Method for estimating a value for friction

Abstract

A method for estimating a value for friction exerted on a power steering system of a vehicle, during turning resulting in the crossing of a straight line angle, the turning being achieved at a substantially uniform speed included within a predefined range, and an angle of a steering wheel included within a predefined range, including: a step (1) of acquiring a plurality of vehicle-related data items, including at least one value of a force of a power steering motor on a rack and pinion, and a value of a force of the steering wheel on the rack and pinion; a step (4) of estimating the value for friction by averaging the sum of the forces of the steering wheel and the forces of the power steering motor on the rack and pinion.

Claims

1. A method for estimating a value of friction exerted on a power steering system of a vehicle, the method comprising: acquiring a plurality of vehicle data, including one or more values of an effort of an assist motor on a rack, and one or more values of a steering wheel effort on the rack, the acquiring being performed during a turning of the vehicle, resulting in a crossing of a straight line angle, said turning being carried out with a substantially uniform speed comprised in a predefined interval, and a steering wheel angle comprised in a predefined interval; estimating a friction value by averaging a sum of the values of the steering wheel effort and the assist motor effort on the rack; and calculating a nominal friction value by recurrence depending on the friction value.

2. The method according to claim 1, wherein a speed of the turning is between 5 and 20°/s.

3. The method according to claim 1, wherein the steering wheel angle is comprised between +/−2° relative to an angle for which the vehicle follows a substantially rectilinear trajectory.

4. The method according to claim 1, wherein a value of an angular spacing relative to a last turning reversal is 5°.

5. The method according to claim 1, further comprising, after acquiring a plurality of vehicle data, a comparison step comparing at least one temperature of a computer, to a range of predefined values.

6. The method according to claim 5, wherein the comparison step emits a validation signal when the data are in the range of predefined values and/or emits an invalidation signal when at least one datum is not in the range of predefined values.

7. The method according to claim 6, wherein the estimating of the friction value is performed when the comparison step emits the validation signal and/or when the comparison step does not emit the invalidation signal.

8. The method according to claim 6, further comprising estimating a reliability coefficient of the vehicle data and the estimated friction value.

9. The method according to claim 8, wherein the calculation of the nominal friction value is weighted by at least the reliability coefficient of the vehicle data and the estimated friction value.

10. The method according to claim 8, wherein the nominal friction value is weighted by an averaging coefficient.

11. The method according to claim 6, wherein the calculating of the nominal friction value is performed when the comparison step does not emit the validation signal, and/or when the comparison step of emits the invalidation signal.

12. The method according to claim 1, wherein the acquiring of a plurality of vehicle data comprises a phase of processing the vehicle data.

Description

(1) The invention will be better understood, thanks to the description below, which relates to an embodiment of the present invention, given by way of non-limiting example and explained with reference to the appended FIG. 1, which is a schematic representation of the steps implemented during the method according to the invention.

(2) FIG. 1 illustrates a method according to the invention implementing an algorithm based on an effort balance being exerted on a power steering system of a vehicle.

(3) The vehicle power steering system comprises a steering wheel connected to a steering column, a rack, an assist motor, a steering computer and two wheels each connected to a link.

(4) The vehicle follows a trajectory which in particular depends on a speed of said vehicle, a rotation speed of the steering wheel and a steering wheel angle. The rotation speed of the steering wheel is also called turning speed.

(5) In the power steering system, the assist motor exerts a force {right arrow over (F)}.sub.motor on the rack. Similarly, when a driver turns the steering wheel to turn the vehicle wheels, the steering wheel exerts a force {right arrow over (F)}.sub.steering wheel via the steering column on the rack. Furthermore, the rack is connected to the wheels by means of links which exerts a force {right arrow over (F)}.sub.link thereon. Finally, the friction being exerted on the steering system is separated into a dry friction force {right arrow over (F)}.sub.dry and a viscous friction force {right arrow over (F)}.sub.viscous.

(6) The dry friction is the friction dependent on the forces applied on the power steering system. It is the minimum friction being exerted on the power steering system. The dry friction will be assimilated to the friction f subsequently.

(7) The viscous friction is dependent on the turning speed of the steering wheel.

(8) Thus, the effort balance is written in a benchmark of the rack:
Ma=F.sub.motor+F.sub.steering wheel−F.sub.link−f−f.sub.viscous

(9) With M: the mass of the power steering system and a: the acceleration being exerted on the power steering system, that is to say the acceleration of the steering wheel rotation.

(10) By assuming that the turning speed is substantially uniform, that is to say that the rotation speed of the steering wheel is substantially constant, the value of the acceleration of the steering wheel being exerted on the power steering system is negligible. Thus, in the above equation, a=0.

(11) In addition, by assuming that the vehicle follows a substantially rectilinear trajectory, the steering wheel angle can be considered equal to the straight line angle. Thus, the effort exerted by the links {right arrow over (F)}.sub.link on the rack is negligible. Thus, in the above equation, F.sub.link=0.

(12) Furthermore, with the assumption of a low turning speed, the viscous friction force {right arrow over (F)}.sub.viscous is also negligible. Thus, in the above equation, f.sub.viscous=0.

(13) Finally, with the above assumptions, the effort balance is written:
F.sub.motor+F.sub.steering wheel=f
Or also,RFe=f

(14) It is deduced that when the turning speed is low and substantially uniform, and that the vehicle follows a substantially rectilinear trajectory, a measurement of the RFe directly provides an estimate of the value of the friction being exerted on the power steering system.

(15) These assumptions are validated when the steering wheel crosses a straight line angle, at a low and constant speed, about the straight line angle.

(16) The crossing of the straight line angle can be a succession of turning.

(17) FIG. 1 represents the method according to the invention based on the algorithm described above implementing a step 1 of acquiring a plurality of vehicle data d, a step 4 of estimating the friction value f, a step 5 of calculating the nominal friction value f.sub.n, a step 2 of comparing the data d, a step 3 of estimating a reliability coefficient p of the vehicle data d and the estimated friction value f.

(18) The step 1 of acquiring a plurality of vehicle data d allows making available, to the different steps of the method, the data d required to carry out the calculation of the friction value f of the power steering system. The step 1 of acquiring a plurality of vehicle data d comprises a phase of processing the vehicle data d. The processing phase in particular removes the dynamic components from the received raw data, by applying a low-pass filter. The step 1 of acquiring a plurality of vehicle data d receives raw data and emits data d. The raw data are the vehicle speed, the steering wheel angle, the turning speed, the force exerted by the assist motor on the rack, the steering wheel force exerted on the rack, and a temperature of the steering computer. The data d are the vehicle speed, the steering wheel angle, the turning speed, the force exerted by the assist motor on the rack, the steering wheel force exerted on the rack, the temperature of the steering computer and also the steering wheel acceleration, the RFe, and an angular derivative of the RFe.

(19) The step 2 of comparing the data d allows determining periods during which conditions are validated. The step 2 of comparing the data d receives as input the data d and compares each of the data d with a specific predefined interval. For example, the turning speed should be comprised in an interval of 5 to 20°/s, the steering wheel angle should be comprised in an interval of +/−2°, the temperature of the computer should be comprised between 0° C. and 60° C. and a value of an angular spacing relative to the last turning reversal should be about 5°. When all these data d are comprised in the corresponding interval, the step 2 of comparing the data d emits a validation signal v.

(20) The step 4 of estimating the friction value f calculates the friction value f by averaging the RFe when the comparison step 2 emits the validation signal v. The step 4 of estimating the friction value f receives as input the data d, and the validation signal v and emits the friction value f.

(21) The step 3 of estimating the reliability coefficient p of the vehicle data d and the estimated friction value f assigns a weight to each datum d and to the friction value f depending on a variation of said datum d and said friction value f. The weight is determined depending on predefined maps and charts. The reliability coefficient 2 of the vehicle data d and the estimated friction value f is then calculated from the assigned weights. The step 3 of estimating the reliability coefficient p of the vehicle data d and the estimated friction value f receives as input the data d, the friction value f and emits the reliability coefficient p of the vehicle data d and the estimated friction value f.

(22) The step 5 of calculating the nominal friction value f.sub.n performs an average weighted by the reliability coefficient p of the vehicle data d and filtered by an averaging coefficient w.

(23) The averaging coefficient w is scalable during a period of vehicle operation. This allows changing a compromise between speed and stability of the calculation of the nominal friction value depending on the period of vehicle operation.

(24) The step 5 of calculating the nominal friction value f.sub.n is performed when the validation signal v disappears. The step 5 of calculating the nominal friction value f.sub.n receives as input the validation signal v, the friction value f, the reliability coefficient p of the vehicle data d and the estimated friction value f and emits the nominal friction value f.sub.n.

(25) The nominal friction value f.sub.n is determined by learning the method since each new calculated value refers to the previous value f.sub.n-1. Thus, the nominal friction value f.sub.n is more accurate than the friction value f.

(26) 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.