are exposed to hazardous chemicals can only be implemented safely and cost-effectively with fiber. Fewer precautions are required when using fiber in such hazardous environments, and less time and money need to be spent on the routing of the transmission line. With copper, by contrast, there is an inherent risk of dangerous Electro-Static Discharges (ESD), which can require expensive shielding or awkward routing of copper wires.

In addition the following benefits of optical fiber can also be realized:

• Greater link lengths without the need for amplification or regeneration of the signals
• Provides galvanic isolation between transmitter and receiver
• Low weight makes routing and handling easier
• Tighter bend radii and greater tolerance of rough handling

Although the maturing of fiber-optic technologies combined with economies of scale, have driven-down equipment and installation costs, the cost-effectiveness of a fiber infrastructure remains dependent on the application. The dominant parameters are the data rate and the link length, as well as environmental factors including fire risks and EMI. For a fair comparison, long-term costs should also be considered. Certainly, the initial costs can be higher for a short, low-data-rate optical link than for an equivalent length of copper. However, long-term savings are likely to accrue from reduced maintenance and the potential for upgrading.

 

Selection and Performance Criteria

Among optical-fiber cables, multi-mode fiber tends to be the most widely used in industrial applications. Advantages include greater tolerances with regard to alignment, optical coupling and cleanliness compared to single-mode fiber. As far as fiber materials are concerned, Plastic Optical Fiber (POF) and Hard Clad Silica (HCS) fiber are both used, though POF is the more common.

Both types have a relatively large core, typically 900µm for POF and 200µm for HCS, which enables efficient coupling of the light into and out of the fiber. A large core also reduces demands on connector alignment so that mechanical tolerances can be relaxed.

A fiber-optic communications link can be characterized by the data rate it can support and the length of the link over which it can operate while maintaining an acceptable bit error rate. Two physical effects determine the link length and data rate that a fiber link is capable of supporting. These are attenuation and dispersion.

Attenuation is dependent on wavelength and is a function of the material properties of the fiber. It is measured in dB/km. Dispersion is a more complex effect that limits the maximum data rate that can be sustained for a given link length. It can be expressed in terms of a bandwidth-distance product, in units of MHz.km, to estimate acceptable combinations of link length and data rate for a given fiber. A figure of 500MHz.km means a data rate of 500MHz can be supported over a link of 1km or a 50MHz data rate can be supported over a 10km link.

The most commonly used optical transmitter in industrial applications is the Light Emitting Diode (LED); the Vertical Cavity Surface Emitting Laser (VCSEL) is less commonly found. Both are easily capable of meeting data-rate and link length requirements for industrial applications, and are available at competitive prices as a result of extremely high production volumes.

A PIN photodiode is commonly used at the receiver. Again, these are available at low cost, and have a wide wavelength-response from 400nm to 1100nm enabling use with LED or VCSEL transmitters in industrial applications.

 

 

Implementing Fiber Links in the Real World

Examining the implementation of a common 5Mbaud link over a few hundred meters provides an illustration of the key design parameters that need to be considered when selecting optical transmission equipment.

Following selection of the cable, transmitters and receiver, these must be integrated with each other and also with the electrical infrastructure at either end. The integration tasks are relatively straight-forward, and are usually aided by manufacturers’ development boards. Avago Technologies, for example, supports its portfolio of fiber-optic products with over 15 evaluation kits, which help engineers quickly set up optical links to finalize the design of the transceivers and verify performance.

A component series such as the Avago HFBRx4xx family of discrete transmitters and receivers provides a good example. These can support link lengths of up to 2.7km or data rates up to 160Mbaud, but the actual data rate and link length achievable depends on the combination of transmitter, receiver and fiber type used. This series is compatible with 50/125, 62.5/125 and 100/140 multi-mode fibers using standard optical connectors.

The various transmitters and receivers in the product family offer different optical launch powers and receiver sensitivities. For the 5Mbaud link, the HFBR-14x4 transmitter and HFBR-24x2 receiver achieve 5Mbaud over a 2km link.

The transmitter is simply driven with a 74521 driver (see Figure 1). The drive current of the LED-based transmitter is determined by a resistor placed between the Vcc of 5V and the input of the driver. To help designers optimize the resistor value for a given set of operating conditions, Avago provides graphs of the link design limits for each product and fiber type.

For example, to operate over a shorter link length of 400m at a data rate of 5Mbaud, the drive current can be reduced from the 60mA nominal required for operation over 2km. Referring to Figure 2, a drive current of approximately 15mA is required for a link length of 400m. Calculation of the required resistor is then:

R=(Vcc – Vf of the LED) / drive current

In this case a 233Ω resistor is required.

In contrast, with the common perception of fiberoptic systems, this circuit is easy to implement.

 

Transceiver for Industrial Fast Ethernet

With the introduction of the Avago AFBR-5978Z transceiver for industrial communications, it is also now possible to implement Fast Ethernet over POF or HCS links. This industrial Fast Ethernet (10/100) transceiver features an integrated digital-monitoring capability and full RoHS compliance. It incorporates a 650nm LED transmitter and PIN-photodiode receiver, and is able to support Fast Ethernet over 50m (POF) and 100m (HCS) links. The electrical inputs and outputs use differential LVPECL signaling to maintain data integrity at the high signal speeds. The digital-monitoring interface provides real-time access to a number of transceiver parameters such as temperature, supply voltage and optical power, to support diagnostic and protection features.

 

Summary

Fiber-optic transmission has now become a viable alternative to copper-based data communications in the factory. Inherent advantages in terms of speed, range, noise immunity and electrical isolation should make fiber the transmission medium of choice for most industrial applications. While cost can still be a limiting factor for short data links operating at low speeds, the added advantages of an upgrade path to longer transmission distances and higher data rates should also be taken into consideration.