Patent ID: 7263031

Claim:
A distance measuring device comprising: an acoustic signal transmitter for transmitting an acoustic signal towards a distant object, an acoustic signal receiver for receiving a reflected acoustic signal reflected from the distant object, a temperature sensor for detecting air temperature, a humidity sensor for detecting air humidity, an amplifier coupled to the receiver for amplifying the reflected acoustic signal to produce an amplified reflected acoustic signal, a comparator coupled to the amplifier for comparing level of the amplified reflected acoustic signal with a reference level and generating a comparator output when the level of the amplified reflected acoustic signal exceeds the reference level, a gain controller for controlling gain of the amplifier and increasing the gain from time of transmitting an acoustic signal towards a distant object until the comparator output is generated, a threshold generator for providing the reference level to the comparator based on both the air temperature detected and the air humidity detected and decreasing the reference level at an exponential rate of decrease from the time of transmitting the acoustic signal until the comparator output is generated, and a controller for determining the distance to the distant object using, only the air temperature detected, velocity of the acoustic signal, and the time of transmitting the acoustic signal until the comparator output is generated.