Abstract:
A method for actuating an electromechanical parking brake device for a brake that can be actuated by an electromechanical actuator comprised of an electric motor and of a reduction gear connected downstream of the electric motor and provided for converting a rotational motion into a translatory motion. In order to guarantee that the electromechanical parking brake device works reliably in all operating states without using a tension force sensor, the method determines and stores, during the actuation of a parking brake device, a mean value of the torque of the electric motor necessary for generating the application force of the brake corresponding to the parking brake actuation and simultaneously determines the actuator position and actuates the electric motor at later points of time in such a way that it generates this torque multiplied by a correction factor kη=&gt;1 so that the exerted tension force is maintained or increased.

Description:
TECHNICAL FIELD 
     The present invention generally relates to a method for actuating electromechanical parking brake devices and more particularly relates to a method for actuating an electromechanical parking brake device for a brake that can be actuated by means of an electromechanical actuator. 
     BACKGROUND OF THE INVENTION 
     An electromechanically operable brake of this type is disclosed in international patent application WO 99/45292. The electromechanical parking brake device of the referenced brake consists of a detent pawl that is operable by means of an electromagnet and can be put into engagement with a gear rim fastened at the rotor of the electric motor. However, said publication does not give any hints with regard to the actuation of the parking brake device. The prior art brake is not provided with a sensor for detecting the tension force so that said force has to be estimated on the basis of other data. 
     BRIEF SUMMARY OF THE INVENTION 
     Therefore it is an object of the present invention to disclose a method for actuating an electromechanical parking brake device guaranteeing a reliable parking brake function in all operating states without using a tension force sensor. 
     This object is achieved according to the invention in that during the activation of the parking brake device a mean value of the torque of the electric motor, which is required for exerting the application force of the brake corresponding to the parking brake operation, is determined and stored while the actuator position is simultaneously detected, and the electric motor is actuated at later points of time in such a fashion that it generates said torque which is multiplied by a correction factor kη=&gt;1 in order to maintain or increase the exerted tension force. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  represents an embodiment of a brake, which can be actuated electromechanically and is provided with an electromechanical parking brake device, and which allows implementing the method according to the invention can be applied, in axial cross-section, 
         FIG. 2  represents the embodiment of the parking brake device used with the brake according to  FIG. 1 , 
         FIG. 3  represents the parking brake device according to  FIG. 2  in its initial position in broken-up representation, 
         FIG. 4  shows the parking brake device according to  FIG. 2  in actuated position in broken-up representation, and 
         FIG. 5  is a flow chart showing the method according to the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The brake that can be actuated in an electromechanical manner, is represented in particular in  FIG. 1 , where the method according to the invention is applicable and which is designed as a floating-caliper disk brake, is principally comprised of an electromechanical actuator and a brake caliper which is indicated only schematically and is slideably arranged on a fixed support (not shown). A pair of friction linings  4  and  5  is arranged in the brake caliper in such a way that they are facing the left and the right side of a brake disc  6 . 
     In the following, the friction lining  4  shown at the right in the drawing is indicated as first friction lining and the other friction lining indicated with reference numeral S is defined as second friction lining. While the actuator can bring the first friction lining  4  into direct engagement with brake disc  6  by means of an actuating element  7 , through the actuator, the second friction lining  5  is pressed against the opposite lateral face of the brake disc  6  by the effect of a reaction force of the brake caliper caused by the actuation of the assembly. The actuator, which is mounted on the brake caliper by way of fastening means (not shown), has a modular design and consists in principle of three subassemblies or modules, respectively, which can be handled independently of each other, i.e. a drive unit  1 , a reduction gear  2  actuating the first friction lining  4  and a second reduction gear  3  inserted, in terms of effect, between the drive unit  1  and the first reduction gear  2 . 
     The above-mentioned drive unit  1  consists of an electric motor  10  provided as a motor which in the example is excited by a permanent magnet and electronically commutated, the stator  9  of which is arranged in an immovable manner in a motor housing  8  and the rotor  11  of which is formed by a ring-type support  13  supporting several segments of permanent magnet  14 . The first reduction gear  2  is inserted, in terms of effect, between the electric motor  10  and said actuating element  7 , the reduction gear being designed as a ball screw  16  to  21  supported in a gear housing  15  which can also be designed in one part with the above-mentioned brake caliper. The ball screw consists herein of a threaded nut  16  and a threaded spindle  17 , several balls  18  being arranged between the threaded nut  16  and the threaded spindle  17  and revolving during the rotational motion of the threaded spindle  17 , thus causing the threaded nut  16  to execute an axial or a translatory motion, respectively. The threaded nut  16  preferably forms said actuating element  7 . The threaded spindle  17  driven by the electric motor  10  by means of the second reduction gear  3 , is formed preferably of three parts consisting of a tubular first spindle part  19  which is in engagement with the threaded nut  16  by means of said balls  18 , a ring-type second spindle part  20  as well as a third spindle part  21 . 
     The arrangement is preferably made in such a way that the rotor  11  of motor  10  drives the third spindle part  21  by inserting the second reduction gear  3  while the threaded nut  16  is supported on the first friction lining  4 . 
     In the embodiment described in the drawing, a reduction of the required motor torque is achieved by means of a suitable integration of a planet gear  30 - 34  forming the second reduction gear  3  mentioned above. The planet gear arranged, in terms of effect, between the rotor  11  and the threaded spindle  17 , consists of a sun wheel  30  preferably formed by an externally toothed area  22  on the rotor  11 , several stepped planet wheels two of which are represented and indicated with reference numerals  31  and  32 , as well as an internally toothed ring  33 . The stepped planet wheels  31 ,  32  mounted in a planet cage  34  are provided with a first stage cooperating with the sun wheel  30  as well as a second stage cooperating with the ring gear  33 , the first stage being formed by toothed wheels  31   a ,  32   a  of a larger diameter and the second stage being formed by toothed wheels  31   b ,  32   b  of a smaller diameter. Said planet cage  34  is preferably designed in such a way that the area between the supporting points of the planet wheels  31 ,  32  and the connecting point of the threaded spindle  17  allows a small axial as well as a radial play and a small offset angle and is formed, e.g. as a lamellar disc or a bellow. An internally toothed area of a cover  23  forming the housing of the planet gear forms the internally toothed ring  33 . 
     The above-mentioned threaded nut  16  of the ball screw is guided or supported in a bowl-type guide member  12 . The threaded nut  16  is supported in the guide member  12  in the area facing the first friction lining  4  by means of a first sliding ring  28  arranged in the guide member  12  as well as in the final area remote from the friction lining  4  by means of a second friction ring  29  arranged on the threaded nut  16 . 
     Furthermore, it can be taken from  FIG. 1  that the second ring-type spindle element  20  is supported on a thrust bearing  26  arranged within the guide member  12  while the third spindle element  21  is connected with the planet cage  34  of the second reduction gear  3  by means of a form-locking plug-in connection. To this effect, the end of the third spindle element  21  is e.g. formed as component of a torx connection or as hexagon, which is pushed into a suitable opening in the planet cage  34 . In this case it is particularly favorable if the form-locking plug-in connection is coupled in a torsion-proof, radially yielding and flexible manner to the planet cage  34 . The coupling is achieved by means of an external ring  51  of a radial bearing  50  provided in the cover  23 . An elastic seal or sealing collar  27  clamped between the threaded nut  16  and the guide member  12  prevents the ingress of dirt into the interior of the ball screw. 
     Furthermore, for a perfect function of the actuation unit according to the invention it is useful that the threaded nut  16  is provided with an axial projection (not shown) on its end remote from the friction lining  4  cooperating with a stop portion formed at the circumference of the second spindle element  20  during the resetting process. By supporting a lateral surface of the projection on the stop portion, a further reset of the threaded nut  16  is reliably prevented so that the two parts  16 ,  20  do not get jammed. 
     In order to detect the actual position of the rotor  11 , a position detection system  46  is provided which is not shown in detail. The position information is then determined by means of a Hall sensor or a magnetoresistive element. 
     In order to be able to realize the function of a parking brake, the actuation unit according to the present invention is provided with electromechanical means (see  FIG. 2 ) allowing in cooperation with the rotor  11  of the electric motor  10  the locking of the latter. In the embodiment shown the electromechanical means is formed by means of an electromechanically actuated freewheel designated by reference numeral  35 , cooperating with a radial bearing  24  in which the rotor  11  is supported. The electrical actuator associated with the freewheel  35  has the form of a mechanical flip-flop, the state of which changes with each short energization. 
     As can be taken in particular from  FIG. 2 to 4 , essential parts of the freewheel  35  are integrated in said radial bearing  24 . To this end, the outer ring  36  as well as the inner ring  37  of the radial bearing  24  are extended on one side in such a way that they delimit a ring area accommodating a clamping member  38 , thus guaranteeing a form-locking connection between the bearing rings  36 ,  37  and the clamping member  38  by the particular configuration of the extended area of the bearing rings  36 ,  37 . The outer ring  36  is preferably provided with a radial recess  39  in its area cooperating with the clamping member  38 , the recess being limited on one side by an inclination or ramp  40 , while the inner ring  37  is provided with a contour  41  corresponding to the contour of the clamping member  38  and limiting a clamping gap together with the recess  39 . The clamping member  38 , which can be designed as a clamping roll or in the form of a ball, is pretensioned by means of a ring-type spring member  42  towards said recess  39 . 
     An electromagnetic actuation unit is used for actuating the freewheel  35 , being designated by the reference numeral  43  in the example illustrated. The actuation unit  43  essentially consists of a bistable electromagnet  44  as well as a tappet  45  cooperating with the armature of the electromagnet  44  displacing the clamping member  38  radially when activating the electromagnet  44 . The tappet  45  is guided in a tubular guide member  47  formed on a ring-type accommodating member  48  arranged in the motor housing  8  and accommodating the bearing outer ring  36 . 
     When activating the parking brake device  35 , the following functional order is provided and described in detail with regard to  FIG. 5 : 
     First the electromechanical brake is applied by corresponding operation of the electric motor  10  until the necessary application force level F park  is reached. The brake is assumed to be in “warm” condition so that the activation of the parking brake device is realized according to the characteristic F-f(φ). The application force F park  to be adjusted is achieved with an actuator position or the rotor position defined with φ park . In point A defined by the coordinates φ park , F park  the parking brake device  35  is locked. At the same time the mean value M park  of the torque produced by the electric motor  10  is determined by measuring the current supplied to the electric motor  10  corresponding to the efficiency=1 of the assembly. The mean value is preferably examined with regard to a lower limit value. If the mean value of the torque is below said limit value, the torque (and thus also the application force) is increased up to this limit value. If the mean value of the torque is above said limit value, the application force adjusted by the control is maintained. The mean value of the torque applied to the brake is stored in a non-volatile memory (EEPROM). Then the mean torque value M park  is multiplied by a correction factor kη&gt;1 in an electronic control unit (not shown), thus defining a higher torque value M 1park . Under the assumption that in this case the ascending branch of the characteristic hysteresis curve depending on the efficiency has to be considered or is used, a higher application force F 1  results from the higher torque value M 1park  which corresponds to a locking point A 1  on the characteristic curve F=f(φ). A changed actuator position φ 1park  corresponds to the additional operation of the actuator according to the mentioned application force increase. 
     It has to be mentioned that the rotor  11  or the bearing inner ring  37 , respectively, is displaced against the clamping direction of the freewheel  35 , i.e. to the left in the drawing, when the brake is applied. When the clamping member  38  is displaced towards the countour  41  by activating the electromagnet  44 , when the parking brake is actuated, the clamping member rolls towards the tapering clamping gap on the above-mentioned ramp  40 . If the current supplied to the electric motor  10  is reduced, the spring force of the applied brake tries to turn the rotor  11  or the bearing inner ring  37  towards the clamping direction. Thus, the parking brake device is securely locked. The locked position of the parking brake device is represented in  FIG. 4 . 
     If the parking brake device is released after a short period of time (while a brake re-application has not yet taken place) the electric motor  10  has to continue for a defined period of time to apply the brake by defining a torque value M rel =k rel *k nη *M park  exceeding considerably the torque M park  defined before and the electromagnet  44  has to be actuated once in order again to move the tappet  45  upwards. The clamping member  38  relieved hereby is pressed into the recess  39  of the bearing outer ring  36  by the force of the spring element  42  by which it is pretensioned and the rotor  11  can freely turn in both directions. A release of the parking brake device can alternatively also be achieved by that the electric motor is operated for a predefined period of time in such a way that it generates its maximum torque M max . 
     The procedure is similar in the case in which the preset application force is reduced to a value denominated F 2park  due to cooling down of the brake with unchanged actuator position φ park , with the parking brake device being locked in point A. Equivalent to this value is an operating state represented by a point A 2  on a characteristic curve F T =f(φ) valid for the cooled condition of the brake and caused by a temperature-dependent displacement of the above-mentioned characteristic curve. The repeated actuation of the electric motor  10  necessary for increasing the required application force to said value F 1 , is represented by the coordinate φ′ park . 
     Within the scope of the present invention, of course several modifications of the method claimed are also possible. Hence, the method can be repeated in previously defined periods of time after the first activation of the parking brake device, if necessary, depending on a temperature difference of the actuator. Here, the actuator temperature difference corresponds preferably to the difference between the actuator temperature during the first activation of the parking brake device or during the last re-application of the brake and the actual actuator temperature estimated e.g. by means of an actuator temperature model. Said correction factor kη depends on the actuator efficiency or on a measured or estimated inclined position of the vehicle, respectively. Besides, it is possible that during the release operation of the parking brake after a further necessary application of the brake a new characteristic curve for actuator position and application force of the actuator is estimated.