Dual-motor hand wheel actuator assembly

Abstract

The invention relates to a double-motor hand wheel actuator assembly for a vehicle. The double-motor hand wheel actuator assembly comprises a housing; a shaft rotatably mounted with respect to the housing and configured to be attached to a steering wheel at one end; a first gear connected with the shaft and configured to rotate together with the shaft; and a first motor and a second motor, each motor having an output to drive a respective output gear, the output gear engaged with the first gear, where each motor includes a permanent magnet motor having a same number of poles and stators, each motor generates a cogging torque of the same mode during a complete mechanical rotation of the motor. The system is arranged such that, in an unpowered state in which both motors are not driven, at least a portion of one phase of each motor is short-circuited such that both motors can provide braking torque when the driver turns the wheels, where the phase comprising the short circuit is selected as a function of a mechanical offset of both motors relative to the output shaft.

Description

Technical Field

[0001] The present invention relates to a dual motor hand wheel actuator assembly. Background Art

[0002] Electric motors are widely used and increasingly common in a variety of applications and industries. For example, in the automotive field, an electric power steering system is known in which an electric motor device applies an assist torque to a portion of the steering system to make it easier for the driver to turn the wheels of the vehicle. The amount of the assist torque is determined based on a control algorithm that receives as input one or more parameters, such as the torque applied by the driver to the steering column to turn the wheels, the vehicle speed, etc.

[0003] Another example of an electric motor in an automotive application is a steer-by-wire system. In normal use, these systems have no direct mechanical connection between the handwheel held by the driver and the steered wheel. The movement of the handwheel by the driver is detected by a sensor and the motor is driven in response to the sensor output to produce the force that steers the wheel. This arrangement is known as a handwheel actuator assembly. Handwheel actuator assemblies are also used or are being actively considered for use in a range of non-automotive applications.

[0004] These systems rely on sensors to transmit user input data on the steering wheel to a control unit, which integrates the user input data with other information (such as vehicle speed and yaw rate) to transmit control signals to the primary motor, which physically actuates the vehicle's steering rack. The control unit also filters out unwanted feedback from the front wheels and provides a response signal to the secondary electric motor at the steering wheel. The secondary motor responds to specific user inputs on the steering wheel to provide the driver with appropriate resistance and feedback to simulate the feel of a conventional steering system.

[0005] In a wire-controlled steering system, if a part of the second component fails or malfunctions, it may cause the vehicle to lose the ability to steer. Therefore, it is desirable to provide a structure for at least temporary fail-safe operation of the second component. US2006 / 0042858A1 discloses a steering device, including a steering assembly, the steering assembly including a steering gear for turning a steerable wheel of a vehicle. The steering assembly includes a steering gear and a first and a second drive unit, each drive unit for actuating the steering gear.

[0006] GB2579374A discloses a steering column assembly for use with a steer-by-wire handwheel actuator. The assembly uses a similar dual drive system, including a first and a second motor, each having an output portion that drives a corresponding output gear. Each output gear drives a first gear that is connected to the shaft of the steering wheel and is configured to rotate the shaft of the steering wheel to provide road feel to the driver. In this example, the dual drive system is used to reduce gear noise, which may occur when the torque and direction of the motor are opposite.

[0007] These applications often use permanent magnet synchronous motors (PMSMs) because of their excellent torque density and dynamic response time. Due to cost and packaging constraints, PMSMs are usually designed with slotted stators.

[0008] The problem with a steer-by-wire arrangement compared to a conventional mechanical steering wheel arrangement is that the wheel is not physically connected to the steering wheel. The steering wheel can only pivot within a given range, so it is important to limit the maximum rotation of the steering wheel to correspond to the maximum degree of pivoting of the steering wheel, otherwise the vehicle driver can turn the steering wheel without limit when the steering wheel is at its maximum steering angle.

[0009] In conventional steering systems, it is standard practice to provide a safety system to prevent the formation of a closed loop in the motor, which would make it difficult to turn the hand wheel. This braking torque is an additional force to the force required to move the wheel relative to the road surface. The combination of these two conditions may result in the wheel being unable to turn.

[0010] For a handwheel actuator in a drive-by-wire system, the main resistance inherent in the system is mechanical friction and electromagnetic drag torque opposing the driver's turning of the wheel. In order to efficiently convert electrical energy into useful mechanical energy, the motor must be designed to have low mechanical friction and low electromagnetic drag torque. In normal operation or in a fault condition where the motor is unpowered (cannot generate any reverse torque), the handwheel will turn very freely and feel very light. Summary of the invention

[0011] The present invention is directed to improving the above-mentioned problems associated with handwheel actuator assemblies.

[0012] According to a first aspect of the invention, a dual motor hand wheel actuator assembly for a vehicle comprises: a housing; a shaft rotatably mounted relative to the housing and configured to be attached to a steering wheel at one end; a first gear connected to the shaft and configured to rotate with the shaft; and a first motor and a second motor, each having an output end driving a corresponding output gear, the output gear being engaged with the first gear, wherein each motor comprises a permanent magnet motor having the same number of poles and stators so that each motor produces the same pattern of cogging torque during a complete mechanical rotation of the motor, and

[0013] It is characterized in that the system is arranged so that in an unpowered state in which both motors are not driven, at least a portion of one phase of each motor is short-circuited, so that when the driver turns the wheel, both motors can provide braking torque, wherein the short-circuited phase is selected based on the mechanical offset of the two motors relative to the output shaft, so that the effect of torque ripple applied to the output shaft by the first motor in the braking torque is at least partially eliminated by the effect of torque ripple applied to the shaft by the second motor in the braking torque.

[0014] Shorting out at least part of one phase of each motor will result in braking torque being provided, ensuring that the driver experiences some resistance when turning the wheel. Without this, the wheel spins with little or no resistance, making the feel of vehicle control very unnatural. By offsetting the motors, the pulsations inherent in the braking torque of one motor are at least partially offset by the braking torque of the other motor, resulting in a smooth and natural feel.

[0015] The offset may be selected to eliminate at least one known harmonic in the brake torque pulsation, which comprises a periodic waveform generated when the motor rotates. It may be selected to cancel the most dominant or most problematic harmonic.

[0016] The assembly may include a controller that forces a short circuit in at least one phase of each motor when the motor is in an unpowered state. The controller may set the position of a switch of a bridge drive circuit for each motor to achieve this.

[0017] The assembly may include a drive stage for each motor, the drive stage comprising a set of switches operable to connect each phase to a positive or negative supply voltage depending on the torque required by the motor and the motor rotor angle, and a controller may place each motor in a state where one or more switches are short-circuited by closing the switches.

[0018] The controller may impose a short circuit current path between the terminal connections of the motors by closing one or more switches of the motor drive stage, or may impose a line fault on each motor by controlling the switches of the bridge. The controller may again impose a three-phase short circuit on each of the two motors by controlling the switches of the bridge. If the star point of the starting point motor is accessible, the short circuit may be introduced by disconnecting the phases from the star point.

[0019] A first aspect of the invention provides smooth motor torque when the motor loses power. Loss of power is an example of a fault condition for a handwheel brake.

[0020] Alternatively or additionally, the present invention also provides an assembly that is capable of continuing to operate a hand wheel actuator when powered but one of the motors fails.

[0021] Thus, in a second aspect, the present invention provides a dual motor hand wheel actuator assembly comprising: a housing; a shaft rotatably mounted relative to the housing and configured to be attached to a steering wheel at one end; a first gear connected to the shaft and configured to rotate with the shaft; and a first motor and a second motor, each having an output end driving a corresponding output gear, the output gear being engaged with the first gear, wherein each motor comprises a permanent magnet motor having the same number of poles and stators so that each motor produces the same pattern of cogging torque during a complete mechanical rotation of the motor, and

[0022] Characterized in that the system includes a control circuit, which is arranged so that when a short-circuit fault occurs in a part of at least one phase of a motor causing torque pulsation at the output end of the motor, a second short-circuit fault is intentionally introduced into at least a part of one phase of another motor, causing the other motor to generate torque pulsation, which at least partially eliminates the torque pulsation generated on the shaft by the first motor.

[0023] The controller may be configured to introduce a single-phase short circuit of the second motor when a single-phase short circuit fault occurs in the first motor.

[0024] The controller may be configured to induce a line short circuit in the second motor when a line short circuit occurs in the first motor.

[0025] By applying the same type of fault to the second motor as occurred in the first motor, similar pulsations will be generated, thus achieving the best cancellation effect.

[0026] However, in certain situations where a fault occurs in the first motor, the type of fault introduced into the second motor may be different in order to better eliminate pulsation.

[0027] The assembly may include means for detecting when a single phase short circuit occurs in one of the two motors and forcing a short circuit of at least one phase of the surviving motor.

[0028] The motors of the components of the first and second aspects of the present invention can be driven by an inverter bridge having multiple switches, and by controlling the switches of the motor drive circuit, fault conditions can be intentionally induced in the two motors to introduce a single FET short circuit, a line short circuit or a three-phase short circuit.

[0029] Two motors can be connected to the shaft so that they both rotate at the same speed and direction when the shaft rotates. This is achieved by using a similar gearing arrangement between the motors and the shaft. So when one motor is at a 0 degree mechanical position, the other motor will also be at 0 degree position, and so on, for every angle X from 0 to 360 degrees.

[0030] The invention of the first and second aspects may also be applied to assemblies where the two motors are offset. Applicants have recognised that where each motor produces a cogging torque, the offset is beneficial because the offset can be chosen such that the cogging torque of each motor can be at least partially cancelled, thereby producing a smoother steering feel.

[0031] In the case of stator misalignment, when a fault occurs in one motor, the exact same fault is simulated in the other motor, and the two resulting torque pulsations cancel at least partially. By exact same, we mean that a fault in phase A, B or C of one motor can be cancelled by creating the same fault in the same phase A, B or C of the second motor. This occurs because the phases are not aligned with respect to the steering axis. When the motors are perfectly aligned with respect to the steering axis, the effect is the same as applying the fault to a different phase.

[0032] In the absence of motor offset, the applicant recognizes that cancellation can best be achieved by failing a phase of the second motor that is different from the phase of the first motor that failed, for example, if phase A of the first motor shorts, then phase B or phase C of the second motor can be best shorted.

[0033] The relative phase offset of the two motors means that for any position of the shaft, the mechanical angles of the two motors are different. For example, when one motor is at zero degrees, the other motor can be at zero plus X degrees, where X is any value between 0 and 360 degrees, such as 0 degrees, 180 degrees, or 360 degrees.

[0034] The offset can be achieved by aligning the stators with the rotors aligned but offset by a non-zero degree relative to each other; or by having the stators in the same alignment but offsetting the rotors by a non-zero degree relative to each other; or by having both the rotor and stator offset by a non-zero degree relative to each other. Looking at it another way, when the motors' torque pulsation and cogging torque patterns vary over the full angular range of 0 to 360 degrees, the two motors may be aligned when the patterns are aligned, but may have a relative phase difference, or phase offset, when the patterns are not aligned. The degree of pattern misalignment can be expressed as an angle, in degrees or radians.

[0035] Applicants have learned that in the case of a single bridge switch phase short circuit, the maximum harmonic amplitude is usually the first order electronic. This corresponds to the third order mechanical for a common 9:6 motor topology. In addition, the second order electronic equivalent to the sixth order mechanical for a 9:6 motor topology is quite high. Therefore, the offset can be selected to specifically eliminate one of these ripple orders.

[0036] The assembly may include a common housing for the two motors, and the offset between the stator or the rotor or both may be achieved by fixing the stators of the two motors in the housing in an appropriate orientation to provide a relative phase difference between the motors, wherein the rotors are connected to the shafts so that they have the same angular electronic position.

[0037] Alternatively, the motor may be fixed to the housing so that the two stators are aligned with no relative phase difference and the rotor engages the gear with a non-zero relative phase difference.

[0038] Preferably, the rotor and stator of each of the two motors are topologically identical. This means that they have the same number of rotor magnets or poles, and the same number of stator magnets or poles. This has the advantage of reducing the bill of materials for the components and simplifying the design of the control and drive circuitry for both motors.

[0039] The dual motor drive assembly may form part of a hand wheel actuator assembly for a vehicle, wherein the shaft includes a fixing member whereby it may be fixed to a steering wheel or yoke.

[0040] The assembly may comprise a common housing for the two motors, and the offset between the stator or rotor or both may be achieved by securing the two motors into the housing in the appropriate orientation.

[0041] Preferably, the two motors are identical, which helps reduce the bill of materials of the components and simplifies the design of the control and drive circuits for the two motors.

[0042] The hand wheel actuator assembly of the first and second aspects of the present invention may be used in an electric power steering system of a vehicle, in particular a steer-by-wire system. BRIEF DESCRIPTION OF THE DRAWINGS

[0043] Specific embodiments of the present invention will now be further described, by way of example only, with reference to the accompanying drawings, in which:

[0044] Figure 1 A perspective view showing an embodiment of a dual motor hand wheel assembly according to the first aspect of the present invention;

[0045] Figure 2 The mechanical connection of the two motors to the common steering shaft is shown in more detail;

[0046] Figure 3 is a block diagram showing the main circuit components of the assembly;

[0047] Figure 4 shows how the motor stator can be oriented at different angular positions to change the phase of any torque ripples in the motor output seen at the steering shaft;

[0048] Figure 5 Shows Figure 1 The relative orientation of the stators of the two motors in the assembly, wherein an optional offset is provided;

[0049] Figure 6 shows a schematic diagram of two motor windings and two drive axles, with one phase of each motor intentionally faulted;

[0050] Figure 7 A curve diagram showing the change of the braking (damping) torque output of one of the two motors under the fault condition is shown;

[0051] Figure 8 shows the total torque seen by the two motors at the steering shaft at different offsets; and

[0052] Fig. 9 is with Figure 6 The corresponding view shows a fault in one phase of one motor during normal operation and a corresponding intentional fault in the other motor. DETAILED DESCRIPTION

[0053] The steer-by-wire dual motor hand wheel transmission 1 comprises an outer elongated metal housing 2, which encloses an elongated space. A shaft 3 connected to a steering wheel (not shown) passes through one end of the metal housing 2, and the end of the shaft is radially supported on a bearing (not shown) located at one end of the housing 2.

[0054] like Figure 2 As shown, the gear 4 is fixed to the end of the shaft 3 and rotates together with the shaft 3 . Figure 2 The shaft is not shown, but the axis of rotation of the shaft is marked by a dotted line 5, which extends vertically through the gear 4. The periphery of the gear 4 is formed as a worm gear, which engages with two identical worms 6, 7 located on opposite sides of the longitudinal axis 5 of the shaft 3. Each worm 6, 7 is connected to the output shaft 8, 9 of a corresponding electric motor 10, 11.

[0055] The axes of the output shafts 8, 9 of the two motors 10, 11 are arranged perpendicular to the rotation axis of the shaft 3, as shown in FIG. Figure 2 and 4 As best shown in , the axes of the two motors can also be tilted relative to each other to reduce the overall size of the assembly. Figure 1 As best shown in FIG. 1 , the motor 10 is received on a two-part extension of the housing 2 extending in the transverse direction.

[0056] Figure 3The circuit of a dual motor hand wheel actuator is shown. Each motor 10, 11 in this example has a three-phase stator winding. These phases are connected to the three arms of the motor drive three-phase bridge. Each arm has a top switch and a bottom switch, the top switch connects the phase to the positive supply voltage when closed, and the bottom switch connects the phase to the negative voltage or ground voltage. The switch opens and closes in response to the pulse width modulated signal output by the controller. The timing of these signals must be set according to the angular position of the motor rotor, so each motor is provided with a position sensor, which measures the angular position of the rotor and transmits it to the controller. The controller receives the motor torque demand signal at the input from a suitable electronic control unit (ECU). These signals cause the motor to apply a resistance torque to the wheel, thereby providing an appropriate steering feel when the driver turns the steering wheel.

[0057] The controller can drive the motors in a variety of ways, but it is preferred that the motors are driven so that one motor opposes the other to eliminate any free play in the system. Typically, one motor will provide a fixed value of low level reverse torque and the other motor will apply a variable torque to set the total torque applied to the hand wheel.

[0058] Specifically, it is preferred that the motors 10, 11 are controlled by an electronic control unit (ECU), which sends torque requirements to the motor controller so that when the input torque level applied by the steering wheel to the shaft 3 is low, the motor acts in the opposite direction on the gear 4 to eliminate backlash. When the input torque level applied by the steering wheel to the shaft 3 is high, the motors 10, 11 act in the same direction on the new wheel to help the shaft 3 rotate.

[0059] By using two independent motors 10, 11, they can be controlled to apply torque to the gear 4 in opposite directions in the first operating mode, thereby eliminating the need to use precision components to control backlash. In addition, by using two independent motors 10, 11, they can be controlled to apply torque to the gear 4 in the same direction in the second operating mode, so that the motor and gear components can be specified to be half the rated value of the required total system torque, thereby reducing the size and cost of the components.

[0060] It is also apparent that, when properly aligned, the use of two motors helps reduce unwanted torque pulsation and cogging torque to levels significantly below that which could be achieved using a single motor of the same design, and also below that which could be achieved using a dual motor assembly with conventional motor alignment.

[0061] The arrangement of the two motors 10 , 11 , the shaft 3 , the worm gears 6 , 7 and the gear 4 together form a dual-motor electrical assembly.

[0062] like Figure 5As shown, the first motor 10 includes a first rotor 20, a first stator 30 and a first output shaft 8, and a first end of the first output shaft 8 is rotatably coupled to the first rotor 20. The first motor 10 also includes a first housing, as shown, the first housing at least partially covers the first rotor 20 and the first stator 30. The first housing 40 is fixed to the housing, which is rigidly mounted to the elongated metal housing 2 or formed integrally with the elongated metal housing 2.

[0063] In this example, the motor has nine stator teeth 31 and the rotor carries six permanent magnets. Each magnet is labeled N for the north pole and S for the south pole, with the north and south poles being alternately arranged around the rotor. Each stator tooth 31 is wound with an electrical conductor 32 so that current flowing through the windings around the teeth generates a magnetic field on the teeth. The coils 32 are connected to form three motor phases, labeled A, B and C in the figure. The motor 10 is driven by an inverter that applies current waveforms to the three phases of the motor in a known manner. When these drive currents are applied to the phases, the electromagnetic interaction between the magnetic field generated by the stator 30 and the magnetic field of the rotor permanent magnets can be used to rotate the rotor 20 and cause the motor 10 to generate torque. This will be applied to the gear 4 through the worm shaft 6 and then to the shaft 3.

[0064] Likewise, the second motor 11 includes a second rotor 120, a second stator 130, and a second output shaft 9, and a first end of the second output shaft is rotatably coupled to the second rotor 120. Figure 1 As shown, the second motor 11 also includes a housing. The housing is also fixed to the gearbox housing 12, which is rigidly mounted to the elongated metal housing 2 or formed integrally with the elongated metal housing 2.

[0065] The second motor 11 has the same rotor 120, stator 130 and coils as the first motor. In this example, the second motor 11 has nine stator teeth 131 and the rotor carries six permanent magnets. The number N for each magnet represents the north pole and S represents the south pole, and the north pole and the south pole are arranged alternately around the rotor. Each stator tooth 131 is wound with an electrical conductor 132 so that current flowing through the windings wound around the teeth will generate a magnetic field on the teeth. The coils are connected to form three motor phases, which are marked A, B and C in the figure. The second motor is driven by a second inverter, which applies current waveforms to the three phases of the motor in a known manner. When these drive currents are applied to the phases, the electromagnetic interaction between the magnetic field generated at the second rotor 120 and the second stator 130 can be used to rotate the rotor and cause the motor to generate torque. This will be applied to the gear 4 through the worm shaft 7 and then to the shaft 3.

[0066] like Figure 1As shown, the first motor 10 and the second motor 11 can be located on diametrically opposite sides of the worm gear 30. Other locations can also be used, but placing them on opposite sides does provide a compact arrangement and facilitates the connection of the motor phases to the drive circuit. Figure 7 Another arrangement is shown, in which the two motors 10 , 11 are arranged such that they face each other but are offset to either side of the shaft 3 .

[0067] In this example, the stators of the two motors 10, 11 may be aligned so that they coincide, but in this first embodiment, they are aligned as Figure 5 As shown, the phase difference between the two is X degrees. The phase difference here means that when the shaft 3 rotates, the same mode of cogging torque generated by the two motors is offset. Figure 5 In the arrangement of , this is achieved by carefully choosing the angular position of the stators of the two motors 10, 11 relative to the gearbox housing 12. When fitting each motor into the gearbox housing, care is also taken to ensure that the two rotors are perfectly aligned with no phase difference between them. This means that for any given position of the shaft 3, the mechanical position of the two motors will be offset by an amount determined by the mechanical phase difference between the two motor stators.

[0068] The applicant has realised that the phase offset has a significant impact on the overall cogging torque present on the shaft 3 due to the combined effect of the cogging torque and the pulsating torque of the two motors acting on the gear 4 . Figure 4 Three possible configurations are shown, producing 0, 30 and 60 degree offsets by changing the orientation of the stator.

[0069] As described below, Figure 1 The components in are configured to operate in two different fault modes. The first fault operation mode applies to the situation where the motor is unpowered, such as a power failure, and the motor cannot be driven to provide any torque regardless of the torque required by the ECU.

[0070] In this mode, if Figure 6 As shown, a short circuit fault is intentionally introduced on one phase of each motor. It is noteworthy that the fault of the first motor occurs on one of the three phases A, B and C, and the fault of the second motor occurs on another phase of the three phases A, B and C. In each case, when the driver turns the wheel, a braking torque will be generated around the closed loop formed by the phase and the short circuit. This will include pulsation. Figure 7The pulsation of a single motor in the case of a single phase short circuit is shown, and how this relates to the shaft position depending on which phase has the short circuit. In this example, the mechanical separation of each phase is 30 degrees. The second motor can also provide the same braking torque and pulsation because the motors are identical, but the controller will select it to be out of phase with the torque pulsation of the first motor, and the amount of difference is optimized to reduce the overall combined torque pulsation. Figure 8 The effect of introducing a short circuit in each different phase of the second motor is shown. As a result, by appropriately selecting the phase in which the fault occurs, the overall braking torque pulsation is reduced, thereby providing a smooth resistance to the movement of the wheels, and different reductions can be obtained when different phases fail.

[0071] From another perspective, the operation in this mode is as follows:

[0072] - If a single switch short circuit occurs, the components are arranged to be connected in parallel as much as possible, forming a line short circuit, which then causes the same fault in the other motor.

[0073] If the assembly cannot close the parallel switch of the faulty motor unit, the assembly can close the switch of another motor of another phase.

[0074] - Preferably, a three-phase short circuit is achieved on both motors by closing all bottom switches, if possible.

[0075] In the second fault operation mode, the motors are still powered, but one of the motors fails, such as a single-phase short circuit or a line fault. The motors can continue to operate, but the failed motor will produce significant pulsations. The difference between a line short circuit and a single switch short circuit is that their torque ripple orders are different. A single switch short circuit has a first-order electronic torque ripple and multiple other orders, such as second order, third order, etc., and the first order is dominant, accounting for 70-80% of the ripple. Line SC has a second-order torque ripple order and is very sinusoidal.

[0076] To compensate for this pulsation, the controller intentionally introduces a fault such as a short circuit or line fault to the second motor. The two motors are then controlled in a normal manner as if the fault did not exist, so that two motor torques, both with pulsation, can be provided to the steering shaft.

[0077] Importantly, in this fault mode, the fault intentionally introduced into the second motor is selected so that the pulsations are out of phase and at least partially cancel.

[0078] If the two motors are identical and connected to the shaft in phase, then in the event that a line fault or a three-phase short circuit fault is not possible, the second fault can be introduced into a different phase than the fault of the first motor.

[0079] like Figure 3As shown, when the two motors are already out of phase, the same type of fault may occur in the second motor as a single switch fault or line fault in the first motor.

[0080] Fig. 9 Two motors are shown, with the first motor having a single switch short circuit fault (as shown in the top portion of the figure) and the second motor having a single phase fault intentionally introduced during normal operation (as shown in the bottom portion of the figure).

[0081] The invention is not limited to the details of the above described embodiments. The motor may have different topologies, in each case using different motor phases to optimize the reduction of cogging torque or torque ripple.

Claims

1. A dual motor hand wheel actuator assembly for a vehicle, comprising: case; a shaft rotatably mounted relative to the housing and configured to be attached to the steering wheel at one end; a first gear connected to the shaft and configured to rotate together with the shaft; and a first motor and a second motor, each having an output driving a respective output gear, the output gear being engaged with the first gear, wherein each motor comprises a permanent magnet motor having the same number of poles and stators so that each motor produces the same pattern of cogging torque over a full mechanical revolution of the motor, and Characterized in that the system is arranged so that in an unpowered state in which both motors are not driven, at least a portion of one phase of each motor is short-circuited so that when the driver turns the steering wheel, both motors can provide braking torque, wherein the phases to be short-circuited are selected based on the mechanical offset of the two motors relative to the output shaft, so that the effect of torque pulsation in the braking torque applied to the output shaft by the first motor is at least partially eliminated by the effect of torque pulsation in the braking torque applied to the shaft by the second motor.

2. The assembly of claim 1, wherein the phases of the two motors into which the short circuit is introduced are selected to eliminate at least one known harmonic in the braking torque ripple, the harmonic comprising a periodic waveform generated when the motors rotate.

3. An assembly according to claim 1 or 2, comprising a controller which forces a short circuit in at least part of one phase of each motor when the motor is in an unpowered state.

4. An assembly according to any one of the preceding claims, wherein the short-circuited parts comprise drive switches of a motor drive stage, for which purpose the controller sets the position of switches of a bridge drive stage of each motor.

5. An assembly according to claim 3 or 4, wherein the controller, when in use, applies a short circuit current path between the terminal connections of the motor by closing a switch of the motor drive bridge, or the controller, when in use, applies a line fault to each motor, or the controller, when in use, applies a three-phase short circuit to each of the two motors.

6. A dual motor hand wheel actuator assembly comprising: case; a shaft rotatably mounted relative to the housing and configured to be attached to the steering wheel at one end; a first gear connected to the shaft and configured to rotate together with the shaft; and a first motor and a second motor, each having an output driving a respective output gear, the output gear being engaged with the first gear, wherein each motor comprises a permanent magnet motor having the same number of poles and stators so that each motor produces the same pattern of cogging torque over a full mechanical revolution of the motor, and Characterized in that the system includes a control circuit, which is arranged so that when a short-circuit fault occurs in at least one phase of a motor causing torque pulsation at the output end of the motor, a second short-circuit fault is intentionally introduced into at least one phase of another motor to cause the other motor to generate torque pulsation, which at least partially eliminates the torque pulsation generated on the shaft by the first motor. 7 . The assembly of claim 6 , wherein the controller is configured to introduce a single-phase short circuit of the second motor when a single-phase short circuit fault occurs in the first motor.

8. An assembly according to claim 6 or 7, wherein the controller is configured to induce a circuit short circuit in the second motor when a circuit short circuit occurs in the first motor.

9. An assembly according to any one of claims 6 to 8, further comprising means for detecting when a single phase short circuit occurs in one of the two motors and forcing at least one phase of the surviving motor to be short circuited.

10. An assembly according to any one of claims 6 to 9, wherein each motor is driven by an inverter bridge having a plurality of switches, and a fault condition is intentionally induced in both motors by controlling the switches of the motor drive circuits to introduce a single FET short, a line short or a three-phase short.

11. An assembly according to any one of claims 6 to 10, wherein two motors are connected to the shaft so that each motor rotates at the same speed and direction as the shaft rotates, by using similar gearing between the motors and the shaft.

12. An assembly according to claim 11, wherein the offset between faults in the two motors is achieved by: mechanically offsetting the stators relative to each other by a non-zero degree with the rotors aligned; or by having the stators in the same alignment but offsetting the rotors by a non-zero degree with respect to each other; or by both the rotor and the stator being offset by a non-zero degree with respect to each other.

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