Optical pulse reflections in the signal paths of a fly-by-light flight control system employing time division multiplexing can interfere with data returning date to the receiver. To determine how reflections of the interrogation pulse may interfere with optical data signals returning from an optical transducer to the flight control computer. Fresnel reflection theory and geometric optics are used to predict the intensity and return times of the pulse reflections at the receiver. A channel with multiple connectors and a 12-bit position transducer is considered as an example of a typical channel in a fly-by-light flight control system. The optical power and return times of the pulse reflections in the example channel are predicted and are compared with the optical power and return time window of the returning data signal. To check the analytical prediction, a dynamic simulation of the example optical channel is used to model the signal behavior. Although the analysis shows that the reflections from the connector interfaces can interfere with the transducer data by arriving at the same time as the data, these spurious signals can be rejected if the optical receiver is designed properly. It is recommended that the Fresnel reflection intensity, the number of disconnects in the interconnect cable and the relative insertion loss of the interconnect cable compared to that of the transducer must be minimized to warrant reliable operation and simpler receiver design.
Fiber optic technology has been implemented within diverse areas of aircraft vehicle management systems, including propulsion and flight controls. At least four different fiber- based technologies have been demonstrated in the laboratory and some have accumulated flight hours while installed in technology demonstration aircraft. Some key technologies developed thus far include Time Division Multiplexing (TDM), Wavelength Division Multiplexing (WDM), dual wavelength analog, and Ladar. Some variations in these technologies have also been shown to have promise, such as transmissive vs. reflective encoders, where the number of interconnect fibers are reduced. TDM technology has been actively developed since 1982 as a result of the US Army sponsored Advanced Digital Optical Control System program. This paper provides an overview of the TDM technology and its status when viewed in light of the key flight control system requirements. Description of the TDM sensor concept, the associated electronics, delay line fiber technology and fiber connector requirements is provided. A comparison with WDM technology is also described.
All-passive fly-by-light technology has been considered to replace the conventional fly-by-wire control systems in aircraft. Both the sensors and their associated interfaces providing data to the flight control computers have been developed. Primarily, two types of electro-optic interfaces and four different types of Fiber Optic technologies have been demonstrated for sensors/switches. Comparison data shows the development of passive TDM flight control technology is near production-ready, but the digital optical position sensors and switches are prohibitively expensive. Two innovative approaches are required for improving producibility and cost of the TDM sensors. Sensors based on other techniques require substantial development to meet system requirements and are still in their infancy. System, device and manufacturing engineers must work closely to implement system requirements and concurrent engineering approaches at early stages if the fiber optic technology is to move from the laboratory to production aircraft.
Several advantages may be realized when implementing fiber optic technology in flight controls. However, the system designer must consider maximum multiplexing of fiber optic sensors while maintaining system reliability in the flight control architecture to fully exploit the technology. Analysis of fiber optic technology conducted at less than the system level may not reveal the full advantages.
A fiber optic speed sensor (FOSS) has been developed, bench tested and rig tested in a real turbine airflow
environment. The FOSS employs an innovative design using a pressure tube and fiber optic microbend transducer in
order to capture turbine blade pass wake frequency. The blade pass frequency can be converted by a signal processor
into turbine rotational speed. The FOSS offers unique potential to meet future requirements for performance (0 to 25
KHz) and environmental tolerance (1200 F temperatures and EMI/EMP threats). Future efforts include development of
the signal processor and environmental/durability testing focused on developing reliable, long life operation in the
hostile environment of an advanced gas turbine engine.
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SC382: Photonics in Communication and Sensing: Smart Design Tips
This course provides sample case histories of previously developed optical communication and fiber optic sensing systems with emphasis on smart design techniques. Engineering techniques that minimize development time are discussed to assist the attendee in bringing a system to market in a timely fashion. Audience specific problems will be addressed and discussed.