Fibre Optic Sensor
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A linear position sensor
An example of a linear position sensor using wavelength division multiplexing is illustrated in the image below. Here a broadband light source, which might be a light-emitting diode, is used to couple light into the system. A single optical fiber is used to carry the light beam up to a wavelength division multiplexing (WDM) element that splits the light into separate fibers that are used to interrogate the encoder card and determine linear position. The boxes on the card represent highly reflective patches, while the rest of the card has low reflectance. The reflected signals are then recombined and separated by a second wavelength division multiplexing element so that each interrogating fiber signal is read out by a separate detector.
Fig 1. A linear position sensor using wavelength division multiplexing decodes position by measuring the presence or absence of a reflective patch at each fiber position as the card slides by via independent wavelength separated detectors.
Fig 2. A linear position sensor using time division multiplexing measure decodes card position via a digital stream of ons and offs dictated by the presence or absence of a reflective patch.
A second common method of interrogating a position sensor using a single optical fiber is to use time division multiplexing methods. In Fig. 2 a light source is pulsed. The light pulse then propagates down the optical fiber and is split into multiple interrogating fibers. Each of these fibers is arranged so that the fibers have delay lines that separate the return signal from the encoder plate by a time that is longer than the pulse duration. When the returned signals are recombined onto the detector, the net result is an encoded signal burst corresponding to the position of the encoded card.
These sensors have been used to support tests on military and commercial aircraft that have demonstrated performance comparable to conventional electrical position sensors used for rudder, flap, and throttle position. The principal advantages of the fiber position sensors are
immunity to electromagnetic interference and overall weight savings.
These sensors have been used to support tests on military and commercial aircraft that have demonstrated performance comparable to conventional electrical position sensors used for rudder, flap, and throttle position. The principal advantages of the fiber position sensors are
immunity to electromagnetic interference and overall weight savings.
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