AUTOMOTIVE FIBER: Plastic optical fiber builds on MOST success

European auto manufacturers have rapidly implemented plastic optical fiber, in part because of the MOST Cooperation standard.

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European auto manufacturers have rapidly implemented plastic optical fiber, in part because of the MOST Cooperation standard. Now the standard is paving the way for plastic optical fiber in other consumer markets.

Paul Polishuk

In just the past three years, European automotive manufacturers have installed 25 million nodes of plastic optical fiber (POF) in more than 40 vehicle models. The Media Oriented Systems Transport or MOST Cooperation (Karlsruhe, Germany; www.mostcooperation.com) is the name for the group of automakers, suppliers, software developers, and end users continuing to develop the optical-fiber standards for higher speeds, higher temperatures, and tighter bend radii within automobiles.

Formed in 1998 by BMW and DaimlerChrysler, among others, the MOST Cooperation was established principally to define the MOST technology-a multimedia fiberoptic network with standard hardware and software interfaces optimized for automotive applications. As the major investment in this standards activity increases and large-volume purchases drive prices down, suppliers and automakers are looking to expand the use of the MOST standard into other areas such as consumer electronics and home networking.

Why POF?

When the European auto manufacturers were looking for a replacement for copper-wire harnesses in automobiles, they wanted a technology that was readily available, easy to terminate, immune to radio frequency and electromagnetic interference, and low in cost. Step-index POF was readily available and even though the losses were high compared to glass, it was a proven commodity and definitely cheap. Further, low-cost light-emitting diodes (LEDs) could be used and simple connectors could be molded. The use of glass was considered but rejected because of its high cost, connector issues, and limited bend radius.

Today, MOST systems are requiring higher data speeds (up to 150 Mbit/s), higher operating temperatures (to 125°C), and tighter bend radii. Though POF suppliers are scurrying to meet these requirements, plastic-clad silica fibers and multimode glass fiber bundles are also being considered.

An area of concern when using POF in automotive applications is the availability of light sources that deliver the needed modulation speeds. Because standard LEDs have inherently low modulation speed, resonant-cavity LEDs have been developed with higher bandwidth and improved coupling efficiency that are capable of modulation speeds of up to 200 Mbit/s and more. Several suppliers, including Firecomms (Cork, Ireland), TrueLight (Hsinchu, Taiwan), and Astri (Hong Kong, China), are now offering these devices commercially.

Automotive applications

The most important use for POF in automobiles is for the crucial purpose of passenger safety. In many instances, safety requires communication between sensors embedded in the vehicle and other devices, such as air bags, that are used to ensure safety. While using POF for other non-safety-oriented applications is becoming more prevalent, automakers typically reserve these uses for the automotive aftermarket-those (usually optional) devices that replace initial factory-installed equipment.1, 2

The first serious intravehicle communication application for POF sensors can be found in BMW automobiles. BMW has developed a POF optical network for internal use operating at 10 Mbit/s-a star network that is used to communicate with the air-bag sensors. This network uses the same components and technology developed for the MOST standard. When a vehicle collision occurs, information from force sensors within the vehicle is interpreted and communicated over the POF network to the individual air bags to control their inflation. This is the first time that POF has been used for such a “mission-critical” application.

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FIGURE 1. Sensors using plastic optical fiber embedded within the framework of an automobile can be used to provide pedestrian protection. The transmission signal changes when a surface-treated zone on the fiber is bent. Analysis of the signal from several parallel fiber strands allows the system to determine the nature of the object impacting the vehicle. The sensor can distinguish between a human and an inanimate object, triggering the hood to lift and softening the impact if a human is struck.
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In addition to protecting vehicle occupants, POF is being used for pedestrian-protection systems. European Union directive 2003/102 mandates pedestrian protection in every new car manufactured from 2005 on, either by passive or active methods. Passive methods include structural measures such as “soft” front ends and sufficient deformation room between the hood and the engine. Active methods can be sensors that identify impact with a pedestrian and then trigger protective action such as using actuators to lift the hood (see Fig. 1).

The basic principle behind the active pedestrian protection systems is a cladding surface treatment of the fiber at discrete zones along the fiber. Bending the fiber in one direction leads to a better transmission whereas bending in the other direction leads to a lower transmission, compared to the straight position. The spatial resolution is achieved by incorporating several fiber strands in parallel and using numerical signal analysis of the treated zones. Due to its high bandwidth, the sensor is able to distinguish between positive and negative bends and can determine the source of an impact with high temporal resolution. High resolution is necessary to distinguish between a human leg, an animal, or a lamp pole-an important distinction in determining whether to lift the hood of a vehicle in the event of a pedestrian or animal collision, or activate air bags or other safety features in the case of a nonpedestrian collision.3, 4

Plastic optical fiber can also be used for seat-occupancy recognition, resulting in cost reduction as well as increased safety. A sensor in the seat identifies whether the seat is occupied. If the seat is empty in the event of a collision, for example, the air bag will not deploy, saving the expense of repair and replacement. Seat-occupancy recognition could also be used to lower the headrests for improved driver visibility if the surrounding seats are not occupied.5

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FIGURE 2. Seat occupancy detection is useful to prevent expensive repair and replacement of air bags in the event of a collision. If the seat is unoccupied, for example, the air bag will not deploy. One available method is the use of special foam that responds to pressure, changing the amount of light transmitted to a plastic optical fiber for occupancy detection.
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One approach for seat-occupancy recognition is the Kinotex cavity sensor from Canpolar East (St. John’s, NF, Canada). The principle behind this method is light-scattering that is dependent on the compression of the scattering medium, such as special rubber foam. The transducer operates by detecting a change in energy intensity in and around an illuminated integrating cavity. Deformation of the integrating cavity by an external influence, such as pressure, results in a localized change to the illumination energy intensity, which can be transmitted by a plastic fiber and then measured (see Fig. 2). In addition to single-point occupancy detection, an embedded array of point sensors within the seat can be used to reveal information on the pressure load distributed over the area, allowing seat-pressurization and/or cushioning to maximize the occupant’s comfort.

Many consumers are probably aware of the use of tactile sensors that can stop a car window if an object is in the way. While many of these sensors are nonoptical, the evanescent field of an optical fiber can also be exploited in this application.

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FIGURE 3. The well-known phenomenon of the "evanescent field" within an optical fiber can be used as a tactile sensor. If an object disrupts the evanescent field, the light transmitted through an optical fiber shifts within its core. This shift can be detected with very high sensitivity, allowing the motion of a window to be stopped if an object is in its path.
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As is well known from theory, the optical rays do not reflect exactly on the boundary between two optical media. In fact, the rays (electromagnetic field) break into the adjacent medium and do not suddenly drop to zero at the core-cladding boundary. They decay instead, exponentially within the cladding. The penetration depth depends on the difference between the respective refractive indices of core and cladding material, the angle of incidence, and the wavelength of the incident light (see Fig. 3). In ray optics, this phenomenon is known as the “GoosHanchen-Shift,” in which the reentry of the ray into the core region is shifted along the geometrical core-cladding interface. The shift due to an object being placed within that evanescent field-such as an object in the path of a car window going up-can be detected and the window motion can be stopped. This type of evanescent sensor offers very high sensitivity without actual physical contact, and can be envisaged as a tactile sensor for robot gripper tools in other consumer and industrial applications.

Beyond the automobile

Today, MOST networks connect multiple devices within automobiles, including car navigation, digital radios, displays, cellular phones, and CD/DVD entertainment systems. In addition, other companies and organizations are creating network systems that utilize POF, such as Byteflight from BMW-a protocol that supports the increasing number of sensors and actuators within cars that improve vehicle safety.

It is easy to make the leap from automobile networking to in-home networking and consumer electronics, as these markets would share many of the same needs for sensing and communication. Based on the positive experience gained with POF systems for vehicle entertainment and safety systems, companies are now confident that POF sensors will meet the most challenging automotive requirements and comfortably move into consumer applications.

REFERENCES

1. O. Ziemann et al., Proc. 14th POF Conf., Hong Kong, China (2005).

2. D. Kalymnios et al., Proc. 13th POF Conf., Nuremberg, Germany (2004).

3. M. Miedrich and H. Schober, ATZ Worldwide107, 15 (March 2005).

4. B. L’Henoret, Proc. 13th POF Conf., Nuremberg, Germany (2004).

5. G. Kodl, Proc. 12th POF Conf., Seattle, WA (2003).

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