Control method and control device for electric four-wheel drive vehicle

Abstract

A control method for an electric four-wheel drive vehicle, the electric four-wheel drive vehicle comprising: a front drive source for driving the front wheels; and a rear drive source for driving the rear wheels independently of the front wheels. The control method for the electric four-wheel drive vehicle obtains a driving direction of the electric four-wheel drive vehicle based on an input state of a shift lever, and obtains an actual moving direction of the electric four-wheel drive vehicle. When the driving direction and the moving direction are different, a basic driving force is set for the front wheels and the rear wheels to inhibit movement in the moving direction. By setting an upper limit value on the driving force generated on one of the front wheels and the rear wheels, the one wheel is driven with a driving force smaller than the basic driving force.

Description

Technical Field

[0001] The present invention relates to a control method and a control device for an electric four-wheel drive vehicle using an electric motor as a driving source. Background Art

[0002] JP2005-033866A describes a system that, when a hybrid vehicle equipped with an engine and an electric motor as its driving source is stopped on an uphill road, generates torque corresponding to the road gradient when the brakes are released, thereby preventing the hybrid vehicle from rolling back. Furthermore, the hybrid vehicle described in JP2005-033866A is four-wheel drive, with separate electric motors connected to the front and rear wheels, each controlled by a separate power drive unit.

[0003] An electric vehicle having an electric motor as a driving source can reduce movement in the downhill direction due to gravity (hereinafter referred to as backslip) when starting from a state where it is stopped on a slope. Specifically, the electric vehicle can reduce backslip by, for example, generating a torque corresponding to the road slope by the electric motor through feedback control, or generating a torque in the forward direction by the electric motor when it is detected that the wheel is rotating in the backward direction. In addition, when starting from a state where it is stopped on a slope, in addition to the case where the vehicle (electric vehicle) is moved forward by taking the foot off the brake pedal and stepping on the accelerator pedal, it also includes the case where the vehicle (electric vehicle) is moved backward by simply taking the foot off the brake pedal and causing it to slip. Moreover, in the case where the electric vehicle is a four-wheel drive vehicle, as long as it is a four-wheel drive vehicle, it is of course reasonable to use both the front and rear wheels to generate torque for reducing backslip.

[0004] However, when the road resistance experienced by the front and rear wheels is different, the motor torque used to reduce backslipping may cause the vehicle to perform actions that are not normally anticipated by the driver or fellow passengers. For example, when the vehicle starts on a slope, some backslipping is expected to occur. However, when one of the front and rear wheels is located on a road surface with relatively low resistance, if the motor torque used to reduce backslipping is generated as described above, there is a possibility that the wheel on the road surface with low resistance will not grip the road surface and will spin in the driving direction (the intended direction of travel). Therefore, the motor torque control used to reduce backslipping may sometimes cause the driver or the like to feel uncomfortable with the vehicle's operating state (hereinafter referred to as "discomfort").

[0005] The discomfort caused by the spinning of some wheels does not occur in conventional four-wheel drive vehicles in which all wheels are mechanically driven in conjunction with each other. Therefore, as described above, the discomfort caused by wheel spinning is a unique issue for four-wheel drive vehicles that can independently control some of the multiple wheels, such as electric four-wheel drive vehicles in which the front and rear wheels are driven by separate electric motors. Summary of the Invention

[0006] An object of the present invention is to provide a control method and a control device for an electric four-wheel drive vehicle, which can implement motor torque control for reducing roll back without causing discomfort to the driver or the like.

[0007] A control method for an electric four-wheel drive vehicle according to one embodiment of the present invention is a control method for an electric four-wheel drive vehicle having a front drive source for driving the front wheels and a rear drive source for driving the rear wheels independently of the front wheels. The method obtains the driving direction of the electric four-wheel drive vehicle based on the input state of a shift lever, and obtains the actual moving direction of the electric four-wheel drive vehicle. When the driving direction is different from the moving direction, a basic driving force is set for the front wheels and the rear wheels to suppress movement in the moving direction. By setting an upper limit value on the driving force generated on one of the front wheels and the rear wheels, the wheel of one of the wheels is driven with a driving force smaller than the basic driving force. BRIEF DESCRIPTION OF THE DRAWINGS

[0008] Figure 1 This is a block diagram illustrating the configuration of an electric four-wheel-drive vehicle that executes the control method of the electric four-wheel-drive vehicle according to the present embodiment.

[0009] Figure 2 This is a flowchart illustrating main processing of the control method of the electric four-wheel drive vehicle according to the present embodiment.

[0010] Figure 3 This is a graph showing an example of an accelerator opening-torque table.

[0011] Figure 4 It is a block diagram showing the configuration of the torque distribution process.

[0012] Figure 5 It is a block diagram showing the configuration of the second correction process.

[0013] Figure 6 This is a map showing the upper limit value set for the torque limit applied during reverse slip.

[0014] Figure 7 This is an explanatory diagram showing an example of a situation in which the electric four-wheel drive vehicle may slide backward.

[0015] Figure 8Graphs showing changes in wheel rotation speed, torque, and accelerator opening in a comparative example in which no torque limit is applied.

[0016] Figure 9 This is a graph showing changes in the number of rotations of the wheels, torque, and accelerator opening when torque limitation is implemented.

[0017] Figure 10 This is an explanatory diagram showing a situation in which the same action as that during backward sliding may be performed.

[0018] Figure 11 It is a block diagram showing the configuration of the second correction process according to the modification. DETAILED DESCRIPTION

[0019] Hereinafter, embodiments of the present invention will be described with reference to the accompanying drawings.

[0020] Figure 1 This is a block diagram illustrating the configuration of an electric four-wheel-drive vehicle that executes the control method of the electric four-wheel-drive vehicle according to the present embodiment.

[0021] In addition, the so-called electric vehicle refers to a vehicle that has an electric motor (hereinafter referred to as the electric motor) as a driving source and is able to travel by generating a driving force caused by the torque generated by the electric motor at one or more wheels. Therefore, among electric vehicles, in addition to so-called electric cars, hybrid vehicles that use both an electric motor and an engine as driving sources are also included. For example, electric vehicles also include hybrid vehicles that use an electric motor as a driving source for either the front wheel or the rear wheel, and an engine as a driving source for the other wheel. In addition, a four-wheel drive vehicle refers to a vehicle that can use four wheels as driving wheels. Therefore, among four-wheel drive vehicles, in addition to vehicles that always use four wheels as driving wheels, vehicles that can switch between two-wheel drive and so-called front-wheel drive or rear-wheel drive are also included. In addition, a four-wheel drive vehicle can control part of the four wheels in conjunction as driving wheels, and sometimes the four wheels are controlled as independently driven driving wheels.

[0022] Therefore, in the present embodiment, the electric four-wheel drive vehicle refers to a vehicle that can travel by generating driving force due to torque generated by an electric motor at some or all of the four wheels.

[0023] like Figure 1 As shown, the vehicle 100 is an electric four-wheel drive vehicle and includes a front drive system fds, a rear drive system rds, a battery 1, and a motor controller 2 (controller).

[0024] The front drive system FDS receives power from the battery 1 and drives the front wheels 9f under the control of the motor controller 2. Specifically, the front drive system FDS includes a front inverter 3f, a front drive motor 4f, a front speed reducer 5f, a front rotation sensor 6f, a front drive shaft 8f, and front wheels 9f.

[0025] The front wheels 9f are a pair of wheels located relative to each other in the front direction of the vehicle 100, among the four wheels of the vehicle 100. The front direction of the vehicle 100 is a predetermined direction determined by, for example, the orientation of the driver's seat. Furthermore, the front wheels 9f function as drive wheels that generate driving force for the vehicle 100 through the front drive system fds.

[0026] The rear drive system rds receives power from the battery 1 and drives the rear wheels 9r under the control of the motor controller 2. Specifically, the rear drive system rds includes a rear inverter 3r, a rear drive motor 4r, a rear speed reducer 5r, a rear rotation sensor 6r, a rear drive shaft 8r, and rear wheels 9r, symmetrically with the front drive system fds.

[0027] The rear wheels 9r are a pair of wheels located relatively rearward of the vehicle 100 among the four wheels of the vehicle 100. The rearward direction of the vehicle 100 is the direction opposite to the forward direction of the vehicle 100. The rear wheels 9r function as drive wheels that generate driving force for the vehicle 100 through the rear drive system rds.

[0028] Battery 1 is connected to the electric motor via an inverter, supplying driving power to the motor through discharge. Battery 1 can also be charged by receiving regenerative power from the electric motor. In the front drive system FDS, battery 1 is connected to the front drive motor 4f via the front inverter 3f. Similarly, in the rear drive system RDS, battery 1 is connected to the rear drive motor 4r via the rear inverter 3r.

[0029] The motor controller 2 is a control device for the vehicle 100. The motor controller 2 is a computer composed, for example, of a central processing unit (CPU), a read-only memory (ROM), a random access memory (RAM), and an input / output interface (I / O interface). Based on vehicle variables of the vehicle 100, the motor controller 2 generates control signals for controlling the front drive motor 4f and the rear drive motor 4r. By controlling the front drive motor 4f and the rear drive motor 4r, the motor controller 2 controls the movement of the front wheels 9f and the rear wheels 9r, respectively.

[0030] Vehicle variables are information indicating the operating state or control state of the vehicle 100 as a whole or of each component constituting the vehicle 100. Vehicle variables can be obtained through detection, measurement, or calculation. For example, the accelerator opening, the gear position signal of the gear lever, the vehicle speed, the yaw rate, the DC voltage value of the battery 1, the steering angle, and the rotor phase, three-phase AC current, electrical angular velocity, rotational speed, number of revolutions, and wheel speed of each electric motor are vehicle variables of the vehicle 100. The motor controller 2 uses these vehicle variables, for example, as digital signal inputs, to control the front drive motor 4f and the rear drive motor 4r.

[0031] The control signals for controlling the front drive motor 4f and the rear drive motor 4r are, for example, PWM signals (Pulse Width Modulation signals) for controlling the currents of these motors. The motor controller 2 generates drive signals for the front inverter 3f and the rear inverter 3r based on the generated PWM signals.

[0032] The front inverter 3f and the rear inverter 3r each have two switching elements (e.g., power semiconductor elements such as IGBTs (Insulated Gate Bipolar Transistors) or MOS-FETs (metal-oxide-semiconductor field-effect transistors)) corresponding to each other. Each of these inverters turns the switching elements on and off based on the drive signals generated by the motor controller 2, thereby converting the DC current supplied from the battery 1 into AC current, and regulating the current supplied to the front drive motor 4f and the rear drive motor 4r, respectively. Furthermore, each inverter inversely converts the AC current generated by the front drive motor 4f and the rear drive motor 4r into DC current through regenerative braking, regulating the current supplied to the battery 1.

[0033] The front drive motor 4f and the rear drive motor 4r are, for example, three-phase AC motors that generate a driving force (torque) by the AC current supplied from the connected inverter. The driving force generated by the front drive motor 4f is transmitted to the front wheel 9f via the front reducer 5f and the front drive shaft 8f. Similarly, the driving force generated by the rear drive motor 4r is transmitted to the rear wheel 9r via the rear reducer 5r and the rear drive shaft 8r. In addition, when the front drive motor 4f and the rear drive motor 4r are driven to rotate by the front wheel 9f and the rear wheel 9r respectively, a regenerative braking force is generated, and the kinetic energy of the vehicle 100 is recovered as electrical energy. In addition, the front drive motor 4f is a driving source (front driving source) that drives the front wheel 9f. Similarly, the rear drive motor 4r is a driving source (rear driving source) that drives the rear wheel 9r independently of the front wheel 9f.

[0034] The front speed reducer 5f and the rear speed reducer 5r are composed of a plurality of gears, for example. Each of these speed reducers reduces the rotation speed of the motor connected thereto and transmits it to the drive shaft, thereby generating a driving torque or a braking torque (hereinafter referred to as torque) proportional to the reduction ratio.

[0035] The front rotation sensor 6f and the rear rotation sensor 6r detect rotor phases of the motors to which they are connected, and output the detected phases to the motor controller 2. The front rotation sensor 6f and the rear rotation sensor 6r are, for example, resolvers or encoders.

[0036] The front current sensor 7f and the rear current sensor 7r detect the current flowing through their respective connected motors and output it to the motor controller 2. In this embodiment, these current sensors detect the three-phase AC current of each motor. Alternatively, the front current sensor 7f and the rear current sensor 7r can be used to detect the current of any two phases and calculate the current of the remaining phase.

[0037] In addition to the front rotation sensor 6f and front current sensor 7f incorporated into the front drive system fds, and the rear rotation sensor 6r and rear current sensor 7r incorporated into the rear drive system rds, the vehicle 100 also includes various other sensors 15. These sensors 15 include, for example, an accelerator position sensor 15a, an acceleration sensor (not shown), a speed sensor (not shown), a yaw rate sensor 15b, a GPS (Global Positioning System) sensor (not shown), and / or a steering angle sensor (not shown). The accelerator position sensor 15a detects the accelerator position APO, which represents the amount of accelerator (not shown) operation. The acceleration sensor detects the longitudinal and / or lateral acceleration of the vehicle 100. The speed sensor detects the vehicle speed V of the vehicle 100. The vehicle speed V represents the overall moving speed (vehicle body speed) of the vehicle 100. The yaw rate sensor 15b detects the yaw rate of the vehicle 100. The GPS sensor detects the position of the vehicle 100. The steering angle sensor detects the steering angle of the steering wheel (not shown). The detection values ​​of the various sensors 15 are input to the motor controller 2. Specifically, the motor controller 2 can acquire detected vehicle variables such as the accelerator opening APO, longitudinal acceleration, lateral acceleration, yaw rate, position information, and steering angle as needed.

[0038] Furthermore, the motor controller 2 can arbitrarily acquire the operation or setting states of various devices (not shown) included in the vehicle 100 as vehicle variables. For example, the motor controller 2 can acquire the input state of the shift lever 16 used to operate the transmission (not shown). In this embodiment, the vehicle 100 is equipped with an automatic transmission, and the shift lever 16 of the vehicle 100 is a so-called selector. Therefore, the motor controller 2 acquires signals indicating gear positions such as park ("P"), reverse ("R"), neutral ("N"), and drive ("D") (hereinafter referred to as shift signals) as the input state of the shift lever 16. This allows the motor controller 2 to determine the driver's intended driving direction of the vehicle 100, etc. For example, if the shift signal SFT indicates the D position, the motor controller 2 determines that the driver's intended driving direction of the vehicle 100 is forward and executes control to drive the vehicle 100 forward. Therefore, if the shift signal SFT indicates the D position, the vehicle 100 is driven forward. Furthermore, when the shift signal SFT is in the R position, the motor controller 2 determines that the driver's intended driving direction of the vehicle 100 is the rearward direction, and executes control for rearward driving of the vehicle 100. Therefore, when the shift signal SFT is in the R position, the driving direction of the vehicle 100 is the rearward direction.

[0039] Figure 2 1 is a flowchart illustrating the main processing of the control method of the electric four-wheel drive vehicle of this embodiment. Figure 2 As shown, the motor controller 2 executes input processing S201, basic torque target value calculation processing S202, torque distribution processing S203, current target value calculation processing S204, and current control processing S205. The motor controller 2 is programmed to execute these processes every predetermined calculation cycle.

[0040] 1. Input processing

[0041] Input processing S201 is a process in which the motor controller 2 obtains vehicle variables by receiving input from various sensors 15, etc. Furthermore, in input processing S201, the motor controller 2 obtains parameters used in subsequent processing that cannot be directly obtained as vehicle variables through calculations using vehicle variables, etc.

[0042] In this embodiment, the motor controller 2 obtains information from various sensors 15, including the accelerator position APO (%), the rotor phase (rad) and three-phase AC current (A) of each motor, the shift position signal SFT of the shift lever 16, the vehicle speed V (km / h), the yaw rate YR (deg / sec), and the DC voltage Vdc (V) of the battery 1. In particular, the motor controller 2 obtains the driving direction of the vehicle 100 based on the shift position signal SFT, which indicates the input state of the shift lever 16.

[0043] Furthermore, in the input process S201 , the motor controller 2 obtains the motor electrical angular velocity ωe [rad / s], the motor rotational speed ωm [rad / s], the motor rotation number Nm [rpm], and the wheel speed ωw [km / h] by the following calculations.

[0044] (1) Motor electrical angular velocity ωe

[0045] The motor controller 2 calculates the electrical angular velocity ωe of each motor by differentiating the rotor phase α with respect to time. Specifically, the motor controller 2 calculates the electrical angular velocity ωef of the front motor by differentiating the rotor phase αf of the front drive motor 4f. Furthermore, the motor controller 2 calculates the electrical angular velocity ωer of the rear motor by differentiating the rotor phase αr of the rear drive motor 4r.

[0046] (2) Motor rotation speed ωm

[0047] The motor controller 2 divides the motor electrical angular velocity ωe by the number of pole pairs of the motor to calculate the motor rotational speed ωm, which is the mechanical angular velocity. Specifically, the motor controller 2 divides the front motor electrical angular velocity ωef by the number of pole pairs of the front drive motor 4f to calculate the front motor rotational speed ωmf. Similarly, the motor controller 2 divides the rear motor electrical angular velocity ωer by the number of pole pairs of the rear drive motor 4r to calculate the rear motor rotational speed ωmr.

[0048] Furthermore, motor controller 2 obtains the actual moving direction of vehicle 100 based on the positive or negative sign of the calculated front motor rotational speed ωmf and / or rear motor rotational speed ωmr. The actual moving direction refers to the direction in which vehicle 100 is actually moving, regardless of the input state of shift lever 16, for example. For example, even if vehicle 100 is facing up a slope and the input state of shift lever 16 is D, if vehicle 100 is actually rolling down the slope, the actual moving direction is the downhill direction, i.e., the rearward direction of vehicle 100. In this embodiment, motor controller 2 obtains the actual moving direction of vehicle 100 based on the positive or negative sign of front motor rotational speed ωmf.

[0049] (3) Motor speed Nm

[0050] The motor controller 2 calculates the motor speed Nm by multiplying the motor rotational speed ωm by the unit conversion factor (60 / 2π). Specifically, the motor controller 2 converts the units of the front motor rotational speed ωmf to calculate the front motor speed Nmf. Similarly, the motor controller 2 converts the units of the rear motor rotational speed ωmr to calculate the rear motor speed Nmr.

[0051] (4) Wheel speed ωw

[0052] The motor controller 2 calculates the front wheel speed ωwf, which is the wheel speed of the front wheel 9f, based on the value obtained by multiplying the front motor rotational speed ωmf by the rolling radius Rf of the front wheel 9f and the gear ratio of the front speed reducer 5f. Similarly, the motor controller 2 calculates the rear wheel speed ωwr, which is the wheel speed of the rear wheel 9r, based on the value obtained by multiplying the rear motor rotational speed ωmr by the rolling radius Rr of the rear wheel 9r and the gear ratio of the rear speed reducer 5r. In this embodiment, a unit conversion factor is applied to the front wheel speed ωwf and rear wheel speed ωwr calculated as described above, converting the units of "m / s" of the front wheel speed ωwf and rear wheel speed ωwr to "km / h."

[0053] 2. Basic torque target value calculation and processing

[0054] The basic torque target value calculation process S202 is a process in which the motor controller 2 calculates a basic torque target value Tm0* based on vehicle variables. The basic torque target value Tm0* is the torque requested by the driver of the vehicle 100 through accelerator operation, etc. (the so-called requested torque). Furthermore, the basic torque target value Tm0* is a target value for the total amount of torque generated by the front drive motor 4f and the torque generated by the rear drive motor 4r.

[0055] More specifically, the motor controller 2 refers to Figure 3 The accelerator position-torque table shown calculates the basic torque target value Tm0* based on the accelerator position APO and the motor rotational speed ωm. When the motor controller 2 refers to the accelerator position-torque table, the front motor rotational speed ωmf, the rear motor rotational speed ωmr, or their average can be used as the motor rotational speed ωm. In this embodiment, the motor controller 2 calculates the basic torque target value Tm0* using the front motor rotational speed ωmf.

[0056] In addition, if Figure 3As shown, within a specified range within the negative range of the motor rotational speed ωm, the basic torque target value Tm0* is a positive value. This setting is used to suppress backward slipping of the vehicle 100 or reduce the amount and / or speed of movement during backward slipping. Specifically, in the basic torque target value calculation process S202, a positive basic torque target value Tm0* is calculated within a specified range within the negative range of the motor rotational speed ωm. This sets the basic driving force that suppresses movement of the vehicle 100 relative to the front wheels 9f and rear wheels 9r in the actual direction of travel during backward slipping. However, if either the front wheels 9f or the rear wheels 9r are on a road surface with low relative road resistance μ (hereinafter referred to as a low μ road surface), as described later, the basic driving force used to reduce the backward slipping may cause the driver to feel uncomfortable with the vehicle's behavior. Therefore, as described later, the motor controller 2 sets an upper limit on the torque allocated to the wheels on the low μ road surface.

[0057] 3. Torque distribution processing

[0058] The torque distribution process S203 is a process in which the motor controller 2 distributes the basic torque target value Tm0* between the target torque values ​​generated by the front drive motor 4f and the target torque values ​​generated by the rear drive motor 4r. In other words, the torque distribution process S203 distributes the basic torque target value Tm0* to the front wheels 9f and the rear wheels 9r.

[0059] Figure 4 : is a block diagram showing the structure of the torque distribution process S203. Figure 4 As shown, the torque distribution process S203 includes a first correction process S410, a distribution ratio multiplication process S420, a second correction process S430, a front torque target value calculation process S440, a feedback process S450, and a ratio restriction process S460.

[0060] (1) First correction process

[0061] In the first correction process S410, the base torque target value Tm0* is corrected before being distributed to the front and rear wheels 9f, 9r, to limit the total amount of driving force generated at the front and rear wheels 9f, 9r. Specifically, in the first correction process S410, the base torque target value Tm0* is multiplied by a predetermined coefficient β to calculate the first torque target value Tm1*.

[0062] The prescribed coefficient β multiplied by the basic torque target value Tm0* represents the amount of restriction on the basic torque target value Tm0*. Therefore, the prescribed coefficient β is, for example, a positive number less than 1. In addition, the prescribed coefficient β is determined based on the yaw feedback torque Ty. The yaw feedback torque Ty is a torque used to reduce the lateral slip of the vehicle 100 by feeding it back to the first correction process S410 as described above. The yaw feedback torque Ty is calculated in the yaw feedback control S411. For example, the yaw feedback torque Ty is calculated based on the steering angle, the yaw rate YR, the deviation between the yaw rate YR and its target value (hereinafter referred to as the yaw rate target value), etc. The yaw rate target value is determined based on, for example, the vehicle speed V and the steering angle.

[0063] In the first correction process S410, the base torque target value Tm0* is limited by a predetermined coefficient β determined based on the yaw feedback torque Ty. This limits the driving force generated by the front wheels 9f and rear wheels 9r according to the yaw rate YR. As a result, the yaw rate YR is reduced, thereby reducing the possibility of the vehicle 100 slipping or spinning in the lateral direction.

[0064] (2) Distribution ratio multiplication processing

[0065] In the distribution ratio multiplication process S420, the first rear torque target value Tm1r* is calculated by multiplying the first torque target value Tm1* by a predetermined distribution ratio κ. In the distribution ratio determination process S421, the distribution ratio κ is determined based on, for example, the driving mode setting or a shift in the center of gravity of the vehicle 100. In this embodiment, a first distribution ratio prioritizing energy efficiency and a second distribution ratio prioritizing driving stability are predefined. In the distribution ratio determination process S421, a distribution ratio is selected from these distribution ratios based on the settings, etc.

[0066] (3) Second correction process

[0067] In the second correction processing S430, various correction processings are performed on the first rear torque target value Tm1r*, and the second rear torque target value Tm2r* is calculated. After being corrected by the feedback processing S450 and the ratio limiting processing S460, the second rear torque target value Tm2r* becomes the rear torque target value Tmr*. The rear torque target value Tmr* is the torque target value allocated to the rear wheel 9r in the first torque target value Tm1*. That is, the rear torque target value Tmr* is the final target value of the torque output by the rear drive motor 4r to the rear wheel 9r. Therefore, the motor controller 2 controls the rear drive motor 4r based on the rear torque target value Tmr*. The various correction processings performed in the second correction processing S430 will be described in detail later. In addition, the torque generated at the rear wheel 9r by the control based on the final rear torque target value Tmr* is the rear torque Tmr (refer to Figure 9) In addition, the second correction process S430 is a feedforward control that determines a target value based on the current vehicle variables and the like.

[0068] (4) Calculation and processing of the front torque target value

[0069] In the front torque target value calculation process S440, the first front torque target value Tm1f* is calculated by subtracting the second rear torque target value Tm2r* from the first torque target value Tm1*. The first front torque target value Tm1f* becomes the front torque target value Tmf* after being corrected by the feedback process S450 and the ratio limitation process S460. The front torque target value Tmf* is the target value of the torque allocated to the front wheel 9f in the first torque target value Tm1*. That is, the front torque target value Tmf* is the final target value of the torque output by the front drive motor 4f to the front wheel 9f. Therefore, the motor controller 2 controls the front drive motor 4f based on the front torque target value Tmf*. In addition, through the control based on the final front torque target value Tmf*, the actual torque generated at the front wheel 9f is the front torque Tmf (refer to Figure 9 ). In addition, the total of the rear torque Tmr and the front torque Tmf is the total torque Tm (refer to Figure 9 ).

[0070] As described later, the second rear torque target value Tm2r* is the value obtained by reducing the first rear torque target value Tm1r* through torque limitation. Therefore, in the front torque target value calculation process S440, the calculation of subtracting the second rear torque target value Tm2r* from the first torque target value Tm1* essentially adds the amount of reduction in rear torque (torque of the rear wheels 9r) due to torque limitation to the front torque (torque of the front wheels 9f). Therefore, the total amount of torque distributed to the front and rear wheels 9f, namely the first torque target value Tm1*, is maintained. As a result, when the driving force generated by the rear wheels 9r is reduced below the base driving force, the reduced driving force (the reduction amount) is added to the driving force generated by the front wheels 9f, driving the front wheels 9f with a driving force greater than the base driving force.

[0071] In this embodiment, the first front torque target value Tm1f* is calculated by subtracting the second rear torque target value Tm2r* from the first torque target value Tm1* in the front torque target value calculation process S440. Therefore, substantially all of the reduction in rear torque due to the torque limit is added to the front torque. However, a portion of the reduction in rear torque due to the torque limit may also be added to the front torque in the front torque target value calculation process S440. For example, the first front torque target value Tm1f* may be calculated by multiplying the first torque target value Tm1* by the allocation ratio "1-κ" and adding a portion of the difference between the first rear torque target value Tm1r* and the second rear torque target value Tm2r* to the result.

[0072] (5) Feedback processing

[0073] In feedback processing S450, feedback torque FBTf (not shown) for the front wheels 9f and feedback torque FBTr (not shown) for the rear wheels 9r are calculated. The feedback torque FBTf for the front wheels 9f is then added to the first front torque target value Tmf1*, while the feedback torque FBTr for the rear wheels 9r is added to the second rear torque target value Tmr2*.

[0074] Feedback torque FBTf is calculated based on, for example, a deviation Δωm (not shown) between the front motor rotational speed ωmf and the rear motor rotational speed ωmr, and / or a deviation Δωmf (not shown) between a target value (estimated value) and an actual value of the front motor rotational speed ωmf. Similarly, feedback torque FBTr is calculated based on, for example, a deviation Δωm between the front motor rotational speed ωmf and the rear motor rotational speed ωmr, and / or a deviation Δωmr (not shown) between a target value (estimated value) and an actual value of the rear motor rotational speed ωmr.

[0075] In this embodiment, feedback torques FBTf and FBTr are calculated based on the deviation Δωm between the front motor rotational speed ωmf and the rear motor rotational speed ωmr. The deviation Δωm represents the difference in the number of revolutions (or rotational speeds) between the front and rear wheels 9f, 9r, and is correlated with the slip of the front and rear wheels 9f, 9r. Therefore, the addition of feedback torques FBTf and FBTr, calculated based on the deviation Δωm, suppresses or reduces slip of the front and rear wheels 9f, 9r, respectively.

[0076] (6) Rate Limiting Processing

[0077] In the ratio limiting process S460, upper limits are set on the change rates of the first front torque target value Tmf1* and the second rear torque target value Tmr2* to which the feedback torque FBTf is added. The ratio limiting process S460 prevents or reduces slip of the front wheels 9f and the rear wheels 9r.

[0078] 4. Current target value calculation and processing

[0079] exist Figure 2 In the target current value calculation process S204, the dq axis current target values ​​for the front drive motor 4f and the rear drive motor 4r are calculated. The dq axis current target values ​​for the front drive motor 4f are calculated based on the front torque target value Tmf* and the DC voltage value Vdc of the battery 1 by referring to a predetermined table. Similarly, the dq axis current target values ​​for the rear drive motor 4r are calculated based on the rear torque target value Tmr* and the DC voltage value Vdc of the battery 1 by referring to a predetermined table.

[0080] 5. Current control processing

[0081] In the current control process S205 , the front drive motor 4 f and the rear drive motor 4 r are driven based on their dq axis current target values ​​to output torques indicated by the front torque target value Tmf* and the rear torque target value Tmr*, respectively.

[0082] Specifically, first, the dq-axis current values ​​are calculated based on the three-phase AC current values ​​and the rotor phase. Next, the dq-axis voltage command values ​​are calculated based on the deviation between the dq-axis current values ​​and the dq-axis current target values ​​calculated in the current target value calculation process S204. Furthermore, the three-phase AC voltage command values ​​are calculated based on the dq-axis voltage command values ​​and the rotor phase. Then, a PWM signal is calculated based on the three-phase AC voltage command values ​​and the DC voltage value Vdc of the battery 1. These dq-axis current values, dq-axis voltage command values, three-phase AC voltage command values, and PWM signals are calculated for the front drive motor 4f and the rear drive motor 4r, respectively. Based on the PWM signals thus calculated, the front drive motor 4f and the rear drive motor 4r are driven with the torques specified by the front torque target value Tmf* and the rear torque target value Tmr*, respectively, by opening and closing the switching elements of the front inverter 3f and the rear inverter 3r.

[0083] Next, the second correction process S430 will be described in detail.

[0084] Figure 5 4 is a block diagram showing the configuration of the second correction process S430. Figure 5 As shown, the second correction process S430 includes a reversal determination process S510, an upper limit value calculation process S520, and a minimum value process S530.

[0085] (a) Reversal determination processing

[0086] In the reverse rotation determination process S510, a determination is made as to whether the actual moving direction of vehicle 100 is reversed relative to the driving direction of vehicle 100. In this embodiment, the driving direction of vehicle 100 is determined by the shift position signal SFT of the shift lever 16. Furthermore, in this embodiment, the actual moving direction of vehicle 100 is determined by the front motor rotational speed ωmf. For example, if vehicle 100 is on a slope, reverse rotation determination process S510 determines whether vehicle 100 is rolling backward or is currently rolling backward. Specifically, it determines whether the driving direction of vehicle 100 indicated by shift position signal SFT differs from the actual moving direction of vehicle 100 indicated by front motor rotational speed ωmf. If the determination result indicates that the driving direction differs from the actual moving direction, a negative front motor rotational speed ωmf is output. For example, if the front motor rotational speed ωmf is negative despite shift position signal SFT being in D, the driving direction of vehicle 100 is forward, while the actual moving direction is rearward. This is the case, for example, when vehicle 100 is rolling backward on an uphill slope. Furthermore, if the front motor rotational speed ωmf is positive despite the shift signal SFT being in the R position, the vehicle 100 is driven in the rearward direction, while actually moving forward. This is a scenario where, for example, the vehicle 100 is sliding backward on a downhill road. Furthermore, in this embodiment, "slipping backward" refers to a situation where the vehicle 100 is moving in the direction opposite to the driving direction. Therefore, slipping backward includes not only situations where the vehicle 100 is moving backward, but also situations where the vehicle 100 is moving forward, as described above.

[0087] More specifically, the inversion determination process S510 includes a multiplication process S511 and an output selection process S512.

[0088] In the multiplication process S511, the front motor rotation speed ωmf is multiplied by "-1." This inverts the sign of the front motor rotation speed ωmf. The calculation result of the multiplication process S511 is used in the output selection process S512.

[0089] In output selection processing S512, either the front motor rotational speed ωmf after sign reversal in multiplication processing S511 or the calculated front motor rotational speed ωmf (the front motor rotational speed ωmf before sign reversal) is selected and output for use in upper limit value calculation processing S520. This selection is made based on the shift position signal SFT of the shift lever 16. Specifically, when the shift position signal SFT of the shift lever 16 is in the R position, the front motor rotational speed ωmf after sign reversal is selected. Furthermore, when the shift position signal SFT of the shift lever 16 is in the D position, the front motor rotational speed ωmf before sign reversal is selected. Therefore, in output selection processing S512, when the vehicle 100 is rolling backward, a negative front motor rotational speed ωmf is output.

[0090] (b) Upper limit value calculation process

[0091] In the upper limit value calculation process S520, the upper limit value T is calculated based on the vehicle speed V and the accelerator opening APO. UL The upper limit value T calculated in the upper limit value calculation process S520 UL It is used in the minimum value processing S530. UL Serves as a torque limit applied to the first rear torque target value Tmr1* during reverse slip.

[0092] Figure 6 The upper limit value T set in the torque limit applied during reverse slip UL The mapping diagram of . Figure 6 As shown, the upper limit value T UL The value of the accelerator opening APO is determined within the range where the vehicle speed V is below zero. Figure 6 The vehicle speed V shown can be directly acquired as a vehicle variable, but the front motor rotational speed ωmf or the rear motor rotational speed ωmr, which is the motor rotational speed ωm, can also be converted into a vehicle speed (rear wheel speed ωwr, which is the wheel speed ωw) and used instead of the vehicle speed V. In the present embodiment, the front motor rotational speed ωmf output as the determination result of the reverse rotation determination process S510 is converted into the vehicle speed V for use.

[0093] The upper limit value T when the vehicle speed V is zero and the accelerator opening APO is zero UL The basic limit value (hereinafter referred to as "T1") is a predetermined value determined to ensure smooth transition without any step difference from the normal torque distributed when the vehicle speed V is positive, that is, the torque distributed when the driving direction coincides with the actual moving direction. This ensures smooth transitions when torque limitation is applied at the start of reverse slip and when normal control is transitioned to after reverse slip.

[0094] When the accelerator opening APO is zero, within a predetermined range where the absolute value of the vehicle speed V is small, the upper limit value T UL It is set to decrease smoothly from the basic limit value T1 along a predetermined ramp C1. The presence of the ramp C1 ensures robustness against fluctuations in the vehicle speed V, that is, against fluctuations in the motor rotational speed ωm. The specific shape of the ramp C1 is predetermined through experiments, simulations, and the like.

[0095] In addition, when the accelerator opening APO is zero and the vehicle speed V is higher than the upper limit value T UL When the range determined by the slope C1 is small and the sliding speed is large, the upper limit value T ULFor example, a predetermined value T2 is determined as a constant value. This predetermined value T2 is a value that suppresses wheel spin and is predetermined by experiments or simulations. In order to suppress wheel spin and exert appropriate driving force at the same time, it is preferable that the upper limit value T2 increases as the vehicle speed V increases. UL Basically, the limit value T1 is set to be smaller, and the torque limit is set to be larger as the vehicle speed V is higher, instead of setting the predetermined value T2.

[0096] When the vehicle speed V is zero, and the accelerator opening APO is within a range of a predetermined threshold value Th1 or less, the upper limit value T UL The predetermined threshold value Th1 is, for example, a value that determines the range in which the accelerator opening APO is substantially zero. In addition, the maximum value T3 is set to be sufficiently larger than the upper limit value T1. UL The value inputted is the first rear torque target value Tm1r*. That is, the upper limit value T UL The torque limit is substantially released in the range of the maximum value T3. Therefore, the torque limit is substantially not implemented at the accelerator opening APO larger than the threshold value Th1. UL The range where the maximum value T3 is obtained is the range where the accelerator opening APO is greater than or equal to the threshold value Th1, regardless of the vehicle speed V. UL It works in a range where the accelerator opening APO smaller than the threshold value Th1 is substantially zero.

[0097] In addition, the upper limit value T UL The accelerator is determined to increase smoothly from the basic limit value T1 along a predetermined slope C2. The presence of slope C2 ensures robustness against fluctuations in accelerator opening APO. The specific shape of slope C2 is predetermined based on experiments, simulations, and the like.

[0098] The upper limit value T is formed within the range where the vehicle speed V is zero or less and the accelerator opening APO is less than the threshold value Th1. UL The curved surface CS is set to have no sharp concavities or discontinuities except for the smooth convex portion formed by the slopes C1 and C2. UL Within the range of the vehicle speed V and the accelerator opening APO, robustness can be ensured with respect to fluctuations in the motor rotation speed ωm and the accelerator opening APO.

[0099] (c) Minimum value processing

[0100] In the minimum value process S530, the first rear torque target value Tmr1* is compared with the upper limit value T calculated in the upper limit value calculation process S520. UL, the smaller value is output as the second rear torque target value Tmr2*. As a result, in the minimum value processing S530, the output first rear torque target value Tmr1* is limited to the upper limit value T UL The second rear torque target value Tmr2* is as follows. That is, in the minimum value process S530, the first rear torque target value Tmr1* is limited to the upper limit value T UL The following torque limits are set. In addition, when the driving direction of the vehicle 100 is the same as the actual moving direction, the upper limit value T UL Since the torque limit is substantially released since the maximum value T3 is reached, the first rear torque target value Tmr1* is directly output as the second rear torque target value Tmr2*.

[0101] As described above, the output first rear torque target value Tm1r* is limited to the upper limit value T by the second correction process S430. UL The second rear torque target value Tm2r* is as follows: As described above, the second rear torque target value Tm2r* output as a result of the second correction process S430 finally becomes the rear torque target value Tmr* through the feedback process S450 and the ratio restriction process S460.

[0102] In addition, in the above description, Figure 4 In the torque distribution process S203 shown, the front wheels 9f and the rear wheels 9r have a priority relationship. For example, the torque allocated to the rear wheels 9r is determined first, and then the torque allocated to the front wheels 9f is determined based on this priority. In addition, the torque allocated to the rear wheels 9r (the second rear torque target value Tmr2*Tc) is set to an upper limit value T using the rotation speed of the front wheels 9f (the front motor rotation speed ωmf). UL In the present embodiment, when the vehicle speed V is calculated from the motor rotation speed ωm, the front motor rotation speed ωmf is used instead of the rear motor rotation speed ωmr.

[0103] However, these precedence relationships and priority relationships are interchangeable, and the motor controller 2 controls the vehicle 100 by changing these precedence relationships and priority relationships as needed. Figure 4 In the torque distribution process S203 shown in FIG. 1 , the torque of the rear wheel 9r is determined. Similarly, the torque of the front wheel 9f is first calculated, and then the torque of the rear wheel 9r is calculated by subtracting the calculated torque of the front wheel 9f from the first torque target value Tm1*, which is the total amount of the distributed torque. In this case, during the calculation of the torque of the front wheel 9f, the torque of the front wheel 9f is subjected to the second correction process S430, and an upper limit value T is set for the torque of the front wheel 9f. ULFurthermore, when performing the second correction process S430 on the torque of the front wheel 9f, the rear motor rotational speed ωmr is used instead of the front motor rotational speed ωmf in the reverse rotation determination process S510 and the upper limit value calculation process S520. Furthermore, when converting the motor rotational speed ωm to obtain the vehicle speed V, the rear motor rotational speed ωmr is used.

[0104] Therefore, in the vehicle 100 of this embodiment, the motor controller 2 determines whether the driving direction of the vehicle 100 is different from the actual moving direction through the reverse rotation determination process S510. Then, if the result of the reverse rotation determination process S510 is that the driving direction of the vehicle 100 is different from the actual moving direction, the torque allocated to one of the front wheels 9f and the rear wheels 9r is limited to the upper limit value T UL The following torque limitation (torque limitation) is implemented. Thus, when the driving direction of the vehicle 100 is different from the actual moving direction, the driving force generated on one of the front wheels 9f and the rear wheels 9r is set to the upper limit value T UL The corresponding upper limit value P UL The limitation of the feedback torque FBTf and the feedback torque FBTr is performed (hereinafter referred to as the driving force limitation). In addition, due to the driving force limitation, the wheel whose driving force is limited is driven with a driving force smaller than the basic driving force generated by the feedback torque FBTf and the feedback torque FBTr.

[0105] Hereinafter, the effects of the vehicle 100 according to the present embodiment will be described.

[0106] Figure 7 1 is an explanatory diagram showing an example of a situation in which an electric four-wheel drive vehicle may slide backward. Figure 7 Vehicle 700 of the illustrated comparative example is a vehicle that performs the same control as vehicle 100 of the above-described embodiment, except that torque limitation in the second correction process S430 is not performed.

[0107] Here, if Figure 7 As shown, a comparative example vehicle 700 is parked on a slope 701, facing forward and uphill. The surface of slope 701 is, for example, a snow-covered road 710, part of which includes a frozen road surface (so-called icy road surface) 711, which has a relatively low μ relative to the snow-covered road 710. For simplicity, it is assumed that the wheels grip the snow-covered road 710, but have difficulty gripping the frozen road surface 711. When a predetermined level of driving force is generated at the wheels, the wheels slip.

[0108] Under the above-described circumstances, when the driver selects the D position with the shift lever 16 and starts the comparative example vehicle 700, the driver's intended driving direction for the comparative example vehicle 700 is forward (i.e., climbing the slope). Furthermore, depending on the inclination angle of the slope 701, before the accelerator pedal is operated, i.e., when the accelerator opening APO is zero, the vehicle 700 may slip backward in the rearward direction A1. In the event of such a slip, the motor controller 2 of the comparative example vehicle 700 generates a basic driving force for the front wheels 9f and rear wheels 9r to propel the vehicle 100 in the driving direction. As a result, under normal road conditions where the wheels are gripping the road surface, the occurrence of the slip is suppressed or the speed of the slip is reduced.

[0109] However, here, the front wheel 9f is on a snowy road surface 710, while the rear wheel 9r is on a frozen road surface 711. There is a difference in road resistance μ between the front wheel 9f and the rear wheel 9r. Therefore, when a basic driving force is generated by the front wheel 9f and the rear wheel 9r, the front wheel 9f gripping the snowy road surface 710 may, for example, balance the force causing the vehicle 100 to slide backward, or may be pulled by the vehicle 100's backward slide, causing it to rotate in the rearward direction A2. This movement of the front wheel 9f is a wheel movement that the driver naturally anticipates.

[0110] On the other hand, since the rear wheels 9r are on a low μ surface, or frozen surface 711, they have no grip. Therefore, in the comparative example vehicle 700, when the basic driving force is generated by the rear wheels 9r, the rear wheels 9r spin in the forward direction A3, which is the driving direction. Furthermore, since almost all of the basic driving force generated by the rear wheels 9r is used for this spin, the spin of the rear wheels 9r occurs rapidly. Furthermore, judging by the vehicle speed V experienced by the driver as a result of the reverse slip, this spin of the rear wheels 9r exceeds the typical driver's expectations. Furthermore, this spin of the rear wheels 9r can occur even when the driver does not actually operate the accelerator pedal, which is relevant to wheel rotation, and the accelerator pedal opening APO is zero. Therefore, the spin of the rear wheels 9r in the comparative example vehicle 700 can sometimes cause discomfort to the driver.

[0111] Figure 8 Graph showing changes in wheel rotation speed, torque, and accelerator opening in a comparative example where no torque limitation is implemented. Figure 8 As shown in (A), when the comparative example vehicle 700 starts to slide backward at time t1, the front wheels 9f and rear wheels 9r of the comparative example vehicle 700 are pulled by the backward slide and rotate, so the front motor rotation speed ωmf and the rear motor rotation speed ωmr decrease. Figure 8As shown in (B), the total torque Tm of the front wheels 9f and the rear wheels 9r increases due to the feedback process S450. In this period (time t1-t2), there is no substantial difference between the front torque Tmf of the front wheels 9f and the rear torque Tmr of the rear wheels 9r.

[0112] However, if the rear wheel 9r on the frozen road surface 711 slips at time t2, Figure 8 As shown in (A), the rear motor rotational speed ωmr increases. Then, eventually, for example, at time t3, although the front wheel 9f is rotating in the actual direction of movement due to the backward slip, the rear motor rotational speed ωmr changes to rotation in the driving direction (forward rotation). Therefore, after time t3, the rotational directions of the front wheel 9f and the rear wheel 9r are reversed.

[0113] After that (after time t3), the rear motor rotation speed ωmr continues to rise, and the idling of the rear wheel 9r cannot be restrained. Figure 8 As shown in (B), in the comparative example vehicle 700, the torque distribution process S203 is executed without applying torque limitation, so there is no substantial difference between the front torque Tmf and the rear torque Tmr between time t2 and time t3 and after time t3.

[0114] Furthermore, the idling of the rear wheel 9r as described above, Figure 8 As shown in (C), it also occurs when the driver does not operate the accelerator at all and the accelerator opening APO continues to be zero.

[0115] on the other hand, Figure 9 Graph showing the changes in wheel rotation speed, torque, and accelerator opening when torque limitation is implemented. Figure 9 As shown in (A), when the vehicle 100 starts to slide backward from time t1, the front wheels 9f and the rear wheels 9r are pulled by the backward slide and rotate, so the front motor rotation speed ωmf and the rear motor rotation speed ωmr decrease. Figure 9 As shown in (B), the total torque Tm increases due to the reverse slip of the vehicle 100, and there is no substantial difference in the front torque Tmf and the rear torque Tmr. These are the same as those of the vehicle 700 of the comparative example.

[0116] However, if the rear wheel 9r is about to slip at time t2, the torque limitation in the second correction process S430 is executed in the vehicle 100. As a result, the second rear torque target value Tm2r* is limited by the upper limit value T UL As a result, if Figure 9 As shown in (B), after time t2, the rear torque Tmr becomes the upper limit value T ULHowever, since the front torque target value Tmf* is calculated by subtracting the rear torque target value Tmr* from the first torque target value Tm1*, the total torque Tm is maintained. Moreover, the reduction in the rear torque Tmr due to the torque limit is compensated by the increase in the front torque Tmf.

[0117] In addition, as described above, when the rear torque Tmr is subject to the upper limit value T UL When the torque is limited, the rear wheel 9r generates driving force within the range of gripping the frozen road surface 711. Therefore, the rear wheel 9r does not slip substantially and maintains the rotation by reverse slip traction like the front wheel 9f. Figure 9 As shown in (A), the rear motor rotational speed ωmr remains substantially the same as the front motor rotational speed ωmf even after time t2. That is, even if the rear wheels 9r do not spin and slightly slip at time t2, the driver can still accept the vehicle 100's movement as a predictable backward slip behavior. Therefore, the driver does not experience any discomfort from the vehicle 100 during a backward slip.

[0118] In addition, the conditions for the accelerator pedal operation are the same as those for the vehicle 700 of the above-mentioned comparative example. Figure 9 As shown in (C), the above-mentioned operation of the vehicle 100 is an operation during a period in which the driver does not operate the accelerator at all and the accelerator opening APO continues to be zero.

[0119] While this example illustrates a situation where the vehicle 100 is facing uphill, control is also performed to change the order and priority of the front wheels 9f and rear wheels 9r when the vehicle 100 is facing downhill. Therefore, even when the vehicle 100 is facing downhill, the vehicle 100 operates in the same manner as described above. Consequently, the driver does not experience any discomfort from the vehicle 100 when sliding backward.

[0120] As described above, the control method of the electric four-wheel drive vehicle of the present embodiment is a control method for an electric four-wheel drive vehicle (vehicle 100) having a front drive source (front drive motor 4f) that drives the front wheels 9f and a rear drive source (rear drive motor 4r) that drives the rear wheels 9r independently of the front wheels 9f. Furthermore, the electric four-wheel drive vehicle (vehicle 100) of the present embodiment obtains the driving direction of the electric four-wheel drive vehicle (vehicle 100) based on the input state (shift signal SFT) of the shift lever 16, and also obtains the actual moving direction of the electric four-wheel drive vehicle (vehicle 100). Furthermore, in the electric four-wheel drive vehicle (vehicle 100) of the present embodiment, when the driving direction differs from the actual moving direction, [i] a basic driving force is set for the front wheels 9f and the rear wheels 9r to suppress movement in the actual moving direction, and [ii] an upper limit is set on the driving force generated by one of the front wheels 9f and the rear wheels 9r (e.g., the rear wheel 9r), thereby driving the one wheel (e.g., the rear wheel 9r) with a driving force less than the basic driving force.

[0121] As a result, even if there is a difference in road resistance between the front and rear wheels 9f, 9r, the front and rear wheels 9f, 9r, will not spin due to the generation of the basic driving force. In particular, the front and rear wheels 9f, 9r, will not rotate in the opposite direction of the slip. Consequently, control is maintained without causing the driver to feel uncomfortable with the vehicle 100's operating state.

[0122] In particular, as in the above-described embodiment, [ii] the second correction process S430 for limiting the upper limit of the driving force generated by one of the front wheels 9f and the rear wheels 9r (e.g., the rear wheel 9r) is a feedforward control based on current vehicle variables, etc. Therefore, the control method of the electric four-wheel drive vehicle of this embodiment can suppress the occurrence of one wheel slipping or spinning before it actually occurs, that is, at the stage when one wheel is about to slip or spin, or can reduce the duration, speed, or other extent of the slipping or spinning.

[0123] For example, when performing the same process as the second correction process S430 through feedback control, it is impossible to suppress slipping or spinning of one wheel unless it actually occurs. Furthermore, when performing the above process through feedback control, an insensitive zone is sometimes provided to delay the effective start of feedback control in order to improve control stability. Furthermore, in feedback control, insensitive zones sometimes occur due to system response delays, etc. Given the existence of such insensitive zones, feedback control cannot suppress actual slipping or spinning of one wheel. Therefore, the second correction process S430 is feedforward control, which can better prevent wheel spinning and prevent the driver from experiencing discomfort compared to the same process performed through feedback control.

[0124] In addition, the control method of the electric four-wheel drive vehicle of the present embodiment sets an upper limit on the driving force generated by the wheel (e.g., the rear wheel 9r) on the side of the front wheel 9f and the rear wheel 9r that is located in the actual direction of movement. When the vehicle 100 slides backward for a certain distance, even if the front wheel 9f and the rear wheel 9r are initially on a grippable road surface such as a snowy road surface 710, there may sometimes be a low μ road surface such as a frozen road surface 711 in front of them. In this case, in many cases, the wheel on the side of the actual direction of movement will reach the low μ road surface before the wheel (e.g., the front wheel 9f) on the other side. Therefore, as in the above-mentioned embodiment, by setting an upper limit on the driving force generated by the wheel on the side of the actual direction of movement, even if the wheel on one side reaches the low μ road surface during the backward slide, the wheel on that side will not accidentally spin. Therefore, the control method of the electric four-wheel drive vehicle of the present embodiment does not cause discomfort to the driver.

[0125] Furthermore, the control method for an electric four-wheel drive vehicle according to this embodiment, when an upper limit is set on the driving force generated by one of the front wheels 9f and the rear wheels 9r (e.g., the rear wheel 9r), drives the other of the front wheels 9f and 9r (e.g., the front wheel 9f) with a driving force greater than the base driving force. Therefore, while the driving force of the wheel with the upper limit set is reduced, the total driving force of the vehicle 100 is reduced. However, at least a portion of the reduced driving force by the wheel is supplemented by the driving force of the other wheel. As a result, even when an upper limit is set on the driving force generated by one wheel, it is easier to prevent the vehicle 100 from slipping. Furthermore, in this embodiment, when the other wheel is driven with a driving force greater than the base driving force, the reduction in driving force of one wheel is supplemented by the base driving force of the other wheel. However, this is not limiting to this example; the driving force of the other wheel may also be a value obtained by adding a driving force greater than the reduction in driving force of the wheel to the base driving force.

[0126] Furthermore, the control method for an electric four-wheel drive vehicle according to this embodiment drives the other wheel (e.g., the front wheel 9f) with a driving force greater than the base driving force by adding at least a portion of the driving force generated by the reduction (reduction amount) of one wheel (e.g., the rear wheel 9r) to the driving force generated by the other wheel. Thus, at least a portion of the driving force reduced by one wheel is directly supplemented by the driving force of the other wheel, thereby easily and reliably suppressing reverse slipping of the vehicle 100.

[0127] More specifically, the control method for an electric four-wheel drive vehicle according to this embodiment maintains a constant total amount of basic driving force by driving the other wheel (e.g., the front wheel 9f) with the driving force obtained by subtracting the driving force generated by one wheel (e.g., the rear wheel 9r) with a set upper limit from the total amount of basic driving force generated by the front wheels 9f and the rear wheels 9r. Therefore, the control method for an electric four-wheel drive vehicle according to this embodiment can impose a limit on the driving force generated by one wheel and suppress reverse slipping of the vehicle 100 in the same manner as without such a limit. In other words, the control method for an electric four-wheel drive vehicle according to this embodiment can suppress the occurrence of reverse slipping because the total amount of basic driving force remains unchanged. Furthermore, the control method for an electric four-wheel drive vehicle according to this embodiment can reduce the speed of reverse slipping even in the event of reverse slipping.

[0128] Furthermore, the control method of the electric four-wheel drive vehicle of this embodiment can calculate the basic driving force based on the rotational speed of the other wheel (e.g., the front wheel 9f) when an upper limit is set on the driving force generated by one of the front wheels 9f and the rear wheels 9r (e.g., the rear wheel 9r). In other words, the upper limit T set for the torque limitation applied during slipping can be calculated based on the wheel speed of the other wheel for which an upper limit is not set. UL As described above, in many cases, one wheel in the actual direction of travel reaches the low-μ road surface before the other wheel. However, the speed of the other wheel gripping the road surface allows accurate understanding of the speed of vehicle 100. As a result, the upper limit of the driving force for one wheel can be appropriately set regardless of the degree of slippage on one wheel, allowing for more reliable control without causing discomfort to the driver.

[0129] Furthermore, the control method for an electric four-wheel-drive vehicle according to this embodiment, when implementing a limit on the driving force generated by one wheel (e.g., the rear wheel 9r), sets the limit within the range where the one wheel grips the road surface. Driving force is generated by the one wheel within a range that does not cause idling and / or reversal of the one wheel, which would cause discomfort to the driver. Therefore, the vehicle 100 can continue to operate stably as a four-wheel-drive vehicle within this range while limiting the driving force generated by one wheel.

[0130] In addition, the control method of the electric four-wheel drive vehicle of this embodiment sets the upper limit value according to the vehicle speed (see Figure 6). Thus, the driving force of one wheel is appropriately limited according to the specific situation of the reverse slip. For example, within a range where the vehicle speed V is relatively low (vehicle speed V is close to zero), the driving force of one wheel is smoothly reduced without any steps compared to the normal driving force distributed when the vehicle speed V is positive. As a result, even if the driving force of one wheel is limited, no shock or vibration will be generated in the vehicle 100, and stable control can be maintained.

[0131] In addition, the control method for an electric four-wheel-drive vehicle according to this embodiment determines, via the motor controller 2, whether the accelerator opening APO is below a predetermined threshold value Th1. If the accelerator opening APO is below the predetermined threshold value Th1, an upper limit is set on the driving force generated at one wheel (e.g., the rear wheel 9r). If the accelerator opening APO is greater than the threshold value Th1, this limit is substantially lifted, and no torque restriction is applied. One of the situations in which the driver is particularly likely to feel uncomfortable with the behavior of the vehicle 100 is when the driver is not operating the accelerator. Specifically, the driver may feel uneasy about the uncontrolled spinning of one wheel even when the driver is not operating the accelerator. In contrast, when the driver is operating the accelerator, since the driver is operating the wheel's rotation, even if the wheel is spinning, the driver is less likely to feel uncomfortable with the vehicle 100. Therefore, as described above, the upper limit on the driving force generated at one wheel is applied while the driver is not operating the accelerator, and the limit is lifted when the driver is clearly operating the accelerator. As a result, in a scene where the driver is particularly likely to feel a sense of discomfort with the vehicle 100 , the spinning of one wheel can be appropriately suppressed.

[0132] Furthermore, the control method for an electric four-wheel-drive vehicle according to this embodiment, while imposing an upper limit on the driving force generated by one wheel (e.g., the rear wheel 9r), removes this limit if the wheel rotates in the opposite direction of the driving direction. If the wheel on the side with the upper limit set for the driving force rotates in the opposite direction of the driving direction, i.e., in the direction causing the vehicle 100 to slip, the wheel will grip the road surface even if it is on a low-μ surface. Therefore, if the wheel on the side with the upper limit set for the driving force grips the road surface, the upper limit on the driving force is removed. This allows the vehicle 100 to continue stable operation as a four-wheel-drive vehicle even while slipping.

[0133] In addition, the control method of the electric four-wheel drive vehicle of this embodiment, as in the above-mentioned embodiment, is particularly suitable for the scenario where the vehicle 100 slides on the slope 701, but the control method of the electric four-wheel drive vehicle of this embodiment is also suitable for other scenarios.

[0134] Figure 10 This is an explanatory diagram showing a situation in which the same action as when sliding backward is possible. Figure 10 As shown, when the vehicle 100 changes direction, the ideal operating mode for the vehicle 100 is for the vehicle 100 to come to a complete stop when the shift lever 16 is switched from the R position to the D position. However, in reality, when such a direction change is performed, the vehicle 100 does not come to a complete stop when the shift lever 16 is switched from the R position to the D position, and the vehicle 100 often moves backward due to inertia. In this operating mode, when the shift lever 16 is switched to the D position, although the vehicle 100 is driving forward, the actual moving direction is the rearward direction A2. Therefore, when the shift lever 16 is switched, for example, if the rear wheels 9r are on a frozen road surface 711, if the control method for an electric four-wheel-drive vehicle according to this embodiment is not implemented, the rear wheels 9r will spin, similar to the situation of sliding backward on a slope 701, and the driver may feel uncomfortable with the vehicle 100's operating state. On the other hand, if the control method for an electric four-wheel-drive vehicle according to this embodiment is implemented, the spinning of the rear wheels 9r can be suppressed, similar to the above-mentioned embodiment. As a result, control is continued without causing the driver to feel uncomfortable with the operating state of vehicle 100 .

[0135] Furthermore, in conventional four-wheel drive vehicles in which the front wheels 9f and rear wheels 9r are all mechanically connected, the structure prevents the wheel (e.g., the rear wheel 9r) from spinning during a reverse slip, which is a problem in the above-described embodiments. Therefore, the control method for an electric four-wheel drive vehicle of this embodiment solves a problem inherent in an electric four-wheel drive vehicle (vehicle 100) having a front drive source (front drive motor 4f) that drives the front wheels 9f and a rear drive source (rear drive motor 4r) that drives the rear wheels 9r independently of the front wheels 9f.

[0136] While the embodiments of the present invention have been described above, the configurations described in the above embodiments and modifications merely represent a portion of application examples of the present invention and are not intended to limit the technical scope of the present invention.

[0137] For example, in the above embodiment, based on Figure 6 The graph shown sets the upper limit value T UL , upper limit value T UL The setting of the accelerator opening APO and the vehicle speed V changes smoothly, but the upper limit value T can also be selected from a plurality of preset candidates. UL , corresponding to the accelerator opening APO and the vehicle speed V switching upper limit value T UL .

[0138] In the above embodiment, when the torque is limited in the second correction process S430, the upper limit value T is determined. UL Mapping diagram (see Figure 6 ), but the upper limit value T of the torque limit may be determined by a method other than using this map. UL .

[0139] For example, Figure 11 4 is a block diagram showing the configuration of the second correction process S430 of the modified example. Figure 11 As shown, in the second correction process S430 of the modified example, an upper limit value switching determination process S521 and an upper limit value selection process S522 are provided instead of the upper limit value calculation process S520.

[0140] The upper limit value switching determination process S521 outputs a value for switching the upper limit value T based on the result of the reverse determination process S510 and the accelerator opening APO. UL An upper limit value switching determination flag is output. More specifically, in upper limit value switching determination processing S521, it is determined whether the front motor rotational speed ωmf input as a result of reverse rotation determination processing S510 is negative due to vehicle 100 rolling backward. Furthermore, in upper limit value switching determination processing S521, it is determined whether accelerator opening APO is zero. Then, in upper limit value switching determination processing S521, an upper limit value switching determination flag is output, which is "true" if the front motor rotational speed ωmf is negative and accelerator opening APO is zero, and "false" otherwise.

[0141] In the upper limit selection process S522, the upper limit value T is selected from the first upper limit value that substantially releases the torque limit and the second upper limit value that is the minimum guaranteed torque based on the upper limit switching determination flag. UL The first upper limit value is, for example, the upper limit value T in the above embodiment. UL The second upper limit is the specified value T2 in the above embodiment. Therefore, in the upper limit selection process S522, the second upper limit is selected when the upper limit switching determination flag is "true", and the first upper limit is selected when the upper limit switching determination flag is "false".

[0142] As mentioned above, if the upper limit value T is used instead UL Mapping diagram (see Figure 6 ), and based on the result of the reverse determination process S510 and the accelerator opening APO, the upper limit value T UL By switching between the first upper limit value and the second upper limit value, the torque limitation of the above embodiment can be easily implemented. UL Mapping diagram (see Figure 6), substantial torque limitation is applied when the accelerator opening APO is below a predetermined threshold value Th1, and substantial torque limitation is released when the accelerator opening APO exceeds the threshold value Th1, thereby adjusting the torque limitation so that substantially no torque limitation is applied. In contrast, the above-described modified example corresponds to a case where the threshold value Th1 is zero. That is, in the above-described modified example, torque limitation is applied when the accelerator opening APO is below a predetermined threshold value (accelerator opening APO is zero), and substantial torque limitation is released when the accelerator opening APO exceeds the threshold value (accelerator opening APO is greater than zero), thereby changing the configuration so that substantially no torque limitation is applied.

Claims

1. A method for controlling an electric four-wheel drive vehicle comprising: a front drive source for driving front wheels; and a rear drive source for driving rear wheels independently of the front wheels, wherein: The control method of the electric four-wheel drive vehicle performs the following control: obtaining a driving direction of the electric four-wheel drive vehicle based on an input state of a gear lever, Obtaining the actual moving direction of the electric four-wheel drive vehicle, When the accelerator opening is zero and the driving direction is different from the moving direction, A basic driving force is set for the front wheel and the rear wheel to suppress movement in the moving direction. In addition, when one of the front wheel and the rear wheel is on a road surface with relatively low road resistance and the other is on a road surface with relatively high road resistance, The other wheel of the front wheel and the rear wheel located on a road surface with high road resistance is driven with the basic driving force or a driving force greater than the basic driving force, By limiting the driving force upper limit value for one of the front and rear wheels located on the road surface with low road surface resistance, the one wheel is driven with a driving force smaller than the basic driving force.

2. The control method of the electric four-wheel drive vehicle according to claim 1, wherein: The upper limit value is set in advance for the driving force generated at the wheel located in the moving direction among the front wheel and the rear wheel.

3. The control method of the electric four-wheel drive vehicle according to claim 1, wherein: The other of the front wheels and the rear wheels is driven with a driving force greater than the basic driving force.

4. The control method of the electric four-wheel drive vehicle according to claim 3, wherein: By adding at least a portion of the driving force of the lowered portion of the one wheel to the driving force generated at the other wheel, the other wheel is driven with a driving force greater than the basic driving force.

5. The control method of the electric four-wheel drive vehicle according to claim 3, wherein: The total amount of the basic driving force is maintained constant by driving the other wheel with a driving force obtained by subtracting the driving force generated at the one wheel for which the upper limit value is set from the total amount of the basic driving force generated at the front and rear wheels.

6. The control method of the electric four-wheel drive vehicle according to claim 3, wherein: The basic driving force is calculated based on the rotational speed of the other wheel.

7. The control method of the electric four-wheel drive vehicle according to claim 1, wherein: The upper limit value is set within a range in which the one wheel grips the road surface.

8. The control method of the electric four-wheel drive vehicle according to claim 1, wherein: The upper limit value is set according to the vehicle speed.

9. The control method of the electric four-wheel drive vehicle according to claim 1, wherein: The restriction is implemented when the accelerator opening is below a predetermined threshold value. The restriction is released when the accelerator opening is greater than the threshold.

10. The control method for an electric four-wheel drive vehicle according to any one of claims 1 to 9, wherein: When the one wheel rotates in a direction opposite to the driving direction, the restriction is released.

11. A control device for an electric four-wheel drive vehicle, the electric four-wheel drive vehicle comprising: a front drive source for driving front wheels; and a rear drive source for driving rear wheels independently of the front wheels, wherein: The control device of the electric four-wheel drive vehicle performs the following control: obtaining a driving direction of the electric four-wheel drive vehicle based on an input state of a gear lever, Obtaining the actual moving direction of the electric four-wheel drive vehicle, When the accelerator opening is zero and the driving direction is different from the moving direction, A basic driving force is set for the front wheel and the rear wheel to suppress movement in the moving direction. In addition, when one of the front wheel and the rear wheel is on a road surface with relatively low road resistance and the other is on a road surface with relatively high road resistance, The other wheel of the front wheel and the rear wheel located on a road surface with high road resistance is driven with the basic driving force or a driving force greater than the basic driving force, By limiting the driving force upper limit value for one of the front and rear wheels located on the road surface with low road surface resistance, the one wheel is driven with a driving force smaller than the basic driving force.

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