Friction-based adaptive cruise control

Abstract

The present disclosure relates to a method for controlling operation of a vehicle. In particular, the disclosure relates to adaptive cruise control. According to a first aspect, the present disclosure relates to a method for controlling operation of a vehicle 1 comprising a secondary brake 13. The method comprises obtaining S1 friction data indicative of friction between the road surface and tires of the vehicle, and determining S2 a braking capacity of the secondary brake based on the obtained friction data and based on current operating conditions of the vehicle. The method also includes controlling S4 the vehicle to automatically actuate the secondary brake in response to the vehicle reaching a brake actuation distance from the vehicle traveling ahead, wherein the brake actuation distance is based on the estimated braking capability of the secondary brake. The disclosure also relates to a corresponding control device and a computer program, and to a vehicle comprising said control device.

Description

TECHNICAL FIELD

[0001] The present disclosure relates to a method for controlling operation of a vehicle. In particular, the present disclosure relates to adaptive cruise control. The present disclosure also relates to a corresponding control device and computer program, and to a vehicle comprising said control device. BACKGROUND

[0002] An adaptive cruise control (ACC) system uses on-board sensors to detect the distance between the host vehicle and a vehicle travelling ahead of the host vehicle, as well as the relative speed difference between the vehicles. The ACC automatically adjusts the speed of the host vehicle to keep it at a pre-set distance or time interval behind the vehicle travelling ahead. More specifically, the adaptive cruise control system generates an automatic intervention in the powertrain and / or brake system of the host vehicle to slow the vehicle as needed to maintain a target following distance. The target following distance is based on a pre-set following distance or time interval, but can also be influenced by other factors such as power efficiency, road conditions and safety requirements.

[0003] A problem with existing ACC systems is that they do not drive very safely on poor roads in adverse weather conditions. As a result, drivers often turn off the ACC because they find it to drive abnormally or unsafely. SUMMARY

[0004] It is an object of the present disclosure to provide a method that enables safe and efficient cruise control also in adverse road conditions. It is a further object to provide a technology that makes the ACC drive in a more human-like way, whereby the driver is discouraged from turning off the cruise control. In particular, it is an object to provide a cruise controller that reduces the use of primary brakes such as friction brakes. These objects, as well as other objects, are at least partly achieved by the method, control device and vehicle according to the independent claims, and the embodiments according to the dependent claims.

[0005] According to a first aspect, the present disclosure relates to a method for controlling operation of a vehicle comprising a secondary brake. The method comprises obtaining friction data indicative of friction between a road surface and a tire of the vehicle, and determining a braking capability of the secondary brake based on the obtained friction data and based on current operating conditions of the vehicle. The method further comprises controlling the vehicle to automatically actuate the secondary brake in response to the vehicle reaching a brake actuation distance to a vehicle travelling ahead, wherein the brake actuation distance is based on the estimated braking capability of the secondary brake. By taking into account the friction and the capability of the secondary brake when determining when to actuate the secondary brake, the driving can be performed in a safer and more smooth way, i.e. it will be performed in a more human-like way.

[0006] In some embodiments, the brake actuation distance is long enough to ensure that the braking force required to maintain a safe distance to the vehicle driving ahead does not exceed the estimated braking capacity of the secondary brake. By taking friction into account when actuating the secondary brake, the risk of unnecessarily using the primary brake is avoided. Thereby, energy loss and wear are reduced. In some embodiments, the safe distance corresponds to the distance at which the primary brake is automatically actuated. Thus, the automatic actuation of the primary brake is avoided.

[0007] In some embodiments, braking forces above a certain level are not considered as a basis for the brake actuation distance. Thereby, exceptional braking forces that are normally rarely required but would instead require a very long brake actuation distance can be ignored. Thus, by ignoring braking forces above a certain level, the brake actuation distance can be kept reasonably long.

[0008] In some embodiments, the certain level corresponds to braking forces applied by the vehicle and / or measured braking forces applied by the vehicle driving ahead during a certain previous time period. Thus, braking forces that have not been applied recently, e.g. during the last 5-10 minutes, can not be considered as a basis for the brake actuation distance.

[0009] In some embodiments, the brake actuation distance is adjusted based on friction data and / or road slope. Thus, the brake actuation distance can be adjusted to take into account road characteristics that can affect braking distance.

[0010] In some embodiments, the brake actuation distance is longer than a configured following distance when the determined braking force falls below a predefined value. Thus, the secondary brake can be automatically actuated before the vehicle reaches the configured following distance, e.g. set by the driver.

[0011] In some embodiments, the method comprises estimating the braking force required to maintain a safe distance to the vehicle driving ahead based on one or more of the operational conditions of the vehicle, and the physical characteristics, braking behaviour, driving speed and acceleration of the vehicle driving ahead. In these embodiments, the method comprises determining the brake actuation distance such that the estimated braking force matches the determined braking capacity of the secondary brake. Thereby, the brake actuation distance can be set such that braking can be performed using only the secondary brake.

[0012] In some embodiments, the brake actuation distance depends on the braking distance of the secondary brake for the determined braking capacity and the current operational conditions. Thus, the brake actuation distance depends on the braking distance of the secondary brake in the current road conditions, thereby determining the brake actuation distance in an improved manner.

[0013] In some embodiments, obtaining friction data comprises one or more of: calculating friction data based on sensor readings and vehicle characteristics, and receiving friction data from other vehicles, from road objects, or from a data storage. Thus, friction data can be obtained in various ways.

[0014] In some embodiments, the current operating conditions comprise one or more of: road slope, temperature, speed, acceleration, historical or predicted braking behavior, map data, axle load, weather conditions, and battery state of charge SoC. Thus, in addition to friction, various parameters influencing the braking capacity of the secondary brakes can be taken into account, thereby obtaining an improved determination of the braking capacity.

[0015] In some embodiments, the control comprises performing a specific measure including increasing the minimum following distance, decelerating, or limiting acceleration when the friction data indicates that a vehicle driving ahead is driving on a road section having a friction below a certain threshold. Thus, additional measures can be taken when the road conditions are extremely poor. Thereby, safety in abnormal road conditions can be improved.

[0016] In some embodiments, the control comprises activating a differential lock when the friction data indicates that an upcoming road section has a friction below a certain threshold. It can be beneficial to activate a differential lock on a wet and slippery road, as it helps to provide better traction and control of the vehicle.

[0017] In some embodiments, the control comprises activating an acceleration limit for the vehicle when the friction data indicates that an upcoming road section has a friction below a certain threshold. Thereby, the driver can maintain better control of the vehicle, thereby reducing the risk of accidents.

[0018] In some embodiments, the control comprises providing an indication to the driver to change lane, where the lane change deactivates the acceleration limit. Thus, the driver can be informed how to remove the acceleration limit.

[0019] In some embodiments, the secondary brakes comprise regenerative brakes and / or auxiliary brakes. Secondary brakes typically cause less power loss and wear than primary brakes, which typically use friction. Thus, by reducing the use of primary brakes, power loss and wear are typically reduced.

[0020] In some embodiments, the determination is based on braking capacity criteria, which limit the braking capacity based on the obtained friction data and based on current operating conditions, and / or wherein the brake actuation distance is determined based on one or more distance criteria. Thus, various algorithms and mathematical models can be used to implement the proposed method.

[0021] According to a second aspect, the disclosure relates to a computer program comprising instructions which, when the computer program is executed by a computer, cause the computer to carry out the method according to the first aspect. According to a third 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.

[0022] According to a fourth aspect, the disclosure relates to a control device configured to carry out the method according to the first aspect.

[0023] According to a fifth aspect, the disclosure relates to a vehicle comprising the system according to the fourth aspect.

[0024] The corresponding effects of the first aspect can be achieved by the second to fifth aspects. BRIEF DESCRIPTION OF DRAWINGS

[0025] The embodiments disclosed herein are illustrated by way of example and not limitation in the figures of the accompanying drawings in which like references indicate corresponding parts, in which:

[0026] Figure 1 A vehicle comprising a secondary brake is shown.

[0027] Figure 2a -b shows a distance considered by an adaptive cruise control system.

[0028] Figure 3 A vehicle obtaining friction data is shown.

[0029] Figure 4a -b is a flowchart of an exemplary method according to the first aspect.

[0030] Figure 5 A control device configured to carry out the proposed method is shown. DETAILED DESCRIPTION

[0031] An adaptive cruise control (ACC) system uses radar, lidar, or other sensors to detect the distance between a vehicle and other vehicles on the road. If the ACC determines that the vehicle is too close to a vehicle driving ahead, it can actuate the brakes to slow the vehicle and maintain a safe distance.

[0032] Heavy vehicles such as trucks typically comprise a primary braking system such as friction brakes, and one or more secondary braking systems separate from the primary braking system. Since secondary brakes are typically not dependent on friction, it is often preferred to use secondary brakes to avoid overheating and wear of the friction brakes. In particular, it is beneficial to use regenerative brakes since they result in less power loss due to motion being converted into electrical energy that can be fed back to the battery. The proposed technology is based on the idea that the ACC can take road conditions, in particular friction, into account in order to optimize braking such that unnecessary use of the primary brakes is avoided.

[0033] Figure 1 A vehicle 1 in which the proposed technology can be implemented is shown. The vehicle 1 can comprise a generalized means of transport, such as a bus, a truck or other similar manned or unmanned vehicle. The shown vehicle 1, here a truck, comprises a propulsion means 11, a primary brake 12, a secondary brake 13, a tire 14, sensors 15 and a control means 10, e.g. implementing an ACC.

[0034] The propulsion means 11 comprises an engine and a drive line that transmits motion to the wheels of the vehicle 1. The engine can comprise one or more of a combustion engine and an electric motor. The primary brake 12 is typically a disc brake or a drum brake, while the secondary brake 13 comprises an auxiliary brake such as an engine brake or a retarder. In electric or hybrid vehicles, the secondary brake 13 can also comprise a regenerative brake that recovers braking energy and converts it into electrical power.

[0035] The vehicle 1 further comprises sensors 15 for monitoring different functions and states of the vehicle 1, providing information to the driver or different systems of the vehicle 1. The sensors 15 can comprise one or more of the following: a speed sensor, a temperature sensor, a position sensor, a motion sensor, a gyroscope, a power sensor, a pressure sensor, a humidity sensor, a rain sensor, etc. Each sensor 15 converts a sensed event or change in a property into a signal or data that is sent to or collected by the control means 10. Such signals or data are sent, e.g., over a CAN (Controller Area Network) or similar of the vehicle 1.

[0036] The plurality of sensors 15 comprises vehicle sensors such as LIDAR, RADAR and image sensors for monitoring the environment of the vehicle 1. The output from the vehicle sensors 15 can provide input to an autonomous control system of the ACC or control means 10 for autonomous driving.

[0037] Figure 2a Distances considered by the adaptive cruise control system are shown. When the ACC is activated, a user, typically the driver, enters a desired following distance d driverThe desired following distance d driver may be defined in meters (e.g. 10 meters) or in time (e.g. 3 seconds). The ACC then configures a target following distance d tar which is the distance between the vehicle 1 and a vehicle 2 driving in front (herein also simply referred to as the followed vehicle 2) that the ACC tries to maintain. The ACC of course takes into account the desired following distance d driver when setting the target following distance d tar , but in some cases there can be a deviation between the desired following distance d driver and the target following distance d tar for safety or energy consumption reasons.

[0038] The ACC is configured to control the propulsion means 11 and the brakes 12, 13 to maintain the target following distance d tar as well as possible while also taking into account driving comfort, wear, energy consumption and safety. To calculate the target following distance d tar , the ACC system is configured to monitor the driving style of the followed vehicle 2 and to determine an expected deceleration value of the followed vehicle 2. The estimated deceleration value can take into account various parameters such as the speed of the followed vehicle 2, map data, vehicle type, etc. In other words, the ACC generally tries to drive as much like a human as possible. When the ACC brakes the vehicle to maintain the target following distance d tar , the secondary brakes are generally used as much as possible. However, if the vehicle 1 is too close to the followed vehicle 2, the primary brakes 12 are also actuated. The distance at which the primary brakes 12 are automatically actuated to avoid a collision is herein referred to as the safety distance d WB .

[0039] The ACC therefore controls the amount of propulsion torque that is applied until the vehicle 1 reaches or is about to reach the target following distance d tar . Below the target following distance d tar , the vehicle is generally allowed to coast for a while even if it exceeds the target following distance d tar , because unnecessary braking generally wastes energy. The zone in which neither torque nor braking force is applied is herein referred to as the power saving zone 4. However, before the vehicle 1 approaches the safety distance d WB , the secondary brakes are generally used to apply braking force in order to avoid using the primary brakes. The distance at which the secondary brakes 13 are automatically actuated to maintain the target following distance d tar is herein referred to as the brake actuation distance d AB .

[0040] The secondary brakes 13 should therefore preferably be activated early enough to ensure that the vehicle 1 never reaches the safety distance d WBhow early the secondary brake 13 must be actuated depends on the braking capability of the secondary brake 13. In other words, the brake actuation distance d AB depends on how much braking torque the secondary brake 13 can generate at a certain point in time. Sometimes, the brake actuation distance d AB can also be adjusted based on map data and friction. However, the inventors have realized that in bad road conditions, such an adjustment cannot provide a balanced following distance. The reason is that the need to take road conditions into account is closely related to other factors such as driving environment and vehicle characteristics. Therefore, it is proposed herein to improve the estimation of the braking capability by taking into account not only vehicle characteristics and map data, but also road conditions, and then to make the brake actuation distance d AB based on the improved estimation of the braking capability. It should be noted that in cases where a large force braking is needed, the wheel brake can be activated before the vehicle has reached the safe distance d WB .

[0041] Figure 2b It is shown how the brake actuation distance d AB can be adjusted based on friction data on a wet road. In this example, the brake actuation distance d AB is longer than the configured following distance d driver set by the driver. This can also affect the target following distance d tar in some scenarios, as it is generally desirable that the power saving zone 4 is not too short. Therefore, in this example, the brake actuation distance d AB and the target following distance d tar have both been increased. This means that on a wet road (when the friction is below a certain limit), the vehicle 1 will start braking earlier, i.e. before it has reached the configured following distance d driver set by the driver.

[0042] The proposed concept of operating a vehicle comprising a secondary brake will now be described with reference to Figures 3-5 .

[0043] Figure 3 It is shown that the vehicle 1 obtains friction data. The friction data can be obtained in various ways, such as by calculating the friction data based on sensor readings and vehicle characteristics. For example, data from one or more of an image sensor, a humidity sensor, a rain sensor, or a temperature sensor can be used to estimate the friction coefficient according to generally known methods. Alternatively, the friction data can be received from other vehicles 2, from road objects, or from a data storage device 5 such as a cloud server accessible by several vehicles. Therefore, the vehicle 1 can be configured to communicate with such objects using vehicle-to-everything (V2X) communication or using other communication technologies.

[0044] Figure 4aA proposed method for controlling operation of a vehicle 1 comprising a secondary brake 13 is shown. In some embodiments, the secondary brake 13 comprises a regenerative brake and / or an auxiliary brake.

[0045] The method is suitable for implementation in a control device, such as the control device 10 of the vehicle 1. Figure 1 The method is typically performed during normal operation of the vehicle 1, Figure 1 such as when ACC is activated to control the speed of the vehicle 1. Alternatively, the method can be performed to control operation of an autonomous vehicle.

[0046] The method can be implemented as a computer program comprising instructions which, when the program is executed by a computer (e.g. a processor in the control device 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 an optical disc) comprising instructions which, when executed by a computer, cause the computer to carry out the method.

[0047] The method comprises obtaining S1 friction data indicative of friction between the road surface 3 and the tyres 14 of the vehicle. In some embodiments, the friction data comprises a friction coefficient of the road on which the vehicle is driving or is about to drive. Thus, the friction data can be indicative of current or expected friction.

[0048] As explained above, the capability of the secondary brake 13 is thereafter determined based on the obtained friction data. The braking capability of the secondary brake 13 is limited by two factors: the maximum force the secondary brake 13 can exert on the tyre, and the maximum force the tyre can exert on the road, where the latter depends on the friction between the tyre and the road surface. In other words, the method further comprises determining S2 the braking capability of the secondary brake 13 based on the obtained friction data and based on current operating conditions of the vehicle 1. In addition to friction, various other parameters associated with the operating conditions can be taken into account when determining S2. For example, the maximum force the secondary brake 13 can exert can depend on temperature, speed, wear, acceleration, and for a regenerative brake, also on battery state of charge SoC. On the other hand, the maximum force the tyre can exert on the road can depend on vehicle properties such as tyre properties, axle load, wear level, humidity, tyre pressure, etc. In other words, in some embodiments, the current operating conditions comprise one or more of tyre properties, temperature, wear level, humidity, pressure, axle load, and battery state of charge SoC.

[0049] Determining S2 is typically performed using a set of rules, such as a formula or a model of the secondary brake 13. In other words, in some embodiments, determining S2 is based on a braking capability criterion which defines the braking capability based on the obtained friction data and based on the current operating conditions.

[0050] In some embodiments, it may be desirable to increase the brake actuation distance d. AB Set to a length sufficient to ensure that the secondary brake 13 can keep the vehicle 1 at a safe distance d. WB A longer distance without using the main brake 12. This means a longer brake actuation distance d. AB The braking distance d will vary based on factors such as speed, gradient, friction, vehicle characteristics, and the driving conditions of the vehicle being followed (2). In other words, in some embodiments, the braking distance d AB Long enough to ensure a safe distance d from the vehicle 2 traveling in front. WB The required braking force does not exceed the estimated braking capacity of the secondary brake 13. This means, for example, that if the speed difference between the vehicle and the vehicle in front is significant, the braking distance increases because stronger braking is required.

[0051] Specifically, it may be desirable to ensure that a safe distance d is maintained when the main brake 13 is automatically actuated. WB The required braking force does not exceed the capacity of the secondary brake 13. This can be achieved by evaluating the driving mode and predicting the braking behavior of the followed vehicle 2. For example, the expected braking value of the followed vehicle 2 can be compared with the braking distance produced by the braking capacity under current driving conditions, such as weather conditions, speed, acceleration, map data, etc. The braking force corresponding to the braking distance is then estimated. In other words, in some embodiments, the method includes estimating the safe distance d for maintaining the vehicle 2 ahead based on the operating conditions of vehicle 1 and one or more of the physical characteristics, braking behavior, speed, and acceleration of the vehicle 2 ahead. WB The required braking force.

[0052] This disclosure proposes a brake actuation distance d AB Based on various parameters, including an estimated braking capacity, the method, in some embodiments, includes determining the S4 brake actuation distance d based on the estimated braking capacity. AB The specific steps are as follows. As explained above, various parameters can be considered when determining S4. Determining S4 typically uses a set of rules, such as those used to calculate the brake actuation distance d. AB The formula is used to execute the operation. Therefore, in some embodiments, the brake actuation distance d is determined based on one or more distance criteria. AB .

[0053] Brake actuation distance d ABThe distance criterion can have several parameters as input, typically depending on the determined braking capability resulting in the current braking distance. Thus, even if the braking capability is known, the braking distance depends on current driving conditions, such as speed (in particular the speed difference between the vehicle 1 and the vehicle 2 driving ahead), slope, etc. Thus, when setting the brake actuation distance d AB , these parameters must be taken into account. In addition, additional adjustments can be performed based on e.g. road conditions and slope to provide a balance between power saving, safety, wear, noise, comfort, etc. Thus, for the determined braking capability and the current operating conditions, the brake actuation distance d AB depends on the braking distance of the secondary brake 13.

[0054] If the braking force needed to maintain the safe distance d WB to the vehicle 2 driving ahead has been estimated, the determination S4 is performed based on this estimate. Thus, in some embodiments, the determination S4 comprises a determination S4a of the brake actuation distance d AB such that the estimated braking force matches the determined braking capability of the secondary brake 13. In other words, the brake actuation distance d AB is chosen to ensure that the braking force needed to maintain the safe distance does not exceed the capability of the secondary brake.

[0055] However, if all possible braking actions of the vehicle 2 being followed should be taken into account, the brake actuation distance d AB may be very long, which can be undesirable for other reasons. This can be solved by only ignoring less likely braking when estimating S3 the braking force needed to maintain the safe distance d WB . One can only run the risk that the primary brake 12 must be actuated in case of a sudden braking, but this is less likely to happen. In other words, in some embodiments, braking forces above a certain level are not considered as a basis for the brake actuation distance.

[0056] What braking should be ignored can be determined based on the driving history of the vehicle 2 being followed. For example, braking forces larger than the braking forces applied within the last 10 minutes can be ignored. This can of course depend on the type of driving and change if the vehicle 1 enters another type of road (e.g. a road with another speed limit) or if the traffic conditions change. In other words, in some embodiments, said certain level corresponds to braking forces applied by the vehicle 1 and / or measured braking forces applied by the vehicle 2 driving ahead during a certain previous time period.

[0057] As explained above, in some cases, the brake actuation distance d AB may be longer than the configured following distance d driver (see Figure 2b). Thus, the vehicle 1 can start braking automatically when it has reached the driver-set configured following distance d driver before. In other words, in some embodiments, the brake actuation distance d AB is longer than the configured following distance d driver .

[0058] The vehicle 1 is then controlled to maintain the target following distance d tar . This involves controlling the drive torque and braking by ACC or alternatively by an autonomous driving function. In other words, the method comprises controlling S5 the vehicle to automatically actuate the secondary brakes in response to the vehicle reaching the brake actuation distance d AB to the vehicle 2 driving ahead, wherein the brake actuation distance d AB is based on the estimated braking capability of the secondary brakes 13.

[0059] The control S5 can also take into account the specific road conditions indicated by the friction data. Figure 4b Details of specific embodiments of the control S5 are shown in more detail. For example, if the friction is very low, the driver can prefer to allow an increase of the determined brake actuation distance d AB and / or the target following distance d tar . This can only be up to the furthest distance the radar is able to detect, or the furthest distance a vehicle can be detected with high reliability vehicle sensors. In some embodiments, this can involve giving the driver a hint to manual driving or ACC disengagement, since the friction is so low that it has to return beyond the brake actuation distance d AB . It is noted that Figure 4b the steps can be performed independently of the preceding steps S2-S4. For example, if the road conditions are very poor, the ACC can initiate a speed limit or cause the following distance to increase by an additional amount. In other words, in some embodiments, the control S5 comprises performing S5a a specific measure including increasing the minimum following distance, decelerating or limiting acceleration when the friction data indicates that the vehicle 2 driving ahead is driving on a road segment with a friction below a certain threshold.

[0060] It can also be feasible for the ACC to take additional measures to increase safety in poor road conditions, e.g. due to low friction or expected low friction. For example, if the friction data indicates that a low friction road segment is coming up, and the map data identifies that the road is not very curved (the curve radius is too small), the vehicle can initiate certain differential locks to help driving on that road segment. In other words, in some embodiments, the control S5 comprises initiating S5b differential locks when the friction data indicates that the upcoming road segment has a friction below a certain threshold.

[0061] Another possibility is to initiate acceleration limits. In other words, in some embodiments, the controlling S5 comprises initiating S5c acceleration limits for the vehicle when the friction data indicates that the upcoming road section has a friction below a certain threshold.

[0062] It can be beneficial for the driver to be able to accelerate some of these acceleration limits are cancelled by initiating an indicator to inform the driver that the lane change will be cancelled. Thus, in some embodiments, the controlling S5 comprises providing an indication to the driver to change lane, wherein the lane change deactivates the acceleration limits.

[0063] Figure 5 A control device 10 is shown, which is configured to perform the presented method of a vehicle comprising a secondary brake 13. The control device 10 can be arranged in the vehicle 1 Figure 1 ). The control device 10 comprises a control circuit to perform the method according to any of the steps, examples or embodiments as described herein. The control device 10 can comprise one or more electronic control units (ECU). For example, the control device 10 can be an electronic control unit ECU of an ACC.

[0064] In more detail, the control device 10 comprises one or more computers 101 and a memory 102. The computer 101 comprises any hardware or hardware / firmware device implemented using processing circuitry, such as but not limited to a processor, a central processing unit (CPU), a controller, an arithmetic logic unit (ALU), a digital signal processor, a microcomputer, a field programmable gate array (FPGA), a system on chip (SoC), a programmable logic unit, a microprocessor, an application specific integrated circuit, or any other device capable of electronically performing operations in a defined manner. In some embodiments, the computer readable medium can be a non-transitory computer readable medium, such as a tangible electronic, magnetic, optical, infrared, electromagnetic, and / or semiconductor system, apparatus, and / or device. The computer readable memory is for example one or more of the memories in the control device 10. Thus, the presented method can be implemented as a computer program. The computer program then comprises instructions which, when the computer program is executed by a computer, cause the computer to carry out the method according to any of the aspects, embodiments or examples as described herein.

[0065] In some embodiments, the control device 10 comprises a communication interface 103 configured to enable wireless communication with non-onboard devices, such as with other vehicles, road objects or with data storage devices, such as a cloud server. This communication can be performed via a controller area network, CAN, or directly via an embedded modem.

[0066] More specifically, the control device 10 is configured to obtain friction data indicative of the friction between the road surface 3 and the tire 14 of the vehicle, and to determine a braking capability of the secondary brake 13 based on the obtained friction data and based on current operating conditions of the vehicle 1. The control device 10 is further configured to control the vehicle to automatically actuate the secondary brake in response to the vehicle reaching a brake actuation distance d AB from the vehicle 2 traveling in front, wherein the brake actuation distance d AB is based on the estimated braking capability of the secondary brake.

[0067] In a further embodiment, the control device is configured to perform the method according to any one of the embodiments described in connection with Figure 4a and 4b The method according to any one of the embodiments described in connection with

[0068] The terms used in the description of the embodiments as shown in the drawings are not intended to limit the described method, control device or computer program. Various changes, substitutions and / or alterations can be made without departing from embodiments of the invention as defined by the appended claims.

[0069] As used herein, the term "or" is to be interpreted as the mathematical OR, i.e. as inclusive disjunction, not as mathematical XOR (XOR). Furthermore, the singular forms "a", "an" and "the" are to be interpreted as "at least one", thus possibly also including multiple entities of the same type, unless explicitly stated otherwise. It will be further understood that the terms "includes", "including", "comprises" and / or "comprising", specify the presence 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 a processor can fulfill the functions of several items recited in a claim.

[0070] The present disclosure is not limited to the preferred embodiments described above. Various alternatives, modifications and equivalents can be used. Therefore, the above embodiments should not be taken as limiting the scope of the present disclosure, which is defined by the appended claims.

Claims

1. A method for controlling operation of a vehicle (1) comprising a secondary brake (13), the method comprising: obtaining (SI) friction data indicative of friction between a road surface (3) and a tyre (14) of the vehicle, determining (S2) braking capability of the secondary brake (13) based on the obtained friction data and based on current operating conditions of the vehicle (1), and - controlling (S5) the vehicle to automatically actuate the secondary brake in response to the vehicle reaching a brake actuation distance (d AB ) to a vehicle (2) travelling ahead, wherein the brake actuation distance (d AB ) is based on an estimated braking capability of the secondary brake (13).

2. The method according to claim 1, wherein said brake actuation distance (d AB ) is long enough to ensure that the braking force required to maintain a safety distance (d WB ) from said vehicle (2) travelling ahead does not exceed the estimated braking capacity of said secondary brake (13).

3. The method according to claim 2, wherein said safety distance (d WB ) corresponds to the distance for which the primary brake (13) is automatically actuated.

4. The method according to claim 2 or 3, wherein a required braking force above a certain level is not considered as a basis for the brake actuation distance.

5. The method according to claim 4, wherein the certain level corresponds to braking force applied by the vehicle (1) and / or measured braking force applied by the vehicle (2) driving ahead during a certain previous time period.

6. The method according to any one of the preceding claims, wherein the brake actuation distance (d AB ) is adjusted based on the friction data and / or road slope.

7. The method according to any of the preceding claims, wherein the brake actuation distance (d AB ) is longer than the configured following distance (d driver ) when the determined braking force drops below a predefined value.

8. The method according to any one of the preceding claims 2-8, wherein the method comprises: - estimates (S3) the braking force required to maintain a safety distance (d WB ) from the vehicle (2) driving in front based on • operating conditions of the vehicle (1), and • one or more of the following: physical properties, braking behaviour, driving speed and acceleration of the vehicle (2) driving ahead, and determining (S4a) the brake actuation distance (4) such that the estimated braking force matches the determined braking capability of the secondary brake (13).

9. The method according to any one of the preceding claims, wherein for a determined braking capacity and the current operating conditions, the actuation distance (d AB ) of the brake depends on the braking distance of the secondary brake (13).

10. The method according to any one of the preceding claims, wherein the current operating conditions comprise one or more of the following: road slope, temperature, speed, acceleration, historical or predicted braking behaviour, map data, axle load, weather conditions and battery state of charge SoC.

11. A computer program comprising instructions which, when the computer program is executed by a computer, cause the computer to carry out the method according to any one of the preceding claims.

12. A computer readable medium comprising instructions which, when executed by a computer, cause the computer to carry out the method according to any one of claims 1-10.

13. A control device (10) configured to operate a vehicle comprising a secondary brake (13), wherein the control device is configured to: obtain friction data indicative of friction between a road surface (3) and a tyre (14) of the vehicle, determine braking capability of the secondary brake (13) based on the obtained friction data and based on current operating conditions of the vehicle (1), and - controlling the vehicle to automatically actuate the secondary brakes in response to the vehicle reaching a brake actuation distance (d AB ) to a vehicle (2) travelling ahead, wherein the brake actuation distance (d AB ) is based on an estimated braking capability of the secondary brakes.

14. The control device (10) according to claim 13, wherein the control device is configured to perform the method according to any one of claims 1-10.

15. A vehicle comprising a control device (10) according to claim 13 or 14.

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