GB2266499A - Hydraulic power-assisted steering system with input force sensor. - Google Patents

Abstract

The system comprises a standard rack and pinion steering system wherein a piston 36 on the intermediate rod 18 is acted on by a pressure difference between two pressure chambers 40, 41, the pressure difference being controlled in a closed loop system by a valve 64 which in turn is controlled by a control unit 66 in response to signals from a steering column torque sensor 60, a vehicle speed sensor 62 and a pressure difference sensor 64. In a modification (Fig. 4) the pressure sensor is replaced by load sensors (64B) in the tie rods 16, 17 and a steering box and drag link may replace the rack and pin ion. The system obviates the need for highly accurate manufacture of the steering valve. <IMAGE>

Description

A POWER ASSISTED STEERING SYSTEM The present invention relates to power assisted steering (PAS) systems and in particular to hydraulically operated power assisted steering systems.

A known hydraulically operated PAS system comprises a rack and pinion steering system in which force is transmitted from a steering wheel via a steering gear input shaft to a pinion and then to the rack on an intermediate rod, and from the intermediate rod to each of two steered wheels via a respective tie rod, steering arm and steering knuckle. The intermediate rod passes through a hydraulic cylinder and has a piston mounted on it which divides the cylinder into two pressure chambers. Hydraulic pressure is supplied to the two pressure chambers from a pump via a valve mounted in the steering gear input shaft. Turning the steering wheel affects the valve such that it directs hydraulic pressure to one or other of the pressure chambers to assist in turning the steered wheels.

It is a problem with such a known system that the valve has to be manufactured very accurately to produce the required level of assistance, and it is particularly difficult to ensure that the characteristics of the valve are symmetrical about the straight ahead position.

It is also known to provide power assistance in the form of an electric motor However such a system suffers from the drawback that it is usually necessary to provide a geared system to achieve sufficient assistance force from a low voltage electric motor, and such a gearbox will increase the inertia effect of the motor affecting returnabilty and power-failed operation.To overcome such inertia effects it is usually necessary to provide an electromagnetic clutch.These features increase the cost and complexity of the system.Furthermore, it is difficult to provide sufficient assistance for large passenger cars.

The present invention provides a power assisted steering system for a vehicle, the system comprising a movable wheel mounting member, a steering gear having an input member and an output member, linking means linking the output member to the wheel mounting member, hydraulic assistance means for applying a force to the wheel mounting member, a force sensor for sensing force applied to the input member, and a control unit for controlling the hydraulic assistance means in response to a force signal from the force sensor.

The force may be a linear force applied to the input member or it may be a torque, for example where the input member is a steering column.

Preferably the system further comprises a speed sensor for producing a speed signal variable with vehicle speed for inputting to the control unit. The PAS system can then be made speed sensitive or made to be effective over a limited range of vehicle speeds, for example speeds up to a fixed value.

Preferably the hydraulic assistance means includes a valve controlled by the control unit, for example via a valve actuator.

Desirably the hydraulic assistance means includes pressure sensor means for producing a pressure signal variable with pressure in the hydraulic means for inputting to the control unit. This allows the control unit to operate in a closed loop system so that the assisting pressure provided by the hydraulic assistance means can be set accurately for any particular input signal or signals to the control unit.

More preferably the hydraulic assistance means includes two pressure chambers for applying hydraulic pressure to opposite sides of a piston, the pressure sensor means being sensitive to the pressure in both the pressure chambers, and the pressure sensor means may be adapted to produce a signal variable with the difference in pressure between the two pressure chambers. This enables the signal produced to be related to the net force exerted on the pistons and therefore the net assisting force produced.

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 diagrammatic representation of a power assisted steering system according to a first embodiment of the present invention, Figure 2 is a diagrammatic representation of part of a power assisted steering system according to a second embodiment of the present invention, and Figure 3 shows the performance characteristics of the system of Figure 1.

Referring to Figure 1, the power assisted steering system is built into a motor vehicle and comprises a pair of stub axles 10, 11 rotatably mounted on kingpins 12, 13 each stub axle having a steering arm 14, 15 fixedly attached to it. Each of the stub axles 10, 11 has a steered wheel 8, 9 mounted on it. The steering arms are coupled by tie rods 16, 17 to opposite ends of an intermediate rod 18.

The intermediate rod 18 has a plurality of teeth 20 on it which form a rack 22.

A steering wheel 24 is attached to a steering column 26 which has a pinion 28 mounted on it which engages with the rack 22. Torque applied to the steering wheel 24 is therefore transmitted through the steering column 26 to the pinion 28, which applies a force via the intermediate rod 18 to the tie rods 16, 17 which in turn apply a torque to the steering arms 14, 15 and the stub axles 10, 11 to turn the steered wheels 8, 9.

The intermediate rod 18 extends through a hydraulic cylinder 30 which has an aperture 32, 33 at each end with a seal 34, 35 making sealing contact with the intermediate rod 18. A piston 36 is formed on the intermediate rod 18 and has a seal 38 around its circumference so that it is a sliding fit in the cylinder 30 and divides the cylinder into two pressure chambers 40, 41.

Each pressure chamber 40, 41 has a port 42, 43 with a hydraulic line 42a, 43a connecting each pressure chamber port to a respective port 44, 45 in a control valve 46.

The control valve has two further ports 48, 49 one which is connected by a hydraulic line 50 directly to a reservoir 52, and one of which is connected by a hydraulic line 55 to a hydraulic pump 56, which is connected via a further hydraulic line 55 to the reservoir 52.

The control valve 46 can connect either one of the pressure chambers 40, 41 to the pump 56 whilst connecting the other pressure chamber 41, 40 directly to the reservoir 52, as shown by the continuous and dotted lines in Figure 1. It can also vary the restriction through which the fluid flows to control the pressure transmitted from the pump 56 to whichever of the pressure chambers 40, 41 it is connected to.

A torque sensor 60 is built into the steering column 26 and produces a torque signal variable with the torque applied to the steering column. A speed sensor 62 is mounted near one of the driven wheels 8 and producers a speed signal variable with the speed of the vehicle. A pressure sensor 64 is connected to both of the hydraulic lines 42a, 43a between the control valve 46 and the pressure chambers 40, 41 and detects the pressure in each of the pressure chambers and produces a pressure signal variable with the difference in pressure between them.

A control unit 66 has three inputs 68, 69, 70 connected to the torque sensor 60, the speed sensor 62 and the pressure sensor 64 respectively. It also has an output 72 connected to a valve actuator 74 which operates the control valve 46.

A map of required pressure difference between the pressure chambers, ie hydraulic steering assistance as a function of vehicle speed and steering column torque or steering effort is held in the memory of the control unit 66.

Figure 3 is a graphic representation of such a map. Under any set of conditions, the control unit measures the values of the speed signal and torque signal, and reads off the desired value of the pressure signal. The valve actuator 74 and control valve 46 are then controlled in a closed loop, the control valve being adjusted until the pressure signal reaches the required value. This adjustment happens in a timescale short enough for the pressure signal to be maintained in the required relationship to the speed signal and torque signal as those two signals vary with time so that the driver perceives instantaneous response.

As can be seen in Figure 3, when the vehicle is travelling at low vehicle speeds such as during parking, the steering assistance increases rapidly with steering effort so the steering feels 'light' and the maximum effort required is quite low. As vehicle speed increases, steering assistance increases less rapidly with steering effort.

This provides a firmer feel to the steering which is desirable at high speeds.

Figure 2 shows part of a PAS system according to a second embodiment of the invention, which includes similar features to the first embodiment which have been numbered accordingly. In the second embodiment, the pressure sensor 64A is connected only to the hydraulic line 54A between the pump 56 and the control valve 46. The pressure sensor 64A produces a signal variable with the pressure output from the pump, which is dependent upon how much restriction the control valve 46 provides between the pump 56 and the pressure chamber 40, 41 to which it is connected.

Various other arrangements are also possible. For example two pressure sensors could be used, each for measuring the pressure in one of the hydraulic lines 42a, 43a between the control valve 46 and the pressure chambers 40, 41. A signal from each pressure sensor could be input to the control unit, which could itself calculate the difference in pressure between the two chambers by comparing the two signals.

The present invention can also be applied to a PAS system which, rather than a rack and pinion steering gear, includes a steering box connected via a drop arm and drag link to one of a pair of steering knuckles the pair of steering knuckles being connected by a track rod so that they turn together.

Figure 4 shows a third embodiment of the invention which uses output force sensing in place of pressure sensing.

Figure 4 is identical to Figure 1, except that the pressure sensor 64 is not present and load sensors 64B are placed in the tie rods 16, 17 and connected to the control unit 66 at inputs 70B. The control unit adds tensile force in one tie rod 16 to the compressive force in the other tie rod 17. Such a system is also well suited to a steering box system utilising one load sensor in the drag link.

Claims (10)

1. A power assisted steering system for a vehicle, the system comprising a movable wheel mounting member, a steering gear having an input member and an output member, linking means linking the output member to the wheel mounting member, hydraulic assistance means for applying a force to the wheel mounting member, a force sensor for sensing force applied to the input member, and a control unit for controlling the hydraulic assistance means in response to a force signal from the force sensor.

2. A system according to claim 1 further comprising a speed sensor for producing a speed signal variable with vehicle speed, for inputting to the control unit.

3. A system according to claim 1 or claim 2 wherein the input member is an input shaft and the force sensor is arranged to sense torque applied to the input shaft.

4. A system according to any foregoing claim wherein the hydraulic assistance means includes a valve which is controlled by the control unit and which controls fluid pressure in the hydraulic assistance means.

5. A system according to any foregoing claim wherein the hydraulic assistance means includes pressure sensor means for producing a pressure signal variable with pressure in the hydraulic assistance means for inputting to the control unit.

6. A system according to claim 5 wherein the hydraulic assistance means includes two pressure chambers for applying hydraulic pressure to opposite sides of a piston, the pressure sensor means being sensitive to the pressure in both of the pressure chambers.

7. A system according to claim 6 wherein the pressure sensor means is adapted to produce a pressure signal variable with the difference in pressure between the two pressure chambers.

8. A system according to any foregoing claim wherein the steering gear comprises a rack and pinion.

9. A system according to any foregoing claim further comprising a second movable wheel mounting member connected to the steering gear output member wherein the hydraulic assistance means is arranged for applying a force to the second wheel mounting member.

10. A power assisted steering system substantially as hereinbefore described with reference to Figures 1 and 3 or Figures 2 and 3 of the accompanying drawings.