Method for compensating trajectory deviation by power steering during braking

Abstract

A method for a power steering system of a motor vehicle, the method transmitting a computer-controlled torque to a steering member for compensating for a deviation in the trajectory of the vehicle when the vehicle is braking, the method calculating in a first step (22) a rough steering torque (32) based on a request to activate the method when braking (20), which takes into account the speed (30) of the vehicle, the yaw angle (24) of the vehicle and a signal representing the force applied by the steering member to the steering wheel (28).

Description

Technical Field

[0001] The invention relates to a method for compensating for course deviations during braking of a motor vehicle by means of a power steering system of the vehicle, and to a power steering system comprising a device for implementing such a compensation method. Background Art

[0002] Some motor vehicles include a computer-controlled power steering system that includes a motor that generates a force or torque when steering the vehicle, which is dependent on the driver's intended torque applied by the driver to the steering wheel as measured by a sensor, in order to provide a portion of the force required to pivot the steering wheel.

[0003] In particular, power steering systems may include one or more electric motors that are controlled by a computer to generate motor torque that can be applied to a steering column or steering box in various ways.

[0004] Furthermore, during braking of the vehicle, due to the asymmetry of the vehicle, such braking may generate forces which tend to cause an offset of the vehicle to one side or the other.

[0005] In particular, due to differences in the operation of the braking system of each wheel, or differences in the control pressure applied thereto, the braking capacity of the wheels may be slightly different on either side of the vehicle.

[0006] Due to manufacturing dispersion, the geometry of the chassis or the operation of the suspension system including springs and shock absorbers may also have slight deviations on one side of the vehicle and the other.

[0007] The mass of the vehicle and the mass of the load in the vehicle may also have a different distribution, particularly if there is a driver on one side of the vehicle, or if passengers are not loaded on the other side.

[0008] The deflections of the vehicle during braking also depend on the type of front and rear chassis used, with some chassis geometries being more sensitive to these deflections than others.

[0009] These various causes may cause the vehicle to deflect during braking, which deflection usually increases during sudden braking or high-speed braking. In particular, emergency braking at high speeds (e.g., speeds above 100 km / h) may cause the vehicle to deflect, which may frighten the driver by creating a feeling of loss of steering accuracy and loss of security.

[0010] The driver should then apply force on the steering wheel to correct the course, which can add uncomfortable pressure, especially in an emergency situation. Summary of the invention

[0011] The present invention is particularly intended to avoid these disadvantages of the prior art.

[0012] To this end, the invention provides a method for a power steering system of a motor vehicle, the method generating a torque controlled by a computer on a steering member for compensating for a deviation of the vehicle's course during braking of the vehicle, the method being characterized in that, starting from a request to activate the method during braking, the method calculates, in a first step, a raw corrective torque taking into account the speed of the vehicle, the yaw rate of the vehicle and a signal representative of the force exerted by the steering member on the steering wheel.

[0013] An advantage of this compensation method is that it uses available signals in a simple and economical way, without adding any equipment to the vehicle, and in a first step derives the magnitude of the course deviation and the required raw corrective torque by taking into account the variations in the yaw rate and the force exerted by the steered wheels on the steering box.

[0014] The second step allows to avoid the amplification of the deviation phenomenon by the correction applied in the wrong direction to preserve the original correction torque. The force generated by the power steering at least partially to resist the deviation of the course is obtained automatically, which gives peace of mind to the driver and improves safety.

[0015] The compensation method according to the present invention may further include one or more of the following features, which may be combined together.

[0016] Advantageously, the activation request is established using an external signal originating from an on-board network of the vehicle and a signal internal to the power steering system comprising information about the operation of the vehicle.

[0017] Advantageously, said first step establishes, by a first mapping, a first corrective torque dependent on the force exerted by said steering member, in order to reduce this exerted force.

[0018] Advantageously, the first step establishes from a second mapping a second corrective torque that is dependent on the yaw rate.

[0019] Advantageously, the yaw rate is filtered in order to attenuate some frequencies.

[0020] Advantageously, said first step establishes, by a third mapping, a third corrective torque dependent on the derivative of said yaw rate in order to reduce said derivative of the yaw rate.

[0021] In this case, advantageously, the method performs a filtering on the derivative of the yaw rate before using the method for the third mapping. By this filtering, some undesired frequencies in this signal are eliminated.

[0022] Advantageously, the method takes into account the speed of the vehicle to establish the first, second and third corrective torques.This consideration allows establishing a dynamic correction that depends on changes in the speed of the vehicle.

[0023] Advantageously, the raw correction torque is weighted by a factor which depends on the time period which has elapsed since the activation request. This factor varies between 0 and 1.

[0024] In this way, the method allows setting the period of time during which the compensation method is applied. In fact, in the event of an excursion phenomenon lasting more than 2 or 3 seconds, the method may preferably deactivate the function gradually so as not to disturb the driver.

[0025] More specifically, the deviation phenomenon associated with braking may only last for a limited time, because in the case of a strong braking of 100 km / h, the vehicle speed becomes about 30 km / h after 2 seconds of braking, and the vehicle stops about 3 seconds after braking. Since the deviation phenomenon is not dangerous at low vehicle speeds, it may be preferable to limit the action of the compensation method for reasons related to the quality of driving perceived by the driver.

[0026] Advantageously, in a second step, the method calculates a safe corrective torque to be applied by the power steering system by setting a limit to the raw corrective torque.

[0027] In this case, advantageously, the safety corrective torque is established by setting an upper and a lower limit to the raw corrective torque, the upper and lower limits each depending on the level and the sign of the signal of the force exerted by the steering member.

[0028] Furthermore, advantageously, said upper or lower limit each comprises a certain value level for the corrective torque in the direction opposite to the direction of the force exerted by the steering member, and a value level lower than said certain value level for the corrective torque in the same direction. In this way, a strong correction in the same direction which amplifies the deflection is avoided.

[0029] Another object of the invention is a power steering system for a motor vehicle comprising a device implementing a compensation method comprising any of the aforementioned characteristics. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] The invention will be better understood and other characteristics and advantages will become more apparent on reading the following description, provided by way of example, with reference to the accompanying drawings.

[0031] Figure 1 A schematic diagram of a steering member of a motor vehicle implementing the compensation method according to the invention is shown.

[0032] Figure 2is a block diagram showing the operation of the compensation method.

[0033] Figure 3 is a map of multiple curves showing the raw corrective torque as a function of the force applied by the steering member; and,

[0034] Figure 4 The operating principle of the second step of the method is demonstrated. DETAILED DESCRIPTION

[0035] Figure 1 A steering box 2 is shown, comprising a rack arranged transversely in the vehicle, each end of which is connected to a front wheel hub 6 by means of tie rods 4 in order to pivot it and thus ensure the steering of the vehicle.

[0036] A steering column equipped with a steering wheel 8 manipulated by the driver, which steering column is connected to the box 2, comprises a sensor 10 for sensing the force exerted by the driver on said column, constituting means for measuring the driver's intention.

[0037] The electric drive 12 comprises an electric motor that generates an auxiliary torque on the steering box 2, which is added to the force generated by the driver on the steering wheel 8. Alternatively, the electric drive 12 may comprise a plurality of independent drive systems working in parallel to achieve a certain redundancy to ensure safety in the event of failure of one of these drives.

[0038] The computer 16 receives the signal output by the force sensor 10 and various information about the operation of the vehicle 14 in order to establish a torque setpoint transmitted to the electric drive 12 in order to exert the assist torque on the steering box 2 .

[0039] In turn, the computer 16 calculates, from the various measurements thus performed, a signal representative of the force exerted by the steering element 2 on the steering wheel 6 , this signal thus being used to establish the torque setpoint for the electric drive 12 .

[0040] Figure 2 The function for estimating the course deviation during braking, which leads to the construction of a signal for requesting the activation of a compensation method, is shown in an initial step 20. In particular, the course deviation estimation function uses external signals originating from the vehicle's on-board network and internal signals of the power steering, conveying different information about the operation of the vehicle.

[0041] In particular, an estimate of the course deviation can be established by detecting high vehicle speeds and strong braking, the estimate of the course deviation being measured, for example, by the braking torque applied to the brakes of the wheels or by a strong negative longitudinal acceleration indicated by an inertial unit. The deviation estimate can also be provided by a signal from a stability control function of the vehicle (called "ESP", for "Electronic Stability Program").

[0042] In this case, there is a significant risk of course deviation and the deviation estimation function sends a signal at level 1 to the correction torque calculation function 18 requesting it to calculate a course correction.

[0043] In a first step 22, the corrective torque calculation function 18 performs the reception of a signal of the force exerted by the steering member 28 and of the yaw rate 24 of the vehicle corresponding to the speed of rotation of the vehicle relative to the vertical axis, provided by an inertial unit provided in particular for the operation of a stability control function of the vehicle, and then performs the calculation of a derivative 26 of this yaw rate. It should be noted that the yaw rate depends on the inclination of the steering wheel and on the linear speed of the vehicle.

[0044] Advantageously, the raw yaw rate is filtered to attenuate some frequencies.

[0045] Then, the first step 22 comprises calculating a first corrective torque from the force applied by the steering member 28 , calculating a second corrective torque from the yaw rate 24 of the vehicle, and calculating a third corrective torque from the derivative of this yaw rate. These three calculated torques are added to form a raw corrective torque 32 .

[0046] The corrective torque, which is dependent on the force applied by the steering member 28 , is calculated by means of a first mapping dependent on this force signal, which may include settings, for example as a function of the speed of the vehicle, in order to optimally adjust this corrective torque.

[0047] In a straight line, the force exerted by the steering member 28 is zero, so that a deviation is achieved which is compensated by the driver by holding the steering wheel, this deviation being detected by a rise in this force, depending on which the corrective torque will tend to decrease to assist the driver.

[0048] Figure 3 A map is shown which comprises three curves 62 , 64 , 66 as a function of the force exerted by the steering member 28 , said curves providing the corrective torque as a function of this force 60 .

[0049] The first curve 62 provides no corrective torque until a steering force threshold is reached, and then rapidly rises in the correction, which then stabilizes at a slightly increasing value. The second curve 64 provides a corrective torque that is directly proportional to the steering force. The third curve 66 provides a corrective torque that has a vertical tangent at the beginning of the steering force 28, and then provides a slope that gradually decreases until it ends at a slightly increasing value.

[0050] For the first curve 62, we obtain a delayed correction after some rise in the force exerted by the steering member 28, for the second curve 64, an immediate proportional correction, and for the third curve 66, an early correction starting significantly from the beginning of the force. These different curves show corrections that become increasingly invasive.

[0051] The corrective torque dependent on the yaw rate is calculated by a second mapping dependent on the yaw rate signal.The derivative of the yaw rate 24 provided by the derivative calculation 26 is filtered in order to attenuate some frequencies and obtain a signal used in a third mapping providing the corrective torque dependent on the derivative.

[0052] If the course deviation is not fully compensated by the corrective torque which is dependent on the force applied by the steering member 28 , an increased yaw rate 24 is obtained, so that the second corrective torque which is dependent on this yaw rate is refined.

[0053] Offset compensation may be performed sequentially, in which case a constant yaw rate should be maintained by correcting for changes in this rate.

[0054] The derivative 26 of the yaw rate detects variations in the yaw rate 24 , taking this variation into account allows stabilizing the vehicle by limiting the variations in this yaw rate. In this way, we try to maintain a constant yaw rate 24 .

[0055] In particular, different corrective torques can be adjusted as a function of the speed of the vehicle, either in a general manner on the raw corrective torque 32 or in an individual manner on each of these corrective torques. These adjustments allow obtaining a raw corrective torque 32 adapted to each speed of the vehicle, since the severity of the vehicle excursion phenomenon during braking increases with this speed.

[0056] Taking the vehicle's speed into account also allows reducing the original corrective torque in a dynamic manner during braking. The reduced speed during braking reduces the magnitude of the vehicle's excursion, and the dynamic correction takes this reduction into account in order to reduce the correction and avoid an excursion compensation that is too intrusive to the driver.

[0057] The raw correction torque is weighted by a factor 23 which depends on the time period which has elapsed since the activation request. This factor varies between 0 and 1.

[0058] The second step 34 includes receiving the raw corrective torque 32 and the signal of the force applied by the steering member 28 to establish a safe corrective torque 36 .

[0059] The sign of the safety corrective torque 36 must provide a force in the opposite direction to the deflection to correct the deflection, so as to avoid the corrective torque amplifying the deflection phenomenon.

[0060] A signal depending on the force applied by the steering member 28, Figure 4 An upper limit 40 and a lower limit 42 are shown on the horizontal axis (including a positive direction oriented to the right) for saturation (including a positive direction oriented upward) of the raw corrective torque 32. These two positive directions apply to the vehicle in the same direction.

[0061] Both the upper limit 40 and the lower limit 42 are symmetrical with respect to the center point O of the curve.

[0062] With respect to the upper limit 40 of the range for positive values, the upper limit is at a high positive level 48a when the force applied by the steering member 28 is negative, or slightly below a first threshold 44a when it is positive. Between the first threshold 44a and a second threshold 46a approaching, the upper limit 40 decreases linearly, and then, above the second threshold, the upper limit is at a low positive level 50a.

[0063] Symmetrically, with respect to the lower limit 42 for the range of negative values, the lower limit is at a high negative level 48b when the force applied by the steering member 28 is positive, or slightly above a first threshold 44b when it is negative. Between the first threshold 44b and a second threshold 46b close to it, the lower limit 42 increases linearly, and then, below the second threshold, the lower limit is at a low negative level 50b.

[0064] In this way, when the safety corrective torque is in the opposite direction to the force applied by the steering member 28, it is ensured that the safety corrective torque 36 has a value saturated by the high level 48a, 48b, or when the safety corrective torque is in the same direction, it is ensured that the safety corrective torque has a value saturated by the low level 50a, 50b, which avoids errors that would significantly amplify the route deviation.

[0065] By using components and sensors already present in the vehicle and with just a few software plugins, we achieve lane deviation compensation in a reliable and economical way, which guarantees driving safety and peace of mind for the driver.

Claims

1. A method for a power steering system of a motor vehicle, the method generating a torque on a steering member (28) controlled by a computer (16) for compensating for a deviation of the course of the vehicle during braking of the vehicle, characterized in that Starting from a request (20) to activate the method during braking, in a first step (22), the method calculates a raw corrective torque (32) taking into account the speed (30) of the vehicle, the yaw rate (24) of the vehicle and a signal representative of the force exerted by the steering member (28) on the steering wheel.

2. The method according to claim 1, characterized in that The activation request (20) of the method uses an external signal originating from an on-board network of the vehicle and a signal internal to the power steering system to establish the activation request, which includes information about the operation of the vehicle.

3. The method according to claim 1 or 2, characterized in that: The first step (22) establishes, by a first mapping, a first corrective torque dependent on the force applied by the steering member (28) in order to reduce the applied force.

4. The method according to claim 3, characterized in that: The first step (22) establishes a second corrective torque dependent on the yaw rate (24) from a second mapping.

5. The method according to claim 4, characterized in that The yaw rate (24) is filtered to attenuate some frequencies.

6. The method according to claim 4, characterized in that The first step establishes a third corrective torque dependent on the derivative (26) of the yaw rate by a third mapping in order to reduce the derivative of the yaw rate.

7. The method according to claim 6, characterized in that Before using the method for the third mapping, the method performs filtering of the derivative (26) of the yaw rate.

8. The method according to claim 6, characterized in that The method takes into account the speed (30) of the vehicle to establish the first, second and third corrective torques.

9. The method according to claim 2, characterized in that: The raw corrective torque (32) is weighted by a coefficient (23) that depends on the time period that has elapsed since the activation request.

10. The method according to claim 1, characterized in that In a second step (34), the method calculates a safety corrective torque (36) applied by the power steering system by setting a limit to the raw corrective torque (32).

11. The method according to claim 10, characterized in that The safety corrective torque (36) is established by setting an upper limit (40) and a lower limit (42) on the raw corrective torque (32), each of which depends on the level and sign of the signal of the force applied by the steering member (28).

12. The method according to claim 11, characterized in that The upper limit (40) or the lower limit (42) each includes a specific numerical level (48a, 48b) for the corrective torque in a direction opposite to the direction of the force applied by the steering member (28), and a numerical level (50a, 50b) below the specific numerical level (48a, 48b) for the corrective torque in the same direction.

13. A power steering system for a motor vehicle, characterized in that: The power steering system comprises means for implementing a method according to any one of claims 1 to 12.

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