Method for friction compensation in a power steering system and associated estimation method

Abstract

A method in which a continuous estimation of the intermediate friction rate is carried out, allowing the integration of the method into a general friction compensation method so as to continuously improve the feel on the steering wheel, particularly for speeds below a determined threshold. Also, a method for friction compensation in an electrical power steering system, characterised in that the compensation method takes into account a continuous estimation of the intermediate friction rate obtained by the estimation method.

Claims

1. A method for compensating for friction of an electric power steering of a vehicle, the method comprising: measuring a first speed by a speed sensor configured to measure a speed of an electric motor of the electric power steering, determining a second speed by measuring, by a torque sensor, a steering wheel torque between a steering wheel and a rack of the electric power steering, time deriving the torque measurement to obtain a torque derivative, and computing the second speed by applying the torque derivative to a stiffness, computing a steering wheel speed by summing the first speed and the second speed, inputting the steering wheel speed to a LuGre model to obtain a continuous estimation of a friction coefficient, computing a continuous estimation of an intermediate friction rate by dividing the continuous estimation of the friction coefficient by a dynamic friction coefficient of the power steering, measured on a test bench, and compensating for friction of the electric power system using the continuous estimation of the intermediate friction rate.

2. The method according to claim 1, further comprising: obtaining at least one estimated dynamic friction amplitude, and modulating the estimated dynamic friction amplitude by the continuous estimation of the intermediate friction rate, obtaining a desired dynamic friction τ.sub.FRC.sup.target amplitude of the electric power steering, computing a difference between the estimated dynamic friction amplitude and the desired dynamic friction amplitude, computing a product of the difference with the continuous estimation of the intermediate friction rate to obtain an amount of friction to be compensated, computing a difference between the amount of friction to be compensated and an estimation of a set of forces opposing a movement of the steering wheel, compensating for the power steering by monitoring the driver torque by a control unit depending on a target driver torque and the measured steering wheel torque.

3. The method according to claim 2, wherein computing the difference between the estimated dynamic friction amplitude and the desired dynamic friction amplitude is performed by using a chart or a pre-established database or a data input.

4. The method according to claim 2, wherein the monitoring of the driver torque is carried out in a closed loop.

5. The method according to claim 3, wherein the monitoring of the driver torque is carried out in a closed loop.

6. The method according to claim 1, wherein the continuous estimation of the friction coefficient is computed using the following equation:
μ=sat.sub.α*(σ.sub.0(V,T)z+σ.sub.1(V,T)ż) where: μ is the continuous estimation of the friction coefficient, a* is the dynamic friction coefficient of the electric power steering, V is a speed of the vehicle, T is a steering temperature, σ.sub.0 is a stiffness, σ.sub.1 is a micro-damping, and z is a state of an internal friction.

Description

(1) The invention will be better understood, thanks to the description below, which relates to an embodiment of the invention, given by way of non-limiting examples and explained with reference to the appended schematic drawings, in which:

(2) FIG. 1 is a diagram representative of the estimation method according to the invention,

(3) FIG. 2 is a diagram representative of an architecture of the controller of the electric power steering implementing the compensation method of the invention,

(4) FIG. 3 is a diagram representative of the compensation method according to the invention,

(5) FIG. 4 is a schematic view of a steering device to which the invention applies,

(6) FIG. 5 is a graph illustrating two hysteresis curves with and without friction compensation.

(7) As illustrated in FIG. 1, the method for continuous estimation of an intermediate friction rate, comprises a step (a) of measuring a first speed by means of a speed sensor of the electric motor of the vehicle. This first speed is represented by the {dot over (θ)}.sub.pinion reference. The estimation method comprises a step (b) of determining a second speed, comprising a sub-step s1 of measuring the steering wheel/driver torque τ.sub.torsion.sup.bar represented by the reference and a sub-step s2 of time derivation of the measurement of the steering wheel torque and in which a gain is applied to the derivative. The estimation method comprises a step (c) of sum of the first and second speeds, represented by the box referenced Σ, a speed called steering wheel speed referred to as is thus obtained {dot over (θ)}.sub.steering.sup.wheel. Step (d) of the estimation method is represented by the box LuGre consisting in simulating a simplified one-dimensional steering model (a mass) and a LuGre friction, whose only input speed is the sum obtained in the previous step (c). Finally, a continuous estimation of the intermediate friction rate of the electric power steering, represented by the reference w, is obtained.

(8) In FIG. 2, a diagram of the controller CPU of the electric power steering of a vehicle is illustrated.

(9) As illustrated in FIG. 3, the compensation method comprises a step E1 of implementing the continuous estimation method, this estimation method is illustrated in detail in FIG. 1 and also taken into account in the friction compensation method.

(10) In FIG. 3, the box referenced FRI represents the estimation of the friction and comprises at least one component which is the continuous estimation of the intermediate friction rate w, obtained by the method illustrated in FIG. 1, and another component which is the estimated dynamic friction amplitude. Furthermore, by means of a chart or a pre-established database, for example a tunable map, the amplitude of the dynamic friction desired for the electric power steering is obtained: this is step E2 of the compensation method which is illustrated in FIG. 3 in the box referenced FRC. In step E3 of the compensation method, the difference between the estimated dynamic friction amplitude and the desired dynamic friction amplitude is made and in step E4, the product of the result of the difference with the continuous estimation of the intermediate friction rate w is made, which allows us to obtain a value X corresponding to the amount of friction to be compensated tcomp. In a step E5, the difference is made between the value X obtained in step E3, with the determined estimation of the set of forces opposing the movement represented by the box referenced RFE and referred to as rack effort estimator. Thanks to this compensation, the hysteresis of the rack force estimator will have a monitored and desired hysteresis which will also impact the hysteresis at the generation of the target torque. In step E6, the target driver torque and the measured driver torque are taken into account for the monitoring of the closed loop torque by the controller CPU illustrated in FIG. 2. In step E6, the action «taking into account» means «comparing and monitoring in closed loop».

(11) By way of example, the electric power steering is at 400N of friction and a feeling at 300N is desired, the friction estimator FRI will identify that the steering is at 400N, and the desired friction value input will be 300N. The difference is therefore 100N, the determined estimation of the set of forces opposing the movement is 400N and 100N is subtracted so that the friction corresponds to the desired value of 300N.

(12) In FIG. 5, a graph is illustrated, representing two curves illustrating the driver steering wheel torque in Nm (ordinates) as a function of the steering wheel angle in degrees (abscissa). The first curve in continuous line illustrates the hysteresis between the driver steering wheel torque and the steering wheel angle without friction compensation. The second curve in dashed line, illustrates the hysteresis with a friction compensation thanks to the methods of the invention. In order to reduce this hysteresis as desired, the area of inversion to + or −15°, when the movement of the steering wheel is reversed, has been adjusted continuously and gradually, as shown in the second discontinuous curve.

(13) In FIG. 4, a steering device is illustrated comprising a power steering allowing implementing the estimation method of the invention as well as the compensation method of the invention. In a manner known per se, and as shown in FIG. 4, said power steering device 1 comprises a steering wheel 3 which allows a driver to maneuver said power steering device 1 by exerting an effort, 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 fastened to said vehicle 2.

(14) Preferably, the ends of said steering rack 6 are each linked to a steering tie rod 8, 9 connected to the stub axle of a steered wheel 10, 11 (respectively a left wheel 10 and a right wheel 11), such that the longitudinal displacement in translation of the rack 6 allows changing the steering angle (yaw angle) of the steered wheels. Moreover, the steered wheels 10, 11 may also preferably be drive wheels.

(15) The power steering device 1 also comprises a motor 12 configured to assist the maneuver of said power steering device 1. The motor will preferably be an electric motor operating in both directions, and preferably a rotary electric motor, of the brushless type.

(16) The power steering device 1 further comprises a steering wheel torque sensor 14 set up especially 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, 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 a motor speed sensor 24 intended to measure the speed of rotation of the motor 12.

(17) Finally, the power steering device 1 also comprises a calculation and control unit 20 configured to implement the estimation method and the compensation method from sensor data 14, 24.

(18) Of course, the invention is not limited to the embodiments described and represented in the appended figures. Changes remain possible, in particular from the point of view of the constitution of the various elements or by substitution of technical equivalents, without departing from the protection field of the invention.