Braking control device for vehicle

1. A braking control device for a vehicle comprising:

a control unit that is configured to respectively control front-wheel braking power, which is braking power to be applied on a front wheel, and rear-wheel braking power, which is braking power to be applied on a rear wheel, based on braking power distribution, which indicates distribution between the front-wheel braking power and the rear-wheel braking power, and requested braking power which indicates a braking power requested value for the vehicle;

a distribution setting unit that is configured to set the braking power distribution; and

a slip judgment unit that is configured to judge that the vehicle is in a rear-wheel-led slip state when the amount of slip of the front wheel is smaller than a slip judgment value while the amount of slip of the rear wheel is equal to or larger than the slip judgment value, wherein

the distribution setting unit executes a distribution shifting process of shifting the braking power distribution to second braking power distribution within a predetermined period if it is judged that the vehicle is in the rear-wheel-led slip state when the braking power distribution is first braking power distribution,

the second braking power distribution is such distribution that the rear-wheel braking power is reduced relative to that observed when the first braking power distribution is set as the braking power distribution, and

when the distribution shifting process is executed, the control unit

respectively derives rear-wheel reference braking power, which is a reference value of the rear-wheel braking power, and front-wheel reference braking power, which is a reference value of the front-wheel braking power, based on the braking power distribution having been shifted and the requested braking power, and

controls the rear-wheel braking power so that the rear-wheel braking power is equal to or smaller than the rear-wheel reference braking power, and controls the front-wheel braking power so that the front-wheel braking power is equal to or larger than the front-wheel reference braking power.

2. The braking control device for a vehicle according to claim 1 , further comprising:

a μ value estimation unit that is configured to calculate, if it is judged that the vehicle is in the rear-wheel-led slip state when the braking power distribution is the first braking power distribution, an estimated road surface μ value, which is an estimated value of a μ value of a road surface, based on a rear-wheel load, which is a vertical component load applied on the road surface from a vehicle body via the rear wheel, and the rear-wheel braking power; and

a rear lock line derivation unit that is configured to derive a rear lock line, which is a line indicating the relationship between the rear-wheel braking power and the front-wheel braking power observed when the rear wheel is locked, based on the estimated road surface μ value thus calculated, wherein

the second braking power distribution is ideal braking power distribution being such braking power distribution that the front wheel and the rear wheel are locked at the same time, and

if the requested braking power is increased even while the distribution shifting process is executed, the control unit

derives the rear-wheel reference braking power, which is a reference value of the rear-wheel braking power, and the front-wheel reference braking power, which is a reference value of the front-wheel braking power, so that points representing the rear-wheel reference braking power and the front-wheel reference braking power are located on the rear lock line, and

controls the rear-wheel braking power so that the rear-wheel braking power is equal to or smaller than the rear-wheel reference braking power, and controls the front-wheel braking power so that the front-wheel braking power is equal to or larger than the front-wheel reference braking power.

3. The braking control device for a vehicle according to claim 1 , wherein, when the distribution shifting process is executed, the control unit

executes a rear-wheel slip reduction process of reducing the amount of slip of the rear wheel by varying the rear-wheel braking power on the basis of the rear-wheel reference braking power, and

when executing the rear-wheel slip reduction process, executes a front-wheel braking correction process of making the front-wheel braking power larger than the front-wheel reference braking power based on a value obtained by subtracting the rear-wheel braking power from the rear-wheel reference braking power.

4. The braking control device for a vehicle according to claim 3 , further comprising a parameter acquisition unit that is configured to acquire a parameter indicating a yawing movement of the vehicle, wherein

in the front-wheel braking correction process, the control unit adjusts the front-wheel braking power so that a difference between the front-wheel reference braking power and the front-wheel braking power may be changed with a change speed according to the parameter.