Method for compensating for friction in a power steering system

Abstract

A method (100) for compensating friction between at least two parts (5, 6, 7), the method being based on a friction model (M), a velocity (v) between at least one part (5) and at least one other part (6) of the at least two parts (5, 6, 7), the velocity (v) being measured (101), or estimated (101') from a measurement, the velocity (v) being an input to the model (M), the model (M) being based on an internal friction state, a time derivative (z) of the internal state being a function of said internal state and the velocity (v) and a first gain, the method comprising a saturation step (102) of the velocity v at the input of the friction model, such that an absolute value of the velocity (v) remains less than a saturation value, the saturation value being a function of the velocity, the first gain, and a second gain. Figure 1

Description

Title of the invention: Method for compensating for friction in a power steering system.

[0001] The present invention relates to the field of power steering systems, and in particular to friction compensation methods.

[0002] It is known to use a friction model in a power steering system, to at least partially compensate for friction and reduce the impact of friction on the driver's feeling. The implementation of this model can nevertheless cause the appearance of instabilities.

[0003] The invention therefore aims to propose a solution to all or part of these problems.

[0004] For this purpose, the present invention relates to a method for compensating for friction between at least two parts, the method being based on a model of the friction, a speed v between at least one part and at least one other part of the at least two parts, the speed v being measured, or estimated from a measurement, the speed v being an input data of the model, the model being based on an internal state z of friction, a time derivative t of the internal state z being a function of said internal state and of the speed v and of a first gain a, the method comprising a step of saturating the speed v at the input of the friction model, so that an absolute value Ivl of the speed v remains lower than a saturation value, the saturation value being a function of the speed, of the first gain and of a second gain;

[0005] According to these provisions, the instabilities of the friction compensation are reduced, or even eliminated.

[0006] According to one embodiment, the invention comprises one or more of the following characteristics, alone or in a technically acceptable combination.

[0007] According to one embodiment, the method is implemented digitally, the speed v at the input of the model being sampled in time according to a sampling period Te, and the saturation value is a function of the sampling period Te.

[0008] According to one mode of implementation, the saturation value is defined by the equation: [Math 1] in which k is the second gain, and g(v) is a function of v, representing the friction in steady state, the steady state being defined by Z = 0.

[0009] According to one embodiment, the second gain is between 0 and 4, in particular between 1 and 3, in particular equal to 2.

[0010] According to one mode of implementation, the time derivative Z of the internal state z satisfies the equation: [Math 2]

[0011] According to one embodiment, the function g(v) is a Stribeck function.

[0012] According to one embodiment, the model is a Lugre model.

[0013] According to these provisions, the instabilities of the friction compensation linked to the digital implementation are reduced or even eliminated, in particular when the function g(v) takes low values, for which the desired level of compensation can be close to zero.

[0014] According to one embodiment, the at least two parts are parts of a power steering system of a vehicle, the speed v at the input of the model being a steering wheel speed defined as a sum of a first speed and a second speed, the first speed being a rotation speed of an electric motor of the power steering system measured by a speed sensor, and the second speed being determined from a steering wheel / driver torque between a steering wheel and a rack of the power steering system, said steering wheel / driver torque being measured by a torque sensor.

[0015] According to one embodiment, the second speed is determined from a steering wheel / driver torque between the steering wheel and the power steering rack, according to the following sub-steps: - measure the steering wheel / driver torque via the torque sensor configured to measure the torque between the steering wheel and the power steering rack, - temporally derive the torque measurement; - calculate the second speed, called torque speed, as a function of a determined stiffness applied to the time derivative of the torque measurement.

[0016] According to one embodiment, another input to the friction model is a friction compensation level, said compensation level being determined by a difference between an estimated friction amplitude and a target friction amplitude.

[0017] According to these provisions, the instabilities of the friction compensation are reduced or even eliminated, in particular when the relative speed becomes high and / or when the desired compensation level is close to zero and in particular when the method is implemented digitally.

[0018] For a better understanding, an embodiment and / or implementation of the invention is described with reference to the attached drawings representing, as by way of non-limiting example, an embodiment or implementation respectively of a device and / or a method according to the invention. The same references in the drawings designate similar elements or elements whose functions are similar.

[0019] [Fig.l] is a schematic presentation of the steps of a method for compensating friction forces according to an embodiment of the invention.

[0020] [Fig.2] is a schematic representation of a steering device to which the method according to the invention is applied.

[0021] In [Fig. 2] is illustrated a steering device 1 comprising a power steering system making it possible to implement the compensation method according to the invention. In a manner known per se, and as can be seen in [Fig. 2], said power steering device 1 comprises a steering wheel 3 which allows a driver to maneuver said power steering device 1 by exerting a force, called “steering wheel torque” T3, on said steering wheel 3. Said steering wheel 3 is preferably mounted on a steering column 4, guided in rotation on the vehicle 2, and which meshes, by means of a steering pinion 5, on a steering rack 6, which is itself guided in translation in a steering casing 7 fixed to said vehicle 2.

[0022] Preferably, the ends of said steering rack 6 are each connected to a steering 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 that the longitudinal translational movement of the rack 6 makes it possible to modify the steering angle (yaw angle) of the steered wheels. The steered wheels 10, 11 may also preferably be drive wheels.

[0023] The power steering device 1 also comprises a motor 12 configured to assist the operation of said power steering device 1. The motor 12 will preferably be an electric motor, with two directions of operation, and preferably a rotary electric motor, of the brushless type.

[0024] The power steering device 1 further comprises a steering wheel torque sensor 14 specially placed within the power steering device 1, for example on the steering column 4, in order to measure the steering wheel torque T3, and having the main, or even exclusive, purpose of providing a measurement of the steering wheel torque T3, regardless of the measurement technology used by said steering wheel torque sensor 14. In addition, the power steering device 1 comprises an engine speed sensor 24 intended to measure the rotational speed of the engine 12.

[0025] Finally, the power steering device 1 also comprises a calculation and control unit 20 configured to implement the compensation method from the data from sensors 14, 24.

[0026] The method 100 according to the invention will now be described with reference to the application context of the power steering presented above as a non-limiting example. Those skilled in the art will understand that the method according to the invention can be implemented in other applications.

[0027] The method 100 according to the invention is in particular applicable to any solution to a technical problem which implements a model M of the friction between at least two parts 5, 6, movable relative to each other, according to a speed v estimated 101' or measured 101 between at least one part 5 and at least one other part 6, the speed v being an input datum of the model, the model involving an internal state z of friction whose time derivative Z of the internal state z is a function of said internal state and of the speed v and of a parameterizable gain 17, that is to say therefore the value can be predetermined.

[0028] According to one embodiment, the speed v is a relative speed between two parts (5, 6). According to a preferred embodiment, the speed v is a speed estimated as a function of at least two speeds vl, v2, measured 101 or estimated 101' at different parts of the power steering 1, the different parts being able, for example, to be the motor 12 of the power steering 1, and the steering wheel 3 of the power steering, as specified below. The speed v, vl, v2 can, for example, be a rotation speed, when the parts are mobile in rotation relative to each other, or a translation speed when the parts are in translation relative to each other, or a combination of the two.

[0029] Thus, for example, the friction model M can be a Lugre model, in which in particular the time derivative Z of the internal state z satisfies the equation:

[0030] [Math.2] M Z — V - <7.$^ -Z in which g(v), is a function of v, representing the friction in steady state, the steady state being defined by [Math.2] Z = 0.

[0031] The function g(v) can in particular be the Stribeck function, representative of a desired level of friction compensation.

[0032] Advantageously, the LuGre model does not present a discontinuity problem. The LuGre model uses an integration principle and has properties for representing friction cycles, while allowing for a certain continuity over intermediate friction rates.

[0033] According to one embodiment, the method is implemented in a manner digital, in the sense that the speed v at the input of the model is sampled in time according to a sampling period Te .

[0034] Thus, in this example of implementation, an oscillatory instability may appear. when the term of the previous equation becomes too large, in particular for values ​​of g(v), that is, for desired friction compensation levels, close to zero.

[0035] To avoid this instability, the method comprises a step 102 of dynamic saturation of the speed v at the input of the friction model M, so that an absolute value Ivl of the speed v remains lower than a saturation value, the value of saturation being a function of speed, gain and a second gain k.

[0036] Thus, in particular in the case of a digital implementation of the method, the saturation value can be defined by the equation: [Math 1] \v\ <kf^

[0037] According to one embodiment, the second gain is between 0 and 4, preferably between 1 and 3, more preferably the second gain is equal to 2.

[0038] According to these provisions, the instabilities of the friction compensation linked to the digital implementation of the method, in particular when the function g(v) takes low values, for which the desired level of compensation can be close to zero, are reduced or even eliminated.

[0039] According to a particular embodiment of the invention, illustrated in more detail in [Fig.l], the speed v at the input of the friction model is a flywheel speed defined as a sum of a first speed vl and a second speed v2.

[0040] For example, the first speed is a speed of the electric motor 12 of the power steering 1 measured 101 by a speed sensor 24, and the second speed being determined 101' from the steering wheel / driver torque T3 between the driving wheel 3 and the rack 6 of the power steering 1, said steering wheel / driver torque T3 being measured by the steering wheel torque sensor 14.

[0041] The determination 101' of the second speed, corresponding to the steering wheel / driver torque T3, according to an estimation method 101' which will be specified below by way of example, makes it possible to take into account the very low stresses of the driver on the steering wheel 3. Indeed, on the very low stresses of the driver, friction present between the steering wheel 3 and the electric motor 12, linked to the various parts and mechanical meshes of the power steering, prevents a stress on the electric motor 12. Thus the first speed is zero.

[0042] Determining a second speed thus makes it possible to take into account and therefore effectively compensate for the friction of the power steering 1 for low driver inputs on the steering wheel 3.

[0043] The sum of the first speed and the second speed makes it possible to take into account both the low stresses of the driver on the steering wheel 3, via the second speed, and the low stresses rising from the rolling surface and directly stressing the rack 6, via the first speed.

[0044] The second speed can for example be determined 101' according to the following sub-steps: - (IF) measure the steering wheel / driver torque T3 via the torque sensor 14 configured to measure the torque T3 between the steering wheel 3 and the rack 6 of the power steering 1, - (S2) temporally derive the measurement of the torque T3; - (S3) obtaining the second speed, called torque speed, according to of a determined stiffness applied to the time derivative of the torque measurement, the application of the determined stiffness to the time derivative of the torque measurement consisting of calculating the product of the determined stiffness by the time derivative of the torque measurement.

[0045] According to one embodiment, a second input data of the friction model may be a desired level of compensation of the friction, said level of compensation being determined by a difference between an estimated amplitude FRI of the friction and a target amplitude of friction FRC. The methods for estimating the amplitude of a friction are well known to those skilled in the art.

[0046] Depending on the method of implementing the friction compensation method described above with reference to [Fig.l], the Stribeck function g(v) can take very low values ​​because the desired level of compensation can be close to zero.

[0047] This makes the occurrence of numerical instabilities in the model much more likely, hence the need to develop the countermeasure according to the invention, based on the relative speed saturation step, the relative speed considered then being a composite flying speed, defined as a sum of the first speed vl and a second speed v2.

[0048] The method according to the invention is however applicable to any situation where a friction model could present numerical instabilities.

Claims

1.

2. Claims Method (100) for compensating for friction between at least two parts (5, 6, 7), the method being based on a model (M) of the friction, a speed (v) between at least one part (5) and at least one other part (6) of the at least two parts (5, 6, 7), the speed (v) being measured (101), and estimated (101') from a measurement, the speed (v) being an input data of the model (M), the model (M) being based on an internal state (z) of friction, a time derivative (t) of the internal state (z) being a function of said internal state and of the speed (v) and of a first gain (°), the method comprising a step of saturating (102) the speed (v) at the input of the friction model, so that an absolute value Ivl of the speed (v) remains lower than a saturation value, the saturation value being a function of the speed, of the first gain and of a second gain, in which the at least two parts are parts of a power steering system (1) of a vehicle (2),the speed v at the input of the model being a steering wheel speed defined as a sum of a first speed (vl) and a second speed (v2), the first speed (vl) being a rotation speed of an electric motor (12) of the power steering (1) measured (101) by a speed sensor (24), and the second speed (v2) being estimated (101') from a steering wheel / driver torque (T3) between a steering wheel (3) and a rack (6) of the power steering (1), said steering wheel / driver torque (T3) being measured by a torque sensor (14)., Method (100) according to claim 1, in which the second speed (V2) is estimated (101') from a steering wheel / driver torque (T3) between the steering wheel (3) and the rack (6) of the power steering (1), according to the following sub-steps: - (SI) measuring the steering wheel / driver torque (T3) via the torque sensor (14) configured to measure the torque (T3) between the steering wheel (3) and the rack (6) of the power steering (1), - (S2) temporally derive the torque measurement (T3); - (S3) calculate the second speed (V2), called torque speed, as a function of a determined stiffness applied to the time derivative of the torque measurement (T3).

3. Method (100) according to one of claims 1 or 2, the method being implemented digitally, the speed v at the input of the model being sampled in time according to a sampling period Te, in which the saturation value is a function of the sampling period Te.

4. The method (100) of claim 3, wherein the saturation value is defined by the equation: [Math 1] 1 ! / in which k is the second gain and g(v), is a function of v, representing the friction in steady state, the steady state being defined by t = 0.

5. Method (1) according to claim 4, wherein the time derivative Z of the internal state z satisfies the equation: [Math 2] M

6. Method (100) according to one of claims 4 or 5, in which the function g(v) is a Stribeck function.

7. A method (100) according to any preceding claim, wherein another input to the friction model is a friction compensation level, said compensation level being determined by a difference between an estimated friction amplitude (FRI) and a predetermined friction target amplitude (FRC).

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