Electric power steering system

1. An electric power steering system comprising

a control unit configured to:

i) compute an assist control amount that is a current amount, based on multiple kinds of state quantities indicating a steering state;

ii) control a motor based on the assist control amount, the motor being a source of assist force to be applied to a steering system of a vehicle;

iii) individually set, for the respective state quantities, limiting values that limit a variation range of the assist control amount, based on the respective state quantities used to compute the assist control amount;

iv) limit a value of the assist control amount using the limiting values;

v) acquire a command value generated by a host control unit to change the assist control amount;

vi) change the assist control amount using the command value before the assist control amount is limited using the limiting values; and

vii) take the command value into account in setting of at least one of the limiting values.

2. The electric power steering system according to claim 1 , wherein:

the command value is a torque command value;

the control unit includes a first converter configured to convert the torque command value to a current amount; and

the control unit is configured to add the current amount obtained through conversion executed by the first converter, to the computed assist control amount, and configured to use a value obtained by adding the torque command value to a steering torque that is one of the multiple kinds of state quantities, when setting the limiting values based on the steering torque.

3. The electric power steering system according to claim 1 , wherein, in a case where the control unit is configured to compute a basic component of the assist control amount and at least one compensation amount for the basic component and configured to compute the assist control amount by adding together the basic component and the at least one compensation amount, the command value is a gain by which the compensation amount is multiplied, and the control unit multiplies, by the gain, each of the limiting values set based on the state quantity that is used as a base for computation of the compensation amount to be multiplied by the gain.

4. The electric power steering system according to claim 1 , wherein, in a case where the control unit is configured to generate each of final limiting values for the assist control amount by adding together multiple kinds of the limiting values individually set for the respective state quantities and configured to limit the value of the assist control amount using the final limiting values, the command value is an additional current command value to be added to the computed assist control amount, and the control unit adds the additional current command value to each of the final limiting values.

5. The electric power steering system according to claim 1 , wherein:

the command value is a steering angle command value;

the control unit includes a second converter configured to convert the steering angle command value to a current amount; and

the control unit is configured to add the current amount obtained through conversion executed by the second converter, to the computed assist control amount, and configured to use a value obtained by adding the steering angle command value to a steering angle that is one of the multiple kinds of state quantities, when setting the limiting values based on the steering angle.