Method for detecting a skidding process of a motor vehicle and control unit

Abstract

Method for detecting a skidding process of a motor vehicle, where a braking force value (p) is determined by a holding assist function. Ssm ) is set on at least one wheel brake of the motor vehicle to prevent the motor vehicle from rolling away after a stopping maneuver and wherein the slipping process is detected by means of an evaluation of a vehicle dynamics parameter (Ψ̇) characterizing the lateral movement of the motor vehicle, characterized in that, in particular in successive calculation cycles, at least one sum (Σ + , Σ - ) is formed via a plurality of measured values ​​of the vehicle dynamics parameter (Ψ̇), and the sliding process is recognized depending on the value of the sum or the values ​​of the sums.

Description

[0001] The invention relates to a method for detecting a skidding process of a motor vehicle, wherein a braking force value is set at at least one wheel of the motor vehicle to prevent the motor vehicle from rolling away after a stationary maneuver, and wherein the skidding process is detected by evaluating a vehicle dynamics parameter that characterizes the lateral movement of the motor vehicle. The invention further relates to an electronic control unit for controlling a braking system of a motor vehicle.

[0002] Hill start assist systems, which support the driver of a motor vehicle when starting on an incline by maintaining the brake pressure required to maintain a standstill until sufficient torque for starting is available from the drive motor, are known from the prior art. In such situations, on surfaces with low friction, the vehicle can slip with locked wheels. This slippage is usually accompanied by a rotational movement of the vehicle due to differing friction coefficients and / or forces at the various wheels. Since such slippage generally increases the risk of an accident, it is a significant advantage to detect it as quickly and reliably as possible in order to deactivate or modify the hill start assist function if necessary and / or to warn the driver.

[0003] From DE 103 22 125 A1, such a method for detecting skidding with subsequent deactivation of a hill start assist function is known. In this method, a characteristic lateral parameter of the vehicle, for example the yaw rate, is evaluated, and skidding is detected when the lateral parameter exceeds a threshold value.

[0004] From DE 10 2007 036 578 A1 a method for determining a slipping process is known in which the yaw rate is also evaluated and a proportion of the yaw rate attributable to steering movements is estimated and included as a disturbance variable in the detection.

[0005] DE 10 2005 045 998 A1 discloses a method for performing a braking operation to ensure that a motor vehicle comes to a standstill. The movement of the motor vehicle and the movement of at least one wheel of the motor vehicle are detected, and the braking force is reduced when movement of the motor vehicle is detected and no rotational movement of the wheel of the motor vehicle is detected.

[0006] DE 10 2005 015 062 A1 concerns a method for determining whether a vehicle has come to a standstill after braking to prevent rolling. It proposes to check the vehicle's operating state using vehicle dynamics parameters and / or vehicle dynamics states and to disable the automatic continuation of the operating state depending on the check result. A special regulation for stability or traction control is introduced.

[0007] Such methods have the disadvantage of proving to be rather unreliable in practice. For example, a brief exceedance of the threshold value by the yaw rate does not necessarily indicate a dangerous skid. Considering the steering angle can also lead to false detections, as, for instance, rapid steering inputs from the driver can cause a discrepancy between the expected and measured yaw rate, resulting in a false positive skid detection result.

[0008] It is therefore an object of the invention to provide a method which enables a more robust detection of a skidding process of the motor vehicle.

[0009] The problem is solved by the method according to claim 1 and by a control unit according to claim 11. Further advantageous embodiments are specified in the dependent claims.

[0010] The invention is based on the idea that a slipping process is detected by means of one or more sums over a plurality of measured values ​​of a vehicle dynamics parameter.

[0011] Preferably, the sum or sums are calculated using a plurality of measured values ​​of the vehicle dynamics parameter, recorded at different, successive times. Particularly preferably, these are measured values ​​recorded at different times from the same measuring device (e.g., the same sensor).

[0012] Advantageously, the yaw rate of the vehicle is used as a vehicle dynamics parameter. If the sliding process is accompanied by a rotational movement, which is usually the case, the yaw rate is not zero. The yaw rate can be used to determine whether the vehicle is undergoing a rotational movement. The yaw rate is preferably measured using a yaw rate sensor. Alternatively, the yaw rate can be calculated using measurements from other sensors, such as lateral acceleration sensors.

[0013] One advantage of using one or more totals is that short-term fluctuations in the measured values, triggered for example by measurement errors or short-term processes that do not pose a hazard, have only a minor influence on the total and thus on the detection of the slipping process.

[0014] In a preferred embodiment, only measured values ​​of the vehicle dynamics parameter that were determined within a predetermined elapsed time period are included in the sum or sums. Particularly preferably, the predetermined elapsed time period corresponds to a predetermined number of previous calculation cycles of the algorithm. A calculation cycle is understood to be a cycle of the algorithm in which measured values ​​are retrieved and calculations are performed.

[0015] Preferably, a first sum is calculated over those measured values ​​of the vehicle dynamics parameter that lie above a first predetermined limit value, and a second sum is calculated over those measured values ​​of the vehicle dynamics parameter that lie below a second predetermined limit value.

[0016] Preferably, the first limit is a positive value and the second limit is a negative value, so that values ​​in a range around zero are not included in either sum. Most preferably, the second limit is equal to the first limit multiplied by minus one.

[0017] Preferably, the difference between the magnitudes of the first and second sums is calculated, and a driving dynamics value is derived from this, or both sums are added together to form a grand total, the magnitude of this grand total is calculated, and a driving dynamics value is derived from this. Preferably, the driving dynamics value corresponds to the magnitude of the difference or the magnitude of the grand total. Since, according to the preferred embodiment in which the first limit is a positive value and the second limit is a negative value, the sums have opposite signs, the results of both calculation methods are identical in this case. A skidding process is detected when the driving dynamics value exceeds a predetermined third limit.

[0018] In a preferred embodiment, when a slipping process is detected, a slipping condition flag (slipping condition status indicator) is set.

[0019] Preferably, a skid is also detected if the vehicle dynamics value exceeds a fourth threshold and a skid was detected in a directly preceding calculation cycle. Particularly preferably, the fourth threshold is smaller than the third threshold. This has the advantage that once a skid has been detected, it continues to be considered detected and a corresponding skid condition flag remains set, even if the vehicle dynamics value falls below the third threshold, as long as it remains above the fourth threshold. The condition for maintaining the detection of the detected skid can thus be achieved with smaller values ​​for the vehicle dynamics value than the entry condition for detection.

[0020] Preferably, a skid is also detected if the vehicle dynamics value is below the fourth threshold and a skid was detected in a directly preceding calculation cycle, and in one of the preceding calculation cycles, which occurred less than a second predefined time interval or predefined second number of calculation cycles prior, the skid was above the fourth threshold. This has the advantage that a skid, once detected, continues to be considered detected even if the vehicle dynamics value is below the third and fourth thresholds, for the predefined second time interval or predefined second number of calculation cycles.

[0021] Maintaining the detection of a previously detected skidding event has the advantage that the detected state changes less frequently. Furthermore, this ensures that a detected skidding event is always considered detected for a predetermined period, thus preventing, for example, the reactivation of a hill start assist function shortly after a skidding event.

[0022] According to a preferred embodiment, it is deemed to be recognized that no slipping process is present if the vehicle dynamics value is below the fourth limit value during a second predetermined time period or a second predetermined number of successive calculation cycles.

[0023] Preferably, the procedure is only carried out when a standstill of the vehicle is detected, and / or a measured pressure in the vehicle's master cylinder is lower than a brake pressure determined by the holding assist function. Here, the brake pressure preferably corresponds to the pressure request by which a braking force value has been set at at least one wheel of the vehicle to prevent it from rolling away. This has the advantage that the procedure is only carried out when needed, i.e., when the vehicle is being held by an assistance system. A prerequisite for the procedure is that the vehicle is being held by a control system by setting a braking force value at at least one wheel.

[0024] Advantageously, a standstill is detected based on measured wheel speeds.

[0025] In a preferred embodiment, the braking force is reduced when a skid is detected. Preferably, an automatic hold of the vehicle by a hill start assist function or a similar assistance system is aborted when a skid occurs. Particularly preferably, the applied brake pressure is reduced gradually. This has the advantage that the driver has time to build up braking force again.

[0026] An embodiment of the invention is explained in more detail with reference to the drawings. These show, in a highly schematic representation: Fig. 1 a skidding process of a motor vehicle; Fig. 2 an exemplary yaw rate profile during a sliding process; Fig. 3 an example yaw rate curve as well as a filtered yaw rate curve; Fig. 4 a schematic flowchart for calculating a sum over a vehicle dynamics parameter according to an exemplary procedure; Fig. 5 a schematic flowchart of the detection of the slipping process according to an exemplary procedure; Fig. 6 exemplary curves of various measured and controlled variables.

[0027] Fig. Figure 1 shows a motor vehicle 101 in which an exemplary procedure can be carried out. The vehicle has been brought to a stop by the driver in position 101a and is prevented from rolling away, for example, by a hill start assist function (HSA) by setting a braking force value on at least one wheel brake. Due to a low coefficient of friction of the road surface, the vehicle begins to skid with locked wheels and follows the movement indicated by arrow 102 until it reaches position 101b. This skidding process is detected by an exemplary procedure.

[0028] In Fig. 2 is an example that is in Fig. Figure 1 shows the corresponding yaw rate Ψ of the motor vehicle. Time is plotted on the x-axis (103) and the yaw rate Ψ̇ on the y-axis (104). The time course of the yaw rate Ψ̇ is represented by line 105.

[0029] Fig. Figure 3 shows an exemplary yaw rate curve during the execution of a method according to the invention. Part a of the figure shows the measured raw data, and part b of the figure shows filtered data for use in the exemplary method. Time is plotted on the x-axis 106, and the measured yaw rate of the vehicle is plotted on the y-axis 108. Line 11 shows the yaw rate of the vehicle as measured by a yaw rate sensor. The measured yaw rate is compared, for example, with a first, positive limit value 109 and a second, negative limit value 110. Only values ​​above the first limit value 109 or below the second limit value 110 are included in an exemplary calculation of the vehicle dynamics value. Data close to zero are removed. Line 112 shows those measured values ​​that meet these criteria and are therefore included in the calculation.

[0030] Fig. Figure 4 shows an exemplary schematic flowchart for calculating a sum over a vehicle dynamics parameter.

[0031] The example procedure begins in block 20. In block 21, the computational variables are first set to their initial values. For example, the variable SLIP_LL stores the result of the last slip detection ("slip detected" or "no slip"). The first sum Σ + and the second sum Σ - are set to zero. Additionally, a counter i is set equal to the number N of calculation cycles to be considered. S set.

[0032] Next, block 22 checks whether the counter i is greater than or equal to zero. As long as this is the case, block 23 displays the stored value of the vehicle dynamics parameter Ψ̇ from the previous calculation cycle i+1 (Ψ̇). i+1 ) to the stored value of the vehicle dynamics parameter of the previous calculation cycle i (Ψ̇ i). This is done to store the saved values ​​of the vehicle dynamics parameter for past calculation cycles in relation to the current calculation cycle. Subsequently, in block 24, the counter i is decremented, and in block 22, it is checked again whether the counter i is greater than or equal to zero.

[0033] If the comparison shows that the counter is less than zero, block 25 checks whether the vehicle is stationary. If so, the stored vehicle dynamics parameter Ψ̇0 of the current calculation cycle is updated in block 27 to the measured value of the vehicle dynamics parameter Ψ̇. mess set. Otherwise, the stored vehicle dynamics parameter of the current calculation cycle Ψ̇0 in block 26 is set to zero.

[0034] The counter i in block 28 is then set equal to the number N of calculation cycles to be considered. Sminus one is set. Block 29 checks whether the counter i is greater than or equal to zero. If this is the case, block 30 checks whether the stored value of the vehicle dynamics parameter for the previous calculation cycle i (Ψ̇) i ) is greater than a first limit ε1 = ε. If this is the case, the stored value of the vehicle dynamics parameter for the previous calculation cycle i in block 32 becomes the first sum Σ + The stored value of the vehicle dynamics parameter for the previous calculation cycle i is added. Otherwise, block 31 checks whether the stored value of the vehicle dynamics parameter for the previous calculation cycle i is less than a second limit value ε2. If this is the case, the stored value of the vehicle dynamics parameter for the previous calculation cycle i is added to the second sum Z- in block 32. For example, the second limit value ε2 is equal to the first limit value multiplied by minus one (ε2 = -ε).

[0035] Then, in block 34, the counter i is decremented, and in block 29 it is checked again whether the counter i is greater than or equal to zero.

[0036] If the comparison shows that the numerator i is less than zero, then all relevant values ​​of the vehicle dynamics parameter Ψ̇ correspond to one of the sums Σ + , Σ - The sums have been added, and the total sum from the first sum Σ is calculated in block 35. + and the second sum Z- was formed and its magnitude calculated. This yields the calculated vehicle dynamics value Ψ̇. SD,calc .

[0037] Block 36 contains the calculated driving dynamics value Ψ̇. SD,calc fixed, so that it allows for the detection of the slipping process, as exemplified in Fig. 5, ready.

[0038] In Fig. Figure 5 shows a schematic flowchart of the detection of the slipping process according to an embodiment of a method according to the invention. The process begins with the transfer of the calculated vehicle dynamics value Ψ̇. SD,calc The process first queries block 41 for the result of the last slip detection, SLIP_LL. If this value is "Slip detected," the procedure branches to block 42; otherwise, it branches to block 45.

[0039] Block 42 checks whether the calculated driving dynamics value Ψ̇ SD,calc is greater than a fourth limit (exit threshold) Ψ̇ exit If this is the case, the procedure branches to block 43; otherwise, it branches to block 45.

[0040] Block 43 checks whether a second counter C is smaller than a second predefined number of calculation cycles N. a If this is the case, the procedure branches to block 44; otherwise, it branches to block 48.

[0041] In block 44, the second counter C is incremented by one; in block 45, the second counter C is set to zero. After either block 44 or block 45 has been executed, block 46 checks whether the second counter C is less than the second predefined number of calculation cycles N. a If this is the case, the procedure branches to block 47; otherwise, it branches to block 48.

[0042] In block 47, the result of the last slip detection SLIP_LL is checked again. If this shows "Slip detected", the procedure branches to block 51; otherwise, it branches to block 48.

[0043] In block 48, the pressure p in the master cylinder is queried and compared with a brake pressure request p determined by a holding assist function. SsmThis is compared. The purpose of this is to determine whether the brake pressure applied to the wheel brakes is caused by the driver actuating the master cylinder (the pressure p in the master cylinder is greater than the brake pressure request p). Ssm ), or whether the holding assist function, for example HSA, requires a brake pressure request p Ssm has set. Provided that the pressure p in the master cylinder is lower than the brake pressure request p. Ssm , so the next step in block 49 is to check whether the calculated driving dynamics value Ψ̇ SD,calc is greater than a third limit (entry threshold) Ψ̇ Grenz of the driving dynamics value. If this is also the case, then in block 51 a slide is considered detected ("slide detected").

[0044] Is the pressure p in the master cylinder greater than the brake pressure requirement pssm or is the calculated driving dynamics value Ψ̇ SD,calc smaller than the third limit Ψ̇ Grenz, so in block 50, no sliding was detected ("no sliding").

[0045] The detected result of the slip detection is then saved and the process starts again from block 20 in the next calculation cycle.

[0046] Fig. Figure 6 shows exemplary curves of various measured and controlled variables. The following are denoted: 1: Flag indicating activation of the Hill Start Assist (HSA) function 2: measured vehicle speed 3: Measured pressure in the main cylinder p 4: Print request pssm 5: Measured vehicle dynamics parameter, for example the yaw rate Ψ̇ mess 6: calculated driving dynamics value Ψ̇ SD,calc 7: Sum of positive values ​​Σ + 8: Sum of negative values ​​Σ - 9: Result of slip detection (slip condition flag) 10: Detected standstill state (standstill flag) Stst.

[0047] The horizontal axis represents time, while the vertical axis represents measured or determined values.

[0048] At time 11, the vehicle is detected as stationary. From this time onward, the standstill flag (progression 1) is one. At time 12, an activation condition for the hill start assist function is met, resulting in a non-zero pressure request p. Ssm (Curve 4) is generated. The pressure p in the master cylinder (Curve 3), controlled by the driver, decreases. The hill start assist function, for example, maintains the pressure in the wheel brakes so that the vehicle remains stationary even on an incline.

[0049] The vehicle begins to slide and rotate, so that the measured yaw rate Ψ̇ mess (Course 5) increases and the first sum Σ +The yaw rate values ​​exceeding the first limit (curve 7) increase. Since the vehicle only rotates in the positive direction in this example, the second sum Z- of the yaw rate values ​​below the second limit (curve 8) is zero. The calculated vehicle dynamics value Ψ̇ SD,calc , which corresponds to the total sum of the first and second sums, therefore also increases (curve 6).

[0050] At time 13, the calculated driving dynamics value exceeds Ψ̇. SD,calc (Course 6) for example the given third limit Ψ̇ Grenz , 17, so that the procedure determines, for example, that a slipping process is occurring. The slip condition flag (progression 9) is not equal to zero from this point onwards.

[0051] For example, the print request p Ssm gradually reduced to zero (curve 4). The hill start assist function is deactivated (activation flag - course 1) when the pressure request p is reached at time 15.Ssm The value is zero. The vehicle therefore starts rolling from time 14 onwards, so that no standstill is detected and the standstill flag is zero (progression 10).

[0052] The driver reacts to the rolling by pressing the brake pedal again himself, thereby increasing the pressure p in the master cylinder to prevent the rolling (procedure 3).

[0053] The determined result of the slip detection 9 is further processed via a second predetermined number of calculation cycles N. a This is maintained until time 16. This prevents the hill start assist function from being reactivated. Symbol Name N S Number of calculation cycles to be considered; predetermined number of calculation cycles ε1 = ε first limit ε2 = -ε second limit Ψ̇ Grenz third limit (entry threshold) N a second predetermined number of calculation cycles i first counter C second counter Ψ̇ exit fourth limit value (exit threshold) Ψ̇ mess measured driving dynamics parameter Ψ̇ i stored driving dynamics parameter p Pressure in the main cylinder p Ssm Brake pressure request Standstill state Ψ̇ SD,calc Driving dynamics value Σ + first sum Σ second sum SLIP result of the slip detection SLIP_LL Result of the last slip detection

Claims

[1] Method for detecting a skidding process of a motor vehicle, where a braking force value (p) is determined by a holding assist function. Ssm ) is set on at least one wheel brake of the motor vehicle to prevent the motor vehicle from rolling away after a stopping maneuver and the slippage process is detected by evaluating a vehicle dynamics parameter (Ψ̇) that characterizes the lateral movement of the vehicle, characterized by , that, especially in successive calculation cycles, at least one sum (Σ + , Σ - ) is formed via a plurality of measured values ​​of the vehicle dynamics parameter (Ψ̇), and the sliding process is recognized depending on the value of the sum or the values ​​of the sums. [2] Method according to claim 1, characterized by , that the vehicle dynamics parameter (Ψ̇) is the yaw rate of the vehicle. [3] Method according to any one of the preceding claims, characterized by, that only measured values ​​of the vehicle dynamics parameter (Ψ̇) are included in the sum (Σ + , Σ - ) or the sums that are entered within a given past time period, in particular within a given number of previous calculation cycles (N S ), were determined. [4] Method according to any one of the preceding claims, characterized by , that a first sum (Σ + ) about those measured values ​​of the vehicle dynamics parameter ̇ (Ψ̇) is formed, which lie above a first given limit (ε1), and a second sum (Σ - ) is formed over those measured values ​​of the vehicle dynamics parameter (Ψ̇) that are below a second predetermined limit value ε2. [5] Method according to claim 4, characterized by , that the difference between the amounts of the first and second sums (Σ + , Σ - ) is formed, with the difference resulting in a driving dynamics value (Ψ̇) SD,calc) is calculated, whereby a skidding process is detected when the vehicle dynamics value (Ψ̇) SD,calc ) above a given third limit (Ψ̇ Grenz ) lies, or that the first and second sum (Σ + , Σ - ) are added to a total sum, from which a driving dynamics value (Ψ̇) is derived from the amount of the total sum SD,calc ) is calculated, whereby a skidding process is detected when the vehicle dynamics value (Ψ̇) SD,calc ) above a given third limit (Ψ̇ Grenz ) lies. [6] Method according to claim 5, characterized by , that a skidding process is also detected when the vehicle dynamics value (Ψ̇ SD,calc ) above a fourth limit (Ψ̇ exit ) and a slippage process was detected in a directly preceding calculation cycle. [7] Method according to claim 6, characterized by , that a skidding process is also detected when the vehicle dynamics value (Ψ̇ SD,calc ) below the fourth limit (Ψ̇ exit ) lies, and - a slippage process was detected in a directly preceding calculation cycle and - in one of the preceding calculation cycles that is less than a second predetermined time interval or a second predetermined number of calculation cycles N a is behind, the driving dynamics value (Ψ̇ SD,calc ) above the fourth limit (Ψ̇ exit ) was. [8] Method according to claim 7, characterized by , that it is considered recognized that no skidding process is taking place if the driving dynamics value (Ψ̇ SD,calc ) during the second specified time period or the second specified number of consecutive calculation cycles (N a ) below the fourth limit (Ψ̇ exit ) lies. [9] Method according to any one of the preceding claims, characterized by, that the procedure is only carried out if a standstill of the motor vehicle is detected, in particular based on the wheel speeds, and / or a measured pressure in the master cylinder (p) of the motor vehicle is less than a brake pressure request (p) determined by the holding assist function Ssm ). [10] Method according to any one of the preceding claims, characterized by , that the braking force value (p Ssm ) is reduced when a slippage process is detected. [11] Electronic control unit for controlling a braking system of a motor vehicle, which performs a holding assist function to prevent the vehicle from rolling away after a stop, by which a braking force value (p) Ssm) is set on at least one wheel brake of the motor vehicle, which performs a detection of a skidding process of the motor vehicle, wherein the skidding process is detected by evaluating a vehicle dynamics parameter (Ψ̇) that characterizes the lateral movement of the motor vehicle, characterized by , that, in order to detect the slippage process, especially in successive calculation cycles, at least one sum (Σ + , Σ - ) is formed via a plurality of measured values ​​of the vehicle dynamics parameter (Ψ̇), and the sliding process is recognized depending on the value of the sum or the values ​​of the sums.

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