GB2324590A

GB2324590A - Brake actuator

Info

Publication number: GB2324590A
Application number: GB9813986A
Authority: GB
Inventors: Robert Alan Anderson
Original assignee: Lucas Industries Ltd
Current assignee: ZF International UK Ltd
Priority date: 1994-10-06
Filing date: 1995-10-04
Publication date: 1998-10-28
Anticipated expiration: 2015-10-04
Also published as: GB9813986D0; GB2324590B

Abstract

A brake actuator (6) for an hydraulic brake-by-wire system comprises a proportional solenoid-operated valve. The valve comprises a housing, a solenoid coil in the housing, an armature (8) movable axially in the housing in response to energisation of the solenoid coil. A valve spool is axially movable in a bore in the housing with respect to a stop (12). A spring (13) urges the spool towards the stop. An inner land of the spool controls communication between the exhaust and outlet passages, and an outer land controls communication between the inlet and outlet passages. The spool has a bore (23) which communicates with the inside of a cap 30. The end portion of the spool is slidably received in the cap which acts as an area determining member. This allows pressure to act on the end of the spool to provide feedback.

Description

IMPROVEMENTS IN ACTUATORS FOR VEHICLE HYDRAULIC BRAKING SYSTEMS OF THE BRAKE-BY-WIRE TYPE This invention relates to actuators for vehicle hydraulic braking systems, and to hydraulic braking systems of the brake-by-wire type incorporating such actuators.

In hydraulic braking systems of the brake-by-wire type the operation of a brake pedal actuates a demand sensing device such as a potentiometer travel transducer or force sensor of which the output, in turn, is adapted to regulate the supply of hydraulic fluid to the brakes through solenoid-operated actuator valves, under the control of an electronic control device.

Such actuator valves normally react to the operating pressure between an inlet and an outlet but do not act to control the actual the magnitude of the operating pressure itself.

According to our invention in a brake actuator for an hydraulic braking system of the brake-by-wire type comprising a solenoid-operated valve comprising a housing, a solenoid coil in the housing, an armature movable axially in the housing in response to energisation of the solenoid coil, a valve spool axially movable in a bore in the housing with respect to a stop defining a retracted position for the spool, a spring urging the spool towards the stop, the spool being adapted selectively to control communication between axially spaced exhaust, inlet and outlet passages in the housing, and being formed at opposite ends of an intermediate portion of reduced diameter with inner and outer shoulders at steps in diameter between the intermediate portion and inner and outer lands of which the inner land controls communication between the exhaust and outlet passages, and the outer land controls communication between the inlet and outlet passages, the spool also being provided in the end remote from the armature with a longitudinally extending bore which is in communication with the bore in the housing through a radial passage in the spool, an area determining member has a working engagement with an end portion of the spool remote from the armature, and energisation of the solenoid coil causing the armature to urge the spool away from the stop into a operative position in which the inner land closes the exhaust passage and the outer land open communication between inlet and the outlet passages, the outlet pressure providing a feed back force on the armature comprising the product of that pressure acting over different effective areas of the spool to subject the spool to a net force in a direction towards the armature member.

The area determining member may comprise a needle slidably received in the longitudinally extending bore. Alternatively the member may comprise a cap having an internal bore in which the end portion is slidably received and an end wall closing the outer end of the internal bore, the end portion being of greater area than the remainder of the spool.

The area determining member is held against movement in a longitudinal direction by a force generated by the applied pressure which acts to hold it against a stop, suitably the end of the bore remote from the armature.

We therefore provide an actuator which is adapted positively to control communication between the inlet passage and the outlet passage, rather than simply to react to such pressure.

Preferably, when the actuator is in an off position, a small clearance is provided between the armature and the spool. This ensures that in this position no load is applied to the armature. The armature can therefore be made as light as possible in order to conserve power, and the armature in unsprung so that no bias resistance force has to be overcome when the actuator is operated by energisation of the coil.

The actuator is provided with a solenoid having an armature which applies a force to the spool of the actuator which is proportional to the current applied to the solenoid. Thus the greater the current applied to the coil the greater will be the force generated.

When the actuator is installed in a vehicle braking system the coil is connected to an electronic controller which emits an energising current of a magnitude dependent upon the driver's braking demand and may take into account vehicle dynamics, suspension, load transfer and the like in as much as these parameters have an effect on the braked wheel. The inlet passage is connected to a source of high pressure fluid, suitably a motor driven pump, the outlet passage is connected to the brake, and the exhaust passage is connected to a reservoir for fluid from which the pump is supplied. Upon driver demand, current is supplied to the solenoid with the result that the armature acts on the spool to control communication between the power source, the reservoir, and the brake.

Two embodiments of our invention are illustrated in the accompanying drawings in which: Figure 1 illustrates a layout of a brake-by-wire hydraulic braking system incorporating a solenoid-operated actuator; Figure 2 is a layout similar to Figure 1 but showing a modified actuator; and Figure 3 is a section on an enlarged scale of the outer end portion of the spool of Figure 2.

In the brake-by-wire braking system illustrated in Figure 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 received signals from transducers responsive to the vehicle dynamics, suspension deflections, and load transfer, in so far as these parameters effect the behaviour of a wheel 4 adapted to be braked by an hydraulic wheel brake 5.

Operation of the brake 5 is controlled by an hydraulic actuator 6.

The actuator 6 incorporates a proportional solenoid-operated valve and comprises a housing 7 in which is located an armature 8 and a solenoid coil (not shown). The coil surrounds the armature 8 which is axially displaceable in the housing 7. The housing 7 includes a forward extension 9 provided with a longitudinally extending bore 10 in which works a spool 11. Although the extension 9 forms a part of the housing 7 it may comprise a separate, independent component, from the component in which the magnet and the coil are located. Normally the spool 11 is urged at its inner end against a stop face 12 adjacent to the armature 8 by means of a compression spring 13 acting on the opposite end of the spool. The housing extension 9 is provided with axially spaced radial exhaust, outlet, and inlet passages 14, 15, 16 respectively. The exhaust passage 14 is connected to a reservoir 17 for fluid, the outlet passage 15 is connected to the brake 5, and the inlet passage 16 is connected to a power source 18, suitably an engine driven pump or a pump driven by an electric motor and which may supply an hydraulic accumulator. In either construction the pump draws fluid from the reservoir 17.

The spool 11 is provided with an intermediate portion 20 of reduced diameter disposed between inner and outer lands 21, 22 respectively. In addition a bore 23 extends inwardly from the outer end of the spool 11 remote from the armature 8 terminating at its inner closed end in a diametrical passage 24 leading into the intermediate portion 20 of reduced diameter.

An area determining member comprising a needle 25 fixed relative to the housing 7 is located in the bore 23.

An inner shoulder 26 is defined at the step in diameter between the inner land 21 and the intermediate portion 20, and an outer shoulder 27 is defined at the step in diameter between the outer land 22 and the intermediate portion 20.

In the inoperative brakes off position shown in the drawings the spool 11 is held against the stop 12 by means of the force in the spring 13 and the armature 8 is located in a rest position in which a small clearance is provided between its outer end and the adjacent end of the spool 11. The land 22 closes the inlet passage 16 to isolate the pressure source 18 from the brake, and communication is provided between the brake 5 and the reservoir 17 passed the land 21.

Upon driver demand the pedal 1 is operated, in turn to act on the travel transducer 10, and an energising current is applied to the solenoid coil of the actuator 6 through the control module 3. The armature 8 is advanced in the housing 10 in proportion to the magnitude of the energising current and acts to displace the spool 11 axially, initially to cause the land 20 to isolate the reservoir 17 from the brake 5, and, subsequently, for the land 22 to place the pressure source 18 in communication with the brake through the outlet passage 15. This pressure also acts on the inner end of the needle 25 to generate on the spool 11 a feed back force which is applied to the armature 8. The feed back force comprises the product of that pressure acting over an area equal to the area of the inner shoulder 26 minus the area of the needle 25. The pressure acting on the needle 25 holds its outer end against a stop, suitably the end of the bore 10 remote from the armature 8.

When the brake pedal is released the spring 13 and the feed back force return the spool 11 to its retracted position in engagement with the stop 12, carrying the armature 8 with it.

Since the armature 8 is unsprung, power is conserved since, during the initial forward movement of the armature, no bias or other return force has to be overcome. The provision of the small clearance between the armature 8 and the spool 11 is simply a manifestation of the absence of the bias force.

In the actuator illustrated in Figures 2 and 3 of the accompanying drawings the needle 25 is replaced by an end cap 30 in the from of a thimble. The end cap 30 has an internal cylindrical bore 31 in which an outer end portion 32 of the spool 11, also of cylindrical outline, is slidably - received. The outer end portion 32 is of a diameter slightly greater than the remainder of the spool 11.

Specifically as illustrated the end portion 32 is increased in diameter by Y to increase the diameter from the normal diameter of X to X+Y.

The outer end of the cylindrical bore 31 is closed by an integral end wall 33, and the compression spring 13 acts between the end wall 33 and the adjacent end of the spool 11.

When the armature 8 is advanced in the housing 10 and acts to displace the spool 11 axially, initially to cause the land 20 to isolate the reservoir 17 from the brake 5, and, subsequently for the land 22 to place the pressure source 18 in communication with the brake through the outlet passage 15, the pressure also acts between the spool 11 and the end cap 30, specifically on the area of the outer end portion 32 of the spool 11 to generate on the spool 11 a feed back force which is applied to the armature 8.

The Y diametrical increase is tailored to suit the reaction requirements of the solenoid, namely the reaction force range for a given pressure range required. Thus: force = PB (TT (X+Y2) - TT (X) 2 4 4 = PB (annular area) where PB = Brake pressure.

This construction has the advantage that by providing the end cap 30, concentricity problems associated with grinding a multi-diameter spool to close tolerances are eliminated.

The feed back force comprises the product of that pressure acting over an area of diameter X+Y, against an equal pressure acting over the area of smaller diameter X in the opposite direction.

Claims

1. A brake actuator for an hydraulic braking system of the brake-bywire type comprising a solenoid-operated valve comprising a housing, a solenoid coil in the housing, an armature movable axially in the housing in response to energisation of the solenoid coil, a valve spool axially movable in a bore in the housing with respect to a stop defining a retracted position for the spool, a spring urging the spool towards the stop the spool being adapted selectively to control communication between axially spaced exhaust, inlet and outlet passages in the housing, and being formed at opposite ends of an intermediate portion of reduced diameter with inner and outer shoulders at steps in diameter between the intermediate portion and inner and outer lands of which the inner land controls communication between the exhaust and outlet passages, and the outer land controls communication between the inlet and outlet passages, the spool also being provided in the end remote from the armature with a longitudinally extending bore which is in communication with the bore in the housing through a radial passage in the spool, in which an area determining member has a working engagement with an end portion of the spool remote from the armature, and in which energisation of the solenoid coil causes the armature to urge the spool away from the stop into an operative position in which the inner land closes the exhaust passage and the outer land opens communication between inlet and the outlet passages, the outlet pressure providing a feed back force on the armature comprising the product of that pressure acting over different effective areas of the spool to subject the spool to a net force in a direction towards the armature and in which the area determining member comprises a cap having an internal bore in which the end portion is slidably received and an end wall closing the outer end of the internal bore, the end portion being of greater area than the remainder of the spool.

2. A brake actuator as claimed in claim 1, in which the area determining member is held against movement in a longitudinal direction by a force generated by the applied pressure which acts to hold it against a stop.

3. A brake actuator as claimed in claim 2, in which the stop comprises the end of the bore remote from the armature.

4. A brake actuator as claimed in any preceding claim in which, a small clearance is provided between the actuator and the spool when the actuator is in an off position.

5. A brake actuator as claimed in any preceding claim in which the armature is unsprung so that no bias resistance force has to be overcome when the actuator is operated by energisation of the coil.

6. A brake actuator as claimed in any preceding claim in which the actuator is provided with a solenoid having an armature which applies a force to the spool which is proportional to the current applied to the solenoid.

7. A vehicle braking system incorporating an actuator as claimed in any preceding claim in which the coil is connected to an electronic controller which emits an energising current of a magnitude dependent upon a pedal force, the inlet passage is connected to a source of high pressure fluid, the outlet passage is connected to the brake, and the exhaust passage is connected to a reservoir for fluid from which the pump is supplied, current being supplied to the solenoid in reponse to driver demand with the result that the armature acts on the spool to control communication between the power source, the reservoir, and the brake.

8. A brake actuator substantially as described herein with reference to and as illustrated in Figure 2 and Figure 3 of the accompanying drawings.

9. A vehicle braking system substantially as described herein with reference to and as illustrated in Figure 2 and Figure 3 of the accompanying drawings.