DE102016201937A1 - Method for determining the initial slope of a braking torque-slip characteristic - Google Patents

Abstract

The invention relates to a method for determining the initial slope of a braking torque-slip characteristic of a measured brake slip at least one vehicle wheel of a vehicle and a recuperative braking torque on this vehicle wheel with the following steps: measuring the wheel speed of the braked vehicle wheel and determining the wheel speed of the braked vehicle wheel from the Wheel speed, measuring the wheel speed of an unbraked vehicle wheel of the vehicle and determining the vehicle speed from the wheel speed, determining the brake slip from the wheel speed of the braked vehicle wheel and the vehicle speed, determining the recuperative braking torque by measuring the electrical power generated by recuperation, and determining the initial slope of the braking torque Slip characteristic from the recuperative braking torque and the specific slip. The invention also relates to a method in which instead of a recuperation braking a braking by means of a retarder is performed. Furthermore, the invention relates to devices for carrying out the method according to the invention.

Description

The invention relates to a method for determining the initial slope of a braking torque-slip characteristic from a brake slip and a braking torque of a vehicle wheel or the wheels of an axle of a vehicle.

For a faultless function of a modern driver assistance system, such as an emergency braking system, it is necessary to estimate a coefficient of adhesion between the tire and the road.

During manual driving, the estimation of the coefficient of adhesion is the responsibility of the driver. An essential part of the driving task is to estimate the road condition and to adapt the driving style or style of driving. With the introduction of highly automated systems and driving functions, the driving task is gradually transferred from the driver to the vehicle. The essential basis for this is to determine the driving environment or the driving conditions as exactly as possible, to describe them and make them available to the system. The maximum adhesion coefficient between the tire and the road surface determines the physical limit for the vehicle's realizable driving dynamics. Thus, the information about the adhesion coefficient forms an essential part of the environment representation. With regard to automated driving reliable information about the road condition or derived from the available maximum adhesion coefficient is an indispensable amount of information to secure the driving function. But even in the area of so-called Driver Assistance Systems and Advanced Driver Assistance Systems, knowledge of the available maximum coefficient of force coefficient results in a significant safety benefit.

From the DE 40 10 507 C1 is a method for monitoring the adhesion between the road and tires of driven vehicle wheels known. In this method, it is assumed that a certain coefficient of friction with associated maximum braking torque can be assigned to different initial gradients. Thus, the wheel slip λ and at the same time an effective Radumfangskraft the driven wheels during stationary driver operating conditions continuously determined. These value pairs from the determined Radumfangskraft and the wheel slip are stored to form a Radumfangskraft wheel slip characteristic field, so that so that the linear part of the slip characteristic is known. The knowledge of the curve increase of the currently effective slip characteristic also provides the knowledge of the maximum effective coefficient of friction or the magnitude of the maximum wheel peripheral force possible in this driving state, since according to this known method there is a substantially fixed relationship between the linear rise and the maximum curve of the slip characteristics. Also the WO 2014/199328 A1 describes a method for friction coefficient estimation and proceeds from the recognition that slip characteristics with a low maximum adhesion coefficient (also called μ-peak) leave the linear region at a lower slip value than is the case with slip characteristics with a higher maximum adhesion coefficient.

Furthermore, also describes the DE 10 2012 217 772 A1 a method for determining the maximum adhesion coefficient by also determining the initial slope of a slip characteristic and from this the maximum adhesion coefficient is determined. For this purpose, first the current traction coefficient on the vehicle wheel and the associated slip is determined by measurements and from this the value of the initial slope is calculated. By comparison with the reference origin line, the range for a maximum adhesion coefficient is determined.

In the area of the initial slope of a slip characteristic, both tire forces and tire slip are very low. In order to obtain a sufficient degree of definition in stationary or quasi-stationary states, a robust and, above all, well-resolving signal is required both for the braking torque or the braking force and for the slip. If the scatter is very large and / or the resolution is low only for one of the two signals, it is no longer possible to achieve an accurate result.

Moreover, from the DE 10 2004 044 788 B4 a method of friction coefficient determination between a tire of a vehicle and a roadway is known in which the wheel speed and the absolute speed of the vehicle are measured, from this wheel speed and the absolute speed of the slip is determined, then a force acting on the vehicle driving or braking torque is measured and Finally, a coefficient of friction between the tire and the roadway is determined by means of an algorithm, are taken into account as the input of the slip and the drive or braking torque.

It is also known to produce a braking torque for braking the vehicle in the energy recovery in vehicles with hybrid drive with internal combustion engine and electric machine as a drive motor or pure electric vehicles with electric drive machine via the electric generator.

In addition, it is also known to use in buses and commercial vehicles so-called retarders as a braking aid, for example. As electrodynamic retarder, also called eddy current brake, are running. To actuate such a retarder, the magnetic coils are energized, wherein the size of the coil current determines the value of the braking torque.

The object of the invention is to provide a method and a device for determining the initial slope of a braking torque-slip characteristic between a braked vehicle wheel or the wheels of an axle and a roadway, with which the braking torque and the brake slip with high resolution in the per thousand range are determined can.

This object is achieved by a method for determining the initial slope of a braking torque-slip characteristic with the features of patent claim 1 and claim 2.

• – Messen der Raddrehzahl des wenigstens einen gebremsten Fahrzeugrades und Bestimmen der Radgeschwindigkeit des gebremsten Fahrzeugrades aus der Raddrehzahl,
• – Messen der Raddrehzahl mindestens eines ungebremsten Fahrzeugrades des Fahrzeugs und Bestimmen der Fahrzeuggeschwindigkeit aus der Raddrehzahl,
• – Bestimmen des Bremsschlupfes aus der Radgeschwindigkeit des wenigstens einen gebremsten Fahrzeugrades und der Fahrzeuggeschwindigkeit,
• – Bestimmung des rekuperativen Bremsmomentes durch Messung des durch Rekuperation in einen Energiespeicher geladenen Ladestroms, und
• – Bestimmung der Anfangssteigung der Bremsmoment-Schlupf-Kennlinie aus dem rekuperativen Bremsmoment und dem bestimmten Schlupf.

In the method according to the invention according to the first-mentioned solution, the following steps are carried out to determine the initial slope of a braking torque-slip characteristic from a measured brake slip of at least one vehicle wheel of a vehicle and a recuperative braking torque on the at least one vehicle wheel:

• Measuring the wheel speed of the at least one braked vehicle wheel and determining the wheel speed of the braked vehicle wheel from the wheel speed,
• Measuring the wheel speed of at least one unrestrained vehicle wheel of the vehicle and determining the vehicle speed from the wheel speed,
• Determining the brake slip from the wheel speed of the at least one braked vehicle wheel and the vehicle speed,
• - Determination of the recuperative braking torque by measuring the charged by recuperation in an energy storage charging current, and
• - Determining the initial slope of the braking torque-slip curve from the recuperative braking torque and the specific slip.

• – Messen der Raddrehzahl des wenigstens einen gebremsten Fahrzeugrades und Bestimmen der Radgeschwindigkeit des gebremsten Fahrzeugrades aus der Raddrehzahl,
• – Messen der Raddrehzahl mindestens eines ungebremsten Fahrzeugrades des Fahrzeugs und Bestimmen der Fahrzeuggeschwindigkeit aus der Raddrehzahl,
• – Bestimmen des Bremsschlupfes aus der Radgeschwindigkeit des wenigstens einen gebremsten Fahrzeugrades und der Fahrzeuggeschwindigkeit,
• – Bestimmung des Retarder-Bremsmomentes durch Messung des einer elektrodynamischen Retarderbremse zugeführten Betriebsstromes, und
• – Bestimmung der Anfangssteigung der Bremsmoment-Schlupf-Kennlinie aus dem bestimmten Retarder-Bremsmoment und dem bestimmten Schlupf.

In the method according to the invention according to the second-mentioned solution, the following steps are carried out to determine the initial slope of a braking torque-slip characteristic from a measured brake slip of at least one vehicle wheel of a vehicle and a retarder braking torque on this vehicle wheel:

• Measuring the wheel speed of the at least one braked vehicle wheel and determining the wheel speed of the braked vehicle wheel from the wheel speed,
• Measuring the wheel speed of at least one unrestrained vehicle wheel of the vehicle and determining the vehicle speed from the wheel speed,
• Determining the brake slip from the wheel speed of the at least one braked vehicle wheel and the vehicle speed,
• - Determination of the retarder braking torque by measuring the fed an electrodynamic retarder brake operating current, and
• - Determining the initial slope of the braking torque-slip characteristic from the determined retarder braking torque and the determined slip.

In the former solution, the recuperation process is used in hybrid or electric vehicles to determine therefrom the initial slope of an initially unknown slip characteristic.

In the second-mentioned solution, the retarder brake is used in commercial vehicles or buses to determine therefrom also the initial slope of an initially unknown slip characteristic.

From the initial slope of an initially unknown slip characteristic can be determined according to an advantageous embodiment of the invention, the slip characteristic associated with this initial slope with a maximum possible braking torque by estimation. With such a maximum possible braking torque and associated slip, a coefficient of friction estimation can also be carried out.

In order to increase the accuracy of the determination of the braking torque, the vehicle deceleration of the vehicle is additionally measured according to further development.

• – der Bremsschlupf jeweils von allen Fahrzeugrädern einer Fahrzeugachse bestimmt wird,
• – aus dem für jedes Fahrzeugrad bestimmten Bremsschlupf die Anfangssteigung der Bremsmoment-Schlupf-Kennlinie bestimmt wird, und
• – aus der für jedes Fahrzeugrad bestimmten Anfangssteigung die zugehörige Schlupfkennlinie mit einem maximal möglichen Bremsmoment geschätzt wird.

In order to determine the associated slip characteristic even with a μ-split braking, it is provided according to a further preferred development of the invention that

• The brake slip is determined in each case by all vehicle wheels of a vehicle axle,
• - From the determined for each vehicle brake brake slip the initial slope of the braking torque-slip characteristic is determined, and
• - From the determined for each vehicle initial slope the associated slip characteristic is estimated with a maximum possible braking torque.

For this purpose, the yaw rate and / or the lateral acceleration of the vehicle are preferably additionally measured for determining the slip characteristic of the vehicle wheels of a vehicle axle, in order to improve the accuracy of the slip characteristic curve.

Devices for carrying out the method according to the invention are given with the features of claims 6 and 7.

The method according to the invention will be described below with reference to exemplary embodiments with reference to a single embodiment 1 which shows a braking torque-slip characteristic field with two slip curves μ1 and μ2. For this purpose, the braking torque M is shown as a function of the wheel slip λ with the coefficient of friction μ as a parameter. The slip curve μ1 shows the course of a Niedrigreibwertes μ _N, while the slip curve μ2 shows the course of a Hochreibwertes μ _H.

A recuperation process of a hybrid or electric vehicle or a braking operation by means of a retarder brake of a commercial vehicle is used to determine an initial slope α of an unknown slip curve μ. If this initial slope corresponds to the value α _N , the associated slip curve μ1 is determined by estimation with which the maximum possible braking torque M _{max_1 is also} known. However, if the initial slope corresponds to the value α _H , the associated slip curve μ 2 is determined by estimation, with which the maximum possible braking torque M _{max - 2 is} known.

From the estimated value of a maximum possible braking torque M _max , a maximum possible coefficient of friction can be calculated by first dividing the maximum braking torque by the effective Council radius. Subsequently, the calculated maximum braking force is divided by the corresponding wheel load. Z

The estimation of the slip curve from an initial slope α is performed by dividing the possible set of curves into a maximum of four classes on the basis of a similar characteristic. The mean initial slope of a class is assigned to a mean possible coefficient of friction of the slip curve of this class.

• – Messen der Raddrehzahl des gebremsten Fahrzeugrades und Bestimmen der Radgeschwindigkeit des gebremsten Fahrzeugrades aus der Raddrehzahl,
• – Messen der Raddrehzahl eines ungebremsten Fahrzeugrades des Fahrzeugs und Bestimmen der Fahrzeuggeschwindigkeit aus der Raddrehzahl,
• – Bestimmen des Bremsschlupfes aus der Radgeschwindigkeit des gebremsten Fahrzeugrades und der Fahrzeuggeschwindigkeit,
• – Bestimmung des rekuperativen Bremsmomentes durch Messung der durch Rekuperation erzeugten Leistung. Aus der erzeugten Leistung der elektrischen Maschine kann unter Berücksichtigung einer Verlustleistung die mechanische Leistung bestimmt werden, mit deren Hilfe das Bremsmoment bestimmt wird, indem die mechanische Leistung durch die Winkelgeschwindigkeit ω des rotierenden Rades dividiert wird.

To determine the initial slope α in a recuperation process of a hybrid or electric vehicle, the following steps are performed:

• Measuring the wheel speed of the braked vehicle wheel and determining the wheel speed of the braked vehicle wheel from the wheel speed,
• Measuring the wheel speed of an unbraked vehicle wheel of the vehicle and determining the vehicle speed from the wheel speed,
• Determining the brake slip from the wheel speed of the braked vehicle wheel and the vehicle speed,
• - Determination of the recuperative braking torque by measuring the power generated by recuperation. From the generated power of the electric machine, taking into account a power loss, the mechanical power can be determined, with the aid of which the braking torque is determined by dividing the mechanical power by the angular speed ω of the rotating wheel.

It is also possible to measure the charging current generated by the recuperation and to determine therefrom with the following formula the braking torque M _Mi on the basis of the torque equation, for example for a synchronous machine: M _Mi = _3/2 × Z _p (M _dE × I _E × I _q + (L _d -L _q ) × I _d × I _q ), where I _{q is} the d current (Statorstrombelag), I _{d is} the d current and I _{E is} the excitation current.

In the special case of a full-pole machine, for example: M _Mi ~ I _E · I _q

In a last method step, the value of an initial slope α of a braking torque-slip characteristic is determined from the value of the recuperative braking torque and the determined slip.

• – Messen der Raddrehzahl des gebremsten Fahrzeugrades und Bestimmen der Radgeschwindigkeit des gebremsten Fahrzeugrades aus der Raddrehzahl,
• – Messen der Raddrehzahl eines ungebremsten Fahrzeugrades des Fahrzeugs und Bestimmen der Fahrzeuggeschwindigkeit aus der Raddrehzahl,
• – Bestimmen des Bremsschlupfes aus der Radgeschwindigkeit des gebremsten Fahrzeugrades und der Fahrzeuggeschwindigkeit,
• – Bestimmen des Retarder-Bremsmomentes durch Messung des einer elektrodynamischen Retarderbremse zugeführten Betriebsstromes, wobei diese Retarderbremse als Wirbelstrombremse mit Magnetspulen aufgebaut ist. Dieser Betriebsstrom wird den Magnetspulen zugeführt, die in Abhängigkeit der Höhe dieses Betriebsstromes ein entsprechendes Magnetfeld erzeugen. Mit zunehmender Stärke eines solchen Magnetfeldes erhöht sich auch das hier durch erzeugte Bremsmoment des Retarders. Aus dem gemessenen Betriebsstrom der Magnetspulen wird das zugehörige Bremsmoment

Mit einem letzten Verfahrensschritt wird ein Wert für eine Anfangssteigung α einer Bremsmoment-Schlupf-Kennlinie aus dem bestimmten Retarder-Bremsmoment und dem ermittelten Schlupf bestimmt.To determine the initial inclination α during a braking operation by means of a retarder of a commercial vehicle, the following steps are carried out:

• Measuring the wheel speed of the braked vehicle wheel and determining the wheel speed of the braked vehicle wheel from the wheel speed,
• Measuring the wheel speed of an unbraked vehicle wheel of the vehicle and determining the vehicle speed from the wheel speed,
• Determining the brake slip from the wheel speed of the braked vehicle wheel and the vehicle speed,
• - Determining the retarder braking torque by measuring the fed to an electrodynamic retarder brake operating current, said retarder brake is constructed as an eddy current brake with magnetic coils. This operating current is supplied to the magnetic coils, which generate a corresponding magnetic field as a function of the magnitude of this operating current. As the strength of such a magnetic field increases, so does the braking torque of the retarder produced by it. From the measured operating current of the solenoid coils, the associated braking torque

With a last method step, a value for an initial slope α of a braking-torque-slip characteristic is determined from the determined retarder braking torque and the determined slip.

Since hybrid and electric vehicles often recuperate and thus operate energy recovery, the availability of the initial slope as information is very high. This also applies to commercial vehicles with retarders, which are used frequently.

When recuperating in the lower part braking range of non-wheeled vehicles, the electric motor is operated as a generator on the drive axle. In terms of the highest possible energy recovery, the aperture is avoided as long as possible by the friction brake as a vehicle brake. The non-driven wheels are thus not slowed down during recuperation and run without slippage, which is why the vehicle speed can be determined from the wheel speed of these wheels. The same applies to the retarder braking in a commercial vehicle.

With the method according to the invention it is ensured that the two parameters, namely the brake slip and the braking torque during recuperation braking or brake slip and the operating current of a retarder during retarder braking to determine the initial slope of a slip characteristic with a maximum possible quality and resolution can be detected. As a result, it is possible to determine from the known initial slope the associated slip curve for estimating the maximum available braking torque.

Also in the case of μ-split braking, the inventive method allows a brake slip in each case for both wheels of a braked axle to determine a single, from this together with the braking torque at these wheels to determine a respective initial pitch α and from this the associated slip characteristic with associated estimate maximum braking torque. In order to improve the accuracy of the slip characteristics, the yaw rate and / or the lateral acceleration of the vehicle are additionally measured and used to determine the slip characteristics.

The obtained maximum braking torques from the estimated slip characteristics are used to adapt vehicle control systems or vehicle assistance systems. Furthermore, they serve as input information for an automated vehicle or for the derivation of a driving strategy.

The information regarding the maximum braking torque can also be used as an input signal for a sensor fusion for determining the local weather, the local road condition and / or a fused friction coefficient estimation.

• – einen elektrischen Antriebsmotor, welcher zur Energierückgewinnung durch Rekuperation zum verschleißfreien Verzögern des Fahrzeugs ausgebildet ist,
• – Raddrehzahlsensoren für die gebremsten und ungebremsten Fahrzeugräder des Fahrzeugs, und
• – eine Recheneinheit zur Bestimmung der Anfangssteigung der Bremskraft-Schlupf-Kurve und Schätzung des maximalen Reibwertes.

An apparatus for carrying out the method according to the invention for determining the initial pitch in a recuperation operation of a hybrid or electric vehicle comprises the following components:

• An electric drive motor, which is designed for energy recovery by recuperation for wear-free deceleration of the vehicle,
• - Raddrehzahlsensoren for the braked and unbraked vehicle wheels of the vehicle, and
• - An arithmetic unit for determining the initial slope of the braking force-slip curve and estimation of the maximum coefficient of friction.

• – eine als Retarder ausgelegte Wirbelstrombremse zum verschleißfreien Verzögern des Fahrzeugs,
• – Raddrehzahlsensoren für die gebremsten und ungebremsten Fahrzeugräder, und
• – eine Recheneinheit zur Bestimmung der Anfangssteigung der Bremskraft-Schlupf-Kurve und Schätzung des maximalen Reibwertes.

A device for carrying out the method according to the invention for determining the initial pitch during a braking operation by means of a retarder of a commercial vehicle comprises the following components:

• An eddy current brake designed as a retarder for the wear-free deceleration of the vehicle,
• - Raddrehzahlsensoren for the braked and unbraked vehicle wheels, and
• - An arithmetic unit for determining the initial slope of the braking force-slip curve and estimation of the maximum coefficient of friction.

QUOTES INCLUDE IN THE DESCRIPTION

This list of the documents listed by the applicant has been generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Cited patent literature

• DE 4010507 C1 [0004]
• WO 2014/199328 A1 [0004]
• DE 102012217772 A1 [0005]
• DE 102004044788 B4 [0007]

Claims (8)

 Method for determining the initial slope of a braking torque-slip characteristic from a measured brake slip of at least one vehicle wheel of a vehicle and a recuperative braking torque at the at least one vehicle wheel, comprising the following steps: Measuring the wheel speed of the at least one braked vehicle wheel and determining the wheel speed of the braked vehicle wheel from the wheel speed, Measuring the wheel speed of at least one unrestrained vehicle wheel of the vehicle and determining the vehicle speed from the wheel speed, Determining the brake slip from the wheel speed of the at least one braked vehicle wheel and the vehicle speed, - Determination of the recuperative braking torque by measuring the electrical power generated by recuperation, and - Determining the initial slope of the braking torque-slip curve from the recuperative braking torque and the specific slip.  Method for determining the initial slope of a braking-torque-slip characteristic from a measured brake slip of at least one vehicle wheel of a vehicle and a retarder braking torque on this vehicle wheel, comprising the following steps: Measuring the wheel speed of the at least one braked vehicle wheel and determining the wheel speed of the braked vehicle wheel from the wheel speed, Measuring the wheel speed of at least one unrestrained vehicle wheel of the vehicle and determining the vehicle speed from the wheel speed, Determining the brake slip from the wheel speed of the at least one braked vehicle wheel and the vehicle speed, - Determination of the retarder braking torque by measuring the fed an electrodynamic retarder brake operating current, and - Determining the initial slope of the braking torque-slip characteristic from the determined retarder braking torque and the determined slip.  Method according to Claim 1 or 2, in which, from the determined initial slope of the braking torque-slip characteristic curve, the associated slip characteristic curve is determined with a maximum possible braking torque with the aim of a friction value estimation by estimation.  Method according to one of the preceding claims, wherein additionally the vehicle deceleration is measured to determine the braking torque.  Method according to one of the preceding claims, in which The brake slip is respectively determined by vehicle wheels of a vehicle axle, - From the determined for each vehicle brake brake slip the initial slope of the braking torque-slip characteristic is determined, and - From the determined for each vehicle initial slope the associated slip characteristic is determined with a maximum possible braking torque by estimation.  Method according to Claim 5, in which the yaw rate and / or the lateral acceleration of the vehicle are additionally measured to determine the slip characteristic of the vehicle wheels of a vehicle axle.  Apparatus for carrying out the method according to one of the preceding claims, comprising: An electric drive motor, which is designed for energy recovery by recuperation for wear-free deceleration of the vehicle, - Raddrehzahlsensoren for the braked and unbraked vehicle wheels of the vehicle, and - An arithmetic unit for determining the initial slope of the braking force-slip curve and estimation of the maximum coefficient of friction. Apparatus for carrying out the method according to one of claims 1 to 6 with: A retarder-type eddy current brake for wear-free deceleration of the vehicle, - Raddrehzahlsensoren for the braked and unbraked vehicle wheels, and - An arithmetic unit for determining the initial slope of the braking force-slip curve and estimation of the maximum coefficient of friction.

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