Estimation device for estimating friction coefficient of road surface and steering device

An estimation device for estimating a friction coefficient of a road surface includes a sensor, a moment calculator, and a friction coefficient estimator. The sensor is configured to detect a force acting on a wheel. The moment calculator is configured to calculate a moment around a vertical axis at a center of a ground contact load of a tire of the wheel based on an output of the sensor. The friction coefficient estimator is configured to estimate the friction coefficient of the road surface with which the tire is in contact, based on the moment.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application claims priority from Japanese Patent Application No. 2020-068210 filed on Apr. 6, 2020, the entire contents of which are hereby incorporated by reference.

BACKGROUND

The disclosure relates to an estimation device for estimating a friction coefficient of a road surface on which a vehicle travels, and a steering device for steering a vehicle.

Vehicles such as automobiles may estimate a friction coefficient of a road surface for the purpose of using the estimated friction coefficient to provide a warning to a user such as a driver when a friction coefficient is low and to control a vehicle.

In the related art, when a tire slips due to either one of (i) steering and (ii) a driving/braking force, the friction coefficient can be estimated based on a degree of slip.

Further, for example, it has been proposed to estimate a friction coefficient of a road surface by performing a fast Fourier transform analysis for information output from an acceleration sensor of a vehicle body even when the vehicle is traveling straight at a constant speed where slip is unlikely to occur.

For example, as a technique of the related art for estimating a friction coefficient of a road surface, Japanese Unexamined Patent Application Publication (JP-A) No. 6-135214 describes determining the friction coefficient of the road surface by detecting a vertical acceleration acting on a spring of a suspension and calculating a power spectral density in a frequency domain.

Further, JP-A No. 2002-039744 describes detecting a front-rear force, a lateral force, a vertical force, and the like acting on a ground contact surface, based on a six component forces acting on a tire, and maintaining a frictional force near a peak value during an anti-lock control and a traction control.

SUMMARY

An aspect of the disclosure provides an estimation device for estimating a friction coefficient of a road surface. The device includes a sensor, a moment calculator, and a friction coefficient estimator. The sensor is configured to detect a force acting on a wheel. The moment calculator is configured to calculate a moment around a vertical axis at a center of a ground contact load of a tire of the wheel on a basis of an output of the sensor. The friction coefficient estimator is configured to estimate the friction coefficient of the road surface with which the tire is in contact, on a basis of the moment.

An aspect of the disclosure provides a steering device. The steering device includes an actuator, a steering controller, and the above-mentioned estimation device. The actuator is configured to steer a steering wheel of a vehicle. The steering controller is configured to control a generation force of the actuator. The steering controller changes control of the actuator according to an estimation result of a friction coefficient by the estimation device.

An aspect of the disclosure provides an estimation device for estimating a friction coefficient of a road surface. The device includes a sensor and circuitry. The sensor is configured to detect a force acting on a wheel. The circuitry is configured to calculate a moment around a vertical axis at a center of a ground contact load of a tire of the wheel on a basis of an output of the sensor. The circuitry is configured to estimate the friction coefficient of the road surface with which the tire is in contact, on a basis of the moment.

DETAILED DESCRIPTION

The technique described in JP-A No. 6-135214 estimates the friction coefficient based on an acceleration on an upper side of the spring of the vehicle, and further, uses a fast Fourier transform process. Therefore, when this technique detects a change in the friction coefficient of the road surface, a time response delay is large. In addition, the device has a complicated configuration in order to perform the fast Fourier transform.

The technique described in JP-A No. 2002-039744 performs control such that a tire generation force approaches a peak value of a frictional force when a tire force is near a limit, such as during anti-lock control and during traction control. This technique does not consider monitoring a friction coefficient of a road surface during normal traveling in which the tire force is relatively small.

It is desirable to provide an estimation device and a steering device capable of appropriately estimating a friction coefficient of a road surface even during normal traveling in which a time response delay is small and a tire force is small.

For example, the estimation device and the steering device of the embodiment are provided in an automobile such as a passenger vehicle and steer front wheels which are steering wheels.

The steering device of the embodiment includes a pinion-assisted electric power steering (EPS) device.

FIG.1is a diagram schematically illustrating configurations of the estimation device and the steering device of the embodiment.

A steering device1includes a steering wheel10, a steering shaft20, a rack shaft30, a rack housing40, a tie rod50, a housing60, a torque sensor70, an actuator unit80, a steering control unit90, and the like.

The steering wheel10is an annular operating member which receives a steering operation in response to a driver rotating the annular operating member.

The steering wheel10is disposed in a vehicle cabin of the vehicle so as to face a driver's seat.

The steering shaft20is a rotation shaft having one end attached to the steering wheel10. The steering shaft20transmits a rotation operation of the steering wheel10to a rack and pinion mechanism which converts the rotation operation into a translational motion.

A universal joint21capable of transmitting the rotation in a state where the steering shaft20is bent is provided in an intermediate portion of the steering shaft20.

A pinion gear22is provided at an end of the steering shaft20on a side opposite to the steering wheel10. The pinion gear22constitutes part of the rack and pinion mechanism.

The rack shaft30is a columnar member disposed so that a longitudinal direction (axial direction) thereof extends along a vehicle width direction.

The rack shaft30is supported so as to be translatable with respect to a vehicle body in the vehicle width direction.

A rack gear31is provided on a portion of the rack shaft30. The rack gear31meshes with the pinion gear22.

The rack gear31is driven by the pinion gear22according to the rotation of the steering shaft20, and thus, the rack shaft30translates (moves straight) along the vehicle width direction.

The rack gear31is disposed offset to either one of right and left sides (commonly, driver's seat side) in the vehicle width direction.

For example, when a vehicle is a so-called right-hand drive vehicle having a right front seat as a driver's seat, the rack gear31is disposed offset to a right side from a center in a neutral state.

The rack housing40is a substantially cylindrical member which accommodates the rack shaft30while supporting the rack shaft30so that the rack shaft30is relatively displaceable along the vehicle width direction.

Rack boots41are provided at both ends of the rack housing40.

The rack boot41is a member which prevents foreign matters such as dust from entering the rack housing40while allowing the tie rod50to relatively displace with respect to the rack housing40.

For example, the rack boot41is formed of a resin-based material such as an elastomer and in a flexible bellows tube shape.

The tie rod50is a shaft-shaped interlocking member which couples an end of the rack shaft30and a knuckle arm61of the housing60to each other and rotates the housing60around a kingpin axis in conjunction with a translational movement of the rack shaft30.

An inner end of the tie rod50in the vehicle width direction is swingably coupled to the end of the rack shaft30via a ball joint51.

An outer end of the tie rod50in the vehicle width direction is coupled to the knuckle arm61of the housing60via a ball joint52.

A turnbuckle mechanism (not illustrated) for toe-in adjustment is provided at a coupling portion between the tie rod50and the ball joint52.

The housing (knuckle)60is a member which accommodates a hub bearing which rotatably supports a wheel W around an axle.

The housing60has the knuckle arm61which projects forward or rearward with respect to the axle.

The housing60is supported so as to be rotatable around the kingpin axis which is a predetermined rotation center axis.

For example, when a front suspension of the vehicle is a MacPherson strut suspension, the kingpin axis is a virtual axis which couples a center of a bearing of a strut top mount to a center of the ball joint which couples a lower portion of the housing60and a transverse link (lower arm) to each other.

The housing60is pushed and pulled in the vehicle width direction by the rack shaft30via the tie rod50, and thus, the housing60rotates around the kingpin axis to steer the wheels W.

The torque sensor70is a sensor which detects a torque acting on the steering shaft20.

The torque sensor70is provided in an intermediate portion of the steering shaft20.

An output of the torque sensor70is transmitted to the steering control unit90.

The actuator unit80is a drive device which rotationally drives a region of the steering shaft20near the pinion gear22to perform power assist during manual driving or a steering operation during self-driving.

The actuator unit80includes a motor81, a gear box82, and the like.

The motor81is an electric actuator which generates a driving force applied to the steering shaft20.

A rotation direction and an output torque of the motor81are controlled by the steering control unit90.

The gear box82includes a reduction gear train which decelerates a rotational output (amplifies torque) of the motor81and transmits the decelerated rotational output to the steering shaft20.

The steering control unit90is a control device which provides command values of the rotation direction and output torque to the motor81.

The steering control unit90sets command values to be applied to the motor81based on a torque input direction and a detected torque value of the torque sensor70during the manual driving of the vehicle.

During the self-driving or during a driving assistance control (for example, lane keep assist) of the vehicle, the steering control unit90sets the command values to be applied to the motor81based on a command provided from an self-driving control device (not illustrated).

In the embodiment, the wheel W is supported by a hub unit described below.

FIGS.2A and2Bare views illustrating the hub unit according to the embodiment.FIG.2Ais a cross-sectional view of the hub unit taken along a vertical plane passing through the axle (rotation center axis of the wheel).FIG.2Bis a view illustrating the hub unit when viewed from b-b ofFIG.2A.

As illustrated inFIGS.2A and2B, a hub unit100includes a hub110, a bearing120, a sensing member130, a mount140, a six-component force detecting device200, and the like.

The hub110is a member to which a rim center of the wheel (not illustrated) is fixed and which rotates around an axle together with the wheel.

The hub110includes a disk111, a center portion112, a drive shaft mount113, an outer cylinder114, a bearing fixing ring115, and the like.

The disk111is substantially concentric with the axle, and is formed in a substantially flat plate shape.

For example, in the disk111, five hub bolts B used for fastening the wheel are arranged on a predetermined pitch circle at equal intervals.

The center portion112is a cylindrical portion projecting outward in the vehicle width direction from a center of the disk111.

The center portion112is inserted into a recess (not illustrated) provided on a rim, and guides the wheel so that the wheel and the hub110can be mounted concentrically when the wheel is mounted.

The drive shaft mount113is a cylindrical portion projecting inward in the vehicle width direction from the center of the disk11.

A spline hole which spline-engages with a spline shaft of a drive shaft (not illustrated) is formed on an inner diameter side of the drive shaft mount113.

A main part of the drive shaft mount113is inserted into an inner diameter side of a sensitive body210of the six-component force detecting device200.

The outer cylinder114is a cylindrical portion which projects inward in the vehicle width direction from an outer peripheral edge of the disk111. The outer cylinder114is substantially concentric with the axle.

The outer cylinder114is a portion to which an outer ring121of the bearing120is fixed.

In order to hold the outer ring121, a portion whose inner diameter increases in a stepwise manner from an inner end in the vehicle width direction to a width substantially the same as a width of the outer ring121is formed on an inner peripheral surface of the outer cylinder114, and the outer ring121is fitted into the portion.

For example, the disk111, the center portion112, the drive shaft mount113, and the outer cylinder114described above are permanently affixed to each other by machining a forged workpiece.

For example, the bearing fixing ring115is an annular portion which is fixed to the inner end of the outer cylinder114in the vehicle width direction by screwing.

The bearing fixing ring115has an outer diameter substantially equal to that of the outer cylinder114and an inner diameter smaller than an outer diameter of the outer ring121of the bearing120. The bearing fixing ring115holds the inner end of the outer ring121assembled to the outer cylinder114in the vehicle width direction and prevents the outer ring121from falling off.

For example, the bearing120is a double-row deep groove ball bearing which rotatably supports the hub110around the axle. The bearing120has the outer ring121having a raceway surface formed on an inner diameter side thereof, an inner ring122having a raceway surface formed on an outer diameter side thereof, steel balls123which are rolling elements assembled between the outer ring121and the inner ring122, and the like.

The sensing member130is a member which is disposed on an inner diameter side of the outer cylinder114of the hub110and to which the inner ring122of the bearing120is fixed.

The sensing member130has a disk131, an outer cylinder132, a bearing fixing ring133, and the like.

The disk131is substantially concentric with the axle and substantially in a flat plate shape. A circular opening into which the drive shaft mount113of the hub110is inserted is formed in the center of the disk131.

A first flange212of the sensitive body210of the six-component force detecting device200, which will be described later, is fastened to an inner peripheral edge of the disk131.

The outer cylinder132is a cylindrical portion which projects inward in the vehicle width direction from the outer peripheral edge of the disk131. The outer cylinder132is substantially concentric with the axle.

The outer cylinder132is a portion to which the inner ring122of the bearing120is fixed.

In order to hold the inner ring122, a portion whose outer diameter decreases in a stepwise manner from an outer end in the vehicle width direction to a width substantially the same as the width of the inner ring122is formed on an outer peripheral surface of the outer cylinder132. This portion is inserted into an inner diameter side of the inner ring122.

For example, the disk131and the outer cylinder132described above are permanently affixed to each other by machining a forged workpiece.

For example, the bearing fixing ring133is an annular portion which is fixed to an outer end of the outer cylinder132in the vehicle width direction by screwing.

The bearing fixing ring133has an outer diameter larger than the inner diameter of the inner ring122. The bearing fixing ring133holds an outer end of the inner ring122assembled to the outer cylinder132in the vehicle width direction to prevent the inner ring122from falling off.

The mount140is a plate-shaped member fixed to the housing60.

For example, four mounting tabs141are provided on the outer peripheral edge of the mount140. The mounting tabs141project toward the outer diameter side, and have bolt holes for fastening the fixing bolts to the housing60.

A circular opening into which the sensitive body210of the six-component force detecting device200is inserted is formed at the center of the mount140. A second flange213of the sensitive body210is fastened to an inner peripheral edge of the circular opening.

The six-component force detecting device200is formed in a substantially cylindrical shape. The six-component force detecting device200includes the sensitive body210which couples the sensing member130and the mount140, plural strain gauges which are provided in the sensitive body210, and a bridge circuit including the strain gauges.

FIG.3is a cross-sectional view of the sensitive body210in the six-component force detecting device200of the embodiment taken along a plane including the central axis.

As illustrated inFIG.3, the sensitive body210includes a cylinder portion211, a first flange212, the second flange213, and the like.

The cylinder portion211is a portion formed in a cylindrical shape having an inner diameter and outer diameter which are substantially constant over a predetermined axial length, and to which the plural strain gauges, which will be described later, are attached (bonded). The cylinder portion211is disposed substantially concentric with the axle.

The first flange212is a flat plate-shaped portion which is provided at one end of the cylinder portion211and which projects from the cylinder portion211to the outer diameter side and the inner diameter side.

The inner peripheral edge of the disk131of the sensing member130is fixed o the first flange212. The first flange212has screw holes212ato which bolts (not illustrated) are fastened.

An intermediate portion214is provided between the cylinder portion211and the first flange212so that an outer diameter and an inner diameter are intermediate therebetween. An outer peripheral surface of the intermediate portion214is stepped from an outer peripheral surface of the cylinder portion211so that an outer diameter of the intermediate portion214is larger than that of the cylinder portion211. An inner peripheral surface of the intermediate portion214is stepped from the inner peripheral surface of the cylinder portion211so that an inner diameter of the intermediate portion214is smaller than that of the cylinder portion211.

An R portion R1is provided between an end surface on the outer diameter side of the first flange212on the second flange213side and the outer peripheral surface of the intermediate portion214.

An R portion R2is provided between the end surface on the outer diameter side of the intermediate portion214on the second flange213side and the outer peripheral surface of the cylinder portion211.

An R portion R3is provided between the end surface on the inner diameter side of the first flange212on the second flange213side and the inner peripheral surface of the intermediate portion214.

An R portion R4is provided between the end surface on the inner diameter side of the intermediate portion214on the second flange213side and the inner peripheral surface of the cylinder portion211.

Of the R portions R1to R4, positions of the R portions R1and R3in the axial direction of the sensitive body210are substantially the same.

The R portions R2and R4are disposed so that a position of the R portion R2in the axial direction of the sensitive body210is offset to be closer to the second flange213side.

The second flange213is a flat plate-shaped portion which is provided at an opposite end of the cylinder portion211to the first flange212and which projects to the outer diameter side and the inner diameter side from the cylinder portion211.

The inner peripheral edge of the mount140is fixed to the second flange213. The second flange213has bolt holes213ainto which bolts (not illustrated) are inserted.

As illustrated inFIGS.2A and2B, the inner peripheral edge of the mount140abuts against a surface of the second flange213on the first flange212side, and is fastened by the bolts inserted into the bolt holes213aof the second flange213from the inside in the vehicle width direction.

An intermediate portion215is provided between the cylinder portion211and the second flange213so that an outer diameter and an inner diameter are intermediate therebetween. An outer peripheral surface of the intermediate portion215is stepped from the outer peripheral surface of the cylinder portion211so that an outer diameter of the intermediate portion215is larger than that of the cylinder portion211. An inner peripheral surface of the intermediate portion215is stepped from the inner peripheral surface of the cylinder portion211so that an inner diameter of the intermediate portion215is smaller than that of the cylinder portion211.

An R portion R5is provided between the end surface on the outer diameter side of the second flange213on the first flange212side and the outer peripheral surface of the intermediate portion215.

An R portion R6is provided between the end surface on the outer diameter side of the intermediate portion215on the first flange212side and the outer peripheral surface of the cylinder portion211.

An R portion R7is provided between the end surface on the inner diameter side of the second flange213on the first flange212side and the inner peripheral surface of the intermediate portion215.

An R portion R8is provided between the end surface on the inner diameter side of the intermediate portion215on the first flange212side and the inner peripheral surface of the cylinder portion211.

Of the R portions R5to R8, positions of the R portions R5and R7in the axial direction of the sensitive body210are substantially the same.

The R portions R6and R8are disposed so that in a position of the R portion R6in the axial direction of the sensitive body210is offset to be closer to the first flange212side.

A thickness t1of the first flange212and a thickness t2of the second flange213are sufficiently larger than a wall thickness t0of the cylinder portion211.

The six-component force detecting device200includes an Fx detection system, an Fy detection system, an Fz detection system, an Mx detection system, an My detection system, and an Mz detection system, each of which includes the bridge circuit including the strain gauges provided in the cylinder portion211of the sensitive body210described above.

The Fx detection system detects a force Fx, in a radial direction (hereinafter, referred to as an x axis direction), acting on the cylinder portion211of the sensitive body210.

The Fy detection system detects a force Fy, in a radial direction (hereinafter, referred to as a y axis direction) that is a direction orthogonal to the x axis direction, acting on the cylinder portion211of the sensitive body210.

The Fz detection system detects a force Fz, in an axial direction (hereinafter referred to as a z axis direction), acting on the cylinder portion211of the sensitive body210.

The Mx detection system detects a moment Mx, around an x axis, acting on the cylinder portion211of the sensitive body210.

The My detection system detects a moment My, around a y axis, acting on the cylinder portion211of the sensitive body210.

The Mz detection system detects a moment Mz, around a z-axis, acting on the cylinder portion211of the sensitive body210.

The Fx detection system, the Fy detection system, the Fz detection system, the Mx detection system, the My detection system, and the Mz detection system described above each has the bridge circuit including the four strain gauges.

FIG.4is a schematic perspective view illustrating an arrangement of the strain gauges in the six-component force detecting device of the embodiment.

FIGS.5A to5Care diagrams illustrating the arrangements of the strain gauges of the force detection systems and configurations of the bridge circuits in the six-component force detecting device of the embodiment.FIGS.5A,5B, and5Cillustrate the Fx detection system, the Fy detection system, and the Fz detection system, respectively.

FIGS.6A to6Care diagram illustrating configurations of the bridge circuits of the moment detection systems in the six-component force detecting device of the embodiment.FIGS.6A,6B, and6Cillustrate the Mx detection system, the My detection system, and the Mz detection system, respectively.

InFIGS.5A to6C, the intermediate portions214,215and the like are not illustrated.

As illustrated inFIGS.4and5A, the Fx detection system includes strain gauges221to224. Each of the strain gauges221to224is a uniaxial strain gauge, and is attached to the outer peripheral surface of the cylinder portion211so that a detection direction thereof is parallel to a central axis direction of the cylinder portion211.

The strain gauge221is disposed in a region on the first flange212side (that is, a region near the intermediate portion214) on the outer peripheral surface of the cylinder portion211.

The strain gauge222is disposed on a straight line which passes through the strain gauge221and which is parallel to the axial direction of the cylinder portion211. The strain gauge222is disposed in a region on the second flange213side (that is, a region near the intermediate portion215) on the outer peripheral surface of the cylinder portion211.

The strain gauge223is disposed at a position which is obtained by shifting a position of the strain gauge222by 180° around the central axis of the cylinder portion211(that is, the positions of the strain gauges222,223are symmetrical about the central axis of the cylinder portion211).

The strain gauge224is disposed at a position obtained by shifting a position of the strain gauge221by 180° around the central axis of the cylinder portion211(that is, the positions of the strain gauges221,224are symmetrical about the central axis of the cylinder portion211).

As illustrated inFIG.5A, in the bridge circuit of the Fx detection system, the strain gauges221to224are sequentially coupled to each other in a loop, a positive electrode and a negative electrode of a power supply are respectively coupled to between the strain gauge222and the strain gauge223, and between the strain gauge221and the strain gauge224, and a potential difference between (i) a portion between the strain gauge221and the strain gauge222and (ii) a portion between the strain gauge223and the strain gauge224is extracted as an output.

The Fy detection system includes strain gauges231to234. Each of the strain gauges231to234is a uniaxial strain gauge, and is attached to the outer peripheral surface of the cylinder portion211so that a detection direction thereof is parallel to the central axis direction of the cylinder portion211.

The strain gauge231is disposed at a position obtained by shifting the position of the strain gauge221of the Fx detection system by 90° around the central axis of the cylinder portion211.

The strain gauge232is disposed at a position obtained by shifting the position of the strain gauge222of the Fx detection system by 90° around the central axis of the cylinder portion211.

The strain gauge231and the strain gauge232are disposed on the same straight line parallel to the axial direction of the cylinder portion211.

The strain gauge233is disposed at a position obtained by shifting the position of the strain gauge232by 180° around the central axis of the cylinder portion211(that is, the positions of the strain gauges232,233are symmetrical about the central axis of the cylinder portion211).

The strain gauge234is disposed at a position obtained by shifting the position of the strain gauge231by 180° around the central axis of the cylinder portion211(that is, the positions of the strain gauges231,234are symmetrical about the central axis of the cylinder portion211).

As illustrated inFIG.5B, in the bridge circuit of the Fy detection system, the strain gauges231to234are sequentially coupled to each other in a loop, the positive electrode and the negative electrode of the power supply are respectively coupled to between the strain gauge232and the strain gauge233, and between the strain gauge231and the strain gauge234, and a potential difference between (i) a portion between the strain gauge231and the strain gauge232and (ii) a portion between the strain gauge233and the strain gauge234is extracted as an output.

The Fz detection system includes strain gauges241to244. Each of the strain gauges241to244is a uniaxial strain gauge, and is attached to the outer peripheral surface of the cylinder portion211so that a detection direction thereof is parallel to the central axis direction of the cylinder portion211.

The strain gauge241is disposed between the strain gauges221and222of the Fx detection system.

The strain gauges242,243, and244are disposed at positions where phases thereof around the central axis of the cylinder portion211are shifted by 90°, 180°, and 270° with respect to the strain gauges241.

Further, as illustrated inFIG.5C, in the bridge circuit of the Fz detection system, the strain gauges241,242,244, and243are sequentially coupled to each other in a loop, the positive electrode and the negative electrode of the power supply are respectively coupled to between the strain gauge241and the strain gauge243, and between the strain gauge242and the strain gauge244, and a potential difference between (i) a portion between the strain gauge241and the strain gauge242and (ii) a portion between the strain gauge243and the strain gauge244is extracted as an output.

As illustrated inFIGS.4and6A, the Mx detection system includes strain gauges251to254. Each of the strain gauges251to254is uniaxial strain gauges, and is attached to the outer peripheral surface of the cylinder portion211so that a detection direction thereof is parallel to the central axis direction of the cylinder portion211.

The strain gauge251is disposed adjacent to the strain gauge231of the Fy detection system in the central axis direction of the cylinder portion211.

The strain gauge252is disposed adjacent to the strain gauge232of the Fy detection system in the central axis direction of the cylinder portion211.

The strain gauge251and the strain gauge252are disposed on the same straight line parallel to the axial direction of the cylinder portion211.

The strain gauge253is disposed at a position obtained by shifting the position of the strain gauge252by 180° around the central axis of the cylinder portion211(that is, the positions of the strain gauges252,253are symmetrical about the central axis of the cylinder portion211).

The strain gauge254is disposed at a position obtained by shifting the position of the strain gauge251by 180° around the central axis of the cylinder portion211(that is, the positions of the strain gauges251,254are symmetrical about the central axis of the cylinder portion211).

As illustrated inFIG.6A, in the bridge circuit of the Mx detection system, the strain gauges251,253,252, and254are sequentially coupled to each other in a loop, the positive electrode and the negative electrode of the power supply are respectively coupled to between the strain gauge251and the strain gauge253, and between the strain gauge252and the strain gauge254, and a potential difference between (i) a portion between the strain gauge251and the strain gauge254and (ii) a portion between the strain gauge253and the strain gauge252is extracted as an output.

The My detection system includes strain gauges261to264. Each of the strain gauges261to264is a uniaxial strain gauge, and is attached to the outer peripheral surface of the cylinder portion211so that a detection direction thereof is parallel to the central axis direction of the cylinder portion211.

The strain gauge261is disposed adjacent to the strain gauge221of the Fx detection system in the central axis direction of the cylinder portion211.

The strain gauge262is disposed adjacent to the strain gauge222of the Fx detection system in the central axis direction of the cylinder portion211.

The strain gauge261and the strain gauge262are disposed on the same straight line parallel to the axial direction of the cylinder portion211.

The strain gauge263is disposed at a position obtained by shifting the position of the strain gauge262by 180° around the central axis of the cylinder portion211(that is, the positions of the strain gauges262,263are symmetrical about the central axis of the cylinder portion211).

The strain gauge264is disposed at a position obtained by shifting the position of the strain gauge261by 180° around the central axis of the cylinder portion211(that is, the positions of the strain gauges261,264are symmetrical about the central axis of the cylinder portion211).

As illustrated inFIG.6B, in the bridge circuit of the My detection system, the strain gauges261,263,262, and264are sequentially coupled to each other in a loop, the positive electrode and the negative electrode of the power supply are respectively coupled to between the strain gauge261and the strain gauge263, and between the strain gauge262and the strain gauge264, and a potential difference between (i) a portion between the strain gauge261and the strain gauge264and (ii) a portion between the strain gauge263and the strain gauge262is extracted as an output.

The Mz detection system includes strain gauges271to274. Each of the strain gauges271to274is a shear strain gauge, and is attached to the outer peripheral surface of the cylinder portion211so that a detection direction thereof is a circumferential direction of the cylinder portion211.

The strain gauge271is disposed between the strain gauges241and242of the Fz detection system.

The strain gauge272is disposed between the strain gauges242and244of the Fz detection system.

The strain gauges272,273are symmetrical about the central axis of the cylinder portion211. The positions of the strain gauges271,274are symmetrical about the central axis of the cylinder portion211.

As illustrated inFIG.6C, in the bridge circuit of the Mz detection system, the strain gauges271,273,274, and272are sequentially coupled to each other in a loop, the positive electrode and the negative electrode of the power supply are respectively coupled to between the strain gauge271and the strain gauge273, and between the strain gauge272and the strain gauge274, and a potential difference between (i) a portion between the strain gauge271and the strain gauge272and (ii) a portion between the strain gauge273and the strain gauge274is extracted as an output.

The strain gauges of each detection system described above are disposed so that a focal point F of the detection system substantially coincides with a center of the wheel (not illustrated) (that is, a center of a tire width on the axle).

In the embodiment, a friction coefficient estimation unit300is provided. The friction coefficient estimation unit300estimates a friction coefficient of a road surface based on an output of the six-component force detecting device200.

For example, even when a vehicle is traveling straight on a flat road surface at a constant speed, a moment MtZaround a vertical axis at a center of a ground contact load of a tire of a wheel FW (tire-ground contact load) acts on the wheel FW due to various acting forces.

For example, the moment MtZis generated by influences of a camber angle with respect to the ground, a conicity torque, ply steer, and the like.

The camber angle with respect to the ground refers to an inclination angle of the rotation center axis of the wheel FW with respect to the road surface.

When the camber angle with respect to the ground is present, the wheel FW generates a lateral force (camber thrust), which affects the moment MtZ.

The conicity refers to a tapered change in a diameter on a tread surface of the tire of the wheel FW.

Even when the conicity is present, a lateral force is generated as in the case where the camber angle with respect to the ground is present, which affects the moment MtZ(conicity torque).

The ply steer refers to a minute lateral force generated even when the camber angle of the wheel FW with respect to the ground and a slip angle of the wheel FW are zero. In a case of a radial tire, the ply steer is mainly generated in a direction (inclination direction of a belt cord) in which the outermost layer tread belt is attached.

It is said that the ply steer is dominated by effects of deformation of the tread surface of the tire according to a movement of an outermost layer belt. The ply steer generates the moment MtZin the same direction on right and left wheels.

The friction coefficient estimation unit300estimates the friction coefficient of the road surface based on the change in the moment MtZdue to these effects.

The friction coefficient estimation unit300is configured to receive the output of the six-component force detecting devices200provided in the right and left front wheels and communicate with the steering control unit90.

Function and operation of the friction coefficient estimation unit300will be described in detail later.

The friction coefficient estimation unit300includes an information notification device301that notifies a user (such as the driver) when the estimated friction coefficient is low.

Examples of the information notification device301include an image display device, an audio output device, and a warning light.

A vehicle speed sensor310that detects a traveling speed (vehicle speed) of a vehicle is coupled to the friction coefficient estimation unit300.

An environment recognition unit320is communicably coupled to the friction coefficient estimation unit300.

The environment recognition unit320recognizes information on an environment, such as a road shape around the host vehicle and various obstacles around the host vehicle, based on outputs of various sensors, road-to-vehicle communication, vehicle-to-vehicle communication, map data, and the like.

For example, a stereo camera device321is coupled to the environment recognition unit320as one of the sensors.

The stereo camera device321includes a pair of imaging devices (cameras), an image processor, and the like. The imaging devices are disposed apart from each other in the vehicle width direction with an imaging range located in front of the vehicle. The image processor performs stereo image processing on the images captured by the cameras.

The environment recognition unit320detects a lateral position in a lane of the host vehicle based on the output of the stereo camera device321and transmits the detected lateral position to the steering control unit90.

The steering control unit90performs a lane keep assist control that controls the actuator unit80so that the lateral position in the lane is within a predetermined range near the lane center.

The environment recognition unit320determines a type of a road surface (for example, from among a dry paved road surface, a wet paved road surface, a snow-packed road, an ice-snow road, and the like) on which the vehicle is currently traveling based on the output of the stereo camera device321.

The friction coefficient estimation unit300calculates (i) a position of a center of a tire-ground contact load and (ii) a moment MtZaround the vertical axis passing through the center of the ground contact load, based on a six-component force F at a wheel center detected by the six-component force detecting device200.

This moment MtZis a value that correlates with the friction coefficient of the road surface.

In one embodiment, the friction coefficient estimation unit300serves as a“moment calculator” and a “friction coefficient estimator”.

Hereinafter, a method for calculating the position of the center of the ground contact load will be described.

FIG.7is a diagram schematically illustrating the wheel center of the wheel, the center of the tire-ground contact load, and an acting force thereof.

First, the six-component forces detected at a wheel center O of the wheel, the six-component forces at the center of the tire-ground contact load, and the like are defined as follows.

Six-component forces observed at the wheel center O:
F=(FX,FY,FZ),M=(MX,MY,MZ)

Six-component forces at the center of the tire-ground contact load:
Ft=(FtX,FtY,FtZ),Mt=(MtX,MtY,MtZ)

It is assumed that the wheel center is the origin and that a position vector of the center of the tire-ground contact load is r=(x, y, z).

It is also assumed that no external force acts except on a tire ground contact surface. Then, Equation 1 is obtained.
F=Ft(1)

Further, from the definition of a moment, Equation 2 is satisfied.
M=r×Ft+Mt(2)

Here, unknown numbers and known numbers are summarized below.It is assumed that a camber angle is in a sufficiently small range. Then, z=−R where R is a tire dynamic radius and a positive value.A point where Mtx=Mty=0 is defined as the center of the ground contact load.

Therefore, there are three unknown numbers, x, y, and Mtz.

When Equation 2 is summarized for unknown numbers, the center of the tire-ground contact load x and y and Mtzat that point can be obtained as in Equation 3.

Next, the operation of the estimation device of the embodiment will be described.

FIG.8is a flowchart of the operations of the estimation device and the steering device of the embodiment.

Hereinafter, the operations will be described step by step.

Step S01: Determine Vehicle Speed, Steering Speed, and Acceleration

The friction coefficient estimation unit300acquires information on a vehicle speed V and a steering speed (that is, a derivative of a steering angle with respect to time) δdot based on information from the steering control unit90and the vehicle speed sensor310.

The vehicle speed V is differentiated with respect to time to obtain a front-rear acceleration Gx of the vehicle.

Alternatively, the front-rear acceleration may be detected using, for example, an acceleration sensor.

As prerequisites for estimating the friction coefficient, the friction coefficient estimation unit300determines whether the vehicle speed V is a predetermined value (for example, 10 km/h) or more, the steering speed δdot is −10 deg/s or more and 10 deg/s or less, and the front-rear acceleration Gx is in −0.5 m/s2or more and 0.5 m/s2or less.

If the conditions are satisfied, the process proceeds to Step S02. If the conditions are not satisfied, a series of processes is ended (returned).

Step S02: Calculate Moment MtZAround Center of Ground Contact Load from Six-Component Wheel Force

The friction coefficient estimation unit300uses the above-mentioned method to calculate (i) coordinate positions x and y of the centers of the tire-ground contact loads of the right and left front wheels and (ii) the moments MtZaround the vertical axes passing through the centers of the tire-ground contact loads, based on the output of the six-component force detecting device200.

For example, the output of the six-component force detecting device200is subjected to a low-pass filter process having a cutoff frequency of 5 Hz.

Here, a third-order Butterworth filter may be used as the low-pass filter.

By performing this low-pass filter process, it is possible to reduce the influence of irregularities such as seams and roughness of the road surface.

After that, the process proceeds to Step S03.

Step S03: Compare Moment MtZwith Threshold Value 1

The friction coefficient estimation unit300compares the moment MtZcalculated in Step S02with a predetermined threshold value 1 (TH1) that is set based on a moment MtZobtained when the vehicle travels on a dry paved road surface which is a reference road surface.

The threshold value 1 is a threshold value used for distinguishing between a dry paved road surface and a wet paved road surface.

When the moment MtZis the threshold value 1 or less, the process proceeds to Step S04; otherwise, a series of processes is ended (returned).

Step S04: Compare Moment MtZwith Threshold Value 2

The friction coefficient estimation unit300compares the moment MtZcalculated in Step S02with a predetermined threshold value 2 (TH2) that is set based on a moment MtZobtained when the vehicle travels on the dry paved road surface which is the reference road surface.

The threshold value 2 is a threshold value used for distinguishing between the wet paved road surface and a snow-packed road surface.

When the moment MtZis the threshold value 2 or less, the process proceeds to Step S05; otherwise, the process proceeds to Step S06.

Step S05: Compare Moment MtZwith Threshold Value 3

The friction coefficient estimation unit300compares the moment MtZcalculated in Step S02with a predetermined threshold value 3 (TH3) that is set based on the moment MtZobtained when the vehicle travels on the dry paved road surface which is the reference road surface.

The threshold value 3 is a threshold value used for distinguishing between the snow-packed road surface and an ice-snow road surface.

When the moment MtZis threshold value 3 or less, the process proceeds to Step S08; otherwise, the process proceeds to Step S07.

Step S06: Warning of Wet Road

The friction coefficient estimation unit300causes the information notification device301to notify the user of information that warns that the road surface on which the vehicle is traveling is a wet road.

Further, the friction coefficient estimation unit300transmits, to the steering control unit90, information indicating that the friction coefficient of the road surface decreases to an equivalent of the wet road. The steering control unit90changes a content of a generation force control of the actuator unit80as necessary.

For example, the steering control unit90performs control that limits a tire force, a steering angle, a steering speed, a yaw rate of a vehicle body, and the like so that none of slip of the tire, an excessive understeer behavior of the vehicle, and an oversteer behavior occurs on a wet road (the same is true for Steps S07and S08).

After that, a series of processes is ended (returned).

Step S07: Warning of Snow-Packed Road

The friction coefficient estimation unit300causes the information notification device301to notify the user of information that warns that the road surface on which the vehicle travels is a snow-packed road.

Further, the friction coefficient estimation unit300transmits, to the steering control unit90, information indicating that the friction coefficient of the road surface decreases to an equivalent of the snow-packed road. The steering control unit90changes the content of the generation force control of the actuator unit80as necessary.

After that, a series of processes is ended (returned).

Step S08: Warning of Ice-Snow Road

The friction coefficient estimation unit300causes the information notification device301to notify the user of information that warns that the road surface on which the vehicle travels is the ice-snow road.

Further, the friction coefficient estimation unit300transmits, to the steering control unit90, information indicating that the friction coefficient of the road surface decreases to an equivalent of the ice-snow road. The steering control unit90changes the content of the generation force control of the actuator unit80as necessary.

After that, a series of processes is ended (returned).

For example, the threshold values 1 to 3 may be set in advance at shipment of a vehicle from a factory. However, for example, the moment MtZis also changed by wear of tire, a change in air pressure, replacement of tire, a change in scrub radius due to a change in rim inset, a minute change in geometry due to aging of suspension bush, and the like.

Therefore, in the present embodiment, for example, the dry paved road surface is used as the reference road surface. The moments MtZat the dry paved road surface are learned sequentially, so that the threshold values 1 to 3 are learned and corrected.

FIG.9is a flowchart of learning and correcting of the threshold values in the estimation device of the embodiment.

Hereinafter, the operations will be described step by step.

Step S11: Determine Road Surface Condition with Stereo Camera

The friction coefficient estimation unit300determines a type of a road surface on which the vehicle is currently traveling based on the information (recognition result of the stereo camera device321) from the environment recognition unit320.

After that, the process proceeds to Step S12.

Step S12: Determine whether Road Surface is Dry Paved Road Surface

The friction coefficient estimation unit300determines whether the road surface on which the vehicle is currently traveling is the dry paved road surface, which is the reference for estimation of a friction coefficient of a road surface.

When the friction coefficient estimation unit300determines that the road surface is the dry paved road surface, the process proceeds to Step S13; otherwise, the process returns to Step S11and subsequent processes are repeated.

Step S13: Make Determination on Vehicle Speed, Steering Speed, and Acceleration

Similar to Step S01, as the prerequisites for estimating the friction coefficient, the friction coefficient estimation unit300determines whether the vehicle speed V is the predetermined value (for example, 10 km/h) or more, the steering speed δdot is −10 deg/s or more and 10 deg/s or less, and the front-rear acceleration Gx is −0.5 m/s2or more and 0.5 m/s2or less.

When the friction coefficient estimation unit300determines that the conditions are satisfied, the process proceeds to Step S14; otherwise, the process returns to Step S11and subsequent processes are repeated.

Step S14: Calculate Moment MtZaround Center of Ground Contact Load from Six-Component Wheel Force

Similar to Step S02, the friction coefficient estimation unit300calculates (i) the coordinate positions x and y of the centers of the tire-ground contact loads of the right and left front wheels and (ii) the moments MtZaround the vertical axes passing the centers of the tire-ground contact loads, based on the outputs of the six-component force detecting devices200.

After that, the process proceeds to Step S15.

Step S15: Set Threshold Values 1 to 3 Based on Moment MtZ

The friction coefficient estimation unit300sets the threshold values 1 to 3 used for estimating the friction coefficient of the road surface, based on the moment MtZobtained in Step S14.

For example, the threshold values 1 to 3 may be set by multiplying the moment MtZby preset coefficients.

After that, a series of processes is ended.

As described above, according to the present embodiment, the following effects can be obtained.(1) By estimating the friction coefficient based on the moment MtZaround the vertical axis at the center of the tire-ground contact load, for example, even during the normal traveling where the tire force is small (for example, when the vehicle is traveling straight at a constant speed), the friction coefficient of the road surface can be estimated appropriately with a small time response delay.(2) By comparing the threshold values 1 to 3 which are set based on the moment MtZacquired when the vehicle is traveling on the dry paved road surface with the latest acquired moment MtZ, the friction coefficient of the currently traveling road surface can be estimated appropriately.(3) By employing the moment MtZobtained when the stereo camera device321recognizes that the vehicle travels on the dry paved road surface as a reference and setting the threshold values 1 to 3 based on the moment MtZ, the determination condition can learned and corrected accurately.(4) By notifying the user when it is estimated that the vehicle is traveling on either one of the wet road, the snow-packed road, and the ice-snow road, the user can recognize a situation where the friction coefficient is low, and thus, the user is prompted to carefully drive the vehicle, and safety can be improved.(5) When the friction coefficient of the road surface is low, for example, the generation control of the actuator unit80is changed so that none of an excessive slip angle and an excessive yaw rate occurs. Thereby, traveling stability of the vehicle is ensured, and the safety can be improved.

The embodiment of the disclosure is not limited to the embodiments described above, but various modifications and changes may be made, which are also within a technical scope of the disclosure.(1) The configurations of the estimation device, the steering device, and the vehicle are not limited to those of the above-described embodiment, but may be appropriately changed.(2) The configuration of the sensor (six-component force detecting device200in the embodiment) for detecting the wheel acting force is an example, and the structure of the sensitive body and the arrangement of the strain sensors may be appropriately changed.(3) In the embodiment, the output of the estimation device is used for controlling the steering device, but the estimation result of the friction coefficient of the road surface may be used for other purposes.

For example, the estimation result may be used for controlling a driving/braking force generator, an anti-lock brake device, and a behavior control device, and the like. Further, when the vehicle is autonomously driven, the estimation result may be used for changing contents of a self-driving scenario including a target travel locus and a target vehicle speed of the self-driving vehicle.(4) A method of recognizing the reference road surface is not limited to the method using the stereo camera device as in the present embodiment, and may be appropriately changed.

For example, when the own vehicle is traveling on a paved road surface based on information on a position of the host vehicle and map data, if a raindrop sensor does not detect raindrops or if it is determined based on weather information that it does not rain, the road surface may be recognized as the dry paved road surface.

A moment due to some acting forces is almost always generated around a vertical axis passing through a center of a ground contact load on a wheel (tire). This moment correlates with a friction coefficient of a road surface.

According to the embodiment of the disclosure, by estimating the friction coefficient based on the moment around the vertical axis at the center of the tire-ground contact load center which correlates with the friction coefficient of the road surface, for example, the friction coefficient of the road surface can be appropriately estimated even during the normal traveling where the tire force is relatively small (for example, when the vehicle is traveling straight at a constant speed).

Further, by estimating the friction coefficient of the road surface based on the acting force on the wheel, for example, the time response delay is small with respect to estimating of the friction coefficient of the road surface based on the behavior on the spring of the vehicle, and further, since a fast Fourier transform process is not used. Therefore, a calculation load can be reduced and a configuration of the device can be simplified.

According to the embodiment of the disclosure, the friction coefficient of the currently traveling road surface can be appropriately estimated based on the moment obtained when the vehicle travels on a reference road surface (for example, a dry paved road surface) and the latest obtained moment.

According to the embodiment of the disclosure, it is possible to appropriately recognize that the vehicle is traveling on the reference road surface, detect the moment when the vehicle travels on the reference road surface, and accurately learn and correct the determination condition.

According to the embodiment of the disclosure, the user can recognize the situation where the friction coefficient is low, and thus, the user is prompted to carefully drive the vehicle, and safety can be improved.

According to the embodiment of the disclosure, when the friction coefficient of the road surface is low, for example, by changing the generation force control of the actuator so that none of an excessive slip angle and yaw rate occurs, traveling stability of the vehicle is ensured, and the safety can be improved.

As described above, according to the embodiment of the disclosure, it is possible to provide the estimation device and the steering device capable of appropriately estimating the friction coefficient of the road surface even during the normal traveling in which the time response delay is small and the tire force is small.