Multi frequency photoelectric detection system

Abstract

Collision avoidance apparatus ( ) is mounted on a host vehicle ( ) to detect the presence of an adjacent vehicle ( ) within a monitored zone ( ). A sensor module ( ) includes a plurality of discrete sensor systems for monitoring unique portions of the monitored zone ( ), each of which comprises an emitter ( ) emitting a fixed frequency modulated beam ( ) of light energy ( ) with a predetermined burst length into its unique portion of the monitored zone ( ), and a receiver ( ) having a field of view ( ), substantially aligned with emitted beam ( ) to produce a fixed optical overlap area (A), and for sensing light energy ( ) and generating a received signal. A frequency generator ( ) produces two or more predetermined different fixed frequencies for use by the emitter ( ) to emit beams ( ) at each frequency. A phase shifter ( ) produces a phase shifted reference signal with a different predetermined phase delay relationship to each fixed frequency. A mixer ( ) mixes each received signal and each phase shifted reference signal to produce demodulation signals from for each frequency. A processor ( ) receives and processes accoupled logic level detection signals generated by a limiter amplifier ( ) for each discrete sensor system at each of the fixed frequencies to determine the presence of a vehicle ( ) in the monitored zone ( ). The fixed frequency and the predetermined phase delay are chosen to produce a null received signal resulting from reflections of the emitted beam from uniformly distributed atmospheric backscatter within the optical overlap area (A).