Motor vehicles – Steering gear – With electric power assist
Reexamination Certificate
2001-07-13
2003-07-29
Morris, Lesley D. (Department: 3611)
Motor vehicles
Steering gear
With electric power assist
C180S443000
Reexamination Certificate
active
06598699
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an electric power steering system for vehicles and, more particularly, to a motor-controlled power steering system for assisting operation of a steering wheel through controlling an electric motor.
2. Description of Related Art
There have been known motor controlled power steering systems and hydraulically controlled power steering systems. Such a power control system actualizes a desired assist characteristic by regulating a controlling quantity for the electric motor or a controlling hydraulic quantity according to a turning torque applied to a steering wheel and/or a steering speed (which is a differential value of turning torque). It is known from, for example, Japanese Unexamined Patent Publication 8-72734 to change the assist characteristic according to a vehicle speed or a yaw rate in addition to a vehicle speed.
In the prior art electric power steering system a motor controlling quantity for the electric motor is determined by multiplying a turning torque applied to a steering wheel, that is detected by a torque sensor such as a torsion bar type torque sensor disposed between a steering wheel and one of front wheels, by a specified assist control gain. The assist control gain is predetermined based on tests conducted on a given vehicle so as to provide a desired assist characteristic.
However, there are possibly variations in assist characteristics of power steering systems. This causes that the feeling of steering varies with the assist characteristic of power steering system. The variation in assist characteristics is due, for example, to variations in inertia and variations in electric motor property and/or variations in friction of parts of reduction gears of steering apparatuses that are disposed between a steering shaft and an electric motor. The variation in friction is predominantly due to manufacturing errors of the parts of the steering apparatus. Specifically, when the part has a magnitude of friction greater than an ordinary magnitude of friction, thrust of the electric motor is spent for the friction even though the electric motor is controlled with a motor controlling quantity that is determined by multiplying the turning torque applied to the steering wheel by the assist control gain. As a result, the electric motor encounters a lack of assist torque for steering, so as to cause a feeling of tottery operation of the steering wheel.
The variation in assist characteristic is further due to assembling errors of, for example, the reduction gear. The reduction gear comprises a worm gear and a worm wheel that have a comparatively large reduction gear ratio. In order for the reduction gear to prevent or significantly reduce backrush and rattling noises, the worm gear is strongly forced against the worm wheel. This causes relatively large variations in friction due to assembling errors because of a high magnitude of friction between the worm gear and the worm wheel forced against each other and has a significant influence on the variations in assist characteristic.
The prior art electric power steering system has the problem of yaw overshooting of front wheels. Since the yaw overshoot cancellation property of wheels (which is referred to the action of wheels such that the front wheels smoothly restore a steering angle of 0 (zero) due to a self-aligning torque without overshooting when letting go a hold on the turned steering wheel) has repercussions on the straight ahead controllability of vehicle, the prior art electric power steering system implements the damping control for braking the electric motor with a controlling quantity of damping that is determined by multiplying a rotational speed of motor by a damping control gain in addition to the assist control. Similarly to the assist control gain, the damping control gain is regulated so that the yaw overshoot cancellation property of wheels is desirable. Therefore, there is the problem that the desired yaw overshoot cancellation property of wheels is not attained. This is due to non-restoration of the steering wheel to the center or neutral position (straight ahead traveling position) or shakes of the steering wheel at the center or neutral position due, for example, to the variations in inertia and/or friction.
In order to eliminate the aforesaid problem, it is valuable to get rid of manufacturing errors of parts of the steering apparatus or to employ high precision parts of the steering apparatus for reducing friction. However, in addition to a change in the magnitude of friction due to expansion or contraction of parts of the steering apparatus that are caused following a change in temperature or a change in magnitude of friction due to aging of parts of the steering apparatus, the assist characteristic and the yaw overshoot cancellation property of wheels possibly change due to a change in gross vehicle weight resulting from a change in loadage. In consequence, it is hard to eliminate the aforesaid problem even when employing high precision parts of the steering apparatus. The utilization of high precision parts causes additional costs of manufacturing the steering apparatus.
SUMMARY OF THE INVENTION
An object of the present invention is therefore to provide an electric power steering system for a vehicle that always provides steering apparatuses with a desired assist characteristic and a desired yaw overshoot cancellation property of wheels.
It is another object of the present invention to provide an electric power steering system which provides a steering apparatus with an improved follow-up property of wheels to operation of the steering wheel.
It is a further object of the present invention to provide an electric power steering system which is provided at low costs.
The above object of the present invention is accomplished by an electric power steering system for a vehicle which performs feedback control of a steering apparatus on the basis of a turning torque applied to a steering wheel in addition to assist control for controlling an electric motor so as to assist a turn of the steering wheel.
According to a preferred embodiment of the present invention, the electric power steering system comprises a torque sensor disposed between the steering wheel and front wheels of the vehicle and operative to detect a turning torque applied to the steering wheel, first control means for determining such a first control quantity as to countervail the turning torque when the electric motor is controlled with the first control quantity, second control means for operating a target rate of change in wheel steering angle on the basis of the turning torque and for determining a second control quantity by subtracting an actual rate of change in wheel steering angle from the target rate of change in wheel steering angle, and motor control means for determining a motor control quantity by adding the first control quantity and the second control quantity together and for controlling the electric motor with the motor control quantity.
The torque sensor detects a turning torque applied to the steering wheel when the steering wheel is turned to steer the vehicle. The first control means determines such a first control quantity as to countervail the turning torque when the electric motor is controlled with the first control quantity, in other words determines a first control quantity representative of an assist torque by multiplying a turning torque by a specified control gain. The second control means determines a target rate of change in wheel steering angle on the basis of the turning torque and then determines a second control quantity by subtracting an actual rate of change in wheel steering angle from the target rate of change in wheel steering angle. The operation of the target rate of change in wheel steering angle may be made based on a vehicle model of a mechanical system of an steering apparatus between the torque sensor and a tire on the front wheel or a vehicle model in which factors such as inertia of an electric motor and a
Sakamoto Kiyoshi
Takehara Shin
Mazda Motor Corporation
Nixon & Peabody LLP
Studebaker Donald R.
Winner Tony
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