Control apparatus for electric power steering system

1. A control apparatus that controls a motor of an electric power steering system to assist a steering effort of an operator, comprising:

a sensor configured to detect a current value of a steering parameter expressing a condition of a steering operation of the operator;

a parameter computer configured to:

store therein a mathematic model expressing, as a plurality of equations, at least a part of the electric power steering system based on the steering parameter and a plurality of physical parameters that express a physical characteristic of the electric power steering system; and

compute, using an online identification method, a current value of at least one of the plurality of physical parameters based on the mathematic model and the current value of the steering parameter;

a target torque computer configured to:

store therein a relationship between change in the at least one of the plurality of physical parameters and a gain for returning a current steering feeling of the operator to a preset steering feeling;

determine a value of the gain based on the relationship and a difference between the current value of the at least one of the plurality of physical parameters and a default value; and

compute, based on the determined value of the gain, a target torque of the motor which compensates for the difference between the current value and the default value of the at least one of the plurality of physical parameters; and

a motor driver configured to drive the motor to generate the target torque so that the steering feeling of the operator is maintained at the preset steering feeling.

2. The control apparatus according to claim 1 , wherein

the online identification method is an on-line recursive least square method.

3. The control apparatus according to claim 1 , wherein

the mathematic model includes a friction coefficient as the at least one of the plurality of physical parameters.

4. A method for controlling a motor of an electric power steering system to assist a steering effort of an operator, comprising:

detecting a current value of a steering parameter expressing a condition of a steering operation of the operator;

storing a mathematic model expressing, as a plurality of equations, at least a part of the electric power steering system based on the steering parameter and a plurality of physical parameters that express a physical characteristic of the electric power steering system; and

computing, using an online identification method, a current value of at least one of the plurality of physical parameters based on the mathematic model and the current value of the steering parameter;

storing a relationship between change in the at least one of the plurality of physical parameters and a gain for returning a current steering feeling of the operator to a preset steering feeling;

determining a value of the gain based on the relationship and a difference between the current value of the at least one of the plurality of physical parameters and a default value;

computing, based on the determined value of the gain, a target torque of the motor which compensates for the difference between the current value and the default value of the at least one of the plurality of physical parameters; and

driving the motor to generate the target torque so that the steering feeling of the operator is maintained at the preset steering feeling.

5. The control method according to claim 4 , wherein

the mathematic model includes a friction coefficient as the at least one of the plurality of physical parameters.

6. The control method according to claim 4 , wherein

the steering system includes a steering wheel, an axis of a pinion, a rack, a torsion bar, and a gear, and

the plurality of equations of the mathematic model are:

T s =J s {umlaut over (θ)} s +C s {dot over (θ)} s +T sn +F Cs ,

T sn =K T (θ s −θ 0 ),

nK tr i a +K T θ s =J{umlaut over (θ)} 0 +C{dot over (θ)} 0 +Kθ 0 +F 0 , and

θ m =nθ 0 ,  (4)

where,

J

=

J

0

+

n

2

⁢

J

m

+

(

p

2

⁢

⁢

π

)

2

⁢

M

R

,

C

=

C

0

+

n

2

⁢

C

m

+

(

p

2

⁢

⁢

π

)

2

⁢

C

R

,

K

=

K

T

+

(

p

2

⁢

⁢

π

)

2

⁢

K

R

,

J s represents a first inertia of the steering wheel,

J 0 represents a second inertia of the axis of the pinion,

J m represents a third inertia of a shaft of the motor,

C s represents a first viscous friction coefficient of the steering wheel,

C 0 represents a second viscous friction coefficient of the axis of the pinion,

C m represents a third viscous friction coefficient of the motor,

C R represents a fourth viscous friction coefficient a fourth viscous friction coefficient of the rack,

K T represents a first spring constant of the torsion bar,

K R represents a second spring constant of the rack,

F Cs represents a first Coulomb friction coefficient of the steering wheel,

F 0 represents a second Coulomb friction coefficient of the axis of the pinion,

n represents a reduction gear ratio of the gear,

p represents a relative stroke between the pinion and the rack,

K tr represents a torque constant of the gear,

M R represents a mass of the rack,

T sn represents a rotary torque applied from the operator,

θ s represents a steering angle of the steering wheel,

T s represents a steering torque applied from the operator,

θ m represents a first rotation angle of the motor,

θ 0 represents a second rotation angle of the axis of the pinion, and

i a represents a motor current flowing through the motor.