DE102018217793A1 - Method for estimating a current coefficient of friction - Google Patents

Abstract

In a method for estimating a current coefficient of friction between a wheel (2) of a vehicle (F) and a roadway (1) while the vehicle (F) is traveling on the roadway (1), the method being suitable for an estimation slip To generate between the wheel (2) and the road (1) and to estimate the current coefficient of friction on the basis of the estimation slip, the wheel (2) is subjected to an estimation wheel torque (T) to generate the estimation slip.

Description

The invention relates to a method for estimating a current coefficient of friction according to the preamble of claim 1. The invention further relates to a control device, a vehicle and a computer-readable medium according to the independent claims.

Autonomous driving and modern driver assistance systems are becoming increasingly important. Autonomous driving systems have to meet different requirements, especially when it comes to driving comfort and safety. In order to meet these requirements, it is necessary for the systems and / or the vehicles to be aware of current circumstances, functional parameters, driving conditions and the like. One parameter, the knowledge of which can help to appropriately control a vehicle according to the current road conditions, is the current coefficient of friction (also: coefficient of friction) between the tires and the road. The more precisely a vehicle control system knows this current coefficient of friction, the more precisely z. B. the acceleration behavior of the vehicle can be adapted to the current road condition. In this respect, there is an urgent need to determine or estimate the current coefficient of friction between vehicle tires and the road surface as well as possible. However, no simple, reliable, safe and comfortable options for determining the current coefficient of friction are known to date.

The object of the invention is to eliminate or at least reduce the disadvantages of the prior art.

The object is achieved by a method for estimating a current coefficient of friction between a wheel of a vehicle and a roadway while the vehicle is traveling on the roadway, the method being suitable for generating an estimation slip between the wheel and the roadway and on the basis of the estimation slip to estimate the current coefficient of friction, wherein the wheel is subjected to an estimation wheel torque to generate the estimation slip.

Estimating is to be understood as an approximate determination, whereby the determination can of course also be very precise and, ideally, it is. The coefficient of friction can also be referred to as the coefficient of friction (symbol: µ). In other words, the term “coefficient of friction” is to be understood in this description as an alternative term for the term “coefficient of friction” (symbol: µ).

In the method, a current coefficient of friction between at least one tire of a vehicle and a roadway is estimated. A wheel in the sense of this description comprises in particular a rim and a tire, the tire being mounted on the rim, the tire comprising rubber and / or a rubber mixture.

Under a vehicle is a motor vehicle such. B. to understand a car, a bus, a truck, a motorcycle or even a three-wheel car. A roadway is to be understood as a road or path that can be paved or unpaved, for example with an asphalt surface or a gravel surface. Furthermore, the roadway can be at least partially covered with snow or ice, can be dry or at least partially wet. Furthermore, the roadway can be at least partially contaminated, in particular by leaves, oil or the like.

A slip is understood to mean the fact that the distance actually traveled per wheel revolution differs from the actual wheel circumference. In other words, when a slip occurs, the wheels turn faster or slower than one would expect given the current vehicle speed, that is, the wheels spin at least partially or at least partially block.

The wheel torque is the torque that is exerted on a wheel of the vehicle. In this description, a wheel torque is to be understood as a positive wheel torque and not braking. However, it is possible that the estimated wheel torque is either temporarily increased or temporarily reduced compared to a current wheel torque that corresponds to the normal running of the vehicle.

The invention is based on the knowledge that a slip on a wheel of the vehicle can be used well to estimate the current coefficient of friction between a tire and the road surface, and that it is possible to temporarily generate such slip for test purposes, namely by Applying an estimated wheel torque to at least one wheel during a certain period of time.

In typical embodiments, the application of the estimated wheel torque to the wheel comprises a temporary increase and / or reduction of a wheel torque of the wheel, the increase and / or reduction being accomplished by means of torque vectoring and / or by means of a suitable control of a wheel drive motor, the wheel drive motor is an electric motor.

The term torque vectoring, which is also referred to as active yaw, is active Understand torque distribution on the wheels of the vehicle. In the case of an electric vehicle, a wheel drive motor is to be understood as a motor which drives a single wheel directly. Torque vectoring or suitable control of an electric wheel drive motor are advantageous because they are particularly simple ways of generating the estimated wheel torque.

In typical embodiments, the method comprises an application step, during which the estimated wheel torque is applied to the wheel, and a detection step, during which the vehicle current vehicle parameters, in particular a vehicle speed and / or a wheel speed and / or the estimation slip, detected. The detection step preferably runs at least partially simultaneously with the application step. The method also includes an estimation step during which the current coefficient of friction is estimated on the basis of the estimation slip.

A characteristic curve field is used to estimate the current coefficient of friction, which field comprises a plurality of characteristic curves. Each characteristic curve relates to a specific road surface condition, each characteristic line indicating the coefficient of friction between a tire and a road surface as a function of the slip between the tire and the road surface. A characteristic curve is therefore a curve in a two-dimensional coordinate system, the slip in percent being plotted on the x-axis and the corresponding coefficient of friction being plotted on the y-axis. The plurality of characteristic curves include a characteristic curve for dry carriageway and / or a characteristic curve for wet carriageway and / or a characteristic curve for snow-covered carriageway and / or a characteristic curve for icy carriageway.

In typical embodiments, the characteristic curve field comprises different characteristic curves for different degrees of moisture in the roadway, as well as a characteristic curve for a roadway with gravel covering and / or with sand covering and / or with other covering. As an alternative to a plurality of characteristic curves, however, it is also conceivable to work with only a single characteristic curve, for example an averaged characteristic curve. In typical embodiments, the characteristic curve used to estimate the current coefficient of friction is decided using the vehicle sensor system. For example, in typical embodiments, sensors installed in the vehicle detect that the road is wet or covered with snow, for example, sensors connected to the vehicle steering system and / or the braking system and / or temperature sensors and / or rain sensors. Based on this information, the corresponding characteristic curve, i.e. B. selected a characteristic curve for snow-covered or wet road. In typical embodiments, the characteristic curves themselves are taken from information available from tire manufacturers. In typical embodiments, the characteristic curves are determined on a test bench in a vehicle-specific manner for the tires currently installed. In other embodiments, standardized, averaged characteristics are used.

In typical embodiments, the method comprises a trigger step during which a trigger signal is generated to start the method. The trigger signal is generated by a central vehicle controller or the like. The trigger signal is generated when a certain time interval has elapsed since the method was last carried out and / or when a dangerous situation has been detected and / or when a favorable opportunity to carry out the method has been detected.

In typical embodiments, the method comprises a start condition test, during which it is checked whether a predefined condition, in particular a safety condition, is fulfilled for the implementation of the method. The process is terminated or at least interrupted if the condition is not met. The condition preferably includes at least one partial condition.

In this context, the term “dangerous situation” is to be understood in such a way that a central vehicle control system recognizes a situation that increases the risk of an accident. An example of such a dangerous situation is e.g. B. detection of black ice or a wet road by a camera oriented in the direction of travel or by sensors. If such a dangerous situation is recognized, the method can then be used to estimate the current coefficient of friction at short notice, and thus to adapt the driving style, in particular the acceleration or the wheel torque distribution, to the weather conditions.

A favorable opportunity is to be understood as a situation in which the estimation slip can be generated in a particularly simple manner or is even generated almost automatically by other driver assistance systems. For example, it is conceivable that in situations in which the wheels of the vehicle are subjected to different wheel torques anyway, such situations can be used to generate the estimation slip without this, for. B. affects driving comfort. This is the case, for example, when a driving dynamics system is taking effect. Such favorable opportunities can also occur, for example, if there are different coefficients of friction for the different tires of the vehicle and if in particular of other driver assistance systems and / or driving dynamics systems must react to this fact.

In typical embodiments, the safety condition includes driving straight ahead. In other words, in typical embodiments, the method is only carried out when the vehicle is currently driving straight ahead. This has the advantage that the method is not carried out during cornering, in which the application of a wheel to the estimated wheel torque could have a particularly unpleasant effect. In typical embodiments, further safety conditions are, for example, the condition “no gas” and / or “brake pedal not depressed”. Alternatively, however, it is also conceivable that the method is preferably carried out precisely when the driver is accelerating because an increased wheel torque is then exerted on the wheels.

In advantageous embodiments, the method applies another estimated wheel torque to another vehicle wheel when a changeover condition is present. The method interrupts the application of the estimated wheel torque to the original wheel and instead acts on another wheel of the vehicle with an estimated wheel torque. Such a switchover has the advantage that it is possible to react to unforeseen problems in the generation of the estimation slip on the wheel of the vehicle originally intended for the method. For example, it is conceivable that, contrary to expectations, sufficient estimation slip cannot be built up on the wheel originally intended for the method. Then, when the switching condition is present, the method can select a wheel of the vehicle other than the wheel to which an estimated wheel torque is applied. However, it is also conceivable that in the presence of a changeover condition, in addition to the first wheel, a further estimation wheel torque is applied. Alternatively, it is of course also possible to abort the method if the estimation slip on the wheel provided for the method cannot be generated or cannot be generated safely without switching to another vehicle wheel.

In advantageous embodiments, the application of the vehicle wheel / vehicle wheels with the estimation wheel torque / the estimation wheel torques takes place according to an application curve, an application duration preferably 1 second to 5 seconds, preferably 2 seconds to 4 seconds, approximately 2.5 Seconds, and / or wherein the load is reduced as soon as a maximum estimation slip is reached, the maximum estimation slip preferably being at most 10%, preferably at most 2%, approximately 1%, and / or wherein at least one estimation Wheel torque is substantially linearly reduced as soon as at least 70%, approximately 80% of the duration of the application has elapsed. The estimated wheel torque at the end of the application duration has the same value as at the beginning of the application. Such exposure times have proven to be particularly advantageous for carrying out the method in practical tests because on the one hand they are long enough to generate a corresponding estimation slip and on the other hand they are not as long, that driving safety and driving comfort would be significantly impaired. Controlling the loading is advantageous because it prevents the wheels from spinning excessively, which has a positive effect on driving comfort. In general, the term “approximately” in this description means a tolerance of +/- 15% or +/- 10%.

In advantageous embodiments, the method, in addition to being suitable for generating the estimation slip by means of the estimation wheel torque, is suitable for generating a braking slip by braking a vehicle wheel. This has the advantage that both the estimate slip and the brake slip can be used to determine the current coefficient of friction, in particular by determining the current coefficient of friction on the basis of the estimate slip and / or the brake slip.

Such a combination of the estimation slip by acting on a wheel with a wheel torque on the one hand and a slip generation by braking on the other hand has different advantages, namely e.g. B. a redundancy of both possibilities for slip generation; a mutual plausibility check of the friction values determined with the different types of slip generation; better accuracy of the friction estimation; as well as a possible increase in driving comfort. This is the case because, depending on the situation, either a brake slip or an estimate slip or a combination of both types of slip can be generated. Such a combination of slip generation by applying a wheel torque on the one hand and slip generation by braking on the other hand is e.g. B. advantageous in situations where a driving dynamics system reacts to different coefficients of friction on different wheels.

The object is further achieved by a control device for estimating a current coefficient of friction between a wheel of a vehicle and a road while the vehicle is traveling on the road. The control device is suitable for generating an estimation slip between the wheel and the road and for estimating the current coefficient of friction on the basis of the estimation slip. The control device is suitable for the wheel Generate the estimation slip with an estimation wheel torque.

The control device advantageously includes computer program code for carrying out the method for estimating a current coefficient of friction according to one of the previously described embodiments. The control device is advantageously part of a vehicle control or a separate control device. The control device advantageously includes a digital control unit.

In advantageous embodiments, the control device is suitable for carrying out a method for estimating a current coefficient of friction according to at least one of the aforementioned embodiments. For this purpose, the control device advantageously comprises suitable components, e.g. B. a trigger device and / or a start condition test device and / or an application device and / or a detection device and / or an estimation device and / or a plurality of communication interfaces for communication with a camera and / or a radar and / or a rain sensor and / or an on-board computer and / or a cloud and / or a central vehicle controller.

At least some of the aforementioned components are advantageously implemented in the control device by means of the computer program code.

In one embodiment of the invention, a vehicle comprises a aforementioned control device.

In one embodiment of the invention, a computer-readable medium comprises computer program code for performing one of the aforementioned methods. However, the term “computer-readable medium” is not exclusively to be understood as hard disks and / or servers and / or memory sticks and / or flash memories and / or DVDs and / or bluerays and / or CDs. In addition, the term “computer-readable medium” is also to be understood to mean a data stream, such as is produced when a computer program product is downloaded from the Internet.

• 1: eine schematische Darstellung eines Fahrzeugs auf einer Fahrbahn, wobei ein Rad mit einem Schätz-Radmoment beaufschlagt wird,
• 2: eine schematische Darstellung einer Ausführungsform des erfindungsgemässen Verfahrens in einem Flussdiagramm, und
• 3: eine Beaufschlag-Kurve, wie sie bei einem typischen Ausführungsbeispiel der Erfindung zur Anwendung kommt.

The invention is briefly explained below with reference to drawings, in which:

• 1 1 shows a schematic illustration of a vehicle on a roadway, an estimated wheel torque being applied to a wheel,
• 2nd : a schematic representation of an embodiment of the inventive method in a flow chart, and
• 3rd : an application curve as used in a typical embodiment of the invention.

1 shows a vehicle F which is on a roadway 1 moves. The vehicle F is in a straight line. A wheel 2nd of the vehicle F , namely the right rear wheel, becomes the one in 1 shown time with an estimated wheel torque, so that the wheel 2nd sets an estimation slip. From this estimation slip on the wheel 2nd is then the current coefficient of friction between the tire on the wheel in the course of the inventive method 2nd and the road surface of the road 1 estimated. Knowing this current coefficient of friction then enables control of the vehicle F , especially in autonomous driving, to adapt to the current coefficient of friction, for example as far as the wheel torque distribution, the acceleration and the like are concerned. Thus, e.g. B. make autonomous driving more comfortable and safe.

2nd shows a flowchart which shows the typical sequence of a method according to the invention in a preferred embodiment. During a trigger step S1 the procedure is initiated. For this purpose, a trigger is made available, for example the fact that the method has not been carried out for a certain time and the current coefficient of friction should therefore be updated. Alternatively, the trigger can be the fact that a changed road condition, for example a snow-covered road, has been detected, which makes it necessary to redetermine the current coefficient of friction.

After the procedure is in the trigger step S1 was triggered, is in the start condition test S2 checks whether there are certain starting conditions for carrying out the method. Such starting conditions can be, for example, driving the vehicle straight ahead, driving without braking and / or driving without gas. Only if the start conditions in the start condition test S2 are recognized as present, the method is continued. The reason for carrying out the start condition test is the fact that the method should only be carried out if the method can be carried out safely and comfortably. In typical embodiments, the start condition test S2 adapted to the vehicle and / or the vehicle type, to the weather conditions and / or to the comfort level of the respective driver.

If the conditions of the start condition test S2 the process is continued, namely with the application step S3 and the acquisition step S4 . As in 2nd represented schematically, the application step are running S3 and the acquisition step S4 in this embodiment at least partially at the same time, during the loading step S3 Information from the acquisition step S4 can be used, and vice versa. In other words, during the charging step S3 In typical embodiments, information is used which is used during the acquisition step S4 were collected.

For example, in typical embodiments during the loading step S3 the application of an estimation wheel torque to one wheel is stopped and an estimation wheel torque is applied to another wheel, if in the context of the detection step S4 it was recognized that the first attempt to generate an estimate slip was unsuccessful. On the other hand, in typical embodiments, the detection step S4 Information from the application step S3 used, for example the height of an estimation wheel torque with which one of the wheels was applied.

Once the capture step S4 is completed, ie as soon as there is an estimation slip, is within the framework of the estimation step S5 a current coefficient of friction is determined on the basis of the estimated slip. For this purpose, a characteristic curve field is used, that is, a plurality of characteristic curves, each characteristic curve indicating a certain coefficient of friction for a certain road condition depending on a certain slip. The characteristic curves are stored in the vehicle and are, for example, either specified by the tire manufacturer or determined on a test bench. This field of characteristics becomes part of the estimation step S5 selected a characteristic curve that corresponds to a current road condition. The current state of the road is determined by means of vehicle sensors, for example by means of steering sensors, which, based on the behavior of the steering, can, for example, infer a snow-covered road surface.

The coefficient of friction is then determined for the selected characteristic curve, which is the estimation slip, which is within the scope of the detection step S4 was determined. As part of a final final step S6 , this current coefficient of friction is then stored in a vehicle memory and is available for various purposes until the process is repeated in a trigger step S1 is triggered. After performing the method again, the current coefficient of friction is then updated again and so on and so on.

In typical embodiments, the method is terminated each time it is recognized, for example in the context of the application step S3 and / or the acquisition step S4 that a new determination of the current coefficient of friction is not possible or not useful or not safe or not comfortable. Such unscheduled discontinuations of the procedure are in 2nd not shown for the sake of clarity.

Is also in 2nd For the sake of clarity, it is not shown that, in typical exemplary embodiments, the method is also suitable for generating a slip by braking and then also estimating the current coefficient of friction using the characteristic field on the basis of the brake slip, and not merely on the basis of the estimate slip . This combination of brake slip and estimate slip can have several advantages, e.g. B. a more reliable and / or more precise and / or more comfortable and / or more flexible estimate of the current coefficient of friction.

3rd shows a typical application curve of an estimation wheel torque in a typical embodiment of the invention. In particular, in the diagram in 3rd the course of estimation wheel moments T over time t applied. Here two loading curves are shown, namely one loading curve 3rd , which is reduced as soon as an estimated slip of 1% is reached, and an application curve 4th without curtailment. The loading curve 4th is shown in dashed lines. From the observation of the loading curves 3rd , 4th in 3rd it becomes clear that at a start time t1 the application of the estimated wheel torque to the wheel T is started. From this start time t1 becomes the estimated wheel torque T linearly increased, so that theoretically an estimated wheel torque after a period of two seconds T would be reached in the amount of 200 Newton meters (Nm). After two seconds, i.e. at the time of decay t3 , then becomes the estimated wheel torque T canceled, linear over a period of 0.5 seconds, so that the estimated wheel torque T at a no-stop time t4 is again 0 Nm. This theoretical estimated wheel torque curve is dashed in the charge curve 4th shown. This course of the curve occurs if an estimated slip of at least 1% is never reached.

In most cases, however, it will be possible to achieve an estimate slip of at least 1%. Then the charge curve takes the one for the charge curve 3rd shown curve. In this case it will also be at a start time t1 the estimated wheel torque T increased linearly, with a gradient of 100 Nm / s. As soon as it has been detected that an estimated slip of 1% has been reached, the estimated wheel torque becomes T to a cut-off value T_1% curtailed. This happens at a cut-off time t2 . Then the estimated wheel torque T on the cutoff value T_1% held until after two seconds the flattening time t3 is reached. Between the time of decay t3 and the zero time t4 , which in this embodiment is 0.5 seconds after the flattening time t3 lies, the estimated wheel torque is successively reduced linearly so that it is at the zero point in time t4 ends or the value 0 Nm assumes.

Reference list

1

roadway

2nd

wheel

3rd

Charge curve (reduced as soon as 1% of the estimation slip has been reached)

4th

Charge curve (without curtailment)

F

vehicle

S1

Trigger step

S2

Start condition test

S3

Acting step

S4

Acquisition step

S5

Estimation step

S6

Final step

T

Estimated wheel torque

T_1%

Shutdown value

t

time

t1

Start time

t2

Resetting time

t3

Deterioration time

t4

No time

Claims (10)

Method for estimating a current coefficient of friction between a wheel (2) of a vehicle (F) and a roadway (1) while the vehicle (F) is traveling on the roadway (1), the method being suitable for estimating a slip between the To produce wheel (2) and the roadway (1) and to estimate the current coefficient of friction on the basis of the estimation slip, characterized in that the wheel (2) is subjected to an estimation wheel torque (T) for generating the estimation slip . Procedure according to Claim 1 , characterized in that applying the estimated wheel torque (T) to the wheel (2) comprises temporarily increasing and / or reducing a wheel torque of the wheel (2), the increasing and / or reducing using torque vectoring and / or is accomplished by means of a suitable control of a wheel drive motor, the wheel drive motor being an electric motor. Method according to one of the preceding claims, characterized in that the method comprises the following: - an application step (S3), during which the wheel (2) is acted upon by the estimation wheel torque (T), and - an acquisition step (S4) , during which the vehicle (F) detects current vehicle parameters, in particular a vehicle speed and / or a wheel speed and / or the estimation slip, the detection step (S4) preferably running at least partially simultaneously with the application step (S3), and - An estimation step (S5) during which the current friction value is estimated on the basis of the estimation slip, wherein a characteristic curve field is used to estimate the current friction value, which comprises a plurality of characteristic curves, each characteristic curve relating to a specific road condition, each characteristic curve the coefficient of friction between a tire and a road surface (1) as a function of the slip between the tire and the road surface (1) indicates. Method according to one of the preceding claims, characterized in that the method comprises the following: - a trigger step (S1), during which a trigger signal for starting the method is generated, the trigger signal being generated by a central vehicle controller or the like and the trigger signal is generated if a certain time interval has elapsed since the method was last carried out and / or if a dangerous situation has been detected and / or if a favorable opportunity to carry out the method has been detected, and - a start Condition test (S2) during which it is checked whether a predefined condition for the implementation of the method is fulfilled, the method being terminated or at least interrupted if the condition is not met. Method according to one of the preceding claims, characterized in that the method acts on another vehicle wheel with a further estimated wheel torque when a changeover condition is present. Method according to one of the preceding claims, characterized in that the application of the vehicle wheel / the vehicle wheels with the estimation wheel torque / the estimation wheel moments takes place in accordance with an application curve, wherein an exposure duration is preferably 1 second to 5 seconds, preferably 2 seconds to 4 seconds, approximately 2.5 seconds, and / or - the exposure being reduced as soon as a maximum estimation slip has been reached, the maximum estimation Slip is preferably at most 10%, preferably at most 2%, approximately 1%, and / or - wherein at least one estimated wheel torque (T) is substantially linearly reduced successively as soon as at least 70%, approximately 80% of the duration of the application has elapsed , the estimated wheel torque (T) at the end of the application duration again having the same value as at the beginning of the application. Method according to one of the preceding claims, characterized in that the method, in addition to being suitable for generating the estimation slip by means of the estimation wheel torque (T), is suitable for generating a braking slip by braking a vehicle wheel. Control device for estimating a current coefficient of friction between a wheel (2) of a vehicle (F) and a roadway (1) while the vehicle (F) is traveling on the roadway (1), the control device being suitable for determining an estimation slip between the Generate wheel (2) and the roadway (1) and estimate the current coefficient of friction on the basis of the estimate slip, characterized in that the control device is suitable for generating the estimate slip (2) with an estimate wheel torque (T) to act on. Vehicle (F) comprising a control device according to Claim 8 . Computer-readable medium, characterized in that the computer-readable medium is computer program code for carrying out a method according to one of the Claims 1 to 7 includes.

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