SE545851C2 - Method and control arrangement for controlling an anti-lock braking function - Google Patents

Abstract

The present disclosure relates to vehicles. According to a first aspect, the disclosure relates to a method for controlling an anti-lock braking function configured to regulate a brake force applied on a front axle of a vehicle to mitigate effects of wheel locking. The method comprises determining S1, based on a steering angle of a front axle of the vehicle, an expected yaw rate of the vehicle and measuring S2, while applying a brake force on the front axle, an actual yaw rate of the vehicle, controlling S4 the antilock braking function to adjust the applied brake force, based on a difference between the measured actual yaw rate and the determined expected yaw rate. The disclosure also relates to a computer program, to a computerreadable medium and to a vehicle.

Description

Technical field The present disclosure relates to braking systems for vehicles. More specifically the disclosure relates to a method and a control arrangement for controlling an anti- lock braking function of a vehicle. The disclosure also relates to a computer program, to a computer-readable medium and to a vehicle.

Background Modern vehicles are equipped with adaptive and relatively advanced braking systems. lt is especially important that the braking systems of heavy vehicles, such as trucks, busses and tractors are flexible since these vehicles are often driven on problematic surfaces. For example, it may be anticipated that today all heavy vehicles are equipped with an anti-lock braking system, ABS, as well as with a redundant braking system.

The ABS system operates by preventing the wheels from locking up during braking, thereby maintaining tractive contact with the road surface and allowing the driver to maintain more control over the vehicle. An ordinary ABS system measures wheel speeds and compares the wheel speeds, with each other and/or with a threshold. lf one or more wheels are determined to be locking (the one with fastest decrease in wheel speed), brake force is reduced on that wheel.

The redundant braking system is a system that provides braking capability in case of failure of a primary (or main) braking system. A typical redundant braking system activates the same brakes as the main braking system butwith another mechanism.

The commonly used redundant braking systems of today rely on that a driver manually pushes the brake pedal to open a valve, whereby a pressure that actuates the brakes is provided. However, when a driver is not present (for example in autonomous vehicles), or unable to push the brake pedal to brake in case of failures, a redundant braking system is arranged to brake the vehicle based on control inputs from electronic systems.

Redundant braking systems typically lack the ABS function due to that signals that are provided by wheel speed sensors are typically also lost when the main or primary braking system fails. With a driver present, it is possible for the driver to resolve a wheel-lock situation by temporarily reducing brake force. However, in electrical vehicles there is typically no protection against wheel lock when braking with the redundant braking system. Hence, there is a risk of loss of steering due to locking of the front wheels. There are also other situations, where the performance of the automatic braking systems used today is not optimal.

Summary lt is an object of the disclosure to alleviate at least some of the drawbacks with the prior art. Thus, it is an object of this disclosure to provide techniques for mitigating consequences of wheel lock in various situations, while maintaining braking efficiency. ln particular, it is an object to avoid insufficient steering ability caused by wheel lock. More specifically, it is an object to provide techniques for controlling an anti-lock braking function that is not dependent on input from wheel speed sensors and that is therefore suitable for use by a redundant braking system.

According to a first aspect, the disclosure relates to a method for controlling an anti- lock braking function configured to regulate a brake force applied on a front axle of a vehicle to mitigate effects of wheel locking. The method comprises determining, based on a steering angle of a front axle of the vehicle, an expected yaw rate of the vehicle and measuring, while applying a brake force on the front axle, an actual yaw rate of the vehicle, controlling the antilock braking function to adjust the applied brake force, based on a difference between the measured actual yaw rate and the determined expected yaw rate. By controlling the antilock braking function based on a difference between actual and expected yaw rate, operation of the antilock braking function can be ensured also when sensor data from wheel speed sensors is lost. ln situations when sensor data from wheel speed sensors is available, the difference may be used in combination with the sensor data, to improve efficiency of the antilock braking function. the vehicle comprises a main brake system and a redundant braking system that takes over when the main braking system is dysfunctional, and the antilock braking function is implemented by the redundant braking system. Thereby, insufficient steering due to wheel lock may be avoided also when the main braking system does not work properly. ln some embodiments, the main braking system utilizes wheel speed sensors for detecting wheel Iocking and wherein the wheel speed sensors are expected to be inoperable upon the main braking system being dysfunctional. The proposed method is independent on wheel speed sensors as the yaw rate is used to detect the wheel Iocking. ln some embodiments, the controlling comprises reducing the brake force applied on the front axle or activating an anti-lock braking system pump, in response to the difference between the actual yaw rate and the expected yaw rate exceeding a level corresponding to insufficient steering ability, such as understeering. By reducing brake force, or activating an ABS pump, insufficient steering may be mitigated also when signals from wheel speed sensors are lost. ln some embodiments, insufficient steering ability corresponds to understeering. Hence, the proposed technique may be used to mitigate understeering caused by wheel lock. ln some embodiments, the level comprises a maximum difference indicative of wheel Iocking on the front axle. A maximum difference that should trigger the anti- lock braking function to adjust the brake force may be set based on calculations, trials simulations etc. ln some embodiments, the method comprises maintaining the reduced brake force or activated anti-lock braking system pump until the difference between the actual yaw rate and the expected yaw rate goes below the level corresponding to insufficient steering ability. The reduction of brake force should continue at least until the difference goes below the level whereby the steering can be expected to be efficient again. ln some embodiments, the method comprises increasing brake force applied on other axles to compensate for reduced brake force on the front axle. By applying brake force on other axles, the total brake force may be maintained. ln some embodiments, the method comprises detecting, using a wheel speed sensor arranged to measure wheel speed of a wheel arranged on the front axle, a wheel speed falling below a level indicative of wheel lock of the wheel. ln these embodiments the method further comprises maintaining the applied brake force on the locked wheel, despite the wheel speed indicating wheel lock, provided that the difference between the actual yaw rate and the expected yaw rate remains below a level representing insufficient steering ability, such as understeering. Thereby, it is possible to maintain brake force, despite the wheels being locked, in situations when no steering is required, such as driving on a straight road. Hence, unnecessary reduction of brake force can be avoided. ln some embodiments, the actual yaw rate is determined using a yaw rate sensor comprising one or more of a gyro, an image sensor, and a position sensor. Hence, various types of sensors may be used to measure the yaw rate. ln some embodiments, the expected yaw rate is determined based on a vehicle model and input steering angle or desired curvature. Hence, the expected yaw rate can typically be determined based on information that is commonly available in vehicles.

According to a second aspect, the disclosure relates to a control arrangement configured to control an anti-lock braking function configured to regulate a brake force applied on a front axle of a vehicle to mitigate effects of wheel locking, the control arrangement being configured to perform the method according to the first aspect.

According to a third aspect, the disclosure relates to a vehicle comprising the control arrangement of the second aspect.

According to a fourth aspect, the disclosure relates to a computer program comprising instructions which, when the program is executed by a computer, cause the computer to carry out the method according to the first aspect.

According to a fifth aspect, the disclosure relates to a computer-readable medium comprising instructions which, when executed by a computer, cause the computer to carry out the method according to the first aspect.

Brief description of the drawinds Fig. 1 i||ustrates a vehicle 1 comprising a redundant braking system.

Fig. 2 i||ustrates how wheel locking may affect steering ability.

Fig. 3 is a flow chart of the proposed method for controlling an anti-lock braking function.

Fig. 4 i||ustrates a control arrangement of the vehicle of Fig. 1 according to an example embodiment.

Detailed description The proposed technique is based on the assumption that locking of front wheels may not always be a problem as long as steering corrections are not required, as the vehicle is intended to continue straight in a current path. However, if steering correction is needed and the front wheels are locked, a wheel lock will result in that it is not possible to steer the vehicle anymore, as turning the front wheel will not have any effect if the wheels are not spinning. ln such a situation it is typically important to mitigate the wheel lock. lnsufficient steering ability associated with locked wheels is illustrated in Fig. 1. Fig. 1 i||ustrates two vehicles 1 denoted (a) and (b). The first vehicle 1(a) (to the left) does not have locked wheels. Hence, a steering angle ala will result in a corresponding yaw rate wla of the vehicle, whereby the vehicle 1(a) follows the road. Per common definition, the steering angle wx is defined as the angle between the front of the vehicle (straight ahead in relation to the vehicle) and the steered wheel direction. Hence, steering angle 0 corresponds to an intention of driving straight forwards. The other vehicle 1(b) (to the right) has locked front wheels. Hence, the steering angle an, will not affect the yaw rate wlb and the vehicle will proceed straight forward, as the wheels are locked.

The proposed technique is based on the insight that wheel locking can be detected by comparing actual and expected yaw rate and that this difference can be utilised by an anti-lock braking function. Thereby, brake force can be reduced on the front axle when needed due to poor steering ability and remain reduced until steering performance is sufficient. This concept may be utilised alone when signals from wheel speed sensors 24 (Fig. 2) are lost, or as a complement to the signals from the wheel speed sensors 24 to optimise performance of the anti-lock braking function.

Fig. 2 conceptually illustrates a vehicle 1, here a truck, where the proposed method for controlling an anti-lock braking function configured to regulate a brake force applied by the front brakes 21 on a front axle 31 of a vehicle 1 to mitigate effects of wheel locking may be implemented. ln some embodiments the vehicle 1 comprises equipment required for autonomous driving such as a navigation system, sensors, and meters etc. ln other embodiments, the vehicle is at least partly manual.

The vehicle 1 comprises a plurality of electrical systems and subsystems. However, for simplicity only some parts of the vehicle 1 that are associated with the proposed method are shown in Fig. 2. Hence, the illustrated vehicle 1 comprises a front axle 31, front wheels 41, two rear axles 32, rear wheels 42, and a braking system Vehicles of today typically comprise two independent braking systems 20, such as a main braking system and a redundant braking system. The redundant braking system is a system that provides braking capability in case of failure of a main braking system. However, for simplicity only one braking system 20 is illustrated in Fig. 2. The proposed technique may be implemented either in one of these braking systems, or in both, in various embodiments as will be described below.

The braking system 20 comprises front brakes 21 acting on the front axle 31, rear brakes 22 acting on rear axles 32, a yaw rate sensor 23 and a control arrangement 10. The front brakes 21 and the rear brakes 22 are electrically controlled brakes of suitable type, such as disc brakes and/or drum brakes.

The yaw rate sensor 23 (or rotational speed sensor) is a sensor that measures a vehicle's angular velocity about its vertical axis. A yaw rate sensor 23 typically comprises a gyro, but it should be appreciated that in this context other sensors such an image sensor or a position sensor may also be used to determine the orientation of the vehicle 1. The yaw rate is for example measured in degrees or radians per second.

The wheel speed sensor 24 is configured to generate signals representative of a wheel-speed of individual front wheels 41 of the vehicle.

The control arrangement 10 is configured to control the braking system 20. More, specifically, the control arrangement 10 implements an anti-lock braking function configured to regulate a brake force applied by the front brakes 21 on a front axle 31 of a vehicle 1 to mitigate effects of wheel locking. For example, the anti-lock braking function is configured to decrease brake force on locked front wheels 41 upon detecting a wheel-lock. The proposed technique is based on the idea of measuring a yaw rate of the vehicle 1 to detect wheel locking. The proposed concept may be useful in different situations and may be embodied in diverse ways. The technique will herein be described with reference to a first embodiment where the proposed technique is used by a redundant braking system and with reference to a second embodiment where the proposed technique is used by a main braking system to avoid ABS intervention when directional change is not needed. These embodiments, serve as examples, which means that other implementations are the vehicle 1 comprises a main braking system and a redundant braking system that takes over when the main braking system is dysfunctional, and the antilock braking function is implemented by the redundant braking system.»š~:>~.

The control arrangement 10 may comprise one or more ECUs. ECU is a generic term that is used in automotive electronics for any embedded system that controls one or more functions of the electrical system or sub systems in a transport autonomous vehicle. A vehicle typically comprises a plurality of ECUs that communicate over a Controller Area Network, CAN. For example, the illustrated ECU comprises a Brake Control Module, BCM, of the vehicle The proposed method for controlling an anti-lock braking function configured to regulate a brake force applied on a front axle 31 of a vehicle 1 to mitigate effects of wheel locking will now be described with reference to Fig. 3. Wheel locking herein refers to a situation when the brakes stop the front wheels 41 from spinning, while the vehicle continues to move forward due to slipping, as explained in Fig. 1. ln the flow chart, steps that are optional are illustrated with dashed lines.

The method is performed by a control arrangement 10 (Fig. 2 and Fig. 4) which is typically arranged on-board. The control arrangement 10 is for example an ECU of a braking system 20 of the vehicle The method may be implemented as a computer program comprising instructions which, when the program is executed by a computer (e.g., a processor in the control arrangement 10 (Fig. 4)), cause the computer to carry out the method. According to some embodiments the computer program is stored in a computer-readable medium (e.g., a memory or a compact disc) that comprises instructions which, when executed by a computer, cause the computer to carry out the method. The method is typically performed during braking operation, such as in case of heavy braking and may be repeated several times, such as every time steering is affected by wheel locking.

The method comprises determining S1, based on a steering angle of a front axle 31 of the vehicle, an expected yaw rate of the vehicle 1. An expected yaw rate herein refers to a yaw rate that the vehicle 1 is expected to have based on vehicle properties, current driving state (such as velocity) and the current steering angle. For a given steering angle it is in practice possible to determine a resulting torque that would be applied and what effect such a torque will have on the vehicle. The calculation may typically be done using various types of modelling of the vehicle dynamics known in the art and used in, for example, autonomous driving. ln other words, in some embodiments, the expected yaw rate is determined based on a vehicle model and input steering angle or desired curvature. The model may be an advanced model for use in autonomous driving or a simple formula taking for example current velocity and steering angle as input. The expected yaw rate may be determined continuously during the driving or at least when a brake force is applied.

The expected yaw rate is then compared with an actual yaw rate, to investigate whether steering is working as expected. ln other words, the method further comprises measuring S2, while applying a brake force on the front axle 31, an actual, (i.e., current, true, factual, or real), yaw rate of the vehicle 1. The actual yaw rate may be determined in different ways based on sensor data from one or more sensors. ln other words, in some embodiments, the actual yaw rate is determined using a yaw rate sensor 23 comprising one or more of a gyro, an image sensor and a position sensor.

The proposed technique is based on the insight that lack of steering ability may be used to detect wheel lock. ln other words, if the actual yaw rate differs significantly from an expected yaw rate this may serve as an indicator of wheel lock, which should trigger the ABS function to reduce brake force applied by the front brakes 21. Stated differently, the method further comprises controlling S4 the antilock braking function to adjust the applied brake force, based on a difference between the measured actual yaw rate and the determined expected yaw rate. lO ln a first example embodiment, the controlling S4 is implemented by a redundant braking system that is activated because a main braking system has failed. ln some embodiments, the main braking system utilizes wheel speed sensors for detecting wheel locking and wherein the wheel speed sensors are expected to be inoperable upon the main braking system being dysfunctional. This embodiment makes it possible to implement an ABS function in the redundant braking system that is independent of data from wheel speed sensors 24. More specifically, in these embodiments the ABS function is configured to reduce brake force in response to detecting that the actual yaw rate differs from the expected yaw rate. ln particular, that the actual yaw rate is less than the expected yaw rate, which is called understeering.

The ABS function may reduce the brake force in different ways. One way is to simply reduce the average brake force level until steering ability is regained. ln other words, in some embodiments, the controlling S4 comprises reducing S4a the brake force applied on the front axle 31. The reduction is performed in response to the difference between the actual yaw rate and the expected yaw rate exceeding a level corresponding to insufficient steering ability, such as understeering. ln this disclosure insufficient steering ability corresponds to a difference between expected and actual yaw rate which is not normal. Normality may be defined by studying the difference over time when the front wheels 41 are not locked. Locked front wheels 41 typically results in that the ability to steer the vehicle is insufficient, such that the vehicle turns much less than expected or does not turn at all, despite the steering angle <> zero. Hence, in some embodiments, insufficient steering ability corresponds to understeering. ln some embodiments, the level comprises a maximum difference indicative of wheel locking on the front axle 31. The exact maximum difference between actual and expected yaw rate that is considered to correspond to understeering is up to implementation and vehicle properties and may be determined based on experiments. For example, a difference of 5°, 7°, 9°, 10°, 15° or 20° degrees may be considered as a maximum difference that should trigger the reduced brake force. ll As an alternative to reducing the brake force level an ABS pump may be activated. An ABS pump works by releasing and then reapplying i.e., 'pumping' the brakes to mitigate the wheel lock. tft/hen e wheel teck is deteeted, the ABS pump pumps the brakes, for example ”lüfis ef times per secehd. This stops the front wheetts) 41 front skidding and hetps keep the driver ih centret of the vehicie. This corresponds to the type of braking that would be performed by a driver but may of course be done at a higher frequency. ln other words, in some embodiments, the controlling S4 comprises activating S4b an anti-lock braking system pump in response to the difference between the actual yaw rate and the expected yaw rate exceeding a level corresponding to insufficient steering ability, such as understeering.

The reduced brake force shall typically be maintained until steering is normal. ln some embodiments, the method comprises maintaining the reduced brake force or activated anti-lock braking system pump until the difference between the actual yaw rate and the expected yaw rate goes below the level corresponding to insufficient steering ability.

To compensate for the reduced brake force additional brake force may be applied on the other axles, that is on the rear axles 32. ln some embodiments, the method comprises increasing brake force applied on other axles (that is rear axles 32) to compensate for reduced brake force on the front axle ln a second embodiment, the proposed method is implemented by a main or primary braking system to avoid ABS intervention when directional change is not needed. The main or primary braking system typically has access to other data, such as wheel speed data. Hence, in some embodiments, the method comprises detecting S3, using a wheel speed sensor 24 arranged to measure wheel speed of a front wheel 41 arranged on the front axle 31, a wheel speed falling below a level indicative of wheel lock of the front wheel 41. A low wheel speed (i.e., below a certain level that depends on the speed of the vehicle) is an indication of a front wheel 41 being looked. The certain level may be determined by experiments to determine what wheel speeds are considered low wheel speed that indicate wheel-Iocking for different speeds of the vehicle. ln ABS systems of today, a low wheel speed ofa front wheel 41 arranged on the front axle 31 will typically trigger reduction of brake force. However, in some situations activation of the ABS is not desirable, as directional change is not needed. lf the wheel speed is used as the single trigger of the ABS, it is not possible to detect such situations. However, with the proposed technique it is possible to maintain the brake force, even if one or more of the front wheels 41 are locked, as long as the vehicle 1 does not need to turn (i.e., deviate from its current direction). ln this way, it is avoided that the antilock braking function is activated in situations when it is not needed because the vehicle is anyway intended to drive straight forward. ln other words, in some embodiments, the method comprises maintaining S4c the applied brake force on the front wheel 41, despite the wheel speed indicating wheel lock, provided that the difference between the actual yaw rate and the expected yaw rate remains below a level representing insufficient steering ability, such as understeering. Stated differently, the method comprises maintaining S4c the applied brake force on the front wheel 41 while steering ability remains sufficient, with respect to a requested steering angle. This may be desirable as activation of the ABS function may cause other problems, such as reduced or undesired braking distance or general discomfort caused by the pumping of brakes. As in the first example embodiment, the level representing sufficient/insufficient driving ability is set depending on vehicle properties etc. The proposed technique may of course also be used by a main braking function for redundancy if one or more of the wheel speed sensors 24 are dysfunctional for any FGGSOH.

Now turning to Fig. 4 which illustrates the control arrangement 10 configured to implement the proposed method in more detail. ln some embodiments, the control arrangement 10 is a “unit” in a functional sense. Hence, in some embodiments the control arrangement 10 is a control arrangement comprising several physical control devices that operate in cooperation.

The control arrangement 10, or more specifically the processor 101 of the control arrangement 10, is configured to cause the control arrangement 10 to perform allaspects of the method described above and below. This is typically done by running computer program code stored in the data storage or memory 102 in the processor 101 of the control arrangement 10. The data storage 102 may also be configured to store semi-static vehicle parameters such as vehicle dimensions.

The control arrangement 10 may also comprise a communication interface 103 for communicating with other control units of the vehicle and/or with external systems, such as with a map database.

More specifically, the control arrangement 10 is configured to determine, based on a steering angle of a front axle of the vehicle, an expected yaw rate of the vehicle 1 and to measure, while applying a brake force on the front axle 2, an actual yaw rate of the vehicle 1. The control arrangement is also configured to control the antilock braking function to adjust the applied brake force, based on a difference between the measured actual yaw rate and the determined expected yaw rate. ln some embodiments, the control arrangement 10 is configured to detect, using a wheel speed sensor 24 arranged to measure wheel speed of a wheel arranged on the front axle 2, a wheel speed falling below a level indicative of wheel lock of the wheel.

The terminology used in the description of the embodiments as illustrated in the accompanying drawings is not intended to be limiting of the described method, control arrangement or computer program. Various changes, substitutions and/or alterations may be made, without departing from disclosure embodiments as defined by the appended claims.

The term “or” as used herein, is to be interpreted as a mathematical OR, i.e., as an inclusive disjunction; not as a mathematical exclusive OR (XOR), unless expressly stated otherwise. ln addition, the singular forms "a", "an" and "the" are to be interpreted as “at least one”, thus also possibly comprising a plurality of entities of the same kind, unless expressly stated otherwise. lt will be further understood that the terms "includes", "comprises", "including" and/ or "comprising", specifies thepresence of stated features, actions, integers, steps, operations, elements, and/ or components, but do not preclude the presence or addition of one or more other features, actions, integers, steps, operations, elements, components, and/ or groups thereof. A single unit such as e.g., a processor may fulfil the functions of several items recited in the claims.

Claims (2)

1. Claims A method for controlling an anti-lock braking function configured to regulate a brake force applied on a front axle of a vehicle (1) to mitigate effects of wheel locking, the method comprising: - determining (S1), based on a steering angle of a front axle of the vehicle, an expected yaw rate of the vehicle (1 ), - measuring (S2), while applying a brake force on the front axle (2), an actual yaw rate of the vehicle (1 ), - controlling (S4) the antilock braking function to adjust the applied brake force, based on a difference between the measured actual yaw rate and the determined expected yaw rate».¿,»g-;_¿3¿;;š\ “r _ wherein the vehicle (1) comprises a main braking system and a redundant braking system that takes over when the main braking system is dysfunctional and wherein the antilock braking function is implemented by the redundant braking system. _ The method of claim wherein the main braking system utilizes wheel speed sensors for detecting wheel locking and wherein the wheel speed sensors are expected to be inoperable upon the main braking system being dysfunctional. _ The method according to any one of the preceding claims, wherein the controlling (S4) comprises: reducing (S4a) the brake force applied on the front axle (2) or activating (S4b) an anti-lock braking system pump, in response to the difference between the actual yaw rate and the expected yaw rate exceeding a level corresponding to insufficient steering ability, such as understeering. _ The method of claim wherein insufficient steering ability corresponds to understeering.The method of claim wherein the level maximum difference indicative of wheel Iocking on the front axle. The method of claim comprising maintaining the reduced brake force or activated anti-lock braking system pump until the difference between the actual yaw rate and the expected yaw rate goes below the level corresponding to insufficient steering ability. The method of any one of claims comprising increasing brake force applied on other axles (3) to compensate for reduced brake force on the front axle (2). “WWThe method according to any one of the preceding claims, wherein the actual yaw rate is determined using a yaw rate sensor (23) comprising one or more of a gyro, an image sensor, and a position sensor. 11+ gmwThe method according to any one of the preceding claims, wherein the expected yaw rate is determined based on a vehicle model and input steering angle or desired curvature. A computer program comprising instructions which, when the program is executed by a computer, cause the computer to carry out the method of any one of the claims 1 tocomputer-readable storage medium comprising instructions which, when executed by a computer, cause the computer to carry out the method of any one of the claims 1 to control arrangement (10) configured to control an anti-lock braking function configured to regulate a brake force applied on a front axle (31) of a vehicle (1) to mitigate effects of wheel locking, the control arrangement (10) being configured to: - determine, based on a steering angle of a front axle of the vehicle, an expected yaw rate of the vehicle (1 ), - measure, while applying a brake force on the front axle (31), an actual yaw rate of the vehicle (1 ), -------- -control the antilock braking function to adjust the applied brake force, based on a difference between the measured actual yaw rate and the determined expected yaw rate_,__g-;« wherein the control arrangement is configured to perform the method according to any one of claims vehicle (1) comprising the control arrangement (10) according to claim