Apparatus and method for estimating a physical quantity and a device and method for an ABS control

In a bandpass filter (12) is a wheel speed for each wheel that has been detected by a wheel speed sensor (10) is input. Signals of frequency bands that do not relate to the unsprung resonance are then removed from the wheel speed signal, and only signals of frequency bands, which refer to the unsprung resonance are output. Means (14) for estimating a road surface mu gradient used an on-line investigation to determine from the signal from the bandpass filter 12, an attenuation ratio of a second order, which is similar to a suspension and tire resonance model resonance model, has been issued. Subsequently, the road surface mu gradient is estimated from the determined damping ratio. The damping ratio of the second-order resonance model corresponds to the road surface mu gradient in the following manner: when it is determined that the damping ratio is small, it is estimated that the road surface gradient mu is large; and if it is determined that the road surface mu-gradient is large, it is estimated that the Damapfungsverhaltnis is small.