Motor vehicles – Steering gear – With fluid power assist
Reexamination Certificate
1999-09-17
2001-06-12
Johnson, Brian L. (Department: 3618)
Motor vehicles
Steering gear
With fluid power assist
C701S041000
Reexamination Certificate
active
06244372
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a power steering apparatus that applies a steering assist force to a steering mechanism by a hydraulic pressure that is generated by a pump driven by an electric motor.
2. Description of Related Art
Power steering apparatuses are known 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.
Drive control of the electric motor is performed, for example, on the basis of the steering angle of the steering wheel. That is, the steering angle is determined on the basis of an output of a steering angle sensor provided in association with the steering wheel, and the driving of the electric motor is controlled on the basis of the steering angle thus determined. More specifically, if the steering angle of the steering wheel is within a motor off range defined around a steering angle midpoint, steering assistance is considered to be unnecessary, so that the electric motor is stopped. On the other hand, if the steering angle of the steering wheel is out of the motor off range, the electric motor is actuated to generate a steering assist force.
Determination of the steering angle midpoint is achieved, for example, by sampling steering angle data outputted from the steering angle sensor and regarding the most frequent steering angle data as corresponding to the steering angle midpoint.
In the prior art described above, the electric motor is actuated in response to detection of a steering angle value falling out of the fixedly defined motor off range. If the motor off range is set wider, an entrapped feeling may occur due to a delay in the actuation of the electric motor, so that the steering feeling is deteriorated. Therefore, the motor off range is generally set narrower. However, this makes the actuation of the electric motor more sensitive to the operation of the steering wheel, resulting in a greater energy loss. That is, the electric motor may needlessly be actuated even when the steering is deflected due to traveling on a rough road or the like.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a power steering apparatus which ensures improved steering feeling for the driver.
It is another object of the present invention to provide a power steering apparatus which features an enhanced energy saving.
The power steering apparatus according to the present invention, which is adapted to generate a steering assist force by a hydraulic pressure generated by a pump driven by an electric motor, comprises: steering angle detection means for sensing a steering angle as measured with respect to a steering angle midpoint; actuation control means for actuating the electric motor in its off state, based on the condition that a change amount of the steering angle sensed by the steering angle detection means exceeds a predetermined actuation threshold; and actuation threshold determining means for determining the actuation threshold in accordance with the steering angle sensed by the steering angle detection means when the electric motor is turned off.
In accordance with one embodiment of the present invention, the actuation threshold determining means determines a first actuation threshold which is to be employed when a steering wheel is operated away from the steering angle midpoint and a second actuation threshold which is employed when the steering wheel is operated toward the steering angle midpoint in accordance with the sensed steering angle so that the sum of the first and second actuation thresholds is virtually constant.
When the steering angle is close to the steering angle midpoint, the steering wheel experiences a substantial amount of change in the steering angle before the steering angle gets out of a play angle range of the steering wheel. That is, when the steering wheel is operated away from the steering angle midpoint, the steering wheel experiences a great amount of change in the steering angle before the steering assist force is required. Where the steering angle is relatively large, conversely, a greater steering assist force is required immediately after the start of the steering operation away from the midpoint.
Where the steering angle is large, a self-alignment force is exerted on a steering mechanism due to a reverse input from wheels to return the steering wheel to the steering angle midpoint. Therefore, the application of the steering assist force is less urgent with a larger steering angle in the steering operation toward the steering midpoint.
In the present invention, the first actuation threshold to be employed for the steering operation away from the midpoint and the second actuation threshold to be employed for the steering operation toward the midpoint are determined in accordance with the steering angle at the deactuation of the electric motor so that the sum (of the absolute values) of the first and second actuation thresholds is virtually constant, and the electric motor is actuated based on the condition that the change amount of the steering angle exceeds the first or second actuation threshold.
Therefore, if the steering angle at the motor deactuation is close to the midpoint, for example, the first actuation threshold for the steering operation away from the midpoint is set at a higher value, and the second actuation threshold for the steering operation toward the midpoint is set at a lower value. On the contrary, if the steering angle at the motor deactuation is relatively large, the first actuation threshold for the steering operation away from the midpoint is set at a lower value, and the second actuation threshold for the steering operation toward the midpoint is set at a higher value.
Thus, the electric motor is not needlessly actuated when the steering angle is close to the steering angle midpoint, and a greater steering assist force can immediately be generated when the steering wheel is operated away from the midpoint at a greater steering angle. For the steering operation toward the midpoint, the actuation threshold is set at a higher value with a greater steering angle, so that power consumption of the electric motor can be reduced by effectively utilizing the self-alignment force. Thus, the enhancement of the energy saving and the improvement of the steering feeling can both be achieved.
The actuation control means may include actuation steering angle calculating means for determining an actuation steering angle at which the electric motor in its off state is to be actuated on the basis of the steering angle sensed by the steering angle detection means and the actuation threshold determined by the actuation threshold determining means, and means for actuating the electric motor in its off state, based on the condition that the steering angle sensed by the steering angle detection means reaches the actuation steering angle determined by the actuation steering angle calculating means.
The actuation threshold determining means is preferably adapted to set the first actuation threshold for the steering operation away from the midpoint at a lower value and the second actuation threshold for the steering operation toward the midpoint at a higher value as the steering angle sensed by the steering angle detection means at the motor deactuation is greater.
The actuation threshold determining means is preferably adapted to set the actuation threshold for the steering operation toward the midpoint at a higher value as the steering angle sensed by the steering angle detection means at the motor deactuation is greater.
In accordance with one embodiment of the present invention, the power steering apparatus further comprises vehicle speed detection means for sensing a vehicle speed, wherein the actuation threshold determining means sets the actuation threshold at a higher value as the vehicle speed s
Kaji Hiroaki
Sakamaki Masahiko
Johnson Brian L.
Koyo Seiko Co. Ltd.
Lerner Avraham H.
Rabin & Champagne, P.C.
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