GB2359794A

GB2359794A - Electric power-assisted steering

Info

Publication number: GB2359794A
Application number: GB0004924A
Authority: GB
Inventors: Peter Hewson
Original assignee: Bayerische Motoren Werke AG; MG Rover Group Ltd
Current assignee: Bayerische Motoren Werke AG; MG Rover Group Ltd
Priority date: 2000-03-02
Filing date: 2000-03-02
Publication date: 2001-09-05
Also published as: GB0004924D0

Description

0 2359794 Vehicle Steerin The present invention relates to vehicle

steering and is particularly, but not exclusively, applicable to electrically power assisted steering (E.P.A.S.) systems.

E.P.A.S. systems generally comprise a steering input shaft to which the steering wheel is attached, a steering rod extending across the vehicle, and movable laterally to change the steering angle, in response to rotation of the steering shaft, and an electric motor which can drive the steering rod in response to the input torque applied to the steering wheel so as to assist with steering. Generally the motor is controlled so that the steering force it applies bears a controlled relationship to the steering force applied to the steering wheel, which is measured by a sensor in the steering shaft.

It can be a problem with E.P.A.S. systems that, when a steering force is applied to the steered wheels by awexternal force, for example when the steered wheels encounter an irregularity in the road surface, this force is not transmitted back to the steering wheel. This is because the electric motor provides a high level of resistance to changes in steering angle under such circumstances. The same also applies with smaller scale inputs, and the result is that the steering feel of the vehicle can be less than satisfactory.

Accordingly the present invention provides a steering system for a vehicle having at least one steerable wheel, the system comprising a driver steering input member, a steering control member connected to the steering input member and movable to change the steering angle of the at least one steerable wheel, a steering assistance means for applying a force to the steering control member, measuring means for measuring the steering input force input by the driver to the input member and the steering force applied to the at least one steered wheel by the steering control member, and control means arranged to determine from said 0 steering input force a target steering force, and to control the steering assistance means so as to bring the steering force towards the target steering force.

Preferred embodiments of the present invention will now be described by way of example only with reference to the accompanying drawing which is a diagrammatic representation of a vehicle steering system according to the invention.

A vehicle has two steerable front wheels 10, 12 steered by means of a steering rod 14 which extends across the vehicle and is coupled to steering arms associated with the wheels by means of tie rods 16, 18. A steering wheel 20 is connected to a steering shaft 22 which in turn is connected to the steering rod 14 by means of a mechanical link 24, for example a rack and pinion mechanism. An electrical power assistance motor 26 is also connected to the steering rod 14 such that it can apply a force to the steering rod 14 in either direction, left or right, under the control of a power steering control unit 28. The control unit receives signals from a steering force input sensor 30 which measures the steering torque input to the steering shaft 22 by the driver using the steering wheel, and from steering force sensors 32 in the tie rods which measure the steering force applied to the wheels by the steering rod 14.

The control of the motor 26 is arranged to compare the steering force in the tie rods 16, 18 with a target steering force which is calculated from the driver's steering input force, to produce an error signal based on the difference between the two, and to control the motor 26 so as to reduce the error signal to zero, or as close to zero as possible. The relationship between the steering input force at the steering wheel and the target steering force at the steered wheels can be a simple magnification factor so that the force input required from the driver is only a certain fraction, for example one tenth, of that which would be required if the 0 power assistance were not present. Alternatively the target steering torque can be dependent on other factors such as the speed of the vehicle. Because the sensors 30, 32 do not distinguish between forces input by the driver turning the steering wheel to change the steering angle, and forces input by the driver to resist steering effects produced by external steering forces tending to turn the steered wheels 10, the power assistance system operates equally well to transmit forces from the wheels to the driver and from the driver to the wheels.

More specifically, if the driver turns the steering wheel 20 to the right the first effect is to produce a torque in the steering shaft 22 which will be sensed by the sensor 30. Because it applies forces to the steering rod 14, the motor 26 can produce a steering torque at the wheels which is ten times that which would be transmitted simply by the rack and pinion 24. This means that the driver can turn the steered wheels 10, 12 with little effort as in a normal E.P.A.S. system. However the closed loop control enables the steering force at the wheels to be more accurately controlled than in a conventional system, thereby providing an improved response.

If a force is input to the wheels 10, 12, for example because one of them has hit an obstacle, or because the vehicle is cornering and the wheels are tending to move back towards their straight-ahead position because of the inherent steering geometry of the vehicle, the motor 26 will apply a force to the steering rod 14 so that a specific fraction of that steering force, one tenth in this example, is applied to the steering shaft through the rack and pinion system 24. This ensures that all such steering forces applied to the wheels 10, 12 are fed back to the driver, which provides a good steering feel.

It will be appreciated that the invention can be varied in many aspects from the specific embodiment described, and can be adapted to work with any 0 mechanical steering system. For example, in a steering system with a centre takeoff rack a single strain gauge could be used to measure the steering force applied to the wheels. Furthermore it can be used in vehicles with any number of steered wheels, including trikes with a single steerable wheel and large vehicles with four.

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Claims

CIAIMS

1. A steering system for a vehicle having at least one steerable wheel, the system comprising a driver steering input member, a steering control member connected to the steering input member and movable to change the steering angle of the at least one steerable wheel, a steering assistance means for applying a force to the steering control member, measuring means for measuring the steering input force input by the driver to the input member and the steering force applied to the at least one steered wheel by the steering control member, and control means arranged to determine from said steering input force a target steering force, and to control the steering assistance means so as to bring the steering force towards the target steering force.

2. A system according to claim 1 for a vehicle having two steerable wheels wherein the measuring means includes two sensors, each for measuring a steering force applied to a respective one of the steered wheels.

3. A system according to claim 1 or claim 2 wherein the steering input member is connected to a rotatable steering shaft and the measuring means is arranged to measure the torque applied to the steering shaft.

4. A system according to any foregoing claim wherein the steering assistance means comprises an electric motor.

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