Steering control device and electric power steering device

Abstract

In the present invention: a target steering torque setting unit sets a target steering torque for a steering mechanism; a steering auxiliary torque calculation unit calculates, on the basis of the deviation between the target steering torque and a steering torque which acts on the steering mechanism, a steering auxiliary torque for causing the steering torque to follow the target steering torque; and a current drive unit controls a motor current which is a current flowing through a motor that imparts the steering auxiliary torque to the steering mechanism. The target steering torque setting unit obtains a base torque that is a basic component of the target steering torque, estimates an instantaneous value of a road surface reaction force on the basis of the steering torque and the current, calculates the absolute value of the road surface reaction force by setting, as a starting point, the instantaneous value at a predetermined steering angle in an increased state of the steering mechanism, and reduces the base torque on the basis of the absolute value of the road surface reaction force.

Description

Steering control device and electric power steering system

[0001] This disclosure relates to a steering control device and an electric power steering device.

[0002] The vehicle behavior instability detection device described in Patent Document 1 includes: a road surface reaction torque actual change amount detection means for detecting the actual change amount of road surface reaction torque per predetermined time of the road surface reaction torque received by the vehicle's tires from the road surface; a road surface reaction torque norm change amount calculation means for calculating the road surface reaction torque norm change amount per predetermined time of the road surface reaction torque in the vehicle's driving state; a steering state detection means for detecting the steering state of the vehicle; and a vehicle behavior instability determination means for determining whether the vehicle's behavior is unstable based on the detected value of the actual change amount of road surface reaction torque, the calculated value of the road surface reaction torque norm change amount, and the steering state detection result. The steering state includes steering speed and steering torque, and the vehicle behavior instability determination means determines that the vehicle is in the process of turning the steering wheel back when the sign of the steering speed and the sign of the steering torque are different, and prohibits determining that the vehicle's behavior is unstable.

[0003] The vehicle steering device described in Patent Document 2 assists control of the vehicle's steering system by driving and controlling a motor that assists the steering force, and includes a target steering torque generation unit that generates a target steering torque for the motor. The target steering torque generation unit generates a target steering torque corresponding to the difference between a torque signal corresponding to the steering angle and vehicle speed and a value obtained by multiplying a physical quantity generated by tire slip by a predetermined proportionality constant.

[0004] Japanese Patent Publication No. 2008-230463, International Publication No. 2020 / 170602

[0005] As described above, the vehicle behavior instability detection device described in Patent Document 1 determines the stability of the vehicle's behavior from the actual change in road surface reaction torque per predetermined time, the reference change in road surface reaction torque, and the steering state. However, the absolute value of the road surface reaction force is not applied in determining stability. The vehicle steering device described in Patent Document 2 provides feedback to the driver that the tire grip has been lost, based on the self-aligning torque calculated from the motor angular velocity, motor angular acceleration, assist torque, and steering torque. However, in order to calculate the self-aligning torque, it is necessary to set the static friction in advance. Therefore, there is a discrepancy between the friction that actually occurs and the static friction that has been set in advance.

[0006] A first aspect of this disclosure has been made to solve the above-mentioned problems, and comprises: a target steering torque setting unit that sets a target steering torque for a steering mechanism; a steering assist torque calculation unit that calculates a steering assist torque to make the steering torque follow the target steering torque based on the deviation between the target steering torque and the steering torque acting on the steering mechanism; and a current drive unit that controls a motor current, which is the current flowing to a motor that provides the steering assist torque to the steering mechanism. The target steering torque setting unit determines a base torque, which is the basic component of the target steering torque, estimates an instantaneous value of road surface reaction force from the steering torque and the current, calculates the absolute value of the road surface reaction force starting from the instantaneous value at a predetermined steering angle in the state of increasing steering of the steering mechanism, and reduces the base torque based on the absolute value of the road surface reaction force.

[0007] A second aspect of this disclosure may be an electric power steering system comprising the motor, a motor rotational angular velocity detection unit for detecting the rotational angular velocity of the motor, and the steering control device described above.

[0008] According to this disclosure, it is possible to provide the driver with an appropriate steering feel in accordance with the road surface conditions.

[0009] This is a schematic diagram showing an example configuration of an electric power steering device according to an embodiment of this disclosure. This is a block diagram showing an example configuration of a control unit according to an embodiment of this disclosure. This is a flowchart illustrating an overview of the operation of the control unit according to an embodiment of this disclosure. This is an explanatory diagram illustrating an overview of the target steering torque setting unit according to an embodiment of this disclosure. This is a block diagram showing an example configuration of the target steering torque setting unit according to an embodiment of this disclosure. This is an explanatory diagram illustrating a method for calculating the absolute value of the estimated road surface reaction force according to an embodiment of this disclosure. This is a diagram showing an example of the calculation of the correction gain according to an embodiment of this disclosure. This is a diagram illustrating the components of steering torque. This is a diagram showing an example of a base torque map. This is a diagram showing an example of hysteresis gain. This is a diagram showing an example of a damper gain map. This is a diagram showing a first example of road surface information control. This is a diagram showing a second example of road surface information control. This is a diagram showing an example of the change in target steering torque over time. This is a diagram showing an example of the correlation between target steering torque and steering angle.

[0010] Embodiments of the present disclosure will be described below with reference to the drawings. Elements common to or corresponding to each figure are denoted by the same reference numerals, and unless otherwise specified, their descriptions will be used accordingly. (Electric Power Steering System) First, an example of the configuration of an electric power steering system according to an embodiment of the present disclosure will be described. Figure 1 is a schematic diagram showing an example of the configuration of an electric power steering system PS according to an embodiment of the present disclosure.

[0011] The electric power steering system PS comprises a steering wheel 1, a steering shaft 2, steering wheels 3, a steering angle sensor 4, a torque sensor 5, a motor 6, a reduction mechanism 7, a vehicle speed sensor 8, a current sensor 9, a motor rotation angle sensor 10, and a control unit 11. The configuration example shown in Figure 1 is a typical hardware configuration for an electric power steering system PS. An electric power steering system PS having such a configuration may be mass-produced and installed in a vehicle.

[0012] The steering wheel 1 can be operated by the driver of the vehicle to give a steering angle to the steering wheels 3 of the vehicle. The steering wheel 1 rotates around the axial direction of the steering shaft 2 in response to steering wheel operation. One end of the steering shaft 2 is connected to the center of the steering wheel 1 and rotates in accordance with the rotation of the steering wheel 1. The steering wheels 3 are provided on both the left and right sides of the vehicle and steer in accordance with the rotation of the steering shaft 2. In this application, the mechanism for steering the steering wheels 3, including the steering wheel 1 and the steering shaft 2, may be referred to as the "steering mechanism" or simply "steering".

[0013] The steering angle sensor 4 is located on the steering wheel 1 and detects the steering angle of the steering wheel 1. The torque sensor 5 is located on the steering shaft 2 and detects the steering torque acting on the steering shaft 2. The torque sensor 5 corresponds to the steering torque detection unit. The motor 6 is connected to the other end of the steering shaft 2 via a reduction mechanism 7 and provides steering assist torque to the steering shaft 2 through its rotation. The steering assist torque is also called "motor assist torque". The vehicle speed sensor 8 detects the vehicle speed, i.e., the vehicle speed. The current sensor 9 detects the current flowing through the motor 6 (sometimes called "motor current" in this application). The motor rotation angle sensor 10 detects the rotation angle of the motor 6.

[0014] The control unit 11 controls the drive of the motor 6 based on the steering angle detected by the steering angle sensor 4, the steering torque detected by the torque sensor 5, the vehicle speed detected by the vehicle speed sensor 8, the current of the motor 6 detected by the current sensor 9, and the rotation angle of the motor 6 detected by the motor rotation angle sensor 10, thereby generating steering assist torque for the steering mechanism. In other words, the control unit 11 calculates the steering assist torque to be applied to the steering shaft 2 based on the steering torque, vehicle speed, current, and rotation angle. The control unit 11 controls the current of the motor 6 required to generate that steering assist torque. The control unit 11 corresponds to a steering control device.

[0015] (Steering control device) Next, the control unit 11 as a steering control device according to this embodiment will be described. Figure 2 is a block diagram showing an example of the configuration of the control unit 11 according to this embodiment. The control unit 11 includes a differentiator 24a, a target steering torque setting unit 22, a torque feedback calculation unit 23, and a current drive unit 12.

[0016] The differentiator 24a receives the rotation angle of the motor 6 from the motor rotation angle sensor 10. The differentiator 24a differentiates the received rotation angle of the motor 6 to calculate the rotational angular velocity of the motor 6 (sometimes referred to as "motor rotational angular velocity" in this application). In the example in Figure 2, the differentiator 24a, together with the motor rotation angle sensor 10, constitutes a motor rotational angular velocity detection unit 24. The motor rotational angular velocity detection unit 24 detects the motor rotational angular velocity.

[0017] The target steering torque setting unit 22 sets a target steering torque for the steering mechanism. The target steering torque setting unit 22 receives motor rotation angular velocity, steering angle of the steering wheel 1, vehicle speed, and motor rotation angular velocity from the steering angle sensor 4, vehicle speed sensor 8, and motor rotation angular velocity detection unit 24, respectively. The target steering torque setting unit 22 calculates the target steering torque for the steering mechanism based on the steering angle, vehicle speed, and motor rotation angular velocity. An example of the configuration of the target steering torque setting unit 22 will be described later.

[0018] The torque feedback calculation unit 23 calculates the steering assist torque based on the deviation between the target steering torque set by the target steering torque setting unit 22 and the steering torque detected by the torque sensor 5. The steering assist torque is required for control to make the steering torque follow the target steering torque. The torque feedback calculation unit 23 corresponds to the steering assist torque calculation unit. The torque feedback calculation unit 23 outputs the calculated steering assist torque to the current drive unit 12. For example, the torque feedback calculation unit 23 may use any of the following controllers for calculating the steering assist torque from the deviation between the target steering torque and the steering torque: a PI controller, a PD controller, or a PID controller. The current drive unit 12 controls the current flowing to the motor 6 in order to generate the steering assist torque calculated by the torque feedback calculation unit 23 in the steering mechanism.

[0019] Furthermore, among the components of the control unit 11, those other than the current drive unit 12, namely the differentiator 24a, the target steering torque setting unit 22, and the torque feedback calculation unit 23, or any combination thereof, may be implemented by a computer system. The computer system comprises a processor and memory. The processor is, for example, a CPU (Central Processing Unit). The memory may include both volatile memory and non-volatile memory. The volatile memory is mainly used as the processor's work area. The non-volatile memory non-temporarily stores various data used in processing by the computer system or acquired data. As the computer system, for example, a microcomputer may be used. Alternatively, other types of arithmetic circuits may be used instead of a processor. The current drive unit 12 is implemented by an analog circuit comprising a plurality of switching elements. As the switching elements, for example, FETs (Field Effect Transistors) may be used.

[0020] Next, an overview of the operation of the control unit 11 according to this embodiment will be described. Figure 3 is a flowchart illustrating an overview of the operation of the control unit 11 according to this embodiment. The process in Figure 3 is repeatedly executed at each preset control cycle. (Step S1) The control unit 11 acquires the steering angle detected by the steering state detection unit 21, the vehicle speed detected by the vehicle speed sensor 8, the motor current detected by the current sensor 9, the steering torque detected by the torque sensor 5, and the motor rotation angle detected by the motor rotation angle sensor 10. Then, the differentiator 24a of the control unit 11 acquires the motor rotation angle and calculates the motor rotation angular velocity by differentiating the acquired motor rotation angle.

[0021] (Step S2) The target steering torque setting unit 22 of the control unit 11 sets the target steering torque using the acquired steering angle, vehicle speed, motor rotation angular velocity, steering torque, and motor current. (Step S3) The torque feedback calculation unit 23 of the control unit 11 calculates the steering assist torque based on the difference between the target steering torque set by the target steering torque setting unit 22 and the steering torque detected by the torque sensor 5. (Step S4) The current drive unit 12 of the control unit 11 controls the current flowing to the motor 6 based on the steering assist torque calculated by the torque feedback calculation unit 23. By controlling the motor current, which is the current flowing to the motor 6, the current drive unit 12 generates the steering assist torque in the steering mechanism.

[0022] Next, an overview of the target steering torque setting unit 22 according to this embodiment will be described. The target steering torque setting unit 22 determines the base torque based on the steering angle detected by the steering state detection unit 21. The target steering torque includes the base torque bt, friction torque ft, and viscous torque vt as components (Figure 8). The base torque is the basic component that constitutes the target steering torque. The viscous torque is the component that gives a viscous feel to steering. The friction torque is the component that gives frictional characteristics to steering.

[0023] The target steering torque setting unit 22 calculates an estimated value of the road surface reaction force (sometimes referred to as "estimated road surface reaction force" in this application) based on the steering torque detected by the torque sensor 5 and the motor current detected by the current sensor 9, using the instantaneous value of the estimated road surface reaction force for each predetermined control cycle. The target steering torque setting unit 22 then monitors the steering state of the steering mechanism based on the steering angle detected by the steering state detection unit 21. In this embodiment, as illustrated in Figure 4, when the steering state is in an increased steering state, the target steering torque setting unit 22 sets the instantaneous value of the estimated road surface reaction force obtained when the detected steering angle becomes a predetermined steering angle (steering angle starting point) as the starting point. The target steering torque setting unit 22 calculates the difference between the instantaneous value (current value) of the estimated road surface reaction force for each control cycle and the starting point as the absolute value of the estimated road surface reaction force. The target steering torque setting unit 22 reduces the base torque based on the absolute value of the estimated road surface reaction force.

[0024] The target steering torque setting unit 22 reduces the base torque, for example, as the absolute value of the estimated road surface reaction force becomes relatively smaller compared to the absolute value of the reference road surface reaction force. The reference road surface reaction force corresponds to the reference value of the reference reaction force. The target steering torque setting unit 22 calculates the reference value of the reference road surface reaction force for each control cycle using a model that has been set in advance based on the steering angle detected by the steering state detection unit 21. When the steering state is in an increased steering state, the target steering torque setting unit 22 sets the instantaneous value of the reference road surface reaction force obtained when the detected steering angle becomes a predetermined steering angle (sometimes called the "steering angle starting point" in this application) as the starting point of the reference road surface reaction force. The target steering torque setting unit 22 then calculates the difference between the instantaneous value of the reference road surface reaction force and the starting point of the reference road surface reaction force for each control cycle as the absolute value of the reference road surface reaction force. The absolute value of this reference road surface reaction force corresponds to the increment from the instantaneous value of the reference road surface reaction force at the steering angle starting point to the instantaneous value of the reference road surface reaction force at the current steering angle.

[0025] Generally, the frictional force between the wheel and the road surface changes when steering is increased. In this embodiment, in the steering increase state, which is the phase in which the absolute value of the steering angle increases, the base torque, which is the basic component of the target steering torque, is corrected using the absolute value of the estimated road surface reaction force obtained by subtracting the starting point from the instantaneous value of the estimated road surface reaction force at each time. Therefore, estimation errors in the estimated road surface reaction force due to fluctuations in frictional force immediately after the start of steering increase are eliminated, and a target steering torque that is more in line with the road surface condition is obtained. In this embodiment, the base torque, which is determined based on the steering angle, may be reduced at each control cycle so as not to fall below the base torque at the start of steering increase. In this application, "road surface reaction force" means road surface reaction torque unless otherwise specified.

[0026] Generally, steering torque decreases as the road surface friction coefficient μ decreases. In this embodiment, the target steering torque setting unit 22 reduces the base torque as the estimated road surface reaction force becomes relatively smaller compared to the reference road surface reaction force. As a result, the target steering torque decreases, and by changing the steering feel given to the driver, changes in road surface conditions can be communicated more accurately.

[0027] Next, an example of the configuration of the target steering torque setting unit 22 according to this embodiment will be described. Figure 5 is a block diagram showing an example of the configuration of the target steering torque setting unit 22 according to this embodiment. The target steering torque setting unit 22 includes a base torque calculation unit 27, a friction torque calculation unit 28, a viscous torque calculation unit 29, a target steering torque calculation unit 30, a road surface information calculation unit 31, and a base torque correction unit 36.

[0028] The base torque calculation unit 27 calculates the base torque based on the steering angle and vehicle speed. When calculating the base torque, the base torque calculation unit 27 uses, for example, a base torque map. The base torque map is a data map that shows the base torque corresponding to the steering angle and vehicle speed. The base torque map illustrated in Figure 9 shows the characteristic that the base torque increases monotonically with increasing steering angle at a constant vehicle speed. However, the rate of increase of the base torque decreases with increasing absolute value of the steering angle, and the base torque converges to a constant value. Furthermore, the base torque map shows the characteristic that the base torque increases monotonically with increasing vehicle speed at a constant base torque. In the example in Figure 9, the absolute value of the base torque with respect to the steering angle increases as the vehicle speed increases.

[0029] The base torque calculation unit 27 may have base torque maps set for each predetermined speed band, and the system may select the base torque map corresponding to the band to which the input speed belongs. For example, the base torque map may be set for each of the three speed bands. The three speed bands correspond to low speed, medium speed, and high speed, as illustrated in Figure 9. The base torque map for each band shows the base torque corresponding to the steering angle for a representative value of the speed in that band. The base torque calculation unit 27 refers to the selected base torque map and calculates the base torque corresponding to the steering angle. The base torque calculation unit 27 outputs the calculated base torque to the base torque correction unit 36.

[0030] The friction torque calculation unit 28 calculates the friction torque based on the steering angle, steering angular velocity, and vehicle speed. The friction torque calculation unit 28 calculates the friction torque using, for example, equation (1). Equation (1) shows that the friction torque is calculated as the product of the sign function sign(θ), the function 1-exp(-abs(dθm)), and the hysteresis gain Tf. The hysteresis gain is also called the friction sensation gain. In equation (1), the sign function sign(θ) gives -1, 0, and 1 when the steering angle θ is negative, 0, and positive, respectively. The function 1-exp(-abs(dθm)) gives a value that increases monotonically as the absolute value of the steering angular velocity dθm increases. The function 1-exp(-abs(dθm)) gives a minimum value of 0 when the steering angular velocity dθm is zero, and asymptotically approaches a maximum value of 1 as the steering angular velocity dθm increases.

[0031] The hysteresis gain Tf is given, for example, as a function of vehicle speed as shown in Figure 10. The hysteresis gain Tf is given by the vehicle speed from zero to a certain critical speed v ｂ As it increases up to the minimum value Tf ｂ It gradually decreases until the critical speed v. On the other hand, the hysteresis gain Tf is when the vehicle speed reaches the critical speed v. ｂ It increases as it increases beyond a certain point, but only when the critical velocity v is reached. ｂ The rate of increase in hysteresis gain Tf for an increase in vehicle speed exceeding v is the critical speed v ｂ The rate of decrease in hysteresis gain Tf for increases in vehicle speed below a certain threshold is more gradual. The friction torque calculation unit 28 may determine the hysteresis gain Tf for vehicle speed using a preset hysteresis gain map. The hysteresis gain map is a data table showing the hysteresis gain Tf for each vehicle speed.

[0032] The hysteresis gain Tf may be a constant value that does not depend on the vehicle speed. In that case, the friction torque calculation unit 28 only needs to have the hysteresis gain Tf pre-set, and a hysteresis gain map is not required. The friction torque calculation unit 28 outputs the calculated friction torque to the target steering torque calculation unit 30.

[0033]

[0034] ​The viscous torque calculation unit 29 calculates the viscous torque based on the steering angular velocity and the vehicle speed. The viscous torque is also called the damper torque. When calculating the viscous torque, the viscous torque calculation unit 29 uses, for example, a damper gain map. The damper gain map is a data map showing the damper gain corresponding to the steering angular velocity and the vehicle speed. The damper gain map illustrated in FIG. 11 has a characteristic that the damper gain increases as the absolute value of the steering angular velocity exceeding the critical angular velocity dθ ｍｃ increases under a certain vehicle speed. The increase rate of the damper gain with respect to the increase in the absolute value of the steering angular velocity tends to decrease as the absolute value of the steering angular velocity increases. However, when the absolute value of the steering angular velocity is within the range from zero to the critical angular velocity dθ ｍｃ , the damper gain becomes zero. Also, the damper gain map shows a characteristic that the damper gain monotonically increases as the vehicle speed increases under the absolute value of a certain steering angular velocity exceeding the critical angular velocity dθ ｍｃ . In the example of FIG. 11, the damper gain with respect to the absolute value of the steering angular velocity exceeding the critical angular velocity dθ ｍｃ increases as the vehicle speed is higher.

[0035] Incidentally, for the viscous torque calculation unit 29, a damper gain map may be set in advance for each predetermined vehicle speed band, and the damper gain map corresponding to the band to which the input vehicle speed belongs may be selected. The damper gain map may be set, for example, for each of three stages of bands. The three stages of bands correspond to the low vehicle speed, medium vehicle speed, and high vehicle speed illustrated in FIG. 11, respectively. Each stage of the damper gain map shows the damper gain corresponding to the steering angular velocity related to the representative value of the vehicle speed of that band. The base torque calculation unit 27 refers to the selected damper gain map and derives the damper gain corresponding to the steering angular velocity. The viscous torque calculation unit 29 multiplies the damper gain derived using the damper gain map by the steering angular velocity to calculate the viscous torque. The viscous torque calculation unit 29 outputs the calculated viscous torque to the target steering torque calculation unit 30.

[0036] Returning to FIG. 5, a configuration example of the target steering torque calculation unit 30 will be described. The target steering torque calculation unit 30 includes addition units 30a and 30b. The addition unit 30a calculates the sum of the corrected base torque input from the base torque correction unit 36 and the added torque input from the addition unit 30b. The addition unit 30a outputs the calculated sum as the target steering torque to the torque feedback calculation unit 23. The addition unit 30b calculates the sum of the frictional torque calculated by the frictional torque calculation unit 28 and the viscous torque calculated by the viscous torque calculation unit 29. The addition unit 30b outputs the calculated sum as the added torque to the addition unit 30a.

[0037] Next, the road surface information calculation unit 31 will be described. The road surface information calculation unit 31 cooperates with the base torque correction unit 36 to realize road surface information control. Road surface information control is a control for calculating an estimated road surface reaction force and reducing the increase amount of the target steering torque with respect to the increase in the steering angle based on the relative change between the calculated estimated road surface reaction force and the reference road surface reaction force.

[0038] Assume a case where the target steering torque corresponding to the steering angle is given as shown by the solid line in FIG. 12 when the road surface information control is not performed. The target steering torque increases monotonically in response to the increase in the steering angle due to the notch steering. On the other hand, assume a case where the road surface information control is started when the frictional force with the road surface decreases and the vehicle starts to slide. In that case, the increase rate of the target steering torque with respect to the increase in the steering angle decreases. In the example of the broken line in FIG. 12, the vehicle starts to slide at the steering angle sa0, and the road surface information control is started. The target steering torque ri02 obtained by executing the road surface information control is lower than the target steering torque ri01 obtained in a state where the road surface information control is not executed.

[0039] The steering torque corresponds to the difference between the road surface reaction force and the steering assist torque. In other words, the road surface information calculation unit 31 can calculate the sum of the steering torque and the steering assist torque as the estimated road surface reaction force. The reference road surface reaction force corresponds to the road surface reaction force in the normal state, that is, the reference value of the road surface reaction force obtained under a standard road surface friction coefficient. In order to derive the reference road surface reaction force, the standard road surface reaction force rigidity measured in advance is set in the road surface information calculation unit 31. The road surface information calculation unit 31 can determine the reference road surface reaction force based on the road surface reaction force rigidity and the steering angle.

[0040] In the example in the upper part of FIG. 13, for a normal road surface, the road surface reaction force increases almost proportionally to the steering angle. Assume a case where the vehicle enters a low-μ road from a normal road surface when the steering angle reaches sa0 due to a cut-in steering. A normal road surface is a road surface having a standard road surface friction coefficient. A low-μ road is a road surface having a road surface friction coefficient significantly lower than the standard road surface friction coefficient. On a low-μ road, the increase rate of the road surface reaction force with respect to the increase amount of the steering angle is lower than that on a normal road surface. A vehicle traveling on a low-μ road tends to slip because the road surface reaction force received by the wheels from the road surface decreases.

[0041] Therefore, when the estimated road surface reaction force relatively decreases with respect to the reference road surface reaction force, the road surface information control is started by the road surface information calculation unit 31. In the example of FIG. 13, the road surface information control is started when the steering angle reaches sa0. At this time, the target steering torque is controlled so that the increase amount corresponding to the increase of the steering angle decreases. By decreasing the target steering torque, the driver can be made aware that the vehicle is slipping.

[0042] In this embodiment, the road surface information calculation unit 31 derives the pre-correction base torque, which is a component of the target steering torque, according to the steering angle, and corrects the pre-correction base torque by executing road surface information control. The road surface information calculation unit 31 may also control the corrected base torque obtained by correcting the pre-correction base torque so that it does not fall below the base torque at the start of reduction. In that case, the steering feel can be improved by reducing or avoiding discomfort for the driver when road surface information control is performed.

[0043] Next, we will describe an example of running a simulation of the target steering torque. In the simulation, we assumed a case where a vehicle traveling at a predetermined speed on a low-μ road surface has its steering angle varied with a constant amplitude over a 5-second period. Figure 14 is a diagram illustrating the time change of the target steering torque. Figure 15 is a diagram illustrating the target steering torque with respect to the steering angle. In both Figures 14 and 15, the solid line shows the target steering torque ri22 obtained by performing road surface information control. The dashed line shows the target steering torque ri21 obtained without performing road surface information control.

[0044] Comparing the two, the difference between having road surface information control and not having it becomes significant when the target steering torque approximates a maximum or minimum value. In other cases, no significant difference was observed between having road surface information control and not having it. This supports the idea that the driver is informed of the condition in which the wheels are more likely to slip on the road surface by suppressing the increase in target steering torque due to an increase in steering angle or the change in steering feel due to a decrease in steering angle. Furthermore, the same degree of hysteresis occurs regardless of whether road surface information control is present or not. This is presumed to be due to control based on base torque, which does not contribute to the hysteresis component.

[0045] Returning to Figure 5, an example of the configuration of the road surface information calculation unit 31 will be described. The road surface information calculation unit 31 includes a road surface reaction force estimation unit 32, a trimming increase determination unit 33, a reference road surface reaction force calculation unit 34, and a correction gain calculation unit 35.

[0046] The road surface reaction force estimation unit 32 calculates the estimated road surface reaction force based on the steering torque detected by the torque sensor 5 and the motor current detected by the current sensor 9. Conversion parameters are pre-set in the road surface reaction force estimation unit 32. The conversion parameters indicate the correlation between the motor current that causes power consumption in the motor 6 and the steering assist torque generated by the motor 6. The road surface reaction force estimation unit 32 converts the motor current into steering assist torque using the pre-set conversion parameters. The road surface reaction force estimation unit 32 calculates the instantaneous value of the estimated road surface reaction force by adding the steering torque and steering assist torque notified from the torque sensor 5.

[0047] The road reaction force estimation unit 32 may also correct the instantaneous value of the estimated road reaction force by subtracting an inertia term from the sum obtained by adding the steering torque and the steering assist torque, and apply the corrected instantaneous value of the estimated road reaction force to the processing described later. The road reaction force estimation unit 32 can calculate the inertia term by multiplying the steering angular acceleration by the inertia of the motor 6 which is set in advance. The road reaction force estimation unit 32 can calculate the steering angular acceleration by taking the second derivative of the steering angle detected by the steering angle sensor 4.

[0048] On the other hand, the road surface reaction force estimation unit 32 monitors the steering state information notified by the steering angle increase determination unit 33. The road surface reaction force estimation unit 32 sets the instantaneous value of the estimated road surface reaction force at the time when the steering angle increase determination unit 33 notifies it of an increase in steering angle as the starting point, i.e., the reference value. At each time after the steering angle increase notification, the road surface reaction force estimation unit 32 calculates the difference obtained by subtracting the starting point from the instantaneous value of the estimated road surface reaction force at that time as the absolute value of the estimated road surface reaction force. In the example in Figure 6, the absolute value of the estimated road surface reaction force corresponds to the increase from the instantaneous value of the estimated road surface reaction force (starting point) at the time when the steering angle of the current instantaneous value of the estimated road surface reaction force (current value) reaches the upper limit of the steering angle near neutral (described later). The upper limit of the steering angle near neutral corresponds to an example of the steering angle starting point. The road surface reaction force estimation unit 32 outputs the calculated absolute value of the estimated road surface reaction force to the correction gain calculation unit 35. When the road surface reaction force estimation unit 32 is notified by the steering angle increase determination unit 33 that the steering angle has returned to neutral, it stops calculating and outputting the absolute value of the estimated road surface reaction force. The road surface reaction force estimation unit 32 then cancels the set starting point and resets a predetermined reference value (for example, zero).

[0049] The road surface reaction force estimation unit 32 may also calculate the absolute value of the estimated road surface reaction force using the following calculation procedure. The road surface reaction force estimation unit 32 differentiates the instantaneous value Tr of the estimated road surface reaction force at each time point with respect to the steering angle θ and calculates the road surface reaction force derivative value dTr / dθ. On the other hand, the road surface reaction force estimation unit 32 receives a steering angle increase notification from the steering angle increase determination unit 33 along with the steering angle start point θ ０ This is notified (see below). The road surface reaction force estimation unit 32 calculates the road surface reaction force derivative value dTr / dθ from the steering angle starting point θ, as illustrated in the first row of the right-hand side of equation (2). ０ From the current steering angle θ ｎ The integral obtained by integrating with respect to the steering angle θ is calculated as the absolute value of the estimated road surface reaction force. Current steering angle θｎ This corresponds to the current steering angle θ. The absolute value of the estimated road surface reaction force calculated using this procedure is equal to the absolute value of the estimated road surface reaction force calculated using the procedure described above, as exemplified in the third row of the right-hand side of equation (2). This value is the instantaneous value of the current estimated road surface reaction force Tr(θ) ｎ ) and the instantaneous value of the estimated road surface reaction force at the start of the increase Tr(θ) ０ This corresponds to the difference between this and ).

[0050]

[0051] Figure 6 shows an example of calculating the absolute value of the estimated road surface reaction force after detecting an increase in steering angle during an increasing steering angle phase, but it is not limited to this example. The road surface reaction force estimation unit 32 calculates the absolute value of the estimated road surface reaction force after detecting an increase in steering angle during a decreasing steering angle phase, following the same procedure as after detecting an increase in steering angle during an increasing steering angle phase. The road surface reaction force estimation unit 32 also stops calculating and outputting the absolute value of the estimated road surface reaction force when it is notified again by the increase determination unit 33 that the steering angle has returned to neutral. The road surface reaction force estimation unit 32 then cancels the set starting point and resets the reference value.

[0052] The steering angle increase determination unit 33 monitors the steering angle detected by the steering angle sensor 4 and determines the steering state. For example, the steering angle increase determination unit 33 determines that steering angle increase is occurring when the absolute value of the detected steering angle is greater than or equal to a predetermined steering angle setting value, or when the absolute value of the steering angular velocity is greater than or equal to a predetermined steering angular velocity setting value. The steering angle setting value is a threshold for detecting effective steering based on the steering angle, and is a positive value significantly greater than zero. The steering angle setting value can be the steering angle corresponding to the steering angle starting point mentioned above. The steering angle increase determination unit 33 calculates the steering angular velocity by differentiating the steering angle with respect to time. The steering angular velocity setting value is a threshold for detecting effective steering based on the steering angular velocity, and is a positive value significantly greater than zero.

[0053] The steering angle increase determination unit 33 may also determine whether the steering angle has been increased by performing the following procedure (sometimes called the "steering angle increase determination procedure 2"). The steering angle increase determination unit 33 determines whether the detected steering angle is other than the vicinity of the neutral steering angle. The vicinity of the neutral steering angle is a predetermined range of steering angles that includes a steering angle of zero (i.e., steering neutral). In this application, the minimum and maximum values ​​of the vicinity of the neutral steering angle are called the "lower limit of the vicinity of the neutral steering angle" and the "upper limit of the vicinity of the neutral steering angle," respectively. The lower limit of the vicinity of the neutral steering angle is a negative value where the steering angle is less than zero. The upper limit of the vicinity of the neutral steering angle is a positive value where the steering angle at which the neutral steering angle occurs is greater than zero. The upper limit of the vicinity of the neutral steering angle and the lower limit of the vicinity of the neutral steering angle can each be the steering angle starting point. The upper limit of the vicinity of the neutral steering angle and the lower limit of the vicinity of the neutral steering angle may be collectively referred to as the "near-neutral steering angle limit." The absolute value of the steering angle near neutral is, for example, 8 to 12 degrees, and is typically around 10 degrees. The steering angle increase determination unit 33 determines whether to increase the steering angle when the steering angle is outside the vicinity of the steering angle neutral and the maximum value of the absolute value of the steering angle is updated, or when the absolute value of the steering angular velocity becomes equal to or greater than the set value of the steering angular velocity.

[0054] When the steering increase determination unit 33 determines that the steering has been increased, it outputs a steering increase notification as steering state information to the road surface reaction force estimation unit 32, the reference road surface reaction force calculation unit 34, and the correction gain calculation unit 35. The steering increase determination unit 33 may also output the steering angle at the time the steering increase was determined to the road surface reaction force estimation unit 32 and the reference road surface reaction force calculation unit 34, using the steering angle as the starting point.

[0055] The steering correction unit 33 determines that the steering wheel should be turned back when the steering angle is not near the neutral position, the absolute value of the steering angle decreases, and the absolute value of the steering angular velocity is equal to or greater than a predetermined set value for the steering angular velocity. When the steering correction unit 33 determines that the steering wheel should be turned back, it outputs a steering correction notification as steering state information to the correction gain calculation unit 35.

[0056] After determining whether the steering angle has been increased, the steering angle determination unit 33 determines whether the steering angle has returned to the neutral position when the steering angle converges to a range near the neutral position. When the steering angle determination unit 33 determines whether the steering angle has returned to the neutral position, it outputs the return to the neutral position as steering state information to the road surface reaction force estimation unit 32, the reference road surface reaction force calculation unit 34, and the correction gain calculation unit 35. When the steering angle determination unit 33 performs the steering angle determination using the steering angle determination procedure 2, it resets the maximum value of the absolute value of the steering angle at that time to a predetermined reference value (for example, zero) when determining whether the steering angle has returned to the neutral position.

[0057] The reference road surface reaction force calculation unit 34 calculates the reference road surface reaction force based on the steering angle detected by the steering angle sensor 4. The reference road surface reaction force calculation unit 34 is pre-set with a standard road surface reaction force stiffness that has been measured in advance. The reference road surface reaction force calculation unit 34 calculates the instantaneous value of the reference road surface reaction force by multiplying the steering angle by the road surface reaction force stiffness set therein.

[0058] The reference road surface reaction force calculation unit 34 calculates the absolute value of the reference road surface reaction force by applying a method similar to the method used to calculate the absolute value from the instantaneous value of the estimated road surface reaction force based on the steering state information notified by the steering increase determination unit 33. That is, the reference road surface reaction force calculation unit 34 sets the instantaneous value of the reference road surface reaction force at the time when the steering increase determination unit 33 notifies it of an increase in steering angle as the starting point. For example, at each time after the steering increase notification, the reference road surface reaction force calculation unit 34 calculates the difference obtained by subtracting the starting point from the current instantaneous value of the reference road surface reaction force at that time as the absolute value of the reference road surface reaction force. Alternatively, the reference road surface reaction force calculation unit 34 may substitute the reference road surface reaction force for the estimated road surface reaction force in equation (2), differentiate the reference road surface reaction force with respect to the steering angle, and integrate the road surface reaction force derivative obtained from the steering angle starting point to the current steering angle to calculate the absolute value of the reference road surface reaction force. The reference road surface reaction force calculation unit 34 outputs the absolute value of the calculated reference road surface reaction force to the correction gain calculation unit 35.

[0059] Furthermore, when calculating the absolute value of the estimated road surface reaction force by subtracting its starting point from the instantaneous value of the current estimated road surface reaction force, the reference road surface reaction force calculation unit 34 stops calculating and outputting the absolute value of the reference road surface reaction force when it is notified by the steering angle adjustment determination unit 33 that the steering angle has returned to neutral. The reference road surface reaction force calculation unit 34 then cancels the set starting point and resets the predetermined reference value.

[0060] The correction gain calculation unit 35 calculates a correction gain based on the estimated road surface reaction force calculated by the road surface reaction force estimation unit 32 and the reference road surface reaction force calculated by the reference road surface reaction force calculation unit 34. The correction gain calculation unit 35 can calculate the correction gain BaseGain as the ratio of the absolute value Test of the estimated road surface reaction force to the absolute value Ta of the reference road surface reaction force, as exemplified in equation (3).

[0061]

[0062] ​The correction gain calculation unit 35 may calculate the correction gain BaseGain according to equation (4) instead of equation (3). By using equation (4), the vehicle speed detected by the vehicle speed sensor 8 is further taken into consideration. That is, equation (4) shows that the correction gain BaseGain is calculated as the difference obtained by subtracting from 1 the product obtained by multiplying the ratio of the road surface reaction force deviation ΔTa to the reference road surface reaction force Ta by the road surface information gain Rinfo_Gain. The road surface reaction force deviation ΔTa corresponds to the difference obtained by subtracting the reference road surface reaction force Ta from the estimated road surface reaction force Test, as exemplified in equation (5).

[0063] The road surface information gain Rinfo_Gain is 0 when the vehicle speed is extremely low, and 1 when the vehicle speed is not extremely low. The correction gain calculation unit 35 can determine that the vehicle speed is extremely low when it is below a predetermined extremely low vehicle speed threshold, and that it is not extremely low when the vehicle speed exceeds the extremely low vehicle speed threshold. The extremely low vehicle speed threshold is, for example, the vehicle speed at which the vehicle can be considered to be stationary in terms of road surface friction (for example, 0.5 to 2 km / h). When the vehicle speed is extremely low, the base torque is not substantially corrected by setting the correction gain to 1. This is because the nature of the load from the road surface is different when the vehicle speed is extremely low compared to other cases. When the vehicle speed is not extremely low, that is, during normal vehicle operation, the correction gain calculated by equation (4) is equal to the correction gain calculated by equation (3).

[0064]

[0065]

[0066] The correction gain calculation unit 35 then controls the output of the correction gain based on the steering state information notified by the steering increase determination unit 33. When a steering increase notification is received, the correction gain calculation unit 35 calculates the correction gain using the above calculation and outputs the calculated correction gain to the base torque correction unit 36. When a steering return notification is received, the correction gain calculation unit 35 maintains (holds) the correction gain at that time and outputs the maintained correction gain to the base torque correction unit 36. At this time, the correction gain calculation unit 35 may stop calculating the correction gain.​​

[0067] The correction gain calculation unit 35 monitors the steering angle detected by the steering angle sensor 4 and records the steering angle at the time when the steering increase determination unit 33 starts receiving a steering increase notification as the steering angle at the start of reduction. As described above, since the base torque changes monotonically in accordance with the fluctuation of the steering angle, the time when the steering increase starts can be considered as the time when the base torque reduction starts due to the correction. When the steering angle returns to the steering angle at the start of reduction, the correction gain calculation unit 35 resets the correction gain to 1 and outputs the reset correction gain to the base torque correction unit 36. The correction gain calculation unit 35 may stop calculating the correction gain by the above calculation until the next steering increase notification is received.

[0068] Next, an example of the configuration of the base torque correction unit 36 ​​will be described. The base torque correction unit 36 ​​comprises a limiting unit 37 and a correction calculation unit 38. The limiting unit 37 receives the base torque from the base torque calculation unit 27, steering state information from the steering increase determination unit 33, and correction gain from the correction gain calculation unit 35. The limiting unit 37 limits the correction gain input from the correction gain calculation unit 35 (sometimes referred to as the "pre-limiting correction gain" in this application) and determines the correction gain after the limiting (sometimes referred to as the "post-limiting correction gain" in this application). The limiting unit 37 stores the base torque obtained from the base torque calculation unit 27 at the time the steering increase determination unit 33 notifies it of a steering increase as the base torque at the start of reduction.

[0069] The limiting unit 37 calculates the lower limit of the correction gain based on the ratio Tb0 / Tb of the base torque Tb0 at the start of reduction to the current base torque Tb, and the limiting ratio R, as illustrated in equation (6). Equation (6) shows that the lower limit of the correction gain is calculated as the product of the limiting ratio R, the ratio Tb0 / Tb, and 1-R. The limiting ratio R is a parameter between 0 and 1 that indicates the degree of correction to the base torque Tb. The lower limit of the correction gain is a real number between Tb0 / Tb and 1. A smaller limiting ratio R indicates a greater degree of correction. That is, when R=0, the lower limit of the correction gain is the ratio Tb0 / Tb. When R=1, the lower limit of the correction gain is 1.

[0070]

[0071] Equation (6) is derived by dividing both sides of equation (7) by the base torque Tb. Equation (7) shows that the corrected base torque, which is the product of the base torque Tb and the lower limit of the correction gain, corresponds to the sum of the product of the difference between the base torque Tb0 at the start of reduction and the base torque before and after correction (Tb - Tb0) and the limiting ratio R. Equation (7) shows that when R = 0, the corrected correction gain after correction becomes the base torque Tb0 at the start of reduction, i.e., the minimum value of the corrected base torque after the start of base torque reduction. When R = 1, the corrected base torque becomes equal to the base torque before correction, and the base torque is not corrected in effect.

[0072] Furthermore, the limiting unit 37 may reset the lower limit of the correction gain to 1 if the pre-limiting correction gain notified by the correction gain calculation unit 35 is 1. In such a case, it is clear that the base torque will not be reduced until the next increase in steering is notified. This includes cases where the steering angle at that point returns to the steering angle at the start of the reduction.

[0073]

[0074] The limiting unit 37 compares the pre-limiting correction gain with the calculated correction gain lower limit and determines whether the pre-limiting correction gain is equal to or greater than the correction gain lower limit. If the limiting unit 37 determines that the pre-limiting correction gain is equal to or greater than the correction gain lower limit, it outputs the pre-limiting correction gain to the correction calculation unit 38 as the post-limiting correction gain. If the limiting unit 37 determines that the pre-limiting correction gain is less than the correction gain lower limit, it outputs the correction gain lower limit to the correction calculation unit 38 as the post-limiting correction gain.

[0075] The correction calculation unit 38 multiplies the uncorrected base torque input from the base torque calculation unit 27 by the limited correction gain input from the limiting unit 37, and outputs the resulting product as the corrected correction gain to the target steering torque calculation unit 30.

[0076] ​Next, an example of calculating the correction gain according to this embodiment will be described. Figure 7 is a diagram showing an example of calculating the correction gain according to this embodiment. The vertical axis shows the correction gain before limiting, and the horizontal axis shows time. In the example in Figure 7, it is assumed that the limiting ratio R is zero, the initial steering angle is zero, the steering is turned inward from time 3 seconds to 4 seconds, then turned back from time 4 seconds to 5 seconds, and finally the steering angle returns to zero.

[0077] Under this assumption, as the steering angle increases due to increased steering input and exceeds the upper limit near the neutral position, the pre-limit correction gain bs01 begins to decrease from its initial value of 1. The pre-limit correction gain bs01 becomes 0.8 when the increased steering input ends. The pre-limit correction gain bs01 is maintained at 0.8 as the steering angle decreases due to reverse steering input, and returns to 1 when the steering angle reaches the upper limit near the neutral position. This steering angle corresponds to the steering angle at the start of reduction, which is the point at which the reduction of base torque begins.

[0078] On the other hand, the lower limit of the correction gain bs02 begins to decrease from its initial value of 1 at time 3.3 seconds as the road surface reaction force begins to decrease. The lower limit of the correction gain bs02 becomes 0.7 when the increasing turn is completed. The lower limit of the correction gain bs02 begins to increase in accordance with the increase in road surface reaction force as the re-turning begins, returns to 1 at time 4.7 seconds, and is maintained at 1 thereafter.

[0079] The post-limit correction gain bs03 is determined by the larger of the pre-limit correction gain bs01 and the lower limit correction gain bs02. From time 3 seconds to 3.4 seconds, the lower limit correction gain bs02 is greater than the pre-limit correction gain bs01, so the lower limit correction gain bs02 is adopted as the post-limit correction gain bs03. From time 3.4 seconds to 4.5 seconds, the pre-limit correction gain bs01 is greater than the lower limit correction gain bs02, so the pre-limit correction gain bs03 is adopted as the post-limit correction gain bs03. From time 4.5 seconds to 5 seconds, the lower limit correction gain bs02 is greater than the pre-limit correction gain bs01, so the lower limit correction gain bs02 is adopted as the post-limit correction gain bs03.

[0080] Note that the lower limit of the correction gain is determined without considering road surface reaction force, unlike the pre-limit correction gain. Therefore, if the decrease in road surface reaction force is more significant during the cutting-in stage, the decrease in the pre-limit correction gain will also be significant. For this reason, there is a higher tendency for the lower limit of the base gain to be adopted as the post-limit correction gain. In the example in Figure 7, at the cutting-in stage, the pre-limit correction gain bs01, calculated assuming a road surface reaction force decrease of about 20%, is dominant as the post-limit correction gain bs03. If we assume that the road surface reaction force decrease is 50%, the lower limit of the correction gain bs02 becomes dominant as the post-limit correction gain bs03.

[0081] In the above explanation, the target steering torque setting unit 22 calculates a correction gain based on the difference between the reference road surface reaction force and the estimated road surface reaction force in the road surface information calculation unit 31, and the base torque correction unit 36 ​​reduces the base torque before correction based on the correction gain to determine the base torque after correction, but the system is not limited to this case.

[0082] When the target steering torque setting unit 22 reduces the uncorrected base torque based on the reference road surface reaction force and the estimated road surface reaction force to determine the corrected base torque, it may calculate the corrected base torque as the product obtained by multiplying the uncorrected base torque by, for example, the ratio of the instantaneous value of the estimated road surface reaction force to the instantaneous value of the reference road surface reaction force, or the ratio of the absolute value of the estimated road surface reaction force to the absolute value of the reference road surface reaction force.

[0083] Alternatively, the target steering torque setting unit 22 may use the product obtained by multiplying the ratio of the absolute values ​​of the estimated road surface reaction force by the base torque before correction as an intermediate value, and set the larger of the intermediate value and the lower limit of the base torque as the corrected base torque. In that case, the target steering torque setting unit 22 can use the value given by equation (7) as the lower limit of the base torque.

[0084] The target steering torque setting unit 22 does not necessarily have to calculate a correction gain when it reduces the pre-correction base torque based on the reference road surface reaction force and the estimated road surface reaction force to determine the corrected base torque. For example, the target steering torque setting unit 22 may reduce the pre-correction base torque so that the corrected base torque becomes the above-mentioned lower limit of base torque when the ratio of the estimated road surface reaction force to the reference road surface reaction force becomes smaller than a predetermined ratio of less than 1, or when the estimated road surface reaction force becomes smaller than a predetermined difference value from the reference road surface reaction force. That is, when the target steering torque setting unit 22 reduces the base torque in accordance with the relative decrease in the estimated road surface reaction force compared to the reference road surface reaction force, it sets the base torque to the lower limit of base torque so that it does not fall below the base torque at the start of the reduction.

[0085] As described above, the steering control device (for example, control unit 11) according to the embodiment of the present disclosure includes: a target steering torque setting unit 22 that sets a target steering torque for the steering mechanism; a steering assist torque calculation unit (for example, torque feedback calculation unit 23) that calculates a steering assist torque to make the steering torque follow the target steering torque based on the deviation between the target steering torque and the steering torque acting on the steering mechanism; and a current drive unit 12 that controls the motor current, which is the current flowing to the motor that provides the steering assist torque to the steering mechanism. The target steering torque setting unit 22 determines the base torque, which is the basic component of the target steering torque, estimates the instantaneous value of the road surface reaction force (for example, estimated road surface reaction force) from the steering torque and current, calculates the absolute value of the road surface reaction force starting from the instantaneous value of the road surface reaction force at a predetermined steering angle (for example, steering angle starting point) in the state of increasing steering angle of the steering mechanism, and reduces the base torque based on the absolute value of the road surface reaction force. In this configuration, the base torque is reduced based on the absolute value of the road surface reaction force calculated from the instantaneous value of the road surface reaction force at a predetermined steering angle when the steering mechanism is turned in an increased direction. Consequently, the target steering torque, which includes the base torque as its basic component, is reduced. Because estimation errors in the road surface reaction force due to fluctuations in the frictional force between the road surface and the wheels immediately after the start of increased steering are eliminated, a target steering torque that is more in line with the road surface conditions can be obtained. By controlling the steering torque to follow this target steering torque, the steering feel given to the driver can be changed, thereby more accurately transmitting changes in the road surface conditions.

[0086] The target steering torque setting unit 22 may determine the steering mechanism is in an increased steering state when the absolute value of the steering angle of the steering mechanism is equal to or greater than a predetermined steering angle, or the absolute value of the steering angular velocity is equal to or greater than a predetermined steering angular velocity. With this configuration, the increased steering state is determined when the steering state is significantly different from the neutral steering angle, or when the steering angle is significantly fluctuating.

[0087] The target steering torque setting unit 22 may calculate the differential value of the road surface reaction force by differentiating the instantaneous value of the road surface reaction force with respect to the steering angle, and then calculate the absolute value of the road surface reaction force by integrating this differential value from a predetermined steering angle to the current steering angle. Generally, when the steering angle changes, the road surface reaction force also changes in accordance with this change. In minute operations where the steering angle hardly changes over time, noise components tend to dominate the time derivative of the road surface reaction force. In contrast, the differential value of the road surface reaction force, which is the derivative of the road surface reaction force with respect to the steering angle, also contains noise components, but its correlation with the change in steering angle is low. By integrating the differential value of the road surface reaction force with respect to the steering angle, the actual fluctuation of the road surface reaction force mainly due to the change in steering angle can be obtained, and the accumulation of errors due to integral noise components is avoided.

[0088] The target steering torque setting unit 22 may include a base torque calculation unit 27 that determines the base torque, a road surface information calculation unit 31 that calculates a correction gain based on the absolute value of the road surface reaction force estimated from the steering torque and motor current, and a base torque correction unit 36 ​​that reduces the base torque based on the correction gain. With this configuration, the base torque, which is the basic component of the target steering torque, is reduced based on the correction gain calculated based on the absolute value of the steering torque and road surface reaction force, and the corrected base torque is determined. Therefore, economical implementation can be achieved by adding the road surface information calculation unit 31 and the base torque correction unit 36 ​​to the existing base torque calculation unit 27.

[0089] The road surface information calculation unit 31 may calculate a correction gain based on the difference between the road surface reaction force (i.e., estimated road surface reaction force) and the reference road surface reaction force, which is a reference value of the road surface reaction force for the steering angle. The base torque correction unit 36 ​​may also reduce the base torque by multiplying it by the correction gain. With this configuration, the correction gain decreases as the estimated road surface reaction force becomes relatively smaller than the reference road surface reaction force, so the corrected base torque decreases. Therefore, the target steering torque decreases in accordance with the decrease in estimated road surface reaction force due to the road surface condition, making the driver aware of changes in the road surface condition without degrading the steering feel.

[0090] The base torque correction unit 36 ​​may set a lower limit of the correction gain, which is the lower limit of the correction gain, so that the base torque does not fall below the base torque at the start of the reduction of the base torque, and may limit the correction gain based on the lower limit of the correction gain. With this configuration, the correction gain at the start of the reduction of the base torque is set as the lower limit of the correction gain, and the base torque before correction is limited based on the set lower limit of the correction gain to obtain the base gain after correction. Therefore, the base torque after correction is limited so that it does not fall below the base torque at the start of the reduction.

[0091] The steering control device according to this embodiment may also include a steering torque detection unit (for example, a torque sensor 5) that detects the steering torque acting on the steering mechanism, and a steering state detection unit 21 that detects the steering angle of the steering mechanism. The electric power steering device according to this embodiment may also include a motor 6, a motor rotational angular velocity detection unit 24 that detects the rotational angular velocity of the motor 6, and the steering control device described above.

[0092] While embodiments of this disclosure have been described above, this disclosure is not limited to these embodiments or their variations. Additions, omissions, substitutions, and other modifications are permitted without departing from the spirit of this disclosure. The directions of arrows shown in block diagrams and other drawings are for illustrative purposes only and do not limit the direction of information, data, signals, etc., during implementation. Furthermore, this disclosure is not limited by the foregoing description, but only by the attached claims.

[0093] The steering torque controlled by the steering control device and electric power steering device PS according to this disclosure allows the driver to accurately perceive changes in road surface conditions.

[0094] PS...Electric power steering system, 1...Steering wheel, 2...Steering shaft, 3...Steering wheel, 4...Steering angle sensor, 5...Torque sensor, 6...Motor, 7...Reduction mechanism, 8...Vehicle speed sensor, 9...Current sensor, 10...Motor rotation angle sensor, 11...Control unit, 12...Current drive unit, 22...Target steering torque setting unit, 23...Torque feedback calculation unit, 24...Motor rotation angular velocity detection unit, 27...Base torque calculation unit, 28...Friction torque calculation unit, 29...Viscous torque calculation unit, 30...Target steering torque calculation unit, 31...Road surface information calculation unit, 32...Road surface reaction force estimation unit, 33...Increased steering determination unit, 34...Reference road surface reaction force calculation unit, 35...Correction gain calculation unit, 36...Base torque correction unit, 37...Limiting unit, 38...Correction calculation unit

Claims

1. A steering control device comprising: a target steering torque setting unit for setting a target steering torque for a steering mechanism; a steering assist torque calculation unit for calculating a steering assist torque to make the steering torque follow the target steering torque based on the deviation between the target steering torque and the steering torque acting on the steering mechanism; and a current drive unit for controlling a motor current, which is the current flowing to a motor that supplies the steering assist torque to the steering mechanism, wherein the target steering torque setting unit determines a base torque, which is the basic component of the target steering torque; estimates an instantaneous value of road surface reaction force from the steering torque and the current; calculates the absolute value of the road surface reaction force starting from the instantaneous value at a predetermined steering angle in the state of increasing steering of the steering mechanism; and reduces the base torque based on the absolute value of the road surface reaction force.

2. The steering control device according to claim 1, wherein the target steering torque setting unit determines the steering mechanism to be turned further when the absolute value of the steering angle of the steering mechanism is equal to or greater than a predetermined steering angle or the absolute value of the steering angular velocity is equal to or greater than a predetermined steering angular velocity.

3. The steering control device according to claim 1 or 2, wherein the target steering torque setting unit calculates the differential value of the road surface reaction force by differentiating the instantaneous value of the road surface reaction force with respect to the steering angle, and calculates the absolute value of the road surface reaction force by integrating the differential value of the road surface reaction force from the predetermined steering angle to the current steering angle.

4. The steering control device according to any one of claims 1 to 3, wherein the target steering torque setting unit comprises: a base torque calculation unit for determining the base torque; a road surface information calculation unit for calculating a correction gain based on the absolute value of the road surface reaction force estimated from the steering torque and the current; and a base torque correction unit for reducing the base torque based on the correction gain.

5. The steering control device according to claim 4, wherein the base torque correction unit reduces the base torque by multiplying the base torque by the correction gain.

6. The steering control device according to claim 4 or 5, wherein the road surface information calculation unit calculates the correction gain based on the difference between the road surface reaction force and a reference road surface reaction force which is a reference value of the road surface reaction force with respect to the steering angle.

7. The steering control device according to any one of claims 4 to 6, wherein the base torque correction unit sets a lower limit value of the correction gain, which is the lower limit of the correction gain, so as not to fall below the base torque at the start of the reduction of the base torque, and limits the correction gain based on the lower limit value of the correction gain.

8. A steering control device according to any one of claims 1 to 7, comprising: a steering torque detection unit for detecting the steering torque acting on the steering mechanism; and a steering state detection unit for detecting the steering angle of the steering mechanism.

9. An electric power steering device comprising: the motor; a motor rotational angular velocity detection unit for detecting the rotational angular velocity of the motor; and a steering control device according to any one of claims 1 to 8.

Images