Method for compensating friction in a power steering system

Abstract

The method (100) for compensating for friction between at least two parts (5, 6, 7) includes a friction model (M) that takes as input data a velocity (v) between at least one part (5) of the at least two parts (5, 6, 7) and at least one other part (6), the velocity (v) being measured (101) or calculated (101') from the measured values, and the friction model (M) is a time derivative of the velocity (v) between at least one part (5) of the at least two parts (5, 6, 7). JPEG2023535464000018.jpg56 is based on a friction model (M) based on an internal state of friction (z) that is a function of an internal state, the velocity (v), and a first gain (σ), and the method includes a step (102) of saturating the velocity (v) input to the friction model such that the absolute value |v| of the velocity (v) remains lower than a saturation value that is a function of the velocity, the first gain, and the second gain.

Description

[Technical Field]

[0001] The present invention relates to the field of power steering systems, and in particular to a method for compensating friction. [Background technology]

[0002] It is known to use friction models in power steering systems to at least partially compensate for friction and reduce its effect on the driver's perception, but implementation of these models can still cause instability. Summary of the Invention [Problem to be solved by the invention]

[0003] The aim of the present invention is to propose a solution to all or some of these problems. [Means for solving the problem]

[0004] To this end, the invention relates to a method for compensating friction between at least two parts, said method being based on a friction model whose input data are the velocities v between at least one of the at least two parts and at least one other part, measured and calculated from the measurements, said model being based on a time derivative JPEG0007812840000001.jpg43 is based on the internal state z of friction, which is a function of the internal state, velocity v, and a first gain σ.

[0005] The method includes saturating the velocity v such that an absolute value |v| of the velocity v remains below a saturation value, the saturation value being a function of the velocity, the first gain, and the second gain.

[0006] These provisions reduce or even eliminate instabilities in the friction compensation.

[0007] According to one embodiment, the present invention comprises one or more of the following features, either alone or in any technically acceptable combination.

[0008] According to one embodiment, the method is digitally processed, and the velocity (v) input to the model is sampled over time according to a sampling period (Te), and the saturation value is a function of the sampling period (Te).

[0009] According to one embodiment, the saturation value is defined by the following formula: JPEG0007812840000002.jpg923 where k is the second gain, g(v) is a function of v and represents the steady-state friction, and the steady-state is Defined by JPEG0007812840000003.jpg411.

[0010] According to one embodiment, the second gain is between 0 and 4, in particular between 1 and 3, and in particular equal to 2.

[0011] According to one embodiment, the time derivative of the internal state z JPEG0007812840000004.jpg11153 satisfies the following formula: JPEG0007812840000005.jpg938 According to one embodiment, the function g(v) is the Stribeck function.

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

[0013] These provisions reduce or even eliminate instabilities in the compensation of frictions associated with digital processing, especially when the function g(v) takes low values, i.e., when the desired compensation level may be close to zero.

[0014] According to one embodiment, the at least two parts are parts of a power steering system of a motor vehicle, and the speed v input to the model is a steering wheel speed defined as the sum of a first speed and a second speed, where the first speed is the rotational speed of an electric motor of the power steering system measured by a speed sensor, and the second speed is calculated from a steering wheel / driver torque between the steering wheel and a rack of the power steering system, where the steering wheel / driver torque is 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 said rack of the power steering system according to the following substeps:

[0016] -Measuring steering wheel / driver torque via a torque sensor configured to measure torque between the steering wheel and rack of a power steering system; - Differentiating the torque measurement with respect to time, - Calculate a second speed, called torque speed, as a function of the determined stiffness applied to the time derivative of the torque measurement.

[0017] According to one embodiment, another input of the friction model is a compensation level for friction, which is defined by the difference between the calculated friction value and a preset target friction value.

[0018] These provisions reduce or even eliminate instabilities in the friction compensation, especially when the relative speed is high and / or the desired compensation level is close to zero, and when the method is processed digitally.

[0019] For a proper understanding, embodiments and / or implementations of the present invention will now be described with reference to the accompanying drawings, which represent, by way of non-limiting examples, respective embodiments or implementations of devices and / or methods according to the present invention, in which the same reference numbers indicate similar or functionally similar elements. [Brief explanation of the drawings]

[0020] [Figure 1] FIG. 1 is a schematic diagram of method steps for compensating for friction according to one implementation of the present invention. [Figure 2] FIG. 2 is a schematic diagram of a steering system to which the method according to the invention is applied. DETAILED DESCRIPTION OF THE INVENTION

[0021] Figure 2 shows a steering device 1 equipped with a power steering system capable of implementing the compensation method according to the present invention. As is well known and can be seen in Figure 2, the power steering device 1 comprises a steering wheel 3 that enables the driver to operate the power steering device 1 by applying a force called "steering wheel torque" T3 to the steering wheel 3. The steering wheel 3 is preferably mounted on a steering column 4 and guided for rotation on the vehicle 2, and engages by a steering pinion 5 with a steering rack 6, which itself is guided for translation within a steering casing 7 fixed to the vehicle 2.

[0022] Preferably, the ends of the steering rack 6 are connected to steering tie rods 8 and 9 connected to the axles of steering wheels 10 and 11 (left steering wheel 10 and right steering wheel 11) so that the steering angle (yaw angle) of the steering wheels can be changed by longitudinal displacement due to parallel movement of the rack 6. Furthermore, the steering wheels 10 and 11 may preferably be drive wheels.

[0023] The power steering device 1 also comprises a motor 12 configured to assist in operating the power steering device 1. The motor 12 is preferably a bidirectional electric motor, and is preferably of the brushless type or a brushless rotary electric motor.

[0024] The power steering device 1 further comprises a steering wheel torque sensor 14, which is particularly mounted in the power steering device 1, for example on the steering column 4, for measuring the steering wheel torque T3, the steering wheel torque sensor 14 having the primary, or in some cases exclusive, purpose of providing a measurement of the steering wheel torque T3, regardless of the measurement technique used in the steering wheel torque sensor 14. Furthermore, the power steering device 1 comprises a motor speed sensor for measuring the rotational speed of the motor 12.

[0025] Finally, the power steering device 1 also comprises a calculation and control unit 20 adapted to carry out a compensation method based on data from the sensors 14,24.

[0026] The method 100 of the present invention will now be described with reference to the power steering system application context mentioned above, by way of non-limiting example, although those skilled in the art will appreciate that the method of the present invention may also be implemented in other applications.

[0027] The method 100 according to the invention is particularly applicable as a solution to the technical problem of implementing a friction model M between at least two parts 5, 6 that are movable relative to each other, the friction model M relating to a calculated 101' or measured 101 velocity v between at least one part 5 and at least one other part 6. The velocity v is an input data of the model, and the model is based on its time derivative JPEG0007812840000006.jpg43 includes an internal state, velocity v, and an internal state of friction z that is a function of a parameterizable gain σ, where the gain σ may be a preset value.

[0028] According to one embodiment, the velocity v relates to the velocities of the two parts (5, 6). In a preferred implementation, the velocity v is calculated as a function of at least two velocities v1, v2 measured 101 or calculated 101' in different parts of the power steering system 1, such as the motor 12 of the power steering system and the steering wheel 3 of the power steering system 1, as will be described below. The velocities v, v1, v2 may be, for example, rotational velocities when the parts are rotatable relative to each other, translational velocities when the parts are translating relative to each other, or a combination of both.

[0029] Therefore, for example, the friction model M is, in particular, the time derivative of the internal state z JPEG0007812840000007.jpg43 may be a Lugrat model that satisfies the following formula: JPEG0007812840000008.jpg938 where g(v) is a function of v and represents the steady-state friction, and the steady-state Defined by JPEG0007812840000009.jpg411.

[0030] The function g(v) may in particular be a Stribeck function, which may represent the desired level of compensation for friction.

[0031] Advantageously, the Lugret model does not suffer from discontinuity problems: it uses integral principles and has the properties to represent the friction cycle while allowing for some continuity in intermediate friction.

[0032] According to one embodiment, the method is processed digitally in the sense that the velocity v input to the model is sampled over time according to a sampling period Te.

[0033] Therefore, in this embodiment, If the JPEG0007812840000010.jpg914 term becomes very large, oscillatory instability may appear, especially if the value of g(v) - the desired level of compensation for friction - approaches zero.

[0034] To avoid this instability, the method includes a step 102 of saturating the velocity (v) input to the friction model so that the absolute value |v| of the velocity v remains below a saturation value, which is a function of the velocity, the gain, and the second gain k.

[0035] Thus, particularly in the digitally processed version of the method, the saturation value may be defined by the following formula: JPEG0007812840000011.jpg923 According to one embodiment, the second gain is between 0 and 4, preferably between 1 and 3, and more preferably the second gain is equal to 2.

[0036] According to these provisions, the instabilities of the friction compensation associated with the digital processing of the method, especially when the function g(v) takes low values, i.e. when the desired compensation level approaches 0, are reduced or even eliminated.

[0037] In a particular implementation of the present invention, as shown in more detail in FIG. 1, the velocity v input to the friction model is the steering wheel velocity, defined as the sum of a first velocity v1 and a second velocity v2.

[0038] For example, the first speed is the speed of the electric motor 12 of the power steering system 1 measured 101 by the speed sensor 24, and the second speed is determined 101' from the steering wheel / driver torque T3 between the steering wheel 3 and the rack 6 of the power steering system 1 measured by the steering wheel torque sensor 14.

[0039] It is possible to take into account very light driver loads at the steering wheel 3 by determining 101' a second speed corresponding to a steering wheel / driver torque T3, for example according to the calculation method 101' specified below. Indeed, at very light driver loads, the friction that exists between the steering wheel 3 and the electric motor 12, associated with the various parts and mechanical engagements of the power steering system, prevents a load on the electric motor 12, and therefore the first speed becomes zero.

[0040] Therefore, by determining the second speed, the low load of the driver on the steering wheel 3 can be taken into account and the friction of the power steering system 1 can be effectively compensated for due to the low load of the driver on the steering wheel 3.

[0041] The sum of the first and second speeds allows for a low load of the driver at the steering wheel 3 via the second speed, and a low load rising from the riding surface and directly urging the rack 6 via the first speed.

[0042] The second speed may be determined 101′, for example, according to the following substeps: (S1) measuring the steering wheel / driver torque T3 via a torque sensor 14 configured to measure the torque T3 between the steering wheel 3 and the rack 6 of the power steering system 1; (S2) Differentiate the measured value of torque T3 with respect to time; (S3) Calculating a second velocity v2, referred to as the torque velocity, as a function of the determined stiffness applied to the time derivative of the torque measurement. Applying the determined stiffness to the time derivative of the torque measurement includes calculating the product of the determined stiffness by the time derivative of the torque measurement.

[0043] According to one embodiment, the second input data of the friction model may be a desired compensation level for friction, which is defined by the difference between the calculated friction value FRI and a preset target friction value FRC. Methods for calculating the magnitude of friction are well known.

[0044] This leads to a high probability of numerical instabilities appearing in the model, which is why it is necessary to develop a countermeasure according to the invention, which is based on a saturation step of the relative velocity.The relative velocity considered is the composite steering wheel velocity, which is defined as the sum of a first velocity v1 and a second velocity v2.

[0045] However, the method according to the present invention is applicable to any situation where the friction model exhibits numerical instabilities.

Claims

1. 1. A method for compensating for friction between at least two parts (5, 6, 7) based on a friction model (M) having as input data velocities (v) between at least one part (5) and at least one other part (6) of the at least two parts (5, 6, 7) measured (101) and calculated (101') from the measured values, comprising: The friction model (M) is a Lugre model, and is a time derivative is based on an internal state of friction (z) that is a function of an internal state, the velocity (v), and a first gain (σ); saturating (102) the velocity (v) input to the friction model so that the absolute value |v| of the velocity (v) remains below a saturation value; the saturation value is a function of the speed, the first gain, and the second gain; the at least two parts are parts of a power steering system (1) of a motor vehicle (2); The velocity (v) input to the model is the velocity of the steering wheel, defined as the sum of a first velocity (v1) and a second velocity (v2); the first speed (v1) is the rotational speed of the electric motor (12) of the power steering system (1) measured (101) by a speed sensor (24); the second speed (v2) is calculated (101') from a steering wheel / driver torque (T3) between the steering wheel (3) and the rack (6) of the power steering system (1) measured by a torque sensor (14); The method is digitally processed; The velocity (v) input to the model is sampled over time according to a sampling period (Te), The saturation value is a function of the sampling period (Te), The saturation value is and is defined by the following formula: k is the second gain, g(v) is a function of v, and represents the steady-state friction defined by The time derivative of the internal state (z) satisfies the following formula, The method of claim 1, wherein the function g(v) is a Stribeck function.

2. 10. The method (100) of claim 1, The second velocity (v2) is measuring (S1) 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 system (1); The measured value of the torque (T3) is differentiated with respect to time (S2); Calculating (S3) the second velocity (v2), called torque velocity, as a function of the determined stiffness applied to the time derivative of the torque measurement; and calculating (101') from the steering wheel / driver torque (T3) between the steering wheel (3) and the rack (6) of the power steering system (1) by the sub-step:

3. 3. The method (100) of claim 1 or 2, Another input to the friction model is the friction compensation level; The method according to claim 1, wherein the compensation level is defined by the difference between a calculated friction value (FRI) and a preset target friction value (FRC).

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