GB2359793A

GB2359793A - A power steering system, which compensates for variations in output torque caused by the universal joints in the steering column, for motor vehicles

Info

Publication number: GB2359793A
Application number: GB0004806A
Authority: GB
Inventors: James Edward Burt
Original assignee: Bayerische Motoren Werke AG; MG Rover Group Ltd
Current assignee: Bayerische Motoren Werke AG; MG Rover Group Ltd
Priority date: 2000-03-01
Filing date: 2000-03-01
Publication date: 2001-09-05
Also published as: GB0004806D0

Description

Vehicle Steerin 2359793 The present invention relates to vehicle steering

systems, and in particular to power assisted steering systems.

Vehicle steering wheels are generally connected to a steering gear by means of a number of shafts, frequently three, which are non-parallel and joined to each other at their ends by means of universal joints. There is a problem with most universal joints that the rotational velocity of the 'input' shaft on one side is not exactly matched by the rotational velocity of the 'output' shaft on the other side of the joint. This means that as the ratio between the velocities of the two shafts varies during rotation of the joint, the ratio of the torques varies also. This means that, in a steering system using such joints, the driver feels a variation in torque needed to rotate the steering wheel as it is rotated.

It is conventional, in a steering linkage with three shafts joined by two universal joints to set the joints up approximately in antiphase so that the variations of torque across them substantially cancel each other out. However for this to work the angle between the upper and middle shafts must be the same as that between the middle and lower shafts. This puts limits on the packaging of the steering system, so that the system used is generally a compromise between steering feel and packaging.

The present invention therefore provides a vehicle steering system comprising an input member and output means including an output member connected to the input member through at least one universal joint such that the ratio of the input torque applied to the input member to the torque applied through the at least one universal joint to the output member varies with the angle of rotation of the input member, a steering angle measuring means for measuring the angle of rotation of the input member, input torque measuring means for measuring the input torque applied to the input member, and assistance means for applying a steering assistance force to the output means which is dependent on the input torque and which varies with the angle of rotation of the input member so as to compensate for the variation in torque transmission rate of the at least one universal joint.

The output means may further include a rack and pinion, or a recirculating ball screw mechanism, or indeed any form of steering mechanism.

The assistance means can be any suitable mechanism. In an EPAS (electric power assisted steering) system it would be an electric motor which could be controlled by a steering angle sensor to give the required variation in assistance rate. In an HPAS (hydraulic power assisted steering) system where the power assistance from a constantly running hydraulic pump is controlled by a hydraulic valve, which can be electronically controlled e.g. to give speed sensitive power steering, the valve could be further controlled to give the variation in assistance required. In an EHPAS (electro-hydrauhc power assisted steering) system, where an electric pump is used to produce hydraulic pressure, the variation in assistance could be produced by control of the pump, but could probably best be produced using an electronically controlled hydraulic valve as in the HPAS system.

Preferred embodiments of the present invention will now be described by way of example only with reference to the accompanying drawings in which:

Figure 1 is a schematic representation of the steering system according to the invention; Figures 2a and 2b show schematically the calculation processes for determining the level of power assistance in conventional systems and the system of Figure 1 respectively; Figure 3 is a graph showing the variation in torque transmission in the system of Figure 1, and Figure 4 is a graph showing the variation in torque transmission rate and gain of parts of the system of Figure 1.

Referring to Figure 1, a vehicle steering system comprises a steering wheel 10 rigidly connected to an upper shaft 12, a middle shaft 14 connected to the upper shaft 12 by an upper universal joint 16 and a lower shaft 18 connected to the middle shaft 14 by a lower universal joint 20. The lower shaft 18 has a pinion 22 at its lower end which is coupled with a steering rack 24 extending transversely across the vehicle having its ends connected to steering arms 26 on the two front wheels 28 in conventional manner.

The upper shaft 12 has a torque sensor 30 associated with it for measuring the torque applied to the upper shaft 12 and hence to the steering wheel 10, and a steering angle sensor 32 which measures the rotational angle of the steering wheel and upper shaft 12. These sensors 30, 32 provide inputs to a power steering control unit 34 which controls the operation of an electric power assistance motor 36. The motor 36 applies a torque to the lower shaft 18, and thereby a steering torque to the front wheels 28, in parallel with that transmitted from the steering wheel through the upper and middle shafts 12, 14 to assist the driver in steering the wheels 28, in a manner which will be described in more detail below.

In terms of system inputs and outputs for the power assistance controller 34, therefore, Figure 2a shows the conventional type of system in which the input torque is simply multiplied by a gain factor to calculate the desired torque assistance. In the system of Figure 1, on the other hand, as shown in Figure 2b, the input torque as measured by the torque sensor and the steering angle as measured by the steering angle sensor are both used to calculate the power assistance which will be provided by the electric motor 36.

Referring now to Figure 3, as mentioned above, each of the universal joints 16, 20 transmits a torque in a ratio which varies with the steering angle of the steering wheel 10. During each rotation the torque transmission ratio of each joint, that is the ratio of the torque applied to it on its input side to the torque it provides on its output side, varies sinusoidally between about 110% and 90%. The two joints 16, 20 are arranged approximately in antiphase so that the total torque transmission rate of the two joints only varies slightly, between about 102% and 98%. This fluctuation is not particularly high, but could be significantly higher, especially if the angle between the upper and middle shafts 12, 14 were more different from the angle between the middle and lower shafts 14, 18.

Referring to Figure 4, the gain of the power assistance system, that is the ratio of the input torque applied to the input shaft plus the torque applied by the motor 36, to the input torque, is arranged to fluctuate with steering angle in a sinusoidal manner which mirrors that of the torque transmission ratio of the universal joints. Therefore as the torque transmission ratio of the joints 16, 20 increases, the assistance from the motor 36 decreases, and vice versa. Therefore the total gain of the system, i.e. the ratio of total output torque applied as a steering torque to the wheels 28 to the input torque provided by the driver to the steering wheel is, in this example, substantially constant.

In practice, a constant gain is not always desired. However the desired gain can be obtained in the same manner as described above by superimposing on the desired gain characteristic a fluctuating factor which depends on steering angle and which cancels out the effects of the varying torque transmission rate of the universal joints.

Claims

1. A vehicle steering system comprising an input member and output means including an outp ut member connected to the input member through at least one universal joint such that the ratio of the input torque applied to the input member to the torque applied through the at least one universal joint to the output member varies with the angle of rotation of the input member, a steering angle measuring means for measuring the angle of rotation of the input member, input torque measuring means for measuring the input torque applied to the input member, and assistance means for applying a steering assistance force to the output means which is dependent on the input torque and which varies with the angle of rotation of the input member so as to compensate for the variation in torque transmission rate of the at least one universal joint.

2. A system according to claim 1 wherein the assistance means includes an electric motor.

3. A system according to claim 1 or claim 2 wherein the output means includes a rack and pinion.

4. A vehicle steering system substantially as hereinbefore described with reference to the accompanying drawings.