METHOD FOR CONTROLLING WIRED STEERING SYSTEMS FOR MOTOR VEHICLES

Abstract

The invention relates to a method for controlling a wired steering system for a motor vehicle (V1) comprising, on the one hand, a steering wheel (V2) electronically connected to a rack and pinion meshing with a set of wheels. The method comprises a first control law based on a nominal variable ratio (steering wheel rotation angle / wheel steering angle), designed, for a given steering wheel rotation angle (V2), to optimize wheel steering as a function of vehicle speed. The method is characterized in that it further comprises a second control law designed to allow, in the event of countersteering during oversteer, full wheel steering with a reduced steering wheel rotation angle (V2), regardless of vehicle speed (V1). (Fig. 1)

Description

Title of the invention: METHOD FOR CONTROLLING STEERING SYSTEMS BY FDS FOR MOTOR VEHICLES

[0001] The invention relates, in general, to the field of steering systems for vehicles and is concerned, more particularly, with an improved method for controlling steering systems by wires also called "Steer-By Wire" as well as a vehicle equipped with such a system.

[0002] Generally, in a wired steering system, the traditional mechanical links between the steering wheel and the wheels are replaced by wired electrical links coupled with electronic assistance means. This equipment simplifies vehicle design, improves safety, and allows for more precise control of the functional components of the steering system.

[0003] In such a system, there is therefore no longer a physical link between the steering wheel and the steering rack of the vehicle. Steering is then ensured by two motorized actuators, a first actuator mounted on the rack which ensures the control of its position (and therefore by extension that of the wheels) to the angular position of the steering wheel and a second actuator mounted on the steering wheel and intended to provide force feedback to the driver.

[0004] In a conventional steering system, the steering ratio is fixed and constant, being defined by mechanics according to the formula: Ratio = Steering wheel rotation angle (in degrees) / wheel steering angle (in degrees relative to the longitudinal axis of the vehicle).

[0005] In contrast, in a "Steer-By-Wire" steering system, this steering ratio is not fixed by the mechanics since there is no longer a physical link between the steering wheel and the wheels. This therefore allows the ratio to be variable and adapted to different driving situations. For a given steering wheel rotation angle, the lower this ratio, the greater the wheel steering angle.

[0006] For example, it becomes possible to make this ratio more direct in the parking phase to decrease the steering wheel rotation angle in order to perform the maneuver, or less direct in the highway driving phase to increase the steering wheel rotation angle in order to improve driving precision.

[0007] Consequently, it becomes possible to continuously vary this ratio according to various parameters, and in particular the vehicle's speed. Thus, such a steering system makes it possible to significantly improve driving assistance by ensuring a variable steering wheel ratio depending on the speed.

[0008] This configuration improves the vehicle's maneuverability at low speeds by providing a low ratio. Indeed, to perform the same maneuver, the lower this ratio, the less the driver needs to turn the steering wheel.

[0009] Simultaneously, the vehicle's stability at high speed is maintained by offering a high ratio. Thus, the driver can turn the steering wheel more sharply without this rotation resulting in abrupt wheel steering and excessive lateral agility.

[0010] For the same steering input, wheel steering is twice as dynamic at 30 km / h as at 100 km / h. For each vehicle speed, a compromise must be found between improved maneuverability and limiting the vehicle's lateral agility:

[0011] Thus, at low speeds (< 20 km / h), the gain in maneuverability can be maximized. Indeed, lateral responsiveness (speed and yaw rate) remains reduced because the vehicle speed is very low: the steering wheel angle / wheel angle ratio can then be low.

[0012] Conversely, at higher speeds (> 20 km / h), the vehicle's speed is high enough to have a significant impact on lateral agility. However, the need for maneuverability is no longer as critical because the driver no longer uses the full steering range: the steering wheel angle / wheel angle ratio can then be increased.

[0013] It has been observed that below 40 km / h, a traditional fixed ratio (even for the most direct, around 13) does not allow the driver's needs to be met with only 180° of steering wheel rotation, whereas a variable ratio always allows a necessary or sufficient turning radius of the wheels to be achieved.

[0014] To inform the driver, a stop is generated by the steering wheel feedback actuator when the wheel steering reaches 100%. This stop prevents the driver from continuing to turn the steering wheel when the wheels are already at their maximum angle. The steering wheel rotation angle at which the stop is activated varies in the same way as the steering ratio.

[0015] In summary and advantageously, the steering wheel rotation angle required to perform a given maneuver becomes much lower than that imposed by a conventional steering system with a fixed ratio.

[0016] As a result, the total angular range applicable to the majority of vehicle driving conditions can therefore be significantly reduced, for example, to only 360° lock-to-lock. This value is considerably lower compared to the 1080° lock-to-lock range of a conventional steering system with a fixed ratio of 16.

[0017] This significant reduction in steering wheel rotation to a single half-turn to the left or right makes it possible to consider new steering wheel shapes that are no longer necessarily circular, since the driver no longer needs to make a hand relay.

[0018] However, in exceptional cases of oversteer (loss of traction on snow-covered roads, for example), the vehicle might require the steering wheel to be rotated more than 180° for the driver to successfully countersteer. However, due to the non-circular shape of a modified steering wheel, its ability to countersteer could be significantly reduced because of the difficulty in turning it beyond 180°, leading to a risk of accidents.

[0019] Indeed, in such circumstances, the driver may need to use 100% of the angular steering range of the wheels during his counter-steering maneuver, including at high speeds.

[0020] In this case, the "nominal" variable ratio no longer meets all the driver's needs. In particular, from 20 km / h, the driver will have to turn the steering wheel more than 180° to reach maximum wheel steering and the steering wheel's counter-steering lock. At 50 km / h, they will have to turn the steering wheel up to 370° before reaching the steering wheel lock. It was therefore considered possible to offer a steering wheel rotation range beyond 180°. However, rotating a non-circular steering wheel more than 180° proves difficult because this shape no longer allows for an ergonomic grip beyond this angular value.

[0021] Furthermore, the steering wheel reaching its full lock during counter-steering is intended to inform the driver that they have reached the maximum counter-steering capacity (100% counter-steering). Under these conditions, if the driver is unable to turn the steering wheel to this lock, they will feel that it is the inability to achieve full wheel steering that prevents them from correcting the oversteer and following the desired trajectory, and will attribute this to the Steer-by-Wire technology.

[0022] Therefore, if one wishes to achieve a full wheel turn (100%) with a steering wheel rotation angle of no more than 180°, one solution would be to maintain a very direct steering ratio, even above 20 km / h, to compensate for oversteer. However, this solution proves problematic for the following reasons.

[0023] Driving at high speed would be dangerous because too direct a steering ratio leads to excessive lateral response even to minor steering inputs. The objective of managing an exceptional oversteer situation would then negatively impact all other driving situations, which is unacceptable.

[0024] Providing a steering wheel stop at 180°, regardless of vehicle speed, can be poorly received by the driver in a situation of severe understeer (above 40 km / h). Indeed, in an understeer situation, the driver's natural reaction is to oversteer. However, beyond a certain understeer threshold, increasing the steering angle becomes ineffective because the vehicle will not turn any further. Nevertheless, the driver will still tend to want to continue rotating the steering wheel to try to follow the desired trajectory. And if the driver was blocked in his attempt to extend the steering wheel rotation (for example via the 180° stop), he may wrongly interpret this blockage as the cause preventing him from following the desired trajectory, even if in reality the vehicle would not have turned any further.

[0025] In conclusion, unlike oversteer where it is truly necessary to achieve full wheel rotation, such rotation is unnecessary in understeer, and even counterproductive because it amplifies the understeer. The "nominal" steering ratio is more appropriate in this case, because the driver does not encounter a steering wheel lock at 180° and spontaneously interrupts the steering wheel rotation without reaching the lock or generating excessive understeer. In this case, if the driver fails to maintain the desired trajectory, they will realize that it is most likely due to excessive vehicle speed and not a steering wheel lock.

[0026] Patent CN11581763 IB describes a method and device for controlling the steer-by-wire system of motor vehicles. This method aims to improve stability and tracking performance by dynamically adjusting the steering ratio based on the actual yaw rate and the target yaw rate. This solution involves detecting the actual yaw rate, determining a target yaw rate based on the actual steering ratio, comparing the yaw rates to identify oversteer or understeer situations, and adjusting the steering ratio accordingly.

[0027] In the event of oversteer, the steering ratio is either reduced if the control device can correct the situation, or increased if it cannot, and in the event of understeer, the steering ratio is reduced to compensate.

[0028] The associated control device includes modules for yaw rate detection, means for determining and adjusting the steering ratio and for steering by wires.

[0029] However, this solution does not involve reducing the steering wheel's rotation range or modifying its shape, in order to improve driving comfort while ensuring optimal control of wheel steering, whether in the event of oversteer or understeer.

[0030] In this context, the invention proposes a compromise solution to satisfy these two opposing constraints (understeer versus oversteer) by distinguishing two laws of the steering ratio (steering wheel rotation angle / wheel steering angle).

[0031] The invention does not relate to the principle of a variable steering ratio, which is already used conventionally by "Steer-by-Wire" systems, but to the switching from a variable ratio called "nominal" in normal driving conditions to a specific ratio in the event of detection of an oversteer situation.

[0032] Thus, the vehicle can be driven with a variable steering ratio offering the best compromise between its maneuverability and stability in nominal driving and understeer, while offering, in exceptional oversteer driving situations, the full travel of the rack to provide a maximum steering angle of the wheels with, for example, only 180° of steering wheel rotation.

[0033] More specifically, the invention proposes a steering system and an associated control method allowing the adoption of a specific ratio offering full wheel steering for a rotation of 180° to 200° of the steering wheel in the very particular case of oversteer while maintaining a variable ratio called "nominal" ensuring the best compromise maneuverability / stability for all other driving situations (in particular in understeer).

[0034] This goal is achieved, according to the invention, by means of a method for controlling a wired steering system for a motor vehicle comprising, on the one hand, a steering wheel electronically connected to a rack in contact with a set of wheels, said method comprising a first control law based on a nominal variable ratio (steering wheel rotation angle / wheel steering angle) intended, for a given steering wheel rotation angle, to optimize the steering of the wheels as a function of the vehicle speed, characterized in that it further comprises a second control law intended to allow, in the event of counter-steering in oversteer, the complete steering of the wheels with a reduced steering wheel rotation angle, regardless of the vehicle speed.

[0035] Thus, the nominal variable ratio is optimal in almost all driving situations and the very direct ratio, which is not suitable for all situations, only comes into play in an oversteer situation.

[0036] According to an advantageous feature of the control method of the invention, the second oversteer law is activated by measuring the difference between the theoretical yaw rate and the actual yaw rate.

[0037] Preferably, the second oversteer law is activated when the actual yaw rate is greater than the theoretical yaw rate.

[0038] According to another feature of the steering method of the invention, the steering wheel rotation angle corresponding to the maximum wheel steering during oversteer is less than 200° and is preferably at most 180°.

[0039] According to a specific implementation of the control method of the invention, the second law is activated during the next passage of the steering wheel to 0° after the detection of oversteer, at the beginning of the counter-steering phase.

[0040] Conversely, the second steering law is deactivated at the next time the steering wheel passes through 0° after the cessation of oversteer.

[0041] Preferably, the first nominal variable ratio control law is activated by default.

[0042] According to a first embodiment of the control method of the invention, the second control law uses a fixed direction ratio.

[0043] According to an alternative embodiment of the steering method of the invention, the second steering law uses a variable steering ratio depending on the vehicle speed and / or the steering wheel angle. This variable ratio is necessarily more direct than the nominal variable ratio of the first steering law.

[0044] Another object of the invention is a motor vehicle equipped with a wire steering system and an on-board computer incorporating a computer program suitable for and intended to control said system by means of a method as defined above.

[0045] Thanks to the steering system control method according to the invention, the vehicle driver is not blocked by any end stop in the rotation of the steering wheel in the event of understeer and, in the event of oversteer, he has the full counter-steering potential with a reduced steering wheel rotation angle, regardless of the vehicle speed.

[0046] The invention provides a significant improvement to Steer-by-Wire (SbW) steering systems, allowing the steering ratio to be varied according to driving situations. Indeed, under normal driving conditions, the steering method uses a nominal steering ratio that optimizes vehicle handling and stability by requiring only a lock-to-lock steering wheel rotation. In the event of oversteer, when dynamic control is crucial, the method of the invention introduces a specific ratio that optimizes wheel steering with only 180° of steering wheel rotation, thus ensuring an effective response without excessive steering wheel rotation, but with a higher wheel steering speed.

[0047] The steering method of the invention combines a "nominal" ratio for normal driving and an "oversteer" ratio for critical situations, thus improving the safety and handling of the vehicle.

[0048] The invention does not require any hardware modification of the wire steering system or the addition of a new physical system and can be implemented with a purely software evolution and at a lower cost.

[0049] Other features and advantages of the invention will become apparent from the following description, with reference to the accompanying figures, for which:

[0050] [Fig.1] is a schematic view of the trajectory of a motor vehicle in a right turn situation with oversteer followed by countersteer and the corresponding steering wheel positions using a preferred implementation method of the steering method of the invention.

[0051] [Fig.2] is a series of three graphs illustrating, from top to bottom, the angle of steering wheel rotation, wheel steering angle and steering ratio in all four first phases of the trajectory of the [Fig.1] at a speed of 60km / h of the vehicle, with and without the implementation of the piloting method of the invention.

[0052] [Fig.3] is a synoptic diagram illustrating one implementation method of the process of steering of the invention during the transition between the nominal steering ratio and the oversteer steering ratio.

[0053] For clarity, identical or similar elements are identified by identical reference symbols throughout the figures.

[0054] Naturally, the method of implementing the control method of the invention, schematically illustrated by the figures above and described below, is given only by way of non-limiting example. It is explicitly provided for within the scope of the invention that different methods can be proposed and combined to offer others.

[0055] The invention relates to the general field of wired steering systems for motor vehicles, also known as "Steer-By-Wire", in which the traditional mechanical links comprising a steering wheel connected to a rack in contact with a wheel assembly are replaced by wired electrical links coupled with electronic assistance means.

[0056] In particular, the invention proposes an improved control method for these systems, making it possible to improve the stability and handling of the vehicle, especially in situations of understeer and oversteer.

[0057] More specifically, a steering method known per se generally comprises a first steering law based on a nominal variable ratio (steering wheel rotation angle / wheel steering angle) designed, for a given steering wheel rotation angle, to optimize wheel steering as a function of vehicle speed. The invention consists of supplementing this existing method by integrating a second steering law designed to allow, in the event of countersteering during oversteer, full wheel steering with a reduced steering wheel rotation angle, regardless of vehicle speed.

[0058] This second oversteer law, which complements the first law, is activated by measuring the difference between the theoretical yaw rate and the actual yaw rate. Preferably, the second oversteer law is activated when the actual yaw rate is greater than the theoretical yaw rate. However, the first steering law at nominal ratio remains activated by default.

[0059] Under the conditions of application of the second law, the steering wheel rotation angle corresponding to the maximum wheel steering during counter-steering in oversteer is then less than 200° and is, preferably, at most 180°.

[0060] According to a specific implementation of the control method of the invention, the second law is activated during the next passage of the steering wheel to 0° after detection of the Oversteer occurs at the beginning of the counter-steering phase. Conversely, this second steering law is deactivated after the oversteer ceases during the next steering wheel turn to 0°.

[0061] The control method of the invention further and preferably provides that the second control law uses a steering ratio (steering wheel rotation angle / wheel steering angle) which is fixed.

[0062] However, according to an alternative embodiment of the steering method of the invention, the second steering law can use a variable steering ratio depending on the speed of the vehicle and / or the steering wheel angle, said ratio being necessarily more direct than the nominal variable ratio of the first steering law.

[0063] Fig. 1 schematically illustrates the behavior of a vehicle VI during the six phases of oversteer with the corresponding angular position of the steering wheel V2.

[0064] Phase 1: Approach of vehicle V1 into the bend from the straight line.

[0065] Phase 2: Turning vehicle VI to the right.

[0066] Phase 3: Onset of oversteer.

[0067] Phase 4: Counter-steering to the left of the steering wheel V2 to stop oversteer.

[0068] Phase 5: Regaining traction of vehicle VI and reducing the rotation angle of the V2 steering wheel to return to the straight line.

[0069] Phase 6: Exit of vehicle V1 in a straight line.

[0070] Oversteer is detected in phase 3. This detection is a necessary but not sufficient condition for activating the so-called "oversteer" ratio. In order to avoid a sudden change in the steering input, the actual switch to the "oversteer" ratio only occurs when the steering wheel V2 reaches 0° during the counter-steering phase, and therefore here at the beginning of phase 4. Once the "oversteer" ratio is applied, the driver is able to achieve 100% of the steering angle with a rotation of 180° to 200° of the steering wheel V2.

[0071] Fig. 2 is a series of three graphs illustrating, respectively from top to bottom, the steering wheel rotation angle V2, the wheel steering angle and the steering ratio in the first four phases 1 to 4 of the trajectory of Fig. 1 as a function of time and at a speed of 60 km / h of the vehicle VI, respectively, with implementation of the steering method of the invention (in dashed lines) and without implementation (in solid lines).

[0072] Figure 2 shows that, without the strategy of switching to the specific "oversteer" ratio proposed by the method of the invention, the "nominal" ratio would only allow 45% of the maximum wheel steering angle to be achieved with 180° of steering wheel rotation V2. However, it turns out that this ratio alone is insufficient to counteract oversteer and regain control of the vehicle VL. Once no more oversteer is detected, and at the next time the steering wheel passes through 0°, the steering ratio law changes again and goes back from "oversteer" to "nominal".

[0073] The motor vehicle V1, equipped with the wire-driven steering system, has an on-board computer incorporating a suitable software program designed to control the system by means of the method of the invention described above. Figure 3 is a block diagram illustrating the steering ratio states and the control commands sent by this software to the wire-driven steering system. Sensors located on the steering wheel V2 and the rack and pinion, and coupled to the computer, advantageously complement the functions of the steering system.

Claims

Demands

1. A method for controlling a wired steering system for a motor vehicle (VI) comprising, on the one hand, a steering wheel (V2) electronically connected to a rack in contact with a wheel assembly, said method comprising a first control law based on a nominal variable ratio, (steering wheel rotation angle / wheel steering angle), intended, for a given steering wheel (V2) rotation angle, to optimize the steering of the wheels as a function of the vehicle speed, characterized in that it further comprises a second control law intended to allow, in the event of counter-steering in oversteer, the full steering of the wheels with a reduced steering wheel (V2) rotation angle, regardless of the vehicle speed (VI).

2. A method according to claim 1, characterized in that the second oversteer law is activated by measuring the difference between the theoretical yaw rate and the actual yaw rate.

3. Method according to the preceding claim, characterized in that the second oversteer law is activated when the actual yaw rate is greater than the theoretical yaw rate.

4. A method according to any one of the preceding claims, characterized in that the steering wheel rotation angle (V2) corresponding to the maximum wheel steering during oversteer is less than 200° and, preferably, is at most 180°.

5. A method according to any one of the preceding claims, characterized in that the second law is activated during the next 0° turn of the steering wheel after detection of oversteer, at the beginning of the counter-steering phase.

6. A method according to any one of the preceding claims, characterized in that the first nominal ratio pilot law is activated by default.

7. A method according to any one of the preceding claims, characterized in that the second steering law is deactivated during the next steering wheel (V2) pass at 0° after the cessation of oversteer

8. A method according to any one of the preceding claims, characterized in that the second steering law uses a fixed steering ratio.

9. A method according to any one of claims 1 to 7, characterized in that the second steering law uses a variable steering ratio in

10. function of vehicle speed (VI) and / or steering wheel angle (V2). Motor vehicle equipped with a wire steering system and an on-board computer incorporating a computer program suitable for and intended to control said system by means of a method according to one of the preceding claims.