JP7582766B2 - Vehicle speed control method and driving control device - Google Patents

Description

The present invention relates to a vehicle speed control method and a driving control device.

Patent document 1 describes a vehicle speed control device that controls vehicle speed based on the maximum speed at which the vehicle can turn a curve by using the friction circle to its limit when the longitudinal acceleration is zero.

JP 2017-43171 A

However, the vehicle speed control device described in Patent Document 1 calculates the maximum speed when the friction circle is used to its limit regardless of the load on the front and rear wheels of the vehicle, so there is a risk that the actual friction limit of the wheels will be exceeded.
An object of the present invention is to set a target vehicle speed profile that prevents the load on the front and rear wheels of a vehicle from exceeding the friction limit of the wheels when the vehicle is traveling on a target traveling trajectory.

A vehicle speed control method according to one aspect of the present invention sets a target driving trajectory for a vehicle, defines ay as the lateral acceleration of the vehicle calculated according to the curvature profile of the target driving trajectory and the vehicle speed of the vehicle, defines μ as the friction coefficient of the road surface, defines m as the mass of the vehicle, defines l as the wheelbase length, defines lf and lr as the lengths from the center of gravity of the vehicle to the front and rear axles, respectively, defines Fzf and Fzr as the loads applied to the front and rear wheels, respectively, defines Fyf and Fyr as the lateral forces of the front and rear wheels, respectively, defines Fxf and Fxr as the longitudinal forces of the front and rear wheels, respectively, generates a profile as a target vehicle speed profile consisting only of vehicle speeds that satisfy the constraint conditions defined by equations (1), (2), (7), and (8) described below, and controls the vehicle speed of the vehicle based on the target vehicle speed profile.
In addition, a vehicle speed control method according to another aspect of the present invention sets a target driving trajectory of a vehicle, defines ay as the lateral acceleration of the vehicle calculated according to the curvature profile of the target driving trajectory and the vehicle speed of the vehicle, defines μ as the friction coefficient of the road surface, defines m as the mass of the vehicle, defines l as the wheelbase length, defines lf and lr as the lengths from the center of gravity of the vehicle to the front and rear axles, respectively, defines Fzf and Fzr as the loads applied to the front and rear wheels, respectively, defines Fyf and Fyr as the lateral forces of the front and rear wheels, respectively, defines Fxf and Fxr as the longitudinal forces of the front and rear wheels, respectively, defines q as an index set to a value greater than or equal to 1 and less than 2, and generates a profile consisting only of vehicle speeds that satisfy the constraint conditions defined by the equations (7), (8), (15), and (16) described below as a target vehicle speed profile, and controls the vehicle speed of the vehicle based on the target vehicle speed profile.

According to the present invention, when the vehicle is driven along a target driving trajectory, a target vehicle speed profile can be set that prevents the load on the front and rear wheels of the vehicle from exceeding the wheel friction limit.

1 is a schematic configuration diagram of an example of a driving control device according to an embodiment; FIG. 13 is a diagram illustrating a first example of a friction circle constraint according to an embodiment. 2 is a block diagram showing an example of a functional configuration of the driving control device according to the embodiment; FIG. 4 is a flowchart of an example of a vehicle speed control method according to an embodiment. FIG. 13 is a diagram illustrating a second example of a friction circle constraint according to an embodiment. FIG. 4 is a block diagram of a modified example of the functional configuration of the driving control device according to the embodiment. FIG. 13 is a diagram illustrating an example of a friction circle constraint based on a braking force distribution ratio Rbf. FIG. 7B is a diagram showing an example of setting the parameter am in FIG. 7A. FIG. 13 is a diagram showing an example of a friction circle constraint based on a driving force distribution ratio Rdf. FIG. 8B is a diagram showing a setting example of the parameter ap in FIG. 8A.

Below, an embodiment of the present invention will be described with reference to the drawings. Note that each drawing is a schematic view and may differ from the actual product. Furthermore, the embodiment of the present invention shown below is an example of an apparatus and method for embodying the technical concept of the present invention, and the technical concept of the present invention does not limit the structure, arrangement, etc. of the components to those described below. The technical concept of the present invention can be modified in various ways within the technical scope defined by the claims.

Please refer to Fig. 1. The host vehicle 1 is equipped with a cruise control device 10 that performs cruise control of the host vehicle 1. The cruise control by the cruise control device 10 may include automatic driving in which the host vehicle 1 is automatically driven without the involvement of a driver based on the driving environment around the host vehicle 1, and driving assistance control such as constant speed cruise control, lane keeping control, and merging assistance control.
The cruise control device 10 includes an object sensor 11 , a vehicle sensor 12 , a positioning device 13 , a map database (map DB) 14 , a controller 15 , an actuator 16 , a steering device 17 , a drive device 18 , and a braking device 19 .

The object sensor 11 includes multiple different types of object detection sensors mounted on the vehicle 1, such as a laser radar, millimeter wave radar, a camera, and LIDAR (Light Detection and Ranging, Laser Imaging Detection and Ranging), that detect objects around the vehicle 1.
The vehicle sensor 12 is mounted on the host vehicle 1 and detects various information (vehicle signals) obtained from the host vehicle 1. The vehicle sensor 12 includes, for example, a vehicle speed sensor that detects the traveling speed (vehicle speed) of the host vehicle 1, a wheel speed sensor that detects the rotational speed of each tire equipped on the host vehicle 1, a three-axis acceleration sensor (G sensor) that detects the acceleration (including deceleration) in three axial directions of the host vehicle 1, a steering angle sensor that detects the steering angle (including the turning angle), a gyro sensor that detects the angular velocity generated in the host vehicle 1, a yaw rate sensor that detects the yaw rate, an accelerator sensor that detects the accelerator opening degree of the host vehicle 1, and a brake sensor that detects the amount of brake operation by the driver.

The positioning device 13 includes a Global Navigation System (GNSS) receiver and receives radio waves from a plurality of navigation satellites to measure the current position of the vehicle 1. The GNSS receiver may be, for example, a Global Positioning System (GPS) receiver. The positioning device 13 may be, for example, an inertial navigation system.
The map database 14 may be a storage device that stores high-precision map data (hereinafter simply referred to as "high-precision map") suitable as a map for automated driving. The high-precision map is map data with higher precision than map data for navigation (hereinafter simply referred to as "navigation map"), and includes lane-by-lane information that is more detailed than road-by-road information.

The controller 15 is an electronic control unit (ECU) that performs driving assistance control of the host vehicle 1. The controller 15 includes a processor 20 and peripheral components such as a storage device 21. The processor 20 may be, for example, a CPU (Central Processing Unit) or an MPU (Micro-Processing Unit).
The storage device 21 may include a semiconductor storage device, a magnetic storage device, an optical storage device, etc. The storage device 21 may include memories such as a register, a cache memory, a ROM (Read Only Memory) used as a main storage device, and a RAM (Random Access Memory).
The functions of the controller 15 described below are realized, for example, by the processor 20 executing a computer program stored in the storage device 21 .
The controller 15 may be formed of dedicated hardware for executing each of the information processes described below.
For example, the controller 15 may include a functional logic circuit configured in a general-purpose semiconductor integrated circuit, or may include a programmable logic device (PLD) such as a field programmable gate array (FPGA).

The actuator 16 operates the steering device 17, the drive device 18, and the braking device 19 in response to a control signal from the controller 15 to generate vehicle behavior of the host vehicle 1. The actuator 16 includes a steering actuator, an accelerator opening actuator, and a brake control actuator.
The steering actuator operates a turning device 17 to control the steering direction and steering amount of the host vehicle 1. The accelerator opening actuator operates a drive device 18 such as an engine and a drive motor to control the longitudinal acceleration of the host vehicle 1. The brake control actuator operates a braking device 19 to control the longitudinal deceleration of the host vehicle 1.

Next, an example of travel control by the controller 15 will be outlined.
In driving assistance controls such as automatic driving, which drives the vehicle 1 automatically, constant speed driving control, lane keeping control, and merging assistance control, the controller 15 generates a target driving trajectory for the vehicle 1 to travel in the future based on information about the surrounding environment of the vehicle 1 detected by the object sensor 11, the current position of the vehicle 1 measured by the positioning device 13, and the map database 14.
Furthermore, the controller 15 generates a target vehicle speed profile, which is a profile of the vehicle speed at which the host vehicle 1 travels along the target travel trajectory.

When generating a target vehicle speed profile, the controller 15 generates a target vehicle speed profile that satisfies both the longitudinal force constraint conditions based on the friction limits of the front wheels of the vehicle 1 and the longitudinal force constraint conditions based on the friction limits of the rear wheels.
For example, the controller 15 may generate a target vehicle speed profile that satisfies the following condition (A) regarding the wheel friction limit.

上式（１）中のＦｙｆ、Ｆｘｆは、それぞれ前輪で発生する横力（前輪横力）及び前後力（前輪前後力）であり、Ｆｚｆは前輪に係る荷重（前輪荷重）である。上式（２）中のＦｙｒ、Ｆｘｒは、それぞれ後輪で発生する横力（後輪横力）及び前後力（後輪前後力）であり、Ｆｚｒは後輪に係る荷重（後輪荷重）である。μは路面の摩擦係数である。
上式（１）は前輪の摩擦限界に関する制約条件であり、前輪横力Ｆｙｆと前輪前後力Ｆｘｆの二乗和を、前輪荷重Ｆｚｆとμとの積の二乗値以下に制限する。すなわち、前輪横力Ｆｙｆと前輪前後力Ｆｘｆの合力の大きさを、前輪のタイヤの摩擦限界（摩擦円）以下に制限する。同様に、上式（２）は後輪の摩擦限界に関する制約条件である。 (A) Constraints on friction limits

In the above formula (1), Fyf and Fxf are the lateral force (front wheel lateral force) and the longitudinal force (front wheel longitudinal force) generated at the front wheels, respectively, and Fzf is the load on the front wheels (front wheel load). In the above formula (2), Fyr and Fxr are the lateral force (rear wheel lateral force) and the longitudinal force (rear wheel longitudinal force) generated at the rear wheels, respectively, and Fzr is the load on the rear wheels (rear wheel load). μ is the friction coefficient of the road surface.
The above formula (1) is a constraint condition related to the friction limit of the front wheels, and limits the sum of the squares of the front wheel lateral force Fyf and the front wheel longitudinal force Fxf to less than or equal to the square value of the product of the front wheel load Fzf and μ. In other words, the magnitude of the resultant force of the front wheel lateral force Fyf and the front wheel longitudinal force Fxf is limited to less than the friction limit (friction circle) of the front wheel tires. Similarly, the above formula (2) is a constraint condition related to the friction limit of the rear wheels.

Therefore, when the front wheel load Fzf is large and the rear wheel load Fzr is small, the upper limit value limiting the magnitude of the front wheel longitudinal force can be set to a large value, and the upper limit value limiting the magnitude of the rear wheel longitudinal force can be set to a small value, compared to when the front wheel load Fzf is small and the rear wheel load Fzr is large.
In addition, when the rear wheel load Fzr is large and the front wheel load Fzf is small, the upper limit value limiting the magnitude of the front and rear forces of the rear wheels can be set to a larger value and the upper limit value limiting the magnitude of the front and rear forces of the front wheels can be set to a smaller value, compared to when the rear wheel load Fzr is small and the front wheel load Fzf is large.

When the vehicle speed changes, the front wheel load Fzf and rear wheel load Fzr fluctuate, and the upper limits that the front wheel lateral force Fyf, front wheel longitudinal force Fxf, rear wheel lateral force Fyr, and rear wheel longitudinal force Fxr can assume change. Therefore, the controller 15 generates a target vehicle speed profile that satisfies the following additional load-related condition (a1) in addition to condition (A).

上式（３）、（４）中のｍは自車両１の質量であり、ｇは重力加速度であり、ｌはホイールベース長であり、ｌｒは車両重心から後輪軸までの長さであり、ｌｆは車両重心から前輪軸までの長さであり、ｈは車両重心の高さであり、ａｘは自車両１の前後方向の加速度（前後加速度）である。
これにより、自車両１の加速度（ａｘ＞０）が大きい場合には小さい場合に比べて、前輪荷重Ｆｚｆが小さく設定され、自車両１の減速度（ａｘ＜０）が大きい場合には小さい場合に比べて、後輪荷重Ｆｚｒが小さく設定される。 Additional condition (a1)

In the above equations (3) and (4), m is the mass of the vehicle 1, g is the acceleration due to gravity, l is the wheelbase length, lr is the length from the vehicle center of gravity to the rear axle, lf is the length from the vehicle center of gravity to the front axle, h is the height of the vehicle center of gravity, and ax is the acceleration of the vehicle 1 in the fore-and-aft direction (fore-and-aft acceleration).
As a result, when the acceleration (ax>0) of the vehicle 1 is large, the front wheel load Fzf is set smaller than when it is small, and when the deceleration (ax<0) of the vehicle 1 is large, the rear wheel load Fzr is set smaller than when it is small.

Furthermore, the longitudinal force for braking the host vehicle 1 is distributed to a front wheel longitudinal force Fxf and a rear wheel longitudinal force Fxr in accordance with the braking force distribution ratio. Also, if the host vehicle 1 is a four-wheel drive vehicle, the longitudinal force for driving the host vehicle 1 is distributed to a front wheel longitudinal force Fxf and a rear wheel longitudinal force Fxr in accordance with the driving force distribution ratio.
Therefore, the controller 15 may generate a target vehicle speed profile that satisfies, in addition to the conditions (A) and (a1), the following additional condition (a2) related to the braking force distribution ratio and/or the driving force distribution ratio.

上式（５）、（６）中のＲは、制動力配分比（減速時、すなわちａｘ＜０である時）又は駆動力配分比（加速時、すなわちａｘ＞０である時）である。 Additional condition (a2)

In the above equations (5) and (6), R is the braking force distribution ratio (during deceleration, i.e., when ax<0) or the driving force distribution ratio (during acceleration, i.e., when ax>0).

横加速度ａｙは、演算対象である目標車速プロファイルによって定まる変数である。そこで、コントローラ１５は、横加速度ａｙの各値に対して上式（１）、（３）、（５）及び（７）を満足する自車両１の前後加速度ａｘ＝Ｆｘｆ／ｍのそれぞれの上限値及び下限値を、前輪の摩擦限界に基づく前輪前後加速度制約Ａｘｆ［ａｙ］として算出する。
同様に、横加速度ａｙの各値に対して上式（２）、（４）、（６）及び（８）を満足する自車両１の前後加速度ａｘ＝Ｆｘｒ／ｍのそれぞれの上限値及び下限値を、後輪の摩擦限界に基づく後輪前後加速度制約Ａｘｒ［ａｙ］として算出する。
前輪前後加速度制約Ａｘｆ［ａｙ］及び後輪前後加速度制約Ａｘｒ［ａｙ］は、それぞれ特許請求の範囲に記載の「第２制約条件」及び「第３制約条件」の一例である。 The front wheel lateral force Fyf and the rear wheel lateral force Fyr in the above equations (1) and (2) are variables determined based on the lateral acceleration ay of the host vehicle 1, as shown in the following equations (7) and (8).

The lateral acceleration ay is a variable determined by the target vehicle speed profile to be calculated. Therefore, the controller 15 calculates the upper and lower limit values of the longitudinal acceleration ax=Fxf/m of the host vehicle 1 that satisfy the above formulas (1), (3), (5), and (7) for each value of the lateral acceleration ay as the front wheel longitudinal acceleration constraint Axf[ay] based on the friction limit of the front wheels.
Similarly, the upper and lower limit values of the longitudinal acceleration ax = Fxr/m of the vehicle 1 that satisfy the above equations (2), (4), (6) and (8) for each value of the lateral acceleration ay are calculated as the rear wheel longitudinal acceleration constraint Axr[ay] based on the friction limit of the rear wheels.
The front wheel longitudinal acceleration constraint Axf[ay] and the rear wheel longitudinal acceleration constraint Axr[ay] are examples of the "second constraint condition" and the "third constraint condition" respectively described in the claims.

2 is a diagram showing an example of the front wheel longitudinal acceleration constraint Axf[ay] and the rear wheel longitudinal acceleration constraint Axr[ay]. A solid line 30 indicates the front wheel longitudinal acceleration constraint Axf[ay], and a dashed dotted line 31 indicates the rear wheel longitudinal acceleration constraint Axr[ay].
The lateral force and longitudinal force that the entire vehicle 1 can generate are within the range of both the upper and lower limits of the front wheel longitudinal acceleration constraint Axf[ay] and the rear wheel longitudinal acceleration constraint Axr[ay].
Therefore, the controller 15 generates a target vehicle speed profile so that the longitudinal acceleration ax of the vehicle 1 does not exceed the region 32 (the hatched region in Figure 2) where the region inside (towards the origin) of the solid line 30 indicating the front wheel longitudinal acceleration constraint Axf [ay] and the region inside (towards the origin) of the dotted line 31 indicating the rear wheel longitudinal acceleration constraint Axr [ay] overlap.

また例えばコントローラ１５は、横加速度ａｙの各値に対して、減速時（ａｘ＜０）の場合の前後加速度の下限値（すなわち負の前後加速度の大きさの上限値）Ａｘ［ａｙ］を次式（１０）に基づいて前輪前後加速度制約Ａｘｆ［ａｙ］及び後輪前後加速度制約Ａｘｒ［ａｙ］から選択してよい。

For example, for each value of lateral acceleration ay, the controller 15 may select the upper limit value Ax[ay] of the longitudinal acceleration during acceleration (ax>0) from the front wheel longitudinal acceleration constraint Axf[ay] and the rear wheel longitudinal acceleration constraint Axr[ay] based on the following equation (9).

For example, the controller 15 may select, for each value of the lateral acceleration ay, the lower limit value of the longitudinal acceleration during deceleration (ax<0) Ax[ay] (i.e., the upper limit value of the magnitude of the negative longitudinal acceleration) from the front wheel longitudinal acceleration constraint Axf[ay] and the rear wheel longitudinal acceleration constraint Axr[ay] based on the following equation (10).

In the following description, the longitudinal acceleration limit value Ax[ay] for each value of the lateral acceleration ay, which is selected so as to be within the range of both the upper and lower limits of the front wheel longitudinal acceleration constraint Axf[ay] and the rear wheel longitudinal acceleration constraint Axr[ay], may be referred to as a "longitudinal acceleration constraint." The longitudinal acceleration constraint is an example of a "first constraint condition" described in the claims.
In this way, the value with the smaller magnitude (absolute value) of the front wheel longitudinal acceleration constraint Axf[ay] and the rear wheel longitudinal acceleration constraint Axr[ay] is selected as the longitudinal acceleration constraint Ax[ay]. Therefore, for example, if a load shift occurs and either the front wheel longitudinal acceleration constraint Axf[ay] or the rear wheel longitudinal acceleration constraint Axr[ay] decreases, the longitudinal acceleration constraint Ax[ay] will also decrease.

Furthermore, as is clear from the above formulas (5) and (6), if the braking force distribution or driving force distribution is biased toward either the front wheels or the rear wheels, the value of one of the front wheel longitudinal force Fxf and the rear wheel longitudinal force Fxr generated for the same longitudinal acceleration ax becomes large, and therefore the longitudinal acceleration ax that satisfies the above formulas (1) and (2) becomes small.
Therefore, for example, when the difference between the braking force distribution ratio to the front wheels and the ideal braking force distribution is large compared to when the difference is small, a small value of the longitudinal acceleration constraint Ax[ay] is set.
Furthermore, when the difference between the driving force distribution to the front wheels and the driving force distribution to the rear wheels is large compared to when the difference is small, a small value of the longitudinal acceleration constraint Ax[ay] is set.

上式（１１）中、Ｓ１及びＳ２は目標車速プロファイルを生成する区間Ｓの始点区間及び終点区間である。 The controller 15 generates, as a target vehicle speed profile, a profile of the vehicle speed v of the vehicle 1 that minimizes the passing time t for passing through the section S on the target driving trajectory while satisfying the above constraints (A), (a1) and (a2).

In the above formula (11), S1 and S2 are the start and end sections of the section S for which the target vehicle speed profile is generated.

The controller 15 judges whether the vehicle speed v satisfies the above constraint conditions (A), (a1), and (a2) based on the curvature profile ρ of the target driving trajectory, the lateral acceleration ay=ρ× ^v2 corresponding to the vehicle speed v, and the longitudinal acceleration constraint Ax[ay]. The controller 15 generates a profile consisting of only the vehicle speed v that satisfies the constraint conditions (A), (a1), and (a2) as the target vehicle speed profile.
In addition, the controller 15 may generate a target vehicle speed profile so as to satisfy, in addition to the constraint conditions (A), (a1) and (a2) relating to the friction limit, the constraint condition (B) on the limit speed determined from the curvature of the road, the constraint condition (C) on the longitudinal jerk jx and/or the constraint condition (D) on the lateral jerk jy.

（Ｃ）前後ジャークｊｘの制約条件
｜ｊｘ｜≦ｊｘｍａｘ …（１３）
上式（１３）中、ｊｘｍａｘはピッチングによって車両挙動を不安定にさせない前後ジャークの大きさの上限値である。
（Ｄ）横ジャークｊｙの制約条件
｜ｊｙ｜≦ｊｙｍａｘ…（１４）
上式（１４）中、ｊｙｍａｘは自車両１のヨー角変化及び横運動が追従できる横ジャークの大きさの上限値である。
コントローラ１５は、生成した目標車速プロファイルに従う速度で自車両１が目標走行軌道を走行するように、アクチュエータ１６を駆動する。 (B) Limit speed constraints determined by the curvature of the road

(C) Constraint condition for forward/rearward jerk jx |jx|≦jxmax ... (13)
In the above equation (13), jxmax is the upper limit of the magnitude of the longitudinal jerk that does not cause the vehicle behavior to become unstable due to pitching.
(D) Constraint condition for lateral jerk jy |jy|≦jymax... (14)
In the above equation (14), jymax is the upper limit of the magnitude of the lateral jerk that the yaw angle change and lateral movement of the vehicle 1 can follow.
The controller 15 drives the actuator 16 so that the host vehicle 1 travels along the target travel path at a speed in accordance with the generated target vehicle speed profile.

Fig. 3 is a block diagram of an example of a functional configuration of the controller 15 which is an example of a cruise control device according to the embodiment. Fig. 4 is a flowchart of an example of a vehicle speed control method according to the embodiment.
The controller 15 includes a trajectory generating unit 40 , a friction coefficient estimating unit 41 , a front wheel friction circle constraint setting unit 42 , a rear wheel friction circle constraint setting unit 43 , a friction circle constraint correcting unit 44 , and a speed profile setting unit 45 .
The trajectory generation unit 40 generates a target driving trajectory along which the vehicle 1 will travel in the future based on information about the surrounding environment of the vehicle 1 detected by the object sensor 11, the current position of the vehicle 1 measured by the positioning device 13, and the map database 14 (step S1).
The friction coefficient estimating unit 41 estimates the friction coefficient μ of the road surface on which the host vehicle 1 is traveling (step S2). For example, the friction coefficient estimating unit 41 may estimate the friction coefficient μ of the road surface in accordance with the longitudinal acceleration ax and the wheel speed of the host vehicle 1.

The front wheel friction circle constraint setting unit 42 sets the front wheel longitudinal acceleration constraints Axf[ay] for each value of the lateral acceleration ay based on the above equations (1), (3), (5) and (7) (step S3).
The rear wheel friction circle constraint setting unit 43 sets the rear wheel longitudinal acceleration constraints Axr[ay] for each value of the lateral acceleration ay based on the above equations (2), (4), (6) and (8) (step S4).

The friction circle constraint correction unit 44 selects either the front wheel longitudinal acceleration constraint Axf[ay] or the rear wheel longitudinal acceleration constraint Axr[ay] for each value of the lateral acceleration ay based on the above formulas (9) and (10) and sets it as the longitudinal acceleration constraint Ax[ay]. The speed profile setting unit 45 generates a target vehicle speed profile so as to satisfy the longitudinal acceleration constraint Ax[ay], the limit speed constraint condition (B) calculated from the curvature of the road, the longitudinal jerk jx constraint condition (C), and the lateral jerk jy constraint condition (D) (step S5).
The actuator 16 controls the steering device 17, the drive device 18 and the braking device 19 so that the host vehicle 1 travels along the target travel path at a speed according to the target vehicle speed profile (step S6), after which the process ends.

(Variation 1)
In the above equations (1) and (2), the sum of the square terms of the front wheel lateral force Fyf and the rear wheel lateral force Fyr and the square terms of the front wheel longitudinal forces Fxf and the rear wheel longitudinal forces Fxr are limited to less than or equal to upper limit values corresponding to the square terms of the front wheel load Fzf and the rear wheel load Fzr, respectively.
Alternatively, as shown in the following equations (15) and (16), the sums of the q-th power terms (i.e., the power terms of the exponent q) of the front wheel lateral force Fyf and the rear wheel lateral force Fyr and the q-th power terms of the front wheel longitudinal forces Fxf and the rear wheel longitudinal forces Fxr may be limited to less than or equal to upper limit values corresponding to the q-th power terms of the front wheel load Fzf and the rear wheel load Fzr, respectively.

5 shows an example of the longitudinal acceleration constraint Ax[ay] when the index q is set to a value equal to or greater than 1 and less than 2. Compared to the above formulas (1) and (2) in which the index q is 2, the longitudinal acceleration constraint Ax[ay] can be set to a small value in an area where both ax and ay are not zero (i.e., a scene in which braking or driving is performed while turning is performed at the same time).
This makes it possible to generate a target vehicle speed profile with a safety margin in a scene where braking and turning are performed simultaneously, or in a scene where driving and turning are performed simultaneously.

(Variation 2)
The controller 15 may set the longitudinal acceleration constraint Ax[ay] for the entire vehicle body, without independently setting the front wheel longitudinal acceleration constraint Axf[ay] for the front wheels and the rear wheel longitudinal acceleration constraint Axr[ay] for the rear wheels.
Fig. 6 is a block diagram of an example of a functional configuration of a modified example of the controller 15. The same reference numerals are used for the same configuration as in Fig. 3. The controller 15 includes a friction circle constraint setting unit 46.
The friction circle constraint setting unit 46 sets a longitudinal acceleration constraint Ax [ay] based on the friction coefficient μ of the road surface.

上式（１７）中、ｂ（≒μｇ）は定数であり、ａｍは、減速時（ａｘ＜０）の前後加速度制約Ａｘ［ａｙ］の下限値（すなわち絶対値の上限値）を調整するためのパラメータである。
例えばパラメータａｍをμｇよりも小さな値に設定することにより、図７Ａに示すように減速時の前後加速度制約Ａｘ［ａｙ］を小さな値に設定できる。
例えば、パラメータａｍを制動力配分比に応じて設定してよい。図７Ｂは、パラメータａｍの設定例の説明図である。パラメータａｍは、前輪への制動力配分比Ｒｂｆに応じて設定される。 For example, the friction circle constraint setting unit 46 may set a longitudinal acceleration constraint Ax[ay] based on the following equation (17) for each value of the lateral acceleration ay.

In the above equation (17), b (≈μg) is a constant, and am is a parameter for adjusting the lower limit value (i.e., the upper limit value of the absolute value) of the longitudinal acceleration constraint Ax[ay] during deceleration (ax<0).
For example, by setting the parameter am to a value smaller than μg, the longitudinal acceleration constraint Ax[ay] during deceleration can be set to a small value as shown in FIG. 7A.
For example, the parameter am may be set according to the braking force distribution ratio Rbf. Fig. 7B is an explanatory diagram of a setting example of the parameter am. The parameter am is set according to the braking force distribution ratio Rbf to the front wheels.

7B, the parameter am may be set to be smaller as the difference between the braking force distribution ratio and the ideal braking distribution ratio Rbi increases. This allows the lower limit value Ax[ay] of the deceleration (ax<0) for decelerating the host vehicle 1 to be set to a smaller value as the difference between the braking force distribution ratio and the ideal braking distribution ratio Rbi increases.
Therefore, the greater the difference between the braking force distribution ratio and the ideal braking force distribution ratio Rbi, the more the longitudinal acceleration constraint Ax[ay] can be set to limit the longitudinal acceleration ax to a smaller value so as not to exceed this, even if the longitudinal force that can be generated at the wheel during deceleration (ax<0) decreases due to the load transfer in the longitudinal direction.

上式（１８）中、ａｐは、減速時（ａｘ＞０）前後加速度制約Ａｘ［ａｙ］の上限値を調整するためのパラメータである。
例えば図８Ａに示すように、パラメータａｐをμｇよりも小さな値に設定することにより、加速時の前後加速度制約Ａｘ［ａｙ］が小さな値に設定される。
例えば、パラメータａｐを駆動力配分比に応じて設定してよい。図８Ｂは、パラメータａｐの設定例の説明図である。パラメータａｐは、前輪への駆動力配分比Ｒｄｆに応じて設定される。 Furthermore, for example, the friction circle constraint setting unit 46 may set a longitudinal acceleration constraint Ax[ay] based on the following equation (18) for each value of the lateral acceleration ay.

In the above equation (18), ap is a parameter for adjusting the upper limit of the longitudinal acceleration constraint Ax[ay] during deceleration (ax>0).
For example, as shown in FIG. 8A, by setting the parameter ap to a value smaller than μg, the longitudinal acceleration constraint Ax[ay] during acceleration is set to a small value.
For example, the parameter ap may be set according to the driving force distribution ratio. Fig. 8B is an explanatory diagram of a setting example of the parameter ap. The parameter ap is set according to the driving force distribution ratio Rdf to the front wheels.

For example, the parameter ap may be set to be smaller as the difference between the driving force distribution to the front wheels and the driving force distribution to the rear wheels increases. This allows the upper limit value Ax [ay] of the acceleration of the host vehicle 1 to be set to a smaller value as the difference between the driving force distributions to the front and rear wheels increases.
Therefore, as the difference between the front and rear driving force distribution increases, a longitudinal acceleration constraint Ax [ay] can be set that limits the longitudinal acceleration ax to a smaller value so as not to exceed this value, even if the longitudinal force that can be generated at the wheels during acceleration (ax>0) becomes smaller.

In addition, in the variant (2), as in the variant (1), the sum of the q-th power term of the lateral force of the wheel and the q-th power term of the longitudinal force may be limited to an upper limit value corresponding to the q-th power term of the load on the wheel (1≦q<2).
For example, the friction circle constraint setting unit 46 may set a longitudinal acceleration constraint Ax[ay] based on the following equation (19) for each value of the lateral acceleration ay.

(Effects of the embodiment)
(1) The controller 15 sets a target driving trajectory for the host vehicle 1, sets a first constraint condition, which is a constraint condition for the longitudinal force that limits the host vehicle 1 in accordance with the loads on the front and rear wheels of the host vehicle 1 and the lateral force of the host vehicle 1, based on the friction limit of the wheels, and generates a target speed profile for driving the target driving trajectory in accordance with the first constraint condition, based on a curvature profile of the target driving trajectory. The actuator 16 controls the vehicle speed of the host vehicle 1 based on the target speed profile.
This makes it possible to set a target vehicle speed profile that prevents the load on the front and rear wheels of the vehicle from exceeding the wheel friction limit when the vehicle is driven along a target driving trajectory.

(2) The first constraint condition may be set according to the braking force distribution or the driving force distribution to the front wheels and the rear wheels.
For example, when the difference between the braking force distribution ratio, which is the ratio of braking force distribution to the front wheels and braking force distribution to the rear wheels, and the ideal braking force distribution is large compared to when the difference is small, the upper limit value of the front and rear forces that decelerate the vehicle 1 may be set to a small value.
Also, for example, when the difference between the driving force distribution to the front wheels and the driving force distribution to the rear wheels is large compared to when the difference is small, the upper limit value of the longitudinal force for accelerating the vehicle 1 may be set to a small value.
This makes it possible to set a target vehicle speed profile that generates a longitudinal acceleration ax that does not exceed the friction limit of the wheels, even if the braking force distribution or driving force distribution becomes biased towards either the front wheels or the rear wheels, and the value of one of the front wheel longitudinal force Fxf and the rear wheel longitudinal force Fxr becomes large.

(3) The first constraint condition may be a condition that limits the sum of the square term of the lateral force and the square term of the longitudinal force to be equal to or less than an upper limit value corresponding to the square term of the load on the wheel.
This makes it possible to set a target vehicle speed profile that generates a longitudinal acceleration ax such that the resultant force of the lateral force and longitudinal force of the wheel does not exceed the grip force of the wheel.
(4) The first constraint condition may be a condition that limits the sum of a power term of the lateral force, whose exponent is a value greater than or equal to 1 and less than 2, and a power term of the longitudinal force, whose exponent is a value, to less than an upper limit value corresponding to the power term, whose exponent is the value of the load on the wheel.
This makes it possible to generate a target vehicle speed profile with a safety margin in a scene where braking and turning are performed simultaneously, or in a scene where driving and turning are performed simultaneously.
(5) The controller 15 may set, based on the friction limits of the front and rear wheels, a second constraint condition which is a constraint condition on the longitudinal force of the front wheels of the vehicle 1 and a third constraint condition which is a constraint condition on the longitudinal force of the rear wheels, the second constraint condition being limited according to the loads on the front and rear wheels of the vehicle 1 and the lateral force of the vehicle 1. The controller 15 may set, as the first constraint condition, the second constraint condition or the third constraint condition which limits the magnitude of the longitudinal force to a smaller value.
This allows the front and rear wheel friction limits to be estimated independently.

(6) When the load on the front wheels is heavy and the load on the rear wheels is light, the upper limit value of the magnitude of the longitudinal force limited by the second constraint condition may be set to a larger value, and the upper limit value of the magnitude of the longitudinal force limited by the third constraint condition may be set to a smaller value, compared to when the load on the front wheels is light and the load on the rear wheels is heavy.
For example, when the acceleration of the host vehicle 1 is large, the load on the front wheels may be set to be smaller than when the acceleration is small.
For example, when the deceleration of the host vehicle 1 is large, the load on the rear wheels may be set to be larger than when the deceleration is small.
This makes it possible to set a target vehicle speed profile that prevents the load on the front and rear wheels of the vehicle from exceeding the wheel friction limit.

1...Vehicle, 10...Drive control device, 11...Object sensor, 12...Vehicle sensor, 13...Positioning device, 14...Map database, 15...Controller, 16...Actuator, 17...Steering device, 18...Drive device, 19...Braking device, 20...Processor, 21...Storage device, 40...Trajectory generation unit, 41...Friction coefficient estimation unit, 42...Front wheel friction circle constraint setting unit, 43...Rear wheel friction circle constraint setting unit, 44...Friction circle constraint correction unit, 45...Speed profile setting unit, 46...Friction circle constraint setting unit

Claims (6)

Set a target driving trajectory for the vehicle;
a curvature profile of the target driving trajectory is ρ, the vehicle speed of the host vehicle is v, the lateral acceleration generated in the host vehicle is ay, the friction coefficient of the road surface is μ, the mass of the host vehicle is m, the wheelbase length is l, the lengths from the center of gravity of the host vehicle to the front and rear axles are lf and lr, respectively, the loads applied to the front and rear wheels are Fzf and Fzr, respectively, the lateral forces of the front and rear wheels are Fyf and Fyr, respectively, and the longitudinal forces of the front and rear wheels are Fxf and Fxr, respectively. A profile consisting only of vehicle speeds that satisfy the constraint conditions defined by the following equations (1) to (5) is generated as a target vehicle speed profile;
controlling a vehicle speed of the host vehicle based on the target vehicle speed profile;
A vehicle speed control method comprising:

Set a target driving trajectory for the vehicle;
a curvature profile of the target driving trajectory, v the vehicle speed, ay the lateral acceleration generated in the vehicle, μ the friction coefficient of the road surface, m the mass of the vehicle, l the wheelbase length, lf and lr the lengths from the center of gravity of the vehicle to the front and rear axles, respectively, Fzf and Fzr the loads applied to the front and rear wheels, respectively, Fyf and Fyr the lateral forces of the front and rear wheels, respectively, Fxf and Fxr the longitudinal forces of the front and rear wheels, respectively, and q an index set to a value greater than or equal to 1 and less than 2, generating a profile consisting only of vehicle speeds that satisfy the constraint conditions defined by the following equations (6) to (10) as a target vehicle speed profile;
controlling a vehicle speed of the host vehicle based on the target vehicle speed profile;
A vehicle speed control method comprising:

3. The vehicle speed control method according to claim 1, wherein the longitudinal forces Fxf and Fxr of the front and rear wheels of the vehicle are calculated based on the following equations (11) and (12), where R is the braking force distribution or driving force distribution to the front and rear wheels and ax is the longitudinal acceleration.

The vehicle speed control method according to any one of claims 1 to 3, characterized in that the loads Fzf and Fzr applied to the front and rear wheels are calculated based on the following equations (13) and (14), where ax is the longitudinal acceleration, g is the gravitational acceleration, and h is the height of the vehicle.

a controller which sets a target driving trajectory of the host vehicle, defines a curvature profile of the target driving trajectory as ρ, a vehicle speed of the host vehicle as v, a lateral acceleration generated in the host vehicle as ay, a friction coefficient of a road surface as μ, a mass of the host vehicle as m, a wheelbase length as l, lengths from the center of gravity of the host vehicle to the front and rear axles as lf and lr, respectively, loads acting on the front and rear wheels as Fzf and Fzr, respectively, lateral forces of the front and rear wheels as Fyf and Fyr, respectively, and longitudinal forces of the front and rear wheels as Fxf and Fxr, respectively, and generates a target vehicle speed profile consisting of only vehicle speeds which satisfy constraint conditions defined by the following equations (15) to (19);
a steering mechanism for controlling a steering angle of the host vehicle based on the target driving trajectory;
a drive device that controls a drive force of the host vehicle based on the target vehicle speed profile;
a braking device that controls a braking force of the host vehicle based on the target vehicle speed profile;
A driving control device comprising:

a controller which sets a target driving trajectory of a host vehicle, defines a curvature profile of the target driving trajectory as ρ, a vehicle speed of the host vehicle as v, a lateral acceleration generated in the host vehicle as ay, a friction coefficient of a road surface as μ, a mass of the host vehicle as m, a wheelbase length as l, lengths from the center of gravity of the host vehicle to the front and rear axles as lf and lr, respectively, loads acting on the front and rear wheels as Fzf and Fzr, respectively, lateral forces of the front and rear wheels as Fyf and Fyr, respectively, longitudinal forces of the front and rear wheels as Fxf and Fxr, respectively, and an index q set to a value greater than or equal to 1 and less than 2, and generates, as a target vehicle speed profile, a profile consisting only of vehicle speeds which satisfy constraint conditions defined by the following equations (20) to (24).
a steering mechanism for controlling a steering angle of the host vehicle based on the target driving trajectory;
a drive device that controls a drive force of the host vehicle based on the target vehicle speed profile;
a braking device that controls a braking force of the host vehicle based on the target vehicle speed profile;
A driving control device comprising:

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