Traction Control System

Abstract

A traction control system for a vehicle having a first wheel driven by a first electric motor, wherein the first electric motor includes a first set of coil windings, the traction control system includes a first controller and a second controller, the first controller being arranged to control current in the first set of coil windings for generating a drive torque for driving the first wheel, the second controller being arranged to determine a maximum wheel speed based on a first slip value of the first wheel and a vehicle speed, and to determine a minimum wheel speed based on a second slip value of the first wheel and the vehicle speed, wherein the second controller is arranged to communicate the maximum wheel speed value, the minimum wheel speed value, and a torque demand value corresponding to the drive torque for driving the first wheel to the first controller, wherein the first controller is arranged to control the current in the first set of coil windings to generate the drive torque based on the maximum wheel speed value, the minimum wheel speed value, and the torque demand value received from the second controller.

Description

Technical Field

[0001] The present invention relates to a traction control system, and more particularly to a traction control system for a vehicle having wheels driven by an electric motor. Background Art

[0002] Traction control systems are designed to enable the vehicle to accelerate, decelerate or turn in response to the driver's intentions, thereby enhancing vehicle controllability and maneuverability.

[0003] Therefore, traction control systems have become ubiquitous on modern vehicles, where these systems are designed to regulate the speed of each wheel so that they remain within an appropriate range relative to the vehicle speed. In modern vehicles, traction control systems typically avoid excessive wheel slip or sliding through a central control unit that actuates the ABS valves and the engine torque transfer system to regulate the torque delivered. In these systems, torque actuation occurs at the vehicle engine or at a centrally located ABS manifold, where the torque generated is transferred to the wheels through the transmission / drivetrain and brake hydraulic lines, respectively.

[0004] However, for traction control systems that centrally implement torque actuation, these systems may suffer from performance limitations due to the lag between the central actuation of torque and the application of the resulting torque to the road surface via the vehicle's wheels.

[0005] In the context of electric vehicle motors, an increasingly popular drive design is the integrated in-wheel motor design, where the electric motor is integrated within the wheel of the vehicle, where the use of an in-wheel motor allows the torque actuation functionality to be moved outward to the wheel itself.

[0006] However, communication between a centrally mounted traction control unit (which regulates torque demand to avoid excessive vehicle slip or wheel spin) and the in-wheel electric motors still imposes limitations on traction control system performance.

[0007] It is desirable to improve this situation. Summary of the invention

[0008] According to one aspect of the present invention, there is provided a traction control system according to the accompanying claims.

[0009] The present invention provides the advantage of allowing a traction control system to be split into a slow loop component running on a central controller, which calculates the appropriate speed limit for each in-wheel motor, and a fast control loop running on an in-wheel motor controller, wherein the slow loop component calculates the appropriate speed limit for each in-wheel motor, and the fast control loop controls the torque actuation produced by the in-wheel motor to maintain the speed limit within the speed limit calculated by the slow loop. BRIEF DESCRIPTION OF THE DRAWINGS

[0010] The present invention will now be described by way of example with reference to the accompanying drawings, in which:

[0011] Figure 1 A vehicle incorporating a traction control system according to an embodiment of the present invention is shown;

[0012] Figure 2 shows an exploded view of an electric motor embodying the present invention;

[0013] Figure 3 A schematic diagram of a control device is shown;

[0014] Figure 4 A graph showing slip ratio for different road conditions is shown. DETAILED DESCRIPTION

[0015] Figure 1 A vehicle 100, such as a car or truck, is shown having four wheels 101, wherein two wheels are located proximal and lateral to the vehicle's forward position, respectively. Similarly, two additional wheels are located proximal and lateral to the vehicle's rearward position, respectively, which is a typical conventional automobile configuration. However, as will be appreciated by those skilled in the art, a vehicle may have any number of wheels.

[0016] The in-wheel motor is incorporated into the wheel 101 at the rear position of the vehicle, as described in detail below. Although the current embodiment describes a vehicle with an in-wheel motor associated with the wheel 101 located at the rear position of the vehicle, as will be appreciated by those skilled in the art, the in-wheel motor may also be located in other wheels. For example, the in-wheel motor may be located in the front two wheels. In addition, although the present embodiment describes the use of an in-wheel motor, other motor configurations may also be used, such as two onboard motors, each of which uses a drive shaft to drive a corresponding wheel.

[0017] The control unit 102 is coupled to the in-wheel motors and a vehicle communication bus, such as a CAN bus (not shown), and controls the torque generated by the in-wheel motors in conjunction with a control device mounted on each of the respective in-wheel motors, as described below.

[0018] For purposes of illustration, an in-wheel motor is a type of in-wheel motor having a set of coils that are part of a stator for attachment to a vehicle and radially surrounded by a rotor with a set of magnets for attachment to a wheel. However, as will be appreciated by those skilled in the art, the present invention is applicable to other types of motors. Typically, the in-wheel motor is configured to provide both drive torque and regenerative braking torque, as desired.

[0019] For the purpose of this example, Figure 2As shown, the in-wheel motor includes a stator 252 (the stator 252 includes a circumferential support 253 used as a heat sink), a plurality of coils 254, two control devices (not shown) (the control devices are mounted on the circumferential support 253 on the rear of the stator for driving the coils), an annular capacitor (not shown) (also called a DC link capacitor) and a lead frame (not shown) described below (which is mounted between the axial edge of the coil and the axial flange formed on the circumferential support for coupling the control device to the coil). The coil 254 is formed on the stator tooth laminations to form a coil winding. A stator cover 256 is mounted on the rear of the stator 252, surrounding the control device and the annular capacitor to form the stator 252, which can then be fixed to the vehicle and does not rotate relative to the vehicle during use.

[0020] like Figure 3 As shown, each control device 400 includes an inverter 410, wherein one of the control devices includes control logic 420, and in this embodiment, the control logic 420 includes a processor for controlling the operation of the two inverters 410. Each inverter is coupled to three sets of coil windings arranged in parallel to form a set of three sub-motors, as described below.

[0021] The ring capacitor is coupled between the inverter 410 and the DC power supply of the motor to reduce the voltage ripple on the power line (also called the DC bus) of the motor and to reduce the voltage overshoot during the operation of the motor. In order to reduce the inductance, the capacitor is preferably installed near the control device 400.

[0022] The rotor 240 includes a front portion 220 and a cylindrical portion 221 forming a cover that substantially surrounds the stator 252. The rotor includes a plurality of permanent magnets 242 arranged around the interior of the cylindrical portion 221. For the purposes of this embodiment, 32 pairs of magnets are mounted inside the cylindrical portion 221. However, any number of magnet pairs may be used.

[0023] The magnets are located adjacent to the coil windings on the stator 252 so that the magnetic field generated by the coils interacts with the magnets 242 disposed around the interior of the cylindrical portion 221 of the rotor 240 to rotate the rotor 240. Since the permanent magnets 242 are used to generate the driving torque for driving the motor, they are often referred to as driving magnets.

[0024] The rotor 240 is attached to the stator 252 by a bearing seat, not shown. The bearing seat may be a standard bearing seat, such as would be used in a vehicle in which the motor assembly is to be installed. The bearing seat comprises two parts, a first part being fixed to the stator and a second part being fixed to the rotor. The bearing seat is fixed to a central portion of the wall of the stator 252 and also to a central portion of the housing wall 220 of the rotor 240. The rotor 240 is thus rotationally fixed to the vehicle to be used therewith by means of a bearing seat at the central portion of the rotor 240. This has the advantage that the rim and the tire can then be fixed to the central portion of the rotor 240 using ordinary wheel bolts, which can then be used to fix the rim to the central portion of the rotor and thus securely fixed to the rotatable side of the bearing seat. The wheel bolts can be installed through the central portion of the rotor into the bearing seat itself. In the case where both the rotor 240 and the wheel are mounted to the bearing seat, there is a one-to-one correspondence between the rotation angles of the rotor and the wheel.

[0025] The rotor also includes a set of magnets (not shown) for position sensing, also known as commutation magnets, which, in conjunction with sensors mounted on the stator, can estimate the rotor flux angle, which the control device uses to modulate the current flow in the coils using space vector pulse width modulation, as described below. The rotor flux angle defines the positional relationship of the drive magnets to the coil windings. Alternatively, instead of a set of separate magnets, the rotor can include a ring of magnetic material with multiple poles that act as a set of separate magnets.

[0026] To allow the commutation magnets to be used to calculate the rotor flux angle, preferably each drive magnet has an associated commutation magnet, wherein the rotor flux angle is derived from the flux angle associated with the commutation magnet group by calibrating the measured commutation magnet flux angle. To simplify the correlation between the commutation magnet flux angle and the rotor flux angle, preferably the group of commutation magnets has the same number of magnets or pole pairs as the group of drive magnet pairs, wherein the commutation magnets and the associated drive magnets are approximately radially aligned with each other. Therefore, for the purposes of this embodiment, the group of commutation magnets has 32 magnet pairs, wherein each magnet pair is approximately radially aligned with the corresponding drive magnet pair.

[0027] The sensor (in this embodiment, a Hall sensor) is mounted on the stator. The sensor is placed in a suitable position so that when the rotor rotates, each commutation magnet forming the commutation magnet ring rotates past the sensor.

[0028] When the rotor rotates relative to the stator, the commutation magnet rotates past the sensor accordingly, and the Hall sensor outputs an AC voltage signal, where the sensor outputs a complete voltage cycle of 360 electrical degrees for each magnet pair passing through the sensor, where the AC voltage signal output by the Hall sensor can be used for rotor position detection and determining the rotor speed (ω).

[0029] To improve position detection, the sensor preferably comprises an associated second sensor positioned 90 electrical degrees from the first sensor.

[0030] In the present embodiment, the motor includes six coil groups, each coil group having three coil sub-groups, which are coupled in a star configuration to form a three-phase sub-motor, resulting in the motor having six three-phase sub-motors, wherein as described above, the corresponding coils of the six coil groups are wound around corresponding stator teeth that form part of the stator. As described below, the operation of the corresponding sub-motor is controlled by one of the two control devices 300. Although the present embodiment describes a motor having six coil groups (i.e., six sub-motors), the motor can also have one or more coil groups with associated control devices. Similarly, each coil group can have any number of coil sub-groups, allowing each sub-motor to have two phases or more.

[0031] Figure 3 The connection between the respective coil groups 60 and the control device 400 is shown, wherein three coil groups 60 are connected to respective three-phase inverters 410 included on the control device 400. As known to those skilled in the art, a three-phase inverter includes six switches, wherein a three-phase AC voltage can be generated by controlled operation of the six switches.

[0032] The six switches are configured as three parallel groups of two switches, where each pair of switches is placed in series and forms a branch of a three-phase bridge circuit.

[0033] However, the number of switches will depend on the number of voltage phases to be applied to the respective sub-motors, where the sub-motors may be configured with any number of phases. Each control device 400 is arranged to communicate with another control device 400 via a communication bus 440 .

[0034] Preferably, the control device 400 has a modular construction. In a preferred embodiment, each control device (also referred to as a power module) includes: a power printed circuit board on which a control printed circuit board is mounted; two power busbars for connecting to a DC battery through a DC line capacitor; three-phase winding busbars for connecting to corresponding coil windings through a lead frame; and a power substrate assembly including an inverter.

[0035] The power printed circuit board includes various other components, including drivers for the inverter switches formed on the power substrate assembly, wherein these drivers are used to convert control signals from the control printed circuit board into a suitable form for operating the switches mounted on the power printed circuit board, but these components will not be discussed in any greater detail.

[0036] One control device 400 comprises a processor 420 for controlling the operation of the inverter switches in both control devices 400. In addition, each control device 400 comprises an interface arrangement to allow communication between the respective control devices 400 via a communication bus 440, wherein one of the control devices 400 is arranged to communicate with a control unit 102 mounted externally to the motor.

[0037] The processor 420 in the corresponding control device 400 is arranged to control the operation of the inverter switch installed in each control device 400 to allow the three-phase voltage power to be supplied to each motor coil group 60, thereby allowing the corresponding coil sub-group to generate a rotating magnetic field. As described above, although the present embodiment describes each coil group 60 as having three coil sub-groups, the present invention is not limited thereto, and it should be understood that each coil group 60 may have one or more coil sub-groups.

[0038] Under the control of the processor, each three-phase bridge inverter 410 is arranged to provide PWM voltage control in the corresponding coil sub-group, thereby generating current in the corresponding coil sub-group to provide the required torque through the corresponding sub-motor.

[0039] PWM control works by using the motor inductance to average the applied pulsed voltage to drive the desired current into the motor coils. Using PWM control, the applied voltage is switched across the motor windings. When the voltage is switched across the motor coils, the current in the motor coils rises at a rate determined by their inductance and the applied voltage. The PWM voltage control is turned off before the current increases beyond the desired value, allowing precise control of the current.

[0040] For a given coil group 60, the three-phase bridge inverter 310 switches are arranged to apply a single voltage phase across each coil subgroup.

[0041] Using PWM switching, multiple switches are arranged to apply an AC voltage across the corresponding coil subsets. The voltage envelope and phase angle of the electrical signal are determined by the modulated voltage pulses.

[0042] The inverter switches may include semiconductor devices such as MOSFETs or IGBTs. In this example, the switches include IGBTs. However, any suitable known switching circuits may be used to control the current.

[0043] The inverter 410 formed on the power supply component in one control device 400 is coupled to three coil groups to form a first group of three sub-motors, and the inverter 410 formed on the power supply component in another control device 400 is coupled to another coil group to form a second group of three sub-motors.

[0044] Both inverters 410 are coupled to respective coil sets via lead frames, wherein each branch of the respective inverter is coupled to the lead frame via respective phase winding busbars. For the purposes of this embodiment, the different voltage phases produced by each inverter branch are labeled W, V, and U.

[0045] As described below, the coil windings are coupled to the lead frame to allow current to flow from a DC power source via a corresponding inverter in the control device to the coil windings to allow drive torque to be generated by the electric motor.

[0046] As described above, the processor is arranged to receive the torque request from the control unit 102 via a CAN interface, however, any form of communication link between the control unit 102 and the corresponding motor drive controller 80 may be used.

[0047] Since each in-wheel motor is directly coupled to the wheel, this allows the torque produced by the respective in-wheel motor to be immediately applied to the wheel, wherein the control circuit can know exactly what torque is produced at any given time within the control device attached to the in-wheel motor. Therefore, the in-wheel motor provides the advantage of having an extremely fast torque response and a speed sensing loop.

[0048] The torque request will typically be initiated by a user of the vehicle 100 to indicate a desire to increase or decrease the acceleration of the vehicle, for example by using a throttle pedal or via a vehicle control unit (which may be incorporated into the control unit 102), which automatically controls the speed / acceleration of the vehicle, which may be, for example, an autonomous vehicle controller providing a level of autonomous driving. The torque request is received by the control unit 102 and forwarded to the corresponding in-wheel motor in the form of a torque demand command.

[0049] An embodiment of performing traction control using the control unit 102 and each control device respectively installed in each in-wheel motor will now be described, wherein during acceleration or braking of the vehicle, traction control is used to maximize the friction coefficient between the vehicle and the road surface by controlling the drive torque applied by the corresponding in-wheel motor.

[0050] As is well known to those skilled in the art, when a vehicle is accelerating or braking, the observed tire angular velocity is different from the vehicle's speed due to slip between the vehicle's tires and the road. The speed difference is usually expressed as a percentage and is called "slip ratio". One equation for determining slip ratio in the absence of vehicle yaw is:

[0051] S=(V vehicle –rω) / V vehicle

[0052] Where V vehicle is the longitudinal velocity of the vehicle;

[0053] r is the wheel radius at the contact point;

[0054] ω is the angular velocity of the tire.

[0055] Although the above equations calculate slip for a vehicle experiencing longitudinal acceleration or deceleration, it will be apparent to one skilled in the art that appropriate equations may also be used to determine slip for a vehicle experiencing combined longitudinal and lateral acceleration.

[0056] Figure 4 , which shows an example of the variation of the coefficient of friction with respect to the slip ratio for an accelerating vehicle under different road conditions. For the acceleration condition, the slip ratio value that provides the maximum coefficient of friction between the tire and the road surface provides the maximum torque transfer between the road and the vehicle. Figure 4 As shown, above a certain slip ratio, the friction coefficient between the road surface and the tire drops dramatically, which can lead to wheel slippage or wheel locking. The purpose of the described traction control system is to avoid this situation.

[0057] As part of its function within the traction control system, the control unit 102 is arranged to determine the speed of the vehicle. For example, the speed of the undriven wheels of the vehicle may be measured, or GPS measurements may be used to determine the speed of the vehicle; for example, or, any suitable means may be used.

[0058] In order to achieve optimal torque transfer between the road and the vehicle under both acceleration and braking, the control unit 102 is configured to use the vehicle speed information to determine a maximum desired slip limit for each wheel under acceleration conditions, and to determine a minimum desired slip limit for each wheel under braking conditions.

[0059] For example, the control unit 102 may be arranged to map the vehicle speed to a maximum / minimum slip ratio, wherein the mapping function may be performed in a variety of ways, such as via a table or using an algorithm.

[0060] Knowing the speed of the vehicle and having a maximum desired slip limit value and a minimum desired slip limit value, using the slip equation above, the control unit 102 is arranged to calculate a maximum speed limit and a minimum speed limit for each wheel driven by the in-wheel motor. In other words, for a given vehicle speed, the maximum speed limit of the wheel (i.e. under acceleration) will result in a slip between the tire mounted on the wheel and the road that will correspond to the maximum desired slip, and the minimum speed limit of the wheel (i.e. under braking) will result in a slip between the tire mounted on the wheel and the road that will correspond to the minimum desired slip. However, any suitable means for determining the maximum speed limit and the minimum speed limit using the maximum desired slip limit value and the minimum desired slip limit value may be used.

[0061] The control unit 102 is arranged to communicate a torque demand request and a maximum speed limit value and a minimum speed limit value associated with each drive wheel to the respective in-wheel electric motor, for example via a CAN bus.

[0062] Once a torque demand request is received by the respective in-wheel electric motor, the in-wheel electric motor is arranged to control the current in the coil windings to produce the requested torque demand, as described above, while monitoring the rotational speed of the rotor.

[0063] Because the mass of the vehicle is greater than the mass of the vehicle's wheels, when torque is applied directly to the wheels, the vehicle's speed changes are typically relatively slow compared to the change in speed of the wheels, which can cause the wheels to enter a slip state.

[0064] Therefore, the rate of update of the maximum speed limit and the minimum speed limit of the vehicle generated by the control unit 102 may be performed relatively slowly compared to the rate of update required for the torque control exerted by the control means of the in-wheel motors.

[0065] The present invention therefore provides the advantage of allowing a slow loop vehicle speed limit to be generated at a remote traction control unit, while a fast loop control torque limit is controlled directly at the wheel via the in-wheel electric motor.

[0066] If the control device for each in-wheel motor determines that the received torque demand request will cause the corresponding in-wheel motor to generate a torque that causes the rotational speed of the rotor for any in-wheel motor to exceed a value corresponding to the maximum speed limit or the minimum speed limit, or to exceed a threshold value that is above the maximum speed limit or below the minimum speed limit, the control device for the corresponding in-wheel motor is arranged to prevent this from occurring, even if this will result in the torque generated by the corresponding in-wheel motor being different from the torque demand value received from the control unit 102.

[0067] Therefore, if the torque demand received by the respective in-wheel motor from the control unit 102 will cause the slip ratio of the wheel coupled to the respective in-wheel motor to exceed the maximum slip limit or the minimum slip limit, the control means associated with the respective in-wheel motor is arranged to limit the torque to a value less than the torque demand received from the control unit 102, thereby preventing the maximum slip limit or the minimum slip limit from being exceeded. Preferably, in this case, the control means associated with the respective in-wheel motor is arranged to generate a drive torque that will cause the wheel speed to substantially correspond to the maximum wheel speed or the minimum wheel speed.

Claims

1. A traction control system for a vehicle having a first wheel driven by a first electric motor, wherein: The first electric motor comprises a first set of coil windings, the traction control system comprises a first controller and a second controller, the first controller being arranged to control current in the first set of coil windings for generating a drive torque for driving a first wheel, the second controller being arranged to determine a maximum wheel speed value based on a first slip value of the first wheel and a vehicle speed and a minimum wheel speed value based on a second slip value of the first wheel and the vehicle speed, wherein the second controller is arranged to communicate the maximum wheel speed value, the minimum wheel speed value and a torque demand value corresponding to the drive torque for driving the first wheel to the first controller, wherein the first controller is arranged to control current in the first set of coil windings to generate the drive torque based on the maximum wheel speed value, the minimum wheel speed value and the torque demand value received from the second controller, wherein the first controller is arranged to control current in the first set of coil windings to generate the drive torque corresponding to the torque demand value unless the first controller determines that the received torque demand value will cause the first wheel to exceed the maximum wheel speed value or the first controller determines that the torque demand value will cause the first wheel to fall below the minimum wheel speed value.

2. The traction control system according to claim 1, wherein: The first slip ratio value is a maximum desired slip ratio.

3. The traction control system according to claim 1, wherein: The second slip ratio value is a minimum desired slip ratio.

4. The traction control system according to claim 1, wherein: The torque demand value is derived from a user input, received from a third controller, or generated by the second controller.

5. The traction control system according to claim 1, wherein: Upon determining that the torque demand value will cause the first wheel to exceed a maximum wheel speed value, the first controller is arranged such that the generated drive torque will not cause the first wheel to exceed a threshold value above the maximum wheel speed value.

6. The traction control system of claim 1, wherein: Upon determining that the torque demand value will cause the first wheel to drop below the minimum wheel speed value, the first controller is arranged such that the drive torque generated will not cause the first wheel to exceed a threshold below the minimum wheel speed value.

7. The traction control system of claim 1, wherein: The first controller is arranged to determine whether the torque demand value will cause the first wheel to exceed the maximum wheel speed value. If the first controller determines that the torque demand value will cause the first wheel to exceed the maximum wheel speed value, the first controller is arranged to generate a drive torque that will cause the wheel speed to correspond to the maximum wheel speed value.

8. A traction control system according to claim 1, wherein the first controller is arranged to determine whether the torque demand value will cause the first wheel to be below the minimum wheel speed value, and if the first controller determines that the torque demand value will cause the first wheel to be below the minimum wheel speed value, the first controller is arranged to generate a drive torque that will cause the wheel speed to correspond to the minimum wheel speed value.

9. The traction control system of claim 1, wherein: The second controller includes means for receiving a vehicle speed value.

10. The traction control system of claim 9, wherein: The means for receiving a speed value is arranged to receive a speed value for a second wheel of the vehicle which is not driven by the driving torque.

11. The traction control system of claim 1 , wherein: The first electric motor is a first in-wheel electric motor.

12. A method for performing traction control for a vehicle having wheels driven by an electric motor, wherein: The electric motor comprises a first set of coil windings, the method comprising transmitting a maximum wheel speed value, a minimum wheel speed value and a torque demand value corresponding to a drive torque for driving a wheel from a second controller to the first controller, wherein the second controller is arranged to determine the maximum wheel speed value based on a first slip value of the wheel and a vehicle speed and to determine the minimum wheel speed value based on a second slip value of the wheel and the vehicle speed, wherein the first controller is arranged to control a current in the first set of coil windings to produce a drive torque based on the maximum wheel speed value, the minimum wheel speed value and the torque demand value received from the second controller, wherein the first controller is arranged to control a current in the first set of coil windings to produce a drive torque corresponding to the torque demand value unless the first controller determines that the received torque demand value will cause the wheel to exceed the maximum wheel speed value or the first controller determines that the torque demand value will cause the wheel to fall below the minimum wheel speed value.

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