The invention relates to a suspension system for a motor vehicle. In

particular it relates to a suspension system for a motor vehicle having front

wheels and rear wheels, the suspension system comprising front suspension

units for supporting vehicle weight on the front wheels and rear suspension

units for supporting vehicle weight on the rear wheels, each suspension unit

of at least one of the front suspension units and the rear suspension units

being operative to adjust the ride height between sprung and unsprung

parts of the vehicle adjacent to each operative suspension unit, and control

means operatively connected to the operative suspension units for

monitoring and maintaining each ride height at a particular setting.

In US δ 452 919 it has been proposed to adapt the control means to

provide for each operative suspension unit a respective up height change

signal to increase the ride height, a respective down height change signal to

decrease the ride height and to monitor the height change signals to detect

the condition where the vehicle becomes at least partially supported by

direct contact between the chassis and the ground. This condition is

sometimes known as vehicle hang-up or belly-out. While the arrangement

described in US 5 452 919 is known to work well, it relies on the vehicle

hang-up occurring before corrective action is taken so progress of the vehicle

over rough terrain can be impeded.

It is an object of the present invention to provide a vehicle suspension

system of the kind described which acts to make severe vehicle hang up less

likely

The present invention provides a suspension system for a motor vehicle

having front wheels and rear wheels, the suspension system comprising

front suspension units for supporting vehicle weight on the front wheels and

rear suspension units for supporting vehicle weight on the rear wheels, each

suspension unit of at least one of the front suspension units and the rear

suspension units being operative to adjust the ride height between sprung

and unsprung parts of the vehicle adjacent to each operative suspension

unit, control means operatively connected to the operative suspension units

for monitoring and maintaining each ride height at a particular setting and

adapted to provide for each operative suspension unit a respective up height

change signal to increase the ride height and a respective down height

change signal to decrease the ride height, and wheel speed monitoring

means for monitoring wheel speeds on at least two of said front and rear

wheels, the control means being operative to provide a corrective up height

change signal m the event of an unrequested height change which is not in

response to a height change signal and there existed before the unrequested

height change a wheel speed signal indicative of vehicle movement

By monitoring wheel speeds it is possible to detect conditions under

which vehicle hang-up is occurring. This enables the suspension system to

take action as hang-up starts and progress of the vehicle over rough terrain

is impeded less.

The corrective height change signal may be applied to each operative

suspension unit. Alternatively, the control means may be operative to

monitor abnormal vehicle activity and to provide the corrective height

change signal to the appropriate suspension unit. For example, the wheel

speed monitoring means may be operative to provide an abnormal wheel

speed signal if the wheel speeds are outside predetermined wheel speed

parameters as an indication of abnormal vehicle activity, the control means

being responsive to the abnormal wheel speed signal to provide the

corrective height change signal to the suspensions unit on the side of the

vehicle with the highest wheel speed. The predetermined wheel speed

parameters may comprise a predetermined difference in wheel speeds. The

abnormal wheel speed signal may be provided by an electronic traction

control system.

In a further example of the control means being operative to monitor

abnormal vehicle activity and to provide the corrective height change signal

to the appropriate suspension unit, the operative suspension units use fluid

pressure to support the vehicle and the control means is operative to

monitor the suspension pressure in each operative suspension unit and

provide an abnormal suspension pressure signal as an indication of

abnormal vehicle activitv if suspension pressure is outside predetermined

suspension pressure parameters the control means being responsive to the

abnormal suspension pressure signal to provide the corrective height change

signal to the suspension unit on the side of the vehicle with the lowest

suspension pressure In this case the predetermined suspension pressure

parameters may comprise a predetermined reduction in suspension pressure

subsequent to the unrequested height change

Preferably, the suspension pressures after the unrequested height

change are averaged and compared with the averaged suspension pressures

before the unrequested height change. Conveniently the suspension

pressures are sensed by a single transducer The predetermined suspension

pressure parameters may also comprise a predetermined pressure difference

between a suspension unit on one side of the vehicle and a suspension unit

on the other side.

Preferably, the control means is responsive to a throttle open signal

indicative of the depression of accelerator pedal or other driver controlled

engine demand device and the corrective height change is inhibited in the

absence of the throttle open signal at least until a further condition is met

Such a further condition may comprise a predetermined time lapse or the

- a

vehicle attaining a predetermined road speed. Furthermore, the control

means may be responsive to an engine low torque signal indicative of a low

engine fuelling rate such that inhibition in the absence of a throttle open

signal is applied only if a low torque signal exits. The inhibition of the

corrective height change may be conditional on the absence of a wheel speed

signal indicating that no wheels are rotating.

The invention also provides a motor vehicle incorporating a suspension

system according to the invention.

Other aspects of the invention will be apparent from the appended

claims and from the following description of the invention which is given by

way of example and with reference to the accompanying drawings, of

which:-

Fig. l is a schematic view of one example of a suspension system

according to the invention;

Fig.2 is a diagram showing components of a valve block and ancillary

items shown in Fig.l.

Fig.3 is a logic diagram illustrating operation of a control means shown

in Fig.l;

Fig.4 is a flow diagram showing a first modification to the diagram

shown in Fig.3; and

Fig.5 is a flow diagram showing a second modification to the diagram

shown in Fig.3

Referring to Figs.l and 2, there is shown a vehicle front axle 11 and a

rear axle 12 Both axles 11, 12 are rigid driving axles and represent

unsprung parts of the vehicle The front axle 11 carries front wheels 13 and

14 and the rear axle 12 carries rear wheels 15 and 16.

A sprung part of the vehicle, represented diagrammatically as a body or

chassis 17, has its weight supported by coil spring suspension units 18 and

19 connected between the chassis 17 and the front axle 11 and by air spring

suspension units 21 and 22 connected to the rear axle 12.

Air pressure in the air springs 21 and 22 is controlled by a valve block

23 through individual pipes 24 and 25. The valve block 23 includes a two

way solenoid valve 23A, 23B for each air spring 21 and 22 which can

connect the respective air spring to a pressure source such as a motor driven

compressor 20, to an exhaust to atmosphere 30 or isolate the respective pipe

24 or 25 so that the mass of air m each air spring remains constant. The

compressor 20 supplies air through a non-return valve 23C and a

regenerative dryer 23D and the exhaust 30 is controlled by a two-way

solenoid valve 23E

Mounted close to each air spring 21 and 22 is a link 26 to an arm 27

carried by a respective height sensor 28, 29 which gives a ride height signal

indicative of the ride height between the sprung and unsprung parts of the

vehicle adjacent the respective air spring 21, 22. Typically the height

sensor 28. 29 is a rotary Hall-effect transducer or a rotary potentiometer

Ride height can be adjusted or maintained at a particular setting by a

control means comprising the solenoid valves 23A, 23B and 23E in the valve

block 23, the compressor 20 and an electronic control unit (ECU) 31 which is

connected to the solenoid valves and the compressor to control the emission

or exhaust to and from each air spring 21. 22.

The height sensors 28 and 29 are connected to the ECU 31 to provide

the ride height signals as described above In addition the ECU 31 is also

connected to wheel velocity transducers 32, 33, 34 and 35 on each wheel 13,

14, 15 and 16 which provide signals of wheel speed to the ECU Provided

optionally are two pressure transducers 36 and 37 in pipes 24 and 25

respectively which can provide a signal indicative of the pressure in each of

the air springs 21 and 22. Alternatively, a single pressure transducer 36A

may be provided in the passage between the solenoid valves 23A and 23B

and the dryer 23D.

In normal use the ECU maintains the ride height at each air spring 21

and 22 within predetermined limits, l e a dead band, and with appropriate

response damping to prevent unnecessarv air consumption when the vehicle

load is constant and the springs are performing their normal function of

isolating the vehicle from the effects of the road surface

Normal system response is undesirable under some driving conditions

particularly some of those conditions which are likely to be encountered

whilst driving Off-road", 1 e off normal paved roads These would include

situations where one or more wheels suffer reduced or no ground contact

pressure the vehicle hang-up or bellv-out previously referred to These

situations occur, for instance when the vehicle is crossing deep ruts or when

reaching the crest of a steep hill. In these abnormal circumstances the ECU

responds in the manner illustrated in Fig.3 At 41 a height change 42 (as

derived from the height sensors 28 and 29) is analysed. If the height change

is not unrequested then the normal function of maintaining the ride height

is continued at 43. On the other hand, if the height change is unrequested,

then the ECU proceeds to analyse at 44 if there was a wheel speed signal

prior to the height change If there was no such wheel speed signal, then

further height changes are inhibited at 45 since this indicates a situation

where the vehicle is stationary and is perhaps being loaded or unloaded or

that there are maintenance or repair functions being carried out on the

vehicle

If there does exist a wheel speed signal prior to the height change then

the logic in the ECU proceeds to ask at 46 whether or not there is abnormal

vehicle activity which would indicate which air spring should be inflated as

indicated at 47. If such abnormal activity cannot be detected then both air

springs are inflated as indicated at 48 In referring to inflation of the air

springs, this would normally be effected by an increase in the datum height

reference for the appropriate air spring so that the system would respond by

moving to an increased ride height.

In a modification to Fig.3 (not shown) the system is simplified by

omitting the abnormal activity query 46 so that if there is a wheel speed

signal prior to a height change at 44 then the ECU 31 responds by

increasing the ride height at both sides of the vehicle, as indicated by line

49

One example of abnormal activity queried at 46 is differential wheels

speed as determined by a difference between wheel speeds on one side of the

vehicle and on the other, i.e. a comparison of the signal derived from the left

hand front wheel speed transducer 32 with that derived from the right hand

front wheel speed transducer 33 or a comparison of the signal derived from

the rear wheel speed transducer 34 and that derived from the right hand

rear wheel speed transducer 35. If there is a speed difference greater than a

predetermined magnitude, the ECU proceeds at 47 to increase the ride

height at the side of the wheel with the greater velocitv

The wheel speed transducers 32, 33 34 and 35 are typically those used

in an anti-lock brake control system (ABS) or an electronic traction control

system (ETC) Hence it may be convenient to query for abnormal activity at

46 by detecting a signal from the ETC control system which itself mav have

an electronic control unit which is integrated with the ECU 31

A further example of a query for abnormal activity at 46 is that there is

a significant suspension pressure difference before and after an unrequested

height change as detected by the optional pressure transducers 35 and 36

If such a pressure difference exists, I e if the difference is more than a

predetermined amount, then the height is increased at the air spring with

the lowest air pressure

An increase in ride height which is not accompanied by a corresponding

increase in suspension pressure is an indication of vehicle hang- up In order

to minimise the effects of normal suspension movements, it is preferable

that the suspension pressures are all averaged both before and after the

unrequested height change

If a single pressure transducer 36A is used, the suspension pressures

are sampled by alternately opening solenoid valves 23A and 23B, preferably

whilst keeping the exhaust solenoid valve 23E closed and the compressor 20

idle.

A further comparison of suspension pressure is to compare the pressure

on either side of the vehicle and, if there is a difference of more than a

predetermined amount, to increase ride height at the side with the lowest

pressure.

It will be appreciated that the queries for abnormal activity described

above are not mutually exclusive and the control means can be designed to

use any or all of them as appropriate.

In a second modification illustrated by Fig.4, instead of proceeding to

increase the ride height at both sides at 48 if there is no abnormal activity

at 46 (line 51), the ECU 31 queries whether or not there is a throttle open or

driver demand signal as indicated at 52. Typically a throttle open/driver

demand signal is derived from a potentiometer or micro-switch connected to

an accelerator pedal. If a throttle open/driver demand signal exists, then

the ECU 31 proceeds to increase the height at both sides of the vehicle at

53. On the other hand, if there is no throttle open/driver demand signal, the

ECU 31 proceeds to query at 54 if any of the wheels 13, 14, 15 or 16 are

rotating. If any wheel is rotating, this indicates that the vehicle is simply

coasting or on the over-run and the ECU responds at 53 to increase the ride

height at both sides. If there are no wheels rotating, an inhibit command is

given at 55 to prevent further inflation or exhaust of the air springs. The

inhibit signal at 55 is maintained for a set time or until the vehicle speed

exceeds a predetermined threshold, e.g. 40 kilometres per hour. The set

time may be in the range 1 to 20 minutes according to the vehicle type and

anticipated driving conditions. 10 minutes being typical for a small sports

utility vehicle with off-road capabilities.

In the third modification, illustrated m Fig.5. the arrangement

described above with reference to Fig.4 is modified by querying at 56

whether engine torque is below a certain magnitude. Engine torque can be

derived from the engine management system, e.g. from the fuelling rate, or

from a drive-by-wire accelerator pedal control system. If low engine torque

is detected, the ECU 31 proceeds to increase the ride height on both sides at

53; if not, it proceeds to query at 52 whether or not there is a throttle

open/driver demand signal as previously described.

Although described above m relation to an air suspension system which

is on only one axle of the vehicle, invention is readily applied to a system

where there are two or more axles fitted with air springs or where there is

independent suspension.

Furthermore although an air suspension system has been described, the

invention is also applicable to hydropneumatic suspension systems which

employ gas springs or other hydraulic accumulators for the controlled

suspension units and the suspension ride height is controlled by increasing

or decreasing a volume of hydraulic fluid which supports the vehicle weight.