The invention relates to vehicle braking systems.

Almost all modern vehicles, particularly motor cars, have hydraulically operated foot brakes operating on all wheels. The systems generally comprise either drum or disc brakes with a brake pedal connected to a piston in a master cylinder whence the hydraulic operating pressure originates. Depression of the brake pedal forces the hydraulic brake fluid along a network of pipes and hoses to wheel cylinders which urge brake calipers or shoes into contact with a brake disc or drum to provide the braking action.

Hydraulic systems have many advantages, such as being self-lubricating (reducing the chance of seizure), having a low rate of wear and low friction, and enabling equal pressures to be exerted on all the brake shoes or calipers, even compensating for unequal wear or adjustment. Further, installation is generally easier than for mechanical systems because of the flexibility of the hoses.

The principal disadvantage of hydraulic braking systems is that a leakage in the system or contamination of the hydraulic fluid used can render the brakes ineffective or in¬ operative.

One solution to this problem is provided by having a tandem master cylinder, and splitting the system into two parts

so that if one part fails the other part can still function. Obviously, in the event of failure of one part braking efficiency is reduced, and there are situations when this can be dangerous.

Sometimes it would be preferable to control the separate brakes individually for example if one tyre has been worn more than its axial counterpart. In standard systems both axial brakes would be operated identically. It would also be useful in braking systems to be able to control the feel at the braking, as some drivers prefer sharp braking and others prefer to have to move the brake pedal some distance before maximum braking is achieved.

Advances in braking technology have been restricted because of the problems in distribution of the fluid in the brake mechanisms.

According to the invention there is provided a vehicle braking system comprising a braking element operable by a driver of the vehicle; control signal generating means responsive to operation of the braking element; means braking at least one wheel of the vehicle in response to control signals generated by the control signal generating means; and reaction means responsive to the control signals to provide a force on the braking element to provide a variable artifical feel to the driver through the braking element.

Preferably the control signals are proportional to a force applied to the braking element by the driver.

In a preferred embodiment the system comprises means for sensing the force applied to the braking element and providing input signals proportional thereto to the control signal generating means, the sensing means preferably comprising an electrical load cell.

Preferably the system comprises means for sensing the position and/or movement of the braking element and providing input signals to the control signal generating means proportional thereto, said sensing means preferably comprising one or more linear variable displacement transducers.

Preferably the reaction means comprises a hydraulic actuator coupled to the braking element, the actuator preferably being a double acting hydraulic actuator having a pair of opposed piston faces.

The actuator is preferably controlled by a servo-valve responsive to the control signals to control the application of hydraulic fluid pressure to the opposed piston faces to operate the actuator.

The braking element can be a brake pedal.

Another preferred embodiment includes error detection means operative to check whether the artifical feel is being provided to the braking element and to provide input signals to the control signal generating means in dependence thereon. On detection or an error by the error detection means outside a pre-set limit, the control signal generating means preferably generates control signals to operate a safety system.

The control signal generating means is preferably a computer which can be programmable to generate control signals dependent on requirements of the driver or the condition of the vehicle.

A vehicle braking system according to the invention will now be described by way of example with reference to the drawings, in which:-

Figure 1 is a schematic representation of the layout of the system;

Figure 2 is a part sectional front elevation of a brake "feel" actuator assembly which is a part of the system of Figure 1;

Figure 3 is a sectional view of Figure 2 on III-III;

Figure 4 is a part sectional front elevation of a distribution valve which is a part of the system of Figure 1; and

Figure 5 is a front elevation of the distribution valve of Figure 3 with parts omitted for clarity.

Referring first to Figure 1 there is shown a braking system 10 for use in a motor vehicle such as a motor car, having steerable wheels (not shown) .

The motor vehicle has four wheels each fitted with standard hydraulically operated disc or drum brakes. Although the brakes themselves are not shown in the drawings, the operable calipers or shoes or each wheel (hereinafter brake mechanism) are attached to a respective servo valve 11, 12, 13,

14, which independently controls the pressure to the brake mechanism at the associated wheel.

The system 10 combines a primary active brake system and a secondary brake override system that provides the braking effort in the event of a system failure or when the active system is turned off. The Primary Active Braking System

The active system is a fully active "feel" system including a braking element in the form of a brake pedal 15 which is physically decoupled from the braking mechanisms. The brake pedal 15 is mounted to the vehicle body to be pivotal about its point of attachment. Attached to the brake pedal 15 is a "feel" actuator 16.

Referring now to Figure 2 also, the feel actuator 16 is a double acting hydraulic actuator comprising a moveable piston 17 having a pair of opposed piston faces and contained in a cylinder 18. Mounted on the piston 17 is a piston head 17a which provides the piston faces. One end of the piston 17 is coupled to the brake pedal 15 by means of a linkage element 20 which includes a load cell 21 (or other suitable device) which is preferably a duplex sensor, for sensing the force applied by a driver of the vehicle to the brake pedal 15. The load cell 21 is electrically connected to a control unit 25, such as a mircoprocessor, and sends input signals thereto relating to the force applied to the brake pedal 15.

Also attached to the linkage element 20 is a rod 22

which enters "a holder 23 of a linear variable differential transformer (LVDT) 24. The LVDT holder 23 is fixed adjacent to the cylinder 18 on a manifold block 75 housing the feel actuator 16. The LVDT 24 converts the mechanical displacement of the linkage element 20 (and therefore the displacement of the brake pedal 15) into an electrical signal which is transmitted to the microprocessor 25 to which the LVDT 24 is electrically connected. Other suitable devices may of course be used for converting the displacement into an electrical signal.

The feel actuator 16 is hydraulically connected to servo valve .28. Hydraulic transfer tubes 9 are provided in the manifold block 75 linking the hydraulic fluid lines from the servo valve 28 to each end of cylinder 18 so that the servo valve 28 may be used to control the movement of the piston 17 by hydraulic pressure to either side of the piston head 17a.

As will be seen in Figure 1 a non-return valve 29a is included in the hydraulic fluid line between the servo valve 28 and feel actuator. 16, for reasons described below. The non-return valve 29a is also connected to the pressure supply line 33 Via an isolating valve 31 described below. The non¬ return valve 29a opens on application of pressure thereto.

The servo valve 28 is connected to hydraulic fluid supply pressure and return lines 33, 34 via an isolating valve 31 which is preferably a solenoid valve. When the solenoid valve 31 is energised the pressure supply line 33 is connected

to the servo valve 28 and when it is de-energised the return line 34 is connected to the servo valve 28. A pressure reducing valve 30 is incorporated in the pressure supply line 33 leading to the isolating valve 31 so that the servo valve 28 uses a pressure lower than the system pressure.

An anti-cavitation valve 27 is also included.

The microprocessor 25 is connected to control the brake servo valves 11, 12, 13, 14 and signals generated by the micro¬ processor 25 control the pressure applied to the four brake mechanisms independently. The servo valves 11, 12, 13, 14 are connected to brake lines 56 which are connected to the brake mechanisms of the front left, rear left, rear right and front brakes respectively, via a distribution valve 35, as shown in Figures 4 and 5.

The servo valves .11, 12, 13, 14 are connected to the hydraulic pressure and return lines 33, 34 via connectors 32. One connector 32 connects a branch of the pressure and return lines to one pair of servo valves 11, 14 and the other connector 32 to the second pair of servo valves 12, 13. The Distribution Valve

The distribution valve 35 comprises a valve manifold 36 having four separate chambers 46, 47, 48, 49 therein. Each chamber 46, 47, 48, 49 is connected via a fluid passage 50 to a respective servo valve 11, 12, 13, 14. The manifold 36 has eight ports 37-44 therein such that each chamber 46, 47, 48, 49 has a pair of ports (37, 41), (38, 42), (39, 43), (40, 44)

therein. One port 37, 38, 39, 40 of each pair is hydraulically connected to a master cylinder 45 via a pair of fluid lines 58, 59. The master cylinder 45 is preferably a tandem master cylinder and has a hydraulic booster 45a attached thereto, the function of which will be explained later.

As shown in Figure 1, one pair of ports 37, 38 feeds

.from one fluid line 58 from the master cylinder whilst the second pair of ports 39, 40 feeds from the second fluid line

59. A balancing valve 60 may be provided in one of the fluid 1

#~3 lines 58,^-59.

The other ports 41, 42, 43, 44 are connected to a brake line 56 leading to a brake mechanism.

Within each chamber 46, 47, 48, 49 is a spool 51 comprising a concave first part 52 which is located in one end σi a concave second part 53, the two parts 52 and 53 being secured together by a screw 55. As shown the two parts 52 and 53 are αjξ. different materials in view of the possibly different fluids acting on the two parts. The spool 51 is slidable within the chamber 46, 47, 48, 49.

A compression spring 54 is also located in each chamber 46, 47, 48, 49, with one end located against an inside face 55 of the second spool part 53 so as to bias the spool 51 outwards to seal off the fluid passage 50 between the chamber 46, 47, 48, 49 and the associated servo valve 11, 12, 13, 14. The length of, the spool 51 is such that when it is biased as mentioned above brake fluid from the master cylinder 45 is free

-- " ^'

to enter/leave the chambers 46, 47, 48, 49 via the ports 37-44.

When hydraulic pressure is applied via pressure line 33 to the end face 57 of the spool part 53 the bias of the spring 54 is overcome and the spool 51 moves to compress the spring 54 and seals off the chambers 46, 47, 48, 49 from the ports 37-40. Secondary Brakinσ System

A secondary braking system is also incorporated, the secondary system being isolated when the primary active system is operating and thus providing no braking effort. However, when the primary active system is turned off or fails the secondary system is selectively activated, for example by a positive decision from the driver who operates a manual switch, or by automatic decision from the system in the event of a failure.

The second system utilises a secondary hydraulic actuator 65 to operate the booster 45a which in turn operates the master cylinder 45 in a known manner to supply pressurised fluid to fluid lines 58, 59.

The actuator 65 comprises a piston 66 having a single head 67 within a cylinder 68. The secondary actuator 65 is hydraulically linked to the feel actuator 16 as follows. A hydraulic fluid line 69 is connected to the cylinder 68 on one side of the piston head 67. After the fluid line 69 leaves the cylinder 68 it divides into two, one branch 69a connecting to the hydraulic fluid return line 34 via a non-return valve 29b, and the second branch 69b leading to a switching valve 70. The

switching valve 70 has a fluid line 71 which splits into two branches 71a, 71b; branch 71a leads to the non-return valve 29a and branch 71b to the cylinder 18 of the feel actuator 16.

The non-return valves 29a, 29b and the switching valve 70 are connected to the pressure supply line 33 via the isolating valve 31. The Mechanical Override

In the event of an undetected failure in the primary active braking system an override mechanism comes into automatic operation. Under normal braking conditions the brake pedal 15 may be depressed over distance A shown in Figure 2. When the braking pedal 15 is subjected to a "panic" force in the event of a failure undetected by the system 10 so that no braking occurs during the normal travel A of the pedal 15, the pedal 15 moves through distance B and operates the override mechanism.

The override mechanism comprises a shaft 76 mounted on the piston 66 of the actuator 65 and located within a cylinder 77, extending between the actuators 16 and 65. Under normal conditions the shaft 76 does not contact the cylinder 77.

An override compression spring 74 is located between the manifold block 75 which houses the actuator 16 and a fixed support 78 on which the cylinder 77 is mounted on a pivotal arm 79; see Figures 2 and 3. The manifold block 75 is mounted in the system 10 to be moveable, but is held, during normal braking, stationary by the spring 74 acting between the block 75

and the support 78.

Figure 2 shows the system in the condition when the override is operating, although the pedal 15 is shown in solid line in its normal position. Primary Active Braking

During primary active braking brake pedal 15 travels through distance A (see Figure 2). The load applied by the driver to the pedal 15 is measured by load cell 21 which sends input signals relating to the load to the microprocessor 25.

A reaction to the driver's demand on the brake pedal 15 is provided by the feel actuator 16. To do ■ this the microprocessor 25 sends control signals to the servo valve 28 which is connected to the feel actuator 16 to apply pressure to either side of the piston head 17a. This is used to provide resistance to movement of the brake pedal 15 and to generally simulate the "feel" of braking. This "feel" may be altered by programming the microprocessor 25 to provide light or heavy braking or whatever the driver prefers.

During primary active braking the isolating valve 31 is energised to connect the servo valve 28 with the pressure line 33. When the isolating valve 31 is energised pressure is also applied to the non-return valves 29a, 29b This causes the non-return valves 29a, 29b to open and the switching valve 70 to close which allows the feel actuator 16 to be driven directly by the servo valve 28 via the fluid lines to either side of the piston head 17a. Any residual fluid pressure in

the master cylinder 45 is vented via fluid line 69 through the open non-return valve 29b to the fluid return line 34.

The feel actuator 16 is capable of simulating a wide range of characteristics depending on the 'feel' required.

The actual braking is carried out by fluid pressure supplied by the four servo valves 11, 12, 13, 14. The microprocessor 25 sends independent control signals to each of the servo valves 11, 12, 13, 14, according to input signals from the load cell 21.

Since the isolating valve 31 is energised the servo valves 11, 12, 13, 14 are connected to the pressure supply line via the connectors 32. The fluid pressure is controlled by each servo valve 11, 12, 13, 14 to operate each brake mechanism independently. The fluid pressure transmitted from the servo valves 11, 12, 13, 14 via fluid passages 50 is sufficient to overcome the preload on the springs 54 causing the spools 51 to move in the chambers 46, 47, 48, 49 to seal them off from the ports 37-40 from the master cylinder 45. The fluid remaining in the chambers 46_-49 is forced out of the chambers 46-49 via the ports 41-44 by movement of the spools 51 to the individual brake mechanisms to operate the brakes independently of each other. Secondary Braking

The secondary braking system may be activated in two ways; either by means of a manual switch (not shown) operated

by the driver to de-energise the isolating valve 31, or as a result of a failure detected in the primary active braking system.

The main failures may be in the form of loss of electrical power to the system of a drop in the system pressure below a pre-set safety limit.

Various safety control loops may be incorporated in the overall braking system which are monitored by the microprocessor 25. An example described earlier in the provision of the LVDT 24 which checks whether the feel actuator 16 is in fact operating according to the control signals sent to the servo valve 28 by the microprocessor 25. The LVDT, load cell 21 and other sensors in the system are preferably duplex sensors. Their signals are duplicated and the signals combined to give a demand and an -error detection signal. Failures in the control loops can be detected by comparing the actual output from the system with a real time model of the system running in parallel. Any detected failures will result in de-energisation of the isolating valve 31.

Loss of the electrical supply will automatically result in de-engerisation of the isolating valve 31. The effect of loss of pressure has a similar effect which will be described later.

When the isolating valve 31 is de-energised it connects to the fluid return line 34. This results in a drop in pressure to the non-return valves 29a, 29b which close and the

and switching valve 70 which opens. This hydraulically disconnects the servo valve 28 from the feel actuator 16 and hydraulically connects the feel actuator 16 to the secondary hydraulic actuator 65 via switching valve 70 which operates the booster 45a and master cylinder 45.

Thus pressure applied to the pedal 15 will be transmitted by movement of the piston 17 with piston head 17a forcing the fluid out of cylinder 18, via the switching valve 70, into the cylinder 68 of the secondary actuator 68 to act on the piston head 66 of the secondary actuator 68. Thus, braking in this mode feels and performs similar to a standard unmodified hydraulic braking system.

Referring to the distribution valve 35, in secondary braking the .-ervo valves 11-14 are connected to the fluid return line 34 and the pressure applied to the spools 51 drops. The fo*ce of springs 54 is sufficient to overcome any residual pressure and force the spools 51 outwards towards the servo valves 11-14 to seal off fluid passages 50 from the chambers 46-49 and the ports 37-40 which communicate with the master cylinder 45 are uncovered.

Thus, as the foot pedal 15 is depressed brake fluid is forced along the fluid lines 58, 59 from the master cylinder 45, into chambers 46-49 and out of ports 41-44 to the brake mechanism via the brake lines 56.

Since ports 37, 40 are connected to the fluid line 58 and ports 38, 39 are connected to line 59 equal pressures are

transferred across each axle pair of brakes.

Loss of system pressure below a certain limit will also allow the springs 54 to recover and return to their original state and thus has the same effect as if the isolating valve 31 had been de-energised. Override Braking

In the event of an undetected failure in the feel system the override mechanism comes into operation. If no braking occurs when the driver depresses pedal 15 through distance A, he will use a panic force and the pedal will move through distance B. The length of the feel actuator cylinder 18 is such that when the brake pedal 15 has reached the end of travel distance A, the piston head 17a reaches the end of the cylinder 18. Further movement of the pedal 15, i.e. through travel distance B on applicaton of a panic force, will cause the piston head 17a to bear on the end face 80 of the cylinder 18, which is attached to the manifold block 75 and thus causes the manifold block 75 to move against and overcome the pre-load of spring 74 so that the whole manifold block 75 moves (to the position shown as Figure 2) . As the manifold block 75 moves toward the support 78 cylinder 77 moves into contact with shaft 76. Further movement of the block 75 caused the shaft 76 to move the piston 66, which activates the brake booster 45a and master cylinder 45 to operate the brakes.

The system is thus very versatile, self-monitoring, fail¬ safe and may perform to a variety of control laws, such as anti-

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lock braking, variable balance, deceleration demand and brake performance feed back to pedal.

Although the system described above operates primarily in dependence upon the load applied to the brake pedal, with appropriate modifications to the system it may be made

"responsi-ve to the position or movement of the brake pedal or any other parameter measurable in the system.