Vehicle braking system

An electronically controlled braking system for a vehicle in which vehicle load measurements, made dynamically, as used to modify the braking demand, individually for each axle of the vehicle and in which, under predetermined conditions of vehicle speed, braking level and operating gradient, the deceleration error formed between braking demand by the driver and measured actual vehicle deceleration is used gradually, over a number of vehicle stops, to form an adaptive factor for correcting the braking demand in order to restore expected braking performance. No correction to the adaptive factor based on the deceleration error is made during a given stopping operation of the vehicle, but a summation of previous errors is arranged to cause a small increment in correction to be made after each stop until, over a number of vehicle stops, the error formed under the predetermined conditions falls to zero. Braking demands by the driver can be arranged to be compensated by introducing a demand offset dependent on the prevailing gradient on which the vehicle is operating.

The present invention relates to electrically controlled braking systems 
for vehicles. 
An electrically controlled braking system is known which includes an input 
transducer producing the driver's braking demands in electrical terms, a 
suitable electronic pressure controller, and electrical relay valves with 
integral pressure transducers on each axle of the vehicle. The system 
includes a pressure control loop taking an input signal from the brake 
pedal transducer which is used to provide a pressure error signal by 
comparison with a pressure transducer output signal, this pressure error 
forming the input to a pressure controller which generates an output 
signal which causes the pressure developed by an electro-pneumatic or 
electro-hydraulic converter to change in a direction such as to reduce the 
pressure error amplitude. 
It is an object of the present invention to provide an electrically 
controlled braking system of this type having improved braking control 
characteristics in relation to known systems. 
In accordance with the present invention, there is provided an 
electronically controlled braking system in which vehicle load 
measurements, made dynamically, are used to modify the braking demand, 
individually for each axle and in which, under preset conditions of speed, 
braking level and operating gradient, the error formed between braking 
demand and measured vehicle deceleration is used gradually, over a number 
of stops, to adapt braking pressures to restore expected braking 
performance. 
Preferably, no correction based on deceleration error is made during a 
vehicle stop but a summation of previous errors is arranged to cause a 
small increment in brake pressure correction to be applied after each stop 
until said error formed under the preset conditions falls to zero. 
Advantageously, braking pressure constants are compensated by introducing a 
demand offset in response to vehicle gradient, said operating gradient 
being measured as an equivalent deceleration, by comparison of an on-board 
deceleration transducer and the corresponding figure generated by 
measuring the rate of change of wheel speeds. 
In the latter system, the gradient correction can be made at low speeds but 
based on data which was obtained at speeds above a preset low limit and 
which is inhibited at very low braking demands. The gradient correction 
can provide a demand offset but is never allowed to exceed the actual 
driver demand in amplitude and at very low demands is reduced to zero. 
Preferably, the adaptive constant slowly built up over a series of vehicle 
stops is used to form a vehicle brake performance figure to be output by 
the system via a diagnostic port on demand. The adaptive constant can be 
monitored against a preset level, brake deficiency being signalled if this 
level is exceeded.

In a compensated braking system a foot pedal transducer generates a first 
signal indicating the braking level desired by the driver and additional 
sensors measure the vehicle axle loads and the operating gradient. The 
system makes appropriate open loop corrections to the brake pressure 
demands being interpreted from the driver pedal input with the aim of 
restoring the vehicle deceleration to be fixed in proportion to the 
driver's demand. 
Referring now to FIG. 1, there is shown a known system which employs a 
pressure control loop 10 taking an input D from a brake pedal transducer 
12 which is used to provide a pressure error signal E by comparison in an 
adder/subtractor 14 with a pressure transducer output signal P, this 
pressure error E forming the input to a pressure controller 16 which 
generates an output signal which causes the pressure developed by an 
electro-pneumatic or electro-hydraulic converter 18 to change in a 
direction such as to reduce the amplitude of the pressure error E. 
Pneumatic or hydraulic pressure medium is stored in a reservoir 20. 
The nature and circuit of such a pressure controller 16 depends upon the 
type of converter 18 employed. Two such converter principles are well 
known, namely an analogue system in which a valve is employed with 
pressure output developed proportional to solenoid current and a digital 
system, as shown in FIG. 1, in which a pair of simpler solenoid valves 
20a, 20b is employed to raise or lower a control chamber pressure by 
selective energisation. One form of pneumatic converter employs a relay 
valve 22 which responds to this control chamber pressure and which 
re-balances into the closed condition when the brake pressures at the 
actuators 24a, 24b for the brakes 26a, 26b (left and right) of the vehicle 
become equal to this control pressure. Such a valve 22 has an advantage in 
that the control chamber pressure responds rapidly to valve opening giving 
a fast control loop which is accurate and responsive. 
FIG. 2 illustrates a compensated braking system in accordance with the 
present invention which comprises closed loop controllers of the type 
shown in FIG. 1 which are separate for each axle or for each wheel and 
which are supplied with pressure demands D by a braking correction 
sub-system 28 such that front and rear systems may receive different 
pressure demands D.sub.F, D.sub.R for equal braking inputs. 
In the present system, as an alternative strategy to closing the 
deceleration loop for the vehicle, the main sources of braking parameter 
change are measured and the pressure demands to the inner closed loop 
pressure systems are adjusted to compensate for these measured changes. 
This leaves the remaining principal sources of system drift within the 
brakes themselves so that errors measured are an indication of brake 
condition and can be used to slowly adapt the relationship between brake 
demand and brake application pressures. As explained in detail below, this 
adaptation is arranged to take place over a sequence of many stops that 
the vehicle makes, but with no adaptive changes taking place during the 
course of each stop. 
The main sources of braking system disturbance are vehicle load and 
operating gradient, both of which can change suddenly, and brake 
deterioration which is much more gradual. The sudden changes require 
compensation by corresponding sudden corrections whilst slow changes can 
be countered by a gradual adaptation over a time period which can extend 
into days or weeks depending on vehicle usage. 
As indicated in FIG. 2, axle load readings L.sub.F, L.sub.R, generated by 
additional transducers 30a, 30b (front and rear) on the vehicle 32, are 
used as correction inputs and for each axle form a pressure constant 
expressed in psi/ton/g. As explained in more detail below in connection 
with FIG. 3, this is achieved by the use of a digital multiplier which 
forms a suitably scaled product of pedal input demand D and axle load 
measurement L, to form the pressure demand figure D.sub.R, D.sub.F. 
The other main disturbance to braking caused by gradient can be determined 
by a comparison between deceleration as sensed by a vehicle decelerometer 
34 and figures generated from speeds sensed by speed sensors 36 at the 
vehicle wheels and differentiated electronically in a gradient 
determination means 38 after being combined to form a vehicle reference 
signal in a manner which is well known in anti-lock systems (see FIG. 5 
and corresponding description). The gradient figure G generated is a 
decelerated error with a sign which indicates uphill or downhill and which 
can be added directly to braking demand D to achieve appropriate 
correction. As explained in detail hereinafter, at very low demand levels 
where check braking is being called, (for example where the vehicle is 
travelling downhill at a constant speed and braking demand is not intended 
to produce any appreciable deceleration), this addition is arranged to be 
inhibited and similar inhibition may be deliberately caused at low speed 
or may be based on gradient signals stored as the speed falls through a 
preset low speed band. Furthermore, at light braking demands, the gradient 
compensation offset is arranged to be reduced so that at no time is it 
allowed to exceed the actual driver demand, so as to prevent input 
cancellation or any step disturbance to braking as demand is gradually 
increased. 
FIG. 3 shows more detail of an example of a compensated braking controller 
in accordance with this invention where compensating input signals are 
derived from load measurements on each axle and gradient signals are 
provided by a gradient detector as in FIG. 2. Only the rear pressure 
channel is illustrated fully in FIG. 3, the front pressure channel being 
essentially the same and therefore being largely omitted for 
simplification of the drawings. 
Front and rear channel demand signals D from the brake demand transducer 12 
are added in respective adders 39a, 39b to the gradient correction signals 
G.sub.C from a correction calculating means 40 and supplied as first 
inputs to respective front and rear digital multipliers 42a, 42b. 
Deceleration demand signals from the demand transducer 12 are also, after 
filtering in a filter 44, compared in an adder/subtractor 46 with 
deceleration feedback signals F from a vehicle decelerometer (not shown) 
to form a deceleration error signal F.sub.E which is supplied to a 
calculation means 48 for providing a long term adaptive constant C for the 
vehicle brakes. As illustrated in more detail in FIG. 4, a controllable 
switch 50 is adapted to permit the passage of the deceleration error 
F.sub.E to the long term adaptive constant calculating means 48 only when 
a control signal S from a gate 52 indicates the receipt of signals from a 
sensor 54 responsive to the demand being greater than a first 
predetermined level, a sensor 56 responsive to the gradient being in a 
zero band, a sensor 58 responsive to the speed being greater than a first 
predetermined threshold, a sensor 60 responsive to the demand being less 
than a second predetermined level and a sensor 62 responsive to the 
vehicle speed being less than a second predetermined threshold. In the 
absence of the signals, the switch inhibits the deceleration error from 
reaching the circuit 48. The switch 50 also inhibits the deceleration 
error on receipt of a signal from an anti-lock detection means 64. 
The long term adaptive constant producing means 48 includes a very slow 
integrating means 66 whose output is connected via a switch 68 to a sample 
averaging means 70, the switch 67 being controlled in response to an End 
of STOP Pulse provided on a line 72 at the end of each vehicle stop. It 
should be pointed out, however, that the block diagrams showing the long 
term adaptive constant being derived from deceleration error are an 
attempt to illustrate in simple terms what in practice would be achieved 
with software. 
By way of example, the integrator 66 of FIG. 4 can be simulated by a 
digital computer using an accumulating memory location which receives the 
addition of processed deceleration errors at regular preset intervals. The 
integrator can be reset at any point in time, to a preset starting point 
such as unity, or a scaled value representing unity. The integral 
correction developed at the end of any stop (or at the low speed point at 
which the correction changes are discontinued) can be determined by 
calculation of the difference between the integrator final reading and the 
stored integrator start figure. 
Thus at the end of each stop, the integrating location can be reset to the 
stored integral start figure. This may be a preset base figure or may 
alternatively be a progressive figure formed from the previous integral 
start level plus a percentage of the integral correction developed during 
the stop. If this is the case, the stored integrator start figure is 
changed after each stop and control is thereby adapted to suit braking 
conditions. Example--Suppose the unity figure is 128. This is the base 
SISF. Integral correction=Integrator reading, In-Stored integrator start 
figure SISF. The routine is as shown below. 
##STR1## 
The resulting long term adaptive constant C, updated at the end of each 
stop, is applied to the second inputs of the multipliers 42a, 42b by way 
of respective load apportioning circuits 74a, 74b adapted to modify the 
long term adaptive constant C in dependence upon the prevailing vehicle 
load figures L.sub.R, L.sub.F, received from the load sensors 30a, 30b and 
supplied via the element 40. 
As indicated in FIG. 5, the prevailing gradient on which the vehicle is 
operating is calculated in a circuit 78 from signals received from a 
vehicle decelerometer 80 and from signals from a wheel speed sensor 82, 
differentiated at 84. Circuit 78 also receives a limiting signal from a 
low-speed threshold device 91 whenever the correction has a tendency to 
exceed the driver's demand. Gradient corrected demand is obtained from the 
adder 39 on a line 43. The gradient is obtained by taking the difference 
between: 
(a) Rate of change of wheel speed, and 
(b) Vehicle decelerometer output. 
This results because, on a gradient, the suspended mass of the 
decelerometer suffers a gravity induced offset (which is algebraically 
additive to any deceleration/acceleration which is taking place. This 
offset is best envisaged in the static condition, pointing downhill, where 
it represents the equivalent deceleration being applied to the vehicle to 
prevent a totally free body adopting an acceleration resulting from the 
gradient induced continuous speed increase. The difference between the two 
signals is therefore in acceleration terms and must be added to the demand 
(deceleration) signals being produced by the driver. On downhill slopes 
these signals are added to the driver demand to generate extra braking, 
and on uphill slopes are subtracted from the demand, as less braking is 
required. It should be noted that at no time is this gradient correction 
allowed to exceed the demand, this being the purpose of amplitude limiter 
81 which receives a reference input from the driver demand and which 
reduces the correction to zero at very low demand levels. Thus, on flat 
ground the two measuring systems always generate equal signals, even under 
deceleration and acceleration conditions. 
Thus, referring back to FIG. 3, input demands D produced by the brake pedal 
are added at the adder 39 to the two gradient correction signals G.sub.C, 
which will be nominally equal but are separate in that they are supplied 
to the two (or more) axle control channels which are provided to maintain 
the accepted split-braking standards. There are certain circumstances in 
which equal correction front and rear is not warranted and the signal 
processing will be effected in the correction calculator which has access 
to the axle load signals. Gradient corrected demands are used to form the 
inputs to each axle multiplier 42 where the adaptive adjustment and load 
corrections are made by multiplying each input by a local Pressure 
Modification Factor (PMF), given by: PMF=Vehicle Adaptive Constant 
C.times.Axle Load Correction Factor. The resulting output, suitably scaled 
for compatibility with the pressure transducer output range, is used to 
form the axle pressure demands D.sub.F or D.sub.R to the pressure 
controllers. One remaining input to the pressure controller can be an 
anti-lock pressure override signal obtained from a separate skid detector 
76, possibly incorporated within the same controller. 
With compensation applied to the brake pressure demands and with no 
override conditions in force from wheel skidding signals, braking will be 
much more repeatable than with non-controlled vehicles. The driver demand 
will be related to achieved vehicle deceleration and comparison of these 
signals, demand and deceleration, can be expected to show good agreement 
unless braking constants change due to brake factor changes. 
By means of the circuitry shown in FIG. 4, deceleration errors are used to 
modify the long term adaptive constant only when: 
i. the vehicle is on level ground and 
ii. the demand is in mid-range, (for example 0.2 to 0.5 g) and 
iii. The speed is in mid-range (for example 20-80 kph) 
to indicate the condition of vehicle braking overall. An accumulation of 
similarly derived figures on each stop is formed and is processed in the 
very slow integrating means 66 to generate the adaptive constant C, 
assessed over a sizeable series of vehicle stops. This adaptive constant 
has a nominal or start value of unity and is gradually modified to correct 
changing brake conditions between stops. This constant is therefore a good 
indicative of brake condition and is regularly updated within the 
controller and stored in electrically alterable non-volatile computer 
memory to provide, at start up or on demand, a brake performance factor 
via a diagnostic output port 86. Internally this constant will be 
monitored and compared against a preset alarm level to signal when braking 
deterioration is such as to merit urgent service attention.