Abstract:
The present invention relates to an electromagnetic valve, which is electrically switched to adopt a throttled position in brake pressure control for reducing valve switching noises.

Description:
TECHNICAL FIELD  
       [0001]     The present invention relates to an electromagnetic valve, in particular for slip-controlled motor vehicle brake systems.  
       BACKGROUND OF THE INVENTION  
       [0002]     DE 43 39 305 A1 discloses an electromagnetic valve of binary operation for use in a slip-controlled motor vehicle brake system, the valve closure member of which remains either in a closed or a fully opened switch position in relation to the valve seat. To avoid the undesirable switching noise of the electromagnetic valve, a hydraulically operated switching piston is arranged in the electromagnetic valve, switching into a position that throttles the valve passage when a defined pressure difference is reached. The effort in construction entailed for noise reduction by hydraulically throttling the pressure fluid is significant.  
         [0003]     In view of the above, it is an object of the invention to improve an electromagnetic valve of the indicated type to the effect that the above-mentioned shortcoming is avoided. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0004]      FIG. 1  is a total view of an electromagnetic valve of the type concerned for use in a slip-controlled brake system.  
         [0005]      FIG. 2  is a diagram for plotting the brake pressure variation and the current variation for the electromagnetic valve according to  FIG. 1 .  
         [0006]      FIG. 3  is another diagram for plotting an alternative brake pressure and current variation for the electromagnetic valve according to  FIG. 1 .  
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0007]      FIG. 1  shows a total view of an electromagnetic valve normally open in its basic position and designed as a two-way/two-position directional seat valve, comprising a cartridge-type valve housing  8  including a spherical valve closure member  9  at a stepped valve tappet  1 . Valve tappet  1  is in contact with a cylindrical magnet armature  10  at the opposite frontal end of the valve closure member  9 . The valve closure member  9  points to a tubular valve seat member  2 , while the oppositely disposed magnet armature  10  faces the magnet core  11  integrated in the valve housing  8 . Fastened to the magnet core  11  is a preferably deepdrawn sleeve  12  in which the magnet armature  10  can align itself and move in an axial direction. A magnet coil  13  is arranged at the periphery of sleeve  12  and is embedded between a yoke-type metal sheet  16  and a magnetic plate  17 .  
         [0008]     In a per se known fashion, the magnet armature  10  moves in the direction of the magnet core  11  during energization of the magnet coil  13  so that the valve closure member  9  shaped at the valve tappet  1  interrupts the pressure fluid connection between a pressure fluid inlet and a pressure fluid outlet channel  14 ,  15  that is normally open in the basic position, in opposition to the effect of a valve spring  4  interposed between the valve tappet  1  and the valve seat member  2 .  
         [0009]     The electromagnetic valve is meant for use in slip-controlled motor vehicle brake systems, and its valve closure member  9  cooperating with the magnet armature  10  is lifted in the basic position from the valve seat member  2  by means of the valve spring  4  that is arranged between the valve tappet  1  and the valve seat member  2 . In the electrically energized valve position, the valve closure member  9  moves in the direction of the valve seat member  2 , and the magnet armature  10  moves in the direction of the magnet core  11 . The special feature is that the magnet coil  13  is energized by means of three different switching current values I 1 , I 2 , I 3  for reducing the valve switching noise. In the electrically non-energized condition of the magnet coil  13 , the first switching current value I 1 =0 so that the valve closure member  9  is completely opened due to the valve spring  4 . In the condition partly energized by means of the second switching current value I 2  which is higher than the first switching current value I 1  but lower than the third switching current value I 3 , the valve closure member  9  opens a throttle cross-section at the valve seat member  2 . To be able to keep this throttle position, it needs a defined geometric design of the valve seat member  2  and the valve tappet  1 . Valve closure member  9  at the valve tappet  1  has a preferably spherical contour with a diameter of 1.8 to 2.2 millimeters for this purpose. This corresponds to a sealing diameter at the valve seat of 0.9 to 1.1 millimeters. The valve seat angle amounts to 120 degrees herein.  
         [0010]     In the fully energized condition, the electromagnetic valve is closed by the effect of the third switching current value I 3 . This permits noise reduction without structural modification of the electromagnetic valve.  
         [0011]     A tandem master cylinder is connected as a brake pressure generator  3  to the pressure fluid inlet channel  14  of the electromagnetic valve illustrated in  FIG. 1 . At the level of valve spring  4 , the pressure fluid outlet channel  15  of the electromagnetic valve is connected to a wheel brake  5 . Connected to said pressure fluid connection that leads to wheel brake  5  is a return line provided with an outlet valve  7  and including a low-pressure accumulator  18  and a pump  19  according to the return delivery principle. Said return line is connected to the pressure fluid inlet channel  14 . The illustrated hydraulic circuit is of a principal nature and serves for general explanations. Deviations herefrom are possible.  
         [0012]     Based on the electrically non-energized condition I 1  of the magnetic coil  13  in which the electromagnetic valve is initially completely open, as shown in the drawings, in a brake pressure control operation the electromagnetic valve is principally switched into a fully energized condition I 3  where it is completely closed. Subsequently, it is opened electrically only in part (condition I 2 ) for noise reduction, and it is switched to re-assume the completely closed condition I 3  only subsequently. Details regarding the control sequence are referred to in the description relating to  FIG. 2 .  
         [0013]     The valve spring  4  is preferably configured as a helical spring and has a progressive spring characteristic curve, the spring force of which is rated so that the valve closure member  9  remains in the partly opened, noise-reducing switching position when the magnet coil  13  adopts its condition partly energized with the second switching current value I 2 .  
         [0014]     For illustrating the hydraulic pressure difference applied to the valve closure member  9  in the partly opened switching position, a means is provided sensing the hydraulic pressure that prevails upstream and downstream of the valve closure member  9 . It is of great significance to determine the pressure difference as exactly as possible by way of appropriate means because in the partly opened condition of the electromagnetic valve, the electric switching current value I 2  that is necessary for the partial opening of the electromagnetic valve will no longer be sufficient to keep the electromagnetic valve open starting from a defined pressure difference.  
         [0015]     As a means for sensing the hydraulic pressure difference, e.g. pressure sensors  6  are well suited that are connected to the brake circuit upstream and downstream of the valve closure member  9 . The pressure sensor signals representative of the pressure difference at the valve closure member  9  are evaluated in an electronic controller  20  actuating the magnet coil  13 .  
         [0016]     According to the illustrated pattern, the electromagnetic valve is inserted into a brake pressure line of a slip-controlled motor vehicle brake system connecting the brake pressure generator  3  to the wheel brake  5  so that alternatively to the pressure sensing by means of pressure sensors  6 , the pressure difference can be sensed by appropriate software in a characteristic field for a pressure model, for what purpose the electronic controller  20  actuating the magnet coil  13  is appropriate. The pressure model represents the pressure variation in the wheel brake  5  and in the brake pressure generator  3 . Advantageously, it is possible to dispense with the comparatively expensive pressure sensor equipment by using the pressure model.  
         [0017]     The pressure model representative of the pressure variation in the wheel brake  5  is computed based on the vehicle-related and brake-specific parameters. Among these parameters is data relating to the vehicle deceleration, the pilot pressure in the brake pressure generator, and the brake pressure increase and brake pressure decrease characteristics. The calculation of the pressure model for the brake pressure generator  3  takes into account the number of the brake pressure increase pulses and/or the duration of the brake pressure increase pulses necessary to complete the desired brake pressure increase by actuating the magnet coil  13 . Further, the pressure model for the wheel brake  5  is included in the calculation of the pressure model for the brake pressure generator  3 .  
         [0018]      FIG. 2  shows a diagram in which, along the ordinate, the brake pressure variation for a slip-controlled wheel brake  5  (cf.  FIG. 1 ) and the three different switching current values I 1 , I 2 , I 3  of the electromagnetic valve known from  FIG. 1  are plotted as a function of time t. The pressure variation rising linearly from the zero point of the axes of coordinates initially represents the slip-free brake pressure increase initiated by the brake pressure generator  3  because the electromagnetic valve is non-energized (I 1 =0). When the allowable brake pressure value (points A-B) is reached and maintained, the magnetic coil  13  is energized by means of the switching current value I 3  that is higher than the switching current values I 1 , I 2 , with the result that the valve closure member  9  adopts its closed position. Simultaneously, the outlet valve  7  connected to the wheel brake  5  (cf.  FIG. 1 ) is switched into the open position so that a rapid pressure reduction commences in wheel brake  5  until point C. After an initially steep pressure reduction, there will be a short phase where the pressure in wheel brake  5  is maintained constant after the closing of outlet valve  7  due to the closed position of the valve closure member  9 , until the reduction of the switching current value I 3  to the switching current value I 2  (point D) that reduces the valve noise. By energizing the magnet coil  13  with a switching current value I 2 , the valve closure member  9  will adopt a throttled position so that the pressure rise in the wheel brake  5  up to point E takes place with a lower pressure rise gradient. Following is a pressure-maintaining phase, to what end the magnet coil  13  is again energized with the maximum switching current value I 3 , with the result that the valve closure member  9  moves to sit on the valve seat member  2 . For the purpose of further throttled pressure increase in the wheel brake  5 , the switching current value I 3  of the magnet coil  13  is reduced in point F to the noise-reducing switching current value I 2 , what causes a further throttled pressure rise until point G. Until point H, a pressure-maintaining phase will follow due to the increase of the electric current of I 2  to the switching current value I 3 . Due to the new reduction of the energization of the magnet coil  13  to the switching current value I 2 , a continued throttled, low-noise pressure rise takes place until point J, which corresponds to the maximum brake pressure value (cf points A, B). Due to the energization of the magnet coil  13  with the switching current value I 3 , the valve closure member  9  will adopt the closed switch position again so that a pressure-maintaining phase follows until point K. When the maximum brake pressure value causes inadmissible brake slip, the outlet valve  7  allows a quick pressure reduction in the wheel brake  5  until point L is reached, which is again succeeded by a phase where the pressure is maintained constant and a phase of throttled pressure increase.  
         [0019]     The brake pressure control operation described herein is based on a so-called current ramp actuation of the electromagnetic valve, whereby lower pressure increase gradients are achieved due to the throttling in the electromagnetic valve, which gradients permit reducing the valve noise and the pedal pulsation during brake pressure control.  
         [0020]     Instead of the initially proposed electromagnetic valve that acts as an inlet valve for a brake system and adopts three different switch positions for noise reduction and minimizing the pedal pulsations with three different current values I 1 , I 2 , I 3 , an electromagnetic valve is disclosed to solve the object at issue (based on the valve construction shown in  FIG. 1 ). The magnet coil  13  of said valve is operated with one single switching current value I 1  in such a fashion that the electromagnetic valve is never closed completely in the electrically energized condition of the magnet coil  13 , but always remains slightly opened so that a pressure fluid connection with a throttle is established between the valve seat  2  and the valve closure member  9  for noise reduction. Consequently, the idea is based on a permanent leakiness at the valve seat member  2  during the energization of the magnet coil  13  with the switching current value I 1  so that the valve closure member  9  will never provide complete sealing at the valve seat member  2 . Consequently, the idea is based on a permanent leakage at the valve seat member  2  during energization of the magnet coil  13  with the switching current value I 1  so that the valve closure member  9  will never fully seal at the valve seat member  2 . This obviates the need for a complicated actuation of the electromagnetic valve and thereby minimizes the valve noise and the pedal pulsations, without detrimental influence on brake pressure control in which the outlet valve  7  is to be included.  
         [0021]     In this respect,  FIG. 3  shows a diagram in which the brake pressure variation for a slip-controlled wheel brake  5  (cf  FIG. 1 ) and the switching current value I 1  of the electromagnetic valve known from  FIG. 1  are plotted along the ordinate as a function of time.  
         [0022]     The pressure variation linearly rising from the zero point initially represents the slip-free brake pressure increase initiated by the brake pressure generator  3  because the electromagnetic valve is non-energized (I=0). When the allowable brake pressure value (point A) is reached, the magnet coil  13  is energized with the switching current value I 1 , with the result that the valve closure member  9  assumes its throttled position. In addition, the outlet valve  7  connected to wheel brake  5  (cf  FIG. 1 ) is switched to adopt its open position so that a rapid pressure reduction commences in wheel brake  5  until point B. After an initially steep pressure reduction, there will be a flat pressure rise in the wheel brake  5  after the outlet valve  7  has closed on account of the throttled position of the valve closure member  9 , until the interruption of the partial current value I 1  (point C). Due to the effect of valve spring  4 , the valve closure member  9  moves from its throttled into the fully open valve switching position, with the result that the pressure gradient rises between points C-D. As soon as the magnet coil  13  is again energized with the partial current value I 1  (point D), the valve closure member will again assume its throttled position, with the result that the further pressure rise in the direction of point E occurs with a flat gradient again. When the pressure reduction phase in wheel brake  5  sets in by the outlet valve  7  customary in slip-controlled brake systems opening, the pressure will drop rapidly until the point F of the characteristic curve because the amount of fluid penetrating the outlet valve  7  is of course considerably greater than in the narrowest throttle cross-section of the electromagnetic valve that acts as an inlet valve. When the outlet valve re-adopts its closed position, the pressure in wheel brake  5  will rise slightly corresponding to the throttled position of the valve closure member  9  until point G. When the energization of the magnet coil  13  is interrupted in point G, the electromagnetic valve will switch back into the unthrottled open position, and a rapid pressure increase takes place in wheel brake  5  until point H. When the electromagnetic valve again switches into the throttled position due to the partial current value I 1 , the flat pressure rise in wheel brake  5  will repeat. Thus, moderation of the valve noise and the pedal pulsations is ensured by the low pressure increase gradients.