Vehicle control device

Abstract

To provide a vehicle control device which suppresses power consumption of a steering actuator according to necessity, and can prevent overheat of the steering actuator.SOLUTION: A vehicle control device comprises: an electric motor 7 for generating steering force for steering rear wheels; a trapezoidal thread 26 for converting a rotation motion generated on the basis of the drive of the electric motor 7 into an axial linear motion for steering the rear wheels. A target steering angle of the rear wheels is set on the basis of steering of the handle by an occupant and a current steering angle of the rear wheels is detected. In a state where the target steering angle of the rear wheels is different from the current steering angle of the rear wheels, the vehicle control device controls to switch alternately between an energization state for supplying a current to the electric motor 7 so that the current steering angle of the rear wheels approximates to the target steering angle and a cutoff state where the current to the electric motor 7 is cut off, on the basis of an accumulated current applied to the electric motor 7.SELECTED DRAWING: Figure 9

Description

Technical Field

[0001] The present invention relates to a vehicle control device for a vehicle capable of controlling the steering angle of a rear wheel.

Background Art

[0002] Conventionally, as a vehicle steering method, there are two-wheel steering (2 Wheel Steering, hereinafter referred to as 2WS) in which only the front wheels are steered based on steering wheel operation by a driver, and four-wheel steering (4 Wheel Steering, hereinafter referred to as 4WS) in which the front and rear wheels are steered respectively. Although 4WS has a problem that the structure becomes more complicated than 2WS, there are also many excellent merits such as reducing the inner wheel difference, being able to reduce the turning radius, preventing slip on an icy road surface, and stable lane change in a high-speed range.

[0003] Here, in a vehicle that steers the rear wheels like the above 4WS, for example, a target steering angle of the rear wheel is set based on the steering of the vehicle's steering wheel, and when the current steering angle of the rear wheel (hereinafter referred to as the actual steering angle) is different from the target steering angle, the steering actuator that is the drive source of the steering mechanism is driven and controlled so as to approach the target steering angle. As the steering actuator, for example, an electric motor is used. However, assuming that steering when the vehicle weight is heavy and straightening when stopped are also performed smoothly, the output (power consumption) of the electric motor is set large, and there is a problem that if a large current flows through the electric motor for a long time, the durability life is reduced due to overheating. Therefore, for example, in Japanese Patent Application Laid-Open No. 2009-298300, when the difference between the actual steering angle and the target steering angle of the rear wheel is equal to or greater than a threshold value, a discontinuous energization mode in which energization is not continuously performed but is alternately turned on and off is shifted to suppress power consumption and prevent overheating of the electric motor. A technique has been proposed.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0005] In Patent Document 1 mentioned above, when the difference between the actual steering angle and the target steering angle of the rear wheels, which is expected to have a particularly large energization amount of the electric motor, is equal to or greater than the threshold value, it shifts to the intermittent energization mode. However, for example, when the driver slowly steers the steering wheel over a long period of time, that is, the difference between the actual steering angle and the target steering angle of the rear wheels is small, but there is a problem that it does not shift to the intermittent energization mode in a situation where the actual steering angle and the target steering angle of the rear wheels continue to differ for a long time. Here, since the heat generation amount of the electric motor is proportional to the square of the time integral of the current flowing through the motor inverter, even if the difference between the actual steering angle and the target steering angle of the rear wheels is small, if the driver continues to slowly steer the steering wheel and the current continues to flow for a long time, the heat generation amount will become extremely large.

[0006] The present invention has been made to solve the above-mentioned conventional problems, and an object of the present invention is to provide a vehicle control device that can suppress the power consumption of the steering actuator and prevent overheating of the steering actuator under necessary circumstances regardless of the steering mode of the driver's steering wheel.

Means for Solving the Problems

[0007] To achieve the above object, a vehicle control device according to the present invention includes an electric steering actuator that generates a steering force for steering the rear wheels, a trapezoidal screw that converts the rotational motion generated based on the drive of the steering actuator into a linear motion in the axial direction for steering the rear wheels, a target steering angle setting means for setting a target steering angle of the rear wheels based on the steering of the steering wheel by the occupant, a steering angle detection means for detecting the current steering angle of the rear wheels, and a power supply control means for controlling the energization state of the steering actuator. The power supply control means performs control to alternately switch between an energization state in which current is supplied to the steering actuator so that the current steering angle of the rear wheels approaches the target steering angle based on the cumulative current supplied to the steering actuator in a state where the target steering angle of the rear wheels is different from the current steering angle of the rear wheels, and a cutoff state in which the current to the steering actuator is cut off. Here, the "cumulative current" is, for example, the time integral of the current supplied to the steering actuator after the start of control to make the steering angle of the rear wheels the target steering angle. Also, "cutting off the current" does not necessarily mean only completely reducing the current supplied to the steering actuator to 0, but may be a state with less current compared to the energization state.

Advantages of the Invention

[0008] According to the vehicle control device according to the present invention having the above configuration, by alternately switching between an energization state in which power is supplied to the steering actuator based on the cumulative current supplied to the steering actuator and a cutoff state in which the current to the steering actuator is cut off, it is possible to suppress the power consumption of the steering actuator and prevent overheating of the steering actuator in a necessary situation regardless of the steering mode of the occupant's steering wheel. Further, since the current steering angle of the rear wheels can be maintained in the cutoff state where the steering actuator is not driven by the trapezoidal screw, the pursuit of the target steering angle can be resumed immediately even immediately after returning to the energization state.

Brief Description of the Drawings

[0009]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

Figure 7

Figure 8

Figure 9

Mode for Carrying Out the Invention

[0010] Hereinafter, the vehicle control device according to the present invention will be described in detail with reference to the drawings based on one embodiment in which it is embodied. First, the vehicle 2 equipped with the vehicle control device 1 according to this embodiment will be described below. FIG. 1 is a schematic configuration diagram of the vehicle 2 according to this embodiment.

[0011] Here, the vehicle 2 may be, for example, an automobile (internal combustion engine automobile) having an internal combustion engine (engine, etc.) as a drive source, an automobile (electric vehicle, fuel cell vehicle, etc.) having an electric motor (motor, etc.) as a drive source, or an automobile (hybrid vehicle) having both of them as drive sources. Also, regardless of the vehicle type, it may be a passenger car, or a commercial large truck, bus, etc. Further, if it has front wheels and rear wheels respectively, it may be a forklift, construction machinery, etc.

[0012] In addition, the vehicle 2 of this embodiment is a vehicle that adopts four-wheel steering (4WS), which controls the steering angles of the front wheels and the rear wheels respectively based on the steering wheel operation by the driver, as the steering method. As the configuration of 4WS, there are a mechanical type in which the steering of the front wheels is mechanically transmitted to the rear wheels via the input gear box on the front wheel side and the steering gear box on the rear wheel side, and an electronically controlled type in which the rear wheels are electronically controlled by controlling an actuator, various valves, etc. based on the steering amount. In this embodiment, an example using the electronically controlled type will be described. That is, it is assumed that the steering and the rear wheels are not mechanically connected but are connected by by-wire (electric communication).

[0013] As shown in FIG. 1, the vehicle 2 includes a vehicle body 3, a steering wheel 4 that is an operation target by the driver, a power steering device 5 that assists the steering of the steering wheel 4, a rear-wheel steering ECU (electronic control unit) 6, and an electric motor (steering actuator) 7 as a drive source. The vehicle 2 also includes a rear-wheel steering device 8 that converts the rotational motion of the electric motor 7 into a linear motion in the axial direction for steering the rear wheels and then steers the rear wheels, and wheels 9A to 9D. In the following description, the left front wheel is 9A, the right front wheel is 9B, the left rear wheel is 9C, and the right rear wheel is 9D. In addition, the vehicle control device 1 includes the power steering device 5, the rear-wheel steering ECU 6, the electric motor 7, the rear-wheel steering device 8, and other control parts related to the control of the wheels 9A to 9D.

[0014] In addition, the "actuator" sometimes refers to a mechanism that includes a drive source such as a motor and transmits or converts the driving force from the drive source, and sometimes refers only to the drive source part separately from the mechanism for transmitting or converting the driving force. In the following description, the latter is used. That is, in this embodiment, only the electric motor 7 in the rear-wheel steering device 8 is referred to and described as the steering actuator.

[0015] The components of the vehicle 2 will be described below. First, the steering wheel 4 (also referred to as the steering wheel) is installed in the driver's seat and is a rudder that changes the traveling direction of the vehicle 2 by the driver gripping and turning it. Basically, when changing the traveling direction to the right, the steering wheel 4 is turned to the right (clockwise), and when changing to the left, the steering wheel 4 is turned to the left (counterclockwise). Incidentally, a power steering device 5 is connected to the steering shaft 11 connected to the steering wheel 4. After being assisted by the power steering device 5, the rack gear and pinion gear 12 at the tip of the steering shaft 11 are driven in accordance with the turning of the steering wheel 4, thereby displacing the steering angles of the front wheels 9A and 9B in a direction corresponding to the turning direction of the steering wheel 4.

[0016] Also, a torque sensor 13 is installed on the steering shaft 11. It detects the steering torque related to the operation of the steering wheel 4 by the driver and transmits it to the power steering device 5 and the rear-wheel steering ECU 6. In addition to the steering torque, the torque sensor 13 can also detect the steering angle and the angular velocity of the steering angle, and these pieces of information are also transmitted. The power steering device 5 controls the torque (assist force for steering) applied to the steering shaft 11 based on the steering torque, steering angle, and angular velocity of the steering angle detected by the torque sensor 13. The rear-wheel steering ECU 6 controls the steering angles of the rear wheels 9C and 9D based on the steering torque, steering angle, and angular velocity of the steering angle detected by the torque sensor 13.

[0017] Incidentally, for the steering angle control of 4WS, there are, for example, in-phase control in which the steering angles of the front wheels and the rear wheels are controlled in the same direction, and reverse-phase control in which the steering angles of the front wheels and the rear wheels are controlled in the opposite direction. Depending on the driver's operation or the vehicle's situation, the vehicle side makes a judgment and appropriately selects which one to execute. As an example, in-phase control is performed to improve stability when changing lanes at high speed or driving on an icy road surface, and reverse-phase control is performed to reduce the turning radius when making a turn at low speed.

[0018] On the one hand, the power steering device 5 is a device that assists in steering the steering wheel 4. It can be roughly divided into a hydraulic type, an electro-hydraulic type, and an electric type. In this embodiment, the electric type will be particularly adopted. Further, the electric type is further divided into a column assist type, a pinion assist type, and a rack assist type according to the position of the motor 16 that assists in steering. Any of these types can be adopted, but in the following description, the column assist type will be taken as an example for explanation.

[0019] In the power steering device 5, the motor 16 is driven after adjusting the current amount according to the steering torque, the steering wheel angle, and the angular velocity of the steering angle detected by the torque sensor 13 described above. The motor 16 is connected to the steering shaft 11 via a worm gear and a wheel gear. When the motor 16 is driven, torque is applied to the steering shaft 11 to assist the driver's steering operation of the steering wheel.

[0020] The rear-wheel steering ECU 6 is a control device that controls the steering angles of the rear wheels 9C and 9D according to the steering torque, the steering wheel angle, and the angular velocity of the steering angle detected by the torque sensor 13 described above. In particular, in this embodiment, current control for supplying the electric motor 7 will also be performed as described later. The rear-wheel steering ECU 6 is connected to the power steering device 5, the electric motor 7, various sensors in the rear-wheel steering device 8, and the torque sensor 13 via an in-vehicle network such as CAN. It is also connected to a vehicle speed sensor, an acceleration sensor, etc. mounted on the vehicle 2. The details of the rear-wheel steering ECU 6 will be described later.

[0021] Further, the electric motor 7 is incorporated as a part of the rear-wheel steering device 8 and is a drive source that generates a steering force for steering the rear wheels. The rear-wheel steering device 8 is a drive mechanism that converts the rotational motion of the electric motor 7 into a linear motion in the axial direction for steering the rear wheels. Hereinafter, the electric motor 7 and the rear-wheel steering device 8 will be described in more detail with reference to FIG. 2. FIG. 2 is a cross-sectional view taken along the rotation axis of the drive shaft (rear-wheel shaft) of the rear-wheel steering device 8.

[0022] As shown in FIG. 2, the rear-wheel steering device 8 includes an ECU folder 22 for housing and fixing the substrate 21 on which the rear-wheel steering ECU 6 is disposed, an electric motor 7, a drive shaft 23 rotatably supported inside the rear-wheel steering device 8 and rotationally driven by the electric motor 7, a planetary gear 25 that reduces the rotational speed and increases the torque with the drive shaft 23 as the input shaft and outputs a drive shaft 24 having the same rotation axis as the drive shaft 23 as the output shaft, a trapezoidal screw 26 that converts the rotational motion of the drive shaft 24 with reduced rotational speed and increased torque into a linear motion in the axial direction and moves the drive shafts 23 and 24 integrally in the axial direction, a resolver 27 that detects the rotation angle of the rotor of the electric motor 7, a stroke sensor 28 that detects the left-right positional deviation with respect to the normal positions of the drive shafts 23 and 24 (the positions of the drive shafts 23 and 24 when the rear wheels face the straight-ahead direction), and a cylindrical housing (case) 29 that houses the above-described members. In the example shown in FIG. 2, the rear-wheel steering ECU 6 is incorporated as a part of the rear-wheel steering device 8, but the rear-wheel steering ECU 6 may be disposed separately from the rear-wheel steering device 8.

[0023] Here, the electric motor 7 is, for example, a brushless motor, and includes a rotor 30 having a cylindrical permanent magnet disposed on the outer peripheral surface and a stator 31 having a plurality of coils disposed around the rotor 30. The rotor 30 is coaxial and integral with the drive shaft 23. When current flows through the coils of the stator 31, the rotor 30 rotates, and accordingly, the drive shaft 23 also rotates.

[0024] Further, the electric motor 7 also includes a motor drive circuit 32. For example, it is an inverter circuit or an H-bridge circuit composed of switching elements (not shown). The on / off state of the switching elements is controlled by a control signal from the rear-wheel steering ECU 6, and electric power corresponding to the control signal is supplied to the electric motor 7. In particular, the rotation direction and torque of the rotor 30 can be controlled. The motor drive circuit 32 includes a motor current sensor 46. The motor current sensor 46 detects the current value flowing from the motor drive circuit 32 to the electric motor 7 and outputs a signal representing the current value to the rear-wheel steering ECU 6.

[0025] In addition, the planetary gear 25 is composed of a combination of a plurality of gears such as a sun gear, planetary gears, a planetary carrier that picks up the revolving motion of the planetary gears, and an internal gear. The gear ratio can be set according to the number of teeth of each gear. In particular, in this embodiment, the drive shaft 23 that is rotationally driven by the electric motor 7 is used as the input shaft, and after reducing the rotational speed and increasing the torque, it is converted into the rotational drive of the drive shaft 24 (output shaft) having the same rotational axis as the drive shaft 23. The drive shafts 23 and 24 are rotatably and integrally supported in the axial direction by bearings 33 and 34 formed at both the left and right ends of the housing 29, and both ends are connected to the rear wheels 9C and 9D via tie rods, knuckle arms, etc. (not shown). The drive shafts 23 and 24 also correspond to the rear wheel shafts.

[0026] Also, the trapezoidal screw 26 is a screw mechanism that converts the rotational drive of the drive shaft 24, which has been decelerated and has increased torque by the planetary gear 25, into a linear motion in the axial direction for steering the rear wheels. Specifically, as shown in FIG. 3, it has a convex portion (thread) 35 with a trapezoidal cross-section formed spirally on the outer periphery of the drive shaft 24 and a nut 37 with a concave portion 36 corresponding to the convex portion 35 also formed spirally on the inner surface. The nut 37 is fixed to the housing 29, that is, the vehicle body 3 of the vehicle. On the other hand, the drive shafts 23 and 24 are rotatably and axially movably supported by bearings 33 and 34 formed at both the left and right ends of the housing 29. Therefore, when the drive shaft 24 rotates, the convex portion 35 and the concave portion 36 engage with each other, and the drive shaft 24 moves axially with respect to the nut 37, that is, the vehicle body 3 of the vehicle. As described above, both ends of the drive shafts 23 and 24 are connected to the rear wheels 9C and 9D via tie rods, knuckle arms, etc., and the rear wheels 9C and 9D are steered by the axial displacement of the drive shafts 23 and 24. That is, it is possible to steer the rear wheels 9C and 9D by the rotational drive of the drive shafts 23 and 24 by the electric motor 7. Note that the left and right movement directions of the drive shafts 23 and 24, that is, the steering directions of the rear wheels 9C and 9D, are determined by the rotational direction of the rotor 30 of the electric motor 7, and the left and right movement amounts of the drive shafts 23 and 24, that is, the steering angles of the rear wheels 9C and 9D, are determined by the rotational angle of the rotor 30.

[0027] Also, as shown in Fig. 4, in the state where the rotation of the drive shaft 24 has stopped, even if an external force is generated in the axial direction with respect to the drive shaft 24, the frictional force generated between the convex portion 35 and the concave portion 36 by the external force, that is, the frictional force acting in the non-rotating direction of the drive shaft 24, is designed to be larger than the external force component acting in the rotating direction of the drive shaft 24. That is, even if a strong external force is generated in the axial direction with respect to the drive shaft 24, the drive shaft 24 does not rotate and does not move in the axial direction. That is, when the electric motor 7 has stopped, even if the vehicle travels in that state, the steering angle of the rear wheels does not return to the straight-ahead direction and the current steering angle is maintained.

[0028] On the other hand, the resolver 27 is a sensor that is disposed in the vicinity of the rotor 30 of the electric motor 7 and detects the rotation angle of the rotor 30 of the electric motor 7. For example, it is composed of a combination of an induction coil and a detection coil facing each other, and the induction coil is disposed with respect to the rotor 30. Then, when the electric motor 7 is driven and the rotor 30 rotates, the induction coil also rotates integrally. When the induction coil rotates, the magnetic field detected by the detection coil changes, and the change amount of the rotation angle of the induction coil, that is, the change amount of the rotation angle of the rotor 30, can be detected by the change amount of the magnetic field. As described above, the steering angles of the rear wheels 9C and 9D are determined by the rotation angle of the rotor 30. Therefore, the rear-wheel steering ECU 6 can detect the steering angle of the rear wheels by detecting the rotation angle of the rotor 30 of the electric motor 7 with the resolver 27. However, since the resolver 27 can only detect the change amount of the steering angle (relative steering angle), the detection result of the stroke sensor 28 described later is also required to detect the current steering angle (actual steering angle) of the rear wheels 9C and 9D.

[0029] On the one hand, the stroke sensor 28 is composed of, for example, a combination of a Hall element and a permanent magnet. The permanent magnet is arranged with respect to the drive shafts 23 and 24, and the Hall element is fixed to the housing 29 side. When the drive shafts 23 and 24 are in the correct position (the position of the drive shafts 23 and 24 when the rear wheels are facing the straight-ahead direction), the Hall element and the permanent magnet are designed to be positioned opposite each other. Then, when the electric motor 7 is driven and further converted into a linear motion in the axial direction by the trapezoidal screw 26 and the drive shafts 23 and 24 are displaced in the left-right direction from the correct position, the magnetic field changes, resulting in a change in the amount of Hall current generated by the Hall element. Since the amount of Hall current changes according to the amount of displacement, by detecting the amount of Hall current, it becomes possible to detect the amount of displacement of the drive shafts 23 and 24 from the correct position. And the rear-wheel steering ECU 6 can specify the initial steering angle value based on the detection signal of the stroke sensor 28 when the ignition is on, and based on the change amount of the steering angle (relative steering angle) from the initial steering angle value obtained based on the output signal from the resolver 27 thereafter, it becomes possible to calculate the actual steering angle of the rear wheels 9C and 9D.

[0030] In addition, the vehicle 2 includes basic components as the vehicle 2 in addition to the components shown in FIG. 1, but only the configuration related to the steering and control of the wheels 9A to 9D and the control related to the configuration will be described.

[0031] Subsequently, the configuration of the vehicle control device 1 will be described in more detail with reference to FIG. 5. FIG. 5 is a block diagram showing the configuration of the vehicle control device 1 according to the present embodiment.

[0032] The vehicle control device 1 according to this embodiment includes the above-described power steering device 5, the rear-wheel steering ECU 6, the electric motor 7, and control portions related to the control of the other wheels 9A to 9D. In particular, the rear-wheel steering ECU 6 is an electronic control unit (ECU: Electronic Control Unit) that performs various controls related to the running of the vehicle, such as the steering angle control of the rear wheels 9C and 9D, and has various means as processing algorithms together with the control portions included in the power steering device 5 and the other vehicle control devices 1. For example, the target steering angle setting means sets the target steering angle of the rear wheels based on the steering of the steering wheel by the occupant. The steering angle detection means detects the current steering angle of the rear wheels. The energization control means controls the energization state of the electric motor 7.

[0033] Specifically, as shown in FIG. 5, it includes an internal storage device such as a CPU 41 as an arithmetic device and a control device, a RAM 42 used as a working memory when the CPU 41 performs various arithmetic processes, a ROM 43 in which, in addition to the control program, a vehicle control processing program (see FIG. 6) described later, etc. are recorded, and a flash memory 44 that stores the program read from the ROM 43. It also has a timer 45 as a means for measuring time. On the other hand, the rear-wheel steering ECU 6 is also connected via an in-vehicle network such as CAN to the power steering device 5, the electric motor 7, the resolver 27, the stroke sensor 28, the torque sensor 13 installed in the vehicle 2, the motor current sensor 46, the vehicle speed sensor 47, the acceleration sensor 48, etc.

[0034] Currently, the vehicle speed sensor 47 is a sensor for detecting the moving distance and vehicle speed of the vehicle 2. It generates a pulse in response to the rotation of the drive wheels of the vehicle 2 and outputs a pulse signal to the rear wheel steering ECU 6. Further, the acceleration sensor 48 detects the acceleration generated in the longitudinal direction (parallel to the traveling direction of the vehicle) and the lateral direction (perpendicular to the traveling direction of the vehicle) with respect to the vehicle body of the vehicle 2 and outputs it to the rear wheel steering ECU 6. Then, the rear wheel steering ECU 6 can calculate the vehicle speed and moving distance of the vehicle by counting the pulses output from the vehicle speed sensor 47, and sets the target steering angles of the rear wheels 9C and 9D based on the steering angle of the steering wheel and the angular velocity of the steering angle detected by the torque sensor 13, the vehicle speed detected by the vehicle speed sensor 47, the acceleration detected by the acceleration sensor 48, and the like. Further, the rear wheel steering ECU 6 can calculate the actual steering angles of the rear wheels 9C and 9D based on the detection results of the resolver 27 and the stroke sensor 28 as described above, and controls the steering angles of the rear wheels 9C and 9D so that the actual steering angles approach the rear wheels 9C and 9D.

[0035] Subsequently, the vehicle control processing program specifically executed by the rear wheel steering ECU 6 in the vehicle control device 1 having the above configuration will be described with reference to FIG. 6. FIG. 6 is a flowchart of the vehicle control processing program according to the present embodiment. Here, the vehicle control processing program is executed after the ACC power supply (accessory power supply) of the vehicle is turned on, and is a program for performing various controls related to the running of the vehicle, such as steering angle control of the rear wheels 9C and 9D. The program shown in the flowchart in FIG. 6 below is stored in the RAM 42, ROM 43, etc. provided in the rear wheel steering ECU 6 and is executed by the CPU 41.

[0036] First, in step (hereinafter abbreviated as S) 1, the CPU 41 acquires, via the CAN, the steering angle of the steering wheel and the angular velocity of the steering angle detected by the torque sensor 13, that is, the steering content of the steering wheel by the occupant. Further, information on the current vehicle behavior such as the vehicle speed detected by the vehicle speed sensor 47 and the acceleration detected by the acceleration sensor 48 is also acquired in the same manner.

[0037] Subsequently, in S2, the CPU 41 sets the target steering angles of the rear wheels 9C and 9D (hereinafter referred to as rear-wheel target steering angles) using the steering angle, the angular velocity of the steering angle, the vehicle speed, the angular velocity, etc. acquired in the above S1. Regarding the rear-wheel target steering angles, it is set after determining whether to perform in-phase control, which controls the steering angles of the front and rear wheels in the same direction, or anti-phase control, which controls the steering angles of the front and rear wheels in the opposite direction. For example, in-phase control is performed to improve stability when the vehicle changes lanes at high speed or travels on an icy road surface, and anti-phase control is performed to reduce the turning radius when making a turn at low speed. Note that the information on the vehicle behavior acquired in the above S1 is used for such determination. Also, instead of automatically determining on the vehicle side whether to execute in-phase control or anti-phase control, the occupant's operation may be used to determine whether to execute in-phase control or anti-phase control.

[0038] In in-phase control, for example, the rear-wheel target steering angle is set to be the same as the steering angle of the front wheels. In anti-phase control, for example, the rear-wheel target steering angle is set so that the steering angle of the rear wheels is about 1 / 10 to 1 / 20 in the opposite direction to the steering angle of the front wheels. However, the above example is just an example, and the optimal rear-wheel target steering angle is set according to the current vehicle situation.

[0039] Note that the processes after S1 are repeatedly executed (for example, every 10 msec) during vehicle travel, and the rear-wheel target steering angle is set based on the steering angle at that time. Here, when the occupant operates the steering wheel to change the traveling direction of the vehicle, the operation is performed such that the steering angle gradually increases or, conversely, gradually decreases. Therefore, as shown in FIG. 7, when the vehicle changes its traveling direction such as during a turn or a lane change, the rear-wheel target steering angle basically does not remain fixed and gradually changes according to the steering operation of the vehicle (the rear-wheel target steering angle changes with the passage of time).

[0040] After that, in S3, the CPU 41 acquires the current steering angle (actual steering angle) of the rear wheels. Here, the actual steering angle of the rear wheels is acquired using the detection results of the resolver 27 and the stroke sensor 28 as described above. Specifically, the initial steering angle is specified based on the detection signal of the stroke sensor 28 when the ignition is on, and the actual steering angle of the rear wheels 9C and 9D is calculated based on the change amount of the steering angle (relative steering angle) from the initial steering angle obtained based on the output signal from the resolver 27 after that.

[0041] Next, in S4, the CPU 41 determines whether the rear-wheel target steering angle set in S2 is different from the actual steering angle of the rear wheels acquired in S3, that is, whether it is necessary to steer the rear wheels (change the steering angle).

[0042] And when it is determined that the rear-wheel target steering angle set in S2 is different from the actual steering angle of the rear wheels acquired in S3, that is, when it is determined that it is necessary to steer the rear wheels (S4: YES), the process proceeds to S5 to perform the steering of the rear wheels. On the contrary, when it is determined that the rear-wheel target steering angle set in S2 is the same as the actual steering angle of the rear wheels acquired in S3, that is, when it is determined that it is not necessary to steer the rear wheels (S4: NO), the process proceeds to S14 without steering the rear wheels (maintaining the current steering angle). In that case, no current is supplied to the electric motor 7, and the drive shafts 23 and 24 do not rotate nor move axially. Incidentally, as shown in FIG. 4, in this embodiment, the rear-wheel steering device 8 has a trapezoidal screw 26, and even if a large external force is applied when the drive of the electric motor 7 is not performed, the current steering angle of the rear wheels (that is, the actual steering angle that matches the rear-wheel target steering angle) is maintained.

[0043] In S5, the CPU 41 reads State, which is a parameter indicating the current control state of the vehicle control device 1, from the RAM 42, and determines whether State is "in feedback control". Note that State is set to switch between two states, "in feedback control" and "current off", according to the cumulative current and the passage of time in the processes after S6 described later. Also, the initial state value when the ACC power is turned on is set to "in feedback control".

[0044] Here, in the vehicle control device 1 of the present embodiment, when the rear wheel target steering angle is different from the current actual steering angle of the rear wheels, control is performed to make the actual steering angle approach the target steering angle after S5. In particular, based on the cumulative current supplied to the electric motor 7, a "power-on state in which the current is supplied to the electric motor 7 so that the current steering angle of the rear wheels approaches the target steering angle" and a "cut-off state in which the current to the electric motor 7 is cut off" are alternately switched.

[0045] As a result, as shown in FIG. 7, conventionally, in a state where the rear wheel target steering angle is different from the current actual steering angle of the rear wheels, for example, when a steering operation is performed and the target steering angle changes with the passage of time, current is continuously supplied to the electric motor 7, so there has been a problem of reducing the durability life of the electric motor 7 due to overheating. Here, since the heat generation amount of the electric motor is proportional to the square of the time integral of the current flowing through the motor inverter, even if the difference between the actual steering angle of the rear wheels and the rear wheel target steering angle is small, if the steering wheel is continuously steered slowly and the current continues to flow for a long time, the heat generation amount becomes very large.

[0046] In the present embodiment, in a state where the rear wheel target steering angle is different from the current actual steering angle of the rear wheels, for example, when a steering operation is performed and the target steering angle changes with the passage of time, current is not continuously supplied to the electric motor 7, and control is performed to alternately switch the above power-on state and cut-off state based on the cumulative current supplied to the electric motor 7. Thereby, it is possible to suppress the heat generation amount even when the steering wheel is continuously steered slowly. However, as shown in FIG. 7, until the cumulative current supplied to the electric motor 7 reaches a preset control start value (until time t0) after the start of the rear wheel steering angle control, that is, at a stage where the heat generation amount is low immediately after the start of driving of the electric motor 7, there is no need to suppress the heat generation amount, so control is performed to continuously supply current to the electric motor 7 in the same manner as in the conventional case. And when State is "in feedback control" in S5, it indicates that it is in the above power-on state at the current time, and when State is "power off", it indicates that it is in the above cut-off state.

[0047] When State is "under feedback control" (S5: YES), the process proceeds to S6. On the other hand, when State is "current off" (S5: NO), the process proceeds to S9.

[0048] In S6, the CPU 41 determines whether or not the cumulative current supplied to the electric motor 7 has reached a threshold value since the energization state to the electric motor 7 was started. As shown in FIG. 7, since control is performed to continuously supply current to the electric motor 7 until the cumulative current supplied to the electric motor 7 reaches a preset control start value (until t0) after starting the steering angle control of the rear wheels, for the period immediately after starting the steering angle control of the rear wheels until it is first determined as YES in S6, the threshold value in S6 is set to a control start value larger than the normal value. The control start value is, for example, the cumulative current supplied to the electric motor 7 until the electric motor 7 reaches a predetermined temperature (for example, 60 degrees). On the other hand, during the control of alternately switching between the energized state and the cut-off state (after t0), it is determined whether or not the cumulative current supplied to the electric motor 7 since the most recent return from the cut-off state to the energized state has reached the threshold value (<control start value). The current value flowing through the electric motor 7 can be detected by the motor current sensor 46. Also, the control start value and the threshold value serving as the determination criteria in S6 can be set as appropriate, and for example, the values can be changed according to the vehicle type, the specifications of the electric motor 7, the driving environment of the vehicle, etc.

[0049] When it is determined that the cumulative current supplied to the electric motor 7 has reached the threshold value since the energization state to the electric motor 7 was started (S6: YES), the process proceeds to S7. On the other hand, when it is determined that the cumulative current supplied to the electric motor 7 has not reached the threshold value since the energization state to the electric motor 7 was started (S6: NO), the process proceeds to S8.

[0050] In S7, the CPU 41 substitutes 0 as the value of "command current (thermal protection)". Note that "command current (thermal protection)" is a value indicating the current value to be supplied to the electric motor 7 at the current time. However, it is not always the case that "command current (thermal protection)" is instructed to the motor drive circuit 32 as a control signal. Instead, after being corrected in S12 described later so that there is no rapid change in current, it is output to the motor drive circuit 32 as a control signal (S13).

[0051] Also, in S7, the CPU 41 reads State, which is a parameter indicating the current control state of the vehicle control device 1, from the RAM 42 and sets "during current off", which indicates that the transition to the cut-off state has occurred. Then, it proceeds to S12.

[0052] On the other hand, in S8, the CPU 41 substitutes, as the value of "command current (thermal protection)", the current value necessary to supply to the electric motor 7 in order to bring the current actual steering angle of the rear wheels closer to the rear wheel target steering angle. Here, the current value substituted in S8 is determined based on the current actual steering angle and the difference between the current actual steering angle and the rear wheel target steering angle. That is, the larger the steering angle, the larger the torque required to further change the steering angle from that steering angle. Therefore, as the actual steering angle increases, the current value necessary to supply to the electric motor 7 for steering also increases. Also, the larger the difference between the actual steering angle and the rear wheel target steering angle, the greater the need to increase the torque in order to quickly approach the rear wheel target steering angle. Therefore, as the difference between the actual steering angle and the rear wheel target steering angle increases, the current value necessary to supply to the electric motor 7 to approach the rear wheel target steering angle also increases. In particular, in this embodiment, feedback control is performed by detecting the actual steering angle in real time using the resolver 27 and the stroke sensor 28, and based on the feedback control, the current value necessary to supply to the electric motor 7 to bring the current actual steering angle of the rear wheels closer to the rear wheel target steering angle is set. However, as in S7, it is not always the case that "command current (thermal protection)" is instructed to the motor drive circuit 32 as a control signal. Instead, after being corrected in S12 described later so that there is no rapid change in current, it is output to the motor drive circuit 32 as a control signal (S13). Then, it proceeds to S12.

[0053] On one hand, in S9, after the CPU 41 transitions to the cutoff state (State = "during current-off"), the timer 45 measures the elapsed time and determines whether the timer value has reached a predetermined time or more. Note that the predetermined time serving as the determination criterion for S9 can be set as appropriate, but in order not to let the occupant notice that the current is being cut off, it is desirable to set it as short as possible within the range where the heat generation of the electric motor 7 can be suppressed. For example, it is set to 100 msec.

[0054] Also, in this embodiment, when the rear-wheel target steering angle changes with the passage of time, as shown in FIG. 8, it is desirable to set the above-mentioned predetermined time for cutting off the current shorter as the change speed becomes faster. The reason for such a setting is that in the state where the current is cut off, the actual steering angle does not change. So, basically, the longer the time for cutting off the current, the wider the difference between the rear-wheel target steering angle and the actual steering angle becomes, and when returning from the cutoff state to the energized state, the actual steering angle will change greatly. However, if the change amount of the actual steering angle becomes large (the step of the broken line indicating the actual steering angle in FIG. 8 becomes large), there is a risk that the occupant will notice that the current is being cut off. Therefore, it is desirable to suppress the change amount. If the change speed of the rear-wheel target steering angle is slow as shown in the left figure of FIG. 8, even if the period for cutting off the current is long, the change amount of the actual steering angle at the time of current recovery can be suppressed, so the current cutoff time can be set long. On the other hand, if the change speed of the rear-wheel target steering angle is fast as shown in the right figure of FIG. 8, if the period for cutting off the current is made long, the change amount of the actual steering angle at the time of current recovery will become large. Therefore, it is necessary to set the current cutoff time as short as possible.

[0055] And when it is determined that the elapsed time since the transition to the cutoff state has reached a predetermined time or more (S9: YES), the process proceeds to S11. On the contrary, when it is determined that the elapsed time since the transition to the cutoff state is less than the predetermined time (S9: NO), the process proceeds to S10.

[0056] In S10, since the CPU 41 continues the interrupted state in which the current to the electric motor 7 is interrupted, 0 is substituted as the value of "command current (thermal protection)". Note that the "command current (thermal protection)" is a value indicating the current value to be supplied to the electric motor 7 at the current time, but the "command current (thermal protection)" is not necessarily instructed to the motor drive circuit 32 as a control signal. Instead, after being corrected in S12 described later so that there is no sudden change in current, it is output to the motor drive circuit 32 as a control signal (S13).

[0057] On the other hand, in S11, the CPU 41 returns from the interrupted state in which the current to the electric motor 7 is interrupted, and substitutes, as the value of the "command current (thermal protection)", the current value necessary to supply the electric motor 7 in order to bring the current actual steering angle of the rear wheels closer to the rear wheel target steering angle. Here, the current value substituted in S11 is determined based on the current actual steering angle and the difference between the current actual steering angle and the rear wheel target steering angle. That is, the larger the steering angle, the larger the torque required to further change the steering angle from that steering angle. Therefore, as the actual steering angle increases, the current value necessary to supply the electric motor 7 for steering also increases. Also, the larger the difference between the actual steering angle and the rear wheel target steering angle, the greater the need to increase the torque in order to quickly approach the rear wheel target steering angle. Therefore, as the difference between the actual steering angle and the rear wheel target steering angle increases, the current value necessary to supply the electric motor 7 to approach the rear wheel target steering angle also increases. In particular, in this embodiment, feedback control is performed by detecting the actual steering angle in real time using the resolver 27 and the stroke sensor 28, and based on the feedback control, the current value necessary to supply the electric motor 7 to bring the current actual steering angle of the rear wheels closer to the rear wheel target steering angle is set. However, similar to S7, the "command current (thermal protection)" is not necessarily instructed to the motor drive circuit 32 as a control signal. Instead, after being corrected in S12 described later so that there is no sudden change in current, it is output to the motor drive circuit 32 as a control signal (S13). Then, the process proceeds to S12.

[0058] Also, in S11, the CPU 41 reads State, which is a parameter indicating the current control state of the vehicle control device 1, from the RAM 42, and sets "During feedback control", which indicates that the device has shifted to the energized state. Then, it proceeds to S12.

[0059] In S12, the CPU 41 calculates the "command current (output value)" to be instructed to the motor drive circuit 32 as the current value supplied to the electric motor 7. Specifically, it is calculated by the following equations (1) and (2) based on the "command current (thermal protection)" set in S7, S8, S10, and S11. Command current change amount = upper limit processing on (command current (thermal protection) - previous value of command current (output value)), that is, when exceeding the upper limit value, the upper limit value is taken as the command current change amount ···· (1) Command current (output value) = previous value of command current (output value) + command current change amount ···· (2)

[0060] According to the above equations (1) and (2), the "command current (output value)" becomes a value as close as possible to the "command current (thermal protection)" within the range where the change amount of the current per unit time does not exceed the upper limit. Also, it is desirable to set the upper limit of the change amount of the current to be as large as possible within the range where no sound or vibration occurs. Here, when the torque of the electric motor 7 changes rapidly, there is a problem that sound and vibration are generated due to the contact of internal components. In S12, by setting an upper limit on the change amount of the current, the generation of such sound and vibration is prevented. Furthermore, as shown in FIG. 9, the upper limit α of the change amount during the process of reducing the current amount when shifting from the energized state to the cut-off state and the upper limit β of the change amount during the process of increasing the current amount when shifting from the cut-off state to the energized state may be different values. For example, for the upper limit α of the change amount when reducing the current amount, a value as large as possible within the range where no sound or vibration occurs is set as a fixed value. On the other hand, for the upper limit β of the change amount when increasing the current amount, it is set considering the difference between the actual steering angle and the rear wheel target steering angle when returning from the cut-off state within the range where no sound or vibration occurs. That is, the upper limit β of the change amount when increasing the current amount is not a fixed value and will change depending on, for example, the change speed of the rear wheel target steering angle or the time (predetermined time of S9) for cutting off the current.

[0061] Thereafter, in S13, the CPU 41 transmits a control signal instructing the supply of current to the electric motor 7 to the motor drive circuit 32 provided in the electric motor 7. Note that the control signal includes the "command current (output value)" as the target value of the amount of current to be supplied. In the motor drive circuit 32 that has received the control signal, the target current value of the electric motor 7 is set to the "command current (output value)", and the current value detected by the motor current sensor 46 is fed back to control the duty ratio of the switching element of the motor drive circuit 32 so that the current value becomes the target current value.

[0062] As a result, particularly when in the energized state (however, even in the off state, the electric motor 7 may be driven immediately after switching from the energized state), as described above, current flows through the stator 31 to drive the electric motor 7, and the rotational motion of the electric motor 7 is converted into a linear motion in the axial direction for steering the rear wheels, thereby steering the rear wheels 9C and 9D. In particular, the actual steering angle is steered so as to become the rear wheel target steering angle. On the other hand, in the off state, the drive of the electric motor 7 is stopped. However, as shown in FIG. 4, in this embodiment, the rear wheel steering device 8 has a trapezoidal screw 26, and even if a large external force is applied when the drive of the electric motor 7 is not performed, the current steering angle of the rear wheels is maintained.

[0063] Thereafter, in S14, it is determined whether or not the vehicle has finished traveling. If not (S14: NO), the process returns to S1 and the travel control of the vehicle is continuously performed. Note that the processes after S1 are repeatedly executed at intervals of, for example, 10 msec during vehicle travel. On the other hand, if the vehicle has finished traveling (S14: YES), the vehicle control processing program is terminated.

[0064] Next, a specific example will be given to explain the vehicle control by the above vehicle control processing program. FIG. 9 shows an example of the transition of the rear wheel target steering angle, the actual steering angle, and the current value supplied to the electric motor 7 when the vehicle control by the above vehicle control processing program is performed. The horizontal axis represents the elapsed time. The example shown in FIG. 9 shows a case where the occupant slowly steers the steering wheel over a long period of time, that is, a case where the rear wheel target steering angle gradually increases in proportion to the elapsed time.

[0065] As shown in FIG. 9, when the occupant steers the steering wheel and a new rear wheel target steering angle is set, the actual steering angle and the rear wheel target steering angle will be different, so it is determined that rear wheel steering is necessary (S4: YES), and the power supply to the electric motor 7 is started. Since the torque required for steering is proportional to the actual steering angle at that time, as shown in FIG. 9, the amount of electric power that needs to be supplied to the electric motor 7 basically increases gradually according to the passage of time (as the steering angle increases). However, in the present embodiment, as described above, when the cumulative current supplied to the electric motor 7 reaches the threshold value (the control start value for the first time) after the energization state of the electric motor 7 is started (S6: YES), the supply of the current to the electric motor 7 is cut off for a predetermined time and shifted to a cut-off state. Thereby, it becomes possible to suppress the heat generation of the electric motor 7 due to the continuous flow of current for a long time. Also, as shown in FIG. 4, in the present embodiment, the rear wheel steering device 8 has a trapezoidal screw 26, and since the current steering angle of the rear wheel can be maintained in the cut-off state where the electric motor 7 is not driven, the follow-up to the rear wheel target steering angle can be resumed immediately even immediately after returning to the energized state. Further, in the cut-off state, the steering angle of the rear wheel becomes fixed and does not synchronize with the occupant's steering wheel operation. However, since the steering wheel and the rear wheels are not mechanically connected and are connected by by-wire (electrical communication), even if they do not completely synchronize with the steering wheel operation, as long as it does not appear visually, the occupant will not notice. In particular, in the present embodiment, the time during which the cut-off state continues is short (for example, 100 msec), and the change amount of the actual steering angle at the time of current return is prevented from becoming large, so that the occupant does not feel discomfort. Since the timing of transitioning to the cutoff state is determined by the cumulative current, the larger the current amount, the shorter the interval for transitioning from the energized state to the cutoff state. On the other hand, as also shown in FIG. 8, the predetermined time during which the cutoff state continues is determined by the change speed of the rear wheel target steering angle. If the change speed is high and the period for cutting off the current is lengthened, the change amount of the actual steering angle at the time of current return will increase. Therefore, the cutoff time of the current is set as short as possible. Also, regarding the change amount of the current (angles α and β in FIG. 9) when transitioning from the energized state to the cutoff state and when transitioning from the cutoff state to the energized state, an upper limit is set, so that the generation of noise and vibration due to a sudden change in the torque of the electric motor 7 can be prevented. Furthermore, especially when transitioning from the cutoff state to the energized state, immediately after returning to the energized state, as shown in FIG. 9, a slightly larger amount of current than the originally required current amount is supplied. Thereby, even in a state where a large difference occurs between the actual steering angle and the rear wheel target steering angle immediately after returning to the energized state, the actual steering angle can be quickly approximated to the rear wheel target steering angle, and the follow-up to the rear wheel target steering angle can be resumed.

[0066] As described in detail above, in the vehicle control device 1 according to the present embodiment and the computer program executed by the vehicle control device 1, there are an electric motor 7 that generates a steering force for steering the rear wheels, and a trapezoidal screw 26 that converts the rotational motion generated based on the drive of the electric motor 7 into a linear motion in the axial direction for steering the rear wheels. The target steering angle of the rear wheels is set based on the steering of the steering wheel by the occupant (S2), and the current steering angle of the rear wheels is detected (S3). When the target steering angle of the rear wheels is different from the current steering angle of the rear wheels, based on the cumulative current supplied to the electric motor 7, a power-on state in which the current is supplied to the electric motor 7 so that the current steering angle of the rear wheels approaches the target steering angle, and a cutoff state in which the current to the electric motor 7 is cut off are alternately switched (S5 to S13). Therefore, regardless of the steering mode of the occupant's steering wheel, it is possible to suppress the power consumption of the electric motor 7 and prevent the electric motor 7 from overheating in a necessary situation. Further, since the current steering angle of the rear wheels can be maintained in the cutoff state in which the electric motor 7 is not driven by the trapezoidal screw, the follow-up to the target steering angle can be restarted immediately even immediately after returning to the power-on state. Further, by repeatedly executing the first control of switching from the power-on state to the cutoff state at the timing when the cumulative current supplied to the electric motor 7 since the start of the power-on state reaches the threshold value, and the second control of returning from the cutoff state to the power-on state after a predetermined time has elapsed since the cutoff state (S5 to S13), the power-on state and the cutoff state are alternately switched. Therefore, by managing the power-on state according to the amount of current flowing through the electric motor 7, it is possible to prevent the electric motor 7 from overheating. Further, when the target steering angle changes with the passage of time, the predetermined time is set shorter as the change speed becomes faster. Therefore, it is possible to prevent a large deviation between the target steering angle and the actual steering angle when returning from the cutoff state to the power-on state, that is, a large change in the actual steering angle when returning from the cutoff state to the power-on state. Further, when switching from the power-on state to the cutoff state and when switching from the cutoff state to the power-on state, control is performed so that the amount of change in the current per unit time supplied to the electric motor 7 is less than the upper limit value. Therefore, it is possible to prevent the generation of noise and vibration due to a sudden change in the torque of the electric motor 7.

[0067] Furthermore, the present invention is not limited to the above-described embodiments, and it goes without saying that various improvements and modifications can be made without departing from the gist of the present invention. For example, in the present embodiment, the cutoff state is defined as a state in which the supply of current to the electric motor 7 is set to 0. However, as long as the heat generation of the electric motor 7 can be suppressed, there is no problem even if a slight current is flowing. That is, the cutoff state may be defined as a state in which the amount of current supplied to the electric motor 7 is reduced compared to the energized state.

[0068] In addition, in the present embodiment, a vehicle employing 4WS that controls the steering angles of the front wheels and the rear wheels respectively based on the steering operation of the steering wheel by the driver is taken as an example. However, as long as the vehicle can steer the rear wheels, it is not necessarily required to adopt 4WS.

[0069] In addition, in the present embodiment, the execution entity of the vehicle control processing program shown in FIG. 4 is the rear-wheel steering ECU 6, which is a dedicated electronic control unit for performing rear-wheel steering. However, part or all of the processing may be performed by an integrated control ECU that controls the entire vehicle. Alternatively, another in-vehicle device such as a navigation device may be the execution entity. Also, part of the processing may be performed by an external server device.

Explanation of Reference Numerals

[0070] 1... Vehicle control device, 2... Vehicle, 3... Vehicle body, 4... Steering wheel, 5... Power steering device, 6... Rear-wheel steering ECU, 7... Electric motor (steering actuator), 8... Rear-wheel steering device, 9A to 9D... Wheels, 13... Torque sensor, 23, 24... Drive shafts, 26... Trapezoidal screw, 27... Resolver, 28... Stroke sensor, 41... CPU

Claims

1. An electric steering actuator that generates a steering force for steering the rear wheels, A trapezoidal screw that converts the rotational motion generated based on the drive of the steering actuator into an axial linear motion for steering the rear wheels, Target steering angle setting means for setting a target steering angle of the rear wheels based on the operation of the steering wheel by the driver, Steering angle detection means for detecting the current steering angle of the rear wheels, A vehicle control device having energization control means for controlling the energization state of the steering actuator, wherein The energization control means supplies current to the steering actuator so that the current steering angle of the rear wheels approaches the target steering angle based on the cumulative current supplied to the steering actuator in a state where the target steering angle of the rear wheels is different from the current steering angle of the rear wheels, and performs control to alternately switch between an energization state and a cutoff state in which the current to the steering actuator is cut off.

2. The energization control means Performs first control to switch from the energization state to the cutoff state at the timing when the cumulative current supplied to the steering actuator since the start of the energization state reaches a threshold value, and Repeatedly executes second control to return from the cutoff state to the energization state after a predetermined time has elapsed since the cutoff state, thereby alternately switching between the energization state and the cutoff state. The vehicle control device according to Claim 1.

3. The energization control means sets the predetermined time shorter as the change speed of the target steering angle increases when the target steering angle changes with the passage of time. The vehicle control device according to Claim 2.

4. The energization control means Controls so that the amount of change in current per unit time supplied to the steering actuator is less than the upper limit value when switching from the energization state to the cutoff state and when switching from the cutoff state to the energization state. The vehicle control device according to any one of Claims 1 to 3.

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