WO2019053025A1

WO2019053025A1 - System and method for a trailer towable by a vehicle

Info

Publication number: WO2019053025A1
Application number: PCT/EP2018/074514
Authority: WO
Inventors: Jeremy Greenwood
Original assignee: Jaguar Land Rover Limited
Priority date: 2017-09-15
Filing date: 2018-09-11
Publication date: 2019-03-21
Also published as: GB201714858D0; DE112018005128T5; GB2566494A

Abstract

The present invention provides a system for use in a trailer towable by a vehicle. The trailer comprises an axle having two wheels and at least one electric motor coupled thereto. The system comprises receiving means configured to receive a surface signal indicative of at least one feature of a surface in the vicinity of the vehicle and/or the trailer. The system also comprises processing means configured to determine, for each of the wheels of the axle of the trailer, a trailer wheel drive force value based on the received surface signal. The system also comprises control means configured to cause the at least one electric motor to apply a force to the or each wheel to which it is coupled in dependence on the determined trailer wheel drive force value.

Description

SYSTEM AND METHOD FOR A TRAILER TOWABLE BY A VEHICLE

TECHNICAL FIELD The present disclosure relates to a system and method for a trailer towable by a vehicle. Aspects of the invention relate to a system, to a method, and to a trailer.

BACKGROUND It is common for a vehicle having motive power means, such as an internal combustion engine, that provides motive power to its wheels to also provide motive power to the wheels of a trailer. This may be in the context of a car towing a caravan or horse box, for example. Trailers may be provided with a mechanical over-run sensor that applies mechanically- coupled friction brakes of the trailer in response to braking, or a decrease in acceleration, of the vehicle. This assists the vehicle with reducing the speed of the trailer as appropriate. Such a mechanical over-run sensor operates only when the vehicle and trailer are moving in a forward direction. This means that a vehicle that is reversing, particularly downhill, or at rest on an uphill gradient, will receive no braking assistance from a trailer with a mechanical over-run sensor.

As the nature of the braking is mechanical as determined by the over-run trailer compared with the tow vehicle of the sensor, then the trailer will apply a destabilising force to the vehicle under braking. This can also result in swaying of the trailer behind the vehicle.

Also, as vehicles become lighter, with increasing numbers of vehicles being provided with motive power at least partially by means of a battery or other energy storage means, it may be the case that such vehicles lack sufficient power or traction to tow a trailer.

It is an aim of the present invention to address one or more disadvantages associated with the prior art. SUMMARY OF THE INVENTION

According to an aspect of the present invention there is provided a system for use in a trailer towable by a vehicle. The trailer comprises an axle having two wheels and at least one electric motor coupled thereto. The system comprises receiving means configured to receive a surface signal indicative of at least one feature of a surface in the vicinity of the vehicle and/or the trailer. The system also comprises processing means configured to determine, for each of the wheels of the axle of the trailer, a trailer wheel drive force value based on the received surface signal. The system also comprises control means configured to cause the at least one electric motor to apply a force to the wheel to which it is coupled in dependence on the determined trailer wheel drive force value. The present invention is advantageous in that it addresses the problem that a trailer can reduce the traction of a vehicle when the vehicle tows the trailer. In particular, in certain situations the tow vehicle may have difficulty in pulling away, or even be unable to pull away, from stationary when a trailer is attached thereto. This may be particularly problematic for a vehicle with a relatively low-powered engine, a hybrid-electric vehicle or an electric vehicle, for example. However, this may be problematic even for a larger vehicle, such as a four-wheel drive vehicle with a relatively large engine, especially in conditions in which the vehicle is on a relatively slippery surface such as wet grass, snow or mud, for example, or where the vehicle is attempting to pull away on a relatively steep gradient. This problem, caused by a general trend in vehicles moving towards smaller engines (for reason of emissions) and taller gearing, is particularly apparent for a vehicle attempting to pull away on a slope because of a lack of available torque from the smaller engine, especially if transmitted through a clutch. The present invention is also advantageous in that it can assist in increasing the speed of a vehicle- trailer combination when a driver of the vehicle demands acceleration of the combination via an accelerator pedal for example. These advantageous effects are achieved by providing drive torque to the wheels of the trailer, via one or more electric machines, in addition to the drive torque provided to the vehicle wheels via a vehicle engine or vehicle energy storage means (e.g. a battery). This may increase the overall drive torque provided to the combination, or simply provide a better distribution of drive torque so as to allow the combination to pull away and/or accelerate.

The processing means may comprise an electronic processor and the receiving means may be an electrical input of the electronic processor, the electrical input being for receiving the surface signal. The system may comprise an electronic memory device electrically coupled to the electronic processor and having instructions stored therein, the processor being configured to access the memory device and execute the instructions stored therein such that it is operable to determine, for each of the wheels of the axle of the trailer, a trailer wheel drive force value based on the received surface signal.

The received surface signal may include a signal from a terrain adjustment subsystem of the vehicle, optionally a Terrain Response®.

The at least one feature of the surface may include an indication of the type of surface terrain.

The at least one feature of the surface may include a level of friction of the surface.

The determined trailer wheel drive force value may be such that the force applied by the at least one electric motor to the wheels is non-zero if the level of friction of the surface is less than a friction threshold value. The at least one feature of the surface may include a gradient of the surface.

The determined trailer wheel drive force value may be non-zero if the gradient of the surface is greater than a gradient threshold value. The receiving means may be configured to receive a speed signal indicative of a speed of the vehicle and/or trailer, and the determined trailer wheel drive force value may be such that the force applied by the at least one electric motor to the wheels is non-zero if the speed of the vehicle and/or trailer is less than or equal to a speed threshold value.

The speed signal may be received from a subsystem of the vehicle. Each wheel of the at least one axle of the trailer may have an associated trailer wheel speed sensor configured to measure a wheel speed of the associated wheel, and the speed signal may be received from the trailer wheel speed sensors.

The receiving means may be configured to receive a signal indicating that movement of the vehicle and/or trailer is being initiated, and the determined trailer wheel drive force value may be such that the force applied by the at least one electric motor to the wheels is applied when movement of the vehicle and/or trailer is being initiated.

The receiving means may be configured to receive the signal indicating that movement of the vehicle and/or trailer is being initiated from the vehicle.

The signal indicating that movement of the vehicle and/or trailer is being initiated may include a vehicle drive force signal indicative of a drive force to be applied to one or more wheels of the vehicle by drive means of the vehicle.

The trailer may comprise a coupling sensor configured to measure a coupling force exerted on the trailer by the vehicle, the receiving means may be configured to receive the signal indicating that movement of the vehicle and/or trailer is being initiated from the coupling sensor.

The trailer may be coupled to the vehicle by means of a mechanical hitch joint, the coupling force being a force at the mechanical hitch joint.

The coupling force may include a magnitude component and a direction component.

The force applied by the at least one electric motor to a first one of the wheels may be different from the force applied to a second one of the wheels.

The system may comprise the at least one electric motor coupled to the wheels of the trailer.

The system may comprise a first electric motor coupled to a first one of the wheels and a second electric motor coupled to a second one of the wheels. The system may comprise energy storage means configured to supply electric power to the at least one electric motor.

According to another aspect of the present invention there is provided a method for use in a trailer towable by a vehicle. The trailer comprises an axle having two wheels and at least one electric motor coupled thereto. The method comprises receiving a surface signal indicative of at least one feature of a surface in the vicinity of the vehicle and/or the trailer. The method also comprises determining, for each of the wheels of the axle of the trailer, a trailer wheel drive force value based on the received surface signal. The method also comprises causing the at least one electric motor to apply a force to the or each wheel to which it is coupled in dependence on the determined trailer wheel drive force value.

According to another aspect of the invention there is provided a trailer for being towed by a vehicle. The trailer comprises an axle having two wheels and at least one electric motor coupled thereto. The trailer comprises a system as described above.

According to another aspect of the invention there is provided a non-transitory, computer-readable storage medium storing instructions thereon that when executed by one or more processors causes the one or more processors to carry out the method described above.

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 schematic plan view of a trailer towed by a vehicle, the trailer having a system according to an embodiment of an aspect of the invention, and showing the inputs to, and outputs from, the system; and,

Figure 2 shows the steps carried out by the system of Figure 1 in a method according to an embodiment of an aspect of the invention.

DETAILED DESCRIPTION

The present invention provides a system for a trailer towable by a vehicle via a hitch. The trailer has at least one axle having two wheels, and each wheel has an electric motor attached thereto. The system is arranged to receive a surface signal indicative of at least one feature of a surface in the vicinity of the vehicle and/or the trailer. The system determines a trailer drive force value for each of the wheels of the trailer in dependence on the received surface signal, and the system then commands the electric motors to apply a force to the respective wheel of the trailer in dependence on the determined trailer drive force value.

Figure 1 shows a schematic plan view of a trailer 10 towed by a vehicle 12. The trailer 10 is connected to the vehicle 12 by means of a hitch coupling 14. The hitch coupling 14 is in the form of a mechanical hitch joint. The hitch 14 comprises a hitch or coupling sensor 15 configured to measure a coupling force exerted on the trailer 10 by the vehicle 12. In particular, the coupling sensor 15 is arranged to measure both over-run and under-run of the trailer 10 on the tow vehicle 12. Over-run occurs when the trailer 10 applies a force in the forward direction to the vehicle 12, for example when the driver of the vehicle 12 applies the vehicle brakes when the trailer 10 and vehicle 12 are travelling in a forwards direction. Under-run occurs when the trailer 10 applies a force in the backward direction to the vehicle 12, for example when the driver of the vehicle 12 applies the vehicle brakes when the trailer 10 and vehicle 12 are reversing. In the described embodiment, the vehicle 12 is a car having a plurality of on-board vehicle control subsystems 16. The vehicle control subsystems 16 also include a vehicle Terrain Response® (TR) system 18. The TR system 18 has a controller operable to adjust the settings of various components and subsystems of the vehicle 12 in dependence on the type of terrain over which the vehicle 12 is travelling. These components and subsystems may include the vehicle suspension, vehicle ride height, anti-lock braking response time, transmission shift schedule, throttle response etc. The driver may input the type of terrain over which the vehicle 12 is travelling into the TR system 18, or the TR system 18 may determine the type of terrain automatically. One or more features of the terrain on which the vehicle 12 and trailer 10 rest is communicated to the trailer 10 via a surface signal 20. For example, the surface signal 20 can include information as to the type of terrain, and/or the friction of the surface, over which the vehicle 12 is travelling. The types of terrain may include tarmac, mud- ruts, grass-gravel-snow, rock crawl and sand.

The vehicle control subsystems 16 also include an electronic stability control system (SCS) 22. The SCS 22 monitors the rotational speed of each of the wheels of the vehicle 12 via individual wheel speed sensors. If the speed of one of the wheels is significantly less than the other wheels under braking of the vehicle 12 then the SCS 22, by means of a controller of the SCS 22, acts to reduce the braking pressure applied to that wheel so as to guard against locking of the vehicle wheels, which can result in skidding and a loss of control of the vehicle 12. The SCS 22 receives data from a plurality of vehicle sensors. For example, the SCS 22 may receive data from a longitudinal acceleration sensor of the vehicle 12 which provides information relating to, inter alia, road pitch. That is, the longitudinal acceleration sensor can provide data relating to the gradient over which the vehicle 12 (and trailer 10) is resting or travelling. The SCS 22 can also generate the surface signal 20 and send the signal 20 to the trailer 10 with information relating to the gradient of the surface over which the vehicle 12 is resting. Also, as mentioned above, the SCS 22 receives speed data from individual wheel speed sensors of the vehicle 12. The SCS 22 can also generate a speed signal 28 and send the signal 28 to the trailer 10 with information relating to the speed of the vehicle 12. The vehicle control subsystems 16 also include an engine control unit (ECU) 24. The ECU 24 receives data from, inter alia, throttle position sensors and controls various aspects of the vehicle engine based on the received sensor data, e.g. air/fuel ratio. The throttle position sensors can provide data indicative that movement of the vehicle 12 is being initiated, for example moving off from rest or increasing the speed of the vehicle 12. The ECU 24 can communicate this throttle position sensor data to the trailer 10 in the form of a vehicle drive force signal 25.

In the described embodiment, the trailer 10 has two wheels 30, 32, one on each side of the trailer 10, and connected by an axle 34. Each of the trailer wheels 30, 32 has an electric motor/generator or machines 36, 38 attached thereto. The electric machines 36, 38 are connected to the axle 34; however, they may be connected to the trailer 10 differently in different embodiments, for example via trailer wheel hub or frame connections. The electric machines 36, 38 are connected to, and powered by, a high- power, low-capacity battery 39 of the trailer 10. The electric machines 36, 38 may be used to apply regenerative braking or tractive torque to the wheels 30, 32 of the trailer 10. In the described embodiment, the electric machines are configured to freewheel during normal driving of the trailer 10 and vehicle 12. The battery 39 is a lithium- titanate rechargeable battery: this type of battery is advantageous because of its fast charging time compared with other types of lithium-ion batteries.

Each of the trailer wheels 30, 32 also has a wheel speed sensor 40, 42 which measures the rotational speed of the respective wheel 30, 32. The trailer 10 also includes a yaw sensor 44 in the form of an accelerometer that is configured to detect the yaw, i.e. the angular velocity about a vertical axis, of the trailer 10.

The trailer 10 includes an electronic control unit or system (ECU) 50 configured to control the operation of the electric machines 36, 38. In particular, the ECU 50 comprises a receiver or receiving means 52, a processor or processing means 54, and a controller or control means 56.

The receiver 52 is configured to receive electronic signals from components and subsystems of both the trailer 10 and the vehicle 12. Specifically, the receiver 52 is configured to receive the vehicle surface signal 20 and the drive force signal 25 from the vehicle 12. Also, the receiver 52 is configured to receive a coupling signal 58 from the coupling sensor 15 indicative of a force exerted on the trailer 12 by the vehicle 10. In addition, the receiver 52 is configured to receive a trailer yaw signal 60 from the trailer yaw sensor 44 indicative of a current level of yaw of the trailer 10. Furthermore, the receiver 52 is configured to receive trailer wheel speed sensor output data 62, 64 from the trailer wheel speed sensors 40, 42.

The processor 54 is configured to calculate a level of drive force that is needed at each trailer wheel 30, 32 in dependence on the received signals 20, 28, 58, 60, 62, 64. In particular, the level of drive force needed at each wheel 30, 32 is calculated so as to assist the vehicle 12 in driving the combination of the vehicle 12 and trailer 10.

The controller 56 is configured to send control signals 66, 68 to the respective electric machines 36, 38 in the form of a level of force to be applied to the trailer wheels 30, 32 by means of the electric machines 36, 38 based on the determined level of drive force that is needed. The target drive force applied to the trailer wheels 30, 32 can be any suitable value relative to the drive force applied to the vehicle wheels.

Figure 2 shows a flow diagram outlining the steps of a method 70 undertaken by the processor 54. At step 72, the ECU 50 receives the surface signal 20. As mentioned above, in the described embodiment the surface signal 20 includes data indicative of various features of the terrain on which the vehicle 12 is resting or travelling including the type of terrain, a level of friction of the terrain, and the gradient of the surface of the terrain. The system 50 can receive the surface signal 20 substantially continuously from the subsystems 16 of the vehicle 12. Note that the surface signal 20 may be received, alternatively or in addition, from sensors or subsystems of the trailer 10.

At step 74, the processor 54 determines a level of friction of the surface on which the vehicle 12 and 10 are resting or travelling based on the received surface signal 20. Similarly, at step 76 the processor 54 determines the gradient of the surface on which the vehicle 12 and 10 are resting or travelling based on the received surface signal 20.

At step 78, the processor 54 determines, for each of the wheels 30, 32 of the axle 34 of the trailer 12, a so-called trailer wheel drive force value based on the received surface signal 20. The trailer wheel drive force value is indicative of a force or a drive torque that is to be applied to each of the wheels 30, 32 by its respective electric machine 36, 38. In the described embodiment, the system 50 is operable to command application of drive force to the trailer wheels 30, 32 in relatively low surface friction conditions in which developing sufficient traction between the vehicle wheels may be problematic, i.e. in conditions in which vehicle slip becomes more likely. The increased load on the vehicle 12 resulting from towing the trailer 10 means that slip between the vehicle wheels and the terrain surface becomes more likely. Therefore, if it is determined at step 74 that a level of surface friction is below a friction threshold value, then at step 78 the determined trailer wheel drive force value is such that the system 50 is operable to cause the electric machines 36, 38 to apply a drive force to the trailer wheels 30, 32. Thus, the trailer 10 provides motive power to drive the vehicle 12 and trailer 10 combination in addition to the motive power provided by the vehicle 12. The level of surface friction may be determined to be above or below the friction threshold based on an identified terrain type: for example, if the identified terrain type is grass-gravel- snow then the level of surface friction may be determined to be below the friction threshold, whereas if the identified terrain type is tarmac then the level of surface friction may be determined to be above the friction threshold.

The friction threshold value may be a constant value or may depend on one or more factors such as the load to be driven (e.g. the mass of the trailer 10 or the mass of the combination of the trailer 10 and vehicle 12) or the power capabilities of the vehicle prime mover (e.g. internal combustion engine, battery).

In the described embodiment, the system 50 is operable to command application of drive force to the trailer wheels 30, 32 when the vehicle 12 and trailer 10 are on a relatively steep gradient in which the combination of the vehicle 12 and trailer 10 may benefit from drive torque being provided by the trailer 10 in addition to that provided by the vehicle 12. In particular, the force may be applied to the trailer wheels 30, 32 if the vehicle 12 and trailer 10 are to travel uphill on the gradient. Therefore, if it is determined at step 76 that the gradient of the surface on which the vehicle 12 and trailer 10 are resting or travelling is greater than a gradient threshold value, then at step 78 the determined trailer wheel drive force value is such that the system 50 is operable to cause the electric machines 36, 38 to apply a drive force to the trailer wheels 30, 32. Thus, the trailer 10 provides motive power to drive the vehicle 12 and trailer 10 combination in addition to the motive power provided by the vehicle 12.

Note that the above description refers to the case in which the vehicle 12 and trailer 10 are resting on a surface with a gradient in the longitudinal direction, i.e. uphill or downhill in the direction of travel of the vehicle 12 and trailer 10. However, the trailer ECU 50 may also receive data relating to a gradient in the transverse direction perpendicular to the direction of travel of the vehicle 12 and trailer 10, and the electric motors 36, 38 may be controlled to provide drive torque in order to assist pull-away in such a case. For example, the electric motor 36, 38 coupled to the trailer wheel 30, 32 on a downslope in the transverse direction may be controlled to provide more drive torque than the other electric motor 36, 38 to the other trailer wheel 30, 32.

It will be understood by the skilled person that the ECU 50 can determine the required drive torque based on any combination of the received signals. For example, the trailer 10 and vehicle 12 may need pull-away assistance on a surface that is both relative steep and slippery.

Similar to the friction threshold value, the gradient threshold value may be a constant value or may depend on one or more factors such as the load to be driven or the power capabilities of the vehicle prime mover. At step 80, the ECU 50 receives the speed signals 28, 62, 64. As mentioned above, in the described embodiment the speed signal 28 includes data indicative of a current speed of the vehicle 12, and the speed signals 62, 64 includes data indicative of a current speed of the trailer 10. The ECU 50 may alternatively be provided with either the speed signal 28 or the speed signals 62, 64. The system 50 is operable to command application of drive force to the trailer wheels 30, 32 when the speed of the vehicle 12 and trailer 10 is relatively low, for example when moving off from rest. Therefore, if the speed signals 28, 62, 64 indicate that the vehicle and trailer speed is below a threshold speed value, then at step 78 the determined trailer wheel drive force value is such that the system 50 is operable to cause the electric machines 36, 38 to apply a drive force to the trailer wheels 30, 32. Thus, the trailer 10 provides motive power to drive the vehicle 12 and trailer 10 combination in addition to the motive power provided by the vehicle 12. The speed threshold value may be a constant value or may depend on one or more factors such as the load to be driven or the power capabilities of the vehicle prime mover.

At steps 82 and 84, the ECU 50 receives the coupling signal 58 and the vehicle drive force signal 25. As mentioned above, in the described embodiment the signals 25, 58 include data relating to initiation of movement of the vehicle 12 and trailer 10, or acceleration of the vehicle 12 and trailer 10. The ECU 50 may alternatively be provided with either the coupling signal 58 or the vehicle drive force signal 25. The system 50 is operable to command application of drive force to the trailer wheels 30, 32 when the signals 25, 58 indicate that the vehicle 12 is initiating movement of the vehicle 12 and trailer 10, or when the vehicle 12 is attempting to increase the speed of the combination of the vehicle 12 and trailer 10. The signals 25, 58 can be used together with the speed signals 28, 62, 64 to determine when the vehicle 12 is moving off from rest. At step 78, the determined trailer wheel drive force value is such that the system 50 is operable to cause the electric machines 36, 38 to apply a drive force to the trailer wheels 30, 32 when the vehicle 12 is moving off from rest or when the vehicle 12 is attempting to accelerate.

The trailer wheel drive force is determined at step 78 based on the combination of the signals 20, 25, 28, 58, 62, 64 and the determinations made at steps 74, 76. It may be beneficial to have intermittent operation of the electric machines 36, 38 (e.g. only in the cases described above) providing drive torque to the trailer wheels 30, 32 so as not to drain the power of the battery 39 unnecessarily. At step 86, the controller 56 causes each of the electric machines 36, 38 to apply a force to the wheel 30, 32 to which it is attached in dependence on the determined trailer wheel drive force value. This is achieved via control signals 66, 68 sent by the controller 56 to the electric machines 36, 38. Note that the determined trailer wheel drive force value for one of the wheels 30 may be different from that of the other trailer wheel 32. That is, the drive force applied to one of the wheels 30 may be different from that applied to the other trailer wheel 32. For example, the coupling signal 58 may indicate that the vehicle 12 is turning a corner such that application of a greater amount of drive torque to one of the wheels is desirable to aid the cornering. Also, the applied drive force can be either in a forward or reverse direction. Indeed, it may be the case that the applied drive force to one of the wheels is in the forwards direction while the applied drive force to the other of the wheels is in the reverse direction. Furthermore, it may be the case that one of the electric machines 36, 38 will be commanded to apply a forward tractive torque, while the other of the electric machines 36, 38 will be commanded to apply a reverse tractive torque. This combination means that the electric motors 36, 38 will not dissipate significant amounts of energy, which in turn means that they may be operated indefinitely without being at risk of overheating.

Note that the present embodiment in which the electric motors 36, 38 are used to provide drive torque to the trailer wheels 30, 32 to provide pull away assist to the vehicle 12 can be used in conjunction with operating the electric motors 36, 38 to provide braking torque to the trailer wheels 30, 32 to provide braking assistance and increased stability to the combination of the trailer 10 and vehicle 12. In such a case, utilising the electric motors 36, 38 to provide drive torque will deplete the stored charge of the battery 39, while utilising the electric motors 36, 38 to provide regenerative braking to the trailer wheels 30, 32 will replenish the stored charge of the battery 39. Many modifications may be made to the above examples without departing from the scope of the present invention as defined in the accompanying claims.

In the described embodiment, the electric machines 36, 38 are powered by the battery 39; however, other energy storage means, such as one or many capacitors, supercapacitor or ultracapacitors, may be used in combination with or instead of the battery.

In the described embodiment, the ESC 22 provides the system 50 with information relating to the gradient of the surface on which the vehicle 12 and trailer 10 are resting; however, in different embodiments surface gradient data may be provided to the system 50 from different subsystems of the vehicle 12 that receive inputs from a longitudinal acceleration sensor or other sensor that provides surface gradient data, or from a sensor on the trailer 10 that provides surface gradient data.

In the described embodiment, the system 50 is operable to command application of drive force to the trailer wheels 30, 32 via the electric machines 36, 38 only to assist the vehicle 12 and trailer 10 to move off from rest or at low speed; however, in different embodiments the system 50 may be operable to command application of such a drive force for any vehicle and/or trailer speed.

In the described embodiment, the system 50 is operable to command application of drive force to the trailer wheels 30, 32 via the electric machines 36, 38 only when the gradient of the surface on which the vehicle 12 is resting or travelling is greater than a gradient threshold value, or when the level of friction of the surface is less than a friction threshold value. However, in different embodiments the system 50 may be operable to command application of such a drive force for any surface gradient or level of surface friction.

Claims

1 . A system for use in a trailer towable by a vehicle, the trailer comprising an axle having two wheels and at least one electric motor coupled thereto, the system comprising: receiving means configured to receive a surface signal indicative of at least one feature of a surface in the vicinity of the vehicle and/or the trailer; processing means configured to determine, for each of the wheels of the axle of the trailer, a trailer wheel drive force value based on the received surface signal; and, control means configured to cause the at least one electric motor to apply a force to the or each wheel to which it is coupled in dependence on the determined trailer wheel drive force value.

A system according to Claim 1 , the received surface signal including a signal from a terrain adjustment subsystem of the vehicle, optionally a Terrain Response® subsystem.

A system according to Claim 1 or Claim 2, wherein the at least one feature of the surface includes an indication of the type of surface terrain.

A system according to any previous claim, wherein the at least one feature of the surface includes a level of friction of the surface.

A system according to Claim 4, the determined trailer wheel drive force value being such that the force applied by the at least one electric motor to the wheels is non-zero if the level of friction of the surface is less than a friction threshold value.

A system according to any previous claim, wherein the at least one feature of the surface includes a gradient of the surface.

7. A system according to Claim 6, wherein the determined trailer wheel drive force value is non-zero if the gradient of the surface is greater than a gradient threshold value.

8. A system according to any previous claim, the receiving means being configured to receive a speed signal indicative of a speed of the vehicle and/or trailer, and the determined trailer wheel drive force value being such that the force applied by the at least one electric motor to the wheels is non-zero if the speed of the vehicle and/or trailer is less than or equal to a speed threshold value.

9. A system according to Claim 8, wherein the speed signal is received from a subsystem of the vehicle.

10. A system according to Claim 8, each wheel of the at least one axle of the trailer having an associated trailer wheel speed sensor configured to measure a wheel speed of the associated wheel, and wherein the speed signal is received from the trailer wheel speed sensors.

1 1 . A system according to any previous claim, the receiving means being configured to receive a signal indicating that movement of the vehicle and/or trailer is being initiated, and the determined trailer wheel drive force value being such that the force applied by the electric motors to the wheels is applied when movement of the vehicle and/or trailer is being initiated.

12. A system according to Claim 1 1 , the receiving means being configured to receive the signal indicating that movement of the vehicle and/or trailer is being initiated from the vehicle.

13. A system according to Claim 12, wherein the signal indicating that movement of the vehicle and/or trailer is being initiated includes a vehicle drive force signal indicative of a drive force to be applied to one or more wheels of the vehicle by drive means of the vehicle.

14. A system according to any of Claims 1 1 to 13, the trailer comprising a coupling sensor configured to measure a coupling force exerted on the trailer by the vehicle, the receiving means being configured to receive the signal indicating that movement of the vehicle and/or trailer is being initiated from the coupling sensor.

15. A system according to Claim 14, the trailer being coupled to the vehicle by means of a mechanical hitch joint, the coupling force being a force at the mechanical hitch joint.

16. A system according to Claim 14 or Claim 15, the coupling force including a magnitude component and a direction component.

17. A system according to any previous claim, wherein the force applied by the at least one electric motor to a first one of the wheels is different from the force applied by the at least one electric motor to a second one of the wheels.

18. A system according to any previous claim, the system comprising the at least one electric motor coupled to the wheels of the trailer.

19. A system according to Claim 18, the system comprising a first electric motor coupled to a first one of the wheels and a second electric motor coupled to a second one of the wheels.

20. A system according to any previous claim, comprising energy storage means configured to supply electric power to the at least one electric motor.

21 . A method for use in a trailer towable by a vehicle, the trailer comprising an axle having two wheels and at least one electric motor coupled thereto, the method comprising: receiving a surface signal indicative of at least one feature of a surface in the vicinity of the vehicle and/or the trailer; determining, for each of the wheels of the axle of the trailer, a trailer wheel drive force value based on the received surface signal; and, causing the at least one electric motors to apply a force to the or each wheel to which it is coupled in dependence on the determined trailer wheel drive force value.

22. A trailer for being towed by a vehicle, the trailer comprising an axle having two wheels and at least one electric motor coupled thereto, the trailer comprising a system according to any of Claims 1 to 20.

23. A non-transitory, computer-readable storage medium storing instructions thereon that when executed by one or more processors causes the one or more processors to carry out the method of Claim 21 .