Vehicle control system

Abstract

The present invention provides a vehicle control device that maximizes the driving force generated by slipping wheels while obtaining a highly accurate estimate of the vehicle's speed. [Solution] An estimated value of the friction coefficient of the wheel in a slipping state is calculated, and the rotational speed of the power source is controlled to maintain the estimated value of the friction coefficient of the wheel near a maximum value, including the maximum value, and an estimated value of the vehicle speed is calculated using the wheel speed in that state. This makes it possible to keep the wheel in a slipping state near a maximum value, including the maximum value of the friction coefficient, and to calculate an estimated value of the vehicle speed while keeping the friction coefficient near a maximum value, including the maximum value. Therefore, it is possible to obtain a highly accurate estimated value of the vehicle speed while maximizing the driving force generated by the slipping wheel.

Description

Technical Field

[0001] The present invention relates to a control device for a vehicle having front, rear, left, and right wheels serving as drive wheels.

Background Art

[0002] A control device for a vehicle including front, rear, left, and right wheels and a power source that generates power serving as the driving force for the wheels is well known. For example, the driving force control device for a four-wheel drive vehicle described in Patent Document 1 is such a device. In this Patent Document 1, when any one of the wheels is in a spinning state, the largest wheel speed among the wheel speeds of the four wheels, excluding those that exceed the previous estimated vehicle body speed, is used as the current estimated vehicle body speed.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] By the way, the technique described in Patent Document 1 assumes driving on a very rough road surface such as a rocky road where any one of the wheels floats from the ground. Therefore, when the vehicle is accelerating while the wheels are slipping, the calculation accuracy of the estimated vehicle body speed may deteriorate.

[0005] The present invention has been made against the background of the above circumstances, and an object thereof is to provide a control device for a vehicle that can obtain an accurate estimated value of the vehicle body speed while maximizing the driving force generated by the wheels in a slip state.

Means for Solving the Problems

[0006] The gist of the first invention is a control device for a vehicle comprising (a) front and rear wheels and a power source that generates power to drive the wheels, the control device comprising (b) a friction coefficient calculation unit that calculates an estimated value of the friction coefficient of the wheels in a slipping state, (c) a drive control unit that controls the rotational speed of the power source so as to maintain the estimated value of the friction coefficient of the wheels in the vicinity of a maximum value including the maximum value, and (d) a vehicle state acquisition unit that acquires an estimated value of the vehicle body speed using the wheel speed in a state in which the estimated value of the friction coefficient of the wheels is maintained in the vicinity of a maximum value including the maximum value. [Effects of the Invention]

[0007] According to the first invention described above, an estimated value of the friction coefficient of the wheel in a slipping state is calculated, the rotational speed of the power source is controlled to maintain the estimated value of the friction coefficient of the wheel in the vicinity of a maximum value including the maximum value, and an estimated value of the vehicle speed is obtained using the wheel speed in that state. This makes it possible to keep the friction coefficient of the wheel in a slipping state in the vicinity of a maximum value including the maximum value, and to obtain an estimated value of the vehicle speed while keeping the friction coefficient in the vicinity of a maximum value including the maximum value. Therefore, it is possible to obtain a highly accurate estimated value of the vehicle speed while maximizing the driving force generated by the slipping wheel. [Brief explanation of the drawing]

[0008] [Figure 1] This diagram illustrates the schematic configuration of a vehicle to which the present invention is applied, and further illustrates the main parts of the control functions and control systems for various control functions in the vehicle. [Figure 2] These are block diagrams illustrating the function of traction control. (a) is a block diagram illustrating the function of traction control under normal conditions. (b) is a diagram illustrating an example of the relationship between slip ratio and driving force. (c) is a block diagram illustrating the function of traction control when all four wheels are slipping. [Figure 3]These diagrams illustrate the search for the μ peak. (a) shows an example of the relationship between slip ratio and tire μ. (b) shows a diagram illustrating the control of the motor rotation speed to search for the μ peak. [Figure 4] This diagram illustrates how to obtain the estimated vehicle speed when slippage occurs in all wheels. (a) is an example of a state where the estimated tire μ is maintained near the μ peak. (b) is a diagram comparing the wheel speed and the actual vehicle speed when the estimated tire μ is maintained near the μ peak. (c) is a diagram comparing the estimated vehicle speed based on the wheel speed and the actual vehicle speed. [Figure 5] This flowchart explains the key aspects of the control operation of an electronic control unit, specifically the control operation to obtain a highly accurate estimated vehicle speed while maximizing the driving force generated by a slipping wheel. [Figure 6] This flowchart explains the control operation for calculating the rotational speed for searching, and is the subroutine corresponding to step S40 in the flowchart of Figure 5. [Figure 7] This figure illustrates an example of a method for maintaining the estimated tire μ of the wheels near the μ peak, which differs from Example 1. (a) is a block diagram illustrating the function of traction control during four-wheel slip, and is a different embodiment from (c) in Figure 2. (b) is a diagram illustrating the control of the motor rotation speed based on the tire μ of the front wheels and the tire μ of the rear wheels. [Figure 8] This flowchart illustrates the essential control operation of an electronic control unit, specifically the control operation to obtain a highly accurate estimated vehicle speed while maximizing the driving force generated by a slipping wheel. This is a different embodiment from the flowchart in Figure 5. [Figure 9] These are the subroutines corresponding to S45 and S48 in the flowchart of Figure 8. (a) is a flowchart explaining the control operation for calculating stiffness. (b) is a flowchart explaining the control operation for performing stiffness feedback. [Modes for carrying out the invention]

[0009] Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. [Examples]

[0010] Figure 1 is a diagram illustrating the schematic configuration of a vehicle 10 to which the present invention is applied. Figure 1 is also a diagram illustrating the main parts of the control functions and control systems for various controls in the vehicle 10. In Figure 1, the vehicle 10 is equipped with front and rear wheels 12 on the left and right sides, and electric motors MG independently provided for each wheel 12. The electric motors MG are the power source of the present invention, which generates power that becomes the driving force Fx of the wheels 12. The driving force Fx of the wheels 12 is a force at the wheels 12, which is the frictional force at the point of contact with the ground at the wheel 12, i.e., the road surface grip force. The wheels 12 include front wheels 12f (left front wheel 12fl, right front wheel 12fr) and rear wheels 12r (left rear wheel 12rl, right rear wheel 12rr). The electric motors MG include the left front electric motor MGfl, the right front electric motor MGfr, the left rear electric motor MGrl, and the right rear electric motor MGrr. Furthermore, the term "front and rear" above refers to the front and rear in the forward and backward direction of the vehicle 10, and the term "left and right" above refers to the left and right relative to the forward direction of the vehicle 10.

[0011] Vehicle 10 is an all-wheel drive vehicle in which the power distribution of the wheels 12 can be adjusted. Since vehicle 10 is equipped with four wheels, one at the front, one at the rear, one at the left, and one at the right, it is also a four-wheel drive vehicle. In this embodiment, all-wheel drive (AWD) and four-wheel drive (4WD) are synonymous. In addition to driving with AWD control (AWD state is also synonymous), which drives all of the wheels 12, vehicle 10 can also drive with two-wheel drive (=2WD) control (2WD state is also synonymous), which drives, for example, only the front wheels or only the rear wheels of the wheels 12.

[0012] The vehicle 10 also includes a drive shaft 14, a gear mechanism 16, a battery 20, and a power control unit (PCU) 22. The drive shaft 14 and the gear mechanism 16 are components that constitute part of a power transmission system that transmits power from the electric motor MG to the wheels 12. The gear mechanism 16 is, for example, a reduction gear. The battery 20 is a rechargeable DC power source and is a high-voltage battery for driving. The battery 20 is electrically connected to the power control unit 22. The power control unit 22 includes, for example, an inverter. The power control unit 22 is electrically connected to the electric motor MG.

[0013] The electric motor MG is a known rotating electric machine, a so-called motor generator. The motor torque Tm of the electric motor MG is controlled by the power control unit 22 controlled by the electronic control device 50, which will be described later.

[0014] The drive shaft 14 includes the left front drive shaft 14fl, the right front drive shaft 14fr, the left rear drive shaft 14rl, and the right rear drive shaft 14rr. The gear mechanism 16 includes the left front gear mechanism 16fl, the right front gear mechanism 16fr, the left rear gear mechanism 16rl, and the right rear gear mechanism 16rr. One side of the drive shaft 14 is connected to the electric motor MG via the gear mechanism 16, and the other side is connected to the wheel 12.

[0015] The power control unit 22 includes the left front PCU 22fl, the right front PCU 22fr, the left rear PCU 22rl, and the right rear PCU 22rr. The power control unit 22 converts DC power from the battery 20 into AC power and supplies it to the motor MG, and converts AC power generated by the motor MG through regenerative braking into DC power and supplies it to the battery 20.

[0016] The vehicle 10 is a vehicle capable of independently controlling the change amount of the driving force Fx of the front, rear, left, and right wheels, and is a front-rear-left-right wheel independently driven electric vehicle, that is, a BEV (Battery Electric Vehicle). In the vehicle 10, the distribution of the driving force to the front, rear, left, and right wheels can be controlled according to the driving state.

[0017] The vehicle 10 further includes an electronic control unit 50 (see "ECU" in the figure) as a controller. The electronic control unit 50 is configured to include a so-called microcomputer having, for example, a CPU, a RAM, a ROM, an input / output interface, and the like. The CPU executes various controls of the vehicle 10 by performing signal processing according to a program stored in the ROM in advance while using the temporary storage function of the RAM. The electronic control unit 50 is the control device of the present invention.

[0018] Various signals based on the detection values by various sensors and the like provided in the vehicle 10 are respectively supplied to the electronic control unit 50. The various sensors and the like are, for example, a motor rotation speed sensor 30, a wheel speed sensor 32, an accelerator opening sensor 34, a steering sensor 36, a G sensor 38, a yaw rate sensor 40, and the like. The various signals and the like are, for example, a motor rotation speed ωm, a wheel speed ωx, an accelerator opening θacc, a steering angle θsw, a steering direction Dsw, a longitudinal acceleration Gx, a lateral acceleration Gy, a yaw rate Ryaw, and the like. The motor rotation speed sensor 30 includes a left front motor rotation speed sensor 30fl, a right front motor rotation speed sensor 30fr, a left rear motor rotation speed sensor 30rl, and a right rear motor rotation speed sensor 30rr. The wheel speed sensor 32 includes a left front wheel speed sensor 32fl, a right front wheel speed sensor 32fr, a left rear wheel speed sensor 32rl, and a right rear wheel speed sensor 32rr.

[0019] The motor rotation speed ωm is the rotation speed of the motor MG. The motor rotation speed ωm includes the left front motor rotation speed ωmfl, the right front motor rotation speed ωmfr, the left rear motor rotation speed ωmrl, and the right rear motor rotation speed ωmrr. The wheel speed ωx is the rotation speed of the wheels 12. The wheel speed ωx includes the front wheel speed ωxf (left front wheel speed ωxfl, right front wheel speed ωxfr), which is the rotation speed of the front wheels 12f, and the rear wheel speed ωxr (left rear wheel speed ωxrl, right rear wheel speed ωxrr), which is the rotation speed of the rear wheels 12r. The accelerator opening θacc is a signal corresponding to the acceleration request amount, which represents the magnitude of the driver's acceleration operation, and is the amount of accelerator operation by the driver. The steering angle θsw is the steering angle of the steering wheel. The steering direction Dsw is the steering direction of the steering wheel. The longitudinal acceleration Gx is the longitudinal acceleration of the vehicle 10. Lateral acceleration Gy is the lateral acceleration of vehicle 10. Yaw rate Ryaw is the angular velocity of rotation of vehicle 10 around its vertical axis.

[0020] The electronic control unit 50 outputs various command signals (e.g., motor control command signal Sm) to each device (e.g., power control unit 22) installed in the vehicle 10. The motor control command signal Sm is a torque instruction value for controlling the motor torque Tm. The motor control command signal Sm includes the left front motor control command signal Smfl, the right front motor control command signal Smfr, the left rear motor control command signal Smrl, and the right rear motor control command signal Smrr.

[0021] The electronic control unit 50 includes a vehicle state acquisition unit 52 and a drive control unit 54 in order to implement various controls in the vehicle 10.

[0022] The vehicle state acquisition unit 52 acquires an estimated vehicle speed Vxe based on the wheel speed ωx. The estimated vehicle speed Vxe is an estimated value of the vehicle speed Vx as the speed of the vehicle 10. For example, the vehicle state acquisition unit 52 acquires the average value of the wheel speeds ωx, or the lowest or second slowest value among the wheel speeds ωx, as the estimated vehicle speed Vxe. When slip occurs in the wheels 12, the estimated vehicle speed Vxe becomes higher than the actual vehicle speed Vxr, resulting in an increase in the vehicle speed Vx. The actual vehicle speed Vxr is the actual value of the vehicle speed Vx. If slip occurs in any of the wheels 12, for example, the vehicle state acquisition unit 52 acquires the estimated vehicle speed Vxe based on the wheel speed ωx of the wheel 12 that is not slipping. The acquisition of the estimated vehicle speed Vxe when slip occurs in all of the wheels 12 will be described later.

[0023] The drive control unit 54 calculates the required drive torque Tdrvdem for the vehicle 10 based on, for example, the accelerator opening θacc and the estimated vehicle speed Vxe. The required drive torque Tdrvdem is the required value of the drive torque Tdrv, which is the torque at the wheels 12. The drive control unit 54 sets the torque distribution ratio for the front and rear left and right wheels based on, for example, multiple drive force-related values ​​such as the accelerator opening θacc, the estimated vehicle speed Vxe, the steering angle θsw, the steering direction Dsw, the longitudinal acceleration Gx, the lateral acceleration Gy, and the yaw rate Ryaw. Based on the required drive torque Tdrvdem and the torque distribution ratio, the drive control unit 54 calculates the required values ​​of the torque for each of the wheels 12 (left front wheel drive torque Tdrvfl, right front wheel drive torque Tdrvfr, left rear wheel drive torque Tdrvrl, right rear wheel drive torque Tdrvrr). The drive torque Tdrv is the sum of the torques of each of the wheels 12. The drive control unit 54 outputs motor control command signals Sm (Smfl, Smfr, Smrl, Smrr) to each of the power control units 22 to achieve the required torque values ​​for each of the wheels 12.

[0024] The vehicle status acquisition unit 52 acquires the respective values ​​of the motor torque Tm based on the motor control command signal Sm output by the drive control unit 54.

[0025] The vehicle state acquisition unit 52 acquires an estimated value of the ground contact load Fz at the wheels 12. For example, the vehicle state acquisition unit 52 calculates the dynamic ground contact load at each of the wheels 12 based on the static ground contact load at each of the wheels 12 predetermined in the vehicle specifications 10, and the estimation of load transfer in the front, rear, left, and right directions. The vehicle state acquisition unit 52 acquires this dynamic ground contact load as an estimated value of the ground contact load Fz at each of the wheels 12. The vehicle state acquisition unit 52 estimates the load transfer in the front, rear, left, and right directions based on, for example, the front-rear acceleration Gx, the left-right acceleration Gy, and the driving torque Tdrv at each of the wheels 12. In this embodiment, the term "ground contact load Fz" refers to an estimated value of the ground contact load Fz. The ground contact load Fz includes the left front wheel ground contact load Fzfl, the right front wheel ground contact load Fzfr, the left rear wheel ground contact load Fzrl, and the right rear wheel ground contact load Fzrr.

[0026] The drive control unit 54 performs known traction control, for example, if slip occurs in the wheel 12, by controlling the motor torque Tm to prevent or suppress the slip of the wheel 12 in order to stabilize the behavior of the vehicle 10.

[0027] Figure 2 is a block diagram illustrating the function of traction control. Figure 2(a) is a block diagram illustrating the function of traction control under normal circumstances, which is executed when slip occurs in any one to three of the wheels 12. Figure 2(b) is a diagram illustrating an example of the relationship between the slip ratio SR and the driving force Fx. Figure 2(c) is a block diagram illustrating the function of traction control during four-wheel slip, which is executed when slip occurs in any of the wheels 12. The slip ratio SR is the value of the increase in wheel speed ωx relative to the estimated vehicle speed Vxe (=(ωx-Vxe) / Vxe).

[0028] In Figure 2(a), block B10 (hereinafter, the block will be omitted) is a vehicle speed acquisition unit included in the vehicle state acquisition unit 52. In B10, the estimated vehicle speed Vxe is acquired based on the wheel speed ωx. B20 is a target slip ratio calculation unit included in the drive control unit 54. In B20, the target slip ratio SRtgt is calculated, which is the target value of the slip ratio SR for the wheel 12 that is in a slipping state. The target slip ratio SRtgt is a predetermined slip ratio SR that can output, for example, the driving force peak as the maximum value of the driving force Fx (see Figure 2(b)). B30 is a target motor rotation speed calculation unit included in the drive control unit 54. In B30, the target motor rotation speed ωmtgt is calculated, which is the target value of the rotation speed of the motor MG that drives the wheel 12 that is in a slipping state, based on the estimated vehicle speed Vxe and the target slip ratio SRtgt. The target motor rotational speed ωmtgt is, for example, the value obtained by converting the wheel speed ωx that achieves the target slip ratio SRtgt using the reduction ratio GR. The reduction ratio GR is the gear ratio in the power transmission path between the motor MG and the wheel 12. B40 is the motor rotational speed F / B control unit included in the drive control unit 54. In B40, the motor MG is controlled by feedback control so that the motor rotational speed ωm becomes the target motor rotational speed ωmtgt.

[0029] If slippage occurs on all four wheels simultaneously, the estimated vehicle speed Vxe, based on the wheel speed ωx, will deviate from the actual vehicle speed Vxr. Therefore, the friction coefficient of the wheel 12, which is considered to be in a slipping state, is estimated, and the motor rotation speed ωm is corrected to control the wheel speed ωx to be near the peak value of the friction coefficient. The estimated vehicle speed Vxe is then obtained based on the wheel speed ωx at this time. For this reason, the electronic control device 50 is further equipped with a friction coefficient calculation unit 56.

[0030] In Figure 2(c), B10-B40 have the same function as B10-B40 in Figure 2(a). B50 is the tire μ estimation unit included in the friction coefficient calculation unit 56. In B50, the estimated tire μ is calculated. The estimated tire μ is the estimated value of the tire μ as the friction coefficient of the wheel 12. The tire μ is the value of the driving force Fx (=Fx / Fz) with respect to the ground contact load Fz on the wheel 12, and is calculated using the following equation (1). In the following equation (1), μ is the tire μ, GR is the reduction ratio, Tm is the motor torque, Im is the rotational inertia mass of the motor MG, ωm is the motor rotational speed, and Fz is the ground contact load. In this way, the friction coefficient calculation unit 56 calculates the estimated tire μ of the wheel 12 which is in a slipping state.

[0031] μ=GR×(Tm-Im×(dωm / dt)) / Fz (1)

[0032] In Figure 2(c), B60 is the rotational speed calculation unit for μ peak search included in the drive control unit 54. In B60, the target motor rotational speed ωmtgt is changed up or down so that the estimated tire μ increases or decreases, and the μ peak is searched for. In other words, in B60, a correction amount for the target motor rotational speed ωmtgt is calculated to change the target motor rotational speed ωmtgt in order to search for the μ peak. The μ peak is the maximum value of tire μ. The μ peak is the tire μ that maximizes the driving force Fx generated by the slipping wheel 12.

[0033] Figure 3 illustrates the search for the μ peak. Figure 3(a) shows an example of the relationship between the slip ratio SR and the tire μ. Figure 3(b) illustrates the control of the motor rotation speed ωm for searching for the μ peak.

[0034] In Figure 3(a), the slip ratio SR corresponding to the μ peak can be seen as having the same value as the slip ratio SR corresponding to the driving force peak (see Figure 2(b)). In Figure 3(b), as shown in sections [1] and [2], the motor rotation speed ωm is increased at a predetermined rate of change α (= "α+" > 0) so that the estimated tire μ can increase or decrease with the μ peak as its peak (see arrows [1] and [2] in Figure 3(a)). When the motor rotation speed ωm exceeds the upper threshold, the predetermined rate of change α is switched from "α+" to "α-". As shown in sections [3] and [4], the motor rotation speed ωm is decreased at a predetermined rate of change α (= "α-" < 0) so that the estimated tire μ can increase or decrease with the μ peak as its peak (see arrows [3] and [4] in Figure 3(a)). When the motor rotation speed ωm falls below a lower threshold, a predetermined rate of change α is switched from "α-" to "α+". The pattern shown in section [1]-section [4] is repeated, and the tire μ is maintained in the vicinity of the μ peak, including the μ peak. The vicinity of the μ peak is a predetermined range of the μ peak where, for example, the driving force Fx generated by the slipping wheel 12 can be judged to be close to the driving force Fx at the μ peak. The predetermined rate of change α is a predetermined value for changing the motor rotation speed ωm within the slip ratio SR range that maintains the tire μ in the vicinity of the μ peak, including the μ peak. "α+" is a predetermined value for increasing the motor rotation speed ωm. "α-" is a predetermined value for decreasing the motor rotation speed ωm. The absolute values ​​of "α+" and "α-" may be the same or different. The upper threshold is a predetermined upper limit motor rotation speed ωmh (= estimated vehicle speed Vxe × reduction ratio GR × k1) corresponding to the upper limit of the slip ratio SR that maintains the tire μ in the vicinity of the μ peak, including the μ peak. "k1" is a predetermined coefficient for calculating the upper threshold. The lower threshold is a predetermined lower limit motor rotation speed ωml (= estimated vehicle speed Vxe × reduction ratio GR × k2) corresponding to the lower limit of the slip ratio SR that maintains the tire μ in the vicinity of the μ peak, including the μ peak. "k2" is a predetermined coefficient for calculating the lower threshold. The motor rotation speed ωm that can be changed at a predetermined rate of change α is calculated as a correction amount for the target motor rotation speed ωmtgt.Furthermore, the predetermined rate of change α may be set to switch when the estimated tire μ falls below a predetermined threshold that is smaller than the μ peak. In this way, the drive control unit 54 controls the motor rotation speed ωm to maintain the estimated tire μ of the wheel 12 in the vicinity of the μ peak that includes the μ peak. For example, the drive control unit 54 maintains the estimated tire μ of the wheel 12 in the vicinity of the μ peak that includes the μ peak by increasing the motor rotation speed ωm so that the estimated tire μ of the wheel 12 can increase or decrease with the μ peak as its peak, and by decreasing the motor rotation speed ωm so that the estimated tire μ of the wheel 12 can increase or decrease with the μ peak as its peak.

[0035] Figure 4 illustrates the acquisition of the estimated vehicle speed Vxe when slip occurs in any of the wheels 12. Figure 4(a) shows an example of a state in which the estimated tire μ of wheel 12 is maintained near the μ peak, including the μ peak. Figure 4(b) compares the wheel speed ωx and the actual vehicle speed Vxr when the estimated tire μ is maintained near the μ peak, including the μ peak. Figure 4(c) compares the estimated vehicle speed Vxe based on the wheel speed ωx and the actual vehicle speed Vxr.

[0036] As shown in Figure 4(a), the control operation of B60 in Figure 2(c) maintains the estimated tire μ of wheel 12, which is in a slipping state, in the vicinity of the μ peak, including the μ peak. As shown in Figure 4(b), when the slip ratio SR is S[%] when the estimated tire μ is considered to be the μ peak, the wheel speed ωx can be approximated by a value that is S[%] higher than the actual vehicle speed Vxr, as shown by the dashed line (=Vxr×(100+s) / 100). As shown in Figure 4(c), the estimated vehicle speed Vxe is a value that is corrected by S[%] for the wheel speed ωx (=ωx×100 / (100+s)). In this way, the vehicle state acquisition unit 52 acquires the estimated vehicle speed Vxe using the wheel speed ωx in the state where the estimated tire μ of wheel 12 is maintained in the vicinity of the μ peak, including the μ peak. The vehicle state acquisition unit 52 obtains the estimated vehicle speed Vxe by correcting the actual value of the wheel speed ωx, that is, the wheel speed ωx detected by the wheel speed sensor 32, by the slip ratio SR corresponding to the μ peak. The vehicle state acquisition unit 52 obtains the estimated vehicle speed Vxe using the wheel speed ωx when the estimated tire μ is maintained near the μ peak during driving in a state where all of the wheels 12 are in a slipping state. In other words, the vehicle state acquisition unit 52 obtains the estimated vehicle speed Vxe based on the estimated tire μ during driving in a state where all of the wheels 12 are in a slipping state.

[0037] Figure 5 is a flowchart illustrating the main part of the control operation of the electronic control device 50. It is a flowchart illustrating the control operation to obtain a highly accurate estimated vehicle speed Vxe while maximizing the driving force Fx generated by the slipping wheel 12, and is, for example, executed repeatedly. Figure 6 is a flowchart illustrating the control operation for calculating the rotational speed for searching, and is a subroutine corresponding to step S40 in the flowchart of Figure 5.

[0038] In Figure 5, first, in S10, which corresponds to the function of the vehicle state acquisition unit 52, it is determined whether the control mode of the vehicle 10 is the vehicle speed Vx estimation mode. The vehicle speed Vx estimation mode is a control mode in which the estimated vehicle speed Vxe is acquired, for example, during driving when all of the wheels 12 are in a slipping state. If the judgment in S10 is negative, this routine is terminated. If the judgment in S10 is positive, in S20, which corresponds to the function of the vehicle state acquisition unit 52, the estimated value of the ground contact load Fz on the wheels 12 is acquired. Next, in S30, which corresponds to the function of the friction coefficient calculation unit 56, the estimated tire μ of the wheels 12 that are in a slipping state is calculated. Next, in S40, which corresponds to the function of the drive control unit 54, the search rotation speed calculation is performed. The search rotation speed calculation calculates a correction amount for the target motor rotation speed ωmtgt in order to control the motor rotation speed ωm to search for the μ peak, for example.

[0039] In Figure 6, the calculation of the rotational speed for searching is performed in S40. First, in S401, it is determined whether the control mode of the vehicle 10 is the search mode. The search mode is, for example, a control mode that searches for the μ peak. If the determination in S401 is negative, this routine is terminated. If the determination in S401 is positive, in S402, the upper limit motor rotational speed ωmh is calculated. Next, in S403, the lower limit motor rotational speed ωml is calculated. Next, in S404, the motor rotational speed ωm is obtained. Next, in S405, a predetermined rate of change α is calculated. Next, in S406, the correction amount for the target motor rotational speed ωmtgt is calculated.

[0040] Returning to Figure 5, following S40, in S50, which corresponds to the function of the drive control unit 54, the target motor rotation speed ωmtgt is adjusted using a correction amount for the target motor rotation speed ωmtgt. Next, in S60, which corresponds to the function of the drive control unit 54, the motor MG is controlled by feedback control so that the motor rotation speed ωm becomes the target motor rotation speed ωmtgt. Next, in S70, which corresponds to the function of the vehicle state acquisition unit 52, the wheel speed ωx is acquired. Next, in S80, which corresponds to the function of the vehicle state acquisition unit 52, the wheel speed ωx is corrected by the slip ratio SR corresponding to the μ peak and converted into the estimated vehicle speed Vxe. Next, in S90, which corresponds to the function of the vehicle state acquisition unit 52, the estimated vehicle speed Vxe is updated.

[0041] As described above, according to this embodiment, the estimated tire μ of the wheel 12, which is in a slipping state, is calculated, and the motor rotation speed ωm is controlled to maintain the estimated tire μ near the μ peak including the μ peak, and the estimated vehicle speed Vxe is obtained using the wheel speed ωx in that state. This makes it possible to keep the tire μ of the wheel 12, which is in a slipping state, near the μ peak including the μ peak, and to obtain the estimated vehicle speed Vxe while keeping the tire μ near the μ peak including the μ peak. Therefore, it is possible to obtain a highly accurate estimated vehicle speed Vxe while maximizing the driving force Fx generated by the slipping wheel 12. The vehicle speed Vx can be estimated with good accuracy even while driving in a slipping state.

[0042] Furthermore, according to this embodiment, the motor rotation speed ωm is increased so that the estimated tire μ of the wheel 12 can increase or decrease with the μ peak as its peak, and the motor rotation speed ωm is decreased so that the estimated tire μ of the wheel 12 can increase or decrease with the μ peak as its peak, thereby maintaining the estimated tire μ in the vicinity of the μ peak that includes the μ peak. In this way, the motor rotation speed ωm is appropriately controlled to maintain the estimated tire μ in the vicinity of the μ peak that includes the μ peak.

[0043] Furthermore, according to this embodiment, the estimated vehicle speed Vxe is obtained by correcting the actual value of the wheel speed ωx for the slip ratio SR corresponding to the μ peak. As a result, the estimated vehicle speed Vxe is appropriately obtained using the wheel speed ωx.

[0044] Furthermore, according to this embodiment, the estimated vehicle speed Vxe is obtained based on the estimated tire μ of the wheels 12 while driving with all 12 wheels in a slipping state. This makes it possible to obtain a highly accurate estimated vehicle speed Vxe even when all four wheels are in a slipping state.

[0045] Next, other embodiments of the present invention will be described. In the following description, parts common to multiple embodiments will be denoted by the same reference numerals and their descriptions will be omitted. [Examples]

[0046] In this embodiment, the method for maintaining the estimated tire μ of the wheel 12 near the μ peak, including the μ peak, differs from that of Embodiment 1 described above. In Figure 1, the left front motor MGfl and the right front motor MGfr are front wheel motors MGf, which are front wheel power sources that generate power that becomes the driving force Fx of the front wheel 12f of the wheel 12 according to the present invention. The left rear motor MGrl and the right rear motor MGrr are rear wheel motors MGr, which are rear wheel power sources that generate power that becomes the driving force Fx of the rear wheel 12r of the wheel 12 according to the present invention. The left front motor rotation speed ωmfl and the right front motor rotation speed ωmfr are the front wheel motor rotation speed ωmf as the rotation speed of the front wheel motor MGf. The left rear motor rotation speed ωmrl and the right rear motor rotation speed ωmrr are the rear wheel motor rotation speed ωmr as the rotation speed of the rear wheel motor MGr.

[0047] Figure 7 illustrates an example of a method for maintaining the estimated tire μ of wheel 12 near the μ peak, including the μ peak, which differs from that of Embodiment 1. Figure 7(a) is a block diagram illustrating the function of traction control during four-wheel slip, which is performed when slip occurs in any of the wheels 12, and is a different embodiment from Figure 2(c). Figure 7(b) illustrates the control of the motor rotation speed ωm based on the tire μ of the front wheel 12f and the tire μ of the rear wheel 12r.

[0048] In Figure 7(a), B10-B50 have the same function as B10-B50 in Figure 2(c). B70 is a stiffness calculation unit included in the drive control unit 54. In B70, the front wheel slip ratio SRf is calculated as the increase in front wheel speed ωxf relative to the estimated vehicle speed Vxe (=(ωxf-Vxe) / Vxe). For the front wheel speed ωxf, for example, one of the left front wheel speed ωxfl and the right front wheel speed ωxfr, or the average of the left front wheel speed ωxfl and the right front wheel speed ωxfr is used. The rear wheel slip ratio SRr is calculated as the increase in rear wheel speed ωxr relative to the estimated vehicle speed Vxe (=(ωxr-Vxe) / Vxe). For the rear wheel speed ωxr, for example, one of the left rear wheel speed ωxrl and the right rear wheel speed ωxrr, or the average of the left rear wheel speed ωxrl and the right rear wheel speed ωxrr is used. In this case, the front-to-rear speed difference Δωxfr, which is the difference between the front wheel speed ωxf and the rear wheel speed ωxr, is defined as the predetermined speed difference. For example, the front wheel speed ωxf is set to a value β (>1.0) times the rear wheel speed ωxr. The predetermined speed difference and β are predetermined values ​​such that the slip ratio SR value S[%] corresponding to the μ peak is between the rear wheel slip ratio SRr value and the front wheel slip ratio SRf value. An estimated stiffness SNe, which is an estimated value of stiffness SN, is calculated. Stiffness SN is the slope of the straight line passing through the operating point of the front wheel 12f and the operating point of the rear wheel 12r, as shown in Figure 7(b). These operating points are the operating points represented by the slip ratio SR and the tire μ. The estimated stiffness SNe corresponds to the difference between the estimated front tire μf and the estimated rear tire μr (= (μf - μr) / (SRf - SRr)) with respect to the difference between the front tire slip ratio SRf and the rear tire slip ratio SRr. The estimated front tire μf is the estimated tire μ of the front wheel 12f. The estimated rear tire μr is the estimated tire μ of the rear wheel 12r.

[0049] In Figure 7(a), B80 is the stiffness F / B control unit included in the drive control unit 54. In B80, the target stiffness SNtgt, which is the target value of stiffness SN, is obtained. The target stiffness SNtgt is a predetermined value that is maintained, for example, in the vicinity of the μ peak for both the estimated front tire μf and the estimated rear tire μr. The target stiffness SNtgt is, for example, a non-negative value. When the motor rotation speed ωm is controlled by feedback control so that the estimated stiffness SNe becomes the target stiffness SNtgt, the amount of change in motor rotation speed ωm is calculated as the correction amount for the target motor rotation speed ωmtgt. As a result, both the estimated front tire μf and the estimated rear tire μr are maintained in the vicinity of the μ peak. In this way, the drive control unit 54 controls the motor rotation speed ωm so as to maintain the estimated tire μ of the wheel 12 in the vicinity of the μ peak, which includes the μ peak. For example, the drive control unit 54 maintains the estimated tire μ of the wheel 12 near the μ peak, including the μ peak, by controlling the rotational speed ωmf of the front motor and the rotational speed ωmr of the rear motor, respectively, so that the front-to-rear wheel speed difference Δωxfr is a predetermined speed difference and the estimated stiffness SNe is a predetermined value (target stiffness SNtgt).

[0050] Figure 8 is a flowchart illustrating the main part of the control operation of the electronic control device 50, and is a flowchart illustrating the control operation to obtain a highly accurate estimated vehicle speed Vxe while maximizing the driving force Fx generated by the slipping wheel 12, and is, for example, executed repeatedly. Figure 8 is a different embodiment from the flowchart in Figure 5. Figure 9 is a subroutine corresponding to S45 and S48 in the flowchart of Figure 8. Figure 9(a) is a flowchart illustrating the control operation for calculating stiffness. Figure 9(b) is a flowchart illustrating the control operation for performing stiffness F / B.

[0051] The flowchart in Figure 8 differs from that in Figure 5 in that S40 is replaced with S45 and S48. The main differences between Figure 8 and Figure 5 will be explained below. In Figure 8, following S30, stiffness calculation is performed in S45, which corresponds to the function of the drive control unit 54. Following S45, stiffness feedback is performed in S48, which also corresponds to the function of the drive control unit 54. Following S48, S50 is performed.

[0052] In Figure 9(a), the stiffness calculation in S45 is performed. First, in S451, the wheel speed ωx (front wheel speed ωxf, rear wheel speed ωxr) is obtained. Next, in S452, the estimated vehicle speed Vxe is obtained. Next, in S453, the slip ratio SR (front wheel slip ratio SRf, rear wheel slip ratio SRr) is calculated. Next, in S454, the estimated tire μ (estimated front tire μf, estimated rear tire μr) is obtained. Next, in S455, the estimated stiffness SNe is calculated.

[0053] In Figure 9(b), the stiffness feedback (F / B) operation in S48 is performed. First, in S481, the target stiffness SNtgt is obtained. Next, in S482, the estimated stiffness SNe is obtained. Next, in S483, the value of the motor rotation speed ωm is calculated by feedback control so that the estimated stiffness SNe becomes the target stiffness SNtgt. Next, in S484, the correction amount for the target motor rotation speed ωmtgt is calculated.

[0054] As described above, according to this embodiment, similar to Embodiment 1 described above, it is possible to obtain a highly accurate estimated vehicle speed Vxe while maximizing the driving force Fx generated by the slipping wheel 12.

[0055] Furthermore, according to this embodiment, the front wheel motor rotation speed ωmf and the rear wheel motor rotation speed ωmr are controlled respectively so that the front-to-rear wheel speed difference Δωxfr is a predetermined speed difference and the estimated stiffness SNe becomes the target stiffness SNTgt, thereby maintaining the estimated tire μ of the wheel 12 in the vicinity of the μ peak including the μ peak. As a result, the motor rotation speed ωm is appropriately controlled to maintain the estimated tire μ in the vicinity of the μ peak including the μ peak.

[0056] Although embodiments of the present invention have been described in detail above with reference to the drawings, the present invention is also applicable to other embodiments.

[0057] For example, in the embodiments described above, the vehicle is not limited to a BEV with independent drive for the front, rear, left, and right wheels. For example, in Embodiment 1 described above, the vehicle may be of a type in which the power source is distributed to each of the four wheels. In Embodiment 2 described above, the vehicle may be a BEV with independent drive for the front and rear wheels. Alternatively, an engine may be used as the power source in addition to or instead of an electric motor.

[0058] Furthermore, although the present invention was described using the example of traction control being performed when all four wheels slip simultaneously in the above-described embodiment, the invention is not limited to this embodiment. For example, the present invention can also be applied when driving with only one wheel in a slipping state. Alternatively, the present invention can be applied at times other than when traction control is being performed.

[0059] It should be noted that the above-described embodiment is merely one example, and the present invention can be implemented in various modified and improved forms based on the knowledge of those skilled in the art. [Explanation of Symbols]

[0060] 10: Vehicle 12: Wheels 12f (12fl, 12fr): Front wheels (left front wheel, right front wheel) 12r (12rl, 12rr): Rear wheels (left rear wheel, right rear wheel) 50: Electronic control unit (control unit) 52: Vehicle status acquisition unit 54: Drive control unit 56: Friction coefficient calculation unit MG: Electric motor (power source) MGf: Electric motor for front wheels (power source for front wheels) MGr: Electric motor for rear wheels (power source for rear wheels)

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