Vehicle

Abstract

A vehicle (1) comprises a front wheel side device (2) that is provided to correspond to a left / right pair of front wheels (11, 12), a rear wheel side device (3) that is provided to correspond to a left / right pair of rear wheels (13,14), and a control device (6) that controls the front wheel side device (2) and the rear wheel side device (3). The control device (6) controls the front wheel side device (2) and the rear wheel side device (3) such that, when the vehicle is decelerating, a braking force that acts on the left / right pair of rear wheels (13, 14) is greater than a braking force that acts on the left / right pair of front wheels (11,12). The rear wheel side device (3) is provided with a differential device (33) for distributing the driving force of a rear wheel side drive source (31) to the left / right pair of rear wheels (13, 14), and a differential limiting device (7) for limiting the differential of the differential device (33). The control device (6) controls the differential limiting device (7) such that a torque bias ratio is greater when it is determined that the traveling road surface is slippery than when it is determined that the traveling road surface is not slippery.

Description

Vehicle

[0001] The present invention relates to a vehicle.

[0002] Conventionally, there has been known a vehicle in which left and right front wheels and left and right rear wheels are each driven by independent electric motors. A front-rear wheel independent drive type electric vehicle described in Patent Document 1 includes a front wheel drive unit having a front MG (motor generator) as a front drive source and a front transmission mechanism, and a rear MG as a rear drive source and a rear MG. and a rear wheel drive unit having a rear transmission mechanism. The output of the front transmission mechanism is distributed to the left and right front wheels by a front differential device, and the output of the rear transmission mechanism is distributed to the left and right rear wheels by a rear differential device.

[0003] Japanese Unexamined Patent Application Publication No. 2023-102684

[0004] When the vehicle decelerates, the front wheel load increases and the rear wheel load decreases due to the inertial force of the vehicle body. Accordingly, the suspension springs on the front wheel side are compressed and the suspensions on the rear wheel side are extended in accordance with the load change, causing a so-called nose dive phenomenon in which the vehicle body leans forward. When the nose dive phenomenon occurs, it may give discomfort to the occupants or cause the loaded luggage to shift.

[0005] This nose dive phenomenon can be mitigated by reducing the braking force on the front wheel side and increasing the braking force on the rear wheel side. However, when the vehicle decelerates, the frictional force between the rear wheel tires and the road surface decreases due to the reduction in rear wheel load, and when the braking force on the rear wheel side approaches the frictional force limit value, vehicle behavior becomes unstable. In particular, when increasing the braking force on the rear wheel side during cornering deceleration, in addition to the braking force, a lateral force against centrifugal force is generated on the rear wheels. Therefore, among the left and right rear wheels, the braking force of the wheel particularly on the inner side of the turn tends to approach the rear wheel friction limit value, which may lead to an oversteering tendency and make it difficult to control the vehicle attitude.

[0006] Therefore, an object of the present invention is to provide a vehicle capable of suppressing destabilization of vehicle behavior while suppressing the nose dive phenomenon.

[0007] To achieve the above objective, the present invention provides a vehicle comprising: a front wheel side device provided corresponding to a pair of left and right front wheels; a rear wheel side device provided corresponding to a pair of left and right rear wheels; and a control device for controlling the front wheel side device and the rear wheel side device, wherein the front wheel side device brakes the pair of left and right front wheels and the rear wheel side device brakes the pair of left and right rear wheels when the vehicle decelerates, wherein the front wheel side device brakes the pair of left and right front wheels by at least one of the frictional braking force from a friction brake and the rotational resistance force of a front wheel side drive source that drives the pair of left and right front wheels when the vehicle decelerates, and the rear wheel side device comprises a rear wheel side drive source that drives the pair of left and right rear wheels and a differential device that distributes the driving force of the rear wheel side drive source to the pair of left and right rear wheels, and The present invention provides a vehicle comprising a differential limiting device that limits the differential action of a differential, wherein during vehicle deceleration, a braking force can be applied to the left and right pair of rear wheels via the differential due to the rotational resistance force of the rear wheel drive source, the differential limiting device can increase or decrease the torque bias ratio of the left and right pair of rear wheels through control by the control device, and the control device controls the front wheel side device and the rear wheel side device so that the braking force acting on the left and right pair of rear wheels is greater than the braking force acting on the left and right pair of front wheels during vehicle deceleration, and controls the differential limiting device so that the torque bias ratio is greater when the road surface is determined to be slippery than when the road surface is determined to be less slippery.

[0008] According to the vehicle of the present invention, it is possible to suppress instability in vehicle behavior while suppressing the nose dive phenomenon.

[0009] Figure 1 is a schematic diagram showing an example of the configuration of a vehicle according to an embodiment of the present invention. Figure 2A is a schematic diagram showing the suspension springs and suspension arms on the front and rear wheel sides, as well as the front wheel load and rear wheel load when the vehicle is traveling at a constant speed. Figure 2B is a schematic diagram showing the suspension springs and suspension arms on the front and rear wheel sides, as well as the front wheel load and rear wheel load when the vehicle is decelerating due to front-wheel bias braking. Figure 2C is a schematic diagram showing the suspension springs and suspension arms on the front and rear wheel sides, as well as the front wheel load and rear wheel load when the vehicle is decelerating due to rear-wheel bias braking. Figure 3A is an explanatory diagram showing the friction circle of the left rear wheel when the vehicle is traveling at a constant speed. Figure 3B is an explanatory diagram showing the friction circle of the left rear wheel when the vehicle is decelerating due to front-wheel bias braking. Figure 3C is an explanatory diagram showing the friction circle of the left rear wheel when the vehicle is decelerating due to rear-wheel bias braking. Figure 4 is a schematic diagram showing the configuration of the rear wheel side device and its surroundings. Figure 5A is a graph showing the amount of extension and compression of the suspension springs of the left front wheel and the right front wheel when the vehicle is decelerated by regenerative braking using rear-wheel bias braking and when the vehicle is decelerated by regenerative braking using front-wheel bias braking. Figure 5B is a graph showing the amount of extension and compression of the suspension springs of the left rear wheel and the right rear wheel when the vehicle is decelerated by regenerative braking using rear-wheel bias braking and when the vehicle is decelerated by regenerative braking using front-wheel bias braking. Figure 6 is a graph showing the change in yaw rate when each wheel is regenerated braking with the accelerator released during cornering.

[0010] [Embodiments] Embodiments of the present invention will be described with reference to Figures 1 to 6. The embodiments described below are shown as preferred specific examples for carrying out the present invention, and some parts specifically illustrate various technically preferred technical matters, but the technical scope of the present invention is not limited to these specific embodiments. In the following description, "left" and "right" refer to the left and right in the vehicle width direction with respect to the forward direction of the vehicle.

[0011] Figure 1 is a schematic diagram showing an example of the configuration of a vehicle 1 according to an embodiment of the present invention. The vehicle 1 is a four-wheel drive vehicle capable of driving a pair of front wheels, namely the left front wheel 11 and the right front wheel 12, and a pair of rear wheels, namely the left rear wheel 13 and the right rear wheel 14.

[0012] Vehicle 1 comprises, as its main components, a vehicle body 10, front wheel side devices 2 corresponding to the left front wheel 11 and right front wheel 12, rear wheel side devices 3 corresponding to the left rear wheel 13 and right rear wheel 14, a steering device 4 for steering the left front wheel 11 and right front wheel 12, a brake fluid pressure generator 5 for generating brake fluid pressure, a control device 6 for controlling the front wheel side devices 2 and rear wheel side devices 3, and a drive battery 15 which is a rechargeable secondary battery. Brake discs 110, 120, 130, and 140 are provided corresponding to the left front wheel 11 and right front wheel 12, and the left rear wheel 13 and right rear wheel 14, respectively. In addition, a navigation device 16 having a touch panel display 160 is installed in the passenger compartment of Vehicle 1.

[0013] The front wheel side device 2 includes an electric motor 21 which is a front wheel side drive source that drives the left front wheel 11 and the right front wheel 12, a transmission 22 which changes the rotation of the output shaft 210 of the electric motor 21, a differential 23 which distributes the driving force of the electric motor 21 transmitted from the transmission 22 to the left front wheel 11 and the right front wheel 12, left and right drive shafts 24 and 25 which connect the left front wheel 11 and the right front wheel 12 to the differential 23, a friction brake 26 which brakes the left front wheel 11, a friction brake 27 which brakes the right front wheel 12, a front wheel side hydraulic circuit 28, and a front wheel side inverter 29.

[0014] The differential 23 of the front wheel side device 2 is a so-called open differential, which does not have a configuration to limit the differential rotation between the left front wheel 11 and the right front wheel 12. The differential 23 has a ring gear 230 driven and connected to the output shaft 210 of the electric motor 21 via the transmission 22, a differential case 231 that rotates integrally with the ring gear 230, a pinion gear shaft 232 fixed to the differential case 231, a pair of pinion gears 233 pivotally supported on the pinion gear shaft 232, and a first side gear 234 and a second side gear 235 that mesh with the pair of pinion gears 233. The pair of pinion gears 233, the first side gear 234, and the second side gear 235 are bevel gears. The left drive shaft 24 is connected to the first side gear 234 in a manner that prevents relative rotation. The right-side drive shaft 25 is connected to the second side gear 235 in a manner that prevents relative rotation.

[0015] The rear wheel side device 3 includes an electric motor 31 which is a rear wheel side drive source that drives the left rear wheel 13 and the right rear wheel 14, a transmission 32 which changes the rotation of the output shaft 310 of the electric motor 31, a differential 33 which distributes the driving force of the electric motor 31 transmitted from the transmission 32 to the left rear wheel 13 and the right rear wheel 14, left and right drive shafts 34 and 35 which connect the left rear wheel 13 and the right rear wheel 14 to the differential 33, respectively, a friction brake 36 which brakes the left rear wheel 13, a friction brake 37 which brakes the right rear wheel 14, a rear wheel side hydraulic circuit 38, and a rear wheel side inverter 39.

[0016] Furthermore, the rear-wheel-side device 3 is equipped with a differential limiting device 7 that limits the differential of the differential 33. The differential 33 itself is an open differential similar to the differential 23 of the front-wheel-side device 2. The differential limiting device 7 has a mechanism 700 and an electric motor 70 that operates the mechanism 700. The mechanism 700 of the differential 33 and the differential limiting device 7 is housed in a differential carrier 30. The differential carrier 30 is filled with lubricating oil to lubricate the mechanism 700 of the differential 33 and the differential limiting device 7. The detailed configuration of the rear-wheel-side device 3 will be described later.

[0017] In this embodiment, the transmissions 22 and 32 of the front wheel side device 2 and the rear wheel side device 3 are reduction gears that reduce the rotation of the output shafts 210 and 310 of the electric motors 21 and 31 at a constant reduction ratio. However, the transmissions 22 and 32 may have multiple stages or continuously variable gear ratios.

[0018] The steering system 4 includes a steering shaft 41 connected to a steering wheel 40 operated by the driver of the vehicle 1, a pinion gear 42 fixed to the steering shaft 41, a rack shaft 43 meshed with the pinion gear 42, and left and right tie rods 44 and 45 pivotably connected between the rack shaft 43 and the left front wheel 11 and the right front wheel 12. When the steering wheel 40 is steered, the pinion gear 42 rotates according to the steering angle, causing the rack shaft 43 to move left and right, and the left front wheel 11 and the right front wheel 12 to turn.

[0019] The brake fluid pressure generator 5 includes an input rod 51 to which the force applied to the brake pedal 50 is input as thrust, an electric brake booster 52 that increases the thrust input to the input rod 51, a master cylinder 53 that increases the pressure of the brake fluid according to the output of the brake booster 52, and a reservoir tank 54 that stores the brake fluid. The brake fluid output by the master cylinder 53 is input to the front wheel hydraulic circuit 28 and the rear wheel hydraulic circuit 38.

[0020] The front friction brakes 26 and 27 are operated by brake fluid supplied from the front hydraulic circuit 28 via brake hoses 281 and 282, pressing the brake pads against the brake discs 110 and 120 to generate frictional force and brake the left front wheel 11 and the right front wheel 12. The rear friction brakes 36 and 37 are operated by brake fluid supplied from the rear hydraulic circuit 38 via brake hoses 381 and 382, ​​pressing the brake pads against the brake discs 130 and 140 to generate frictional force and brake the left rear wheel 13 and the right rear wheel 14.

[0021] The drive battery 15 supplies DC voltage to the front-wheel inverter 29 and the rear-wheel inverter 39. When the vehicle 1 decelerates, the front-wheel device 2 and the rear-wheel device 3 regenerate braking the left front wheel 11 and the right front wheel 12, as well as the left rear wheel 13 and the right rear wheel 14. The power obtained from this regenerative braking charges the drive battery 15. This extends the driving range of the vehicle 1.

[0022] The control device 6 controls the front wheel side device 2 and the rear wheel side device 3 based on vehicle information. The vehicle information includes the detected steering angle of the steering wheel 40, the detected value of the brake pedal sensor that detects the amount of depression of the brake pedal 50, the detected value of the accelerator pedal sensor that detects the amount of depression of the accelerator pedal 100, the detected values ​​of the wheel speed sensors that detect the rotational speeds of the left front wheel 11 and the right front wheel 12, and the left rear wheel 13 and the right rear wheel 14, respectively, as well as the detected values ​​of various sensors such as the yaw rate sensor.

[0023] The control device 6 controls the front wheel side device 2 by sending electrical signals to the front wheel side hydraulic circuit 28 and the front wheel side inverter 29, and controls the rear wheel side device 3 by sending electrical signals to the rear wheel side hydraulic circuit 38 and the rear wheel side inverter 39. The electrical signals that the control device 6 sends to the front wheel side hydraulic circuit 28 and the rear wheel side hydraulic circuit 38 are excitation currents for adjusting the opening degree of control valves that increase or decrease the friction braking force generated between the friction brakes 26, 27, 36, and 37 and the brake discs 110, 120, 130, and 140.

[0024] The electrical signals that the control device 6 sends to the front wheel inverter 29 and the rear wheel inverter 39 are, for example, PWM (Pulse Width Modulation) signals that turn multiple switching elements on and off. Alternatively, if the front wheel inverter 29 and the rear wheel inverter 39 themselves have a function to generate PWM signals, the control device 6 may send an electrical signal indicating the current to be supplied to the front wheel electric motor 21 and the rear wheel electric motor 31 from the front wheel inverter 29 and the rear wheel inverter 39, for example via a CAN (Controller Area Network). The electric motors 21 and 31 are three-phase AC motors, and the front wheel inverter 29 and the rear wheel inverter 39 each have multiple switching elements connected in a three-phase bridge configuration.

[0025] When vehicle 1 is accelerating, the electric motor 21 of the front wheel side device 2 and the electric motor 31 of the rear wheel side device 3 generate positive torque in accordance with the current supplied from the front wheel side inverter 29 and the rear wheel side inverter 39. When vehicle 1 is decelerating, the electric motor 21 of the front wheel side device 2 and the electric motor 31 of the rear wheel side device 3 function as generators, and are capable of generating negative torque as a rotational resistance force. The positive torque is a driving force that increases the rotation speed of the left front wheel 11 and the right front wheel 12, and the left rear wheel 13 and the right rear wheel 14, while the negative torque is a regenerative braking force that decelerates the rotation of the left front wheel 11 and the right front wheel 12, and the left rear wheel 13 and the right rear wheel 14.

[0026] The electric motor 21 of the front wheel unit 2 generates rotational resistance by generating regenerative power from the rotational force of the left front wheel 11 and the right front wheel 12 during deceleration. The electric motor 31 of the rear wheel unit 3 generates rotational resistance by generating regenerative power from the rotational force of the left rear wheel 13 and the right rear wheel 14 during deceleration.

[0027] Vehicle 1, under control performed by the control device 6, brakes the left front wheel 11 and right front wheel 12, as well as the left rear wheel 13 and right rear wheel 14, using the front wheel side device 2 and the rear wheel side device 3 during deceleration. The front wheel side device 2 brakes the left front wheel 11 and right front wheel 12 during deceleration using at least one of the frictional braking force from the friction brakes 26 and 27 and the rotational resistance force from the electric motor 21. At low vehicle speeds, the control device 6 primarily brakes the left front wheel 11 and right front wheel 12 using the frictional braking force from the friction brakes 26 and 27, and at medium and high vehicle speeds, primarily brakes the left front wheel 11 and right front wheel 12 using the regenerative braking force of the electric motor 21 as rotational resistance force. This is because at low vehicle speeds, sufficient braking force cannot be applied to the left front wheel 11 and right front wheel 12 by regenerative control of the electric motor 21.

[0028] Furthermore, for example, if it is necessary to rapidly decelerate or stop the vehicle 1 at high speed, the front wheel-side device 2 uses a combination of frictional braking force from the friction brakes 26 and 27 and rotational resistance force from the electric motor 21 to brake the left front wheel 11 and the right front wheel 12. In this way, the control device 6 controls the front wheel-side device 2 so that the braking force of the left front wheel 11 and the right front wheel 12, which is the sum of the frictional braking force from the friction brakes 26 and 27 and the regenerative braking force from the electric motor 21, is of an appropriate magnitude according to the vehicle information.

[0029] Similarly, with respect to the rear wheel side device 3, at low vehicle speeds, the left rear wheel 13 and the right rear wheel 14 are braked mainly by the frictional braking force of the friction brakes 36 and 37, and at medium and high vehicle speeds, the left rear wheel 13 and the right rear wheel 14 are braked mainly by the regenerative braking force of the electric motor 31 as rotational resistance force.

[0030] The control device 6 controls the front wheel-side device 2 and the rear wheel-side device 3 so that, when the vehicle is decelerating, the braking force acting on the left rear wheel 13 and the right rear wheel 14 is greater than the braking force acting on the left front wheel 11 and the right front wheel 12. Furthermore, when the control device 6 brakes the left front wheel 11 and the right front wheel 12 and the left rear wheel 13 and the right rear wheel 14 using the regenerative braking force of the electric motors 21 and 31 as rotational resistance force, the control device 6 controls the front wheel-side device 2 and the rear wheel-side device 3 so that the braking force due to the rotational resistance force generated by the electric motor 31 of the rear wheel-side device 3 is greater than the braking force due to the rotational resistance force generated by the electric motor 21 of the front wheel-side device 2. This makes it possible to mitigate the nose dive phenomenon, in which the front of the vehicle body is lower than the rear of the vehicle body, and improves the riding comfort of the driver and other occupants.

[0031] Next, referring to Figures 2A, 2B, and 2C, we will explain the relationship between the magnitude of the braking force of the left front wheel 11 and the right front wheel 12, and the left rear wheel 13 and the right rear wheel 14, and the nose dive phenomenon.

[0032] Figures 2A, 2B, and 2C are schematic diagrams showing the suspension spring 171 and suspension arm 172 on the front wheel side, the suspension spring 181 and suspension arm 182 on the rear wheel side, and the front wheel load NF and rear wheel load NR. Figure 2A shows the state when vehicle 1 is traveling at a constant speed on a level road surface 9. Figure 2B shows the state in which vehicle 1 is decelerated by front-wheel bias braking, in which a greater braking force is applied to the left front wheel 11 and right front wheel 12 than to the left rear wheel 13 and right rear wheel 14. Figure 2C shows the state in which vehicle 1 is decelerated by rear-wheel bias braking, in which a greater braking force is applied to the left rear wheel 13 and right rear wheel 14 than to the left front wheel 11 and right front wheel 12. In Figures 2B and 2C, the magnitude of the braking force BF acting on the left front wheel 11 and right front wheel 12, and the braking force BR acting on the left rear wheel 13 and right rear wheel 14 are represented by the length of the dashed arrows. Furthermore, in Figures 2A, 2B, and 2C, the magnitudes of the front wheel load NF and rear wheel load NR are represented by the length of the arrows.

[0033] Figure 2A shows, as an example, the case where the front wheel load NF and rear wheel load NR are equal when vehicle 1 is traveling at a constant speed. When vehicle 1 decelerates, pitching occurs as the front part of the vehicle body 10 sinks and the rear part of the vehicle body 10 lifts up because the center of gravity 101 of vehicle 1 is above the contact surface of the left front wheel 11 and right front wheel 12 and the left rear wheel 13 and right rear wheel 14. As a result, the suspension spring 171 compresses on the left front wheel 11 and right front wheel 12 side and the front wheel load NF increases, while the suspension spring 181 extends on the left rear wheel 13 and right rear wheel 14 side and the rear wheel load NR decreases. Here, pitching refers to the rotation of the vehicle body 10 around the left-right axis center passing through the center of gravity 101 of vehicle 1.

[0034] The braking force BF acting on the left front wheel 11 and the right front wheel 12 acts to extend the rear suspension spring 181, while the braking force BR acting on the left rear wheel 13 and the right rear wheel 14 acts to suppress the extension of the rear suspension spring 181. As a result, by decelerating the vehicle 1 with rear-wheel biased braking, pitching of the vehicle body 10 is suppressed compared to when the vehicle 1 is decelerated with front-wheel biased braking. This mitigates the nose dive phenomenon and reduces discomfort and anxiety for the driver and passengers of the vehicle 1.

[0035] However, when vehicle 1 is decelerated by rear-wheel braking, the frictional force between the left rear wheel 13 and the right rear wheel 14 and the road surface 9 is used to decelerate vehicle 1, making it difficult to secure the lateral force of the tires necessary for turning vehicle 1. As a result, the left rear wheel 13 and the right rear wheel 14 are prone to skidding during deceleration turns. This will be explained in more detail next with reference to Figures 3A, 3B, and 3C.

[0036] Figure 3A is an explanatory diagram showing the friction circle FC of the left rear wheel 13 when vehicle 1 is traveling at a constant speed on the road surface 9. Figure 3B is an explanatory diagram showing the friction circle FC of the left rear wheel 13 when vehicle 1 is decelerating while turning left due to front-wheel braking. Figure 3C is an explanatory diagram showing the friction circle FC of the left rear wheel 13 when vehicle 1 is decelerating while turning left due to rear-wheel braking. In Figures 3A, 3B, and 3C, the size of the friction circle FC represents the frictional force between the tire of the left rear wheel 13 and the road surface 9. In Figures 3A, 3B, and 3C, the X-axis direction represents the longitudinal force of the tire, and the Y-axis direction represents the lateral force of the tire, with the maximum lateral force FY that can be secured within the friction circle FC indicated by an arrow.

[0037] As shown in Figures 3A, 3B, and 3C, the size of the friction circle FC decreases as the rear wheel load NR decreases. In other words, when the vehicle 1 is decelerated by rear-wheel bias braking, the longitudinal tire force FX in the deceleration direction borne by the left rear wheel 13 becomes larger compared to when the vehicle 1 is decelerated by front-wheel bias braking, while the friction circle FC becomes smaller, and the lateral tire force FY that can be secured within the friction circle FC becomes smaller.

[0038] Here, if the rear wheel side device 3 does not have a differential limiting device 7 that suppresses differential rotation between the left rear wheel 13 and the right rear wheel 14, the differential device 33 itself is an open differential, so the maximum value of the longitudinal force that can be applied to the left rear wheel 13 and the maximum value of the longitudinal force that can be applied to the right rear wheel 14 will be about the same. If slip occurs in one of the wheels, the left rear wheel 13 or the right rear wheel 14, it will become impossible to apply longitudinal force to the other wheel. For example, when decelerating and turning to the left, the maximum value of the longitudinal force that can be applied to the right rear wheel 14, which is the outer wheel of the turn, will be the same as that of the left rear wheel 13, where the load decreases on the inner wheel side of the turn. As a result, the braking force of the left rear wheel 13 and the right rear wheel 14 will not be able to sufficiently decelerate the vehicle 1, and oversteer will also be more likely to occur. This phenomenon is particularly noticeable when the road surface friction coefficient is low and slip is likely to occur.

[0039] In this embodiment, the rear wheel side device 3 is equipped with a differential limiting device 7. When the vehicle decelerates, the differential limiting device 7 limits the differential of the differential 33, thereby ensuring the stability of the vehicle's behavior while decelerating the vehicle 1 through rear-wheel bias braking. In other words, in this embodiment, by limiting the differential of the differential 33 with the differential limiting device 7, for example, when decelerating while turning to the left, even if the tire of the left rear wheel 13 reaches its grip limit, a greater longitudinal force from the tire than that of the left rear wheel 13 is distributed to the right rear wheel 14. This allows a larger regenerative braking force from the electric motor 31 than in the conventional system to be applied, enabling smooth deceleration of the vehicle 1. Furthermore, the longitudinal force from the tire of the right rear wheel 14 in the deceleration direction, which becomes the outer wheel during turning, can suppress the occurrence of oversteer. Next, a specific example of the configuration of the rear wheel side device 3 will be described in detail.

[0040] Figure 4 is a schematic diagram showing an example of the configuration of the rear wheel side device 3 and its surroundings. As shown in Figure 4, the left rear wheel 13 and the right rear wheel 14 each have metal wheels 131, 141 and rubber tires 132, 142 mounted on the wheels 131, 141. The contact surfaces 132a, 142a of the tires 132, 142 are in contact with the road surface 9 of the vehicle 1.

[0041] The left rear wheel 13 is rotatably supported by the left hub unit 81, and the right rear wheel 14 is rotatably supported by the right hub unit 82. The hub units 81 and 82 each have hub rings 811 and 821 to which the wheels 131 and 141 are fixed, and outer rings 812 and 822 that support the hub rings 811 and 821, respectively. Wheel speed sensors 813 and 823 are attached to the outer rings 812 and 822, respectively, to detect the rotational speed of the left rear wheel 13 and the right rear wheel 14 by their relative rotation with the hub rings 811 and 821.

[0042] The drive shafts 34 and 35 each have intermediate shafts 341 and 351, inboard sliding constant velocity joints 342 and 352, and outboard fixed constant velocity joints 343 and 353. The inboard sliding constant velocity joints 342 and 352 are, for example, tripod-type constant velocity joints. The outboard fixed constant velocity joints 343 and 353 are, for example, ball-type constant velocity joints. The fixed constant velocity joints 343 and 353 are respectively connected to hub wheels 711 and 721 of hub units 81 and 82.

[0043] Similar to the differential device 23 of the front wheel side device 2, the differential device 33 includes a ring gear 330 drivingly connected to an output shaft 310 of the electric motor 31 via a transmission 32, a differential case 331 that rotates integrally with the ring gear 330, a pinion gear shaft 332 fixed to the differential case 331, a pair of pinion gears 333 pivotally supported by the pinion gear shaft 332, and a first side gear 334 and a second side gear 335 meshing with the pair of pinion gears 333. The pair of pinion gears 333, the first side gear 334, and the second side gear 335 are bevel gears.

[0044] The sliding constant velocity joint 342 of the left drive shaft 34 is connected to the first side gear 334. The sliding constant velocity joint 352 of the right drive shaft 35 is connected to the second side gear 335. The differential device 33 distributes the driving force input from the transmission gear 321 of the transmission 32 to the ring gear 330 to the left rear wheel 13 and the right rear wheel 14 via the left and right drive shafts 34 and 35.

[0045] The differential limiting device 7 comprises an electric motor 70 controlled by a control device 6, a clutch hub 71 whose relative rotation with respect to a differential case 331 is restricted, a clutch drum 72 whose relative rotation with respect to a first side gear 334 is restricted, a friction clutch 73 disposed between the clutch hub 71 and the clutch drum 72, a pressing member 74 that presses the friction clutch 73, a ball cam 75 that applies cam thrust force for pressing the friction clutch 73 to the pressing member 74, a thrust bearing 76 disposed between the pressing member 74 and the ball cam 75, a return spring 77 that biases the pressing member 74 in a direction separating from the friction clutch 73, a reduction gear 78 that reduces rotation of a motor shaft 701 which is an output rotation shaft of the electric motor 70, and a counter gear 79 meshed with the reduction gear 78.

[0046] The clutch hub 71, the clutch drum 72, the friction clutch 73, the pressing member 74, the ball cam 75, the thrust bearing 76, the return spring 77, the reduction gear 78, and the counter gear 79 constitute a mechanism section 700 of the differential limiting device 7. The control device 6 controls the electric motor 70 by outputting a PWM (pulse width modulation) signal to an inverter 61, and causes the electric motor 70 to operate the mechanism section 700. The inverter 61 switches the direct-current voltage of an auxiliary battery 19 in accordance with the PWM signal, and supplies a drive current to the electric motor 70.

[0047] The reduction gear 78 has a large-diameter gear portion 781 and a small-diameter gear portion 782. The large-diameter gear portion 781 meshes with a gear portion 702 provided at an end of the motor shaft 701. The small-diameter gear portion 782 meshes with the counter gear 79. The ball cam 75 has a fixed cam member 751 fixed to a differential carrier 30, a rotating cam member 752 meshed with the counter gear 79, a plurality of cam balls 753 disposed between the fixed cam member 751 and the rotating cam member 752, and a retainer 754 that retains the plurality of cam balls 753.

[0048] As the motor shaft 701 rotates, the rotating cam member 752 rotates relative to the fixed cam member 751, causing the cam ball 753 to roll on the cam surfaces 751a and 752a of the fixed cam member 751 and the rotating cam member 752. The cam surfaces 751a and 752a are inclined with respect to the axial direction of the fixed cam member 751 and the rotating cam member 752, and the rolling of the cam ball 753 on the cam surfaces 751a and 752a generates cam thrust, causing the rotating cam member 752 to move axially relative to the fixed cam member 751. As the rotating cam member 752 moves away from the fixed cam member 751, the thrust bearing 76 and the pressing member 74 move axially together with the rotating cam member 752, and the pressing member 74 presses against the friction clutch 73.

[0049] The friction clutch 73 has a plurality of inner clutch plates 731 that are axially movable and non-rotatable with respect to the clutch hub 71, and a plurality of outer clutch plates 732 that are axially movable and non-rotatable with respect to the clutch drum 72. When the friction clutch 73 is pressed in the axial direction, a frictional force is generated between the plurality of inner clutch plates 731 and the plurality of outer clutch plates 732.

[0050] As described above, the clutch hub 71, which engages with multiple inner clutch plates 731, has its relative rotation with respect to the differential case 331 restricted, and the clutch drum 72, which engages with multiple outer clutch plates 732, has its relative rotation with respect to the first side gear 334 restricted. Therefore, when frictional force is generated between the multiple inner clutch plates 731 and the multiple outer clutch plates 732, the relative rotation between the differential case 331 and the first side gear 334 is restricted, and this also restricts the relative rotation between the differential case 331 and the second side gear 335. In other words, the frictional force generated in the friction clutch 73 becomes a differential limiting force that restricts the differential of the differential 33. That is, the differential limiting device 7 limits the differential of the differential 33 by the differential limiting force generated by the friction clutch 73.

[0051] Here, the differential of the differential gear 33 refers to the differential between the first side gear 334 and the second side gear 335. Furthermore, since the first side gear 334 rotates at the same speed as the left rear wheel 13 and the second side gear 335 rotates at the same speed as the right rear wheel 14, the differential of the differential gear 33 is synonymous with the differential between the left rear wheel 13 and the right rear wheel 14.

[0052] The frictional force generated in the friction clutch 73 is proportional to the torque generated by the electric motor 70. In other words, the differential limiting device 7 can increase or decrease the torque bias ratio (TBR) of the left rear wheel 13 and the right rear wheel 14 through control by the control device 6. Here, the torque bias ratio is the value obtained by dividing the maximum braking force that can be applied to the first wheel by the maximum braking force that can be applied to the second wheel, when the wheel that can be subjected to a larger braking force (braking force or driving force) of a pair of left and right wheels is designated as the first wheel and the other as the second wheel. For example, if the torque bias ratio is 4, four times the braking force of the second wheel can be applied to the first wheel.

[0053] The control device 6 determines whether the road surface 9 is slippery or not, and controls the differential limiter 7 so that the torque bias ratio is greater when it is determined that the road surface 9 is not slippery than when it is determined that the road surface 9 is not slippery. The determination of whether the road surface 9 is slippery or not can be made by, for example, one of the following determination methods, or a combination of these determination methods.

[0054] The first determination method is a method for determining whether the road surface 9 is slippery or not based on the slip ratio of at least one of the left front wheel 11, the right front wheel 12, the left rear wheel 13, and the right rear wheel 14. The slip ratio is obtained, for example, during braking, by subtracting the vehicle speed calculated from the rotational speed of the wheels from the actual vehicle speed and dividing the result by the actual vehicle speed. Since a low road surface friction coefficient results in a high slip ratio, if the slip ratio is above a predetermined value, it can be determined that the road surface 9 is slippery.

[0055] The second determination method is to determine whether the road surface 9 is slippery or not based on a comparison of the steering angle and vehicle speed with the actual yaw rate. If the road surface friction coefficient is low, the turning performance of the vehicle 1 will be low and the resulting yaw rate will be small. Therefore, if the actual yaw rate detected by the yaw rate sensor is lower than the yaw rate expected based on the steering angle and vehicle speed by a predetermined value or more, it can be determined that the road surface 9 is slippery.

[0056] The third determination method involves detecting the condition of the road surface 9 using an on-board camera or laser sensor. When the road surface 9 is slippery due to rain or snow, the reflectivity of light on the road surface 9 increases, allowing the on-board camera or laser sensor to determine whether or not the road surface 9 is slippery.

[0057] In addition to these methods, the slipperiness of the road surface 9 can also be determined by the frequency of operation of the wipers, anti-lock braking system, or traction control system. This is because the frequency of operation of these systems increases when the road surface 9 is slippery.

[0058] If the control device 6 determines that the road surface 9 is slippery, it increases the torque generated by the electric motor 70 and increases the torque bias ratio of the left rear wheel 13 and the right rear wheel 14. In this case, the torque bias ratio is, for example, between 2.3 and 4.0. Furthermore, if the control device 6 can determine the road surface friction coefficient, which is an index value of the slipperiness of the road surface 9, in multiple stages, the torque bias ratio may be increased in stages according to the stage of the road surface friction coefficient, with the smaller the road surface friction coefficient. Even in this case, the control device 6 increases the torque bias ratio when the road surface friction coefficient is lower than the intermediate stage (when the road surface 9 is determined to be slippery) compared to when the road surface friction coefficient is higher than the intermediate stage (when the road surface 9 is determined to be less slippery).

[0059] Furthermore, if the control device 6 determines that the road surface 9 is not slippery, it may choose not to supply current to the electric motor 70, and may also choose to generate a smaller torque in the electric motor 70 than when the road surface 9 is deemed slippery. The torque bias ratio when no current is supplied to the electric motor 70 is equivalent to that of a typical open differential, for example, 1.0 to 1.2. By reducing the differential limiting force when the road surface 9 is deemed not slippery, deterioration of initial steering responsiveness can be prevented.

[0060] The control device 6 generates a differential limiting force using the friction clutch 73 when regenerative braking is performed on the left rear wheel 13 and the right rear wheel 14 by the electric motor 31 of the rear wheel side device 3. In other words, the control device 6 may always generate a differential limiting force using the friction clutch 73 while it is determined that the road surface 9 is slippery, or it may generate torque in the electric motor 70 to generate a differential limiting force using the friction clutch 73 when regenerative braking is performed on the left rear wheel 13 and the right rear wheel 14 by the electric motor 31 of the rear wheel side device 3.

[0061] Furthermore, the control device 6 may also change the rate of increase or decrease of the torque bias ratio of the left rear wheel 13 and the right rear wheel 14 according to a setting value made by the vehicle driver (driver of vehicle 1). In this case, the vehicle driver sets the rate of increase or decrease of the torque bias ratio by operating icons displayed as a UI (user interface) on the touch panel display 160 of the navigation device 16, for example.

[0062] (Verification results of vehicle behavior) Figures 5A and 5B are graphs showing examples of changes in the amount of extension and contraction of the suspension springs corresponding to each wheel before and after the start of braking, when the vehicle 1 is driven straight at a constant speed with the left side of the vehicle heavier than the reference state, and when the vehicle 1 is decelerated by regenerative braking using rear-wheel bias braking, and when the vehicle 1 is decelerated by regenerative braking using front-wheel bias braking. Here, the amount of extension and contraction of the suspension springs refers to the change in spring length from the reference state. The ratio of braking force on the front wheels and rear wheels (front wheels:rear wheels) is 7:3 in the case of front-wheel bias braking and 4:6 in the case of rear-wheel bias braking. The torque bias ratio of the left rear wheel 13 and the right rear wheel 14 was set to 4.0.

[0063] Figure 5A shows the expansion and contraction amounts of the suspension springs for the left front wheel 11 and the right front wheel 12. Figure 5B shows the expansion and contraction amounts of the suspension springs for the left rear wheel 13 and the right rear wheel 14. In Figures 5A and 5B, the solid line shows the case of rear-wheel biased braking, and the dashed line shows the case of front-wheel biased braking. The expansion and contraction amounts of the suspension springs shown on the vertical axis of the graph are represented by - (minus) for compression and + (plus) for extension. The horizontal axis of the graph is the time axis, with time T １ The point indicates the start of regenerative braking. In addition, in Figures 5A and 5B, the amount of extension and contraction of the suspension spring before the start of regenerative braking is shown by a dashed line parallel to the time axis.

[0064] As shown in the graphs in Figures 5A and 5B, there is no significant difference in the compression amount of the suspension springs of the left front wheel 11 and the right front wheel 12 between the front-wheel bias braking case and the rear-wheel bias braking case. However, the extension amount of the suspension springs of the left rear wheel 13 and the right rear wheel 14 is smaller in the rear-wheel bias braking case than in the front-wheel bias braking case. In other words, by applying a greater braking force to the left rear wheel 13 and the right rear wheel 14 than to the left front wheel 11 and the right front wheel 12, the amount of lift of the rear part of the vehicle body 10 is reduced, and the nose dive phenomenon is suppressed.

[0065] Figure 6 is a graph showing an example of the change in yaw rate when the accelerator pedal is released (to zero) during cornering, and regenerative braking is performed on each wheel. The horizontal axis of the graph is the time axis, with time T. ２１ This indicates the point at which the turn began, and time T ２２ The value indicates the point in time when the accelerator is released. In the graph of Figure 6, the solid line shows the change in yaw rate when rear-wheel biased braking is performed with a braking force ratio of 4:6 between the front and rear wheels in the vehicle 1 of this embodiment which has a differential limiting device 7. In the graph of Figure 6, the dashed line shows the change in yaw rate when rear-wheel biased braking is performed with a braking force ratio of 4:6 between the front and rear wheels in the case where the rear-wheel device 3 does not have a differential limiting device 7. In addition, the dashed line in the graph of Figure 6 shows the change in yaw rate when front-wheel biased braking is performed with a braking force ratio of 7:3 between the front and rear wheels in the case where the rear-wheel device 3 does not have a differential limiting device 7.

[0066] As shown by the dashed line in the graph of Figure 6, when rear-wheel braking is performed without a differential limiting device 7 in the rear-wheel-side device 3, the inner wheel slips during a turn, and braking force is no longer applied to the outer wheel during a turn. After releasing the accelerator, the vehicle enters a spin state, and the yaw rate increases rapidly. Also, as shown by the dashed line in the graph of Figure 6, when front-wheel braking is performed without a differential limiting device 7 in the rear-wheel-side device 3, the load on the front wheels in the longitudinal direction of the vehicle increases, making it impossible to generate sufficient lateral force on the tires, resulting in an understeer state with insufficient yaw rate.

[0067] In contrast, as shown by the solid line in the graph of Figure 6, when rear-wheel bias braking is performed while turning in the vehicle 1 of this embodiment, which has a differential limiting device 7, understeer and oversteer are suppressed, and the vehicle exhibits relatively stable behavior.

[0068] (Effects of the Embodiment) According to the embodiment of the present invention described above, the nose dive phenomenon can be suppressed by performing rear-wheel bias braking, and the rear-wheel side device 3 is equipped with a differential limiting device 7 that limits the differential of the differential 33, and the control device 6 controls the differential limiting device 7 so that the torque bias ratio is larger when the road surface 9 is determined to be slippery than when the road surface 9 is determined to be less slippery, thereby making it possible to suppress instability of vehicle behavior even when decelerating during cornering.

[0069] (Note) The present invention has been described above based on embodiments, but these embodiments do not limit the invention as defined in the claims. It should also be noted that not all combinations of features described in the embodiments are necessarily essential for solving the problem of the invention. Furthermore, the present invention can be implemented by modifying it as appropriate, without departing from its spirit, by omitting some components or adding or substituting components. It can also be implemented by modifying it as follows, for example.

[0070] In the above embodiment, a case where the front wheel side device 2 does not have a differential limiting device has been described. However, the invention is not limited to this, and a differential limiting device similar to the differential limiting device 7 described above may also be provided for the differential 23 of the front wheel side device 2. Furthermore, an engine that uses fuel such as gasoline may be used as the drive source for the front wheels instead of the electric motor 21. Moreover, the present invention can also be applied to a vehicle in which the front wheel side device 2 does not have a drive source such as an electric motor 21 and drives only the rear wheels.

[0071] Furthermore, although the above embodiment describes a case where the driver steers the steering wheel 40, the present invention is not limited to this, and can also be applied to fully autonomous vehicles without a steering wheel, or to vehicles equipped with a steer-by-wire steering system that does not have a steering shaft connecting the steering wheel and the rack shaft, and instead uses a motor to steer the front wheels according to the angle of the steering wheel. In addition, the friction brake is not limited to a hydraulic type, but may also be an electric type, for example, one that presses the brake pads against the brake disc by the rotation of a motor.

[0072] Furthermore, in the above embodiment, the case in which the pressing member 74 is pressed against the friction clutch 73 by the cam thrust of the ball cam 75 which is operated by the torque of the electric motor 70 has been described. However, the configuration of the mechanism that presses the friction clutch 73 is not limited to this, and the friction clutch 73 may be pressed by an electric linear motion mechanism such as a linear solenoid, or by a hydraulic linear motion mechanism such as a hydraulic piston.

[0073] 1...Vehicle 11...Left front wheel 12...Right front wheel 13...Left rear wheel 14...Right rear wheel 2...Front wheel side equipment 21...Electric motor (front wheel side drive source) 26, 27...Friction brakes 3...Rear wheel side equipment 31...Electric motor (rear wheel side drive source) 33...Differential 36, 37...Friction brakes 6...Control device 7...Differential limiting device 9...Road surface

Claims

1. A vehicle comprising a front wheel side device provided corresponding to a pair of left and right front wheels, a rear wheel side device provided corresponding to a pair of left and right rear wheels, and a control device that controls the front wheel side device and the rear wheel side device, wherein the front wheel side device brakes the pair of left and right front wheels and the rear wheel side device brakes the pair of left and right rear wheels when the vehicle decelerates, wherein the front wheel side device brakes the pair of left and right front wheels by at least one of the frictional braking force from the friction brake and the rotational resistance force of the front wheel side drive source that drives the pair of left and right front wheels, wherein the rear wheel side device comprises a rear wheel side drive source that drives the pair of left and right rear wheels, a differential device that distributes the driving force of the rear wheel side drive source to the pair of left and right rear wheels, and a differential limiting device that limits the differential of the differential device, wherein when the vehicle decelerates, it is possible to apply braking force to the pair of left and right rear wheels via the differential device by the rotational resistance force of the rear wheel side drive source, and the differential limiting device is capable of increasing or decreasing the torque bias ratio of the pair of left and right rear wheels by control by the control device. The control device controls the front wheel side device and the rear wheel side device so that the braking force acting on the left and right rear wheels is greater than the braking force acting on the left and right front wheels when the vehicle decelerates, and controls the differential limiting device so that the torque bias ratio is greater when the road surface is determined to be slippery than when the road surface is determined to be less slippery.

2. The vehicle according to claim 1, wherein the rear wheel drive source is an electric motor that generates rotational resistance force by generating regenerative power from the rotational force of the left and right pair of rear wheels when the vehicle decelerates.

3. The vehicle according to claim 1 or 2, wherein the control device changes the rate of increase or decrease of the torque bias ratio of the left and right rear wheels according to a value set by the vehicle driver.

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