Electric motor and steering wheel actuator assembly including motor

Abstract

The invention relates to an electric motor for use in a steering assembly of a vehicle, said motor comprising a stator with a plurality of phase windings and a rotor with a plurality of magnetic poles and connected to a shaft; the stator comprises a yoke having a plurality of teeth, each tooth comprising a stem and a tooth tip located at an end of the stem closest to the rotor, and the motor further comprises a sleeve of electrically conductive material located in a position that is otherwise an air gap between the inwardly facing tip of the stator tooth and the rotor, and wherein the sleeve is configured to be electrically conductive with respect to the stator. The sleeve includes an electrically conductive material that provides a magnetic flux path that extends axially along the motor bore such that rotation of the rotor generates an eddy current within the sleeve that resists rotation of the rotor. The invention also relates to a steering wheel actuator assembly of a steer-by-wire vehicle.

Description

Technical Field

[0001] The present invention relates to electric motors, and more particularly to those suitable for use in a steering wheel actuator assembly suitable for use in a steer-by-wire system for a vehicle. Background Art

[0002] Electric motors are widely used and increasingly common in automotive applications. For example, it is known to provide an electric power steering system 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 steer the wheels of the vehicle. The magnitude of the assist torque is determined based on a control algorithm that receives as input one or more parameters, such as the torque applied to the steering column by the driver turning the wheel, the vehicle speed, etc.

[0003] Another example of an electric motor in an automotive application is a steer-by-wire system. During normal use, these systems have no direct mechanical connection to the steering wheel moved by the driver and the wheels being steered. These systems rely on sensors to transmit user input data at 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 is also used to filter out unwanted feedback from the front wheels and provide a response signal to a secondary electric motor coupled to the steering wheel. The secondary electric motor responds to specific user inputs at the steering wheel to provide the driver with appropriate resistance and feedback to simulate the feel of a traditional steering system. The secondary electric motor is connected to the shaft supporting the steering wheel through a gear set, and these three parts together form the steering wheel actuator assembly.

[0004] The motor in a steer-by-wire system is typically constructed with a stator that includes laminations stacked together in an axial direction. This is both a cost-effective way to manufacture the motor and prevents current from flowing axially through the stator steel by applying an electrically insulating coating between each lamination in the stack. This reduces energy losses within the motor that would otherwise result from current flowing axially in the stator steel. This prior art motor construction exhibits low levels of drag torque by eliminating or severely limiting current flowing in the axial direction.

[0005] The term "drag torque" as used in this specification refers to the torque generated by a motor as it rotates, which opposes any torque applied to drive the motor. A motor with all stator winding phases open circuited and a rotor with nearly zero drag torque can turn with only a small amount of external torque applied, while a motor with very high drag torque will require an equally high or higher external torque to rotate the rotor.

[0006] In a steer-by-wire system, malfunction or failure of a portion of an assembly may impair the steering ability of the vehicle. Therefore, it is desirable to provide the assembly with a structure for providing at least temporary fail-safe operation. US2006 / 0042858A1 discloses a steering device including a steering assembly, the steering assembly including a steering wheel actuator. The steering wheel actuator includes a steering column for supporting a steering wheel, a gear mechanism and two motors, each motor for providing torque to the steering column. Single motor steering wheel actuator assemblies are also known.

[0007] When the steer-by-wire system is powered and functioning correctly, the HWA applies torque to the shaft carrying the steering wheel which, in most use cases, resists the driver's turning of the steering wheel shaft. For much of the system's operation, a motor or motors within the steering wheel actuator operate as a controlled resistance to the driver. This can be used to give the driver a sense of what is happening at the interface between the road wheels and the road surface, which would otherwise be lost if the mechanical link from the steering wheel to the road wheels was removed.

[0008] It is not desirable for the steering wheel to be able to rotate freely, and a small amount of resistance to movement that the driver can always overcome is always desirable. Preferably, this resistance to movement is either constant or proportional to the steering wheel speed to achieve a good steering feel. In the prior art, it is known to provide resistance to movement by absorbing energy within the motor drive electronics using a principle known as braking torque. For example, see Hendershot & Miller ISBN978-0-9840687-0-8, page 507, where they talk about the braking torque of a direct start permanent magnet motor caused by current flowing through an external short circuit. When there is at least one closed circuit through the motor phase (the current generated when the motor rotates will flow in this closed circuit), the braking torque is generated. This requires active control of the motor drive circuit to operate (usually close) the appropriate phase switch and force a closed circuit in the motor.

[0009] Applicants have appreciated that it is desirable that this resistance should be present even if power is removed from the system. Where the resistance is achieved by applying an appropriate current to the motor, the resistance will be removed if there is a fault in the motor that prevents current from being supplied or generated. Applicants have also appreciated that it is desirable to have minimal electrical connections to the motor and avoid the use of auxiliary windings and connected controllers to create resistance to movement in the event of a fault in the system. Summary of the invention

[0010] It is an object of the present invention to provide a motor and a steering wheel actuator assembly incorporating such a motor which provide a desired level of resistance to rotation when the motor is de-energized.

[0011] According to a first aspect, the present invention provides an electric motor for use in a steering assembly of a vehicle, the motor comprising a stator and a rotor, the stator having a plurality of phase windings, and the rotor having a plurality of magnetic poles and connected to a shaft; and

[0012] The stator comprises a yoke having a plurality of teeth, each tooth comprising a trunk and a tooth tip, wherein the tooth tip is located at the end of the trunk closest to the rotor,

[0013] And the motor also includes a sleeve of conductive material located in what would otherwise be an air gap between the inwardly facing tips of the stator teeth and the rotor, and wherein the sleeve includes conductive material that provides a magnetic flux path extending axially along the motor bore so that rotation of the rotor generates eddy currents within the sleeve that resist rotation of the rotor.

[0014] The sleeve may optionally be magnetically permeable so that it generates hysteresis losses which can supplement eddy current losses. Eddy currents generate a resistive torque that is proportional to speed, whereas hysteresis generates a constant resistive torque. Thus, supplementary hysteresis losses contribute to increased drag at low speeds.

[0015] The motor of the present invention generates a desired level of eddy current and hysteresis losses when the rotor is turned without power being supplied to the motor, and thus provides a relatively high resistance to rotation achieved by the sleeve of conductive material in the air gap between the stator and the rotor. The use of a simple sleeve of conductive material provides a design well suited for mass production of electric motors.

[0016] The steering assembly may include a steering wheel actuator for a steer-by-wire system.

[0017] The present invention encompasses motors in which the conductive components fit within the magnetic air gap of the machine, inside the stator bore. In a permanent magnet machine, the flux reversals to which the conductive components are subjected generate eddy currents which generate losses which in turn generate work resisting torque against anything that would cause the motor shaft to rotate.

[0018] The electrically conductive bushing may comprise a solid-walled tubular sleeve which fits into the air gap between the rotor and the stator.

[0019] The rotor may be located concentrically within the stator, each sharing a common axis.

[0020] The sleeve has an inner peripheral wall having a uniform radius at all points, and an outer peripheral wall also having a constant radius, so that the sleeve has a uniform thickness at all points.

[0021] In one alternative, the thickness of the sleeve may vary around the circumference of the sleeve to define geometric features such as ribs or grooves on the inner and outer circumferential surfaces.

[0022] The outer surface of the sleeve may include a plurality of ribs, each rib extending radially outwardly into the space between adjacent tips of the stator teeth.

[0023] The inner diameter / bore of the stator sleeve does not have to be perfectly circular, but may have features such as protrusions, recesses, flats, radii, chamfers, or any other significant geometric structure that can help keep the magnetic flux density as sinusoidal as possible to reduce the ripple component of the damping torque or torque pulsation of the machine during normal operation.

[0024] In addition to the geometry, part or all of the sleeve may be made of a magnetically permeable material. In addition to the above contributions to hysteresis losses, careful selection of shape, permeability, and saturation flux density can help achieve an optimal flux density to reduce the torque ripple or cogging torque of the motor without significantly affecting the torque capability. The material can be selected to have a permeability and saturation flux density that best serves the application.

[0025] The relative permeability can be as low as 18 for some soft magnetic composites, or as high as 4000 for electrical steels. The saturation flux density can vary from 0.9T to over 1.5T.

[0026] The height of the sleeve measured in the axial direction of the motor may be substantially the same as the height of the stator. In this way, the sleeve may extend from one end of the stator to the other end. Relative to the stator, the sleeve may be shorter or longer than this.

[0027] The sleeve may include a plurality of cut-outs extending from the inner peripheral wall to the outer peripheral wall of the sleeve, wherein strips of material are defined between the cut-outs, which provide an axial conductive path. These cut-outs may allow air to flow from one side of the sleeve to the other, which may help cool the motor.

[0028] The sleeve may have a generally circular inner and outer surface, with the outer surface matching the inner diameter of the stator steel. The sleeve may be interference fit within the air gap in contact with the stator teeth. A conductive adhesive may be used between the tooth tips and the outer surface of the sleeve to secure the sleeve in place.

[0029] The stator may be coated with a lacquer which may aid adhesion. If the sleeve material is too far in the noble metal sequence compared to the stator electrical steel, the sleeve may or may not be tinned or otherwise passivated to prevent galvanic corrosion from occurring.

[0030] The stator sleeve may be located within an air gap having a length that may range from a low of 0.4 mm to a high of 1.5 mm as measured radially from the center of rotation. The air gap length is not necessarily limited to this range and should be determined by the electromagnetic performance requirements of the machine, the resistivity of the selected sleeve material, the target damping torque, and other performance aspects.

[0031] The length of the sleeve (also measured in the radial direction) can be between 0.2 mm and 0.6 mm. The radial length need not be limited to this range, depending on the desired performance. The length of the sleeve can be between 20% and 50% of the air gap.

[0032] The sleeve may comprise a metal or metal alloy and may be homogeneous. For example, the sleeve may be cast, or may be pressed or rolled from a uniform block of material. The sleeve material may be different from the stator tooth material.

[0033] The sleeve may comprise a copper sleeve, or may comprise an aluminum sleeve, or may be any other conductive material that provides the properties required for the application or for ease of manufacture or to reduce cost. For example, since the goal is to maximize braking torque, as high a conductivity as possible is beneficial, which makes copper the most useful option per unit mass and per unit cost, but aluminum alloys may also be useful to further reduce cost or prevent galvanic corrosion that might otherwise occur.

[0034] The lower the resistance of the stator, the greater the losses in the sleeve, and therefore the greater the resistive torque felt when the shaft rotates.

[0035] The stator sleeve may typically have a resistivity of less than 5e-8 Ωm.

[0036] The sleeve has a resistivity of about 3.8e-8 Ωm when made of aluminum alloy or 1.724e-8 Ωm when made of copper. Any alloy material that provides the necessary electrochemical or mechanical properties to prevent corrosion and enhance manufacturability may have a resistivity that deviates from these values. The radial length or thickness of the sleeve may be increased or decreased accordingly to achieve the desired amount of braking torque.

[0037] The motor stator may be configured such that at a rotational speed of 1000 rpm, the effective loss per unit mass of the motor stator assembly excluding the copper windings is at least 50 watts per kilogram of electromagnetically active motor material.

[0038] The socket may provide the primary source of drag for the motor, although the socket may be combined with other functions to provide a controlled amount of drag.

[0039] The stator teeth may comprise stacked laminations of steel sheets.

[0040] The motor may be configured to provide at least 60% of the resistance to rotation of the rotor in at least one speed range, or at least 70%, or 80%, or substantially all of the drag torque over a non-zero speed range. The motor may provide all or substantially all of the resistance to rotation of the output shaft, by which we mean at least an order of magnitude greater than any resistance provided by mechanical friction present in the motor. Resistance to rotation is typically amplified by the gear set.

[0041] Taking into account the transmission structure between the motor and the output shaft (if provided), the motor may be configured to provide a drag torque of at least 2 Nm over a substantial non-zero rotational speed range of the output shaft.

[0042] At a rotational speed of the shaft of 180 degrees per second, the drag torque of the motor may provide at least 60% or at least 70% or more resistance to shaft rotation. The motor may provide all or substantially all resistance to shaft rotation, by which we mean at least an order of magnitude greater than any resistance provided by mechanical friction present.

[0043] According to a second aspect of the present invention, there is a steering wheel actuator assembly for a steer-by-wire vehicle, the steering wheel actuator assembly comprising:

[0044] case;

[0045] a shaft rotatably mounted relative to the housing;

[0046] One or more motors, each having a stator with a plurality of phase windings and a rotor with a plurality of magnetic poles connected to a shaft;

[0047] control circuitry adapted to control the flow of current to or from the or each motor to cause a net torque to be applied to the shaft during normal operation; and

[0048] wherein at least one of the motors comprises a yoke having a plurality of teeth, each tooth comprising a stem and a tooth tip, the tooth tip being located at an end of the stem closest to the rotor;

[0049] And the motor also includes a sleeve of conductive material, which is located in the position of the air gap between the tips of the stator teeth and the rotor, and wherein the sleeve includes conductive material, which provides a magnetic flux path extending axially along the motor bore so that rotation of the rotor generates eddy currents in the sleeve, and the eddy currents resist rotation of the rotor.

[0050] The mechanical assembly may include a gearbox including a first gear fixed relative to the shaft and a second gear fixed relative to the output of the motor, rotation of the first gear causing rotation of the second gear.

[0051] The two gears may mesh directly or may be connected to each other via a belt. If the mechanism is part of a steering wheel actuator assembly, the steering wheel actuator may include a second gear connected to the shaft and arranged to rotate with the shaft; and a second motor having an output end driving a respective second output gear, the second output gear engaging the first gear and thereby engaging the shaft.

[0052] The second motor may also produce significant drag torque, so that when the motors are unpowered, the sum of the drag torques from the two motors provides considerable resistance to rotation of the steering wheel.

[0053] Alternatively, the second motor may have a more conventional construction using electrical steel as the rotor so that it does not contribute significantly to the overall resistance to rotation when no power is applied. Thus, the first motor may provide much greater drag torque than the second motor, for example at least twice as much drag torque.

[0054] The motor may comprise a brushless permanent magnet type motor comprising a rotor and a stator having a number of windings surrounding regularly circumferentially spaced teeth.

[0055] The shaft can be connected to the steering wheel directly through a spline connector on the end of the shaft, which fits into the internal spline connector of the steering wheel. Therefore, the shaft will rotate at the same speed as the steering wheel. If it is connected directly to the shaft, the motor will also rotate at the same speed. If the motor is connected to the shaft through a gearbox, the motor will rotate at a different speed than the steering wheel.

[0056] Alternatively, the shaft can be connected to the steering wheel through a gearbox. In this case, the rotating speed of the shaft may be different from the rotating speed of the steering wheel.

[0057] The motor may form part of a steering wheel actuator assembly for a steer-by-wire vehicle, wherein the motor rotor is connected to a shaft which in turn is connected to a steering wheel of the vehicle, and the steering wheel actuator assembly is configured such that when the control circuit is de-energized or disconnected and the steering wheel rotates 180 degrees per second, the motor combination as a whole provides a drag torque of at least 50% of the resistance to rotation of the shaft and a torque at the steering wheel of at least 3 Nm.

[0058] The steering wheel actuator assembly may comprise two motors of the first aspect of the invention, the rotor of each motor being connected to a shaft which in turn is connected to the steering wheel.

[0059] By providing one or more motors, at least one of which provides a significant and useful level of drag torque, the driver must apply at least 3 Nm at a steering wheel speed of 180 degrees per second to maintain a constant rotational speed of the steering wheel of 180 degrees per second, as this drag must be overcome before any additional torque can be used to accelerate the steering wheel. This level of drag is generally considered acceptable in vehicle steering wheel actuator applications. If the drag is too low, the steering feel is too light and may be too easy to turn at high speeds (including stability); but if the drag is too high, the drag may make the steering too heavy and the driver may have difficulty turning the wheel and steering the vehicle. When in electrical contact with the stator laminations, the role of the conductive metal coating is to increase the conductivity between adjacent laminations so that it approaches the conductivity of solid magnetic components (such as stator stacks), thereby promoting the generation of larger eddy currents than is normally the case with insulating laminations in the laminations.

[0060] The present invention provides damping of otherwise uncontrolled rotation without electronics, additional mechanical parts or complexity, which prevents the steering wheel from free spinning when power is removed, the motor is disconnected, or under certain fault conditions that render the control unit inoperable electrically. Removing the need to dissipate energy in the drive circuit protects the circuit from damage due to heat buildup, as is known in the art, and shifts the heat loss to the motor where it can be better managed.

[0061] According to a third aspect, the present invention provides a method of assembling a motor having the features of the first aspect of the invention, the method comprising inserting a sleeve into the interior of a stator using a cold-drop process, wherein the sleeve is cooled to a suitably low temperature at which it contracts so that its outer diameter is small enough to be able to slide loosely into the stator bore, and then the sleeve is allowed to heat back to room temperature at which time it will expand to achieve an interference fit with the stator.

[0062] This is actually the reverse of the typical hot drop process, where for example the housing can be heated and dropped outside the stator steel OD. As the housing cools it shrinks and eventually achieves an interference fit with the stator OD. This process can be used, but it is more difficult because the windings etc. around the stator may not be able to withstand the high temperatures required. BRIEF DESCRIPTION OF THE DRAWINGS

[0063] Only one embodiment of the present invention will now be described by way of example, with reference to the accompanying drawings of the present invention and as illustrated in the drawings of the present invention:

[0064] Figure 1The key mechanical components of one embodiment of a steering wheel actuator assembly according to one aspect of the present invention are shown, which includes a pair of motors, each of which may fall within further aspects of the present invention;

[0065] Figure 2 Another embodiment of a steering wheel actuator assembly according to an aspect of the present invention is shown;

[0066] Figure 3 shows the general arrangement of an electronic control unit which controls two motors of a dual motor drive assembly according to the first aspect of the invention;

[0067] Figure 4 shows the layout of a steer-by-wire system comprising a dual motor drive assembly according to the first aspect of the invention;

[0068] Figure 5 It shows that Figure 1 a cross-sectional view of the configuration of a motor used in the mechanism; and

[0069] Figure 6 shows when the rotor is turning while the motor is isolated from the power supply Figure 5 Eddy current density of a simple sleeve inside a motor. DETAILED DESCRIPTION

[0070] Figure 1 A steering wheel actuator (HWA) assembly for a vehicle according to a first aspect of the invention is shown. The example is a dual motor assembly having two motors, each connected to a common shaft via respective gearboxes. The invention may be implemented with a single motor and may also be implemented with a direct connection of the motor rotor to the shaft in the absence of a gearbox. The motor has special characteristics and embodies the first aspect of the invention. By describing the motor in relation to one potential use in a steering wheel actuator assembly, the advantages of these motors over conventional motors in such an application may be readily appreciated.

[0071] The assembly 1 comprises a first motor 10 with a rotor 101 and a stator 102 and a second motor 11 with a rotor 111 and a stator 112, the first motor 10 being connected to a first worm 6 and the second motor 11 being connected to a second worm 7. Each worm 6, 7 comprises a threaded shaft arranged to engage with a gear 4 connected to a steering column shaft 3, so that torque can be transmitted from the worm 6, 7 to the gear 4 connected to the steering column shaft 3. The gear 4 is operatively connected to the driver's steering wheel (not shown) via the steering column shaft 3. In this example, each of the two motors 10, 11 is a brushless permanent magnet type motor, and each motor comprises a rotor 101, 111 and a stator 102, 112, which has a plurality of windings, which surround teeth regularly spaced apart in the circumferential direction. The arrangement of the two motors 10, 11, the shaft 3, the worm 6, 7 and the gear teeth 4 together forms a dual motor electrical assembly.

[0072] Each of the two motors 10, 11 is controlled by an electronic control unit (ECU) 20. The ECU 20 controls the level of current applied to the windings and therefore the level of torque produced by each motor 10, 11.

[0073] In this example, the two motors 10, 11 are similar in design and produce similar maximum torque levels. However, it is within the scope of the present disclosure to have an asymmetric design in which one motor 10, 11 produces a higher torque level than the other motor 10, 11.

[0074] One of the functions of the steering wheel actuator (HWA) assembly is to provide feedback force to the driver to provide proper steering feel. This can be achieved by controlling the torque of the motors 10, 11 based on signals from the steering wheel actuator (such as column angle) and signals from other systems in the vehicle (such as vehicle speed, rack angle, lateral acceleration and yaw rate).

[0075] The use of two motors 10, 11 is advantageous in eliminating squeaks. If a single electric motor is used in the torque feedback unit instead, the motor can be kept in locking contact with the transmission structure by means of springs. However, under certain driving conditions, the action of the springs is not strong enough, which allows the gears to "squeak" during sinusoidal movements or sharp changes in position of the steering column.

[0076] The use of two motors 10, 11 that can be actively controlled (as in this embodiment) improves the problems associated with using a single motor. In this arrangement, both motors 10, 11 are controlled by the ECU 20 to provide torque feedback to the steering column and ensure that the worms 6, 7 of both motors 10, 11 are in constant contact with the gear 4 to minimize squeaks. Using two motors 10, 11 in this way also allows for active management of friction and therefore feedback forces to the driver.

[0077] like Figure 1 As shown in the figure, the motors 10, 11 are received in and fixed to the transversely extending two-piece extension of the housing 2. The worm 6, 7 of each motor is supported relative to the housing by two sets of bearings. The first set of bearings 41 supports the first end of each worm 6, 7 at the distal end of its respective motor 10, 11, while the second set of bearings 42 supports the second end of each worm 6, 7 at the proximal end of its respective motor 10, 11.

[0078] Figure 2 The axis of rotation of the shaft 3 is shown, marked by a dashed line 5, extending vertically through the gear 4. The periphery of the gear 4 is formed as a worm gearing which meshes with two identical worms 6, 7, respectively, located on opposite sides of the longitudinal axis 5 of the shaft 3. Each worm 6, 7 is connected to an output shaft 8, 9 of an electric motor 10, 11, respectively.

[0079] 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 and the axes of the two motors may also be tilted relative to each other to reduce the overall size of the assembly.

[0080] The motors 10, 11 are controlled by an electronic control unit (ECU) 20 so that when the input torque applied to the shaft 3 by the steering wheel is at a low level, the motors 10, 11 act on the gear 4 in opposite directions to eliminate backlash. When the input torque applied to the shaft 3 by the steering wheel is at a higher level, the motors 10, 11 act on the gear 4 in the same direction to help the shaft 3 rotate. Here, the motors 10, 11 acting in "one direction" is used to indicate the direction of the torque applied to the gear 4 by the motors 10, 11.

[0081] The use of two separate motors 10, 11 controllable in a first operating mode to apply torque to the gear 4 in opposite directions eliminates the need for precision components to control backlash. In addition, the use of two separate motors 10, 11 controllable in a second operating mode to apply torque to the gear 4 in the same direction allows the motors 10, 11 and gear components 4, 6, 7 to be sized at half the required total system torque, thereby reducing the size and cost of the drive assembly 1.

[0082] exist Figure 1 and Figure 2 In the embodiment shown in FIG, the worms 6, 7 engage with the diametrically opposed portion of the gear 4. The threads of the worms 6, 7 are respectively in the same direction, i.e. they are both left-hand threads. The motors 10, 11 are arranged so that they are located on the same side of the gear 4 (both motors 10, 11 are located on one side of an imaginary plane perpendicular to the axis of the worms 6, 7 and passing through the center point of the gear 4). Figure 2 Taking the perspective shown in as an example, driving the two motors 10 and 11 clockwise will apply torque to the gear 4 in opposite directions, wherein the motor 10 applies a clockwise torque to the gear 4 and the motor 11 applies an opposite counterclockwise torque to the gear 4.

[0083] Figure 2 Another embodiment of the steering wheel actuator assembly 1 according to the first aspect of the present invention is shown. Figure 1 and Figure 2 The embodiment shown in is substantially similar, the only difference being the positioning of the motors 10, 11. Components and functional units which are equivalent or identical in function and / or structure to those of the preceding embodiment have the same reference numerals and are not described separately. Figure 1 The relevant explanation applies in a similar manner to Figure 3 , except for the positioning of the two motors 10 and 11.

[0084] exist Figure 2 In the embodiment, the worms 6 and 7 are engaged with the opposite parts of the gear 4, and the threads of the worms 6 and 7 are respectively in the same direction, that is, they are both right-hand threads in this example. The motors 10 and 11 are configured so that they are located on opposite sides of the gear 4 (the motor 10 is located on one side of a virtual plane perpendicular to the axes of the worms 6 and 7 and passing through the center point of the gear 4, and the motor 11 is located on the other side of the virtual plane).

[0085] The application of torque in the clockwise direction by the driver causes the steering wheel 26 and the steering column shaft 3 to rotate about the dotted line 5. This rotation is detected by a sensor (not shown). The first motor 10 is then controlled by the ECU 20 to apply torque in the opposite direction. In the first operating mode, the second motor 11 is actuated by the ECU 20 to apply a bias torque 32 in the opposite direction to the torque 30 of the first motor 10 to reduce transmission mechanism abnormal noise. Alternatively, in the second operating mode, the second motor 11 is actuated by the ECU 20 to apply a torque 34 in the same direction as the torque 30 of the first motor 10 to increase the feedback torque to the steering column shaft 3.

[0086] The net result of the torques produced by the first and second motors 10, 11 results in a feedback torque being applied to the steering column shaft 3 and the steering wheel 26 to provide the driver with a road feel. In this example, the feedback torque is applied in the opposite direction to that applied by the driver to the steering wheel 26. In this way, the "squeak" produced between the worm 6, 7 and the gear 4 can be eliminated or greatly reduced.

[0087] Figure 3 A portion of a HWA assembly 80 is disclosed, showing the general arrangement of an electronic control unit (ECU) 20 that controls each of the two motors 10, 11. The ECU 20 may include a steering wheel actuator (HWA) control system 21 and first and second motor controllers 22, 23 that control the first and second motors 10, 11, respectively. A reference demand signal is input to the HWA control system 21, which distributes torque demands to each of the first and second motors 10, 11. These motor torque demands are converted into motor current demands and transmitted to the first and second motor controllers 22, 23. Each motor 10, 11 provides operational feedback to their respective motor controllers 22, 23. The HWA control system 21 is configured to calculate the magnitude of mechanical friction using the motor torque demands. In another embodiment, the HWA control system 21 may be implemented by separate ECUs for the first and second motor controllers 22, 23.

[0088] Figure 4 The general layout of a steer-by-wire system 100 for a vehicle is shown, which includes a steering wheel actuator (HWA) assembly 80 according to the first aspect of the present invention. The HWA assembly 80 supports the driver's steering wheel 26 and measures the driver's demand, which is typically a steering angle. The steering controller 81 converts the driver's demand into a position demand, which is sent to the front axle actuator (FAA) 82. The FAA 82 controls the steering angle of the road wheels to achieve the position demand. The FAA 82 can feed back the operating status and measurements to the steering controller 81.

[0089] The steering controller 81 combines the feedback from the FAA 82 with other information measured in the vehicle, such as lateral acceleration, to determine a target feedback torque that should be felt by the vehicle driver. This feedback requirement is then sent to the HWA control system 21 and provided by controlling the first and second motors 10, 11 via the first and second motor controllers 22, 23, respectively.

[0090] Figure 4 A steering controller 81 is shown that is physically separate from the HWA control system 21 and the FAA 82. Alternatively, a different architecture may be used within the scope of the present disclosure in which one or more of these components are physically connected to each other. For example, the functionality of the steering controller 81 may be physically implemented in the HWA controller 21, the FAA 82, or another control unit in the vehicle, or in some combination of all three. Alternatively, the control functions caused by the HWA controller 21 and the FAA 82 may be implemented in part or in whole in the steering controller 81.

[0091] If a fault occurs in the motor windings preventing any current from flowing through the motor, or the motor becomes disconnected from the control electronics, or a fault occurs in the motor drive stage or in the control system, including a loss of power to the steering wheel assembly, then it becomes impossible for the driver to control the rotation of the steering wheel to provide feedback. Figure 1 The motor of the steering wheel actuator assembly is configured to ensure that the rotation of the wheel disc is damped in this case. This is beneficial because it feels more natural to the driver and also helps the driver avoid steering inputs at excessive rates by damping the driver's actions.

[0092] In conventional prior art steering wheel actuator assemblies, motors are manufactured using high performance electrical steel for the stator, as it is generally desirable to reduce drag torque levels and the energy losses that result therefrom. Further reductions are obtained by using a laminated stator in which the electrical steel sheets are kept separated by interleaved layers of insulating material.

[0093] exist Figure 1 In the embodiments and Figure 2In an embodiment of the invention, the two motors are identical and each is configured to provide a significant level of drag torque when the driver causes the motor to spin in an unpowered state by rotating the steering wheel. This ensures that the steering wheel does not spin freely in a fault condition where power is removed from the motor or where the motor has an internal fault (meaning that the current in the windings does not produce any torque in the motor caused by the motor). An additional benefit is that more energy is dissipated within the motor than with a low drag torque motor and therefore less has to be done for the electronics to provide controlled drag, with heat being dissipated within the motor rather than from the electronics.

[0094] Those skilled in the art will appreciate that the present invention can be implemented where only one of the motors provides a significant drag torque, while the other motor is a conventional motor with low drag torque used in prior art steering wheel actuators.

[0095] By drag torque we mean the torque produced as a result of energy conversion within the motor stator while the motor is spinning. Mechanical energy from the driver causes the rotor to spin. As the rotor spins, the rotor and stator interact magnetically, producing a changing magnetic flux within the stator. This causes eddy currents and hysteresis losses, and as these currents pass through the resistive material that makes up the stator, electrical energy is converted to heat. Thus, mechanical energy is converted to heat and produces drag torque.

[0096] exist Figure 5 The following table shows the Figure 1 and Figure 2 A plan view of a first structure of a motor 200 of one or both of the motors 1, 11. The motor 200 includes a rotor 202 and a stator 201. The stator 201 includes a circular yoke 203 and a set of teeth 204, each tooth protruding radially away from the yoke 203. The end of the tooth farthest from the yoke 203 is widened to form an arc-shaped tip 205. These tips and the rotor 202 together form an air gap.

[0097] The yoke body can be a continuous tube as shown or can include a set of arcuate sections, each arcuate section supporting two or more teeth. Wire windings are wound around the teeth between the outer yoke body 202 and the tooth tips, and these wire windings are connected together to form a set of motor phases, for example, divided into three separate phases. Each phase can then be supplied with current by the motor drive circuit, and the modulation of the current controls the movement of the motor.

[0098] Each tooth 204 extends axially from the upper end to the lower end below the stator. The rotor 202 fits into the gap defined by the tips of these teeth 203 and its axis is common to the axis of the stator yoke. The rotor carries a set of permanent magnets 206.

[0099] The stator 201 in this example comprises a stack of laminations in the form of metal sheets, which may be stamped or otherwise cut from a large sheet of metal. An insulating coating may be provided on the surface of each lamination to help prevent corrosion of the metal sheets.

[0100] The stator teeth 204 and the rotor 202 define an air gap. In this example, the air gap is about 1.2 mm. A sleeve 207 of conductive material is located in the air gap, the sleeve length being about 0.4 mm. The outer surface of the sleeve abuts the tooth tips and thus occupies one third of the air gap, leaving a remaining true air gap of 0.8 mm.

[0101] The sleeve 207 in this example is a copper tube with a perfectly cylindrical inner and outer bore. In a variant, the sleeve may be provided with various grooves or ribs either on the inner or outer bore or on both, extending axially along the sleeve. In the case of providing external ribs, these external ribs may extend into the circumferential space between adjacent teeth 204.

[0102] Figure 6 It shows how eddy currents form in the sleeve because the sleeve has low electrical resistance and is relatively thick compared to the air gap length. These eddy currents provide resistance to the rotation of the rotor, helping to damp the motion of the motor when power is lost. This effect occurs because the sleeve will be stationary when the rotor is rotating, and so will be subject to flux reversals as the south and north poles pass radially "underneath" the sleeve, even when there is no power to the machine. If the tube is made of a conductive material, eddy currents and therefore heat losses will occur due to the flux reversals described above. This will be felt as a damping torque against the force turning the rotor shaft. By careful selection of the (usually high) conductivity, thickness, and other geometric features, the machine can be designed to match a specific level of damping torque.

Claims

1. An electric motor used in a steering assembly of a vehicle, the motor comprising a stator and a rotor, the stator having a plurality of phase windings, and the rotor having a plurality of magnetic poles and connected to a shaft; and in, The stator comprises a yoke having a plurality of teeth, each tooth comprising a stem and a tooth tip, the tooth tip being located at an end of the stem closest to the rotor, And the motor also includes a sleeve of conductive material located in what would otherwise be an air gap between the inwardly facing tips of the stator teeth and the rotor, and wherein the sleeve includes conductive material that provides a magnetic flux path extending axially along the motor bore so that rotation of the rotor generates eddy currents within the sleeve that resist rotation of the rotor.

2. The motor according to claim 1, wherein: The electrically conductive sleeve comprises a solid-walled tubular sleeve which fits into the air gap between the rotor and the stator.

3. The motor according to claim 1 or 2, wherein: The sleeve has an inner peripheral wall having a uniform radius at all points, and an outer peripheral wall also having a constant radius, so that the sleeve has a uniform thickness at all points.

4. The motor according to claim 1 or 2, wherein: The outer surface of the sleeve includes a plurality of ribs, each rib extending radially outwardly into the space between adjacent tips of the stator teeth.

5. A motor according to any one of the preceding claims, wherein: The height of the sleeve measured in the axial direction of the motor is substantially the same as the height of the stator.

6. A motor according to any one of the preceding claims, wherein: The radially outwardly facing surface of the sleeve is interference fit with the radially inwardly facing surface of the tip of the tooth within the air gap.

7. A motor according to any one of the preceding claims, wherein: The inner sleeve comprises a magnetically permeable material.

8. A motor according to any one of the preceding claims, wherein: The stator is coated with paint where the stator contacts the sleeve.

9. A motor according to any one of the preceding claims, wherein: If the sleeve material is too far along in the noble metal series compared to the stator material, the sleeve is tinned or otherwise passivated to prevent galvanic corrosion.

10. A motor according to any one of the preceding claims, wherein: The sleeve comprises metal or a metal alloy.

11. The motor according to claim 9, wherein: The sleeve is a copper sleeve.

12. A steering wheel actuator assembly for a steer-by-wire vehicle, the steering wheel actuator assembly comprising: case; a shaft rotatably mounted relative to the housing; One or more motors, each having a stator with a plurality of phase windings and a rotor with a plurality of magnetic poles connected to a shaft; control circuitry adapted to control the flow of current to or from the or each motor to cause a net torque to be applied to the shaft during normal operation; and wherein at least one of the motors comprises a yoke having a plurality of teeth, each tooth comprising a stem and a tooth tip, the tooth tip being located at an end of the stem closest to the rotor; And the motor also includes a sleeve of conductive material, which is located in the position of the air gap between the tips of the stator teeth and the rotor, and wherein the sleeve includes conductive material, which provides a magnetic flux path extending axially along the motor bore so that rotation of the rotor generates eddy currents in the sleeve, and the eddy currents resist rotation of the rotor.

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