A control system

Abstract

Aspects of the present invention relate to a control system for controlling a braking mechanism. The control system may increase a braking torque applied to one axle of a vehicle when the wheels of another axle lock. This may provide a sufficient braking force overall and, when the vehicle is on a slope, may prevent the vehicle from rolling back down along the slope.

Description

[0001] LRP13500W01

[0002] A CONTROL SYSTEM

[0003] TECHNICAL FIELD

[0004] The present disclosure relates to a control system. Aspects of the invention relate to a control system, to a 5 vehicle, and to a method of controlling a braking system of a vehicle.

[0005] BACKGROUND

[0006] When a braking torque is applied to the wheel, there is a possibility that a wheel may stop rolling and begin sliding over a surface, which is known as wheel locking. Since sliding friction between a wheel and the ground is less than sticking friction, this may reduce the braking force experienced by the vehicle. While it is conventional to attempt to avoid locking of a wheel in order to increase a braking force, this may not be appropriate at low speeds.

[0007] It is an aim of the present invention to address one or more of the disadvantages associated with the prior art.

[0008] 15

[0009] SUMMARY OF THE INVENTION

[0010] Aspects and embodiments of the invention provide a control system, a vehicle, a method of controlling a braking system and computer readable instructions as claimed in the appended claims.

[0011] 20 According to an aspect of the present invention there is provided a control system for controlling a braking system of a vehicle comprising a first axle and a second axle, the braking system comprising a braking mechanism arranged to exert a braking torque on the second axle, the control system comprising one or more processors collectively configured to: receive a brake request signal indicative of a required braking torque; determine a raw braking torque for application on the second axle based on the brake request signal; receive 25 a first axle speed signal indicative of a rotational speed of the first axle; receive a vehicle speed signal indicative of a speed of the vehicle; determine whether a loss-of-traction condition exists for the first axle in dependence on the first axle speed signal; and determine whether the speed of the vehicle is below a vehicle speed threshold; wherein, upon determining that a loss-of-traction condition exists on the first axle when the speed of the vehicle is below the vehicle speed threshold, the one or more processors are collectively configured to 30 configure the braking command to cause the braking mechanism to exert a modified braking torque on the second axle, the modified braking torque being greater than the raw braking torque.

[0012] The one or more processors may be further collectively configured to: output a braking command, the braking command being configured to cause the braking mechanism to exert the raw braking torque on the second axle.

[0013] In some cases, while a vehicle is being braked, such as being brought to rest, one of the vehicle axles may stop rotating before the vehicle has stopped. This may result in a reduction of traction of the vehicle and, if an insufficient amount of torque is applied to the other axle, the vehicle may not come to rest in a desired time. In 40 addition, when the vehicle is on a slope, the vehicle may roll backwards rather than stop.LRP13500W01

[0014] The braking mechanism may comprise an electric machine arranged to provide a braking torque and to generate electrical energy and / or may comprise foundation brakes, for example friction brakes arranged to generate a frictional brake torque and / or induction brakes arranged to generate eddy current braking torque.

[0015] 5 The braking system may further comprise a further braking mechanism arranged to exert a braking torque on the first axle. The braking command may also be configured to cause the further braking mechanism to exert a braking torque on the first axle, which may be a further raw braking torque based on the brake request signal or may be a further modified braking torque based at least partially on such a further raw braking torque.

[0016] The one or more processors may be collectively configured to: receive a gradient signal indicative of a gradient of a surface on which the vehicle is positioned; determine whether the gradient of the surface is greater than a gradient threshold; and configure the braking command to cause the braking mechanism to exert the modified braking torque on the second axle in dependence on the gradient of the surface being greater than the gradient threshold.

[0017] 15

[0018] Locking of wheels is more likely when the vehicle is on a steep slope, since the vehicle weight may be less equally distributed between the wheels of the vehicle on such a slope. The defined control system may bring a vehicle to rest on such a slope. Further, on a steep slope there is a greater chance of a vehicle rolling back as it stops. In this case, increasing a braking torque on an unlocked axle may be particularly beneficial. In 20 particular, where a vehicle may be facing up or down a slope, the vehicle may exhibit uneven weight on front and rear axles, making locking of one of the front and rear axles more likely. The gradient signal may therefore be indicative of a gradient of a surface on which the vehicle is positioned in a longitudinal direction of the vehicle.

[0019] 25 The first axle speed signal may comprise an indication of a rotational speed of a first wheel on the first axle and an indication of a rotational speed of a second wheel on the first axle, and the one or more processors may be collectively configured to determine the rotational speed of the first axle in dependence on the indication of the rotational speed of the first wheel on the first axle and the indication of the rotational speed of the second wheel on the first axle.

[0020] 30

[0021] By considering both wheels on the first axle separately, the control system may modify the braking torque more appropriately, and by not modifying the braking torque when one wheel is rotating and the other is locked or underrotating.

[0022] The one or more processors may be collectively configured to determine the rotational speed of the first axle as being the greater of the rotational speed of the first wheel on the first axle and the rotational speed of the second wheel on the first axle.

[0023] In this case, the control system may only modify the second braking torque when both wheels of the first axle 40 are underrotating or locked. This may avoid the second raw braking torque being unnecessarily modified, since a single rotating wheel of the first axle may still provide a necessary braking force.LRP13500W01

[0024] The one or more processors may be collectively configured to determine whether the rotational speed of the first axle is below a first axle speed threshold, and to determine that a loss-of-traction condition exists on the first axle when the rotational speed of the first axle is below the first axle speed threshold.

[0025] 5 The one or more processors may be collectively configured to receive a second axle speed signal indicative of a rotational speed of the second axle and to determine the first axle speed threshold based on the second axle speed.

[0026] In this way, the under rotation or locking of the first axle may be determined more accurately. In particular, since both axles should cease rotation simultaneously, the braking torque should be modified only if there is a significant speed difference between the two axles. It will also be understood that the first axle speed threshold may be considered as an axle speed difference threshold, which may be compared to a difference in speed between the first and second axles. The one or more processors may therefore equally be configured to determine a difference between the first and second axle speeds and to compare the difference to a difference 15 threshold.

[0027] The one or more processors may be collectively configured to: receive a vehicle control signal indicating an activation of a vehicle stability control event, and configure the braking command to cause the braking mechanism to exert the raw braking torque based on receiving the vehicle control signal indicating the 20 activation of the vehicle stability control event, irrespective of whether a loss-of-traction condition exists forthe first axle in dependence on the first axle speed signal when the speed of the vehicle is below the vehicle speed threshold.

[0028] The vehicle stability control event may be the activation of an anti-lock braking system (ABS) ortraction control.

[0029] 25 In cases where a vehicle loses traction following activation of ABS ortraction control, there is a risk of a conflict between the two systems. In particular, in situations where ABS is required, it may be desired to reduce a braking torque to allow an underrotating or locked wheel to begin rotating in order to increase a braking force. By preventing the modification of the raw braking torque in this situation, the possibility of a conflict between different control systems or control loops may be reduced.

[0030] 30

[0031] The one or more processors may be collectively configured to: receive a stationary state signal indicating whether the vehicle is in a stationary state, determine an elapsed time since the vehicle was last stationary, and configure the braking command to cause the braking mechanism to exert the raw braking torque when the elapsed time since the vehicle was last stationary is below an elapsed time threshold, irrespective of whether a loss-of-traction condition exists for the first axle in dependence on the first axle speed signal when the speed of the vehicle is below the vehicle speed threshold.

[0032] In some cases, a wheel may underrotate during an acceleration of a vehicle, such as the beginning of movement after being stationary. In this case, it would be undesirable to bring the vehicle to rest. The control 40 system may therefore determine whether the vehicle has been in a stationary state, i.e. in a park or neutral gear or at zero speed, recently and may not modify the raw braking torque based on this assessment.LRP13500W01

[0033] The braking system may further comprise a further braking mechanism arranged to exert a braking torque on the first axle, and the one or more processors may be collectively configured to determine a first raw braking torque for application on the first axle and a second raw braking torque for application on the second axle, and the braking command may be configured to cause the further braking mechanism to exert a respective braking 5 torque on the first axle and the braking mechanism to exert a respective braking torque on the second axle, upon determining that a loss-of-traction condition exists on the first axle when the speed of the vehicle is below the vehicle speed threshold, the one or more processors may be collectively configured to configure the braking command to cause the further braking mechanism to exert a further modified braking torque on the first axle, the further modified braking torque being greater than the first raw braking torque.

[0034] The control system may also increase the braking torque applied to the front axle. This may reduce the prospect of the front axle beginning to rotate again and may thereby bring the vehicle to rest more quickly and more steadily.

[0035] 15 The one or more processors may be collectively configured to configure the braking command to cause a ratio of the further modified braking torque exerted on the first axle to the modified braking torque exerted on the first axle to be equal to a ratio of the first raw braking torque to the second raw braking torque.

[0036] In this case, the brake balance may not be affected by this control scheme. This may improve the stability of 20 the vehicle as it is brought to rest.

[0037] The one or more processors may be collectively configured to determine the modified braking torque based on an earlier braking torque exerted by the braking mechanism and a maximum allowable rate of change of braking torque.

[0038] 25

[0039] In this way, a step change in braking torque may be avoided. This may improve the comfort of a user of the vehicle.

[0040] The one or more processors may be collectively configured to: receive a pedal signal from a pedal position 30 sensor, the pedal signal comprising information indicative of a position of a pedal, and determine the rate of change of braking torque limit based on the position of the pedal.

[0041] The pedal signal may be a brake pedal signal from a brake pedal position sensor comprising information indicative of a position of a brake pedal. In cases where a user of the vehicle is activating a brake pedal, a higher deceleration may be prioritised over user comfort and so the vehicle may be decelerated more quickly. The one or more processors may be collectively configured to receive the brake request signal from a drive torque request pedal position sensor, which may also be referred to as an accelerator pedal position sensor. For example, the torque request pedal being depressed less than a particular amount may be interpreted as a request to generate a braking torque.

[0042] 40LRP13500W01

[0043] According to another aspect of the present invention, there is provided a braking system for a vehicle, the vehicle comprising a first axle and a second axle, the braking system comprising: a braking mechanism arranged to exert a braking torque on the second axle, and the control system of the first-mentioned aspect.

[0044] 5 The braking system may further comprise a further braking mechanism arranged to exert a braking torque on the first axle.

[0045] The braking system may further comprise a common actuator arranged to actuate the braking mechanism such that a ratio of the braking torque exerted on the first and second axles is fixed.

[0046] In this way, known brake mechanisms, such as brake mechanisms controlled by a single actuator or by two actuators, may be modified to incorporate the control system more easily.

[0047] According to a further aspect of the present invention, there is provided a vehicle comprising: a first axle, a 15 second axle, a braking mechanism arranged to exert a braking torque on the first and second axles, and a control system according to the first-mentioned aspect.

[0048] According to a still further aspect of the present invention, there is provided a method of controlling a braking system of a vehicle comprising a first axle and a second axle, the braking system comprising a braking 20 mechanism arranged to exert a braking torque on the second axle, the method comprising: receiving a brake request signal indicative of a required braking torque; determining a raw braking torque for application on the second axle based on the brake request signal; and outputting a braking command, the braking command being configured to cause the braking mechanism to exert the raw braking torque on the second axle, the method further comprising: receiving a first axle speed signal indicative of a rotational speed of the first axle; 25 receiving a vehicle speed signal indicative of a speed of the vehicle; determining whether a loss-of-traction condition exists for the first axle in dependence on the first axle speed signal; and determining whether the speed of the vehicle is below a vehicle speed threshold; and upon determining that a loss-of-traction condition exists on the first axle when the speed of the vehicle is below the vehicle speed threshold, configuring the braking command to cause the braking mechanism to exert a modified braking torque on the second axle, the 30 modified braking torque being greater than the raw braking torque.

[0049] According to a yet still further aspect of the present invention, there are provided computer readable instructions which, when executed by one or more processors, cause the one or more processors to perform the method of the still further aspect.

[0050] Within the scope of this application it is expressly intended that the various aspects, embodiments, examples and alternatives set out in the preceding paragraphs, in the claims and / or in the following description and drawings, and in particular the individual features thereof, may be taken independently or in any combination. That is, all embodiments and / or features of any embodiment can be combined in anyway and / or combination, 40 unless such features are incompatible. The applicant reserves the right to change any originally filed claim orLRP13500W01

[0051] file any new claim accordingly, including the right to amend any originally filed claim to depend from and / or incorporate any feature of any other claim although not originally claimed in that manner.

[0052] BRIEF DESCRIPTION OF THE DRAWINGS

[0053] 5 One or more embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings, in which:

[0054] Figure 1 shows a schematic representation of a vehicle according to the present invention;

[0055] Figure 2 shows a system diagram of a vehicle according to the present invention;

[0056] Figure 3 is a flowchart illustrating a method of controlling a braking system according to the present invention; Figure 4 is a flowchart illustrating a further method of controlling a braking system according to the present invention;

[0057] Figure 5 is a flowchart illustrating a further method of controlling a braking system according to the present invention;

[0058] Figure 6 is a flowchart illustrating a method of determining an axle speed for use in a method according to the 15 present invention; and

[0059] Figure 7 is a flowchart illustrating a method of outputting a stationary signal flag for use in a method according to the present invention.

[0060] DETAILED DESCRIPTION

[0061] 20 Figures 1 and 2 show a vehicle 10. The vehicle 10 includes four wheels 11a, 11b, 11c, 11d, and four braking mechanisms 14a, 14b, 14c, 14d, each braking mechanism being arranged to exert a braking torque on a respective wheel. The braking mechanisms 14a, 14b, 14c, 14d may collectively form a braking system.

[0062] The wheels 11a, 11b, 11c, 11 d are arranged in two axles, a front axle formed of wheels 11a, 11b, and a rear 25 axle formed of wheels 11c, 11d. Each braking mechanism 14a, 14b, 14c, 14d and each wheel 11a, 11b, 11c, 11d may be individually controlled, such that a braking torque exerted on each wheel may be determined. Alternatively, the braking mechanisms 14a, 14b, 14c, 14d may be commonly controlled, such that there is a fixed brake balance between the front and rear axles and so the braking torques exerted on each wheel have a fixed ratio, while the magnitude of the braking forces may be increased and decreased.

[0063] 30

[0064] Each braking mechanism 14a, 14b, 14c, 14d may include a foundation brake 16, which may be a friction brake, such as a disc brake or drum brake, or may be an alternative form of foundation brake 16, such as an eddy current brake. The braking mechanisms 14 may also include electric machines 18, which may be arranged to exert a drive torque on the wheels 11 and may also be arranged to generate a braking torque and to act as a generator to generate electrical energy during a braking event.

[0065] It will be understood that the braking mechanisms may each comprise only friction brakes or only electric machines and that in some cases, electric machines or friction brakes may be shared between wheels. In particular, wheels on a common axle, such as the front wheels 11a, 11b may share a common electric machine, 40 which may provide a braking torque to both of the front wheels 11a, 11b at the same time.LRP13500W01

[0066] The vehicle 10 may have a common actuator 22 that may actuate one or more foundation brakes 16. The common actuator 22 may be a brake master cylinder 22. The brake master cylinder 22 may provide a pressurised fluid to the foundation brakes 16 to actuate the foundation brakes 16. The brake balance may be determined by relative sizes of brake pistons within each foundation brake 16.

[0067] 5

[0068] The vehicle 10 may also include wheel speed sensors 20, which may each sense the speed of a respective wheel 11a, 11b, 11c, 11d. The wheel speed sensors 20 may each output a wheel speed signal to a controller 12, the wheel speed signal including information indicative of the speed of rotation of the wheel to which the speed sensor 20 is coupled. The speed sensor 20 may, in some cases, be integrated within the electric machine 18 and may be, for example, an electrical sensor, that determines a rate of electrical power generation within the electric machine 18 during a braking event. Further, speed sensors 20 may be provided on only a subset of the wheels where it is not necessary to determine a speed of each wheel individually.

[0069] The vehicle 10 also comprises control pedals 24, 26 for controlling the vehicle 10. The control pedals 24, 26 15 include a brake pedal 24, and an accelerator pedal 26, which may also be referred to as a gas pedal 26 or a drive torque request pedal 26. A driver may depress the respective pedals and a respective pedal sensor may provide an input to a control system 12 to indicate the position of the pedal. A brake request signal may be received from a brake pedal 24 in response to depression of the brake pedal 24, or may be received from the accelerator pedal 26 in response to the accelerator pedal 26 being depressed less than a threshold value. 20

[0070] The vehicle 10 may also include a gradient sensor 28, which may be an inertial measurement unit (IMU) 28 and may be arranged to provide an indication of a gradient of a surface on which the vehicle is positioned, which may be a gradient in a longitudinal direction of the vehicle, i.e. along the direction of travel of the vehicle.

[0071] 25 In some cases, the control system 12 may also receive an input from an advanced driver assistance system (ADAS), which may provide a braking request signal.

[0072] There is also illustrated a control system 12 for controlling the braking system including the braking mechanisms 14a, 14b, 14c, 14d. The control system 12 comprises one or more controllers.

[0073] 30

[0074] The control system 12 comprises processing means 12a and memory means 12b. The processing means 12a may be one or more electronic processing devices which operably execute computer readable instructions. The memory means 12b may be one or more memory devices. The memory means 12b may be electrically coupled to the processing means 12a. The memory means 12b is configured to store instructions, and the processing means 12a is configured to access the memory means 12b and to execute instructions stored thereon.

[0075] The control system 12 is configured to receive speed data from the speed sensors and to determine a required braking torque for each of the braking mechanisms 14a, 14b, 14c, 14d. The control system 12 may then output 40 a braking command to cause the foundation brakes 16 and / or the electric machines 18 to generate a required braking torque.LRP13500W01

[0076] The controller comprises an input means and an output means. The input means may comprise an electrical input of the controller. The output means may comprise an electrical output of the controller. The input is arranged to receive a speed signal from the speed sensors 20. The speed signal is an electrical signal which 5 is indicative of a rotational speed of a respective wheel. The output is arranged to output a braking command signal that is indicative of a required braking torque for controlling the respective braking mechanisms 14a, 14b, 14c, 14d.

[0077] Figures 3 and 4 illustrate methods 100, 200 of controlling a braking system of a vehicle, such as the vehicle 10 illustrated in Figure 1. The method may be performed by the control system 12 illustrated in Figure 2 . In particular, the memory 12b of the control system 12 may comprise computer readable instructions which, when executed by the processor 12a, perform the methods.

[0078] Figure 3 shows a method 100 of controlling a braking system.

[0079] 15

[0080] At step 102, the control system may receive a brake request signal. The brake request signal may be received from a brake pedal or a brake pedal position sensor, from an accelerator pedal or accelerator pedal position sensor, and / or from advanced driver assistance system. The brake request signal may indicate a requirement to decelerate the vehicle at a particular rate.

[0081] 20

[0082] At step 104, the control system may determine a required braking torque, which may also be referred to as a raw braking torque. The raw braking torque may be the braking torque required to achieve the deceleration requested at step 102 under normal conditions, such as with all wheel maintaining traction and not being locked. This may include determining a specific raw braking torque for each wheel or a level of actuation of a 25 master brake cylinder. The control system may also determine at step 104 whether the braking torque should be generated by the foundation brakes, by the electric machines or by a combination thereof.

[0083] At step 106, the control system may output a braking command signal. The braking command signal may be configured to cause one or more of the braking mechanisms 14a, 14b, 14c, 14d to generate the required 30 braking torque. For example, the braking command signal may be output to an electric machine in order to cause the electric machine to generate a braking torque or the braking command signal may be output to an actuator of a brake master cylinder to cause the brake master cylinder to generate a particular pressure within a foundation braking system.

[0084] Following the outputting of the braking command signal at step 106, one or more brake mechanisms may generate a braking torque in order to decelerate the vehicle.

[0085] Figure 4 illustrates a further method 200 for controlling a braking system that may be carried out by the control system 12. The method 200 begins at step 100 by carrying out the method 100 described above with reference 40 to Figure 3.LRP13500W01

[0086] Following the outputting of the braking command signal at step 106, the vehicle will be in a braking event where at least one of the braking mechanisms may be generating a braking torque to decelerate the vehicle.

[0087] During the braking event, at step 204, the control system receives a first axle speed signal. The axle speed 5 signal may include information indicative of a rotational speed of a first axle of the vehicle, which may be a front axle of the vehicle. The axle speed signal may be received from a single speed sensor arranged to sense an average speed of multiple wheels on an axle or may be determined based on more than one speed signal including information indicative of respective speeds of more than one wheel on an axle, as described below with reference to Figure 6.

[0088] Some vehicles may have a brake balance that means that a greater braking torque is exerted on front wheels than on rear wheels. Consequently, when a vehicle is braking while facing an upward slope, the front wheels may be more likely to lock than rear wheels. The first axle may therefore be the front axle.

[0089] 15 At step 206, the control system may receive a vehicle speed signal including information indicative of the speed of the vehicle. The vehicle speed signal may be output by a vehicle speed sensor and may be based on sensed rotational speeds of one or more of the vehicle wheels. At step 206, the vehicle speed may be received from a sub-program ora further controller, which may determine the vehicle speed based on speed signals received from one or more of the vehicle speed sensors. The vehicle speed may be determined based on an average 20 rotational speed of the vehicle wheels or on a greatest speed of the rotational speeds of the vehicle wheels.

[0090] Generally, receiving a vehicle speed signal at step 206 may comprise receiving a plurality of wheel speed signals, each wheel speed signal including a rotation speed of a specific wheel, and determining a vehicle speed based on the one or more wheel speeds.

[0091] 25 Steps 204 and 206 may take place simultaneously or in either order.

[0092] At step 208, the vehicle speed received or determined at step 206 is compared to a vehicle speed threshold. The vehicle speed threshold may be, for example, 10 mph or less. By determining that the vehicle speed is below a threshold, it may be determined that the brake request signal is intended to bring the vehicle to rest.

[0093] 30 Further, it may be determined that a locked wheel or a loss of traction is unlikely to result in vehicle instability such as skidding, since the vehicle is likely to be brought to rest shortly.

[0094] At step 210, it is determined whether a loss of traction condition exists for the first axle, whose speed has been received at step 204. The loss of traction condition may be determined based on the first axle speed being below a first axle speed threshold. The first axle speed threshold may be based on the vehicle speed, and the first axle speed being below the first axle speed threshold may therefore indicate that the wheels of the first axle are not rolling such that there is a static frictional engagement between the wheels and a surface on which the vehicle is travelling.

[0095] 40 At step 210, it may be determined that the first axle speed received at step 204 is below the first axle speed threshold value.LRP13500W01

[0096] Steps 208 and 210 may be carried out in either order or simultaneously.

[0097] Based on the determination that the vehicle speed is below the vehicle speed threshold, and that a loss of 5 traction condition exists due to the first axle speed being below the first axle speed threshold, the braking torque may be modified at step 212.

[0098] The braking torque may be modified at step 212 to increase a braking torque on a second axle of the vehicle. This may result in the wheels of the second axle generating an increased braking force, meaning that the vehicle may still provide the desired overall braking force and so may decelerate as required. In some cases, in particular where a common actuator controls all vehicle brakes, the braking torque generated by all braking mechanisms may be increased at step 212.

[0099] The modified braking torque may be determined based on an earlier braking torque exerted by the braking 15 mechanism and an allowable rate of change of braking torque. The earlier braking torque may be a braking torque applied during step 106. Based on the allowable rate of change of braking torque, the modified braking torque may be increased overtime, until the modified braking torque becomes equal to a final braking torque.

[0100] The allowable rate of change of braking torque may be determined based on the provenance of the brake 20 request signal received at step 102. For example, where the brake request signal is received from a brake pedal, the allowable rate of change of braking torque may be greater as braking speed may be prioritised over user comfort. Alternatively, if the brake request signal is received from an accelerator pedal or ADAS, passenger comfort may be prioritised and a lower allowable rate of change of braking torque may be selected.

[0101] 25 The control system may configure the braking command signal to cause the braking mechanisms to generate the modified braking torque.

[0102] It will also be understood that, where the vehicle speed is above the vehicle speed threshold and / or the wheel speed is above the wheel speed threshold, the controller or control system may carry out a normal braking 30 process according to the method 100.

[0103] Figure 5 shows a further method 250 of controlling a braking system. The methods 200 and 250 of Figures 4 and 5 includes several steps in common. The common steps, which may be the same in both methods, are not described here again in detail for brevity. In particular, steps 100, 204, 206, 208, 210 and 212 may be substantially same in the methods 200 and 250.

[0104] At step 252, the control system may receive a gradient signal including information indicative of a gradient of a surface on which the vehicle is located. The gradient signal may be received from a gradient sensor. The gradient sensor may be an inertial measurement unit (IMU), such as the IMU 28. In particular, the gradient 40 signal may include information indicative of the gradient of the surface on which the vehicle is located in aLRP13500W01

[0105] longitudinal direction of the vehicle, as this may determine the normal reaction forces between the wheels of the front and rear axles and the ground.

[0106] At step 254, the gradient may be compared to a gradient threshold. If the gradient is less than the gradient 5 threshold, then the method may return to step 100 and may proceed with normal braking and without modifying the raw braking torque. Otherwise, the method may continue to step 208, and so on in order to adjust the required braking torque.

[0107] When a vehicle is on a surface having a high gradient, there may be a greater normal reaction force between the ground and the downhill wheels than between the ground and the uphill wheels, relative to when the vehicle is on flat ground. Therefore, it may be more likely that the uphill wheels will lock than the downhill wheels. This may be particularly felt when a vehicle is facing uphill, as there is typically a greater braking torque exerted on front wheels of a vehicle than on rear wheels, which is known as a forward brake bias or a forward brake balance.

[0108] 15

[0109] Moreover, when a vehicle is braking on a surface having a high gradient, there may be a risk of the vehicle rolling backwards due to a loss of traction of one or more of the wheels. This may cause damage to the vehicle drivetrain. Therefore, it may be advantageous in this situation to increase braking torque on the rolling wheels, such that an overall braking force on the vehicle is maintained and such that a rollback of the vehicle does not 20 occur. Therefore, if it is determined that the gradient of the surface on which the vehicle lies is greater than a threshold value, the method may proceed to step 208.

[0110] At step 210, the speed of the first axle may be compared to an axle speed threshold. The axle speed threshold may be determined at step 258, based on a second axle speed received at step 256.

[0111] 25

[0112] At step 256, the control system may receive a second axle speed signal, including information indicative of a speed of the second axle. The second axle may be the rear axle or the downhill axle. The second axle speed may be determined as set out below with respect to Figure 6.

[0113] 30 The first axle may be the front axle or uphill axle and the second axle may be the rear axle or downhill axle.

[0114] However, both axles may be assessed for locking at low speed and so either axle may be considered as the first axle and either axle may be considered as the second axle. In effect, the method may be carried out twice simultaneously, with each axle being treated as the first and second axle in the respective methods.

[0115] At step 258, the wheel speed threshold may be determined based on the second axle speed and may be determined by multiplying the second axle speed by a particular factor, such as 0.8. As will be envisaged by the skilled person, other values may be suitable here, such as 0.6, 1.0 or 0.5, 0.4, amongst many others. In this way, the wheel speed threshold may be used to determine whether the first axle speed is below the second axle speed by greater than a particular amount, which may in turn be determinative of a loss of traction 40 condition.LRP13500W01

[0116] As described above, it is determined at step 210 that the first axle speed is below the first axle speed threshold, the method may proceed to later steps, otherwise the method may move to step 100 and may continue normal braking.

[0117] 5 At step 262, it is determined whether a vehicle control signal has been received indicating an activation of a vehicle stability control event. The vehicle stability control event may be an activation of ABS ortraction control. If it is determined that a vehicle stability control event has occurred and a vehicle control signal has been received, the method may return to step 100 and may continue normal braking. The braking system may, in this case, exert the raw braking torque irrespective of whether a loss-of-traction condition exists. This may avoid a conflict between multiple control systems, since ABS may attempt to prevent locking of wheels, while the present invention may allow locking of wheels and may increase torque on other wheels. Therefore, by testing for the presence of a vehicle stability control event, a more robust control system may be provided.

[0118] At step 264, it may be determined whether the vehicle has recently been in a stationary state. If the vehicle 15 has recently been a stationary state, which may be determined by the vehicle being in a park or neutral gear, or a vehicle speed reading zero, then the method may return to step 100 and may carry out normal braking. The braking system may, in this case, exert the raw braking torque irrespective of whether a loss-of-traction condition exists. This may avoid the control system increasing braking torque during a vehicle start or during a pull-away event, where it is desired to gradually increase the speed of the vehicle by reducing a braking 20 torque. However, if the vehicle has not recently been stationary, the method may proceed to step 212 and may modify the raw braking torque accordingly.

[0119] The control system may then modify the required braking torque and may output a braking command signal at step 212 to cause braking mechanisms to generate the required braking torque as described above.

[0120] 25

[0121] Figure 6 illustrates a method 260 of determining an axle speed. The method 260 may be carried out within step 204 shown in Figures 4 and 5. To determine an axle speed, the method may firstly receive a first wheel speed at step 302 and a second wheel speed at step 304, each of the first and second wheel speeds being a rotational speed of a wheel on the relevant axle. It will be understood that the two wheel speeds may be 30 received in either order or simultaneously.

[0122] At step 306, the control system may select the higher of the first and second wheel speeds received at steps 302 and 304 as the axle speed. This may result in an axle speed being determined as a locked axle only when both wheels are locked. When a single wheel is locked, the axle overall may still provide a sufficient braking force that rollback is unlikely and a total vehicle braking force may be sufficiently closely to the desired vehicle braking force that modification of an initial or raw braking torque is not required.

[0123] At step 308, the axle speed may be output for use in a braking system control method, such as the method 200 or the method 250 illustrated in figures 4 and 5 respectively.

[0124] 40LRP13500W01

[0125] Figure 7 shows a method 264 of determining whether the vehicle has been stationary recently, which may be used at step 264 of the method 250 shown in Figure 5.

[0126] At step 402, the control system may receive a stationary signal indicating that the vehicle is in a stationary 5 state. A stationary signal may be determined based on a vehicle speed that determined to be nil or zero, a parking brake being engaged or the vehicle gearbox being in neutral.

[0127] At step 404, a time period may elapse. This method step may have the form of a wait function.

[0128] 10 At step 406, the time since the stationary signal has occurred may be compared to a time threshold, which may be five seconds or 10 seconds for example. If it is determined that the time since the stationary signal was received is below the threshold, the system may move to step 408 and may output a stationary signal flag, determining that the vehicle has recently been stationary. The method steps 404 and 406 may then be repeated.

[0129] Alternatively, if it is determined that the time since the receipt of the stationary signal is greater than the threshold, then the method may move to step 410, where the stationary signal flag may not be output, and it may be determined that the vehicle has not recently been stationary.

[0130] 20 It will be appreciated that various changes and modifications can be made to the present invention without departing from the scope of the present application.

Claims

LRP13500W01CLAIMS1. A control system for controlling a braking system of a vehicle comprising a first axle and a second axle, the braking system comprising a braking mechanism arranged to exert a braking torque on the second 5 axle, the control system comprising one or more processors collectively configured to:receive a brake request signal indicative of a required braking torque;determine a raw braking torque for application on the second axle based on the brake request signal; andoutput a braking command, the braking command being configured to cause the braking mechanism to exert the raw braking torque on the second axle,wherein the one or more processors are further collectively configured to:receive a first axle speed signal indicative of a rotational speed of the first axle;receive a vehicle speed signal indicative of a speed of the vehicle;determine whether a loss-of-traction condition exists for the first axle in dependence on the first axle speed signal; anddetermine whether the speed of the vehicle is below a vehicle speed threshold; wherein, upon determining that a loss-of-traction condition exists on the first axle when the speed of the vehicle is below the vehicle speed threshold, the one or more processors are collectively configured to configure the braking command to cause the braking mechanism to exert a modified braking torque on the second axle, the modified braking torque being greater than the raw braking torque.

2. The control system of claim 1 , wherein the one or more processors are collectively configured to:receive a gradient signal indicative of a gradient of a surface on which the vehicle is positioned; determine whether the gradient of the surface is greater than a gradient threshold; and configure the braking command to cause the braking mechanism to exert the modified braking torque on the second axle in dependence on the gradient of the surface being greater than the gradient threshold.

3. The control system of claim 1 or 2, wherein the first axle speed signal comprises an indication of a rotational speed of a first wheel on the first axle and an indication of a rotational speed of a second wheel on the first axle, and the one or more processors are collectively configured to determine the rotational speed of the first axle in dependence on the indication of the rotational speed of the first wheel on the first axle and the indication of the rotational speed of the second wheel on the first axle.

4. The control system of claim 3, wherein the one or more processors are collectively configured to determine the rotational speed of the first axle as being the greater of the rotational speed of the first wheel on the first axle and the rotational speed of the second wheel on the first axle.

5. The control system of any preceding claim, wherein the one or more processors are collectively configured to determine whether the rotational speed of the first axle is below a first axle speed threshold, andLRP13500W01to determine that a loss-of-traction condition exists on the first axle when the rotational speed of the first axle is below the first axle speed threshold.

6. The control system of claim 5, wherein the one or more processors are collectively configured to 5 receive a second axle speed signal indicative of a rotational speed of the second axle and to determine the first axle speed threshold based on the second axle speed.

7. The control system of any preceding claim, wherein the one or more processors are collectively configured to:receive a vehicle control signal indicating an activation of a vehicle stability control event, and configure the braking command to cause the braking mechanism to exert the raw braking torque based on receiving the vehicle control signal indicating the activation of the vehicle stability control event, irrespective of whether a loss-of-traction condition exists forthe first axle in dependence on the first axle speed signal when the speed of the vehicle is below the vehicle speed threshold.

8. The control system of any preceding claim, wherein the one or more processors are collectively configured to:receive a stationary state signal indicating whether the vehicle is in a stationary state, determine an elapsed time since the vehicle was last stationary, andconfigure the braking command to cause the braking mechanism to exert the raw braking torque when the elapsed time since the vehicle was last stationary is below an elapsed time threshold, irrespective of whether a loss-of-traction condition exists forthe first axle in dependence on the first axle speed signal when the speed of the vehicle is below the vehicle speed threshold.9 The control system of any preceding claim, wherein the braking system further comprises a further braking mechanism arranged to exert a braking torque on the first axle, andwherein the one or more processors are collectively configured to determine a first raw braking torque for application on the first axle and a second raw braking torque for application on the second axle, and the braking command is configured to cause the further braking mechanism to exert a respective braking torque on the first axle and the braking mechanism to exert a respective braking torque on the second axle, wherein upon determining that a loss-of-traction condition exists on the first axle when the speed of the vehicle is below the vehicle speed threshold, the one or more processors are collectively configured to configure the braking command to cause the further braking mechanism to exert a further modified braking torque on the first axle, the further modified braking torque being greater than the first raw braking torque.

10. The control system of claim 9, wherein the one or more processors are collectively configured to configure the braking command to cause a ratio of the further modified braking torque exerted on the first axle to the modified braking torque exerted on the first axle to be equal to a ratio of the first raw braking torque to the second raw braking torque.LRP13500W0111 . The control system of any preceding claim, wherein the one or more processors are collectively configured to determine the modified braking torque based on an earlier braking torque exerted by the braking mechanism and a maximum allowable rate of change of braking torque.5 12. The control system of claim 11 , wherein the one or more processors are collectively configured to:receive a pedal signal from a pedal position sensor, the pedal signal comprising information indicative of a position of a pedal, anddetermine the rate of change of braking torque limit based on the position of the pedal.

13. A vehicle comprising:a first axle,a second axle,a braking mechanism arranged to exert a braking torque on the first and second axles, and the control system of any preceding claim.

14. A method of controlling a braking system of a vehicle comprising a first axle and a second axle, the braking system comprising a braking mechanism arranged to exert a braking torque on the second axle, the method comprising:receiving a brake request signal indicative of a required braking torque;determining a raw braking torque for application on the second axle based on the brake request signal; andoutputting a braking command, the braking command being configured to cause the braking mechanism to exert the raw braking torque on the second axle,the method further comprising:receiving a first axle speed signal indicative of a rotational speed of the first axle; receiving a vehicle speed signal indicative of a speed of the vehicle;determining whether a loss-of-traction condition exists for the first axle in dependence on the first axle speed signal; anddetermining whether the speed of the vehicle is below a vehicle speed threshold; and upon determining that a loss-of-traction condition exists on the first axle when the speed of the vehicle is below the vehicle speed threshold, configuring the braking command to cause the braking mechanism to exert a modified braking torque on the second axle, the modified braking torque being greater than the raw braking torque.

15. Computer readable instructions which, when executed by one or more processors, cause the one or more processors to perform the method of claim 14.16

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