SUMMARY OF THE INVENTION It is an object of the present invention to achieve a slip level of a wheel that improves the stability and steering of a vehicle. To this end, all of the wheels are unstable, all the wheels are in a state of low pressure for a long time, The recovery flag is set for all the wheels. When all of the recovery flags are set, vibration of the four-wheel drive shaft is present, and the reference speed is not in a negative correction state over a long period of time, the slope of the reference speed of the vehicle is limited to a fixed relatively small slope. When one or more of the recovery flags is set, it is possible to take the flatness of the vehicle reference speed by gradually applying the calculated vehicle acceleration, and other treatments containing a longer pressure maintenance and a delay of the pressure increase. Only when a specific wheel is completely stabilized, the recovery flag is reset individually for each wheel. The presence of the vibration of the four-wheel drive shaft is determined based on the difference between the sum of the speeds of the front baruchas and the sum of the speeds of the rear wheels. This difference is also filtered to determine whether the vehicle is a quadratic gyro.
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