Electromagnetic Valve

Abstract

There is disclosed an electromagnetic valve incorporated as the pressure medium flow regulator of an antiskid system. The valve body of the valve is connected with the magnetic armature to control the cross-sectional area of pressure medium flow through the valve in a continuous sequence proportional to the magnetic force resulting from the energizing current produced by the electronic circuitry of the antiskid system. By changing the energizing current, it is possible to change the pressure difference between the pressure medium source and the pressure-operated brake. Thus, the hydraulic antiskid control is more accurate and smoother than is achieved with previously employed open/close antiskid hydraulic control valves.

Description

BACKGROUND OF THE INVENTION

This invention relates to an electromagnetic valve for use in antiskid systems, the valve body thereof being mechanically connected with the magnetic armature such that the valve body blocks, opens or diverts the passage of pressure medium flow through the valve when the magnet winding is energized.

It is a decisive factor in all antiskid systems to keep the switching times of regulator-operated electromagnetic valves at a value as low as possible. The switching times are determined, among other things, by the mass of the magnetic armature and by the amount of friction to be overcome when the armature is accelerated, as well as by the magnetic resistance in the closed magnetic circuit. In known plunger-type armature valves, the plunger-type armature represents a relatively large mass. In order to provide this type of armature with a minimum amount of friction, it is necessary to provide, besides the operating air gap, a radial air gap located between the armature and the magnetic casing.

Furthermore, it is a particular desired feature in antiskid systems that the cross-sectional area of the valve control orifice changes continuously and in proportion to the magnetic force in order to achieve a control more accurate and smooth than could be achieved with the open/close valves known to date.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide an electromagnetic valve which avoids the above-mentioned drawbacks, which is of simple design, easy to mount, affords rapid switching and, if possible, permits a continuous control of the hydraulic pressure.

A feature of the present invention is the provision of an electromagnetic valve for use in an antiskid brake system comprising a magnet housing having a first longitudinal axis, a magnet winding disposed in the housing coaxial of the axis, a magnetic return path disposed in the magnet housing magnetically coupled to the winding, a sleeve of non-magnetic material disposed in the return path coaxial of the first axis, a first chamber disposed in the magnet housing adjacent the sleeve and the return path, a valve body disposed coaxial of the first axis, at least a portion of the valve body projecting into the first chamber, at least one armature plate disposed in the first chamber in an engaged relationship with the portion of the valve body projecting into the first chamber, and a spring disposed in the first chamber in an engaged relationship with the valve body and the armature plate to bias the valve body and the armature plate in a rest position, the valve body controlling the flow of a brake pressure medium through the valve between at least a brake pressure medium inlet and a brake pressure medium outlet when the winding is energized and the armature plate is attracted toward the return path.

Another feature of the present invention is the provision of armature plates arranged to leave a free space for a central opening and having their inner edges grip the under surface of a collar provided directly on the valve body or on the movable supporting member of the valve body. The armature plates are slightly inclined with their outer edges abutting the magnetic return path.

A further feature of the present invention is the provision of a diaphragm spring provided with openings and including a central aperture. This spring has its inner edge engaged in a groove provided on the valve body or on the valve body support and its outer edge firmly connected to the housing. This spring biases both the valve body and the armature plates in their rest positions.

Still another feature of the present invention is the provision of a star-shaped diaphragm spring having its inner edge in engagement with a groove in an annular element mounted on the valve body or on the valve body support.

Still a further feature of the present invention is the provision that the characteristic curve of the diaphragm spring is adapted to the characteristic curve of the electromagnet.

According to another feature of this invention, the valve body is a valve slide controlling in a known manner the flow of pressure medium between the connectors of the pressure-medium source of the pressure-operated device and of a pressure medium reservoir, and that the valve slide is pressure-balanced in rest position, i.e., when there is flow of pressure medium between the pressure-medium source and the pressure-operated device.

Still a further feature of this invention is the provision of a duct interconnecting the chambers accommodating the ends of the valve slide.

Still another feature of this invention is that the duct interconnecting the chambers accommodating the ends of the valve slide also communicates with the pressure-operated device in one embodiment and with the pressure medium source in another embodiment.

A further feature of this invention is that the valve slide is divided to form two pistons. In one embodiment, these pistons are mechanically connected and pressure-balanced in rest position at their outer and inner ends by the pressure of the pressure-operated device and by the source pressure, respectively.

In a further embodiment, the pistons forming the valve slide are pressure-balanced in rest position at their outer ends by the source pressure and at their inner adjacent ends by the pressure of the pressure-operated device.

In accordance with still another feature of this invention, when the electromagnet is energized, the valve slide can be actuated by source pressure in a direction opposite to that of the magnetic force, and by pressure of the pressure-operated device in the direction of the magnetic force, the actuating surfaces for both pressures being of equal size.

In accordance with a further feature of the invention, the energizing current of the electromagnet and consequently the pressure difference between the pressure medium source and the pressure-operated device are continuously variable.

BRIEF DESCRIPTION OF THE DRAWING

Above-mentioned and other features and objects of this invention will become more apparent by reference to the following description taken in conjunction with the accompanying drawing, in which:

FIG. 1 is a longitudinal cross-sectional view of a first embodiment of an electromagnetic valve in accordance with the principles of the present invention showing a seat valve which is closed when the magnet coil is deenergized;

FIG. 2 is a transverse cross-sectional view of the seat valve taken along line A-B of FIG. 1;

FIG. 3 is a longitudinal cross-sectional view of a second embodiment of an electromagnetic valve in accordance with the principles of the present invention showing a seat valve which is opened when the magnet coil is deenergized;

FIG. 4 is a longitudinal cross-sectional view of a third embodiment of an electromagnetic valve in accordance with the principles of the present invention showing a 3/2 directional control valve; and

FIG. 5 and 6 are further embodiments of an electromagnetic valve in accordance with the principles of the present invention showing 3/2 directional control valves.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to FIG. 1, there is located in a housing 2 made of magnetizable material the magnet coil 3 in an insulated relationship with housing 2. Housing 2, together with the magnet core 9 surrounded by coil 3, forms a magnetic return path 10 into which there is embedded a sleeve 4 made of non-magnetic material. In the housing chamber 11 below magnetic return path 10, there are situated armature plates 1 held by means of retaining clips 6. Armature plates 1 have an annular central opening. It can be seen from FIG. 2 that there are two trapezoidal armature plates resting against each other at their bases and that a semicircular aperture is cut into each base. At the circular opening, the inner edges of armature plates 1 grip the under surface of a collar 12 of the supporting member 13 for the valve body 7. Supporting member 13 is provided with clearance for motion by virtue of a centric recess 14 in the magnet core 9 into which supporting member 13 is projecting. At the opposite end of axially symmetrical supporting member 13, a U-shaped cross-sectional annular element 16 is located on the top surface of a collar 15. Retaining clips 6 each have one end gripped by one arm of annular element 16, while the other ends of clips 6 engage a groove in armature plates 1. The inner edge of a star-shaped diaphragm spring 5 includes a central opening gripped by the second arm of annular element 16, while the outer ends of spring 5 engage an annular groove in the wall of housing 2. When magnet coil 3 is deenergized, diaphragm spring 5 keeps member 13 supporting the valve body 7 and armature plates 1 spaced apart from the electromagnet. In this state, armature plates 1 assume a slightly inclined position.

Valve body 7 is jointed to its supporting member 13 in a well-known way and, when in rest position, has its closing member resting against the valve seat of the outlet 17 provided in the adjacent housing wall. Preferably, the same housing wall also includes the inlet 18 terminating in housing chamber 11. Inlet 18 is covered with a filter 8 to provide a protection against dirt. A thread 19 provided at housing 2 permits the valve to be screwed into a cooperating device such as a pressure medium conduit. If current flows through coil 3 of the electromagnet, the induced magnetic field will cause armature plates 1 to be attracted by magnet core 9 and magnetic return path 10, respectively, armature plates 1 thus causing supporting member 13 and the associated valve body 7 to follow against the force of diaphragm spring 5, so that the valve-closing member will be lifted from its seat and the valve will be opened.

FIG. 3 shows an embodiment of a seat valve constructed in accordance with this invention which is opened with the electromagnet being deenergized. A substantial number of parts of FIG. 3 are identical to those of the seat valve of FIG. 1 and, therefore, like parts have been assigned like reference numerals in both FIGS.

From housing chamber 11, a blind-end bore 20 extends into magnet core 9, the outlet 21 extending laterally from bore 20 through magnetic return path 10. Before outlet 21 branches off from bore 20, a valve seat 22 is disposed on a stepped portion of blind-end bore 20. In contrast to the embodiment of FIG. 1, the positions of supporting member 13' and of valve body 7' jointed thereto are just reversed. The closing member of valve body 7' projects into blind-end bore 20 of magnet core 9, thus lying opposite valve seat 22. The inner edges of armature plates 1 grip the under surface of collar 12' and are kept in position by means of retaining clips 6 in the manner described with respect to FIG. 1. With one end of diaphragm spring 5 engaging annular element 16 at collar 15' of supporting member 13' and with the other end of diaphragm spring 5 engaging a groove in housing chamber 11, the diaphragm spring 5 via supporting member 13' keeps valve body 7' spaced apart from valve seat 22 and armature plates 1 apart from magnetic return path 10. This embodiment also provides for a protective filter 8 covering inlet 23 terminating in housing chamber 11.

In rest position, when the valve is open, current flow through magnet coil 3 will cause armature plates 1 and consequently supporting member 13' and valve body 7' to be attracted so that the closing member of the valve body 7' rests on its valve seat 22, thus interrupting the connection between inlet and outlet.

In the two seat valves described above, the magnetic leakage field is reduced by virtue of magnetic return path 10, thus permitting more efficient use to be made of the magnetic forces and consequently allowing utilization of a smaller electromagnet. The non-magnetic sleeve 4 embedded in magnetic return path 10 avoids the occurrence of a magnetic short circuit and induces the lines of magnetic flux to run through armature plates 1. In order to prevent armature plates 1 from further sticking to the magnet core following interruption of the energizing current, a non-adhesive plate (not shown) made of non-magnetic material has been provided between magnet core 9 and armature plates 1. Since it is known from experience that the adhesives used did not stick, the non-adhesive plate, while resting on armature plates 1 may be clamped under collar 12 of supporting member 13, or, in the shape of an expanding cap, be squeezed into centric recess 14 of magnet core 9.

In contrast to a plunger-type armature, armature plates 1 enjoy the advantage of moving under extremely low friction during switching and of having less mass. In a valve constructed in accordance with the present invention, there is only one air gap in the magnetic circuit, namely, the working air gap. The radial air gap necessary in frictionless suspended plunger-type armatures in not required. The flat design of the armature also enables diaphragm springs to be mounted in a preferable manner. The diaphragm springs permit high forces to be controlled more readily than is the case with spiral springs.

It has been proven that valves constructed in accordance with the present invention have very short switching times even if no special magnetic materials are used.

FIG. 4 shows the arrangement of the magnetic armature already described together with a slide valve incorporating, in the embodiment shown, further decisive advantages particularly for use in antiskid control systems.

The housing of the slide valve contains a cylinder bore 30 enlarging at its one end to form a chamber 32 via a stepped portion 31. In its middle area, cylinder bore 30 enlarges once more into a wide groove 33. Groove 33 connects in its whole width with a port 34 of the slide valve. Cylinder bore 30 contains two slidably mounted pistons 36 and 37 coupled to each other via a mechanical connection 35. The diameter of piston 37 is smaller than the diameter of piston 36. Piston 36 is sealed to the inner wall of cylinder bore 30 and projects into enlarged chamber 31. Piston 36 is secured against sliding out by means of a circlip 38. By means of a recess provided in the lateral surface of piston 36, an annular chamber 39 is created permanently communicating with a second port 40 of the slide valve. That end of piston 36 which is connected with the second piston 37 is reduced to the diameter of piston 37, thus producing within the area of groove 33 a collar 41 on piston 36. Collar 41 corresponding in length to the width of groove 33. The other end of piston 37 is tightly confined within cylinder bore 30 reduced to the diameter of piston 37. This diameter reduction of bore 30 creates annular chamber 42 communicating with the third port 43 of the 3/2 directional control valve. The edges of groove 33 or of its port 34, respectively, together with the associated adjacent edges of collar 41 on piston 36, form the leading edges 44 and 45 through which, under common sliding movements of pistons 36 and 37, groove 33 and thus port 34 communicate with annular chamber 42 and the port 43, on the one hand, and with annular chamber 39 and port 40, on the other hand. At the end of piston 37 projecting out of cylinder bore 30, a first collar 46 is provided resting against the housing when the valve is in rest position. A second collar 47 establishes the connection with the electromagnetic control system described above. In this FIG., also, like parts have been assigned like reference numerals.

Housing 2 of the electromagnet is screwed into a recess of the valve casing in which cylinder bore 30 terminates. The thus limited housing chamber 11 into which the end of the piston 37 projects, communicates, for the purpose of obtaining pressure balance, with the port 34 of groove 33 via a duct 48, and further with chamber 32 situated in front of piston 36. The armature plates 1 with their inner edges grip the upper surface of collar 47 on the end of piston 37 and are secured by means of retaining clips 6 which are fixed to annular element 16. The inner edge of diaphragm spring 5 also engages annular element 16 resting against collar 46 while the outer edges of spring 5 engage a groove of housing wall 2 of the electromagnet. Diaphragm spring 5 keeps both armature plates 1 and pistons 36 and 37 in their illustrated home or rest positions.

As mentioned before, this valve is to be use preferably for antiskid control systems. For this purpose, port 43 connects with the brake actuation system, i.e., with the master cylinder, while port 34 connects with the wheel brake cylinder and port 40 with the reservoir. In the event of wheel locking danger, the electronic control system will transmit a current pulse to the electromagnet. This pulse is continuously variable in accordance with the variation of the output variable relative to its rated value.

In the illustrated valve position, wherein the electromagnet is deenergized, the brake can be operated as usual. The brake pressure is transmitted through port 43, annular chamber 42 and the port 34 to the wheel brake cylinder.

Pressure P.sub.1 in the master cylinder is equal to pressure P.sub.2 in the wheel brake cylinder. Since pressure P.sub.2, via duct 48, also acts on the outer front surfaces of pistons 36 and 37, these pistons are hydraulically balanced. If the electronic control unit of the antiskid system detects danger of wheel lock, current will flow through coil 3 of the electromagnet and cause armature plates 1 to be attracted towards magnetic return path 10, thereby also moving the mechanically connected pistons 36 and 37 in the same direction. In this process, leading edges 44 first interrupt the pressure-medium connection between the master cylinder and the wheel brake cylinder, whereupon the leading edges 45 open the connection from port 34 of the wheel brake cylinder through annular chamber 39 to port 40 leading to the reservoir, thus permitting pressure in the wheel brake to drop. Pressure P.sub.1 of the master cylinder exerts the force P.sub.1 .times. A on the annular surface A of collar 41 at piston 36, whereas, caused by the flow conditions, pressure P.sub.2 in the wheel brake cylinder exerts the opposing force P.sub. 2 .times. A. Pressure P.sub.2 in the wheel brake cylinder is now continuously decreasing until the resulting hydraulic force (P.sub.1 - P.sub.2) .times. A acting on pistons 36 and 37 is equal to the magnetic force determined by the coil current, disregarding the friction and spring forces. Variation of the coil current, which as mentioned above is continuously variable dependent on the output variable, involves a change of the pressure difference P.sub.1 - P.sub.2. The characteristic curve of the diaphragm spring 5 is preferably adapted to that of the electromagnet in order to achieve independence of the resulting force from the stroke and to obtain an improved controller action.

Thus, in a pressure regulator constructed in accordance with the present invention, the pressure difference between the brake control valve or the master cylinder and the wheel brake cylinder is adjusted in proportion to the magnetic force. Via the coil current, the control pressure thus represents a further item for the electronic control unit.

FIG. 5 shows a further improved embodiment of the slide valve constructed in accordance with the present invention. This embodiment also has one end of the cylinder bore 50 enlarged to form a chamber 51 into which projects the tightly confined smaller piston 52. Piston 53, which is of larger diameter than piston 52, is confined within that area of cylinder bore 50 enlarged by a stepped portion and rests against the adjacent surface of piston 52, around which end an annular chamber 54 is formed. By means of a recess provided in the lateral surface of piston 53, an annular chamber 55 is created permanently communicating with one port 56 which, when used in an antiskid control system, leads to the reservoir. A further recess provided in bore 50 creates an annular chamber 57. A collar 58 is provided on piston 53 having a width to the width of a groove 59. This groove 59 connects with the second port 60 of the valve which port leads to the wheel brake cylinder. The edges of groove 59, together with the associated adjacent edges of collar 58, form the leading edges 61 and 62 of the slide valve. The annular chamber 57 communicates with chamber 11 via recesses 63 provided in the valve housing. Chamber 11 receives piston 53 which is connected with the electromagnetic control system in the manner described with reference to FIG. 4. Via one of these recesses 63, the annular chamber 57 and chamber 11 communicate by means of a duct 64 with chamber 51 and the port 65 leading to the master cylinder, in order to achieve pressure balance. The groove 59 connects with the annular chamber 54 at the piston 52 via a longitudinal bore 66 and a cross bore 67 provided in piston 53.

Basically, the mode of operation of the embodiment of FIG. 5 is the same as that described with reference to FIG. 4. In addition, the embodiment of FIG. 5 has the following advantages.

The pistons are interchanged and pressure P.sub.1 of the master cylinder always acts on the outer end surfaces of pistons 52 and 53, while pressure P.sub.2 of the wheel brake cylinder acts on the inner end surfaces of pistons 52 and 53. This avoids the necessity for providing a mechanical means for connecting the pistons. Therefore, when the magnet is energized only the friction forces of piston 53 have to be overcome and piston 52 is made to follow hydraulically by virtue of pressure P.sub.1 of the master cylinder.

FIG. 6 represents a slide valve operating according to the same principles as described above but showing a further simplified embodiment provided with an integrally formed valve slide. In the valve casing 70, there is accommodated the cylinder 71 with the cylinder bore 72. The valve slide 75 is tightly and slidably received in cylinder bore 72 terminating in the housing chamber 74 which communicates with the master cylinder via a port 73. A recess 76 provided in the wall of cylinder 71 connects cylinder bore 72 with a further housing chamber 77 which communicates with the wheel brake cylinder via a port 78. The collar 79 is produced by a recess in valve slide 75. The width of collar 79 corresponds to the width of recess 76 provided in the cylinder wall. Those edges of collar 79 and recess 76 which are adjacent to each other form the leading edges 80 and 81 of the valve. The annular chamber 82 is created by the above-mentioned recess in valve slide 75 and communicates with the housing port 84 via a cross bore 83 in the cylinder wall. Port 84 leads to the reservoir. Cylinder 71 projects into a housing chamber 85 into which is screwed housing 86 of the electromagnet. Valve slide 75 connects with the pressure-balanced magnetic armature 87 and is biased in rest position by means of a spring 88. Via a duct 89 provided in valve casing 70, housing chamber 77 is connected with housing chamber 85 containing the valve-slide front surface facing the electromagnet. In this way, valve slide 75 is pressure-balanced in rest position. Brake pressure is thus free to be transmitted from the master cylinder through housing chamber 74, cylinder bore 72, recess 76 in the cylinder wall, and housing chamber 77, to the wheel brake cylinder. If the electromagnet receives a current pulse in the event of wheel locking danger, magnetic armature 87 will be attracted towards the magnet core 90, with the valve slide 75 being displaced. Leading edges 80 first interrupt the pressure-medium connection between the master cylinder and the wheel brake cylinder, whereupon leading edges 81 will open up the connection between wheel brake cylinder and reservoir. The master cylinder pressure P.sub.1 acts on the front surface of collar 79 of the valve slide 75, while, via duct 89, the wheel brake cylinder pressure P.sub.2 acts on the front surface of valve slide 75 facing the electromagnet. As described above, a pressure difference is then produced at valve slide 75 which is proportional to the continuously variable coil current. With the magnet force being of the same magnitude, the cross-sectional area of the valve slide in FIG. 6 is bound to be approximately the same as the annular surface A in FIG. 4.

It is to be understood that the functional aspects of the pressure regulating valve described herein are not limited to the inventive construction of an electromagnet embodying armature plates. Therefore, FIG. 6 shows an embodiment of the invention incorporating a plunger-type armature.

It is to be understood further that the advantages mentioned in connection with the description of the seat valves of FIGS. 1 and 2 and relating to the special arrangement of the armature plates also refer to slide valves. These advantages consist in affording valve-slide movement under extremely low friction during switching, a small mass of the armature, the omission of the radial air gap with the magnetic armature being mounted under a minimum amount of friction, and the possibility of using diaphragm springs.

It is in the particular use in antiskid systems that the slide valves of FIGS. 4, 5 and 6 include still further decisive advantages. There is firstly, as has already been mentioned previously, the pressure difference between the brake control valve and, respectively, the master cylinder and the wheel brake cylinder, which difference is continuously variable in proportion to the magnet current. Since 3/2 directional control valves are used, each control circuit requires but one valve. The phase in the antiskid control cycle during which the pressure is maintained at a constant value and which had to be created in 3/2 directional control valves by "clicking", poses no problems in slide valves. The valve constructed in accordance with this invention provides for more freedom in selecting the most expedient control method and is, among other things, the prerequisite for a continuous control system no longer producing sharply defined phases for pressure decrease, pressure stabilization and pressure increase, but producing a pressure curve smoothly swinging near its ideal value. This is achieved by permanently interrogating the output variable in a timing screen and by modifying the coil current of the electromagnet according to the variation exceeding the permissible deviation with a downward or upward tendency. Thus, the invention relates to an analog valve instead of a digital valve, i.e., control is via the current intensity and not via the duration of voltage application.

In the valves of FIGS. 4, 5 and 6, the steepness of the curve slopes (dp/dt) is not predetermined as is the case with seat valves, but variable. The pressure decrease .DELTA. p is not ignored, since the pressure controlled by the brake control valve (master cylinder) also plays a governing role.

Since the pistons or slides, respectively, are hydraulically balanced in normal or rest position, only the friction forces and the small spring force have to be overcome when switching on. This results in short response times. The slide valves combine the advantage of low resistance to flow with accurate control possibilities, thus enabling the valves to be installed directly in the brake line.

While we have described above the principles of our invention in connection with specific apparatus, it is to be clearlly understood that this description is made only by way of example and not as a limitation to the scope of our invention as set forth in the objects thereof and in the accompanying claims.

Claims

We claim:

1. An electromagnetic valve for use in an antiskid brake system comprising:

a magnet housing having a first longitudinal axis;

a magnet winding disposed in said housing coaxial of said axis;

a magnetic return path disposed in said magnet housing magnetically coupled to said winding;

a sleeve of non-magnetic material disposed in said return path coaxial of said first axis;

a first chamber disposed in said magnet housing adjacent said sleeve and said return path;

a valve body disposed coaxial of said first axis, at least a portion of said valve body projecting into said first chamber;

a plurality of armature plates disposed in said first chamber, said armature plates having a central opening coaxial of said axis, the outer edges of said armature plates abutting said return path and being slightly inclined away from said return path, and the inner edges of said armature plates engaging said portion of said valve body projecting into said first chamber to move said valve body when said winding is energized and said armature plates adjacent said inner edges are attracted toward said return path;

a spring disposed in said first chamber in an engaged relationship with said valve body and said armature plates to bias said valve body and said armature plates in a rest position;

said valve body controlling the flow of a brake pressure medium through said valve between at least a brake pressure medium inlet and a brake pressure medium outlet when said winding is energized and said armature plates are attracted toward said return path;

said spring including

a diaphragm spring having openings therethrough and a central aperture coaxial of said first axis, the inner edge of said diaphragm spring adjacent said central aperture engaging a groove provided in said portion of said valve body projecting into said first chamber and the outer edge of said diaphragm spring being firmly connected to said magnet housing surrounding said first chamber.

2. A valve according to claim 1, wherein

said groove is provided by a U-shaped cross-sectioned annular element mounted on said portion of said valve body projecting into said first chamber.

3. A valve according to claim 2, wherein

said diaphragm spring is star shaped.

4. A valve according to claim 3, wherein

the characteristic curve of said diaphragm spring is adapted to the characteristic curve of an electromagnet including said winding, said return path, said sleeve and said armature plates.

5. A valve according to claim 4, further including

a valve housing connected to said magnet housing,

said valve housing havig a second longitudinal axis which is an extension of said first axis;

a pressure medium source inlet connection extending through the wall of said valve housing;

a pressure medium reservoir outlet connection extending through the wall of said valve housing; and

a pressure medium wheel brake cylinder outlet connection extending through the wall of said valve housing; and

wherein

said valve body includes

a valve slide extending from said first chamber into said valve housing coaxial of said second axis, said valve slide being disposed in a cooperative relationship with said source inlet connection, said reservoir outlet connection and said wheel brake cylinder outlet connection to control the flow of pressure medium between these three connections, said valve slide being pressure balanced in its rest position when the flow of pressure medium is between said source inlet connection and said wheel brake cylinder outlet connection.

6. A valve according to claim 5, further including

a second chamber disposed in said valve housing embracing the end of said valve slide remote from said first chamber; and

a duct disposed in said valve housing to interconnect said first and second chambers.

7. A valve according to claim 6, wherein

said duct is connected to said wheel brake cylinder outlet connection.

8. A valve according to claim 6, wherein

said duct is connected to said source inlet connection.

9. A valve according to claim 6, wherein

said valve slide is formed by two pistons in tandem relationship.

10. A valve according to claim 9, wherein

said two pistons are mechanically connected together,

said two pistons being pressure-balanced in their rest position by the pressure present at said wheel brake cylinder outlet connection being applied to the remote ends of said two pistons and by the pressure at said source inlet connection being applied to the adjacent ends of said two pistons.

11. A valve according to claim 9, wherein

said two pistons can move freely with respect to each other,

said two pistons being pressure-balanced in their rest position by the pressure at said source inlet connection being applied to the remote ends of said two pistons and by the pressure at said wheel brake cylinder outlet connection being applied to the adjacent ends of said two pistons.

12. A valve according to claim 6, wherein

when said winding is energized to provide a magnetic force said valve slide is actuated in a direction opposite to that of said magnetic force by pressure from said source inlet connection being applied to a first actuating surface and is actuated in the direction of said magnetic force by pressure from said wheel brake cylinder outlet connection being applied to a second actuating surface.

13. A valve according to claim 12, wherein

said first and second actuating surfaces are equal in size.

14. A valve according to claim 12, wherein

a current energizing said winding and consequently the pressure difference between said source inlet connection and said wheel brake cylinder outlet connection are continuously variable.