Vehicle hydraulic braking systems of the brake-by-wire type

A vehicle hydraulic braking system of the brake-by-wire type is disclosed in which proportional solenoid-operated valves (12, 13) are adapted to control the pressure in wheel brake actuators of the brakes (8, 9) on a single axle of a vehicle, and a balance valve (20) is provided to enable individual hydraulic brake circuits to be hydraulically connected so that the pressure applied to each brake is guaranteed to be substantially the same, a first one (13) of the proportional valves (12, 13) is adapted to be isolated so that the hydraulic circuit is fed by a second, single, proportional valve (12). Applying both brakes under the control of a single proportional valve ensures that any fluid leakages are balanced since substantially equal brake-applying pressures are generated. An alternative vehicle hydraulic braking system of the brake-by-wire type is also disclosed in which a control module (3) is provided which is adapted to select between at least a first control mode (104) in which more than one of the proportional valves (12, 13) is adapted to be controlled in parallel and a second control mode (103) in which only one of the proportional valves (12, 13) is operated to control the pressure in the wheel brake actuators.

This invention relates to hydraulic braking systems of the brake-by-wire 
type in which operation of a brake-pedal actuates a demand sensing device, 
such as a potentiometer or the like, of which the output, in turn, is 
adapted to regulate a supply of hydraulic fluid to the brake through 
solenoid-operated actuator valves, and under the control of an electronic 
control device. 
Each solenoid-operated actuator valve comprises a proportional solenoid 
valve of the kind in which, by varying the current supply to the coil of 
the valve, fluid flow through the valve can be varied such that the 
pressure generated in a closed hydraulic circuit connected to the output 
port of the valve can be caused to rise at a variable rate, remain 
constant in a hold position, or fall at a variable rate. 
In systems of the kind set forth in which the proportional solenoid valve 
is adapted to control the pressure in a wheel brake actuator, and a 
balance valve is provided to enable individual hydraulic brake circuits, 
for example on a single axle of a vehicle, to be hydraulically connected 
so that the pressure applied to each brake is guaranteed to be 
substantially the same, it is difficult to balance fluid leakage in 
proportional solenoid valves feeding a single hydraulic circuit. 
This problem can be overcome by isolating one proportional valve from the 
hydraulic circuit and using the other proportional valve to feed fluid to 
the hydraulic circuit. 
According to a first aspect of our invention in a vehicle hydraulic braking 
system of the brake-by-wire type in which proportional solenoid-operated 
valves are adapted to control the pressure in wheel brake actuators of the 
brakes on a single axle of a vehicle, and a balance valve is provided to 
enable individual hydraulic brake circuits to be hydraulically connected 
so that the pressure applied to each brake is guaranteed to be 
substantially the same, characterised in that a first one of the 
proportional valves is adapted to be isolated so that the hydraulic 
circuit is fed by a second, single, proportional valve. 
Applying both brakes under the control of a single proportional valve 
ensures that any fluid leakages are balanced since substantially equal 
brake-applying pressures are generated. 
Preferably, an isolator valve is provided at the output of the said first 
one of the proportional valves so as to enable said proportional valve to 
be isolated. The said first proportional valve may be reconnected through 
the isolator valve to the brake actuator(s) by opening the isolator valve. 
This may be advantageous when, for example, it is desired to generate 
individual brake pressures at each brake. 
Alternatively, said first proportional valve can be placed in the hold 
condition, thus isolating the first proportional valve from the hydraulic 
circuit and preventing that valve from supplying fluid under pressure to 
the hydraulic circuit and allowing total control of the axle brake 
actuators from the other proportional valve. Exiting the hold condition 
automatically reconnects the valve to the hydraulic circuit. 
Simultaneously to connecting the first, isolated, valve to the brakes, the 
balance valve may be closed to hydraulically isolate the individual 
hydraulic circuits thus allowing independent brake pressures to be 
generated. 
Most preferably, said isolator valve and said balance valve may comprise a 
single three port two way valve (3/2) operable between a first position in 
which said first proportional valve is isolated from the brakes whilst the 
second proportional valve feeds both brakes, and a second position in 
which both the first and second proportional valves are connected to 
respective brake actuators whilst the common connection between the 
individual hydraulic brake circuits is closed thus allowing individual 
wheel brake pressures to be generated. 
Alternatively, said balance valve may comprise a two port-two way (2/2) 
valve connected between the individual hydraulic circuits which works in 
combination with the hold position of the proportional valves. 
According to a second aspect of our invention, in a vehicle hydraulic 
braking system of the brake-by-wire type in which proportional 
solenoid-operated valves are adapted to control the pressure in wheel 
brake actuators of the brakes on a single axle of a vehicle under control 
of a control means, and a balance valve is provided to enable individual 
hydraulic brake circuits to be hydraulically connected so that the 
pressure applied to each brake is guaranteed to be substantially the same, 
said control means is adapted to select between at least a first control 
mode in which more than one of the proportional valves is adapted to be 
controlled in parallel, and characterised in that said control mode is 
further adapted to select between said first control mode and a second 
control mode in which only one of said proportional valves is adapted to 
control the pressure in said wheel brake actuators. 
Preferably, when said second control mode is selected, during a first 
braking operation a first one of said proportional valves is operated to 
control the brake pressure whilst the output of a second one of said 
proportional valves is isolated from the hydraulic circuit, and during a 
second subsequent braking operation, said second proportional valve is 
operated whilst the output of the first proportional valve is isolated 
from the hydraulic circuit. The choice of which valve controls the brake 
pressure thus "flip-flops" between the first and second valves on each 
operation in the second mode. This switching preferably occurs on 
alternate braking operations. 
Preferably, the proportional valves are isolated from said hydraulic 
circuit by placing the proportional valve in the hold position. 
Alternatively, the proportional valve may be isolated from the hydraulic 
circuit using an isolating valve, such as a 3/2 position valve. 
Preferably, said first control mode is selected at high braking duties or 
when a rapid dumping of pressure is required, and said second control mode 
is selected during low braking duties. This is advantageous as it ensures 
that the additional volume of the combined use of valves is available 
which allows higher duties to be achieved than if just one valve is 
operated. 
Preferably, in the first control mode, both first and second proportional 
valves may be operated by the same current, preferably using a single 
control algorithm. This is advantageous as it prevents the valves from 
fighting against each other to control the pressure at the brakes. 
In addition to the two main control modes of the second aspect of the 
invention described hereinbefore, the braking system may be adapted to 
select between one of three additional brake pressure demand modes. The 
first mode may correspond to zero brake pressure demand. The second mode 
may correspond to normal brake demand pressure in which the braking rates 
of both actuators on the axle is substantially the same. The third mode 
may correspond to the case where differing brake pressure demands are made 
by the brake actuators. In the second mode, the balance valve may be 
opened to guarantee a substantially matched pressure across the axle, thus 
achieving the balanced brake pressures. In the third mode, the balance 
valve may be closed to isolate the hydraulic circuits containing each 
brake actuator, and both actuators may then be controlled individually by 
applying a respective control signal to each respective proportional 
control valve. 
A braking system in accordance with the first aspect, and a braking system 
in accordance with the second aspect of the present invention, are 
illustrated in the accompanying drawings in which:

A brake-by-wire braking system in accordance with one aspect of the 
invention is shown in FIG. 1. 
In the brake-by-wire type braking system illustrated in FIG. 1 of the 
drawings a brake pedal 1 is adapted to operate a travel transducer 2 in 
the form of a potentiometer of which the output is fed into an electronic 
control module 3 which also receives signals from transducers responsive 
to the vehicle dynamics, suspension deflections, and load transfer, in so 
far as these parameters affect the behaviour of a pair of front wheels 4 
and 5, and a pair of rear wheels 6 and 7. Each wheel is adapted to be 
braked by a respective brake 8,9,10, and 11 from a fluid power system 16 
under the control of the pedal. 
Operation of each brake is controlled by a respective proportional 
solenoid-operated valve 12,13,14,15. The proportional valves 12 and 13 are 
arranged to control the pressure in the front wheel brakes in conjunction 
with a solenoid-operated balance valve 20, and the proportional valves 14 
and 15, in conjunction with a solenoid-operated balance valve 21, are 
adapted to control the pressures applied to the brakes 10 and 11. 
Whilst each balance valve 20 and 21 is in its closed position the valve 12 
controls the pressure applied to the brake 8, and the valve 13 controls 
the pressure applied to the brake 9. 
It is likely that valves 12 and 13 will exhibit different characteristics 
such that, under a range of unspecified conditions, the pressures in the 
actuators of the brakes 8 and 9 will be different, either in a steady 
state or whilst the pressures are changing during brake application. This 
may lead to vehicle instability. To overcome this problem, the balance 
valve 20 may be moved to its open position, hydraulically connecting the 
output ports of the valves 12 and 13. In this condition both valve 12 and 
valve 13 are capable of delivering fluid to control the pressures in the 
actuators of the brakes 8 and 9 simultaneously. To avoid the possibility 
of the aforementioned differing valve characteristics causing valves 12 
and 13 to fight each other for control of the brake pressure, it is 
desirable to drive the valve 12 or the valve 13 such that it supplies no 
fluid to its output port and to deliver all the fluid for controlling the 
brake pressure from the output side of the other valve. 
A similar mode of operation can be experienced for operating the brakes 10 
and 11 on the rear axle by using the proportional valves 14 and 15 and the 
balance valve 21. 
The proportional solenoid valves 12 and 13 are arranged to control the 
pressure in the actuators of the front wheel brakes 8 and 9 under the 
control of a control algorithm. The algorithm can operate in either a 
first or second control mode and selects between the two modes in response 
to braking demand or other variables. 
In the first control mode, the balance valve 20 is open during normal 
braking operation. To avoid the possibility of the aforementioned 
differing valve characteristics causing the valves 12 and 13 under the 
control of independent control algorithms, fighting each other for control 
of the actuator pressures, the two valves 12, 13 are both supplied with 
the same current under the control of one control algorithm. 
A further benefit obtained during the first control mode of operation of 
this arrangement is that any random variations in solenoid valve 
performance will cause one of the valves 12 and 13 to open fractionally 
earlier and close fractionally later than the other. This makes the 
delivery of fluid more progressive and thus makes the system more 
controllable. This benefit may further be extended by deliberately 
supplying one valve with slightly more current than the other in order to 
make the difference between the points at which each of the valves opens 
more predictable. 
The proportional valves 14 and 15 are also controlled in a similar manner 
thereby obtaining the advantages detailed above with respect to solenoid 
valves 12 and 13. 
In the second control mode, only one of the proportional valves 12, 13 is 
used to control the pressure in the actuators of the front wheel brakes 8, 
9 whilst the output of the other actuator 12, 13 is controlled so that its 
output is held (i.e. in the held position as hereinbefore described). For 
ease of description, the valve 12, 13 which has its output held is 
referred to here as a `slave` valve, whilst the other is referred to as a 
`master`. 
On alternate braking operations (i.e. each time the brake pedal is 
depressed indicating a new brake demand), the first solenoid valve 12, 13 
is the `master` whilst the second 12, 13 is the `slave` and vice versa. 
Thus, on each braking operator, the role of the two valves is swapped over 
under the control of the control means. 
To better understand the two control modes, reference is made to the 
control algorithm shown in the schematic of FIG. 3. 
The first and second control modes are indicated in the schematic. The 
first control mode is labelled 104 in the figure and is selected primarily 
at high braking duties. The second control mode is labelled 103 in the 
figures and is selected primarily at low braking duties. During normal 
operation 102, the balance valve 20 between the axles is opened, so that 
regardless of which control mode 103, 104 is selected, brake pressure 
applied to each wheel brake is guaranteed to be substantially the same. In 
an alternate operational mode 105, when the pressure demand for each wheel 
brake is different the control mode 105 may be adapted to control each 
proportional valve separately as well as closing the valve 20 to allow 
independent pressures to be generated at each wheel brake. On exiting a 
brake operation to a zero braking condition 101, the control means 
alternates which of the proportional valves acts in the hold position i.e. 
as the slave valve for the next braking operation. This is represented by 
the block 106. All these modes are shown in FIG. 3. 
FIG. 4 shows an example of a brake operation illustrating the various 
control modes selected for each brake condition. The figure shows a plot 
of brake pressure demand PD (assumed to be equal for each wheel brake), 
demand rate RD, and control mode selected against a common time axis for a 
typical brake demand pattern. 
Initially, from zero brake demand 101 at time to, the driver applies the 
brake pedal to start a brake operation which spans from t.sub.0 to 
t.sub.5. Initially, brake demand is low and so the second control mode 103 
is selected in which only a single proportional valve 12, 13 is used to 
increase brake pressure whilst the other valve is maintained in a pressure 
hold condition. In the example, it is assumed that the valve 12 is acting 
as the master and valve 13 is the slave. At time t.sub.1, the pressure 
demand has increased to the threshold level for selecting the first 
control mode 104. Both proportional valves 12, 13 are thus controlled in 
parallel from time t.sub.1 to t.sub.2 to meet the high demand. From time 
t.sub.2 to t.sub.3 braking demand has decreased and so the second control 
mode 103 is again selected. 
At time t.sub.3, brake demand rate DR starts to decrease at a high rate, 
and so the first control mode 104 is reselected to meet the demand. 
Finally, from t.sub.4 to t.sub.5, when demand is again low, the second 
control mode 103 is selected. It will be noted that throughout this single 
brake operation, the same valve 12 always acts as the master whilst valve 
13 acts as the slave during the second control mode, and the balance valve 
20 is open to allow equal braking pressure. 
From time t.sub.6 to t.sub.9, following a period of zero braking demand 
101, a second braking operation is commenced as the brake pedal is 
depressed. This time, the role of the proportional valves 12, 13 is 
swapped over, and valve 13 acts as the master, whilst valve 12 acts as the 
slave. This swapping occurs on every subsequent braking operation to 
ensure approximately equal usage of both valves 12, 13 and prevent a valve 
seizing up due to constant use as a slave valve. It also ensures 
substantially even wear of the two valves. Selection of the first and 
second control modes 103, 104 is again made in response to a measure of 
brake demand rate RD as for the first operation, the second mode being 
selected from t.sub.6 to t.sub.7, the first mode from t.sub.7 to t.sub.8 
and the second from t.sub.8 to t.sub.9. 
A braking system in accordance with at least the first aspect of the 
invention is illustrated in the layout of FIG. 2 of the drawings. The 
balance valves 20 and 21 in the layout of FIG. 1 are omitted, and the 
front wheel brake circuit incorporates a 3-port balance valve 30 and the 
rear wheel brake circuit incorporates a similar 3-port balance valve 31. 
Each balance valve 30,31 is arranged in each respective circuit so that 
while the balance valve is in its open position, opposite from that shown 
in the drawing, the proportional valve 12 controls the pressure in the 
actuator of the front brake 8, and the proportional valve 13 controls the 
pressure in the actuator for the brake 9 on the other front wheel 5. 
Since it is likely that the valves 12 and 13 will have different 
characteristics such that, under a range of unspecified conditions the 
pressures in the actuators of the brakes 8 and 9 will be different, either 
in a steady state or rather pressures are changing during brake 
application, this may lead to vehicle instability. To overcome this 
drawback, the balance valve 30 may be moved to its closed position, as 
shown, thereby hydraulically isolating the output port of the valve 13 
from its associated brake actuator 9. Indeed the output port from the 
proportional valve 13 is blocked in this position. However with the 
balance valve 30 in the open position as shown, the output port from the 
proportional valve 12 is connected in the circuit such as to control the 
pressure in the actuators for both brakes 8 and 9. 
In this condition only the valve 12 requires any control action, and the 
valve 13 can be left un-energised, thereby avoiding the need for a 
complicated control strategy to maintain it in its hold position. 
A further advantage of this arrangement is a reduction of system operating 
current during all the normal, non APS/CDC brake applications since the 
proportional valve B need not be energised. 
The balance valve 31 can operate in a similar manner in conjunction with 
proportional valve 14 and 15.