Abstract:
The present invention discloses a valve assembly for controlling a differential pressure, including a housing, an actuating device, a first, a second, and a third port, a first sealing seat, a second sealing seat, and a valve member whose interaction with the first sealing seat controls a first connection between the first and the second port, and whose interaction with the second sealing seat controls a second connection between the second and the third port. In order to ensure an optimal analog controllability, according to the present invention, the sealing seats are axially opposite each other and have equal effective diameters, and the valve member is arranged between the sealing seats.

Description:
TECHNICAL FIELD 
     The present invention relates to valve assemblies and more particularly relates to a valve assembly for controlling a differential pressure. 
     BACKGROUND OF THE INVENTION 
     A preferably pneumatic valve assembly of this type which is used especially for controlling the differential pressure in vacuum brake force boosters of automotive vehicle brake systems is disclosed in German published patent application No. 39 43 003. The three-way/three-position directional control valve disclosed in this application has two coaxially arranged sealing seats which cooperate with a sealing surface of an elastic valve member. The first sealing seat which is interposed between the first and the second port in terms of effect is designed on a valve piston that is elastically suspended in a valve housing. The valve piston is axially displaceable in the direction of the valve member by means of an electromagnet. The second sealing seat which is inserted between the second and the third port in terms of effect is provided in the interior of the valve housing. The valve piston houses an actuating sleeve which is arranged coaxially relative to the first sealing seat, is captivated preferably on the valve piston and movable into a force-transmitting connection with the valve member. The prior art valve suffers from the disadvantage of an abrupt rise of the actuating force which is to be produced by the electromagnet in the presence of a predeterminable travel of the valve piston. This abrupt rise of the actuating force is due to the effect of a pressure difference acting on an annular surface on the sealing surface limited by the two sealing seats and exerts an adverse influence on the controllability of a valve of this type. Another shortcoming is the comparatively high magnetic force level which is induced by the above-mentioned abrupt rise in force and requires high current values for the actuation of the electromagnet. 
     Therefore, an object of the present invention is to provide a valve assembly of the type mentioned hereinabove which operates on a low level of force which is to be produced by an actuator and the controllability of which is ensured without problems. Another objective is that the valve assembly is easy to manufacture in series production and comprises individual components which are inexpensive to make, thereby avoiding complicated manufacturing measures to the extent possible. 
     According to the present invention, this object is achieved because the sealing seats are arranged axially opposite each other and have equal effective diameters, and because the valve member is arranged between the sealing seats. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic cross-section taken through a first design of the valve assembly of the present invention in an inoperative state. 
     FIG. 2 is a view of a second design of the valve assembly of the present invention in a representation which corresponds to FIG.  1 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The valve assembly of an electronically controllable brake force booster which is schematically shown in the attached drawing has a generally rotation-symmetrical design and includes a housing  1  which has a first port  2 , a second port  3 , and a third port  4 . The first port  2  is associated with a vacuum chamber (not shown) of the brake force booster that is connected to an appropriate vacuum source. The second port  3  opens into a working chamber (which is also not shown), and the third port  4  serves for the supply of the ambient atmosphere. In the interior of the housing  1 , a plate-shaped valve member  5  is axially slidably arranged which is operable by an electromagnetic actuating device  7  by way of a tubular extension  6 . The electromagnetic actuating device  7  comprises a coil  8  and an armature  9  on which the above-mentioned extension  6  abuts under bias of a first spring  10 . In this arrangement, the valve member  5  is favorably arranged between a first sealing seat  11  and a second sealing seat  12  which are provided on each one sleeve  13 ,  14  that is slidable within limits in the housing  1 . The two sealing seats  11 ,  12  are provided by each one sealing surface made of an elastic material and cooperate with each one preferably annular sealing edge  15 ,  16  on the valve member  5 , and the arrangement is preferably made so that the sealing edges  15 ,  16  have the same diameter. The sleeves  13 ,  14  include each one annular collar  17 ,  18  which is movable into abutment with each one stop  19 ,  20  configured in housing  1 . An axial preload of the sleeves  13 ,  14  in the direction of the stop  19 ,  20  is applied by each spring  21 ,  22 . Openings  23 ,  24  are furnished in the collars in order to provide optimal flow conditions both during evacuation and ventilation of the working chamber (not shown) of the brake force booster connected to the second port  3 . 
     In the inactive position of the valve device of the present invention shown in FIG. 1, or when the electromagnetic actuating device  7  is inoperative, the first sleeve  13  bears against the stop  19 , biased by spring  21 , that means at an axial distance from the sealing edge  15  so that a pneumatic connection exists between the first port  2  and the second port  3  and leads through the interior  25  of the housing  1 . The second sealing surface bears against the annular sealing edge  16  of the valve member  5  under the bias of the second spring  22  so that the second sealing seat  12  and, thus, the third port  4  are closed. 
     When electric current is applied to the coil  8 , a force, directed to the left in the drawing, is exerted on the armature  9  and causes axial displacement of the valve member  5  towards the first sealing seat  11 . Biased by spring  22 , the second sealing seat  12  follows the movement of the valve member  5  until the collar  18  of the second sleeve  14  moves into abutment with the stop  20 . Upon continued movement of the valve member  5  to the left, the sealing edge  15  moves into abutment with the first sealing surface so that the first sealing seat  11  is closed and the connection between the first port  2  and the second port  3  is interrupted. The condition in which both sealing seats  11 ,  12  are closed is referred to as ‘pneumatic pressure retaining phase’. 
     Upon an increase of the current which flows through the coil  8 , the first sleeve  13  is moved to the left in opposition to the spring  21 , and the second sealing edge  16  is lifted from the second sealing seat  12 . Opening of the second sealing seat  12  causes opening of a connection between the second port  3  and the third port  4  so that the above-mentioned (non-illustrated) working chamber of the brake force booster is ventilated. The condition of pressure increase in the working chamber is referred to as ‘pneumatic pressure increasing phase’. 
     However, when the current supplied to the coil  8  is decreased, starting from the ‘pressure retaining phase’, the armature  9  and, thus, the valve member  5  are moved to the right, as viewed in the drawing, under the effect of the first compression spring  10 . During this movement, the second sleeve  14  is displaced in opposition to the effect of the spring  22 , into the position shown in FIG. 1, and the first sealing edge  15  is lifted from the first sealing surface, and the first sealing seat  11  is opened. The result is that the connection between the ports  2  and  3  is opened so that there occurs evacuation of the working chamber and, thus, decrease of the pressure prevailing therein. The condition of the pressure decrease in the working chamber is referred to as ‘pneumatic pressure decreasing phase’. 
     The magnetic force which acts upon the armature  9  is a function of the current which flows through the coil  8 . By variation of the current supplied to the coil  8 , the position of the armature  9  in the axial direction of the valve assembly can be adjusted so that any pressure variation desired can be adjusted by a defined alternation between increasing, retaining and decreasing phase. The energization of coil  8  can be effected by appropriate signals which are produced by an electric control unit (not shown). 
     In the second design variation of the subject matter of the present invention as shown in the embodiment of FIG.  2 . In FIG. 2 a valve member  50  is stationarily arranged in the valve housing  1  and has two opposed sealing surfaces  27 ,  28  which cooperate with the above-mentioned sealing seats, that have been assigned reference numerals  110 ,  120  in FIG.  2 . The valve assembly shown in FIG. 2 is actuated by way of a bowl-shaped actuating element  29  which is movable into engagement with an armature  38  of an electromagnetic actuating device  70 , on the one hand, and with sleeves  130 ,  140  which form sealing seats  110 ,  120 , on the other hand, with a view to the axial displacement of the sleeves with respect to a stationary valve member  50 . The transmission of force between the armature  38  and the actuating element  29  is carried out by means of a tubular, radially inward axial extension  30  that is spaced axially from the armature  38 . A variation of the distance between armature  38  and extension  30  permits adjusting the desired magnetic force level which is required in the operation of the valve assembly of the present invention. The force is transmitted between the actuating element  29  and the sleeves  130 ,  140  by way of radially outward areas  31 ,  32  of the actuating element  29  which have each one radially inwardly directed collar  33 ,  34  provided on the sleeve  130  or  140 . The preload of the sleeves  130 ,  140  in the direction of the valve member  50 , which is necessary for a proper functioning, is applied by springs or poppet spring units  35 ,  36 . Preferably, the rating of the parts  130 ,  140  and  50  is chosen so that, in the inactive position, the first sealing seat  110  is retained on the sleeve  130  by abutment of its collar  33  on the actuating element  29  or  31  at an axial distance from the valve member  50 , while the second sealing seat  120  is in sealing abutment with the valve member  50  by way of the above-mentioned preload of the second sleeve  140 , and the radial collar  34  of the second sleeve  140  is out of engagement from the actuating element  29  or  32 . 
     The armature  38  of the design shown in FIG. 2 is favorably supported in a bearing bush or slide bush  37  having an axial overall length which is smaller than that one of the armature  38 . The armature  38  cooperates with a stop  41 , and the distance between the armature end, shown on the right in the drawing, and the stop  41  corresponds to the actuating travel S of the armature  38 .