Control system for controlling a powertrain and a foundation brake of a vehicle

Abstract

Aspects of the present invention relate to transitioning a vehicle into a hold condition where foundation brakes are used to hold the vehicle at rest. This transition may occur when a powertrain of the vehicle is used to decelerate the vehicle rather than foundation braking. The transition may reduce energy consumption by the electric machine. Controlling the transition from being held by a powertrain to being held by foundation brakes may improve passenger comfort.

Description

[0001] CONTROL SYSTEM FOR CONTROLLING A POWERTRAIN AND A FOUNDATION BRAKE OF A VEHICLE

[0002] TECHNICAL FIELD

[0003] The present disclosure relates to a transition into vehicle hold. Aspects of the invention relate to a control system for controlling a powertrain and a foundation brake of a vehicle, a vehicle including the control system, a method for controlling a powertrain and a foundation brake of a vehicle, and computer readable instructions which, when executed on a computer, are arranged to perform the method.

[0004] BACKGROUND

[0005] When bringing a vehicle to rest, some vehicles and driving modes use a powertrain of the vehicle to generate a torque to decelerate the vehicle. Once at rest, the powertrain may be used to hold the vehicle stationary. However, using the powertrain to apply a constant torque to hold the vehicle stationary may cause overheating and / or excess wear of the associated components, as well as requiring energy to be continuously employed.

[0006] It is an aim of the present invention to address one or more of the disadvantages associated with the prior art.

[0007] SUMMARY OF THE INVENTION

[0008] Aspects and embodiments of the invention provide control systems, vehicles, and methods as claimed in the appended claims.

[0009] According to an aspect of the present invention there is provided a control system for controlling a powertrain and a friction brake of a vehicle, the powertrain comprising a power source and a drivetrain, the power source arranged to apply a torque to the drivetrain, the control system comprising one or more processors collectively configured to: output a friction brake demand signal to cause the friction brake to generate a friction brake torque; receive a friction brake torque signal, the friction brake torque signal containing information indicative of the applied friction brake torque; compare the applied friction brake torque to a friction brake threshold value; determine a power source torque demand based on the comparison of the friction brake torque and the friction brake threshold value; and output a power source torque demand signal to cause the power source to generate a torque equal to the power source torque demand.

[0010] The power source torque demand signal may be used to reduce / remove the torque generated by the power source once the friction brakes are applied to hold the vehicle stationary. This may help to prevent the power source overheating and / or reduce wear and tear of the system.

[0011] The friction brake threshold value may be 75% of the holding torque requirement. The value may be calibrated depending on road conditions and / or vehicle performance requirements.

[0012] The power source may be an electric machine, for example in a hybrid electric vehicle or a pure electric vehicle. The friction brake demand signal may be arranged to cause the friction brake torque to increase at a first rate. The power source torque demand signal may be arranged to cause the power source to reduce the generated torque at a second rate. The first rate may be greater than the second rate.

[0013] The rate at which the friction brakes are applied and the power source torque is reduced may impact the perceived smoothness of transition for the vehicle passenger. Specifying that the power source torque is reduced at a slower rate than the friction brakes are applied match the figures shown in the inventor meeting and may improve smoothness of transition.

[0014] The one or more processors may be collectively configured to receive a road condition signal including information indicative of a road condition. The one or more processors may be collectively configured to determine the first and / or second rate based on road conditions.

[0015] Determining the first and / or second rate of claim 3 based on a road condition allows each rate to be calibrated dependent on e.g. Road gradient, road surface etc...

[0016] The friction brake demand signal may be arranged to cause the friction brake torque to increase at a constant rate. The power source torque demand signal may be arranged to cause the power source torque to reduce at a constant rate.

[0017] Applying the brakes results in the friction brake torque increased at a constant rate. Similarly, the power source torque should preferably be reduced at a constant rate. Altering the torque from each of the friction brake and power source at a constant rate provides a convenient way to calibrate and / or quantify the rate of change of torque.

[0018] The one or more processors may be collectively configured to receive a vehicle speed signal including information indicative of a speed of the vehicle; determine, based on the speed of the vehicle, a stationary time of the vehicle; determine whether the stationary time exceeds a stationary time threshold; and based on a determination that the stationary time exceeds the stationary time threshold, generate the hold decision signal.

[0019] This provides a process for determining when the vehicle is held stationary such that the hold decision signal is generated.

[0020] The one or more processors may be collectively configured to compare the speed of the vehicle to a speed threshold value, and determine the stationary time based on a time for which the speed of the vehicle is less than the speed threshold value. The stationary time threshold may be 0.6 second.

[0021] This allows the speed threshold value indicating that the vehicle is stationary to be calibrated and / or quantified.

[0022] The powertrain may comprise: a first axle; a first friction brake arranged to apply a first friction brake torque to the first axle; a second axle; and a second friction brake arranged to apply a second friction brake torque to the second axle. The powertrain may be arranged to drive the first axle with a first axle torque and to drive the second axle with a second axle torque. The one or more processors may be collectively configured to: receive a first axle torque signal, the first axle torque signal containing information indicative of the first axle torque at a first time; receive a second axle torque signal, the second axle torque signal containing information indicative of the second axle torque at the first time; determine an axle torque ratio based on the ratio between the first axle torque at the first time and the second axle torque at the first time; determine a brake torque ratio based on the axle torque ratio; determine a required first friction brake torque and a required second friction brake torque based on the brake torque ratio; and output the friction brake demand signal to cause the first friction brake to apply the first friction brake torque in accordance with the required first friction brake torque and to cause the second friction brake to apply the second friction brake torque in accordance with the required second friction brake torque.

[0023] Determining the ratio between the axle torque applied to the front and rear axle allows the friction brakes on the front and rear axle to be applied in the same ratio, such that the pitch of the vehicle will not change during transition and the perceived smoothness of the transition for the passenger will be improved.

[0024] The friction brake demand signal may be arranged to cause the first friction brake torque and the second friction brake torque to be applied simultaneously.

[0025] Applying friction brake torques to the front and rear axles simultaneously may improve perceived smoothness of the transition into vehicle hold by the friction brakes.

[0026] The friction brake torque signal may contain information indicative of the applied first friction brake torque and of the applied second friction brake torque. The one or more processors may be collectively configured to: compare the applied first friction brake torque to a first friction brake threshold value; compare the applied second friction brake torque to a second friction brake threshold value; determine a first axle torque demand based on the comparison of the applied first friction brake torque to the first friction brake threshold value; and determine a second axle torque demand based on the comparison of the applied second friction brake torque to the second friction brake threshold value. The power source torque demand signal may be arranged to cause the powertrain to drive the first axle with a first axle torque equal to the first axle torque demand and to cause the powertrain to drive the second axle with a second axle torque equal to the second axle torque demand.

[0027] Determining the ratio between the axle torque applied to the front and rear axle allows the friction brakes on the front and rear axle to be applied in the same ratio, such that the pitch of the vehicle will not change during transition and the perceived smoothness of the transition for the passenger will be improved.

[0028] The friction brake demand signal may be arranged to cause the first friction brake torque and the second friction brake torque to be applied simultaneously. Applying friction brake torques to the front and rear axles simultaneously may improve perceived smoothness of the transition into vehicle hold by the friction brakes.

[0029] The friction brake torque signal may contain information indicative of the applied first friction brake torque and of the applied second friction brake torque. The one or more processors are collectively configured to: compare the applied first friction brake torque to a first friction brake threshold value; compare the applied second friction brake torque to a second friction brake threshold value; determine a first axle torque demand based on the comparison of the applied first friction brake torque to the first friction brake threshold value; and determine a second axle torque demand based on the comparison of the applied second friction brake torque to the second friction brake threshold value. The power source torque demand signal may be arranged to cause the powertrain to drive the first axle with a first axle torque equal to the first axle torque demand and to cause the powertrain to drive the second axle with a second axle torque equal to the second axle torque demand.

[0030] This provides a process to reduce each of the first and second axle torques dependent on when the friction brake torque associated with that axle passes a friction brake threshold for that axle.

[0031] The one or more processors may be collectively configured to determine the brake torque ratio such that the ratio of the first friction brake torque to the second friction brake torque is equal to the ratio of the first axle torque at the first time to the second axle torque at the first time.

[0032] This provides a specific case wherein the axle torque ratio is equal to the ratio of the first axle torque to the second axle torque at the first time, the brake torque ratio is equal to the axle torque ratio and the brake torque ratio is equal to the ratio between the first and second friction brake torques. Applying the friction brakes in the same ratio as the axle torques may improve smoothness of transition.

[0033] According to another aspect of the invention, there is provided a vehicle comprising a power source, a friction brake, and the control system of the aspect described above.

[0034] According to a further aspect of the invention, there is provided a method for controlling a powertrain and a friction brake of a vehicle, the powertrain comprising a power source and a drivetrain, the power source arranged to apply a torque to the drivetrain, the method comprising: while the vehicle is held stationary by a torque generated by the power source, receiving a hold decision signal; outputting, based on receiving the hold decision signal, a friction brake demand signal to cause the friction brake to generate a friction brake torque; receiving a friction brake torque signal, the friction brake torque signal containing information indicative of the applied friction brake torque; comparing the applied friction brake torque to a friction brake threshold value, determining a power source torque demand based on the comparison of the friction brake torque and the friction brake threshold value; and outputting a power source torque demand signal to cause the power source to generate a torque equal to the power source torque demand. According to another further aspect of the invention, there is provided computer readable instructions which, when executed by a computer, are arranged to perform a method according to the above-described further aspect.

[0035] 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 any way and / or combination, unless such features are incompatible. The applicant reserves the right to change any originally filed claim or 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.

[0036] BRIEF DESCRIPTION OF THE DRAWINGS

[0037] One or more embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings, in which:

[0038] Figure 1 shows a vehicle in accordance with all embodiments of the invention;

[0039] Figure 2 shows a schematic diagram of a vehicle, including a control system, a powertrain and a foundation brake, in accordance with an embodiment of the invention;

[0040] Figure 3 shows a flowchart illustrating a method in accordance with an embodiment of the invention;

[0041] Figure 4 shows a graph of torque against time for a power source and a foundation brake of a vehicle;

[0042] Figure 5 shows a flowchart illustrating a method in accordance with a further embodiment of the invention;

[0043] Figure 6 shows a schematic diagram of a vehicle, including a control system, two powertrains and two foundation brakes, in accordance with a further embodiment of the invention;

[0044] Figure 7 shows a flowchart illustrating a method in accordance with a still further embodiment of the invention;

[0045] Figure 8 shows a graph of torque against time for two power sources and two foundation brakes of a vehicle; and

[0046] Figure 9 shows a graph of torque against time for two power sources and two foundation brakes of a vehicle.

[0047] DETAILED DESCRIPTION

[0048] This invention relates to a control system for transitioning a vehicle into a hold condition where foundation brakes are used to hold the vehicle at rest. This transition may occur when a powertrain of the vehicle is used to decelerate the vehicle rather than foundation braking such as, for example, in single pedal driving (SPD) modes of operation. In some cases, both the powertrain and the foundation braking may be used to decelerate the vehicle before the vehicle transitions to being held at rest by the foundation brakes alone. It is beneficial to use foundation braking to hold the vehicle stationary instead of the powertrain to reduce wear and / or heating of the powertrain and associated components. Controlling the transition from being held by a powertrain to being held by foundation brakes may improve passenger comfort. Foundation braking here refers to applying foundation brakes, which may include any system which decelerates the vehicle by converting kinetic energy of the vehicle into heat, as opposed to a regenerative brake, such as may be provided by an electric machine. The foundation brakes may be friction brakes, to decelerate the vehicle. The foundation brake torque may be caused by a foundation brake pressure, a foundation brake force or equivalent. Foundation brake pressure or force is the pressure or force exerted on (for example) a brake drum by a brake shoe, or on a brake pad by a brake calliper. A greater foundation brake pressure or force will result in a greater foundation brake torque if the wheel is turning, resulting in a greater deceleration of the vehicle. If the vehicle is held stationary by a torque applied to the wheels by a powertrain, it will be appreciated that applying the foundation brake will effectively result in a foundation brake torque despite the wheel not rotating. This foundation brake torque opposes any potential rotation of the wheel due to for example the vehicle being held stationary on a non-zero gradient. Therefore, a reference to a particular foundation brake torque may be considered as equivalent to a reference to a foundation brake pressure that correlates to the particular brake torque when the wheel is rotating.

[0049] When applying the foundation brakes to hold a vehicle stationary, the foundation brakes may be to provide a maximum foundation brake torque to the wheels. In this case, the foundation brake pressure may be selected to be equivalent to a foundation brake torque that is greaterthan a torque required to hold the vehicle stationary during expected normal use.

[0050] A vehicle 10 in accordance with an embodiment of the present invention is described herein with reference to the accompanying Figure 1 . As shown in Figure 1 , the vehicle 10 comprises a control system 100 configured to control a foundation brake 120 and a powertrain 130 of the vehicle 10. The vehicle 10 may be a hybrid electric vehicle having an electric machine and an internal combustion engine both arranged to drive the wheels of the vehicle or may be a battery electric vehicle powered by an electric machine only. The vehicle 10 may be a mild hybrid electric vehicle (MHEV) or a plug-in hybrid electric vehicle (PHEV).

[0051] The control system 100 as illustrated in Figure 2 comprises one or more controllers 160. In Figure 2, the control system 100 comprises one controller 160, although it will be appreciated that this is merely illustrative. The controller 160 comprises processing means 182 and memory means 184. The processing means may be one or more electronic processing devices 182 which operably executes computer-readable instructions. The memory means 184 may be one or more memory devices 184. The memory means 184 is electrically coupled to the processing means 182. The memory means 184 is configured to store instructions, and the processing means 182 is configured to access the memory means 184 and execute the instructions stored thereon.

[0052] The controller 160 comprises an input means 186 and an output means 188. The input means 186 may comprise an electrical input of the controller 180 arranged to receive data such as a foundation brake torque signal 124 from the foundation brake 120. The output means 188 may comprise an electrical output of the controller 180 arranged to output commands, such as a power source torque demand signal 142 to cause the power source 140 to generate a required torque. The powertrain 130 comprises at least one power source 140 arranged to apply a torque 152 to a drivetrain 150 as illustrated in Figure 2. The power source 140 may be an electric machine arranged to generate a torque. The control system 100 is configured to receive a hold decision signal 102, indicating that the vehicle 10 is to be held stationary by the foundation brake 120. The hold decision 102 may be generated by a separate hold decision controller 101 as illustrated in Figure 2, or the hold decision controller 101 may be part of the control system 100, and the hold decision signal 102 may be received by one part of the control system as it is determined by another part.

[0053] The hold decision controller 101 may be configured to receive a vehicle speed signal 112 comprising information indicative of a vehicle speed. The vehicle speed signal 112 may be output by a vehicle speed sensor 1 11 which may be separate from the control system 100. The vehicle speed sensor 111 may measure a vehicle wheel speed and determine a linear vehicle speed based on this data. Alternatively, the vehicle speed sensor may be arranged to measure a power source speed and to determine a linear vehicle speed based on the power source speed and a known torque ratio applied by the drivetrain 150.

[0054] The control system 100 may also be configured to receive a road surface condition signal 104 comprising information indicative of a condition of a surface which the vehicle 10 is on. The road surface condition signal 104 may be output by a road surface condition controller 103. The road surface controller 103 may form part of the control system 100. The condition of the surface on which the vehicle 10 is on may be provided by a user of the vehicle 10, for example, via an input to a human machine interface (HMI), or it may be determined using for example signals from a traction control system and / or an anti-lock braking system. Such signals may indicate wheel slippage which may indicate that the road surface condition is loose or low friction.

[0055] The control system 100 may further receive inputs 106, 108, 1 10 from an accelerator pedal 105, a brake pedal 107 and an inertial measurement unit (IMU) 109 respectively. An accelerator pedal signal 106 comprises information indicative of an acceleration request provided by a driver of the vehicle 10 using the accelerator pedal 105. In some cases, such as SPD modes of operation, the accelerator pedal 105 may also be used by the driverto request a deceleration. A brake pedal signal 106 comprises information indicative of a deceleration request provided by the driver of the vehicle using the brake pedal 107. A gradient signal 110 comprises information indicative of the road gradient on which the vehicle 10 is on. Determining the gradient of the road may comprise using the IMU to determine the proper acceleration of the vehicle 10 and comparing this value to an estimated vehicle acceleration determined using the vehicle mass and the torque generated by the power source 140. Proper acceleration is the difference between the acceleration due to gravity and a measured acceleration in the direction of gravity and indicates travel up or down a slope.

[0056] A method 200 for controlling a foundation brake 120 and a powertrain 130 according to an embodiment of the invention will now be described with reference to Figures 3 and 4. Figure 3 is a flowchart illustrating a method 200 for controlling a foundation brake 120 and a powertrain 130 according to an embodiment of the invention. Figure 4 is a graph 300 showing an example of a power source torque profile 310 and a foundation brake torque profile 320 when the method 200 is performed by the control system 100, and shows how the torque generated by the power source 140 and the foundation brake 120 vary with time The method 200 is a method of controlling a foundation brake 120 and a powertrain 130 of a vehicle 10, such as the vehicle 10 illustrated in Figure 1. The method 200 may be performed by the control system 100 illustrated in Figure 2. In particular, the memory 184 may comprise computer-readable instructions which, when executed by the processing means 182, perform the method 200 according to an embodiment of the invention.

[0057] Upon receiving a hold decision signal 102 at the first step 210, the control system 100 outputs a foundation brake demand signal 122 at step 220 at a time indicated by the dashed hold decision line HD in Figure 4. The foundation brake demand signal 122 causes the foundation brake to be applied, generating a foundation brake torque to oppose rotation of the wheel. The foundation brake demand signal 122 may cause the foundation brake torque to be increased at a first rate. The first rate may be constant as illustrated by the constant rate of increase in the foundation brake torque profile 320.

[0058] At step 230 the control system 100 is configured to receive a foundation brake torque signal 124. The foundation brake torque signal 124 comprises information indicative of the foundation brake torque applied by the foundation brake 120. Optionally, the foundation brake torque signal 124 may comprise information indicative of the foundation brake pressure and / or the foundation brake force as described previously.

[0059] At step 240 a foundation brake threshold value T1 is determined. For example, the foundation brake threshold may be a threshold value of foundation brake pressure and / or foundation brake force. The foundation brake threshold value may be a predetermined non-variable value. Alternatively, the foundation brake threshold value may be determined based on one or more of the mass of the vehicle, the gradient of the road and a torque generated by the power source 140. The foundation brake threshold value may be determined at any time relative to the hold decision signal being received and the foundation brake signal being output, i.e. step 240 may take place before or after steps 210, 220 and 230.

[0060] As one example, the control system 100 may be configured to receive a hold signal 144 as illustrated in Figure 2. The hold signal 144 comprises information indicative of a holding torque required to hold the vehicle 10 stationary. Although shown in Figure 2 as being output by the power source 140, it will be appreciated that the hold signal 144 may be generated by any vehicle system including the control system 100. The foundation brake threshold may be determined based on the holding torque required to hold the vehicle stationary. By way of non-limiting example, the foundation brake threshold may be 70% of the holding torque required to hold the vehicle stationary.

[0061] At step 250, the foundation brake torque is compared to the foundation brake threshold value T1. It will be appreciated that comparing the foundation brake torque to a foundation brake threshold value may comprise comparing a foundation brake pressure or a foundation brake force to the foundation brake threshold value. The foundation brake threshold is shown in Figure 4 as a threshold torque value T1. The foundation brake torque may be compared to the foundation brake threshold value at regular time intervals to continually monitor the foundation brake torque applied. At step 260 a power source torque demand is determined based on the comparison of the foundation brake torque to the foundation brake threshold value. For example, if the foundation brake torque is below the foundation brake threshold value, the control system 100 will determine that the power source torque demand should be maintained at its current level. If the foundation brake torque is above the foundation brake threshold, the control system 100 will determine that the power source torque demand should be decreased to zero. Optionally, the power source torque demand may be determined based on one or more of a requested acceleration, a requested deceleration, the mass of the vehicle, the gradient of the road and a torque generated by the power source 140.

[0062] In the case illustrated by Figure 4, the vehicle 10 is initially held stationary by a constant torque generated by the power source 130, indicated by the horizontal section of the power source torque profile 310. The power source torque demand therefore remains constant whilst the foundation brake torque is less than the foundation brake threshold value T1. Once the foundation brake torque exceeds the threshold torque value T1 at a time indicated by the dashed line S1 , the control system 100 determines that the power source torque demand should decrease. In this example, the power source torque demand signal 142 causes the power source 140 to reduce the torque 152 generated by the power source 140.

[0063] A power source torque demand signal 142 containing information indicative of the power source torque demand is then output at step 270. The power source torque demand signal 142 causes the power source 140 to generate a torque 152 equal to the power source torque demand. The power source torque demand signal 142 may cause the power source torque 152 to be reduced at a second rate. The second rate may be constant as illustrated by the constant rate of decrease in the power source torque profile 310. The first rate may be greater than the second rate. That is to say, the foundation brake torque may be increased at a greater rate than the rate at which the power source torque 152 is reduced.

[0064] The rate at which the foundation brake 120 is applied and the rate at which the power source torque 152 is reduced may affect perceived smoothness of the transition for a vehicle passenger. For example, the condition of the surface which the vehicle 10 is on may also be used to determine the first rate and / or the second rate. It may be beneficial to vary the first rate and / or the second rate dependent on the surface condition to improve smoothness of transition. Reducing the power source torque 152 at a lower rate than the rate at which the foundation brakes 120 are applied may improve smoothness of transition and hence vehicle user comfort.

[0065] A method 400 for generating the hold signal 102 will now be described with reference to Figure 5. It will be appreciated that this method may precede or replace step 210 of Figure 3. As described previously, the hold decision signal 102 may be generated based on the vehicle being held stationary by the powertrain 130. The hold decision controller 101 receives a vehicle speed signal 112 comprising information indicative of a vehicle speed at step 410. The vehicle speed signal 112 may be output by a vehicle speed sensor 111 which may be separate to the control system 100. The vehicle speed sensor 111 may measure a vehicle wheel speed and determine a linear vehicle speed based on this data. Alternatively, the vehicle speed sensor may be arranged to measure a power source speed and determine a linear vehicle speed based on the power source speed and a known torque ratio applied by the drivetrain 150.

[0066] At step 420, the hold decision controller 101 determines a stationary time period of the vehicle 10 based on the vehicle speed. Here the stationary time period is defined as the period of time for which the vehicle 10 is determined to be stationary. To determine when the vehicle 10 is stationary, the vehicle speed may be compared to a vehicle speed threshold value. By way of non-limiting example, the vehicle speed threshold value may be 1 kilometre per hour. The stationary time period is defined as the continuous period of time for which the vehicle speed is less than the vehicle speed threshold value. Where the vehicle speed sensor is arranged to measure a power source speed, the vehicle may be determined to be stationary when the power source speed is below a power source speed threshold value. By way of non-limiting example, the power source speed threshold value may be 5 revolutions per minute. In this case, the stationary time period is defined as the continuous period of time for which the power source speed is less than the power source speed threshold value.

[0067] At step 430 the stationary time period is compared to a stationary time period threshold. If the stationary time period exceeds the stationary time period threshold, the vehicle is determined to be stationary. In this case, the hold decision signal 102 is generated at step 440 to initiate transition from hold by the powertrain 130 to hold by the foundation brake 120. The stationary time period threshold may be 1 second, preferably 0.8 seconds, further preferably 0.6 seconds. The stationary time period may be used to determine whether overheating of the power source 140 is likely and the stationary time period threshold can be selected to avoid this.

[0068] Figure 6 illustrates a schematic diagram of a vehicle 20 according to a further embodiment of the invention. The vehicle 20 comprises the control system 100 as described with reference to Figure 2. In this respect, the internal features of the control system 100 are the same as illustrated in Figure 2 and their discussion is not repeated in detail for brevity. It will also be appreciated that the control system 100 shown in Figure 6 may be configured to receive signals 102-112 as described with reference to Figure 2.

[0069] The vehicle 20 comprises a first axle 160 and a second axle 170. The first axle 160 is driven by a first power source 140a and a first drivetrain 150a to form a first powertrain 130a. The first powertrain 130a is arranged such that the first power source 140a applies a first power source torque 152a to the first drivetrain 150a, and the first drivetrain 150a applies a first axle torque 162 to the first axle 160. The second axle 160 is driven by a second power source 140b and a second drivetrain 150b to form a second powertrain 130b. The second powertrain 130b is arranged such that the second power source 140b applies a second power source torque 152b to the second drivetrain 150b, and the second drivetrain 150b applies a second axle torque 172 to the second axle 170. Alternatively, a single power source 140 and drivetrain 150 may be used to drive both the first axle 160 and the second axle 170 to form a single powertrain 130. Any reference made hereinafter to a first powertrain 130a and a second powertrain 130b may also apply to a vehicle comprising a single powertrain 130 only. The vehicle 20 also comprises a first foundation brake 120a and a second foundation brake 120b. The first foundation brake 120a is arranged to apply a first foundation brake torque 126a to the first axle 160. The second foundation brake 120b is arranged to apply a second foundation brake torque 126b to the second axle 170. It will be appreciated that in alternative embodiments, a single foundation brake system 120 may be used to apply a first foundation brake torque 126a to the first axle 160 and a second foundation brake torque 126b to the second axle 170.

[0070] The control system 100 is configured to receive a first axle torque signal 164 and a second axle torque signal 174. The first axle torque signal 164 comprises information indicative of the first axle torque 162 applied to the first axle 160 at a first time. The second axle torque signal 174 comprises information indicative of the second axle torque 172 applied to the second axle 170 at the first time. Although shown as being output by the first axle 160 and the second axle 170, it will be appreciated that the first axle torque signal 164 and the second axle torque signal 174 may be generated and / or output by any vehicle system. It will be appreciated that the first and second axle torques 164, 174 may be output by sensors coupled to the respective drivetrains 150a, b or the respective power sources 140a, b.

[0071] Providing a vehicle 20 comprising a first axle 160 with a first foundation brake 120a and a second axle 170 with a second foundation brake 120b allows different foundation brake torques to be applied to each axle. A method 500 of applying a different foundation brake torque to a first axle 160 and a second axle 170 will here be described with reference to Figure 7. It will be appreciated that the method 500 may replace step 220 in Figure 3.

[0072] At step 510, the control system 100 receives the first axle torque signal 164 and the second axle torque signal 174.

[0073] At step 520 the control system 100 determines an axle torque ratio based on the ratio between the first axle torque 162 at the first time and the second axle torque 172 at the first time. The axle torque ratio may be equal to the ratio between the first axle torque 162 at the first time and the second axle torque 172 at the first time.

[0074] At step 530 the control system 100 determines a brake torque ratio based on the axle torque ratio. For example, the brake torque ratio may be equal to the axle torque ratio.

[0075] At step 540, a required first foundation brake torque and a required second foundation brake torque is determined based on the brake torque ratio.

[0076] At step 550 the control system outputs the foundation brake signal 122, optionally wherein the foundation brake signal 122 comprises a first foundation brake signal 122a received by the first foundation brake 120a and a second foundation brake signal 122b received by the second foundation brake 120b. The first foundation brake signal 122a causes the first foundation brake 120a to apply the first foundation brake torque 126a in accordance with the required first foundation brake torque. The second foundation brake signal 122b causes the second foundation brake 120b to apply the second foundation brake torque 126b in accordance with the required second foundation brake torque.

[0077] By applying the front and rear foundation brakes 120a, 120b in the same ratio as the front and rear axle torques 164, 174 prior to transition, the pitching of the vehicle 10 during transition may be reduced and / or the pitch of the vehicle 10 may be kept relatively constant. This may improve perceived smoothness of transition for a user of the vehicle 10.

[0078] In some cases, the ratio of the first foundation brake torque 126a to the second foundation brake torque 126b may be a constant predetermined value. This ratio may account for a difference in the relative size of the first foundation brake 120a and the second foundation brake 120b. This ratio may be referred to as a fixed brake ratio. In this case, the first powertrain 130a and the second powertrain 130b may be controlled such that during a deceleration, the ratio of the first axle torque 162 to the second axle torque 172 is equal to the fixed brake ratio. Applying a first axle torque 162 and a second axle torque 172 at a ratio equal to the fixed brake ratio allows the first foundation brake 120a and the second foundation brake 120b to be applied at the same fixed brake ratio. This may improve perceived smoothness for a user of the vehicle 10 during transition even when the ratio of the first foundation brake torque 126a to the second foundation brake torque 126b cannot be varied.

[0079] Figure 8 is a graph 600 showing an example of a first axle torque profile 610a, a first foundation brake torque profile 620a, a second axle torque profile 610b and a second foundation brake torque profile 620b according to the above-described embodiment. Each profile shows how the torque applied to the axle or generated by the foundation brake varies over time. The control system 100 is here configured such that upon receiving the hold demand signal 102 indicated by the dashed hold decision line HD, the foundation brake signals 122a, 122b cause the first foundation brake 120a and the second foundation brake 120b to be applied simultaneously. This is illustrated in Figure 8 by the shared inclined section of the first foundation brake torque profile 620a and the second foundation brake torque profile 620b.

[0080] The first foundation brake torque 126a is compared to a first foundation brake threshold value T1 a. Although shown as a torque value, it will be appreciated that the first foundation brake threshold value T1 a may be a pressure or force value as described previously. A first axle torque demand is determined based on the comparison between the first foundation brake torque 126a and the first foundation brake threshold value T1 a. A first power source demand signal 142a is output by the control system 100 to cause the first powertrain 130a to drive the first axle 160 with a first axle torque 162 equal to the first axle torque demand. In the illustrated example, once the first foundation brake torque 126a exceeds the first foundation brake threshold value T1 a at a time S1 a, the first power source demand signal 142a causes the first axle torque 162 to reduce at a constant rate.

[0081] The second foundation brake torque 126b is compared to a second foundation brake threshold value T1 b. Although shown as a torque value, it will be appreciated that the second foundation brake threshold value T 1 b may be a pressure or force value as described previously. A second axle torque demand is determined based on the comparison between the second foundation brake torque 126b and the second foundation brake threshold value T1 b. A second power source demand signal 142b is output by the control system 100 to cause the second powertrain 130b to drive the second axle 170 with a second axle torque 172 equal to the second axle torque demand. In the illustrated example, once the second foundation brake torque 126b exceeds the second foundation brake threshold value T1 b at a time S1 b, the second power source demand signal 142b causes the second axle torque 172 to reduce at a constant rate.

[0082] It will be appreciated that the control system 100 may be configured to perform the above comparisons and following outputs simultaneously such that the first power source demand signal 142a and the second power source demand signal 142b are output simultaneously.

[0083] As illustrated in Figure 8, the ratio between the first foundation brake torque 126a and the second foundation brake torque 126b may be equal to the ratio of the first axle torque 162 at the first time to the second axle torque 172 at the first time. This is illustrated by the horizontal sections of each of the axle torque profiles 610a, 610b and the foundation brake profiles 620a, 620b which show the same ratio of first to second torques before and after transition.

[0084] Figure 9 is a graph 700 showing a further example of a first axle torque profile 710a, a first foundation brake torque profile 720a, a second axle torque profile 710b and a second foundation brake torque profile 720b. It will be appreciated that the method described with reference to Figure 8 also applies to Figure 9 and will not be repeated here.

[0085] Figure 9 illustrates a further embodiment wherein the first foundation brake 120a and the second foundation brake 120b are applied at different rates. That is to say, the first foundation brake torque 126a increases at a different rate to the second foundation brake torque 126b. The rate at which the first foundation brake 120a and the second foundation brake 120b are applied may be determined such that the time S1 a at which the first foundation brake torque 126a exceeds the first foundation brake threshold value T1 a coincides with the time S1 b at which the second foundation brake torque 126b exceeds the second foundation brake threshold value T1 b. This allows reduction of the first axle torque 162 and the second axle torque 172 to begin simultaneously. This is illustrated by the first axle torque profile 710a and the second axle torque profile 710b beginning to decrease at the same shared time S1 a, S1 b.

[0086] In the illustrated example, the rate at which the first axle torque 162 and the second axle torque 172 are decreased are different. The rates at which the first axle torque 162 and the second axle torque 172 are decreased are here determined such that the two torque profiles meet an axle torque threshold value T2 simultaneously. The axle torque threshold value T2 may be 0 newton metres as illustrated in Figure 9. Although shown as decreasing at different rates, it will be appreciated that the first axle torque 162 and the second axle torque may be reduced at the same rate.

[0087] 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

CLAIMS1 . A control system for controlling a powertrain and a foundation brake of a vehicle, the powertrain comprising a power source and a drivetrain, the power source being arranged to apply a torque to the drivetrain, the control system comprising one or more processors collectively configured to: receive a hold decision signal; output, in response to the hold decision signal, a foundation brake demand signal to cause the foundation brake to generate a foundation brake torque; receive a foundation brake torque signal, the foundation brake torque signal comprising information indicative of the applied foundation brake torque; compare the applied foundation brake torque to a foundation brake threshold value; determine a power source torque demand based on the comparison of the foundation brake torque and the foundation brake threshold value; and output a power source torque demand signal to cause the power source to generate a torque equal to the power source torque demand.

2. The control system of claim 1 , wherein the one or more processors are collectively configured to: receive a power source hold signal comprising information indicative of a holding torque required to hold the vehicle stationary; determine the foundation brake threshold value based on the holding torque required to hold the vehicle stationary.

3. The control system of claim 1 or 2, wherein the foundation brake demand signal is arranged to cause the foundation brake torque to increase at a first rate; wherein the power source torque demand signal is arranged to cause the power source to reduce the generated torque at a second rate; and wherein the first rate is greater than the second rate.

4. The control system of claim 3, wherein the one or more processors are collectively configured to: receive a surface condition signal comprising information indicative of a condition of a surface which the vehicle is on; and determine the first and / or second rate based on the surface condition.

5. The control system of any of claims 2 to 4, wherein the foundation brake demand signal is arranged to cause the foundation brake torque to increase at a constant rate; and / or wherein the power source torque demand signal is arranged to cause the power source torque to reduce at a constant rate.

6. The control system of any preceding claim, wherein the one or more processors are collectively configured to: receive a vehicle speed signal comprising information indicative of a speed of the vehicle; determine, based on the speed of the vehicle, a stationary time period of the vehicle; determine whether the stationary time period exceeds a stationary time period threshold; and based on a determination that the stationary time period exceeds the stationary time period threshold, generate the hold decision signal.

7. The control system of claim 6, wherein the one or more processors are collectively configured to: compare the speed of the vehicle to a speed threshold value, and determine the stationary time period based on a time for which the speed of the vehicle is less than the speed threshold value.

8. The control system of claim 6 or 7, wherein the stationary time period threshold is 0.6 second.

9. The control system of any preceding claim, wherein the one or more processors are collectively configured to: receive a first axle torque signal, the first axle torque signal comprising information indicative of a first axle torque of a first axle of the powertrain at a first time; receive a second axle torque signal, the second axle torque signal comprising information indicative of a second axle torque of a second axle of the powertrain at the first time; determine an axle torque ratio based on the ratio between the first axle torque at the first time and the second axle torque at the first time; determine a brake torque ratio based on the axle torque ratio; determine, based on the brake torque ratio, a required first foundation brake torque for a first foundation brake arranged to apply a brake torque to the first axle and a required second foundation brake torque for a second foundation brake arranged to apply a brake torque to the second axle; and output the foundation brake demand signal to cause the first foundation brake to apply the first foundation brake torque in accordance with the required first foundation brake torque and to cause the second foundation brake to apply the second foundation brake torque in accordance with the required second foundation brake torque.

10. The control system of claim 9, wherein the foundation brake demand signal is arranged to cause the first foundation brake torque and the second foundation brake torque to be applied simultaneously.11 . The control system of claim 9 or 10, wherein the foundation brake torque signal contains information indicative of the applied first foundation brake torque and of the applied second foundation brake torque, wherein the one or more processors are collectively configured to: compare the applied first foundation brake torque to a first foundation brake threshold value; compare the applied second foundation brake torque to a second foundation brake threshold value; determine a first axle torque demand based on the comparison of the applied first foundation brake torque to the first foundation brake threshold value; and determine a second axle torque demand based on the comparison of the applied second foundation brake torque to the second foundation brake threshold value, and wherein the power source torque demand signal is arranged to cause the powertrain to drive the first axle with a first axle torque equal to the first axle torque demand and to cause the powertrain to drive the second axle with a second axle torque equal to the second axle torque demand.

12. The control system of claim 9, 10 or 1 1 wherein the one or more processors are collectively configured to determine the brake torque ratio such that the ratio of the first foundation brake torque to the second foundation brake torque is equal to the ratio of the first axle torque at the first time to the second axle torque at the first time.

13. A vehicle comprising a powertrain, a foundation brake, and the control system of any preceding claim, the powertrain comprising a power source and a drivetrain, the power source being arranged to apply a torque to the drivetrain.

14. A method for controlling a powertrain and a foundation brake of a vehicle, the powertrain comprising a power source and a drivetrain, the power source arranged to apply a torque to the drivetrain, the method comprising: while the vehicle is held stationary by a torque generated by the power source, receiving a hold decision signal; outputting, based on receiving the hold decision signal, a foundation brake demand signal to cause the foundation brake to generate a foundation brake torque; receiving a foundation brake torque signal, the foundation brake torque signal comprising information indicative of the applied foundation brake torque; comparing the applied foundation brake torque to a foundation brake threshold value, determining a power source torque demand based on the comparison of the foundation brake torque and the foundation brake threshold value; and outputting a power source torque demand signal to cause the power source to generate a torque equal to the power source torque demand.

15. Computer readable instruction which, when executed by a computer, are arranged to perform a method according to claim 14.

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