Abstract:
A method for controlling a motor vehicle electrohydraulic braking system. The method being activatable in a “brake-by-wire” operating mode, having a pressure supply device ( 2 ), which can be activated by an electronic control and a regulation unit, can be connected to a hydraulically actuatable wheel brake. Wherein the pressure supply device ( 2 ) is formed by a cylinder-piston assembly ( 43 ). The piston ( 45 ) can be actuated by an electromechanical actuator ( 44 ), the control unit carries out the steps of determining a pressure target value (P V,Soll ) for the pressure supply device ( 2 ), performing a pressure control operation or an actuator position control operation on the pressure supply device ( 2 ) in accordance with the magnitude of the pressure target value (P V,Soll ).

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This invention relates to German Patent Application Nos. 101010029900.6, filed Jun. 10, 2010, 102010029898.0, filed Jun. 10, 2010, 102010029897.2, filed Jun. 10, 2010, 102011076952.8, filed Jun. 6, 2011, and PCT/EP2011/059320, filed Jun. 7, 2011. 
     SUMMARY OF THE INVENTION 
     The invention relates to a method for controlling a braking system for motor vehicles and to a control circuit. 
     BACKGROUND AND SUMMARY OF THE INVENTION 
     “Brake-by-wire” braking systems are becoming ever more widespread in the motor vehicle industry. Such braking systems often comprise a pedal decoupling unit, which is inserted ahead of a brake master cylinder, and, as a result, a brake pedal actuation by the driver in the “brake-by-wire” operating mode does not lead to direct actuation of the brake master cylinder by the driver. Instead, the brake master cylinder is actuated by an electrically controllable pressure supply device, i.e. is “externally” actuated, in the “brake-by-wire” operating mode. In order to give the driver a pleasant pedal feel in the “brake-by-wire” operating mode, braking systems generally comprise a brake pedal feel simulation device. In these braking systems, the brake can be actuated on the basis of electronic signals, even without the active intervention of the vehicle driver. These electronic signals can be output by an electronic stability program ESC or an adaptive cruise control system ACC, for example. 
     International Patent Application WO 2008/025797 A1 discloses a braking system of the above-referenced kind. In order to be able to dispense with expensive temporary storage of hydraulic actuating energy, which is unfavorable in terms of energy, the proposal is that the pressure medium required for electric control of the pressure fed into an intermediate space used for actuating the brake master cylinder should be held ready in the unpressurized form in the pressure supply device and subjected to a higher pressure when required. For this purpose, the pressure supply device is, for example, formed by a cylinder-piston assembly, the piston of which can be actuated by an electromechanical actuator. No method for controlling the braking system, in particular the pressure supply device, is described. 
     It is therefore the object of the present invention to provide a method for controlling an electrohydraulic “brake-by-wire” braking system having an electrically controllable pressure supply device comprising a cylinder-piston assembly, the piston of which can be actuated by an electromechanical actuator. 
     This object is achieved by a method in accordance with this invention. 
     It is advantageous if the method according to the invention is performed in a motor-vehicle braking system which can be activated either by the vehicle driver or independently of the vehicle driver in a “brake-by-wire” operating mode, preferably being operated in the “brake-by-wire” operating mode, and having the capacity for operation in at least one fallback operating mode in which only operation by the vehicle driver is possible. 
     An actuator position control operation is preferably performed when a mechanical end stop of the actuator is supposed to be detected. 
     It is likewise preferred that the pressure or actuator position control operation should be followed by an actuator speed control operation, in which the current actuator speed is adjusted to the target value for the actuator speed output by the pressure or actuator position control operation. 
     It is advantageous if the motor torque output by the motor torque feedforward function is taken into account in the actuator speed control operation. 
     According to a development of the invention, the pressure target value, advantageously the driver&#39;s required pressure target value, is formed from the sum of a first target pressure component and a second target pressure component. 
     The second target pressure component is preferably determined in accordance with the brake pedal actuation speed and a pedal speed threshold. 
     The pedal speed threshold is particularly preferably determined using a predetermined functional relationship from a pedal position/travel. 
     It is advantageous if the pedal speed threshold is selected in accordance with the vehicle speed. Thus, the value calculated for the pedal speed threshold using the functional relationship can additionally be multiplied by a function of the vehicle speed. 
     According to a preferred embodiment, a quotient of the brake pedal speed and the pedal speed threshold is calculated, and the second target pressure component is determined in accordance with the magnitude of the quotient, wherein the second target pressure component is calculated from the quotient and the first target pressure component, advantageously when the quotient is greater than one. 
     According to another preferred embodiment, a pressure gradient, in particular an expected pressure gradient, is determined, and a pressure control operation or an actuator position control operation or a combined pressure/actuator position control operation is performed on the pressure supply device in accordance with the magnitude of the pressure target value and/or the magnitude of the pressure gradient. 
     It is preferred if the pressure target value is used to determine a first component target value for the actuator rotational speed in an actuator position controller and is used to determine a second component target value for the actuator speed in a pressure controller, and if a target value for the actuator speed in a speed control operation on the pressure supply device is determined from the first and second component target values. 
     The target value for the actuator speed is preferably determined from the first and second component target values by weighted addition. As a particularly preferred option, the respective weighting factor is determined in accordance with the expected pressure gradient. As a very particularly preferred option, the weighting factors are determined from the pressure gradient using at least one predetermined function. 
     Exclusive pressure control of the pressure supply device is preferably performed if the pressure target value is greater than zero bar and the pressure gradient is less than a predetermined, positive, first value. 
     Exclusive actuator position control of the pressure supply device is preferably performed if the pressure target value is greater than zero bar and the pressure gradient is greater than a predetermined second value and, in particular, there is no brake control intervention. 
     It is advantageous if combined pressure/position control of the pressure supply device is performed if the pressure target value is greater than zero bar and the pressure gradient is greater than a predetermined first value and less than a predetermined second value and, in particular, there is no brake control intervention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Further preferred embodiments of the invention will become apparent from the dependent claims and from the following description with reference to schematic figures, of which: 
         FIG. 1  shows an illustrative basic structure of a known braking system, 
         FIG. 2  shows an illustrative control structure for implementing the method according to the invention, 
         FIG. 3  shows an illustrative embodiment of a brake pressure target value calculation, 
         FIG. 4  shows an illustration relating to the determination of a brake pedal speed threshold, 
         FIG. 5  shows a flow diagram relating to the determination of a dynamic target pressure component, 
         FIG. 6  shows a flow diagram relating to the determination of the brake pressure target value, 
         FIG. 7  shows an illustrative embodiment of a pressure controller according to the invention with downstream rotational speed control, 
         FIG. 8  shows an illustrative target rotational speed limiting operation, 
         FIG. 9  shows an illustrative embodiment of an actuator position controller according to the invention with downstream rotational speed control, 
         FIG. 10  shows a development of the pressure/actuator position controller shown in  FIGS. 7 and 9 , and 
         FIG. 11  shows an illustrative representation of a weighting function that can be used in the method according to the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The braking system illustrated in  FIG. 1  consists essentially of an actuating device  1 , a pressure supply device  2 , wherein the actuating unit and the pressure supply device form a brake booster, and a brake master cylinder or tandem master cylinder  3 , which is effectively inserted downstream of the brake booster and the pressure spaces (not shown) of which can be connected to the chambers of a first pressure medium reservoir  18 , said chambers being at atmospheric pressure. On the other hand, the pressure spaces are connected to wheel brake circuits I, II, which supply the wheel brakes  5 - 8  of a motor vehicle with hydraulic pressure medium via a known ABS or ESP hydraulic unit or a controllable wheel brake pressure modulation module. The wheel brake pressure modulation module  4  is assigned an electronic control and regulation unit  41 . The actuating device  1 , which is arranged in a housing  20 , to which the tandem master cylinder  3  is attached, can be activated by means of a brake pedal  9 , which is effectively connected to a first piston  11  of the actuating device  1  by an actuating rod  10 . The actuating travel of the brake pedal  9  is detected by means of a travel sensor  19 , which is preferably of redundant design and which detects the travel of the first piston  11 . However, the same purpose can also be served by using a rotation angle sensor which detects the rotation angle of the brake pedal  9 . The first piston  11  is arranged in a second piston  12 , delimiting a pressure chamber  14  that accommodates a compression spring  15 , the latter bringing the first piston  11  to bear on the second piston  12  when the brake pedal  9  is unactuated. As an alternative or in addition, a pedal return spring can be provided in the region of the push rod  10  or of the brake pedal  9 . In the unactuated state of the actuating device  1 , the pressure chamber  14  is connected to a chamber  38  of a second pressure medium reservoir  38 ,  39 , which is assigned to the actuating device  1 . The second piston  12  interacts with a third piston  13 , which can form the primary piston of the tandem master cylinder  3 , wherein a pressure intensifying piston  16  is arranged between the second piston  12  and the third piston  13  in the example illustrated. Bounded between the second piston  12  and the pressure intensifying piston  16  is an interspace  21 , the admission to which of a hydraulic pressure holds the second piston  12  against a stop  22  formed in the housing  20 , while the pressure intensifying piston  16  and hence the primary piston  12  of the tandem master cylinder are acted upon to give a pressure build-up in the tandem master cylinder  3 . A movement of the pressure intensifying piston  16  resulting from this loading is detected by means of a second travel sensor  23 . Moreover, the second piston  12  delimits a hydraulic chamber  17  in the housing  20 , the function of this chamber being explained in the text which follows. A first line  34  is connected to the hydraulic chamber  17 , said line being connected via a normally open (NO) shutoff valve  33  to a second line  35 , which is connected to the abovementioned pressure chamber  14 . 
     It can furthermore be seen from  FIG. 1  that the abovementioned pressure chamber  14  is connected via a connecting line  24  that can be shut off to a hydraulic simulator chamber  25 , which is delimited by a simulator piston  26 . In this arrangement, the simulator piston  26  interacts with a simulator spring  27  and with an elastomer spring  28  arranged in parallel with the simulator spring  27 . In this arrangement, the simulator chamber  25 , the simulator piston  26 , the simulator spring  27  and the elastomer spring  28  form a pedal travel simulator, which gives the vehicle driver the accustomed pedal feel corresponding to a conventional brake pedal characteristic when the braking system is actuated. This means that, when the brake pedal travel is small, the resistance rises slowly and, when the brake pedal travel is relatively large, it increases disproportionately. To damp the movement of the simulator piston  26 , damping means (not shown), e.g. pneumatic damping means, can be provided. The hydraulic connecting line  24  between the simulator chamber  25  and the pressure chamber  14  and the chamber  38  of the second pressure medium reservoir is shut off by a movement of the second piston  12  in the actuating direction of the brake master cylinder  3 , thereby switching off the pedal travel simulator in terms of its effect. The first piston  11 , the spring  15 , the hydraulic chamber  14 , the hydraulic connection  24 , the simulator chamber  25 , the simulator piston  26 , the simulator springs  18  and  27  and the damping means (not shown) together form the simulation device which, together with a chamber  38  at atmospheric pressure in the second pressure medium reservoir is assigned to a first brake booster pressure medium circuit, which is completely separate from the wheel brake circuits I, II. 
     The abovementioned electrohydraulic pressure supply device  2  consists essentially of a hydraulic cylinder-piston assembly  29  and of an electromechanical actuator  30 , which is formed, for example, by an electric motor with a reduction gear which provides a translatory movement of a hydraulic piston  31 , resulting in a hydraulic pressure build up in a pressure space  36  of the hydraulic cylinder-piston assembly  29 . The electromechanical actuator  30  is supplied with power by an electric energy storage device, which is provided with the reference sign  49 . The movement of the piston  31  is detected by means of a travel sensor, which is provided with the reference sign  32 . On the one hand, the pressure space  36  is connected to the interspace  21  and, on the other hand, can be connected by means of a normally open (NO) 2/2-way valve  37  to a chamber  39  at atmospheric pressure in the second pressure medium reservoir. In this arrangement, the pressure supply device  2 , the interspace  21  and the chamber  39  of the second pressure medium reservoir are assigned to a second brake booster pressure medium circuit, which is completely separate both from the first brake booster pressure medium circuit and from the wheel brake circuits I, II. A pressure sensor  40  is used to detect the pressure supplied by the pressure supply device  2  and prevailing in the interspace  21 . 
     The abovementioned shutoff valve  33  makes it possible to shut off the chamber  17  from the pressure chamber  14 , thereby preventing a movement of the second piston  12  in the actuating direction. The chamber  17 , the first pressure medium line  34 , the shutoff valve  33 , the second pressure medium line  35 , the pressure chamber  14 , the connecting line  24 , the simulator chamber  25  and the second pressure medium reservoir  38  form a second brake booster pressure medium circuit, which is completely separate from the first brake booster pressure medium circuit and from the two wheel brake circuits I, II. Said elements are assigned a dedicated electronic control unit  42 , which interacts with the abovementioned electronic control and regulation unit  41  and serves to detect sensor data, to process said data, to exchange data with other control units (not shown) present in the vehicle, to activate the electromechanical actuator  30  and to activate the brake lights of the vehicle. 
     The operation of the braking system described above is known, for example, from the international patent application of the applicant cited above in respect of the prior art and does not need to be explained in detail in the text which follows. 
     The basic structure of a control system that can be used in the braking system illustrated in  FIG. 1  is shown schematically in  FIG. 2 . It consists essentially of the function blocks “driver requirement detection”  100 , “target value selection”  200 , “controller selection”  300  and a “pressure/actuator position control”  400 , which is followed by an “actuator speed control”  500 . In the case of an actuator having an electric motor, the “actuator speed control”  500  corresponds to the rotational speed control of the electric motor. 
     The actuator speed/rotational speed can be calculated from the actuator position (block  90 : “rotational speed calculation”). 
     The functional unit “driver requirement detection”  100  determines the driver requirement from the sensors assigned to the pedal unit and, from this, calculates a signal for the target booster pressure P V,Soll,Drv  of the linear actuator. 
     Depending on the embodiment of braking system, one or more sensor signals are available here to represent the driver requirement. In the illustrative braking system described in connection with  FIG. 1 , the pedal position is determined in a redundant manner (signal X Ped ) and the pedal force produced by the driver is determined by means of a pressure sensor (signal P Drv ). In the example, the driver requirement detection unit  100  thus has two physically independent items of information for the driver actuation representing the driver&#39;s braking requirement for target value generation of the required braking force intensification by means of the actuator. 
     The output variable of the functional unit “driver requirement detection”  100  is a pressure target value (pressure target value of the actuator, signal P V,Soll,Drv ) determined on the basis of the driver pedal actuation, said value corresponding at least statically to the brake pressure in the wheel brakes as long as there are no interventions by the higher-ranking pressure control system (e.g. antilock system, vehicle dynamics control system or the like), e.g. ESP pressure control system (ESP: Electronic Stability Program). 
     As an option, the functional unit “driver requirement detection”  100  is supplied with the vehicle speed V Kfz . The pressure target value P V,Soll,Drv  can then be additionally modified in accordance with the vehicle speed V Kfz . 
     The functional unit “driver requirement detection”  100  is described in greater detail in connection with  FIG. 3 . As already mentioned above, the functional unit “driver requirement detection”  100  determines the driver requirement from the sensors assigned to the pedal unit and, from this, calculates a signal for the pressure target value P V,Soll,Drv  of the actuator. 
     Improved driver requirement detection is achieved by additionally taking into account the pedal depression speed V Ped . In contrast to known braking assistant functions, which adjust to the maximum pressure as soon as the trigger criteria, which are decisively determined by the pedal depression speed, are met, account is now taken here of the degree to which the trigger threshold is exceeded. 
       FIG. 3  shows a basic structure of an illustrative driver requirement detection unit, which is expanded by the calculation and superimposition of a dynamic pressure component. The function block  101  “driver requirement calculation” illustrated in  FIG. 3  supplies a static pressure target value component P V,Soll,Drv,Stat  based on known functions/methods. For example, the static pressure target value component P V,Soll,Drv,Stat  can be determined from one or more variables by means of a model, using a predetermined functional relationship f. Thus, for example, the static pressure target value component can be calculated from the pedal position/actuation X Ped  using a function P V,Soll =f(X Ped ) or, more generally, from the pedal position X Ped , the pedal force (or corresponding pressure) P Drv , and the vehicle speed V Kfz  using a function P V,Soll =f(X Ped , P Drv , V Kfz ). 
     The pedal depression speed V Ped  can be determined, for example, from the pedal position X Ped  or the time variation thereof (function block  103  “calculation of pedal speed”). 
     The function block “calculation of dynamic pressure component”  102  determines, essentially from the pedal speed V Ped , a dynamic pressure target value component P V,Soll,Drv,Dyn , which depends decisively on the extent to which the pedal speed threshold has been exceeded. As can be seen from  FIG. 3 , the two pressure target value components P V,Soll,Drv,Stat  and P V,Soll,Drv,Dyn  are added together in an adder  103  to give a pressure target value P V,Soll,Drv . 
     The block diagram shown in  FIG. 4  shows the formation of the pedal speed threshold V Ped,Limit  mentioned in the previous paragraph. The speed threshold V Ped,Limit  can be defined as a preset value or can be determined in accordance with the pedal travel X Ped  in the form using a functional relationship V Ped,Limit =f sw (X Ped ). Here, the functional relationship f sw (X Ped ) can be defined in the form of a static equation (function f sw ) or, alternatively, as a table. 
     As is apparent from  FIG. 4 , it is also possible additionally to scale the speed threshold V Ped,Limit  that has been defined or determined on the basis of the pedal travel X Ped  in accordance with the vehicle speed V Kfz . Scaling is performed in a multiplier, which is indicated by the letter X and is provided with the reference sign  104 . By this means, it is possible, for example, to ensure that this dynamic pressure component does not take effect or takes effect only in an attenuated way at relatively low vehicle speeds or when stationary, while coming fully into play when traveling, depending on the design criterion. 
     The formation, illustrated in  FIG. 4 , of the pedal speed threshold V Ped,Limit  is represented in  FIG. 5  by function block  110 . As a measure of the extent to which the current pedal speed threshold V Ped,Limit  determined is exceeded, said threshold being determined in block  110  in accordance with the pedal travel X Ped , the quotient of the pedal speed V Ped  and the pedal speed threshold V Ped,Limit  is formed (block  111 ):
 
 Q   P,Dyn   =V   Ped   /V   Ped,Limit  
 
     In the case where the current pedal travel X Ped  represents the maximum pedal travel X Ped,Max  occurring during the current brake actuation, this value is adopted as the new maximum X Ped,Max  in function block  112 . In the following block  113 , the current maximum value determined for the pedal travel X Ped,Max  is used to calculate a reference travel X Ped,Ref , which is obtained by subtracting a tolerance threshold ε X,Ref  from X Ped,Max . The reference travel X Ped,Ref  represents a travel threshold that is relevant for the reduction of the dynamic target pressure component when the brake is released. 
     In enquiry block  114 , a check is made to determine whether the quotient Q P,Dyn &gt;1. If this condition is met, a dynamic pressure component is calculated in accordance with
 
 P   V,Soll,Drv,Dyn,aktuell =( Q   P,Dyn −1)* P   V,Soll,Drv,Stat  
 
(block  115 ).
 
     In the case where this component P V,Soll,Drv,Dyn,aktuell  represents the maximum of the dynamic pressure component, said maximum being calculated during the current brake actuation, this value is adopted as a new maximum P V,Soll,Drv,Dyn,Max  (function block  116 ). In function block  117 , the maximum thus determined for the dynamic pressure component is assigned to the variable P V,Soll,Dyn , which represents the signal for the dynamic pressure component, which, as the output variable of function block  102 , is then superimposed by addition on the target booster pressure P V,Soll,Drv,Stat  (see  FIG. 3 ). 
     When the brake is released (i.e. V Ped &lt;0), this dynamic pressure component P V,Soll,Drv,Dyn  is reduced again to the value 0. For reasons of comfort, this reduction in the dynamic pressure target value takes place in accordance with the pedal travel, more specifically in such a way that, from the reference travel X Ped,Ref  determined in block  113  during the brake actuation, said travel depending on the maximum pedal travel detected during the braking operation, a reduction is carried out in a linear manner with the travel X Ped  to the value X Ped =X Ped,Dyn,0 , reducing it to zero.  FIG. 5  illustrates this procedure in the form of a flow diagram (blocks  118 - 122 ). 
     If it is ascertained in enquiry block  114  that the quotient Q P,Dyn ≦1, then the brake actuation concerned is one which does not require any further increase in the dynamic pressure component, or the brake is being released. First of all, a check is made in enquiry block  118  to determine whether the pedal travel X Ped  is greater than the lower threshold X Ped,Dyn,0 . If this is not the case, this lower threshold has been undershot and the dynamic pressure target value is set to zero, P V,Soll,Dyn =0 (block  122 ). If the pedal travel X Ped  is greater than the lower threshold X Ped,Dyn,0 , a check is made in enquiry block  119  to determine whether the upper threshold, given by the reference travel X Ped,Ref , which is necessary for the reduction of the dynamic pressure target value, has been undershot. If this is the case, then, in function block  120 , the maximum value P V,Soll,Drv,Dyn,Max  determined during the braking operation is reduced in a linear manner with the pedal travel X Ped  and assigned to the signal P V,Soll,Drv,Dyn,aktuell . If this value is less than the dynamic pressure target value P V,Soll,Drv,Dyn  (k−1) of the preceding sampling step (k−1), then, in function block  121 , this value P V,Soll,Drv,Dyn,aktuell  is assigned to the variable P V,Soll,Dyn , which represents the signal for the dynamic pressure component, and this in turn is then superimposed by addition, as the output variable of function block  102 , on the target booster pressure P V,Soll,Drv,Stat  (see  FIG. 3 ). 
     If it is ascertained in enquiry block  119  that the current pedal travel X Ped  is greater than or equal to the reference travel X Ped,Ref , the dynamic pressure target value P V,Soll,Drv,Dyn  (k−1) of the preceding sampling step (k−1) is retained unaltered. 
     When the brake pedal is depressed rapidly, the above-described procedure for taking into account the pedal speed V Ped  leads to a shift in the relationship P V,Soll =f(X Ped ) and P V,Soll =f(X Ped , P Drv , V Kfz ) to ward higher booster and hence also higher brake pressures, this effect being all the more pronounced, the faster the driver actuates the brake pedal. In the case of fast brake pedal actuation, the target booster pressure P V,Soll,Drv  in the linear actuator is already reached at shorter brake pedal travels X Ped  than is the case with a slow actuation. This leads to an increase in the dynamic response of the braking system combined with more rapid response from the brake to driver actuation (shortening of the response time). In the case of a slow pedal actuation (V Ped  short), or (0&lt;Q P,Dyn &lt;1), this dynamic pressure component is not present, and therefore the determination of the target booster pressure in this case can be designed primarily according to comfort criteria. If rapid brake responses are required by the driver, this is achieved by means of the component P V,Soll,Drv,Dyn . 
     It is likewise advantageous in terms of actuating comfort and ensuring predictable behavior that the dynamic component is not reduced abruptly when the brake or brake pedal is released but in a linear manner with the pedal travel back to the value 0. 
     In addition to the above-described target value P V,Soll,Drv  based on driver pedal actuation, the higher-ranking pressure control system can also demand a pressure target value P V,Soll,ESC  in accordance with its control strategy (ABS (antilock system), TCS (traction control system), ESP or the like). A target value selection is therefore performed in block  200  ( FIG. 2 ). The output variable of this function block  200  is the resulting pressure target value P V,Soll . An illustrative target value selection is shown in  FIG. 6 . In enquiry block  201 , a check is made to determine whether a control signal Req ESC =1. If this condition is met, a check is made in enquiry block  202  to determine whether the inequality P V,Soll,ESC &gt;P V,Soll,Drv  is satisfied. In the case of an active demand, the pressure target value P V,Soll  is obtained from the maximum value of the two values P V,Soll,ESC  and P V,Soll,Drv  (see function blocks  203  and  204 ). 
     If the abovementioned condition is not met, there is no pressure demand from the higher-ranking control system, and therefore the signal P V,Soll,Drv  is output as the target value for actuator control (see function block  205 ). 
     Controller selection in function block  300  is performed in accordance with the pressure target value P V,Soll  determined (see  FIG. 2 ). If the target pressure P V,Soll &gt;0 bar, pressure/actuator position control (function block  400 ,  FIG. 2 ) is activated (selection parameter S=1), which sets the desired pressure. At the same time, the hydraulic connection between the actuator and the reservoir is interrupted (e.g. by energizing the normally open control valve  37  arranged between the cylinder-piston assembly  30  and the reservoir  39  (activation signal S Cmd, BV ), which valve is therefore closed). The actuator position controller (block  400 ) is activated or a switch is made from the pressure controller to the position controller (selection parameter S=0) as soon as the target pressure is P V,Soll =0 bar and the current booster pressure P V,Ist  is less than a predefined minimum pressure threshold P V,Ist,Min . In this case, the abovementioned valve  37  is also opened again to enable the actuator to draw in an additional volume of fluid from the reservoir  39  if required. Here, the target value for the actuator position corresponds to the zero position of the actuator, which is to be approached with a defined actuator speed and in which the actuator is in an unactuated state. In this position, the braking system does not build up any brake pressure. 
     Actuator position control is likewise activated, in the context of an initialization routine when starting the program, in order to determine the zero position of the actuator (X Akt,0 ) by detection of the mechanical rear end position (X Akt,Mech,0 ). For this purpose, the position target value is ramped down slowly with the reservoir valve  37  open until the linear actuator reaches its rear end position. In this case, the movement of the actuator comes to a halt and the motor torque rises sharply. These two criteria are evaluated in order to detect X Akt,Mech,0 . Once this has been done, the zero position of the actuator X Akt,0 =X Akt,Mech,0 +ΔX Akt,0  is adopted, likewise with the reservoir valve BV open. The offset value ΔX Akt,0  represents a defined safety clearance, which is intended to prevent the actuator from striking against the rear end position during normal operation of the brake control system (e.g. due to undershooting by the control system). By means of the selection parameter S, either the actuator position controller or the booster pressure controller is activated in block  400 . Both controllers have a target value for the actuator speed as an output variable, this corresponding in the example to the motor speed ω Akt,Soll . 
     The pressure controller is activated if there is a braking demand and a defined booster pressure P V,Soll  is to be set. An illustrative embodiment of a pressure controller  401  with a downstream actuator rotational speed controller  501  is illustrated schematically in  FIG. 7 . The pressure controller  401  adjusts the deviation ΔP, formed in a subtraction element  409 , between the requested target booster pressure P V,Soll  and the currently prevailing actual booster pressure P V,Ist  by specifying a target speed ω Akt,Soll,DR,Ctrl . A controller with a proportional action is sufficient for the controller response. To increase the dynamic response of the pressure controller, two feedforward functions can be used: speed feedforward and motor torque feedforward. 
     The speed feedforward function determines a target pressure speed from the pressure target value P V,Soll  by differentiation (function block  402 : calculation of target pressure change), which, weighted with an intensification factor K Prs,1  (function block  403 ), superimposes an additional component ω Akt,Soll,DR,FFW  on the output variable of the pressure controller ω Akt,Soll,DR,Ctrl . The two rotational speed target value components ω Akt,Soll,DR,FFW , ω Akt,Soll,DR,Ctrl  are added together in an adder  404  and fed to a limiting function  405  for limitation to the minimum or maximum permissible target rotational speed (ω Min , ω Max ). Said minimum and maximum values for the rotational speed target values ω Min , ω Max  are calculated in a rotational speed target value calculation module  406 , to which the signal X Akt  representing the actuator travel is fed as an input variable. 
     The target value, limited in this way, for the rotational speed of the actuator is described by ω Akt,Soll,DR =ω Akt,Soll  and, when pressure control is activated by the function block “controller selection”  300  ( FIG. 2 ) with S=1, represents the output variable ω Akt,Soll  of the function block “pressure/actuator position control”  400 . 
     The second feedforward component for increasing the dynamic response of the controller comprises the calculation and direct stipulation of the motor torque M Akt,PV  corresponding to the pressure target value P V,Soll  by function block  407  (“calculation of feedforward torque”), to which the abovementioned system variables P V,Soll , P V,Ist  and the output variable of the rotational speed target value calculation module  406  are fed as input variables. With the aid of the intensification factor K Prs,2  (where K Prs,2  is between 0 and 1) (function block  408 ), it is possible to define the weighting of this torque feedforward component; in this case, a value of K Prs,2 =1 signifies a 100% weighting. The output variable of the torque feedforward function, which simultaneously also supports the rotational speed controller, is then the signal M Akt,PV,FFW , which is processed in the rotational speed control unit  500  described below. 
     As can furthermore be seen from  FIG. 7 , the output signal ω Akt,Soll  of the rotational speed limiting function  405  is fed to a subtraction element  507 , in which the actual value of the actuator rotational speed ω Akt  is subtracted from ω Akt,Soll . The result Δω of the subtraction is fed as an input variable to an actuator rotational speed controller  501 , the output variable M Akt,Soll,Ctrl  of which represents a target value for the abovementioned actuator torque, with an addition of the target value to the output value M Akt,Pv,FFW  of the abovementioned function block  408  being performed in an adder  502 . The result of the addition is finally subjected to a torque limitation function  503 , the output variable M Akt,Soll  of which represents the torque target value. The signal characteristics, which represent the dependence of the actuator rotational speed limiting values ω Min , ω Max  on the actuator position X Akt,Mech , are illustrated in  FIG. 8 . 
     In normal operation of the brake and of the pressure control system, the actuator is in a position in which no limitation of the target rotational speed (especially in the direction of “brake actuation”) is active in the control system (i.e. ω Max =ω Akt,Max ). In this case, the motor torque M Akt,PV  is determined from the target value P V,Soll  for the booster pressure. When the actuator position approaches the mechanical front end position, the rotational speed limiting function  503  is activated. Since it must be assumed in this case that the pressure target value P V,Soll  demanded cannot be set, the motor torque M Akt,PV  is then additionally determined on the basis of the current actual pressure value P V,Ist . The resulting feedforward torque to be output to the rotational speed control system is then obtained from a weighted superimposition of the two component torque target values, wherein the weighting of the value determined from the pressure target value decreases, the greater the limitation, while the weighting of the variable determined from the actual pressure value increases to the same extent. 
     In normal operation of the braking system, the actuator position controller is activated with S=0 when the brake is supposed to be released (see description of “controller selection”  300 ,  FIG. 2 ). An illustrative basic structure  460  of the position controller  420  with a downstream actuator rotational speed controller  501  is illustrated in  FIG. 9  in the form of a block diagram. Here too, a speed feedforward function  421 ,  422  is superimposed in parallel on the position controller  420  in order to boost the dynamic response. Since, in this controller mode, the pressure target value P V,Soll =0 bar, no torque feedforward is required here, for which reason this value is set to a defined value of M Akt,PV,FFW =0 Nm (function block  504 ). 
     The task of the rotational speed controller  501 , which generally has a proportional-integral (PI) action, is to ensure as rapid and accurate as possible setting of the target rotational speed ω Akt,Soll  demanded and compensation of the load torques acting on the actuator, said torques being caused, in the case of the actuator, essentially by the pressure set. 
     To improve the controller structure described above, the function blocks “controller selection”  300  and “pressure/actuator position control”  400  are expanded in order to improve the pressure controller behavior in respect of the maximum pressure build up dynamics, especially in the case of a rapid pressure build up. For this purpose, combined pressure/position control is performed. By way of example, in certain braking situations (e.g. in the case of a rapid pressure build up), both controllers are simultaneously active and make a contribution, weighted by a factor, to the controller output, motor target rotational speed ω Akt,Soll . 
       FIG. 10  shows a block diagram of an illustrative combined pressure/position control system. The pressure target value P V,Soll  is used to determine an actuator travel target value X Akt,Soll  corresponding to the pressure target value in function block  301  by means of a pressure model. Various embodiments of actuator position controllers  460  and pressure controllers  450  are conceivable. As an option, the actual actuator position X Akt  is fed to the pressure controller  450  as an input variable (in order to determine a rotational speed limiting target value). 
     The two controllers  460 ,  450  operate in parallel and supply controller outputs ω Akt,Soll,LR  and ω Akt,Soll,DR  for the actuator rotational speed in accordance with the controller algorithm provided. The resulting controller output as a rotational speed target value ω Akt,Soll  for the lower-ranking rotational speed controller is then obtained in the adder  304  by addition of the two component target values, which are multiplied by a weighting factor λ Pos  and λ Druck  respectively. The two weighting factors are determined in a function block  306  “determination of controller weighting factor”, upstream of which there is a function block  305 . Here, it is advantageous if the following applies to the two weighting factors: λ Druck +λ Pos =1. The weighting factors λ Druck  and λ Pos  determine to what extent the individual controllers contribute. 
     The function block “simulation of pressure controller dynamic response”  305  serves to determine a value for the pressure gradient dP V,Ist,Sim /dt on the basis of the current pressure target value P V,Soll  and a model for the dynamic behavior of the closed pressure control circuit, in particular taking into account the maximum possible pressure gradient. 
     If a rapid pressure build up is required on the basis of the input information and if there are no control interventions by the higher-ranking ESC control system (STATUS ESC =0), the controller output of the actuator position controller  460  is weighted with a large weighting factor λ Pos ≈1. As a result, the actuator moves in a controlled manner to a position value X Akt,Soll , which corresponds approximately to the target booster pressure P V,Soll  demanded, irrespective of the pressure information P V,Ist  (and hence without being influenced by the backpressure information). In the case of decreasing values of dP V,Ist,Sim /dt, the parameter λ Pos  becomes smaller, while λ Druck  increases in a corresponding manner. As a result, the pressure controller  450  is more heavily weighted and can ensure the steady-state accuracy of the overall control system on the basis of the available pressure information. 
     In the case of slow pressure changes and in the case of a pressure reduction, the parameter λ Pos  approaches the value 0, in which case only the pressure controller  450  is then active and sets the pressure target value demanded with greater accuracy on the basis of the measured pressure information. By means of this measure, it is ensured that the pressure controller behavior in the case of a rapid pressure build up is improved in respect of the maximum pressure build up dynamic response in comparison with a simple pressure controller. 
     An illustrative mode of operation of the function block “determination of controller weighting factor”  306  illustrated in  FIG. 10  can be described by:
     λ Pos =1 and hence λ Druck =0, i.e. only actuator position controller  460     when P V,Soll =0 and P V,Ist &lt;P V,ε  (release brake, X Akt,Soll =0)
 
or
   when P V,Soll &gt;0 and dP V,Ist,Sim /dt&gt;dP V,ε2  and STATUS ESC =0,   λ Pos =0 and hence λ Druck =1, i.e. only pressure controller  450  when STATUS ESC &lt;&gt;0 (intervention by a higher-ranking pressure control system)
 
or
   when P V,Soll &gt;0 and dP V,Ist,Sim /dt&lt;dP V,ε1  with predetermined parameter dP V,ε1 &gt;0,   0&lt;λ Pos =f(dP V,Ist,Sim /dt)&lt;1 and λ Druck =1−λ Pos , i.e. combined pressure/actuator position control   when STATUS ESC =0 and P V,Soll &gt;0 and dP V,ε1 &lt;dP V,Ist,Sim /dt&lt;dP V,ε2  with predetermined parameters dP V,ε1  and dP V,ε2 .   

       FIG. 11  shows an illustrative definition of a function f for determining the weighting factor λ Pos  of the actuator position controller  460  using the pressure gradient dP V,Ist,Sim /dt determined:
 
λ Pos   =f ( dP   V,Ist,Sim   /dt ).
 
     In this specification and claims, variable subscripts are used from original text. The following variable subscripts may also be expressed and understood as: AKT —current value, Soll—nominal or target, Druck—pressure, ist—present. 
     While the above description constitutes the preferred embodiment of the present invention, it will be appreciated that the invention is susceptible to modification, variation and change without departing from the proper scope and fair meaning of the accompanying claims.