In this paper, a new method for estimating the tire pressure of a vehicle and a pneumatic monitor based on this method are proposed. Most of the monitors presently available are of an air pressure switch type and of a loaded radius type. The former is higher in manufacturing cost, and the latter has difficulty in detecting natural leaks causing all tires to be underinflated. In our proposed method, the variation of the tire stiffness which depends on the pressure is estimated by using a disturbance observer. The tire stiffness is one of the parameters constructing a system. The disturbance observer estimates parameter variations of the system as a disturbance and we can detect the variation value by using the least mean square method. Our method is realized at lower cost because it needs only wheel speed sensors already provided for an anti-lock brake system (ABS), and detects underinflated tires in every situation including a puncture and natural leaks. The monitor is installed on a vehicle, and its performance is confirmed by experiments.
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