METHOD DESIGNED TO SLAVE THE POSITION OF A VEHICLE STEERING RACK TO A POSITION SETPOINT ON THE BASIS OF DYNAMIC CONSTRAINTS IMPOSED ON A MOVEMENT OF THE VEHICLE

Abstract

A method for controlling an assist motor of a power steering system, power steering system including the assist motor configured to apply a motor torque on a rack, and at least one steering computer, method being designed to servo-control a position of the rack to a position setpoint, method including: a maneuvering step in which the assist motor exerts a motor torque on the rack according to a motor torque setpoint; wherein the method also includes: servo-control step in which the steering computer determines a speed setpoint of the rack according to the position setpoint and to the position of the rack; limitation step in which the steering computer issues a limited speed setpoint which is lower than or equal to a maximum speed threshold; control step in which the steering computer determines the motor torque setpoint according to the limited speed setpoint and to a speed of the rack.

Claims

1. A method for controlling an assist motor of a power steering system, said power steering system comprising the assist motor configured to apply a motor torque on a rack, and at least one steering computer, the method being designed to servo-control a position of the rack to a position setpoint, the method comprising: a maneuvering step in which the assist motor exerts a motor torque on the rack according to a motor torque setpoint; wherein the method also comprises: a servo-control step in which the steering computer determines a speed setpoint of the rack according to the position setpoint and to the position of the rack; a limitation step in which the steering computer issues a limited speed setpoint which is lower than or equal to a maximum speed threshold; a control step in which the steering computer determines the motor torque setpoint according to the limited speed setpoint and to a speed of the rack.

2. The method according to claim 1, wherein the servo-control step comprises a comparison phase in which a position deviation is calculated according to the position of the rack and to the position setpoint, then a correction phase in which the speed setpoint is determined so as to reduce the position deviation.

3. The method according to claim 1, wherein the control step comprises a deviation phase in which a speed deviation is calculated according to the speed of the rack and to the limited speed setpoint, then a compensation phase in which the motor torque setpoint is determined so as to reduce the speed deviation.

4. The method according to claim 1, wherein the limitation step determines the maximum speed threshold of the rack according to at least one kinematic constraint exerted on the vehicle.

5. The method according to claim 1, wherein the limitation step determines the limited speed setpoint according to a maximum acceleration threshold;

6. The method according to claim 5, wherein the maximum acceleration threshold is determined according to at least one kinematic constraint exerted on the vehicle.

7. The method according to claim 1, wherein the position of the rack is obtained by either: a position sensor, an angular measurement of a steering wheel, a mathematical calculation using an angular measurement of a shaft of the assist motor.

Description

[0047] The invention will be better understood, thanks to the description below, which relates to several embodiments according to the present invention, given as non-limiting examples and explained with reference to the appended schematic drawings, in which:

[0048] FIG. 1 is a schematic representation of a method for controlling an assist motor according to the prior art;

[0049] FIG. 2 is a schematic representation of a method for controlling an assist motor according to the invention;

[0050] FIG. 3 is a schematic representation of a position of the rack, of a maximum speed threshold and of a maximum acceleration threshold, over time;

[0051] FIG. 4 is a representation of a response of the rack to a position setpoint according to a first embodiment;

[0052] FIG. 5 is a representation of a response of the rack to a position setpoint according to a second embodiment;

[0053] FIG. 6 is a representation of a response of the rack to a position setpoint according to a third embodiment;

[0054] FIG. 7 is a schematic representation of a steering system of a vehicle.

[0055] The invention relates to a method 1 for managing an assist motor 12 of a power steering system 100 for a vehicle 21, and more particularly for a motor vehicle 21 intended for the transport of persons.

[0056] In a manner known per se, and as shown in FIG. 7, said power steering system 100 comprises a steering wheel 30 which allows a driver to maneuver said power steering system 100 by exerting a force on said steering wheel 30.

[0057] Said steering wheel 30 is preferably mounted on a steering column 40, guided in rotation on the vehicle 21, and which meshes, by means of a steering pinion 50, with a rack S, which is itself guided in translation in a steering casing 7 fastened to said vehicle 21.

[0058] Preferably, each of the ends of said steering rack S is connected to a steering tie rod 8, 9 connected to the steering knuckle of a steered wheel 10, 11 (respectively a left wheel 10 and a right wheel 11), such so that the translational longitudinal displacement of the rack S allows modifying the steering angle (yaw angle) of the steered wheels.

[0059] Moreover, the steered wheels 10, 11 can preferably also be drive wheels.

[0060] The power steering system 100 also comprises the assist motor 12 intended to provide a motor torque C.sub.M, to assist the maneuver of said power steering system 100.

[0061] The assist motor 12 will preferably be an electric motor, with two directions of operation, and preferably a rotary electric motor, of the brushless type. The assist motor 12 can come into engagement, where necessary via a reducer of the gear reducer type, either on the steering column 40 itself, to form a so-called “single pinion” mechanism, or directly on the rack S, for example by means of a second pinion 13 distinct from the steering pinion 50 which allows the steering column 40 to mesh with the rack S, so as to form a so-called “double pinion” mechanism, as illustrated in FIG. 7, or else by means of a ball screw which cooperates with a corresponding thread of said rack S, at a distance from said steering pinion 50.

[0062] The steering system further comprises a steering computer 20 which implements the method 1 according to the invention. More specifically, the steering computer 20 receives the information of a position X.sub.S of the rack S either by means of a position sensor, or by an angular measurement θ3 of the steering wheel 30 which is carried out by an angle sensor 23, or by a mathematical calculation using an angular measurement θ12 of a shaft 24 of the assist motor 12.

[0063] The steering system 100 comprises the control method 1, as represented in FIG. 2.

[0064] The method 1 for controlling the assist motor 12 of the power steering system 100 comprises a servo-control step 2 in which the steering computer 20 determines a speed setpoint T.sub.VS of the rack S according to a position setpoint T.sub.X and to the position X.sub.S of the rack S.

[0065] More specifically, the servo-control step 2 comprises a comparison phase in which a position deviation is calculated according to the position X.sub.S of the rack S and to the position setpoint T.sub.X.

[0066] The comparison phase calculates the position deviation by subtracting the position X.sub.S of the rack S from the position setpoint T.sub.X.

[0067] Then, the servo-control step 2 comprises a correction phase in which the speed setpoint T.sub.VS is determined so as to reduce the calculated position deviation. The servo-control step 2 is therefore a first closed-loop regulation which determines the speed setpoint T.sub.VS so that the position X.sub.S of the rack S is equal to the position setpoint T.sub.X.

[0068] The method 1 according to the invention then comprises a limitation step in which the steering computer issues a limited speed setpoint T.sub.VSL.

[0069] The speed setpoint T.sub.VS is conditioned by speed and acceleration kinematic constraints exerted on the vehicle 21. For this purpose, the speed setpoint T.sub.VS is bounded by a maximum speed threshold SV.sub.S and its derivative with respect to time, in other words its acceleration, is bounded by a maximum acceleration threshold SA.sub.S. The maximum speed threshold SV.sub.S and the maximum acceleration threshold SA.sub.S depend on at least one kinematic constraint.

[0070] The method 1 also comprises a control step 5 in which the steering computer 20 determines a motor torque setpoint T.sub.CM according to the limited speed setpoint T.sub.VSL and to a speed V.sub.S of the rack S.

[0071] The control step comprises a deviation phase in which a speed deviation is calculated by subtracting the speed V.sub.S of the rack S from the limited speed setpoint T.sub.VSL.

[0072] The control step also comprises a compensation phase in which the motor torque setpoint is determined so as to reduce the speed deviation.

[0073] The control step 5 corresponds to a second closed-loop regulation positioned in parallel with the first regulation loop. Indeed, the control step 5 determines the motor torque setpoint T.sub.CM by using as input the limited speed setpoint T.sub.VSL which is the output of the first control loop, and by using the speed V.sub.S of the rack S which is measured at the same time as the position X.sub.S of the rack S.

[0074] The control step 5 modifies the motor torque setpoint T.sub.CM, so as to impose that the speed V.sub.S of the rack S is substantially equal to the limited speed setpoint T.sub.VSL.

[0075] Finally, the method 1 comprises a maneuvering step 3 in which the assist motor 12 exerts the motor torque C.sub.M on the rack S according to the motor torque setpoint T.sub.CM.

[0076] The method 1 is designed to servo-control a position X.sub.S of the rack S to a position setpoint T.sub.X, which is carried out by the servo-control step 2, while keeping a speed V.sub.S of the rack S at a value lower than a threshold which is carried out by the step of limitation 4 and control 5.

[0077] Thus, the method 1 according to the invention allows, on the one hand, keeping an actual trajectory of the vehicle 21 by means of the control of the position X.sub.S of the rack 5, close to the reference trajectory indicated by the position setpoint T.sub.X, and on the other hand keeping the speed V.sub.S of the rack S close to the limited speed setpoint T.sub.VSL.

[0078] FIG. 3 illustrates the position X.sub.S of the rack S, the maximum speed threshold SV.sub.S and the maximum acceleration threshold SA.sub.S according to the time t.

[0079] In FIG. 3, the speed setpoint T.sub.VS has a value higher than the maximum speed threshold SV.sub.S and a derivative with respect to time higher than the maximum acceleration threshold SA.sub.S. Thus, the limitation step 4 determines that the limited speed setpoint T.sub.VSL is constrained on the one hand by the maximum speed threshold SV.sub.S and on the other hand by the maximum acceleration threshold SA.sub.S.

[0080] In FIG. 3, the maximum acceleration threshold SA.sub.S is constant and equal to the value A.sub.max over a first duration I, equal to the value-A.sub.max over a third duration III and equal to zero over a second duration II comprised between the first duration I and the third duration III. Thus, the maximum speed threshold S.sub.VS is a straight line with a slope coefficient constant over the first I and third III durations, and zero over the third duration III.

[0081] Finally, the position X.sub.S of the rack evolves slowly over the first duration I and the third duration III then more rapidly over the second duration II.

[0082] FIGS. 4 to 6 illustrate a response of a rack S controlled with the method 1 according to the invention.

[0083] More specifically, in the response illustrated in FIG. 4, the maximum speed threshold SV.sub.S is chosen equal to 70 mm/s, and there is no determined value for the maximum acceleration threshold SA.sub.S. In addition, the method imposes a position setpoint T.sub.X, in the form of a ramp.

[0084] FIG. 4 shows that the position X.sub.S of the rack S follows the position setpoint T.sub.X with a slight offset whereas the speed V.sub.S of the rack follows the limited speed setpoint T.sub.VSL with the exception of regulation overruns. The motor torque setpoint T.sub.CM decreases automatically when the speed V.sub.S of the rack exceeds the limited speed setpoint T.sub.VSL. Thus, the method 1 according to the invention guarantees that the speed V.sub.S of the rack S does not exceed the limited speed setpoint T.sub.VSL with the exception of regulation errors.

[0085] In the response illustrated in FIG. 5, the maximum speed threshold SV.sub.S is chosen equal to 20 mm/s, and there is no determined value for the maximum acceleration threshold SA.sub.S. In addition, the method imposes a position setpoint T.sub.X, in the form of a step.

[0086] FIG. 5 shows that the position X.sub.S of the rack S joins the position setpoint T.sub.X following a slope, whereas the speed V.sub.S of the rack S follows the limited speed setpoint T.sub.VSL with the exception of regulation overruns. The response of the position X.sub.S of the rack S is independent of the trajectory to be traveled. The rack S reaches the desired position T.sub.X while complying with the constraint of the maximum speed threshold SV.sub.S. Thus, the method 1 according to the invention guarantees that the speed V.sub.S of the rack does not exceed the limited speed setpoint T.sub.VSL even when the distance to be traveled is significant.

[0087] In the response illustrated in FIG. 6, the maximum speed threshold SV.sub.S is chosen equal to 20 mm/s, and the maximum acceleration threshold SA.sub.S is chosen equal to 156 mm/s.sup.2. In addition, the method imposes a position setpoint T.sub.X, in the form of a step.

[0088] FIG. 6 shows that the position X.sub.S of the rack S joins the position setpoint T.sub.X following a curve, whereas the speed V.sub.S of the rack S follows the limited speed setpoint T.sub.VSL with the exception of regulation overruns. The rack S reaches the desired position T.sub.X while complying with the constraint of the maximum speed threshold SV.sub.S and of the maximum acceleration threshold SA.sub.S, with the exception of regulation errors. Thus, the method 1 according to the invention guarantees that the speed V.sub.S of the rack S does not exceed the limited speed setpoint T.sub.VSL even when the distance to be traveled is significant, and that the acceleration of the rack remains lower than the maximum acceleration until the end of the movement.

[0089] Of course, the invention is not limited to the embodiments described and represented in the appended figures. Modifications are still possible, in particular with regards to the construction of the various elements or by substitution with technical equivalents, yet without departing from the scope of protection of the invention.