Method for estimating coefficients of friction, computer program, controller, vehicle, in particular utility vehicle

Abstract

Method (100) for estimating coefficients of friction for a vehicle (300a), in particular utility vehicle (300b), which can be driven by an electric device (200), the method (100) comprising the steps of: operating (110), using a torque (T), a wheel (270) of the vehicle (300a), in particular utility vehicle (300b), that is arranged on an underlying surface (260); ascertaining (120) the slip (S) of the wheel (270); applying (130) a temporally predetermined excitation torque (ET) to the wheel (270), the excitation torque (ET) being applied (130) to the wheel (270) periodically with a frequency (F); ascertaining (140) a change in slip (DS) depending on the excitation torque (ET), the change in slip (DS) being ascertained (140) taking into account the frequency (F); and ascertaining (150) a coefficient of friction (MU) on the basis of the change in slip (DS).

Description

[0001] Method for estimating the coefficient of friction, computer program, control unit, vehicle, in particular commercial vehicle

[0002] The invention relates to a method for estimating the coefficient of friction for a vehicle, in particular a commercial vehicle, driven by an electric drive. The invention also relates to a computer program and / or computer-readable medium, a control unit for a vehicle, in particular a commercial vehicle, and a vehicle, in particular a commercial vehicle.

[0003] Controlling electric drive systems in vehicles, particularly commercial vehicles such as e-trucks, e-buses, and e-semi-trailers, is known from the state of the art. An electric drive is installed, for example, in a towing vehicle and / or an electrified trailer. In electrically powered vehicles, particularly commercial vehicles, the electric drive is primarily intended to be used for traction, traction assistance, and / or recuperation, i.e., regenerative braking. For ease of description, commercial vehicles are also referred to as vehicles below.

[0004] Automated driving functions are known for vehicles, particularly for traction, traction assistance, and / or recuperation. For example, a traction function (Automatic Traction Control, ATC), a braking function (Anti-lock Braking System, ABS), and a stability function (Electronic Stability Control, ESC) are known as automated driving functions. Automated driving functions require reliable information about the road surface traveled by the vehicle and the associated coefficient of friction in order to be carried out effectively and reliably. The maximum power that can be transmitted by the wheels is highly dependent on the coefficient of friction of the road surface. If information regarding the coefficient of friction is missing, the respective automated driving function cannot intervene optimally from the start of the driving maneuver. Information regarding the road surface is not taken into account when carrying out the current automated driving functions.This can lead to situations where the vehicle's tires lock or spin despite, for example, ABS and ATC. If the traction, braking, and stabilization functions do not use friction coefficient estimation, it cannot always be prevented on a road surface with a low friction coefficient that tires lock or spin, resulting in a critical driving situation. Brief tire locking followed by a "run-up phase" with reduced braking force leads to an increase in stopping distance. If the electric drive system configured for regenerative braking is involved in braking, the recuperation power is limited during these phases, and the recuperation potential is not sufficiently utilized.

[0005] In electrically powered vehicles, maximum use of recuperation is crucial for energy-efficient vehicle operation. Here, too, it is helpful to have information about the road's friction coefficient, for example, to be able to adjust the optimal slip on an electrically powered axle and thus enable recuperation without locking the tires.

[0006] In order to enable reliable operation of automated driving functions and effective recuperation, it is therefore desirable to determine the friction coefficient of the road surface in advance in order to be able to carry out the automated driving functions and / or recuperation taking the road friction coefficient into account.

[0007] The currently known systems for determining the road friction coefficient are mainly based on additional sensors such as camera systems, moisture sensors, noise sensors, which on the one hand cause additional costs and on the other hand cannot distinguish between all road surfaces.

[0008] Camera-based systems can estimate the road's coefficient of friction. These systems monitor the road ahead of the vehicle and require additional processing units to evaluate the image data. Different ambient and lighting conditions, such as sun and shadow, pose a problem. For example, water and shadow are difficult to distinguish. The road's coefficient of friction cannot be measured; it can only be analyzed and estimated from the camera image. These systems still require improvement.

[0009] With the help of additional sensors in the wheel housing area, the tire's rolling noise can be analyzed to estimate the coefficient of friction using acoustic sensors. For example, rolling noise on a wet road differs significantly from that on a dry road. The acoustic sensors can also be used in conjunction with moisture sensors. These systems also require an evaluation unit that identifies the road surface based on the different rolling noise levels and allows an estimate of the coefficient of friction.

[0010] The coefficient of friction can also be determined using defined, wheel-specific test braking. However, these test brakings lead to wear on the brakes and tires and a temporary increase in fuel and / or energy consumption. Furthermore, the friction brake cannot be precisely controlled, and the relationship between brake pressure and braking force depends on many factors and can vary during the journey. Test braking cannot be performed continuously, meaning the road surface can only be checked sporadically.

[0011] The methods for determining the coefficient of friction need to be improved in that there is no direct measurement of the coefficient of friction of the road surface and / or the measurement is not energy-neutral and causes increased wear on the brakes and tires.

[0012] A vehicle's drive can be used to estimate the road surface's friction coefficient. US 2007 / 0061061 A1 discloses a method for determining road surface conditions. A specific vehicle acceleration or deceleration is induced by applying torque to a driven wheel. Speeds of the driven wheel and a non-driven wheel are measured. A tire-road friction coefficient and slip are calculated based on the wheel speeds.

[0013] Similarly, US 2010 / 0131165 A1 discloses a method for real-time identification of a maximum tire-road friction coefficient through induced wheel acceleration / deceleration. One axle is subjected to a specific torque at a specific frequency. Another axle is subjected to a coordinated torque to keep the vehicle acceleration or deceleration within a range intended by a driver and prevent any adverse effects on the driver.

[0014] However, determining the coefficient of friction involves various disturbances that can influence the speed of the driven wheel. This results in a relatively high signal-to-noise ratio. Thus, a relatively large slip must be generated by a corresponding torque to reliably determine the coefficient of friction. This torque must then be compensated on another axle.

[0015] The object of the invention is to enrich the state of the art and to enable a reliable and effective determination of the road friction coefficient using an electric drive and thereby to support and improve the already existing functions for friction coefficient detection.

[0016] According to the invention, a method for estimating the coefficient of friction for a vehicle, in particular a commercial vehicle, driven by an electric drive is provided. The method comprises the following steps: operating, with a torque, a wheel of the vehicle, in particular a commercial vehicle, arranged on a surface; 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 at a frequency; determining a change in slip as a function of the excitation torque, wherein the determination of the change in slip takes into account the frequency; and determining a coefficient of friction based on the slip and the change in slip.

[0017] The method according to the invention is for estimating or determining a coefficient of friction. The coefficient of friction of the road surface, or the coefficient of friction, is the ratio of a frictional force and a contact force acting between the vehicle wheel and the surface on which the wheel is mounted. The frictional force is a force acting tangentially to a contact surface between the wheel and the surface. The contact force is a force acting normal, i.e., perpendicular, to the contact surface.

[0018] It is proposed to determine the road surface's coefficient of friction by means of torque excitation from the electric drive. The torque excitation with the excitation torque enables a continuous evaluation of the slip change induced by the torque excitation while driving. For this purpose, the wheel is operated with a torque, the so-called steady-state torque. Slip is established by operating the wheel with this torque. In addition to the torque, the wheel is subjected to the periodic excitation torque. The excitation torque leads to the measurable slip change, which can be used to estimate the road surface's coefficient of friction. The excitation torque is predetermined in time by the frequency.

[0019] It was recognized that, compared to a combustion engine and / or a friction brake, the electric drive offers high control dynamics and precise knowledge of the torque applied. This enables the proposed method for determining the road surface's coefficient of friction. Using the electric drive and high-frequency torque excitation, minimal slip changes are induced and evaluated, thus determining the road surface's coefficient of friction.

[0020] The drive and braking torque required for acceleration, traveling at a constant speed, and / or deceleration of the vehicle is referred to below as "stationary torque." Depending on the magnitude of the "stationary torque" and the coefficient of friction of the road surface, "stationary slip" occurs. This "stationary slip" is also dependent on the steering angle, sideslip angle, and the cornering forces of the tire. The "stationary torque" can be distinguished from the excitation torque because the frequency is assigned to the excitation torque, whereby the frequency can optionally be fixed and constant. The "stationary torque" can vary over time to produce different accelerations and / or decelerations of the wheel.

[0021] Due to the precise controllability and high dynamics of the electric drive, it can be used to estimate the road surface's friction coefficient. No additional sensors are required to evaluate the friction coefficient, allowing for effective and cost-effective determination. This method allows the slip behavior at different tire forces to be directly "measured," thus determining the road surface's friction coefficient.

[0022] The frequency of the periodic excitation torque is preferably in the range of 0.1 Hz to 20 Hz, preferably from 1 Hz to 5 Hz. Thus, the excitation torque has a frequency that is advantageous for determining the coefficient of friction. Below 0.1 Hz, vibration modes of the vehicle can be excited, which are perceived as rocking of the vehicle and negatively affect the vehicle's handling. Above 20 Hz, the signal-to-noise ratio (SNR) when determining the slip change decreases to such an extent that a clear determination of the slip change is no longer reliably possible.

[0023] The slip change is preferably determined using a filter. Since the frequency of the excitation torque is known, the slip change can be evaluated specifically for this frequency, thereby minimizing influences from measurement noise and / or other short-term events. Alternatively or additionally, the slip change is determined using a Fourier analysis of the slip change. The time-dependent slip change caused by the periodic excitation torque is Fourier transformed to determine a slip change spectrum. The slip change spectrum is frequency-dependent. By applying a filter to the slip change spectrum, influences from measurement noise and / or other short-term events can be minimized.

[0024] Preferably, the filter is applied to the slip with respect to a predetermined interval comprising the frequency. This allows the filter to be specifically adjusted to evaluate the slip change for the interval. The interval is selected such that the slip change due to the excitation torque has a frequency within the interval. For example, the interval for this purpose includes the frequency of the excitation torque.

[0025] Preferably, several wheels of the vehicle, in particular a commercial vehicle, are subjected to the periodic excitation torque and a predetermined phase shift. It has been recognized that a phase shift of the excitation is advantageous for wheel-individual drives. 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, in particular by 180°, the excitation has little or no influence on the total torque acting on the vehicle and the resulting vehicle acceleration. If several electrically driven axles, each with a central drive per electrically driven axle, are involved in the vehicle drive, the phase shift can be axle-specific.This allows higher excitation amplitudes to be achieved without any negative effect on the vehicle dynamics compared to a central drive with only one drive axle, thus simplifying the determination of the slip change.

[0026] Preferably, the slip change is determined using a lock-in amplifier. This improves the signal-to-noise ratio of the determined slip change. The lock-in amplifier receives the slip change as a measurement signal and the excitation torque as a reference signal with a known frequency. The lock-in amplifier determines the product of the slip change and the excitation torque for a specific phase shift. This allows the slip change to be effectively and reliably amplified with respect to the excitation torque to suppress interference frequencies.

[0027] The method preferably further comprises the steps of determining an operating point based on the torque and the slip. A specific surface exhibits a specific relationship between the coefficient of friction and slip. The relationship between the coefficient of friction and slip can be represented in a coefficient of friction curve that can be assigned to the surface. The coefficient of friction can be determined from the ratio of the vehicle load acting on a wheel to a propulsion force acting on the wheel. The determined slip and the coefficient of friction thus result in a specific operating point that can be assigned to one or more coefficient of friction curves and thus to surfaces. The load acting on the wheel is the normal force, which results from the product of a mass times a location factor. Furthermore, the slip can be determined by considering a driven wheel and an undriven wheel of the vehicle.

[0028] The method preferably further comprises the steps of determining a gradient of the coefficient of friction as a function of the slip. It has been recognized that each friction coefficient curve attributable to a specific surface exhibits a specific relationship between the gradient of the coefficient of friction and slip, i.e., a local increase for a specific slip of a friction coefficient curve is characteristic of a surface. If an excitation torque is present, the slip value on the electrically driven axle changes at the same frequency. Using the excitation, the gradient at the operating point of the friction coefficient curve can be determined. If the torque excitation leads to only a slight change in slip, a high gradient results for the friction coefficient curve.

[0029] Preferably, the method further comprises the steps of: assigning a friction coefficient curve corresponding to the surface based on the gradient of the friction coefficient. The gradient or its dependence on the slip is a typical variable that can be assigned to a specific surface. Alternatively or additionally, a friction coefficient curve corresponding to the surface is assigned based on a stochastic variable of the friction coefficient and / or the slip. The stochastic variable can be, for example, a variance, a standard deviation and / or an error of the determined friction coefficient and / or the slip. It was recognized that the friction coefficient and / or the slip can be measured with varying degrees of accuracy depending on the surface or roadway and is therefore subject to different fluctuations that can be detected and classified using the stochastic variable.Alternatively or additionally, it is possible to assign the friction coefficient curve corresponding to the surface based on a curve fit taking into account the operating point and the gradient of the friction coefficient curve at the operating point. For this purpose, a functional relationship of the friction coefficient curve defined by parameters can be assumed, with mathematical optimization being carried out to determine the parameters taking into account the determined operating point and the determined gradient. It is possible to record a plurality of operating points at a predetermined time interval in order to perform a fit of the friction coefficient curve taking into account the plurality of operating points. Based on the determined parameters, the friction coefficient curve can be assigned to the surface.

[0030] Preferably, the method further comprises the steps of determining a maximum coefficient of friction of the ground based on the coefficient of friction. If the slip changes comparatively significantly due to the excitation, the maximum value of the coefficient of friction is typically almost reached. Using the "stationary operating point" and the determined gradient at this operating point, the current operating point can be assigned to a characteristic coefficient of friction curve. In addition, the maximum coefficient of friction of the road surface can be estimated using interpolation.

[0031] Preferably, the excitation torque has an amplitude, wherein the amplitude is selected such that a slip limit is not exceeded. The amplitude of the torque excitation is thus advantageously selected such that the excitation is not noticeable by the driver and / or has a negative impact on vehicle stability. For example, this prevents critical slip values ​​from being reached on road surfaces with a low coefficient of friction, e.g. ice, using the excitation torque alone. Additionally or alternatively, the excitation torque has an amplitude, wherein the amplitude is selected such 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 resulting torque from the “stationary” torque and the periodic torque excitation does not change sign.This can prevent an unwanted tooth flank change in a vehicle's transmission. Alternatively or additionally, the excitation torque has an amplitude, with the amplitude being selected to take vehicle stability and / or efficiency into account. This can prevent excitation by the electric drive at an inefficient operating point, thus suppressing a reduction in the efficiency of the electric drive.

[0032] Preferably, the slip and the slip change are determined by a plurality of wheel speed sensors and / or taking into account information from the electric drive relating to a speed. The vehicle's existing wheel speed sensors can be used to evaluate the slip. Advantageously, the slip and the slip change are determined by a wheel speed sensor arranged on a driven axle and a wheel speed sensor arranged on a non-driven axle. In wheel-specific drives, the speed signal can be the speed-related information from the electric drive and can be used, in particular, for plausibility checks, since this may have a higher resolution and faster sampling rate.Additionally or alternatively, the slip and slip change are determined by two wheel speed sensors mounted on a driven axle, optionally on different sides of the vehicle, in order to determine different friction coefficients for each wheel. This allows the friction coefficient to be taken into account specifically for each wheel and / or to identify a "critical wheel," i.e., the wheel operating closer to the slip limit.

[0033] According to one aspect of the invention, a computer program and / or a computer-readable medium is provided. The computer program and / or the computer-readable medium comprise instructions which, when the program or instructions are executed by a computer, cause the computer to perform the method according to the invention and / or steps thereof. Optionally, the computer program and / or the computer-readable medium comprises instructions which, when the program or instructions are executed by a computer, cause the computer to perform the method steps described as advantageous or optional in order to achieve an associated technical effect.

[0034] According to one aspect of the invention, a control unit for a vehicle, in particular a commercial vehicle, is provided. The control unit is configured to carry out the method according to the invention. Optionally, the control unit is configured to carry out the method steps described as advantageous or optional in order to achieve an associated technical effect.

[0035] According to one aspect of the invention, a vehicle, in particular a commercial vehicle, is provided with a control unit according to the invention. Optionally, the control unit of the vehicle and / or the vehicle are configured to perform the method steps described as advantageous or optional in order to achieve an associated technical effect.

[0036] Further advantages and features of the invention as well as its technical effects emerge from the figures and the description of the preferred embodiments shown in the figures.

[0037] Fig. 1 is a schematic representation of a flow chart of a method according to an embodiment of the invention;

[0038] Fig. 2 two schematic representations of friction coefficient curves; and

[0039] Fig. 3 is a schematic representation of an overview of a vehicle, in particular a commercial vehicle, according to an embodiment of the invention.

[0040] Figure 1 shows a schematic representation of a flowchart of a method 100 according to an embodiment of the invention. In particular, Figure 1 shows a method 100 for estimating the coefficient of friction for a vehicle 300a, in particular a commercial vehicle 300b, that can be driven by an electric drive 200. The vehicle 300a, in particular a commercial vehicle 300b, is referred to below as vehicle 300a, 300b. The vehicle 300a, 300b is described in more detail with reference to Figure 3.

[0041] The method 100 according to Figure 1 begins with operating 110, with a torque T, a wheel 270 of the vehicle 300a, 300b arranged on a surface 260. Operating 110 is an acceleration, travel at a constant speed, or deceleration of the wheel 270. The torque T for accelerating or decelerating, i.e., the drive or braking torque used for accelerating, traveling at a constant speed, and decelerating the vehicle 300a, 300b, is also referred to as the "stationary" torque T.

[0042] The slip S of the wheel 270 is determined 120. The slip S is determined as a "stationary" slip S depending on the level of the "stationary" torque T and a friction coefficient MU of the road surface. This "stationary" slip S is additionally dependent on an optional steering angle, a sideslip angle and lateral guidance forces of the tire or the wheel 270. Figure 2 shows exemplary friction coefficient curves 400, i.e. the relationship between the friction coefficient MU and the slip S for different road surfaces or surfaces 260. The surface 260 on which the vehicle 300a, 300b travels and the corresponding friction coefficient curve 400 are not known during travel and can change during travel.

[0043] In Figure 1, the wheel 270 is subjected 130 to a temporally predetermined excitation torque ET, wherein the application 130 of the wheel 270 to the excitation torque ET occurs periodically at a frequency F. The periodic excitation torque ET can be idealized as a time-dependent angular function, as a time-dependent rectangular function, and / or as the sum of such functions of the excitation torque ET. The frequency F of the periodic excitation torque ET is in the range from 0.1 Hz to 20 Hz, preferably from 0.5 Hz to 5 Hz.

[0044] The excitation torque ET has an amplitude A, wherein the amplitude A is selected such that a slip limit ST is not exceeded, a sign of a sum of torque T and excitation torque ET is equal to a sign of torque T, and vehicle stability and / or efficiency are taken into account. The application 130 of a plurality of wheels 270 of the vehicle 300a, 300b occurs with the periodic excitation torque ET and with a predetermined phase shift DP. As a result, the excitation torques ET assigned to the various wheels 270 are predetermined in time by the frequency and the phase shift DP. The plurality of wheels 270 can be assigned to one or more axles of the vehicle 300a, 300b. The phase shift DP is, for example, 180°.

[0045] Subsequently, a slip change DS is determined 140 as a function of the excitation torque ET. If, in addition, a high-frequency torque excitation with the excitation torque ET is applied to the "stationary" torque T, the slip S on the electrically driven axle changes by the slip change DS at the same frequency. The slip change DS is determined 140 taking into account the frequency F and using a filter P and / or a Fourier analysis of the slip change DS. The filter P is applied to the slip S with regard to a predetermined interval I comprising the frequency F. The slip change DS is determined 140 using a lock-in amplifier 280. The Fourier analysis of the slip change DS allows a measurement signal associated with the slip change DS to be examined for a specific frequency.In particular, the measurement signal can be examined for the frequency F and / or an interval I around the frequency F.

[0046] The determination 120 of the slip S and the determination 140 of a slip change DS is carried out by a plurality of wheel speed sensors 220 and is optionally verified by the electric drive 21.

[0047] This is followed by determining 150 a coefficient of friction MU based on the slip change DS. The slip change DS causes a change in the tangential force or propulsion force at a contact surface between the wheel 270 and the ground 260. Thus, the slip change DS results in a change in the coefficient of friction MU.

[0048] This is followed by determining 155 an operating point 210 based on the torque T and the slip S. The operating point 210 is a "stationary" operating point 210 and is a point on a friction coefficient curve 400 (see Figure 2), which can be determined based on the "stationary" torque T and the associated "stationary" slip S. With the help of the excitation by the excitation torque ET and the slip change DS, the gradient in the operating point 210 of the friction coefficient curve 400 can be determined, since the friction coefficient MU changes due to the slip change DS.

[0049] This is followed by a determination 156 of a gradient D of the friction coefficient MU as a function of the slip S. If the torque excitation by the excitation torque ET leads only to a small change in slip DS, a high gradient D results for the friction coefficient curve 400.

[0050] An assignment 157 of a friction coefficient curve 400 corresponding to the ground 260 takes place on the basis of the gradient D of the friction coefficient MU and on the basis of a stochastic variable of the friction coefficient MU and / or the slip S. With the aid of the “stationary” operating point 210 and the determined gradient or the gradient D in the operating point 210, the current operating point 210 can be assigned to a characteristic friction coefficient curve 400.

[0051] Determination 160 of a maximum friction coefficient MM of the surface 260 based on the friction coefficient MU. With a high gradient D, the current operating point 210 is far from the maximum friction coefficient MM, which indicates a high friction coefficient MU of the road surface. However, if the slip S changes significantly due to the excitation, the maximum friction coefficient MM is almost reached. In addition, the maximum friction coefficient MM of the road surface can be estimated using interpolation. Due to the direct relationship between torque excitation, change in the slip value DS, and the friction coefficient MU of the road surface, it can be determined at any time whether the current operating point 210 is already close to the slip limit ST or not.

[0052] Those skilled in the art will recognize that the steps of method 100 can also be performed in a different order than that shown. Steps of the method can also occur simultaneously. For example, the operation 110 of wheel 270 arranged on ground 260 with torque T and the application 130 of wheel 270 with the temporally predetermined excitation torque ET can occur at any time and, in particular, simultaneously. The determination 120 of slip S can occur at any time after the operation 110 of wheel 270 arranged on ground 260 with torque T.

[0053] Figure 2 shows two schematic representations (Figure 2 (A) and Figure 2 (B)) of friction coefficient curves 400.1, 400.2, 400.3, 400.3, 400.4, 400.5, 400.6.

[0054] Figure 2 (A) shows six different friction coefficient curves 400.1, 400.2, 400.3,

[0055] 400.3, 400.4, 400.5, 400.6. Each of the friction coefficient curves 400.1, 400.2, 400.3, 400.3,

[0056] 400.4, 400.5, 400.6 show the relationship between the coefficient of friction MU and slip S for a specific subsurface 260. The friction coefficient curve 400.1 shows the relationship between the coefficient of friction MU and slip S for dry asphalt. The friction coefficient curve 400.2 shows the relationship between the coefficient of friction MU and slip S for wet asphalt. The friction coefficient curve 400.3 shows the relationship between the coefficient of friction MU and slip S for crushed stone and / or gravel. The friction coefficient curve 400.4 shows the relationship between the coefficient of friction MU and slip S for wet crushed stone and / or gravel. The friction coefficient curve 400.5 shows the relationship between the coefficient of friction MU and slip S for snow. The friction coefficient curve 400.6 shows the relationship between the coefficient of friction MU and slip S for ice.

[0057] Each of the friction coefficient curves 400.1, 400.2, 400.3, 400.3, 400.4, 400.5, 400.6 has a unimodal shape with a maximum friction coefficient MM at a certain slip S (see Figure 2 (B)), wherein the slope or gradient D (see Figure 2 (B)) of each of the friction coefficient curves 400.1, 400.2, 400.3, 400.3, 400.4, 400.5, 400.6 is positive and comparatively large for a smaller slip S and negative and comparatively small for a larger slip S, as also described with reference to Figure 2 (B).

[0058] Figure 2 (B) shows a section for a selection of the friction coefficient curves 400.1, 400.2, 400.5 as shown in Figure 2 (A). An operating point 210 is shown for one of the friction coefficient curves 400.5. The operating point 210 results from the "stationary" torque T and the associated friction coefficient MU and slip S. At the operating point 210, a gradient triangle for determining the gradient D is shown. The gradient D is the local gradient of the friction coefficient curve 400.5. The excitation torque ET induces a slip change DS and a change in the friction coefficient MU. The gradient D can thus be determined based on the slip change DS and the change in the friction coefficient MU. The gradient D and the operating point 210 provide information about the friction coefficient curve 210 and enable the assignment 157 of the friction coefficient curve 400.5 corresponding to the respective surface 260 based on the gradient D of the friction coefficient MU.

[0059] The maximum friction coefficient MM is shown for the friction coefficient curve 400.5. The gradient D of the friction coefficient curve 400.5 is positive and comparatively large for a smaller slip S and negative and comparatively small for a larger slip S.

[0060] Figure 3 shows a schematic representation of an overview of a vehicle 300a, in particular a commercial vehicle 300b, according to an embodiment of the invention. The vehicle 300a, 300b according to Figure 3 will be described with reference to the description of Figures 1 and 2.

[0061] As shown in Figure 3, the vehicle 300a, 300b is arranged on a ground 260. The wheels 270 are arranged on the ground 260. A contact surface (not shown) is arranged between the wheels 270 and the ground 260. The wheels 270 and the ground 260 make contact at the contact surface. Forces can act between the wheels 270 and the ground 260 through the contact surface. In particular, a normal force can act perpendicular to the contact surface, which depends on the mass of the vehicle 300a, 300b and the number and geometry of the wheels 270. In addition to the normal force, a tangential force may act, which depends on the dynamics of the respective wheel 270, in particular on propulsion and / or braking or the corresponding torque T and / or the excitation torque ET in the case of a driven wheel 270. The ratio of normal force to tangential force describes the coefficient of friction MU of the ground 260.The torque T causes a slip S and the excitation torque ET causes a slip change DS. The slip S is the ratio of a rotational speed R of a driven wheel 270 to a rotational speed R of a non-driven and positively rotating wheel 270. The vehicle 300a, 300b is configured to carry out the method 100 described with reference to Figure 1. For this purpose, the vehicle 300a, 300b has, as shown in Figure 3, a control unit 250, an electric drive 200, the plurality of wheels 270 and the plurality of wheel speed sensors 220. The control unit 250 is connected to the electric drive 200 and the wheel speed sensors 220 in such a way as to carry out the method 100 according to Figure 1.

[0062] The control unit 250 is configured to control the electric drive 200 to apply the torque T and the excitation torque ET to the wheels 270. For this purpose, the control unit 250 can transmit 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. For this purpose, the control unit 250 transmits the instantaneous torque request TR to the electric drive 200. The torque request TR consists of a superposition of a "stationary" torque request for the "stationary" torque T with the additionally superimposed excitation torque ET.The frequency F, the amplitude A, the phase shift DP, and the slip limits ST are stored in the control unit 250 and serve to "generate" the excitation torque ET, which is added to the "stationary" torque T. The electric drive 200 receives an instantaneous value of a total torque as the sum of the torque T and the excitation torque ET, which then changes periodically due to the superimposed excitation torque ET. The electric drive 200 is configured to apply the torque T and the excitation torque ET to the wheel(s) 270 based on the signal from the control unit 250.

[0063] Each of the wheel speed sensors 220 is configured to measure the rotational speed R of one of the wheels 270. Using multiple wheel speed sensors 220, the slip S and the slip change DS can be determined. For this purpose, one of the wheel speed sensors 220 is configured to determine the rotational speed R of a non-driven wheel 270, and one of the wheel speed sensors 220 is configured to determine the rotational speed R of a driven wheel 270. The wheel speed sensor 220 is connected to the control unit 250 to transmit the rotational speeds R of the wheels 270 to the control unit 250. The control unit 250 is configured to determine the slip S and the slip change DS based on the rotational speeds R of the wheels 270.

[0064] The control unit 250 is further configured to form a lock-in amplifier 280. For this purpose, the control unit 250 can provide the interval I and the filter P to the lock-in amplifier 280.

[0065] The control unit 250 further comprises a data processing device (not shown) and a memory (not shown). For example, the friction coefficient curves 400 shown in Figure 2 can be stored in the memory. For each set of substrates 260, a friction coefficient curve 400 and / or a friction coefficient gradient curve is stored in the memory in order to effectively implement the method 100 according to Figure 1. The friction coefficient curve 400 can be obtained by measurement and / or modeled heuristically.

[0066] Reference symbol (part of the description)

[0067] 100 procedures

[0068] 110 Operating a bicycle

[0069] 120 Determining the slip

[0070] 130 Applying excitation torque

[0071] 140 Determining a slip change

[0072] 150 Determining a coefficient of friction

[0073] 155 Determining an operating point

[0074] 156 Determining a gradient

[0075] 157 Assigning a friction coefficient curve

[0076] 160 Determining a maximum coefficient of friction

[0077] 200 electric drive

[0078] 210 operating point

[0079] 220 wheel speed sensor

[0080] 250 control unit

[0081] 260 Underground

[0082] 270 wheels

[0083] 280 lock-in amplifiers

[0084] 300a vehicle

[0085] 300b commercial vehicle

[0086] 400 Friction coefficient curve

[0087] A Amplitude

[0088] D Gradient

[0089] DP phase shift

[0090] DS slip change

[0091] ET Excitation torque

[0092] F Frequency

[0093] MM maximum friction coefficient

[0094] MU friction coefficient P filter

[0095] R speed

[0096] S slip

[0097] ST slip limit

[0098] T torque

[0099] TR torque requirement

Claims

Patent claims 1. Method (100) for estimating the coefficient of friction for a vehicle (300a) driven by an electric drive (200), in particular a commercial vehicle (300b), the method (100) comprising the steps: - Operating (1 10), with a torque (T), of a wheel (270) of the vehicle (300a), in particular a commercial vehicle (300b), arranged on a surface (260); - Determining (120) the slip (S) of the wheel (270); - Applying (130) to the wheel (270) with a time-predetermined excitation torque (ET), wherein the application (130) to the wheel (270) with the excitation torque (ET) is periodic at a frequency (F); - Determining (140) a slip change (DS) as a function of the excitation torque (ET), wherein the determination (140) of the slip change (DS) is carried out taking into account the frequency (F); and - Determining (150) a coefficient of friction (MU) based on the slip (S) and the change in slip (DS).

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

3. Method according to claim 1 or 2, wherein - the determination (140) of the slip change (DS) is carried out using a filter (P) and / or by Fourier analysis of the slip change (DS).

4. Method according to claim 3, wherein - the filter (P) is applied to the slip (S) with respect to a predetermined interval (I) encompassing the frequency (F).

5. Method according to any one of the preceding claims, wherein - the application (130) of several wheels (270) of the vehicle (300a), in particular a commercial vehicle (300b), with the periodic excitation torque (ET) and with a predetermined phase shift (DP) is carried out.

6. Method according to any one of the preceding claims, wherein - the determination (140) of the slip change (DS) is carried out using a lock-in amplifier (280).

7. Method according to any of the preceding claims, further comprising the step of: - Determining (155) an operating point (210) based on the torque (T) and the slip (S).

8. The method of claim 7, further comprising the step of: - Determining (156) a gradient (D) of the coefficient of friction (MU) as a function of the slip (S).

9. The method of claim 8, further comprising the step of: - Assigning (157) a friction coefficient curve (400) corresponding to the substrate (260) based on the gradient (D) of the coefficient of friction (MU) and / or based on a stochastic quantity of the coefficient of friction (MU) and / or the slip (S).

10. Method according to any of the preceding claims, further comprising the step: - Determine (160) a maximum coefficient of friction (MM) of the substrate (260) based on the coefficient of friction (MU). 1 1. Method according to one of the preceding claims, wherein - the excitation torque (ET) has an amplitude (A), wherein the amplitude (A) is chosen such that a slip limit (ST) is not exceeded, a sign of a sum of torque (T) and excitation torque (ET) is equal to a sign of the torque (T) and / or consideration is given to vehicle stability and / or efficiency.

12. Method according to any one of the preceding claims, wherein - the determination (120) of the slip (S) and the determination (140) of a change in slip (DS) is carried out by a plurality of wheel speed sensors (220) and / or taking into account information relating to a speed (R) of the electric drive (21 ).

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

14. Control unit (250) for a vehicle (300a), in particular a commercial vehicle (300b), wherein the control unit (250) is configured to perform the method (100) according to any one of claims 1 to 12.

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

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