Abstract:
The present invention relates to a brake actuation unit comprising a cylinder/piston arrangement which acts on at least one brake lining which can be brought into frictional engagement with a brake disk, a hydraulic pump which is hydraulically connectable with a pressure chamber of the cylinder/piston arrangement in order to move the cylinder of the cylinder/piston arrangement relative to the piston of the cylinder/piston arrangement, and a motor for driving the hydraulic pump.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority to German Patent Application 195 42 657.6, filed Nov. 15, 1995. 
     BACKGROUND OF THE INVENTION 
     The present invention relates to a brake actuation unit comprising a cylinder/piston arrangement which acts on at least one brake lining which can be brought into frictional engagement with a brake disk, a hydraulic pump which is hydraulically connectable with a pressure chamber of the cylinder/piston arrangement in order to move the cylinder of the cylinder/piston arrangement relative to the piston of the cylinder/piston arrangement, and a motor for driving the hydraulic pump. 
     From EP 286 504 B1 a hydraulic brake actuation device with an electric control system is known which comprises a body and a brake piston which is suitable for sliding in the body parallel to its axis and which together with it defines a main control chamber which is connected to a pump via a pressure line. The pump is driven via an output shaft of the electric motor. The pump is connected to a brake fluid reservoir via an inlet line. A circuit line which is controlled by a solenoid valve selectively connects the main control chamber with the reservoir. The reservoir is formed in the body. The electric motor is secured on said body. The pump has a variable volume pump chamber which is formed in said body and which is partially limited by one end of a plunger which is supported in said body and the opposite end of which is connected with the output shaft of the electric motor via a mechanism. This mechanism converts the rotational movement of the shaft into a reciprocating movement of the plunger. An inlet valve in the inlet line as well as an outlet valve are arranged in the return line in the body. The axis of the output shaft of the electric motor is aligned perpendicularly to the axis of the plunger. A crankshaft is supported by the shaft and rests resiliently against the body in order to keep the plunger permanently resting against the crankshaft. 
     In view of the fact that this brake actuation unit has a radial piston pump which, due to heavy pressure pulsations, does not enable adequate controllability, this arrangement is not suited for a sensitive pressure modulation, e.g. for an antislip control system. Moreover, this brake actuation unit requires an additional solenoid valve for relieving the hydraulic pressure, which results in an increased number of components. It is also disadvantageous that the pump is directly connected with the brake piston. 
     SUMMARY OF THE INVENTION 
     The invention is consequently based on the object to provide an improved brake actuation unit which is suited for the application of an antislip control system, a driving dynamics control system, a vehicle-to-vehicle ranging control system, a hill hold control system or an antiblock control system. 
     In order to solve this problem, the hydraulic pump can be brought into a pressure build-up, a pressure holding, and a pressure relief position. 
     Further characteristics and embodiments are the subject matter of subclaims. 
    
    
     DESCRIPTION OF THE DRAWINGS 
     The invention will be explained with reference to drawings in which: 
     FIG. 1 shows an embodiment of an electrically controlled brake actuation unit according to the invention; 
     FIG. 2 schematically shows a particularly advantageous braking system in which an electrically controlled brake actuation unit according to the invention is employed for one axle of a vehicle; and 
     FIGS. 3 a   1 ,  3   a   2 ,  3   b   1 ,  3   b   2 ,  3   c   1 , and  3   c   2  schematically show construction and function of a particularly advantageous embodiment of a pump for an electrically controlled brake actuation unit according to the invention. 
     FIG. 3 d  is a schematic longitudinal section of a pump for an electrically controlled brake actuation unit according to the invention, the pump including an electromagnetic adjusting mechanism for adjusting the inclination angle of a supporting disk relative to a cylinder drum. 
     FIG. 4 is a schematic cross-sectional view of a wobble-plate machine which can be used as a pump for an electrically controlled brake actuation unit according to the invention. 
     FIG. 5 is a schematic cross-sectional view of a gear pump which can be used as a pump for an electrically controlled brake actuation unit according to the invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1 shows an electrically controlled brake actuation unit  10  according to the invention. The brake actuation unit  10  comprises a piston/cylinder arrangement  42  which is formed by a brake cylinder  42   b  which accommodates a brake piston  42   c  in an axially sliding manner. The brake piston  42   c  acts upon a first brake block  4   a  which, together with a second brake block  4   b,  acts upon two opposite surfaces of a brake disk  8 . A wheel brake caliper  2  which is integrally connected with the brake cylinder  42   b  straddles the brake disk  8  and secures the first and second brake block  4   a,    4   b.    
     The hydraulic sealing of the brake piston  42   c  against the brake cylinder  42   b  is effected by a seal  6   a  which is arranged in a truncated cone shaped circumferential groove of the brake cylinder  42   b.  The seal  6   a  also serves for pulling back or resetting the brake piston  42   c  in order to avoid a sliding of the brake blocks  4   a,    4   b  against the brake disk  8  with the brake actuation unit  10  being not activated. This resetting operation is also referred to as “rollback”. In addition, a seal  6   b  is provided for sealing the cylinder/piston arrangement  42  against dust and moisture. 
     In order to modulate the brake pressure in the pressure chamber  42   a,  a pump  50  is mounted on the bottom of the brake cylinder  42   b  by means of fastening elements  50   a.  The pump  50  is driven by an electric motor  40 . For this purpose the drive shaft  64  of the pump  50  is transmission coupled to the electric motor  40  via a coupling element  12 . The electric motor  40  is controlled by an electronic control unit (not shown) via an electric connecting line  40   a.    
     A brake fluid reservoir  46  which radially surrounds the coupling element  12  is arranged between the electric motor  40  and the brake cylinder  42   b.  The brake fluid reservoir  46  is attached to a housing  42   d  of the piston/cylinder arrangement  42  by means of connecting elements  46   a,  while the electric motor  40  is attached to the brake fluid reservoir  46  by means of connecting means  40   b.  However, it is also possible to attach the electric motor  40  directly to the housing  42   d  of the cylinder/piston arrangement  42  and to arrange the brake fluid reservoir  46  in such a manner that it surrounds the electric motor  40  radially in order to save installation space in this manner. 
     The pump  50  is connected with the brake fluid reservoir  46  via a first channel  50   b  and with the pressure chamber  42   a  via a second channel  50   c.  In order to modulate the brake pressure the pump  50  can be switched between two directions of delivery, which is effected by means of reversing the direction of rotation of the electric motor  40 . In order to increase the brake pressure in the pressure chamber  42   a  the pump  50  is operated in the one direction so that brake fluid is drawn from the brake fluid reservoir  46  via the first channel  50   b  and delivered into the pressure chamber  42   a  via the second channel  50   c.  In order to decrease the brake pressure in the pressure. chamber  42   a  the pump  50  is operated in the other direction so that brake fluid is drawn from the pressure chamber  42   a  via the second channel  50   c  and returned into the brake fluid reservoir  46  via the first channel  50   b.    
     The pump  50  is arranged in the pressure chamber  42   a  in such a manner that it does not contact the brake piston  42   c.  Therefore, the mechanical properties of the brake, in particular, the “rollback”, are not affected, for example, by additional frictional forces acting on the brake piston  42   c.    
     The operation of the inventive electrically controlled brake actuation unit  10  will be explained below in more detail with reference to a brake system. For this purpose, FIG. 2 gives a schematic representation of a particularly advantageous brake system which is obtained by using the inventive electrically controlled brake actuation unit for the wheel brakes of one axle of a vehicle. 
     The electrically controlled brake actuation unit  10  comprises a pump  50  which is driven by an electric motor  40  for the actuation of a piston/cylinder arrangement  42 . The pump  50  is connected with a brake fluid reservoir  46 . In addition, a pressure limiting valve  44  is arranged in parallel to the pump  50 , which upon exceeding a predetermined pressure in the pressure chamber  42   a  of the piston/cylinder arrangement  42  drains the excess brake fluid directly into the brake fluid reservoir  46 . 
     The brake system is a so-called “brake-by-wire” system. This means that a parameter which is related to an actuation of the brake pedal  30  by the driver (e.g. pedal travel, pedal force or pedal actuation speed) is sensed by means of a sensor arrangement  32 . The sensor arrangement  32  supplies a corresponding input signal to an electronic control system (not shown) for evaluation in order to provide electrical control signals for brake actuation units  10 . This enables normal braking operations and antiblock control operations to be carried out. A brake system of this type also offers the possibility to drive the brake actuation units  10  automatically, i.e. independent of an actuation of the brake pedal  30 , so that, amongst others, antislip control, driving dynamics control, vehicle-to-vehicle ranging control and hill hold control can be performed. 
     The conversion of the electrical signals, which are provided by the electronic control system (not shown) for the modulation of the brake pressure, is effected in the brake actuation unit  10  by means of the electric motor  40  which drives the pump  50 . The direction of rotation of the electric motor  40  can be reversed so that the pump  50  can be switched for two directions of delivery. In the first delivery direction the pump  50  delivers brake fluid from the reservoir  46  into the pressure chamber  42   a  of the cylinder/piston arrangement  42  in order to build up the brake pressure. In the second delivery direction the pump  50  returns brake fluid from the pressure chamber  42   a  of the piston/cylinder arrangement  42  into the brake fluid reservoir  46  so that the brake pressure is relieved. 
     A quasi-constant brake pressure in the pressure chamber  42   a  of the cylinder/piston arrangement  42  can be adjusted by a defined time sequence of pressure build-up and pressure relief phases. In order to maintain a brake pressure in the pressure chamber  42   a  of the piston/cylinder arrangement  42  actually constant it is, among other things, possible to separate the pressure chamber  42   a  of the piston/cylinder arrangement  42  from the pump  50  for the duration of the pressure relief phases. The pump  50  can be shut off for the duration of the pressure relief phase so that electric energy is saved. This proves to be particularly advantageous with pressure relief phases with a long duration which, for example, occur with constant deceleration of the vehicle during downhill drives. In order to save an additional solenoid valve and primarily the required control electronics, the use of a pump  50  which is designed to be mechanically self-locking is advantageous which separates the pressure chamber  42   a  of the piston/cylinder arrangement  42  from the pump  50  with the electric motor  40  shut off in order to keep the brake pressure constant. Thus, as used in this application, the term “self-locking” when used with respect to a pump means that the pump is designed as one of the types of pumps known where pressure at a discharge (outlet) of the pump will not be readily relieved through the pump when the electric motor for the pump is shut off. 
     The electric control signals which are supplied by the electronic control system (not shown) to the electric motor  40  are current, voltage and pulse width modulation signals which are capable not only of reversing the direction of rotation, but also of varying the speed of the electric motor  40  so that not only the direction of delivery but also the delivery quantity of the pump  50  is adjustable. The gradients of the pressure build-up and the pressure relief are variable via the adjustment of the delivery quantity so that a very good controllability for driving the brake actuation unit  10  is obtained. In order to carry out the pressure modulation on a closed control loop, a sensor arrangement  48  is provided in the brake actuation unit  10 , which senses the actually prevailing pressure in the pressure chamber  42   a  of the piston/cylinder arrangement  42 . The sensing of the pressure actually prevail in the piston/cylinder arrangement  42  also serves as a safety monitoring operation of the brake actuation unit  10 . 
     The above described brake actuation unit  10  already represents an independent system which is coupled with a wheel brake in the vehicle and only requires to be electrically driven by an electronic control system (not shown). In view of this, the hydraulic connection  10   a  indicated in FIG. 2 is not mandatory for a “brake-by-wire” system. The hydraulic connection  10   a,  however, permits further supplementary functions to increase the efficiency and the system safety, which result in considerable advantages of the inventive brake actuation unit  10  compared to a so-called “dry” brake actuation means. The term “dry” brake actuation unit in this context refers to such a unit where an electric motor drives a spindle which immediately acts upon a wheel brake, i.e. which does not comprise any hydraulic components, in particular brake fluid. 
     On the one hand, the hydraulic connection  10   a  provides the possibility of hydraulically coupling the actuation units  10  of the wheel brakes of one axle of the vehicle. For this purpose, a solenoid valve  38  is arranged between the connections  10   a,  which in the electrically inactive condition connects the connections  10   a  to one another and in the electrically active condition closes the connections  10   a  against one another. When the connections  10   a  are connected, the pressure chamber  42   a  of the piston/cylinder arrangements  42  are hydraulically short-circuited so that the same pressure level is obtained in the pressure chambers  42   a  of the piston/cylinder arrangements  42 , which compensates differences in the control behaviour of the individual actuations units  10  which can occur due to manufacturing tolerances. This is particularly advantageous in the case of a normal braking operation because the same braking behaviour as with a conventional hydraulic brake system results at the wheels of an axle so that the stability of the vehicle is ensured. With an antiblock control system it is also possible to set a “select low” control mode at the rear rear axle of the vehicle in a simple manner. If, however, an individual regulation of the brake pressure in the wheel brakes is required which, among other things, is the case for an anti-block control system, an antislip control system or a driving dynamics control system, the hydraulic connection between the pressure chambers  42   a  of the piston/cylinder arrangements  42  is separated by electrically controlling the solenoid valve  38 . 
     When a pump  50  is used which is designed to be mechanically self-locking in order to maintain a constant pressure level in the pressure chamber  42   a  of the piston/cylinder arrangement  42  with the electric motor  40  switched off, the case may occur due a mechanical or electrical defect that a brake pressure prevailing in the pressure chamber  42   a  of a piston/cylinder arrangement  42  can no longer be relieved. For counteracting such a safety-critical driving condition, a hydraulic connection to the pump  50  of the opposite actuation unit  10  can be made in such a case in order to return the brake fluid. The prerequisite for this is that the brake fluid reservoir  46  has a sufficient capacity. It is also possible with the solenoid valve  38  in the opened position to build up the brake pressure in the pressure chambers  42   a  of both piston/cylinder arrangements  42  via only one pump  50  in order to carry out at least normal braking operations, provided that a sufficient fluid volume is available. 
     On the other hand, it is possible to couple the actuation unit  10  with a so-called “push-through” system via the hydraulic connection  10   a  for a hydraulic emergency operation. The hydraulic emergency system comprises a brake pressure transducer  34  which is mechanically acutated via the brake pedal  30 . A solenoid valve  36  is arranged between the brake pressure transducer  34  and the actuation units  10  of at least one axle of the vehicle. In the electrically inactive condition, the solenoid valve  36  makes a connection between the brake pressure transducer  34  and the actuation unit  10  so that in the case of a failure of the electrical supply voltage in the vehicle, a brake actuation by the hydraulic emergency system is possible. Otherwise, the solenoid valve  36  closes the connection between the brake pressure transducer  34  and the actuation unit  10  with the electrical supply voltage in the vehicle being available and the electrical system being defect-free so that the brake actuation is effected exclusively by the electrical system. 
     Construction and function of a particularly advantageous embodiment of the pump  50  will be explained with reference to FIGS. 3 a   1  through  3   c   2 . Each of FIGS. 3 a   2 ,  3   b   2 , and  3   c   2  shows a schematical longitudinal section of the pump  50 , while each of FIGS. 3 a   1 ,  3   b   1 , and  3   c   1  shows a section along line C-D of FIGS. 3 a   2 ,  3   b   2 , and  3   c   2 , respectively. 
     As can be seen from FIGS. 3 a   1  and  3   a   2 , the pump  50  is designed as an axial piston pump. The axial piston pump is essentially rotation symmetrical with respect to an axis A. Therefore, a housing  52  has a circular cylindrical hole  54  which accommodates a cylinder drum  56  so as to be rotatable about the axis A. Through holes  66 ,  68  are arranged in the cylinder drum  56  parallel to the axis A on a circle K in which two pistons  60 ,  62  are slidably accommodated. The pistons  60 ,  62  bear against an inclined disk  58  which is arranged stationary in the bottom of the circular cylindrical hole  54 . The cylinder drum  56  is (integrally) connected with a drive shaft  64  which penetrates the inclined disk  58  and the housing  52  in the direction of the axis A. The drive shaft  64  is transmission coupled with an electric motor  40  for driving the axial piston pump. 
     Such an axial piston pump is also referred to as an “inclined disk machine” because the drive shaft  64  and the cylinder drum  56  are arranged equi-axial, the supporting (inclined) disk  58  is stationary, and the cylinder drum  56  is driven by the electric motor  40  via the drive shaft  64 . Contrary to this, axial piston pumps with a stationary cylinder drum and a driven supporting disk are referred to as “wobble-plate machines”. “Wobble-plate machines (e.g., pumps) are conventional in the art. FIG. 4 illustrates a wobble-plate pump  51 , including a stationary cylinder drum  57  with axially slidable pistons  61  and  63 , and a rotatory supporting disk  59  which is arranged transversely to the drum  57 . The disk  59  can be driven by a motor  40  via a drive shaft  65  to actuate the pistons  61 , and  63  to cause the pump  51  to pump.” 
     A lid  72  is securely connected with the housing  52  for closing the housing  52  in a tight manner on its top. The lid  72  has a first and a second connection  74 ,  76  both of which are formed as through holes in parallel to the axis A on the circle K. The two connections  74 ,  76  communicate with the pressure chamber  42   a  of one cylinder/piston arrangement  42 . 
     Between the cylinder drum  56  and the lid  72  a control disk  70  is arranged to be rotatable about the axis A. As a centering means with respect to the axis A the control disk  70  is provided with a circumferential edge  78  which is radially guided along the circumference of the cylinder drum  56 . 
     The control disk  70  and the pistons  60 ,  62  define pressure spaces  108 ,  110  in the holes  66 ,  68  of the cylinder drum  56 . In order to increase the volume of the pressure spaces  108 ,  110  each of the pistons  60 ,  62  is designed as a hollow cylinder. 
     On the face associated with the cylinder drum  56 , the control disk  70  is provided with a first and a second arc-shaped groove  80 ,  82  which are arranged on the circle K. A first and a second hole  84 ,  86  are arranged at either end of the first arc-shaped groove  80  on the circle K and penetrate the control disk  70 . The control disk  70  also comprises a centrally arranged circular cylindrical chamber  88  on the face associated with the cylinder drum  56 ; said chamber is connected with the second arc-shaped groove  82  via another groove  90 . 
     The centrally arranged circular cylindrical chamber  88  in the control disk is connected via a central hole  92  which is arranged in the cylinder drum  56  or the drive shaft  64 , respectively, as well as via radial hole  94  with an inner space  112  which communicates with a brake fluid reservoir  46  via a connection  96  arranged at the housing  52 . 
     A radially outwardly directed driving pin  98 , which projects into a recess  102  of the lid  72 , is attached to the control disk  70 . The driving pin  98  is coupled with a spring arrangement  100  which consists of two identical, though oppositely acting spring elements which are supported in the recess  102  of the lid  72  in a stationary manner. When the cylinder drum  56  is not driven, as shown in FIGS. 3 a   1  and  3   a   2 , no sliding friction forces are transmitted from the upper face of the cylinder drum  56  to the lower face of the control disk  70  and thus to the driving pin  98  so that the control disk  70  is held by the spring arrangement  100  in the basic position shown in FIGS. 3 a   1  and  3   a   2 . 
     In the basic position of the control disk  70  as shown in FIG. 3 a,  the first and the second connection  74 ,  76  are closed so that a pressure level prevailing in the pressure chamber  42   a  of the cylinder/piston arrangement  42  is maintained constant. The setting of the so-called “pressure holding phase” is thus achieved by simply switching off the electric motor  40 . 
     The setting of a so-called “pressure build-up phase” is shown in FIGS. 3 b   1  and  3   b   2 . For this purpose, the cylinder drum  56  is driven by the electric motor  40  in a first sense of direction I (here counter-clockwise). Sliding friction forces are then transmitted from the upper face of the cylinder drum  56  to the lower face of the control disk  70  and thus to the driving pin  98 , which counteract the spring forces applied to the driving pin  98  by the spring arrangement  100 . The spring arrangement  100  is dimensioned in such a manner that the sliding frictional forces which are effective upon the drive of the cylinder drum  56  are always sufficient for a rotation of the control disk  70  against the spring force of the spring arrangement  100  about the axis A until the driving pin  98  abuts a first stop  104  and the control disk  70  assumes the position illustrated in FIGS. 3 b   1  and  3   b   2 . 
     The arrangement of the first and the second arc-shaped groove  80 ,  82  in the control disk  70  is dimensioned in such a manner that in the position of the control disk  70  shown in FIGS. 3 b   1  and  3   b   2  the first through hole  84  arranged in the first arc-shaped groove  80  extends equi-axially to the first connection  74  and the second connection  76  is closed. The drive of the cylinder drum  56  causes the two pistons  60 ,  62  to carry out mutually opposed reciprocating movements. While one of the pressure chambers  108 ,  110  communicates with the second arc-shaped groove  82 , the associated piston  60 ,  62  carries out a downward stroke in order to draw in brake fluid from the brake fluid reservoir  46 . The suction path thereby extends over the connection  96 , the inner space  112 , the holes  94  and  92 , the chamber  88 , the groove  90 , as well as the second arc-shaped groove  82 . After the suction operation, the respective pressure chamber  108 ,  110  is connected with the first arc-shaped groove  80 , while the associated piston  60 ,  62  carries out an upward stroke in order to supply the pressure chamber  42   a  of the cylinder/piston arrangement  42  with the brake fluid which has been drawn into the pressure chamber  108 ,  110 , with the delivery path leading via the first arc-shaped groove  80 , the first through hole  84  and the first connection  74 . 
     FIGS. 3 c   1  and  3   c   2  show the setting of a so-called “pressure relief phase”. For this purpose, the cylinder drum  56  is driven by the electric motor  40  in a second sense of direction II (here clockwise). Due to the sliding friction forces which occur between the contact faces of the cylinder drum  56  and the control disk  70 , the driving pin  98  abuts a second stop  106  so that the control disk  70  assumes the position shown in FIGS. 3 c   1  and  3   c   2 . 
     In the position of the control disk  70  as shown in FIGS. 3 c   1  and  3   c   2 , the second through hole  86  which is arranged in the first arc-shaped groove  80  extends equiaxially with the second connection  76  and the first connection  74  is closed. While one of the two pressure chambers  108 ,  110  communicates with the first arc-shaped groove  80 , the associated piston  60 ,  62  carries out a downward stroke in order to draw in brake fluid from the pressure chamber  42   a  of the cylinder/piston arrangement  42 . The suction path thereby extends over the second connection  76 , the second through hole  86 , as well as the first arc-shaped groove  80 . After the suction operation, the respective pressure chamber  108 ,  110  is connected with the second arc-shaped groove  82 , while the associated piston  60 ,  62  carries out an upward stroke in order to return the brake fluid which has been drawn into the pressure chamber  108 ,  110  into the brake fluid reservoir  46 . The return delivery is thereby made via the second arc-shaped groove  82 , the groove  90 , the chamber  88 , the holes  92  and  94 , the inner space  112 , as well as the connection  96 . 
     With the embodiment of an axial piston pump as shown in FIGS. 3 a   1  through  3   c   2  the inclination angle of the inclined (supporting) disk  58  relative to the cylinder drum  56  is constant. The delivery volume of the pump can be modified by varying the inclination angle. In the case of an inclination angle equal to zero, i.e. the inclined (supporting) disk  58  is arranged perpendicularly to the axis A of the cylinder drum  56 , no fluid will be delivered so that a pressure holding phase can also be set in this manner. The control disk  70  can therefore be replaced by an inclined (supporting) disk  58 , the inclination of which can be adjusted via an adjusting mechanism, with the adjusting mechanism positioning the inclined (supporting) disk  58  perpendicularly to the axis A of the cylinder drum  56  when the electric motor  40  is switched off in order to save electric energy during the pressure holding phases. Preferably, the hydraulic pump includes an adjustment mechanism adapted to precisely control the delivery capacity of the hydraulic pump within a tolerance of about 1 bar of pressure 
     It is also possible to actuate such an adjusting mechanism in an electromagnetic manner, which also applies to the control disk  70 , the adjustment of which is effected by utilizing the sliding friction forces. Such an electromagnetic actuation is then to be coupled electrically with the existing electrical control means of the electric motor  40  in order to save the expenditure for an additional electrical control means. The current supplied to the electric motor  40  can therefore simultaneously energize the solenoid of such an electromagnetic actuation means. The electromagnetic actuation means is then to be designed in such a manner that in the de-energized condition, i.e. with the electric motor  40  not driven, the pressure holding phase is set by straight positioning the inclined (supporting) disk  58  or by positioning the control disk  70  according to FIGS. 3 a   1  and  3   a   2 . Due to the fact that the pressure build-up and pressure relief phases are set as a function of the delivery direction of the pump  50  which results from the sense of rotation of the electric motor  40  and thus from the direction of the current, the respective inclination of the inclined (supporting) disk  58  or the positioning of the control disk  70  can be set according to FIGS. 3 b   1  and  3   b   2  or FIGS. 3 c   1  and  3   c   2  with reference to the current direction. FIG. 3 d  illustrates an axial piston pump  50 ′ including an adjustable supporting disk  58 ′. The inclination angle Ø of the supporting disk  58 ′ can be adjusted via an adjusting mechanism. In the illustrated embodiment, the adjusting mechanism includes an electromagnetic actuator  3 . Preferably, the control of the electromagnetic actuator  3  is coupled with the control of the electric motor  40  (FIG. 1) of the pump  50 ′. 
     Finally, it should be mentioned that the pump  50  for the inventive brake actuation unit need not necessarily be limited to the axial piston pump type but that other types, in particular, gear pumps, can be employed as well. Gear pumps are conventional in the art. FIG. 5 illustrates a gear pump  120  including a housing  122  and a pair of meshing, rotatable gears  124  and  126 . The gears  124  and  126  can be driven to rotate by a motor  40 . The gear pump  120  is adapted to pump brake fluid from a brake fluid reservoir  46  to a pressure chamber  42   a  of a cylinder/piston arrangement  42 .