Vehicle travel assist device

A vehicle travel assist device installed on a vehicle includes: a vehicle travel control device that controls travel of the vehicle; and a travel state acquisition device that acquires travel state information indicating a travel state of the vehicle. The vehicle travel control device: detects that a first wheel of the vehicle climbs over a difference-in-level, based on the travel state information; acquires a first driving force required for the first wheel to climb over the difference-in-level or a change in the travel state when the first wheel passes the difference-in-level, as reference information, based on the travel state information; estimates a second driving force required for a second wheel of the vehicle to climb over the difference-in-level, based on the reference information; and generates the estimated second driving force when the second wheel passes the difference-in-level after the first wheel.

BACKGROUND

Technical Field

The present disclosure relates to a vehicle travel assist device that assists vehicle travel. In particular, the present disclosure relates to a vehicle travel assist device that controls a driving force when a vehicle passes a difference-in-level.

Background Art

A technique that assists vehicle travel by automatically controlling the vehicle travel is known. Such the technique is exemplified by parking assist that automatically parks a vehicle at a target position.

Patent Literature 1 discloses a method that controls a braking force and a driving force of a vehicle to park the vehicle at a target position. When a wheel comes into contact with a difference-in-level and stops, the driving force is increased in order to climb over the difference-in-level. More specifically, the driving force is increased so as to compensate for decrease in a vehicle speed caused by the wheel coming into contact with the difference-in-level. Then, it is determined whether or not the wheel coming into contact with the difference-in-level leaves the ground. When it is determined that the wheel coming into contact with the difference-in-level leaves the ground, the driving force is gradually decreased.

LIST OF RELATED ART

Patent Literature 1: Japanese Unexamined Patent Application Publication No. JP-2013-049389

SUMMARY

According to the technique disclosed in Patent Literature 1, when the wheel comes into contact with the difference-in-level and stops, the driving force is increased. At this time, a suitable driving force required for the wheel to climb over the difference-in-level is not known. Therefore, the driving force may be gradually increased in order to prevent abrupt acceleration of the vehicle. In that case, however, a stop time until the wheel starts to move becomes longer. In other words, a time during which the vehicle does not move despite the increase in the driving force becomes longer. As seen from the above, when the driving force required for the wheel to climb over the difference-in-level is not known, control of the driving force becomes inefficient.

An object of the present disclosure is to provide a technique that can efficiently control a driving force when a wheel passes a difference-in-level during vehicle travel control.

A first aspect provides a vehicle travel assist device installed on a vehicle. The vehicle travel assist device includes:

a vehicle travel control device configured to control travel of the vehicle; and

a travel state acquisition device configured to acquire travel state information indicating a travel state of the vehicle.

The vehicle travel control device is further configured to:

detect that a first wheel of the vehicle climbs over a difference-in-level, based on the travel state information;

acquire a first driving force required for the first wheel to climb over the difference-in-level or a change in the travel state when the first wheel passes the difference-in-level, as reference information, based on the travel state information;

estimate a second driving force required for a second wheel of the vehicle to climb over the difference-in-level, based on the reference information; and

generate the estimated second driving force when the second wheel passes the difference-in-level after the first wheel.

A second aspect further has the following feature in addition to the first aspect.

The vehicle travel control device estimates the second driving force before the second wheel reaches the difference-in-level.

A third aspect further has the following feature in addition to the first or second aspect.

When the first wheel reaches and stops at the difference-in-level, the vehicle travel control device increases a driving force at a first increase rate.

When the second wheel reaches and stops at the difference-in-level, the vehicle travel control device increases the driving force to the second driving force at a second increase rate higher than the first increase rate.

A fourth aspect further has the following feature in addition to any one of the first to third aspects.

The vehicle travel assist device further includes a difference-in-level position estimation device configured to acquire difference-in-level position information indicating a position of the difference-in-level.

The vehicle travel control device estimates the second wheel that reaches the difference-in-level after the first wheel, based on the difference-in-level position information and the travel state information.

A fifth aspect further has the following feature in addition to the fourth aspect.

The travel state information includes vehicle position information indicating a position of each wheel of the vehicle.

The difference-in-level position estimation device is further configured to:

detect that two wheels of the vehicle pass the difference-in-level, based on the travel state information;

acquire respective positions of the two wheels when passing the difference-in-level as a first passing position and a second passing position, based on the vehicle position information; and

assume a position of a line connecting the first passing position and the second passing position as the position of the difference-in-level.

A sixth aspect further has the following feature in addition to the fourth or fifth aspect.

A first load is a load applied to the first wheel concurrently passing the difference-in-level.

A second load is a load applied to the second wheel concurrently passing the difference-in-level.

The vehicle travel control device is further configured to:

estimate the second wheel and the second load based on the difference-in-level position information and the travel state information; and

estimate the second driving force based on the reference information, the first load, and the second load.

A seventh aspect further has the following feature in addition to the sixth aspect.

The travel state information includes travel control information indicating a driving force controlled by the vehicle travel control device.

When the first wheel reaches and stops at the difference-in-level, the vehicle travel control device is further configured to:

increase the driving force;

acquire the first driving force required for the first wheel to climb over the difference-in-level, as the reference information, based on the travel control information; and

estimate the second driving force based on the first driving force and a ratio of the first load and the second load.

An eighth aspect further has the following feature in addition to the sixth aspect.

The travel state information includes vehicle state information indicating a speed of the vehicle.

When the first wheel passes the difference-in-level without stopping, the vehicle travel control device is further configured to:

acquire a change in the speed when the first wheel passes the difference-in-level, based on the vehicle state information;

estimate a height of the difference-in-level based on the first load and the change in the speed; and

estimate the second driving force based on the height of the difference-in-level and the second load.

According to the present disclosure, information when the first wheel climbs over the difference-in-level is acquired as the reference information. Then, the second driving force required for the second wheel to climb over the difference-in-level is estimated from the reference information. Since the required second driving force is estimated, it is possible to efficiently control the driving force when the second wheel passes the difference-in-level after the first wheel.

EMBODIMENTS

Embodiments of the present disclosure will be described below with reference to the attached drawings.

1. Outline of Vehicle Travel Assist Device

FIG. 1is a conceptual diagram for explaining an outline of a vehicle travel assist device10installed on a vehicle1according to the present embodiment. The vehicle1is provided with a plurality of wheels5. More specifically, the vehicle1is provided with a left front wheel5FL, a right front wheel5FR, a left rear wheel5RL, and a right rear wheel5RR. In the following description, the left front wheel5FL and the right front wheel5FR may be collectively referred to as a “front wheel”, and the left rear wheel5RL and the right rear wheel5RR may be collectively referred to as a “rear wheel”.

As shown inFIG. 1, the vehicle travel assist device10includes a travel state acquisition device100and a vehicle travel control device200. The travel state acquisition device100acquires travel state information300indicating a travel state of the vehicle1. The travel state of the vehicle1is exemplified by a position, a speed (a vehicle speed), an acceleration, a steering angle, a driving force, a braking force, a surrounding situation, and the like. The vehicle travel control device200executes “vehicle travel control” that controls travel of the vehicle1, based on the travel state information300. The vehicle travel control includes driving force control, braking force control, and steering control.

The vehicle travel assist device10assists (supports) the travel of the vehicle1through the vehicle travel control. For example, the vehicle travel assist device10executes vehicle guidance control that automatically moves the vehicle1and stops it at a target stop position. Such the vehicle guidance control is utilized, for example, when parking the vehicle1at a desired parking position. It should be noted that the vehicle travel control according to the present embodiment is not limited to the vehicle guidance control.

A difference-in-level may exist on a travel path of the vehicle1during the vehicle travel control. In this case, the vehicle travel control device200executes the vehicle travel control (especially, the driving force control) such that the wheel5appropriately passes the difference-in-level.

Here, “the wheel5passing the difference-in-level” means that the wheel5reaches (i.e. comes into contact with) the difference-in-level and further climbs over the difference-in-level. A “passing period” in which the wheel5passes the difference-in-level is a period from when the wheel5reaches (i.e. comes into contact with) the difference-in-level to when the wheel5climbs over the difference-in-level. Two wheels5may concurrently pass the difference-in-level. That is, a set of the left front wheel5FL and the right front wheel5FR or a set of the left rear wheel5RL and the right rear wheel5RR may concurrently pass the difference-in-level. “Two wheels5concurrently passing the difference-in-level” means that respective passing periods of the two wheels5at least partially overlap with each other.

It should be noted in the present embodiment that a shape of the difference-in-level is not limited in particular. For example, the shape of the difference-in-level includes a step shape, a slope shape, and a bump shape.

FIG. 2is a conceptual diagram for explaining climbing over a difference-in-level DL. As the vehicle1moves, the plurality of wheels5reach the difference-in-level DL in series. A wheel5that reaches the difference-in-level DL relatively early (i.e. a preceding wheel) is hereinafter referred to as a “first wheel5-1”. A wheel5that reaches the difference-in-level DL relatively late (i.e. a subsequent wheel) is hereinafter referred to as a “second wheel5-2”.

As an example, let us consider a case where the right rear wheel5RR and the left rear wheel5RL concurrently pass the difference-in-level DL and thereafter the right front wheel5FR and reaches the difference-in-level DL. In this case, the right rear wheel5RR and the left rear wheel5RL each is the first wheel5-1, and the right front wheel5FR is the second wheel5-2.

As another example, let us consider a case where the right rear wheel5RR, the left rear wheel5RL, and the right front wheel5FR pass the difference-in-level DL in this order. In this case, the right rear wheel5RR is the first wheel5-1, and the right front wheel5FR is the second wheel5-2. The left rear wheel5RL is the second wheel5-2with respect to the right rear wheel5RR (the first wheel5-1), and is the first wheel5-1with respect to the right front wheel5FR (the second wheel5-2).

The first wheel5-1reaches the difference-in-level DL earlier than the second wheel5-2. That is, after the first wheel5-1climbs over the difference-in-level DL, the second wheel5-2reaches the difference-in-level DL. According to the present embodiment, information regarding the driving force when the first wheel5-1climbs over the difference-in-level DL is retained as “reference information”. Then, the reference information is utilized for the driving force control when the second wheel5-2passes the difference-in-level DL. It is thus possible to efficiently control the driving force when the second wheel5-2passes the difference-in-level DL.

FIG. 3is a timing chart showing an example of the driving force control according to the present embodiment. A horizontal axis represents time, and a vertical axis represents the driving force.

At a time t1a, the first wheel5-1reaches and stops at the difference-in-level DL. The vehicle travel control device200increases the driving force. Here, the driving force may be gradually increased in order to suppress abrupt acceleration of the vehicle1. At a time t1b, the first wheel5-1starts to move and climb up the difference-in-level DL. At a time t1c, the first wheel5-1climbs over the difference-in-level DL. A period from the time t1ato the time t1cis a first passing period T1required for the first wheel5-1to pass the difference-in-level DL. After the first wheel5-1passes the difference-in-level DL, the driving force may be decreased in order to suppress unnecessary acceleration.

The driving force that is actually required for the first wheel5-1to climb over the difference-in-level DL is hereinafter referred to as a “first driving force F1”. In the example shown inFIG. 3, the first driving force F1is used as the “reference information”.

A driving force required for the second wheel5-2to climb over the difference-in-level DL is hereinafter referred to as a “second driving force F2”. After the first wheel5-1climbs over the difference-in-level DL, the vehicle travel control device200estimates the second driving force F2from the first driving force F1(the reference information). When a ratio of loads respectively applied to the first wheel5-1and the second wheel5-2is known, the second driving force F2can be estimated based on the ratio and the first driving force F1. Then, when the second wheel5-2passes the difference-in-level, the vehicle travel control device200generates the estimated second driving force F2.

In the example shown inFIG. 3, at a time t2a, the second wheel5-2reaches and stops at the difference-in-level DL. The vehicle travel control device200increases the driving force. At this time, there is no need to gradually increase the driving force in order to suppress abrupt acceleration of the vehicle1. Since the second driving force F2required for the second wheel5-2to climb over the difference-in-level DL is already estimated, it is possible to “quickly” increase the driving force to the second driving force F2. In other words, it is possible to set a second increase rate when increasing the driving force to the second driving force F2to be higher than a first increase rate when increasing the driving force to the first driving force F1. The vehicle travel control device200increases the driving force to the first driving force F1at the first increase rate, and increases the driving force to the second driving force F2at the second increase rate higher than the first increase rate.

At a time t2b, the second wheel5-2starts to move and climb up the difference-in-level DL. At a time t2c, the second wheel5-2climbs over the difference-in-level DL. A period from the time t2ato the time t2cis a second passing period T2required for the second wheel5-2to pass the difference-in-level DL. Since the driving force can be quickly increased to the second driving force F2, the second passing period T2is shorter than the first passing period T1.

FIG. 4shows a modification example of the driving force control shown inFIG. 3. A timing to increase the driving force is not limited to at or after the timing when the second wheel5-2reaches the difference-in-level DL. In the example shown inFIG. 4, the vehicle travel control device200increases the driving force from a time t2pbefore the second wheel5-2reaches the difference-in-level DL. The driving force control as shown inFIG. 4also is possible if the timing when the second wheel5-2reaches the difference-in-level DL can be predicted, or approach of the second wheel5-2to the vicinity of the difference-in-level DL can be detected.

FIG. 5shows still another example of the driving force control according to the present embodiment. The first wheel5-1does not necessarily stop at the difference-in-level DL. In the example shown inFIG. 5, the first wheel5-1passes the difference-in-level DL without stopping. In this case, the second driving force F2can be estimated by a method different from the ones described inFIGS. 3 and 4.

More specifically, the travel state such as the vehicle speed changes when the first wheel5-1passes the difference-in-level DL. The larger the difference-in-level DL is, the larger the change in the travel state is. That is to say, the change in the travel state when the first wheel5-1passes the difference-in-level DL reflects a height (size) of the difference-in-level DL. Therefore, the vehicle travel control device200can estimate the second driving force F2required for the second wheel5-2to climb over the difference-in-level DL, based on the change in the travel state. That is, in the example shown inFIG. 5, the change in the travel state is used as the “reference information”.

A situation where the second wheel5-2passes the difference-in-level DL is similar to that in the example shown inFIG. 3. At the time t2a, the second wheel5-2reaches and stops at the difference-in-level DL. The vehicle travel control device200increases the driving force. Since the second driving force F2required for the second wheel5-2to climb over the difference-in-level DL is already estimated, it is possible to “quickly” increase the driving force to the second driving force F2.

FIG. 6shows a modification example of the driving force control shown inFIG. 5. As in the case of the foregoingFIG. 4, the vehicle travel control device200increases the driving force from the time t2pbefore the second wheel5-2reaches the difference-in-level DL.

The processing by the vehicle travel control device200described above can be summarized as follows. First, it is necessary to detect that the first wheel5-1climbs over the difference-in-level DL. Based on the travel state information300, the vehicle travel control device200can detect that the first wheel5-1climbs over the difference-in-level DL.

Subsequently, the vehicle travel control device200acquires the first driving force F1required for the first wheel5-1to climb over the difference-in-level DL, as the reference information. Alternatively, the vehicle travel control device200acquires the change in the travel state when the first wheel5-1passes the difference-in-level DL, as the reference information. In either case, the vehicle travel control device200can acquire the reference information based on the travel state information300.

Subsequently, the vehicle travel control device200estimates the second driving force F2required for the second wheel5-2to climb over the difference-in-level DL, based on the reference information. Preferably, the vehicle travel control device200estimates the second driving force F2before the second wheel5-2reaches the difference-in-level DL. Then, when the second wheel5-2passes the difference-in-level DL, the vehicle travel control device200generates the estimated second driving force F2.

As described above, according to the present embodiment, information when the first wheel5-1climbs over the difference-in-level DL is acquired as the reference information. Then, the second driving force F2required for the second wheel5-2to climb over the difference-in-level DL is estimated from the reference information. Since the required second driving force F2is estimated, it is possible to efficiently control the driving force when the second wheel5-2passes the difference-in-level DL.

As a comparative example, let us consider a case where the second driving force F2is not estimated. According to the comparative example, when the driving force is increased so that the second wheel5-2climbs over the difference-in-level DL, the driving force may become unnecessarily large. In that case, the second wheel5-2dashes up the difference-in-level DL and the vehicle1accelerates abruptly. By contrast, according to the present embodiment, the necessary second driving force F2is estimated and the vehicle1is driven by the estimated second driving force F2. It is thus possible to suppress the abrupt acceleration of the vehicle1.

As another comparative example, the driving force may be gradually increased in order to suppress the abrupt acceleration of the vehicle1. In that case, however, a stop time until the second wheel5-2starts to move becomes longer. In other words, a time during which the vehicle1does not move despite the increase in the driving force becomes longer. This causes increase in load applied to a driving device, reduction in fuel economy, increase in noise, and the like. By contrast, according to the present embodiment, the required second driving force F2is estimated, and it is thus possible to quickly increase the driving force to the second driving force F2. This contributes to reduction in load applied to the driving device, increase in fuel economy, reduction in noise, and the like.

Hereinafter, the vehicle travel assist device10according to the present embodiment will be described in more detail.

2. Concrete Example of Vehicle Travel Assist Device

FIG. 7is a block diagram showing a configuration example of the vehicle travel assist device10according to the present embodiment. The vehicle travel assist device10includes a sensor group30, a travel device50, and a control device70.

The sensor group30includes a vehicle state sensor31and a surrounding situation sensor32.

The vehicle state sensor31detects a state of the vehicle1. The state of the vehicle1is exemplified by a wheel speed, a vehicle speed, an acceleration (a longitudinal acceleration, a lateral acceleration, and a vertical acceleration), a steering angle, a suspension stroke amount, and the like. The vehicle state sensor31includes a wheel speed sensor, a vehicle speed sensor, a variety of acceleration sensors, a steering angle sensor, a stroke sensor, and the like. The vertical acceleration sensor and the stroke sensor are provided at a position of each wheel5, for example. The vehicle state sensor31may further include a GPS (Global Positioning System) device that measures a position and an orientation of the vehicle1.

The surrounding situation sensor32detects a situation around the vehicle1. For example, the surrounding situation sensor32includes a camera, a sonar, a LIDAR (Laser Imaging Detection and Ranging), and the like. Using the surrounding situation sensor32makes it possible to perceive (recognize) space and objects around the vehicle1.

The travel device50includes a driving device51, a braking device52, a turning device53, and a transmission device54. The driving device51is a power source that generates the driving force. The driving device51is exemplified by an engine, an electric motor, and an in-wheel motor. The braking device52generates a braking force. The turning device53turns the wheel5. For example, the turning device53includes a power steering device (e.g., EPS: Electric Power Steering).

The control device (i.e. a controller)70is a microcomputer provided with a processor71and a memory device72. The control device70is also called an ECU (Electronic Control Unit). A control program is stored in the memory device72. A variety of processing by the control device70is achieved by the processor71executing the control program stored in the memory device72.

For example, the control device70(the processor71) executes the vehicle travel control by appropriately controlling an operation of the travel device50. The vehicle travel control includes driving force control, braking force control, steering control, and gear control. The driving force control is performed through the driving device51. The braking force control is performed through the braking device52. The steering control is performed through the turning device53. The gear control is performed through the transmission device54. It can be said that the control device70and the travel device50constitute the “vehicle travel control device200” shown inFIG. 1.

Moreover, the control device70(the processor71) executes a variety of information processing. More specifically, the control device70(the processor71) acquires a variety information and stores the acquired information in the memory device72. The control device70reads necessary information from the memory device72to execute a variety of information processing.

FIG. 8is a block diagram showing an example of a variety of information used in the present embodiment. The travel state information300, difference-in-level position information400, and driving control information500are stored in the memory device72. The travel state information300indicates the travel state of the vehicle1. The difference-in-level position information400indicates a position of the difference-in-level DL on a travel path of the vehicle1. The driving control information500is information used in the driving force control relating to the difference-in-level passing. Hereinafter, acquisition and utilization of the variety of information will be described in detail.

The control device70acquires the travel state information300indicating the travel state of the vehicle1. As shown inFIG. 8, the travel state information300includes vehicle state information310, surrounding situation information320, travel control information330, target information340, and vehicle position information350.

The vehicle state information310indicates the state of the vehicle1. The control device70acquires the vehicle state information310based on a result of detection by the vehicle state sensor31. The state of the vehicle1is exemplified by the wheel speed, the vehicle speed, the acceleration (the longitudinal acceleration, the lateral acceleration, and the vertical acceleration), the steering angle, the suspension stroke amount, and the like. As to the vertical acceleration and the suspension stroke amount, their values at the position of each wheel5are calculated.

When the vehicle state sensor31includes the GPS device, the vehicle state information310may include position information regarding the vehicle1that is acquired by the GPS device.

The surrounding situation information320indicates the situation around the vehicle1. The control device70acquires the surrounding situation information320based on a result of detection by the surrounding situation sensor32. For example, the surrounding situation information320includes image information obtained by the camera. Moreover, the surrounding situation information320includes object information regarding a surrounding object (e.g. a wall) measured by the sonar and the LIDAR. The object information indicates a relative position of the surrounding object (i.e. a distance to the surrounding object). The object information may further indicate a relative velocity.

The difference-in-level DL near the vehicle1may be detected by the surrounding situation sensor32. In that case, the surrounding situation information320may include information indicating a relative position of the detected difference-in-level DL.

The travel control information330indicates a control amount of the travel device50controlled by the control device70(i.e. the vehicle travel control device200). For example, the travel control information330indicates the driving force and the braking force that are controlled by the control device70.

FIG. 9is a conceptual diagram for explaining the target information340. The control device70(i.e. the vehicle travel control device200) can execute the vehicle guidance control that moves the vehicle1and stops it at a target stop position PT. For example, the vehicle guidance control is utilized when parking the vehicle1at a desired parking position. When such the vehicle guidance control is executed, the target information340indicating the target stop position PT is created.

The target stop position PT is beforehand set manually or by the control device70. For example, the control device70automatically determines an appropriate target stop position PT, based on the above-described surrounding situation information320. Alternatively, the control device70displays information indicating space and objects around the vehicle1on an HMI (Human Machine Interface), based on the surrounding situation information320. Then, a user of the vehicle1refers to the displayed information to designate a desired target stop position PT.

After the target stop position PT is set, the control device70may generate a target path TP from a current position of the vehicle1to the target stop position PT. The target path TP is defined, for example, in a coordinate system whose origin is at the target stop position PT. When the target path TP is generated, the target information340indicates the target stop position PT and the target path TP. The control device70(i.e. the vehicle travel control device200) executes the vehicle travel control such that the vehicle1travels along the target path TP.

FIG. 10is a conceptual diagram for explaining the vehicle position information350. The vehicle position information350indicates a position of the vehicle1and a position of each wheel5. The positions of the vehicle1and each wheel5are defined in a predetermined coordinate system. For example, a coordinate system whose origin O is at the above-described target stop position PT is used as the predetermined coordinate system. However, the predetermined coordinate system is not limited to that.

InFIG. 10, a vehicle position PV[x, z, θ] represents the position of the vehicle1. For example, an intermediate position between the left rear wheel SRL and the right rear wheel5RR is used as the vehicle position PV. Wheel positions Pfl, Pfr, Prl, and Prr represent respective positions of the left front wheel5FL, the right front wheel5FR, the left rear wheel5RL, and the right rear wheel5RR. A wheelbase Lh and a track width Tr are known parameters. Using the vehicle position PV and the known parameters, the wheel positions Pfl, Pfr, Prl, and Prr are expressed by the following Equations (1) to (4), respectively.

The control device70calculates and updates the vehicle position PV and the wheel positions Pfl, Pfr, Prl, and Prr based on the vehicle state information310. More specifically, the vehicle state information310includes the steering angle and the wheel speed. Based on the steering angle and the wheel speed, the control device70can calculate an amount of movement of the vehicle1to sequentially calculate and update the vehicle position PV. When the vehicle position PV is updated, the wheel positions Pfl, Pfr, Prl, and Prr also are updated in accordance with the above Equations (1) to (4).

As another example, when the vehicle state information310includes the position information regarding the vehicle1that is acquired by the GPS device, the control device70may utilize the position information. As still another example, the control device70may calculate and update the vehicle position PV based on the relative position with respect to the surrounding object (e.g. a wall) indicated by the surrounding situation information320.

3-6. Travel State Acquisition Device100

As shown inFIG. 7, it can be said that the sensor group30and the control device70constitute the “travel state acquisition device100”.

The difference-in-level position information400indicates a position of the difference-in-level DL on the travel path of the vehicle1. Such the difference-in-level position information400is useful for the vehicle travel control. Hereinafter, an example of a method of acquiring the difference-in-level position information400will be described.

4-1. First Example

Based on the travel state information300, the control device70detects that any wheel5passes the difference-in-level DL and identifies the wheel5that passes the difference-in-level DL.

FIG. 11is a conceptual diagram for explaining an example of a method of detecting the difference-in-level passing. A horizontal axis represents time, and a vertical axis represents the vertical acceleration at the position of each wheel5. The vertical acceleration is obtained from the vehicle state information310. When the vertical acceleration at a position of a certain wheel5exceeds a determination threshold Gth, the control device70determines that said certain wheel5passes the difference-in-level DL. In this manner, it possible by referring to the vertical acceleration to detect that any wheel5passes the difference-in-level DL and identify the wheel5that passes the difference-in-level DL. The stroke amount may be taken into consideration instead of or together with the vertical acceleration.

As another example, the control device70may detect the difference-in-level passing based on changes in the vehicle speed and the longitudinal acceleration indicated by the vehicle state information310. As still another example, the control device70may detect the difference-in-level passing and identify the wheel5that passes the difference-in-level DL, based on a change in the camera's field of vision indicated by the surrounding situation information320.

It should be noted that there are not only cases where the wheel5climbs up the difference-in-level DL but also cases where the wheel5goes down the difference-in-level DL. Climbing up and going down are distinguishable based on the change in the vehicle speed or the change in the camera's field of vision.

After identifying the wheel5that passes the difference-in-level DL, the control device70acquires the wheel position of the wheel5when passing the difference-in-level DL, based on the vehicle position information350. The wheel position of the wheel5when passing the difference-in-level DL is hereinafter referred to as a “passing position”. For example, the wheel position at a timing when the wheel5comes into contact with the difference-in-level DL is used as the passing position. As another example, an average of the wheel positions during the passing period may be used as the passing position.

The control device70acquires the passing positions regarding at least two different wheels5by the above-described method. The two wheels5may pass the difference-in-level DL at different timings or may concurrently pass the difference-in-level DL.

FIG. 12shows respective passing positions Pa and Pb of two wheels5aand5b. A first passing position Pa[xa, za] is the passing position of the wheel5a. A second passing position Pb[xb, zb] is the passing position of the wheel5b. The first passing position Pa and the second passing position Pb are different from each other. Information of the first passing position Pa and the second passing position Pb is stored in the memory device72. The control device70assumes a position of a line connecting the first passing position Pa and the second passing position Pb as the position of the difference-in-level DL. The position of the difference-in-level DL in the above-mentioned predetermined coordinate system is expressed by the following Equation (5).

As described above, according to the first example, the control device70detects that the two wheels5aand5bpass the difference-in-level DL and then estimates the position of the difference-in-level DL based on the first passing position Pa and the second passing position Pb. Using the passing position at which the wheel5actually passes the difference-in-level DL makes it possible to estimate the position of the difference-in-level DL with high accuracy.

4-2. Second Example

It is also possible that the surrounding situation sensor32(e.g. the LIDAR) detects the difference-in-level DL. In that case, the surrounding situation information320includes relative position information of the detected difference-in-level DL. According to the second example, the control device70calculates the position of the difference-in-level DL in the predetermined coordinate system based on the relative position information of the difference-in-level DL included in the surrounding situation information320. In the case of the second example, it is possible to acquire the position of the difference-in-level DL even before the two wheels5aand5bpass the difference-in-level DL.

It can be said that the control device70serves as a “difference-in-level position estimation device”. Based on the travel state information300, the difference-in-level position estimation device estimates the position of the difference-in-level DL on the travel path of the vehicle1to acquire the difference-in-level position information400.

More specifically, in the case of the first example, the difference-in-level position estimation device detects that the two wheels5aand5bpass the difference-in-level DL, based on the travel state information300. Then, the difference-in-level position estimation device acquires respective wheel positions of the two wheels5aand5bwhen passing the difference-in-level DL as the first passing position Pa and the second passing position Pb, based on the vehicle position information350. Then, the difference-in-level position estimation device assumes the position of the line connecting the first passing position Pa and the second passing position Pb as the position of the difference-in-level DL.

In the case of the second example, the difference-in-level position estimation device estimates the position of the difference-in-level DL based on the surrounding situation information320.

5. Driving Force Control Relating to Difference-in-Level Passing

Next, let us describe the driving force control relating to the difference-in-level passing. In the driving force control, the driving control information500shown inFIG. 8is acquired and utilized. The driving control information500includes reference information510and second driving force information520.

5-1. First Example

FIG. 13is a flow chart showing a first example of the driving force control relating to the difference-in-level passing. In the first example, let us consider the case described in the foregoingFIGS. 3 and 4where the first wheel5-1reaches and stops at the difference-in-level DL.

Based on the travel state information300, the vehicle travel control device200detects that any wheel5stops due to the difference-in-level DL. For example, when the vehicle1does not move despite generation of the driving force, the vehicle travel control device200determines that any wheel5stops due to the difference-in-level DL.

The vehicle travel control device200increases the driving force in order to climb over the difference-in-level DL. Here, the driving force may be gradually increased in order to suppress abrupt acceleration of the vehicle1. The increase rate of the driving force at this time is the first increase rate described above.

Due to the increase in the driving force, the wheel5climbs over the difference-in-level DL. Based on the travel state information300, the vehicle travel control device200detects that the wheel5climbs over the difference-in-level DL and identifies the wheel5that climbs over the difference-in-level DL (seeFIG. 11and the first example 4-1 in Section 4). The wheel5identified here is the first wheel5-1.

After the first wheel5-1climbs over the difference-in-level DL, the vehicle travel control device200acquires the reference information510based on the travel state information300. The acquired reference information510is stored in the memory device72.

The reference information510indicates the first wheel5-1identified in Step S10. When a single first wheel5-1passes the difference-in-level DL, the reference information510indicates the single first wheel5-1. When two first wheels5-1concurrently pass the difference-in-level DL, the reference information510indicates the two first wheels5-1.

In addition, the reference information510includes the passing position at which the first wheel5-1passes the difference-in-level DL in Step S10. The vehicle travel control device200can acquire the passing position based on the vehicle position information350(see the first example 4-1 in Section 4).

Furthermore, the reference information510includes the first driving force F1required for the first wheel5-1to climb over the difference-in-level DL in Step S10. The vehicle travel control device200can acquire the first driving force F1based on the travel control information330.

The vehicle travel control device200repeats the above-described Steps S10and S20until at least two first wheels5-1pass the difference-in-level DL. The two first wheels5-1may pass the difference-in-level D at different timings or may concurrently pass the difference-in-level DL.

After the two first wheels5-1pass the difference-in-level DL, the vehicle travel control device200estimates the position of the difference-in-level DL by the method described inFIG. 12to acquire the difference-in-level position information400. More specifically, the two first wheels5-1correspond to the wheels5aand5bshown inFIG. 12. The reference information510includes the first passing position Pa of the wheel5aand the second passing position Pb of the wheel5b. The vehicle travel control device200assumes a position of a line connecting the first passing position Pa and the second passing position Pb as the position of the difference-in-level DL (see the first example 4-1 in Section 4). The acquired difference-in-level position information400is stored in the memory device72.

Subsequently, the vehicle travel control device200estimates the second driving force F2required for the second wheel5-2to climb over the difference-in-level DL, based on the reference information510. Preferably, the vehicle travel control device200estimates the second driving force F2before the second wheel5-2reaches the difference-in-level DL.

Let us consider a “first load W1” and a “second load W2” for explaining estimation of the second driving force F2. The first load W1is a load applied to the first wheel5-1concurrently passing the difference-in-level DL. The first load W1depends on the number of the first wheel5-1concurrently passing the difference-in-level DL (the number is hereinafter referred to as a first wheel number N1). The first load W1in a case of N1=2 is naturally larger than the first load W1in a case of N1=1. Similarly, the second load W2is a load applied to the second wheel5-2concurrently passing the difference-in-level DL. The second load W2depends on the number of the second wheel5-2concurrently passing the difference-in-level DL (the number is hereinafter referred to as a second number of wheels N2).

There is a correlation between the first driving force F1and the first load W1. Similarly, there is a correlation between the second driving force F2and the second load W2. There is a relationship between the first load W1, the second load W2, the first driving force F1, and the second driving force F2as expressed by the following Equation (6).

In Equation (6), a first mass M1is a mass equivalent to the first load W1(W1=M1×g, g=gravitational acceleration). A second mass M2is a mass equivalent to the second load W2(W2=M2×g). A ratio of the first mass M1and the second mass M2is equal to a ratio of the first load W1and the second load W2. As indicated by Equation (6), the second driving force F2can be calculated based on the first driving force F1and the ratio of the first load W1and the second load W2.

As mentioned above, the reference information510indicates the first wheel5-1identified in Step S10and the first driving force F1. Therefore, the vehicle travel control device200can acquire the first load W1and the first driving force F1based on the reference information510.

The second load W2is acquired as follows. The vehicle travel control device200first estimates the second wheel5-2that will reach the difference-in-level DL after the first wheel5-1. More specifically, the vehicle travel control device200estimates the second wheel5-2based on the travel state information300and the difference-in-level position information400. The vehicle position information350indicates the latest wheel position of each wheel5. The vehicle state information310indicates the steering angle and the wheel speed. The difference-in-level position information400indicates the position of the difference-in-level DL. Based on such the information, it is possible to detect that another wheel5different from the first wheel5-1is likely to reach the difference-in-level DL after the first wheel5-1. The said another wheel5is the second wheel5-2.

When the target path TP is set, the vehicle travel control device200executes the vehicle travel control such that the vehicle1travels along the target path TP. In this case, the vehicle travel control device200may estimate the second wheel5-2in consideration of the target path TP (i.e. the target information340) as well.

The number of the second wheel5-2concurrently passing the difference-in-level DL, that is, the second wheel number N2may be one or two. The second load W2is the load applied to the second wheel5-2concurrently passing the difference-in-level DL. The vehicle travel control device200can estimate the second wheel5-2and the second load W2based on the travel state information300and the difference-in-level position information400.

Then, the vehicle travel control device200calculates the second driving force F2in accordance with the above-described Equation (6). That is, the vehicle travel control device200estimates the second driving force F2based on the first driving force F1and the ratio of the first load W1and the second load W2. The second driving force information520indicates the estimated second driving force F2. The second driving force information520is stored in the memory device72.

When the second wheel5-2passes the difference-in-level DL after the first wheel5-1, the vehicle travel control device200generates the second driving force F2indicated by the second driving force information520(seeFIGS. 3 and 4).

In the example shown inFIG. 3, the second wheel5-2reaches and stops at the difference-in-level DL at the time t2a. The vehicle travel control device200“quickly” increases the driving force to the second driving force F2. The increase rate of the driving force at this time is the second increase rate higher than the above-mentioned first increase rate. At the time t2b, the second wheel5-2starts to move and climb up the difference-in-level DL. At the time t2c, the second wheel5-2climbs over the difference-in-level DL. Since the driving force can be quickly increased to the second driving force F2, the second passing period T2is shorter than the first passing period T1.

In the example shown inFIG. 4, the vehicle travel control device200increases the driving force from the time t2pbefore the second wheel5-2reaches the difference-in-level DL. For example, when the second wheel5-2enters a predetermined range before the position of the difference-in-level DL, the vehicle travel control device200starts to increase the driving force. The latest position of the second wheel5-2is obtained from the vehicle position information350. The position of the difference-in-level DL is obtained from the difference-in-level position information400.

In order to suppress shock when the second wheel5-2passes the difference-in-level DL, the vehicle travel control device200may substantially slow down the vehicle1before the second wheel5-2reaches the difference-in-level DL.

5-2. Second Example

In a second example, let us consider the case described inFIGS. 5 and 6where the first wheel5-1passes the difference-in-level DL without stopping. The flow chart is the same as in the case of the first example described above (seeFIG. 13). An overlapping description with the first example will be omitted as appropriate.

The first wheel5-1passes the difference-in-level DL without stopping. Based on the travel state information300, the vehicle travel control device200detects that a wheel5passes the difference-in-level DL and identifies the wheel5that passes the difference-in-level DL (seeFIG. 11and the first example 4-1 in Section 4). The wheel5identified here is the first wheel5-1.

After the first wheel5-1passes the difference-in-level DL, the vehicle travel control device200acquires the reference information510based on the travel state information300. In the second example, the reference information510does not include the first driving force F1. Instead of the first driving force F1, the reference information510indicates a change in the travel state when the first wheel5-1passes the difference-in-level DL.

For example, when the first wheel5-1passes the difference-in-level DL, the vehicle speed (i.e. a kinetic energy) changes. The vehicle speed is obtained from the vehicle state information310. Based on the vehicle state information310, the vehicle travel control device200acquires the change in the vehicle speed (the kinetic energy) when the first wheel5-1passes the difference-in-level DL.

The vehicle travel control device200estimates the position of the difference-in-level DL to acquire the difference-in-level position information400, as in the case of the first example.

As in the case of the first example, the vehicle travel control device200estimates the second wheel5-2and the second load W2based on the travel state information300and the difference-in-level position information400. Furthermore, the vehicle travel control device200estimates the second driving force F2based on the reference information510.

In the second example, the vehicle travel control device200first estimates a height h of the difference-in-level DL based on the change in the travel state indicated by the reference information510. Since the change in the travel state becomes larger as the difference-in-level DL becomes larger, it is possible to estimate the height h of the difference-in-level DL from the change in the travel state.

For example, the change in the travel state is the change in the vehicle speed (the kinetic energy). From a standpoint of the law of conservation of energy, the following Equation (7) is satisfied.
[Equation 7]
½M(Vs2−Vo2)×C=M1×g×h(7)

In Equation (7), M is a total mass of the vehicle1, M1is the first mass equivalent to the above-described first load W1regarding the first wheel5-1(W1=M1×g, g=gravitational acceleration), Vs is the vehicle speed after the first wheel5-1passes the difference-in-level DL, Vo is the vehicle speed before the first wheel5-1passes the difference-in-level DL, and C is a correction coefficient representing energy dissipation. In a case of climbing up the difference-in-level DL, the correction coefficient C is equal to or smaller than −1 (C≤−1). In a case of going down the difference-in-level DL, the correction coefficient C is equal to or larger than 1 (C≥1). The following Equation (8) is obtained from Equation (7).

The vehicle travel control device200estimates the height h of the difference-in-level DL in accordance with Equation (8). That is to say, the vehicle travel control device200estimates the height h of the difference-in-level DL based on the first load W1and the change in the vehicle speed. Furthermore, based on the height h of the difference-in-level DL and the second load W2, the vehicle travel control device200estimates the second driving force F2required for the second wheel5-2to climb over the difference-in-level DL.

FIG. 14is a conceptual diagram for explaining a method of estimating the second driving force F2. InFIG. 14, the driving force F causes the second wheel5-2to climb up the difference-in-level DL of the height h. M2is the second mass equivalent to the above-described second load W2regarding the second wheel5-2(W2=M2×g, g=gravitational acceleration), “r” is a dynamic rolling radius of the second wheel5-2, w is an angle of rotation of the second wheel5-2from a passing start point (i.e. a contact point), ξ is a slope of a road surface that is calculated by the acceleration sensor, Mo is a mass equivalent to a load applied to wheels5-X other than the second wheel5-2, and DA is a direction in which the second wheel5-2proceeds. Using an acceleration A in the direction DA, a mass M (M=M2+Mo) of the vehicle1, and the driving force F, an equation of motion is expressed by the following Equation (9).

As to an angle φ, the following Equations (11) and (12) are satisfied.

The angle ψ is expressed by the following Equation (13). Here, “s” is a moving distance of a center of the second wheel5-2from the passing start point (contact point).

The following Equations (14) and (15) are obtained from Equations (10) to (13).

As indicated in Equations (14) and (15), the driving force F depends on the acceleration A, the height h of the difference-in-level DL, and the second load W2(i.e. the second mass M2). By using Equations (14) and (15), the vehicle travel control device200estimates the second driving force F2required for the second wheel5-2to climb over the difference-in-level DL. The acceleration A is set to a desired value. For example, when the acceleration A is zero, no acceleration and deceleration occurs when the second wheel5-2passes the difference-in-level DL.

As in the case of the first example, when the second wheel5-2passes the difference-in-level DL, the vehicle travel control device200generates the second driving force F2indicated by the second driving force information520

5-3. Third Example

FIG. 15is a flow chart showing a third example of the driving force control relating to the difference-in-level passing. In Step S1, the vehicle travel control device200acquires the difference-in-level position information400based on the surrounding situation information320(see the second example 4-2 in Section 4). In the case of the third example, the above-described Step S30is not necessary, and the passing position may be eliminated from the reference information510. The other Steps S10, S20, S40, and S50are the same as in the case of the first or second example described above.

5-4. Fourth Example

When the wheel5passes the difference-in-level DL, the vehicle travel control device200may correct the vehicle position PV given by the vehicle position information350in consideration of the height h of the difference-in-level DL. The height h of the difference-in-level DL is calculated from the above-described Equation (8). When the wheel5passes the difference-in-level DL, the vehicle travel control device200may regenerate the target path TP. These processing examples can be applied to any of the first to third examples described above.

Next, let us consider a case where the vehicle travel control device200cancels (aborts) passing the difference-in-level DL. The vehicle travel control device200executes the vehicle travel control such that the vehicle1travels toward the target stop position PT. Depending on a relationship between the target stop position PT and the position of the difference-in-level DL on the travel path, the vehicle1may go beyond the target stop position PT when a certain wheel5passes the difference-in-level DL. In that case, the vehicle travel control device200executes “difference-in-level passing cancel processing” in advance so as to prevent the certain wheel5from passing the difference-in-level DL. The wheel5subject to determination of whether or not to prevent from passing the difference-in-level DL is hereinafter referred to as a “subject wheel”.

6-1. First Example

FIG. 16is a conceptual diagram for explaining a first example of the difference-in-level passing cancel processing. The first wheel5-1has already passed the difference-in-level DL, while the second wheel5-2does not yet reach the difference-in-level DL. In the present example, the second wheel5-2is the subject wheel.

The vehicle travel control device200determines (predicts) whether or not the vehicle1(i.e. the vehicle position PV) goes beyond the target stop position PT when the second wheel5-2passes the difference-in-level DL. More specifically, after the first wheel5-1passes the difference-in-level DL, the above-described Step S30is executed and thus the position of the difference-in-level DL is estimated and the difference-in-level position information400is acquired. The vehicle position PV and the position of the second wheel5-2are given by the vehicle position information350. Based on the difference-in-level position information400and the vehicle position information350, it is possible to determine (predict) whether or not the vehicle position PV goes beyond the target stop position PT when the second wheel5-2passes the difference-in-level DL.

For example, the vehicle travel control device200predicts the vehicle position PV at a timing when the second wheel5-2passes the difference-in-level DL, based on the difference-in-level position information400and the vehicle position information350. The vehicle position PV predicted is hereinafter referred to as a “predicted vehicle position PVp”. When the predicted vehicle position PVp goes beyond the target stop position PT, the vehicle travel control device200determines that the vehicle position PV goes beyond the target stop position PT when the second wheel5-2passes the difference-in-level DL. As another example, in the situation where the driving force is increased for the difference-in-level passing, it is not always possible to stop the vehicle1immediately after the second wheel5-2climbs over the difference-in-level DL. Therefore, when the predicted vehicle position PVp is within a predetermined range before the target stop position PT, the vehicle travel control device200determines that the vehicle position PV goes beyond the target stop position PT when the second wheel5-2passes the difference-in-level DL.

When determining that the vehicle position PV goes beyond the target stop position PT when the second wheel5-2passes the difference-in-level DL, the vehicle travel control device200changes the target stop position PT to the near side so as to prevent the second wheel5-2from passing the difference-in-level DL. After that, the vehicle travel control device200moves the vehicle1to the post-change target stop position PT. The vehicle1stops at the target stop position PT and is prevented from going beyond the target stop position PT. Accordingly, for example, the vehicle1is prevented from colliding with a wall or the like.

FIG. 17is a flow chart showing the first example of the difference-in-level passing cancel processing. Steps S10to S30are the same as those described inFIG. 13. The first wheel5-1passes the difference-in-level DL, and the vehicle travel control device200acquires the reference information510and the difference-in-level position information400.

In Step S100, the vehicle travel control device200estimates the second wheel5-2based on the travel state information300and the difference-in-level position information400. Then, based on the difference-in-level position information400and the vehicle position information350, the vehicle travel control device200determines whether or not the vehicle1(i.e. the vehicle position PV) goes beyond the target stop position PT when the second wheel5-2(i.e. the subject wheel) passes the difference-in-level DL.

If it is determined that the vehicle1does not go beyond the target stop position PT (Step S100; No), then the processing proceeds to Step S40. Steps S40and S50are the same as those described inFIG. 13. After Step S50, the processing proceeds to Step S60. In Step S60, the vehicle travel control device200executes the vehicle travel control to move the vehicle1to the target stop position PT and stop the vehicle1at the target stop position PT. Then, the vehicle assist control ends.

On the other hand, if it is determined that the vehicle1goes beyond the target stop position PT (Step S100; Yes), then the processing proceeds to Step S110. In Step S110, the vehicle travel control device200changes the target stop position PT to the near side so as to prevent the second wheel5-2from passing the difference-in-level DL. After that, the processing skips Steps S40and S50and proceeds to Step S60. In Step S60, the vehicle travel control device200moves the vehicle1to the post-change target stop position PT. The vehicle1stops at the target stop position PT and is prevented from going beyond the target stop position PT. Accordingly, for example, the vehicle1is prevented from colliding with a wall or the like.

6-2. Second Example

FIG. 18is a flow chart showing a second example of the difference-in-level passing cancel processing. An overlapping description with the first example shown inFIG. 17will be omitted as appropriate. If it is determined in Step S100that the vehicle1goes beyond the target stop position PT (Step S100; Yes), then the processing proceeds to Step S120.

In Step S120, the vehicle travel control device200generates the braking force to stop the vehicle1before the second wheel5-2passes the difference-in-level DL. In other words, the vehicle travel control device200forces the vehicle1to stop by executing the braking control. The target stop position PT may be maintained or canceled. In either case, Steps S40to S60are not performed. The vehicle1stops before reaching the target stop position PT. Then, the vehicle assist control ends. Such the processing can also achieve the same effects as in the case of the first example.

6-3. Third Example

Even before the first wheel5-1does not pass the difference-in-level DL, the position of the difference-in-level DL may be estimated (seeFIG. 15and the third example 5-3 in Section 5). In this case, it is also possible to determine whether or not the vehicle1goes beyond the target stop position PT when the first wheel5-1passes the difference-in-level DL. That is to say, the first wheel5-1can also be the subject wheel.

FIG. 19is a flow chart showing the difference-in-level passing cancel processing in a summarized manner. In Step S200, the difference-in-level position estimation device acquires the difference-in-level position information400. In Step S210, the vehicle travel control device200determines (predicts) whether or not the vehicle1goes beyond the target stop position PT when the subject wheel passes the difference-in-level DL, based on the difference-in-level position information400and the vehicle position information350.

If it is determined that the vehicle1does not go beyond the target stop position PT (Step S210; No), then the processing proceeds to Step S220. In Step S220, the subject wheel passes the difference-in-level DL.

On the other hand, if it is determined that the vehicle1goes beyond the target stop position PT (Step S210; Yes), the processing proceeds to Step S230. In Step S230, the vehicle travel control device200changes the target stop position PT to the near side so as to prevent the subject wheel from passing the difference-in-level DL. Alternatively, the vehicle travel control device200generates the braking force to stop the vehicle1before the subject wheel passes the difference-in-level DL.

FIG. 20shows a situation where the wheel5goes down the difference-in-level DL. Climbing up and going down are distinguishable based on the change in the vehicle speed or the change in the camera's field of vision. Based on the travel state information300, the vehicle travel control device200detects that any wheel5goes down the difference-in-level DL and identifies the wheel5that passes the difference-in-level DL. The identified wheel5is the first wheel5-1. Then, the vehicle travel control device200estimates the position of the difference-in-level DL and estimates the second wheel5-2by the same method as in the case of climbing up.

In order to suppress shock when the second wheel5-2passes the difference-in-level DL, the vehicle travel control device200may substantially slow down the vehicle1before the second wheel5-2reaches the difference-in-level DL. The vehicle travel control device200may preliminarily increase a brake pressure before the second wheel5-2reaches the difference-in-level DL so as to be able to generate the braking force immediately after the going down. The vehicle travel control device200may execute the braking force control during a period when the second wheel5-2goes down the difference-in-level DL.