Method for controlling a braking system for motor vehicles

Abstract

A method for controlling a braking system for motor vehicles having a hydraulically actuatable wheel brake which can be actuated by an electronically controllable pressure supply device having a cylinderpiston arrangement with a hydraulic pressure chamber, the piston of which can be displaced by an electromechanical actuator such that a pre-defined desired pressure value can be adjusted, a position of the pressure supply device being detected, and an actual pressure (P.sub.v, act) of the pressure supply device being determined by a measurement device. To allow braking pressure to build dynamically at low or medium desired pressure values, manipulated variables (.sub.act,soll,M.sub.act,soll) are formed for the electromechanical actuator (1) on the basis of the pre-defined desired pressure value and of an actual pressure value (P.sub.V,Dach), determined on the basis of the desired pressure value (P.sub.v,soll) by weighted addition of the actual pressure (P.sub.v,act) and of a model pressure (P.sub.v,Mod) calculated from a current position (X.sub.act) of the pressure supply device.

Claims

1. A method for controlling a braking system for motor vehicles, the method comprising the steps of: providing a hydraulically actuable wheel brake which can be actuated by means of an electronically controllable pressure supply device having a cylinder-piston arrangement with a hydraulic pressure chamber, and a piston which is configured to be displaced by an electromechanical actuator, with the result that a predefined setpoint pressure value (P.sub.V,Setp) in the hydraulic pressure chamber can be adjusted, detecting a position (X.sub.act) of the pressure supply device, determining an actual pressure (P.sub.V, act) of the pressure supply device by a measuring device, forming manipulated variables (.sub.act,Setp, M.sub.act,Setp) for an electromechanical actuator formed on the basis of the predefined setpoint pressure value (P.sub.V,Setp) and an acquired pressure value (P.sub.V, acq), and determining the acquired pressure value (P.sub.V, acq) as a function of the setpoint pressure value (P.sub.V,Setp) by weighted addition of the actual pressure (P.sub.V, act) and of a model pressure (P.sub.V,Mod) calculated from a the position (X.sub.act) of the pressure supply device.

2. The method as claimed in claim 1, further comprising the step of determining or selecting the acquired pressure value (P.sub.V, acq) as a function of the time derivative of the predefined setpoint pressure value (dP.sub.V,Setp/dt).

3. The method as claimed in claim 1 further comprising the step of determining a weighting factor () for the weighted addition as a function of the setpoint pressure value (P.sub.V,Setp).

4. The method as claimed in claim 3, further comprising the step of determining the weighting factor () for the weighted addition as a function of the time derivative of the predefined setpoint pressure value (dP.sub.V,Setp/dt).

5. The method as claimed in claim 1 further comprising the step of determining the acquired pressure value (P.sub.V, acq) is equal to the actual pressure (P.sub.V,act) if the setpoint pressure value (P.sub.V,Setp) is above a predefined pressure threshold value (P.sub.2).

6. The method as claimed in claim 2 further comprising the step of comparing the time derivative of the predefined setpoint pressure value (dP.sub.V,Setp/dt) with a predefined threshold value (S.sub.1, S.sub.2) if the setpoint pressure value (P.sub.V,Setp) is below the predefined pressure threshold value (P.sub.2).

7. The method as claimed in claim 6 further comprising the step of determining the acquired pressure value (P.sub.V, acq) is equal to the model pressure (P.sub.V,Mod) if the time derivative of the predefined setpoint pressure value (dP.sub.V,Setp/dt) is above the predefined threshold value (S.sub.2) and if the setpoint pressure value (P.sub.V,Setp) is below a predefined pressure threshold value (P.sub.1).

8. The method as claimed in claim 1 further comprising the step of calculating the model pressure (P.sub.V,Mod) according to a predefined characteristic curve or function (f(X.sub.act)) which represents the dependence, characterizing the braking system, of the actual pressure (P.sub.V, act) on the position (X.sub.act) of the pressure supply device.

9. The method as claimed in claim 1 further comprising the step of feeding the setpoint pressure value (P.sub.V,Setp) and the acquired pressure value (P.sub.V, acq) to a pressure controller with a speed controller connected downstream, wherein a speed pilot controller is provided, the output variable (.sub.add) of which is determined as a function of the current position (X.sub.act) of the pressure supply device.

10. A control device for a hydraulically actuatable wheel brake, the control device comprising: an electronically controllable pressure supply device having a cylinder-piston arrangement with a hydraulic pressure changer, the electronically controllable pressure supply device being configured to actuate the hydraulically actuatable wheel brake, a piston of the cylinder-piston arrangement of which can be displaced by an electromechanical actuator, with the result that a predefined setpoint pressure value (P.sub.v, Setp) in the hydraulic pressure chamber can be adjusted, a detecting device configured to detect a position (X.sub.act) of the pressure device, a measuring device configured to determine an actual pressure (P.sub.v, act) of the pressure supply device and feed the actual pressure (P.sub.v, act) to a pressure controller, the control device forming manipulated variables (.sub.act,Setp, M.sub.act,Setp) for the electromechanical actuator as a function of an acquired pressure value (P.sub.V, acq) and a predefined setpoint pressure value (P.sub.V,Setp), the control device comprising the pressure controller and a speed controller which is connected downstream of the pressure controller, and wherein manipulated variable (.sub.act,Setp) which represents a setpoint actuator speed value and an actual actuator speed value (.sub.act) are fed as input variables to the speed controller, in that means are provided which determine the acquired pressure value (P.sub.V, acq), and feed it to the pressure controller.

11. The control device as claimed in claim 10, further comprising: the pressure controller is configured to output a first setpoint actuator speed value (.sub.act Setp,DR,Ctrl), the pressure controller is configured to determine a second setpoint actuator speed value (.sub.act,Setp,DR,FFW) from the setpoint pressure value (P.sub.V,Setp), the time derivative of the setpoint pressure value, and the pressure controller is configured to determine a third setpoint actuator speed value (.sub.add) at least from the position (X.sub.act) of the pressure supply device, and the manipulated variable (.sub.act,Setp), which represents the setpoint actuator speed value, for the speed controller is determined on the basis of the first, the second and the third setpoint actuator speed values (.sub.act,Setp,DR,Ctrl,.sub.act,Setp,DR,FFW,.sub.add).

12. A braking system for motor vehicles comprising: at least one hydraulically actuable wheel brake an electronically controllable pressure supply device for actuating the wheel brake, the pressure supply device comprises a cylinder-piston arrangement with a hydraulic pressure chamber, a piston of which can be displaced by an electromechanical actuator, a measuring device for determining an actual pressure (P.sub.V, act) of the pressure supply device, having means for determining a position (X.sub.act) of the pressure supply device and an electronic open-loop and closed-loop control unit for controlling the electromechanical actuator, a control device, the control device configured to form manipulated variables (.sub.act,Setp, M.sub.act,Setp) for the electromechanical actuator as a function of an acquired pressure value (P.sub.V, acq) and a predefined setpoint pressure value (P.sub.V,Setp), and the control device a pressure controller and a speed controller which is connected downstream of the pressure controller, wherein manipulated variable (.sub.act,Setp) which represents a setpoint actuator speed value and an actual actuator speed value (.sub.act) are fed as input variables to the speed controller, in that means are provided which determine the acquired pressure value (P.sub.V, acq), and feed it to the pressure controller, falls below a configured low limit.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) In the drawings, in each case schematically:

(2) FIG. 1 shows a basic circuit diagram of an exemplary braking system for carrying out a method according to the invention,

(3) FIG. 2 shows a control device for carrying out a method for controlling a braking system,

(4) FIG. 3 shows an exemplary control device for carrying out a method according to the invention,

(5) FIG. 4 shows an exemplary embodiment of the method according to the invention,

(6) FIG. 5 shows an exemplary embodiment of the method according to the invention, and

(7) FIG. 6 shows an exemplary embodiment of the method according to the invention.

FURTHER DESCRIPTION OF THE INVENTION

(8) The method according to the invention preferably relates to a control concept for the improved (more rapid) adjustment of predefined setpoint pressures by means of a piston which is driven by an electric motor in an active, externally actuated braking system. In particular, the improvement in the pressure buildup dynamics in the case of relatively low and medium pressure requirements is considered here. A development which is also presented deals with particularly taking into account large brake lining clearances when improving the pressure buildup dynamics.

(9) FIG. 1 shows the simplified principle of an active braking system for a controlled wheel of a hydraulically braked vehicle. The method according to the invention is preferably carried out in an active, externally actuable braking system in which the driver makes a pressure request, for example by means of brake pedal travel, and said pressure request is implemented electronically using a pressure supply device 50, for example including an electric motor or actuator 1, a suitable gear mechanism 2 and a piston 3 which bounds a hydraulic pressure chamber 4, in that the piston 3 moves by a travel X.sub.act from a position of rest 15 into a position 14, with the result that a specific volume of the brake fluid is displaced from the pressure chamber 4 via the line 5 and an initially opened inlet valve 6 into the brake line 8, and therefore into the wheel brake 9. Therefore, after the brake lining clearance has been overcome a brake pressure is generated in the wheel brake 9. A braking pressure reduction can occur in that the piston 3 is moved back again in the direction of the position of rest 15. However, a rapid braking pressure reduction such as is required, for example, in the case of ABS control, is also possible by means of the valve combination 6, 7 in that the inlet valve 6 is closed and the outlet valve 7 is opened for a specific time. Brake fluid then flows out of the wheel brake 9 via the line 8 through the outlet valve 7 and therefore via the line 10 in the brake fluid container 11. This measure for reducing pressure is appropriate, in particular, when the pressure chamber 4 serves a plurality of wheel brakes in parallel.

(10) Furthermore, for example a measuring device 31 is provided by means of which a position of the pressure supply device 50, which is characteristic of a position or orientation or location of the actuator 1 and therefore the piston 3 of the pressure supply device 30, is detected. The measuring device 31 can detect, for example, a rotor orientation angle of the electric motor 1 or a spindle position of a rotation-translation gear mechanism or else the travel X.sub.A act of the piston 3 from its position of rest 15. Alternatively, the position of the pressure supply device 50 can also be determined from other variables, for example on the basis of a model.

(11) Furthermore, for example a pressure measuring device 32 is provided by means of which the actual pressure P.sub.v,act, i.e. the pressure in the pressure chamber 4 of the pressure supply device 50, is measured.

(12) For reasons of safety and of rapid detection of faults, the measurement variable X.sub.act and/or the actual pressure measurement variable P.sub.V,act are/is advantageously determined redundantly. For this purpose, the corresponding measuring device 31, 32 can be of intrinsically safe design or two redundant measuring devices can be correspondingly provided.

(13) Basically, the braking system which is illustrated in FIG. 1 can be extended with any desired number of wheel brakes 9 in that a plurality of lines 5 are led to the wheel circuits, wherein each wheel circuit preferably has an individual valve pair 6, 7.

(14) In order to form multiple circuits of the system for safety reasons, a plurality of pistons 3 and a plurality of pressure chambers 4 can be provided. For a passenger car a dual circuit is appropriate, wherein in each case two wheel brakes are connected to one of two pressure chambers.

(15) Numerous improvements and different embodiments of the principle, for example in terms of the selection of the valves, are conceivable compared to the simplified illustration of the system in FIG. 1. For example a master brake cylinder can also be arranged between the hydraulic pressure chamber 4 and the wheel brake or brakes 9, with the result that the pressure generated in the pressure chamber 4 is fed to a hydraulic intermediate space, for example in an actuation device, as a result of which the master brake cylinder is actuated.

(16) The invention is concerned with the object of setting suitable pressures in the pressure chamber 4.

(17) The need to set a predefined pressure or pressure profile using a control method arises whenever the driver requests a general braking pressure for all the wheels of the motor vehicle by means of actuation of the brake pedal or if this pressure request is made by means of an assistance function ACC (adaptive cruise control), HSA (hill start assist), HDC (hill descent control) etc., or if a special wheel-specific brake control function becomes active, such as, for example, ABS (anti-lock braking system), TCS (traction control system) or ESP (electronic stability program).

(18) In all cases the pressure of the pressure chamber 4 is to be advantageously set in such way that the wheel with the highest braking pressure request can be reliably supplied with the necessary pressure. With respect to the dynamics of the pressure or pressure profile to be set, within the scope of the available dynamics of the actuator 1 the shortest possible time delay between the pressure request being made and the pressure which is set in the admission pressure chamber 4 is to be aimed for. This applies, in particular, even if the actuator 1 is located in its position of rest 15 at the start of the pressure request and therefore must firstly overcome the brake lining clearance in order to set the requested pressure. In this context, the actuator firstly forces a volume, dependent on the size of the wheel brakes 9 being used and of the set brake lining clearance, out of the pressure chamber 4 into the wheel brakes, in order to apply the brake linings, for example to the brake disk. However, during this process, braking pressure is not yet built up in the wheel brakes 9. In particular by taking into account the request for a response behavior which is as good as possible, the request to implement a pressure buildup which is as rapid as possible within the scope of the available actuator dynamics, even in the case of low pressure requests, also occurs here (independently of the size of the clearance which has been set).

(19) With respect to the control behavior for setting the requested admission pressure this means that for the pressure control it is necessary to provide measures so that the time between the making of a pressure request and the start of the pressure buildup in the wheel brakes under consideration is as short as possible. This applies, in particular, to rapid pressure buildup requests to a low to medium pressure level. When a linear controller approach is used it may be the case that the full available dynamics of the linear actuator are not utilized in this setpoint pressure range.

(20) A basic structure of a control device 200 for setting a requested pressure or pressure profile is shown in FIG. 2. It shows a pressure controller 20, to which an actuator speed controller (advantageously engine rotational speed controller) 21 is subordinated through the intermediate connection of further circuit elements (23-25). In this context, the result P.sub.v of a subtraction, carried out in a subtraction element 19, of the requested setpoint pressure value P.sub.V,Setp from the actual pressure value P.sub.V,act which is currently present, is fed to the pressure controller 20. The output variable of the pressure controller 20 is the setpoint value for the actuator rotational speed .sub.act,Setp,DR,Crl, which is transferred to the rotational speed controller 21 as an input variable while taking into account predefined minimum and maximum actuator rotational speed values .sub.Min, .sub.Max. The setpoint pressure value P.sub.V,Setp occurs on the basis of the requests described in the preceding sections. The actual pressure value P.sub.V,act is preferably measured by means of a pressure sensor (32 in FIG. 1). This pressure sensor 32 senses the pressure at the outlet of the pressure chamber 4. It is to be noted here that, in particular, the dynamic pressures which occur at the valves located between the pressure chamber 4 and the wheel brakes 9 (for example 6), which occur in the case of rapid pressure buildups, are also measured as a result.

(21) A proportionally active controller (P controller) is usually sufficient as controller transmission behavior. In order to increase the pressure controller dynamics, for example a speed pilot control 22 is provided. The latter determines from the requested setpoint pressure value P.sub.V,Setp by differentiation (over time) a setpoint pressure speed which superimposes an additional component .sub.act,Setp,DR,FFW (.sub.act,Setp,DR,FFW=K.sub.p*dP.sub.V,Setp/dt), the setpoint actuator rotational speed of the pressure controller 20 .sub.act,Setp,DR,Ctrl, weighted with a gain factor (K.sub.p). The two setpoint rotational speed components are added together in an adder element 23 and fed to a limiting function 24 for limiting to the minimum or maximum permissible setpoint rotational speed (.sub.Min, .sub.Max). The output variable of the pressure controller 20, 24 is the setpoint value for the motor rotational speed .sub.act,Setp, which is transferred as an input variable to the rotation speed controller 21, 25.

(22) The limited setpoint actuator rotational speed value .sub.act,Setp is compared with the actual actuator rotational speed value .sub.act in a further subtraction element 25 in order to form a setpoint actuator rotational speed value difference .sub.Setp. The setpoint actuator rotational speed value difference .sub.Setp is fed as an input variable to the abovementioned rotational speed controller 21, the output variable of which corresponds to a setpoint value M.sub.act,Setp,Ctrl of the torque which is to be applied by the actuator. The setpoint torque value M.sub.act,Setp,Ctrl is finally limited to the minimum or maximum permissible torque value M.sub.min,M.sub.max in a second limiting module 26, and provides the setpoint torque value M.sub.act,Setp for the electric motor. A further input variable for the rotational speed controller which usually has proportional-integrating (PI) behavior is the actual rotational speed .sub.act of the actuator, which is preferably acquired from the actuator position X.sub.act (measuring device 31 in FIG. 1) which is available by means of measuring equipment for commutation purposes, for example.

(23) The abovementioned dynamic pressures act in a disruptive fashion on the pressure control and, in order to avoid vibrations, result in a somewhat cautious adjustment of the pressure controller. As long as no limiting function is active for the pressure controller, it is operated in its linear range, which has the result that small control errors also give rise to only small setpoint rotational speeds. In both cases, the full available dynamics of the linear actuator 50 are therefore not used, in particular, for rapid pressure buildup requests from the unactuated state to a low present to medium pressure level.

(24) FIG. 3 illustrates an exemplary control device 201 for carrying out a method according to the invention which overcomes the abovementioned disadvantages and significantly improves the control behavior.

(25) For example, the actual pressure P.sub.V,act which is sensed by a sensor is not basically considered as an actual pressure value for the pressure control but instead an acquired value P.sub.V, acq is used. For example, the actual pressure value.

(26) P.sub.V,acq in the module is determined for the calculation of the pressure information (block 40).

(27) For example the following relationship applies to the pressure signal P.sub.V, acq.
P.sub.V,acq=(1)*P.sub.V,act+*P.sub.V,Model(1)

(28) The actual pressure value P.sub.V,acq is obtained from an addition, weighted by the factor (=0, . . . , 1), of the two pressure signals P.sub.V,act and P.sub.V,Model, wherein one signal represents the already mentioned measured pressure signal P.sub.V,act. The second signal P.sub.V,Model is a calculated model pressure which is obtained on the basis of the measured travel X.sub.act, wherein a static characteristic curve or function (f(X.sub.act)), which represents the dependence, characterizing the braking system, of the actual pressure (P.sub.V,act) on the position (X.sub.act) of the pressure supply device 50, is calculated as a model. By means of the weighting factor it is determined which signal component (in the form of the actual pressure value P.sub.V,Dach) is fed with which intensity to the pressure control.

(29) For example, the (predefined) characteristic curve P.sub.V,Model=f(X.sub.act) is equal to zero to a predefined position limiting value X.sub.0 (i.e. for X.sub.actX.sub.0, P.sub.V,Model=0) and then increases as X.sub.act becomes larger.

(30) The weighting factor is acquired as a function of the requested setpoint pressure P.sub.V,Setp and advantageously additionally as a function of the requested setpoint pressure gradient dP.sub.V,Setp/dt, which is obtained by means of the time derivative of the setpoint pressure P.sub.V,Setp.

(31) In the text which follows, an exemplary embodiment of the method according to the invention is described, in particular an exemplary determination of a weighting factor .

(32) If the setpoint pressure P.sub.V,Setp is higher than a first predefined pressure value P.sub.2(P.sub.V,Setp>P.sub.2), control is always carried out with the actual pressure P.sub.V,act sensor signal, i.e. the weighting factor is zero, =0, and P.sub.V,acq=P.sub.V,act.

(33) If the setpoint pressure P.sub.V,Setp is equal to or lower than the first predefined pressure value P.sub.2(P.sub.V,SetpP.sub.2), the requested setpoint pressure profile dP.sub.V,Setp/dt is additionally considered.

(34) If the setpoint pressure profile dP.sub.V,Setp/dt exceeds a predefined second threshold S.sub.2, dP.sub.V,Setp/dt>S.sub.2, therefore applies so that becomes=1 and the pressure control forms the control error by means of the abovementioned model variable P.sub.V,Model.

(35) In order to avoid undesired sharp transitions from P.sub.V,Act to P.sub.V,Model (or the other way around), intermediate values are also predefined for the factor .

(36) Precise details on the exemplary embodiment for determining the weighting factor , as it can be implemented in the module 40, is illustrated in FIG. 4. Here, a first parameter .sub.1 is determined in block 42 on the basis of a predefined characteristic curve as a function of the setpoint pressure P.sub.V,Setp. In block 43, a second parameter .sub.2 is determined on the basis of a further predefined characteristic curve as a function of the setpoint pressure derivative dP.sub.V,Setp/dt. The weighting factor is determined in block 44 as a product of the two parameters .sub.1 and .sub.2 (=.sub.1*.sub.2). In block 45, the actual pressure value P.sub.V,acq is calculated on the basis of the weighting factor and the relationship in equation (1).

(37) If the setpoint pressure P.sub.V,Setp is higher than a first predefined pressure value P.sub.2 (P.sub.V,Setp>P.sub.2), .sub.1=0 applies, and therefore =0 (independently of .sub.2). If the setpoint pressure P.sub.V,Setp is equal to or lower than a second predefined pressure value P.sub.1, .sub.1=1 applies. For setpoint pressure values P.sub.V,Setp between the pressure values P.sub.1 and P.sub.2, .sub.1 decreases, for example, linearly with P.sub.V,Setp. If the setpoint pressure profile dP.sub.V,Setp/dt undershoots a predefined first threshold S.sub.1 (i.e. only a slow pressure buildup is requested), .sub.2=0 applies, and therefore =0, i.e. control is carried out with the actual pressure P.sub.V,act sensor signal. If the setpoint pressure profile dP.sub.V/.sub.Setp/dt exceeds a predefined second threshold S.sub.2 (i.e. a very rapid pressure buildup is requested), .sub.2=1 applies. For small setpoint pressures P.sub.V,Setp (i.e. P.sub.V,Setp<P.sub.1), =1 (maximum value) then applies, i.e. control is carried out with the model signal P.sub.V,Model with the result that measured actual pressures P.sub.V,Act which are influenced by dynamic effects (are too large) do not influence the pressure control. In order to avoid undesired sharp transitions during the acquisition of the weighting factor , intermediate values for the parameters .sub.1 and .sub.2 and therefore for the factor are acquired in the range between P.sub.1 and P.sub.2 (for P.sub.V,Setp) or between S.sub.1 and S.sub.2 (for P.sub.V,Setp/dt).

(38) The pressure value P.sub.2 can be, for example, several 10s of bar. The threshold S.sub.2 can be, for example, in the range of 1 or several 100s of bar/sec. The requested setpoint pressure profile dP.sub.V,Setp/dt is determined in Block 41. An exemplary method for determining dP.sub.V,Setp/dt is illustrated in FIG. 5. The requested setpoint pressure profile dP.sub.V,Setp/dt is preferably acquired using a differentiating filter (block 41b), wherein a rise limiting function (optional block 41a) is advantageously additionally introduced, with the result that step-shaped changes to the pressure request P.sub.V,Setp result in a finite setpoint pressure gradient dP.sub.v,Setp/dt. In block 41a a step-shaped pressure request P.sub.V,Setp (continuous line in the right-hand diagram P.sub.V,Setp as a function of the time t) is converted into a pressure request P.sub.V,Setp with, for example, a linear rise (dashed line in the right-hand diagram P.sub.V,Setp as a function of the time t).

(39) As a result of the described measures, in the case of slow pressure requests the pressure control always operates with the measured pressure sensor signal, while in the case of rapid pressure requests it is initially controlled up to a certain pressure level using the model signal, and the sensor signal is only used to adjust the static target pressure, as a result of which the steady-state accuracy of the pressure control is achieved again.

(40) The transition of the pressure control signal P.sub.V,acq from the model signal P.sub.V,Model to the measured pressure value P.sub.V,act is defined by the weighting factor , which is acquired, for example, in accordance with the arrangement illustrated in FIG. 4. As a result, in particular the influence of the dynamic pressures mentioned at the beginning, which influence acts in a disruptive way on the pressure control, is minimized, which gives rise to an increase in the pressure buildup dynamics. Furthermore, owing to this the parameterization of the pressure controller can be performed significantly more strongly with respect to improved control behavior.

(41) For improved response behavior, in particular when wheel brakes with an increased brake lining clearance are used, in order to overcome this clearance more quickly an additional extension of the controller structure illustrated in FIG. 2 is preferably used, which extension relates to an extension of the speed pilot control and is illustrated by dashed lines in FIG. 3 (block 22a and connections). In addition to the already mentioned pilot control rotational speed .sub.act, Setp,DR,FFW), which is obtained on the basis of the setpoint pressure gradient (block 22), an additional engine rotational speed component .sub.add is provided, for example, as a pilot control variable (block 22a) which depends, for example, on the size of the clearance (X.sub.L) to be overcome. This additional engine rotational speed component .sub.add is superimposed additively in the case of a requested pressure buildup of the pilot control rotational speed .sub.act, Setp,DR,FFW, as long as the clearance (X.sub.L) has not yet been overcome. An exemplary characteristic curve of the engine rotational speed component .sub.add as a function of the currently measured actuator travel X.sub.act is illustrated in FIG. 6. The limiting value X.sub.L advantageously corresponds to the value of the position limiting value X.sub.0 of the model used as a basis (see the characteristic curve P.sub.V,Model=f(X.sub.act) above). The size of the additional rotational speed component .sub.add can also be predefined (additionally) as a function of the requested setpoint pressure gradient dP.sub.V,Setp/dt.

(42) 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.