Abstract:
In a method for setting the clamping force exerted by a parking brake, which is generated at least partially by an electromechanical braking device and, if needed, in supplementary fashion by a hydraulic braking device, in the provision of the hydraulic clamping force, a boost pressure to be generated in the hydraulic braking device is increased with respect to a switch-off pressure at the switch-off time that corresponds to the hydraulic clamping force.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a method for setting the clamping force exerted by a parking brake in a vehicle. 
     2. Description of the Related Art 
     Published German patent document DE 103 61 042 B3 describes an electromechanical parking brake having an electric brake motor as actuator, which upon activation adjusts a brake piston, which bears a brake lining, axially in the direction of a brake disk. The magnitude of the electromechanical brake force may be adjusted via the supply of current to the brake motor. 
     It is furthermore known to couple electromechanical parking brakes with a hydraulic vehicle brake, in that the brake piston of the parking brake additionally has hydraulic pressure applied to it such that the total clamping force to be set is composed of an electromechanical portion and a hydraulic portion. The hydraulic portion of the clamping force is provided as needed and in supplementary fashion to the electromechanical portion. For a precise setting of the clamping force it is necessary to know the hydraulic clamping force boost as precisely as possible. 
     BRIEF SUMMARY OF THE INVENTION 
     The present invention is based on the objective of providing, by simple measures, the clamping force in a parking brake having an electromechanical braking device with additional hydraulic boost at sufficient accuracy. 
     The method according to the present invention relates to an electromechanical parking brake in a vehicle, which has an electric actuator, via which a clamping force is able to be generated for arresting the vehicle at standstill. The electromechanical actuator is preferably an electric brake motor, the rotational motion of which is translated into an axial actuating motion of a brake piston which bears a brake lining and is pressed against a brake disk. Fundamentally, however, other electric actuators could also be considered as well, for example electromagnetic actuators. 
     The clamping force is fundamentally set at least in part by the electromechanical actuator. As required and in supplementary fashion, a portion of the clamping force may also be generated by a hydraulic braking device, the hydraulic braking pressure of which preferably also acts on the brake piston and thereby boosts the electromechanical clamping force. If the hydraulic braking device becomes active, then the total clamping force is composed of an electromechanical portion and a hydraulic portion. 
     The hydraulic braking device is preferably the regular hydraulic vehicle brake. 
     In a simultaneous activation of the electrical actuator for generating an electromechanical clamping force and of the hydraulic braking device for generating a hydraulic clamping force, these brake systems come to influence each other reciprocally. The forward motion of the brake piston enlarges the space for the hydraulic volume, which is associated with a drop in hydraulic pressure. According to the present invention, in a supplementary hydraulic clamping force boost, a boost pressure to be produced in the hydraulic braking device is generated, which is increased by an application-specific pressure value with respect to the pressure at the time of switch-off. At the switch-off time, the electrical actuator is switched off, thereby freezing the total clamping force generated, a control element of the actuator being locked or persisting in its current position. 
     The increase of the boost pressure to be provided by the hydraulic braking device by the application-specific pressure value compensates for the pressure drop that occurs during the blocking motion of the brake piston in the direction of the brake disk due to the volume enlargement. The pressure value by which the boost pressure is increased takes on a constant value, independent of the absolute pressure level in the hydraulic braking device. This makes it possible to ascertain in advance the pressure value for the respective parking brake by which the hydraulic pressure typically decreases in the forward motion of the brake piston. Since the pressure drop for the respective parking brake is always constant, the corresponding pressure value may be taken into account independently of the currently prevailing pressure level by addition to the switch-off pressure, which provides the hydraulic clamping force portion. For example, if a specific hydraulic clamping force portion of the total clamping force is requested, the hydraulic clamping force portion being determined from the hydraulic switch-off pressure, then the pressure value may be added to the known switch-off pressure in order to obtain the boost pressure that must be generated by the hydraulic braking device. 
     One expedient variant provides for a hydraulic initial pressure applied by the driver by operating the brake pedal to be also subtracted from the switch-off pressure to be provided. In order to attain the desired switch-off pressure, it is only necessary to generate a boost pressure that is reduced by the value of the initial pressure. 
     The hydraulic boost pressure may either be provided or generated already prior to the generation of the electromechanical clamping force or only after activating the electromechanical braking device. If the boost pressure is already set prior to activating the electromechanical braking device, then overall a greater time period is available for the pressure buildup, which is associated with a lower hydraulic pump performance and lower noise generation. If, on the other hand, the hydraulic boost pressure is generated only after activating the electromechanical braking device, then indeed only a shorter time period is available for generating the pressure, which is associated with a higher pump performance and a greater noise generation. However, the pressure buildup may be scheduled in an idling or free travel phase of the electromechanical braking device, whereby the overall time period for providing the total clamping force is reduced. 
     It may be expedient to take pressure tolerances into account when providing the hydraulic boost pressure, which stem from model or measurement inaccuracies for example. The boost pressure is increased by the value of the pressure tolerance in order to ensure that the required switch-off pressure is attained. 
     The method according to the present invention runs in a closed-loop or open-loop control unit in the vehicle, which may be a component of the parking brake system. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a section through an electromechanical parking brake for a vehicle, in which the clamping force is generated via an electric brake motor. 
         FIG. 2  shows a diagram showing a time-dependent curve of the current, the voltage and the motor speed during the process of applying the parking brake as well as a hydraulic pressure from a hydraulic braking device, the hydraulic pressure being provided prior to the generation of the electromechanical clamping force. 
         FIG. 3  shows a diagram corresponding to  FIG. 2 , the hydraulic pressure only being generated in a free travel phase of the electromechanical braking device during the blocking process. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  shows an electromechanical parking brake  1  for arresting a vehicle at a standstill. Parking brake  1  includes a brake caliper body  2  having a caliper  9  that reaches over a brake disk  10 . As a controlling element, parking brake  1  has an electric motor as brake motor  3 , which rotates a spindle  4 , on which a spindle component  5  is rotationally mounted. In a rotation of spindle  4 , spindle component  5  is axially adjusted. Spindle component  5  moves within a brake piston  6 , which bears a brake lining  7 , which is pressed by brake piston  6  against brake disk  10 . On the opposite side of brake disk  10 , there is another brake lining  8 , which is held in a stationary manner on caliper  9 . 
     Within brake piston  6 , spindle component  5  is able to move axially forward in the direction of brake disk  10  when spindle  4  is rotated or is able to move axially backward until reaching a stop  11  when spindle  4  is rotated in the opposite direction. In order to generate a clamping force, spindle component  5  applies force on the inner front side of brake piston  6 , whereby brake piston  6  supported in parking brake  1  in axially displaceable fashion is pressed with brake lining  7  against the facing side of brake disk  10 . 
     If necessary, the parking brake may be boosted by a hydraulic vehicle brake such that the clamping force is composed of an electromotive portion and a hydraulic portion. In the hydraulic boost, the backside of brake piston  6  facing the brake motor has pressurized hydraulic fluid applied to it. 
       FIGS. 2 and 3  respectively show a diagram of current curve  1 , voltage U and rotational speed curve n of the electric brake motor as a function of time for a brake application process. The brake application process begins at time t 1  in that an electrical voltage is applied and the brake motor is supplied with current in a closed electric circuit. At time t 2 , voltage U and motor speed n have reached their maximum. The phase between t 2  and t 3  represents the idling phase, in which current  1  is at a minimum level. This is followed, beginning at time t 3  and ending at time t 4 , by the force buildup phase, in which the brake linings contact the brake disk and are pressed with increasing clamping force F against the brake disk. At time t 4 , the electrical brake motor is switched off by opening the electric circuit such that in the further course the speed n of the brake motor drops to zero. 
     The point of rise of the force coincides with the phase of the force buildup at time t 3 . The force buildup or the curve of clamping force F may be ascertained, for example, on the basis of the curve of current  1  of the brake motor, which fundamentally has the same curve as the electromechanical clamping force. Starting from the low level during the idling phase between t 2  and t 3 , the current curve rises sharply at the beginning of time t 3 . This rise in the current may be detected and used to determine the point of rise of the force. Fundamentally, however, the curve of the force buildup may also be determined from the voltage or rotational speed curve or from any combination of the signals of current, voltage and rotational speed. 
       FIGS. 2 and 3  additionally shows the curve for a boost pressure p U , which is generated by the hydraulic braking device in order to generate, in addition to the electromechanical clamping force, a hydraulic clamping force, which is added to the electromechanical clamping force to form the total clamping force. A hydraulic pressure is set via the vehicle brake, which acts on the backside of the brake piston and boosts the electromechanically provided clamping force. The boost pressure p U , which must be generated by the hydraulic braking device in order to attain a specific requested hydraulic clamping force, is accordingly
 
 p   U   =p   t,off   +Δp+p   tol   −p   initial  
 
additively composed of a switch-off pressure p t,off , a pressure value Δp and a tolerance pressure p tol . In addition, the initial pressure p initial  in the hydraulic system is taken into account, which is applied by the driver via a brake pedal operation at the time of the blocking of the electric brake motor; the initial pressure p initial  is subtracted since via the boost pressure only the difference between the initial pressure and the absolute pressure level to be attained must be generated. The switch-off pressure p t,off  is the pressure acting on the brake piston at the time when the electric brake motor is switched off, which corresponds directly to the hydraulic clamping force. Δp represents a constant pressure value, which characterizes the pressure drop occurring in the hydraulic system due to the forward motion of the brake piston during the blocking process and the associated volume enlargement. Pressure drop Δp is always constant for the respective vehicle brake independently of the pressure level, and lies for example in a value range between 10 bar and 20 bar. Model and measurement inaccuracies may be taken into account via the pressure tolerance p tol .
 
     The curve of the boost pressure p U  shown in  FIGS. 2 and 3  also corresponds to switch-off pressure p t,off . As may be gathered from  FIG. 2 , in a first variant of an embodiment the boost pressure p U  is set already prior to activating the electric brake motor. For this purpose, boost pressure p U  rises already at a time t 0 , which is before time t 1 , at which the application process starts in the electromechanical braking system. Boost pressure p U  is reached already at time t 1  and is maintained until time t 3 , at which the force buildup occurs in the electromechanical braking system. Boost pressure p U  drops until time t 4 —the switch-off time—, which is due to the volume enlargement resulting from the advancing motion of the brake piston. The pressure drop in the curve of p U  between time t 3  and time t 4  corresponds to the pressure value Δp. 
     In the variant of an embodiment shown in  FIG. 3 , boost pressure p U  in the hydraulic braking device is generated only during the idling phase between time t 2  and time t 3 . The pressure buildup begins after time t 2 , and ends prior to the termination of the idling phase at time t 3  once the full boost pressure p U  is attained. In the force buildup phase between time t 3  and time t 4 , as in  FIG. 2 , the pressure level of boost pressure p U  drops again due to the volume enlargement. Switch-off pressure P t,off  is reached at time t 4 .