JP7239238B2 - automotive steering gear - Google Patents

Description

TECHNICAL FIELD The present invention relates to a steering device for automobiles.

The toe angle is the angle at which the front ends of a pair of left and right tires are directed inward or outward with respect to the traveling direction of the vehicle. When the toe angle is positive, the front end of the tire faces inward, resulting in "toe-in," which improves straight running stability. When the toe angle is negative, the front end of the tire faces outward, resulting in "toe-out," which enhances turning performance. Some sports cars are set to toe-out, but in general, one or both of the front wheels and rear wheels are set to toe-in in order to improve straight-line stability.

For example, Patent Literature 1 discloses a steering system provided with toe angle control means in an independently steered vehicle capable of independently steering left and right tires. In this steering system, when the vehicle speed is equal to or higher than a predetermined value, the road surface friction coefficient is estimated based on the current value of the steering actuator.

International publication WO2019/189096

In the prior art of Patent Document 1, in an independent steering vehicle, the external tire force (for example, self-aligning torque) generated when the toe angle is controlled is estimated from the current generated in the steering actuator, and further, from the characteristics with respect to the tire external force. It can be understood as estimating the road surface friction coefficient.

However, this prior art does not consider the effects of road gradients such as cant. If there is cant on the road surface, lateral force due to the cant acts on the tire in addition to the force related to the road surface friction coefficient. .

The present invention was created in view of the above points, and its object is to suppress the influence of road surface conditions such as cant in an independently steered vehicle and to reduce the estimation error of the road surface friction coefficient. is to provide

In the automotive steering apparatus according to the present invention, one or more rows of tires (91-94) out of two or more rows of left and right tires in the vehicle front-rear direction are capable of independently steering the left and right wheels. It is mounted on a rudder vehicle (100). This automobile steering apparatus includes a plurality of steering actuators (31-34) for driving each tire to be driven in a steering direction, a toe angle control section (25), a tire external force detection section (29), and a road surface friction coefficient estimator (26).

The angle at which the front ends of the pair of left and right tires face inward with respect to the direction of travel of the vehicle is positive, and the angle at which the front ends of the left and right tires face outward is negative. The toe angle control unit outputs a drive signal to the steering actuator so as to execute "periodic steering" in which the toe angle is periodically changed at a predetermined steering frequency and angular amplitude.

The tire external force detection unit detects a tire external force applied to the tire steered by the steering actuator. The road surface friction coefficient estimator performs frequency analysis of the tire external force and estimates the road surface friction coefficient based on the relationship between the tire external force and the road surface friction coefficient at the steering frequency.

For example, if the road has a certain cant, the external force generated on the tire due to the cant is a DC component. Therefore, by frequency-analyzing the tire external force due to periodic steering and estimating the road surface friction coefficient using only the steering frequency component, it is possible to suppress the error due to the road surface condition.

1 is a block diagram of an automotive steering system according to one embodiment; FIG. (a) A toe angle of 0, (b) a diagram showing a toe-in state. The figure which shows the lateral force and SAT (tire external force) which act on a tire. FIG. 5 is a diagram showing estimation of a road surface friction coefficient based on SAT at a constant toe angle (comparative example); The figure which shows the tire external force by a cant. FIG. 4 is a diagram showing the change over time of SAT during running with a constant toe angle; 4 is a flow chart of road surface friction coefficient estimation according to one embodiment. Waveform diagrams of toe angle and SAT in periodic steering. The figure explaining the influence on SAT by acceleration/deceleration during periodic steering. The SAT waveform diagram in periodic steering with road surface friction coefficient μ=0.4. The figure which shows the frequency-analysis result of SAT in periodic steering. The figure which shows the estimation (this embodiment) of the road surface friction coefficient based on the SAT maximum value in the steering frequency extracted by the frequency analysis.

DETAILED DESCRIPTION OF THE INVENTION An automotive steering apparatus according to an embodiment of the present invention will be described below with reference to the drawings. The automotive steering apparatus of the present embodiment is mounted on an independently steered vehicle in which one or more rows of left and right tires in two or more rows in the longitudinal direction of the vehicle can be independently steered by the left and right wheels. be done.

Conventionally, in a general vehicle, a pair of left and right tires are mechanically connected via a link, and the tires are steered by steering. In the future, it is expected that the vehicle will develop into a steer-by-wire in which the steering and the link of the pair of left and right tires are mechanically separated, and an independent steering vehicle in which the pair of left and right tires can be steered independently. In the steer-by-wire, the left and right tires are interlocked and move only in the same phase, whereas in the independent steering vehicle, the left and right wheels can be freely steered.

An example of merits of an independently steered vehicle will be described. In a conventional vehicle, the steering angles of the left and right tires are always the same when turning. However, geometrically, unless the steering angle on the inner side of the turn is greater than the steering angle on the outer side of the turn, the tires will skid. On the other hand, in the case of an independently steered vehicle, the left and right rudder angles can be freely set according to the turning radius, so it is possible to reduce the running resistance caused by tire skidding.

(one embodiment)
With reference to FIG. 1, the configuration of an automotive steering system 20 according to one embodiment will be described. The independently steered vehicle 100 shown in FIG. 1 is a four-wheeled vehicle having two rows of left and right pairs of tires in the longitudinal direction of the vehicle. Both of the front row tires 91 and 92 and the rear row tires 93 and 94 can be independently steered. be. The automotive steering device 20 includes a plurality of (for example, four) steering actuators 31 to 34, a toe angle controller 25, a road surface friction coefficient estimator 26, an acceleration/deceleration detector 27, a vehicle state detector 28, and a tire external force sensor. A detection unit 29 is provided.

A plurality of steering actuators 31-34 drive each tire 91-94 in the steering direction. The four-wheel independent steering vehicle 100 has a steering actuator FL31 and a steering actuator FR32 that drive the left and right tires 91 and 92 in the front row, and a steering actuator RL33 and a steering actuator RL33 that drive the left and right tires 93 and 94 in the rear row. An RR 34 is provided.

The toe angle control unit 25 outputs drive signals to the steering actuators 31 to 34 so as to execute "periodic steering" in which the toe angle is periodically changed at a predetermined steering frequency f and an angular amplitude θamp. . In this specification, the "toe angle" is defined as "the angle at which the front ends of the left and right tires face inward with respect to the traveling direction of the vehicle is positive, and the angle at which the front ends of the left and right tires face outward is negative. is defined as the "deflected angle".

In other words, in conventional general vehicles, the toe angle is initially set during alignment adjustment when the vehicle is stopped. It is possible to change the toe angle and perform periodic steering. Further, in this embodiment, the toe angle control section 25 can individually set the steering frequency f and the angle amplitude θamp of each row. However, it is not assumed that the left and right pairs of tires in each row are cyclically steered asymmetrically. The front row tires 91 and 92 are periodically steered by symmetric drive signals output by the left and right steering actuators 31 and 32 in the front row. Rear-row tires 93 and 92 are cyclically steered by symmetric drive signals output from the left and right steering actuators 33 and 34 in the rear row.

The toe angle is defined as positive when the front ends of the pair of left and right tires face inward, and the toe angle is negative when the front ends of the pair of left and right tires face inward. When toe-in, straight running stability is high, but turning performance is low. When toe-out, turning performance is high, but straight running stability is low. As shown in FIG. 2, a vehicle is normally adjusted to "toe-in" in which the front ends of a pair of left and right tires face slightly (1 [deg] or less) inward in a straight running state in order to improve straight running stability.

As shown in FIG. 3, as a contradiction to the stability of the vehicle, the tire lateral force dependent on the road surface friction coefficient μ is generated because the direction of the tires deviates from the traveling direction. Note that the road surface friction coefficient μ is a coefficient representing the friction between the road surface and the tire. Since the running resistance increases due to this tire lateral force, it is basically desirable to set the toe angle to 0 [deg].

Next, the tire external force detection unit 29 detects the tire external force applied to the tires 91-94 steered by the steering actuators 31-34. The tire external force means self-aligning torque (hereinafter "SAT") or tire lateral force that tends to return the tire in the direction opposite to the steering direction. Hereinafter, the tire external force will be described as SAT. The description of "SAT" is appropriately interpreted as "tire external force". As shown in FIG. 3, SAT is a function of wheel load F, road friction coefficient μ, and toe angle θ. The tire external force detection unit 29 can detect the SAT from the load currents of the steering actuators 31 to 34 and the force sensors and torque sensors provided in the steering mechanism.

By the way, since there is a correlation between the SAT and the road surface friction coefficient μ, the road surface friction coefficient μ can be estimated by detecting the SAT. When the vehicle is driven with the toe angle set to a constant value (for example, 2 [deg]), SAT occurs according to the road surface friction coefficient μ, as shown by the solid line in FIG. When traveling on an ideally flat road, a constant SAT is detected, and the road surface friction coefficient μ can be accurately estimated from the detected SAT. Note that specific numerical values of SAT are not shown in each figure. However, in order to clearly indicate that it is a torque-dimensional quantity, the axis of each figure is marked with "SAT [Nm]".

Patent Document 1 (International Publication No. WO2019/189096) discloses a technique for estimating a road surface friction coefficient in an independently steered vehicle. It can be understood that this prior art estimates the external tire force (for example, SAT) generated when the toe angle is controlled from the current generated in the steering actuator, and further estimates the road surface friction coefficient from the characteristics with respect to the external tire force.

However, an actual road cannot be flat because it has a cant, which is a cross slope for better drainage, and has ruts. Kant is defined as ±2% by the Road Structure Ordinance. As shown in FIG. 5, when the cant is provided, the tire is always subjected to an external tire force in one direction. Due to the influence of cant, the SAT changes in the range from the dashed line (cant -2%) to the dashed line (cant +2%) in Fig. 4, and there is a maximum error of 19% in the road surface friction coefficient μ estimated from the same SAT value. occur. Therefore, the estimation of the road surface friction coefficient based on the SAT at a constant toe angle will be treated as a comparative example with respect to the present embodiment.

FIG. 6 shows changes in SAT over time during running with a constant toe angle. Compared to the SAT at 0% cant, the SAT at cant -2% is slightly larger, and the SAT at cant +2% is slightly smaller. This difference is the SAT detection error due to Kant. However, the SAT generated in the tire maintains a constant value while the vehicle is running on a road with a constant cant after starting, and can be regarded as a low-frequency stationary component. Therefore, by periodically changing the toe angle at a predetermined steering frequency f, it is possible to distinguish between the SAT due to cant and the SAT due to the road surface friction coefficient μ due to periodic steering.

In this embodiment, based on this point of view, the toe angle control unit 25 outputs drive signals to the steering actuators 31 to 34 so as to execute periodic steering as described above. Then, the road friction coefficient estimator 26 performs frequency analysis on the SAT detected by the tire external force detector 29, and estimates the road friction coefficient μ based on the relationship between the SAT and the road friction coefficient μ at the steering frequency f.

Returning to FIG. 1, the automotive steering apparatus 20 of the present embodiment further includes an acceleration/deceleration detection section 27 and a vehicle state detection section 28 . The acceleration/deceleration detection unit 27 detects the rate of change in speed per time when the vehicle is accelerating or decelerating, and notifies the road surface friction coefficient estimation unit 26 of it. The velocity change rate during acceleration corresponds to positive acceleration, and the velocity change rate during deceleration corresponds to negative acceleration.

The vehicle state detection unit 28 detects the vehicle state due to vehicle behavior or external force, and notifies the road surface friction coefficient estimation unit 26 of the detected vehicle state. The vehicle behavior means the active behavior of the vehicle, and the external force means the force of disturbance acting on the tires due to road ruts, swells, crosswinds, and the like. For example, the vehicle state detection unit 28 detects roll, yaw, lateral acceleration, vehicle height variation, and the like using a yaw rate sensor, lateral acceleration sensor, height sensor, and the like.

The road surface friction coefficient estimator 26 determines whether or not the road surface friction coefficient μ can be estimated based on the information from the acceleration/deceleration detector 27 and the vehicle state detector 28 . The road friction coefficient estimator 26 estimates the road friction coefficient μ only when the absolute value of the speed change rate is equal to or less than a predetermined change rate threshold and the vehicle state satisfies a predetermined stability condition.

Next, estimation of the road surface friction coefficient according to the present embodiment will be described with reference to the flow chart of FIG. 7 and FIGS. 8 to 12. FIG. In the description of the flowchart, the symbol "S" means step. In S51, the acceleration/deceleration detector 27 detects acceleration or deceleration of the vehicle. In S52, it is determined whether the absolute value of the speed change rate is equal to or less than the change rate threshold. If NO in S52, the process returns to START without proceeding to the next step.

If YES in S52, the vehicle state detector 28 detects vehicle states such as roll, yaw, and lateral acceleration in S53. In S54, it is determined whether or not a stability condition, which is a criterion for determining that the vehicle is in a stable state, is satisfied. For example, when the detected value of a yaw rate sensor or lateral acceleration sensor mounted on the vehicle is equal to or less than a predetermined threshold value, it is determined that the stability condition is satisfied. Other parameters reflecting the vehicle state are not limited to parameters for which it is determined that "the lower the numerical value, the more stable the vehicle state". If NO in S54, the process returns to the start without proceeding to the next step. If YES in S54, the process proceeds to S55.

In S55, the toe angle control unit 25 sets the steering frequency f and the angular amplitude θamp of the periodic steering. In S56, the toe angle control unit 25 drives the steering actuators 31-34 at the set steering frequency f and angular amplitude θamp. In S57, the tire external force detection unit 29 detects SAT as the tire external force.

FIG. 8 shows changes in toe angle and SAT during periodic steering. The steering conditions are vehicle speed V = 80 [km/Hr], road surface friction coefficient μ = 1.0, steering frequency f = 0.5 Hz, center toe angle θc = 2 [deg], angle amplitude θamp = 2 [deg]. deg]. After one second, the toe angle θ changes periodically within the range of 2±2 [deg], and the SAT also changes periodically accordingly. Here, the toe angle θ in the periodic steering is set so as to always be 0 or more, in other words, so that even the minimum value is not negative. Also, the positive amplitude SATp and the negative amplitude SATn of SAT do not match, and in this example, the negative amplitude SATn is slightly larger than the positive amplitude SATp. This is considered to be the influence of Kant.

Furthermore, as shown in FIG. 9, when the vehicle accelerates or decelerates during cyclic steering, an extra external force different from SAT is generated on the tire due to the toe angle of cyclic steering and the road surface friction coefficient μ. Similarly, when the vehicle condition is unstable and the vehicle is swaying from side to side, an extra external force is generated on the tires. Therefore, in S52 and S54 described above, the road surface friction coefficient μ is not estimated when the vehicle is accelerating or decelerating or when the vehicle state is not stable.

In S58, the road surface friction coefficient estimator 26 confirms the maximum value of SAT and the distortion of the waveform. FIG. 10 shows changes in toe angle and SAT in periodic steering when the road surface friction coefficient μ is relatively small. The steering conditions are vehicle speed V=80 [km/Hr], road surface friction coefficient μ=0.4, steering frequency f=0.5 Hz, angle amplitude θamp=0.5, 1, 2 [deg]. . After one second, the SAT changes periodically as the toe angle θ changes.

In FIG. 10, when the angular amplitude θamp=0.5 [deg], the SAT maximum value is smaller than the lower limit suitable for detection. In that case, if the angular amplitude θamp is increased to 1 [deg], the SAT maximum value becomes large and detection becomes easy. On the other hand, if the angular amplitude θamp is increased too much to 2 [deg], the tire slips and the SAT waveform is distorted.

In S59, it is determined whether the maximum value of SAT and distortion are within proper ranges. Specifically, it is determined whether the maximum value of SAT is equal to or higher than the lower limit, and whether the number of inflection points and the width of distortion are within a predetermined value. If NO in S59, the toe angle control unit 25 resets the steering frequency f and the angular amplitude θamp of the periodic steering in S60. Thereafter, steps S56 to S59 are repeated until YES is determined in S59. Thus, the toe angle control section 25 changes the steering frequency f or the angular amplitude θamp of the periodic steering according to the detected SAT.

If YES is determined in S59, the road surface friction coefficient estimator 26 performs frequency analysis of the SAT in S61 and extracts the maximum value of the SAT at the steering frequency f. FIG. 11 shows the result of frequency analysis of the SAT data by FFT. A peak appears at 0.5 Hz, which is the steering frequency f. A difference in cant (0±2%) affects only the DC component near 0 Hz and has little effect on the peak value (ie, maximum value).

In S62, the road surface friction coefficient estimator 26 estimates the road surface friction coefficient μ based on the relationship between the SAT maximum value at the steering frequency f and the road surface friction coefficient μ. As shown in FIG. 12, the road surface friction coefficient μ is estimated based on the SAT maximum value at 0.5 Hz obtained in FIG. The maximum error due to the cant difference (0±2%) in this embodiment is about 6%, which is less than one-third of the maximum error of 19% in the comparative example shown in FIG.

(Effect of this embodiment)
(1) The toe angle control unit 25 of the automotive steering device 20 outputs drive signals to the steering actuators 31-34 so as to execute periodic steering. The road surface friction coefficient estimator 26 frequency-analyzes the SAT detected by the tire external force detector 29, and estimates the road surface friction coefficient μ based on the relationship between the SAT and the road surface friction coefficient μ at the steering frequency f. For example, if the road has a certain cant, the external force generated on the tire due to the cant is a DC component. Therefore, by frequency-analyzing the SAT due to periodic steering and estimating the road surface friction coefficient μ using only the steering frequency component, the error due to the road surface state can be suppressed.

(2) The toe angle control unit 25 changes the steering frequency f or the angular amplitude θamp of the periodic steering according to the SAT. When the road surface friction coefficient μ is relatively small, the SAT caused by periodic steering is small and slippery. Therefore, the toe angle control unit 25 increases the angular amplitude θamp or decreases the steering frequency f (in other words, lengthens the steering period) to slowly steer the vehicle. Thus, by setting the turning frequency f or the angular amplitude θamp of the periodic turning to an appropriate value, the detected SAT can be increased and the accuracy of estimating the road surface friction coefficient μ can be improved.

(3) The automotive steering device 20 includes an acceleration/deceleration detector 27 that detects the speed change rate during acceleration or deceleration of the vehicle. Estimation of the road surface friction coefficient μ is performed when the rate of change is equal to or less than the change rate threshold. When the vehicle accelerates or decelerates, extra external force is generated on the tires. Therefore, the road friction coefficient estimator 26 can reduce errors by estimating the road friction coefficient μ only in a stable state where the absolute value of the speed change rate is equal to or less than a predetermined change rate threshold.

(4) The automobile steering device 20 includes a vehicle state detector 17 that detects the vehicle state caused by vehicle behavior or external force, and the road surface friction coefficient estimator 26 detects the road surface friction when the vehicle state satisfies a predetermined stability condition. An estimation of the coefficient μ is performed. When the vehicle state is unstable and the vehicle is swaying from side to side, an extra external force is generated on the tires. Therefore, the road surface friction coefficient estimator 26 can reduce the error by estimating the road surface friction coefficient μ only when the vehicle state is stable.

(5) The toe angle in periodic steering is set to be always greater than or equal to zero. When the vehicle is toe-in, the straight running stability is high. Therefore, by setting the variation range of the toe angle so that it is always toe-in, it is possible to improve the stability in straight running. In other words, the vehicle is prevented from becoming unstable due to a temporary toe-out.

(6) In the automotive steering apparatus 20 mounted on the vehicle 100 in which the front and rear tires can be independently steered, the toe angle control unit 25 controls the steering frequency f or the angular amplitude of the periodic steering of each row. θamp can be set individually. For example, the toe angle control unit 25 does not change the toe angles of the front row and the rear row in the same phase, but shifts the phase by 180 [deg] so that the toe angle of one of the rows always increases, thereby improving the straight running stability. can be secured.

(Other embodiments)
(a) In the independently steerable vehicle 100 shown in FIG. 1, both the front row tires 91 and 92 and the rear row tires 93 and 94 can be independently steered, but only the front row or rear row tires can be independently steered. may be Further, the automotive steering apparatus of the present invention may be mounted on a trailer having six or more wheels in which three or more rows of tires can be independently steered in the longitudinal direction of the vehicle.

In this case, the toe angle control unit 25 can individually set the steering frequency f or the angular amplitude θamp of the periodic steering of each row according to the priority of vehicle stability and reduction of running resistance. is preferred. For example, in a vehicle such as a trailer that has a large weight difference between the front and rear, the angular amplitude θamp of the heavy front row may be made relatively small, and the angular amplitude θamp of the light rear row may be made relatively large.

(b) The waveform of the periodic turning is not limited to a sine wave, and may be a triangular wave, a rectangular wave, a square wave, or the like, as long as the waveform is periodic. Alternatively, half-wave rectification or full-wave rectification may be used to use only the waveform on the positive side with respect to the center of amplitude. By using only the rectified waveform on the positive side, it becomes easier to set the toe angle in periodic steering so that it is always 0 or more.

(c) In the above embodiment, the road friction coefficient estimator 26 estimates the road friction coefficient μ based on the maximum value of SAT at the steering frequency f after frequency analysis. However, the road surface friction coefficient μ may be estimated not only based on the maximum value of SAT, but also based on a predetermined ratio to the maximum value, such as 80% of the maximum value.

As described above, the present invention is not limited to such an embodiment, and can be embodied in various forms without departing from the spirit of the present invention.

The controller and techniques described in this disclosure may be implemented by a dedicated computer provided by configuring a processor and memory programmed to perform one or more functions embodied by the computer program. may be Alternatively, the controls and techniques described in this disclosure may be implemented by a dedicated computer provided by configuring the processor with one or more dedicated hardware logic circuits. Alternatively, the control units and techniques described in this disclosure can be implemented by a combination of a processor and memory programmed to perform one or more functions and a processor configured by one or more hardware logic circuits. It may also be implemented by one or more dedicated computers configured. The computer program may also be stored as computer-executable instructions on a computer-readable non-transitional tangible recording medium.

20... Automobile steering device,
25 toe angle control unit,
26 road surface friction coefficient estimator, 27 acceleration/deceleration detector,
28... vehicle state detector, 29... tire external force detector,
31-34 ... steering actuator,
91-94 tires, 100 independently steered vehicles.

Claims (6)

One or more rows of tires (91-94) out of two or more rows of left-right pairs of tires in the longitudinal direction of the vehicle are mounted on an independently steered vehicle (100) in which the left and right wheels can be steered independently,
a plurality of steering actuators (31-34) for driving each tire to be driven in a steering direction;
Assuming that the angle at which the front sides of the pair of left and right tires are directed inward with respect to the traveling direction of the vehicle is positive, and the angle at which the front sides of the left and right tires are directed outward is negative, the angle symmetrically deflected by the steering actuator is defined as the toe angle. a toe angle control unit (25) for outputting a drive signal to the steering actuator so as to periodically change the angle at a predetermined steering frequency and angular amplitude;
a tire external force detection unit (29) for detecting a tire external force applied to the tire steered by the steering actuator;
a road surface friction coefficient estimator (26) for frequency-analyzing the tire external force and estimating the road surface friction coefficient based on the relationship between the tire external force and the road surface friction coefficient at the steering frequency;
An automotive steering device comprising: 2. The automotive steering system according to claim 1, wherein the toe angle control section changes the steering frequency or angular amplitude of the periodic steering according to the tire external force. An acceleration/deceleration detection unit (27) that detects a rate of change in speed per time during acceleration or deceleration of the vehicle,
3. The automotive steering apparatus according to claim 1, wherein the road friction coefficient estimator estimates the road friction coefficient when the absolute value of the speed change rate is equal to or less than a predetermined change rate threshold. A vehicle state detection unit (28) that detects a vehicle state due to vehicle behavior or external force,
The automotive steering apparatus according to any one of claims 1 to 3, wherein the road surface friction coefficient estimator estimates the road surface friction coefficient when the vehicle state satisfies a predetermined stability condition. 5. The automotive steering system according to claim 1, wherein the toe angle in the periodic steering is set to always be 0 or more. Mounted on a vehicle that can independently steer two or more rows of tires in the longitudinal direction of the vehicle,
6. The automotive steering apparatus according to claim 1, wherein the toe angle control unit can individually set the steering frequency or angular amplitude of the periodic steering of each row.

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