Abstract:
A process for operating the brake arrangement of a vehicle is presented, which comprises an electrically controllable service brake system, which is set to generate brake forces independently of driver actuation, and 
     which comprises an electrically controllable parking brake system, which is set to generate brake forces and maintain these forces. So that the parking brake system or its electromechanical actuating unit only needs to cope with relatively small load situations, it is proposed that, when, for certain operating conditions, the parking brake system has to maintain brake forces which are greater than the brake forces it is able to generate itself, the service brake system generates the additionally required brake forces.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This application is a continuation of International Application No. PCT/EP2005/000878 filed Jan. 28, 2005, the disclosures of which are incorporated herein by reference, and which claimed priority to German Patent Application No. DE 10 2004 004 992.0 filed Jan. 30, 2004, the disclosures of which are incorporated herein by reference. 

   BACKGROUND OF THE INVENTION 
   The invention relates to a process for operating the brake arrangement of a vehicle, which brake arrangement comprises an electrically controllable service brake system, which is set to generate brake forces independently of driver actuation, and which comprises an electrically controllable parking brake system, which is set to generate brake forces and maintain these forces. 
   The service brake system enables the driver to gradually reduce the speed of the vehicle during its operation or to bring the vehicle to a standstill. Modern vehicles are today equipped with an electrically controllable service brake system in order to enable brake functions which are independent of driver actuation, i.e. automatic brake functions, such as drive slip control (ASR) or driving dynamics control (ESP) to be carried out in addition to the anti-locking control function (ABS). To this end, the service brake system comprises, in known manner, a correspondingly constructed electrohydraulic control unit, an electronically controllable brake booster or it is constructed as a so-called “brake-by-wire” system. 
   By contrast, the parking brake system also enables the vehicle to be held on a road with an incline, and particularly kept stationary when the driver is absent, using mechanical means. The aim today is also to construct the parking brake system such that its electrically controllable, as already known for example as an “electric parking brake (EPB)”. With this, the driver only needs to actuate an electrical control element by way of which at least one electromechanical actuating unit is activated to actuate the actual wheel brakes. Whilst, with the so-called “cable puller”, only a central electromechanical actuating unit is used, which replaces the conventional parking brake actuating element (lever or pedal) and acts on the wheel brakes concerned in conventional manner by way of cables, there are also EPB systems with generally two peripheral electromechanical actuating units which are constructed or integrated directly on the wheel brakes concerned. 
   Therefore, DE 101 50 803 A1 discloses a wheel brake which can be hydraulically actuated in conventional manner for the functional range of the service brake system. To this end, the wheel brake has a brake piston which acts on at least one friction lining and can be displaced by means of a hydraulic pressure introduced into a hydraulic chamber to clamp the at least one friction lining against a rotary element connected in torsion-resistant manner to a wheel of the vehicle in order to generate a brake force. So that the functions of the parking brake system can also be carried out, an electromechanical actuating unit is furthermore integrated in the wheel brake, and this has a gear unit driven by an electric motor, which acts on the brake piston in order to clamp and mechanically fix this against the rotary element connected in torsion-resistant manner to a wheel of the vehicle to generate a brake force, to which end the gear unit is of a self-locking construction. 
   A brake arrangement having a wheel brake such as that known from DE 101 50 803 A1 is disadvantageous in that the electromechanical actuating unit provided for the parking brake system also has to be designed for load situations such as those for which very high brake forces have to be generated and maintained, for example to safely hold a fully loaded vehicle stationary on a road with a gradient of 30% and steeper. The result of this is that the technology of the electromechanical actuating unit has to be relatively complex, making the brake arrangement significantly more expensive. 
   BRIEF SUMMARY OF THE INVENTION 
   The object of the invention, therefore, is to provide a process for operating the brake arrangement mentioned above, by means of which the parking brake system or its electromechanical actuating unit only needs to cope with relatively small load situations, in order to prevent the disadvantages mentioned above. 
   To achieve this object, the process according to the invention proposes that when, for certain operating conditions, the parking brake system has to maintain brake forces which are greater than the brake forces it is able to generate itself, the service brake system generates the additionally required brake forces. 
   The advantage of the invention is that the brake forces to be generated by the parking brake system can be limited. Limited, for example, to the extent that it is only necessary to cover a simple load situation in which a vehicle with a small load is to be held stationary on a substantially level road. It is thus possible to use an electric motor with relatively low power and a gear unit with a relatively low reduction/transmission ratio for the electromechanical actuating unit, which is not only advantageous in terms of cost but also means that the overall size of the electromechanical actuating unit is reduced to more compact dimensions. 
   The invention furthermore advantageously makes use of the above-mentioned feature that the gear unit of the electromechanical actuating unit of the parking brake system is of a self-locking construction. As a result, the parking brake system can maintain higher brake forces than it, or its electromechanical actuating unit, is able to generate. Therefore, although the brake forces which can be generated by the parking brake system are limited, it is still suitable for load situations in which a vehicle with a full load, for example, has to be safely held stationary on a road with a gradient of 30% and steeper. 
   The additionally required brake forces are provided by the service brake system, which is present in any case, with the invention likewise advantageously making use of its above-mentioned feature of being able to generate brake forces automatically, i.e. independently of driver actuation. 
   Since the service and parking brake systems are electrically controllable, one or more electronic control units are present which detect operating conditions, for example the loaded condition of the vehicle and/or the gradient of the road and/or the temperature of the wheel brakes, by way of corresponding sensor means or mathematical models. There is therefore also the advantage that the additional brake forces to be provided by the service brake system can be variably and individually adjusted according to particular operating conditions or load situations. This has a very favourable effect on the total collective load to be applied by the brake arrangement and, in individual cases, e.g. when the vehicle is to be held stationary on a level road, can even mean that no additional brake forces have to be provided by the service brake system. 
   It is preferably provided for the service brake system to withdraw the additionally required brake forces after the parking brake system has reached the brake forces which it is able to generate. On the one hand, with the withdrawal of the additionally required brake forces, an increase in the self-locking effect of the parking brake system or its electromotive actuating unit is achieved, which is particularly conducive to safety when the vehicle is to be held stationary for example on a road with a steep gradient. On the other hand, the service brake system is then force-free during the stationary or parking phase of the vehicle, which is a legal stipulation for a hydraulic service brake system owing to the risk of leakages. 
   Even if the parking brake system can maintain higher brake forces than it is able to generate owing to its self-locking effect, the forces which it, or its electromechanical actuating unit, is able to generate are generally sufficient to overcome the self-locking forces to release the parking brake system. In particular cases, when it is moreover necessary to overcome reaction forces, for example because the vehicle has been parked on a road with a very steep gradient, it is therefore possible to provide for the service brake system to generate predetermined brake forces before the parking brake system withdraws the brake forces maintained by it. Provision can also be made here for the predetermined brake forces to be generated by the service brake system to be variably and individually adjustable according to particular operating conditions, e.g. the loaded condition of the vehicle and/or the gradient of the road. 
   In principle, it is possible to provide for the brake forces to be generated by the service and parking brake system at the same time when the parking brake system is activated. 
   In any case, for example in the event that only a relatively small additional brake force component has to be provided by the service brake system, e.g. because the vehicle is unloaded and/or the road has a flat gradient, it is possible to provide for the service brake system to generate the additionally required brake forces after the parking brake system has generated predetermined brake forces. 
   Likewise, for example in the event that a relatively large additional brake force component has to be provided by the service brake system, e.g. because the vehicle is loaded and/or the road has a steep gradient, it is possible to provide for the service brake system to generate the additionally required brake forces before the parking brake system generates brake forces. 
   It can furthermore be the case that, when the parking brake system is activated, brake forces are already generated by the service brake. For example because the service brake system is either already actuated by the driver or within the framework of an automatic brake function, such as “hill hold” or “auto-hold”, in order to hold the vehicle stationary for example on a road with an incline. In these cases, it is possible to provide for the service brake system to at least maintain brake forces which are in any case already generated as the additionally required brake forces. 
   It goes without saying that the invention also relates to a brake arrangement of a vehicle which is operated according to the process according to the invention. 
   Other advantages of this invention will become apparent to those skilled in the art from the following detailed description of the preferred embodiments, when read in light of the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  an exemplary embodiment of a brake arrangement which can be operated according to the process according to the invention; 
       FIG. 2  a force/time graph of a first exemplary embodiment of the process according to the invention upon actuation of the parking brake system; 
       FIG. 3  a force/time graph of a second exemplary embodiment of the process according to the invention upon actuation of the parking brake system; 
       FIG. 4  a force/time graph of a third exemplary embodiment of the process according to the invention upon actuation of the parking brake system; 
       FIG. 5  a force/time graph of a fourth exemplary embodiment of the process according to the invention upon actuation of the parking brake system; and 
       FIG. 6  a force/time graph of an exemplary embodiment of the process according to the invention when the parking brake system is released, 
   

   these being simplified schematic illustrations and the reference numerals used each denoting the same components and having the same significance. 
   DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1  shows an exemplary embodiment of a brake arrangement, which can be operated according to the inventive process, for a wheel brake RB. The brake arrangement comprises a service brake arrangement BBA which acts hydraulically on a wheel brake RB by way of a brake line BL. A parking brake system FBA with an electromotive actuating unit is integrated in the wheel brake. 
   To control/regulate the service and parking brake system, a common electronic control unit ECU is provided, in which, or in the computer unit of which, the inventive process is preferably implemented as software. A person skilled in the art will understand that the service and parking brake system can also be controlled/regulated starting from separate electronic control units, which exchange data by way of a communications system, e.g. CAN-bus. 
   The wheel brake RB illustrated here in a longitudinal section with reference to its longitudinal axis A has a housing  10  in which a brake piston  11  is received such that it is displaceable coaxially to the longitudinal axis A. A sealing arrangement  12  seals the brake piston  11  with respect to the housing  10  to form a hydraulic chamber  13  in the housing  10 . The hydraulic chamber  13  is connected to the brake line BL so that the service brake system BBA can introduce hydraulic pressure for displacing the brake piston  11 . The brake piston  11  acts directly on a first friction lining  14  and, by way of a brake calliper  17  constructed on the housing  10 , directly on a second friction lining  15  according to the floating calliper principle. Arranged between the first and second friction lining  14 ,  15 , there is a rotary element  16  or a brake disc which (not illustrated in more detail) is connected in torsion-resistant manner to a wheel of the vehicle. If a displacement of the brake piston  11  is effected as a result of a hydraulic pressure introduced into the hydraulic chamber  13 , the friction linings  14 ,  15  are clamped against the side faces of the rotary element  16  to generate a brake force. 
   The electromotive actuating unit of the parking brake system FBA has an electric motor  20  which can be electrically controlled by the electrical control unit ECU by way of control signals s 6 . The electric motor  20  drives a gear unit which functions in the manner of a nut/spindle arrangement  21 ,  22  and by means of which the rotary movement of the electric motor  20  is converted into a longitudinal movement for a control element  23 . The control element  23  can be displaced coaxially to the longitudinal axis A, is arranged inside the hydraulic chamber  13  and acts on the base  18  of the brake piston  11 . Thus, upon activation of the electric motor  20 , a displacement of the brake piston  11  is effected so that, to generate a brake force, the friction linings  14 ,  15  are clamped against the side faces of the rotary element  16 . If the activation of the electric motor  20  is withdrawn or stopped, the brake force generated by clamping the friction linings  14 ,  15  is mechanically retained since the gear unit functioning in the manner of a nut/spindle arrangement  21 ,  22  is of a self-locking construction for the parking brake function. A withdrawal of the brake force to release the parking brake function is only possible by re-activating the electric motor  20  in the opposite direction of rotation, during which the control element  23  is moved away from the base  18  of the brake piston  11 . 
   The service brake system BBA illustrated as a hydraulic circuit diagram for a wheel brake RB can be electrically activated by the electronic control unit ECU by way of control signals S 1  to S 5 . Here, the electromagnetically controllable valve arrangements  31  to  34  are each shown in their electrically unactuated starting position. The pump  35  can be controlled by way of an electromotive drive M. 
   In the event of conventional braking, where the driver actuates a brake pedal  39 , a hydraulic pressure is generated in a brake pressure transmitter unit  30  and is introduced into the hydraulic chamber  13  of the wheel brake RB by way of the brake line BL as a result of the open check valves  32  and  34 . 
   In order to modulate the hydraulic pressure introduced into the hydraulic chamber  13 , for example for an ABS brake system as a result of a time alternation between pressure reduction, pressure maintaining and pressure build-up phases, the electronic control unit ECU controls the transfer valve  31 , the check valve  32  and the pump  35  by way of the control signals S 1 , S 2  and S 5  as follows: To reduce the pressure, both the transfer valve  31  and the check valve  32  are activated so that hydraulic fluid admitted into the hydraulic chamber  13  is discharged into the low pressure store  36 . To maintain the pressure, only the check valve  32  is activated so that the volume of hydraulic fluid admitted into the hydraulic chamber  13  remains unaltered. To build up the pressure, neither the transfer valve  31  nor the check valve  32  is activated, so that hydraulic fluid is again admitted into the hydraulic chamber  13 . During the pressure modulation, the pump  35  is at least sometimes activated to convey hydraulic fluid which has been discharged into the low pressure store  36  back into the brake line BL. 
   To perform automatic brake functions, i.e. brake functions which are independent of driver actuation, such as ESP, the electronic control unit ECU firstly activates the transfer valve  33 , the check valve  34  and the pump  35  by way of the control signals S 3 , S 4  and S 5 . Thus, the suction side of the pump  35  is connected to the reservoir  37  of the brake pressure transmitter unit  30  so that it can remove hydraulic fluid there to introduce it into the hydraulic chamber  13  of the wheel brake RB by way of the brake line BL as a result of the open check valve  32 . If pressure modulation is also required here, this can take place as described above on the part of the electronic control unit ECU by further activating the transfer valve  31  and the check valve  32  by way of the control signals S 1  and S 2 . 
   Information relating to the operating conditions of the vehicle which is collected by corresponding sensor means (not illustrated in more detail) is supplied by way of input signals E 1  to the electronic control unit ECU for processing. This includes the speeds of the rotary elements  16  or the associated wheels of the vehicle to detect, amongst other things, whether the vehicle is stationary, the gradient of the road on which the vehicle is to be held stationary, and the loaded condition of the vehicle. A person skilled in the art will understand that some sensor means, e.g. gradient sensors, can also be integrated in the electronic control unit to gain advantages in terms of costs and susceptibility to failure. 
   By way of the input signals E 2 , a corresponding control means (not illustrated), which can be operated by the driver, informs the electronic control unit ECU that the activation of the parking brake system FBA is desired to hold the vehicle stationary. Moreover, within the framework of a “hill hold” or “auto hold” function, it is also possible to activate the parking brake system FBA independently of the will of the driver, i.e. automatically, for example when, after a particular time period has been exceeded, the vehicle is no longer to be held stationary by the service brake system BBA but by the parking brake system FBA. 
   It is also optionally possible to provide a pressure sensor  38  which detects the pressure generated in the hydraulic chamber  13  or brake line BL and communicates this to the electronic control unit ECU by way of input signals E 3 . The pressure generated in the hydraulic chamber  13  is namely proportional to the brake force which is generated when the friction linings  14 ,  15  are clamped against the side faces of the rotary element  16  and is therefore of a level relevant for regulating/controlling the brake arrangement. In addition, or when a pressure sensor  38  is not present, the brake force can be determined by mathematical models. In the parking brake system FBA, this is based for example on the power input of the electric motor  20 ; in the service brake system BBA this is based for example on evaluation of the activation times for the valve arrangements  31  to  34  and the pump  35 . 
     FIG. 2  shows a first exemplary embodiment of the process according to the invention upon actuation of the parking brake system with reference to a force/time graph. At the time T 1 , the activation of the parking brake system FBA is requested so that this begins to build up a brake force electromechanically, which is limited to a value F_FBA,IST as illustrated in the dot-and-dash curve. In the interval between the times T 1  and T 2 , the electronic control unit ECU evaluates the current operating condition (load, gradient of the road, etc.) of the vehicle to determine the brake force F_HALTE,SOLL, which is at least necessary for holding the vehicle stationary for this operating condition. Since the necessary brake force F_HALTE,SOLL is greater here than the brake force F_FBA,IST which can be generated by the parking brake system FBA itself, the additionally required brake force F_BBA,SOLL is generated hydraulically by automatically activating the service brake system BBA, as illustrated in the dotted curve. To keep the actuating time as brief as possible, the time T 2  can be selected shortly after the time T 1  and can be dependent, for example, on whether the parking brake system FBA has generated a predetermined brake force F_FBA,VOR which is smaller than or equal to the brake force F_FBA,IST (F_FBA,VOR&lt;=F_FBA,IST) which can be generated by the parking brake system FBA itself. The additional brake force component F_BBA,SOLL is variable and is substantially the result of the (absolute) difference between the necessary brake force F_HALTE,SOLL and the brake force F_FBA,IST which can be generated, which is preferably increased to be safe, for example by adding a value F_OFFSET. This consequently gives:
   F   —   BBA,SOLL=|F   —   HALTE,SOLL−F   —   FBA,IST|+F   —   OFFSET,    
or, by multiplication with a value F_FAKTOR, which is greater than one (F_FAKTOR&gt;1.0), this consequently gives
   F   —   BBA,SOLL=|F   —   HALTE,SOLL−F   —   FBA,IST|*F   —   FAKTOR.    
   The result of this is that the overall brake force generated on the rotary element  16  runs somewhat above the threshold for the necessary brake force F_HALTE,SOLL, as illustrated in the continuous curve. At the time T 3 , the activation of the parking brake system FBA is withdrawn which, owing to the gear unit  21 ,  22  being of a self-locking construction, has no influence on the overall brake force. The same applies when, at the time T 4 , i.e. after the parking brake system FBA has reached the brake forces which it is able to generate, the activation of the service brake system BBA is withdrawn. Here, the hydraulic fluid which was previously admitted into the hydraulic chamber  13  is discharged so that the wheel brake RB remains hydraulically set without force in the stationary or parking phase beginning at time T 5 . 
   In the second exemplary embodiment shown in  FIG. 3 , the additionally required brake force F_BBA,SOLL is hydraulically generated upon a request for activation of the parking brake system FBA at time T 1  by automatically activating the service brake system BBA, as illustrated in the dotted curve. Then, at time T 2 , the brake force F_BBA,IST is built up electromechanically by the parking brake system FBA, as illustrated in the dot-and-dash curve, to produce the overall brake force on the rotary element  16  (as illustrated in the continuous curve), which is somewhat above the threshold for the necessary brake force F_HALTE,SOLL (as shown in  FIG. 2 ). As shown in  FIG. 2 , the activation of the parking brake system FBA is also withdrawn here at the time T 3  and the activation of the service brake system BBA is withdrawn at the time T 4 , without this effecting the curve for the overall brake force for the reasons mentioned above. 
   The third exemplary embodiment shown in  FIG. 4  shows the case when, before the request for activation of the parking brake system FBA at the time T 1 , a brake force F_BBA,IST, is already hydraulically initiated by the service brake system BBA, be it because the driver has actuated the brake pedal  39  and/or because an automatic brake function, e.g. “hill hold” or “auto hold” is executed. Here, the brake force F_BBA,IST which is generated in any case by the service brake system BBA is maintained as an additionally required brake force F_BBA,SOLL from the time T 1 . Otherwise, the process is effected analogously to  FIG. 3 . 
   By contrast with  FIG. 4 , the fourth exemplary embodiment according to  FIG. 5  shows the case when the brake force F_BBA,IST generated in any case by the service brake system BBA is not sufficient as an additionally required brake force. Therefore, by automatically activating the service brake system BBA, as illustrated in the dotted curve, there is an increase in the hydraulic brake force component to F_BBA,SOLL at the time T 2   a . Otherwise the process is effected analogously to  FIG. 3  or  4 . 
   With reference to a force/time graph,  FIG. 6  shows an exemplary embodiment of the process according to the invention when the parking brake system is released. It is assumed here that the vehicle is in the stationary or parked phase, which begins according to  FIGS. 2 to 5  from the time T 5 . At the time T 7 , if there is a request to withdraw the parking brake system FBA, the electromechanical brake force component is withdrawn, as illustrated in the dot-and-dash curve, as a result of which the overall brake force acting on the rotary element  16  decreases as illustrated in the continuous curve until the wheel brake RB is again set such that it is completely, i.e. electromechanically and hydraulically, without force from the time T 9 . If, to withdraw the electromechanical brake force component, it is necessary to overcome self-locking forces, then it can be optionally provided for a predetermined brake force F_BBA,VOR to be generated at the time T 6  by the service brake system BBA before the parking brake system begins to withdraw the brake forces maintained by it, as illustrated in the dot-and-dash curve. The predetermined brake force F_BBA,VOR is then preferably withdrawn at the time T 8 , i.e. before the overall force acting on the rotary element  16  has dropped completely. It also applies for the predetermined brake force F_BBA,VOR that, if at all necessary, it can be variably adjusted according to the current operating condition (load, gradient of the road, etc.). 
   In accordance with the provisions of the patent statutes, the principle and mode of operation of this invention have been explained and illustrated in its preferred embodiment. However, it must be understood that this invention may be practiced otherwise than as specifically explained and illustrated without departing from its spirit or scope.