Abstract:
The invention relates to a method for providing haptic information to the driver of a motor vehicle, equipped with a brake-by-wire brake system, about the operating state of the brake system, in which method the brake pedal characteristic is generated by a pedal travel simulator, in the form of a functional relationship between the brake pedal opposing force and the brake pedal travel, and is modified as a function of an operating state. In order to provide the driver with the haptic information channel via which electronically controlled information can be communicated to the driver during braking, without the impression of a defect arising, the invention proposes that a basic brake pedal characteristic is generated by a passive simulator spring which assigns pedal travel, on which pedal return travel transporting the haptic information is superimposed, to a given pedal force in an unchanging, constant relationship.

Description:
Cross Reference to Related Applications 
       [0001]    This application is the U.S. National Phase Application of PCT/EP2013/055761, filed Mar. 20, 2013, which claims priority to German Patent Application Nos. 10 2012 204 564.3, filed Mar. 22, 2012, 10 2013 203 710.4, filed Mar. 5, 2013 and 10 2013 204 778.9, filed Mar. 19, 2013, the contents of such applications being incorporated herein by reference. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention relates to a method for providing haptic information to the driver of a motor vehicle, which is equipped with a brake-by-wire brake system, about the operating state of the brake system, in which the brake pedal characteristic is generated in the form of a functional relationship between the brake pedal opposing force and the brake pedal travel, by a simulator, and is modified as a function of the operating state, and a device for carrying out the method according to the invention. 
       BACKGROUND OF THE INVENTION 
       [0003]    Document DE 34 09 705 A1, which is incorporated by reference, discloses a method for providing feedback about disruptions or faults during a braking process during which a fault reaction-actuating device acts on the brake pedal travel simulator in such a way that a change in the brake pedal characteristic predefined by the brake pedal travel simulator occurs and remains until the provided function of the by-wire system is available again. Furthermore, it is proposed to change the brake pedal characteristic preferably in such a way that rhythmic pulsating of the brake pedal occurs. For this purpose, an electromagnetic actuator which is actuated with the pulsating signal of an electronic oscillatory circuit is to engage with a locking projection in a mechanism with which the simulator characteristic can be modified. The locking projection is designed to enter into a locking operative connection with a sliding piece on which a spring is supported by means of which part of the pedal force is transmitted. A technical teaching such as how pulsating of the brake pedal is to be generated by means of periodic activation of the locking is not provided. In order to permit periodic pedal movement, once an additional pedal travel has been enabled it must be capable of being reset again. A corresponding pedal actuating device and its supply with actuating energy would accordingly still have to be added. 
         [0004]    German laid-open patent application DE 10 2006 026 873 A1, which is incorporated by reference, describes an activation device for a by-wire brake system. In order to provide the driver with haptic feedback about a relatively low coefficient of adhesion or friction at the vehicle wheels or a start of ABS control, the brake pedal axle is provided with an electrical, hydraulic or pneumatic rotary drive which generates pulsations at the brake pedal. In this context, the amplitude of the pulsations can be changed in a range between approximately 0 mm and approximately 5.0 mm, and the frequency thereof between approximately 1 Hz and approximately 10 Hz. 
         [0005]    Finally, document DE 10 2004 040 616 A1, which is incorporated by reference, discloses a method for providing haptic information to the driver of a motor vehicle, which is equipped with a brake-by-wire brake system, about the operating state of the brake system, in which a brake pedal characteristic is generated in the form of a functional relationship between the brake pedal opposing force and the brake pedal travel, by a simulator, and is modified as a function of the operating state, as well as a device for carrying out the method. An opposing force, which counteracts the activation force applied to the brake pedal by the driver&#39;s foot is generated by an active pedal travel simulator, specifically in such a way that the driver senses this as a limitation of the comfort and/or functioning of the brake system. 
         [0006]    Such limitation of the comfort and a sensed limitation of functioning are not acceptable if the driver is to be informed about the operating state of an intact brake system. 
       SUMMARY OF THE INVENTION 
       [0007]    An aspect of the present invention proposes a method which makes available a haptic information channel, in particular, with simple electronic means, by means of which haptic information channel information can be communicated to the driver in an electronically controlled fashion during a braking operation without the impression of a defect being produced. 
         [0008]    This is achieved according to the invention by a method for providing haptic information to the driver of a motor vehicle, which is equipped with a brake-by-wire brake system, about the operating state of the brake system, in which a brake pedal characteristic is generated in the form of a functional relationship between the brake pedal opposing force and the brake pedal travel, by a simulator, and is modified as a function of the operating state wherein a basic brake pedal characteristic is generated by a passive simulator spring which assigns a pedal travel to a given pedal force in an unchangingly constant relationship, on which pedal travel a pedal return travel which transports the haptic information is superimposed. It is particularly advantageous here that in order to apply the opposing force predominantly components of the brake-by-wire brake system are used which are present in any case for correct operation of the brake system. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]    In the figures: 
           [0010]      FIG. 1  shows a hydraulic circuit diagram of an exemplary brake system for carrying out the method according to an aspect of the invention, 
           [0011]      FIG. 2  shows the brake system according to  FIG. 1  in a first state in order to clarify the method according to an aspect of the invention, and 
           [0012]      FIG. 3  shows the brake system according to  FIG. 1  in a second state in order to clarify the method according to an aspect of the invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0013]    The brake system illustrated in the drawing comprises essentially a hydraulic activation unit  2  which can be activated by means of an activation or brake pedal  1 , a travel simulator or simulation device  3  which interacts with the hydraulic activation unit  2 , a pressure medium reservoir container  4  which is assigned to the hydraulic activation unit  2  and is at atmospheric pressure, an electrically controllable pressure supply device  5 , an electronic open-loop and closed-loop control unit  12  and an electrically controllable pressure modulation device for setting wheel-specific brake pressures. 
         [0014]    The brake modulation device (not denoted here in more detail) comprises, for example, per wheel brake  8 ,  9 ,  10 ,  11  of a motor vehicle which is not illustrated, an inlet valve  6   a - 6   d  and an outlet valve  7   a - 7   d,  which are hydraulically connected in pairs to one another via center ports and to the wheel brakes  8 ,  9 ,  10 ,  11 . The inlet ports of the inlet valves  6   a - 6   d  are supplied, by means of brake circuit supply lines  13   a,    13   b,  with pressures which in a “brake-by-wire” operating mode are derived from a system pressure which is present in a system pressure line  38  which is connected to a pressure space  37  of the electrically controllable pressure supply device  5 . Connected in parallel with each of the inlet valves  6   a - 6   d  is a non-return valve  50   a - 50   d  which opens toward the brake circuit supply lines  13   a ,  13   b.  In a non-boosted fallback operating mode, the brake circuit supply lines  13   a,    13   b  are supplied with the pressures of the pressure spaces  17 ,  18  of the activation unit  2  via hydraulic lines  22   a,    22   b.  The outlet ports of the outlet valves  7   a - 7   d  are connected in pairs to the pressure medium reservoir container  4  via return flow lines  14   a,    14   b.  In order to detect the pressure present in the system pressure line  38 , a pressure sensor  19  which is of preferably redundant design is provided. 
         [0015]    The hydraulic activation unit  2  has, in a housing  21 , two pistons  15 ,  16  which are arranged one behind the other and which bound hydraulic chambers or pressure spaces  17 ,  18  which form, together with the pistons  15 ,  16 , a dual-circuit master brake cylinder or a tandem master cylinder. The pressure spaces  17 ,  18  are connected, on the one hand, to the pressure medium reservoir container  4 , via radial bores formed in the pistons  15 ,  16  and corresponding pressure equalization lines  41   a,    41   b  (via the return flow line  14   a  and  14   b ) wherein said bores and lines  41   a ,  41   b  can be shut off by a relative movement of the pistons  17 ,  18  in the housing  21  and, on the other hand, to the already specified brake circuit supply lines  13   a,    13   b  by means of the hydraulic lines  22   a,    22   b.  In this context, a parallel connection of a diagnostic valve  28  which is open in the currentless state, with a non-return valve  27  which closes toward the pressure medium reservoir container  4 , is contained in the pressure equalization line  41   a.  The pressure spaces  17 ,  18  accommodate resetting springs (not denoted in more detail) which position the pistons  15 ,  16  in an initial position when the master brake cylinder  2  is not activated. A piston rod  24  couples the pivoting movement of the brake pedal  1  owing to a pedal activation to the translatory movement of the first (master cylinder) piston  15  whose activation travel is detected by a travel sensor  25 , preferably of redundant design. The corresponding piston travel signal is as a result a measure of the brake pedal activation angle. It represents a braking request of a vehicle driver. 
         [0016]    An isolating valve  23   a,    23   b  is arranged in each of the line sections  22   a,    22   b  connected to the pressure spaces  17 ,  18 , which isolating valve  23   a,    23   b  is embodied as an electrically activated 2/2-way valve which is preferably open in the currentless state. The hydraulic connection between the pressure spaces  17 ,  18  and the brake circuit supply lines  13   a,    13   b  can be shut off by means of the isolating valves  23   a,    23   b.  A pressure sensor  20  which is connected to the line section  22   b  detects the pressure built up in the pressure space  18  as a result of displacement of the second piston  16 . 
         [0017]    The travel simulator  3  can be hydraulically coupled to the master brake cylinder  2  and is composed essentially of a simulator chamber  29 , a simulator spring chamber  30  and a simulator piston  31  which disconnects the two chambers  29 ,  30  from one another. The simulator piston  31  is supported on the housing  21  by an elastic element (for example a spring) which is arranged in the simulator spring chamber  30  and is advantageously prestressed. The simulator chamber  29  can be connected to the first pressure space  17  of the tandem master brake cylinder  2  by means of an electrically activated simulator release valve  32 . When a pedal force is predefined and the simulator release valve  32  is activated, pressure medium flows from the master brake cylinder pressure space  17  into the simulator chamber  29 . A non-return valve  34  which is arranged hydraulically in an anti-parallel fashion with respect to the simulator release valve  32  permits, independently of the switched state of the simulator release valve  32 , approximately unimpeded flowing back of the pressure medium from the simulator chamber  29  to the master brake cylinder pressure space  17 . 
         [0018]    The electrically controllable pressure supply device  5  is embodied as a hydraulic cylinder-piston arrangement or a single-circuit electrohydraulic actuator whose piston  36  can be activated by a schematically indicated electric motor  35  with intermediate connection of a rotational translatory gear mechanism (also illustrated schematically). A rotor position sensor which serves to detect the rotor position of the electric motor  35  and is indicated in a merely schematic fashion is denoted by the reference symbol  44 . In addition, a temperature sensor can also be used to sense the temperature of the motor winding. The piston  36  bounds a pressure space  37 . 
         [0019]    The actuator pressure which is generated by the force effect of the piston  36  on the pressure medium which is enclosed in the pressure space  37  is fed into the system pressure line  38  and detected by the system pressure sensor  19 . In the “brake-by-wire” operating mode, the system pressure line  38  is connected to the brake circuit supply lines  13   a,    13   b  via the sequence valves  26   a,    26   b.  In this way, in the case of normal braking, wheel brake pressure is built up and reduced for all the wheel brakes  8 ,  9 ,  10 ,  11 . When pressure is reduced, the pressure medium which has been previously moved into the wheel brakes  8 ,  9 ,  10 ,  11  from the pressure space  37  of the actuator  5  flows back again into the pressure space  37  of the actuator  5  in the same way. In contrast, in the case of braking with wheel brake pressures which are regulated differently on a wheel-specific basis using the modulation valves  6   a - 6   d,    7   a - 7   d,  the pressure medium portion which is discharged via the outlet valves  7   a - 7   d  flows into the pressure medium reservoir container  4 . Subsequent sucking of pressure medium into the pressure space  37  is possible as a result of the piston  36  moving back with the sequence valves  26   a,    26   b  closed, by virtue of the fact that pressure medium can flow out of the container  4  into the actuator pressure space  37  via a suction valve  52  which is embodied as a non-return valve which is open in the direction of flow to the actuator. 
         [0020]    The components  2 ,  3 ,  5 ,  6   a - 6   d,    7   a - 7   d,    12 ,  19 ,  20 ,  22   a,    22   b,    23   a ,  23   b,    25 ,  26   a,    26   b,    27 ,  28 ,  32 ,  34 ,  38 ,  41   a,    41   b,    44 ,  46 ,  52  mentioned above are preferably combined to form an electro-hydraulic module (electrohydraulic activation unit) which is provided with the reference symbol  60 . The electronic open-loop and closed-loop control unit  12  is used for actuating the electrically activated components of the module  60 , in particular the valves  6   a - 6   d,    7   a - 7   d,    23   a,    23   b,    26   a,    26   b,    28 ,  32  and of the electric motor  35  of the pressure supply device  5 . The signals of the sensors  19 ,  20 ,  25  and  44  are also processed in the electronic open-loop and closed-loop control unit  12 . 
         [0021]    In a normal braking function of the brake system (“brake-by-wire” operating mode), the master brake cylinder  2 , and therefore the vehicle driver, is disconnected from the wheel brakes  8 ,  9 ,  10 ,  11  by the closed isolating valves  23   a,    23   b,  and the brake circuit supply lines  13   a,    13   b  are connected via the opened sequence valves  26   a,    26   b  to the first pressure supply device  5  which makes available the system pressure for the activation of the wheel brakes  8 ,  9 ,  10 ,  11 . The simulation device  3  is activated by the opened simulator release valve  32 , so that the pressure medium volume which is expelled in the master brake cylinder  2  as a result of the activation of the brake pedal  1  by the driver is taken up by the simulation device  3 , and the simulation device  3  communicates a customary brake pedal sensation to the vehicle driver. 
         [0022]    In a fallback operating mode of the brake system, for example in the event of a failure of the electrical energy supply of the brake system, the simulation device  3  is switched off by the simulator release valve  32  which is closed in a currentless state, and the pressure supply device  5  is disconnected from the brake circuit supply lines  13   a,    13   b  by the sequence valves  26   a ,  26   b  which are closed in the currentless state. The master brake cylinder  2  is connected via the lines  22   a,    22   b  to the isolating to valves  23   a,    23   b,  open in the currentless state, by the brake circuit supply lines  13   a,    13   b,  and therefore to the wheel brakes  8 ,  9 ,  10 ,  11 , so that the vehicle driver can build up pressure directly in the wheel brakes  8 ,  9 ,  10 ,  11  by activating the brake pedal  1 . 
         [0023]    In a simulator brake system (in the “brake-by-wire” operating mode), the pedal  1  is hydraulically disconnected from the wheel brakes  8 - 11 . The pedal reactions which are unavoidable in conventional brake systems during wheel brake pressure regulating activities of the brake system therefore do not occur. In order nevertheless to provide the driver with haptic pedal feedback in selected operating states, it is proposed, for example, that this pedal reaction be superimposed temporarily under electronic control on the pedal characteristic which is predefined by the simulator (for example by means of the spring in the space  30 ). 
         [0024]    In order to carry out a method for generating an electronically controlled pedal reaction, for example at least one isolating valve ( 23   a : isolating valve of the pressure circuit,  23   b : isolating valve of the floating circuit) is opened in a pulsed fashion. As a result, a pressure medium volume pulse is conducted from the system pressure side (line  13   a  or  13   b ) of the isolating valve to the driver&#39;s pressure side (line  22   a  or  22   b ) of the isolating valve which generates the desired pedal pulsation. 
         [0025]    The method preferably provides that the system pressure is raised (for example by means of the pressure supply device  5 ) if the pressure difference composed of the system pressure and master cylinder pressure is not sufficient, not present or is negative. 
         [0026]    Finally, the isolating valve  23   b  which is assigned to the floating circuit is preferably used to generate the pedal reaction. This has the advantage that after the pressure medium volume which is additionally introduced as a volume pulse has fulfilled its function as a pedal pulse generator it is reduced again via the equalization connection  100  of the floating circuit in the direction of the container  4 , and the steady-state pedal characteristic therefore remains unchanged. 
         [0027]      FIG. 2  and  FIG. 3  illustrate the brake system in  FIG. 1  in various states. Volume flows and pressure conditions which are relevant for the function are illustrated by way of example in  FIG. 2  and  FIG. 3 . 
         [0028]    In the case of normal braking without ABS and in the case of ABS braking (“brake-by-wire” operating mode), a state such as is illustrated in  FIG. 2  frequently occurs. The system pressure P sys  in the system pressure circuit  40  is higher than the driver pressure P Driver  in the driver pressure circuit  41  since the system pressure is usually increased by an order of magnitude compared to the unboosted driver pressure. 
         [0029]    The brake pedal feedback is generated, for example, using the isolating valve  23   b  which is continuously energized, and therefore closed during braking in the case of a normal, active operating mode of the brake system. The isolating valve  23   b  disconnects the driver pressure circuit  41  from the system pressure circuit  40  here in the active operating mode with the result that brake fluid volume and pressure cannot flow from one circuit in the respective other circuit. Without the exemplary method which is described below for setting a suitable haptic brake pedal feedback for the driver in an active brake system during ABS control, the driver would therefore also be disconnected from the active system pressure or the wheel brake circuits during ABS control and would not obtain any haptic feedback via the brake pedal  1 . In contemporary, conventional ABS/ESC brake systems (for example with a vacuum brake booster and feedback pump) the driver senses the ABS pressure fluctuations in the brake pedal  1  during ABS control owing to feedback pump strokes and valve switching. 
         [0030]    As has already been explained in relation to  FIG. 1 , in order to generate haptic brake pedal feedback during ABS braking in an active brake system a pressure or volume pulse ΔV  42  is preferably fed from the system pressure circuit  40  into the driver pressure circuit  41  by briefly opening the isolating valve  23   b.  Said pressure pulse or volume pulse ΔV  42  generates a force pulse acting on the floating piston  16 , which passes on the force pulse via the first return spring, arranged in the first pressure space of the master brake cylinder, and the brake fluid in the first pressure space  17  to the first piston  15  via the pressure rod  24  to the brake pedal  1  and therefore to the driver&#39;s foot which is located thereon during the braking. As a result, by briefly opening the isolating valve  23   b  a pressure pulse or volume pulse ΔV, and as a result thereof, a force pulse acting on the brake pedal  1  or the driver&#39;s foot is generated. The driver therefore feels in his foot a force pulse and the driver&#39;s foot is as a result moved dynamically by several mm or fractions of mm since the pedal opposing force of the driver&#39;s foot force is overcome by the force pulse. A haptic feedback via the brake pedal  1  is therefore possible without the pedal basic characteristic being changed. 
         [0031]    The intensity of the force pulse is preferably varied over the opening time of the isolating valve  23   b  since in this way the size of the volume pulse ΔAV  42  is determined. 
         [0032]    Furthermore, the frequency of the force pulses in the pedal is preferably varied by varying the pause time between the valve openings. 
         [0033]    In order to make the haptic feedback suitable and subject to be plausible for the driver, the frequency and intensity of the feedback are preferably set or varied suitably. 
         [0034]    According to one development of the method according to the invention, the frequency and intensity of the brake pedal feedback are adapted to the frequencies and amplitudes of the wheel brake pressure modulations in the wheel brake circuit. 
         [0035]    The ABS pressure modulations at all the wheel brakes are preferably taken into account for this purpose. 
         [0036]    The frequency of the brake pedal feedback is preferably selected as a function of a duration of one or more ABS pressure build up cycles up to the ABS pressure reduction. 
         [0037]    In the case of a relatively high ABS control frequency of the pressures in the wheel pressure circuits of the active system, a relatively high frequency of the volume pulses ΔV for the brake pedal feedback is set. This results in a plausible relationship between the behavior of the vehicle or ABS control and the haptic pedal feedback for the driver. 
         [0038]    In terms of vehicle movement dynamics it can be particularly valuable for the driver to know if he has reached the adhesion limit of the tires to the road when the vehicle is being braked, that is to say whether he has reached entry into the ABS control. It is therefore possible, for example, for the coefficient of friction μ of the carriageway to be determined by the driver in wet or winter road conditions by braking as far as the ABS control since this estimation of the coefficient of friction μ of the carriageway is only possible with difficulty by visual assessment and this can also vary greatly. Furthermore, this is appropriate also to communicate the adhesion limit of the tires to the driver in the case of cornering with braking by communicating the entry into the ABS control. It is therefore possible for the haptic brake pedal feedback to be particularly important for the driver when ABS control is entered. For example, for the start of ABS control the brake pedal feedback has a relatively high frequency and is activated with suitable intensity in order to communicate the entry into ABS in an optimum way to the driver. For example, specifically for ABS entry in the case of cornering with braking the brake pedal feedback also has a relatively high frequency and is activated with a suitably high intensity. 
         [0039]    This ensures that the driver can learn an assignment of the pulsation to the driving state and is therefore able to draw conclusions about the operating state of his vehicle from the pedal behavior. 
         [0040]    In the case of ABS braking at a low coefficient of friction μ of the carriageway or at a very high activation force of the driver on the brake pedal, a state (pressure relationship) which is illustrated in  FIG. 3  can result. In this context, the system pressure P sys  in the system pressure circuit  40  is lower than the driver pressure P Driver  in the driver pressure circuit  41 . As a result, opening of the isolating valve  23   b  causes a volume pulse ΔV  43  to flow from the driver pressure circuit  41  into the system pressure circuit  40 . Since the brake fluid volume flows pulse by pulse from the driver pressure circuit  41  into the system pressure circuit  40 , in this context as a result of the driver&#39;s foot force on the brake pedal on the floating piston  16 , the primary piston  15  and therefore the brake pedal  1  would move further and drop in the activation direction. This could unjustifiably give the impression of a defect in the brake system to the driver. Furthermore, a drop in brake pedal in the case of failure of the active system, that is to say switching over into the hydraulic fallback level (in which all the electromagnetic valves are non-energized) could bring about a safety-critical state of the vehicle since when the brake pedal is already depressed there would be no or very little brake fluid volume available in the master brake cylinder as a reserve for the now active fallback level. 
         [0041]    In the situation illustrated in  FIG. 3 , the system pressure is therefore preferably raised above the value of the driver pressure (by means of the pressure supply device). The system pressure is therefore not kept at the level of the greatest ABS wheel pressure request but instead raised above the value of the driver pressure in order to achieve a volume flow, as illustrated in  FIG. 2  from the system pressure circuit  40  into the driver pressure circuit  41 , and therefore prevent dropping of the brake pedal  1 . 
         [0042]    An advantage of the invention is that no additional actuation system such as, for example, an additional actuator is required in the pedal simulator unit or on the brake pedal for the function of the haptic brake pedal feedback. This function can be implemented purely by means of software in the system which is shown by way of example in  FIG. 1 .