USE OF AN ASSIST MOTOR OF A POWER STEERING SYSTEM IN ORDER TO GENERATE TEST CYCLES ACCORDING TO A FORCE ASCERTAINING CYCLE

A method for characterizing a power steering system for empirically determining at least one property of the system, the power steering system including at least one steering wheel, a steering mechanism provided with a rack, and at least one assist motor, the method including, outside a steering phase during which the power steering system is assigned to the driving of a vehicle in order to cause the vehicle to follow a trajectory which is determined as a function of the situation of the vehicle with respect to its environment, a step (a) of automatically activating the assist motor, during which step a computer is used to automatically generate and apply to the assist motor, without requiring any external action on the steering wheel, an activation instruction that follows one or more cycles referred to as pre-established exploration cycles for measuring.

The present invention concerns the characterization methods intended to empirically determine at least one property of a power steering system, such as for example the position of the end-of-stroke stops of a steering rack or the frequency-response characteristics of the power steering system, during the fine-tuning or the calibration of said system in factory.

The known characterization methods require a human operator installing the power steering system on a test bench, then the latter maneuvering the steering wheel according to pre-established special maneuver cycles so that sensors and recorders equipping the test bench could observe the reactions of the steering system and measure the indicator parameters which then allow quantifying the pursued property.

Of course, such manual maneuvers are sometimes quite tedious, and often relatively inaccurate, to the extent that the operator cannot exert an accurate speed or force setpoint, and in particular a constant value setpoint, in a reliable and repeatable manner, or else he could for example be mistaken about the direction of maneuver during a cycle, which may distort the estimate of the pursued property.

Moreover, while it is possible, in absolute terms, to consider replacing the operator with a robotized arm that actuates the steering wheel, such a solution is particularly complex and expensive to implement, in particular because it is necessary, at each test, to install and couple the robotized arm to the steering wheel, and to materially reconfigure the robotized arm and the test bench according to the model of the tested steering system.

Consequently, the objects assigned to the invention aim at overcoming the aforementioned drawbacks and at providing a method for characterizing a power steering system which allows for a quick, reliable and low-cost characterization of said power steering system.

The objects assigned to the invention also aim at providing a new method for characterizing a power steering system which has a great versatility, as said method adapts in a simple manner to many models of power steering systems and/or allows completely characterizing several properties of the same power steering system.

The objects assigned to the invention are achieved by means of a method for characterizing a power steering system intended to empirically determine at least one property of said power steering system, called «pursued property», said power steering system comprising at least one heading definition device, such as a steering wheel, which allows defining the orientation, called «steering angle» of the power steering system, a steering mechanism provided with at least one movable member, such as a rack, whose position adapts so as to correspond to the selected steering angle, as well as at least one assist motor arranged so as to be able to drive said steering mechanism, said method being characterized in that it comprises, besides a piloting phase during which the power steering system is dedicated to driving of a vehicle in order to make said vehicle follow a path that is determined according to the situation of said vehicle with respect to its environment, a step (a) of automatically activating the assist motor, during which a calculator is used to automatically generate and apply to the assist motor, without requiring any external action on the heading definition device, an activation setpoint which follows one or several pre-established cycle(s) called «exploration cycles», a measurement step (b), according to which is measured, during the exploration cycle(s) or on completion of said exploration cycle(s), at least one physical parameter, called «indicator parameter», which is specific to the response supplied by the power steering system to the automatic activation of the assist motor and which is characteristic of the pursued property, then an analysis step (c), during which the pursued property is quantified from the measurement(s) of the indicator parameter.

Advantageously, the invention thus uses the assist motor itself as a (unique) means to activate the steering mechanism according to the selected exploration cycle(s), without it being necessary to use an auxiliary drive means, and in particular an auxiliary motor, external to the steering system.

Thus, an operator or a robotized arm is no longer necessary.

Furthermore, the automation of the exploration cycles advantageously allows applying to the assist motor, during the phases where the steering system is characterized, particularly accurate setpoints, much more accurate than during manual maneuvers, and in particular predetermined speed, acceleration or force setpoints that are constant over predetermined periods or over displacement distances of the movable member, which allows accurately measuring the indicator parameter(s), without the activation of the power steering system constituting by its very nature a potential source of error that would be related to an excessive and uncontrolled variability of the setpoint with respect to the target ideal exploration cycle.

Hence, the characterization of the pursued property is particularly accurate and repeatable.

Furthermore, the invention allows in particular equipping the power steering system, irrespective of the model of said system, with an onboard calculation module which contains a complete set of characterization functions, for example in the form of a library file stored in a non-volatile memory of said module, such that the power steering system will be intrinsically provided with the tools that are necessary to the characterization thereof, and more generally to the characterization of several ones of its properties.

Hence, the fine-tuning and the calibration of said power steering system will be greatly facilitated.

The invention concerns a method for characterizing a power steering system1intended to empirically determine at least one property of said power steering system1, specific to said system, called «pursued property».

As shown inFIG. 1, said power steering system1comprises at least one heading definition device2which allows defining the orientation, called «steering angle» A1, of the power steering system.

Preferably, the heading definition device2will comprise a steering wheel2which enables a driver (human) to freely define said steering angle A1so as to ensure a manual piloting of a vehicle equipped with the power steering system1.

Said steering system also comprises a steering mechanism3provided with at least one movable member4, such as a rack4, whose position P4adapts so as to correspond to the selected steering angle A1.

For convenience, the movable member4may therefore be assimilated to a rack in what follows.

In a manner known per se, said movable member4, and more particularly the rack4, may preferably be mounted movable and guided in translation within a steering casing.

Thus, the steering mechanism3allows modifying the orientation of an orientable member5, such as a steered wheel5, displaced by the rack4, in order to direct a vehicle on which said power steering system1is embedded.

In a manner known per se, the steering mechanism3may comprise steering tie rods6each linking one end of the rack4to a yaw-orientable steering knuckle and carrying the corresponding steered wheel5.

The power steering system1also comprises at least one assist motor7arranged so as to be able to drive said steering mechanism3.

Preferably, said assist motor7will consist of an electric motor, with two directions of operation, so as to be able to drive the steering mechanism3indifferently to the left or to the right, for example a brushless motor.

Although the use of a linear motor7is not excluded, a rotary motor7will be preferred.

The assist motor7is placed, through a calculator comprising a first onboard module8, that is to say integrated to the system1, called «assist module»8, under the dependence of the heading definition apparatus2.

Preferably, the heading definition apparatus2may serve to define a steering angle setpoint A2, which may typically be defined, in the case where the apparatus2comprises a steering wheel2or is formed by a steering wheel2, by the angular position P2of said steering wheel2.

Alternatively or complementarily to the supply of a steering setpoint A2, the heading definition apparatus2may supply a force datum T2, called «steering wheel torque», which corresponds to the force exerted by the driver on said heading definition apparatus2, and more particularly to the torque exerted by the driver on the steering wheel2.

Said steering wheel torque T2may be measured by a torque sensor9associated to the steering wheel2.

According in particular to the steering angle setpoint A2and/or where appropriate according to the «steering wheel torque» T2exerted by the driver on said heading definition apparatus2, the assist module8defines, according to an assist law stored in said assist module8, an assist force setpoint (assist torque setpoint) T7applied thereby to the assist motor7, in order to make the actual steering angle A1of the system1, and consequently the yaw angle of the wheels5, coincide with the orientation defined by the heading definition apparatus2.

Of course, other parameters, and in particular dynamic parameters of the vehicle, such as the longitudinal speed of the vehicle, may be taken into consideration by the assist law.

It should be noted that the invention may preferably apply to a power steering system within which the steering wheel2is mechanically linked to the rack4and therefore mechanically linked, at least indirectly, to the assist motor7, for example through a steering column10carrying said steering wheel2and provided with a pinion11which meshes on the rack4.

In this manner, the steering wheel2is an integral part of the steering mechanism3, and can transmit a manual steering force and/or a steering movement to the movable member (rack)4, and conversely, be driven by the assist motor7.

Alternatively, it is quite possible to consider applying the invention to a power steering system called «steer-by-wire», within which there is no drive mechanical linkage between the steering wheel2and the movable member (rack)4driven by the assist motor7, but only an electric link which transmits the steering angle setpoint A2and/or the steering wheel torque information T2to the assist module8which, in turn, servo-controls the assist motor7.

The assist motor7may be coupled to the rack4by any suitable mechanism, and in particular by a motor pinion12, possibly distinct from the pinion11of the steering column, and which directly meshes on the rack4, as illustrated inFIG. 1, or by a ball screw, or else through a reducer placed on the steering column10so as to form a so-called «single-pinion» mechanism.

Whether considering a mechanical linkage steering or a steer-by-wire, the heading definition apparatus2intervenes during a phase called «piloting phase», during which the power steering system1is effectively dedicated to driving of a vehicle, in order to make said vehicle follow a path that is determined according to the situation of said vehicle with respect to its environment.

According to the invention, the method comprises, besides such a piloting phase, that is to say at the time where the steering system1, and more generally the vehicle, is not in a traffic situation, and that it is not therefore necessary to take into account the environment of said vehicle to define a vehicle path adapted to such an environment, or to necessary comply with a particular path to ensure safety of the vehicle and of its occupants, a step (a) of automatically activating the assist motor7, during which a calculator13is used to automatically generate and apply to the assist motor7, without requiring any external action on the heading definition device2, an activation setpoint which follows one or several pre-established cycle(s) called «exploration cycles» CY, a measurement step (b), according to which is measured, during the exploration cycle(s) CY or on completion of said exploration cycle(s) (CY), at least one physical parameter, called «indicator parameter», which is specific to the response supplied by the power steering system1to the automatic activation of the assist motor7and which is characteristic of the pursued property, then an analysis step (c), during which the pursued property is quantified from the measurement(s) of the indicator parameter.

Although it is not excluded to punctually use a calculator13external to the power steering system1, that would be electrically connected to said system1when it is desired to proceed with the characterization of the latter, said calculator13may preferably be an integral part of the power steering system1, and therefore of the vehicle equipped with said system1, and form to this end a second onboard module, called «characterization module»13.

Preferably, the first module, namely the assist module8used for assisting steering during the piloting phase, and the second module, namely the characterization module13intended to monitor the automated process of characterizing the power steering system1off the piloting phase will co-exist within the same calculator onboard the vehicle.

Advantageously, the invention allows intrinsically using the assist motor7embedded in the power steering system1as an exclusive drive source to drive the steering mechanism3during the characterization, without requiring an external active movement source, such as the manual force of an operator or an external additional motor, that would be distinct from the assist motor7(and for example integrated to a robotized arm).

Hence, more generally, the characterization according to the invention may advantageously be carried out without it being necessary to mechanically act in an active way, whether manually or by an external motor, on the power steering system1, and more particularly on the steering mechanism3, from the outside, and more particularly without it being necessary to actuate, whether manually or by an external motor, any of the movable mechanical members, such as the steering wheel2, an apparent end of the rack4, or possibly a steering tie rod6or a wheel5linked to said rack4, that form a mechanical interface between said power steering system1, respectively said steering mechanism3, and the outside thereof.

Hence, the actuation of the steering mechanism3for the characterization according to the invention may be carried out in a standalone, easy manner and at a lesser cost, by exclusively exploiting drive means (assist motor7), and where appropriate control means (characterization module13), that are intrinsically present in the power steering system1.

Moreover, it should be noted that it is possible to provide for using one or several passive external load(s), such as for example blocking wedges, springs and/or dampers, that are coupled to either one or both of the mechanical interfaces of the power steering system1(steering wheel2or ends of the rack4, for example) in order to simulate a particular behavior of the steering system1and thus access to the pursued property.

Nonetheless, these external loads will be passive, that is to say, unlike the assist motor7, they will not intrinsically bring in energy to the power steering system, but will rather serve to dissipate all or part of the energy imparted to the steering mechanism3by said assist motor7or to modify the distribution of said energy over time and through said steering mechanism3.

As indicated hereinabove, the characterization method according to the invention takes place off any piloting phase of a vehicle, in a test situation that may be qualified as “virtual” situation, since said situation does not require complying with a particular path or with a particular dynamic behavior of the vehicle, and therefore allows characterizing the power steering system1as such, irrespective of the influence of the vehicle, by de-correlating the use of said power steering system1from the use of the vehicle itself, and consequently without imposing on the characterization method restrictions related to safety of said vehicle or of the occupants of the latter.

Thus, the method according to the invention will be particularly suited to the characterization in factory, off traffic, typically on a test bench, of a vehicle equipped with a power steering system1, or even of a power steering system1alone, before assembly of said system1on a vehicle, and for example of a power steering system1on which the wheels5, and where appropriate the steering tie rods6have not yet been installed.

Since step (a) of automatic activation for the characterization takes place off a vehicle piloting phase, it is advantageously possible to control the assist motor7by means of an exploration cycle CY, and therefore of an activation setpoint, whose nature, form and duration, defined according to a predetermined activation diagram («pattern»), will be arbitrarily and freely selected, so as to be able to determine the pursued property, in an optimum manner, and without having to comply with a compulsory path of a vehicle, and in particular without having to take into consideration safety of the vehicle, of the occupants of said vehicle, or of the persons or objects present in the environment of said vehicle.

In practice, it will therefore be possible to define and apply the exploration cycles CY, and more generally the activation setpoint applied to the assist motor7during the characterization method, without the need for acquiring (and in particular measuring) or taking into consideration parameters representative of the dynamics specific to the vehicle with respect to its environment, that is to say parameters representative of the behavior specific to the vehicle within a reference frame external to said vehicle, amongst which in particular the longitudinal speed of the vehicle, the lateral acceleration of said vehicle, the yaw speed of said vehicle, or the distance of the vehicle from an obstacle or from an external reference (for example a white line delimiting the traffic lane) detected within said external reference frame.

In this manner, said exploration cycles will not be subjected to any restriction related to such parameters representative of the dynamics of the vehicle, and, in practice, will not therefore require for their definition and their application, any external information input related to such parameters, and in particular any visual information input.

Thus, it will be possible to activate the assist motor7without having to input information concerning parameters representative of the dynamics of the vehicle within its environment, which information input would be carried out either by the senses (in particular tactile and visual) of a human driver, who would react afterwards to this information by manually actuating the steering wheel2, or through an automatic acquisition process (for example by means of a camera or a radar, in particular laser, infrared or ultrasonic) which would be implemented by an automatic piloting module.

At most, said exploration cycles may possibly be dimensioned so as to comply with some material limitations inherent to the design of the power steering system1itself, such as for example the maximum torque that the assist motor7can output (and therefore the maximum electric current that said assist motor7can tolerate without damage).

As illustrated inFIG. 2, the exploration cycle may preferably include at least one sign change, which corresponds to a reversal of the direction of activation of the assist motor7, so as to activate said assist motor7to the right, and then to the left (or vice versa).

Thus, a so-called «elementary» exploration cycle may preferably comprise a positive alternation and a negative alternation.

Nonetheless, it is of course possible to alternatively use an elementary cycle comprising one single alternation, with a constant sign, for example positive, in order to load the assist motor7only in one direction, to the right or on the contrary to the left, if this is enough to define the pursued property.

Of course, each elementary exploration cycle CY may be repeated as many times as necessary, preferably identically, without exceeding a predetermined number of iterations Ni.

Where appropriate, the repetition of the exploration cycles CY will allow multiplying, during the successive cycles, the measurements of the same indicator parameter, for example at the rate of at least one, and even exactly one, measurement of said indicator parameter per cycle.

By thus using a plurality of successive measurements of the same indicator parameter over several cycles to quantify the pursued property, and for example by using to this end an arithmetic average or a weighted average of the different measurements of said indicator parameter over several cycles, and even a selection of said measurements excluding values deemed to be doubtful, it is advantageously possible to improve the accuracy and the reliability of the analysis step (c), during which the pursued property is quantified from said indicator parameter, respectively from said average.

Of course, during the measurement step (b), the reactions of the power steering system1, and more particularly of the steering mechanism3, to the mechanical constraints created by the activation of the assist motor7, are observed by measuring and possibly recording as many indicator parameters as necessary to determine the pursued property from said observed response.

In particular, it is possible to measure, as needed, one or several indicator parameter(s) among: the position P7(and therefore the displacements) of the shaft of the assist motor7, the position (and therefore the displacements) P4of the movable member4(rack) or the position P2(and therefore the displacements) of the steering wheel2, preferably expressed in the reference frame of the assist motor7, the speed P7′, P4′, P2′ and in particular the angular speed (preferably expressed in the reference frame of the motor7, while taking into consideration the possible mechanical transmission ratios) of either one of these components7,4,2, the force T7delivered by the assist motor7, the steering wheel torque T2, or a resisting force T4exerted by an external element on the movable member (rack)4against the assist motor7.

For convenience of the description, it is possible to add in what follows the suffix «_mes» to explicitly refer to an indicator parameter (measured or assessed) associated to a given quantity, in particular when it is necessary to explicitly differentiate the effective value measured by said indicator parameter from a corresponding setpoint value. Nonetheless, for convenience of the description, it is generally possible to assimilate the indicator parameter (measured effective value) to the corresponding setpoint.

Preferably, the method allows determining at least one pursued property, and even more preferably several (at least two) pursued properties, among:a stiffness K characteristic of the elasticity of a portion of the steering mechanism3,a temperature rise or a thermal evolution pattern of the assist motor7,an endurance property characterized by a wear indicator, such as a degree of wear of the steering mechanism3or of the assist motor7, as a function of a number Ni of back-and-forth cycles CY performed by the steering mechanism3.

These different possibilities provided by the invention will be detailed hereinafter.

According to a first possibility of the invention, during the activation step (a), a force exploration cycle CY_force, or a succession of several force exploration cycles CY_force, is applied where each force exploration cycle CY_force servo-controls the force T7of the assist motor7, and more particularly servo-controls the torque T7of the assist motor7, according to at least one non-zero force setpoint (torque setpoint) T7, T7_1, T7_2.

An example of an elementary force exploration cycle CY_force is illustrated inFIG. 2.

Preferably, said force exploration cycle CY_force comprises at least one first alternation20, conventionally positive, which activates the assist motor7to the right.

Preferably, the force exploration cycle CY_force also comprises, thereafter, a second alternation120, with an opposite sign, and therefore negative, which activates the motor7in the opposite direction, herein conventionally to the left.

Preferably, the first alternation20, respectively the second alternation, comprises an ascending phase (in absolute value)22,122herein over a time frame [t2; t3], respectively [t6; t7], preferably in the form of a ramp, during which the torque setpoint (force setpoint) passes from a zero value to a peak value T7_1, T7_2, then possibly a plateau hold phase23,123during which said torque setpoint (force setpoint) is held at said peak value T7_1, T7_2for a predetermined duration, herein over the time frame [t3; t4], respectively [t7; t8], then a descending phase (in absolute value)24,124, preferably in the form of a ramp, herein over a time frame [t4; t5], respectively [t8; t9], during which the torque setpoint (force setpoint) is brought back to zero.

For convenience and safety of programming, the peak value T7_1is preferably expressed as a percentage of the acceptable maximum torque (acceptable maximum force) T7_max that the assist motor7can output.

In order to ensure a perceptible activation of the assist motor7, while avoiding a damage of said motor7, the peak value T7_1is strictly comprised between 0% and 100% of the acceptable maximum torque (acceptable maximum force) T7_max, and preferably comprised between 30% and 90%, or more preferably between 50% and 80% of said acceptable maximum torque.

Preferably, we choose T7_2=−T7_1, so as to apply alternations20,120with a substantially symmetrical amplitude, to the right and to the left.

It is also possible to provide for one or several rest phase(s)21[t1; t2],121[t5; t6],25[t9; t10], during which a substantially zero torque setpoint is held, which may serve for example to calibrate the sensors during the cycle.

Moreover, the respective durations of the different phases of the cycle, and in particular of the plateau hold phases23,123, will be selected to be long enough so as to sufficiently stabilize the power steering system1, and more particularly the steering mechanism3and the assist motor7, in a stable regime, preferably permanent, and thus accurately measure the desired indicator parameter(s), such as a the effective position P7_mes of the assist motor7, the effective position P4_mes of the rack (or that of the steering wheel P2_mes), as well as the effective assist torque (motor torque) T7_mes or an effective force (typically an axial tensile or compressive force) T4_mes exerted on the rack4. For example, it is possible to choose to this end hold times that are equal to or longer than the 95% response time to a step-type setpoint.

During the application of the force exploration cycle(s) CY_force, it is possible to block a movable member4of the steering mechanism, for example it is possible to block the rack4, against the assist motor7.

To this end, it is possible to use a blocking wedge (or any similar locking device), which immobilizes one end of the rack4, or possibly which immobilizes the steering tie rod6or the wheel5, with respect to a fixed frame on which the steering casing is also fastened. Thus, the rack4will be immobilized with respect to said steering casing.

Advantageously, it is then preferably possible to measure, during the measurement step (b), at least one force indicator parameter T7_mes, T4_mes, representative of the forces subjected to the blocked movable member4, as well as at least one displacement indicator parameter P7_mes, P4_mes, representative of the relative displacement P7_mes-P4_mes performed by the assist motor7against said blocked movable member4, in order to quantify, during the analysis step (c), an elastic stiffness property, also called «flexibility» property, of the corresponding portion of the steering mechanism3.

Thus, the stiffness K of a portion of the steering mechanism3may be assessed by applying a force exploration cycle CY_force.

More particularly, it is possible to measure, in the same reference frame, for example the reference frame of the assist motor7, the (angular) position P7_mes reached by the shaft of said motor7under the assist torque setpoint T7(also called «motor torque» setpoint), and that more particularly when the peak value: T7=T7_1is applied, with regards to the position P4of the blocked rack4, which is almost invariant because of the blocking ensured by the wedge.

The motor torque T7will correspond, while taking into account a possible reduction ratio, to the force exerted by the motor7on the rack4, that is to say to the force T4subjected to the rack4, and which is compensated, when the rack4is in a static equilibrium, by the retaining force exerted by the blocking wedge against said rack.

For example, said motor torque T7may be assessed by means of a torque sensor integrated to the assist motor7which measures an effective motor torque T7_mes, or else by knowing the magnitude of the power supply current that crosses the assist motor7.

The force T4subjected to the rack, such as said force results from the stresses exerted on said rack4, causes, by elastic deformation, a differential displacement P7-P4between the shaft of the motor7and the rack4.

Of course, this force T4may be determined by any other equivalent means, for example by means of a strain gauge that would be glued on the rack.

The differential displacement ΔP=p7−P4being due to the intrinsic elasticity of the components and of the mechanical linkages that connect the assist motor7to the rack4, it is therefore possible to assess the stiffness K of this mechanism portion as being equal, at a given time, to the ratio of the motor torque T7, T7_1to the differential displacement P7−P4, as:

Of course, it is alternatively possible to block the entire movable member4,2driven by the assist motor7, other than the rack, so as to allow studying the elasticity of the corresponding portion, comprised between said motor7and said blocked member4,2.

Thus, according to another variant of implementation based on a similar principle, it is possible, if the steering system1comprises a steering wheel2mechanically connected to the steering mechanism3through a steering column10, and therefore prone to be driven in rotation by the assist motor7, to block the steering wheel2while the force exploration cycle CY_force is applied to the assist motor7.

The relative movement ΔP=P7−P2of the shaft of the assist motor7relative to the blocked steering wheel2is then essentially due to the elasticity of the torque sensor9placed on the steering column10, and more particularly the elasticity of a torsion bar integrated to said torque sensor9, whose stiffness K may herein be determined from the expression:

It should be noted that it is possible, according to a variant of application of the force exploration cycle CY_force, to carry out a thermal test of the assist motor7by using a force exploration cycle CY_force (in particular as described hereinabove with reference toFIG. 2), or by using a succession of several force exploration cycles CY_force, in particular repeated a predefined number of iterations Ni.

To this end, during said force exploration cycle(s), it is possible to measure, as an indicator parameter, the temperature of the assist motor7.

For example, this measurement may aim at determining the reached maximum temperature as a function of the applied peak torque T7_1and/or as a function of the duration of application of said force.

In particular, it is possible for example to choose to apply one single alternation20, comprising a long plateau phase23, during which the force setpoint T7is persistently held at a constant torque value T7_1, which may for example get close to the acceptable maximum torque T7_max, and for example represent up to 80%, 90%, 95%, or 100% of said acceptable maximum torque T7_max, for a duration equal to or longer than 15 seconds, and for example comprised between 15 s and 300 s, or beyond, in order to activate the assist motor7according to an uninterrupted permanent regime.

Alternatively, it is possible for example to apply a series of elementary force exploration cycles CY_force each comprising either one single alternation, or two opposite alternations20,120, preferably with equal values and plateau durations, and by defining, or varying over several tests, the ratio, or «duty cycle», between the activation duration (and more particularly the cumulated duration of the plateau hold phases23,123) and the cumulated duration of the cycles (including the activation phases and the rest phases21,121,25).

Preferably, in any case, whether applying one or several alternation(s), and/or repeating or not the force exploration cycle CY_force, it is possible to block a movable member4, and in particular the rack4, in order to be sure that the assist motor7reaches the peak torque T7_1, or its maximum torque T7_max, which typically corresponds to its short-circuit current, rapidly and with a small amplitude of displacement.

Moreover, the characterization method may also include, during the activation step (a), a safeguarding substep (a1), during which the motor torque setpoint T7applied to the assist motor7is clipped in order to keep said torque setpoint below (in absolute value) a predetermined safety threshold T7_safe, said safety threshold T7_safe being adjusted, and more particularly reduced, when approaching a limit position Xlim that should not be exceeded, and for example when approaching an end-of-stroke stop S1, S2.

To this end, a function, called «safeguarding function», is used which defines, as illustrated inFIG. 3, in a reference frame associating a steering wheel torque T7(in ordinate) to a value representative of the position P7, P4, P2of the steering mechanism, and more preferably representative of the position P4of the rack4, on the one hand an authorized domain D1(blank inFIG. 3) and, on the other hand, a prohibited domain D2(hatched inFIG. 3), whose boundary corresponds to the safety threshold T7_safe.

It should be noted that, in each considered direction of displacement (to the right, respectively to the left), the safety threshold T7_safe is lowered (that is to say its absolute value decreases), from a safety position Xsafe that precedes the limit position Xlim in the considered direction of displacement, and preferably until becoming zero when said limit position Xlim is reached.

To this end, the safeguarding function may form a ramp decreasing from the safety position Xsafe down to the limit position Xlim.

Thus, it is possible to force a progressive slow-down of the steering mechanism3to avoid exceeding the limit position Xlim, and more particularly hitting against the stop S1(when the used exploration cycle does not aim at determining the position of said stop, of course), when getting close to said limit position Xlim.

However, since it is not necessary to brake the mechanism3when getting away from the limit position Xlim, the safety threshold T7_safe may directly return back to its maximum value (plateau value), as illustrated by the rectangular corner like shaped boundary of the authorized domain D1inFIG. 3.

Preferably, the limit position Xlim is defined as a percentage, for example comprised between 75% and 100%, and more particularly between 80% and 95% of the position of the corresponding end-of-stroke stop S1, S2.

Of course, the invention also concerns as such a power steering system1allowing implementing all or part of the aforementioned characterization methods.

Thus, the invention concerns more particularly a power steering system1which comprises a characterization module13forming a complete characterization «toolbox», containing and allowing implementing an exploration cycle selectively among a plurality of available exploration cycles, and that in particular in order to facilitate the automatic calibration and fine-tuning of the system1in factory.

Thus, the invention concerns a power steering system1intended to equip a vehicle and comprising at least one heading definition device2, such as a steering wheel, which enables a driver to define a steering angle A1of the power steering system, a steering mechanism3provided with at least one movable member4, such as a rack, whose position P4adapts so as to correspond to the selected steering angle A1, as well as at least one assist motor7arranged so as to be able to drive said steering mechanism3, said power steering system1including on the one hand a first onboard module8, called «assist module»8, which contains a first set of functions called «assist laws», which allow generating, when the power steering system1is dedicated to driving of a vehicle, piloting setpoints towards the assist motor7, in order to make said vehicle follow a path that is determined according to the situation of said vehicle with respect to its environment, and on the other hand a second onboard module13, called «characterization module»13, which contains a second set of functions, called «characterization functions», distinct from the assist laws, and which allow implementing, during a period where the power steering system is not dedicated to driving of a vehicle, and automatically, a characterization method intended to empirically determine at least one property of said power steering system, called «pursued property».

Like the assist module8, the characterization module13preferably consists of an electronic or computer module.

As indicated hereinabove, said characterization method comprises a step (a) of automatically activating the assist motor7, during which the second onboard module13automatically generates and applies to the assist motor7, without requiring any external action on the heading definition device2, an activation setpoint T7, V7, P7which follows one or several pre-established cycle(s) called «exploration cycles» CY, in order to enable a measurement step (b), according to which is measured, during the exploration cycle(s) CY or on completion of said exploration cycle(s) CY, at least one physical parameter, called «indicator parameter» P7_mes, T7_mes, P4_mes, T2_mes, V2_mes, etc., which is specific to the response supplied by the power steering system1to the automatic activation of the assist motor7and which is characteristic of the pursued property, then an analysis step (c), during which the pursued property is quantified from the measurement(s) of the indicator parameter.

Hence, the characterization module13, as well as the assist module8, will preferably be integrated to the steering system1, and in particular integrated to an onboard calculation module which may be used in a standalone manner.

The characterization functions, and more particularly the exploration cycles CY that these characterization functions automatically implement, may advantageously be stored in a non-volatile memory of the characterization module13, for example in the form of libraries of functions (dll files) programmed in said characterization module13and/or mappings («maps»).

Thus, the characterization module13will contain a plurality of pre-established exploration cycles CY, for example so as to allow selectively activating, besides the vehicle piloting phase, a cycle CY selected from the exploration cycles described in the foregoing.

Preferably, the second onboard module (characterization module)13includes a stiffness characterization function which uses a force exploration cycle CY_force which applies a non-zero torque setpoint T7to the assist motor7, whereas a movable member4,2of the steering is blocked against the assist motor, and which measures the displacement performed by said assist motor7against said blocked movable member4,2, in order to determine a stiffness K characteristic of the elasticity of a corresponding portion of the steering mechanism3.

Preferably, the characterization module13will also comprise a selector allowing selecting and executing either one of said available characterization functions, separately from the other characterization functions and assist functions, and thus control automatically, and in a standalone manner, the assist motor7for characterization, independently of the piloting of the vehicle.

Of course, the invention is not limited to the sole variants described in the foregoing, those skilled in the art being in particular able to freely isolate or combine together the aforementioned features, or substitute them with equivalents.