A BRAKE SYSTEM FOR A VEHICLE

Abstract

A braking system is described in which a synchronous electric machine, mechanically coupled to one or more wheels of a vehicle, is used to provide a controllable braking torque if one or more components in the powertrain (for example, the battery or the inverter supplying power to the synchronous electric machine) are in a fault condition. In such a fault condition, the electric machine generates uncontrolled recovered currents when the rotor rotates. A braking resistor is used to dissipate the uncontrolled recovered currents, and one or more switches in the current flow path are used to modulate the power through the braking resistor, thus creating a controllable braking system.

Description

FIELD OF THE INVENTION

[0001] The present invention relates to a control device for electric or hybrid vehicles with a fail-safe dynamic braking function. BACKGROUND OF THE INVENTION

[0002] Regenerative braking has become a key technology in electric and hybrid vehicles, significantly improving energy efficiency and extending the range of these vehicles. By converting kinetic energy into electrical energy during deceleration and braking, regenerative braking systems help minimize energy waste and promote sustainable transport solutions.

[0003] Regenerative braking solutions for normal driving conditions have received considerable attention, as this is the most likely scenario and requires inventive steps to maximize battery power, reduce mass, and pursue improved reliability and / or reduced maintenance. Normal driving conditions typically involve one or more multi-phase traction motors driving the wheels through one or more axles, or perhaps in-wheel motors where each wheel is driven separately and coordinated via a central control system, a battery as the power source, an inverter controller to convert the DC voltage supplied by the battery into the required multi-phase voltage, often three-phase, and the vector currents required by a multi-phase motor.Combustion engine / electric hybrids typically feature a similar set of electrical systems, typically combined with the hardware and software of the combustion engine.

[0004] Regenerative braking is typically controlled by an inverter that converts the vehicle's kinetic energy into a DC voltage and returns power to the battery. Such regeneration occurs, for example, when driving downhill and / or applying the brake pedal to reduce speed. In both cases, at least the vehicle or wheel speed, brake and accelerator pedal positions provide the necessary inputs to inform the inverter of the torque and speed requirements. In both examples, the inverter attempts to recover energy and, if possible, recharge the battery.

[0005] Under normal driving conditions, there are situations where the battery is fully charged and cannot absorb regenerative energy. In these cases, mechanical braking is an obvious choice to limit speed and maintain control of electric / hybrid vehicles, while the electric motor continues to generate voltage, or energy, that must be safely dissipated. One way to absorb generated power that cannot be used to recharge a traction battery is to place the motor in a controlled active short circuit (ASC). ASC is achieved in a controlled manner by switching on the switches of the upper bridge (or a lower bridge), for example, of a 3-phase inverter, so that they are kept in a conducting state, thereby short-circuiting the stator windings of a 3-phase motor.US2014001987A1 teaches this approach, where an inverter uses vector control to meet a braking torque demand under normal operating conditions when the battery power supply is full, e.g., after overnight charging, and a downgrade is encountered at the beginning of a drive cycle, releasing the potential for regenerative braking that cannot be deployed.

[0006] In contrast, uncontrolled ASC is of interest as a 'safe operating condition' in which a controller or inverter is inoperative, i.e., in a fault state. In this case of uncontrolled ASC, large transient stator currents can be generated if the current condition with stable ASC circulation differs significantly from the current condition before switching. US2020186058A1 teaches a method to mitigate the transient and also draws the motor engineer's attention to the fact that an uncontrolled ASC provides the highest braking torque at low speed and lower braking torque at high speed. Switching to an uncontrolled ASC at high motor speed, i.e., >2 000 rpm, helps protect the inverter and power supply, but does little to reduce the motor speed in an emergency.

[0007] In cases where the battery storage is full or unavailable and a drive motor is no longer able to dissipate regenerated energy internally, there remains a need to dissipate converted kinetic energy, and a method in which regenerated energy is dissipated into resistors is a solution.

[0008] US2014001987 illustrates the use of modulated regenerative and controlled active short-circuit methods for regenerative braking. For normal braking torque requirements and without a fault condition, US2014001987 teaches three possible combinations of energy dissipation: primarily controlled ASC, with secondary alternatives of regenerative braking and resistive energy dissipation, depending on the battery condition. By relying on pulse-width modulation (PWM) and vector control of the ASC, US2014001987 teaches that resistive dissipation can be used as a "backup for short-circuit braking" and that, consequently, relatively small amounts of energy are dissipated, allowing for a simpler heat dissipation design.

[0009] JPH10150702A follows a similar approach, but instead of increasing the number of parts by introducing a power dissipation resistor, JPH10150702A teaches that a non-operational motor is selected from a clutch motor or an auxiliary motor based on the connection states of a first clutch and a second clutch. The selected motor acts as a resistive power dissipation device, injecting only d-axis current (field current) into the motor's three-phase coils, with regenerated energy being dissipated as copper loss in the motor. Since no q-axis current (torque current) flows in the motor's three-phase coils, no torque is developed, and there is no influence on the power output other than the requested braking.

[0010] The integration of controlled active short-circuiting methods complements regenerative braking by efficiently dissipating excess energy that cannot be stored or recycled. This modulated approach, which alternates between regenerative, controlled short-circuiting, and resistive dissipation modes, is effective as long as heat generated in stator coils can be dissipated to ambient air via stator cooling, that field currents are not so high as to demagnetize permanent magnet rotor machines, and, most importantly, that the motor control unit and the interface to the traction battery are functioning correctly.

[0011] However, in cases where there is a fault in the control system or battery storage, or more commonly during controlled emergency braking from high speed, an additional backup braking principle is required. Especially for inverter or control system fault conditions and cases of controlled emergency braking, mechanical brakes are retained in vehicles to provide additional braking power and dissipate kinetic energy as heat, usually through brake discs and calipers.

[0012] Mechanical brakes are retained not because electric motors lack sufficient braking torque to reduce a vehicle's kinetic energy, but because the complexity of achieving safe and reliable electrodynamic braking is more than offset by the relative simplicity of mechanical brakes. In short, although mechanical brakes are not perfect, it is accepted that their reliability exceeds their shortcomings, so they continue to be used as a backup for electrodynamic braking.

[0013] We have discussed the advantages of controlled ASC braking and some disadvantages of uncontrolled ASC braking. It is well known among electric motor engineers that ASC is a safe condition to fall back to in the event of an inverter or controller failure, and recourse to ASC is usually from the unsafe condition of uncontrolled regeneration. Uncontrolled regeneration is considered by motor engineers to be one of the most severe fault conditions and is the condition in which the upper and lower (high and low) inverter switches are left open or pushed open—the so-called 6-switch-open (6SO) condition for a 3-phase inverter and associated rectifier diodes in freewheeling mode, allowing the current generated in the motor / generator to flow unimpeded to the primary battery power source.The primary power source must be removed from the circuit to avoid damage, but other systems are also at risk.

[0014] While with ASC, braking torque decreases with increasing rotor speed, uncontrolled regenerative braking at high rotor speeds can lead to significant safety problems due to uncontrolled high braking torques as well as excessive and damaging voltages and currents. Control system errors that result in uncontrolled regenerative braking are among the most serious safety failures and can lead to loss of vehicle control and serious accidents, especially because the effects are more extreme at higher speeds. It is understandable that, compared to uncontrolled regenerative braking, uncontrolled ASC is indeed a "safe state."

[0015] We therefore identified the need for an electric braking system suitable for a vehicle when one or more modules in the powertrain are in a fault condition, thus creating a risk that the electric machine will generate uncontrolled regenerative currents.

[0016] In reviewing prior art approaches addressing electric and hybrid vehicle braking by combining electrodynamic and mechanical systems, we have found that removing at least a portion of the mechanical auxiliary brake and replacing it with a fully electrodynamic braking system offers weight savings and, counterintuitively, safety benefits. The electrodynamic braking system of the present invention is fault-tolerant, does not rely on inverter motor control, does not rely on access to a battery for propulsion, and significantly reduces mechanical complexity and weight while improving braking dynamics and safety. Other objects and features of the present invention will become apparent from the following description with reference to the accompanying drawings. SUMMARY OF THE INVENTION

[0017] The present invention is defined by the appended independent claims. Furthermore, advantageous embodiments are also defined by the appended dependent claims.

[0018] We describe an electric braking system for a vehicle comprising: an electric machine mechanically coupleable to one or more wheels of a vehicle; an inverter for generating multi-phase AC output voltages from positive and negative DC input voltages to power the electric machine, the positive and negative DC voltages being provided by respective positive and negative DC bus bars, the inverter comprising: a plurality of switches for each phase connected between the DC input voltages and a respective AC output; and a controller for controlling each of the switches using pulse width modulation (PWM) over a plurality of PWM periods to generate the multi-phase AC output voltages for the electric machine;a safe-state braking system comprising: a controller having an output for controlling one or more of the switches in the inverter; and a resistor connected between the positive and negative DC bus bars, the resistor connected in series with a controllable switch, wherein the controller of the safe-state braking system is configured, during a fault condition: to control one or more of the switches in series with the resistor and the switches of the inverter to use uncontrolled regenerated currents from the electric machine to apply a controlled braking torque to the rotor;

[0019] By using such an arrangement, uncontrolled regenerated currents generated by the electric machine when one or more components in the driveline are in a fault condition can be used in a controlled manner to provide a controllable braking torque for the rotor of the electric machine and thus for any wheel mechanically coupled to the electric machine.

[0020] In a first implementation, the controller of the safe state braking system may be configured to control one or more of the switches of the inverter into an open-circuit mode in which all of the plurality of switches of the inverter are open, and the controller is configured to control the switch in series with the resistor to alternate between an open state and a closed state at a first frequency, and wherein the controlled braking torque is provided by the uncontrolled regenerated currents flowing through anti-parallel diodes connected to respective switches of the inverter and through the resistor.

[0021] This implementation uses the switch in series with the resistor to modulate the uncontrolled regenerative currents generated by the electric machine to provide the controlled braking torque on the rotor of the electric machine.

[0022] The controller of the safe-state braking system may be configured to vary a period of time for the first frequency spent in each of the open state and the closed state. The controller of the safe-state braking system may be configured to vary the period of time spent in each of the open state and the closed state between a first state and a second state, wherein in the first state, 100% of the period is in the closed state and 0% of the period is in the open state, and in the second state, 0% of the period is in the closed state and 100% of the period is in the open state.

[0023] The proportion of time spent in each of the open and closed states may depend on speed data indicating a speed of a rotor of the electric machine, and braking data may indicate a desired braking torque to be applied to the electric machine to slow the speed of the rotor of the machine. The speed of the rotor of the electric machine may be proportional to a speed of a wheel mechanically coupled to the rotor of the electric machine. The desired braking torque may be proportional to a braking torque or force applied by a user to a brake pedal.

[0024] In a first method of the first implementation, the controller of the safe state braking system may be configured to: measure a braking torque applied to the electric machine; compare the measured braking torque applied to the electric machine with the desired braking torque of the braking data; and adjust the braking torque applied to the electric machine to be within a threshold of the desired braking torque of the braking data by controlling the proportion of time spent in each of the open state and the closed state.

[0025] The controller can measure the braking torque applied to the electric machine based on a voltage across the resistor, a current flowing through the resistor and an angular velocity of the rotor.

[0026] The safe state braking system controller may be configured to receive speed data indicative of a speed of one or more other wheels of the vehicle, and wherein the safe state braking system controller is configured to adjust the braking torque applied to the electric machine to bring the speed of the respective wheel to within a threshold of the speed of one or more of the other wheels.

[0027] The controller may reduce the torque if the speed data indicates that the respective wheel is rotating at a speed greater than or less than the threshold speed of one or more of the other wheels. Alternatively, the safe-state braking system controller may be configured to adjust the braking torque applied to the electric machine to maintain a speed of the respective wheel that is less than a speed of one or more of the other wheels and within the threshold.

[0028] In a second method of the first implementation, the safe-state braking system controller may determine the proportion of time spent in each of the open and closed states by comparing the electric machine speed data and the braking data with a model that defines values ​​for the time spent in each of the open and closed states for a plurality of values ​​of respective speed data and braking data. The model may include a lookup table.

[0029] In a second implementation, the controller of the safe state braking system may be configured to: close the switch in series with the resistor; and control one or more of the switches of the inverter to alternate between an open-circuit mode in which all of the plurality of switches of the inverter are open and a short-circuit mode in which one or more of the plurality of switches in the inverter are closed, at a first frequency, and wherein the controlled braking torque is provided by the uncontrolled regenerated currents flowing through anti-parallel diodes connected to respective switches of the inverter when it is in the open-circuit mode and through the resistor.

[0030] This second implementation uses the switches in the inverter that toggle between open-circuit mode and short-circuit mode to modulate the uncontrolled regenerative currents generated by the electric machine to provide the controlled braking torque on the electric machine rotor.

[0031] In the second implementation, the safe-state braking system controller may be configured to vary a period of time for the first frequency spent in each of the idle modes and the short-circuit modes. The safe-state braking system controller may be configured to vary the period of time spent in each of the idle modes and the short-circuit modes between a first state and a second state, wherein in the first state, 100% of the period is in short-circuit mode and 0% of the time is in idle mode, and in the second state, 0% of the period is in short-circuit mode and 100% of the period is in idle mode.

[0032] The proportion of time spent in each of the idle modes and the short-circuit modes may depend on speed data indicating a speed of a rotor of the electric machine and on braking data indicating a desired braking torque to be applied to the electric machine to slow the speed of the machine's rotor. The speed of the electric machine's rotor may be proportional to a speed of a wheel mechanically coupled to the electric machine's rotor. The desired braking torque may be proportional to a braking torque or force applied by a user to a brake pedal.

[0033] In a first method of the second implementation, the safe state braking system controller may be configured to: measure a braking torque applied to the electric machine; compare the measured braking torque applied to the electric machine to the desired braking torque of the braking data; and control the proportion of time spent in each of the idle modes and the short circuit modes to adjust the braking torque applied to the electric machine to be within a threshold of the desired braking torque of the braking data.

[0034] In the first method of the second implementation, the controller can measure the braking torque applied to the electric machine based on a voltage across the resistor, a current flowing through the resistor, and an angular velocity of the rotor.

[0035] The safe-state braking system controller may be configured to receive speed data indicative of a speed of one or more other wheels of the vehicle, and wherein the safe-state braking system controller is configured to adjust the braking torque applied to the electric machine to bring the speed of the respective wheel to within a threshold speed of one or more of the other wheels. The controller may reduce the torque if the speed data indicates that the respective wheel is rotating at a speed greater than or less than the threshold speed of one or more of the other wheels.

[0036] Alternatively, the safe state braking system controller may be configured to adjust the braking torque applied to the electric machine to maintain a speed of the respective wheel that is less than a speed of one or more of the other wheels and within the threshold.

[0037] In a second method of the second implementation, the safe-state braking system controller may determine the proportion of time spent in each of the idle modes and the short-circuit modes by comparing the electric machine speed data and the braking data to a model that defines values ​​for the time spent in each of the idle modes and the short-circuit modes for a plurality of values ​​of respective speed data and braking data. The model may include a lookup table.

[0038] We also describe a vehicle comprising: a plurality of wheels; at least one electric machine, the at least one electric machine mechanically coupled to one or more of the plurality of wheels to drive the one or more of the plurality of wheels; an inverter for generating multi-phase AC output voltages from positive and negative DC input voltages to power a respective electric machine, the positive and negative DC voltages provided by respective positive and negative DC bus bars, the inverter comprising: for each phase, a plurality of switches connected between the DC input voltages and a respective AC output;and a controller for controlling each of the switches using pulse width modulation (PWM) over a plurality of PWM periods to generate the multi-phase AC output voltages for the electric machine; a safe-state braking system comprising: a controller having an output for controlling one or more of the switches in the inverter; and a resistor connected between the positive and negative DC bus bars, the resistor connected in series with a controllable switch, wherein the controller of the safe-state braking system is configured, during a fault condition: to control one or more of the switches in series with the resistor and the switches of the inverter to use uncontrolled regenerated currents from the electric machine to apply a controlled braking torque to the rotor;

[0039] By using such an arrangement, uncontrolled regenerated currents generated by the electric machine when one or more components in the driveline are in a fault condition can be used in a controlled manner to provide a controllable braking torque to the rotor of the electric machine and thus to any wheel mechanically coupled to the electric machine.

[0040] In a first implementation, the controller of the safe state braking system may be configured to control one or more of the switches of the inverter into an open-circuit mode in which all of the plurality of switches of the inverter are open, and the controller is configured to control the switch in series with the resistor to alternate between an open state and a closed state at a first frequency, and wherein the controlled braking torque is provided by the uncontrolled regenerated currents flowing through anti-parallel diodes connected to respective switches of the inverter and through the resistor.

[0041] This implementation uses the switch in series with the resistor to modulate the uncontrolled regenerative currents generated by the electric machine to provide the controlled braking torque on the rotor of the electric machine.

[0042] In the first implementation, the safe-state braking system controller may be configured to vary a period of time spent in each of the open state and the closed state for the first frequency. The safe-state braking system controller may be configured to vary the period of time spent in each of the open state and the closed state between a first state and a second state, wherein in the first state, 100% of the period is in the closed state and 0% of the time is in the open state, and in the second state, 0% of the period is in the closed state and 100% of the period is in the open state.

[0043] The proportion of time spent in each of the open and closed states may depend on speed data indicating a speed of a rotor of the electric machine and on braking data indicating a desired braking torque to be applied to the electric machine to slow the speed of the rotor of the machine. The speed of the rotor of the electric machine may be proportional to a speed of a wheel mechanically coupled to the rotor of the electric machine. The desired braking torque may be proportional to a braking torque applied by a user to a brake pedal.

[0044] In a first method of the first implementation, the safe state controller of the safe state braking system may be configured to: measure a braking torque applied to the electric machine; compare the measured braking torque applied to the electric machine to the desired braking torque of the braking data; and adjust the braking torque applied to the electric machine to be within a threshold of the desired braking torque of the braking data by controlling the proportion of time spent in each of the open state and the closed state.

[0045] The safe state controller can measure the braking torque applied to the electric machine based on a voltage across the resistor, a current flowing through the resistor, and an angular velocity of the rotor.

[0046] The safe-state braking system controller may be configured to receive speed data indicative of a speed of one or more of a plurality of wheels, and wherein the safe-state braking system controller is configured to adjust the braking torque applied to the electric machine to bring the speed of the respective wheel to within a threshold speed of one or more of the other plurality of wheels. The torque may be reduced if the speed data indicates that the respective wheel is rotating at a speed greater than or less than the threshold speed of one or more of the other plurality of wheels.

[0047] Alternatively, the safe state braking system controller may be configured to adjust the braking torque applied to the electric machine to maintain a speed of the respective wheel that is less than a speed of one or more of the other plurality of wheels and within the threshold.

[0048] In a second method of the first implementation, the safe-state braking system controller may determine the proportion of time spent in each of the idle modes and the short-circuit modes by comparing the electric machine speed data and the braking data with a model that defines values ​​for the time spent in each of the idle modes and the short-circuit modes for a plurality of values ​​of respective speed data and braking data. The model may include a lookup table.

[0049] In a second implementation, the controller of the safe state braking system may be configured to: close the switch in series with the resistor; and control one or more of the switches of the inverter to alternate between an open-circuit mode in which all of the plurality of switches of the inverter are open and a short-circuit mode in which one or more of the plurality of switches in the inverter are closed, at a first frequency, and wherein the controlled braking torque is provided by the uncontrolled regenerated currents flowing through anti-parallel diodes connected to respective switches of the inverter when it is in the open-circuit mode and through the resistor.

[0050] This second implementation uses the switches in the inverter that toggle between open-circuit mode and short-circuit mode to modulate the uncontrolled regenerative currents generated by the electric machine to provide the controlled braking torque on the electric machine rotor.

[0051] The safe-state braking system controller may be configured to vary a period of time for the first frequency spent in each of the idle modes and the short-circuit modes. The safe-state braking system controller may be configured to vary the period of time spent in each of the idle modes and the short-circuit modes between a first state and a second state, wherein in the first state, 100% of the period is in short-circuit mode and 0% of the time is in idle mode, and in the second state, 0% of the period is in short-circuit mode and 100% of the period is in idle mode.

[0052] The proportion of time spent in each of the idle mode and the short-circuit mode may depend on speed data indicating a speed of a rotor of the electric machine and on braking data indicating a desired braking torque to be applied to the electric machine to slow the speed of the rotor of the machine. The speed of the rotor of the electric machine may be proportional to a speed of one or more of the respective wheels of the vehicle that are mechanically coupled to the respective electric machine. The desired braking torque may be proportional to a braking force, power, or torque applied by a user to a brake pedal.

[0053] In a first method of the second implementation, the safe state braking system controller may be configured to: measure a braking torque applied to the electric machine; compare the measured braking torque applied to the electric machine to the desired braking torque of the braking data; and control the proportion of time spent in each of the idle mode and the short circuit mode to adjust the braking torque applied to the electric machine to be within a threshold of the desired braking torque of the braking data.

[0054] The controller measures the braking torque applied to the electric machine based on a voltage across the resistor, a current flowing through the resistor and an angular velocity of the rotor.

[0055] The safe-state braking system controller may be configured to receive speed data indicative of a speed of one or more of a plurality of wheels of the vehicle, and wherein the safe-state braking system controller is configured to adjust the braking torque applied to the electric machine to bring the speed of the respective wheel to within a threshold speed of one or more of the other plurality of wheels. The controller may reduce the torque when the speed data indicates that the respective wheel is rotating at a speed greater than or less than the threshold speed of one or more of the other plurality of wheels.

[0056] Alternatively, the safe state braking system controller may be configured to adjust the braking torque applied to the electric machine to maintain a speed of the respective wheel that is less than a speed of one or more of the other plurality of wheels and within the threshold.

[0057] In a second method of the second implementation, the safe-state braking system controller may determine the proportion of time spent in each of the idle modes and the short-circuit modes by comparing the electric machine speed data and the braking data to a model that defines values ​​for the time spent in each of the idle modes and the short-circuit modes for a plurality of values ​​of respective speed data and braking data. The model may include a lookup table.

[0058] In any of the above embodiments, the first frequency may be at or around 10 kHz.

[0059] In any of the above embodiments, the safe state braking system may comprise a controllable supply switch in one or both of the DC busbars supplying a DC voltage to the inverter, and wherein the controller of the safe state braking system is configured to open the supply switch during the fault condition to electrically disconnect the respective one or both DC busbars from the input of the inverter.

[0060] A fault condition can include a condition where one or more components of the inverter and / or a DC power source feeding the DC busbars are in a faulty state. For example, if the inverter is in a faulty state, it may be due to a problem with the low-voltage supply that provides power to various components in the controller. Additionally, inverter fault conditions can occur if there is a CANbus fault, where various components communicating via the vehicle's CANbus lose communication with each other, resulting in the inverter controller being unable to control the switches correctly. There may be a fault with the microcontrollers within the inverter or inverter controller that prevents the switches from switching correctly.There may be a fault in the position sensor data sent to the controller (defective sensor or open wiring), or there may be a fault in the switch gate control within the power electronics, preventing the power electronics within the controller from switching the switches correctly. Other fault conditions are also possible that would render the inverter and / or the DC power source supplying the DC busbars inoperable or in a fault state that would prevent normal operation of the devices.

[0061] In any of the above embodiments, the resistor may be configured to have a resistance of less than 5 Ω, preferably less than 2.5 Ω, more preferably in the range of 1 Ω. The resistor may have a negative temperature coefficient.

[0062] The control of the safe state braking system can be independent of the control of the inverter.

[0063] In the vehicle, each wheel can be mechanically coupled to a respective electric motor.

[0064] It is possible that in the vehicle one or more of the wheels mechanically coupled to a respective electric machine are not coupled to a mechanical braking system.

[0065] In any of the above embodiments, the electric machine may be an axial flux machine.

[0066] We also describe a braking system for a vehicle comprising: a synchronous electric machine mechanically couplable to one or more wheels of a vehicle; a rectifier electrically coupled to the synchronous electric machine for generating positive and negative DC output voltages; and a safe-state braking system comprising: a resistor electrically connected to the rectifier and a switch in the current flow path between the synchronous electric machine and the resistor;and a controller configured to control the switch, wherein during a fault condition the synchronous electric machine generates uncontrolled regenerated currents and the safe state braking system controller is configured to modulate the uncontrolled regenerated currents flowing through the resistor to apply a controlled braking torque to a rotor of the synchronous electric machine.; LIST OF FIGURES Fig. shows a power supply system for an electric machine; Fig. shows a power supply system for an electric machine according to the present invention; Fig. shows an exemplary uncontrolled regeneration current flowing in the circuit when the switch SWR is closed and the inverter switches SW1-6 are in an open-circuit state; Fig. shows an exemplary uncontrolled regeneration current flowing in the circuit when the switch SWR is closed and the inverter switches SW1-6 are in an active short-circuit state; Fig. shows a representation of the different operating points between the first state (100% ASC; 0% 6SO) and the second state (100% 6SO; 0% ASC); and Fig. shows an example of the shaft braking force (in kW) at different rotor rpm at different ASC levels (and thus the complementary 6SO components) for different resistance values ​​(1 Ω and 2.5 Ω). DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0067] Automobiles are equipped with a controlled power source and braking systems, allowing the driver to maintain control of the vehicle at both low and high speeds. Typically, vehicles are equipped with a 60:40 front-rear braking force split, which applies increased force to the front wheels when the vehicle decelerates.

[0068] In electric vehicles, the use of electrodynamic braking provides a means of assisting in slowing the vehicle through regenerative braking and transferring generated power to the power source battery.

[0069] The regenerative forces provided by modern electric machines, such as axial flux machines, have reached a stage where they can exceed the braking performance of many mechanical braking systems. This opens up the possibility of providing vehicles in which some or all of the wheels are mechanically coupled to respective electric machines, thus dispensing with any mechanical braking systems. For example, vehicles may be provided with one or more electric machines for each wheel. The rear wheels may be arranged to be driven by the respective electric machine and also provide braking torque from the respective electric machine. The front wheels may be driven by the electric machine while also receiving braking torque from the electric machine and may also be provided with a mechanical braking system.In some cases, all wheels may be arranged so that they only have a braking torque exerted by the respective electric machines, i.e. no mechanical braking systems are coupled to any of the wheels in the vehicle.

[0070] However, this poses a problem because a controllable braking torque must be generated to slow the vehicle even if one or more components in the powertrain are in a faulty state, for example a fault in the battery pack that supplies power to the inverter coupled to the respective electric machines, or the inverter itself is in a faulty state.

[0071] The basic principle of the braking system and method described here is the use of a synchronous electric machine, mechanically coupled to one or more wheels of a vehicle, as a means of providing controllable braking torque if one or more of the components in the driveline (for example, the battery or the inverter supplying power to the synchronous electric machine) is in a fault condition. In such a fault condition, the electric machine generates uncontrolled regenerated currents as the rotor rotates. A braking resistor is used to shunt the uncontrolled regenerated currents, and one or more switches in the current path are used to modulate the current through the braking resistor, thus creating a controllable braking system. Means for rectifying the uncontrolled regenerated currents from the electric machine may be used.

[0072] In one implementation, the present invention provides a braking system suitable for a vehicle that is capable of providing controllable braking torque to the electric machine even when portions of the powertrain or other aspects of the vehicle's systems are in a fault condition. The braking system includes an electric machine that is mechanically coupled to one or more wheels of a vehicle. An inverter generates multi-phase AC output voltages from a DC power source to power the electric machine (for example, when the inverter controller uses pulse width modulation to control a plurality of switches to generate the multi-phase AC output voltages). The braking system also includes a safe-state braking system that includes a resistor connected in series with a switchable switch between the DC busbars.A separate controller is configured to control one or more of the switches in series with the resistor and the inverter switches during a fault condition (for example, in the battery or the inverter) to use uncontrolled regenerated currents from the electric machine to apply a controlled braking torque to the rotor.

[0073] In one implementation, the braking torque is controlled by placing the inverter's switches in an open state and alternating the switch in series with the resistor between an open and a closed state at a first frequency. If the resistor has a sufficiently low resistance and a sufficiently high rating, this provides a current flow path through which the uncontrolled regenerated currents can flow and be dissipated as heat when the switch is closed. Alternating between the closed (in which braking torque is generated) and the open (in which little to no braking torque is generated) states allows the torque to be modulated and the torque value to be controlled.

[0074] In another alternative implementation, the braking torque is controlled by closing the switch in series with the resistor and alternately controlling one or more of the inverter switches into an open-circuit mode and a closed-circuit mode. If the resistor has a sufficiently low resistance and a sufficiently high rating, it provides a current flow path through which the uncontrolled regenerated currents can flow when the inverter switches are in open-circuit mode and be dissipated as heat. Switching between the open-circuit mode (where braking torque is generated) and the closed-circuit mode (where little to no braking torque is generated) allows the torque to be modulated and the torque value to be controlled.

[0075] The present invention therefore aims to reduce the speed of an electric vehicle safely and quickly without the need for additional mechanical brakes by using the circuit of uncontrolled regenerative power via a load resistor to dissipate the energy and provide a controllable braking torque.

[0076] The present invention is particularly effective when applied to axial flux motors due to the high torque characteristics of this topology. However, radial flux motors can offer similar advantages, albeit with less effective motor braking, which can be enhanced by a gearbox.

[0077] Electric machines of the present invention can be operated as motors or generators: the former for propulsion and the latter when regenerative braking is applied to adjust the speed of the vehicle or to locally adjust the torque at a wheel.

[0078] The present invention is particularly advantageous for vehicles in which an electric motor is mechanically coupled to one or more wheels of a vehicle. Examples include wheel hub motors and wheel-mounted motors on steering knuckles. However, motors connected to wheels via a gear reducer or other transmission components can also benefit from the present invention.

[0079] With reference to Fig. This shows a prior art arrangement for driving an electric machine 100. The electric machines are multi-phase synchronous AC machines fed by an inverter 110 for generating multi-phase AC output voltages U, V, W from a drive energy source 112, for example, a battery with positive and negative DC busbars. An internal combustion engine driving a generator and converter (not shown) can also feed positive and negative DC busbars, or other power sources such as fuel cells or one or more super / ultracapacitors.

[0080] The inverter 110 includes a plurality of switches SW1-6 connected between the DC input voltages and a respective AC output for each phase, with a controller (not shown) controlling the switches using pulse width modulation (PWM) over a plurality of PWM periods to generate the multi-phase AC output voltages U, V, W for the electric machine 100.

[0081] Although the figures show a three-phase output powering a three-phase electrical machine (e.g., an axial flux machine) from a two-level three-phase inverter, the present invention is not limited to two-level three-phase inverter configurations and three-phase electrical machines. Other multi-phase voltages and electrical machine configurations may be used. Likewise, other inverters with multi-level topologies may be used. However, for simplicity, we refer only to three-phase voltages implemented with a three-phase two-level inverter configuration.

[0082] During normal operation, inverter 110 generates the required output voltages and currents using PWM to control the switches to supply electric machine 100 based on a torque request provided to the inverter controller, for example, by an accelerator pedal in a vehicle. During braking, for example, when the user applies force to the vehicle's brake pedal, the inverter may be configured to utilize regenerative currents induced in the electric machine to apply braking torque to the electric machine and thus to any wheels mechanically coupled to the electric machine. These regenerated currents may be passed to the battery pack.

[0083] As discussed above, the regenerative forces provided by modern electric machines, such as axial flux machines, have reached a stage where they can exceed the braking performance of many mechanical braking systems. This opens up the possibility of providing vehicles in which some or all of the wheels, mechanically coupled to respective electric machines, can be provided without any mechanical braking systems.

[0084] However, in scenarios where some or all wheels are not equipped with a mechanical braking system, the system must be able to perform its braking function even if one or more modules in the powertrain (for example, the battery pack or the respective inverter) are in a fault condition.

[0085] In a fault condition, such as if the inverter were to fail and the electric machine rotor was still rotating, back EMF could still be generated in the electric machine and generated currents could still flow. For example, if the inverter switches were left open and the back EMF generated by the electric machine was greater than the voltage from the DC power source or across DC capacitor 114, an uncontrolled generated current would flow through one or more of the anti-parallel diodes associated with switches SW1-6 to the DC capacitor and power source. This would cause a sudden and uncontrolled regenerative (braking) torque to be applied to the rotor, and thus to the wheel mechanically coupled to the rotor, until the voltages equalize. This uncontrolled regenerative torque could be hazardous in vehicles.

[0086] In state-of-the-art systems, one method to reduce or mitigate the uncontrolled torque is to short-circuit all upper switches (SW1, 3, 5) or all lower switches (SW2, 4, 6), which causes the uncontrolled regenerated currents caused by the back EMF from the electric machine to be fed back into the electric machine. This method generates little to no useful braking torque, resulting in a safer system, but one in which no useful braking torque is possible.

[0087] The present invention utilizes the properties of the uncontrolled regenerative currents in a powertrain fault condition to generate a controlled braking torque.

[0088] Fig. shows an arrangement according to the present invention.

[0089] The arrangement of the present invention thus additionally provides a means for enabling a safe, electrodynamically controllable braking state if one or more components are in a faulty state. These faults may be due to one or more components in the drivetrain being in a faulty state, for example, one or both of the inverters or even parts of the stator of the electric machine, for example, the position sensor of the electric machine, a phase imbalance, or a partial short circuit.

[0090] The safe-state braking system is provided by an independent controller 150, separate from the inverter controller 140 (which controls switches SW1-6 to generate the multi-phase output voltage, as described above), having an output for controlling one or more of the switches SW1-6 in the inverter, and a resistor R connected between the positive and negative DC busbars. Resistor R is connected in series with a controllable switch SWR connected to the safe-state braking system controller 150.

[0091] The resistor R preferably has a low resistance, preferably less than 5 Ω, more preferably less than 2.5 Ω, and most preferably in the range of 0.5-1 Ω. Preferably, the resistor is capable of dissipating power in the range of hundreds of kW. Preferably, the resistor has a negative temperature coefficient, although this is not required. The purpose of the resistor is to dissipate energy from the uncontrolled regenerated currents of the electric machine. To give an example of the energy involved, the energy required to reduce the speed of a vehicle traveling at 320 km / h to less than 100 km / h may be in the range of 2 MJ over a period of 3 s or even less. The resistor is preferably sized to safely accommodate such large energy transfers without compromising the performance of surrounding materials or components.The resistor can be cooled actively or passively.

[0092] As discussed above, the basic principle of the braking system and method described herein involves the use of a synchronous electric machine, which generates uncontrolled regenerated currents when the rotor is rotating and components in the driveline are in a fault condition. A braking resistor R is used to shunt the uncontrolled regenerated currents, and one or more switches in the current flow path (SWR and / or one or more of SW1-6) are used to modulate the power across the braking resistor R. When current flows through the braking resistor R, a braking torque is generated on the rotor (and thus any wheel mechanically coupled to the rotor), creating a fault-tolerant, controllable braking system.

[0093] In a first implementation of the braking system, the safe-state braking system controller 150 is configured to control one or more of the inverter's switches SW1-6 into an open-circuit mode in which all of the inverter's plurality of switches are open. The controller 150 is configured to control the switch SWR in series with the resistor R to alternate between an open state and a closed state at a first frequency. This is preferably in the range of 10 kHz, although the method is not limited to this value.

[0094] Fig. shows an example of an uncontrolled regeneration current flowing in the circuit when the switch SWR is closed and the inverter switches SW1-6 are in an open-circuit state.

[0095] The controlled braking torque is provided by the uncontrolled regenerated currents flowing through anti-parallel diodes connected to the respective switches SW1-6 of the inverter and through the resistor. Since the resistor R has a very low resistance (preferably in the range of 0.5-1 Ω), the resistor dissipates the energy as heat. With this arrangement, braking powers of over 500 kW are possible.

[0096] If the switch remained permanently closed in this arrangement, a continuous and large braking torque would be applied to the rotor. To control the amount of torque applied to the rotor, the present invention utilizes a modulation technique whereby the safe-state braking system controller 150 is configured to vary a period of time for the first frequency spent in each of the open and closed states. The controller is capable of varying the period or duty cycle of the open and closed states similar to a pulse modulation principle to vary the braking torque applied to the rotor.

[0097] At the two extremes, the controller controls the time spent in each of a first state, where 100% of the time is in the closed state and 0% of the time is in the open state (maximum braking torque is generated), and a second state, where 0% of the time is in the closed state and 100% of the time is in the open state. In the open state, braking torque generation is significantly reduced because the uncontrolled regenerated current cannot flow through the low-value resistor R.

[0098] In some implementations, the amount of time spent in each of the open state and the closed state of the switch SWR depends on speed data indicating a speed of a rotor of the electric machine and on braking data indicating a desired braking torque to be applied to the electric machine to slow the speed of the rotor of the machine.

[0099] The rotational speed of the rotor of the electric machine may be proportional to a rotational speed of a wheel that is mechanically coupled to the rotor of the electric machine, and this data may be provided, for example, by an anti-lock braking system (ABS) sensor (not shown in the figures).

[0100] The desired braking torque can be proportional to the braking torque applied by a user to a brake pedal. This can be in the form of a simple voltage proportional to the force applied to the brake pedal, or in another form familiar to knowledgeable readers.

[0101] There are two main methods for controlling the SWR switch, which are discussed below.

[0102] In the first method, a simple control algorithm is specified by the safe-state braking system controller 150. In this method, the controller 150 measures a braking torque applied to the electric machine and compares the measured braking torque applied to the electric machine with the desired braking torque supplied by the braking data. The controller 150 then adjusts the braking torque applied to the electric machine to be within a threshold of the desired braking torque from the braking data. The controller achieves this by controlling the proportion of time spent in each of the open and closed states.

[0103] The threshold value makes it possible to generate a braking torque that either corresponds to the desired braking torque or is within a tolerance of the desired braking torque.

[0104] Such an implementation also allows other functions to be implemented even when part of the powertrain is in a fault state. For example, ABS functionality can be applied to the wheel mechanically coupled to the electric motor.

[0105] The controller 150 may be configured to receive speed data indicative of a speed of one or more other wheels of the vehicle. In the situation where the controller 150 determines that the controlled wheel is rotating at a different speed than one or more of the other wheels of the vehicle, the controller 150 may adjust the braking torque applied to the electric machine to bring the speed of the respective wheel to within a threshold of the speed of one or more of the other wheels.

[0106] That is, if the controller 150 determines that the speed of the controlled wheel is slowing down, indicating that the wheel is losing traction (e.g., locking up, i.e., slipping or sliding, indicating that a skid is likely), the controller 150 may reduce the braking torque applied to the wheel, thus preventing the loss of traction. Once the speed data indicates that the wheels are rotating at the same or a similar speed, the controller 150 may increase the braking torque again to achieve the desired braking torque, as required by the braking data.

[0107] Such an arrangement also allows for a known and desired degree of wheel slip. Wheel slip occurs when the tire associated with the wheel loses traction with the road surface. A complete loss of traction is skidding and generally undesirable. However, in some scenarios, some wheel slip (but not a complete loss of traction) may be beneficial. The described method essentially uses the controller 150 to adjust the braking torque applied to the electric machine to maintain a speed of the respective wheel that is less than a speed of one or more of the other wheels and within the threshold.

[0108] Various methods can be used to determine or measure the braking torque generated at the rotor. However, the controller can determine the braking torque applied to the electric machine based on a voltage across the resistor R, a current flowing through the resistor R, and an angular velocity of the rotor.

[0109] In a second method for controlling the switch SWR in series with the resistor R, the controller 150 may determine the proportion of time spent in each of the open state and the closed state by comparing the electric machine speed data and the braking data to a model that defines values ​​for the time spent in each of the open state and the closed state for a plurality of values ​​of respective speed data and braking data.

[0110] In its simplest form, the model may include a look-up table (LUT) that defines multiple values ​​of speed and brake data and the required time fraction for each of the open and closed states. The controller must then look up the respective speed and brake data entries and implement the required time fraction.

[0111] In a second, alternative implementation of the safe-state braking system, if one or more modules in the powertrain are in a fault state, the safe-state braking system controller 150 is configured to close the switch SWR in series with the resistor R and control one or more of the inverter switches SW1-6 to alternate at the first frequency between an open-circuit mode, in which all of the plurality of inverter switches are open, and a short-circuit mode, in which one or more of the plurality of switches in the inverter are closed. As mentioned above, this first frequency is chosen to be at or around 10 kHz, although other frequencies may be used.The controlled braking torque is provided by the fact that in no-load mode the uncontrolled regenerated currents flow through anti-parallel diodes connected to respective switches of the inverter and through the resistor.

[0112] In state-of-the-art systems, the switches are often placed in an ASC (Active Short Circuit) configuration, where either the upper switches (SW1, 3, or 5) are shorted, or the lower switches (SW2, 4, 6) are shorted when an inverter is in a safe state, i.e., when the inverter is in a fault condition due to a fault in the inverter or the battery pack. This arrangement provides a 'safe state' in which zero power is generated to prevent uncontrolled regeneration from the motor (uncontrolled regeneration could damage the battery pack or generate an uncontrolled amount of regenerative braking torque at the wheel). In this scenario, where there is no mechanical braking system at the wheel, this is unacceptable, as no braking force at the wheel would be possible in a fault condition.

[0113] The ASC method also injects large currents into the electric machine (sometimes more than 1 000 A in the Id axis), which can lead to heating and possible demagnetization of the electric machine.

[0114] The second implementation provides a solution where the safe-state braking system controller also controls the inverter switches to an open configuration. In the open configuration, current generated by the electric machine can flow in the circuit, but through the resistor R as a means of dissipating the energy in the form of heat. Thus, a controlled regenerative force, and hence a braking force, can be provided to the wheel mechanically coupled to the electric machine. The regenerative torque at the rotor, and thus at the wheel mechanically coupled to the electric machine rotor, is proportional to the current and voltage generated by the electric machine, as well as the electric machine's speed. We will discuss this below.

[0115] Fig. shows an arrangement in which the safe-state braking controller 150 (due to a fault condition) has closed switch SWR in series with resistor R and the lower switches (SW2, 4, 6) are in an ASC configuration. The safe-state braking system controller 150 sends a control signal to one or more of the inverter switches SW1-6 to close either one, two, or all of the upper switches SW1, 3, 5 or one, two, or all of the lower switches SW2, 4, 6. Under no circumstances is a conduction condition permitted, such as when switches SW1 and SW2 are closed. As shown in Fig. As shown by thick black lines, in the case shown the switches SW2, 4, 6 are closed in an active short circuit mode.

[0116] In the ASC configuration, the torque is relatively small, especially at medium to high speeds. Therefore, the safe-state braking controller is also configured to use an open-switch configuration to provide controllable torque force to enable controlled braking.

[0117] As mentioned above with reference to Fig. As discussed above, this figure shows the configuration in which the safe-state braking controller 150 has opened switches SW1-6. This is sometimes referred to as an SO configuration, or 6SO, when all six switches of this particular implementation have been controlled to the open state. In this state, the switches conduct current through their respective bypass diodes along a circuit of the respective coupled phase windings of the electric machine when the back EMF from the uncontrolled regenerated voltages is large enough to overcome the voltages already present in the circuit.

[0118] As above, in the open state, the safe state braking controller 150 has closed the switch SWR in series with the resistor R (due to a fault condition). Since generated current flows through the respective bypass diodes, Fig. a path for the current generated by one of the phase coils (between the U and V phase windings, as shown in this figure) of the electric machine, through SW1 and SW4 and through the series resistor R. In this state, the generated current has a flow path, the excess energy is dissipated in the resistor R (in the form of heat), and a torque is generated that can be used to apply a braking force to the respective wheel mechanically coupled to this electric machine. Other phase coils have different current paths through their respective diodes if the back EMF generated by the electric machine is large enough.

[0119] The torque generated in this open configuration is approximately as follows: Torque(6SO)=Vres×Ires2π×w wherein: Vres and Ires are the voltages and currents generated in the electric machine; w is the angular velocity of the wheel.

[0120] In the open switch configuration, using an electric machine implementation and a suitable resistor with a resistance of approximately 1 Ω, braking powers of over 500 kW can be achieved.

[0121] However, if only the open switch configuration is used and the SWR switch is continuously closed, even in combination with the resistor, this does not provide a controllable means to bring a vehicle to a safe stop when one or more modules are in a fault condition.

[0122] If the SWR switch remained permanently closed and the inverter switches remained in the 6SO configuration, a continuous and large braking torque would be applied to the rotor. To control the amount of torque applied to the rotor, the present invention utilizes a modulation technique whereby the safe-state braking system controller 150 is configured to vary a period of time for the first frequency spent in each of the open-circuit and short-circuit modes. The controller is capable of varying the period or duty cycle of the open-circuit and short-circuit modes similar to a pulse modulation principle to vary the braking torque applied to the rotor.

[0123] At the two extremes, the controller controls the time spent in each of a first state, where 100% of the time is in short-circuit mode and 0% of the time is in no-load mode (where minimum braking torque is generated), and a second state, where 0% of the time is in short-circuit mode and 100% of the time is in no-load mode (where maximum braking torque is generated). In short-circuit mode, braking torque generation is significantly reduced because the uncontrolled regenerated current cannot flow through the low-value resistor R (since the uncontrolled regenerated currents will take the path of least resistance through the windings of the electric machine).

[0124] In some implementations, the amount of time spent in each of the idle mode and the short circuit mode of the inverter switches SW1-6 depends on speed data indicating a speed of a rotor of the electric machine and braking data indicating a desired braking torque to be applied to the electric machine to slow the speed of the rotor of the machine.

[0125] As above, the speed of the rotor of the electric machine may be proportional to a speed of a wheel that is mechanically coupled to the rotor of the electric machine, and this data may be provided, for example, by an anti-lock braking system (ABS) sensor.

[0126] As above, the desired braking torque can be proportional to the braking torque applied by a user to a brake pedal. Again, this can be in the form of a simple voltage proportional to the force applied to the brake pedal, or in another form familiar to knowledgeable readers.

[0127] Similar to the first implementation described above, there are two main methods for controlling switches SW1-6 to achieve the desired braking torque.

[0128] In the first method, a simple control algorithm is specified by the safe-state braking system controller 150. In this method, the controller 150 measures a braking torque applied to the electric machine and compares the measured braking torque applied to the electric machine with the desired braking torque supplied by the braking data. The controller 150 then adjusts the braking torque applied to the electric machine to be within a threshold of the desired braking torque from the braking data. The controller achieves this by controlling the amount of time spent in each of the idle modes and the short-circuit modes.

[0129] The threshold value makes it possible to generate a braking torque that either corresponds to the desired braking torque or is within a desired tolerance of the desired braking torque.

[0130] As in the first implementation described above, such a method also allows other functions to be implemented even when part of the powertrain is in a fault state. For example, ABS functionality can be applied to the wheel mechanically coupled to the electric motor.

[0131] The controller 150 may be configured to receive speed data indicative of a speed of one or more other wheels of the vehicle. In the situation where the controller 150 determines that the speed of the controlled wheel is rotating at a different speed than one or more of the other wheels of the vehicle, the controller 150 may adjust the braking torque applied to the electric machine to bring the speed of the respective wheel to within a threshold of the speed of one or more of the other wheels.

[0132] That is, if the controller 150 determines that the speed of the controlled wheel is slowing down, indicating that the wheel is losing traction (e.g., locking up, i.e., slipping or sliding, indicating that a skid is likely), the controller 150 may reduce the braking torque applied to the wheel, thus preventing the loss of traction. Once the speed data indicates that the wheels are rotating at the same or a similar speed, the controller 150 may increase the braking torque again to achieve the desired braking torque, as required by the braking data.

[0133] Such an arrangement also allows for a known and desired degree of wheel slip. Wheel slip occurs when the tire associated with the wheel loses traction with the road surface. A complete loss of traction is skidding and generally undesirable. However, in some scenarios, some wheel slip (but not a complete loss of traction) may be beneficial. The described method inherently uses the controller 150 to adjust the braking torque applied to the electric machine to maintain a speed of the respective wheel that is less than a speed of one or more of the other wheels and within the threshold.

[0134] Various methods can be used to determine or measure the braking torque generated at the rotor. However, the controller can determine the braking torque applied to the electric machine based on a voltage across the resistor R, a current flowing through the resistor R, and an angular velocity of the rotor.

[0135] In a second method for controlling the switches of the inverter SW1-6, the controller 150 may determine the proportion of time spent in each of the open modes and the short circuit modes by comparing the electric machine speed data and the braking data to a model that defines values ​​for the time spent in each of the open state and the closed state for a plurality of values ​​of respective speed data and braking data.

[0136] In its simplest form, the model may include a look-up table (LUT) that defines multiple values ​​of speed and braking data and the required time fraction for each of the idle and short-circuit modes. The controller must then look up the respective entries for speed and braking data and implement the required time fraction.

[0137] Thus, the present invention implements a method wherein the safe state braking system controller 150 is configured to control one or more of the switches of the inverter SW1-6 to alternate between the open circuit mode and the short circuit mode at a first frequency.

[0138] By switching between idle mode and short circuit mode, the braking torque can be controlled and transferred to the wheel in a controlled manner, even if one or more of the modules in the driveline are in a fault condition.

[0139] The safe-state braking system controller varies a period of time for the first frequency spent in each of the idle and short-circuit modes. Thus, the regenerative torque can be represented as follows: Brake torque=TVres×Ires2π×w+(1−T)×TASC where T is the duty cycle of the idle mode and the active short circuit mode.

[0140] T ASC is the torque at active short circuit, which in most cases is close to zero (but not always, and it increases when the angular velocity of the wheel is low).

[0141] In most cases, especially when the vehicle is moving fast, the equation for calculating torque can be simplified to: Brake torque=TVres×Ires2π×w

[0142] Thus, the safe-state braking system controller can vary the time spent in each of the idle and closed-circuit modes between a first state and a second state. In the first state, 100% of the time is in short-circuit mode (i.e., generating near-zero torque force) and 0% of the time is in idle mode. In the second state, 0% of the time is in short-circuit mode and 100% of the time is in idle mode (i.e., generating maximum regenerative torque). The controller can vary the time periods in each of the idle and closed-circuit modes at different points between these two states at the extremes of operating conditions.

[0143] Fig. shows a representation of the different operating points between the first state (100% ASC; 0% 6SO) and the second state (100% 6SO; 0% ASC).

[0144] Fig. shows the shaft braking force (in kW) at different rotor rpm at different ASC values ​​(and thus the complementary 6SO proportions) for different resistance values ​​(1 Ω and 2.5 Ω). As can be seen, in the first state (100% ASC; 0% 6SO), the torque output is at or near 0, and in the second state (100% 6SO; 0% ASC), the torque output is at a maximum. By varying the ratio of ASC and 6SO between these two states, a variable and controllable amount of braking force can be applied to the wheel using the uncontrolled regenerative currents from the electric machine.

[0145] With reference to the Fig.The safe-state braking system may also include a controllable feeder switch 130 in one or both of the DC busbars that supply a DC voltage to the inverter. For simplicity, it is shown on only one supply rail in these figures. The controller 150 may be configured to open the feeder switch during the fault condition to electrically disconnect the respective one or both DC supply rails from the input of the inverter. This is beneficial for protecting the battery pack from unwanted currents flowing back into the pack during the fault condition, or for protecting the inverter and other driveline components from the battery pack when the battery pack is in the fault condition.

[0146] In the scenario where a vehicle includes a plurality of electric machines (e.g., one per wheel), each electric machine is driven by a respective inverter, and each of these inverters is provided with a safe-state braking system, as described above. Thus, the system may be provided with respective feeder switches 130, each capable of isolating respective inverters and electric machines from the battery pack 112. Since each of the safe-state braking systems can operate independently, it is possible to isolate only the faulty powertrain from the battery pack (assuming the battery pack is not in a fault state).

[0147] In each of the above implementations, the safe-state braking system controller 150 is independent of the respective inverter controllers 140. This allows for the implementation of a fault-tolerant safe-state braking system should the inverter controller be the module in the powertrain that is in a fault state. The safe-state braking system controller 150 can be provided by an FPGA or an ASIC; preferably, a component with minimal or no software that can meet ASIL D functional safety requirements.

[0148] As described above, the electric machine described above can be any synchronous electric machine, such as a radial flux machine or an axial flux machine. The preferred implementations use an axial flux machine due to its advantageous performance for a given weight and volume, but the above implementations and methods are not limited to axial flux machines.

[0149] Undoubtedly, many other effective alternatives will occur to those skilled in the art. It is understood that the invention is not limited to the described embodiments and includes modifications that would be obvious to those skilled in the art and fall within the scope of the appended claims. QUOTES CONTAINED IN THE DESCRIPTION

[0000] This list of documents submitted by the applicant was generated automatically and is included solely for the convenience of the reader. This list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions. Cited patent literature

[0000] US 2014001987A1

[0005] US 2020186058A1

[0006] US 2014001987

[0008]

Claims

[1] An electric braking system for a vehicle, comprising: an electric machine that can be mechanically coupled to one or more wheels of a vehicle; an inverter for generating multi-phase AC output voltages from positive and negative DC input voltages to power the electric machine, the positive and negative DC voltages being provided by respective positive and negative DC busbars, the inverter comprising: a plurality of switches for each phase connected between the DC input voltages and a respective AC output; and a controller for controlling each of the switches using pulse width modulation (PWM) over a plurality of PWM periods to generate the multi-phase AC output voltages for the electric machine; a braking system with a safe condition, which includes: a controller having an output for controlling one or more of the switches in the inverter; and a resistor connected between the positive and negative DC bus bars, the resistor being connected in series with a controllable switch, wherein the safe state braking system controller is configured to, during a fault condition: control one or more of the switch in series with the resistor and the switch of the inverter to use uncontrolled regenerated currents from the electric machine to apply a controlled braking torque to the rotor. [2] The electric braking system of claim 1, wherein the controller of the safe state braking system is configured to control one or more of the switches of the inverter to an open-circuit mode in which all of the plurality of switches of the inverter are open, and the controller is configured to control the switch in series with the resistor to alternate between an open state and a closed state at a first frequency, and wherein the controlled braking torque is provided by the uncontrolled regenerated currents flowing through anti-parallel diodes connected to respective switches of the inverter and through the resistor. [3] The electric braking system of claim 2, wherein the safe state braking system controller is configured to vary a period of time for the first frequency spent in each of the open state and the closed state. [4] The electric braking system of claim 3, wherein the safe state braking system controller is configured to vary the amount of time spent in each of the open state and the closed state between a first state and a second state, wherein in the first state 100% of the time is in the closed state and 0% of the time is in the open state, and in the second state 0% of the time is in the closed state and 100% of the time is in the open state. [5] An electric braking system according to claim 3 or 4, wherein the proportion of time spent in each of the open state and the closed state depends on speed data indicative of a speed of a rotor of the electric machine and braking data indicative of a desired braking torque to be applied to the electric machine to slow down the speed of the rotor of the machine. [6] Electric braking system according to claim 5, wherein the rotational speed of the rotor of the electric machine is proportional to a rotational speed of a wheel which is mechanically coupled to the rotor of the electric machine. [7] An electric braking system according to claim 5 or 6, wherein the desired braking torque is proportional to a braking torque applied by a user to a brake pedal. [8] An electric braking system according to claim 5, 6 or 7, wherein the safe state braking system controller is configured to: to measure a braking torque exerted on the electric machine; to compare the measured braking torque applied to the electric machine with the desired braking torque from the braking data; and to adjust the braking torque applied to the electric machine to be within a threshold of the desired braking torque of the braking data by controlling the proportion of time spent in each of the open and closed states. [9] The electric braking system of claim 8, wherein the safe condition controller measures the braking torque applied to the electric machine based on a voltage across the resistor, a current flowing through the resistor, and an angular velocity of the rotor. [10] An electric braking system according to claim 8 or 9, wherein the safe state braking system controller is configured to receive speed data indicative of a speed of one or more other wheels of the vehicle, and wherein the safe state braking system controller is configured to adjust the braking torque applied to the electric machine to bring the speed of the respective wheel to within a threshold of the speed of one or more of the other wheels. [11] An electric braking system according to claim 10, wherein the torque is reduced when the speed data indicates that the respective wheel is rotating at a speed greater than or less than the threshold speed of one or more of the other wheels. [12] The electric braking system of claim 10, wherein the safe state braking system controller is configured to adjust the braking torque applied to the electric machine to maintain a speed of the respective wheel that is less than a speed of one or more of the other wheels and within the threshold. [13] An electric braking system according to claim 5, 6 or 7, wherein the safe state braking system controller determines the proportion of time spent in each of the open state and the closed state by comparing the electric machine speed data and the braking data with a model that defines values ​​for the time spent in each of the open state and the closed state for a plurality of values ​​of respective speed data and braking data. [14] An electric braking system according to claim 13, wherein the model comprises a look-up table. [15] The electric braking system of claim 1, wherein the safe state braking system controller is configured to: close the switch in series with the resistor; and to control one or more of the switches of the inverter to switch between an open-circuit mode in which all of the plurality of switches of the inverter are open and a short-circuit mode in which one or more of the plurality of switches in the inverter are closed, at a first frequency, and wherein the controlled braking torque is provided by the uncontrolled regenerated currents flowing through anti-parallel diodes connected to respective switches of the inverter when it is in idle mode and through the resistor. [16] The electric braking system of claim 15, wherein the safe state braking system controller is configured to vary a period of time for the first frequency spent in each of the idle modes and the short circuit modes. [17] The electric braking system of claim 16, wherein the safe state braking system controller is configured to vary the amount of time spent in each of the idle modes and the short modes between a first state and a second state, wherein in the first state 100% of the time is in the short mode and 0% of the time is in the idle mode, and in the second state 0% of the time is in the short mode and 100% of the time is in the idle mode. [18] An electric braking system according to claim 16 or 17, wherein the proportion of time spent in each of the idle modes and the short circuit modes depends on speed data indicative of a speed of a rotor of the electric machine and braking data indicative of a desired braking torque to be applied to the electric machine to slow the speed of the rotor of the machine. [19] An electric braking system according to claim 18, wherein the rotational speed of the rotor of the electric machine is proportional to a rotational speed of a wheel that is mechanically coupled to the rotor of the electric machine. [20] An electric braking system according to claim 18 or 19, wherein the desired braking torque is proportional to a braking torque applied by a user to a brake pedal. [21] An electric braking system according to claim 18, 19 or 20, wherein the safe state braking system controller is configured to: to measure a braking torque exerted on the electric machine; to compare the measured braking torque applied to the electric machine with the desired braking torque from the braking data; and to control the proportion of time spent in each of the idle modes and the short circuit modes in order to adjust the braking torque applied to the electric machine so that it is within a threshold of the desired braking torque of the braking data. [22] The electric braking system of claim 21, wherein the controller measures the braking torque applied to the electric machine based on a voltage across the resistor, a current flowing through the resistor, and an angular velocity of the rotor. [23] An electric braking system according to claim 21 or 22, wherein the safe state braking system controller is configured to receive speed data indicative of a speed of one or more other wheels of the vehicle, and wherein the safe state braking system controller is configured to adjust the braking torque applied to the electric machine to bring the speed of the respective wheel to within a threshold of the speed of one or more of the other wheels. [24] An electric braking system according to claim 23, wherein the torque is reduced when the speed data indicates that the respective wheel is rotating at a speed greater than or less than the threshold speed of one or more of the other wheels. [25] The electric braking system of claim 23, wherein the safe state braking system controller is configured to adjust the braking torque applied to the electric machine to maintain a speed of the respective wheel that is less than a speed of one or more of the other wheels and within the threshold. [26] An electric braking system according to claim 18, 19 or 20, wherein the safe state braking system controller determines the proportion of time spent in each of the idle modes and the short circuit modes by comparing the electric machine speed data and the braking data with a model that defines values ​​for the time spent in each of the idle modes and the short circuit modes for a plurality of values ​​of respective speed data and braking data. [27] An electric braking system according to claim 26, wherein the model comprises a look-up table. [28] An electric braking system according to any one of claims 2 to 27, wherein the first frequency is at or around 10 kHz. [29] An electric braking system according to any preceding claim, wherein the safe state braking system comprises a controllable supply switch in one or both of the DC busbars supplying a DC voltage to the inverter, and wherein the controller of the safe state braking system is configured to open the supply switch during the fault condition to electrically disconnect the respective one or both of the DC busbars from the input of the inverter. [30] An electric braking system according to any preceding claim, wherein a fault condition comprises a condition in which one or more components of the inverter and / or a DC power source feeding the DC busbars are in a fault condition. [31] An electric braking system according to any one of the preceding claims, wherein the resistor is configured to have a resistance of less than 5 Ω, preferably less than 2.5 Ω, more preferably in the range of 0.5 Ω to 1 Ω. [32] An electric braking system according to any preceding claim, wherein the resistor has a negative temperature coefficient. [33] An electric braking system according to any preceding claim, wherein the safe state braking system control is independent of the inverter control. [34] Electric braking system according to one of the preceding claims, wherein the electric machine is an axial flux machine. [35] Vehicle comprising: a variety of wheels; and an electronic braking system according to any one of the preceding claims, wherein the vehicle further comprises: at least one electric machine mechanically coupled to one or more of the plurality of wheels to drive the one or more of the plurality of wheels. [36] A vehicle according to claim 35, wherein one or more of the wheels mechanically coupled to a respective electric machine are not coupled to a mechanical braking system. [37] Braking system for a vehicle, comprising: a synchronous electric machine that can be mechanically coupled to one or more wheels of a vehicle; a rectifier electrically coupled to the synchronous electric machine to generate positive and negative DC output voltages; and a braking system with a safe condition, which includes: a resistor electrically connected to the rectifier and a switch in the current flow path between the synchronous electric machine and the resistor; and a controller configured to control the switch, wherein during a fault condition, the synchronous electric machine generates uncontrolled regenerated currents and the safe state braking system controller is configured to modulate the uncontrolled regenerated currents flowing through the resistor to apply a controlled braking torque to a rotor of the synchronous electric machine.

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