Vehicle suspension system

A vehicle suspension system incorporating suspension units (46-48) associated with respective vehicle wheels and selectively controllable to vary the ride height of the vehicle. Vehicle speed sensors and height sensors are responsive to departure of the ride height from a predetermined height datum which is a function of the vehicle speed and to provide a signal related thereto to control movement of at least one suspension unit thereby to tend to move at least a part of the vehicle to said predetermined height datum. Timer means are provided so that the movement of a suspension unit in response to a signal from a height sensor is inhibited until a signal of at least a predetermined magnitude has persisted for at least a prescribed part of a prescribed period of time.

BACKGROUND OF THE INVENTION 
This invention relates to a vehicle suspension and in particular, although 
not exclusively, to a suspension for an untracked wheeled vehicle. It 
relates also to a vehicle incorporating the suspension of the invention 
and to method of controlling a vehicle suspension. 
The invention is directed particularly but not exclusively to a vehicle of 
a kind having an air or like gas suspension. 
The invention seeks to provide a suspension which offers a facility to 
achieve good ride characteristics related to the operating condition of 
the vehicle. 
SUMMARY OF THE INVENTION 
In accordance with one of its aspects the present invention provides a 
vehicle suspension comprising in. combination vehicle speed sensing means, 
suspension units associated with respective vehicle wheels and selectively 
controllable to vary the ride height of the vehicle, each suspension unit 
incorporating an electrical height sensing device responsive to departure 
of the ride height from a predetermined height datum which is a function 
of the vehicle speed and to provide a signal related thereto to control 
movement of at least one suspension unit thereby to tend to move at least 
a part of the vehicle to said predetermined height datum and timer means 
whereby the movement of a suspension unit in response to a signal from a 
height sensing device is inhibited until a signal of at least a prescribed 
magnitude has persisted for at least a prescribed part of a prescribed 
period of time. 
The suspension units each may be gas, e.g pneumatic suspension units and 
valve means responsive to signals from the height sensing device may be 
provided for selective control of gas to or from each suspension unit. 
Each suspension unit may incorporate a damping device and the damping 
device may be of a kind which incorporates an electrical height sensing 
device to provide a signal related to departure of the ride height from a 
predetermined height datum. An example of a suitable damper unit is a 
linear variable differential transformer type unit such as described in 
the specification of UK Patent Application GB 2027207A. Alternatively 
electrical height sensing devices such as of the types comprising 
ultrasonic displacement sensors or potentiometers may be employed. 
The invention further provides a variable sensitivity system in which the 
period for which the timer means inhibits movement of a suspension unit in 
response to a signal from a height sensor is a function of the magnitude 
of that signal. The sensitivity variation may be substantially 
continuously variable whereby for example the time delay is a function of 
an averaged value of the signal from the height sensor or it may be of a 
stepped kind in which when the magnitude of the signal exceeds a 
prescribed magnitude there is no time delay or only a short period of time 
delay whereas when the magnitude of the height departure signal is below a 
prescribed magnitude the timer means causes a delay of at least a 
prescribed magnitude. 
It is envisaged that the system may have two sensitivity modes, which may 
be known as insensitive and sensitive modes respectively. The sensitive 
mode may be invoked for example during initial start up of the vehicle to 
result in speedy supply of gas to gas suspension units thereby to place 
the vehicle at approximately the desired ride height. That initial desired 
ride height may be a height lying within a prescribed band. After a 
prescribed period following initial start up the system may change to an 
insensitive mode of operation in which the ride height information is used 
to position the vehicle more accurately to a particular height. The later 
positioning may be achieved by the signals from the height sensors 
indicating departure from a prescribed desired height; if that signal 
persists continuously for at least a specified period of time or at least 
a specified proportion of a prescribed period of time, that signal may be 
regarded as indicative of a valid error situation and control means 
associated with the suspension system may operate to adjust the vehicle 
height by admitting or exhausting gas from one or more of the suspension 
units. 
More particularly, the vehicle suspension may comprise an electronic 
control unit having an algorithm adapted to collate and store relevant 
input data as a vehicle traverses a road and to eliminate irrelevant input 
data thereby to ensure that the system does not respond inappropriately to 
transient inputs. In one example the algorithm may be adapted to decide 
which of the four height sensors has the largest error relative to a 
datum, to decide whether the error is above or below the datum and whether 
the error is outside pre-set limits. If the error is greater than the 
pre-set limits a timer may be initiated to set a first time delay T1. If 
the direction of error should change during the time T1, the timer may be 
adapted to be re-set. However, if the timer times out, i.e. there is no 
intermediate change of direction, then the electronic control unit may 
cause gas to be admitted or exhausted from one or more of the suspension 
units. 
The magnitude of T1 may have two values dependent upon whether the system 
is to be operating in a sensitive or an insensitive mode. The selection of 
mode may be achieved automatically having regard to the vehicle speed 
and/or for example to the magnitude of height error from a prescribed 
datum. The system may be arranged to select the sensitive (i.e. rapid 
response) mode automatically when a change of ride height state of at 
least a certain magnitude has been demanded. The demand might arise 
automatically having regard to information from the height sensor means or 
as a result of a manual information input to the electronic control unit 
(e.g when changing from a manual height override situation to an automatic 
normal mode of operation). The insensitive mode may be invoked 
automatically by the vehicle attaining a height lying within a prescribed 
error band from the desired ride height. In that case the timer T1 has a 
value much greater than when in the sensitive mode. The purpose of this 
greater time is to ensure that the system does not respond inappropriately 
to transient road input data. Thus operation of control equipment to admit 
or exhaust gas from the suspension units is inhibited unless reasonably 
necessary, and energy to maintain a supply of compressed gas is conserved. 
The vehicle suspension may have associated therewith an electronic control 
unit which operates automatically to control the suspension units in 
response to received information and which also operates in response to a 
manual override. The system may be adapted to facilitate manual override 
for example to put the suspension in a kneel condition in which the 
vehicle is lowered; the electronic control unit may be programmed to allow 
a kneel condition to be achieved only if the vehicle is stationary and/or 
the hand (park) brake is applied to resist vehicle movement. 
The manual override may be operable to a high profile condition in which 
the ride height is selected by the driver to be greater than normally 
would prevail in automatic operation of the system, The electronic control 
unit may be arranged to allow entry to the high profile mode only if the 
vehicle speed is below a prescribed threshhold, e.g 35 miles per hour, and 
it may furthermore be arranged to revert automatically to a standard ride 
height mode in the event of the vehicle speed exceeding a prescribed 
figure, which may be aforementioned speed which must not be exceeded for 
entry into the high profile mode, or another datum speed. 
The system may also incorporate a manual override which locks the 
suspension to a standard ride height in the case of a vehicle used for 
towing. Alternatively, sensor means may be provided automatically to 
detect when the vehicle is coupled to a load for towing and automatically 
to inhibit significant departures from a standard ride height, e.g to a 
high or low ride height at least at such times as the vehicle is in 
motion. 
The electronic control unit may be adapted automatically to lower the ride 
height of the vehicle from a standard to a lower height when the vehicle 
speed exceeds a prescribed magnitude, e.g 60 miles per hour. The control 
unit may be adapted also to return to a normal ride height if the vehicle 
speed falls below a prescribed figure for at least a prescribed period 
and/or to revert forthwith to a normal, standard mode if there is a 
significant reduction of speed to below another prescribed speed.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
The following table lists major components of the system. 
The electronically controlled air suspension system as fitted to a four 
wheel, four wheel drive vehicle comprises the following major components. 
Front Air Spring Assembly 
Rear Air Spring Assembly 
Electronic Control Unit 
Reservoir Assembly 
Air Harness 
Mounting Bracket and Ancilliary Equipment, comprising: 
(i) Air Supply System comprising: 
Mounting Brackets 
Compressor Assembly 
Air Dryer Assembly 
Isolation Mounts 
(ii) Assembly 10--Valve Block & Air (see FIG. 1) Service Unit comprising: 
Solenoid Operated Air Distribution Valve Block 11 
Air Service Unit 12 incorporating: 
Non return Valve 
Pressure Switch 
Filter 
Schrader Connection 
Air Pipe Connector 
Damper--Height Sensor Assembly (see FIG. 2 ) 
Driver Operated Functional Switches 
The suspension spring media is provided by four air springs which replace 
the conventional coil springs. 
The air springs provide a variable rate spring which achieve near constant 
ride frequency for all load conditions. Thus a significant improvement in 
overall ride is attainable. 
The high pressure (13 bar) compressed air is provided by a 12V D.C. wobble 
piston Compressor which incorporates life of the vehicle brushes, radio 
frequency interference suppression and thermal trip protection. 
To reduce air induction noise and protect the compressor cylinder bore, a 
serviceable air intake silencer/filter is fitted. 
The compressed air passes through a oneway valve (incorporated into the air 
service unit), to a 5L Reservoir which stores air at a nominal 13 bar 
pressure (see FIG. 3). 
The reservoir, in addition, acts as a drop tank for the moisture that the 
compressed air will contain. Consequently, the reservoir must be drained 
periodically using the drain plug provided. 
The compressed air is taken from the reservoir to the Air Service Unit 
which has a replaceble main air filter and a pressure switch. The purpose 
of the Pressure Switch is to maintain the system pressure between set 
limits by switching the compressor on and off via an E.C.U. controlled 
relay. 
Air is then passed to the solenoid (six) operated air distribution valve 
block. The purpose of the valve block is to control the flow of air to, 
and from, the four air springs dependent upon system demands as determined 
by the inputs to the E.C.U.. 
Air flow to, and from, the air springs is controlled via pilot (air) 
operated high flow diaphragm valves. The pilot air is controlled by the 
six solenoid operated valves. 
The pilot air line incorporates non-return valves in order to prevent loss 
of pilot pressure, and therefore air spring pressure, if there is a total 
loss of reservoir pressure. 
For air to be admitted to any air spring the main air feed solenoid valve 
must be energized in addition to the relevant air spring solenoid or 
solenoids. 
Conversely, for air to be exhausted from any spring the exhaust solenoid 
valve must be energized in addition to the relevant air spring solenoid or 
solenoids. 
A silencer is fitted to the exhaust port of the valve block. 
Air is passed through a dryer prior to being fed through the high flow 
diaphragm valves and into the air springs. 
Conversely, air exhausted from the air springs passes through the dryer in 
the reverse direction prior to exhaust to atmosphere via the exhaust 
valve. 
Moisture is removed from the air as it passes vertically upwards through 
the dryer dessicant. The dessicant in the lower portion of the dryer 
becomes `wet`. 
During exhaust the dry air from the air springs passes vertically downwards 
through the `wet` dessicant absorbing moisture prior to venting to 
atmosphere. 
This purging action regenerates the air dryer. 
As stated, the purpose of the system is to provide four height sensor 
modes. The height of the vehicle, relative to the axles, is provided by 
sensors incorporated in each damper. 
A variable inductive height sensor is incorporated in the dust cover of 
each damper 30 (see FIG. 2) and provides a signal whose level is dependent 
upon damper displacement. 
The suspension is controlled by an eight bit microprocessor Electronic 
Control Unit which operates in one of the several states shown below 
status chart. During each state the E.C.U. maintains the requested ride 
height by adjusting the volume of air in an air spring attached to each 
wheel. 
The controller determines engine rotation by measurement of the period of a 
phase of the vehicle alternator. If this period is greater than a required 
value the engine shall be considered stopped and all suspension functions 
will be inhibited except that level vehicle on parking mode. 
This is to prevent the compressor drawing a large current from the battery 
when the alternator is not charging. 
The controller calculates a value of vehicle speed by measuring the period 
between pulses from a speed sensor. The value is used to determine an 
allowable transition from one suspension state to another. 
The controller adjusts the height of each suspension unit in accordance 
with the demanded state. 
To raise the height the appropriate air spring valves are selected together 
with the main air feed inlet valve. To lower the height the appropriate 
air spring valves are selected with the exhaust valve. 
When raising the height, the rear of the vehicle shall be raised first by 
approximately 70% of the required height change followed by raising the 
front to 70% of the required change. The remaining 30% change will be 
achieved by individual adjustment of each suspension unit. 
Lowering of the vehicle will be complimentary to raising, with the front of 
the vehicle being lowered first. 
This will ensure that when the headlamps are illuminated there is no 
inconvenience to other road users. 
However, when lowering to the kneel position all air valves are opened at 
the same time to achieve a fast response. 
The E.C.U. accepts switch inputs for the following functions: 
Handbrake 
Footbrake 
Door Switch (open/closed) except tailgate 
Reservoir pressure 
Up 
Down 
Inhibit 
The handbrake switch is used to control the kneel demand. Kneel cannot be 
entered unless: vehicle speed is zero, all doors closed, engine running 
and HANDBRAKE APPLIED. 
Door (except tailgate) switch (open/closed) is used to control kneel 
demand. Kneel cannot be entered unless: vehicle speed is zero, ALL DOORS 
CLOSED, engine running and handbrake applied. 
When the FOOTBRAKE is on, and for a period of one second after it is off, 
all height levelling is suspended. 
The purpose of this inhibit is to prevent the system reacting to transient 
suspension movement caused by weight transfer during braking. 
This inhibit function is removed after a period of 80 seconds regardless of 
footbrake state. 
When the E.C.U. detects an output from the PRESSURE SWITCH indicating low 
pressure, then the E.C.U. operates the pump relay until the pressure 
switch indicates normal pressure. The pump relay will not operate unless 
the engine speed is greater than 500 r.p.m.. 
The HIGH PROFILE ride state is driver selected by depressing the momentary 
UP switch. High profile raises the vehicle body by approximately 45 mm at 
road speeds below 40 m.p.h.. If road speed exceeds this figure the E.C.U. 
automatically reverts to standard ride height. High profile then has to be 
re-selected, if required, and is dependent upon conditional requirements 
(see FIG. 4). 
Lowering the vehicle from high profile to standard ride height is achieved 
by depressing the momentary down switch twice. 
The above are dependent upon fulfilment of all conditional requirements. 
The self latching inhibit switch is engaged to maintain the suspension at 
the standard ride height. That is, automatic height adjustment and the up 
and down momentary switches are inoperative (inhibited). This should be 
engaged when towing. 
Note that engaging the inhibit switch will automatically return the vehicle 
to standard ride height from any other height mode. 
General freeze state is entered at any time on the E.C.U. detecting a 
passenger door opening. All height control is suspended in this state with 
all height control valves closed. 
The E.C.U. will maintain the system in this state until the criterion for 
entering any state are met. 
The freeze state is intended as a SAFETY factor and thus should not be 
artificially overriden. 
A levelling mode on parking is incorporated into the E.C.U. functionality 
wherein the vehicle will continue to level to the lowest corner, for 
approximately 10 seconds, after the vehicle is exited and all doors 
closed. 
The Electronic Control Unit incorporates Fault Recovery Strategies to 
minimise the effect of a sensor, or actuator failure. 
A serial data link is provided to allow diagnostics information to be 
displayed and to set height sensor datums at the end of vehicle build or 
service. 
The valve means of FIG. 5 comprises a valve manifold unit 40 having a 
central manifold chamber and four direct acting electrically operated 
solenoid valves 42-45 selectively controllable to allow pressurised air to 
be admitted to a respective air spring 46-49 or exhausted therefrom. 
The manifold also incorporates three other electrically operated solenoid 
valves 50-52. One valve is a supply valve to allow pressurised air to be 
admitted to the chamber 41 from an external reservoir via a non-return 
valve 54. Another valve 51 is a first exhaust valve and interconnects with 
a second exhaust valve 52 via a first common flow path 55, an externally 
mounted regenerative drier 56 and a second common flow path 57. 
An externally located compressor 58 connects via passage 58' with the 
second common flow path 57. The valve manifold also incorporates an 
exhaust passage 59 from the valve 52, an auxiliary supply passage 60 
having a one-way valve 61 and interconnecting the reservoir 53 with a 
supply passage part 62 of the first common flow passage 55, and a non 
return valve 63 in that other part of the passage 55 between the supply 
part 62 and the first exhaust valve 51. 
The valve means also incorporates other conventional items such as a safety 
relief valve 64, pressure switch 65 to initiate operation of the 
compressor 58 and air inlet filter 66. 
In the aforedescribed apparatus the pressure in chamber 41 is controlled by 
operation of the valves 50,51 and is selected in accordance with the 
instantaneous requirements of a particular spring 46-49 which is put in 
communication with the chamber via 49 a respective valve 42-45. 
When the reservoir pressure falls below a predetermined level and valves 51 
and 52 are not in an exhaust mode the compressor 58 supplies filtered air 
to the reservoir 53 via the second common flow 57, drier 56, supply 
passage 62, non-return valve 61 and line 60. 
When it is required to exhaust the chamber 41 any operation of the 
compressor is inhibited and the exhaust valves 51,52 are opened to allow 
air to flow via passage 55 and non-return valve 63 to pass regeneratively 
through drier 56 and then passage 57 and valve 52 to the exhaust line 59. 
The embodiment of FIG. 5 may be modified by replacing the second exhaust 
valve 52 with a pilot operated type valve 70 as shown schematically in 
FIG. 6, and the valve as shown in cross-section in FIG. 7. Parts common 
with FIG. 5 bear corresponding reference numerals. 
The valve 70 has a main exhaust path via passage 71, which leads from 
passage 57 and over a first face of a valve diaphragm 72 to an exhaust 
plenum 73 connected to the exhaust line 59. 
The valve diaphragm 72 is biased closed against passage 71 by a compression 
spring 74 and its face opposite the first face is acted on also by 
pressure of gas in the pilot chamber 75. The pressure in the pilot chamber 
is under the control of an electrically actuated solenoid valve plunger 
76. When plunger 76 is against a first valve seat 77 as shown in FIG. 7 
the second valve seat 78 is open and a pilot exhaust line 79 results in 
the pilot chamber 75 being at atmospheric pressure. When the plunger 76 
lies against the seat 78 the chamber is exposed to the pressure in the 
first common flow path 62 with which it interconnects via a pilot feed 
line 80. 
In use of the valve of FIG. 7, as incorporated in the valve means of FIG. 5 
in place of the second exhaust valve 52, during normal exhaust modes the 
plunger 76 is seated against seat 77 so that exhaust occurs only via the 
drier and passage 71, thereby obtaining regenerative drying of the drier 
by all of the exhausting gas. In this position chamber 75 is open to 
atmosphere and gas exhausting via passage 71 is able to counter the effect 
of bias spring 74 and thereby lift the diaphragm from the seat of passage 
71. 
In the case of the compressor being called upon to supply pressurized air 
to the reservoir the solenoid of valve plunger 76 is operated to lift the 
plunger from seat 77 to bear against seat 78, thus closing chamber 75 from 
the pilot exhaust line. The pressure in chamber 75 will thus be that in 
line 62 which is at or substantially equal to atmospheric pressure because 
of the presence of the reservoir's one-way valve 61 and the facility for 
line 62 to exhaust via passsage 71 when the compressor is not operational. 
Hence on initial start up of the compressor 58 the pressure in line 57 is 
at or substantially equal to atmospheric pressure. The compressor is not 
required to suffer the strain and wear associated with start up against a 
significant pressure head. 
Upon initial start up some gas will tend to exhaust via passage 71 against 
the bias of spring 74, but progressive increase of pressure in line 62 
will act via line 80 and chamber 75 to cause the diaphragm 72 to be 
closed, it being noted that the area of diaphragm exposed to the pressure 
in chamber 75 is much greater than that area within the valve seat at the 
end of passage 71. Thus after a momentary initial start up period the line 
71 is closed by action of the pressure built up in chamber 75 and the 
reservoir can be recharged.