Damping control device and damping control method for vehicle

1. A damping control device for a vehicle, comprising:

a control force generating device configured to generate vertical control force between a vehicle body of the vehicle and at least one wheel suspended from the vehicle body by a suspension; and

an electronic control unit configured to reduce, by controlling the control force generating device to change the control force, vibration of the vehicle body that is caused by vertical vibration occurring in the wheel in response to vertical road surface displacements while the vehicle is traveling, the vertical vibration being transmitted to the vibration of the vehicle body via the suspension, wherein:

the suspension is configured to generate, in a vertical direction between the wheel and the vehicle body, a suspension force that is based on a relative displacement related value related to a vertical relative displacement between the wheel and the vehicle body;

the suspension force to be actually generated by the suspension is larger than a theoretical suspension force proportional to the relative displacement related value as an amplitude of the vibration occurring in the wheel due to the road surface displacements decreases, and the suspension force is theoretically represented by an expression including a spring term proportional to the relative displacement;

a magnitude of a component corresponding to the spring term in the suspension force to be actually generated by the suspension is larger than a magnitude of the spring term as the amplitude of the vibration occurring in the wheel due to the road surface displacements decreases; and

the electronic control unit is configured to

calculate an amplitude index indicating an amplitude of road surface displacement related values related to the road surface displacements in a predetermined sampling zone,

determine a control gain to increase, as the amplitude indicated by the amplitude index decreases, a magnitude of a target control force that is a target value of the control force at a timing when a predetermined period has elapsed from a current time,

calculate, based on the control gain and the road surface displacement related values at a predicted passing position where the wheel is predicted to pass at the timing when the predetermined period has elapsed from the current time, the target control force in a direction in which a component of the suspension force that corresponds to a value related to a vertical displacement of the wheel at the predicted passing position is canceled out,

control the control force generating device to generate the control force to agree with the target control force at the timing when the wheel passes through the predicted passing position, and

calculate the target control force at a magnitude of a value obtained by multiplying together the control gain and the road surface displacements determined based on the road surface displacement related values at the predicted passing position, and in a direction in which a component of the spring term that corresponds to the vertical displacement of the wheel at the predicted passing position is canceled out.

2. The damping control device according to claim 1 , wherein:

the electronic control unit is configured to

calculate time series variations of the road surface displacement related values in the sampling zone based on a speed of the vehicle at the current time and the road surface displacement related values in the sampling zone,

calculate a first amplitude index indicting an amplitude of road surface displacement related values in a predetermined first frequency band of the time series variations,

calculate a second amplitude index indicating an amplitude of road surface displacement related values in a predetermined second frequency band of the time series variations that has a minimum frequency equal to or higher than a maximum frequency of the first frequency band,

determine the control gain to increase as the amplitude indicated by the first amplitude index decreases, and to increase as the amplitude indicated by the second amplitude index decreases, and

calculate the target control force by multiplying together the control gain and the road surface displacement related values at the predicted passing position; and

an increase amount of the control gain through a decrease in the amplitude indicated by the second amplitude index is set larger than an increase amount of the control gain through a decrease in the amplitude indicated by the first amplitude index.

3. The damping control device according to claim 1 , wherein the electronic control unit is configured to:

calculate time series variations of the road surface displacement related values in the sampling zone based on a speed of the vehicle at the current time and the road surface displacement related values in the sampling zone;

calculate a first amplitude index indicating an amplitude of road surface displacement related values in a predetermined first frequency band of the time series variations;

calculate a second amplitude index indicating an amplitude of road surface displacement related values in a predetermined second frequency band of the time series variations that has a minimum frequency equal to or higher than a maximum frequency of the first frequency band;

determine the control gain to decrease as a ratio of the amplitude indicated by the second amplitude index compared to the amplitude indicated by the first amplitude index increases; and

calculate the target control force by multiplying together the control gain and the road surface displacement related values at the predicted passing position.

4. The damping control device according to claim 1 , wherein:

the suspension force is theoretically represented by an expression including a damping term proportional to a speed of the relative displacement;

a magnitude of a component corresponding to the damping term in the suspension force to be actually generated by the suspension is larger than a magnitude of the damping term as the amplitude of the vibration occurring in the wheel due to the road surface displacements decreases; and

the electronic control unit is configured to calculate the target control force at a magnitude of a value obtained by multiplying together the control gain and a time derivative of the road surface displacements determined based on the road surface displacement related values at the predicted passing position, and in a direction in which the component of the damping term that corresponds to a speed of the vertical displacement of the wheel at the predicted passing position is canceled out.

5. The damping control device according to claim 1 , wherein:

the suspension force is theoretically represented by an expression including a spring term proportional to the relative displacement and a damping term proportional to a speed of the relative displacement;

a magnitude of a component corresponding to the spring term in the suspension force to be actually generated by the suspension is larger than a magnitude of the spring term as the amplitude of the vibration occurring in the wheel due to the road surface displacements decreases;

a magnitude of a component corresponding to the damping term in the suspension force to be actually generated by the suspension is larger than a magnitude of the damping term as the amplitude of the vibration occurring in the wheel due to the road surface displacements decreases; and

the electronic control unit is configured to

determine a first control gain and a second control gain to increase the target control force as the amplitude indicated by the amplitude index decreases,

calculate first target control force at a magnitude of a value obtained by multiplying together the first control gain and a time derivative of the road surface displacements determined based on the road surface displacement related values at the predicted passing position, and in a direction in which the component of the damping term that corresponds to a speed of the vertical displacement of the wheel at the predicted passing position is canceled out,

calculate second target control force at a magnitude of a value obtained by multiplying together the second control gain and the road surface displacements determined based on the road surface displacement related values at the predicted passing position and in a direction in which the component of the spring term that corresponds to the vertical displacement of the wheel at the predicted passing position is canceled out, and

control the control force generating device to generate resultant force of the first target control force and the second target control force at the timing when the wheel passes through the predicted passing position.

6. A damping control method for reducing, by controlling a control force generating device to change vertical control force, vibration of a vehicle body of a vehicle that is caused by vertical vibration occurring in at least one wheel suspended from the vehicle body by a suspension in response to vertical road surface displacements while the vehicle is traveling, the vertical vibration being transmitted to the vibration of the vehicle body via the suspension, the control force generating device being configured to generate the control force between the wheel and the vehicle body,

the suspension being configured to generate, in a vertical direction between the wheel and the vehicle body, a suspension force that is based on a relative displacement related value related to a vertical relative displacement between the wheel and the vehicle body,

the suspension force to be generated by the suspension being larger than a theoretical suspension force proportional to the relative displacement related value as an amplitude of the vibration occurring in the wheel due to the road surface displacements decreases, and,

a magnitude of a component corresponding to the spring term in the suspension force to be actually generated by the suspension is larger than a magnitude of the spring term as the amplitude of the vibration occurring in the wheel due to the road surface displacements decreases,

the damping control method comprising:

calculating an amplitude index indicating an amplitude of road surface displacement related values related to the road surface displacements in a predetermined sampling zone;

determining a control gain to increase, as the amplitude indicated by the amplitude index decreases, a magnitude of a target control force that is target value of the control force at a timing when a predetermined period has elapsed from a current time;

calculating, based on the control gain and the road surface displacement related values at a predicted passing position where the wheel is predicted to pass at the timing when the predetermined period has elapsed from the current time, the target control force in a direction in which a component of the suspension force that corresponds to a value related to a vertical displacement of the wheel at the predicted passing position is canceled out; and

controlling the control force generating device to generate the control force to agree with the target control force at the timing when the wheel passes through the predicted passing positon; and

calculating the target control force at a magnitude of a value obtained by multiplying together the control gain and the road surface displacements determined based on the road surface displacement related values at the predicted passing position, and in a direction in which a component of the spring term that corresponds to the vertical displacement of the wheel at the predicted passing position is canceled out.