Data processing: vehicles – navigation – and relative location – Vehicle control – guidance – operation – or indication – Vehicle subsystem or accessory control
Reexamination Certificate
1999-03-17
2001-08-07
Cuchlinski, Jr., William A. (Department: 3661)
Data processing: vehicles, navigation, and relative location
Vehicle control, guidance, operation, or indication
Vehicle subsystem or accessory control
C701S036000, C701S041000, C701S043000, C180S416000, C180S443000, C180S404000, C180S413000, C180S446000, C180S400000, C318S140000, C318S430000, C318S432000, C318S434000, C318S469000, C318S488000, C388S811000
Reexamination Certificate
active
06272410
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an electric power steering system for the current feedback control of a motor.
2. Description of the Prior Art
The electric power steering system of the prior art carries out current feedback control based on a difference between a current instruction value computed based on the steering state of a steering wheel and the travelling state of a vehicle and a motor drive current value supplied from a battery to a motor for providing assist force to a system for transmitting power from the steering wheel to the wheels, paying attention to the excellent controllability of output torque for the drive current of the motor. That is, the electric power steering system has been designed to ensure quick response for the controllability of the drive current of the motor as the current control characteristics of current feedback control so as to provide a good steering feeling.
In the current feedback control of the prior art, to ensure the response of current feedback control in order to give a good steering feeling to a driver, the gain of current feedback control is enhanced, the control range of a PID controller for current feedback control is widened, or further the response frequency of a current detection sensor is raised. However, these have the following defects.
When the gain of current feedback control is enhanced, there is an upper limit of the gain due to the control range of the PID controller, an internal response delay of a current detection circuit, or the like. When the gain exceeds the upper limit, in the loop transfer characteristics of current feedback control, both a gain margin and a phase margin decrease and the response of current feedback control becomes instable, whereby the motor drive current oscillates shortly. To prevent such inconvenience and optimize control response, it is desired that the phase margin of the loop transfer function of current feedback control should be set to 40 to 60° and the gain margin should be set to 10 to 20 dB. However, when the thus predetermined control characteristics of a controller, that is, the cross frequency of the loop transfer function of current feedback control is almost equal to the resonance frequency of a mechanical system, even if the loop transfer function of current feedback control itself is stable, a current oscillation triggered by mechanical resonance occurs, or a current oscillation is caused by a kind of induction phenomenon that the current oscillation induces torque oscillation or noise. This problem becomes marked when relatively large assist force is required for the motor, that is, when the motor stops while outputting a torque by stopping and holding the steering wheel at a turned position as in the case of turning the steering wheel while the vehicle stops, or when the rotation speed of the motor is extremely low by turning the steering wheel very slowly. Therefore, to prevent oscillation induced by the mechanical resonation frequency and the cross frequency of current feedback control, the cross frequency is made much lower than the resonance frequency of the mechanical system (that is, the gain of current feedback control is made small) to ensure the stability of current control at the sacrifice of the follow-up properties of current control, thereby preventing the oscillation of the output torque of the motor.
As alternative means of stabilizing the drive current control of the motor, it is conceivable to design current feedback control having quick response which can suppress current oscillation at the resonance frequency of the mechanical system without losing the follow-up properties of current control by making the cross frequency of current feedback control much higher than the resonance frequency of the mechanical system. However, to realize this means, the response range of the PID feedback controller must be widened. To this end, an extraordinary high-performance controller is required according to control specifications such as the response of torque control, for example, a high-speed CPU is required when the improvement of the response of hardware such as a current detection circuit and PID control are carried out by software. This is not wise for the construction of a system.
SUMMARY OF THE INVENTION
The present invention has been made to solve the above problem and it is an object of the present invention to provide an inexpensive electric power steering system which can provide a good steering feeling while keeping the cross frequency of current feedback control low and can prevent output torque oscillation and the generation of noise by setting the gain of current feedback control to a large value when the motor drive current must be controlled at quick response as in the case of a sudden change in current instruction value and to a low value when follow-up properties to the current instruction value are not required so much.
According to a first aspect of the present invention, there is provided an electric power steering system for carrying out current feedback control according to a difference between a current instruction value computed based on a steering state of a steering wheel and a traveling state of a vehicle and a motor drive current value supplied from a battery to a motor for providing assist force to a system for transmitting power from the steering wheel to the wheels, wherein the system has a first operation expression and a second operation expression as operation expressions for current feedback control, the first operation expression is selected when an index for selecting an operation expression is larger than a first predetermined value, the second operation expression is selected when the index is smaller than a second predetermined value, and the current feedback control is carried out by correcting the difference between the current instruction value and the current detection value using the selected operation expression.
According to a second aspect of the present invention, there is provided an electric power steering system, wherein the first predetermined value and the second predetermined value are equal to each other.
According to a third aspect of the present invention, there is provided an electric power steering system, wherein the product of the difference between the current instruction value and the motor drive current detection value and a first correction gain is obtained in the first operation expression and the product of the difference between the current instruction value and the motor drive current detection value and a second correction gain is obtained in the second operation expression.
According to a fourth aspect of the present invention, there is provided an electric power steering system, wherein a timer is provided to judge the index, the operation expression used for computation is switched to a first or second operation expression based on the judgment result of the index, and then the first or second operation expression is retained for a time set by the timer.
According to a fifth aspect of the present invention, there is provided an electric power steering system, wherein a timer is provided to judge the index, the first operation expression is selected when the index is judged to be larger than the first predetermined value for a predetermined time set by the timer and the second operation expression is selected when the index is judged to be smaller than the second predetermined value for a predetermined time set by the timer.
According to a sixth aspect of the present invention, there is provided an electric power steering system, wherein the difference between the current instruction value and the motor drive current detection value is represented by &egr;, the first correction gain is represented by K1, the second correction gain is represented by K2, the predetermined value for judging the index is represented by C, (&egr;×K1) is used as the first operation expression when the absolute value of &egr; is smaller than the predetermin
Kifuku Takayuki
Okanoue Takahiro
Cuchlinski Jr. William A.
Mancho Ronnie
Mitsubishi Denki & Kabushiki Kaisha
Sughrue Mion Zinn Macpeak & Seas, PLLC
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