Method for estimating friction value, computer program, controller, vehicle, in particular commercial vehicle

Abstract

The invention relates to a method (100) for estimating a friction value for a vehicle (300a), in particular a commercial vehicle (300b), which can be driven by an electric drive (200), comprising the following steps: operating (110) wheels (270) of the vehicle (300a), in particular a commercial vehicle (300b), which are arranged on the ground (260), with a torque (T); ascertaining (120) a slip rate (S) of the wheel (270); applying (130) a temporally predetermined excitation torque (ET) to the wheel (270), the excitation torque (ET) being periodically applied (130) to the wheel (270) at a frequency (F); ascertaining (140) a slip rate change (DS) dependent on the excitation torque (ET), the ascertaining (140) of the slip rate change (DS) taking into account the frequency (F); and ascertaining (150) the friction value (MU) by means of the slip rate change (DS).

Description

Technical Field

[0001] The invention relates to a method for estimating friction values ​​for a vehicle, in particular a commercial vehicle, which can be driven by an electric drive. The invention also relates to a computer program and / or a computer-readable medium, a control unit for a vehicle, in particular a commercial vehicle, and a vehicle, in particular a commercial vehicle. Background Art

[0002] The control of vehicles, especially commercial vehicles (such as electric trucks, electric buses, electric semi-trailers) by electric drives is known from the prior art. For example, the electric drive is installed in a tractor vehicle and / or an electrified trailer. In electrically driven vehicles, especially commercial vehicles, the electric drive is mainly used for traction, for traction support and / or for regeneration, i.e. for regenerative braking. In the following, for the sake of easier description, commercial vehicles are also referred to as vehicles.

[0003] Automated driving functions are known for vehicles and in particular for traction, traction support and / or regeneration. For example, traction functions (Automatic Traction Control, ATC (Automatic Traction Control System)), braking functions (Anti-Blockiersystem, ABS (Anti-lock Braking System)) and stability functions (Electronic Stability Control, ESC (Electronic Stability Control System)) are all well known as automated driving functions. Automated driving functions require reliable information about the lane on which the vehicle is traveling and the friction values ​​associated therewith in order to be able to perform them effectively and reliably. The maximum force that can be transmitted by the wheels is highly dependent on the friction value of the lane. If information about the friction value is missing, the respective automated driving function cannot intervene optimally at the beginning of the driving action. Information about the road surface is not taken into account for performing the current automated driving function. Therefore, it may happen that the tires of the vehicle still lock or slip despite, for example, ABS and ATC.

[0004] If the traction, braking and stabilization functions do not use friction value estimation, tire locking or slipping and thus critical driving situations cannot always be prevented on roads with low friction values. Brief tire locking and the subsequent "slip phase" with reduced braking force lead to longer braking distances. If an electric drive set up for regenerative braking participates in the braking, the regenerative power is limited in these phases and the regenerative potential is not fully utilized.

[0005] In electrically driven vehicles, making the most of regeneration is crucial for energy-efficient operation of the vehicle. Knowledge of the friction values ​​of the road is also helpful in this regard, for example in order to be able to control the optimum slip ratio on the electrically driven axle and thus enable regeneration without locking the tires.

[0006] In order to enable reliable functioning of automated driving functions and efficient regeneration, it is therefore worthwhile to know the friction value of the roadway in advance, so that the automated driving functions and / or regeneration can be carried out taking into account the friction value of the roadway.

[0007] Currently known systems for determining roadway friction values ​​are mainly based on additional sensors such as camera systems, moisture sensors and noise sensors, which on the one hand entail additional costs and on the other hand are not able to distinguish all roadway coatings from one another.

[0008] Camera-based systems are able to estimate the friction value of the roadway. These systems monitor the roadway in front of the vehicle and require an additional evaluation unit in order to evaluate the image data. Different ambient and light conditions, such as due to sunlight and shadows, are problematic here. For example, it is difficult to distinguish water from shadows. The friction value of the road cannot be determined by measurement technology, but can only be analyzed and estimated from the camera image. These systems still need to be improved.

[0009] The rolling noise of the tires can be analyzed by means of additional sensors in the area of ​​the wheel arches in order to estimate the friction value using acoustic sensors. For example, the rolling noise on a wet road surface is significantly different from the rolling noise on a dry road surface. Acoustic sensors can also be used in conjunction with moisture sensors. These systems also require an evaluation unit that distinguishes the road surface coating from the different rolling noises and can estimate the friction value.

[0010] Friction values ​​can also be determined by means of defined, wheel-specific test brakes. However, these test brakes lead to wear of the brakes and tires and to a temporary increase in fuel and / or energy consumption. In addition, friction brakes cannot be adjusted precisely and the ratio between brake pressure and braking force depends on a number of factors and can change during driving. Test brakes cannot be carried out permanently, so that only occasional checks of the roadway coating are possible.

[0011] These methods for determining friction values ​​still need to be improved, since the friction values ​​of the roadway are not measured directly and / or are not measured in an energy-neutral manner and lead to increased wear on the brakes and tires.

[0012] The drive of the vehicle can be used to estimate the friction value of the roadway. US 2007 / 0061061 A1 discloses a method for determining road surface properties. Here, a specific vehicle acceleration or vehicle deceleration is induced by using the torque acting on the driven wheels. The speeds of the driven wheels and the undriven wheels are measured. The tire-road friction coefficient and the slip rate are calculated using the speeds of these wheels.

[0013] Similarly, US 2010 / 0131165 A1 discloses a method for identifying the maximum tire-road friction coefficient in real time by inducing wheel acceleration / wheel deceleration. Here, a specific torque is applied to an axle at a specific frequency. Other axles are loaded with torque coordinated therewith so as to keep the vehicle acceleration or vehicle deceleration within the range expected by the driver and prevent adverse effects on the driver.

[0014] However, in order to determine the friction coefficient, various disturbance variables that may influence the speed of the driven wheels need to be added. This results in a relatively high signal-to-noise ratio. Therefore, a relatively large slip must be generated by a corresponding torque in order to reliably determine the friction coefficient. The torque must then be compensated again on the other axles. Summary of the invention

[0015] The object of the present invention is to enrich the prior art and to enable a reliable and efficient determination of roadway friction values ​​by means of an electric drive and thereby to support and improve existing functions for friction value recognition.

[0016] According to the present invention, a method for estimating friction values ​​for a vehicle, in particular a commercial vehicle, which can be driven by an electric drive is provided. The method comprises the following steps: operating a wheel of a vehicle, in particular a commercial vehicle, which is arranged on the ground, with a torque; determining the slip of the wheel; applying a temporally predetermined excitation torque to the wheel, wherein the excitation torque is applied to the wheel periodically with a certain frequency; determining a slip change that depends on the excitation torque, wherein the slip change is determined while taking the frequency into consideration; and determining the friction value with the help of the slip and the slip change.

[0017] The method according to the invention is used to estimate or determine the friction value. The friction value or friction coefficient of the roadway is the ratio of the friction force to the contact force acting between the wheels of the vehicle and the ground on which the wheels are located. The friction force is a force acting tangentially to the contact surface between the wheel and the ground. The contact force is a force acting normal to the contact surface, that is, perpendicularly.

[0018] It is proposed to perform the determination of the friction value of the roadway by means of a torque excitation by an electric drive, wherein the torque excitation by means of an excitation torque enables a permanent evaluation of the slip change caused by the torque excitation during driving. For this purpose, the wheels are operated with a torque, the so-called steady-state torque. The slip occurs by operating the wheels with this torque. In addition to this torque, a periodic excitation torque is applied to the wheels. The excitation torque leads to a detectable slip change, by means of which the friction value of the roadway can be estimated. The excitation torque is predetermined in time by the frequency.

[0019] It has been recognized that electric drives have higher control dynamics and more precise knowledge of the torque achieved compared to internal combustion engines and / or friction brakes. This makes possible the proposed method for determining the friction value of a roadway. Here, a minimum slip change is initiated and evaluated with the aid of an electric drive and a high-frequency torque excitation, and thus the friction value of the roadway is determined.

[0020] The driving and braking torques required for acceleration, steady-state driving and / or deceleration of a vehicle are referred to below as "steady-state torques". Depending on the size of the "steady-state torque" and the friction value of the roadway, a "steady-state slip" occurs. This "steady-state slip" is additionally dependent on the steering angle, the floating angle and the lateral guiding force of the tires. The "steady-state torque" can be distinguished from the excitation torque because a frequency is assigned to the excitation torque, wherein the frequency is optionally fixed and remains constant. The "steady-state torque" can be variable in time in order to cause different accelerations and / or decelerations of the wheels.

[0021] Due to its precise controllability and high dynamics, the electric drive can be used to estimate the friction value of the roadway. No additional sensors are required to evaluate the friction value, which makes it possible to determine the friction value effectively and economically. With this method, the slip behavior of the tire under different forces can be directly, that is to say non-indirectly, "measured" and thus the friction value of the roadway can be determined.

[0022] Preferably, the frequency of the periodic excitation torque is in the range of 0.1 Hz to 20 Hz, preferably in the range of 1 Hz to 5 Hz. The excitation torque thus has a frequency that is advantageous for determining the friction value. Below 0.1 Hz, vibration modes of the vehicle may be excited that are perceptible as shaking of the vehicle and negatively affect the driving of the vehicle. Above 20 Hz, the signal-to-noise ratio (SNR) when determining the slip change decreases, so that a clear determination of the slip change cannot be made reliably.

[0023] Preferably, the slip change is determined using a filter. Since the frequency of the excitation torque is known, the slip change can be evaluated in a targeted manner using this frequency, thereby minimizing the influence of measurement noise and / or other short-term events. Alternatively or additionally, the slip change is determined by means of a Fourier analysis of the slip change. In this case, the slip change over time due to the periodic excitation torque is Fourier transformed in order to determine the frequency spectrum of the slip change. The frequency spectrum of the slip change is frequency-dependent. By applying a filter to the frequency spectrum of the slip change, the influence of measurement noise and / or other short-term events can be minimized.

[0024] Preferably, the filter is applied to the slip ratio taking into account a predetermined interval with a frequency. Thus, the filter can be set to this interval in a targeted manner to evaluate the slip ratio change. The interval is selected such that the slip ratio change caused by the excitation torque has a frequency within this interval. For example, the interval has the frequency of the excitation torque.

[0025] Preferably, multiple wheels of a vehicle, in particular a commercial vehicle, are loaded with a periodic excitation torque and with a predetermined phase shift. It is recognized that phase shifting the excitation is advantageous in a drive device that is wheel-specific. In particular, a phase shift of 180° is advantageous. The wheels can be assigned to one or more drive axles of the vehicle. Due to the phase shift of 180°, in particular, the excitation has little or no effect on the total torque acting on the vehicle and the resulting vehicle acceleration. If multiple electrically drivable axles participate in driving the vehicle using a central drive device that each electrically drivable axle has, the phase shift can be performed with respect to the axles. As a result, compared with a central drive device with only one driven axle, a higher excitation amplitude can be achieved without negatively affecting the vehicle dynamics, so as to simplify the acquisition of slip changes.

[0026] The slip change is preferably determined using a lock-in amplifier. The signal-to-noise ratio of the determined slip change can thus be improved. The lock-in amplifier receives the slip change as a measurement signal and the excitation torque as a reference signal with a known frequency as input. The lock-in amplifier determines the product of the slip change and the excitation torque for a specific phase shift. The slip change can thus be amplified efficiently and reliably with respect to the excitation torque in order to suppress interference frequencies.

[0027] Preferably, the method further comprises the following steps: determining the operating point with the aid of torque and slip. A specific ground surface has a specific relationship between friction value and slip. The relationship between friction value and slip can be reflected in a friction value curve that can be matched to the ground surface. The friction value can be determined by the ratio of the vehicle load acting on the wheel to the propulsion force acting on the wheel. Therefore, a specific operating point is derived from the known slip and friction value, which can be matched to one or more friction value curves and thus to the ground surface. The load acting on the wheel is a normal force, which is obtained by multiplying the mass by the product of the gravity coefficient. In addition, the slip can also be determined by observing the driven wheels and the undriven wheels of the vehicle.

[0028] Preferably, the method further comprises the following step: determining the gradient of the friction value as a function of the slip. It is recognized that each friction value curve that can be assigned to the ground has a specific relationship between the gradient of the friction value and the slip, i.e. a local rise of the friction value curve for a specific slip is characteristic of the ground. If an excitation torque is given, the slip of the electrically drivable axle changes at the same frequency. With the aid of this excitation, the slope of the friction value curve at the operating point can be determined. If the torque excitation leads only to a small slip change, a higher gradient of the friction value curve is obtained.

[0029] Preferably, the method further comprises the following steps: assigning a friction value curve corresponding to the ground with the help of the gradient of the friction value. The gradient or the dependence of the gradient on the slip rate is a typical parameter that can be assigned to a specific ground. Alternatively or additionally, the friction value curve corresponding to the ground is assigned with the help of a statistical parameter of the friction value and / or a statistical parameter of the slip rate. For example, the statistical parameter can be the variance, standard deviation and / or error of the friction value and / or the slip rate that is known. It has been recognized that the friction value and / or the slip rate can be measured well and differently according to the ground or the roadway, and therefore experience different shaking, which can be detected and classified with the help of statistical parameters. Alternatively or additionally, it is possible to assign the friction value curve corresponding to the ground with the help of curve fitting, taking into account the working point and the gradient of the friction value curve at the working point. For this purpose, a functional relationship of the friction value curve defined by parameters can be assumed, wherein the mathematical optimization for obtaining the parameters is performed taking into account the known working point and the known gradient. It is possible to record a plurality of operating points at time-predetermined intervals in order to carry out the fitting of the friction value curve taking into account a plurality of operating points. With the aid of the known parameters, the friction value curve can be assigned to the ground surface.

[0030] Preferably, the method further comprises the following step: the maximum friction value of the ground is determined using the friction value. If the slip rate changes relatively greatly due to the excitation, the maximum friction value is typically almost reached. With the help of the "steady-state operating point" and the gradient determined at this operating point, the current operating point can be assigned to a representative friction value curve. In addition, the maximum friction value of the roadway can also be estimated using interpolation methods.

[0031] Preferably, the excitation torque has an amplitude, wherein the amplitude is selected so that the slip rate limit is not exceeded. Therefore, the amplitude of the torque excitation is advantageously selected so that the excitation cannot be felt by the driver and / or does not have a negative impact on vehicle stability. For example, it is avoided that in the case of a road surface covering with a low friction value (such as an ice surface), the critical slip rate value cannot be reached by the excitation torque alone. In addition or alternatively, the excitation torque has an amplitude, wherein the amplitude is selected so that the sign of the sum of the torque and the excitation torque is the same as the sign of the torque. In other words, the torque synthesized by the "steady-state" torque and the periodic torque excitation does not undergo a sign change. Undesirable tooth flank switching in the transmission of the vehicle can thus be prevented. Alternatively or additionally, the excitation torque has an amplitude, wherein the amplitude is selected to take into account vehicle stability and / or efficiency. It is thus possible to avoid excitation by the electric drive at an inefficient operating point in order to suppress a reduction in the efficiency of the electric drive.

[0032] Preferably, the slip and the change in slip are determined by a plurality of wheel speed sensors and / or taking into account information about the speed of the electric drive. Existing wheel speed sensors of the vehicle can be used to evaluate the slip. Advantageously, the slip and the change in slip are determined by wheel speed sensors arranged at the driven axle and wheel speed sensors arranged at the undriven axle. In wheel-specific drives, the speed signal can be information about the speed of the electric drive and is used in particular for plausibility checks because the information may have a higher resolution and a faster sampling rate. Additionally or alternatively, the slip and the change in slip are determined by two wheel speed sensors arranged at the driven axle, which are optionally on different sides of the vehicle, so that different friction values ​​for each wheel can be determined. Therefore, the friction value can be considered specifically for each wheel and / or a "critical wheel", i.e. a wheel that runs closer to the slip limit, can be identified.

[0033] According to one aspect of the present invention, a computer program and / or computer readable medium is provided. The computer program and / or computer readable medium includes instructions that, when the program or instructions are implemented by a computer, cause the computer to perform the method and / or its steps according to the present invention. Optionally, the computer program and / or computer readable medium includes instructions that, when the program or instructions are implemented by a computer, cause the computer to perform the method steps described as advantageous or optional so as to achieve the technical effects associated therewith.

[0034] According to one aspect of the present invention, a controller for a vehicle, in particular a commercial vehicle, is provided. The controller is configured to perform a method according to the present invention. Optionally, the controller is configured to perform method steps described as advantageous or optional in order to achieve the technical effects associated therewith.

[0035] According to one aspect of the present invention, a vehicle, in particular a commercial vehicle, is provided, the vehicle having a controller according to the present invention. Optionally, the controller of the vehicle and / or the vehicle is set up to perform the method steps described as advantageous or optional in order to achieve the technical effects associated therewith. BRIEF DESCRIPTION OF THE DRAWINGS

[0036] Further advantages and features of the present invention and their technical effects are found in the drawings and in the description of the preferred embodiments shown in the drawings.

[0037] Figure 1 A schematic diagram showing a simplified flow chart of a method according to an embodiment of the present invention;

[0038] Figure 2 Two schematic diagrams showing friction value curves; and

[0039] Figure 3 The schematic diagram shows an overview of a vehicle, in particular a commercial vehicle, according to one embodiment of the present invention. DETAILED DESCRIPTION

[0040] Figure 1 A schematic diagram of a simplified flow chart of a method 100 according to an embodiment of the present invention is shown. In particular, Figure 1 A method 100 for estimating friction values ​​for a vehicle 300a, in particular a commercial vehicle 300b, which can be driven by an electric drive 200 is shown. The vehicle 300a, in particular the commercial vehicle 300b, is referred to below as vehicle 300a, 300b. Figure 3 The vehicles 300a, 300b are described in more detail.

[0041] according to Figure 1The method 100 first starts with operating 110 the wheels 270 of the vehicles 300a, 300b arranged on the ground 260 with a torque T. The operation 110 is to accelerate, drive or decelerate the wheels 270. The torque T for acceleration or deceleration, i.e. the driving torque or braking torque for accelerating, driving and decelerating the vehicles 300a, 300b, is also referred to as the “steady-state” torque T.

[0042] The slip S of the wheel 270 is known 120. Depending on the magnitude of the “steady-state” torque T and the friction value MU of the roadway, the slip S occurs as a “steady-state” slip S. In addition, this “steady-state” slip S optionally also depends on the lateral guiding force of the tire or wheel 270 and the steering angle, the floating angle. Figure 2 An exemplary friction value curve 400 is shown, ie the relationship between friction value MU and slip S for different roadways or surfaces 260. The surface 260 on which the vehicle 300a, 300b is traveling and the friction value curve 400 corresponding thereto are unknown during driving and may change during driving.

[0043] exist Figure 1 In the embodiment of the present invention, a wheel 270 is loaded 130 with an excitation torque ET predetermined in time, wherein the excitation torque ET is periodically loaded 130 with a certain frequency F. The periodic excitation torque ET can be ideally realized as a trigonometric function with respect to time, as a rectangular function with respect to time and / or as a sum of such functions of the excitation torque ET. The frequency F of the periodic excitation torque ET is in the range of 0.1 Hz to 20 Hz, preferably in the range of 0.5 Hz to 5 Hz.

[0044] The excitation torque ET has an amplitude A, wherein the amplitude A is selected such that the slip ratio limit ST is not exceeded, the sign of the sum of the torque T and the excitation torque ET is the same as the sign of the torque T and vehicle stability and / or efficiency are taken into account.

[0045] Multiple wheels 270 of vehicles 300a, 300b are loaded with a periodic excitation torque ET and with a predetermined phase shift DP. As a result, the excitation torque ET assigned to different wheels 270 is predetermined in time by frequency and phase shift DP. Multiple wheels 270 can be assigned to one or more axles of vehicles 300a, 300b. The phase shift DP is, for example, 180°.

[0046] Subsequently, the slip change DS is determined 140 as a function of the excitation torque ET. If a torque excitation with a high frequency of the excitation torque ET is additionally provided to the "steady-state" torque T, the slip S on the electrically driven axle will change at the same frequency according to the slip change DS. The slip change DS is determined here taking into account the frequency F and using a filter P and / or Fourier analysis of the slip change DS. The filter P is applied to the slip S taking into account a predetermined interval I with the frequency F. The slip change DS is determined 140 using a lock-in amplifier 280. By Fourier analysis of the slip change DS, a measurement signal belonging to the slip change DS can be checked for specific frequencies. In particular, the measurement signal can be checked for the frequency F and / or for an interval I around the frequency F.

[0047] The slip S is determined 120 and the slip change DS is determined 140 by means of a plurality of wheel speed sensors 220 and optionally undergoes a plausibility check by means of the electric drive 21 .

[0048] The friction value MU is determined 150 by means of the slip change DS. The slip change DS causes a change in the tangential force or propulsion force on the contact surface between the wheel 270 and the ground 260. Therefore, the change in the friction value MU is derived from the slip change DS.

[0049] An operating point 210 is determined 155 using torque T and slip S. Operating point 210 is a “steady-state” operating point 210 and is a friction value curve 400 (see Figure 2 ), which can be determined using the “steady-state” torque T and the associated “steady-state” slip S. Using the excitation by the excitation torque ET and the slip change DS, the slope of the friction value curve 400 at the operating point 210 can be determined, since the friction value MU changes due to the slip change DS.

[0050] The gradient D of the friction value MU is determined 156 as a function of the slip S. If the torque excitation via the excitation torque ET leads only to a small slip change DS, a higher gradient D of the friction value curve 400 results.

[0051] With the help of the gradient D of the friction value MU and with the help of the statistical variables of the friction value MU and / or the statistical variables of the slip S, a friction value curve 400 corresponding to the ground surface 260 is assigned 157. With the help of the “steady-state” operating point 210 and the slope or gradient D determined at the operating point 210, the current operating point 210 can be assigned to the representative friction value curve 400.

[0052] The maximum friction value MM of the ground surface 260 is determined 160 with the help of the friction value MU. When the gradient D is high, the current operating point 210 is far from the maximum friction value MM, which can be inferred that the friction value MU of the roadway is high. However, if the slip rate S changes significantly due to the excitation, the maximum friction value MM is almost reached. In addition, the maximum friction value MM of the roadway can be estimated with the help of interpolation. Through the direct relationship between the torque excitation, the slip rate value change DS and the friction value MU of the roadway, it can be known at any time whether the current operating point 210 has approached the slip rate limit ST.

[0053] The person skilled in the art recognizes that the steps of method 100 can also be carried out in a different order than that shown. The steps of the method can also be carried out synchronously, i.e. at the same time. For example, operating 110 a wheel 270 arranged on a ground surface 260 with a torque T and loading 130 the wheel 270 with a temporally predetermined excitation torque ET can be carried out at any time and in particular simultaneously. After operating 110 a wheel 270 arranged on a ground surface 260 with a torque T, the slip S can be determined 120 at any time.

[0054] Figure 2 Two schematic diagrams showing friction value curves 400.1, 400.2, 400.3, 400.4, 400.5, 400.6 ( Figure 2 (A) and Figure 2 (B)).

[0055] Figure 2 (A) shows six different friction value curves 400.1, 400.2, 400.3, 400.3, 400.4, 400.5, 400.6. Each of the friction value curves 400.1, 400.2, 400.3, 400.3, 400.4, 400.5, 400.6 shows the relationship between the friction value MU and the slip rate S for a special ground surface 260. Here, the friction value curve 400.1 shows the relationship between the friction value MU and the slip rate S for dry asphalt. The friction value curve 400.2 shows the relationship between the friction value MU and the slip rate S for wet asphalt. The friction value curve 400.3 shows the relationship between the friction value MU and the slip rate S for crushed stone and / or gravel. The friction value curve 400.4 shows the relationship between the friction value MU and the slip rate S for wet crushed stone and / or wet gravel. Friction value curve 400.5 shows the relationship between friction value MU and slip rate S for snow. Friction value curve 400.6 shows the relationship between friction value MU and slip rate S for ice.

[0056] Each of the friction value curves 400.1, 400.2, 400.3, 400.4, 400.5, 400.6 has a single peak shape, which has a maximum friction value MM at a specific slip rate S (see Figure 2 (B)), wherein the slope or gradient D of each friction value curve 400.1, 400.2, 400.3, 400.3, 400.4, 400.5, 400.6 (see Figure 2 (B) is positive and relatively large when the slip ratio S is small, and negative and relatively small for a large slip ratio S, as also referred to Figure 2 (B) as described.

[0057] Figure 2 (B) shows the Figure 2 (A) is a section for selected friction value curves 400.1, 400.2, 400.5. Here, an operating point 210 is shown for one of the friction value curves 400.5. The operating point 210 is derived from the "steady-state" torque T and the friction value MU and slip S associated therewith. At the operating point 210, a slope triangle for determining the gradient D is shown. The gradient D is the local slope of the friction value curve 400.5. The excitation torque ET causes a change in slip DS and a change in the friction value MU. Therefore, the gradient D can be determined with the help of the change in slip DS and the change in friction value MU. The gradient D and the operating point 210 provide information about the friction value curve 210 and enable the friction value curve 400.5 corresponding to the respective ground surface 260 to be assigned 157 with the help of the gradient D of the friction value MU.

[0058] The maximum friction value MM is shown for friction value curve 400.5. The gradient D of friction value curve 400.5 is positive and relatively large when the slip ratio S is small, and is negative and relatively small when the slip ratio S is large.

[0059] Figure 3 A schematic diagram shows an overview of a vehicle 300a, in particular a commercial vehicle 300b, according to an embodiment of the present invention. Figure 3 The vehicles 300a and 300b refer to Figure 1 and Figure 2 Description given.

[0060] like Figure 3As shown in FIG. , vehicles 300a, 300b are arranged on a ground surface 260. Here, wheels 270 are arranged on the ground surface 260. A contact surface (not shown) is arranged between the wheels 270 and the ground surface 260. The wheels 270 and the ground surface 260 are in contact with each other at the contact surface. Through the contact surface, forces can act between the wheels 270 and the ground surface 260. In particular, a normal force can act perpendicularly to the contact surface, which depends on the mass of the vehicles 300a, 300b and the number and geometry of the wheels 270. In addition to the normal force, a tangential force can also act, which depends on the dynamics of the respective wheels 270, in particular on the propulsion and / or braking in the case of the driven wheels 270 or the corresponding torque T and / or excitation torque ET. The ratio of the normal force to the tangential force describes the friction value MU of the ground surface 260. The torque T causes the slip S, and the excitation torque ET causes the slip change DS. Slip S is the ratio of rotational speed R of driven wheel 270 to rotational speed R of wheel 270 which is not driven and is driven in a positively locking manner.

[0061] The vehicles 300a, 300b are configured to perform a reference Figure 1 The method 100 is described. To this end, Figure 3 As shown in FIG. 1 , the vehicle 300a, 300b has a controller 250, an electric drive device 200, a plurality of wheels 270 and a plurality of wheel speed sensors 220. The controller 250 is connected to the electric drive device 200 and the wheel speed sensor 220 so as to execute the Figure 1 Method 100.

[0062] The controller 250 is configured to control the electric drive 200 for applying a torque T and an excitation torque ET to the wheels 270. To this end, the controller 250 can send a torque request TR to the electric drive 200, which defines the torque T and the excitation torque ET, wherein the amplitude A of the excitation torque ET, the phase shift DP of the excitation torque ET, the frequency F of the excitation torque ET and the slip limit ST are taken into account for the torque request TR. To this end, the controller 250 sends an instantaneous torque request TR to the electric drive 200. The torque request TR consists of a "steady-state" torque request for the "steady-state" torque T superimposed with the additionally superimposed excitation torque ET. The frequency F, the amplitude A, the phase shift DP and the slip limit ST are stored in the controller 250 and are used to "generate" the excitation torque ET added to the "steady-state" torque T. The electric drive 200 receives an instantaneous value of the total torque as the sum of the torque T and the excitation torque ET, which then changes periodically by the superimposed excitation torque ET. Electric drive 200 is designed to apply torque T and excitation torque ET to one or more wheels 270 by means of signals from control unit 250 .

[0063] Each wheel speed sensor 220 is configured to measure the rotation speed R of one of the wheels 270. The slip ratio S and the slip ratio change DS can be determined by means of the plurality of wheel speed sensors 220. To this end, one of the wheel speed sensors 220 is configured to determine the rotation speed R of the undriven wheel 270, and one of the wheel speed sensors 220 is configured to determine the rotation speed R of the driven wheel 270. The wheel speed sensors 220 are connected to the controller 250 to send the rotation speed R of the wheel 270 to the controller 250. The controller 250 is configured to obtain the slip ratio S and the slip ratio change DS by means of the rotation speed R of the wheel 270.

[0064] Furthermore, the controller 250 is also configured to form a lock-in amplifier 280. To this end, the controller 250 can provide the interval I and the filter P to the lock-in amplifier 280.

[0065] The controller 250 also has a data processing device not shown and a memory not shown. For example, the memory may store Figure 2 For a large number of ground surfaces 260, the friction value curve 400 and / or the friction value slope curve are stored in the memory respectively, so that the friction value curve 400 and / or the friction value slope curve can be efficiently performed according to the ground surface 260. Figure 1 Method 100. Friction value curve 400 can be determined by measurement technology and / or heuristically modeled.

[0066] List of reference numerals (part of the description)

[0067] 100 Methods

[0068] 110 running wheels

[0069] 120 Get the slip rate

[0070] 130 Loading excitation torque

[0071] 140 Obtaining slip rate changes

[0072] 150 Obtain the friction value

[0073] 155 Know the working point

[0074] 156 Gradient

[0075] 157 Friction value curve

[0076] 160 Get the maximum friction value

[0077] 200 Electric drive unit

[0078] 210 working points

[0079] 220 wheel speed sensor

[0080] 250 Controller

[0081] 260 Ground

[0082] 270 wheels

[0083] 280 Lock-in Amplifier

[0084] 300a Vehicle

[0085] 300b Commercial Vehicle

[0086] 400 Friction Value Curve

[0087] A-Amplitude

[0088] D Gradient

[0089] DP Phase Shift

[0090] DS slip ratio change

[0091] ET excitation torque

[0092] F Frequency

[0093] MM maximum friction value

[0094] MU friction value

[0095] P Filter

[0096] R speed

[0097] S slip rate

[0098] ST slip rate boundary

[0099] T Torque

[0100] TR torque requirement

Claims

1. A method (100) for estimating a friction value for a vehicle (300a), in particular a commercial vehicle (300b), which can be driven by an electric drive (200), the method (100) comprising the following steps: - operating (110) a wheel (270) of the vehicle (300a), in particular a commercial vehicle (300b), which is arranged on the ground (260), with a torque (T); - obtaining (120) a slip rate (S) of the wheel (270); - the wheel (270) is loaded (130) with a temporally predetermined excitation torque (ET), wherein: Periodically applying (130) the excitation torque (ET) to the wheel (270) at a certain frequency (F); - determining (140) a slip change (DS) as a function of the excitation torque (ET), wherein the slip change (DS) is determined (140) taking into account the frequency (F); and - Using the slip (S) and the slip change (DS), a friction value (MU) is determined (150).

2. The method according to claim 1, wherein: The frequency (F) of the periodic excitation torque (ET) is in the range from 0.1 Hz to 20 Hz, preferably in the range from 0.5 Hz to 5 Hz.

3. The method according to claim 1 or 2, wherein: - determining (140) the slip change (DS) using a filter (P) and / or by means of a Fourier analysis of the slip change (DS).

4. The method according to claim 3, wherein: - applying the filter (P) to the slip (S) taking into account a predetermined interval (I) with the frequency (F).

5. A method according to any one of the preceding claims, wherein: - a plurality of wheels (270) of the vehicle (300a), in particular a commercial vehicle (300b), are loaded (130) with a periodic excitation torque (ET) and with a predetermined phase shift (DP).

6. A method according to any one of the preceding claims, wherein: - determining (140) the slip change (DS) using a lock-in amplifier (280).

7. The method according to any one of the preceding claims, further comprising the following steps: - Using the torque (T) and the slip (S), an operating point (210) is determined (155).

8. The method according to claim 7, further comprising the following steps: - determining (156) a gradient (D) of the friction value (MU) as a function of the slip (S).

9. The method according to claim 8, further comprising the following steps: - assigning (157) a friction value curve (400) corresponding to the ground surface (260) by means of a gradient (D) of the friction value (MU) and / or by means of a statistical variable of the friction value (MU) and / or a statistical variable of the slip (S).

10. The method according to any one of the preceding claims, further comprising the following steps: - using the friction value (MU) to determine (160) a maximum friction value (MM) of the ground surface (260).

11. A method according to any one of the preceding claims, wherein: - the excitation torque (ET) has an amplitude (A), wherein the amplitude (A) is selected such that: a slip ratio limit (ST) is not exceeded, the sign of the sum of the torque (T) and the excitation torque (ET) is the same as the sign of the torque (T), and / or taking into account vehicle stability and / or efficiency.

12. A method according to any one of the preceding claims, wherein: The slip (S) is determined (120) and the slip change (DS) is determined (140) by means of a plurality of wheel speed sensors (220) and / or taking into account information about the speed (R) of the electric drive (21).

13. A computer program and / or a computer-readable medium, comprising instructions which, when the program or instructions are executed by a computer, cause the computer to execute the method (100) according to any one of claims 1 to 12 and / or the steps of the method (100) according to any one of claims 1 to 12.

14. A control unit (250) for a vehicle (300a), in particular a commercial vehicle (300b), wherein: The controller (250) is configured to carry out the method (100) according to any one of claims 1 to 12.

15. A vehicle (300a), in particular a commercial vehicle (300b), comprising a control unit (250) according to claim 14.

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