Braking methods for a hybrid vehicle

Abstract

Hybrid electric vehicle that includes the following: a drivetrain configured to deliver drivetrain torque to the vehicle wheels; Friction brakes configured to apply a braking torque to the vehicle wheels; an internal combustion engine configured to supply positive torque to the drivetrain; an electric motor configured to alternately supply both positive and negative torque to the drivetrain; and a control system programmed to respond to a request to transition from positive wheel torque to negative wheel torque by controlling the power unit and / or electric motor to reduce the drivetrain torque from a positive initial value to a positive threshold value with a first rate of change. Controlling the brakes to apply a negative braking torque, while the engine and / or electric motor are controlled to reduce the drivetrain torque from a positive threshold to a negative threshold with a second rate of change, the magnitude of which is less than the magnitude of the first rate of change, in order to mitigate backlash disturbances, and Controlling the brakes to increase the braking torque to zero, while controlling the electric motor to reduce the drivetrain torque from the negative threshold to a negative setpoint with a third rate of change, the magnitude of which is higher than the magnitude of the second rate of change.

Description

TECHNICAL AREA

[0001] The present disclosure relates to the field of hybrid vehicle control systems. In particular, the disclosure relates to a method for reducing backlash disturbances during the transition of wheel torque from positive to negative torque. BACKGROUND

[0002] Under normal vehicle operation, a power source, such as an internal combustion engine, generates power that is transmitted to the vehicle's wheels via a drivetrain. However, under certain conditions, power flows in the opposite direction through the drivetrain. For example, when a vehicle decelerates during engine compression braking, power flows from the vehicle's wheels to the internal combustion engine. The drivetrain often includes a gear train that either increases torque and decreases speed, or increases speed and decreases torque. When the direction of torque reverses, there can be a momentary interruption in torque as shafts rotate to bring opposing flanks of the gear teeth into contact. This is known as backlash.During this time, some energy can accumulate in the powertrain components, which is released as an audible noise or a torque disturbance, which vehicle occupants may find disturbing.

[0003] The extent of the disturbance correlates strongly with the rate of change of torque during the regenerative braking cycle. To mitigate the impact of a regenerative braking cycle, a powertrain control system can limit the rate of change of torque during the transition from positive to negative. This is usually not perceived by vehicle occupants because the magnitude of the negative torque associated with engine braking is much lower than that associated with the positive torque associated with normal driving. However, the magnitude of negative torque associated with regenerative braking in a hybrid vehicle can be much greater than that of typical engine compression braking.

[0004] The relevant state of the art is represented by the publications DE 10 2013 104 654 A1 and DE 10 2013 104 656 A1. BRIEF SUMMARY OF THE REVELATION

[0005] A hybrid electric vehicle comprises a powertrain, an internal combustion engine, an electric motor, friction brakes, and a control unit. The powertrain delivers drive torque to the vehicle wheels, while the brakes deliver braking torque to the vehicle wheels. The powertrain may include a variable-ratio transmission, a driveshaft, a differential, and left and right axles. The internal combustion engine delivers positive torque to the powertrain, while the electric motor alternately delivers both positive and negative torque to the powertrain. The control unit is programmed to respond to a request to transition from positive to negative wheel torque in a manner designed to minimize powertrain backlash disturbances.The control unit directs the engine and / or electric motor to rapidly reduce the drivetrain torque from a positive initial value to a low positive threshold. Then, the control unit directs the brakes to apply a negative braking torque, while the engine and / or electric motor are controlled to slowly reduce the drivetrain torque from the low positive threshold to a low negative threshold. Finally, the control unit releases the brakes, while the electric motor is controlled to rapidly reduce the drivetrain torque to the negative setpoint.

[0006] A method for operating a vehicle involves periodically activating a power unit and motor to supply drivetrain torque and activating the friction brakes to supply braking torque. The drivetrain torque corresponds to a maximum value derived from the net wheel torque demand (NWTD) and a regenerative torque limit (RTL). The braking torque corresponds to a minimum value derived from zero and NWTD minus RTL. The RTL and NWTD can be calculated by a drivetrain control unit and transmitted via CAN bus to a brake control unit. In response to the NWTD changing from positive to negative, the RTL is gradually reduced from a positive threshold to a negative threshold to minimize drivetrain play. BRIEF DESCRIPTION OF THE DRAWINGS Fig. Figure 1 is a schematic representation of a hybrid vehicle powertrain with a step-change transmission. Fig. Figure 2 is a diagram representing a torque transition that is most likely to produce an unpleasant noise and torque disturbance during a game run. Fig. Figure 3 is a diagram representing a torque transition that reduces the noise and torque disturbance of the clearance process, but most likely produces an undesirable delay in the supply of the negative wheel torque. Fig. Figure 4 is a diagram illustrating a torque transition that reduces the noise and torque disturbance of the game cycle and the delay of Fig. 3 partially reduces. Fig. Figure 5 is a diagram that represents a torque transition which reduces the noise and torque disturbance of the clearance process without delaying the supply of negative wheel torque. Fig. Figure 6 is a flowchart for a procedure for controlling the drivetrain of Fig. 1 and the friction brakes, from a positive torque to a negative torque according to Fig. 5 to move on. DETAILED DESCRIPTION

[0007] Embodiments of the present disclosure are described here. It is understood, however, that the disclosed embodiments are purely exemplary of the invention and that other embodiments can be designed in various and alternative forms. The figures are not necessarily to scale; some features may be exaggerated or minimized to show details of particular components. The specific structural and functional details disclosed herein should therefore not be interpreted as limiting, but merely as a representative basis for teaching a person skilled in the art how the present invention can be used in various ways.It is obvious to a person skilled in the art that various features illustrated and described with reference to any one of the figures can be combined with features illustrated in one or more other figures to create embodiments not explicitly illustrated or described. The combinations of illustrated features provide exemplary embodiments for typical applications. However, various combinations and modifications of the features, consistent with the teachings of this disclosure, may be desirable for specific applications or implementations.

[0008] With reference to Fig. Figure 1 shows a schematic diagram of a hybrid electric vehicle (HEV - Hybrid Electric Vehicle) 10 according to an embodiment of the present disclosure. Fig. Figure 1 illustrates exemplary relationships between the components. The physical positioning and orientation of the components within the vehicle may vary. The HEV 10 comprises a powertrain 12. The powertrain 12 includes a power machine 14 that drives a transmission 16, which can be referred to as a modular hybrid transmission (MHT). As described in more detail below, the transmission 16 includes an electric machine, such as an electric motor / generator (M / G) 18, an associated traction battery 20, a torque converter 22, and a multi-stage automatic transmission or gear transmission 24.

[0009] Both the power machine 14 and the M / G 18 are drive sources for the HEV 10. The power machine 14 generally represents a power source, which may include an internal combustion engine, such as one powered by gasoline, diesel, or natural gas. The power machine 14 generates power and corresponding torque, which is supplied to the M / G 18 when a disconnect clutch 26 between the power machine 14 and the M / G 18 is at least partially engaged. The M / G 18 can be implemented by any of several types of electric machines. For example, the M / G 18 can be a permanent magnet synchronous motor. Power electronics 56 prepares direct current (DC) energy supplied by the battery 20 according to the requirements of the M / G 18, as described below. For example, power electronics can supply the M / G 18 with three-phase alternating current (three-phase AC current).

[0010] When the disconnect clutch 26 is at least partially engaged, power can flow from the power unit 14 to the M / G 18 or from the M / G 18 to the power unit 14. For example, the disconnect clutch 26 can be engaged and the M / G 18 can be operated as a generator to convert rotational energy supplied by a crankshaft 28 and an M / G shaft 30 into electrical energy to be stored in the battery 20. The disconnect clutch 26 can also be disengaged to disconnect the power unit 14 from the rest of the drive train 12, so that the M / G 18 can act as the sole power source for the HEV 10. The shaft 30 extends through the M / G 18. The M / G 18 is continuously driven by the shaft 30, while the power unit 14 is only driven by the shaft 30 when the disconnect clutch 26 is at least partially engaged.

[0011] The M / G 18 is connected to the torque converter 22 via shaft 30. The torque converter 22 is therefore connected to the power unit 14 when the disconnect clutch 26 is at least partially engaged. The torque converter 22 comprises a pump impeller attached to the M / G shaft 30 and a turbine attached to a transmission input shaft 32. The torque converter 22 thus provides a hydraulic coupling between shaft 30 and the transmission input shaft 32. The torque converter 22 transmits power from the pump impeller to the turbine when the pump impeller rotates faster than the turbine. The magnitude of the turbine torque and the pump impeller torque generally depends on the relative rotational speeds. If the ratio of pump impeller speed to turbine speed is sufficiently high, the turbine torque will be a multiple of the pump impeller torque.A torque converter bypass clutch 34 can also be provided, which, when engaged, frictionally or mechanically couples the impeller and turbine of the torque converter 22, thereby allowing a more efficient power flow. The torque converter bypass clutch 34 can be operated as a starting clutch to provide smooth vehicle acceleration. Alternatively, in applications that do not include a torque converter 22, a starting clutch similar to the disconnect clutch 26 can be provided between the M / G 18 and the gear unit 24.

[0012] The gear unit 24 can include gear sets (not shown) that are selectively shifted into different gear ratios by selectively engaging friction elements, such as clutches and brakes (not shown), to generate the desired multiple discrete or stepped gear ratios. The friction elements are controllable by a switching scheme that connects and disconnects specific elements of the gear sets to control the ratio between a transmission output shaft 36 and the transmission input shaft 32. Based on various vehicle and environmental operating conditions, the gear unit 24 is automatically shifted from one ratio to another by an associated control unit, such as a powertrain control unit (PCU) 50. The gear unit 24 then supplies powertrain output torque to the output shaft 36.

[0013] It is understood that the hydraulically controlled gear transmission 24 using a torque converter 22 is merely one example of a gear transmission or transmission arrangement; with the embodiments of the present disclosure, any multi-stage gear transmission that receives input torque(s) from a power unit and / or a motor and then supplies torque to an output shaft at the various gear ratios can be used. The gear transmission 24 can, for example, be implemented in an automated manual transmission (AMT) which includes one or more servomotors for moving / rotating shift forks along a shift rail to select a desired gear ratio.

[0014] As in the representative embodiment of Fig. As shown in Figure 1, the output shaft 36 is connected to a differential 40. The differential 40 drives a pair of wheels 42 via corresponding axles 44 connected to the differential 40. The differential transmits approximately the same amount of torque to each wheel 42 while allowing slight differences in rotational speed, for example, when the vehicle is cornering. Various types of differentials or similar devices can be used to distribute torque from the drivetrain to one or more wheels. In some applications, the torque distribution can vary depending on, for example, the specific operating mode or operating condition.

[0015] The powertrain 12 also includes an associated control 50, such as a powertrain control unit (PCU). Although depicted as a single control, the control 50 may be part of a larger control system and may be controlled by various other control units distributed throughout the vehicle 10, such as a vehicle system controller (VSC). It is therefore understood that the powertrain control unit 50 and one or more other control units can collectively be referred to as a "controller" that, in response to signals from various sensors, controls various actuators to perform functions such as starting / stopping the engine 14, operating the motor / gear unit 18 to provide wheel torque or charge the battery 20, selecting or scheduling transmission shifts, etc.The controller 50 can include a microprocessor or a central processing unit (CPU) that interacts with various types of computer-readable storage devices or media. Computer-readable storage devices or media can include volatile and non-volatile storage in, for example, read-only memory (ROM), random-access memory (RAM), and keep-alive memory (KAM). KAM is a persistent or non-volatile memory that can be used to store various operating variables while the CPU is powered off.The computer-readable storage devices or media can be implemented using any of a number of known storage devices, such as PROM (programmable read-only memory), EPROM (electrical PROM), EEPROM (electrically erasable PROM), flash memory, or any other electrical, magnetic, optical, or combination storage devices capable of storing data, some of which represent executable instructions used by the controller in controlling the power machine or vehicle.

[0016] The controller communicates with various power units / vehicle sensors and actuators via an input / output interface (I / O interface), which can be implemented as a single integrated interface providing various raw data or signal conditioning, processing, and / or conversion, short-circuit protection, and the like. Alternatively, one or more dedicated hardware or firmware chips can be used to condition and process specific signals before they are fed to the CPU. As in the representative embodiment of Fig. As generally shown, the controller 50 can communicate signals to and / or from the engine 14, the disconnect clutch 26, the M / G 18, the starting clutch 34, the gearbox 24, and the power electronics 56. The average person will recognize various functions or components controllable by the controller 50 in each of the subsystems identified above, although these are not explicitly shown. Representative examples of parameters, systems, and / or components that can be actuated directly or indirectly by control logic executed by the controller include fuel injection timing, quantity, and duration; throttle position; spark plug ignition timing (in spark-ignition engines); intake / exhaust valve timing and opening times; and front-end accessory drive (FEAD) components, such as...An alternator, an air conditioning compressor, battery charging, regenerative braking, M / G operation, clutch pressures for the disconnect clutch 26, the starting clutch 34, and the gear train of the transmission 24, and the like. Sensors transmitting an input through the I / O interface can be used to indicate, for example, turbocharger boost pressure, crankshaft position (PIP), engine speed (RPM), wheel speeds (WS1, WS2), vehicle speed (VSS), coolant temperature (ECT), manifold absolute pressure (MAP), accelerator pedal position (PPS), ignition switch position (IGN), throttle valve position (TP), ambient air temperature (TMP), exhaust oxygen content (EGO) or any other exhaust component concentration or presence, intake airflow (MAF), transmission gear, ratio, or mode, transmission oil temperature (TOT), transmission turbine wheel speed (TS), torque converter bypass clutch 34 status (TCC), and deceleration or shift mode (MDE).

[0017] Control logic or functions performed by the controller can be represented by flowcharts or similar diagrams in one or more figures. These figures provide representative control strategies and / or logic that can be implemented using one or more processing strategies, such as event-driven, interrupt-driven, multitasking, multithreading, and the like. Thus, various depicted steps or functions can be performed in the shown sequence or in parallel, or in some cases, omitted. Although not explicitly shown, it is obvious to the average person that one or more of the depicted steps or functions can be performed repeatedly, depending on the specific processing strategy used.Similarly, the processing sequence is not strictly necessary to achieve the features and benefits described here, but is given for better illustration and description. The control logic can primarily be implemented in software, executed by a microprocessor-based control unit of a vehicle, power machine, and / or powertrain, such as the Control Unit 50. Of course, the control logic can be implemented in software, hardware, or a combination of both in one or more control units, depending on the specific application. If implemented in software, the control logic can be stored in one or more computer-readable storage devices or media containing data representing code or instructions that are executed by a computer to control the vehicle or its subsystems.The computer-readable storage devices or media may include one or more of a number of known physical devices that use electrical, magnetic and / or optical storage to hold executable instructions and associated calibration information, operating variables and the like.

[0018] An accelerator pedal 52 is used by the vehicle's driver to provide a torque request, power request, or driving command for propelling the vehicle. Generally, pressing and releasing the pedal 52 generates an accelerator pedal position signal, which can be interpreted by the control unit 50 as a request for more or less power. Based on at least one input from the pedal, the control unit 50 applies torque from the power unit 14 and / or the M / G 18. The control unit 50 also controls the timing of gear shifts in the gear transmission 24, as well as the engagement or disengagement of the disconnect clutch 26 and the torque converter bypass clutch 34. Like the disconnect clutch 26, the torque converter bypass clutch 34 can be modulated over a range between the engaged and disengaged positions.This generates variable slip in the torque converter 22 in addition to the variable slip generated by the hydrodynamic coupling between the pump impeller and the turbine. Alternatively, the torque converter bypass coupling 34 can be operated as locked or open, depending on the specific application, without using a modulated operating mode.

[0019] To drive the vehicle with the motor 14, the disconnect clutch 26 is at least partially engaged in order to transmit at least part of the motor torque through the disconnect clutch 26 to the motor / gearbox 18 and then from the motor / gearbox 18 through the torque converter 22 and the gear unit 24. The motor / gearbox 18 can assist the motor 14 by providing additional power to rotate the shaft 30. This operating mode can be referred to as a “hybrid mode” or an “electric motor assist mode”.

[0020] When the vehicle is driven by the M / G 18 as the sole power source, the power flow remains the same, except that the disconnect clutch 26 is released to decouple the motor 14 from the rest of the drivetrain 12. During this time, combustion in the motor 14 may be deactivated or otherwise shut down to save fuel. The traction battery 20 transfers stored electrical energy via wiring 54 to the power electronics 56, which may, for example, include an inverter. The power electronics 56 converts DC voltage from the battery 20 into AC voltage usable by the M / G 18. The controller 50 controls the power electronics 56 to convert voltage from the battery 20 into AC voltage, which supplies the M / G 18 to deliver positive or negative torque to the shaft 30. This operating mode can be described as a "pure electric operating mode".

[0021] The M / G 18 can function as a motor in any operating mode, providing drive power to the drivetrain 12. Alternatively, the M / G 18 can function as a generator, converting kinetic energy from the drivetrain 12 into electrical energy to be stored in the battery 20. For example, the M / G 18 can act as a generator while the power unit 14 provides drive power to the vehicle 10.

[0022] A brake pedal 60 is used by the vehicle's driver to provide a command for the requested negative torque to decelerate the vehicle. Similar to the accelerator pedal 52, pressing and releasing the brake pedal 60 generates a brake pedal position signal, which can be interpreted by the controller 50 as a request for varying degrees of negative torque. Based on at least one input from the brake pedal 60, the controller 50 applies torque from the friction brakes 62 and / or the M / G 18. When the M / G 18 acts as a generator during regenerative braking, energy is transported back from the vehicle through the gear transmission 24 and converted into electrical energy for storage in the battery 30.

[0023] It goes without saying that the in Fig. The schematic representation shown in Figure 1 is purely exemplary and in no way intended to be limiting. Other configurations have been considered that utilize the selective engagement of both a power unit and a motor for transmission through the gearbox. For example, the M / G 18 can be offset from the crankshaft 28, an additional motor can be provided for starting the power unit 14, and / or the M / G 18 can be positioned between the torque converter 22 and the gear unit 24. Other configurations are also conceivable without deviating from the scope of protection of this disclosure.

[0024] When negative wheel torque is applied, the control unit 50 determines how much negative torque is to be generated using the M / G 18 acting as a generator and how much is to be generated using the friction brakes 62. The control unit sets a regenerative torque limit (RTL) at any given time, indicating the maximum negative torque that the M / G 18 is capable of generating at the wheels. This limit can be based on a number of factors, including battery state of charge, vehicle speed, and gear ratio. For example, a fully charged battery may affect the limit to prevent overcharging. Furthermore, the limit may be set to zero at very low vehicle speeds to improve brake feel.To maximize energy recovery, the controller can allocate as much of the target braking torque as possible to the powertrain, as indicated by the regenerative torque limit. The remainder of the negative torque is assigned to the friction brakes. The controller 50 can include a powertrain controller and a brake system controller, which communicate via a CAN (Controller Area Network). In such a configuration, the RTL (Return To Load) and NWTD (Non-Weighted Torque Data) can be calculated by the powertrain controller and transmitted to the brake controller.

[0025] Fig. Figure 2 is a diagram illustrating the behavior of an MHT powertrain during a transition from positive to negative torque. The dotted line 100 indicates the net wheel torque demand (NWTD). The NWTD is based on the accelerator pedal position 52, the brake pedal position 60, and the vehicle speed. The torque demand drops rapidly after the accelerator pedal 52 is released and the brake pedal 60 is applied at time 102. The dashed line 104 represents the regenerative torque limit. It can be seen that the RTL is always below zero. As the vehicle decelerates between time 106 and time 108, the limit gradually decreases from a low value determined by the battery state of charge to zero. The powertrain's contribution to wheel torque is shown by the solid line 110.This line follows the torque demand line 100 until shortly after time 106, where the drivetrain torque is reduced as the vehicle decelerates, according to the regenerative torque limit 104. In this document, drivetrain torque means the sum of the torques supplied to the drive wheels by the drivetrain from the internal combustion engine 14 and the M / G 18, after any torque multiplication by the transmission 24 and / or any axle drive ratio. In the diagram, the lines are shown slightly offset from each other so that both lines are visible even if they coincide in practice. Finally, the strong solid line 112 represents the wheel torque contributed by the friction brakes 62. Torque from the friction brakes is always zero or negative. The braking torque is zero until shortly after time 106, where friction braking is applied as the vehicle decelerates.In this document, braking torque means the sum of the torques applied to all wheels by the friction brakes. When the vehicle is moving forward, the braking torque is always either zero or negative. Therefore, releasing the brakes can be described as increasing the braking torque.

[0026] In Fig. 2. The drivetrain torque 110 transitions rapidly from positive values ​​to negative values ​​at 114. Gears in the gearbox 24 and differential 40 transmit torque in a positive direction through contact between tooth flanks for positive torque. As the torque transitions to a negative torque, the opposite side of each tooth transmits the torque. During the transition, the driven gear must rotate slightly relative to the driving gear to bring the opposite tooth flanks into contact. This is referred to as backlash. In addition to gear meshes, connections, such as splined connections, can contribute to backlash in a drivetrain. The drivetrain input may accelerate during the backlash passage, producing an audible noise and / or torque disturbance.The extent of the disturbance correlates strongly with the rate of change of torque during the game cycle.

[0027] Fig. Section 3 represents a transition using a modified control strategy designed to reduce torque disturbance and audible noise associated with the powertrain slack cycle. The powertrain torque drops rapidly until a small positive torque value is reached at 116. For example, the torque can be reduced from a powertrain torque of more than 100 Nm in approximately 250 milliseconds. The small positive torque value is chosen to be just large enough to ensure that the torque remains positive across the powertrain, taking into account both parasitic powertrain losses and control accuracy. For example, the small positive value might be in the range of 10 Nm. The powertrain torque then drops slowly until a small negative torque value is reached at 118.For example, the transition can be achieved by reducing the torque to zero for a period of approximately 200-500 milliseconds. The small positive torque value is chosen to ensure that the torque is negative throughout the entire drivetrain. Since the rate of change during the transition is low (as an absolute value), the extent of the disturbance is drastically reduced. Unfortunately, this modified procedure introduces a delay between the time the torque request is reduced and the time the reduced torque is actually delivered to the wheels.

[0028] Fig. Figure 4 represents a partial solution to the delay 122. The controlled friction brake torque 112 can include both an open-loop term (sometimes called a feed-forward term) and one or more closed-loop terms (sometimes called feedback terms). As discussed above, the open-loop term can be calculated by subtracting the regenerative torque limit 104 from the net wheel torque requirement 100. The closed-loop terms can be calculated based on the difference between the net wheel torque requirement 100 and the actual net wheel torque. Closed-loop terms can be, for example, proportional to this difference (called a P-term), proportional to a derivative of this difference (called a D-term), or proportional to an integral of this difference (called an I-term).The closed-loop terms can, in response to the deficit during deceleration, lead to some application of the friction brakes at 124, but never enough to eliminate the deficit. After deceleration, the closed-loop terms eventually return to zero. A P- or I-term quickly returns to zero (or a positive value), whereas an I-term only returns to zero after the net wheel torque has been below the demand (higher as an absolute value) for some time. Although the in . Fig. The results shown in 4 are in Fig. Although the results shown in section 3 may be preferred, the applied wheel torque still deviates from the driver's intention for a period of time, leading to customer dissatisfaction.

[0029] Fig. Figure 5 presents the results of an improved control strategy. In response to the release of the accelerator pedal at 102, the control strategy increases the regenerative torque limit 104 to a small positive value. This causes the friction brakes to be applied as the net wheel torque request 100 falls below this small value to generate the requested negative wheel torque, as shown in Figure 126. During the clearance phase between 116 and 118, the regenerative torque limit 104 corresponds to the powertrain torque. After the clearance phase, the control strategy rapidly reduces the regenerative torque limit to its normal value. As the regenerative torque limit 104 falls, the powertrain torque 110 follows the limit 104, and the friction brake torque increases to zero.The rate of change of the regenerative torque limit is set so that the drivetrain torque and friction brake torque can be changed quickly enough to keep pace. Throughout the entire process, the supplied net torque closely follows the net wheel torque requirement of 100, while mitigating the adverse effects of backlash.

[0030] Fig. Figure 6 is a flowchart for a method for controlling the drivetrain torque and the friction brake torque to achieve the in Fig. The result shown in Figure 5 is as follows. This procedure is performed by controller 50 at regular intervals. Initially, a mode variable is set to "constant driving". At 130, the net wheel torque demand (NWTD) is calculated based on the accelerator pedal position, brake pedal position, and vehicle speed. NWTD is positive when the accelerator pedal is depressed with the brake pedal released, and negative when the brake pedal is depressed with the accelerator pedal released. When both are released, the NWTD may be slightly negative to simulate the engine braking behavior of non-hybrid powertrains. Some filtering may be applied to limit the rate of change of the NWTD. At 132, a baseline regenerative torque limit (BRTL) is calculated based on the vehicle speed, gear ratio, and battery state of charge.The BRTL indicates the highest negative torque that the M / G 18 is capable of generating at the wheels without overloading the battery or impairing the braking feel.

[0031] If the mode is still constant speed at 134, the control unit checks at 136 whether the accelerator pedal has been released. If the accelerator pedal is still depressed, the regenerative torque limit is set to the BRTL at 138. If the accelerator pedal has been released, the mode is updated to Transition 1 at 140. The Transition 1 mode corresponds to the time interval between 102 and 116 in Fig. 5, during which the accelerator pedal was released, but the NWTD is still positive. While the mode corresponds to Transition 1, the RTL is set to a small positive value, Threshold 1, at 142. Block 142 can be reached either immediately after the transition to the Transition 1 state, or because the mode at 144 corresponded to the Transition 1 state and the control at 146 determines that it should remain in the Transition 1 state. If the NWTD is near point 116 in Fig. When the RTL falls below 5, the controller changes the mode to Transition 2 at 148. During Transition 2 mode, as determined at 150, the RTL at 152 drops by a small amount, inkr1, at each time step. The calibratable value inkr1 is chosen to be small enough to effectively reduce the game cycle time. If the RTL at 118 in Fig. 5 and at 154 in Fig. If the RTL falls below a small negative value, threshold 2, the controller changes the mode to transition 3 at 156. In transition 3 mode, the RTL either falls immediately after the mode change or, as in 158, falls faster than at 160. The calibratable value inkr2 is significantly higher than inkr1. As soon as the RTL falls below the BRTL, as in 162, the mode is updated to braking at 164. In braking mode, the RTL is set equal to the BRTL at 166.

[0032] Once the RTL has been determined, the applied drivetrain torque and the applied friction brake torque are calculated at points 168 and 170, respectively. If the RTL is below the NWTD (more negative than it is), the drivetrain torque is controlled to supply the requested torque, and the brake torque is controlled to zero. If the RTL is above the NWTD, the drivetrain is controlled to supply the RTL, and the friction brakes are controlled to supply the remainder. The friction brake command may include one or more closed-loop terms in addition to the open-loop terms calculated at point 170; however, these terms are nominally zero.

[0033] Although exemplary embodiments are described above, it is not intended that these embodiments describe all possible forms encompassed by the claims. Instead, the terms used in the description serve to describe rather than limit the scope, and it is understood that various modifications can be made without departing from the intent and scope of the disclosure. As described above, the features of the different embodiments can be combined to form further embodiments of the invention that may not be explicitly described or illustrated.Although various embodiments may have been described as offering advantages or being preferred over other embodiments or implementations of the prior art with respect to one or more desired properties, it is apparent to the person skilled in the art that compromises can be made between one or more features or properties in order to achieve the desired system characteristics, which depend on the specific application and implementation. Thus, embodiments described as less desirable than other embodiments or implementations of the prior art with respect to one or more properties are not outside the scope of protection of the disclosure and may be desirable for certain applications. Key to symbols

[0034] Fig. 6: 132 Calculating the BRTL based on vehicle speed, gear ratio and battery charge level 134 Mode = Constant speed? Yes No 136 Accelerator pedal enabled? 140 Mode = Transition 1 142 RTL = Threshold 1 130 Computational determination of the NWTD based on accelerator pedal position, brake pedal position and vehicle speed, filtering required 144 Mode = Transition 1? 148 Mode = Transition 2 150 Mode = Transition 2? 152 RTL = RTL - inkr1 154 RTL < Threshold 2? 158 Mode = Transition 3? 160 RTL = RTL - inkr2 156 Mode = Transition 3 164 Mode = Brakes 170 Controlled friction brake torque = min(0, NWTD - RTL) 168 Controlled drivetrain torque = max(NWTD, RTL)

Claims

[1] Hybrid electric vehicle comprising the following: a drivetrain configured to deliver drivetrain torque to the vehicle wheels; Friction brakes configured to apply a braking torque to the vehicle wheels; an internal combustion engine configured to supply positive torque to the drivetrain; an electric motor configured to alternately supply both positive and negative torque to the drivetrain; and a control system programmed to respond to a request to transition from positive wheel torque to negative wheel torque by controlling the power unit and / or electric motor to reduce the drivetrain torque from a positive initial value to a positive threshold value with a first rate of change. Controlling the brakes to apply a negative braking torque, while the engine and / or electric motor are controlled to reduce the drivetrain torque from a positive threshold to a negative threshold with a second rate of change, the magnitude of which is less than the magnitude of the first rate of change, in order to mitigate backlash disturbances, and Controlling the brakes to increase the braking torque to zero, while controlling the electric motor to reduce the drivetrain torque from the negative threshold to a negative setpoint with a third rate of change, the magnitude of which is higher than the magnitude of the second rate of change. [2] Vehicle according to claim 1, wherein the drive train comprises a transmission with a variable gear ratio. [3] Vehicle according to claim 2, wherein the drive train further comprises: a drive shaft that is rigidly coupled to an output of the gearbox; a differential that has an input rigidly coupled to the drive shaft; and a first and a second axle, which are rigidly coupled to the first and second outputs of the differential respectively, and each is rigidly coupled to one of the vehicle's wheels. [4] Vehicle according to claim 1, wherein the control system is further programmed to apply an increasing negative braking torque by controlling the brakes in this manner, while the electric motor is controlled to increase the drive train torque to zero, in order to react to the fact that a vehicle speed falls below a threshold value. [5] Vehicle according to claim 1, wherein the braking torque is increased to zero at a third rate, so that a total wheel torque remains constant. [6] Method for transitioning from a positive wheel torque to a negative wheel torque in a hybrid electric vehicle, comprising the following: Reducing a positive powertrain torque to a positive threshold; Applying a braking torque while further reducing the drivetrain torque by a zero height with a reduced rate of change of the drivetrain torque; and Increasing the braking torque in conjunction with further reducing the negative drivetrain torque to maintain a constant negative total wheel torque. [7] The method of claim 6, further comprising: In response to a vehicle speed falling below a threshold, a decreasing braking torque is applied while the drivetrain torque is increased to zero. [8] Method according to claim 6, wherein the initial positive drivetrain torque is generated by an internal combustion engine. [9] Method according to claim 6, wherein the initial positive drivetrain torque is generated by an electric motor. [10] Method for operating a vehicle comprising the following: at regular intervals, controlling a power unit and a motor to supply a drivetrain torque corresponding to a maximum value derived from a net wheel torque requirement (NWTD) and a regenerative torque limit (RTL), and Activating the friction brakes to apply a braking torque corresponding to a minimum value of zero and a difference between the NWTD and the RTL; and In response to the NWTD changing from positive to negative, reduce the RTL at a rate from a positive threshold to a negative threshold to mitigate one drivetrain play cycle. [11] The method of claim 10, further comprising reducing the RTL to the negative threshold by further reducing the RTL at a faster rate. [12] The method of claim 10, further comprising gradually increasing the RTL to zero in response to a vehicle speed falling below a speed threshold. [13] Method according to claim 10, further comprising sending the RTL and the NWTD via a Controller Area Network from a powertrain control to a brake control.

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