Method for performing a braking operation

Abstract

Method for carrying out a braking operation for a motor vehicle, wherein in order to ensure that the motor vehicle is stationary, a braking force is generated by actuating a braking device of the motor vehicle, wherein a state of movement of the motor vehicle (1) and a state of movement of at least one wheel of the motor vehicle (1) are detected, wherein the braking force is reduced if a movement of the motor vehicle (1) is detected and no rotational movement of the wheel of the motor vehicle (1) is detected, characterized in that the braking device of the motor vehicle (1) is released if a movement of the motor vehicle (1) and, at the same time, no rotational movement of the wheel of the motor vehicle (1) is detected.

Description

[0001] The invention relates to a method for carrying out a braking operation for a motor vehicle, wherein a braking force is generated by actuating a braking device of the motor vehicle in order to ensure that the motor vehicle comes to a standstill.

[0002] In motor vehicles equipped with an externally controlled service brake system or an externally controlled parking brake, it is common practice to hold the vehicle stationary by controlling the service brake system or the parking brake using a control unit. This function, also referred to as the hold function, increases driver comfort by relieving them of the burden of manually operating the vehicle brakes while the vehicle is stationary.

[0003] The hold function is activated automatically when a standstill of the vehicle is detected, or due to an activation signal that the driver triggers when the vehicle is detected to be at a standstill.

[0004] If the vehicle begins to slide while the Hold function is activated, which can happen even on relatively slight gradients on surfaces with low friction coefficients, such as icy or snowy surfaces, it has been shown that the vehicle's maneuverability is severely limited due to the locked wheels. Thus, the effect of the Hold function makes it impossible or very difficult for the driver to regain control of the vehicle in the event of a slide, significantly compromising vehicle safety.

[0005] The subsequently published DE 10 2005 015062 A1 discloses a hill start assist system in which the vehicle is automatically further braked to simplify starting on an incline. If vehicle movement is detected by evaluating acceleration sensors, the braking system is released and a special control system is initiated. Therefore, it is an object of the present invention to further develop a method of the type mentioned above in such a way that vehicle safety is increased.

[0006] According to the invention, this object is achieved by a method according to patent claim 1.

[0007] Accordingly, it is provided that a method of the type mentioned at the outset is carried out in such a way that a movement state of the motor vehicle and a movement state of at least one wheel of the motor vehicle are detected and that the braking force is reduced if a movement of the motor vehicle is detected and essentially at the same time no rotational movement of the wheel of the motor vehicle is detected.

[0008] The invention advantageously provides for detecting a skid of the motor vehicle by detecting that the motor vehicle is moving without at least one wheel of the motor vehicle being rotated. Furthermore, the invention is based on the idea of reducing the braking force demand, which is determined in the control unit as part of the hold function, when a skid of the motor vehicle is detected, in order to restore the maneuverability of the motor vehicle, which would be significantly limited if the wheels were locked.

[0009] According to the invention, it is provided that the braking device of the motor vehicle is released when a movement of the motor vehicle and essentially no rotational movement of the wheel of the motor vehicle is detected at the same time.

[0010] In this way, the hold function is terminated or canceled if a skidding of the motor vehicle is detected, so that the operation of the braking device can be carried out entirely by the driver in order to regain control of the motor vehicle.

[0011] In a preferred embodiment of the invention, the movement state of the motor vehicle is detected based on signals from at least one acceleration sensor.

[0012] In an advantageous embodiment of the invention, the acceleration sensor is a longitudinal acceleration sensor and / or a lateral acceleration sensor.

[0013] In a further advantageous embodiment of the invention, the acceleration sensor is a yaw acceleration sensor.

[0014] In a likewise advantageous embodiment of the invention, it is provided that the movement state of the motor vehicle is detected based on signals from a yaw rate sensor.

[0015] Preferably, in one embodiment of the invention, it is provided that a movement of the motor vehicle is detected when an acceleration value determined by means of an acceleration sensor and / or a yaw rate value determined by means of the yaw rate sensor changes at least by a predetermined amount compared to a value present when the motor vehicle is stationary.

[0016] Preferably, in a further embodiment of the invention, it is provided that a movement of the motor vehicle is detected if an acceleration value determined by means of an acceleration sensor and / or a yaw rate value determined by means of the yaw rate sensor deviates for a predetermined period of time from the value present during the standstill of the motor vehicle.

[0017] Preferably, it is also provided that a movement of the motor vehicle is detected if an acceleration value determined by means of an acceleration sensor and / or a yaw rate value determined by means of the yaw rate sensor retains its sign during the predetermined period of time.

[0018] Due to the measures described above, minor movements of the motor vehicle, which are caused, for example, by a movement of the occupants of the motor vehicle, are not taken into account, so that incorrect detection of a sliding movement of the motor vehicle during such vehicle movements is avoided.

[0019] In a preferred embodiment of the invention, it is provided that the braking force is reduced and / or the braking device is released if a movement of the motor vehicle is detected at a point in time and no rotational movement of the wheel of the motor vehicle is detected within a period of time beginning at this point in time.

[0020] In one embodiment of the invention, it is expediently provided that the braking device of the motor vehicle is a service braking system of the motor vehicle.

[0021] In an equally expedient embodiment of the invention, it is provided that the braking device of the motor vehicle is a parking brake device.

[0022] Further advantages, special features and expedient developments of the invention emerge from the subclaims and the following representation of preferred embodiments with reference to the single figure.

[0023] The Fig. 1 shows a schematic representation of a motor vehicle with a service brake system and a parking brake system.

[0024] The Fig. The motor vehicle schematically illustrated in Figure 1 is designated as a whole by the reference number 1. It has a service brake system, which is preferably designed as a hydraulic brake system.

[0025] The driver of motor vehicle 1 can operate the braking system using an actuating device 2, which may be, for example, a brake pedal. The actuating device 2 acts via a brake booster 3 on a master brake cylinder 4, in which a braking pressure can be generated that can be transmitted to the wheel brakes 5 of motor vehicle 1.

[0026] In addition, the service brake system can be actuated by a control unit 6, which controls an actuator with which a brake pressure can also be built up in the wheel brakes 5. In the Fig. The exemplary embodiment of the service brake system shown in Figure 1 comprises a hydraulic unit 7 having a motor-driven pump with which a brake pressure in the wheel brakes 5 can be built up or increased. Furthermore, the hydraulic unit 7 contains valves that can be controlled to maintain a brake pressure in the wheel brakes 5, preferably for each individual wheel, or to reduce the brake pressure in the wheel brakes 5. Such service brake systems are generally known to those skilled in the art as components of vehicle dynamics control systems, such as ESP systems (ESP: Electronic Stability Program), and are already present in a large number of motor vehicles.

[0027] For the externally controlled design of a service brake system, a so-called active brake booster can also be provided, which is controlled by the control unit 6 and increases the brake pressure in the master brake cylinder. In a further embodiment of the invention, the service brake system can be designed as an electro-hydraulic brake system, in which the driver's braking commands are electronically detected and electronically transmitted to a hydraulic unit, which then adjusts the brake pressure according to the braking commands. Furthermore, the service brake system can also be an externally controlled pneumatic brake system, for example.

[0028] In addition to the service brake system, the motor vehicle 1 in one embodiment of the invention also has an electromechanical or electrohydraulic parking brake device, which is controllable by the control unit 6. In the Fig. In the exemplary embodiment of the parking brake device shown in Figure 1, an actuator 8 is provided that acts on the wheel brakes 5 on the rear wheels 9 of the motor vehicle 1. This can, for example, be an electromechanical parking brake device in which the actuator 8 is designed as an electric motor that actuates the wheel brakes 5 of the rear wheels 9 via cables. In principle, however, any externally controllable parking brake device is suitable for use within the scope of the present invention.

[0029] In addition, the motor vehicle 1 in the Fig. 1, wheel speed sensors (not shown) are provided, from whose signals the vehicle speed can be determined. Furthermore, the motor vehicle 1 has a lateral acceleration sensor, a yaw rate sensor and / or a yaw acceleration sensor, and preferably a longitudinal acceleration sensor.

[0030] The starting point of the present invention is the hold function already mentioned at the beginning, in which the motor vehicle 1 is held at a standstill automatically, ie controlled by the control unit 6.

[0031] In one embodiment of the invention, the hold function is activated by an activation signal generated by the driver while the motor vehicle 1 is stationary. This signal can be triggered, for example, by operating a switch or by briefly increasing the actuating force exerted on the actuating device 2 of the service brake system once or several times. The brief increase in the actuating force leads to a brief increase in the brake pressure in the master brake cylinder 4, which is registered by a pressure sensor that detects the brake pressure within the master brake cylinder 4.

[0032] In a further preferred embodiment of the invention, the hold function is automatically activated when the control unit 6 detects that the motor vehicle 1 is stationary based on the signals from the wheel speed sensors. This is the case when the determined vehicle speed drops below a predetermined low threshold value of, for example, 1 km / h.

[0033] After activation of the hold function, the service brake system of the motor vehicle or the parking brake device is actuated by means of the control unit 6 in order to generate a braking force which keeps the motor vehicle 1 at a standstill.

[0034] Actuation of the service brake system is preferred, as it can generate braking force and reduce existing braking force with greater dynamics and less noise than the parking brake system, thus making brake actuation more comfortable for the driver. To actuate the service brake system, a brake pressure value is calculated by means of the control unit 6 and transmitted as a pressure request to the hydraulic unit 7 and / or the active brake booster, which implements the pressure request and sets the brake pressure value in the wheel brakes 5 of the motor vehicle 1.

[0035] In order to hold the motor vehicle at a standstill using the service brake system, it can be provided, particularly when the driver activates the hold function while the motor vehicle 1 is stationary, to adopt the brake pressure present in the wheel brakes 5 and set by the driver as a pressure request from the control unit 6. By closing the valves via which the wheel brakes 5 are connected to the master brake cylinder 4, this pressure request is implemented and the brake pressure set by the driver is thus maintained.

[0036] In a further embodiment of the invention, it can also be provided that the control unit 6 calculates a pressure requirement from the longitudinal inclination angle of the motor vehicle 1 and a value for its mass, initially a braking force which represents the slope force F acting on the motor vehicle 1 Hang= mg sin(α) (m: vehicle mass). The pressure requirement or the value of the brake pressure to be set is then determined such that the previously calculated braking force is achieved based on the brake pressure.

[0037] The longitudinal inclination angle of the motor vehicle can be determined using an inclination angle sensor, which can detect the longitudinal inclination angle of the motor vehicle 1, and / or using the longitudinal acceleration sensor. When stationary, sin(α) = -a sensor / g, where α is the longitudinal inclination angle of the motor vehicle 1, a Sensor is the longitudinal acceleration of motor vehicle 1 measured by the longitudinal acceleration sensor, and g is the acceleration due to gravity. When motor vehicle 1 is traveling forward, the longitudinal inclination angle is sin(α) = (dv / dt-a Sensor ) / g and correspondingly when reversing sin(α) = - (dv / dt+a Sensor) / g, where dv / dt is the rate of change of the vehicle speed determined from the signals of the wheel speed sensors.

[0038] The mass of the motor vehicle 1 can be determined, for example, during previous braking operations based on the set braking force and the resulting vehicle deceleration, taking advantage of the fact that a relationship between the braking force and the vehicle deceleration is influenced by the mass of the motor vehicle 1. Alternatively, it can be provided to use, for example, the value corresponding to the permissible total weight of the motor vehicle 1 for the vehicle mass.

[0039] If actuation of the parking brake device is intended to ensure the stationary state of the motor vehicle 1, the actuator 8 of the parking brake device is controlled by the control unit 6 in such a way that the parking brake device is activated. Preferably, the parking brake device is fully applied, so that a safe stationary state of the motor vehicle 1 can be ensured.

[0040] After activation of the hold function, it is provided within the scope of the invention to monitor the standstill of the motor vehicle 1 and to detect any skidding of the motor vehicle 1. A skidding of the motor vehicle 1 can occur if it is stopped on a gradient. The ground can slope downwards in the transverse direction of the motor vehicle 1, so that the motor vehicle begins to skid in the transverse direction, or in the longitudinal direction, so that it begins to skid in the longitudinal direction. Furthermore, the motor vehicle 1 can be stopped at an angle on a slope, so that the ground slopes downwards in both the transverse and longitudinal directions. Depending on the position of the vehicle, different acceleration values and / or the yaw rate change in the event of a skid.

[0041] The skidding of motor vehicle 1 can be caused, in particular, by the fact that, due to the nature of the ground, only a low frictional force acts on motor vehicle 1 or the locked wheels. For example, on icy or snow-covered ground, as well as on gravel or similar surfaces, motor vehicle 1 may begin to skid even on a relatively slight gradient. If higher frictional forces act on motor vehicle 1, a skidding movement may occur on a steep gradient.

[0042] Within the scope of the invention, a slippage of the motor vehicle 1 is detected when the state of movement of the motor vehicle 1 changes without a rotational movement of the wheels of the motor vehicle 1 being detected.

[0043] The motion state of motor vehicle 1 is determined using the existing acceleration sensors and the yaw rate sensor. As previously mentioned, longitudinal and lateral acceleration sensors, as well as a yaw acceleration sensor, can be used as acceleration sensors. At least one yaw rate sensor and, as a rule, also one lateral acceleration sensor are usually already part of conventional ESP systems and are therefore already present in a large number of motor vehicles.

[0044] When the motor vehicle 1 is stationary, the signals from the longitudinal and lateral acceleration sensors represent the slope accelerations directed in the longitudinal and transverse directions of the motor vehicle 1, respectively. A movement of the motor vehicle 1 is detected based on these signals if the acceleration value determined from these signals changes.

[0045] Preferably, the acceleration values are initially monitored by the control unit 6 for a predetermined period of time beginning with the activation of the hold function, and a constant acceleration value established within this period is recorded and stored. These are the acceleration values corresponding to the slope accelerations, which can be determined after a settling phase of the acceleration sensors. In one embodiment of the invention, a movement of the motor vehicle 1 is detected if an acceleration value determined after this period from the signals of the longitudinal or lateral acceleration sensor changes by at least a predetermined amount compared to the stored value.Alternatively or additionally, it is provided that a movement of the vehicle is detected if an acceleration value determined after the period deviates from the corresponding stored acceleration value for a predetermined period of time and retains its sign.

[0046] In this way, minor movements of the motor vehicle 1, which may be caused, for example, by a movement of the motor vehicle 1 caused by the occupants of the motor vehicle 1 and which may lead to a slight or brief change in the acceleration values, are not taken into account. This prevents false detection of a skidding movement of the motor vehicle 1 due to such minor vehicle movements.

[0047] The values for the yaw rate and yaw acceleration determined using the yaw rate sensor and the yaw acceleration sensor while the motor vehicle is stationary are each zero. Once these values have been recorded and stored during the stationary state following a transient phase of the sensors, a movement of the motor vehicle is again detected using the signals from the yaw rate sensor and / or the yaw acceleration sensor if the value of the yaw rate or yaw acceleration determined from the signals changes. To prevent false detection of a skidding movement, one embodiment of the invention provides that a movement of the motor vehicle 1 is detected in particular when the magnitude of the yaw rate or the magnitude of the yaw acceleration each exceeds a predetermined threshold value.Alternatively or additionally, it may also be provided that a skidding movement is detected if the value of the yaw rate or the value of the yaw acceleration is not equal to zero for a predetermined period of time and has the same sign.

[0048] The movement state of the motor vehicle's wheels is detected or monitored using signals from the wheel speed sensors. Typically, the wheel speed sensors transmit pulses to the control unit 6, with the wheel having been rotated by a predetermined angular amount between the occurrence of two pulses. A rotational movement of a wheel of the motor vehicle 1 is detected if at least one pulse from the corresponding wheel speed sensor is received in the control unit 6 within a predetermined time interval. If, instead of pulses from the wheel speed sensors, values of the vehicle speed and / or the wheel speeds are transmitted to the control unit 6, a rotational movement of a wheel of the motor vehicle 1 is detected if the vehicle speed or the corresponding wheel speed is not equal to zero.

[0049] If a movement of the motor vehicle 1 is detected in the manner described above based on the signals of at least one of the acceleration sensors mentioned or based on the signals of the yaw rate sensor and if there is no rotational movement of the wheels of the motor vehicle 1 detected in the manner described, slipping is detected in the control unit 6.

[0050] If it is provided that pulses from the wheel speed sensors are transmitted to the control unit 6, a skidding movement is detected in particular when, based on the signals of at least one of the sensors provided for detecting the movement state of the motor vehicle 1, a movement of the motor vehicle 1 has been detected at a point in time, and no pulse from the wheel speed sensors is received during a predetermined period beginning at this point in time. If a value of the vehicle speed is evaluated in the control unit 6, it is provided that a skidding movement is detected when, in the manner described above, a movement of the motor vehicle 1 is detected and the vehicle speed has the value zero at essentially the same time.

[0051] If a skidding movement of the motor vehicle 1 is detected in the control unit 6 when the hold function is activated, the invention provides that the braking force generated by the service brake system or by the parking brake device is reduced to zero, thus ending or aborting the hold function. For this purpose, the brake pressure within the service brake system is reduced to zero by corresponding control commands from the control unit 6 if the service brake system is applied. If the motor vehicle 1 is braked by the parking brake device, the control unit 6 controls the actuator 8 of the parking brake device such that the parking brake device is completely released.

[0052] Extensive driving tests have shown that this approach often enables the driver of motor vehicle 1 to regain control of the vehicle 1 during a skid by steering movements and automatic brake interventions. This is because the maneuverability of the vehicle is severely limited when the service brake or parking brake is applied due to the locked wheels, and this maneuverability is restored by releasing the service brake or parking brake, which is no longer effective anyway.

Claims

[1] Method for carrying out a braking operation for a motor vehicle, wherein, in order to ensure that the motor vehicle is stationary, a braking force is generated by actuating a braking device of the motor vehicle, wherein a movement state of the motor vehicle (1) and a movement state of at least one wheel of the motor vehicle (1) are detected, wherein the braking force is reduced if a movement of the motor vehicle (1) is detected and no rotational movement of the wheel of the motor vehicle (1) is detected, characterized by that the braking device of the motor vehicle (1) is released when a movement of the motor vehicle (1) and at the same time no rotational movement of the wheel of the motor vehicle (1) is detected. [2] Method according to claim 1, characterized by that the state of movement of the motor vehicle (1) is detected on the basis of signals from at least one acceleration sensor. [3] Method according to claim 2, characterized bythat the acceleration sensor is a longitudinal acceleration sensor and / or a lateral acceleration sensor. [4] Method according to claim 2, characterized by that the acceleration sensor is a yaw acceleration sensor. [5] Method according to one of the preceding claims, characterized by that the state of motion of the motor vehicle (1) is detected using signals from a yaw rate sensor. [6] Method according to one of the preceding claims, characterized by that a movement of the motor vehicle (1) is detected when an acceleration value determined by means of an acceleration sensor and / or a yaw rate value determined by means of the yaw rate sensor changes at least by a predetermined amount compared to a value present while the motor vehicle (1) is at a standstill. [7] Method according to one of the preceding claims, characterized bythat a movement of the motor vehicle (1) is detected if an acceleration value determined by means of an acceleration sensor and / or a yaw rate value determined by means of the yaw rate sensor deviates for a predetermined period of time from the value present while the motor vehicle (1) is at a standstill. [8] Method according to one of the preceding claims, characterized by that a movement of the motor vehicle (1) is detected if an acceleration value determined by means of an acceleration sensor and / or a yaw rate value determined by means of the yaw rate sensor retains its sign during the predetermined period of time. [9] Method according to one of the preceding claims, characterized bythat the braking force is reduced and / or the braking device is released if a movement of the motor vehicle (1) is detected at a time and no rotational movement of the wheel of the motor vehicle (1) is detected within a period beginning at this time. [10] Method according to one of the preceding claims, characterized by that the braking device of the motor vehicle (1) is a service braking system (2, 3, 4, 5, 7) of the motor vehicle (1). [11] Method according to one of the preceding claims, characterized by that the braking device of the motor vehicle (1) is a parking brake device (5, 8).

Images