Patent ID: 7963157

Claim:
An apparatus for detecting a decrease in a tire air pressure, comprising: a rotation velocity information detection means for periodically detecting tire rotation velocity information for the respective wheels of a vehicle; a frequency characteristic estimate means for estimating, based on the rotation velocity information obtained by the rotation velocity information detection means, a frequency characteristic of the rotation velocity information; and a judgment means for judging, based on the estimated frequency characteristic, a decrease in the air pressure of the tire, wherein the frequency characteristic estimate means includes, with regard to a time-series signal including the rotation velocity information, a parameter estimate means for performing a first step of subjecting parameters of a three-order or more linear model to a time-series estimate; an input signal estimate means for performing a second step of estimating, based on the estimated linear model and the rotation velocity information that is an output signal of the linear model, an input signal to the linear model; a parameter identification means for performing a third step of identifying, based on the estimated input signal and the rotation velocity information, parameters of a linear model caused to have a lower order of a second order; and a resonant frequency estimate means for performing a fourth step of estimating, based on the parameters identified based on the model caused to have a lower order of the second order, a resonant frequency of the tire in a torsional direction, and wherein the judgment means is configured to judge a tire having a decreased air pressure based on the estimated resonant frequency of the tire in the torsional direction.