Motor vehicles – Steering gear – With electric power assist
Reexamination Certificate
2003-03-10
2004-11-02
Morris, Lesley D. (Department: 3611)
Motor vehicles
Steering gear
With electric power assist
Reexamination Certificate
active
06810986
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to electric power steering apparatus and method for controlling the electric power steering apparatus and, more particularly, to an improved electric power steering apparatus and control method therefor which can effectively reduce steering loads on a human operator to permit an enhanced steering feel by imparting a steering torque assist of an electric motor to the steering system of a motor vehicle such as a passenger car.
BACKGROUND OF THE INVENTION
The electric power steering apparatus are assistance apparatus to provide an automatic steering torque assist by activating a steering assisting motor in response to manual operation of a steering wheel by a human vehicle operator. In the electric power steering apparatus, the steering assisting motor is controlled by a motor control unit, using a steering torque signal output from a steering torque detector section that detects steering torque applied by the vehicle operator to a steering shaft and a vehicle velocity signal output from a vehicle velocity detector section, so as to reduce steering force to be manually applied by the vehicle operator. The motor control unit sets target values of motor currents to be supplied to the steering assisting motor, on the basis of the steering torque signal and vehicle velocity signal, and thereby generates target motor current signals representative of the thus-set target current values. Then, the motor control unit determines a difference or offset between the target motor current signals and motor current signals fed back from a motor current detector section that detects motor currents caused to actually flow through the steering assisting motor, and it then performs a proportional/integral compensation process (PI control) on the determined offset to thereby generate motor control signals for controlling the motor.
Among the conventional electric power steering apparatus is one employing a brushless steering assisting motor, and such an electric power steering apparatus employing the brushless steering assisting motor (hereinafter also referred to as a “brushless motor”) can afford a stable steering torque assist, because the absence of voltage drops between a brush and a commutator can prevent reduction and variation in motor output power. Further, since the brushless motor presents a smaller moment of inertia than the brush-equipped motor, the electric power steering apparatus with such a brushless motor can afford a good steering feel when the steering wheel is turned during high-speed straight travel of the vehicle.
With the electric power steering apparatus employing the brushless motor, it is necessary to control an electric current to be supplied to the motor in accordance with a current rotational angle of the motor. Thus, it has been conventional for the electric power steering apparatus to include, as a feedback-controlling detector unit, a rotational angle detector section for detecting a current rotational angle (electrical angle) of the motor, in addition to the motor current detector section. Output signals from the motor current detector section and rotational angle detector section are fed back to the motor control unit, which in turn generates motor control signals for driving the brushless motor on the basis of PWM (Pulse Width Modulation) control (i.e., PWM-driving the brushless motor) in accordance with the motor currents and rotational angle. Here, the “rotational angle”, i.e. “electrical angle”, of the steering assisting motor is a rotational angle of a rotor detected via magnets provided on the rotor and a magnetism-detecting element provided near the magnet and on the basis of positions of the magnets. Specifically, at an end of the rotor, there are provided a plurality of (e.g., four) magnets, each presenting a pair of N and S magnetic poles, along the circumference of the rotor in such a manner that eight N and S poles are arranged alternately at equal intervals along the circumference of a predetermined portion of the rotor. As the rotor of the steering assisting motor rotates, magnetism produced by the N and S poles alternately acts on the magnetism-detecting element. In the electric power steering apparatus thus constructed, each full rotation of the rotor through a 360° mechanical angle produces an electrical angle of four cycles; that is, each quarter turn of the rotor corresponds to a 360° electrical angle. Rotational position of the rotor is determined by the electrical angle, and the phases of the motor currents to be supplied to the brushless motor are controlled with reference to the electrical angle.
In the electric power steering apparatus employing the brushless motor, the feedback-based motor control processing carried out by the motor control unit comprises for major steps of: (1) detecting an electrical angle of the rotor; (2) detecting motor currents (e.g., three-phase motor currents) and performing three-phase-to-two-phase conversion (three-phase-to-dq conversion) of the detected motor currents; (3) performing PI control; and (4) performing two-phase-to-three-phase conversion (dq-to-three-phase conversion). These four steps can be further classified into a first group of steps pertaining to feedback control of the motor currents (step (2) and step (3)), and a second group of steps pertaining to control for generating motor control signals using the detected electrical angle (step (1) and step (4)). The motor control unit is implemented by a microcomputer, and thus the four major steps of the feedback-based motor control processing is executed on the basis of computer software.
If the microcomputer (CPU) of the motor control unit is a high-performance microcomputer capable of operating at high speed, the above-mentioned four major steps can always be executed appropriately with no inconveniences even when the steering assisting motor rotates at high speed. However, if the microcomputer (CPU) is of relatively low performance and only capable of operating at low speed, then there would arise the problem that, when the steering assisting motor rotates at high speed, the three-phase sinusoidal waveforms of the motor control signals would deform considerably so that the motor control performance is undesirably lowered.
SUMMARY OF THE INVENTION
In view of the foregoing prior art problems, it is an object of the present invention to provide an improved electric power steering apparatus equipped with a brushless steering assisting motor and control method therefor which can reliably avoid degradation in motor control performance even where a microcomputer of low operating speed is employed in a motor control unit.
In order to accomplish the above-mentioned object, the present invention provides an electric power steering apparatus which includes, as a steering assisting motor, a brushless motor capable of being PWM-driven with motor currents of at least three phases to give a steering torque assist to a steering system of a motor vehicle. The electric power steering apparatus of the present invention further comprises: a steering torque detector section for detecting steering torque applied through operation of a steering wheel and generating a steering torque signal indicative of the detected steering torque; a target current setting section for setting target currents on the basis of at least the steering torque signal generated by the steering torque detector section; a motor current detection section for detecting the motor currents to be supplied to the brushless motor and generating motor current signals indicative of the detected motor currents; an electrical angle detector section (rotational angle detection section) for detecting an electrical angle of the brushless motor and generating an electrical angle signal indicative of the detected electrical angle; a motor control unit. The motor control unit comprises: an offset calculation section for calculating offsets between the target currents and the motor currents on the basis of the motor current signals and generating offse
Honda Giken Kogyo Kabushiki Kaisha
Merchant & Gould P.C.
Morris Lesley D.
Royal, Jr. Paul
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