Fiber-Optic Bus Lines
Fiber-optic Bus Lines
Introduction
Transmission of data, voice and images involves ever increasing volumes of data. Fiber-optic technology (already in use in tele-communications and industrial installations) is capable of handling such large volumes of data while at the same time offering additional advantages.
High data transmission rates in copper wires cause electromagnetic interference. This can interfere with other vehicle systems. Compared with copper wires, fiber-optic lines require less space and are lighter in weight for the same transmission band width. In contrast with copper wires, which carry digital or analog voltage signals as the means of transmitting data, fibre-optic busses transmit light. BMW uses plastic (POF-polymer optical fibers) cables manufactured by Dow-Corning.
Advantages of Plastic Fiber-optic Lines
Because of the large cross section of the fibers, the positioning of the fiber core does not present a significant technical problem in production.
Relatively Low Sensitivity to Dust.
Even with the most careful precautions, dust can find its way onto the surface and alter the amount of light entering/leaving the optical fibre. Small amounts of contamination do not result in communication failure.
Easy to Work With.
The fiber (core) is approximately 1 mm thick and easier to handle than glass optical fiber with a thickness of approximately 62.5 micro mm, for example. In comparison with a glass fiber-optic cable, it is considerably easier to handle. The simplest way of putting it is that glass breaks and plastic doesn't.
Processing.
The optical fiber material used by BMW is polymethyl methacrylate
(PMMA) which, in comparison with glass optical fibers, is relatively easy to cut, smooth down or melt which offers major advantages with regard to the production of wiring looms and in repair work.
Economical Costs.
All of the above characteristics of plastic fiber-optic cables ultimately make them relatively cheap to connect to transmitters and receivers (connectors, casings). The ease with which the fiber ends can be prepared makes assembly and installation in the field in particular very economical to perform.
Structure of a Fiber-optic line
Principle of Light Transmission
The electrical signal generated by the control unit is converted into an optical signal by an internal transmitter module and sent along the fiber optic bus. The fiber core carries the light beam.
In order to prevent the light escaping, the fibre core is enclosed by a cladding layer. The cladding is reflective and reflects the light back into the core, thus making it possible to transmit light along the fibre.
In this way, the light passes along the optical fibre and is ultimately converted back into an electrical signal by a receiver module in side the recipient control unit.
Light Attenuation
Attenuation refers to the reduction in strength of a signal. The light transmitted along the optical fiber becomes weaker the further it has to travel. Attenuation is usually measured in decibel units (dB). In fiber optic cables attenuation is measured in terms of the number of decibels per unit of length (foot/meter etc..) The less the attenuation per unit distance the more efficient the cable.
Average attenuation in the E65/E66 fiber optic bus lines:
- .5dB at each connector
- .3dB for each meter of cable
Special considerations when handling fibre-optic cables
Excessive attenuation can be caused by the following reasons:
- Bends in the fiber-optic cable with a radius smaller than 50mm.
- Kinks in the fiber-optic cable.
- Squashed or compressed fiber-optic cable.
- Damaged insulation on fiber-optic cable.
- Stretched fiber-optic cable.
- Dirt or grease on the exposed cable ends.
- Scratches on the exposed cable ends.
- Overheated fiber-optic cable.
Bending radius
The plastic fibre-optic cable should not be bent to a radius of less than 50 mm. That is roughly equivalent to the diameter of a soft-drink can. Bending the plastic fiber-optic cable any tighter than that can impair its function or even damage it irreparably.
Light can escape at points where the cable is bent too tightly.
This is caused by the fact that the light beam strikes the interface between core and cladding at too steep an angle and is not reflected.
Fibre-optic cables must not under any circumstances be kinked when fitted because this damages the cladding and the fibre core. The light is then partially dispersed at the point where the fibre is kinked and transmission loss results. Even just kinking an optical fibre once very briefly is enough to damage it!
Compression Points and Stretching
Compression points must also be avoided because they can permanently deform the light-conducting cross section of the optical fiber.
Light is then lost during transmission.
Pulling fibre-optic cables too tight can also cause compression points as it increases the lateral pressure on the cable. The effect is exaggerated at higher temperatures because the cable straps then become tighter as well. The fiber-optic cable is then constricted, the attenuation increases at that point and function is severely impaired.
Overstretching of fibre-optic cables, caused by pulling for example, can destroy them,
Stretching reduces the cross-sectional area of the fibre core. Restricted passage of light (attenuation) is the result.
Abrasion Points
In contrast with copper wires, abrasion of fiber-optic cables does not cause a short circuit.
Instead, loss of light or intrusion of external light occurs. The system then suffers interference or fails completely.
Dirty or Scratched Fiber-optic Cable Ends
Another potential source of problems are dirty or scratched cable ends. Although the ends of the cables are protected against accidental contact, they can still be damaged by incorrect handling.
Dirt on the end of an optical fiber will prevent the light entering/exiting the fiber. The dirt absorbs the light and the attenuation is then too great.
Scratches on the cable ends scatter light striking them so that the amount of light reaching the receiver is reduced.
Overheated Fibre-Optic Cables
The demands of the new technology even have to be taken into account in subsequent repair work such as painting. As fiber-optic cables are damaged by heat, care must be taken to ensure that the maximum permissible temperature of 85 °C is not exceeded when drying paintwork.
Notes on Working With Fiber-optic Cables
Extreme care is required when working on vehicle wiring harnesses, unlike copper wiring damage does not immediately result in a fault but may come to the owner's notice later.
Notes on Repairing Fiber-optic Cables
The MOST fiber-optic cable may be repaired once between each control unit. The byteflight cannot be repaired at present. A special crimping tool will be available to the service department for crimping the new "optical fiber sleeves." The precise procedure are described in the instructions for using the crimping tool.