This application is based on and incorporates herein by reference Japanese Patent Application No. 2000-173340 filed Jun. 9, 2000.
The present invention relates to a raindrop detecting method and apparatus that compensate for temperature-dependent output variations.
A conventional raindrop detecting apparatus detects raindrops by emitting a light from a light emitting device and receiving a reflected light by a light receiving device, because the reflected light varies with raindrops. The light emitting level of the light emitting device and the output signal level of the light receiving device change with ambient temperature. As a result, the relation between the raindrop and the output signal level of the light receiving device varies with ambient temperature, thus causing error in the raindrop detection.
JP-A-11-326186 proposes to compensate for this temperature-dependent detection output variation by adjusting the gain of an amplifier used in the light receiving device. However this adjustment is not satisfactory, because it is made uniformly or evenly to a plurality of apparatus units of the same type, when the amplifier is assembled in a raindrop detecting apparatus.
It is an object of the present invention to provide a raindrop detecting method and apparatus, which is capable of compensating for detection output variations from unit to unit even after manufacture of the apparatus.
According to the present invention, a raindrop detecting apparatus has a raindrop sensor, a temperature sensor, a processor and an EEPROM. A temperature characteristic data is defined by a first temperature data of the temperature sensor and a first output data of the raindrop sensor produced at a first temperature condition, and stored in the EEPROM. The processor calculates a raindrop detection threshold and compares an output data of the raindrop sensor with the raindrop detection threshold to detect a raindrop, while converting at least one of the characteristic data and the output data in correspondence with a temperature data produced at a raindrop detection time.
Preferably the characteristic data is defined and stored before the apparatus is installed in a vehicle. The processor updates the raindrop detection threshold by using the stored characteristic data, a second temperature data and a second output data produced at a second temperature condition, when an engine is started under no-raindrop condition.