Method for determining current friction coefficient of brake

Abstract

The invention relates to a method for determining the current coefficient of friction of at least one brake of a motor vehicle during coasting or traction operation of the motor vehicle, comprising at least the following steps: briefly applying a defined brake pressure to the brake whose coefficient of friction is to be determined, the generated braking torque is compensated by adapting to a driving torque during coasting or traction operation of the motor vehicle, the braking torque is determined from a required change in the driving torque during coasting or traction operation of the motor vehicle, and the current friction coefficient is calculated from a defined braking pressure and the determined braking torque.

Description

Field of Technology

[0001] The present invention relates to a method for determining a current coefficient of friction of at least one brake of a motor vehicle during coasting or towing operation of the motor vehicle. Background Art

[0002] In electronic driving assistance systems (such as ABS, ESP, torque vectoring functions), the braking pressure of the corresponding braked wheel is adjusted. However, the actual coefficient of friction, which is decisive for the braking effect in addition to the braking pressure, of the brake is unknown to the system.

[0003] However, due to different influencing factors such as weather, wear, dirt, etc., the coefficient of friction varies strongly, so that the braking effect may also vary strongly. In current solutions, the coefficient of friction is not detected. Therefore, possible variations cannot be taken into account. It is assumed that the coefficient of friction of the brake is always the same value. If the coefficient of friction is stored as variable in the system, this is based on empirical values, for example empirical values from previously performed measurements during the service life of the friction pair. Short-term variations of the coefficient of friction cannot be taken into account. Summary of the Invention

[0004] The object of the present invention is to describe a method that allows determination of the coefficient of friction of a brake during coasting or towing operation of a motor vehicle.

[0005] The above requirement can be covered by the subject matter of independent claim 1 of the present invention. Advantageous embodiments of the present invention are described in the dependent claims.

[0006] The features, details and possible advantages of the device according to embodiments of the present invention are discussed in detail below.

[0007] In principle, the method according to the present invention for determining the coefficient of friction of a brake of a motor vehicle during vehicle travel can be used not only in motor vehicles having an internal combustion engine as a drive unit but also in motor vehicles having an electric motor as a drive unit - the only prerequisite being the feasibility of detecting the driving torque of the corresponding drive unit. However, in an electric motor operating in motor mode, the torque can be determined more precisely than in an internal combustion engine, which in turn corresponds to the method according to the present invention enabling precise determination of the coefficient of friction of the brake.

[0008] The method according to the present invention can be applied not only to drum brakes but also to disc brakes, only that depending on the brake variant, the "external" influence on the coefficient of friction is different.

[0009] A method for determining the current friction coefficient of at least one brake of a motor vehicle during the coasting or towing operation of the motor vehicle according to the present invention comprises at least the following steps:

[0010] - Briefly load the brake for which the friction coefficient is to be determined with a defined braking pressure,

[0011] - Compensate for the generated braking torque by adapting the driving torque during the coasting or towing operation of the motor vehicle,

[0012] - Determine the braking torque from the required change in the driving torque during the coasting or towing operation of the motor vehicle,

[0013] - Calculate the current friction coefficient from the defined braking pressure and the determined braking torque.

[0014] The brake for which the friction coefficient is to be determined can be arranged on the drive axle and / or on the non-drive axle.

[0015] In a preferred embodiment variant of the present invention, the current friction coefficient of the brake is determined when the motor vehicle is moving at a constant speed.

[0016] In another preferred embodiment variant of the present invention, the current friction coefficient of the brake of the motor vehicle is determined during regenerative braking.

[0017] Preferably, the method is initiated based on long-term and / or short-term effects on the friction coefficient.

[0018] Long-term effects on the friction coefficient are, for example, longer parking times, expired inspection intervals, or services with brake lining replacement or brake disc replacement, etc.

[0019] Short-term effects on the friction coefficient relate to all effects that rapidly change the friction coefficient, such as through specific weather conditions or the brake disc temperature due to a change in power input during braking, etc.

[0020] The method can be initiated by a software loop, wherein preferably after the motor vehicle is put into operation, it is checked via the software loop whether a long-term and / or short-term effect on the friction coefficient has occurred since the method was last executed. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] The present invention is described below by way of example with reference to the accompanying drawings.

[0022] Figure 1 A schematic diagram showing a software loop for determining when to execute the method according to the present invention is shown. DETAILED DESCRIPTION

[0023] The following describes a method for determining the friction coefficient of at least one brake of a motor vehicle during the coasting or towing operation of the motor vehicle according to the present invention, with the motor serving as the drive unit.

[0024] In principle, the method can be divided into two implementation variants, which, however, can both be applied simultaneously and independently of each other in the motor vehicle:

[0025] 1. Determining the friction coefficient of at least one brake provided on the drive axle of the motor vehicle, and

[0026] 2. Determining the friction coefficient of at least one brake provided on the non-drive axle of the motor vehicle.

[0027] Therefore, in the motor vehicle, it can be proposed to determine the friction coefficient of at least one brake on the drive axle of the motor vehicle, determine the friction coefficient of at least one brake on the non-drive axle of the motor vehicle, or determine the friction coefficient of at least one brake on the non-drive axle and the drive axle of the motor vehicle.

[0028] In the first implementation variant, the friction coefficient of at least one brake provided on the axle that also serves as the drive axle is determined. This can be not only the front axle but also the rear axle of the motor vehicle. In a four-wheel drive vehicle with a drive system provided on each axle, the method can be applied independently of each other to the brakes on all drive axles. In an electric vehicle with a motor provided on each wheel, the friction coefficient of the brake of each wheel can even be measured individually.

[0029] In the second implementation variant, the friction coefficient of at least one brake provided on the axle that does not serve as the drive axle but "rolls freely together" is determined. In this case, the brake whose friction coefficient is to be determined and the drive unit are not provided on the same axle. The non-drive axle is braked via the brake, and the increased resistance torque must be compensated by the drive on the drive axle. It should be noted that in addition to transmission losses, wheel slip also affects the measurement result here.

[0030] For the moment of determining the friction coefficient, in principle, two scenarios can be considered - the friction coefficient of the brake can be determined during coasting or during towing operation.

[0031] During towing operation, it is meaningful for the motor vehicle to move at a constant speed and with a constant load at the moment of measurement, so that as accurate a calculation as possible can be performed. This is the case, for example, during operation with a speed controller. Here, the motor should be speed-adjustable so that when a braking torque is introduced via the brake to be measured, the motor maintains its speed and thereby increases the torque by a compensation amount corresponding to the magnitude of the braking torque to maintain the speed.

[0032] In the case of coasting operation, the measurement can be performed, for example, during regenerative braking. This is advantageous in cases where a constant deceleration is involved so that an exact calculation can be achieved. This is the case, for example, when driving downhill at a constant speed. In this case, pure regenerative braking reduces a certain proportion. This proportion is provided instead by the operating brake to be measured. The magnitude of the reduction of the regenerative brake corresponds to the magnitude of the braking torque generated by the operating brake. With the aid of this value and the applied brake pressure, the friction coefficient can be calculated again. The state of constant deceleration can be detected, for example, by an acceleration sensor in the motor vehicle, GPS data, wheel speed or a combination of all the mentioned possibilities.

[0033] In principle, losses in terms of the vehicle's travel are taken into account every time the friction coefficient is measured, whether during steady travel (traction operation) or during regenerative braking (coasting), since each actuation of the brake reacts on the drive and reduces efficiency. The measurement process is therefore only carried out as often as necessary. For reasons of efficiency, it makes sense to carry out the measurement during regenerative braking, since in this way overall less energy is lost than when a drive torque must be actively applied by means of a motor to compensate for the braking torque. The measurement is particularly efficient when the braking is so strong that the brakes must be used anyway, since the regenerative braking effect is no longer sufficient. In this case, the measurement can be carried out without losing more energy.

[0034] When the method is executed is determined by a software loop which detects as many influences as possible, in particular on the friction coefficient of the brake, and initiates the execution of the method when necessary. Figure 1 A schematic diagram of such a software loop is shown in . The software loop checks both possible short-term and long-term changes in the friction coefficient of the brake. After the motor vehicle has been put into operation, in a first step it is checked whether events have occurred that have a long-term effect on the friction coefficient since the last execution of the method. This includes, for example, detecting longer parking times, expired inspection intervals or services involving replacement of brake linings or brake discs. In addition, brake use and wear are monitored, which can be estimated based on how much energy has been applied to the brakes. The braking energy can be determined via the (calculated) braking torque and the measured wheel speed. If one of the influences mentioned here is detected, the software loop issues an instruction for executing the method ( Figure 1 ).

[0035] In a further branch of the software loop, the short-term effects are now checked. This includes the effects of all rapid changes in the coefficient of friction, such as specific weather conditions or the brake disc temperature due to a change in power input during braking. In addition, parameters are considered in order to estimate whether there is a higher probability for situations that require precise braking intervention or for which precise braking intervention is advantageous. This includes the current driving style, the route section or the traffic. If a change in the friction situation or a future critical situation is expected, the method ( Figure 1 ) is executed.

[0036] Before the instructions for executing the method are implemented, it must first be checked whether the motor vehicle is in a critical situation or whether an unsafe situation could occur due to a light braking during the execution of the method. If this is the case, the execution of the method is not permitted. Possible critical situations include, for example: when a driving assistance system such as ABS or ESP is activated, when the road surface is detected as not being sufficiently even and non-slip, and in the case of sub-zero temperatures and slippery road conditions, in the case of heavy rainfall or in the case of extreme cornering, etc. If no critical situation is recognized, the method can finally be executed. At the end of the software loop, the result "current coefficient of friction" is processed in the system - the parameter "current coefficient of friction" of the brake is corrected to a new value ( Figure 1 ).

[0037] The invention is characterized in that not only is the coefficient of friction detected during the current driving of the motor vehicle in order to improve the braking effect, but in addition data should be collected within the scope of the service life of the motor vehicle, from which a profile of the change in the coefficient of friction can be created depending on different parameters. The data can be statistically evaluated such that the trend of how the coefficient of friction changes depending on different influences can be seen. Then, with the aid of the evaluated data, the software loop can predict the coefficient of friction very precisely without having to continuously measure. The software can be said to "learn" from past coefficient-of-friction determinations and can gain knowledge from the information stored in the database. In addition, the information collected can be stored as a data packet in the "cloud". There, all motor vehicles of the same construction type of the manufacturer can not only feed in information in order to increase the data diversity and accuracy, but also download data in order to always be equipped with the latest data set for predicting the coefficient of friction ( Figure 1 )

Claims

1. A method for determining a current friction coefficient of at least one brake of a motor vehicle during coasting or towing operation of the motor vehicle, the method comprising at least the following steps: - Briefly load the brake for which the friction coefficient is to be determined with a defined braking pressure, - Compensate for the generated braking torque by adapting the driving torque during coasting or towing operation of the motor vehicle, - Determine the braking torque from the required change in the driving torque during coasting or towing operation of the motor vehicle, - Calculate the current friction coefficient from the defined braking pressure and the determined braking torque.

2. The method according to claim 1, It is characterized in that The brake for which the friction coefficient is to be determined is arranged on the drive axle.

3. The method according to claim 1 or 2, It is characterized in that The brake for which the friction coefficient is to be determined is arranged on a non-drive axle.

4. The method according to claim 1, 2 or 3, It is characterized in that Determine the current friction coefficient of the brake when the motor vehicle is moving at a constant speed.

5. The method according to claim 1, 2 or 3, It is characterized in that Determine the current friction coefficient of the brake during regenerative braking.

6. The method according to any one of the above claims, It is characterized in that Start the method based on long-term and / or short-term effects on the friction coefficient.

7. The method according to any one of the above claims, It is characterized in that Start the method by means of a software loop.

8. The method according to claim 7, It is characterized in that Check via the software loop after the motor vehicle has been put into operation whether a long-term and / or short-term effect on the friction coefficient has occurred since the method was last executed.

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