Fog detector and method of placing detector in vehicle

A fog detector placed in a vehicle interior that is segregated from an exterior by a translucent protective wall, the fog detector comprising:

• an emitting/receiving device, the emitting/receiving device includes a light emitting element and a light receiving element, the light emitting element and the light receiving element are placed in such a manner that the light axes of the elements cross each other;
• a signal processing device, the signal processing device provides transmission signals having predetermined pulse width to the light emitting element, the signal processing device being arranged to detects a signal level of the received signals provided from the light receiving element, and
• a determination processing device that determines the density of fog based on detected signals detected by the signal processing device, wherein the detected signals are detected at a timing delayed from the transmission signals for the time that is needed for the emitted light from the light emitting element to reach the light receiving element via reflection at a crossing point of the light axes,
• wherein the emitting/receiving device is placed in such a manner that the light axes of the emitting/receiving device pass through the protective wall, the crossing point of the light axes is provided at an exterior of the vehicle, a difference distance between a light pathway length of light waves that roundtrip the crossing point of the light axes and a light pathway length of light waves that reflects at the protective wall is ΔR;
• in such condition a pulse width “Tp” of the transmission signals satisfies the following formula, in which “c” is the speed of light:
• ΔR/c>Tp.

A fog detector placed in a vehicle interior that is segregated from an exterior by a translucent protective wall, the fog detector comprising:

• an emitting/receiving device that includes a light emitting element for emitting a light waves and a light receiving element for receiving light waves, wherein the light emitting element and the light receiving element are placed in such a manner that the light axes of the elements crosses at a crossing point of the light axes;
• a signal processing device that provides a transmission signal having a predetermined frequency to the light emitting element for emitting the light waves and performs a synchronous detection of a received signal corresponding to the received light waves provided from the light receiving element so as to extract a signal corresponding to a reflected light waves reflected from the crossing point of the light axes, wherein the synchronous detection is performed using a reference signal having the same frequency as the transmission signal and the same phase as the reflected light waves reflected from the crossing point of the light axes;
• and
• a determination processing device that determines whether or not fog exists based on results of the synchronous detection performed by the signal processing device
• wherein the emitting/receiving device is placed in such a manner that the light axes of the emitting/receiving device pass through the protective wall, the crossing point of the light axes is provided at the exterior of the vehicle, the difference distance between a light pathway length of light waves that roundtrip the crossing point of the light axes and a light pathway length of light waves that reflect at the protective wall is ΔR;
• in such condition the wavelength of the transmission signals λ satisfies the following formula:
• ΔR=λ/4.

A placement method of a fog detector placed in a vehicle interior that is segregated from an exterior by a translucent protective wall, the fog detector having:

• an emitting/receiving device that includes a light emitting element for emitting light waves and a light receiving element for receiving light waves, wherein the light emitting element and the light receiving element are placed in such a manner that the light axes of the elements cross at a crossing point of the light axes;
• a signal processing device that provides a transmission signal having a predetermined frequency to the light emitting element for emitting the light waves and performs a synchronous detection of a received signal corresponding to the received light waves provided from the light receiving element so as to extract a signal corresponding to a reflected light waves reflected from the crossing point of the light axes, wherein the synchronous detection is performed using a reference signal having the same frequency as the transmission signal and the same phase as the reflected light waves reflected from the crossing point of the light axes;
• and
• a determination processing device that determines whether or not fog exists based on results of the synchronous detection performed by the signal processing device wherein the method comprises:
• placing the emitting/receiving device in such a manner that the light axes of the emitting/receiving device pass through the protective wall;
• having a crossing point of the light axes at an exterior of the vehicle;
• and
• having the placement position of the emitting/receiving devices satisfy the following formula, in which “ΔR” refers to a difference distance between a light pathway length of light waves that roundtrip the crossing point of the light axes and a light pathway length of light waves that reflect at the protective wall, and “λ,” refers to the wavelength of the transmission signals:
• ΔR=λ/4.