Brake system

Abstract

1. A braking system for a vehicle having a first electric motor arranged to apply a regenerative braking torque to a first wheel and a friction braking device arranged to apply a friction braking torque to the first wheel, the braking system comprising: a controller arranged to generate, in response to receiving a torque request, a first control signal for applying regenerative braking torque to the first wheel by the first electric motor, the first control signal providing an indication of a regenerative braking torque value, and a second control signal for applying the friction braking torque to the first wheel by the friction braking device, the second control signal providing an indication of a friction braking torque value, wherein, in determining an estimate of the friction braking torque to be applied to the first wheel, the controller adjusts the regenerative braking torque applied to the first wheel by the first electric motor based on a difference between the friction braking torque value indicated by the second control signal and the estimated friction braking torque.

Description

[Technical Field]

[0001] The present invention relates to a braking system, in particular to a braking system for a vehicle having wheels driven by an electric motor. [Background technology]

[0002] Typically, the electric motor drive system used to drive the vehicle is also used to provide regenerative braking. However, due to the vehicle's braking requirements, the electric motor drive system cannot provide all of the necessary braking torque. Therefore, braking systems for electric vehicles typically use a combination of friction braking and regenerative braking. Summary of the Invention [Problem to be solved by the invention]

[0003] However, torque-intensive braking systems can be subject to performance limitations due to the time lag between the application of the braking torque intensive and the application of the generated braking torque to the vehicle.

[0004] In the context of electric vehicle motors, the integrated in-wheel electric motor design, where the electric motor is integrated within the wheel of the vehicle, is an increasingly popular drive design; the use of an in-wheel motor allows torque actuation functions to be directed towards the wheel itself.

[0005] In-wheel motors also offer the advantage of providing an instantaneous speed control loop running on the in-wheel motor controller, which controls the torque actuation produced by the in-wheel motor, allowing for rapid torque modulation.

[0006] However, integrating an instantaneous speed control loop operating with an in-wheel electric motor and a friction braking system to ensure that an optimum level of braking torque is applied at all times can be problematic.

[0007] It is hoped that this situation will be improved. [Means for solving the problem]

[0008] According to one aspect of the present invention there is provided a braking system as set out in the claims.

[0009] The present invention provides the advantage of allowing the braking system to match different levels of braking between friction braking and regenerative braking based on certain operating modes of the vehicle, for example, whether or not antilock brakes are engaged. [Brief explanation of the drawings]

[0010] The invention will now be described, by way of example only, with reference to the accompanying drawings, in which: [Figure 1] 1 is a diagram showing a vehicle incorporating a traction control system according to an embodiment of the present invention; [Figure 2] 1 is an exploded perspective view of a motor embodying the present invention; [Figure 3] FIG. 2 is a schematic diagram of a control device. [Figure 4] FIG. 2 illustrates a braking system controller according to an embodiment of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0011] 1 shows a vehicle 100, such as a car or lorry, having four wheels 101, two of which are located at the front of the vehicle, on the near side and off side, respectively. Similarly, two additional wheels are located at the rear of the vehicle, on the near side and off side, respectively, as is typical of conventional automotive configurations. However, as will be appreciated by those skilled in the art, a vehicle may have any number of wheels.

[0012] Each wheel includes a friction brake for applying a friction braking torque to the respective wheel.

[0013] Incorporated within the wheels 101 at the rear of the vehicle are in-wheel electric motors, as described in more detail below. While the current embodiment describes a vehicle having in-wheel electric motors associated with the wheels 101 located at the rear of the vehicle, those skilled in the art will appreciate that in-wheel electric motors may be located on other wheels. For example, in-wheel electric motors may be located on the two front wheels. Furthermore, while the present embodiment describes the use of in-wheel electric motors, other electric motor configurations may also be used, such as two inboard (body) mounted electric motors, each using a drive shaft to drive a respective wheel.

[0014] A control unit 102 coupled to the in-wheel electric motors and to a vehicle communication bus, e.g., a CAN bus (not shown), is arranged to control, in conjunction with controllers attached to each in-wheel electric motor, the drive and braking torques (i.e., regenerative braking torques) generated by the in-wheel electric motors, as described below.

[0015] As described below, the control unit 102 is arranged to generate wheel braking torque requests that are used to generate friction braking torque applied by friction braking and regenerative braking torque applied by the in-wheel electric motors. The torque requests are typically initiated by a user of the vehicle 100 indicating a desire to increase or decrease vehicle acceleration, for example using a throttle pedal and / or brake pedal, or via a vehicle control unit that may be incorporated within the control unit 102 that automatically controls vehicle speed / acceleration, such as an autonomous vehicle controller that provides a level of autonomous driving.

[0016] For purposes of explanation, an in-wheel electric motor is of the type having a set of coils that are part of a stator for mounting to the vehicle and radially surrounded by a rotor that has a set of magnets for mounting to the wheel. However, as will be appreciated by those skilled in the art, the present invention is applicable to other types of electric motors. Typically, an in-wheel electric motor is configured to provide both driving torque and regenerative braking torque, as required.

[0017] For purposes of this embodiment, as shown in FIG. 2 , the in-wheel electric motor includes a stator 252 with circumferential supports 253 that function as a heat sink, a plurality of coils 254, two controllers (not shown) mounted to the circumferential supports 253 at a rear portion of the stator for driving the coils, an annular capacitor (not shown), also referred to as a DC link capacitor, and a lead frame (not shown) described below mounted between the axial ends of the coils and an axial flange formed on the circumferential supports for coupling the controllers to the coils. The coils 254 are formed on stator tooth laminations to form the coil windings. A stator cover 256 is attached to the rear of the stator 252 and encloses the controllers and annular capacitor to form the stator 252, which is fixed to the vehicle and does not rotate relative to the vehicle during use.

[0018] 3, each controller 400 includes an inverter 410, and one of the controllers includes control logic 420, which in this embodiment includes a processor, for controlling the operation of both inverters 410. Each inverter is coupled to three sets of coil windings arranged electrically in parallel to form three sub-motors, as described below.

[0019] The annular capacitor (capacitor) is coupled between the inverter 410 and the electric motor's DC power supply to reduce voltage ripple in the electric motor's power supply lines (also known as DC bus bars) and to reduce voltage overshoot during electric motor operation. The capacitor is preferably mounted adjacent to the controller 400 to reduce inductance.

[0020] The rotor 240 includes a front portion 220 and a cylindrical portion 221 that forms a cover that substantially surrounds the stator 252. The rotor includes a plurality of permanent magnets 242 disposed inside the cylindrical portion 221. In this embodiment, 32 pairs of magnets are mounted inside the cylindrical portion 221, although any number of magnet pairs may be used.

[0021] The magnets are adjacent to the coil windings of the stator 252 so that the magnetic field generated by the coils interacts with magnets 242 located inside the cylindrical portion 221 of the rotor 240 to rotate the rotor 240. The permanent magnets 242 are typically referred to as drive magnets because they are used to generate the drive torque to drive the electric motor.

[0022] The rotor 240 is attached to the stator 252 by a bearing block (not shown). The bearing block can be a standard bearing block such as that used in the vehicle to which this motor assembly is attached. The bearing block consists of two parts: a first part fixed to the stator and a second part fixed to the rotor. The bearing block is fixed to the central portion of the wall of the stator 252 and also to the central portion of the housing wall 220 of the rotor 240. Thus, the rotor 240 is rotatably fixed to the vehicle via the bearing block at its central portion. This has the advantage that a wheel rim and tire can be fixed to the central portion of the rotor 240 using normal wheel bolts, thereby firmly securing the wheel rim to the rotatable side of the bearing block. The wheel bolts may pass through the central portion of the rotor and attach to the bearing block itself. Because both the rotor 240 and the wheel are attached to the bearing block, there is a one-to-one correspondence between the rotor and the wheel in terms of rotation angle.

[0023] The rotor also includes a set of position-sensing magnets (not shown), known as commutating magnets, which, in combination with sensors mounted on the stator, can estimate the rotor flux angle, which is used by a controller to control current flow in the coils using space vector pulse-width modulation, as described below. Alternatively, instead of a single set of independent magnets, the rotor can include a ring of magnetic material with multiple poles that acts as a single set of independent magnets.

[0024] To enable the rotor flux angle to be calculated using the commutating magnets, preferably each drive magnet has an associated commutating magnet, and the rotor flux angle is derived from the flux angle associated with the set of commutating magnets by calibrating the measured commutating magnet flux angle. To simplify the correlation between the commutating magnet flux angle and the rotor flux angle, preferably the set of commutating magnets has the same number of magnets or magnet pole pairs as the set of drive magnet pairs, and the commutating magnets and associated drive magnets are generally radially aligned with each other. Thus, for purposes of this embodiment, the set of commutating magnets has 32 magnet pairs, each magnet pair generally radially aligned with a respective drive magnet pair.

[0025] The sensor, which in this embodiment is a Hall sensor, is mounted on the stator and positioned so that as the rotor rotates, each of the commutating magnets forming the commutating magnet ring rotates past the sensor.

[0026] As the rotor rotates relative to the stator, the commutating magnets rotate past the sensors, causing the Hall sensors to output an AC voltage signal. The sensor outputs a complete 360° voltage cycle for each magnet pair that passes the sensor. The AC voltage signal output by the Hall sensors can be used to both sense rotor position and determine rotor speed (ω).

[0027] To improve position detection, the sensor preferably has an associated second sensor positioned 90 electrical degrees from the first sensor.

[0028] In this embodiment, the electric motor has six coil sets, each having three coil subsets, which are joined in a Y-configuration to form a three-phase sub-motor, resulting in six three-phase sub-motors, with each coil of the six coil sets wound on an individual stator tooth forming a portion of the stator, as described above. Operation of each sub-motor is controlled via one of two controllers 300, described below. While this embodiment describes an electric motor having six coil sets (i.e., six sub-motors), a motor may similarly have one or more coil sets with associated controllers. Similarly, each coil set may have any number of coil subsets, thereby allowing each sub-motor to have two or more phases.

[0029] 3 shows the connections between each coil set 60 and the control device 400, with the three coil sets 60 connected to respective three-phase inverters 410 included in the control device 400. As is well known to those skilled in the art, a three-phase inverter includes six switches, and a three-phase AC voltage is generated by the controlled operation of the six switches.

[0030] However, the number of switches depends on the number of voltage phases applied to each sub-motor, and the sub-motors can be configured to have any number of phases. Each controller 400 is arranged to communicate with the other controllers 400 via a communication bus 440.

[0031] Preferably, the controller 400 is of modular construction. In a preferred embodiment, each controller, known as a power module, includes a power printed circuit board with a control printed circuit board mounted thereon, two power bus bars for connection to a DC battery via DC link capacitors, three-phase winding bus bars for connection to respective coil windings via lead frames, and a power board assembly including an inverter.

[0032] The power supply printed circuit board includes various other components, including drivers for the inverter switches formed on the power supply board assembly, which are used to convert control signals from the control printed circuit board into a form suitable for operating the switches mounted on the power supply printed circuit board, but these components will not be described in further detail.

[0033] One of the controllers 400 includes a processor 420 for controlling the operation of the inverter switches of both controllers 400. Additionally, each controller 400 includes an interface arrangement that allows communication between the respective controllers 400 via a communication bus 440, with one controller 400 being arranged to communicate with a control unit 102 mounted externally to the electric motor.

[0034] A processor 420 in each controller 400 is arranged to control the operation of inverter switches mounted in each controller 400 to supply a three-phase voltage to each electric motor coil set 60, thereby enabling each coil subset to generate a rotating magnetic field. As described above, in this embodiment, each coil set 60 is described as having three coil subsets, but the present invention is not limited thereto, and each coil set 60 may have one or more coil subsets.

[0035] Under the control of the processor, each three-phase bridge inverter 410 is arranged to provide PWM voltage control across a respective coil subset, thereby generating current flow in the respective coil subset to provide the torque required for the respective sub-motor.

[0036] PWM control works by using the motor's inductance to average the applied pulse voltage and allow the necessary current to flow through the motor coils. PWM control switches the voltage applied to the motor's windings. While the voltage across the motor coils is switching, the current rises at a rate determined by the motor's coil inductance and the applied voltage. PWM voltage control turns off before the current increases more than necessary, allowing for precise current control.

[0037] For a given coil set 60, the three-phase bridge inverter 310 switches are arranged to apply a single voltage phase to each of the coil subsets.

[0038] Using PWM switching, multiple switches are arranged to apply an alternating voltage across each of the coil subsets, with the voltage envelope and phase angle of the electrical signal determined by the modulated voltage pulses.

[0039] The inverter switches may include semiconductor devices such as MOSFETs or IGBTs. In this embodiment, the switches are IGBTs. However, any suitable known switching circuit may be employed to control the current.

[0040] An inverter 410 formed in the power supply assembly of one control device 400 is coupled to three coil sets to form a first set of three sub-motors, and an inverter 410 formed in the power supply assembly of the other control device 400 is coupled to another coil set to form a second set of three sub-motors.

[0041] Both inverters 410 are coupled to their respective coil sets via lead frames, and each leg of each inverter is coupled to the lead frame via a respective phase winding bus bar. In this embodiment, the different voltage phases generated by each inverter leg are designated W, V, and U.

[0042] The coil windings are coupled to a lead frame as described below, and current is passed from a DC power source through inverters in the control device to the coil windings, enabling the electric motor to generate drive torque.

[0043] As mentioned above, the processor is arranged to receive torque requests from the control unit 102 via a CAN interface, although any form of communication link between the control unit 102 and each motor drive controller 80 may be used.

[0044] Since each in-wheel electric motor is directly connected to a wheel, the torque generated by each in-wheel electric motor can be instantly applied to the wheel, and the torque generated at any time can be accurately known by a control circuit in a control device attached to the in-wheel electric motor. Therefore, in-wheel electric motors offer the advantage of having both an extremely fast torque response and a speed detection loop.

[0045] As mentioned above, the torque request is typically initiated either by a user of the vehicle 100 indicating a request to increase or decrease the vehicle's acceleration, e.g., using the throttle pedal and / or brake pedal, or via a vehicle control unit, which may be incorporated within the control unit 102, that automatically controls the vehicle's speed / acceleration, e.g., via an autonomous vehicle controller that provides a level of autonomous driving. The torque request is received by the control unit 102 and forwarded directly to the respective in-wheel electric motors in the form of a torque request command and / or a brake system controller.

[0046] Next, an embodiment of a brake system controller 600 according to the present invention will be described with reference to FIG.

[0047] The braking torque demand is communicated from the control unit 102 to a braking system controller 600 shown in FIG. 4, which includes a motor torque limit input 610, a torque margin value 620, a braking torque estimate input 630, an output motor torque demand 640, a filter 650, a torque to pressure converter 670, a motor torque limit function 670, and a brake pressure demand output 660.

[0048] The braking torque request received by the controller 600 indicates the total braking torque that needs to be applied to the wheels having in-wheel electric motors and friction braking.

[0049] The value assigned to the motor torque limit indicates the maximum regenerative braking torque that can be applied by the in-wheel electric motors. As described below, the effective motor torque limit can be modified using a torque margin value 620, which is subtracted from the motor torque limit, thereby providing a mechanism to limit the amount of regenerative braking that can be applied by the in-wheel electric motors. Based on the motor torque limit, the braking torque request is separated into a friction braking torque component and a regenerative braking torque component. For example, if the total braking torque request is 1000 Nm and the motor torque limit (i.e., the maximum regenerative braking torque that can be applied to the in-wheel electric motors) is 300 Nm, the controller will issue a friction braking torque request of 700 Nm, with the remaining braking torque being applied by the in-wheel electric motors via regenerative braking.

[0050] The friction braking torque demand is converted to a brake pressure demand using a filter 650 and a torque-to-pressure converter, which provides the friction braking pressure demand based on the characteristics of the braking system.

[0051] To correct for variations between the friction braking torque demand generated by controller 600 and the actual friction braking torque applied by friction braking, an estimate of the applied friction braking torque is determined by any suitable means, for example, an estimate of the actual brake pressure combined with a torque-to-pressure model, and compared to the friction braking torque demand. Controller 600 is configured to compensate for the difference between the estimated braking torque and the friction braking torque demand by adjusting the regenerative braking torque demand transmitted to the in-wheel electric motors; the fast torque response of the in-wheel electric motors allows this to be achieved without a significant degradation in braking performance.

[0052] To maximize the efficiency of the braking system and obtain the maximum regenerative current from the in-wheel electric motors during braking, it is desirable for as much braking as possible to be performed by the in-wheel electric motors. However, when using the in-wheel electric motors to generate that maximum regenerative current, if the estimated friction braking torque is less than the friction braking torque demand generated by controller 600, this difference in braking torque cannot be corrected using the in-wheel electric motors but must be corrected by varying the friction braking torque demand, which is applied more slowly than would be achieved using the in-wheel electric motors.

[0053] To address this issue, the torque demand is varied for different operating modes of the brake controller.

[0054] In a preferred embodiment, the controller 600 is configured to operate in two modes of operation: in a first mode of operation corresponding to a normal braking mode, the controller 600 is arranged to emphasize maximizing regenerative efficiency, and in a second mode of operation corresponding to an anti-lock braking system mode, the controller 600 is arranged to emphasize braking efficiency.

[0055] In the first operating mode, the emphasis is on maximizing regenerative braking efficiency by setting a low torque margin value, which, as described above, is used to modify the value assigned to the motor torque limit. For example, by setting the torque margin value to zero, the value assigned to the motor torque limit is used to determine the friction braking torque request value without modification from the torque margin value. For example, if the braking torque request value is 1000 Nm and the motor torque limit value is 300 Nm, the controller 600 generates a motor braking torque request value of 300 Nm and a friction torque request value of 700 Nm. As described above, fluctuations between the braking torque estimate and the friction braking torque request value are accommodated by adjusting the friction braking torque request value, and during normal braking operations, the degradation of brake response time due to adjusting the friction braking is not noticeable.

[0056] In the second mode of operation, emphasis is placed on maximizing braking efficiency at the expense of regenerative braking efficiency, and a higher torque margin value is set than in the first mode, in which the torque margin value is subtracted from the motor torque limit value. As a result, for a given braking torque demand, the higher torque margin value in the second mode of operation results in a higher friction braking demand relative to the motor braking torque demand, compared to the lower torque margin value in the first mode of operation.

[0057] For example, by setting the torque margin value to 100 Nm, a reduced motor torque limit is used to determine the friction braking torque request. As an example, if the braking torque request is 1000 Nm, the motor torque limit is 300 Nm, and the torque margin value is 100 Nm, the controller generates a motor braking torque request of 200 Nm (i.e., 300-100) and a friction braking torque request of 800 Nm. As a result, because the electric motor still has the ability to provide additional regenerative braking, any fluctuations between the braking torque estimate and the friction braking request can be resolved by adjusting the motor braking torque up to a value of 100, thereby taking advantage of the fast torque response provided by the electric motor to quickly apply any braking corrections. To ensure that the motor torque limit is not exceeded, the motor torque limit function 670 monitors the motor braking torque request to ensure that it is equal to or less than the motor torque limit.

[0058] The controller 600 can switch between the first and second operating modes using any suitable means, for example, via a control signal from the control unit 102 indicating that an ABS braking response is required, or via detection of a wheel of the vehicle driven by an in-wheel electric motor having a wheel speed below a minimum wheel speed.

[0059] To improve the driving experience, a filter can be used to filter the commanded friction braking torque value, and the filtering characteristics of the filter change between the first operating mode and the second operating mode.

[0060] For example, in a preferred embodiment, the filter is configured to apply friction braking torque more smoothly in the first mode of operation compared to the second mode of operation.

[0061] Preferably, the controller is arranged to adjust the regenerative braking torque applied to the first wheel by the first electric motor to maintain the speed of the first wheel at or above a minimum wheel speed, the minimum wheel speed being determined based on a first slip ratio value for the first wheel and vehicle speed.

[0062] Additionally, in a preferred embodiment, the control unit 102 provides traction control functionality, and the control unit 102 is configured to determine the speed of the vehicle. For example, the speed of the non-driven wheels of the vehicle may be measured, or GPS measurements may be used to determine the speed of the vehicle, although any suitable means may be used.

[0063] To achieve optimal torque transfer between the road surface and the vehicle during both acceleration and braking, the control unit 102 is configured to use the vehicle speed information to determine a maximum required slip ratio limit for each wheel under acceleration conditions and a minimum required slip ratio limit for each wheel under braking conditions.

[0064] For example, the control unit 102 may be configured to map vehicle speed to a maximum / minimum slip ratio, and the mapping function may be performed in any number of ways, such as via a table or using an algorithm.

[0065] Knowing the vehicle speed and having the maximum and minimum required slip ratio limits, the control unit 102 is configured to calculate the maximum and minimum speed limits for each wheel driven by an in-wheel electric motor. In other words, for a given vehicle speed, the highest wheel speed limit (i.e., acceleration state) will cause slip between the tire mounted on the wheel corresponding to the maximum required slip ratio and the road surface, and the lowest wheel speed limit (i.e., braking state) will cause slip between the tire mounted on the wheel corresponding to the minimum required slip ratio and the road surface. However, any suitable means for determining the maximum and minimum speed limits using the maximum and minimum slip ratio limits can be used.

[0066] The controller 102 is configured to communicate the torque demand, maximum speed limit and minimum speed limit associated with each drive wheel to the respective in-wheel electric motor and / or controller, as described above, which in a preferred embodiment is integrated into the controller 400 forming part of the in-wheel electric motor.

[0067] When each in-wheel electric motor receives a torque demand, it is arranged to control the flow of current in its coil windings to generate the demanded torque demand, as described above, while monitoring the rotational speed of its rotor.

[0068] Since the mass of the vehicle is large compared to the mass of the wheels, in a situation where torque is applied directly to the wheels, the change in speed of the vehicle is relatively slow compared to the change in speed of the wheels, causing the wheels to slip.

[0069] As a result, the update rate of the vehicle's maximum and minimum speed limits generated by the control unit 102 can be relatively slow compared to the update rate required for torque control applied by the controller of the in-wheel electric motor.

Claims

1. 1. A braking system for a vehicle having a first electric motor arranged to provide a regenerative braking torque to a first wheel, and a friction braking device arranged to provide a friction braking torque to the first wheel, a controller arranged to generate, in response to receiving a torque request, a first control signal for applying regenerative braking torque by the first electric motor to the first wheel, the first control signal providing an indication of a regenerative braking torque value, and a second control signal for applying the friction braking torque to the first wheel by the friction braking device, the second control signal providing an indication of a friction braking torque value; and wherein the controller, when determining the estimated value of the friction braking torque to be applied to the first wheel, adjusts the regenerative braking torque applied to the first wheel by the first electric motor based on a difference between the friction braking torque value indicated by the second control signal and the estimated friction braking torque.

2. 2. The braking system of claim 1, wherein the controller is configured to adjust the regenerative braking torque applied by the first electric motor to compensate for a difference between the friction braking torque value indicated by the second control signal and the estimated friction braking torque.

3. The braking system of claim 1 , wherein the controller is configured to operate in a first mode of operation or a second mode of operation based on a received control signal.

4. 4. The braking system of claim 3, wherein said first mode of operation is a normal braking mode of operation and said second mode of operation is an antilock braking system (ABS) mode of operation.

5. 4. The braking system of claim 3, wherein the controller is configured to vary the indicated regenerative braking torque and the indicated friction braking torque based on whether the braking system is operating in the first mode of operation or the second mode of operation.

6. 4. The brake system according to claim 3, wherein the controller is configured to change the ratio of the indicated regenerative braking torque to the indicated friction braking torque by changing a torque margin value with respect to a regenerative torque limit value of the first electric motor when switching between the first operating mode and the second operating mode.

7. 7. The braking system of claim 6, wherein a lower torque margin value is configured to provide increased regenerative current and lower indicated friction braking torque relative to a higher torque margin value for the same received torque demand.

8. 4. The braking system of claim 3, wherein the controller includes a filter that filters the indicated friction braking torque value.

9. 9. The braking system of claim 8, wherein the filtering characteristics of the filter change between the first mode of operation and the second mode of operation.

10. 10. The braking system of claim 9, wherein the filter is configured to provide a smoother application of the friction braking torque in the first mode of operation relative to the second mode of operation.

11. 2. The brake system of claim 1, wherein the controller is configured to adjust the regenerative braking torque applied by the first electric motor to the first wheel to maintain the speed of the first wheel above a minimum wheel speed.

12. 12. The braking system of claim 11, wherein the minimum wheel speed is determined based on a first slip ratio value of the first wheel and vehicle speed.

Images