Steering control device, steering control method, steering control program, and steering system

Abstract

In one embodiment of a steering control device, a steering control method, a steering control program, and a steering system according to the present invention: an attenuation ratio and a natural angular frequency are set so that the gain and phase of a yaw rate, which is the behavior of a vehicle with respect to a steering operation by a driver, serve as a target gain and a target phase; a target yaw rate is obtained from the operation amount of a steering input member, a steady yaw rate gain based on vehicle speed, the attenuation ratio and the natural angular frequency, and a vehicle model provided in advance; and a motor for applying a steering force to the steering wheel of the vehicle is controlled so that the target yaw rate is reached. Due to this configuration, optimum steering control can be implemented in consideration of responsiveness and convergence with respect to vehicle behavior.

Description

Steering control device, steering control method, steering control program, and steering system

[0001] The present invention relates to a steering control device, a steering control method, a steering control program, and a steering system.

[0002] The steering control device of Patent Document 1 is a steering control device that controls the steering of steered wheels based on an operation amount resulting from operation of an operating member, and includes a dynamic characteristic information generation unit that generates dynamic characteristic information indicating target dynamic characteristics of the vehicle from operation amount information that indicates the operation amount based on non-linear calculation, a steering angle calculation unit that calculates a target steering angle based on a vehicle model and on the dynamic characteristic information generated by the dynamic characteristic information generation unit, and a steering angle control unit that controls a drive device so that the target steering angle and actual steering angle match.

[0003] Japanese Patent Application Laid-Open No. 2021-059267

[0004] In a typical two-wheel steering vehicle equipped with a steering system in which a steering input member such as a steering wheel is mechanically coupled to the steered wheels, the response of the vehicle to the driver's steering operation becomes more sensitive and delayed as the vehicle travels at higher speeds. This requires the driver to perform driving operations while artificially compensating for the vehicle's response characteristics, which increases the operational burden on the driver at high speeds.

[0005] Therefore, an object of the present invention is to provide a steering control device, a steering control method, a steering control program, and a steering system that can perform optimal steering control in a steer-by-wire system, taking into account the responsiveness and convergence of vehicle behavior.

[0006] In one aspect, the steering control device, steering control method, steering control program, and steering system according to the present invention set a damping ratio and a natural angular frequency to set a gain and a phase of a yaw rate, which is the vehicle behavior of the vehicle in response to a steering operation by a driver of the vehicle, to a target gain and a target phase, determine a target yaw rate generated in the vehicle from a first physical quantity related to the operation amount of a steering input member and a steady-state yaw rate gain of the vehicle that is based on a second physical quantity related to the vehicle speed, the set damping ratio and natural angular frequency, and a predetermined vehicle model, and control a motor that applies a steering force to the steered wheels of the vehicle so as to achieve the target yaw rate.

[0007] According to the present invention, in a steer-by-wire system, it is possible to perform optimal steering control that takes into consideration the responsiveness and convergence of vehicle behavior.

[0008] 1 is a schematic diagram showing a vehicle equipped with a steer-by-wire steering system; FIG. 2 is a functional block diagram of a steering control device; FIG. 3 is a diagram showing the correlation between vehicle speed v and steady-state yaw rate gain k; FIG. 4 is a diagram showing the correlation between vehicle speed v and target yaw rate differential value gain K3; FIG. 5 is a diagram showing the correlation between vehicle speed v and damping ratio ζ; FIG. 6 is a diagram showing the correlation between vehicle speed v and natural angular frequency ωn; FIG. 7 is a diagram showing the correlation between steering operation frequency and yaw rate gain; FIG. 8 is a diagram showing the correlation between steering operation frequency and yaw rate phase; and FIG. 9 is a diagram showing the step response of yaw rate when damping ratio ζ and natural angular frequency ωn are varied while maintaining constant steering stability capacity ζωn.

[0009] Hereinafter, embodiments of a steering control device, a steering control method, a steering control program, and a steering system according to the present invention will be described with reference to the drawings. Figure 1 is a schematic diagram showing one aspect of a vehicle 100 equipped with a steering system 200. Vehicle 100 is a four-wheeled automobile equipped with a pair of left and right front wheels 101, 102 and a pair of left and right rear wheels 103, 104.

[0010] Steering system 200 has steering input device 300 to which a steering operation by the driver of vehicle 100 is input via steering input member 310, steering device 400 including steering motor 410 as a road wheel actuator that applies a steering force to front wheels 101, 102 that are steered wheels of vehicle 100, and steering control device 500 that controls steering input device 300 and steering device 400. Here, steering input member 310 and front wheels 101, 102 that are steered wheels are mechanically separated, and steering system 200 is a steer-by-wire steering system that changes the steering angle (in other words, tire angle) of front wheels 101, 102 that are steered wheels by controlling steering motor 410 based on an electrical signal of the driver's steering operation.

[0011] The steering input device 300 includes a steering input member 310, a steering reaction force imparting device 320, and an operation angle sensor 330. The steering input member 310 is an operator that accepts steering operations by the driver, and in this embodiment, a steering wheel is used, but it may also be a dial-type or stick-type steering input member.

[0012] The steering reaction force imparting device 320 is a means for biasing the steering input member 310 toward the neutral position (in other words, the straight-ahead position of the vehicle 100), and applies a pseudo steering reaction force to the steering input member 310 using a motor or the like. The operation angle sensor 330 detects the operation angle θ, which is the steering operation amount of the steering input member 310 such as a steering wheel, that is, the rotation angle of the steering shaft 310A, as a first physical quantity related to the operation amount of the steering input member 310.

[0013] Steering device 400 includes steering motor 410 and steering mechanism 420. Steering mechanism 420 converts the rotational motion of steering motor 410 into linear motion of rack shaft 421, for example by a rack-and-pinion system, thereby changing the steering angle of front wheels 101, 102 connected to rack shaft 421.

[0014] Furthermore, steering device 400 has a steering angle sensor 430 that detects steering angle δ of front wheels 101, 102, and a motor control device 440 that drives and controls steering motor 410. Motor control device 440 acquires a steering angle command δtg signal that indicates the target steering angle output by steering control device 500, and controls steering motor 410 so that the actual steering angle δ detected by steering angle sensor 430 approaches steering angle command δtg.

[0015] The target control amount for steering motor 410 is not limited to steering angle command δtg, and may be any command for a state amount correlated with the tire angle of front wheels 101, 102, such as a target stroke amount for rack shaft 421. Furthermore, steering angle sensor 430 can detect the stroke amount of rack shaft 421, the rotation angle of steering motor 410 (for example, the rotation angle of the pinion shaft), and the like, as physical amounts correlated with steering angle δ. Furthermore, steering control device 500 can be a system that acquires a signal of steering angle δ output by steering angle sensor 430, and outputs a control signal for steering motor 410.

[0016] The steering control device 500 is an electronic control device equipped with an MCU (Micro Controller Unit) 510 having one or more processors that execute a steering control method and a steering control program. The steering control device 500 controls the operation of the steer-by-wire steering system 200 by controlling the steering reaction force imparting device 320 and the steering device 400 (steering motor 410).

[0017] In detail, MCU 510 calculates and processes various signals acquired from the outside to determine the target steering reaction force and the steering angle command δtg, outputs a signal of the target steering reaction force to steering reaction force application device 320, and outputs a signal of the steering angle command δtg to motor control device 440 of steering device 400. Note that MCU 510 can also be referred to as a microcomputer, a processor, a processing device, an arithmetic device, etc.

[0018] The MCU 510 acquires signals output by sensors that detect the running state of the vehicle 100, information indicating the driving characteristics of the driver of the vehicle 100, and the like, for use in controlling the steering system 200. The vehicle 100 is equipped with sensors that detect the running state of the vehicle 100, such as wheel speed sensors 621-624 that detect wheel speeds WS1-WS4, which are the rotational speeds of the wheels 101-104, an acceleration sensor 630 that detects acceleration αx in the longitudinal direction of the vehicle 100, and a yaw rate sensor 640 that detects a yaw rate γac generated in the vehicle 100, and the MCU 510 acquires the detection signals of these sensors. Here, the MCU 510 calculates the vehicle speed v of the vehicle 100 (in other words, the running speed) as the running state of the vehicle 100, based on the wheel speeds WS1-WS4 (second physical quantities related to the vehicle speed of the vehicle 100) detected by the wheel speed sensors 621-624.

[0019] Vehicle 100 also includes a drive mode selection switch 650 that allows the driver to specify the driving characteristics of vehicle 100, and MCU 510 acquires a signal indicating the state of the drive mode specified by drive mode selection switch 650. The drive mode is associated with the output characteristics of the drive source of vehicle 100, and includes multiple modes such as sport mode, comfort mode, racing mode, and normal mode, and is configured so that the driver can arbitrarily select one by operating drive mode selection switch 650.

[0020] Here, MCU 510 has a function of controlling steering angle δ so that the gain and phase of yaw rate γ, which represents the vehicle behavior of vehicle 100 in response to the steering operation of steering input member 310 by the driver, become target gain and target phase. In particular, MCU 510 sets damping ratio ζ and natural angular frequency ωn [rad / s] to achieve the target gain and target phase, and calculates a target yaw rate generated in vehicle 100 from the steering operation amount (operation angle θ) of steering input member 310, a steady-state yaw rate gain of vehicle 100 based on vehicle speed v of vehicle 100, the damping ratio ζ and natural angular frequency ωn, and a pre-given vehicle model. MCU 510 then obtains steering angle command δtg (target control amount), which is the control amount of steering motor 410 that results in the target yaw rate, and outputs a signal equivalent to steering angle command δtg.

[0021] 2 is a block diagram showing steering angle control by MCU 510. Steady-state yaw rate gain calculation unit 511 acquires a signal of vehicle speed v [km / h] obtained from the output of wheel speed sensors 621-624, and calculates steady-state yaw rate gain k based on the acquired signal of vehicle speed v.

[0022] 3 shows one aspect of the correlation between the vehicle speed v and the steady-state yaw rate gain k in the steady-state yaw rate gain calculation unit 511. In the example of the characteristics shown in FIG. 3, in the region where the vehicle speed v is equal to or less than a predetermined speed v1, the steady-state yaw rate gain k gradually increases as the vehicle speed v increases, and in the region where the vehicle speed v exceeds the predetermined speed v1, the steady-state yaw rate gain k gradually decreases as the vehicle speed v increases, and when the vehicle speed v is near the predetermined speed v1, the steady-state yaw rate gain k reaches a local maximum value (maximum value). The predetermined speed v1 is set to a high speed of, for example, about 90 km / h.

[0023] The vehicle response characteristic setting unit 512 sets the damping ratio ζ and the natural angular frequency ωn, which represent the response characteristics of the vehicle 100, to arbitrary constant values ​​regardless of the vehicle speed v. For example, the vehicle response characteristic setting unit 512 sets the damping ratio ζ to 1.0 and the natural angular frequency ωn to arbitrary constant values ​​according to the vehicle characteristics. As will be described later, the damping ratio ζ and the natural angular frequency ωn are constants for setting the gain and phase of the yaw rate, which is the vehicle behavior of the vehicle 100 in response to the steering operation of the driver of the vehicle 100, to target gain and target phase, and the damping ratio ζ and the natural angular frequency ωn are arbitrarily determined based on the required yaw rate response characteristics.

[0024] Note that the damping ratio ζ is over-damped when ζ > 1 and under-damped when 1 > ζ > 0, so the damping ratio ζ is basically set to 1.0, which is the critical damping state, but is not limited to ζ = 1.0. Furthermore, since the natural angular frequency ωn [rad / s] is expressed as ωn = 2π fn when the natural frequency is fn [Hz], the vehicle response characteristic setting unit 512 can set the natural frequency fn instead of the natural angular frequency ωn.

[0025] The target yaw rate calculation unit 513 acquires signals of the operation angle θ, the vehicle speed v, the steady-state yaw rate gain k, the damping ratio ζ, and the natural angular frequency ωn, and calculates the target yaw rate γ based on these signals. In detail, the target yaw rate calculation unit 513 is a conversion processing unit that receives the steering angle δ calculated from the operation angle θ and the steering gear ratio N, and the vehicle speed v, and converts the transfer function G1(s) when outputting the target yaw rate γ into a second-order delay system represented by Equation 1.

[0026] As shown in Equation 1, the transfer function G1(s) is determined using the steady-state yaw rate gain k, the damping ratio ζ, and the natural angular frequency ωn.

[0027] In this embodiment, the steering gear ratio N is the ratio of the steering operation amount (operation angle θ) of the steering input member 310 to the steering angle δ of the steered wheels, and is defined as "steering gear ratio N = operation angle θ / steering angle δ". Therefore, the steering angle δ is calculated as "steering angle δ = operation angle θ / steering gear ratio N". Note that the steering gear ratio is set to a smaller value (so-called quick ratio) as the vehicle speed v is lower, for example.

[0028] The target yaw rate differential value calculation unit 514 acquires signals of the operation angle θ, the vehicle speed v, the steady-state yaw rate gain k, the damping ratio ζ, and the natural angular frequency ωn, and calculates the target yaw rate differential value γ (dot) based on these signals. In detail, the target yaw rate differential value calculation unit 514 is a conversion processing unit that receives the steering angle δ calculated from the operation angle θ and the steering gear ratio N, and the vehicle speed v, and converts the transfer function G2(s) when outputting the target yaw rate differential value γ (dot) into a second-order delay system represented by Equation 2.

[0029] As shown in Equation 2, the transfer function G2(s) is determined using the steady-state yaw rate gain k, the damping ratio ζ, and the natural angular frequency ωn.

[0030] Comparator 515 determines the deviation between target yaw rate γ calculated by target yaw rate calculator 513 and yaw rate estimated value γest output by virtual vehicle model 516 as control deviation e (e=γ−γest). Adder 517 is a functional unit for calculating a steering angle command δtg (target steering angle), and determines the steering angle command δtg by summing up the following various signals:

[0031] The control deviation e calculated by the comparison unit 515 is integrated by an integration unit 518 (in other words, an integration operator), and the integral value of the control deviation e is further multiplied by an integral value gain K5, and "the integral value of the control deviation e × K5" is output to an addition unit 517. In addition, the target yaw rate differential value γ (dots) calculated by the target yaw rate differential value calculation unit 514 is multiplied by a target yaw rate differential value gain K3, and "the target yaw rate differential value γ (dots) × K3" is output to the addition unit 517.

[0032] 4 is a diagram illustrating the correlation between the target yaw rate derivative gain K3 and the vehicle speed v. The target yaw rate derivative gain K3 rapidly decreases to a predetermined value as the vehicle speed v increases in a low vehicle speed range where the vehicle speed v is lower than a predetermined speed, and maintains the predetermined value in a medium to high vehicle speed range where the vehicle speed v exceeds the predetermined speed.

[0033] The target yaw rate γ calculated by the target yaw rate calculation unit 513 is multiplied by a target yaw rate gain K4, and "target yaw rate γ×K4" is output to the adder 517. Furthermore, the adder 517 receives a value obtained by multiplying the yaw rate estimated value γest output by the virtual vehicle model 516 by a yaw rate estimated value gain K1 (yaw rate estimated value γest×K1), and a value obtained by multiplying the vehicle body sideslip angle estimated value βest output by the virtual vehicle model 516 by a vehicle body sideslip angle estimated value gain K2 (vehicle body sideslip angle estimated value βest×K2).

[0034] Then, adder 517 outputs the sum (see Equation 3) of the “yaw rate estimated value γest×K1,” “vehicle body sideslip angle estimated value βest×K2,” “target yaw rate differential value γ (dot)×K3,” “target yaw rate γ×K4,” and “integral value of control deviation e×K5” as steering angle command δtg to steering device 400 and virtual vehicle model 516.

[0035] Steering device 400 drives and controls steering motor 410 so that actual steering angle δ becomes steering angle command δtg. Furthermore, virtual vehicle model 516 uses a vehicle model such as a two-wheel model that is given in advance to determine yaw rate estimate γest and vehicle body sideslip angle estimate βest, which represent the behavior of vehicle 100 when vehicle speed v and steering angle command δtg are given.

[0036] The equation of motion of the two-wheel model is generally expressed by Equation 4. From the equation of motion of Equation 4, the differential value of the yaw rate γ and the differential value of the vehicle body sideslip angle β have a relationship as shown in Equation 5.

[0037] The yaw rate estimated value γest and vehicle body sideslip angle estimated value βest output by virtual vehicle model 516 are used for the turning angle command δtg, and the turning angle command δtg is corrected in accordance with the deviation between the yaw rate estimated value γest and the target yaw rate γ. In this way, in the turning angle control by MCU 510, model following control (MFC) is used, which causes the output of virtual vehicle model 516 to follow.

[0038] If a two-wheel model is used for the virtual vehicle model 516 for model following control and the target yaw rate responds with a second-order delay, when the steady-state yaw rate gain k that varies according to the vehicle speed v, and the damping ratio ζ and natural angular frequency ωn for achieving the target gain and phase are arbitrarily set, a state equation such as that shown in Equation 6 can be established. The asterisk in Equation 6 indicates a target value.

[0039] For each variable in the state equation of Equation 6, weights Q and R are set using the evaluation function shown in Equation 7, and the gain for each variable is calculated using linear quadratic regulator design (LQR design). The sum of the calculated gain for each variable and each variable becomes the turning angle command δtg, and the turning angle command δtg is determined according to the above-mentioned equation 3.

[0040] The settings of the damping ratio ζ and the natural angular frequency ωn will be described in detail below. Fig. 5 is a diagram illustrating an example of the correlation between the damping ratio ζ and the vehicle speed v. Here, the characteristic shown by the solid line in Fig. 5 indicates the setting characteristic of the damping ratio ζ in the vehicle response characteristic setting unit 512, which sets the damping ratio ζ to a constant value (for example, ζ = 1.0) regardless of the vehicle speed v.

[0041] 5 indicates the correlation between vehicle speed v and damping ratio ζ in a two-wheel steering vehicle equipped with electric power steering (EPS) in which steering input member 310 is mechanically connected to steered front wheels 101, 102. In the case of a two-wheel steering vehicle equipped with electric power steering, the vehicle characteristic is that damping ratio ζ decreases as vehicle speed v increases, but vehicle response characteristic setting unit 512 sets damping ratio ζ to 1.0 regardless of vehicle speed v so that damping ratio ζ does not decrease as vehicle speed v increases.

[0042] 6 is a diagram illustrating an example of the correlation between the natural angular frequency ωn and the vehicle speed v. Here, the characteristic shown by the solid line in Fig. 6 indicates the setting characteristic of the natural angular frequency ωn in the vehicle response characteristic setting unit 512, and the vehicle response characteristic setting unit 512 sets the natural angular frequency ωn to a constant value regardless of the vehicle speed v.

[0043] The characteristic shown by the dotted line in Fig. 6 indicates the correlation between the vehicle speed v and the natural angular frequency ωn in a two-wheel steering vehicle equipped with electric power steering. In the case of a vehicle equipped with electric power steering, the natural angular frequency ωn decreases as the vehicle speed v increases, as a vehicle characteristic.

[0044] The characteristic shown by the dashed dotted line in Figure 6 indicates the correlation between vehicle speed v and stable handling capacity ζωn in a two-wheel steering vehicle equipped with electric power steering. Stable handling capacity ζωn is the value obtained by multiplying damping ratio ζ by natural angular frequency ωn. In the case of a vehicle equipped with electric power steering, the damping ratio ζ and natural angular frequency ωn decrease as the vehicle speed v increases, so the stable handling capacity ζωn also decreases as the vehicle speed v increases.

[0045] Fig. 7 is a diagram showing the correlation between the steering operation frequency and the yaw rate gain for each vehicle speed in a two-wheel steering vehicle equipped with electric power steering, and Fig. 8 is a diagram showing the correlation between the steering operation frequency and the phase for each vehicle speed in a two-wheel steering vehicle equipped with electric power steering. As shown in Fig. 7 and Fig. 8, in the case of a two-wheel steering vehicle equipped with electric power steering, the damping ratio ζ and the natural angular frequency ωn become smaller as the vehicle speed v increases, so that the yaw rate gain becomes excessive for fast steering at high vehicle speeds, as shown in Fig. 7, and the phase lag becomes excessive for fast steering at high vehicle speeds, as shown in Fig. 8.

[0046] For this reason, in the case of a two-wheel steering vehicle equipped with electric power steering, the resonance peak of the yaw rate increases at high speeds, requiring the driver to steer within a small steering angle range so as not to turn the steering input member 310 too much. Also, in the case of a two-wheel steering vehicle equipped with electric power steering, the phase lag of the yaw rate increases at high speeds, requiring the driver to predict the behavior of the vehicle 100 and perform steering operations that artificially compensate for the phase lag. Therefore, in two-wheel steering vehicles equipped with electric power steering, maneuverability and stability at high speeds tend to deteriorate.

[0047] In contrast to this, the steering control of the present application utilizes the characteristics of steer-by-wire, which can arbitrarily control the tire angle of front wheels 101, 102 in response to the steering operation of steering input member 310, and controls the damping ratio ζ and natural angular frequency ωn to be constant regardless of vehicle speed v, for setting the gain and phase of the yaw rate in response to the steering operation of the driver of vehicle 100 to target gain and target phase, by model following control using a virtual vehicle model. This makes it possible to suppress the occurrence of resonance peaks in the high-speed range, as shown in Fig. 7, and to reduce the phase delay, as shown in Fig. 8.

[0048] Therefore, the operability of the driver at high speeds is improved and the burden of driving operations can be reduced. In particular, in an emergency avoidance driving situation, when avoiding an obstacle and when returning to the original lane, the yaw rate actually generated in the vehicle 100 converges and follows the yaw rate expected by the driver without overshooting, so that unnecessary steering operations such as corrective steering are not required and obstacle avoidance can be performed with ease.

[0049] Incidentally, by arbitrarily setting the damping ratio ζ and the natural angular frequency ωn to constant values ​​regardless of the vehicle speed, it is possible to variably control the yaw rate response characteristics, and the system is not limited to a configuration in which the damping ratio ζ and the natural angular frequency ωn are fixed values, but can make the damping ratio ζ and the natural angular frequency ωn variable depending on conditions such as the running state of the vehicle 100 and the driving characteristics of the driver. In other words, steering control device 500 (vehicle response characteristics setting unit 512) can set the damping ratio ζ and the natural angular frequency ωn to achieve yaw rate convergence characteristics that are suitable for the running state of vehicle 100 or the driver, in order to set the yaw rate gain and phase, which are the vehicle behavior of the vehicle in response to the steering operation of the driver, to the target yaw rate gain and target phase.

[0050] Furthermore, by making the damping ratio ζ and the natural angular frequency ωn variable according to conditions such as the driving state of the vehicle 100 and the driving characteristics of the driver, it is possible to set response and convergence characteristics that are more appropriate for the driving state of the vehicle 100 and the driving characteristics of the driver. Note that the driving state of the vehicle 100 includes the setting state of the steering gear ratio N according to the vehicle speed v, the steering operation state such as primary steering and secondary steering, and the driving characteristics of the driver include the driver's skill level and preferences for the response of vehicle behavior to driving operations.

[0051] Here, when the damping ratio ζ and the natural angular frequency ωn are variable, the turning control device 500 (vehicle response characteristic setting section 512) can vary the damping ratio ζ and the natural angular frequency ωn so that the stable handling capacity ζωn, which is the value obtained by multiplying the damping ratio ζ and the natural angular frequency ωn, remains constant. If the stable handling capacity ζωn is constant, the convergence follows an envelope, and the convergence time remains the same even if the damping ratio ζ and the natural angular frequency ωn are variable.

[0052] Therefore, after adapting the natural angular frequency ωn based on the damping ratio ζ = 1.0, if it is desired to improve the responsiveness relative to the base specifications, the damping ratio ζ is set to a value within the range of ζ < 1.0 that provides the required responsiveness, and the natural angular frequency ωn is adapted so that the value of the stable handling capacity ζωn is the same as that of the base specifications even when the damping ratio ζ is set to ζ < 1.0. This makes it possible to obtain a higher responsiveness than that of the base specifications that is suited to the driving state or driver of the vehicle 100, while maintaining the same convergence time as that of the base specifications.

[0053] Figure 9 illustrates an example of transient response to a step input when the damping ratio ζ is gradually reduced from 1.0 while the natural angular frequency ωn is gradually increased so that the stable handling capacity ζωn is the same as in the case of a base specification where the damping ratio ζ = 1.0. As shown in Figure 9, when the damping ratio ζ is reduced to improve responsiveness, if the natural angular frequency ωn is increased so that the stable handling capacity ζωn is the same, the convergence time will be the same even if the responsiveness is changed by varying the damping ratio ζ. Furthermore, if the damping ratio ζ and the natural angular frequency ωn are varied within a range where the convergence time is the same, it is easy to understand whether the response and convergence characteristics are within appropriate ranges, and this is expected to reduce the man-hours required for adjusting the damping ratio ζ and the natural angular frequency ωn at the design and development stage of steering control device 500 (vehicle response characteristic setting unit 512).

[0054] Steering control device 500 (vehicle response characteristic setting section 512) can vary damping ratio ζ and natural angular frequency ωn, for example, in accordance with the drive mode specified by the driver using drive mode selection switch 650. In other words, the drive mode specified by the driver represents the driving characteristics of the driver of vehicle 100, more specifically, the driver's level of skill and preference for the response of vehicle behavior to driving operations. For example, when sport mode or racing mode is selected as the drive mode, it can be inferred that the driver is more skilled or that the driver prefers quicker response of vehicle behavior to driving operations than when comfort mode or normal mode is selected.

[0055] Therefore, when the sport mode or racing mode is selected, the steering control device 500 (vehicle response characteristic setting section 512) can make the damping ratio ζ smaller to increase the response compared to when the comfort mode or normal mode is selected. By changing the damping ratio ζ in this way, the response characteristic can be set to be more suited to the driving characteristics of the driver, improving the steering operability of the driver.

[0056] It should be noted that when changing the damping ratio ζ and the natural angular frequency ωn in accordance with the driving characteristics of the driver, if the natural angular frequency ωn is changed in accordance with the change in the damping ratio ζ so that the stable handling capacity ζωn does not change, it is possible to change the responsiveness while maintaining the same convergence time. Furthermore, the determination of the driver's driving characteristics is not limited to determination based on the specified drive mode, and steering control device 500 (vehicle response characteristic setting section 512) can estimate the driver's skill level and the driver's preferred reaction speed, for example, from a history of accelerator operation speed, accelerator operation amount, etc.

[0057] Furthermore, steering control device 500 (vehicle response characteristic setting unit 512) can vary damping ratio ζ so as to lower the yaw rate gain (in other words, responsiveness) from secondary steering onwards compared to primary steering. If the responsiveness during vehicle behavior correction from secondary steering onwards is made the same as when turning the primary steering further, the difficulty of steering increases, and secondary steering, which is steering to the opposite side while the primary behavior is occurring, makes it difficult to return to the original lane in an emergency avoidance or the like. Therefore, by lowering the responsiveness from secondary steering onwards compared to primary steering and relatively lowering the responsiveness during steering back (secondary steering) to a level that allows the driver to deal with the behavior correction, it is possible to achieve both emergency avoidance and behavior convergence.

[0058] In particular, experienced drivers tend to understand and predict vehicle characteristics and turn back (secondary steering) faster than turning further (primary steering) when correcting behavior through secondary steering. Therefore, if the base response is increased based on the driver's expertise, making the response after secondary steering the same as when turning further in primary steering would likely cause significant disruption in vehicle behavior, making it difficult to correct behavior after secondary steering.

[0059] Therefore, the process of lowering the responsiveness at secondary steering and onward compared to that at primary steering can be performed on the condition that the damping ratio ζ is set to less than 1.0 as a base specification based on the driving characteristics of the driver. In other words, when the damping ratio ζ is set to less than 1.0 as a base specification, for example, the process of returning the damping ratio ζ to 1.0 at secondary steering and onward can be performed. Also, even when the damping ratio ζ and the natural angular frequency ωn are changed due to the primary steering and secondary steering, the natural angular frequency ωn can be changed in accordance with the change in the damping ratio ζ so that the steering stability capacity ζωn does not change.

[0060] Furthermore, when the steering gear ratio N is set to a smaller value as the vehicle speed v decreases, if the steering gear ratio is set to a quick ratio at low vehicle speeds, the yaw rate gain is likely to become large and steering operability will deteriorate. Therefore, when the steering gear ratio N is set to a small value at low vehicle speeds, steering control device 500 (vehicle response characteristic setting section 512) can change the damping ratio ζ and the natural angular frequency ωn in accordance with vehicle speed v so that the phase is more delayed than when the steering gear ratio N is set to a large value at high vehicle speeds. Even in such variable settings of the damping ratio ζ and the natural angular frequency ωn in accordance with vehicle speed v, the natural angular frequency ωn can be changed in accordance with the change in damping ratio ζ so that the handling stability capacity ζωn does not change.

[0061] The technical ideas described in the above embodiments can be used in appropriate combinations as long as no contradictions arise. Furthermore, although the contents of the present invention have been specifically described with reference to preferred embodiments, it is obvious that a person skilled in the art can adopt various modified embodiments based on the basic technical ideas and teachings of the present invention.

[0062] For example, the drive mode is not limited to those associated with the output characteristics of the drive source of the vehicle 100, and may be a mode dedicated to steering characteristics that specifies the gain and phase of the yaw rate in response to the steering operation of the driver (in other words, the responsiveness and convergence of the yaw rate in response to the steering operation of the driver). In other words, the system can be configured so that the driver can arbitrarily specify the settings of the damping ratio ζ and the natural angular frequency ωn.

[0063] 100...vehicle, 101, 102...front wheels (steered wheels), 200...steering system, 300...steering input device, 310...steering input member, 330...operation angle sensor, 400...steering device, 410...steering motor, 500...steering control device, 510...MCU, 621-624...wheel speed sensor, 630...acceleration sensor, 640...yaw rate sensor, 650...drive mode selection switch

Claims

1. A steering control device having one or more processors and provided on a vehicle having a steering input member that accepts steering operation by a driver of the vehicle, and a motor that applies a steering force to steered wheels of the vehicle that are mechanically separated from the steering input member, wherein the steering control device: obtains a first physical quantity related to the operation amount of the steering input member and a second physical quantity related to the vehicle speed, sets a damping ratio and a natural angular frequency to make the gain and phase of a yaw rate, which is the vehicle behavior of the vehicle in response to the steering operation by the driver, a target gain and a target phase, calculates a target yaw rate generated in the vehicle from the first physical quantity, a steady-state yaw rate gain of the vehicle based on the second physical quantity, the set damping ratio and natural angular frequency, and a predetermined vehicle model, obtains a target control variable that is the control variable of the motor that will result in the target yaw rate, and outputs a signal corresponding to the target control variable to the motor.

2. A steering control device according to claim 1, wherein the damping ratio is set to 1 regardless of the vehicle speed.

3. A steering control device according to claim 1, wherein the damping ratio and the natural angular frequency are variably set in accordance with the running state of the vehicle or the driving characteristics of the driver of the vehicle.

4. A steering control device according to claim 3, wherein, when the damping ratio is ζ and the natural angular frequency is ωn, the damping ratio ζ and the natural angular frequency ωn are varied so that ζωn remains constant.

5. A steering control device according to claim 3, wherein the driving characteristics of the driver are determined based on a drive mode that indicates the driving characteristics of the vehicle designated by the driver.

6. A steering control device according to claim 3, wherein the damping ratio and the natural angular frequency are set so that the gain of the yaw rate is lower in the secondary steering than in the primary steering.

7. A steering control device as claimed in claim 3, wherein a steering gear ratio, which is the ratio of the amount of operation of the steering input member to the steering angle of the steered wheels, is varied so as to become smaller as the vehicle travel speed decreases, and when the steering gear ratio is small, the damping ratio and the natural angular frequency are set so as to delay the phase of the yaw rate more than when the steering gear ratio is large.

8. A steering control method implemented by a steering control device having one or more processors and provided to a vehicle having a steering input member that accepts steering operation by a driver of the vehicle, and a motor that applies a steering force to steered wheels of the vehicle that are mechanically separated from the steering input member, the steering control method comprising: acquiring a first physical quantity related to the operation amount of the steering input member and a second physical quantity related to the vehicle speed; setting a damping ratio and a natural angular frequency to make the gain and phase of a yaw rate, which is the vehicle behavior of the vehicle in response to the steering operation by the driver, a target gain and a target phase; determining a target yaw rate generated in the vehicle from the first physical quantity, a steady-state yaw rate gain of the vehicle based on the second physical quantity, the set damping ratio and natural angular frequency, and a predetermined vehicle model; obtaining a target control variable that is the control variable of the motor that will result in the target yaw rate; and outputting a signal corresponding to the target control variable to the motor.

9. A steering control program executed by a steering control device having one or more processors and provided to a vehicle having a steering input member that accepts steering operation by a driver of the vehicle, and a motor that applies a steering force to steered wheels of the vehicle that are mechanically separated from the steering input member, the steering control program comprising: acquiring a first physical quantity related to the operation amount of the steering input member and a second physical quantity related to the vehicle speed; setting a damping ratio and a natural angular frequency to make the gain and phase of a yaw rate, which is the vehicle behavior of the vehicle in response to the steering operation by the driver, a target gain and a target phase; determining a target yaw rate generated in the vehicle from the first physical quantity, a steady-state yaw rate gain of the vehicle based on the second physical quantity, the set damping ratio and natural angular frequency, and a predetermined vehicle model; obtaining a target control variable that is the control variable of the motor that will result in the target yaw rate; and outputting a signal corresponding to the target control variable to the motor.

10. A steering system comprising: a steering input member that accepts steering operation by a vehicle driver; a motor that applies a steering force to steered wheels of the vehicle that are mechanically separated from the steering input member; and a steering control device having one or more processors, the steering control device acquiring a first physical quantity related to the operation amount of the steering input member and a second physical quantity related to the vehicle speed, setting a damping ratio and a natural angular frequency to make the gain and phase of a yaw rate, which is the vehicle behavior of the vehicle in response to the steering operation of the driver, a target gain and a target phase, determining a target yaw rate generated in the vehicle from the first physical quantity, a steady-state yaw rate gain of the vehicle based on the second physical quantity, the set damping ratio and natural angular frequency, and a predetermined vehicle model, acquiring a target control variable that is the control variable of the motor that will result in the target yaw rate, and outputting a signal corresponding to the target control variable to the motor.

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