Motor vehicles – Steering gear – With electric power assist
Reexamination Certificate
2000-01-31
2001-12-11
Swann, J. J. (Department: 3611)
Motor vehicles
Steering gear
With electric power assist
C180S443000, C701S041000, C701S042000, C701S044000, C073S781000, C073S847000, C073S862321
Reexamination Certificate
active
06328128
ABSTRACT:
BACKGROUND OF THE INVENTION
FIELD OF THE INVENTION
The present invention relates to an electric power steering apparatus.
An electric power steering apparatus for a vehicle, for instance, is disclosed in Japanese Patent Unexamined Publication No. Hei. 8-207798. In the apparatus, while the wheels of the vehicle are steered, a steering torque is detected so as to assist the manual power imparted to a steering wheel. In such a structure, an input shaft receives a steering force from the steering wheel and an output shaft transmits the steering force to the wheels. The input shaft and the output shaft are connected by a torsion bar, and the steering torque necessary for control can be detected by detecting the torsion of such a torsion bar.
In a case where the wheel has come into contact with the shoulder of a road, or the steering wheel has been rotated to a rack end during sudden steering, a large force is transmitted between the input shaft and the output shaft, so that there is a possibility that the large force is applied to the torsion bar. In such a case, if the torsion bar undergoes plastic deformation beyond its elastic limit, the zero reference point of the steering torque changes, so that the control function of the electric power steering apparatus is hampered. Accordingly, a limiting member is generally provided for limiting the relative rotation of the input shaft and the output shaft, thereby functioning to prevent plastic deformation from occurring in the torsion bar even if a large force is applied to it.
Such a limiting member comprises a projecting portion and a recessed portion, the projecting portion being formed on the output shaft and having a cruciform cross section, and the recessed portion being formed on the input shaft and having a corresponding configuration to the projecting portion. During the normal operation, the projecting portion and the recessed portion are not engaged with each other and allow the twisting of the torsion bar. On the other hand, when the relative rotation of the output shaft and the input shaft has become larger than a limit angle, the projecting portion is engaged in the recessed portion, thereby functioning to prevent the excessive twisting of the torsion bar.
Further, although the electric power steering apparatus has been used mainly for subcompact passenger cars with relatively light weight, the electric power steering apparatus has also come to be used for compact passenger cars with heavier weight. In such a compact passenger car with heavier weight, the force from the road surface and the steering force tend to become large, so that there is a possibility that the torsion bar becomes twisted to its limit angle, and therefore striking sound occurs frequently due to the collision between the projecting portion of the output shaft and the recessed portion of the input shaft.
In contrast, in the case of an electric hydraulic power steering apparatus which is conventionally used for a compact passenger car and the like, viscosity resistance is produced so as to prevent sudden relative rotation of the input shaft and the output shaft, so that such striking sound has not presented a particular problem.
To suppress such striking sound, it is conceivable to set the limit angle to a large degree such that even if the torsion bar is twisted at a larger angle, the projecting portion of the output shaft and the recessed portion of the input shaft do not collide against each other. However, if the torsion bar is twisted with a large degree, an output signal from a torque sensor increases (or decreases) corresponding therewith, so that there is a possibility that the output signal exceeds a threshold for detecting an abnormality of the torque sensor. If such an output signal, which is outputted from the torque sensor when the torsion bar is twisted with a large degree, exceeds the threshold, an erroneous diagnosis is possibly made that the torque sensor is abnormal although its control circuit is normal, thereby issuing an error signal.
SUMMARY OF THE INVENTION
Accordingly, in view of the above-described problems, an object of the present invention is to provide an electric power steering apparatus which is capable of suppressing the striking sound more effectively.
To this end, in accordance with a first aspect of the present invention, there is provided an electric power steering apparatus comprising:
a housing;
an output shaft for outputting a steering force so as to steer vehicle wheels;
an input shaft for transmitting the steering force from a steering wheel to the output shaft;
a torsion member for connecting the input shaft to the output shaft while being resiliently twisted in correspondence with a steering-torque applied to the input shaft;
a motor attached to the housing for imparting an auxiliary steering force to the output shaft;
a power transmitting mechanism, which couples a rotating shaft of the motor with the output shaft so as to transmit the auxiliary steering force from the motor to the output shaft;
a detecting device for detecting a torsion amount of the torsion member and also outputting a detection signal corresponding to the torsion amount;
a control unit for controlling the motor on the basis of the detection signal from the detecting device; and
a limiting member for limiting a relative rotation between the output shaft and the input shaft so as to inhibit the torsion of the torsion member from becoming a limit angle or more,
wherein the detecting device outputs a first detection signal converted by a first conversion ratio with respect to the torsion when the torsion of the torsion member is within a predetermined angle which is smaller than the limit angle, and the detecting device outputs a second detection signal converted by a second conversion ratio which is lower than the first conversion ratio when the torsion of the torsion member has exceeded the predetermined angle.
In the above-mentioned apparatus according to the present invention, it is advantageous that the control unit discriminates that an abnormality has occurred in the detecting device when the detection signal outputted from the detecting device has exceeded an upper threshold or dropped below a lower threshold, the upper and lower thresholds being set outside a range of the detection signal outputted from the detecting device in its normal state where the torsion of the torsion member is within the limit angle.
Further, it is advantageous that the torque, to which the torsion member is subjected when the torsion member is twisted up to the limit angle, is −15 N·m or less or 15 N·m or more.
Furthermore, it is advantageous that the detecting device comprises a torque sensor of a non-contact type.
In the above-described structure, it is preferable that the output characteristic of the first detection signal converted by the first conversion ratio depicts a substantially straight line, and the output characteristic of the second detection signal converted by the second conversion ratio depicts a quadratic curve, a sine curve or a periodic curve similar to the sine curve.
In accordance with the above-described first aspect of the present invention, if the torsion of the torsion member is within the predetermined angle, the detecting device outputs a first detection signal converted by the first conversion ratio with respect to the torsion, and if the torsion of the torsion member has exceeded the predetermined angle, the detecting device outputs a second detection signal converted by the second conversion ratio which is lower than the first conversion ratio. Therefore, in the control range of the electric power steering apparatus, that is in the normal control range where the relative angle of the input shaft and the output shaft is within a predetermined angle, it is possible to use a relatively high detection signal converted by the first conversion ratio with respect to the torsion of the torsion member. Further, even in a case where the torsional rigidity of the torsion member is increased to prevent the striking sound of the limiti
Divine David
NSK Ltd.
Sughrue Mion Zinn Macpeak & Seas, PLLC
Swann J. J.
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