Motor vehicles – Steering gear – With fluid power assist
Reexamination Certificate
1999-05-12
2001-06-26
DePumpo, Daniel G. (Department: 3611)
Motor vehicles
Steering gear
With fluid power assist
C180S446000, C701S041000
Reexamination Certificate
active
06250418
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 such as a brushless motor, and a steering assist force is generated by the power cylinder in accordance with the rotation speed of the electric motor.
The electric motor is supplied with a driving current via a driving circuit, which is operative on the basis of a driving pulse signal applied from a controller such as an electronic control unit (ECU). The controller generates a driving pulse signal of a high duty ratio when a load on the motor is high, and generates a driving pulse signal of a low duty ratio when the load is small. Thus, a pulse width modulation (PWM) control is conducted to drive the electric motor at a constant rotation speed irrespective of the magnitude of the load. In this case, the detection of the magnitude of the load is achieved, for example, by detecting a motor current.
When a target rotation speed of the motor is low and the load is high, the PWM control is conducted under a high electric current condition. However, if this state continues for a long time, electronic components in the controller suffer from a remarkable temperature rise thereby being damaged. This is because the PWM control under the high electric current condition maximizes the switching loss of a switching device, a steady-state loss, and a loss due to a flywheel current.
In order to eliminate the switching loss and the loss due to the flywheel current under the high current condition, it is a conventional practice to set the PWM duty to 100% and discontinue the pulse width modulation control when the load on the electric motor exceeds a predetermined level. This approach is advantageous in that the electronic components in the controller can be protected and less expensive electronic parts can be employed as the components of the controller.
However, if the load gradually increases to exceed a certain level, the PWM duty is abruptly increased to 100%, whereby the steering assist force to be applied to the steering mechanism is suddenly increased. This gives a driver such an impression that the steering is suddenly lightened, resulting in an unsatisfactory steering feeling.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a power steering apparatus which ensures an improved steering feeling particularly when a load is high.
In accordance with one feature of the present invention, there is provided a power steering apparatus, which is adapted to generate a steering assist force by a hydraulic pressure of a pump driven by an electric motor (
27
), and comprises: a pulse width modulation control circuit (
30
,
31
) for controlling the electric motor through a pulse width modulation control; a load detection circuit (
14
) for detecting a load applied on the electric motor; and a control circuit (
30
) for progressively increasing the PWM duty of the pulse width modulation control circuit up to 100% if the load on the electric motor detected by the load detection circuit is not lower than a predetermined level (TH).
It is noted that reference characters in parentheses hereinbefore and hereinafter represent the corresponding components to be described in the following embodiment.
With this arrangement, if the load on the electric motor is not lower than the predetermined level, the PWM duty is not abruptly but progressively increased up to 100%. Therefore, the steering assist force is not abruptly but progressively increased. Thus, a satisfactory steering feeling can be ensured.
In addition, the pulse width modulation control is discontinued with a PWM duty set to 100% when the load is high. Therefore, electronic components constituting the control circuit and the like will not suffer from a remarkable temperature rise.
In accordance with another feature of the present invention, there is provided a power steering apparatus which comprises: a pulse width modulation control circuit (
30
,
31
) for controlling the electric motor through a pulse width modulation control; a mountain road traveling detection circuit (
40
,
41
) for judging whether or not a vehicle is in mountain road traveling; and a control circuit (
30
, S
1
, S
3
) for progressively increasing the PWM duty of the pulse width modulation control circuit up to 100% if the mountain road traveling detection circuit judges that the vehicle is in the mountain road traveling.
During the mountain road traveling, the load on the electric motor is kept high. For this reason, the PWM duty is set to 100% when the vehicle is in the mountain road traveling, in accordance with the present invention. Thus, the PWM duty is prevented from being continuously kept at a high level, whereby electronic components constituting the control circuit and the like are prevented from suffering from a remarkable temperature rise. Further, the PWM duty is not abruptly changed but progressively increased to 100%, so that a satisfactory steering feeling can be ensured.
The two features described above may be employed in combination. More specifically, the PWM duty of the pulse width modulation control circuit may progressively be increased up to 100% if at least one of the following conditions is satisfied: the load on the electric motor is not lower than the predetermined level; and the vehicle is in the mountain road traveling.
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: 5711394 (1998-01-01), Fujii et al.
patent: 5816360 (1998-10-01), Spillner
patent: 5829547 (1998-11-01), Fujii et al.
patent: 5904222 (1999-05-01), Liubakka et al.
patent: 5936379 (1999-08-01), Matsuoka
patent: 5954152 (1999-09-01), Kada et al.
patent: 5995890 (1999-11-01), Permuy
patent: 6052647 (2000-04-01), Parkinson et al.
patent: 6073721 (2000-06-01), Grabowski
Matsuoka Hirofumi
Shimizu Yoshinobu
DePumpo Daniel G.
Koyo Seiko Co. Ltd.
Rabin & Champagne PC
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