Motor vehicles – Steering gear – With fluid power assist
Reexamination Certificate
1999-06-07
2001-04-17
Dickson, Paul N. (Department: 3618)
Motor vehicles
Steering gear
With fluid power assist
C701S041000
Reexamination Certificate
active
06216814
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a power steering apparatus which applies a steering assist force to a steering mechanism by a hydraulic pressure generated by a pump driven by an electric motor.
2. Description of Related Art
Power steering apparatuses are conventionally utilized which assist operation of a steering wheel by supplying a working oil from an oil pump to a power cylinder coupled to a steering mechanism. The oil pump is driven by an electric motor, and a steering assist force is generated by the power cylinder in accordance with the rotation speed of the electric motor. The steering assist force is not required when the steering wheel is not turned. Therefore, the electric motor is off in a straight travel steering state (a non-steering state) where the steering wheel assumes a position virtually at a steering angle midpoint and, in response to detection of a change in any of steering-related conditions, the electric motor is actuated.
However, a recent trend is that a standby driving operation is performed to drive the electric motor at a low voltage in the non-steering state to ensure a smooth rise of the hydraulic pressure.
For the electric motor control, it is necessary to determine a time point at which a steering operation is started in the non-steering state.
The electric motor control may be achieved, for example, by sensing an electric current flowing through the electric motor and increasing a driving voltage when the electric current exceeds a threshold. However, this control method suffers from such a drawback that the hydraulic pressure rise cannot keep up with an abrupt steering operation thereby resulting in a bad steering feeling.
In conventional motor-driven pump hydraulic power steering apparatuses, the driving voltage of the electric motor is set higher in the non-steering state, or a steering angle sensor for sensing a steering angle is employed for improvement of a response time of the hydraulic pressure rise. However, the former approach trades off the power consumption, and the latter approach requires an expensive component.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a power steering apparatus which is capable of quickly detecting a steering state even at the start of an abrupt steering operation without deterioration in steering performance.
The power steering apparatus according to the present invention comprises a control circuit which is capable of switching a control mode between a first mode (assist mode) in which a first voltage is applied to an electric motor when a steering operation is performed and a second mode (standby mode) in which a second voltage lower than the first voltage is applied to the electric motor when the steering operation is not performed, wherein the control circuit includes means for judging (a) if an electric current sensed by an electric current sensor is higher than a first threshold in the standby mode and (b) if a change rate of the electric current sensed by the electric current sensor is higher than a second threshold in the standby mode, and the control circuit switches the control mode from the standby mode to the assist mode if at least one of the conditions (a) and (b) is satisfied.
FIG. 1
is a graph showing a change in the motor current I before and after the steering operation is started, in which the first threshold (electric current threshold) is indicated by I
1
, the second threshold (change rate threshold) is indicated by an inclined straight line, an electric current differential is indicated by &Dgr;I, and a time differential is indicated by &Dgr;t. Before the start of the steering operation, the motor current I is lower than the first threshold I
1
. After the start of the steering operation, however, the motor current I increases to exceed the first threshold I
1
at a certain time point, and the change rate of the motor current exceeds the second threshold at another time point.
In accordance with the present invention, when the steering operation is started in the standby mode, the control mode is switched from the standby mode to the assist mode on the basis of a logical OR between the condition that the electric current sensed by the electric current sensor is higher than the first threshold I
1
and the condition that the change rate of the electric current is higher than the second threshold.
When the steering operation is abruptly started, the time point at which the electric current change rate &Dgr;I/&Dgr;t exceeds the second threshold comes earlier than the time point at which the electric current I exceeds the first threshold I
1
. Thus, the start of the abrupt steering operation can be detected more quickly.
Where the time point at which the electric current I exceeds the first threshold I
1
comes earlier than the time point at which the electric current change rate &Dgr;I/&Dgr;t exceeds the second threshold, unlike the case shown in
FIG. 1
, it is also possible to promptly detect the start of the steering operation.
Thus, the power steering apparatus of the present invention can promptly detect the abrupt steering operation without the use of an expensive steering angle sensor to ensure a satisfactory steering feeling at the start of the steering operation.
The foregoing and other objects, features and effects of the present invention will become more apparent from the following description of the preferred embodiments with reference to the attached drawings.
REFERENCES:
patent: 4875539 (1989-10-01), Abukawa et al.
patent: 5460235 (1995-10-01), Shimizu
patent: 5737223 (1998-04-01), Matsuoka
patent: 5758741 (1998-06-01), Tomioka
patent: 5996724 (1999-12-01), Shimizu et al.
Fujita Kazuhiko
Nakajima Tsuyoshi
Dickson Paul N.
Koyo Seiko Co. Ltd.
Lerner Avraham H.
Rabin & Champagne PC
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