(a) Technical Field
The present invention relates to motor driven power steering, more particularly, to a system for compensating for disturbance of a motor for motor driven power steering, which compensates for the disturbance by utilizing a closed loop based input value prediction model unit.
(b) Description of the Related Art
Recently, certain vehicles have been equipped with a motor driven power steering (MDPS) system in which the steering force of a steering wheel is changed according to a travelling speed.
A permanent magnet synchronous motor typically is used as a motor in the motor driven power steering system, and the permanent magnet synchronous motor is generally used for three kinds of usage, that is, torque control, speed control, and position control by using a current control.
The torque control of the motor is a basic control process of the speed control and the position control, and is commonly referred to as current control because a current is directly proportional to a torque.
Torque control, that is, current control, is generally performed by a PI controller and a linear feedback control, but has a disadvantage in that control stability deteriorates due to the influence of disturbance, such as back electromotive force (back-emf).
When a permanent magnet is rotated, a change in magnetic flux induced in a stator and a current of the stator interact with each other, so that non-linearity is generated by back electromotive force, and the non-linearity may change a motor model or may be interpreted as a disturbance.
Accordingly, non-linearity caused by the disturbance, such as back electromotive force, may be predicted based on parameter values of a motor circuit and by measuring a current and a speed of the motor, but in order to compensate for non-linearity, it is necessary to design an additional controller.
Accordingly, in order to compensate for the influence of back electromotive force, a feed-forward and feed-back control method is mainly used, but the method has a problem in that the method is sensitive to a change of unpredicted parameters, and a motor speed needs to be accurately measured.
When a speed of the motor is frequently changed, such as the MDPS system, back electromotive force is changed to influence a voltage input into the motor, and the current is undesirably controlled, so that compensation for back electromotive force is necessary for accurate control of the motor current.
However, the existing method of compensating for disturbance, such as back electromotive force, compensates for the disturbance adequately when a speed of the motor is not changed, but does not compensate for the disturbance, such as back electromotive force, when a speed of the motor is rapidly changed.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.