Method for automatic calibration of a camshaft sensor in order to correct a reluctor runout

A method for automatic calibration of a camshaft sensor for a motor vehicle engine. The sensor includes a processing module configured to generate, from a raw signal indicative of the variations in a magnetic field which are caused by a rotation of a target and measured by a primary cell, an output signal indicative of the moments at which teeth of the target pass past the primary cell. The sensor further includes two secondary measurement cells. The calibration method therefore makes it possible to determine two different switching thresholds for each tooth from a differential signal indicative of a difference in magnetic field measurement by the secondary cells. Also disclosed are a camshaft sensor implementing such a method, and a motor vehicle including such a sensor.

This application is the U.S. National Phase Application of PCT International Application No. PCT/FR2019/050725, filed Mar. 28, 2019, which claims priority to French Patent Application No. 1852946, filed Apr. 5, 2018, the contents of such applications being incorporated by reference herein.

FIELD OF THE INVENTION

The present invention relates to the field of sensors for motor vehicles. In particular, the invention relates to a method for automatically calibrating a motor vehicle camshaft sensor.

BACKGROUND OF THE INVENTION

A camshaft sensor is, for example, used in a motor vehicle to determine which stroke of the combustion cycle is taking place in a cylinder of the engine (admission stroke, compression stroke, combustion stroke, or exhaust stroke). Such information for example allows the computer to determine at what moment and into what cylinder fuel is to be injected.

Such a camshaft sensor generally comprises a target (for example a metal disk, the periphery of which is toothed), a magnetic-field generator (for example a permanent magnet), a magnetic-field measurement cell (for example a Hall-effect cell or a magneto-resistive cell) and an electronic signal-processing module.

The teeth of the target are generally all of the same height, but may have spacings (gaps) and lengths that are not all identical so as to code the angular position of the target.

Thus, the rotation of the target and the passing of the various teeth past the magnetic-field generator will generate variations in the magnetic field measured by the measurement cell, which variations can be analyzed in order to recognize the various teeth of the target and to decode the angular position of the target and, ultimately, the angular position of the camshaft rigidly connected to the target.

The measurement cell supplies the processing module with a raw signal indicative of the intensity of the magnetic field measured. The processing module then generates, from this raw signal, an output signal indicative of the moment at which the various teeth of the target pass past the measurement cell.

This output signal is, for example, an electrical signal comprising a succession of square waveforms, each square form corresponding to the passing of a tooth past the measurement cell. Each square form comprises a rising front and a falling front corresponding more or less to the passing of the mechanical fronts of the tooth past the measurement cell.

In general, each rising and falling front of the output signal (namely each transition of the electrical signal) is determined from a switching threshold that is predefined for the raw signal. In other words, the output signal exhibits a rising front when the raw signal passes above the switching threshold, and the output signal exhibits a falling front when the raw signal passes below the switching threshold. Conventionally, a switching threshold corresponding to approximately 75% of the amplitude of the raw signal is used (what is meant by the “amplitude of the raw signal” is the difference between a maximum value and a minimum value observed for said raw signal).

It is possible, for example, to define a fixed switching threshold which does not change value during operation of the sensor. However, such a solution is particularly imprecise insofar as the minimum and maximum values of the raw signal may change significantly during the operation of the sensor, notably as a function of the temperature.

It is therefore known practice in the prior art to update the value of the switching threshold for each new revolution of the target, as a function of the minimum and maximum values of the raw signal which are observed during said revolution of the target. The updated value of the switching threshold is then used for the next revolution of the target. Such a solution improves the precision of the sensor.

However, the precision of the sensor is generally also impacted by deficiencies in the geometry of the target (for example if not all of the teeth have exactly the same height). The consequence of such deficiencies is that the magnitude of the gap between the measurement cell and a tooth of the target is not the same for each tooth. The raw signal then adopts different maximum and minimum values for each tooth, and a switching threshold that is defined as optimal for one of the teeth may prove to be entirely inappropriate for another tooth.

It is therefore known practice in the prior art to determine a different switching threshold for each tooth of the target. Each switching threshold for each tooth of the target may be updated for each new revolution of the target in order to be used for the next revolution of the target. Such a solution improves the precision of the sensor still further.

Nevertheless, the various solutions of the prior art are not always able to achieve the precision required by certain motor manufacturers for a camshaft sensor.

SUMMARY OF THE INVENTION

It is an aim of the present invention to overcome all or some of the disadvantages of the prior art, notably those set out hereinabove, by proposing a method for the automatic calibration of the camshaft sensor using two different switching thresholds for each tooth of the target rather than a single switching threshold per tooth.

To this end, and according to a first aspect, the present invention proposes a method for the automatic calibration of the camshaft sensor for a motor vehicle engine. The sensor comprises:a toothed target,a primary measurement cell configured to supply a raw signal indicative of the variations in a magnetic field which are induced by a rotation of the target, anda processing module configured to supply, from this raw signal, an output signal indicative of the moments at which the teeth of the target pass past the primary cell.

The calibration method comprises steps of:determining a local minimum of the raw signal as a space separating two teeth of the target passes past the primary cell,determining a first local maximum of the raw signal in the vicinity of a falling front of said raw signal corresponding to the end of the passage of a tooth of the target past the primary cell,determining a first switching threshold for the generation of the output signal as a function of the values of the first local maximum and of the local minimum.

The calibration method is notable in that the value of said first local maximum is determined from a differential signal indicative of a difference in magnetic-field measurement by two secondary cells. Furthermore, the method comprises steps of:determining, from said differential signal, a second local maximum of the raw signal in the vicinity of a rising front of said raw signal corresponding to the start of the passage of said tooth of the target past the primary cell,determining a second switching threshold as a function of the values of the second local maximum and of the local minimum,generating said output signal from the raw signal, from the first switching threshold and from the second switching threshold.

Thus, for each passage of a tooth of the target past the primary cell, two distinct switching thresholds are determined. Such measures notably make it possible to determine with greater precision the moments of a rising front and of a falling front of the output signal corresponding respectively to the moments marking the beginning and end of the passage of the mechanical fronts of said tooth as the tooth passes past the primary cell.

Advantageously, the two switching thresholds for one tooth may be determined as a function of the observations made in respect of said tooth in a previous revolution, so that they are adapted to suit the specific characteristics of said tooth (a potential geometric deficiency and the effects of target runout).

In particular modes of implementation, an aspect of the invention may furthermore include one or more of the following features, taken alone or in any technically feasible combination.

In particular implementations, the first local maximum corresponds to a value adopted by the raw signal when the differential signal adopts a first predetermined value, and the second local maximum corresponds to a value adopted by the raw signal when the differential signal adopts a second predetermined value, as said tooth of the target passes past the secondary cells.

In particular implementations:the first local maximum corresponds to a value adopted by the raw signal in the vicinity of said falling front when the differential signal has a negative gradient and adopts a first predetermined value Dfedefined by:
Dfe=Dm−(Dm−Dmin)×Kfethe second local maximum corresponds to a value adopted by the raw signal in the vicinity of said rising front when the differential signal has a negative gradient and adopts a second predetermined value Dredefined by:
Dre=Dm+(Dmax−Dm)×Kre

in which:

Dmaxand Dmincorrespond respectively to a maximum value and to a minimum value of the differential signal as the teeth of the target pass past the secondary cells,

Dmis a value defined by:

Kreand Kfeare two factors comprised between 0 and 1.

In particular implementations, said local minimum corresponds to a value adopted by the raw signal when the differential signal adopts a predetermined value as a space separating two teeth of the target passes past the secondary cells.

In particular implementations, the local minimum corresponds to the value adopted by the raw signal when the differential signal adopts a value Dmdefined by:

where Dmaxand Dminare respectively a maximum value and a minimum value for the differential signal as the teeth of the target14pass past the secondary cells.

In particular implementations, the secondary cells are positioned one on either side of the primary cell, at equal distances from the primary cell, and at a distance from the center of the toothed target that is the same as that of the primary cell.

According to a second aspect, the invention relates to a camshaft sensor for a motor vehicle engine, comprising:a toothed target,a primary measurement cell configured to supply a raw signal indicative of the variations in a magnetic field which are induced by the rotation of the target, anda processing module configured to supply, from said raw signal, an output signal indicative of the moments at which the teeth of the target pass past the primary cell.

Furthermore, the sensor comprises two secondary measurement cells and the processing module is configured to:generate a differential signal indicative of a difference in magnetic field measurement by said two secondary cells,determine, from said differential signal, a first local maximum of the raw signal in the vicinity of a falling front of said raw signal corresponding to the end of the passage of a tooth of the target past the primary cell,determine, from said differential signal, a second local maximum of the raw signal in the vicinity of a rising front of said raw signal corresponding to the start of the passage of said tooth of the target past the primary cell,determine a local minimum of the raw signal as a space separating two teeth of the target passes past the primary cell,determine a first switching threshold as a function of the values of the first local maximum and of the local minimum,determine a second switching threshold as a function of the values of the second local maximum and of the local minimum,generate said output signal from the raw signal, from the first switching threshold and from the second switching threshold.

In some particular embodiments, the invention may furthermore comprise one or more of the following features, taken alone or in any technically feasible combination.

In particular embodiments, the first local maximum corresponds to a value adopted by the raw signal when the differential signal adopts a first predetermined value, and the second local maximum corresponds to a value adopted by the raw signal when the differential signal adopts a second predetermined value, as said tooth of the target passes past the secondary cells.

In particular embodiments:the first local maximum corresponds to a value adopted by the raw signal in the vicinity of said falling front when the differential signal has a negative gradient and adopts a first predetermined value Dfedefined by:
Dfe=Dm−(Dm−Dmin)×Kfethe second local maximum corresponds to a value adopted by the raw signal in the vicinity of said rising front when the differential signal has a negative gradient and adopts a second predetermined value Dredefined by:
Dre=Dm+(Dmax−Dm)×Kre

in which:

Dmaxand Dmincorrespond respectively to a maximum value and to a minimum value of the differential signal as the teeth of the target pass past the secondary cells,

Dmis a value defined by:

Kreand Kfeare two factors comprised between 0 and 1.

According to a third aspect, the present invention relates to a motor vehicle comprising a camshaft sensor according to any one of the above embodiments.

In these figures, references that are identical from one figure to the next denote identical or analogous elements. For the sake of clarity, the elements that are shown are not necessarily to the same scale, unless stated otherwise.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

As indicated previously, an aspect of the present invention seeks to improve the precision of a motor vehicle engine camshaft sensor.

FIG. 1schematically depicts one example of a conventional camshaft sensor10. This sensor10comprises a target14, a magnetic-field generator11, a primary measurement cell12, and an electronic signal-processing module13.

In one example considered and described entirely nonlimitingly, the target14consists of a metal disk the periphery of which is toothed, the magnetic-field generator11is a permanent magnet, and the primary cell12for measuring the magnetic field is a Hall-effect cell. As illustrated inFIG. 1, the primary measurement cell12is positioned at the level of the magnetic-field generator11.

It should be noted that, according to another example, the magnetic field measured by the measurement cell may be formed by the target itself, which, as appropriate, is made of a magnetic material. In such an instance, the target is “magnetically” toothed, which means to say that the geometry of the periphery of the target exhibits an alternation of North poles (equivalent to the teeth in the example ofFIG. 1) and south poles (equivalent to the spaces in the example ofFIG. 1).

The target14is fixed to a camshaft spindle in such a way that the disk of the target14and the camshaft spindle are coaxial. In other words, in an ideal situation, namely in the absence of any lack of precision in the mounting of the target14on the camshaft, the axis of the camshaft spindle and the axis of the target14coincide, and both pass through the center15of the target14.

The teeth D1, D2, D3of the target14have respective lengths l1, l2and l3, and are separated from one another by spaces S1, S2, S3of respective lengths s1, s2and s3. In order to code for the angular position of the target, the lengths l1, l2, l3, s1, s2, s3of the teeth D1, D2, D3and of the spaces S1, S2, S3are not all identical. The teeth D1, D2, D3generally all have the same height, but deficiencies in the manufacturing of the target14may nevertheless cause slightly different values to be observed for the respective heights h1, h2, h3of the teeth D1, D2, D3.

It should be noted that, in the example considered, the target14comprises three teeth D1, D2, D3, but an aspect of the invention also applies to sensors10of which the target14comprises a different number of teeth. In particular, an aspect of the invention is applicable to a target14comprising at least one tooth.

The rotation R of the target14and the successive passage of the various teeth D1, D2, D3past the magnetic-field generator11lead to variations in the magnetic field measured by the primary cell12. In effect, the magnetic field varies as a function of the magnitude of the gape separating the magnetic-field generator11and the target14.

The primary measurement cell12supplies the processing module13with a raw signal indicative of the intensity of the magnetic field measured. The processing module13is, for example, configured to generate, from this raw signal, an output signal indicative of the moments at which the various teeth D1, D2, D3of the target14pass past the primary measurement cell12. The output signal may then make it possible to recognise the moments at which the various teeth D1, D2, D3of the target14pass past the primary measurement cell12and, ultimately, the angular position of the camshaft secured to the target.

In order to do that, the processing module13comprises for example one or more processors and storage means (electronic memory) in which a computer program product is stored, in the form of a set of program code instructions to be executed in order to implement the various steps needed for generating said output signal from the raw signal. Alternatively or in addition, the processing module13comprises programmable logic circuits of FPGA, PLD, etc. type, and/or one or more specialized integrated circuits (ASIC), and/or discrete electronic components, etc., suitable for implementing these steps. In other words, the processing module13comprises means configured by software and/or by hardware to implement the operations necessary for generating said output signal from the raw signal.

FIG. 2Aschematically depicts a portion of a raw signal20, indicative of the variations in the magnetic field measured by the primary cell12. The intensity B of the magnetic field is represented on the ordinate axis while the time t is represented on the abscissa axis.

The portion of the raw signal20depicted inFIG. 2Acorresponds for example to a passage of the tooth D1of the target14past the primary measurement cell12. The raw signal20thus exhibits a main square form with a rising front21corresponding to the start of the passage of the tooth D1past the primary cell12, and a falling front22corresponding to the end of the passage of the tooth D1past the primary cell12. The rising front corresponds to a sharp increase in the magnetic field caused by the sharp decrease in the magnitude of the gap e as the tooth D1begins to pass past the primary cell12(the transition from a space S3to a tooth D1). The falling front corresponds to a sharp decrease in the magnetic field caused by the sharp increase in the magnitude of the gap e as the tooth D1completes its passage past the primary cell12(the transition from a tooth D1to a space S1). Between the rising front21and the falling front22, the signal20adopts a value that is more or less constant assuming that the magnitude of the gap remains substantially identical throughout the time taken for the tooth D1to pass past the primary cell12. Such an assumption assumes a height h1that is more or less constant over the entire length l1of the tooth D1, and the imbalance of any runout presented by the target14.

FIG. 2Bschematically depicts a portion of an output signal30generated by the processing module13from the raw signal20.

This output signal30is for example an electrical signal adopting a positive value (for example 5V) when a tooth D1, D2, D3is facing the primary cell12, and a zero value (0V) when a space S1, S2, S3is facing the primary cell12. The electrical voltage V of the output signal30is represented on the ordinate axis and the time t is represented on the abscissa axis.

The output signal30thus comprises a succession of square waveforms, each square form corresponding to the passage of a tooth D1, D2, D3of the target14past the primary measurement cell12. Each square form comprises a rising front31and a falling front32corresponding more or less to the passing of the mechanical fronts of a tooth D1, D2, D3past the measurement cell. The portion of the raw signal30depicted inFIG. 2Bcorresponds for example to a passage of the tooth D1past the primary cell12.

In general, each rising front31and falling front32of the output signal30(namely each transition of the electrical signal) is determined from a switching threshold S that is predefined for the raw signal20. In other words, the output signal30exhibits a rising front31when the raw signal20passes above the switching threshold S, and the output signal30exhibits a falling front32when the raw signal20passes below the switching threshold S.

Conventionally, the switching threshold S is calculated for example on the basis of a percentage of an amplitude A of the raw signal20equal to the difference between a local maximum Hcand a local minimum Lcwhich are observed for said raw signal20. The switching threshold S conventionally corresponds to a value chosen in a range comprised between 70% and 80% of the amplitude A, preferably around 75% of the amplitude A. In other words, for a factor K comprised between 0 and 1, generally comprised between 0.7 and 0.8 and preferably equalling 0.75, the switching threshold S is conventionally defined by:
S=Lc+A×K

It is known practice, for example, to determine, for each new revolution of the target14, for the tooth D1, a local minimum Lcof the raw signal20preceding a rising front21, and a local maximum Hcpreceding a falling front22, so as to update the value of a switching threshold S to be used to generate the output signal30during the next revolution of the target14.

It is also known practice, for example, to detect a local minimum Lc(or, respectively, a local maximum Hc) when the raw signal20varies by a value that is greater (in terms of absolute value) than a predefined constant C after its gradient has become positive (or, respectively, negative).

This can be repeated for each revolution of the target14and for each tooth D1, D2, D3of the target14so as to obtain, for the next revolution of the target14, a value for the switching threshold S to be used. This may be the one same switching threshold S to be used for all the teeth D1, D2, D3(the value of this threshold being calculated for example as a function of a mean, minimum or maximum value of the local minima Lcand/or of the local maxima Hcobserved for the teeth D1, D2, D3), or else it may be a switching threshold S that is different for each tooth D1, D2, D3(the value of this threshold being calculated for example as a function of the local maximum Hcand of the local minimum Lcwhich are observed for each tooth D1, D2, D3).

However, such a solution does not provide sufficient precision in the event of the target14exhibiting runout. What is meant by the “target exhibiting runout” is a deficiency associated with the fact that the axis of the target14does not coincide perfectly with the axis of the camshaft spindle to which the target14is attached. Such a deficiency may lead to greater or lesser variations in the raw signal20over a portion separating a rising front21from a falling front22.

FIG. 3Aschematically depicts the effect that significant runout has on the portion of the raw signal20corresponding to the passage of the tooth D1past the primary cell12. Because of the runout, the magnitude of the gap e between the tooth D1and the magnetic-field generator11can vary significantly as the tooth D1passes past the primary measurement cell12. The greater the length l1of the tooth D1, the greater this variation may be.

As illustrated inFIG. 3A, the local maximum Hcof the raw signal20detected in the vicinity of the falling front22is significantly greater than a local maximum of the raw signal20in the vicinity of the rising front21. The result of this is that the switching threshold S calculated as a function of the difference between the local maximum Hcand the local minimum Lcis appropriate for determining the falling front32of the output signal30, but by contrast not at all appropriate for determining the rising front31thereof.

FIG. 3Bdepicts the portion of the output signal30generated from the switching threshold S thus calculated. It is clearly apparent that the rising front31of the output signal30is triggered belatedly in comparison with the start of the passage of the tooth D1past the primary cell12. That then leads to imprecision of the sensor10in determining the moment at which the tooth D1passes past the primary measurement cell12and, ultimately, to imprecision in the estimate of the angular position of the camshaft attached to the target14.

It should be noted that the problem of target runout leading to variations in the raw signal20over a portion separating a rising front21from a falling front22may also be generated or amplified by the fact of a height h1, h2, h3of a tooth D1, D2, D3not being constant over the length l1, l2, l3of said tooth D1, D2, D3. Such a phenomenon may be brought about by deficiencies in the manufacturing of the target14.

As illustrated inFIG. 3A, target runout leads to a modulation of the raw signal20, leading to asymmetry in the portion of the raw signal20that corresponds to the passage of a tooth D1, D2, D3past the primary cell12. This modulation is generally periodic and follows the frequency of rotation of the target14. In other words, the asymmetry illustrated inFIG. 3Ain respect of the tooth D1, repeats for each revolution of the target14.

The remainder of the description concerns itself with describing a camshaft sensor and a method for calibrating said sensor to make it possible to correct the aforementioned problem of target runout.

FIG. 4schematically depicts such a camshaft sensor10′. In addition to the elements already described with reference toFIG. 1, the sensor10′ depicted inFIG. 4also comprises two secondary magnetic-field measurement cells12a,12b.

In the example considered and as illustrated schematically inFIG. 4, the two secondary cells12a,12bare arranged one on either side of a primary cell12′, at equal distances from the primary cell12′ and at a distance from the center15of the toothed target14that is equal to the distance separating the primary cell12′ from the center15of the target14.

The processing module13′ is configured to generate, from the magnetic-field measurements taken by the secondary cells12a,12b, a differential signal indicative of a difference in the magnetic field measured by said secondary cells12a,12b.

FIG. 5Aschematically depicts a portion of a raw signal20indicative of the variations in the magnetic field measured by the primary cell12′ upon the passage of the tooth D1. This portion of the raw signal20is similar to that previously described with reference toFIG. 3A.

Furthermore, the corresponding portion of the differential signal40indicative of the difference in magnetic field measured by the secondary cells12a,12bis also depicted inFIG. 5A.

In the portion illustrated inFIG. 5A, the differential signal40reaches a local minimum41when the secondary cell12ais already facing the space S3separating the tooth D3from the tooth D1whereas the secondary cell12bis still facing the tooth D3(at this moment, the secondary cell12aactually measures a weak magnetic field because the magnitude of the gap ea separating the magnetic-field generator11′ and the target14at the level of the secondary cell12ais large, whereas conversely at the same moment the secondary cell12bmeasures a strong magnetic field because the magnitude of the gap ebseparating the magnetic-field generator11′ and the target14at the level of the secondary cell12bis small). The differential signal40reaches a local maximum42when the secondary cell12ais already facing the tooth D1(magnitude of gap ea small) whereas the secondary cell12bis still facing the space S3separating the tooth D3from the tooth D1(magnitude of gap eblarge). The differential signal40then reaches a new local minimum43when the secondary cell12ais already facing the space S1separating the tooth D1from the tooth D2whereas the secondary cell12bis still facing the tooth D1.

The differential signal40thus exhibits a succession of local minima and maxima as the various teeth D1, D2, D3of the target14pass past the secondary cells12a,12bduring a revolution of the target14.

It is therefore possible, from this differential signal40, to determine, for a given tooth D1, D2, D3of the target14, two distinct switching thresholds rather than one single switching threshold.

For example, and as illustrated inFIG. 5A, as the tooth D1passes past the various primary12′ and secondary12a,12bmeasurement cells, it is possible to determine, from the differential signal40, a first switching threshold Sfefor the falling front22of the raw signal20corresponding to the end of the passage of the tooth D1past the primary cell12′, and a second switching threshold Srefor the rising front21of the raw signal20, corresponding to the start of the passage of the tooth D1past the primary cell12′.

Determining a first switching threshold Sfefor the falling front22and a distinct second switching threshold Srefor the rising front21makes it possible to correct the problem generated by the target runout.

In one particular implementation, the processing module13′ is configured for example to determine, for the tooth D1during one revolution of the target14:a value Dmaxcorresponding to a mean value for the local maxima42observed for the differential signal40for the various teeth D1, D2, D3,a value Dmincorresponding to a mean value for the local minima41,43observed for the differential signal40for the various teeth D1, D3, D3,a value Dmdefined by:

where Kfeis a predetermined factor comprised between 0 and 1, and preferably comprised between 0.125 and 0.25,a second predetermined value Dredefined by:
Dre=Dm+(Dmax−Dm)×Kre

where Kreis a predetermined factor comprised between 0 and 1, and preferably comprised between 0.125 and 0.25,a local minimum L for the raw signal20upon the passage of a space S3a value of a first local maximum Hfe, as being the value adopted by the raw signal20in the vicinity of the falling front22when the differential signal40has a negative gradient and adopts the first predetermined value Dfe,a value of a second local maximum Hre, as being the value adopted by the raw signal20in the vicinity of the rising front21when the differential signal40has a negative gradient and adopts the second predetermined value Dre,a first amplitude Afecorresponding to the difference between the values of the first local maximum Hfeand of the local minimum L,a second amplitude Arecorresponding to the difference between the values of the second local maximum Hreand of the local minimum L,

the first switching threshold Sfedefined by:
Sfe=L+Afe×K

the second switching threshold Sredefined by:
S=L+Are×K

where K is a predefined factor comprised between 0 and 1, preferably between 0.7 and 0.8, and more preferably still, approximately equal to 0.75.

FIG. 5Aschematically depicts the values of the first local maximum Hfe, of the second local maximum Hre, of the first amplitude Afe, of the second amplitude Are, of the first switching threshold Sfe, and of the second switching threshold Srewhich are determined for a passage of the tooth D1past the primary cell12′. These operations can obviously be repeated for the other teeth D1, D2, D3of the target14so as to determine two different switching thresholds for each of them.

FIG. 5Billustrates how the first switching threshold Sfeand the second switching threshold Sreare used, in the next revolution of the target14, to generate the output signal30. As the tooth D1passes past the primary cell12′, the output signal30exhibits a rising front31(which means to say that the electrical signal switches from a low state at 0V to a high state at 5V) at the moment at which the raw signal20measured by the primary cell12′ passes above the second switching threshold Sre. The output signal30exhibits a falling front32(transition from the high state to the low state) at the moment at which the raw signal20measured by the primary cell12′ passes below the first switching threshold Sfe.

It can be seen fromFIG. 5Bthat the output signal30indicates with precision the instant at which the tooth D1passes past the primary cell12′, and does so in spite of the asymmetry of the corresponding portion of the raw signal20due to the target runout.

Once again, it should be noted that these operations can be repeated for the various teeth D1, D2, D3of the targets14using the switching thresholds determined for each of them.

It is appropriate to note that other methods can be employed for determining the values Dminand Dmax. For example, in order to determine Dmin, rather than using a mean value of the local minima41,43observed for the differential signal40for the various teeth D1, D2, D3during one revolution of the target14, it is conceivable to use a maximum value or a minimum value of said local minima41,43. The same goes for the determination of Dmax, which can be determined as being the maximum value or the minimum value of the local maxima42observed for the differential signal40for the various teeth D1, D2, D3during a revolution of the target14. Also, a particular choice of method for determining Dmin, Dmax, Dm, the first predetermined value Dfeor the second predetermined value Dremerely represents a variant of an aspect of the invention.

The values Dmin, Dmax, Dm, Kre, Kfe, the first predetermined value Dfeor the second predetermined value Dremay potentially be determined according to the positioning of the secondary cells12a,12bwith respect to the primary cell12′, notably if the secondary cells12a,12bare not situated at the same distance from the primary cell12′.

In the example considered, the values Dmin, Dmax, Dm, the first predetermined value Dfeor the second predetermined value Dreare updated for each new revolution of the target14. However, there is nothing to prevent the values, in other examples, from being predetermined and from not varying during operation of the sensor10′, or else from being updated less frequently, for example when the target14has reached a certain number of revolutions, or else when a predetermined period of time has elapsed. It is advantageous for these values to be updated regularly, because they can vary, as a function of the temperature for example, during the course of operation of the sensor.

The same goes for the values of the first local maximum Hfe, of the second local maximum Hre, of the local minimum L, of the first amplitude Afe, of the second amplitude Are, of the first switching threshold Sfe, and of the second switching threshold Sre: these can be determined for each tooth D1, D2, D3and updated for each new revolution of the target14, or else they can be updated less frequently.

The local minimum L may for example be detected, in a known way, similarly to that which has been described with reference toFIG. 3A, when the raw signal20varies by a value that is greater (in terms of absolute value) than a predefined constant C after its gradient has become positive in the vicinity of the rising front21. It is thus possible to define such a local minimum L for each tooth D1, D2, D3for each new revolution of the target14.

According to other examples, it is also conceivable to use the mean (or maximum or minimum) value of the collection of local minima L observed for the raw signal20for the various teeth D1, D2, D3during a revolution of the target14. Here again, this value can be updated for each revolution of the target14, or else less frequently.

In particular implementations, a local minimum L can be detected using the differential signal40. For example, and as illustrated inFIG. 5A, the value of the local minimum L of the raw signal20during the passage of the space S3separating the tooth D3and the tooth D1corresponds to the value adopted by the raw signal20at the moment at which the differential signal reaches the value Dm.

Thus, in the example illustrated inFIG. 5A:the local minimum L of the raw signal20is determined as being the value of the raw signal20at the moment at which the differential signal40adopts the value Dmas it passes from a local minimum41to a local maximum42,the first local maximum Hfeis determined as being the value of the raw signal20at the moment at which the differential signal40adopts the first predetermined value Dfeas it passes from a local maximum42to a local minimum43,the second local maximum Hreis determined as being the value of the raw signal20at the moment at which the differential signal40adopts the second predetermined value Dreas it passes from a local maximum42to a local minimum43.

It is appropriate to note that runout presented by the target14has less of an impact on those portions of the raw signal20that correspond to the passage of a space S1, S2, S3past the primary cell12′ than it does on those portions of the raw signal20that correspond to the passage of a tooth past the primary cell12′. In other words, the asymmetry caused by the target runout and observed on a portion of the raw signal20corresponding to the passage of a tooth past the primary cell12′ is not generally observed on a portion of the raw signal20corresponding to the passage of a space S1, S2, S3past the primary cell12′. According to the teaching of an aspect of the invention, it is still nevertheless conceivable to define, from the differential signal40, two distinct local minima Lreand Lfecorresponding, respectively, to a local minimum of the raw signal20in the vicinity of the rising front21and to a local minimum of the raw signal20in the vicinity of the falling front22. The first switching threshold Sfeand the second switching threshold Srecan then for example be determined thus:
Sre=Lre+(Hre−Lre)×K
Sfe=Lfe+(Hfe−Lfe)×K

The description above clearly illustrates that, through its various features and the advantages thereof, the an aspect of present invention achieves the set aims. In particular, the calibration method according to an aspect of the invention makes it possible to determine with greater precision in the moments of a rising front31and of a falling front32of the output signal30corresponding respectively to the moments marking the beginning and end of the passage of the mechanical fronts of a tooth D1, D2, D3as said tooth D1, D2, D3passes past the primary cell12′. The first switching threshold Sfeand the second switching threshold Srefor one tooth D1, D2, D3are advantageously determined as a function of the observations made in respect of said tooth in a previous revolution, so that they are adapted to suit the specific characteristics of said tooth (potential deficiencies of geometry and the effects of target runout).