Patent ID: 7333048

Claim:
A periphery monitoring system comprising a Doppler sensor, a signal output from the Doppler sensor being subjected to an FFT analysis such that a predetermined frequency range is divided into a predetermined number of frequency bands, each frequency band having a predetermined bandwidth, a frequency level being calculated for each of the frequency bands, a movement of a mobile object around an installation location of the Doppler sensor being monitored based on the frequency levels of the frequency bands, the system further comprising: a calculation section for calculating a total sum of the frequency levels of all of the frequency bands at predetermined time intervals; a reference level setting section for setting a reference level based on the total sum calculated by the calculation section; an abnormal level setting section for setting an abnormal level by adding a predetermined level to the reference level set by the reference level setting section; and a determination section for determining whether the periphery state is abnormal or normal based on a comparison between the total sum calculated by the calculation section and the abnormal level set by the abnormal level setting section, wherein the determination section is configured such that if the total sum exceeds the abnormal level and falls to or below the abnormal level before a first set period passes since the exceeding of the abnormal level, the determination section determines the periphery status to be abnormal, and if, before the determination of abnormality, the total sum does not exceed the abnormal level or the total sum exceeds the abnormal level but does not fall to or below the abnormal level even after the first set period passes since the exceeding of the abnormal level or if, after the determination of abnormality, the total sum does not exceed the abnormal level continuously for a second set period or more, the determination section determines the periphery status to be normal, and the reference level setting section is configured such that, if the total sum exceeds the abnormal level but does not fall to or below the abnormal level even after the first set period passes since the exceeding of the abnormal level, the reference level setting section updates the reference level to a new reference level which is set based on a total sum calculated during the first set period.