Ultrasonic surveillance system provided with erroneous operation preventing function

Abstract From the Canadian patent

An ultrasonic surveillance system which emits pulse- -like ultrasonic waves to an area under surveillance which statistically compares a reflected-wave pattern established by receiving ultrasonic waves reflected from the under surveillance with a predetermined reference pattern, and which produces an alarm signal when these two patterns are not in agreement. In order to exclude ultrasonic noise having a relatively small amplitude from the reflected-wave pattern, a difference component between a pattern of frequencies of emitted ultrasonic waves and a pattern of the other frequencies is used as the reflected pattern. The frequency components other than those of emitted ultrasonic waves are …