Digital Angular Position Sensor Using Wavelength Division Multiplexing

Future aircraft will use fly-by-light control systems with fiber-linked optical sensors for such measurands as temperature, pressure, and linear and angular position. We describe a digital optical sensor which was developed to transmit the angular position of such slowly rotating parts as a throttle or fuel flow control valve on an aircraft. The sensor employs a reflective code plate with ten channels providing a resolution of 0.35 degrees. Two light-emitting diodes with overlapping spectra are used as light sources. A single micro-optic multiplexer-demultiplexer composed of a GRIN rod lens and a miniature grating is used to disperse the spectrum and recombine the spectral components from each channel after reflection by the code plate. We discuss the results of preliminary environmental tests of this unit. We have operated the sensor for brief periods of time between -60°C and +125°C without adverse effects. Preliminary vibration tests indicate that the unit will work properly at the maximum vibration levels expected in a jet-engine environment. Use of a reference track on the code plate in conjunction with an automatic gain control in the electronic circuit compensates for large changes in transmitted light intensity.