To enhance estimation accuracy of the friction factor of a road surface by suppressing effects of noises, etc., included in a measured value of a sensor.
By establishing a function (μ-s function) analogous to the characteristic (μ-s characteristic) of road-surface friction factors μ relative to slip ratios λ, a road-surface friction factor μqu corresponding to a slip ratio is computed (7e). An estimated wheel speed xwpr corresponding to the computed friction factor μqu is computed based on a wheel speed xw, driving/braking torque u, the computed friction factor μqu, a wheel load W, and the moment I of inertia of wheels, and an error e is computed by subtracting the estimated speed xwpr from the wheel speed xw (7f). A calibration parameter θpr for calibrating the μ-s function is computed based on the wheel speed estimation error e, the computed friction factor μqu, the wheel load W, and the moment I of the wheels (7j). The parameter μpr is corrected by being converged in the proximity of a plurality of calibration parameters θi (i=1, 2, etc.), previously established relative to various road surface conditions (7g to 7i). The computed friction factor θqu is calibrated by the parameter θpr to find an estimated road-surface friction factor θpr (7k).
KAWABE TAKETOSHI
SHIN SEIICHI
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