GB2456350A

GB2456350A - Electric in-wheel drive arrangement

Info

Publication number: GB2456350A
Application number: GB0800587A
Authority: GB
Inventors: Martin Boughtwood
Original assignee: PML FLIGHTLINK Ltd; QED Group Ltd
Current assignee: PML FLIGHTLINK Ltd; QED Group Ltd
Priority date: 2008-01-14
Filing date: 2008-01-14
Publication date: 2009-07-15
Anticipated expiration: 2028-01-14
Also published as: GB0800587D0; GB2456350B

Abstract

A torque drive and control system for a vehicle having a plurality of driven wheels is disclosed. The vehicle comprises a plurality of in-wheel electric motors for mounting within each respective driven wheel of the type having a plurality of coils (54, 56, 58, 74A, 74B, 740 see figs 3 and 4) forming a stator 252 radially surrounded by a plurality of magnets forming a rotor 240. A control circuit 80 within each electric motor for controlling a switching of voltage applied to coils (54, 56, 58, 74A, 74B, 74C) of that motor is provided. This controls the accelerating or braking torque provided by the motor. Means associated with each respective electric motor for detecting the speed of rotation of the motor and means for transmitting the speed of each motor to each other motor in the vehicle are provided. Each control circuit 80 is configured to adjust the torque provided by each respective motor in response to the detected speed of rotation of the motor and the speed of rotation of at least one other motor. A vehicle (see fig 9) may comprise a separate electric motor in each wheel, wherein torque is controlled to improve vehicle handling.

Description

ELECTRIC IN-WHEEL DRIVE ARRANGEMENT

FIELD OF THE INVENTION

The invention relates to electric vehicles, and particularly to a vehicle with in-wheel electric motors.

BACKGROUND OF THE INVENTION

Electric vehicles of various types powered by electric motors are known to the skilled person. In recent years, developments in electric motors have allowed in-wheel electric motors to be proposed for use in road vehicles. Motors of the type having a set of coils being part of the stator for attachment to a vehicle, radially surrounded by a rotor carrying a set of magnets for attachment to a wheel are now being used in electric vehicle. These motors are sometimes referred to as pancake" motors and may be three phase or, more recently, multiphase designs.

Advances in electric motor drive arrangements for vehicles are generally concentrated on the areas of power and efficiency to try and produce a vehicle having sufficient acceleration performance and range to be a realistic alternative to technologies such as the internal combustion engine.

SUMMARY OF THE INVENTION

We have appreciated that in-wheel electric motors can provide greater control over a vehide than arrangements such as internal combustion engine drive with mechanical brakes. We have further appreciated that a new type of in-wheel motor drive that we have developed can be used in a vehicle in a new manner to improve vehicle handling.

The invention is defined in the accompanying independent claims, with preferred features set out in the dependent claims. An embodiment of the invention is a vehicle with separate in-wheel electric motors for each driven wheel arranged so that the torque provided by each electric motor at any instant is correctly matched to the prevailing driving conditions. This contrasts with earlier known ideas which only disclose managing torque in the event that a wheel loses grip in a form of traction control.

The invention may be embodied in a torque control method or by a computer program for operating a torque control method described above. The computer program for implementing the invention can be in the form of a computer program on a carrier medium. The carrier medium could be a storage medium, such as a solid state, magnetic, optical, magneto-optical or other storage medium. The carrier medium could be a transmission medium such as broadcast, telephonic, computer network, wired, wireless, electrical, electromagnetic, optical or indeed any other transmission medium.

BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding of the invention and to show how the same may be carried into effect reference is now made by way of example to the accompanying drawings in which: Figurel is an exploded view of a motor as used in an embodiment of the invention; Figure 2 is an exploded view of the motor of Figure 1 from an alternative angle; Figure 3 schematically shows schematically shows an example arrangement for a three phase motor as used in an embodiment of the invention; Figure 4 schematically shows an example arrangement of coils in one of the coil sub-sets shown in Figure 3 as used in an embodiment of the invention; Figure 5 schematically shows the coils of the embodiment in relation to the magnets; Figure 6 schematically a control circuit; Figure 7 is a circuit diagram of the switching arrangement; and Figure 8 schematically shows a common control device; Figure 9 schematically shows a vehicle embodying the invention.

DETAILED DESCRIPTION

The embodiment of the invention described is vehicle having in-wheel electric motors. The motors are of the type having a set of coils being part of the stator for attachment to a vehicle, radially surrounded by a rotor carrying a set of magnets for attachment to a wheel.

It is of importance that the electric motors are able to provide sufficient torque to both drive the vehicle and distribute torque between the wheels.

Accordingly, the type of motor used in the embodiment will first be described, followed by the general principles of operation of the vehicle.

The in-wheel electric motors as used in the embodiment are shown in Figures 1 and 2. The assembly can be described as a motor with built in electronics and bearing, or could also be described as a hub motor or hub drive as it is built to accommodate a separate wheel.

Referring first to Figure 1, the assembly comprises a stator 252 comprising a rear portion 230 forming a first part of the housing of the assembly, and a heat sink and drive arrangement 231 comprising multiple coils and electronics to drive the coils as well as a heat sink. The coil drive arrangement 231 is fixed to the rear portion 230 to form the stator 252 which may then be fixed to a vehicle and does not rotate during use.

A rotor 240 comprises a front portion 220 and a cylindrical portion 221 forming a cover, which substantially surrounds the stator 252. The rotor includes a plurality of magnets arranged around the inside of the cylindrical portion 221.

The magnets are thus in close proximity to the coils on the assembly 231 so that magnetic fields generated by the coils in the assembly 231 generate a force on the magnets arranged around the inside of the cylindrical portion 221 of the rotor 240 thereby causing the rotor 240 to rotate.

The rotor 240 is attached to the stator 252 by a bearing block 223. The bearing block 223 can be a standard bearing block as would be used in a vehicle to which this motor assembly is to be fitted. The bearing block comprises two parts, a first part fixed to the stator and a second part fixed to the rotor. The bearing block is fixed to a central portion 233 of the wall 230 of the stator 252 and also to a central portion 223 of the housing wall 220 of the rotor 240. The rotor 240 is thus rotationally fixed to the vehicle with which it is to be used via the bearing block 223 at the central portion 225 of the rotor 240. This has a significant advantage in that a wheel rim and tyre can then be fixed to the rotor 240 at the central portion 225 using the normal wheel bolts to fix the wheel rim to the central portion of the rotor and consequently firmly onto the rotatable side of the bearing block 223. The wheel bolts may be fitted through the central portion 225 of the rotor through into the bearing block itself. A first advantage of this arrangement is that the whole assembly may be simply retrofitted to an existing vehicle by removing the wheel, bearing block and any other components such as the braking arrangement. The existing bearing block can then fitted inside the assembly and the whole arrangement fitted to the vehicle on the stator side and the normal rim and wheel fitted to the rotor so that the rim and wheel surrounds the whole motor assembly. Accordingly, retrofitting to existing vehicles becomes very simple.

A second advantage is that there are no forces for supporting the vehicle on the outside of the rotor 240, particularly on the circumferential wall 221 carrying the magnets on the inside circumference. This is because the forces for carrying the vehicle are transmitted directly from the suspension fixed to one side of the bearing block (via the central portion of the stator wail) to the central portion of the wheel surrounding the rotor fixed to the other side of the bearing block (via the central portion of the rotor wall). This means that the circumferential wall 221 of the rotor is not subject to any forces that could deform the wall thereby causing misalignment of the magnets. No complicated bearing arrangement is needed to maintain alignment of the circumferential rotor wall.

Figure 2 shows an exploded view of the same assembly as Figure 1 from the opposite side showing the stator 252 comprising the rear stator wall 230 and coil and electronics assembly 231. The rotor 240 comprises the outer rotor wall 220 and circumferential wall 221 within which magnets 242 are circumferentially arranged. As previously described, the stator 252 is connected to the rotor 240 via the bearing block at the central portions of the rotor and stator wails.

Additionally shown in Figure 1 are circuit boards 80 carrying control electronics. Due to their kite shape these circuit boards can be referred to as kite boards. Additionally in Figure 2 a V shaped seal 350 is provided between the -4..

circumferential waIl 221 of the rotor and the outer edge of the stator housing 230.

Further, in Figure 2, a magnetic ring 227 comprising a commutation focusing ring and a plurality of magnets is provided for the purpose of indicating the position of the rotor with respect to the stator to a series of sensors arranged on the circuit boards 80 of the stator 252.

Figures 3 and 4 schematically show an example of the configuration of the electric motor of figures 1 and 2 as used in the embodiment of this invention. The motor 40 shown in Figure 3 is a three phase motor. The motor therefore has three coil sets. In this example, each coil set includes eight coil sub-sets. The coil sub-sets of each coil set are labelled 44, 46 and 48, respectively in Figure 3.

Each coil set includes pairs of coil sub-sets, which are arranged opposite each other around the periphery of the motor 40. However, it should be noted that there is no express need for each coil sub-set to have a corresponding coil sub-set located opposite from it on the opposite side of the periphery of the motor 40.

Each coil sub-set can be connected to a respective control device. The terminals for each coil sub-set of each coil set are labelled 54, 56 and 58, respectively in Figure 3. While the arrangement shown in Figure 3 includes a larger number of coil sub-sets, this does not significantly increase the size and bulk of the switching means which are used to operate the motor, as would be the case if the increased number of coil sub-sets were connected together in series. Instead, it is merely necessary to provide an additional control device incorporating relatively small switching devices for each additional coil sub-set.

These control devices are sufficiently small such that they can be located adjacent to their corresponding coil sub-sets.

As described above, each coil sub-set can include one or more coils. In this example, each coil sub-set includes three coils as is shown schematically in Figure 4. In Figure 4, these three coils are labelled 74A, 74B and 74C. The three coils 74A, 74B and 74C are alternately wound such that each coil produces a magnetic field which is anti-parallel with its adjacent coil/s for a given direction of current flow. As described above, as the permanent magnets of the rotor of the motor 40 sweep across the ends of the coils 74A, 74B and 74C, appropriate switching of the currents in the coils can be used to create the desired forces for providing an impulse to the rotor. As is shown schematically in Figure 4, each coil in a coil sub-set can be wound in series.

The reason that the coils 74A, 74B and 74C within each subset are wound in opposite directions to give antiparallel magnetic fields can be understood with respect to Figure 5 which shows the arrangement of the magnets 242 on the rotor surrounding the coils 44, 46 and 48 of the stator. For simplicity, the arrangement is shown as a linear arrangement of magnets and coils, but it will be understood that in the embodiment of the invention described the coils will be arranged around the periphery of the stator with the magnets arranged around the inside of the circumference of the rotor, as already described.

The magnets 242 are arranged with alternate magnetic polarity towards the coils 44, 46 and 48. Each subset of three coils 74A, 74B and 74C thus presents alternate magnetic fields to the alternate pole faces of the magnets.

Thus, when the left-hand coil of a subject has a repelling force against a North Pole of one of the magnets, the adjacent central coil will have a repelling force against a South Pole of the magnets and so on.

As shown schematically in Figure 5, the ratio of magnets to coils is eight magnets to nine coils. The advantage of this arrangement is that the magnets and coils will never perfectly align. If such perfect alignment occurred, then the motor could rest in a position in which no forces could be applied between the coils and the magnets to give a clear direction as to which sense the motor should turn. By arranging for a different number of coils and magnets around the motor, there would always be a resultant force in a particular direction whatever position the rotor and motor come to rest A particular benefit of the independent control of the coil subsets by the separate control devices is that a larger than normal number of phases can be arranged. For example, rather than a three phase motor, as described in Figure 3, higher numbers of phases such as twenty-four phase or thirty-six phase are possible with different numbers of magnets and coils. Ratios of coils to magnets, such as eighteen coils to sixteen magnets, thirty-six coils to thirty-two magnets and so on, are perfectly possible. Indeed, the preferred arrangement, as shown in Figures 1 and 2 is to provide 24 separate control "kite" boards 80, each controlling three coils in a sub-set. Thereby providing a twenty-four phase motor.

The use of a multiphase arrangement, such as twenty-four phases, provides a number of advantages. The individual coils within each sub-set can have a larger inductance than arrangements with lower numbers of phases because each control circuit does not have to control large numbers of coils (which would require controlling a large aggregate inductance). A high number of phases also provides for lower levels of ripple current. By this it is meant that the profile of the current required to operate the motor undulates substantially less than the profile from, say a three-phase motor. Accordingly, lower levels of capacitance are also needed inside the motor. The high number of phases also minimises the potential for high voltage transients resulting from the need to transfer large currents quickly through the supply line. As the ripple is lower, the impact of the supply cabling inductance is lower and hence there is a reduction in voltage transient levels. When used in a braking arrangement, this is a major advantage, as in hard braking conditions, several hundred kilowatts need to be transferred over several seconds and the multiphase arrangement reduces the risk of high voltage transients in this situation.

The relative arrangement of magnets and coils, shown in Figure 5 can be repeated twice, three times, four times or indeed as many times as appropriate around 360 mechanical degrees of the rotor and stator arrangement. The larger the number of separate sub-sets of coils with independent phases, the lower the likelihood of high voltage transients or significant voltage ripple.

Figure 6 shows an example of a control device 80 in accordance with an embodiment of this invention. As described above, the control device 80 includes a number of switches which may typically comprise one or more semiconductor devices. The control device 80 shown in Figure 6 includes a printed circuit board 82 upon which a number of components are mounted. The circuit board 82 includes means for fixing the control device 80 within the motor, for example, adjacent to the coil sub-set which it the controls -directly to the cooling plate. In the illustrated example, these means include apertures 84 through which screws or suchlike can pass. In this example, the printed circuit board is substantially wedge-shaped. This shape allows multiple control device 80 to be located adjacent each other within the motor, forming a fan-like arrangement.

Mounted on the printed circuit board 82 of the control device 80 there can be provided terminals 86 for receiving wires to send and receive signals from a 92 control device as described below.

In the example shown in Figure 6, the control device 80 includes a number of switches 88. The switches can include semiconductor devices such as MOSFETs or IGBTs. In the present example, the switches comprise IGBTs.

Any suitable known switching circuit can be employed for controlling the current within the coils of the coil sub-set associated with the control device 80. One well-known example of such a switching circuit is the H-bridge circuit. Such a circuit requires four switching devices such as those shown in Figure 6. The wires (e.g. copper wires) of the coil sub-sets can be connected directly to the switching devices 88 as appropriate, and interconnections between the switching devices 88 can be formed on the printed circuit board 82. Since the switching devices 88 can be located adjacent the coil sub-sets as described above, termination of the wires of the coil sub-sets at the switching devices 88 is made easier.

As shown in Figure 7, the control device includes semiconductor switches arranged in an H-bridge arrangement. The H-bridge is of course known to those skilled in the art and comprises four separate semiconductor switches 88 connected to a voltage supply (here 300 volts) and to ground. The coils of each sub-coil are connected across the terminals 81 and 83. Here a sub-coil 44 is shown connected across the terminals. Simplistically, to operate the motor and supply a voltage in one direction, switches 88A and 88D are closed and the other switch is left open, so that a circuit is made with current in one direction. To operate the motor this current direction is changed in harmony with the alternating magnetic polarity passing the coil. To change the direction of rotation of the motor, the timing and polarity of the current flow in the coil is changed to cause the resulting forces in the opposite direction. The direction of current flow in the coil is reversed when switches 88B and 88C are closed and the other two switches are left open. In practice, the technique of pulse width modulating is used to pulse width modulate the signal applied to the gate of the semiconductor switches to control the voltage applied to the coils. The braking arrangement operates in a manner not known in the prior art and will be described after describing the overall control arrangement A shown in Figure 8, a common control device 92 can be used to coordinate the operations of the multiple control devices 80 provided in the motor.

In Prior motors, in which synchronization of the magnetic fields produced by the coils of each coil sub-set is automatically achieved by virtue of the fact that they *1I are connected in series. However, where separate power control is provided for each coil sub-set, automatic synchronization of this kind does not occur.

Accordingly, in accordance with an embodiment of this invention, a common control device 92 such as that shown In Figure 8 can be provided to ensure correct emulation of a polyphase system incorporating series-connected coils. As described above in relation to Figure 7, terminals 86 can be provided at the multiple control devices 80 to ailow interconnections 90 to be formed between the multiple control devices 80 and the common control device 92.

The interconnections 90 can pass signals between the common control device 92 and the control devices 80 such as timing/synchronization signals for appropriate emulation of a potyphase series-connected system.

In an alternative embodiment, each control unit can operate independently, without the need of a central control device. For example, each control unit can have independent sensors to detect a position of a rotor of the motor, which would dispense with the need to provide synchronisation signals of the kind described above. Instead, each control unit would receive a demand signal enabling it to control the voltage applied to its associated coils in isolation.

It is stressed that the preferred embodiment does not require any form of central control device for the operation of each wheel incorporating a motor.

Preferably, each motor is self-contained and, within each motor, the control circuits 80 are self-contained and depend upon nothing other than a torque demand signal to operate. This means that the elements are able to continue to function and to deliver demanded torque levels, irrespective of any other failures within the total drive system. In a system incorporating a plurality of wheels each having a motor, each motor incorporates all the intelligence needed to manage its actIons. Each motor understands its position on the vehicle and controls its actions accordingly. Preferably, each motor is further provided with information regarding the other motors such as the speed, torque and status and are based on each motor's knowledge of its position on the vehicle and the state and status of the other motors it can determine the optimum level of torque that it should apply for a given demanded torque. Even without this other information, though, the motor can continue to respond to a demanded torque.

As discussed above, motors constructed according to an embodiment of this invention can allow for highly responsive torque control. The use of separate motors for each wheel of a vehicle can allow for increased flexibility in handling torque control for the vehicle. Moreover, the short response times for torque control afforded by a motor according to an embodiment of this invention can enhance this flexibility.

A schematic view of a vehicle embodying the invention is shown in Figure 9 including showing possible forces on the vehicle and wheels at a given instant.

As previously described, each wheel of a vehicle can be controlled by its own motor and corresponding drive software, thereby allowing each motor to handle its own torque control. This means that each motor can handle, for example, a skid situation independently of the other wheels. Moreover, the fast response times (e.g. within a single PWM period of, for example 50 l.tS) afforded by embodiments of this invention can allow intricate control of the torque applied to each wheel independently, for increased effectiveness in handling.

To provide the appropriate toque, various parameters are determined, such as speed of rotation, angle of turn and acceleration. These could be provided by sensors. A speed sensor can be provided by measurement of the back EMF provided by the coils to determine the angular velocity of the wheel.

Similarly, the acceleration sensor may be provided by determining the rate of change of angular velocity of the motor by measuring the back EMF from the coils. As an alternative, separate magnetic sensors may be provided within each motor. The angle of turn of the wheel may be measured by a separate sensor mounted within the suspension arrangement of the wheel to determine the angle of the wheel with respect to the vehide body.

Whilst the measurement of speed of each wheel, direction of turn of the vehicle and acceleration of each wheel could be determined by separate sensors, these are preferably determined by logic within each of the control circuits 80 shown in Figure 6 and in conjunction with communicating information between the control circuits of each wheel within a network, such as shown in Figure 8.

The controllers within each wheel determine the speed of rotation of that wheel by monitoring the back EMF, by separate sensors or otherwise. In addition, the angular acceleration is determined using one of the same techniques. The controllers within each wheel receive parameters indicating the speed of rotation and rate of angular acceleration of the other wheels in the vehicle, as well as the indication of the position of each wheel in the vehicle. Within each control circuit, control logic can then determine based on the angular velocity of each wheel, the position of each wheel and the angular acceleration of each wheel whether the wheel has traction or is entering a skid condition, If it is determined that the wheel is likely to be about to skid or in the process of skidding, then a traction control mode operates to reduce the torque applied to that wheel by the motor.

On the other hand, if the wheel is not determined to be skidding then the control logic remains in a torque share mode in which the torque provided by an electric motor to a given wheel may be increased where the control logic determines that a wheel is on the outside of a turn and so should receive an increase in the torque provided.

The logic within each control device for providing the appropriate torque settings may be referred to as torque share logic. The logic is able to operate by determining an angle turn of a vehicle in the manner described above. As an alternative, the controller area network linking the motors together may distribute the angular speed of each wheel to controllers of other wheels and the torque share logic may determine the appropriate torque for a wheel based on the relative speeds of the wheels alone, without the step of determining the angle of turn of the vehicle, The torque share logic can determine the absolute or relative torque required of a wheel and switch power to the coils accordingly, either as an absolute level, but more preferably as a relative level. For example, the torque share logic could determine a torque difference needed in comparison to another wheel as a percentage of the RPM of the two wheels up to a maximum percentage difference. The maximum percentage difference would ensure that each wheel operates within a sensible range of torque difference in comparison to other wheels.

Whilst not essential to the invention, a master controller shown as the common control device 92, in Figure 8, can retain information received from each wheel as to speed, torque and acceleration and then provide instruction signals to the controllers within each wheel to specify the appropriate torque to provide.

As previously explained, it is the relative velocities of the wheels that are crucial, to determining the appropriate torque to provide within the limits of traction. The master controller could operate to determine an average vehicle speed based on the average velocities of the wheels at a given time (taking into account any known slip of the wheels) and act as a master controller to instruct the controllers within the wheels how torque should be distributed amongst the wheels of the vehicle. The function as to the percentage difference of torque to apply may be a function of the average speed of the vehicle, as well as the speed of each of the wheels.

It is important to stress that unlike known arrangements, the invention operates by distribution of torque amongst wheels not velocity. The motor itself, as already described, is arranged to provide torque control rather than speed control.

In accordance with an embodiment of this invention, motor control is by a high speed continuous range torque loop which can also provide traction control.

This can allow the response to be smoother and the achieved grip to be greater than a mechanically modulated torque management system. The motor drives can be networked together by a controller area network (CAN). This can allow information regarding, for example, skid events to exchanged between the motor drives for coordinated action to be taken. In one example, this information includes acceleration data indicative of the angular acceleration of each wheel. A sharp increase in angular acceleration can be interpreted as a wheel slip of a wheel skid.

According to an embodiment of the invention, sensors such as internal magnetic angle sensors can be provided in the motor of each wheel or in the wheeis themselves. These sensors can detect the angular velocity of each wheel. By taking the first derivative of the angular velocity determined by the sensors, the angular acceleration of each wheel can be determined for wheel. In another embodiment, wheel torque requirements can be detected by comparing each wheel speed with that of the other wheels. As described above, wheel torque requirement could be detected by detecting changes in the angular velocity of a wheel.

There are a number of ways in which the torque applied to a wheel can be reduced for regaining traction. For example, a combination of a calculated step reduction in torque followed by a linear reduction could be applied until it is detected that traction been regained. Alternatively, the torque could be dropped to zero or a very low value. The time taken for the wheel to stabilise back to the average vehicle speed could then be determined. This would give enough information to find the grip coefficient of the tyre as the rotational inertia of the wheel is known in advance. In turn, this measurement can then be used to modulate the torque produced in the wheel motor.

As described above, the motor drives of a vehicle can be networked together by, for example, a controller area network (CAN). Networking of this kind can allow the motor drives to communicate for providing improved awareness of each motor drive as to the overall condition of the vehicle. For example, in such a configuration, the motor drives can provide for the maintenance of left/right torque balance across the four wheels of, for example, a car. This can allow a significant left/right imbalance, which could alter the steering direction of the car or even spin it around, to be corrected for.

Claims

1. A torque drive and control system for a vehicle having a plurality of driven wheels, comprising: a plurality of in-wheel electric motors for mounting within each respective driven wheel of the type having a plurality of coils forming a stator radially surrounded by a plurality of magnets forming a rotor; a control circuit within each electric motor for controlling a switching of voltage applied to coils of that motor to thereby control the accelerating or braking torque provided by the motor; means associated with each respective electric motor for detecting the speed of rotation of the motor; means for transmitting the speed of each motor to each other motor in the vehicle; wherein each control circuit is configured to adjust the torque provided by each respective motor in response to the detected speed of rotation of the motor and the speed of rotation of at least one other motor.

2. A torque drive and control system for a vehicle having a plurality of driven wheels, comprising: a plurality of in-wheel electric motors for mounting within each respective driven wheel of the type having a plurality of coils forming a stator radially surrounded by a plurality of magnets forming a rotor a control circuit within each electric motor for controlling a switching of voltage applied to coils of that motor to thereby control the accelerating or braking torque provided by the motor; means associated with each respective electric motor for detecting the speed of rotation of the motor; means for determining an angle of turn of the vehicle; wherein each control circuit is configured to adjust the torque provided by each respective motor in response to the detected speed of rotation of the motor and the angle of turn of the vehicle.

3. A torque drive and control system according to daim I or 2, wherein each motor comprises a plurality of control circuits, each control circuit being connected to a respective sub-set of the coils. (

4. A torque drive and control system according to claim 1, 2 or 3, wherein each control circuit is operable to perform torque control independently of other control circuits in the system according to predetermined rules.

5. A torque drive and control system according to any preceding claim, wherein each control circuit includes torque share logic arranged to instruct the motor the appropriate torque to provide.

6. A torque drive and control system according to any preceding claim, wherein each control circuit is configured to increase the torque provided by the motor if it is determined that the motor is on the outside of a turn in comparison to a wheel determined to be on the inside of a turn.

7. A vehicle comprising: a plurality of wheels, each wheel being independently powered by a respective motor; and a torque drive and control system of any preceding claim.

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