GB2349368A

GB2349368A - Vehicle suspensions

Info

Publication number: GB2349368A
Application number: GB9909830A
Authority: GB
Inventors: Jose-Luis Martins Dos Sa Lopes
Original assignee: MG Rover Group Ltd
Current assignee: MG Rover Group Ltd
Priority date: 1999-04-29
Filing date: 1999-04-29
Publication date: 2000-11-01
Also published as: GB9909830D0

Abstract

In an active roll control system for a vehicle, anti-roll bars 26, 30 are provided with actuators 28, 32 which can provide a torque in the anti-roll bars to control vehicle body roll. The actuators can also provide a pulsed variation in torque to the anti-roll bars and the resulting variation in load on the wheels is used to drive up the rotational speed of the wheels if they are in deep slip and no braking torque is being applied to them.

Description

Vehicle Suspensions The present invention relates to anti-lock braking systems (ABS), and in particular to the use of vehicle suspension systems in co-ordination with the vehicle brakes so as to improve the ABS.

It is known in ABS systems, in situations where a wheel is slipping even though its brake has been completely released, to use the power train to apply a driving torque to the wheel so as to being its speed back up to a desirable level. However this requires the co-ordination of the engine management system with the suspension control system which can be inconvenient.

Therefore the present invention provides a vehicle suspension system comprising an actuator arranged to produce an actuator force between a sprung part of the vehicle and a part of the suspension which can move relative to said sprung part thereby to allow vertical travel of a wheel, measuring means arranged to measure slip of said wheel, and control means arranged to produce a controlled variation in said actuator force in the event of predetermined slip conditions so as to control said slip.

The sprung part of the vehicle may, for example, be the vehicle body, and said part of the vehicle may comprise a suspension arm, or a beam axle.

The present invention can be applied to any form of active suspension.

Accordingly the actuator may comprises a hydraulic strut arranged to produce forces in a vertical direction between a wheel of the vehicle and a sprung part of the vehicle, or an air spring in a fully active air suspension system. Alternatively the actuator may be arranged to urge two wheels of the vehicle in opposite vertical directions, as in an active roll-control system.

Preferred embodiments of the 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 vehicle including a suspension system according to a first embodiment of the invention, Figure 2 is a diagrammatic representation of a vehicle including a suspension system according to a second embodiment of the invention.

Referring to Figure 1, a vehicle has two steerable front wheels 10,12 mounted on suspension arms 14,16 which are pivotably mounted on the vehicle body at their inner ends to allow vertical movement of the wheels 10,12 relative to the vehicle body. Two non-steerable rear wheels 18,20 are similarly mounted on corresponding suspension arms 22,24 at the rear of the vehicle. The front suspension arms 14,16 are interconnected by an antiroll torsion bar 26 which resiliently opposes relative vertical movement of the two front wheels 10,12. The torsion bar 26, which has its central lateral part rotatably mounted on the vehicle body or chassis by mounting bushes, forms part of an assembly including a hydraulic actuator 28 which is only shown schematically and can be operated to apply a torque between the two ends of the torsion bar 26 thereby tending to move the suspension arms 14, 16, and hence the two front wheels 10,12, in opposite vertical directions relative to the vehicle body. The actuator can take a variety of forms which are known, such as a hydraulic strut acting between the end of the torsion bar and the body, or a torsional actuator in the central lateral part of the torsion bar applying a torque between the two ends. A corresponding antiroll bar 30 with an associated actuator 32 acts between the rear suspension arms 22,24 to control articulation of the rear wheels 18,20.

The actuators 28,32 are controlled by a hydraulic control valve block 34 which can connect them up in known manner to a hydraulic pump 36 and reservoir 38 so as to apply torque in either direction independently to each anti-roll bar 26,30.

The valve block 34 is controlled by an electronic suspension control unit 40, which receives signals from a pair of lateral accelerometers 42,44 mounted on the vehicle body which measure the acceleration of the vehicle body in the lateral direction, i. e. the horizontal direction perpendicular to the normal direction of travel at separate points on the vehicle body. The control unit uses these signals to control vehicle roll, but is also capable of causing cyclical or pulsed variations in the actuator force, the purpose of which will be described below.

Each of the wheels 10,12,18,20 has a brake associated with it comprising a brake disk 52,54,56,58 and a hydraulically operated brake calliper 60,62,64,66. The braking torque at each wheel is controlled by a hydraulic modulator unit 68 under the control of an electronic braking control unit 70. This control unit 70 receives signals from wheel speed sensors 72,74,76,78 associated with each of the wheels which enable it to determine in known manner the level or magnitude of rotational slip of each of the wheels, and whether the slip is positive, i. e. the wheels are rotating too fast for the ground speed of the vehicle, or negative, i. e. the wheels are rotating too slowly for the ground speed of the vehicle. It is also in communication with the suspension control unit 40.

In operation the braking control unit 70 performs an anti-lock function.

It does this by monitoring, during braking, the wheel speeds and determining from them the level of slip of each of the wheels. If the slip of any the wheels is negative and greater than a predetermined level this implies that the braking torque is too high for the coefficient of friction between the tyre and the surface over which it is travelling. The braking torque to the slipping wheel is therefore reduced to allow the wheel speed to increase. The level of slip is thereby controlled and is limited to a level where traction is maximized.

In some cases the coefficient of friction between the wheel and the surface over which is travelling is so low that, even if the brake is completely released, the friction is not sufficient to drive the wheel speed back up again to the desired level quickly enough. Under these circumstances, if the brake has been completely released and the wheel is still slipping by an undesirable amount, the brake control unit 70 sends a signal to the suspension control unit 40 which applies a pulsed oscillation in the hydraulic pressure supplied to the roll control actuator 28,32 associated with the slipping wheel. This pulsation will cause a pulsation in the torque between the two halves of the torsion bar, thereby causing the load on the wheel to oscillate about the level at which it would be without the pulsation.

Since the wheel is slipping anyway the periods of reduced load will not greatly affect the wheel speed, but the periods of higher load will, under certain conditions, be sufficient to cause the wheel speed to increase again so that the wheel slip decreases towards the desired level. In this way the suspension system is used to enhance the functioning of the ABS system.

Referring to Figure 2, in a second embodiment of the invention a vehicle has a fully active independent suspension, each of the two front wheels 110, 112 and the two rear wheels 118,120 being mounted on a suspension arm 114, 116,122,124 which pivots about its inboard end. The vertical movement of each wheel is controlled by a coil spring (not shown) and a hydraulic strut 126,128,130,132, the struts being connectable via a valve block 134 to a hydraulic pump 136 and to a reservoir. The valve block 134 is controlled by an electronic control unit 140 which controls the hydraulic pressure supplied to each of the struts 126,128,130,132 in response to signals from various sensors. These comprise: a pair of lateral accelerometers 142,144 which measure the lateral acceleration at different points of the vehicle body, height sensors 146 which measure the height of each of the wheels 110,112,118,120 relative to the vehicle body, and road speed sensors 150, only one of which is shown, which measure the road speed of each of the wheels 110,112,118,120, and hence allow the control unit to determine the road speed of the vehicle and the level of slip of each of the wheels.

The control unit 140 includes a map which determines what pressure is supplied to each of the struts 126,128,130,132 in response to the full range of possible inputs from the sensors 142,144,146,150. This control includes the usual functions to control pitch and roll of the vehicle and to keep the vehicle level under various loads. However, it also includes a function which allows the pressure supplied to the struts 126,128,130,132 to be pulsed when certain conditions are detected. The vehicle also includes a brake system corresponding to that shown in the system of Figure 1, but it is not shown in Figure 2.

The operation of the system in the second embodiment is essentially the same as in the first embodiment, except that instead of pulsing the hydraulic pressure to the roll-control actuators, the pressure to the hydraulic struts is pulsed. This has the advantage over the first embodiment that each of the wheels can have its suspension pulsed independently.

It will be appreciated that air springs in a fully active air suspension system, or even gas struts in a gas-operated levelling system, could also be used to the same effect instead of the hydraulic struts.

Claims

CLAIMS 1. A vehicle suspension system comprising an actuator arranged to produce an actuator force between a sprung part of the vehicle and a part of the suspension which can move relative to said sprung part thereby to allow vertical travel of a wheel, measuring means arranged to measure slip of said wheel, and control means arranged to produce a controlled variation in said actuator force in the event of predetermined slip conditions so as to control said slip.
2. A system according to claim 1 wherein the controlled variation in said actuator force is a cyclical variation.
3. A system according to claim 1 or claim 2 wherein the controlled variation in said actuator force is a pulsed variation.
4. A system according to any foregoing claim wherein said predetermined slip conditions include a wheel slip of at least a predetermined magnitude.
5. A system according to any foregoing claim wherein the control means is also arranged to control the level of braking of the wheel.
6. A system according to claim 5 wherein the control means is arranged to control the actuator force and the braking so as to bring the slip of the wheel towards a predetermined desired level of slip.
7. A system according to claim 5 or claim 6 wherein the control system is arranged to provide the controlled variation in the actuator force if the slip of the wheel is higher than a desired level and a brake associated with the wheel is completely released.
8. A system according to any foregoing claim wherein the actuator comprises a strut arranged to produce forces in a vertical direction between a wheel of the vehicle and a sprung part of the vehicle.
9. A system according to any foregoing claim wherein the actuator is arranged to urge two wheels of the vehicle in opposite vertical directions.
10. A system according to claim 9 wherein the actuator is a roll-control actuator, said two wheels being on opposite sides of the vehicle.
11. A system according to any foregoing claim wherein said actuator controls vertical wheel travel of a wheel or the wheels at one end of the vehicle only.
12. A system according to claim 11 further comprising a second actuator arranged to control vertical movement of a wheel or the wheels at the other end of the vehicle only, the two actuators being controllable independently.
13. A system according to any one of claims 1 to 8 including four actuators each associated with a respective wheel of the vehicle, wherein each actuator can be controlled so as to provide an independently controlled variation in actuator force for its respective wheel.
14. A vehicle suspension system substantially as hereinbefore described with reference to the accompanying drawings.