Abstract:
In order to increase the operational safety of a device which is used in a brake system of the “brake-by-wire” type and has the purpose of activating and deactivating a pedal travel simulator, an articulated connection which permits an all around pivoting movement of a piston rod is provided between a piston and the piston rod which activates the latter.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is the U.S. national phase application of PCT International Application No. PCT/EP2007/055046, filed May 24, 2007, which claims priority to German Patent Application No. DE 10 2007 010 513.6, filed Mar. 5, 2007, the contents of such applications being incorporated by reference herein. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The invention relates to an activation unit for a motor vehicle brake system of the “brake-by-wire” type, which has:
       a) a brake booster which can be activated either by means of an input element which can be brought into a force-transmitting connection with a brake pedal or as a function of a driver&#39;s request or independently of the driver&#39;s will by means of an electronic control unit, with means which permit decoupling of a force-transmitting connection between the brake pedal and brake booster in the “brake-by-wire” operating mode being provided between the brake pedal and the brake booster,   b) a master brake cylinder which is connected downstream of the brake booster,   c) means for sensing a driver&#39;s request or the activation travel of the brake pedal,   d) a pedal travel simulator which interacts with the brake pedal and by means of which a restoring force which acts on the brake pedal in the “brake-by-wire” operating mode can be simulated independently of activation of the brake booster, and   e) a hydraulic activation and deactivation device which activates the pedal travel simulator in the “brake-by-wire” operating mode and deactivates it outside the “brake-by-wire” operating mode, and which is formed by a cylinder-piston arrangement whose pressure space is connected by means of a disconnectable connection to a pressure medium volume accommodating element and on whose piston a simulator housing is supported.       
 
         [0009]    2. Description of the Related Art 
         [0010]    Such an activation unit is known, for example, from the applicant&#39;s international patent application WO 2006/084864 A1. The support of the abovementioned simulator housing on the activation and deactivation device is provided in the abovementioned activation unit by means of a piston rod via which the movement of the brake pedal or of the simulator housing is transmitted to the piston outside the “brake-by-wire” operating mode. The piston rod is rigidly connected to the piston and guided by it. The piston has, at its end facing away from the pressure space, an annular face which, in the unactivated position of the arrangement, bears against a metallic stop ring. However, it is felt to be disadvantageous that when the brake pedal is released suddenly high acceleration values occur at the stop, and said high acceleration values may cause undesired play between the piston rod and the piston of the cylinder-piston arrangement. 
       SUMMARY OF THE INVENTION 
       [0011]    An object of the present invention is therefore to propose suitable measures which permit effective disconnection of the piston rod from the piston while maintaining satisfactory functioning. 
         [0012]    This object is achieved according to aspects of the invention in that a connection whose degree of freedom=3 is provided between the piston and the piston rod. 
         [0013]    In order to concretize the inventive idea, in one advantageous development of the subject matter of the invention the connection between the piston and the piston rod is embodied as an articulated connection in which the piston rod has a spherical articulation head, and the piston is provided with a correspondingly formed head receptacle. 
         [0014]    In the description below, the activation unit according to aspects of the invention will be explained in more detail with reference to the appended drawing, in which: 
     
    
     
       BRIEF DESCRIPTION OF THE PREFERRED EMBODIMENTS 
         [0015]      FIG. 1  shows an inventive activation unit of a motor vehicle brake system of the “brake-by-wire” type in a three-dimensional illustration; and 
           [0016]      FIG. 2  shows a sectional illustration of the activation and deactivation device which is used in the activation unit shown in  FIG. 1  and is assigned to the pedal travel simulator. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0017]    The activation unit illustrated in  FIG. 1  is composed of a brake booster, preferably an underpressure brake booster  1 , a master brake cylinder, preferably a tandem master cylinder  2 , which is connected downstream of the brake booster  1  and to whose pressure spaces (not illustrated) wheel brakes of the motor vehicle are connected, with the intermediate connection of a hydraulic open-loop and closed-control unit (not illustrated either), and a pressure medium reservoir vessel  3  which is assigned to the master brake cylinder  2 . A brake pedal  4  is used by the driver to activate the brake booster  1 , with a pedal travel simulator  5  (only indicated), which interacts with the brake pedal  4 , in particular in the “brake-by-wire” operating mode and which conveys the usual brake pedal sensation to the driver, being provided. A driver&#39;s deceleration request or the activation travel of the brake pedal  4  is sensed by means of at least one sensor device whose signals are fed to an electronic control unit (not shown). The output signals of the electronic control unit can be used to activate, inter alia, an electromagnet which is assigned to the brake booster  1  and which permits a pneumatic control valve to be activated independently of the driver&#39;s will, said control valve controlling the supply of air to the brake booster  1 . The control valve is activated by the driver by means of an input element  7  which is connected to the brake pedal  4  in a force-transmitting fashion. The coupling of the brake pedal  4  to the input element  7  is preferably embodied in such a way that decoupling of the force-transmitting connection between the brake pedal  4  and the input element  7  is ensured in the “brake-by-wire” operating mode. A travel sensor  9  is used to sense the travel of a movable wall, which applies the boosting force of the brake booster  1 , or the travel of an output element of the brake booster  1  which transmits its output force to a first piston (not illustrated) of the master brake cylinder  2 . 
         [0018]    The pedal travel simulator  5 , which is located outside the force flux between the brake pedal  4  and the brake booster  1  in the exemplary embodiment shown and by which, as already mentioned, a resetting force which acts on the brake pedal  4  can be simulated independently of activation of the brake booster  1  in the “brake-by-wire” operating mode, is embodied in such a way that an activation and deactivation device  10  (only indicated schematically in  FIG. 1 ) can be used to activate said pedal travel simulator  5  when the force-transmitting connection between the brake pedal  4  and the brake booster  1  is decoupled in the “brake-by-wire” operating mode, and to deactivate said pedal travel simulator  5  outside the “brake-by-wire” operating mode. A simulator housing  9  is supported on the activation and deactivation device  10  via a piston rod  21 . 
         [0019]    As is apparent, in particular, from  FIG. 2 , the activation and deactivation device  10  is formed essentially by a piston-cylinder arrangement  20  and a hydraulic pressure medium accommodating element  40 , which are arranged in a housing  41 . The housing  41  is supported on a pedal block (not illustrated). The piston  22  of the piston-cylinder arrangement  20 , which is connected in a force-transmitting fashion to the simulator housing  9  by means of the activation rod  21  mentioned in conjunction with  FIG. 1 , bounds a pressure space  23  which is connected, by means of a hydraulic connection  24  illustrated by dashed lines, to the pressure medium accommodating element  40  which, in the example shown, is embodied as a low pressure accumulator with a piston  42  which is prestressed by means of a spring  43 . The low pressure accumulator  40  is preferably configured here in such a way that it has a pressure medium reserve volume  44  for compensating changes in temperature or leaks. The two pistons  22 ,  42  are preferably of identical design here in terms of geometry and material despite their differing functions. A check valve  34 , which can be activated electromagnetically and which permits the abovementioned connection  24  to be shut off, is inserted into the hydraulic connection  24 . The hydraulic pressure in the pressure space  23  of the piston-cylinder arrangement  20  can be determined by means of a pressure sensor  35 . 
         [0020]    As is also apparent in  FIG. 2 , the piston rod  21  is embodied in two parts and is composed of a first piston rod part  211 , which is assigned to the piston  22  of the cylinder-piston arrangement  20 , and of a second piston rod part  212 , which is coupled to the previously mentioned simulator housing  9 . The piston rod parts  211 ,  212  have, at their ends lying opposite one another, sections  211   a,    212   a  with a relatively large diameter, with an annular face, turned away from the first section  211   a,  of the second section  212   a  bearing against a damping element  33 . The damping element  33 , which is composed of suitable elastic material in the illustrated embodiment by means of an O ring, is accommodated by a pot shaped securing part  32  which is attached to the first piston rod part  211  or its section  211   a  with a relatively large diameter by means of a positively locking connection. This connection can be implemented, for example, by shearing the securing part  32 . 
         [0021]    In order to permit an all round pivoting movement of the piston rod  21 , which movement is necessary for the activation process and for compensating tolerances, the piston rod part  211  has, at its end turned toward the piston  22 , a spherical articulation head  26  which interacts with a head receptacle  27 , correspondingly formed on the piston  22 , or forms an articulated connection with said head receptacle  27 . The articulation head  26  is adjoined in the axial direction by a ball segment shaped section  28 , with the articulation head  26  and the section  28  having a common center point. In the unactivated position of the cylinder-piston arrangement  20 , the ball segment shaped section  28  bears against a correspondingly shaped stop element  29  with prestress of a spring  25 . The stop element  29 , which is secured in the housing of the cylinder-piston arrangement  20  by means of a wire closure  31 , accommodates a stripping element  30  which protects the running surface of the piston  22  against soiling. 
         [0022]    The function of the activation unit described is known from patent application WO 2006/084864 A1 described at the beginning, so that it is not necessary to describe it.