Vehicle with braking system having hydraulic shear device

Abstract

A vehicle including a first braking system (e.g. hydraulically-operated disc brakes 106), for selectively braking road wheels 102, 104; a second braking system 116 including a first energy sink 118; and a control system 128. Energy sink 118 includes a hydraulic shear device 120, which may include: an input shaft 122; housing 134; a series of axially spaced rotating plates 136 interleaved with stator plates 138; and hydraulic fluid 148 at least partially immersing the plates. The control system 128 determines that emergency braking is required based on input signals 114 received from the vehicle (e.g. from brake pedal 108), and hence outputs a control signal 132 to the second braking system to couple it to (rear) wheels 102, such that vehicle forward kinetic energy is converted into heat in the first energy sink 118 via rotational movement of the wheels 102 (e.g. via shaft 122 rotating plates 136 within hydraulic fluid, which causes drag). The hydraulic fluid 148 may be an electrorheological fluid or magnetorheological fluid, with the device 120 including a field generator (150; fig. 5) to vary fluid viscosity. There may also be a flywheel (162, fig. 7) and / or electric machine (166, fig.7; 168, fig 8).

Description

TECHNICAL FIELD The present disclosure relates to a vehicle with a braking system having a hydraulic shear device. Aspects of the invention relate to a vehicle, a method of controlling a braking system, and computer readable instructions for controlling a braking system. BACKGROUND It is known to provide braking systems for vehicles. For example, friction brakes, such as disc brakes, convert linear kinetic energy of the vehicle into heat. Engine braking and electrical regeneration are other methods that can be used to slow a vehicle. Depending upon the circumstances, braking can be constrained by a maximum energy conversion rate of the braking system and other methods. These challenges can be exacerbated by particular braking circumstances, such as emergency braking, especially from high speeds and / or when the vehicle is relatively heavy. Another potential issue is brake dust that can be generated by friction brakes, especially under hard or emergency braking. It is an aim of the present invention to address one or more of the disadvantages associated with the prior art. SUMMARY OF THE INVENTION Aspects and embodiments of the invention provide a vehicle, a method of controlling a braking system, and computer readable instructions for controlling a braking system. According to an aspect of the present invention there is provided a vehicle comprising: a plurality of road wheels; a first braking system for selectively braking a first subset of the road wheels; a second braking system comprising a first energy sink, the first energy sink comprising a hydraulic shear device having an input couplable to a second subset of the road wheels, the hydraulic shear device being configured to convert at least some of the forward kinetic energy of the vehicle into heat; and a control system for controlling the second braking system, the control system comprising one or more processors collectively configured to: receive one or more input signals from the vehicle; determine, based at least partly on the one or more input signals, that emergency braking is required; and responsive to the determination that emergency braking is required, output a control signal to the second braking system; the second braking system being configured to, responsive to the control signal, couple the second braking system to the second subset of the road wheels, such that kinetic energy of the vehicle is transferred into the first energy sink via rotational movement of the second subset of the road wheels. Selectively coupling the second braking system to the second subset of the road wheels may offer significant additional braking capacity in emergency braking circumstances, and the use of a hydraulic shear device may offer a speed sensitive and / or relatively low-impact way of sinking the forward kinetic energy of the vehicle. The generation of brake dust may be reduced. The first subset of the road wheels may comprise a pair of front wheels arranged at a forward end of the vehicle, while the second subset of the road wheels may comprise a pair of rear wheels arranged ata rear end of the vehicle. The control system may comprise one or more controllers collectively comprising at least one electronic processor having an electrical input for receiving an input signal; and at least one memory device electrically coupled to the at least one electronic processor and having instructions stored therein; and wherein the at least one electronic processor is configured to access the at least one memory device and execute the instructions thereon so as to: receive the one or more input signals from the vehicle; determine that emergency braking is required; and responsive to the determination that emergency braking is required, output the control signal to the second braking system. Optionally, the control system may be configured such that the second braking system is used only when emergency braking is required. This may enable decoupling of ordinary and emergency braking systems, which may offer advantages due to each system being optimised for its task. Optionally, the hydraulic shear device may comprise an electrorheological and / or magnetorheological fluid, and a field generator for generating an electrical and / or magnetic field responsive to which the electrorheological and / or magnetorheological fluid changes its viscosity. This may offer increased control over, and / or magnitude of, braking. Optionally, the field generator may comprise an input for receiving a control signal from the control system. Optionally, the hydraulic shear device may comprise a torque converter having an output, the vehicle comprising a second energy sink coupled to the output. This arrangement may allow torque from the road wheels to be coupled to the second energy sink in a shockabsorbing manner via the torque converter. Optionally, the second energy sink may be configured to store kinetic energy received via the output. Optionally, the second energy sink may comprise a flywheel. Optionally, the second energy sink may comprise: an electric machine configured to convert kinetic energy into electrical energy; and an electrical storage system for storing the electrical energy. Optionally, the electrical storage system may comprise a chemical battery and / or a capacitor-based storage system. Optionally, the vehicle may comprise a brake control configured to output a brake signal responsive to user operation of the brake control, wherein the one or more input signals comprise a braking input at least partly based on the brake signal, the braking input being indicative of a force applied to, a displacement of, and / or rate of change of position of, the brake control. Optionally, the control system may be configured to determine that emergency braking is required based at least in part by determining that the braking input is indicative of a braking force, displacement, and / or rate of change of position exceeding a first threshold. Optionally, the control system may be configured to determine the first threshold based on one or more factors comprising one or more of: a speed input indicative of a speed of the vehicle; a mode input indicative of a current vehicle drive mode; a first temperature input indicative of a temperature of one or more components within the first braking system; a second temperature input indicative of a temperature of one or more components within the second braking system; a rotational speed input indicative of a rotational speed of one or more components of the second braking system; and a capacity input indicative of a current capacity of the electrical storage system, if used. According to a further aspect of the present invention there is provided a method for controlling a second braking system of the vehicle of any preceding claim, the method comprising: receiving one or more input signals from the vehicle; determining, based on the one or more input signals, that emergency braking is required; and responsive to the determination that emergency braking is required, outputting a control signal to the second braking system. According to a further aspect of the present invention there are provided computer readable instructions which, when executed by a computer, are arranged to perform a method according to the preceding aspect. According to a further aspect of the present invention there is provided a vehicle comprising: a braking system comprising a first energy sink, the first energy sink comprising a hydraulic shear device having an input couplable to a second subset of the road wheels, the hydraulic shear device being configured to convert at least some of the forward kinetic energy of the vehicle into heat; and a control system for controlling the second braking system, the control system comprising one or more processors collectively configured to: determine that emergency braking is required; and responsive to the determination that emergency braking is required, output a control signal to the second braking system; the second braking system being configured to, responsive to the control signal, couple the second braking system to the second subset of the road wheels, such that kinetic energy of the vehicle is transferred into the first energy sink via rotational movement of the second subset of the road wheels. 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. BRIEF DESCRIPTION OF THE DRAWINGS One or more embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings, in which: Figure 1 is a front perspective view of a vehicle in accordance with an embodiment of the invention; Figure 2 is a schematic representation of the vehicle of Figure 1; Figures 3 to 8 show block diagrams of implementations of second braking systems and other components; Figure 9 shows a block diagram of a control system including examples of input factors; and Figure 10 is a flowchart of a method in accordance with an embodiment of the invention. All Figures are schematic and do not necessarily reflect the relative or absolute positions, or the scale, of the illustrated components. DETAILED DESCRIPTION A vehicle 100 in accordance with an embodiment of the present invention is described herein with reference to the Figures 1 and 2. The vehicle 100 can take the form of a road vehicle, which is a vehicle that is capable of being driven on a road. Non-hmiting examples of such vehicles include cars, SUVs, trucks, vans, and the like. Such vehicles can also have offroad uses. For example, some SUVs and trucks are designed with occasional or even frequent offroad use in mind. As shown in Figure 2, the vehicle 100 includes four road wheels, including rear wheels 102 and front wheels 104. In the implementation of Figure 2, the front wheels 104 are driven by a prime mover, such as an electric machine (not shown) or combustion engine (not shown). The front wheels 104 are steerable under the control of a driver and / or autonomous or semi-autonomous controller (not shown). In other implementations, any other number and arrangement of road wheels can be provided, and any combination of the road wheels can be driven and / or steerable. The vehicle 100 includes a first braking system comprising hydraulically operable disc brakes 106 mounted for selectively braking each of the rear wheels 102 and the front wheels 104. The vehicle 100 comprises a brake control 107 in the form of a user-operable brake pedal 108 mounted within the vehicle 100 for operation by the driver, and a brake controller 110. The brake controller 110 is configured to receive a brake signal 112 from the brake pedal 108, the brake signal 112 being indicative of movement of the brake pedal 108 responsive to driver operation of the brake pedal 108. For example, the brake signal 112 can be indicative of an absolute position of the brake pedal 108, a displacement of the brake pedal 108 from its uppermost position (i.e., the brake pedal’s position when the driver’s foot is not in contact with the brake pedal 108), and / or a force applied to the brake pedal 108. The brake signal 112 can take any suitable form. For example, brake signal 112 can be analogue (for example, a variable voltage) or digital (for example, a binary value). The brake controller 110 is configured to generate a braking input 114 that is at least partly based on the brake signal 112. The brake controller 110 can be configured to generate the braking input 114 by processing the brake signal 112 in any suitable way. Such processing can include filtering (e.g. low pass filtering to remove noise and / or to smooth braking inputs) and / or determining a rate of change of position of brake pedal 108 over time. The braking input 114 can take any suitable form. For example, the braking input 114 can be analogue (for example, a variable voltage) or digital (for example, a binary value), and can be indicative of a force applied to, a displacement of, and / or rate of change of position of, the brake pedal 108, for example. The braking signal 112 and / or the braking input 114 can be used as an input for controlling hydraulic operation of the disc brakes 106 during braking. Vehicle 100 comprises a second braking system 116. The second braking system 116 comprises a first energy sink 118, which in turn comprises a hydraulic shear device 120 having an input in the form of an input shaft 122. The hydraulic shear device 120 comprises housing 134, a series of axially-spaced rotating plates 136 attached to the input shaft 122, and a series of axially-spaced stator plates 138 mounted within the housing 134. A hydraulic fluid 148 fills at least some of the housing 134 such that at least a portion of the rotating plates 136 and stator plates 138 are immersed within the hydraulic fluid 148. The rotating plates 136 are interleaved with the stator plates 138. Spacing between the rotating plates 136 and the stator plates 138 is selected such that, when the second braking system is in operation, hydraulic shear caused by relative movement between the rotating plates 136 and the adjacent stator plates 138 causes significant drag, as described in more detail below. The rotating plates 136 and / or the stator plates 138 can optionally include one or more vanes or other features (not shown) that increase hydraulic shear and / or otherwise increase hydraulic coupling and hence drag between the rotating plates 136 and / or the stator plates 138. The second braking system 116 is couplable to a second subset of the road wheels, which in the case of vehicle 100 includes both rear wheels 102. The input shaft 122 is connected to a differential 124, and the rear wheels 102 are connected to the differential 124 by way of half shafts 126. The vehicle 100 includes a control system 128 for controlling the second braking system 116. The control system 128 shown comprises a controller 129, although it will be appreciated that this is merely illustrative and that the control system 128 may comprise a plurality of controllers 129 which are communicatively coupled to control the braking system 116. The controller 129 comprises processing means 130 in the form of one or more electronic processing devices (such as a microprocessor) that operably execute computer-readable instructions, and memory means 131 in the form of one or more memory devices. The processing means 130 is electrically coupled to memory means 131. The memory means 131 is configured to store instructions, and the processing means 130 is configured to access the memory means 131 and execute the instructions stored thereon. The control system 128 comprises an input means in the form of an electrical input 133 of the controller 129 configured to receive an input signal in the form of the braking input 114 from the brake controller 110. The brake input 114 is updated periodically or continuously, to reflect changing conditions including the value of the brake signal 112. The control system 128 also comprises an output means in the form of an electrical output 135 of the controller 129. Based on at least the braking input 114, the control system 128 determines whether emergency braking is required. This determination can be based on any suitable factor or combination of factors. For example, the control system 128 can determine that emergency braking is required based at least in part by determining that the braking input 114 is indicative of a braking force, displacement, and / or rate of change of position exceeding a first threshold. Where more than one of the braking force, displacement, and / or rate of change of position forms part of the determination, different thresholds can apply to each. The control system 128 can also determine that emergency braking is required based on a current deceleration rate, for example as measured by one or more accelerometers or other sensor(s) (not shown) within the vehicle. For example, if the deceleration exceeds a particular force or a predetermined rate of change of velocity, it can be determined that emergency braking is required. Any or all of the threshold(s) can be predetermined. For example, it can be determined that emergency braking is required if the displacement ofthe brake pedal 108 exceeds a displacement threshold. Such a displacement threshold can be chosen so as to be indicative of unusually hard braking by the driver. Alternatively, or in addition, it can be determined that emergency braking is required if the rate of change of position of the brake pedal 108 exceeds a rate of change threshold. Such a rate of change threshold can be chosen so as to be indicative of unusually quickly applied braking by the driver. Alternatively, the threshold(s) can be variable. For example, at relatively low speeds, the second braking system may offer reduced, or even no, benefit in terms of stopping distance compared to the first braking system. The threshold(s) may therefore be configurable based on the speed at which the vehicle is travelling. In another example, in wet weather or on a loose surface such as gravel, the second braking system may offer reduced, or even no, benefit in terms of stopping distance compared to the first braking system. The threshold(s) may therefore be configurable based on road surface conditions. In yet other implementations, factors such as those described herein can be used to prevent the second braking system from being engaged. For example, if the vehicle’s speed is lower than a threshold, the control system can prevent the second braking system from being engaged. Additionally or alternatively, if the vehicle’s speed is lower than a threshold, the input signal to the control system can be prevented from indicating that emergency braking is required. Figure 9 shows a number of other exemplary factors that can be used in the determination of the threshold(s). These factors are given as examples only, and other factors will suggest themselves to the skilled person. Also, any particular implementation may not necessarily use all of the factors shown in Figure 9. The factors shown in Figure 9 include: a speed input 180 indicative of a speed of the vehicle. The speed input 180 can be provided by way of any suitable vehicle-based sensor, such as a GPS unit (not shown), speedometer (not shown), accelerometer (not shown), or any combination of such sensors. The speed input 180 can also be received from an external source by way of a wireless communication with the vehicle, for example; a mode input 182 indicative of a current vehicle drive mode. The mode input 182 can be provided by way of a manual selector (not shown) operable by the driver, or can be generated automatically by a system (not shown) within the vehicle. Non-exhaustive examples of such drive modes include wet weather mode, allwheel drive, loose surface (e.g., gravel) mode, offroad mode, sand driving mode, eco mode, low emission mode, fault detected mode, and snow driving mode. It will be appreciated that in some implementations, the vehicle does not operate in discrete modes (or at least only operates in a limited number of modes). Instead, data from sensors, controllers, and / or other sources are fed into a control algorithm running on one or more controllers, which adjusts the sorts of settings that would otherwise be controlled via a discrete mode. For example, instead of separate sand, snow and gravel modes, the vehicle may adjust parameters to suit the measured characteristics of the surface on which it is driving, without entering any particular mode. Alternatively, a general low friction mode can be employed, and the particular parameters within that mode adjusted to suit the driving conditions, without necessarily employing specific low friction modes such as sand mode, snow mode, or gravel mode. a first temperature input 184 indicative of a temperature of one or more components within the first braking system. The first temperature input 184 can be indicative of, for example, an estimated or measured temperature of one or more discs, pads, or other components, within a disc brake system. The first temperature being above a threshold may, for example, be indicative of reduced braking capacity of the first braking system. It may therefore be preferable to set a lower threshold(s) for determining that emergency braking is required; a second temperature input 186 indicative of a temperature of one or more components within the second braking system. The second temperature input 186 can be indicative of, for example, an estimated or measured temperature of hydraulic fluid within the hydraulic shear device of the first energy sink. The second temperature being above a threshold may, for example, be indicative of reduced energy sinking capacity of the second braking system. It may therefore be preferable to set higher threshold(s) for determining that emergency braking is required; a rotational speed input 188 indicative of a rotational speed of one or more components of the second braking system. The rotational speed input 188 can be indicative of a rotational speed of a flywheel (such as flywheel 162), carrier (such as carrier 158), electric machine (such as electric machine 166 or 168), shaft or other component of the second braking system. The rotational speed being above a threshold may, for example, be indicative of a reduced energy sinking capacity of the second braking system. It may therefore be preferable to set higher threshold(s) for determining that emergency braking is required; and a capacity input 190 indicative of a current capacity of the electrical storage system, if used. The capacity input 190 can be indicative of the remaining capacity available in the electrical storage system (such as battery pack 172). The capacity being above a threshold may indicate a reduced energy sinking capacity of the second braking system. It may therefore be preferable to set a higher threshold(s) for determining that emergency braking is required. Although the various inputs 180-190 are shown as being input to the control system 128 to determine the first threshold, in other implementations, they may be input to another controller (not shown), which interprets them and outputs to the control system 128 a signal indicative of (or for use in determining) the first threshold. In yet other implementations, instead of modifying a threshold, any of the described or other factors can be used to filter whether the second braking system is used when emergency braking is required, and / or to modulate the amount of braking that the second braking system applies. Once the control system 128 has determined that emergency braking is required, it outputs a control signal 132 to the second braking system 116 by way of the electrical output means 135. “Outputs a control signal” in this context can include changing a value of the control signal. The control signal 132 can take any suitable form. For example, the control signal 132 can be analogue (for example, a variable voltage) or digital (for example, a binary value). In the implementation of Figure 2, the control signal 132 is a binary value, corresponding to whether the second braking system 116 is to be engaged. In other implementations, the control signal can represent a variable related to an amount by which the second braking system 116 is to be engaged. Such a variable can be useful when the second braking system 116 itself is controllable in a continuous or at least multi-level manner. Responsive to the control signal 132 indicating that emergency braking is required, the second braking system 116 is coupled to the rear wheels 102 via the differential 124 and half shafts 126. The coupling can take any suitable form, many particular implementations of which are described below in relation to Figures 3 to 8. As an example, the control signal 132 can control a clutch or other torque transmission mechanism (not shown) disposed between the first energy sink 118 and the rear wheels 102. Alternatively, the road wheels can be permanently linked to the second braking system 116, and the coupling can take the form of engaging or otherwise causing the second brake system 116 to operate. Once the second braking system 116 is coupled to the rear wheels 102 as a result of the control signal 132, rotational movement of the rear wheels 102 causes corresponding rotation of the half shafts 126, which drive rotation of the input shaft 122 through the differential 124. The rotating plates 136 are rotated on input shaft 122, causing hydraulic shear between the rotating plates 136 and the adjacent stator plates 138. The hydraulic shear causes significant drag, which brakes the input shaft 122, and hence the rear wheels 102. In this way, kinetic energy of the vehicle 100 is converted into heat within the first energy sink 118. The first braking system (i.e., disc brakes 106) can be operated alongside the second braking system 116. In some implementations, the second braking system 116 is configured to be either on or off, and cannot modulate the amount of braking it provides. In that case, first braking system can contribute significantly to the total braking power, but can also modulate the braking power underthe control ofthe driver and / or the vehicle’s systems. For example, the first braking system can comprise an anti-lock braking function, which can operate to modulate the braking of each individual wheel, taking into account the braking power provided by the second braking system 116. Although the rear wheels 102 are couplable to the second braking system 116 in the implementation of Figure 2, in other implementations, any other subset of the road wheels can be couplable to the second braking system, alternatively or in addition to the first braking system. For example, due to forward transfer ofthe vehicle’s mass during hard braking, the rear wheels 102 can offer less grip than the front wheels 104. It may therefore be desirable in certain implementations to couple the front wheels 104 to the second braking system 116. In other implementations, both the front wheels 104 and the rear wheels 102 can be coupled to the second braking system 116. In yet other implementations, more than one second braking system can be provided, each being couplable to any one or more ofthe road wheels. Where more than one second braking system is employed, they may be of different types. In the implementation of Figure 2, a prime mover (not shown) drives the front wheels 104. In other implementations, differential 124, and hence the rear wheels 102, can be driven by a prime mover. This can be as an alternative, or in addition, to the driving ofthe front wheels 104 by the prime mover. In yet other implementations, different prime movers drive one or more ofthe front and / or rear wheels. Turning to Figure 3, there is shown a schematic of a further implementation of a second braking system 216, illustrated in the context of several other components. Several of the components in the Figure 3 implementation correspond with those described in relation to the Figure 2 implementation, and corresponding reference signs are used to indicate those components in both implementations. Figure 3 shows an example of how the first energy sink 118 and the rear wheels 102 can be coupled when emergency braking is required. A clutch 140 is disposed between the differential 124 and the first energy sink 118. An input shaft 142 of clutch 140 is coupled to the differential 124 and an output shaft 144 of the clutch 140 is connected to the input shaft 122 of the hydraulic shear device 120. The clutch 140 comprises an actuator (not shown) that is responsive to the control signal 132. The clutch 140 can take any suitable form, including a dry or wet clutch. In ordinary driving, when emergency braking is not required, the clutch 140 is not engaged. The input shaft 142 is driven by the differential 124, but the output shaft 144 remains undriven due to the clutch 140 not being engaged. When the control system 128 determines that emergency braking is required and outputs the corresponding control signal 132 to the actuator, the clutch 140 engages, causing the input shaft 142 to drive the output shaft 144. This transfers torque from the differential 124 to the input shaft 122 of the hydraulic shear device 120. Turning to Figure 4, there is shown a schematic of a further implementation of a second braking system 316, illustrated in the context of several other components. Several of the components in the Figure 4 implementation correspond with those described in relation to the Figures 2 and 3 implementations, and corresponding reference signs are used to indicate those components in all implementations. Figure 4 shows an example of the integration of a prime mover in the form of an electric machine in the form of a brushless DC motor 146 disposed between the differential 124 and the clutch 140. The motor 146 is powered by a battery pack (not shown) under the control of drive / control circuitry (not shown). The motor 146 is arranged such that the input shaft 142 of the clutch 140 is also the drive shaft of the motor 146. In ordinary driving, when emergency braking is not required, the clutch 140 is not engaged. The motor 146 can drive the rear wheels 102 via the input shaft 142 and the differential 124, and conversely, can be operated in a regenerative mode to convert kinetic energy of the vehicle 100 into electrical current that can be used to recharge the battery pack. The output shaft 144 remains undriven due to the clutch 140 not being engaged. When the control system 128 determines that emergency braking is required and outputs the corresponding control signal 132 to the actuator, the clutch 140 engages, causing the input shaft 142 to drive the output shaft 144. This transfers torque from the differential 124 to the input shaft 122 of the hydraulic shear device 120, for example as described above in relation to Figure 2. The motor 146 can optionally be used for regeneration during emergency braking, and can optionally be controlled to modulate the amount of regeneration (and hence the total braking through the rear wheels 102) so as to maintain traction. Turning to Figure 5, there is shown a schematic of a further implementation of a second braking system 416, illustrated in the context of several other components. Several of the components in the Figure 5 implementation correspond with those described in relation to the Figures 2, 3, and 4 implementations, and corresponding reference signs are used to indicate those components in all implementations. In Figure 5, the hydraulic fluid 148 takes the form of a magnetorheological fluid, the viscosity of which increases when a magnetic field is applied to it. The hydraulic shear device 120 includes a magnetic field generator 150, which is arranged and configured such that it can apply a magnetic field to at least some of the hydraulic fluid 148. The magnetic field generator 150 includes an input for receiving the control signal 132. The spacing of the rotating plates 136 and the stator plates 138 in second braking system 416 is wider than that in second braking systems 116, 216, and 316, such that drag between the rotating plates 136 and the stator plates 138 is minimal when the magnetic field generator 150 is not operative. In ordinary driving, when emergency braking is not required, the magnetic field generator 150 does notoperate. The input shaft 122 causes the rotating plates 136 to rotate within the housing 134, but because the magnetic field generator 150 is not operating, the viscosity of the magnetorheological fluid is relatively low. When the control system 128 determines that emergency braking is required and outputs the control signal 132, magnetic field generator 150 responds to the control signal 132 by generating a magnetic field across at least some of the magnetorheological fluid. The magnetic field causes the viscosity of the magnetorheological fluid to increase significantly, which correspondingly increases the shear force between the rotating plates 136 and the stator plates 138. This brakes the input shaft 122, and hence the rear wheels 102. In this way, kinetic energy of the vehicle 100 is converted into heat within the first energy sink 118. Optionally, the magnetic field generator 150 is configured such that the magnetic field strength, and / or the volume of the magnetorheological field across which the magnetic field is applied, can be varied. This variation can be responsive to a value of the control signal 132, and / or can be varied independently by control / drive circuitry (not shown) that controls / drives the magnetic field generator. Optionally, a clutch similar to the clutch 140 can be positioned between the second braking system 416 and the differential 124. This arrangement reduces the losses within the second braking system 416 in the arrangement of Figure 5 during ordinary driving. In an alternative implementation of the arrangement of Figure 5, the hydraulic fluid 148 can take the form of an electrorheological fluid, and the magnetic field generator 150 can be replaced by an electrical field generator. The operation of such an arrangement is similar to that of second braking system 416, except that an electrical (rather than magnetic) field is generated when emergency braking is required. Turning to Figure 6, there is shown a schematic of a further implementation of a second braking system 516, illustrated in the context of several other components. Several of the components in the Figure 6 implementation correspond with those described in relation to the Figures 2, 3, 4, and 5 implementations, and corresponding reference signs are used to indicate those components in all implementations. In Figure 6, the second braking system 516 comprises the first energy sink 118 and a second energy sink 152. The hydraulic shear device of the first energy sink 118 takes the form of a torque converter 154. The input shaft 122 and the rotating plates 136 are similar to those of the second braking system 116, 216, 316, and 416. However, instead of having stator plates 138, the torque converter 154 has a series of axially-spaced plates 156 that extend radially inwardly from a carrier 158 that is mounted for rotation relative to the housing 134. The carrier 158 is coupled to an output in the form of an output shaft 160. The plates 156 are interleaved with the rotating plates 136. Spacing between the plates 156 and the rotating plates 136 is selected such that, when the second braking system is in operation, hydraulic shear caused by relative movement between the plates 156 and the adjacent rotating plates 136 causes significant drag. The hydraulic shear converts some of the kinetic energy into heat, and also causes coupling between the input shaft 122 and the output shaft 160, as described in more detail below. The output shaft 160 is connected to the second energy sink 152. The second energy sink 152 can take any suitable form, examples of which are described below with reference to Figures 7 and 8. In ordinary driving, when emergency braking is not required, the second braking system 516 is not coupled to the rear wheels 102. Once the control system 128 has determined that emergency braking is required, it outputs the control signal 132 to the second braking system 116. First energy sink 118 is coupled to the rear wheels 102, for example as described above in relation to the implementation of Figure 2. Rotation of the input shaft 122 causes hydraulic shear as a result of relative movement between the rotating plates 136 and the plates 156. The hydraulic shear converts some of the kinetic energy into heat, and also causes torque to be applied to the carrier 158, and hence to the output shaft 160. The resultant rotation of the output shaft 160 transfers torque into the second energy sink 152, where it is converted into heat or another form of energy such as electricity or rotational kinetic energy. The second energy sink can take any suitable form. For example, in Figure 7 there is shown a schematic of a further implementation of a second braking system 616, illustrated in the context of several other components. Several of the components in the Figure 7 implementation correspond with those described in relation to the Figures 2, 3, 4, 5, and 6 implementations, and corresponding reference signs are used to indicate those components in all implementations. In Figure 7, the second energy sink 152 comprises a flywheel 162 mounted for rotation within a housing 164. The flywheel 162 is driven by the output shaft 160, either directly or by way of a gearbox (not shown). The flywheel 162 stores kinetic energy as it accelerates responsive to the torque provided by the output shaft 160 of the first energy sink 118. The flywheel 162 can optionally be configured to convert some of its rotational kinetic energy into another form of energy. This conversion can happen continuously during the emergency braking, and / or after the emergency braking has stopped. For example, the flywheel 162 can be formed from one or more discs (not shown) of conductive material arranged to spin adjacent to one or more magnets (not shown), thereby forming an eddy current brake. Alternatively, or in addition, the flywheel 162 can be at least partially immersed in a fluid and can optionally include one or more vanes or other drag-increasing features (not shown) that interact with the fluid to cause hydraulic shear. The hydraulic shear causes drag, which in turn converts rotational kinetic energy of the flywheel 162 into heat that can be dispersed within the hydraulic fluid. Optional stator features (not shown) inside the housing 164 can interact with the flywheel 162 to increase hydraulic shear and / or otherwise increase hydraulic coupling and hence drag between the flywheel 162 and the housing 164. As shown in Figure 7, the flywheel 162 can optionally be coupled (or couplable) to an electric machine 166. This can be the same electric machine (e.g., electric machine 146) that is used to drive one or more road wheels 102, 104 of the vehicle 100, in which case the electricity generated by the flywheel 162 can be converted to electricity that can be used to charge a battery pack 172, such as a chemical battery and / or capacitor-based storage. If the flywheel 162 is directly or permanently coupled to the electric machine 166, the electric machine 166 can be controlled so as to modulate the amount of electricity generated, and hence the total amount of energy converted by the second braking system 116. Alternatively, or in addition, the flywheel 162 can be selectively coupled to the electric machine 166 by way of a further clutch (not shown), which can take any suitable form, including any of types described above. In such an implementation, the further clutch can be controlled by the control system 128 such that it selectively couples the flywheel 162 to the electric machine 166 as required. Turning to Figure 8, there is shown a schematic of a further implementation of a second braking system 616, illustrated in the context of several other components. Several of the components in the Figure 6 implementation correspond with those described in relation to the Figures 2, 3, 4, and 5 implementations, and corresponding reference signs are used to indicate those components in all implementations. In the implementation of Figure 8, the second energy sink 152 comprises an electrical storage system comprising an electric machine 168, charging circuitry 170, and a battery pack 172. The battery pack 172 can include chemical battery storage cells such as lithium-based rechargeable batteries, and / or capacitor-based storage, for example. Other battery types and / or electrical storage technologies can also be used. The electric machine 168 is coupled to be driven by the output shaft 160 of the torque converter 154. An electrical output of the electric machine 168 is electrically coupled to the charging circuitry 170, an electrical output of which is coupled to the battery pack 172. The charging circuitry 170 is configured to convert the electrical output of the electric machine 168 into a form usable to charge the battery pack 172. The electric machine 168 can optionally be the same electric machine (e.g., electric machine 146) that is used to drive one or more road wheels 102, 104 of the vehicle 100. If the output shaft 160 of the torque converter 154 is permanently or directly coupled to the electric machine 168, the electric machine 168 and charging circuitry 170 can be controlled so as to modulate the amount of electricity generated, and hence the total amount of energy converted by the second braking system 116. Alternatively, or in addition, the output shaft 160 can be selectively coupled to the electric machine 168 by way of a further clutch (not shown), which can take any suitable form, including any of types described above. The electric machine 168 can operate only as means for converting kinetic energy into electrical energy for storage in the battery pack 170 via the second braking system 716. Optionally, it can also operate as a prime mover of the vehicle 100 (as per the description of electric machine 146) and / or be operable to provide regenerative braking during ordinary driving, such as when the second braking system is not being used for emergency braking. Operation of the charging circuitry 168 is controlled by the control system 128, although in other implementations operation of the charging circuitry 168 can also, or alternatively, be controlled by another controller (not shown) within the vehicle 100. In ordinary driving, when emergency braking is not required, the second braking system 716 is not coupled to the rear wheels 102. Once the control system 128 has determined that emergency braking is required, it outputs the control signal 132 to the second braking system 716. First energy sink 118 is coupled to the rear wheels 102, for example as described above in relation to the implementation of Figure 2. Rotation of the input shaft 122 causes hydraulic shear as a result of relative movement between the rotating plates 136 and the plates 156. The hydraulic shear converts some of the kinetic energy into heat, and also causes torque to be applied to the carrier 158, and hence the output shaft 160. The resultant rotation of the output shaft 160 transfers torque to the electric machine 168, which generates electricity via the charging circuitry 170 for storage in the battery pack 172 as described above. Together, the first energy sink 119 and the second energy sink 152 can be configured to absorb more energy than would be possible with just the first energy sink 119. The hydraulic shear device 120 in the first energy sink 118 can also provide a shock-absorbing function that protects the second energy sink 152 and intermediate drive components from the sudden application of torque when emergency braking is initiated. When the second braking system comprises a torque converter (such as torque converter 154), the hydraulic fluid within the torque converter can optionally comprise a magnetorheological or electrorheological fluid. In a similar manner to that described above in relation to Figure 5, a suitable magnetic or electric field generator can be disposed so as to apply a magnetic or electrical field across at least some of the magnetorheological or electrorheological fluid responsive to the control signal 132 being indicative of emergency braking being required. When the second braking system comprises a torque converter (such as torque converter 154), the torque converter can be configured to absorb an average and / or maximum power, which may be based at least in part on a mass of the vehicle and / or the amount of forward kinetic energy that may need to be converted into heat in the event emergency braking is required. The remainder of the power input to the torque converter is passed through to the second energy sink. Optionally, the torque converter can be configured to maximise the energy absorbed (for a given set of packaging or other design constraints and / or requirements of the torque converter, for example) with the remainder being passed to the second energy sink. Alternatively, if the torque converter is configured to absorb energy in a controlled variable manner (e.g., under the control of a controller within the vehicle), the amount of energy it absorbs can be varied, for example in dependence on the current energy absorption capacity of the second energy sink. The energy absorption capacity can be measured in any suitable way, depending upon the form in which it sinks energy. For example, if the second energy sink is a chemical battery, the voltage of the battery can be determined to indicate its remaining capacity, although temperature can also be used as a factor (e.g., if the battery is too hot, it may be desirable to reduce or stop further charging). For a mechanical sink such as a flywheel, the rate of rotation can indicate how much capacity is available. The skilled person will appreciate that there are other ways of measuring energy sink capacity. Optionally, where the vehicle includes a controller for allowing autonomous or semi-autonomous braking, one of the inputs to the control system 128 can be a signal from that controller. For example, where the vehicle is equipped with one or more sensors, such as lidar sensor(s), radar sensor(s), ultrasonic sensor(s), and visual sensor(s) such as camera(s) that can detect that the vehicle is approaching an obstacle, the information from such sensor(s) can be processed to determine whether braking is required. If so, but the required level of braking is not considered to be an emergency, then the first braking system can be controlled to brake the car. However, if it is determined that emergency braking is required, then the control system 128 is provided with a suitable input, and the second braking system (optionally in conjunction with the first braking system) is employed to brake the vehicle. The second braking system can be configured such that it is used only for emergency braking. That is, the second braking system is not engaged unless an emergency braking situation is identified, for example as described above. Optionally, the second braking system can also be used in non-emergency situations. For example, the second braking system can be usable to assist the first braking system, particularly in scenarios when periods of long and / or hard braking are detected. For example, where the vehicle is being driven and braked aggressively over a period of time, or is driving down a long descent, the disc brakes 106 of the first braking system can be at risk of overheating, which can reduce braking power. The second braking system can be configured to provide supplementary braking in such a scenario. Although described in the context of a vehicle under the control of a driver, it will be appreciated that embodiments of the invention have application in autonomous, semi-autonomous, and driver-assisted vehicles. For example, where the vehicle is configured with one or more sensors to determine its surroundings and one or more systems for determining when braking is required based on information from sensor(s), the input signal to the control system can include an input signal based on information from the sensors. That input signal can take the form of an emergency braking signal, in which case the second braking system can be engaged, for example as described above. Alternatively, the signal can take the form of information including any of the factors described above, optionally including information from, or based on information from, the sensor(s). The control system can then determine, based on those mput(s), that emergency braking is required, in which case the second braking system can be engaged, for example as described above. Wherever shafts are referred to, it will be appreciated that alternative and / or additional drive mechanisms may be employed to transfer torque between components, such drive mechanisms including, but not being limited to, chain drive mechanisms, gear drive mechanisms, fluid drive mechanisms, transmission systems, and the like. Turning to Figure 10, there is shown a flowchart illustrating a method 192 for controlling a second braking system, such as the second braking system 116, according to an embodiment of the invention. The method 192 can be implemented within a vehicle, such as the vehicle 100 illustrated in Figure 1. The method 192 may be performed by the control system 128 illustrated in Figure 2. In particular, the memory means 131 may comprise computer-readable instructions that, when executed by the processing means 130, perform the method 192 according to an embodiment of the invention. The method 192 includes receiving 194 one or more input signals from the vehicle. The one or more input signals can include, for example, the braking input 114. The method includes determining 196, based on the one or more input signals, that emergency braking is required, and responsive to the determination that emergency braking is required, outputting 198 a control signal (such as control signal 132) to the second braking system. 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

1. A vehicle comprising:a plurality of road wheels ;a first braking system for selectively braking a first subset of the road wheels;a second braking system comprising a first energy sink, the first energy sink comprising a hydraulic shear device having an input couplable to a second subset of the road wheels, the hydraulic shear device being configured to convert at least some forward kinetic energy of the vehicle into heat; anda control system for controlling the second braking system, the control system comprising one or more processors collectively configured to:receive one or more input signals from the vehicle;determine, based at least partly on the one or more input signals, that emergency braking is required; andresponsive to the determination that emergency braking is required, output a control signal to the second braking system;the second braking system being configured to, responsive to the control signal, couple the second braking system to the second subset of the road wheels, such that kinetic energy of the vehicle is transferred into the first energy sink via rotational movement of the second subset of the road wheels.

2. The vehicle of claim 1, wherein the control system is configured such that the second braking system is used only when emergency braking is required.

3. The vehicle of any preceding claim, wherein the hydraulic shear device comprises an electrorheological and / or magnetorheological fluid, and a field generator for generating an electrical and / or magnetic field responsive to which the electrorheological and / or magnetorheological fluid changes its viscosity.

4. The vehicle of claim 3, wherein the field generator comprises an input for receiving a control signal from the control system.

5. The vehicle of any preceding claim, wherein the hydraulic shear device comprises a torque converter having an output, the vehicle comprising a second energy sink coupled to the output.

6. The vehicle of claim 5, wherein the second energy sink is configured to store kinetic energy received via the output.

7. The vehicle of claim 6, wherein the second energy sink comprises a flywheel.

8. The vehicle of any one of claims 4 to 7, wherein the second energy sink comprises:an electric machine configured to convert kinetic energy into electrical energy; andan electrical storage system for storing the electrical energy.

9. The vehicle of claim 8, wherein the electrical storage system comprises a chemical battery and / or a capacitor-based storage system.

10. The vehicle of any preceding claim, comprising a brake control configured to output a brake signal responsive to user operation of the brake control, wherein the one or more input signals comprise a braking input at least partly based on the brake signal, the braking input being indicative of a force applied to, a displacement of, and / or rate of change of position of, the brake control.

11. The vehicle of claim 10, wherein the control system is configured to determine that emergency braking is required based at least in part by determining that the braking input is indicative of a braking force, displacement, and / or rate of change of position exceeding a first threshold.

12. The vehicle of claim 11, wherein the control system is configured to determine the first threshold based on one or more factors comprising one or more of:a speed input indicative of a speed of the vehicle;a mode input indicative of a current vehicle drive mode;a first temperature input indicative of a temperature of one or more components within the first braking system;a second temperature input indicative of a temperature of one or more components within the second braking system;a rotational speed input indicative of a rotational speed of one or more components of the second braking system; anda capacity input indicative of a current capacity of the electrical storage system, if used.

13. A method for controlling a second braking system of the vehicle of any preceding claim, the method comprising:receiving one or more input signals from the vehicle;determining, based on the one or more input signals, that emergency braking is required; and responsive to the determination that emergency braking is required, outputting a control signal to the second braking system.

14. Computer readable instructions which, when executed by a computer, are arranged to perform a method according to claim 13.

Images