vehicle

Abstract

To provide a vehicle having a pair of right and left and drive wheels, which are mutually independently driven to rotate, and a turn wheel capable of being turned, and being characterized in that a criterion for control of the turn wheel by a turning apparatus is properly determined.SOLUTION: When a vehicle 10 is caused to make a steady-state turn by turning or driving a pair of drive wheels 22 with a difference in a turning speed kept between the drive wheels, a turn angle θ of a turn wheel 24 turned with a self-aligning torque is acquired using a turn angle sensor. Based on the turn angle, turn angle calibration for determining a criterion for the turn angle in turn control is carried out. For example, when a super pivot turn is adopted as the steady-state turn, the turn angle calibration can be carried out even in a narrow place. For example, the vehicle is caused to make the steady-state turn rightward or leftward at a turn radius r, then, a midpoint of the turn angles of the rightward and leftward turns is regarded as a criterion, thereby an accurate criterion can be designated.SELECTED DRAWING: Figure 3

Description

[Technical Field]

[0001] The present invention relates to a vehicle having a pair of left and right drive wheels that are rotationally driven independently of each other and steerable wheels. [Background technology]

[0002] When steering a vehicle to turn, it is important to determine a reference steering angle for the wheels in the steering control of the wheels. For example, in a vehicle equipped with an electric power steering system, as disclosed in the following Patent Document, when the vehicle is put into an idle stop state by an idle stop mechanism, the reference steering angle set before the idle stop is used to perform steering control after the idle stop is released. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Laid-Open No. 2010-260464 Summary of the Invention [Problem to be solved by the invention]

[0004] However, with the technology described in the above patent document, there is a risk that the reference for the steering angle itself may become distorted if an external force acts on the vehicle during idling stop, changing the steering angle of the vehicle's wheels. In other words, in order to continue to grasp the reference for the steering angle, it is necessary to maintain power supply to sensors for detecting the steering angle even when the vehicle is not in operation, i.e., when the ignition switch is turned off. In other words, there is room for improvement regarding the reference for the steering angle in steering control, and by making some improvement, it is possible to improve the practicality of vehicles that perform steering control. The present invention has been made in consideration of such circumstances, and an object of the present invention is to provide a vehicle that is highly practical. [Means for solving the problem]

[0005] In order to solve the above problem, the vehicle of the present invention comprises: A pair of left and right drive wheels that are rotated independently of each other; A steerable wheel; a steering device for steering the steered wheels; a steering angle sensor for detecting a steering angle of the steered wheels; a controller that executes drive control for rotationally driving each of the pair of drive wheels and steering control for steering the steered wheels by the steering device; A vehicle equipped with The controller: In a state where the vehicle is made to make a steady turn by driving the pair of drive wheels to rotate with a rotational speed difference, the steering angle of the steered wheels turned by the self-aligning torque is acquired by a steering angle sensor, and steering angle calibration is performed to determine a reference steering angle for the steering control based on the steering angle. [Effects of the Invention]

[0006] According to the vehicle of the present invention, by performing the steering angle calibration, it is possible to appropriately determine the reference steering angle for steering control, and to perform appropriate steering control. Even in a situation where the steered wheels are turned by an external force while the vehicle is not in operation, the steering angle calibration can be performed when the vehicle starts operating.

[0007] The "vehicle" of the present invention (hereinafter sometimes referred to as "this vehicle") is a so-called left-right wheel differential turning vehicle. In other words, the "pair of left and right drive wheels" are arranged coaxially and are both non-steerable wheels (wheels that are not steered), and this vehicle can turn by setting a difference in their rotational speeds. Furthermore, the turning radius of this vehicle can be changed by changing the rotational speed ratio of the left and right drive wheels. In extreme cases, this vehicle can make a pivot turn by stopping the rotation of only one of the left and right drive wheels, and can make a super pivot turn by rotating both wheels in opposite directions at the same speed. Note that this vehicle may be a vehicle driven by a person or an autonomous vehicle.

[0008] It is desirable to provide an independent wheel drive unit for each pair of left and right drive wheels. The wheel drive unit is a device that drives the wheels to rotate at any speed, and an electric motor, for example, can be used as the drive source. In this case, in consideration of simplifying the structure of the vehicle, it is desirable to use an in-wheel motor type wheel drive unit in which the electric motor is provided inside the drive wheels.

[0009] The "steerable wheels" can be considered auxiliary wheels in this vehicle. To ensure the vehicle's ability to prevent tipping over, it is desirable that the steerable wheels be positioned so that their rotational axes are shifted in the longitudinal direction from the rotational axes of the left and right drive wheels. It is desirable that the steerable wheels be freely steerable by external force. They are preferably similar to so-called swivel casters. Specifically, it is desirable that the steerable wheels be positioned so that the intersection of the steering axis (kingpin axis) with the road surface is separated from the center of the steerable wheel's contact patch, i.e., so that a caster trail exists. By employing steerable wheels with such a configuration, when the vehicle is turning, the steerable wheels turn in a direction perpendicular to the line connecting the vehicle's turning center and the center of the steerable wheel's contact patch.

[0010] A "steering device" can be thought of as a device that actively steers steered wheels. In other words, it applies a steering force to the steered wheels that counteracts the self-aligning torque, and uses that steering force to steer the steered wheels to a desired steering angle or maintain that steering angle. By using the steering device to steer the steered wheels to a steering angle that corresponds to the turning radius of the vehicle and maintaining that steering angle, even if an external force acts on the vehicle body, the lateral force acting on the steered wheels from the road surface counteracts that force, allowing the vehicle to make stable turns. Simply put, this allows the vehicle to make stable turns even in the face of external disturbances. Incidentally, it is desirable for the steering device to be configured so that, when not functioning, the steered wheels can be freely steered by the self-aligning torque.

[0011] The "steering angle sensor" may directly detect the steering angle of the steered wheels, or may indirectly detect the steering angle. Regarding the latter, for example, if the driving source of the steering device is an electric motor such as a brushless DC motor, the electric motor has a motor rotation angle sensor (e.g., a resolver) for detecting the rotation phase in order to switch the energized phase. If the motor rotation angle of the electric motor and the steering angle of the steered wheels are related by a predetermined gear ratio, such as the gear ratio of a reducer, the steering angle of the steered wheels can be detected by integrating the motor rotation angle detected by the motor rotation angle sensor. In this case, the motor rotation angle sensor is a so-called relative rotation angle sensor, and functions as a steering angle sensor that indirectly detects the steering angle.

[0012] The "controller" can be considered to be a control device for the vehicle, and is configured to include, for example, a computer, a wheel drive device, a driver (drive circuit) for the steering device, etc. The controller may be configured to execute, for example, "drive control" and "steering control." Drive control can be considered to be control over the forward movement, reverse movement, turning, and radius of the vehicle, and steering control can be considered to be control over the turning angle of the steered wheels, more specifically, control that determines the turning angle of the steered wheels based on the turning radius determined by the drive control, and steers the steered wheels to that turning angle or maintains the steered wheels at that turning angle.

[0013] "Steering angle calibration" is a process for determining a reference steering angle when steering control is performed; in other words, a steering angle reference calibration process. Simply put, it is a process for determining the steering angle that the steered wheels should take when the vehicle is traveling straight, i.e., the neutral angle (steering angle = 0°). By driving the left and right drive wheels to rotate at a set rotational speed ratio, the vehicle turns around a fixed turning center, i.e., performs steady-state turning. While performing this steady-state turning, the steering device is not functioned, and the self-aligning torque steers the steered wheels to a steering angle that corresponds to the steady-state turning, specifically, the turning radius. For example, if the steering angle that corresponds to the turning radius is known structurally for the vehicle, it is possible to determine the reference for steering control, i.e., the neutral angle, based on that steering angle.

[0014] The steady-state turning in steering angle calibration may be a turning of any turning radius, but considering the space for steady-state turning, a small turning radius is desirable. Taking this into consideration, it is desirable to perform a yaw turn as the steady-state turning, and more preferably, a super-yaw turn. Since this vehicle is a left-right wheel differential turning vehicle as described above, super-yaw turns are possible.

[0015] If the longitudinal axis of a vehicle is defined as an axis that passes exactly midway between the left and right drive wheels and is perpendicular to the rotation axes of those drive wheels, then when there is only one steerable wheel, it is desirable that the steerable wheel, or more specifically, its steering axis, be located on the longitudinal axis of the vehicle, taking left and right balance into consideration. In a vehicle configured as described above, in the steering angle calibration, the left and right wheels are each made to perform a steady turn with the same turning radius, and the midpoint of the steering angle of the steered wheels obtained in each of the left and right steady turns can be determined as the reference steering angle for steering control. Such steering angle calibration makes it possible to accurately determine the reference steering angle. Furthermore, it is also possible to perform a steady turn with the same turning radius multiple times on each of the left and right wheels, and determine the reference steering angle for steering control by averaging the midpoints of the multiple steering angles obtained in the multiple turns. Such steering angle calibration makes it possible to more accurately determine the reference steering angle.

[0016] In the steering angle calibration, the reference steering angle may be determined as the steering angle that should be achieved when the vehicle is traveling straight in the steering control. When the reference steering angle is determined in this way, the determined reference steering angle may be reset when the vehicle exhibits a deviation behavior when traveling straight. Furthermore, after resetting, the steering angle calibration may be performed again.

[0017] This vehicle is suitable for use as a towing vehicle (tractor) for towing a towed vehicle (trailer). When a towing vehicle turns while towing a towed vehicle, it is subjected to an external force (moment) caused by towing the towed vehicle. The action of this external force likely prevents the vehicle from making the intended turn. Specifically, the force in the vehicle's width direction likely prevents the vehicle from making the intended turn with the intended turning radius. As described above, this vehicle is able to make stable turns even while towing a towed vehicle by countering the external force with the lateral force acting from the road surface on the steered wheels. Note that, because the moment caused by towing the towed vehicle acts on the towing vehicle's joint with the towed vehicle, it is desirable to position the steered wheels at this joint. Note that, in the case of a towing vehicle, it is desirable to perform steering angle calibration when the towed vehicle is not being towed. [Brief explanation of the drawings]

[0018] [Figure 1] 1 is a perspective view showing a state in which a towing vehicle, which is a vehicle of an embodiment, is towing a towed vehicle. FIG. [Figure 2] 1A and 1B are a side view and a bottom view of a vehicle according to an embodiment of the present invention; [Figure 3] 5A to 5C are schematic diagrams illustrating the operation of the vehicle during turning and the state of the steered wheels when steering angle calibration is being performed in the embodiment. [Figure 4] FIG. 2 is a block diagram illustrating the functional configuration of a controller provided in the vehicle of the embodiment. [Figure 5] 4 is a flowchart of steering angle calibration executed in the vehicle of the embodiment. [Figure 6] 10 is a flowchart of another steering angle calibration that can be performed in the vehicle of the embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0019] The following describes in detail a vehicle as an embodiment of the present invention, with reference to the drawings. In addition to the following embodiment, the present invention can be embodied in various forms, including those described in the above section "Modes of the Invention," and various modifications and improvements based on the knowledge of those skilled in the art. [Example]

[0020] [A] Vehicle configuration The vehicle in this embodiment is a towing vehicle 10 as shown in Figure 1, which tows a towed vehicle 12. Referring to Figure 2(a), a side view of the towing vehicle 10 traveling straight ahead, and Figure 2(b), a bottom view, the towing vehicle 10 has a body 20 that is generally pentagonal in plan view. Incidentally, in these figures, the towing vehicle 10 and towed vehicle 12 are shown with the left side facing forward.

[0021] The towing vehicle 10 has a pair of left and right drive wheels 22L, 22R and a steerable wheel 24 disposed behind the drive wheels 22L, 22R. Hereinafter, the left drive wheel 22L may be referred to as the left drive wheel 22L, and the right drive wheel 22R may be referred to as the right drive wheel 22R. When there is no need to distinguish between left and right, they may be collectively referred to as drive wheels 22. The rotation axes of the left drive wheel 22L and the right drive wheel 22R are aligned, and this aligned axis may be hereinafter referred to as the drive axis DL. The left drive wheel 22L and the right drive wheel 22R are each driven by an independent wheel drive unit 26. Each wheel drive unit 26 is driven by an electric motor (hereinafter sometimes referred to as a "drive motor") as a drive source. The wheel drive unit 26 is supported by the vehicle body 20 and disposed within the wheel of each drive wheel 22. In other words, it is a so-called in-wheel motor type drive unit. The in-wheel motor type wheel drive device 26 may be a known device, and a description of the configuration of the wheel drive device 26 will be omitted here.

[0022] The steerable wheels 24 are arranged in the form of swivel casters. More specifically, the casters 28 include a steering shaft 30 held on the vehicle body 20 so as to be rotatable about a vertical axis, and a fork 32 fixed to the lower end of the steering shaft 30. The steerable wheels 24 are held at the lower ends of the forks 32 via shafts 34 so as to be rotatable about a horizontal axis. The center axis of the steering shaft 30 functions as the steering axis KL, i.e., the kingpin axis of the steerable wheels 24. If an axis extending in the fore-and-aft direction exactly midway between the left driving wheel 22L and the right driving wheel 22R in the vehicle width direction is called the vehicle center axis CL, then, as shown in FIG. 2(b), when the towing vehicle 10 is traveling straight, the steering wheels 24 are located on the vehicle center axis CL in a bottom view (plan view) and are oriented along the direction of the vehicle center axis CL.

[0023] Towing vehicle 10 has a steering device 36 inside vehicle body 20 for actively steering steerable wheels 24. Specifically, steering device 36 is configured to include a steering motor 38, which is an electric motor serving as a drive source, and a reducer 40 that reduces the rotation of steering motor 38 and transmits it to steering shaft 30. By supplying current to the coil of steering motor 38, that is, by operating steering device 36, a steering force is applied to caster 28, and steered wheels 24 are steered left and right as shown by the two-dot chain lines in FIG. 2(b). On the other hand, when current is not supplied to the coil of steering motor 38, that is, when steering device 36 is not operating, steerable wheels 24 can be freely steered by an external force. Note that steering motor 38 is a brushless DC motor and includes a motor rotation angle sensor 42 (e.g., a resolver) for detecting its own rotation phase.

[0024] The towing vehicle 10 is an unmanned, autonomous vehicle. Although not shown, the vehicle body 20 includes a controller for controlling its travel and operation. The controller is primarily a computer and also includes a drive motor for the wheel drive unit 26 and a driver (drive circuit) for the steering motor 38 for the steering unit 36. Furthermore, for autonomous travel, the towing vehicle 10 is equipped with a camera and LiDAR for monitoring the surroundings, a communication device for wireless communication with the control center, a wheel speed sensor for detecting the rotational speed of the drive wheels, a position sensor for detecting its own traveling position, and longitudinal acceleration sensors, lateral acceleration sensors, and yaw rate sensors for monitoring its own movement and attitude, all of which are connected to the controller. The towing vehicle 10 travels within a business building, and the position sensor is configured to detect its traveling position using a beacon. The controller also stores map data for the business building. The functions of these sensors and other related devices related to autonomous travel are already known, so a detailed description of them will be omitted.

[0025] A brief explanation of the towed vehicle 12 will now be provided. The towed vehicle 12 comprises a platform 50 for carrying items, a pair of left and right swivel casters 52 attached to the front portion of the underside of the platform 50, a pair of left and right fixed casters (casters that do not steer) 54 attached to the rear portion, and a coupler 56 fixed to the front of the platform 50 for coupling to the towing vehicle 10. The front end of this coupler 56 is coupled to a coupler 58 disposed at the rear portion of the body 20 of the towing vehicle 10. The coupler 58 functions to allow the towing vehicle 10 and towed vehicle 12 to rotate relative to each other. As can be seen from FIG. 2(a), the axis of this relative rotation coincides with the steering axis KL described above.

[0026] [B] Vehicle turning movement As described above, the towing vehicle 10 of this embodiment is a vehicle that travels by independently driving the left and right drive wheels 22 by the corresponding wheel drive devices 26, and moves straight by rotating the drive wheels 22 at the same rotational speed. More specifically, by rotating the drive wheels 22 forward at the same rotational speed, the towing vehicle 10 moves forward in a straight line, and by rotating the drive wheels 22 backward at the same rotational speed, the towing vehicle 10 moves backward in a straight line. Note that this towing vehicle 10 is not provided with a mechanical brake device, and although a detailed description will be omitted, the drive motor functions as a generator to perform regenerative braking. When a large braking force is required, reverse braking is performed by rotating the drive motor in the reverse direction.

[0027] The turning of the towing vehicle 10 is achieved by creating a difference in the rotational speed of the left and right drive wheels 22. Referring to FIG. 3(a), which shows a schematic top view of the turning operation of the towing vehicle 10, when the towing vehicle 10 is turned left as shown in the figure, the rotational speed of the right drive wheel 22R, which is the turning outer wheel (hereinafter sometimes referred to as the "right wheel rotational speed") v WR However, the rotation speed of the left driving wheel 22L, which is the turning inner wheel (hereinafter, sometimes referred to as the "left wheel rotation speed") v WL The driving wheels 22 are driven to rotate in a higher position. At this time, the turning center TC of the towing vehicle 10 is located on the driving axis DL in a plan view. The turning radius r, which is the distance between the turning center TC and the center point of the left driving wheel 22L and the right driving wheel 22R on the driving axis DL, is calculated by the left wheel rotation speed v WL and the right wheel rotation speed v WR It depends on the wheel speed ratio R, which is the ratio of

[0028] During the above-mentioned turning, the steerable wheels 24 are steered by the steering device 36 as follows. If, in a plan view, a line that passes through the center of the steerable wheels 24 in the width direction and is perpendicular to the rotation axis of the steerable wheels 24, i.e., a line that indicates the direction of the steerable wheels 24, is defined as a steering wheel direction line SL, then the steerable wheels 24 are steered in a direction where the steering wheel direction line SL is perpendicular to a line connecting the center of the steerable wheels 24 and the turning center TC. In the present tractor 10, in a plan view, the steering axis line KL is located on the vehicle center axis line CL, so the angle between the steering wheel direction line SL and the vehicle center axis line CL can be defined as the steering angle θ of the steerable wheels 24. The steering device 36 steers the steerable wheels 24 so that the steering angle θ is based on the turning radius r.

[0029] [C] Control performed by the controller The controller of the towing vehicle 10 performs travel control related to the travel of the towing vehicle 10, drive control of the drive wheels 22 by the wheel drive device 26, and steering control of the steerable wheels 24 by the steering device 36. These controls will be explained below in order with reference to the functional block diagram of the controller 100 shown in FIG.

[0030] i) Driving control The controller has a driving control unit 102, and driving control is performed by the driving control unit 102. The towing vehicle 10 drives in accordance with instructions wirelessly sent from an external control device. The driving control unit 102 is connected to a communication device 103, and determines a driving route by referring to map data it possesses in accordance with instructions, such as "go this way and that," sent from the control device via the communication device 103. The driving control unit 102 then issues commands to a drive control unit 112 and a steering control unit 114 (described later) to automatically drive the towing vehicle 10 along the determined driving route based on the position of the towing vehicle 10 acquired by a position sensor 104, surrounding information acquired by a surrounding monitoring device 106 such as a camera or LiDAR, and attitude and behavior information of the towing vehicle 10 acquired by an acceleration sensor 108 such as a lateral acceleration sensor and a longitudinal acceleration sensor, and a yaw rate sensor 110. A known method may be used for this automatic driving, and a description of this method will be omitted.

[0031] ii) Drive control The drive control is performed by the drive control unit 112. The above-mentioned commands from the travel control unit 102 include "move forward or backward at a vehicle travel speed (hereinafter sometimes referred to as "vehicle speed") v", "slow down or stop", and "turn right or left with a turning radius r". Based on these commands, the drive control unit 112 calculates the target drive wheel rotation speed v of each of the left and right drive wheels 22. W * For details, the target left wheel rotation speed v WL * , target right wheel rotation speed v WR * Incidentally, when the towing vehicle 10 needs to turn, the wheel speed ratio R is determined based on the turning radius r, and the target left wheel rotation speed v is also determined based on the wheel speed ratio R. WL * , target right wheel rotation speed v WR * Determine.

[0032] The towing vehicle 10 has a drive wheel rotation speed v of each of the left and right drive wheels 22. W (Left wheel rotation speed v WL , right wheel rotation speed v WR The drive control unit 112 has a drive wheel rotation speed sensor 116 that detects the target drive wheel rotation speed v for each of the left and right drive wheels 22. W * Drive wheel rotation speed v W The deviation of the driving wheel rotation speed Δv W and determine the drive wheel rotation speed deviation Δv W Based on this, the torque to be applied to each drive wheel 22, that is, the drive torque Tq that the drive motor 118 of each wheel drive device 26 should generate, is calculated. D Determine the driving torque Tq D The instruction about the driving current I is transmitted to the driving driver 120 which is the driver of the driving motor 118 corresponding to each driving wheel 22. Each driving driver 120 controls the driving current I based on the instruction. D to the corresponding drive motor 118.

[0033] iii) Steering control The steering control is performed by the steering control unit 114. The command from the traveling control unit 102 is to "turn right or left with a turning radius r." Based on this command, the steering control unit 114 sets a target steering angle θ, which is the steering angle θ of the target steered wheels 24. * The steering control unit 114 determines the target steering angle θ * The steering angle deviation Δθ, which is the deviation of the actual steering angle θ from the steering angle θ, is specified, and the torque to be applied to the steered wheels 24, i.e., the steering torque Tq, which is the torque to be generated by the steering motor 38 of the steering device 36, is calculated based on the steering angle deviation Δθ. S Determine the steering torque Tq S The instruction about the steering current I is transmitted to the steering driver 122, which is the driver of the steering motor 38. The steering driver 122 controls the steering current I based on the instruction. S is supplied to the steering motor 38.

[0034] Here, the detection of the actual steering angle θ in the towing vehicle 10 will be explained. The towing vehicle 10 does not have a steering angle sensor that directly detects the steering angle θ of the steered wheels 24. In the towing vehicle 10, the steering angle θ is detected using a motor rotation angle sensor 42 that detects the rotation phase of the steering motor 36 in order to switch the energized phase. In other words, the motor rotation angle sensor 42 functions as a steering angle sensor that indirectly detects the steering angle θ. To explain in more detail, the steering angle θ and the motor rotation angle θ M There is a specific ratio between these values ​​depending on the reduction ratio of reducer 40, etc. As a reference for steering angle θ, a steering angle θ0 (hereinafter referred to as "reference steering angle") at which steering angle θ becomes 0° is set, and steering control section 114 determines motor rotation angle θ corresponding to this reference steering angle θ0. M The reference motor rotation angle θ M0 and store the reference motor rotation angle θ M0 Based on this, the motor rotation angle θ detected by the motor rotation angle sensor 42 in response to the rotation of the steering motor 38 is calculated. M The actual steering angle θ is determined based on the ratio, and this steering angle θ is used for the steering control.

[0035] [D] Steering angle calibration When the towing vehicle 10 stops operating and power is no longer supplied to the controller and the steering motor 38, the motor rotation angle sensor 42 detects the motor rotation angle θ M In such a state, if an external force acts on the steered wheels 24 or the vehicle body 20, the motor rotation angle θ M cannot be detected, and the reference motor rotation angle θ M0 Even if the reference motor rotation angle θ is stored, the value will be inaccurate. In other words, when the towing vehicle 10 stops operating, there is a high possibility that the reference motor rotation angle θ will also become inaccurate. Therefore, in the towing vehicle 10, when the towing vehicle 10 starts operating, the reference motor rotation angle θ M0 , steering angle calibration is executed as a process for setting the reference steering angle θ, that is, a process for calibrating the reference for the steering angle θ. Specifically, the steering angle calibration is executed by calibration unit 124 of the controller.

[0036] To give an overview of steering angle calibration, the towing vehicle 10 is made to make a right or left pivot turn when not towing the towed vehicle 12 and the steering device 36 is not functioning. A pivot turn is what is known as a "turn on the spot," as shown in Figures 3(b) and 3(c), and is a turn in which the turning center TC is located exactly midway between the left and right drive wheels 22, i.e., a turn in which the turning radius r is 0. The towing vehicle 10 can be made to turn even in a very narrow space.

[0037] As shown in FIG. 3(b), when the steered wheels 24 are steered by the self-aligning torque during a left turn, the steering angle θ L The motor rotation angle θ corresponding to ML As shown in FIG. 3(c), the steering angle θ when the steered wheels 24 are steered by the self-aligning torque during a right turn is R The motor rotation angle θ corresponding to MR Then, the motor rotation angle θ M(Hereafter, "motor rotation angle midpoint θ MC ") is calculated by dividing the reference motor rotation angle θ M0 and the reference motor rotation angle θ M0 The steering angle θ corresponding to this is set as the reference steering angle θ0.

[0038] More specifically, the calibration unit 124 performs the steering angle calibration by executing a steering angle calibration process as shown in the flowchart of Fig. 5. The steering angle calibration process will be described in detail below with reference to the flowchart.

[0039] In the process according to the above flowchart, calibration unit 124 first calculates the current motor rotation angle θ of steering motor 38 in step 1 (hereinafter abbreviated as “S1”; the same applies to the other steps). M and converts this value into the reference motor rotation angle θ M0 It is provisionally set as:

[0040] Next, in S2, the calibration unit 124 controls the wheel drive device 26 to make a leftward pivot turn of the towing vehicle 10 as shown in Fig. 3(b). In S3, it is determined whether the pivot turn has been performed a number of times or more that is set as the number of times at which the steering angle θ of the steered wheels 24 is considered to be sufficiently stable. If the pivot turn has been performed, in S4, the motor rotation angle θ at that time is determined. M The steering angle θ when making a left pivot turn L The motor rotation angle θ corresponding to M The left motor rotation angle θ ML Then, in a similar manner, in S5, the wheel drive unit 26 is controlled to make the towing vehicle 10 make a rightward pivot turn, as shown in FIG. 3(c). In S6, it is determined whether the pivot turn has been made the number of times set above or more, and if so, in S7, the motor rotation angle θ at that time is obtained. M The steering angle θ when making a right pivot turn R The motor rotation angle θ corresponding to M The right motor rotation angle θ MR Obtain as.

[0041] In this steering angle calibration process, in order to obtain an accurate reference steering angle θ0, the left motor rotation angle θ ML and the right motor rotation angle θ MR The acquisition of the left motor rotation angle θ is regarded as one turning set, and the turning set is performed multiple times. Then, in the next step S8, the calibration unit 124 determines whether the number of turning sets has been set or more. If the number of turning sets has not been reached, the calibration unit 124 performs further turning sets from S2 onwards. If the number of turning sets has been reached the set number of times, in step S9, the left motor rotation angle θ ML and right motor rotation angle θ MR The midpoint (the exact center angle) of the motor rotation angle is MC In step S10, the average of these is calculated as the average motor rotation angle midpoint θ MC - AVE It is calculated as follows.

[0042] Then, in S11, the calibration unit 124 calculates the calculated average motor rotation angle midpoint θ MC - AVE The reference motor rotation angle θ M0 In step S12, the reference motor rotation angle θ M0 The reference steering angle θ0 is set based on the above. In other words, this steering angle calibration process is designed to determine the reference for the steering angle θ as the steering angle θ that should be achieved when the towing vehicle 10 is traveling straight in steering control. By performing this steering angle calibration process, it is possible to accurately determine the reference for the steering angle θ in steering control, and because this steering angle calibration process is performed using a pivot turn, it can be performed in a fairly narrow space.

[0043] The steering angle calibration process is a process of performing a plurality of left and right swivel turns to set the reference steering angle θ0, but instead of this process, the following simple process may be performed. Specifically, one of the left and right swivel turns is performed until the detection value of the motor rotation angle sensor 42 stabilizes, and the stabilized detection value is used as a reference for the known motor rotation angle θ to be realized in the swivel turn. M According to this process, it is possible to simply set the reference for the steering angle θ in the steering control.

[0044] Even if the steering angle calibration described above is performed, there is a possibility that the reference steering angle θ0 may deviate for some reason while the vehicle is traveling. This deviation will cause the towing vehicle 10 to exhibit deflection when traveling straight. This changing behavior can be detected, for example, by a yaw rate sensor provided in the towing vehicle 10. Specifically, for example, when the towing vehicle 10 is traveling straight, that is, when the steering angle θ of the steered wheels 24 is maintained at the reference steering angle θ0 during steering control, it is possible to determine that the reference steering angle θ0 has deviated if the detected yaw rate is greater than a set value and this large state continues for a set time.

[0045] In the present towing vehicle 10, if it is determined that a deviation has occurred in the reference steering angle θ0 while the vehicle is traveling, the reference for the steering angle θ is reset by, for example, performing a steering angle calibration process as shown in the flowchart in FIG. 6. In this process, the calibration unit 124 first stops steering control in S21. In other words, it disables the steering device 36. In the following S22, it is determined whether the vehicle is traveling in a steady turning state, that is, a turning state in which the turning radius r is not changing. If it is determined that the vehicle is in a steady turning state, in S23, the motor rotation angle sensor 42 of the steering motor 38 calculates the motor rotation angle θ corresponding to the steering angle θ of the steered wheels 24 that have been turned by the self-aligning torque. M Then, in S24, the motor rotation angle θ M Whether the motor rotation angle θ is stable or not MIt is determined whether the angle has changed by more than a set angle that is set to a fairly small value.

[0046] Motor rotation angle θ M If the turning radius r is stable, the calibration unit 124 acquires the turning radius r of the current steady turning in S25. Subsequently, in S26, the turning angle θ of the steered wheels 24 corresponding to the turning radius r is estimated based on the turning radius r, and in S27, the motor rotation angle θ corresponding to the turning angle θ is calculated based on the estimated turning angle θ. M Then, in S28, the motor rotation angle θ M In other words, the current detected value of the motor rotation angle sensor 42 is set as the estimated value. M0 In other words, the calibration of the steering angle θ in the steering control is performed. After the calibration, the steering control is resumed in S29. [Explanation of symbols]

[0047] 10: Towing vehicle (vehicle) 12: Towed vehicle 20: Vehicle body 22: Driving wheel 22L: Left driving wheel 22R: Right driving wheel 24: Steering wheel 26: Wheel drive unit 28: Caster 36: Steering unit 38: Steering motor 42: Motor rotation angle sensor (steering angle sensor) 58: Coupler 100: Controller 102: Travel control unit 110: Yaw rate sensor 112: Drive control unit 114: Steering control unit 120: Drive driver 122: Steering driver 124: Calibration unit TC: Turning center r: Turning radius θ: Steering angle θ0: Reference steering angle θ L : Steering angle θ when making a left pivot turn R :Rudder angle θ when turning right M : Motor rotation angle θ M0 : Reference motor rotation angle

Claims

1. A pair of left and right drive wheels that are driven to rotate independently of each other; A steerable wheel; a steering device for steering the steered wheels; a steering angle sensor for detecting a steering angle of the steered wheels; a controller that executes drive control for rotationally driving each of the pair of drive wheels and steering control for steering the steered wheels by the steering device; A vehicle equipped with The controller: The vehicle is configured such that, in a state where the vehicle is made to make a steady turn by rotationally driving the pair of drive wheels with a rotational speed difference, a steering angle of the steered wheels turned by self-aligning torque is obtained by a steering angle sensor, and a steering angle calibration is performed to determine a reference steering angle for the steering control based on the steering angle.

2. The controller: The vehicle according to claim 1 , wherein the steady turning is performed by performing a sharp turn in the steering angle calibration.

3. The controller:

3. The vehicle according to claim 2, wherein in the steering angle calibration, the vehicle is made to steadily turn with the same turning radius on each of the left and right sides, and a midpoint of the steering angle of the steered wheels acquired in each of the left and right steady turning states is determined as a reference steering angle in the steering control.

4. The controller:

4. The vehicle according to claim 3, wherein the steering angle calibration is configured to determine a reference steering angle for the steering control by having the vehicle perform steady-state turning a plurality of times with the same turning radius on each of the left and right sides, and averaging midpoints of the plurality of steering angles obtained by the plurality of executions.

5. 5. A vehicle according to claim 1, wherein the vehicle is a vehicle for towing a towed vehicle.

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