Motor vehicles – Steering gear – With electric power assist
Reexamination Certificate
2001-12-12
2004-04-27
Morris, Lesley D. (Department: 3611)
Motor vehicles
Steering gear
With electric power assist
Reexamination Certificate
active
06725965
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention generally relates to an electric power steering system of a vehicle. More particularly, this invention relates to an electric power steering system that remains stable even when subjected to forces, such as vibrations received from road surface, from a motor or engine or other source.
2. Discussion of the Related Art
A power steering system is mounted to an automobile (or any vehicle) to assist the steering by reducing the steering force required by a driver. Electric power steering systems, as compared to conventional non-electric systems, are smaller in size and have lighter weight. Also, the number of constituent parts is relatively small.
An electric power steering system generally comprises a torque sensor for outputting a signal corresponding to a value of the steering torque of the driver, an electric motor (a kind of drive means) for driving an assist apparatus which assists the steering force, and an electronic control unit (ECU) for determining current flowing through the electric motor in accordance with the output signal of the torque sensor. That is, as the driver turns the steering wheel, the torque sensor outputs a signal corresponding to a value of the steering torque generated. The ECU determines the assist current and the electric motor drives the assist apparatus based on the torque corresponding to the assist current.
However, the problem is that such a torque sensor usually detects not only the steering torque generated by the driver's steering operation, but also vibration received from the road surface or generated by an internal combustion engine or motor or other source of vibration. If such vibration is detected by the torque sensor, then that vibration produces a noise component that is also included in the assist current. That is, the assist force is varied also in relation to the vibration. In this case, the steering becomes unstable.
Accordingly, there is a need for an electric power steering system that is not influenced by vibration and which remains stable. The present invention satisfies this need and provides related advantages as well.
SUMMARY OF THE INVENTION
It is a purpose of the present invention to provide an electric power steering system capable of keeping the steering stable even when a torque sensor detects vibration forces other than those input by the driver, such as received from road surface, from vibrations generated from a motor or engine, or from other sources of vibrations.
As used herein, the term “stable,” when used in reference to the electric power steering system, refers to the system only responding to the input of the driver and not to external forces, such as vibrations.
To achieve the above purpose, a first aspect of the present invention provides an electric power steering system having a torque sensor for outputting a signal corresponding to a value of a steering torque, a derivative means for outputting a signal obtained by differentiating the output signal of the torque sensor, a driving means for electrically driving an assist apparatus which assists a steering force, and an assist current determining means for determining current flowing to the drive means in accordance with the output signal of the torque sensor and the output signal of the derivative means. The electric power steering system further has gain control means for setting a gain of the derivative means to zero or reducing the gain when the output signal of the torque sensor is smaller than a predetermined value.
According to the electric power steering system of the first aspect, the signal corresponding to the steering torque is output by the torque sensor, and the signal obtained by differentiating the output signal of the torque sensor is output by the derivative means. The assist apparatus for assisting the steering force is electrically driven by the drive means, but the current flowing to the drive means is determined by the assist current determining means in accordance with the output signal of the torque sensor and the output signal of the derivative means. When the output signal of the torque sensor is smaller than the differential value, the gain of the derivative means is set to zero or reduced by the gain control means. Thus, even if the torque sensor detects vibrations received from the road surface or vibrations generated from the motor or engine, this noise component is prevented from being included in the assist current, so as to prevent the assist force from varying with the vibration. Thus, it is possible to prevent the steering from being unstable so that the vehicle drives straight.
As described above, the torque sensor detects the vibration, but if the vibrations detected by the torque sensor are high frequency, the time between the instant when the steering torque is detected and the instant when the steering force is assisted becomes relatively long, and the assist force is adversely applied in a direction opposite from a direction to which the steering torque is applied. However, even in such a case, it is possible to prevent the steering from being unstable.
A second aspect of the present invention provides an electric power steering system having a torque sensor for outputting a signal corresponding to a value of a steering torque, a derivative means for outputting a signal obtained by differentiating the output signal of the torque sensor, a driving means for electrically driving an assist apparatus which assists a steering force, and an assist current determining means for determining current flowing to the drive means in accordance with the output signal of the torque sensor and the output signal of the derivative means. The electric power steering system further has second gain control means for setting a gain of the derivative means to zero or reducing the gain when the output signal of the derivative means is smaller than a predetermined value.
According to the electric power steering system of the second aspect, the signal corresponding to the steering torque is output by the torque sensor, and the signal obtained by differentiating the output signal of the torque sensor is output by the derivative means. The assist apparatus for assisting the steering force is electrically driven by the drive means, but the current flowing to the drive means is determined by the assist current determining means in accordance with the output signal of the torque sensor and the output signal of the derivative means. When the output signal of the derivative means is smaller than the differential value, which occurs when the variations amount per unit time of the steering torque are smaller than the predetermined value, the gain of the derivative means is set to zero or reduced by the second gain control means even if a steering torque equal to or greater than the predetermined magnitude is applied. Thus, even if the torque sensor detects vibrations received from the road surface or vibrations generated from the motor, it is possible to prevent that noise component from being included in the current flowing to the drive means, so as to prevent the assist force from being varied in terms of vibrations. Thus, it is also possible to prevent the steering from being unstable when the vehicle is turning.
As described above, the torque sensor detects the vibrations, but if the vibrations detected by the torque sensor are high frequency, the delay time (the time between the instant when the steering torque is detected and the instant when the steering force is assisted) becomes relatively long, and the assist force is adversely applied in a direction opposite from a direction to which the steering torque is applied. However, even in such a case, it is possible to prevent the steering from being unstable.
According to a third aspect of the present invention, in the electric power steering system of the second aspect, the system further has gain control means for setting a gain of the derivative means to zero or reducing the gain when the output signal of the to
Ishikawa Hisazumi
Kani Akira
Kogiso Yoshinori
Kayaba Industry Co. Ltd.
Morris Lesley D.
Royal, Jr. Paul
The Maxham Firm
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