Control apparatus for four-wheel drive vehicle

A control apparatus configured to control a four-wheel drive vehicle configured to drive right and left front wheels and right and left rear wheels includes an electronic control unit. The electronic control unit calculates a vehicle body speed based on rotation speeds of the wheels and a cumulative value of accelerations in a longitudinal direction of the vehicle. The accelerations are detected by an acceleration sensor. The electronic control unit calculates the vehicle body speed based on the cumulative value of the accelerations. The electronic control unit calculates a correction value based on a lowest rotation speed among the rotation speeds of the wheels under a predetermined condition. The electronic control unit performs correction by using the correction value to make the vehicle body speed closer to a vehicle body speed conversion value of the lowest rotation speed.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority to Japanese Patent Application No. 2019-193466 filed on Oct. 24, 2019, incorporated herein by reference in its entirety.

BACKGROUND

1. Technical Field

The present disclosure relates to a control apparatus configured to control a four-wheel drive vehicle configured to drive right and left front wheels and right and left rear wheels.

2. Description of Related Art

A vehicle body speed of a vehicle (vehicle speed) is determined based on detection values from rotation speed sensors configured to detect rotation speeds of wheels. In a four-wheel drive vehicle, an accurate vehicle body speed cannot be determined based on rotation speeds of four wheels, for example, when all the four wheels slip during acceleration. Therefore, various proposals are made to obtain a highly accurate vehicle body speed even during acceleration or the like.

In Japanese Unexamined Patent Application Publication No. 2002-127881 (JP 2002-127881 A), when an accelerating state of a vehicle is detected, coefficients of road friction are calculated for four wheels, and an optimum coefficient of road friction is selected from among the calculated coefficients of road friction. An upper limit acceleration value (upper limit value of an acceleration at which the vehicle can be accelerated maximally at the optimum coefficient of road friction) is calculated based on the selected coefficient of road friction. Then, a vehicle body speed is calculated by limiting a selected wheel speed based on the calculated upper limit acceleration value. For example, the coefficient of road friction is estimated based on an arithmetic expression using a driving torque of a wheel, inertia of the wheel, a load on the wheel, and an angular acceleration of the wheel.

Japanese Unexamined Patent Application Publication No. 2019-55682 (JP 2019-55682 A) describes a four-wheel drive vehicle including a first coupling apparatus and a second coupling apparatus. The first coupling apparatus transmits a driving force to a left rear wheel. The second coupling apparatus transmits the driving force to a right rear wheel. During acceleration, a coupling torque of one of the first and second coupling apparatuses is set larger than zero, and a coupling torque of the other is set to zero. Then, a vehicle body speed is calculated based on a rotation speed of the right or left rear wheel whose coupling torque is set to zero.

SUMMARY

In JP 2002-127881 A, the coefficients of road friction cannot always be determined accurately unless the wheels slip. Further, the upper limit acceleration value calculated based on any coefficient of road friction may contain a significant deviation. In JP 2019-55682 A, the driving force to be transmitted to the right or left rear wheel needs to be zero during acceleration. Thus, there is a possibility that stable and sufficient acceleration performance cannot be exerted. In the related-art four-wheel drive vehicles, it is difficult to determine a highly accurate vehicle body speed without impairing the acceleration performance particularly during acceleration in which the wheels are likely to slip.

According to the present disclosure, a highly accurate vehicle body speed can be determined without impairing the acceleration performance.

An aspect of the present disclosure relates to a control apparatus configured to control a four-wheel drive vehicle configured to drive right and left front wheels and right and left rear wheels. The control apparatus includes an electronic control unit. The electronic control unit is configured to calculate a vehicle body speed based on rotation speeds of the wheels and a cumulative value of accelerations in a longitudinal direction of the vehicle. The accelerations are detected by an acceleration sensor. The electronic control unit is configured to calculate the vehicle body speed based on the cumulative value of the accelerations. The electronic control unit is configured to calculate a correction value based on a lowest rotation speed among the rotation speeds of the wheels under a predetermined condition. The electronic control unit is configured to perform correction by using the correction value to make the vehicle body speed closer to a vehicle body speed conversion value of the lowest rotation speed.

With the configuration described above, a highly accurate vehicle body speed can be determined without impairing the acceleration performance.

DETAILED DESCRIPTION OF EMBODIMENTS

Embodiment

An embodiment of the present disclosure is described with reference toFIG. 1toFIG. 3B. The following embodiment is described as a preferred specific example for carrying out the present disclosure. Although various preferred technical matters are partially exemplified in detail, the technical scope of the present disclosure is not limited to the specific modes.

Configuration of Four-Wheel Drive Vehicle

FIG. 1is a schematic diagram illustrating an example of the configuration of a four-wheel drive vehicle1according to the embodiment of the present disclosure. The four-wheel drive vehicle1is configured to drive right and left front wheels and right and left rear wheels. In this embodiment, the four-wheel drive vehicle1includes a right front wheel12and a left front wheel11as main driving wheels to which a driving force is constantly transmitted, and includes a right rear wheel14and a left rear wheel13as auxiliary driving wheels to which the driving force is transmitted depending on vehicle conditions. The right front wheel12, the left front wheel11, the right rear wheel14, and the left rear wheel13are supported by hubs (not illustrated) so as to be rotatable relative to a vehicle body10.

The four-wheel drive vehicle1includes an engine15, a transmission16, and a driving force transmission system2. The engine15serves as a drive source. The transmission16changes a speed of rotation of an output shaft of the engine15. The driving force transmission system2transmits a driving force of the engine15obtained through the speed change in the transmission16to the right and left front wheels12and11and the right and left rear wheels14and13. Examples of the drive source also include an electric motor and a so-called hybrid system including an engine and an electric motor in combination.

The driving force transmission system2includes right and left drive shafts22and21on the front wheel side, right and left drive shafts24and23on the rear wheel side, a front differential3, a rear differential4, a propeller shaft20, a driving force transmission apparatus5, and a control apparatus6. The propeller shaft20transmits the driving force in a longitudinal direction of the vehicle. The driving force transmission apparatus5transmits the driving force to the right rear wheel14and the left rear wheel13. The control apparatus6controls the driving force transmission apparatus5. In this embodiment, the driving force transmission apparatus5is arranged between the propeller shaft20and the rear differential4. The driving force transmission apparatus5is configured to adjust the driving force to be transmitted from the propeller shaft20to the right rear wheel14and the left rear wheel13.

The front differential3includes a front differential case31, a pinion shaft32, a pair of pinion gears33and33, and first and second side gears34and35. The pinion shaft32rotates together with the front differential case31. The pinion gears33and33are rotatably supported by the pinion shaft32. The first and second side gears34and35mesh with the pinion gears33and33with their gear axes set orthogonal to each other. The front differential3distributes the driving force to the right front wheel12and the left front wheel11. The right and left drive shafts22and21on the front wheel side are coupled to the second and first side gears35and34so as not to be rotatable relative to the second and first side gears35and34, respectively.

The driving force output from the transmission16is transmitted to the front differential case31of the front differential3, and is transmitted from the front differential case31to the propeller shaft20via a gear mechanism25. Examples of the gear mechanism25include a pair of hypoid gears including a ring gear251and a pinion gear252meshing with each other. The ring gear251rotates together with the front differential case31. The pinion gear252is provided atone end of the propeller shaft20. For example, the other end of the propeller shaft20is coupled to the driving force transmission apparatus5via a joint spider (not illustrated).

The driving force transmission apparatus5includes a bottomed cylindrical housing51, an inner shaft52, a multi-disc clutch53, a cam mechanism54, an electromagnetic clutch55, and an electromagnetic coil56. The driving force is input from the propeller shaft20to the housing51. The inner shaft52is supported so as to be coaxially rotatable relative to the housing51. The multi-disc clutch53includes a plurality of clutch plates arranged between the housing51and the inner shaft52. The cam mechanism54generates a pressing force for pressing the multi-disc clutch53. The electromagnetic clutch55transmits an actuation force for actuating the cam mechanism54. The electromagnetic coil56is supplied with an exciting current from the control apparatus6.

When the electromagnetic coil56is energized, a magnetic force is generated to engage the electromagnetic clutch55. With the electromagnetic clutch55, the rotational force of the housing51is partially transmitted to a pilot cam541of the cam mechanism54. The cam mechanism54includes the pilot cam541, a main cam542, and a plurality of cam balls543. The pilot cam541and the main cam542are rotatable relative to each other in a predetermined angle range. The cam balls543are rollable between the pilot cam541and the main cam542. The pilot cam541and the main cam542have cam grooves inclined with respect to their circumferential directions. The cam balls543roll along the cam grooves.

The main cam542is axially movable but not rotatable relative to the inner shaft52. When the pilot cam541rotates relative to the main cam542with the rotational force transmitted by the electromagnetic clutch55, the cam balls543roll along the cam grooves, and the main cam542moves away from the pilot cam541. Then, the multi-disc clutch53is pressed, and the clutch plates are brought into frictional contact with each other. Thus, the driving force is transmitted between the housing51and the inner shaft52. The driving force to be transmitted by the multi-disc clutch53changes depending on the magnitude of the current supplied to the electromagnetic coil56.

A pinion gear shaft26having a gear portion261at one end is coupled to the inner shaft52of the driving force transmission apparatus5so as not to be rotatable relative to the inner shaft52. The gear portion261of the pinion gear shaft26meshes with a ring gear40fixed to a rear differential case41of the rear differential4.

The rear differential4includes the rear differential case41, a pinion shaft42, a pair of pinion gears43and43, and first and second side gears44and45. The pinion shaft42rotates together with the rear differential case41. The pinion gears43and43are rotatably supported by the pinion shaft42. The first and second side gears44and45mesh with the pinion gears43and43with their gear axes set orthogonal to each other. The rear differential4distributes the driving force to the right rear wheel14and the left rear wheel13. The right and left drive shafts24and23on the rear wheel side are coupled to the second and first side gears45and44so as not to be rotatable relative to the second and first side gears45and44, respectively.

The control apparatus6includes a controller61, a storage62, and a switching power supply63. The control apparatus6may include the electronic control unit (ECU). The controller61includes a central processing unit (CPU: arithmetic processor). The storage62includes a non-volatile memory. The controller61functions as a vehicle body speed calculator611and a driving force controller612such that the CPU executes programs stored in the storage62. The vehicle body speed calculator611calculates a vehicle body speed, which is a movement speed of the vehicle body10relative to a road. The driving force controller612controls, by using the vehicle body speed, a driving force to be transmitted to the right rear wheel14and the left rear wheel13. The switching power supply63includes switching elements such as transistors. The switching power supply63switches a voltage of a direct-current power supply such as a battery through pulse width modulation (PWM) control of the controller61to supply the voltage to the electromagnetic coil56of the driving force transmission apparatus5.

The controller61is configured to acquire detection values from wheel speed sensors71to74and a detection value from an acceleration sensor75via an internal communication network such as a controller area network (CAN). The wheel speed sensors71to74detect rotation speeds of the right front wheel12, the left front wheel11, the right rear wheel14, and the left rear wheel13. The acceleration sensor75detects an acceleration in the longitudinal direction of the four-wheel drive vehicle1(longitudinal G-force).

Examples of the wheel speed sensors71to74include magnetic wheel speed sensors provided on the hubs. The wheel speed sensor includes a magnetic encoder and a magnetic sensor. The magnetic encoder rotates together with the wheel. The magnetic sensor detects a magnetic force of the magnetic encoder. In the magnetic encoder, a plurality of N poles and a plurality of S poles are alternately arranged along a rotational direction. The magnetic sensor outputs a pulse signal having a period that depends on a wheel speed. For example, a capacitive sensor or a piezoresistive sensor may be used as the acceleration sensor75as appropriate.

The controller61serving as the vehicle body speed calculator611calculates the vehicle body speed based on a cumulative value of the accelerations in the longitudinal direction of the vehicle, which are detected by the acceleration sensor75, and the rotation speeds of the wheels, which are detected by the wheel speed sensors71to74. The controller61serving as the driving force controller612controls the driving force transmission apparatus5based on slip ratios of the wheels. The slip ratio is determined from “(R·ω−V)/R·ω” during driving or “(V−R·ω)/V” during braking, where V represents the vehicle body speed, R represents a tire radius of the wheel, and a represents the rotation speed of the wheel (angular velocity). By keeping the slip ratios of the wheels within an appropriate range, the driving force is efficiently transmitted to a road while suppressing a slip of the wheels. Thus, the four-wheel drive vehicle1can travel stably.

Method for Detecting Vehicle Body Speed

Next, description is given of a method for calculating the vehicle body speed by the vehicle body speed calculator611. An overview of this calculation method is as follows. A vehicle body speed is calculated based on a cumulative value of accelerations in the longitudinal direction of the vehicle, which are detected by the acceleration sensor75. A correction value is calculated based on a lowest rotation speed among the rotation speeds of the wheels under a predetermined condition. Correction is performed by using the correction value to make, as a calculation result, the vehicle body speed closer to a vehicle body speed conversion value of the lowest rotation speed. In this embodiment, the predetermined condition is that a difference between the cumulative value of the accelerations in the longitudinal direction of the vehicle and the vehicle body speed conversion value of the lowest rotation speed is smaller than a predetermined value. The vehicle body speed conversion value can be determined by multiplying the rotation speed of the wheel (rad) by the tire radius. In this embodiment, the correction value is calculated based on a difference between a vehicle body speed calculated in a previous control period (previous value of vehicle body speed) and a vehicle body speed conversion value of a lowest rotation speed of a wheel in a current control period. The method for calculating the vehicle body speed is described below in more detail with reference toFIG. 2.

FIG. 2is a flowchart illustrating a specific example of an arithmetic process to be executed by the controller61as the vehicle body speed calculator611. After the four-wheel drive vehicle1is started, that is, after an ignition is turned ON, the controller61repeatedly executes the process of this flowchart in every predetermined control period (for example, 5 ms) to calculate vehicle body speeds. When the four-wheel drive vehicle1is started, each cumulative value described later is initialized to zero. That is, each cumulative value is a sum obtained after the four-wheel drive vehicle1is started and power supply to the control apparatus6is started.

In the flowchart illustrated inFIG. 2, the controller61first acquires detection values from the wheel speed sensors71to74and a detection value from the acceleration sensor75(Step S1). Next, the controller61stores a cumulative value of detection values from the acceleration sensor75in a previous control period as a variable VAC(Step S2), and then calculates a cumulative value of detection values from the acceleration sensor75in a current control period (Step S3). That is, the detection value acquired from the acceleration sensor75in Step S1is added to the variable VACto obtain a new cumulative value of detection values from the acceleration sensor75. The detection value acquired from the acceleration sensor75in Step S1is hereinafter referred to as a longitudinal G-force detection value D. The cumulative value of the detection values from the acceleration sensor75that is calculated in Step S3is hereinafter referred to as a longitudinal G-force cumulative value S1.

Next, the controller61determines whether all the detection values from the wheel speed sensors71to74are smaller than a threshold A (Step S4). The threshold A corresponds to a wheel speed in a case where the four-wheel drive vehicle1travels at an extremely low speed of, for example, 1 km/h or lower. That is, the result of the determination in Step S4is positive (Yes) when all the rotation speeds of the right and left front wheels12and11and the right and left rear wheels14and13are lower than the threshold A indicating that the four-wheel drive vehicle1is traveling at the extremely low vehicle speed.

When the result of the determination in Step S4is negative (No), the controller61extracts a wheel having a lowest rotation speed from among the right and left front wheels12and11and the right and left rear wheels14and13, and sets the rotation speed of the extracted wheel as a variable ωSLOW(Step S5). Next, the controller61determines a vehicle body speed conversion value VSPEED, which is a conversion of the variable ωSLOWinto a vehicle body speed, by multiplying the variable ωSLOWby a predetermined conversion coefficient K (Step S6). The controller61determines whether an absolute value of a difference between the vehicle body speed conversion value VSPEEDand the variable VACthat is the cumulative value of the detection values from the acceleration sensor75in the previous control period is smaller than a threshold B (Step S7). The threshold B is such a small value that the result of the determination in Step S7is positive (Yes) when the variable VACand the vehicle body speed conversion value VSPEEDare substantially equal to each other.

When the result of the determination in Step S7is positive (Yes), the controller61calculates a correction value C by subtracting the vehicle body speed conversion value VSPEEDfrom a vehicle body speed determined in the previous control period (previous value V0of vehicle body speed) and multiplying a value obtained through the subtraction by again coefficient KG(Step S8). The gain coefficient KGis a positive constant smaller than 1. When the result of the determination in Step S7is negative (No), the correction value C is set to 0 (zero)(Step S9).

Next, the controller61calculates a difference cumulative value S2by adding up a value (difference) obtained by subtracting the correction value C determined in Step S8or S9from the longitudinal G-force detection value D acquired in Step S1(Step S10). That is, the value of (longitudinal G-force detection value D−correction value C) in the current control period is added to a difference cumulative value S2calculated in Step S10of the previous control period to obtain a new difference cumulative value S2. Then, the controller61adds the difference cumulative value S2to the previous value V0of the vehicle body speed that is determined in the previous control period to determine a vehicle body speed V1that is an estimated value of a vehicle body speed in the current control period (=previous value V0of vehicle body speed+difference cumulative value S2) (Step S11).

When the result of the determination in Step S4is positive (Yes), the controller61converts the threshold A into a vehicle body speed by multiplying the threshold A by the predetermined conversion coefficient K, and sets the obtained vehicle body speed conversion value as the vehicle body speed V1that is an estimated value of the vehicle body speed in the current control period (Step S12). The vehicle body speed V1determined in Step S11or S12is used as a previous value V0of the vehicle body speed in the process of Step S11of a next control period.

According to the process illustrated in this flowchart, when the absolute value of the difference between the variable VACand the vehicle body speed conversion value VSPEEDis small in the determination of Step S7, that is, when the vehicle body speed conversion value VSPEEDis presumed to be an actual vehicle body speed because at least one of the right and left front wheels12and11and the right and left rear wheels14and13does not slip, in other words, when the vehicle body speed conversion value VSPEEDis reliable, the correction process of Step S10is performed by using the correction value C calculated in Step S8. Therefore, the deviation component contained in the longitudinal G-force cumulative value S1does not continue to increase due to, for example, accumulation of detection deviations of the acceleration sensor75. Further, erroneous correction based on a rotation speed of a slipping wheel is prevented. Thus, the vehicle body speed can be determined with high accuracy.

In Step S7, the difference between the cumulative value of the accelerations in the longitudinal direction of the vehicle in the previous control period (V) and the vehicle body speed conversion value of the lowest rotation speed of the wheel (VSPEED) is compared to the predetermined threshold B. For example, when the extracted wheel having the lowest rotation speed slips and the rotation speed of this wheel abruptly increases after the calculation of the vehicle body speed V1in the previous calculation period, the correction value C can be set to zero (Step S9) to prevent correction. In other words, it is possible to prevent erroneous correction to make the vehicle body speed V1closer to a vehicle body speed conversion value of the rotation speed of the slipping wheel.

In Steps S8, S10, and S11, the vehicle body speed V1is calculated as a value obtained by summing the previous value V0of the vehicle body speed and the cumulative value obtained by applying, to the longitudinal G-force detection value D, the correction value C determined by using the gain coefficient KGsmaller than 1 (longitudinal G-force detection value D−correction value C). Therefore, even if the difference between the longitudinal G-force cumulative value S1and the vehicle body speed conversion value VSPEEDis large, the vehicle body speed V1can be made gradually closer to the vehicle body speed conversion value VSPEED. Thus, an abrupt fluctuation of the vehicle body speed V1is suppressed.

FIG. 3AandFIG. 3Billustrate comparison between a calculation result of a vehicle body speed estimated by the calculation method according to this embodiment and a calculation result of a vehicle body speed according to a comparative example.FIG. 3Ais a layout diagram illustrating inclination of a test course9where a test vehicle8travels. An upper part ofFIG. 3Bis a graph illustrating the vehicle body speed calculated by the calculation method according to this embodiment. A lower part ofFIG. 3Bis a graph illustrating the vehicle body speed calculated by a calculation method according to the comparative example.

The test vehicle8has a vehicle speed measurement apparatus81configured to detect an accurate vehicle body speed based on a rotation speed of a wheel811that rotates on a road. The configuration of the test vehicle8is the same as the configuration illustrated inFIG. 1.

The test vehicle8travels along the test course9from a start point S to a goal G. The test course9includes a first flat road91extending from the start point S, a second flat road93extending behind the goal G, and a slope92between the first flat road91and the second flat road93. A driver of the test vehicle8depresses an accelerator pedal to accelerate the test vehicle8from the start to a point before the top of the slope92, then depresses a brake pedal to apply a brake to the test vehicle8while the test vehicle8passes through the top of the slope92, and then depresses the accelerator pedal again to accelerate the test vehicle8.

The method for calculating a vehicle body speed according to the comparative example is as follows. (1) When rotation speeds of all wheels are smaller than a predetermined threshold indicating a traveling condition at an extremely low speed, the threshold is converted into a vehicle body speed. (2) When absolute values of the amounts of change in the rotation speeds of the wheels per unit time are smaller than a predetermined value and therefore the rotation speeds of the wheels are substantially constant, a lowest rotation speed among the rotation speeds of the four wheels is converted into a vehicle body speed. (3) During braking, a highest rotation speed among the rotation speeds of the four wheels is converted into a vehicle body speed. (4) In a case other than the cases (1) to (3), a cumulative value of detection values of accelerations in the longitudinal direction of the vehicle is compared to the vehicle body speed conversion value of the lowest rotation speed of the wheel. When an absolute value of a difference obtained through the comparison is smaller than a predetermined threshold, the vehicle body speed conversion value is set as a vehicle body speed. When the absolute value of the difference is equal to or larger than the predetermined threshold, the cumulative value of the detection values of the accelerations in the longitudinal direction of the vehicle is set as a vehicle body speed.

When the vehicle body speed is estimated by the calculation method according to this embodiment as illustrated in the upper part ofFIG. 3B, the calculation value of the vehicle body speed highly agrees with an actual value of the vehicle body speed that is measured by the vehicle speed measurement apparatus81. Thus, a highly accurate vehicle body speed is obtained. In the calculation method according to the comparative example, the calculation result of the vehicle body speed varies significantly depending on the cases (1) to (4), and also deviates significantly from the actual value of the vehicle body speed.

According to this embodiment, a highly accurate vehicle body speed can be determined, and the driving force transmission apparatus5can appropriately control the driving force to be transmitted to the right and left rear wheels14and13. Thus, the acceleration performance and the traveling stability of the four-wheel drive vehicle1can be increased.

Supplementary Note

Although the present disclosure is described above based on the embodiment, the embodiment is not intended to limit the claimed disclosure. It should be noted that all combinations of the features described in the embodiment are not essential for the solution of the disclosure to the problem.

The present disclosure may be modified as appropriate by partially omitting, adding, or replacing components without departing from the spirit of the present disclosure. For example, the embodiment described above is directed to the case where the driving force transmission apparatus5is arranged between the propeller shaft20and the rear differential4and the driving force of the engine15arranged on the front wheel side is distributed to the right and left front wheels12and11and the right and left rear wheels14and13. The configuration of the four-wheel drive vehicle to which the present disclosure is applied is not limited to this configuration. For example, the rear differential4may be omitted, and two driving force transmission apparatuses5may be arranged in association with the right rear wheel14and the left rear wheel13.

The present disclosure may be applied to a four-wheel drive vehicle having a configuration in which the driving force transmission apparatus5is omitted and a center differential is provided to distribute the driving force of the engine15to the right and left front wheels12and11and the right and left rear wheels14and13at predetermined distribution ratios.

The present disclosure may be applied to a four-wheel drive vehicle having a configuration in which right and left front wheels are driven by an engine and right and left rear wheels are driven by an electric motor. The present disclosure may also be applied to a four-wheel drive vehicle having a configuration in which wheels are driven by in-wheel motors.