METHOD FOR IMPROVING THE CONTROL BEHAVIOR OF AN ELECTRONIC MOTOR VEHICLE BRAKING SYSTEM

Abstract

The invention relates to a method for improving the control behavior of an electronic motor vehicle braking system which comprises at least a slip control function. Wheel dynamic information which is evaluated as a criterion for initiating a control intervention is used individually for each wheel and is compared with control thresholds for a pressure reduction phase, a pressure maintenance phase, and a pressure buildup phase for generating corresponding braking torques by means of a vehicle braking system. According to the invention, the expected acceleration change of a vehicle wheel is calculated from a pressure change at said wheel, said pressure change being caused by a control intervention; the actual acceleration change at the vehicle wheel, said acceleration change being caused by the pressure change, is determined from measured wheel speeds as wheel dynamic information; and the control behavior of the slip control is adapted when the actual acceleration change deviates from the expected acceleration change by a defined degree such that the deviation is minimized.

Claims

1. A method for improving the control behavior of an electronic motor vehicle braking system, comprising at least the function of a slip control, in which analyzed wheel dynamics information is used for each individual wheel as a criterion for initiating a control intervention and is compared with control thresholds for a pressure-decrease phase, a pressure-maintenance phase and a pressure-increase phase for generating corresponding braking moments by means of a vehicle braking system, characterized in that the expected acceleration change (j.sub.exp) at a vehicle wheel is calculated from a pressure change at this wheel effected by means of a control intervention, the actual acceleration change (j.sub.act) effected by the pressure change at the vehicle wheel is determined from measured wheel speeds as wheel dynamics information, and in the case of a deviation of the actual acceleration change (j.sub.act) from the expected acceleration change (j.sub.exp) by a defined amount, the control behavior of the slip control is adjusted in such a manner that the deviation becomes minimal.

2. The method as claimed in claim 1, characterized in that the defined amount is given by a tolerance band defined around the value of the expected acceleration change (j.sub.exp)

3. The method as claimed in claim 1, characterized in that the defined amount is given by a value range, the values of which are compared with the quotient of the actual acceleration change (j.sub.act) and the expected acceleration change (j.sub.exp).

4. The method as claimed in one of the preceding claims, characterized in that the control behavior is adjusted by means of a change of the controller amplification factors and/or by means of a change of the controller frequency and/or by means of a change in the pulse height relating to the pressure request.

5. The method as claimed in one of the preceding claims, characterized in that the pressure change signals used for determining the expected acceleration change (j.sub.exp) and the wheel speed signals used for determining the actual acceleration change (j.sub.act) are temporally synchronized to one another.

Description

[0016] According to an advantageous embodiment of the invention, the defined amount is given by a tolerance band defined around the value of the expected acceleration change. If the value of the actual acceleration change lies outside this tolerance band, the deviation is too large, so that a control intervention becomes necessary.

[0017] An alternative embodiment of the invention provides that the defined amount is given by a value range, the values of which are compared with the quotient of the actual acceleration change and the expected acceleration change.

[0018] According to a further preferred development of the invention, the control behavior is adjusted by means of a change of the controller amplification factors and/or by means of a change of the controller frequency and/or by means of a change in the pulse height relating to the pressure request.

[0019] As the signals for calculating the expected acceleration change and the signals for determining the actual acceleration change must physically relate to the same point in time, it is provided according to a development that the pressure change signals used for determining the expected acceleration change and the wheel speed signals used for determining the actual acceleration change are temporally synchronized to one another.

[0020] The method according to the invention is explained in more detail in the following with reference to the attached FIG. 1. This FIG. 1 shows the curves occurring at a vehicle wheel for the speed v, the braking pressure p and the expected and actual acceleration change, which represents a jerk value j as a differential variable.

[0021] The following is based on an ABS control of a motor vehicle braking system using an ABS controller, using which the braking behavior of the vehicle wheels of a vehicle is controlled and an excessive slip is prevented in the process.

[0022] FIG. 1 shows a section from such an ABS control of a vehicle wheel, which is in an unstable phase, that is to say in a braking phase with slip onset.

[0023] Thus, at time t.sub.1, a pressure-decrease phase is introduced by the ABS controller, with the consequence that the braking pressure p and thus also the wheel speed v decreases in accordance with the representation according to FIG. 1.

[0024] On the basis of the pressure change at the vehicle wheel, an expected acceleration change is calculated as jerk value j.sub.exp from the gradient of the pressure p by means of a braking pressure model. As the pressure p for t<t.sub.1 and t>t.sub.3 is constant, the value zero results for the expected acceleration change j.sub.exp. With the start of the pressure decrease, that is to say shortly after the time t.sub.1, this calculated signal j.sub.exp changes to a positive value, which remains constant due to the constant gradient of the pressure until the end of the pressure decrease at time t.sub.3, in order to subsequently drop again to zero.

[0025] The actual acceleration change is determined as jerk value j.sub.act and calculated from the wheel speed v as 2nd derivative. As long as the speed curve v is curved to the right, the actual jerk j.sub.act shows negative values and becomes positive if the speed curve v transitions to a left curvature (cf. FIG. 1).

[0026] During the entire ABS control, the actual jerk value fact is compared with the expected jerk value j.sub.exp.

[0027] Generally, these two variables j.sub.exp and j.sub.act only differ slightly within a predetermined amount, i.e. the ABS control behaves as expected.

[0028] However, if the situation illustrated in FIG. 1 by way of example, that the difference between these two values j.sub.exp and j.sub.act exceeds a defined amount at time t.sub.2, occurs, this means that the acceleration change j.sub.act that is set is substantially smaller than the expected acceleration change j.sub.exp.

[0029] The defined amount is given by a tolerance band defined around the value of the expected acceleration change j.sub.exp. A fault is assumed if the actual acceleration change j.sub.act lies outside this tolerance band. Therefore, a large control deviation is imminent, which may lead to a slip onset which will last for a long time and can only be corrected over a plurality of control loops.

[0030] The cause of such a fault, that is to say the deviation of the actual acceleration change j.sub.act from the expected acceleration change j.sub.exp beyond the defined amount may for example lie in the wheel load decreasing, a decreasing highway coefficient of friction being present (due to a change in the highway surface) and/or the tire coefficient of friction decreasing because of the slip. The cause may also lie on the actuator side of the vehicle braking system, for example a smaller coefficient of friction of the lining may be present than was assumed during the calculation of the expected jerk value j.sub.exp, as a result of which the braking-moment decrease is actually smaller than expected. The defined amount for detecting a fault may also be specified by a value range, the values of which are compared with the quotient of the actual acceleration change j.sub.act and the expected acceleration change j.sub.exp. If this quotient lies outside this value range, a fault is assumed.

[0031] If a deviation of the actual acceleration change j.sub.act from the expected acceleration change j.sub.exp by the defined amount detected in the above-mentioned manner is then present, the ABS controller intervenes and carries out a correction at time t.sub.2, in that for example by means of a parameter switch, the pressure request is corrected such that the pressure decrease is continued in a timely manner and consequently at a time t.sub.3, the actual jerk value j.sub.act follows the expected jerk value j.sub.exp within the defined amount, as is illustrated in FIG. 1. From this time t.sub.3, the deviation between these two values j.sub.exp and j.sub.act is only slight and therefore no longer lies outside the defined amount.

[0032] Also illustrated in FIG. 1 is the situation where an early extension of the pressure-decrease phase is not carried out in spite of the deviation of the two variables j.sub.exp and j.sub.act lying outside the defined amount. The pressure decrease would then be ended at time t.sub.2 (cf. FIG. 1, braking pressure curve p, illustrated dashed), with the consequence that the slip also increases further, that is to say the slip onset is extended (cf. FIG. 1, speed curve v, illustrated dashed). The ABS control would only react to this slip onset with a pressure decrease at a later point in time, that is to say after a decrease pause, with the consequence that the speed v would fall further after the time t.sub.2.

[0033] However, using the method according to the invention, the pressure decrease is continued or even amplified because of the deviation between the expected jerk value j.sub.exp and the actual jerk value j.sub.act lying outside the defined amount.

[0034] In the case of a deviation of the two jerk values j.sub.exp and j.sub.act lying outside the defined amount, the control behavior of the ABS controller can also consist in changing the controller amplification factors, particularly the I part (also the other parts), reducing the pause time of the control, that is to say increasing the control frequency or changing the pulse height.

[0035] When determining or calculating the variables j.sub.exp and j.sub.act it is to be ensured that the associated signals with regards to the pressure change and the acceleration change are synchronized to one another, and that they do not physically relate to the same point in time, even if they are calculated simultaneously in the ABS controller.

[0036] As the mathematical derivation in the ABS controller is for the most part determined computationally by means of a differentiation between the respective signal variable at the current point in time and the signal variable at a preceding point in time, there is a time delay here which adds up over multiple derivations. The actually “oldest” signal is therefore relevant during the synchronization and older values of the physically newer signals must be made available by means of intermediate storage. If one of the signals used is filtered, the other signals used are correspondingly also to be filtered in the same way.

[0037] The above-described exemplary embodiment relates to the case of “pressure decrease”, in order to allow re-acceleration. Naturally, it is also valid for the case of “pressure increase”, in order to limit the re-acceleration.

[0038] The illustrated method is suitable not only for ABS control, if the vehicle wheel is in the unstable region of the p-slip curve, but also generally for a slip control in the unstable region of a vehicle wheel.