Abstract:
The invention relates to a wheel brake apparatus having an electric motor, a rotation/translation conversion mechanism that can be driven to rotate by the electric motor, a hydraulic working piston that can be moved by the rotation/translation conversion mechanism, and a brake lining piston that can be hydraulically moved by the working piston. In order to permit an emergency actuation of the wheel brake apparatus in the event of a leak in the hydraulics, the invention proposes embodying the wheel brake apparatus so that the brake lining piston can also be mechanically moved by the working piston. This can be achieved by virtue of the fact that the two pistons are disposed coaxially.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The invention relates to a wheel brake apparatus and a method for operating the wheel brake apparatus including an electric motor and a rotation/translation conversion mechanism.  
           [0003]    2. Description of the Prior Art  
           [0004]    A wheel brake apparatus of the type with which this invention is concerned is disclosed by DE 195 29 664 A1. The known wheel brake apparatus has an electric motor which can drive a rotation/translation conversion mechanism. The rotation/translation conversion mechanism of the known wheel brake apparatus is embodied as a helical gear. The rotation/translation conversion mechanism can move a hydraulically acting working piston in a working cylinder. The working cylinder communicates with a cylinder in which a brake lining piston is movably contained. The movement of the working piston in the working cylinder moves the brake lining piston in its cylinder and because the cylinder diameters are different, a hydraulic path reduction and force increase can be produced. The brake lining piston can press a frictional brake lining against a brake body non-rotatably connected to a vehicle wheel, for example a brake disk or brake drum, in order to produce a brake force or a brake moment. The known wheel brake apparatus combines an electromechanical drive mechanism with a hydraulic drive mechanism.  
         OBJECT AND SUMMARY OF THE INVENTION  
         [0005]    In the wheel brake apparatus according to the invention, the rotation/translation conversion mechanism can be brought into a mechanical connection with the brake lining piston and as a result, the brake lining piston can be moved. This has the advantage that a parking brake function of the wheel brake apparatus can be produced which is independent of the hydraulics. A brake force built up in the parking brake function remains unchanged for a long time since the brake force is built up in an exclusively mechanical fashion and as a result, leakage losses in the hydraulics are prevented from reducing the brake force. Another advantage of the wheel brake apparatus according to the invention is its ability to be actuated in an exclusively mechanical fashion, for example in the event of a leak in the hydraulics of the wheel brake apparatus. This permits a mechanical emergency braking operation in the event of a malfunction in the hydraulics. In comparison to an exclusively electromechanical wheel brake apparatus, the wheel brake apparatus according to the invention has the advantage that it can easily be embodied with two or more brake lining pistons and can thus be inexpensively designed, for example in the form of a fixed yoke brake device.  
           [0006]    According to one embodiment, a movable element of the rotation/translation conversion mechanism can be moved into contact with the brake lining piston and in this manner, the brake lining piston can be mechanically moved in order to press the frictional brake lining against the brake body. The movable element can be a spindle of a rotation/translation conversion mechanism embodied as a helical gear.  
           [0007]    Another possibility for mechanically moving the brake lining piston is to provide the working piston so that it can be moved into contact with the brake lining piston.  
           [0008]    In a preferred embodiment of the invention, the wheel brake apparatus has a valve that can be opened and closed, which is connected to the working cylinder and to the cylinder of the brake lining piston. When the valve is open, a hydraulic operational connection between the working piston and the brake lining piston is disengaged and the brake lining piston can be moved mechanically with the rotation/translation conversion mechanism. Moreover, opening the valve makes possible for the wheel brake apparatus to be released in the event of a malfunction in its electromechanical drive mechanism. The valve is preferably open in a normal position.  
           [0009]    According to one modification, the wheel brake apparatus of the invention has a pressure sensor for measuring the hydraulic pressure. The pressure sensor can be used, for example, to determine a brake force of the wheel brake apparatus since the brake force is at least approximately proportional to the hydraulic pressure.  
           [0010]    According to a further embodiment, the wheel brake apparatus according to the invention has a rotation angle sensor for the rotor of the electric motor or the rotation/translation conversion mechanism. The rotation angle sensor can measure a rotation angle of the rotor of the electric motor or of a rotating part of the rotation/translation conversion mechanism, in complete rotations and/or in fractions of a rotation. Since the rotation angle is proportional to a displacement path of the rotation/translation conversion mechanism, a displacement path of the rotation/translation conversion mechanism can therefore be determined.  
           [0011]    The correlation of the rotation angle measured with the rotation angle sensor and the hydraulic pressure measured with the pressure sensor permits the proper functioning of the wheel brake apparatus to be monitored. When the wheel brake apparatus is functioning properly, these two values maintain a particular proportion to each other in every operating state of the wheel brake apparatus. During operation of the wheel brake apparatus according to the invention, if the proportion of the two values to each other diverges significantly from their proportion during proper operation wheel brake apparatus, then this indicates a malfunction.  
           [0012]    Instead of the rotation angle sensor, the wheel brake apparatus can also have a path sensor for the displacement path of the rotation/translation conversion mechanism.  
           [0013]    According to another modification, the rotation/translation conversion mechanism of the wheel brake apparatus according to the invention is embodied as self locking-free so that when the electric motor is without current, a pressing force of the frictional brake lining against the brake body decreases to a negligible value. As a result, it is possible for the wheel brake apparatus to be released in the event of a failure of the power supply of the electric motor.  
           [0014]    For the parking brake function, in order to maintain a brake force without current once the brake force has been exerted, a brake is provided with which the rotor of the electric motor or the rotation/translation conversion mechanism can be locked in place. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWING  
       [0015]    The invention will be better understood and further objects and advantages thereof will become more apparent from the ensuing detailed description of preferred embodiments taken in conjunction with the drawing, which shows a partially simplified schematic representation of an axial section through a wheel brake apparatus according to the invention.  
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0016]    The wheel brake apparatus  10  according to the invention shown in the drawing has a brake lining piston  12 , which is movably contained in a cylinder  14 . The brake lining piston  12  can press a frictional brake lining, which is not shown and is in contact with the brake lining piston  12 , against a brake body, likewise not shown, for example a brake disk, in order to produce a brake moment or a brake force in an intrinsically known manner.  
         [0017]    In order to move the brake lining piston  12 , the wheel brake apparatus  10  has a working piston  16 , which is movably contained in a working cylinder  18 . The working piston  16  is disposed coaxial to the brake lining piston  12 ; it has a smaller diameter than the brake lining piston  12 . The working cylinder  18  and the cylinder  14  of the brake lining piston  12  are hydraulic cylinders which are operationally connected to each other. In the exemplary embodiment shown, the working cylinder  18  and the cylinder  14  of the brake lining piston  12  are of one piece with each other; the working cylinder  18  coaxially adjoins the cylinder  14  of the brake lining piston  12 . The working cylinder  18  is connected to a brake fluid storage tank  20  which is operationally connected to the working cylinder  18  and therefore also the cylinder  14  of the brake lining piston  12  by means of a solenoid valve  22 . The solenoid valve is embodied as a 2/2-way solenoid valve that is open in its currentless normal position. In addition, a pressure sensor  24  is connected to the working cylinder  18  and can measure a hydraulic pressure in the working cylinder  18  and the cylinder  14  of the brake lining piston  12 .  
         [0018]    The working piston  16  is a one-piece component of a spindle  26  of a helical gear  28 . The helical gear  28  constitutes a rotation/translation conversion mechanism, or screw link actuator. Instead of the helical gear  28 , another kind of rotation/translation conversion mechanism can also be used, for example a recirculating ball transmission or a planetary roller screw drive (not shown). For rotational securing, the spindle  26  has a slide key or spring  30 , which rests in an axially parallel groove  32  of the spindle  26  and in an axially parallel groove  34  in a housing  36  of the wheel brake apparatus  10 .  
         [0019]    In order to axially move the spindle  26  together with a working piston  16  that is of one piece with it, the helical gear  28  has a nut  38  which engages with the spindle  26 . The nut  38  is rotatably supported in the housing  36  by an angular roller bearing  40  and is supported axially against the housing  36  by this angular roller bearing  40 . The screw link actuator  28  is self locking-free, i.e. axial pressure in the spindle  26  can set the nut  38  into rotation and move the spindle  26  axially.  
         [0020]    The nut  38  of the helical gear  28  is of one piece with a gear  42 , which meshes with a smaller diameter gear  44 . The two gears  42 ,  44  constitute a toothed wheel-work. The smaller gear  44  is non-rotatably connected to a motor shaft  46  of an electric motor  48 . The electric motor  48  has an integrated rotation angle sensor, not visible in the drawing, for its motor shaft  46 . A magnetically actuatable brake is flange-mounted to the electric motor  48  and will be referred to below as a magnetic brake  50 . The magnetic brake  50  has a stable brake position in which it fixes the motor shaft  46  in place. In order to release the magnetic brake  50 , it is supplied with current. The magnetic brake  50  can be designed as bistable, i.e. it remains without current both in the braked position and in a released position and is supplied with current only for switching between the braked position and the released position and vice versa. Magnetic brakes of this kind are intrinsically known to the specialist in a number of embodiments, both in a monostable and bistable design, and therefore the construction of the magnetic brake  50  requires no further discussion at this point.  
         [0021]    The toothed wheel-work  42 ,  44  is a mechanical reduction gear; a moment transmission from the electric motor  48  to the nut  38  of the helical gear  28  occurs here. Because of the smaller diameter of the working piston  16 , a hydraulic reduction of the displacement path and an increase of the force of the working piston  16  against the brake lining piston  12  also take place.  
         [0022]    Function of the Wheel Brake Apparatus  
         [0023]    In order to actuate the wheel brake apparatus  10 , the magnetic brake  50  is released, the solenoid valve  22  is closed, and the electric motor  48  is supplied with current in a brake-applying direction. The toothed wheel-work  42 ,  44  sets the nut  38  of the helical gear  28  into rotation and moves the spindle  26 , together with the working piston  16  that is of one piece with it, toward the brake lining piston  12 . The working piston  16  displaces brake fluid from the working cylinder  18  into the cylinder  14  of the brake lining piston  12  and therefore moves the brake lining piston  12 . This produces a force transmission from the working piston  16  to the brake lining piston  12  in the proportion of their two diameters. In an intrinsically known manner, the brake lining piston  12  presses the frictional brake lining, not shown, against the brake body, not shown, and therefore exerts a brake moment or a brake force on the brake body.  
         [0024]    In order to reduce the brake force and to restore the wheel brake apparatus  10 , the electric motor  48  is supplied with current in a reverse rotation direction, as a result of which the toothed wheel-work  42 ,  44  and the helical gear  28  move the working piston  16  and therefore also the brake lining piston  12  back. The hydraulic pressure in the working cylinder  18  and in the cylinder  14  of the brake lining piston  12  is measured by the pressure sensor  24 . Since the hydraulic pressure is proportional to a force with which the brake lining piston  12  presses the frictional brake lining against the brake body, the pressing force of the frictional brake lining against the brake body and therefore the brake force of the wheel brake apparatus  10  can be determined from the hydraulic pressure measured by the pressure sensor and can be regulated in relation to a reference value.  
         [0025]    Parking Brake Function, Emergency Brake Function  
         [0026]    In order to be used as a parking brake, when the magnetic brake  50  is released, the electric motor  48  is supplied with current and the working piston  16  is moved in the above-described manner until it strikes against the brake lining piston  12 . The solenoid valve  22  thus remains open so that brake fluid displaced by the working piston  16  flows out into the brake fluid storage tank  20 . With the working piston  16  resting against the brake lining piston  12 , the brake lining piston  12  is moved further until the frictional brake lining, not shown, rests against the brake body. Through a continued supply of current to the electric motor  48 , the working piston  16  resting against the brake lining piston  12  presses the frictional brake lining against the brake body and thus produces a brake force. The magnetic brake  50  is brought into its braking position and locks the motor shaft  46  in place. The supply of current to the electric motor  48  can be stopped; the magnetic brake  50  uses the toothed wheel-work  42 ,  44  to keep the nut  38  of the helical gear  28  from rotating and therefore also locks the spindle  26  and the working piston  16  in place. The brake force exerted due to the supply of current to the electric motor  48  is retained when the electric motor  48  is without current. The brake force is exerted and maintained mechanically through the contact of the working piston  16  against the brake lining piston  12 , without the hydraulic action of the brake fluid, so that once applied, the brake force is not reduced by possible leakage losses.  
         [0027]    An emergency brake function is possible in the same way as the parking brake function. If the hydraulic transmission of the movement of the working piston  16  to the brake lining piston  12  fails, for example due to the lack of brake fluid or due to leakage, the working piston  16  can be moved into contact with the brake lining piston  12  and the brake lining piston  12  can be mechanically displaced by means of the working piston  16  resting against it.  
         [0028]    Function Monitoring  
         [0029]    The hydraulic pressure measured with the pressure sensor  24  and the rotation angle of the motor shaft  46  measured by the integrated rotation angle sensor of the electric motor  48  retain a particular relationship to each other in every operating state of the wheel brake apparatus  10 . The term rotation angle is understood to mean a number of complete rotations and/or a fraction of a rotation. For example, when the solenoid valve  22  is closed, if the electric motor  48  is supplied with current in order to build up a brake force and therefore its motor shaft  46  is set into rotation, then the working piston  16  and the brake lining piston  12  are thus displaced. Only a slight amount of hydraulic pressure builds up before the frictional brake lining comes into contact with the brake body. As soon as the frictional brake lining comes into contact with the brake body, the hydraulic pressure increases with further rotation of the motor shaft  46 . This dependence of the hydraulic pressure on the rotation angle of the motor shaft  46  is used to monitor the function of the wheel brake apparatus  10 . To that end, the actually prevailing hydraulic pressure and the rotation angle of the motor shaft  46  are measured and compared to reference values associated with a properly functioning wheel brake apparatus  10 . If the measured values diverge from the reference values by more than a fixed, permissible tolerance, this indicates a malfunction of the wheel brake apparatus  10 . This function monitoring can also take place when a motor vehicle equipped with the wheel brake apparatus  10  according to the invention is not running. The function monitoring of the wheel brake apparatus  10  can, for example, be automatically executed when the engine of the vehicle is started, before driving begins.  
         [0030]    A heating of the brake fluid during braking can be determined in the following way: if the brake fluid in the working cylinder  18  and in the cylinder  14  of the brake lining piston  12  heats up due to frictional heat during a braking maneuver, then the brake fluid expands. The hydraulic pressure is higher than the known hydraulic pressure which would prevail at the same rotation angle of the motor shaft  46  with cold brake fluid. The increased hydraulic pressure permits the temperature increase to be detected or also calculated. As a result, a warning can be given well before a critical temperature of the hydraulic fluid is reached.  
         [0031]    Even when driving while not using the brakes, the brake fluid can be checked for heating. This can be the case when there is insufficient air play or an absence thereof, i.e. when the frictional brake lining continuously rests against the brake body due to a mechanical malfunction. In order to test for such a temperature increase of the brake fluid when the wheel brake apparatus  10  is not being actuated, the solenoid valve  22  is closed and the hydraulic pressure is measured by the pressure sensor  24 . If the brake fluid heats up, then the brake fluid expands and the hydraulic pressure increases.  
         [0032]    Determination of the Air Play  
         [0033]    The air play determination takes place during the above-described actuation of the wheel brake apparatus  10  through the monitoring of the hydraulic pressure by means of the pressure sensor  24 . Until the frictional brake lining contacts the brake body, an increase in the hydraulic pressure is slight and the hydraulic pressure remains virtually constant. As soon as the frictional brake lining rests against the brake body, the hydraulic pressure increases. The number of rotations of the motor shaft  46  can be used to determine the displacement path of the brake lining piston  12  and therefore the air play up until the pressure increase, i.e. until the frictional brake lining comes into contact with the brake body. If the air play is too great, for example due to a wear on the brake lining, then it is adjusted by virtue of the fact that the motor shaft  46  is turned back less during the release of the wheel brake apparatus  10  than it was turned in the brake-applying direction when the wheel brake apparatus  10  was applied.  
         [0034]    The distance of the working piston  16  from the brake lining piston  12  can be determined in the following manner: by supplying current to the electric motor  48  while the solenoid valve  22  is open, the working piston  16  is moved toward the brake lining piston  12 . The working piston  16  thus displaces brake fluid from the working cylinder  18 , as a result of which the hydraulic pressure increases slightly. As soon as the working piston  16  strikes against the brake lining piston  12  and moves it, the larger diameter brake lining piston  12  aspirates brake fluid into the working cylinder  18  and the cylinder  14  of the brake lining piston  12 , the hydraulic pressure drops to a negative pressure. The number of rotations of the motor shaft  46  until the drop in the hydraulic pressure when the working piston  16  strikes against the brake lining piston  12  can be used to determine the displacement path that the working piston  16  has traveled and thereby the starting distance between the working piston  16  and the brake lining piston  12  and can be adjusted when the working piston  16  is reset.  
         [0035]    Releasing in the Event of a Malfunction  
         [0036]    In the event of a malfunction, the wheel brake apparatus  10  can be released in two ways. On the one hand, when the wheel brake apparatus  10  is being actuated, the solenoid valve  22  can be opened and as a result, the wheel brake apparatus  10  can be released even in the event of a jammed helical gear  28 , for example. If the working piston  16  is resting against the brake lining piston  12 , then the second possibility for releasing the wheel brake apparatus  10  lies in releasing the magnetic brake  50 . When the wheel brake apparatus  10  is being actuated and the magnetic brake  50  is released, the brake lining piston  12  moves the working piston  16  and the spindle  26  of the self locking-free screw link actuator  28  back until the brake force exerted by the frictional brake lining on the brake body has decreased to a residual brake force.  
         [0037]    The foregoing relates to a preferred exemplary embodiment of the invention, it being understood that other variants and embodiments thereof are possible within the spirit and scope of the invention, the latter being defined by the appended claims.