Power steering apparatus

Details Power steering apparatus Power steering apparatus

1999-05-20

2001-05-08

Johnson, Brian L. (Department: 3618)

Motor vehicles

Steering gear

With fluid power assist

C180S421000, C180S441000, C180S446000, C701S041000

Reexamination Certificate

active

06227328

ABSTRACT:

This is the national stage of International Application PCT/JP98/03943, with an international filing date of Sep. 2, 1998 and Japanese application 9-262365 filed Sep. 26, 1997.
TECHNICAL FIELD
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.
BACKGROUND 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 rotational speed of the electric motor. Since the steering assist force is not required when the steering wheel is not turned, a so-called “stop-and-go control” is conducted, wherein the electric motor is off when the steering assumes a straight travel steering state virtually at a steering angle midpoint and, in response to detection of a steering angle change greater than a predetermined level, the electric motor is actuated.
The detection of the steering angle midpoint is, for example, based on an output of a steering angle sensor provided in association of the steering mechanism. The steering sensor is adapted to detect a steering angle change with respect to an initial steering angle, i.e., a relative steering angle. The initial steering angle corresponds to a steering angle observed when an ignition key switch is actuated. Accordingly, the initial steering angle does not always correspond to the steering angle midpoint, but often corresponds to a steering angle at which the steering wheel is offset from the steering angle midpoint. Therefore, steering angle data are sequentially sampled for determination of frequency of occurrence of steering angle data, and the most frequent steering angle data is defined as steering angle data corresponding to the steering angle midpoint, for example.
On the other hand, a greater steering assist force is required at low speed traveling than at high speed traveling. There has been a proposal that the electric motor is actuated in response to a slight steering angle change (e.g., 3 degrees) when the vehicle speed is lower than 10 km/h, and actuated in response to a greater steering angle change (e.g., 9 degrees) when the vehicle speed is not lower than 10 km/h. Thus, a catch-up (caught feeling) can be suppressed particularly at the low speed traveling.
However, no steering assist force is required when the steering angle is close to the steering angle midpoint. Accordingly, if the electric motor is actuated in response to a slight steering angle change when the vehicle speed is lower than 10 km/h, the steering assist is provided in excess, resulting in an energy loss.
It may be considered to employ a steering angle based electric motor actuation control in combination with a vehicle speed based electric motor actuation control.
However, the steering angle midpoint is not determined immediately after the startup of the vehicle engine, but the determination thereof becomes possible after the vehicle travels a certain distance. Hence, the absolute steering angle cannot be detected during a certain period immediately after the startup of the engine, so that the electric motor actuation control cannot be performed on the basis of the steering angle.
For this reason, the electric motor is actuated in response to a slight steering angle change at the low speed traveling irrespective of the steering angle. Therefore, improvement in the steering feeling comes first with the energy saving sacrificed to some extent.
DISCLOSURE OF THE INVENTION
It is an object of the present invention to solve the aforesaid technical problems and to provide a power steering apparatus which ensures enhancement of the energy saving without deterioration of the steering feeling during the period in which the detection of the steering angle is impossible.
In accordance with the present invention, there is provided a power steering apparatus adapted to generate a steering assist force by a hydraulic pressure generated by a pump driven by an electric motor, the apparatus comprising: steering angle detection means (steering angle sensor
11
in
FIG. 1
, and Steps S
1
, S
4
in
FIG. 2
) for detecting an absolute steering angle on the basis of a steering angle midpoint and a relative steering angle; first control means (CPU
31
and the like in
FIG. 1
, and Steps S
6
, S
8
to S
10
in
FIG. 2
) for actuating the electric motor in response to that the steering angle detection means detects an absolute steering angle of not smaller than a first threshold value; and second control means (CPU
31
and the like in
FIG. 1
, and Steps S
6
, S
11
, S
9
, S
10
in
FIG. 2
) for actuating the electric motor in response to that a steering angle change becomes not smaller than a second threshold value which is smaller than the first threshold value when absolute steering angle detection is not carried out by the steering angle detection means.
With this arrangement, where the absolute steering angle detection is carried out, the electric motor is actuated in response to that the absolute steering angle reaches the relatively large first threshold value. Therefore, the electric motor is not actuated in response to a slight steering angle change when the steering angle is close to the steering angle midpoint, whereby the energy saving can be enhanced.
On the other hand, when the absolute steering angle detection is not carried out, the electric motor is actuated in response to that the steering angle change reaches the relatively small second threshold value. Therefore, the catch-up can effectively be prevented which is liable to occur particularly when the steering angle is relatively large. Thus, the steering feeling can be improved.
By thus employing different threshold values for the absolute steering angle detection period and for the absolute steering angle non-detection period, the energy saving can be enhanced without deterioration of the steering feeling.
The state where the steering angle detection means carries out the absolute steering angle detection may be a state where the steering angle midpoint has been determined. Upon determination of the steering angle midpoint, the detection of the absolute steering angle can be achieved by employing the steering angle midpoint and the relative steering angle.
The power steering apparatus may further comprise means for actuating the electric motor on condition that the absolute steering angle reaches a relatively large third threshold value (which may be equal to the first threshold value) if the absolute steering angle is at a level within a predetermined range around the steering angle midpoint, and actuating the electric motor in response to that the steering angle change reaches a fourth threshold value (which may be equal to the second threshold value) which is smaller than the third threshold value if the absolute steering angle is at a level out of the predetermined range around the steering angle midpoint, when the absolute steering angle detection is carried out by the steering angle detection means. With this arrangement, the electric motor is not actuated needlessly when the steering angle is close to the steering angle midpoint, while the electric motor is actuated in response to a slight steering angle change when the steering angle is relatively large. Thus, the catch-up can be prevented.
The power steering apparatus may further comprise vehicle speed detection means (vehicle speed sensor
12
in FIG.
1
and Step S
1
in
FIG. 2
) for detecting a vehicle speed, and means (Step S
5
, S
6
in
FIG. 2
) which permits switching between an absolute steering angle based motor actuation control and a steering angle change based motor actuation control in accordance with whether or not the absolute steering angle detection is carried out by the steering angle detection means, when the vehicle speed detected

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