Steering system for a vehicle

Abstract

In a steer-by-wire steering system for a vehicle, a control unit (16) controls operation of a steering actuator to make a steered angle of a wheel be in a prescribed relationship with a steering angle, and controls operation of a reaction force actuator (15) including a pair of reaction force motors (14) to make a reaction force be a value (Tt) corresponding to a steered state of the wheel, wherein the control unit is provided with a failure detection unit (36) that detects a failure of the reaction force motors, and when a failure of one of the reaction force motors is detected, gradually reduces an output (Tta) of the other reaction force motor to a prescribed limit value (TL).

Description

Technical Field

[0001] This invention relates to a steer-by-wire system for vehicles. Background Technology

[0002] A known steer-by-wire system for a vehicle includes: a steering member, such as a steering wheel configured for operation by a driver; and a steering mechanism mechanically separated from the steering member and configured to change the steering angle of the wheels. The steering mechanism is driven by a steering actuator that generates a driving force to change the steering angle of the wheels. A reaction force actuator applies a reaction force to the steering member in response to steering operation.

[0003] In such a steer-by-wire system, it is known to install a pair of steering motors at different locations on the steering mechanism to prevent steering force from becoming unavailable even if either steering motor fails (Patent Document 1). In this system, under normal operating conditions, the control unit determines the target output values ​​of the two steering motors by distributing the required steering force between them at a predetermined ratio. When one steering motor fails, the control unit changes the distribution ratio, thereby disabling the output of the failed steering motor and increasing the output of the other steering motor, or the healthy steering motor. As a result, steering is performed only by the healthy steering motor.

[0004] List of cited references

[0005] Patent documents

[0006] [Patent Document 1] JPH10-218000A Summary of the Invention

[0007] Technical issues

[0008] However, the steering system disclosed in Patent Document 1 only has one reaction force motor as the reaction force actuator. Therefore, when the reaction force motor fails, the steering reaction force applied to the steering component may suddenly decrease. In this case, the driver tends to over-turn the steering component, causing the vehicle to steer beyond the driver's expectations.

[0009] Here, it is conceivable to apply the concept of using two electric motors in the steering actuator described in Patent Document 1 to a reaction force actuator. In this case, if one reaction force motor fails, the output of the other reaction force motor will increase to compensate for the loss of the failed reaction force motor. As a result, the driver can operate the vehicle using the remaining reaction force motor without any inconvenience. However, if the remaining reaction force motor may also fail during cornering due to a sudden loss or reduction of reaction force, the driver may turn the steering wheel, causing the vehicle to steer in a way that exceeds the driver's expectations.

[0010] Against this background, the main objective of this invention is to prevent over-operation of the steering components in the event of a failure of the reaction force motor in a wire steering system, thereby preventing the vehicle from turning beyond the driver's expectations.

[0011] Solution to the problem

[0012] To achieve this objective, the present invention provides a steering system (1) for a vehicle (2), the steering system for the vehicle comprising: a steering member (10) configured to receive a steering operation; a steering mechanism (11) mechanically separated from the steering member and configured to steer a wheel (3); a steering angle sensor (21) detecting a steering angle (β) of the steering member; a steering angle sensor (32) detecting a steering angle (α) of the wheel; a steering actuator (13) configured to provide a driving force to the steering mechanism; and a reaction force actuator (15) comprising at least two... The system includes a reaction force motor configured to apply a reaction force to the steering member in response to the steering operation; and a control unit (16) that controls the operation of the steering actuator to make the steering angle have a predetermined relationship with the steering angle, and controls the operation of the reaction force actuator to make the reaction force a value (Tt) corresponding to the steering state of the wheel, wherein the control unit is provided with a fault detection unit (36) configured to detect faults in the reaction force motor, and when all reaction force motors except one are detected to be faulty, gradually reduce the output (Tta) of the remaining reaction force motors to a predetermined limit value (TL).

[0013] Therefore, even if one reaction force motor fails, the remaining reaction force motor will also fail during cornering. As the output of the other reaction force motor gradually decreases to the specified limit, the sudden loss of steering reaction force can be avoided, thus preventing the driver from operating the steering components in an excessive manner.

[0014] Preferably, during normal operation of the reaction force actuator, the control unit controls the reaction force motor to share the output of the reaction force actuator at a predetermined distribution ratio.

[0015] Therefore, regardless of which reaction motor fails, the resulting reduction in reaction force is the same, and the decrease in reaction force can be minimized at this time.

[0016] Preferably, the specified limit value is equal to or greater than the value (e.g., 1 Nm) that allows the steering member to be driven by the reaction force actuator when no external force is applied to the steering member.

[0017] Therefore, the reaction force actuator can drive the steering component using the output of the remaining reaction force motor that has not failed. Thus, when the driver releases the steering component during a turn, the control unit drives the reaction force motor, causing the steering component to return to the neutral position, similar to when the wheels and steering component return to the neutral position via self-aligning torque. Once both the steering angle and the turned angle are zero, the driver can easily recognize that the vehicle is moving straight forward from the neutral position of the steering component.

[0018] Preferably, the steering component includes a steering wheel (19) rotatable about the axis of the steering shaft (18), and the specified limit value is less than the value required to eliminate the torque (e.g., 6.35 Nm) corresponding to the load of the driver's arm placed on the steering wheel.

[0019] Therefore, after the control unit reduces the output of the remaining reaction force motor to a specified limit, if the driver applies a load to the steering wheel corresponding to the weight of only one arm, the steering wheel will rotate in the direction of the applied load. Since the weight of the arm is sufficient to rotate the steering wheel, the driver is required to apply some force to the arm to keep the steering wheel stationary. Therefore, to keep the steering wheel stationary, the driver is required to grip the steering wheel with both hands, or, if gripping the steering wheel with only one hand, to exert some effort in gripping it. Thus, even if the output of the other reaction force motor drops to a specified limit, the other reaction force motor malfunctions, and the reaction force completely disappears, it can prevent the driver from over-turning the steering wheel.

[0020] Preferably, the control unit gradually reduces the output of the residual reaction force motor at a reduction rate (R) that decreases as the vehicle speed (V) increases.

[0021] Because the rate of decrease in the output of the reaction motor becomes smaller at high speeds than at low speeds, it prevents the steering components from operating in ways the driver does not want. Therefore, it improves the vehicle's ability to travel straight.

[0022] Preferably, the control unit gradually reduces the output of the residual reaction force motor at a smaller rate when the vehicle is turning than when the vehicle is traveling straight.

[0023] When a vehicle turns, the driver applies a force to the steering mechanism. Therefore, when the steering reaction force decreases rapidly during a turn, the balance between the driver-applied steering force and the steering reaction force is disrupted, potentially causing the steering mechanism to be operated too quickly. However, with this arrangement, because the rate of decrease in the output of the steering reaction motor is small during a turn, the driver may have time to become familiar with the reduction in steering reaction force, thus avoiding unintentional rapid movement of the steering mechanism.

[0024] Preferably, the control unit gradually reduces the output of the residual reaction force motor only when the absolute value of the steering angle (|β|) is greater than a predetermined value (βth).

[0025] If the output of the reaction motor is reduced while the vehicle is traveling straight, and then the vehicle turns (the steering angle increases beyond a predetermined value), the driver may over-operate the steering mechanism because the steering reaction force is less than the driver expects. With this arrangement, since the output of the reaction motor does not decrease substantially when the vehicle is traveling straight, over-operation of the steering mechanism during subsequent turns is prevented.

[0026] Preferably, the steering system for the vehicle further includes a vibration device (15) for vibrating the steering member, wherein the control unit is configured to control the operation of the vibration device such that the vibration device is activated to vibrate the steering member when the vehicle is traveling straight and the output of the residual reaction force motor gradually decreases.

[0027] Therefore, by vibrating the steering components when the vehicle is traveling straight, the driver can be made aware that the steering reaction force is decreasing.

[0028] Preferably, the control unit drives the vibration device only when the vehicle is traveling straight, and controls the steering actuator so as not to change the steering angle when driving the vibration device.

[0029] Therefore, when the vehicle is traveling straight, it prevents the vehicle from swerving due to vibrations in the steering components.

[0030] Beneficial effects of the invention

[0031] Therefore, based on the above structure, in the event of a failure of all reaction motors except for one reaction motor in the steer-by-wire system, over-operation of the steering components is prevented, thereby preventing the vehicle from turning beyond the driver's expectations. Attached Figure Description

[0032] [ Figure 1 ] Figure 1 This is a schematic diagram of a steering system according to an embodiment of the present invention.

[0033] [ Figure 2 ] Figure 2 This is a flowchart of the reaction torque control executed by the control unit.

[0034] [ Figure 3 ] Figure 3 This is a timing diagram illustrating the typical operating modes of the steering system.

[0035] [ Figure 4A ] Figure 4A The gain graph is shown, which depends on the rate of decrease in vehicle speed.

[0036] [ Figure 4B ] Figure 4B The gain plot is shown, which depends on the rate of decrease of the steering angle.

[0037] [ Figure 5 ] Figure 5 This is a gain diagram depending on the rate of reduction of the steering angle according to an alternative embodiment of the present invention. Detailed Implementation

[0038] The following describes a steering system 1 for a vehicle 2 according to an embodiment of the present invention. Figure 1 As shown, the steering system 1 includes a steer-by-wire (SBW) system. The vehicle 2 equipped with the steering system 1 is a four-wheeled vehicle with left and right front wheels 3 and left and right rear wheels (not shown in the figure). The left and right front wheels 3 are controlled by the vehicle body 8 via their respective steering knuckles 7. Figure 1 (Only the lower outline of the front wheel 3 is shown in the diagram.) The steering system 1 provides support, allowing the steering angle α of the front wheel 3 to be changed, thus enabling the front wheel 3 to function as a steerable wheel. The steering angle α refers to the angle of the front wheel 3 relative to the fore-and-aft direction in the top view. Therefore, the steering system 1 changes the steering angle α of the front wheel 3.

[0039] The steering system 1 includes a steering member 10 operably mounted on the vehicle body 8 and a steering mechanism 11 for steering the front wheels 3. The steering mechanism 11 is provided with a steering actuator 13, which includes two steering motors 12 (a first steering motor 12A and a second steering motor 12B) providing driving force. Hereinafter, the two steering motors 12 may be collectively referred to as steering motor 12 without distinction. The steering member 10 is provided with a reaction force actuator 15, which includes two reaction force motors 14 (a first reaction force motor 14A and a second reaction force motor 14B) providing reaction torque T. Hereinafter, the reaction force motors 14 may be collectively referred to as reaction force motor 14 without distinction. The operation of the steering actuator 13 and the reaction force actuator 15 is controlled by a control unit 16. The steering actuator 13 and the reaction force actuator 15 are thus configured as a redundant system comprising two steering motors 12 and two reaction force motors 14, respectively. The control unit 16 can also be a redundant system that provides multiple control units 16.

[0040] The steering member 10 receives steering input from the driver. The steering member 10 includes a steering shaft 18 rotatably supported by the vehicle body 8 and a steering wheel 19 disposed at the end of the steering shaft 18. The steering shaft 18 is rotatably supported by a steering column 20 disposed on the vehicle body 8, and the rear end of the steering shaft 18 protrudes rearward from the steering column 20. The steering wheel 19 is connected to the rear end of the steering shaft 18 so as to rotate integrally with the steering shaft 18.

[0041] Each of the two reaction force motors 14 includes an electric motor connected to the steering shaft 18 via a gear mechanism. When the reaction force actuator 15 is driven by supplying power to the reaction force motor 14, the output (torque) of the reaction force motor 14 is transmitted to the steering shaft 18 as a rotational force (torque). The reaction force actuator 15 applies a torque as a reaction torque T to the steering member 10 in response to steering operation.

[0042] The steering system 1 is also provided with a steering angle sensor 21, which detects the rotation angle of the steering shaft 18 about its central axis as the steering angle β. The steering angle sensor 21 may be a rotary encoder, which is known per se. Furthermore, the steering system 1 is provided with a torque sensor 22, which detects the torque applied to the steering shaft 18 as the steering torque Ts. The torque sensor 22 detects the steering torque Ts applied to the portion of the steering shaft 18 located between the steering wheel 19 and the reaction torque actuator 15. The steering torque Ts is determined by the operating torque applied by the driver to the steering wheel 19 and the reaction torque T applied by the reaction torque actuator 15 to the steering shaft 18. The torque sensor 22 may include a torque sensor known per se, such as a magnetostrictive torque sensor or a strain gauge; alternatively, the steering torque may be estimated based on the current value flowing through the electric motor of the reaction torque actuator 15.

[0043] The steering system 1 also includes a pair of first rotation angle sensors 23 that detect the rotation angle θ of the first reaction force motor 14A and the second reaction force motor 14B, respectively. Each of the first rotation angle sensors 23 may include a known solver or rotary encoder.

[0044] The steering mechanism 11 has a rack 26 extending in the lateral direction of the vehicle. The rack 26 is supported by a gearbox 27 (not shown) to allow movement in the lateral direction of the vehicle. The left and right ends of the rack 26 are connected to steering knuckles 7, which support the left and right front wheels 3 via corresponding tie rods 30. When the rack 26 moves in the lateral direction of the vehicle, the steering angle α of the front wheels 3 is changed. The steering mechanism 11 is mechanically separated from the steering member 10.

[0045] Each of the two steering motors 12 includes an electric motor. When the steering actuator 13 operates by supplying power to at least one of the steering motors 12, the output (torque) of the one or more electric motors is converted into a lateral force that serves as the driving force on the rack 26. As the rack 26 moves laterally, the steering angle α of the left and right front wheels 3 changes accordingly.

[0046] The steering system 1 has a pair of second rotation angle sensors 31 (31A, 31B) that respectively detect the rotation angle θ of the two steering motors 12. Each second rotation angle sensor 31 can be a known solver or rotary encoder. Furthermore, the steering system 1 has a driven angle sensor 32 that detects the driven angle α of the front wheels 3. In this embodiment, the driven angle sensor 32 is a rack travel sensor that detects the rack position or the position of the rack 26 in the lateral direction and detects the driven angle α of the front wheels 3 relative to the rack position.

[0047] Control unit 16 is an electronic control unit including a CPU, memory, and storage device for storing programs. Steering angle sensor 21, torque sensor 22, two first rotation angle sensors 23, two second rotation angle sensors 31, and a steering angle sensor 32 are connected to control unit 16. Based on signals from these sensors, control unit 16 acquires the steering angle β, steering torque Ts, rotation angle θ of the reaction force motor 14, rotation angle θ of the steering motor 12, and steering angle α. Furthermore, control unit 16 is connected to vehicle speed sensor 33 and lateral acceleration sensor 34, and acquires the vehicle speed V and lateral acceleration Gy of the vehicle body 8.

[0048] Control unit 16 is connected to reaction force actuator 15 and steering actuator 13 to control reaction force actuator 15 (two reaction force motors 14) and steering actuator 13 (two steering motors 12). Control unit 16 controls steering actuator 13 according to steering angle β and controls reaction force actuator 15 according to steering angle α.

[0049] The control actions of the control unit 16 in SBW mode are described in detail below. The control unit 16 calculates a target steering angle αt that has a predetermined relationship with the actual steering angle β detected by the steering angle sensor 21. The control unit 16 can calculate the target steering angle αt (αt = β × K) by, for example, multiplying the steering angle β by a predetermined gear ratio K. The gear ratio K can be, for example, 0.01 to 0.5, and preferably 0.125. Then, the control unit 16 calculates a first current value A1 to be supplied to the steering actuator 13 based on the deviation Δα (= αt - α) between the target steering angle αt and the actual steering angle α, such that the steering angle α matches the target steering angle αt. That is, the control unit 16 performs feedback control of the steering actuator 13 based on the deviation Δα. The control unit 16 distributes the first current value A1 between the first steering motor 12A and the second steering motor 12B at a predetermined distribution ratio (e.g., 50%:50%). The first current value A1 supplied to the steering actuator 13 increases as the deviation Δα increases, and the output of the steering actuator 13 increases as the first current value A1 increases, resulting in an increase in the rate of change of the steering angle α.

[0050] The control unit 16 calculates the target reaction torque Tt to be generated by the reaction motor 14 based on the steering state of the front wheels 3, and in particular on the deviation Δα. The larger the deviation Δα of the steering angle α, the larger the target reaction torque Tt. The target reaction torque Tt can be calculated by multiplying Δα by a predetermined coefficient.

[0051] Control unit 16 distributes a target reaction torque Tt between the first reaction motor 14A and the second reaction motor 14B. The distributed target reaction torques of the first reaction motor 14A and the second reaction motor 14B are represented by Tta and Ttb, respectively. When no fault is detected in the reaction motor 14, the distributed reaction torque Tta of the first reaction motor 14A and the distributed reaction torque Ttb of the second reaction motor 14B are calculated, such that they have an equal distribution ratio of, for example, 50%:50%. Then, control unit 16 calculates a second current value A2 to be supplied to each reaction motor 14 based on the calculated corresponding distributed reaction torques Tta and Ttb. The current value to be supplied to the reaction motor 14 (distributed between the first reaction motor 14A and the second reaction motor 14B) can be determined by referring to a predetermined mapping for the target reaction torque Tt.

[0052] Control unit 16 supplies a second current value A2 to reaction force motor 14, and generates an output (torque) in reaction force actuator 15. The output of reaction force actuator 15 is applied to steering shaft 18 as a reaction torque T against the driver's input. As a result, the driver can receive a reaction force (resistance) against steering operation from steering wheel 19.

[0053] Additionally, the control unit 16 can apply a small rotational vibration to the steering wheel 19 by adding a component with alternating signs at high frequency to the second current value A2 supplied to the reaction force motor 14. This rotational vibration of the steering wheel 19 serves as an alarm to the driver holding the steering wheel 19. At this time, the reaction force actuator 15 functions as a vibration device to vibrate the steering member 10. The control unit 16 controls the reaction force actuator 15 to function as a vibration device.

[0054] Furthermore, the control unit 16 includes a fault detection unit 36 ​​for detecting a fault in either the steering motor 12 or the reaction motor 14. For example, when a reaction motor 14 fails, the available reaction torque is reduced by half. Thus, it can be determined that a reaction motor 14 has failed. When a reaction motor 14 is determined to have failed, the fault detection unit 36 ​​sequentially supplies torque command values ​​to the first reaction motor 14A and the second reaction motor 14B, causing the reaction motors 14 to generate reaction torque T respectively. By determining whether each reaction motor 14 has generated the corresponding reaction torque, the fault detection unit 36 ​​can identify which reaction motor 14 has failed.

[0055] Figure 2This is a flowchart of the reaction torque control executed by control unit 16. When control unit 16 is activated, it begins to execute the reaction torque control process described below. First, control unit 16 acquires the output of each sensor (step ST1). Then, control unit 16 calculates the target reaction torque Tt (total value) jointly generated by the reaction motors 14 based on the deviation Δα of the steering angle α (step ST2). Control unit 16 determines whether the fault detection unit 36 ​​has detected a fault in either reaction motor 14 (step ST3). When no fault is detected in the reaction motor 14 (ST3: No), control unit 16 sets the distribution ratio of the two reaction motors 14 to 50%:50%, which is the normal distribution ratio described above (step ST4).

[0056] Subsequently, the control unit 16 calculates the second current value A2 to be supplied to the reaction force motor 14, and distributes the second current value A2 between the two reaction force motors 14 according to the aforementioned distribution ratio (step ST5). The control unit 16 supplies the calculated second current value A2 to the reaction force motor 14 (step ST6). As a result, the output of the reaction force actuator 15 is applied to the steering member 10 as a reaction torque T. After outputting the second current value A2 in step ST6, the control unit 16 repeats the above process.

[0057] On the other hand, when any fault of the reaction motor 14 is detected in step ST3 (ST3: Yes), the control unit 16 determines whether the first reaction motor 14A has failed (step ST7). When a fault of the first reaction motor 14A is detected (ST7: Yes), the control unit 16 sets the distribution ratio of the first reaction motor 14A to 0 (step ST8). When no fault of the first reaction motor 14A is detected (ST7: No), the control unit 16 sets the distribution ratio of the second reaction motor 14B to 0 (step ST9). Subsequently, the control unit 16 gradually reduces the distribution ratio of the other reaction motor 14 that has not failed (hereinafter referred to as "healthy reaction motor 14") from a normal distribution ratio of 50% (step ST10). In step ST10, the output of the healthy reaction motor 14 gradually decreases before the control flow enters step ST5. Again, the second current value is calculated in step ST5, and the second current value A2 is output in step ST6. Repeat this process until the output of the reaction force actuator 15 reaches its limit value and remains at that value.

[0058] When the control unit 16 performs reaction torque control in this manner, the reaction torque T, which is the output of the reaction force motor 14, changes as described below. Figure 3 This is a timing diagram illustrating an example of the operation of steering system 1. (Example) Figure 3As shown, one of the normally operating reaction motors 14A and 14B (the first reaction motor 14A in the illustrated example) malfunctions at time t1. The fault detection unit 36 ​​of the control unit 16 identifies the malfunctioning reaction motor 14 at time t2. Upon identification of the malfunctioning reaction motor 14, the control unit 16 sets the output of the malfunctioning reaction motor 14 to 0 and begins to gradually reduce the output of the normal reaction motor 14 from 50% to a predetermined limit value TL (a descent process). As the descent process proceeds, the output of the healthy reaction motor 14 reaches the limit value TL at time t3. After time t3, the control unit 16 maintains the output of the healthy reaction motor 14 at the limit value TL. The limit value TL can be any value, as long as it is below the normal distribution ratio of the healthy reaction motor 14, or 50%.

[0059] Thus, when a malfunction of the reaction motor 14 is detected, the control unit 16 gradually reduces the output of the healthy reaction motor 14 to a predetermined limit value TL. As a result, the output of the reaction motor 14 gradually decreases to the predetermined limit value TL at time point t3, and the driver becomes familiar with the reduction in reaction torque, ensuring that even if the healthy reaction motor 14 malfunctions during cornering maneuvers of the vehicle 2, the reaction torque T will not be suddenly lost. Therefore, excessive steering wheel turning 19 due to the lack of reaction torque T is prevented.

[0060] The limit value TL is expressed herein as a percentage, or more specifically as a percentage relative to the normal reaction torque value. Typically, the limit value TL is equal to or greater than the value at which the reaction force actuator 15 can rotatably drive the steering wheel 19 in the absence of any external force, such as the steering torque applied to the steering wheel 19 by the driver. Since the steering member 10 generates rotational friction during rotational operation, the limit value TL is preferably 1 Nm or greater, as a value capable of overcoming this frictional resistance. Preferably, the limit value TL is less than the output of the reaction force motor 14 required to counteract the torque applied to the steering wheel 19 when a load corresponding to the weight of the driver's arm acts on the steering wheel 19. Assuming a load of 3.5 kg force on the driver's arm and a radius of 0.18 m for the steering wheel 19, the torque applied to the steering wheel 19 by the weight of the driver's arm is 6.35 Nm. Therefore, the limit value TL is preferably 1 Nm or greater and less than 6.35 Nm.

[0061] By setting the limit value TL to 1 Nm or greater in this manner, the reaction force actuator 15 can drive the steering member 10 through the output of the healthy reaction force motor 14. Therefore, when the driver releases the steering wheel 19 during a turn, the control unit 16 can drive the reaction force actuator 15 to return the steering member 10 to the neutral position in a manner similar to how the self-aligning torque returns the front wheels 3 and the steering member 10 to the neutral position. Furthermore, since both the steering angle β and the turned angle α become 0°, the driver can easily recognize that the vehicle is moving straight from the neutral position of the steering wheel 19.

[0062] Furthermore, by setting the limit value TL to less than 6.35 Nm, if the driver applies a load corresponding to the weight of one arm to the steering wheel 19 while the output of the health reaction motor 14 is controlled to the limit value TL in the control unit 16, the steering wheel 19 will rotate in the direction of the load applied by the arm. In other words, the weight of one arm applied to the steering wheel 19 is sufficient to turn the steering wheel 19. Therefore, when the driver attempts to drive the vehicle straight, he needs to grip the steering wheel 19 with both hands. If the driver grips the steering wheel 19 with only one arm and attempts to drive the vehicle straight, he needs to exert some effort on the arm gripping the steering wheel 19. Therefore, even if the reaction motor 14 malfunctions and the reaction torque T is reduced to the maximum extent, it prevents the driver from excessively turning the steering wheel 19.

[0063] The gradual reduction process or ramp-down process is described below. When gradually reducing the output of the health reaction motor 14, the control unit 16 selects a reduction rate R and gradually reduces the output of the health reaction motor 14 according to the selected reduction rate R. The reduction rate R used here should be interpreted broadly. In this embodiment, the gradual reduction of the reaction torque is linear, or the reaction torque decreases in a fixed increment per unit time. The larger the reduction rate R, the larger the increment of the reduction in reaction torque per unit time becomes. In other words, the larger the reduction rate R, the shorter the time it takes for the output of the reaction motor 14 to decrease from 50% to the limit value TL. The control unit 16 can set the reduction rate R to a variable value rather than a constant value. For example, the reduction rate R can be calculated such that the control unit 16 sets a standard reduction rate of 5% / second as the standard value of the reduction rate R and changes the reduction rate R RS according to various parameters indicating the dynamic state of the vehicle 2.

[0064] More specifically, the control unit 16 may calculate the reduction rate R as described below. Figure 4A and Figure 4B It is a gain graph used to set the reduction rate; Figure 4A The vehicle speed gain G1 is shown. Figure 4B The steering angle gain G2 is shown. (Example) Figure 4AAs shown, the vehicle speed gain G1 is set such that the higher the vehicle speed V, the smaller the vehicle speed gain G1. Figure 4B As shown, the steering angle gain G2 is set such that the larger the absolute value of the steering angle β, |β|, the smaller the steering angle gain G2. When the absolute value of the steering angle β, |β|, is equal to or less than the predetermined value βth, or when the vehicle is traveling substantially straight, the steering angle gain G2 is set to 1. The control unit 16 sets the vehicle speed gain G1 based on the vehicle speed V from the vehicle speed gain mapping, and sets the steering angle gain G2 based on the steering angle β from the steering angle gain mapping. The control unit 16 calculates the reduction rate R by multiplying the standard reduction rate RS by the vehicle speed gain G1 and the steering angle gain G2. Therefore, the reduction rate R during cornering is less than the reduction rate R during the straight-traveling state of vehicle 2.

[0065] Thus, as the vehicle speed V increases, the control unit 16 sets a smaller rate of decrease R in the output of the health reaction motor 14. As a result, the rate of decrease R in the output of the reaction motor 14 at high speeds is smaller than the rate of decrease R at low speeds, thereby preventing the steering wheel 19 from turning against the driver's intention. In particular, the driver gradually becomes accustomed to a state where the effort required for steering operations is significantly reduced. Therefore, the vehicle 2 can be driven straight in a stable manner.

[0066] Because the driver applies steering torque to the steering wheel 19 during the turning of vehicle 2, if the reaction torque T decreases rapidly during the turning, the balance between the driver's steering torque and the reaction torque T may not be maintained, potentially causing the steering wheel 19 to over-turn. In this embodiment, as... Figure 4B As shown, the control unit 16 selects a smaller rate of decrease R in the output of the health reaction motor 14 during cornering compared to the period of straight driving. Therefore, because the rate of decrease R in the output of the reaction motor 14 is smaller during cornering, the driver can become familiar with the reduction in reaction torque T and can avoid unintentional over-turning of the steering wheel 19.

[0067] If one reaction motor 14 fails, and the output of the other reaction motor 14 continues to decrease while the vehicle 2 is traveling straight, the driver may over-steer during the subsequent turn because the reaction torque T may be less than expected (where the absolute value of the steering angle β is equal to or greater than the predetermined value βth).

[0068] Therefore, in this embodiment, the control unit 16 controls the reaction force actuator 15 so that the steering member 10 can be used as a vibration device. For example, when a reaction force motor 14 fails, a vibration component can be superimposed on the output of the normal reaction force motor 14. As a result, the driver can be notified of the failure of a reaction force motor 14, the decrease in reaction torque T can be identified, and the driver can be prompted to take necessary measures.

[0069] Control unit 16 can drive the reaction force actuator 15, which serves as a vibration device for steering member 10, only when vehicle 2 is traveling straight. Control unit 16 controls steering actuator 13 such that the steering angle α remains unchanged when driving reaction force actuator 15, which serves as a vibration device. More specifically, control unit 16 can perform low-pass filtering on the steering angle β used to calculate the target steering angle αt of steering actuator 13. This prevents the target steering angle αt from changing in response to small changes in steering angle β. However, the control method for steering actuator 13 is not limited to this. By controlling steering actuator 13 in this way, control unit 16 prevents vehicle 2 from swerving due to vibration of steering member 10 when vehicle 2 is traveling straight.

[0070] Figure 5 An alternative embodiment of the invention is shown. In this alternative embodiment, the steering angle gain G2 is substantially zero when the steering angle β is equal to or less than a predetermined value βth, or when the vehicle is traveling substantially straight. Once the absolute value |β| of the steering angle β exceeds the predetermined value βth, the steering angle gain G2 begins to increase as the absolute value |β| of the steering angle β increases. Therefore, in this alternative embodiment, the rate of decrease R of the reaction torque T is zero when the steering angle β is equal to or less than the predetermined value βth, or when the vehicle is traveling substantially straight.

[0071] As a result, the output of the reaction motor 14 does not decrease during straight-line driving, thus preventing the steering wheel 19 from being over-turned at the start of subsequent cornering maneuvers. On the other hand, the rate of decrease R of the reaction torque can be relatively large when the vehicle turns to a certain extent. If the reaction torque T gradually decreases during cornering, the driver can become familiar with the decrease in the reaction torque T.

[0072] In this alternative embodiment, the steering wheel 19 can also vibrate when the vehicle is traveling straight, but in this alternative embodiment, the reaction torque is not reduced when traveling straight, so the control unit 16 does not need to operate the reaction force actuator 15 as a vibration device. Conversely, if the reaction force actuator 15 continues to operate as a vibration device when the vehicle is traveling straight, the reaction force actuator 15 may overheat, and this situation can be avoided.

[0073] The invention has been described according to specific embodiments, but the invention is not limited to these embodiments and can be modified in various ways without departing from the scope of the invention. In the above embodiments, the reaction force actuator 15 includes a pair of reaction force motors 14. However, the reaction force actuator 15 may also include three or more reaction force motors 14. In this case, when a failure is detected in all but one reaction force motor, the control unit 16 gradually reduces the target reaction torque Tt of the remaining normally operating reaction force motors 14.

[0074] Furthermore, in the aforementioned embodiments, the control unit 16 typically sets the distribution ratio of the two reaction motors 14 to 50%:50%, but it could also be 60%:40%, 70%:30%, or similar ratios. In this case, when one reaction motor 14 fails, the control unit 16 can set the distribution ratio of the normal reaction motor 14 to 50%, and then gradually reduce the output to a predetermined limit value TL. Alternatively, when one reaction motor 14 fails, the control unit 16 can gradually reduce the output of the normal reaction motor 14 from its current output value. Moreover, the distribution ratio of the two steering motors 12 is not limited to 50%:50%.

[0075] Furthermore, the reduction rate R can be defined as the amount of reduction proportional to each current value, and in this case, the reduction rate R can be represented by an exponential decay constant.

[0076] Furthermore, as long as the spirit of the invention is not departed from, the specific configuration and arrangement, quantity, angle, procedure, etc., of each component and part can be appropriately changed. On the other hand, not all components shown in the above embodiments are indispensable, and they can be appropriately selected.

[0077] List of reference numerals

[0078] 1: Steering system 2: Vehicle

[0079] 3: Front wheel 10: Steering component

[0080] 11: Steering mechanism 13: Steering actuator

[0081] 14: Reaction Motor 14A: First Reaction Motor

[0082] 14B: Second reaction force motor; 15: Reaction force actuator

[0083] 16: Control unit; 18: Steering shaft

[0084] 19: Steering wheel 21: Steering angle sensor

[0085] 32: Steering angle sensor; 33: Vehicle speed sensor

[0086] 34: Lateral acceleration sensor; 36: Fault detection unit

[0087] R: Reduction rate; T: Reaction torque (reaction force)

[0088] TL: Limit value; Tt: Target reaction torque

[0089] Tta, Ttb: Distribute reaction torque (output)

[0090] V: Vehicle speed; α: Steering angle

[0091] β: Steering angle; βth: Predetermined value

[0092] |β|: Absolute value of the steering angle

Claims

1. A steering system for a vehicle, the steering system for a vehicle comprising: A steering component configured to receive a steering operation; A steering mechanism, mechanically separated from the steering member and configured to steer the wheels; A steering angle sensor that detects the steering angle of the steering component; A steering angle sensor that detects the steering angle of the wheel; A steering actuator configured to provide driving force to the steering mechanism; A reaction force actuator, the reaction force actuator comprising at least two reaction force motors and configured to apply a reaction force to the steering member in response to the steering operation; as well as The control unit controls the operation of the steering actuator to make the steering angle have a predetermined relationship with the steering angle, and controls the operation of the reaction force actuator to make the reaction force a value corresponding to the steering state of the wheel. The control unit includes a fault detection unit configured to detect faults in the reaction force motors. Upon detecting faults in all but one reaction force motor, the unit gradually reduces the output of the remaining reaction force motors to a predetermined limit value. The control unit gradually reduces the output of the residual reaction force motor at a reduction rate (R) that decreases as the vehicle speed increases.

2. The steering system for a vehicle according to claim 1, wherein, During normal operation of the reaction force actuator, the control unit controls the reaction force motor to share the output of the reaction force actuator at a predetermined distribution ratio.

3. The steering system for a vehicle according to claim 1, wherein, The specified limit value is equal to or greater than the value that allows the steering component to be driven by the reaction force actuator when no external force is applied to the steering component.

4. The steering system for a vehicle according to claim 3, wherein, The steering component includes a steering wheel that can rotate about the axis of the steering shaft, and the specified limit value is less than the value required to eliminate the load corresponding to the weight of the driver's arm placed on the steering wheel.

5. The steering system for a vehicle according to any one of claims 1 to 4, wherein, The control unit gradually reduces the output of the residual reaction force motor at a smaller rate when the vehicle is turning than when the vehicle is going straight.

6. The steering system for a vehicle according to any one of claims 1 to 4, wherein, The control unit gradually reduces the output of the residual reaction force motor only when the absolute value of the steering angle is greater than a predetermined value (βth).

7. The steering system for a vehicle according to any one of claims 1 to 4, wherein, The steering system for the vehicle also includes a vibration device for vibrating the steering component, wherein the control unit is configured to control the operation of the vibration device such that the vibration device is activated to vibrate the steering component when the vehicle is traveling straight and the output of the residual reaction force motor gradually decreases.

8. The steering system for a vehicle according to claim 7, wherein, The control unit drives the vibration device only when the vehicle is traveling straight, and controls the steering actuator so as not to change the steering angle when driving the vibration device.

Images