Method for automatic calibration of a camshaft sensor for a motor vehicle

Abstract

A method for automatic calibration of a camshaft sensor for a motor vehicle, allowing reduction of the fluctuations on the output signal of the sensor. The method proposes comparing, on each target rotation, the new maximum values of the magnetic field of each tooth to the maximum values of the same teeth from the preceding target rotation. The switching thresholds are only calculated with the new maximum values if these differ from the maximum values of the preceding target rotation. Moreover, the invention proposes using a single minimum value of the magnetic field, i.e. the absolute minimum value on a target rotation in order to calculate the switching thresholds.

Claims

1. A method for automatically calibrating a motor vehicle engine camshaft sensor, said engine comprising at least one camshaft, a toothed coded target associated with this camshaft and a magnetic field sensor placed near the target to detect magnetic field variations induced by the passage of the teeth of the target near the sensor, said sensor delivering an electrical signal indicative of teeth and troughs of the target according to a predetermined switching threshold dependent on the amplitude of the magnetic field, said method continuously measuring the value of the magnetic field, said method comprising: during a first revolution of the target: Step 1: measuring a maximum value and a minimum value for the magnetic field (B) for each tooth, Step 2: calculating an amplitude of the magnetic field for said teeth, and calculating a switching threshold for each tooth according to the amplitude thus calculated, Step 3: measuring an absolute minimum value for the magnetic field over a full revolution of the target, Step 4: storing the maximum values and the absolute minimum value in a memory, then, for each further revolution of the target: Step 5: measuring and storing a new maximum value of the magnetic field for each tooth, and a new absolute minimum value over the full revolution of the target, Step 6: comparing the absolute value of the difference between the new maximum value and the maximum value stored in the memory for the previous revolution of the target which are associated with that same said tooth and the absolute value of the difference between the maximum value of that same said tooth, and the absolute minimum value for the previous revolution of the target, if the absolute value of the difference between the new maximum value and the maximum value from the previous revolution of the target is higher than a percentage of the absolute value of the difference between the maximum value and the absolute minimum value for the previous revolution of the target, namely, if
|BmaxiBmaxi|>K|BmaxiBmin| where: K is a factor comprised between 0.003 and 0.1 (namely between 0.3% and 10%), Bmaxi is the maximum value of the magnetic field stored in the memory during the previous revolution of the target for a given tooth, Bmaxi is the new maximum value of the magnetic field for a further revolution of the target for that same given tooth, Bmin is the absolute minimum value of the magnetic field stored in the memory during the previous revolution of the target, then: Step 7: calculating the switching threshold with the new maximum value and with the absolute minimum value stored in the memory from the previous revolution, and replacing the maximum values stored in the memory from the previous revolution of the target with the new maximum values, else: Step 8: calculating the switching threshold with the maximum value and with the absolute minimum value both stored in the memory from the previous revolution of the target, Step 9: repeating steps 5 to 8 for each new revolution of the target.

2. The automatic calibration method as claimed in claim 1, wherein the first revolution of the target is performed each time power is applied to the camshaft sensor.

3. A camshaft sensor calibrated by the method according to claim 1.

4. A motor vehicle comprising the camshaft sensor according to claim 3.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Other features and advantages of the present invention will become more apparent from reading the following description, provided by way of non-limiting example and with reference to the appended drawings, in which:

(2) FIG. 1 is a schematic view in cross section, depicting a camshaft sensor and its associated target,

(3) FIG. 2 illustrates an example of curves of the variation in magnetic field perceived by a sensor associated with a target during the first revolution of the target,

(4) FIG. 3 illustrates an example of curves of the variation in magnetic field perceived by a sensor associated with the target during a revolution after the first revolution of the target.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

(5) According to the embodiment described and depicted in FIGS. 1 to 3, a camshaft sensor 10 comprises a ferromagnetic element 11 and a magnetic field detection means 12 (for example a Hall-effect cell). This sensor 10 delivers a digital signal to a central processor 13.

(6) A target 14 associated with this sensor 10 takes the form of a metal disk 15 firmly attached to a camshaft 16. This target bears, on its periphery, a plurality teeth D.sub.1, D.sub.2, D.sub.3 (3 in the example depicted) of different heights h1, h2, h3 and of variable lengths l.sub.1 to l.sub.3 and variable spacings (troughs) s.sub.1 to s.sub.3. These variable lengths and variable spacings in the way known per se constitute a coding.

(7) The way in which a sensor 10 plus target 14 assembly works is described hereinafter.

(8) When the target 14 is rotationally driven (arrow F FIG. 1) by the camshaft 16, sensor 10 perceives a series of variations in the magnetic field B indicative of the length l of the teeth D.sub.1, D.sub.2, D.sub.3 moving past it and of their spacings s.sub.1, s.sub.2, s.sub.3. The curve thus obtained, for example during the first revolution of the target, is depicted in FIG. 2.

(9) In this FIG. 2, the abscissa axis indicates the angles of the engine cycle varying from 0 to 360 and the ordinate axis indicates the value B of the magnetic field perceived (field normalized as a function of air gap). As indicated in FIG. 2, the teeth D.sub.1, D.sub.2, D.sub.3 are not of the same height h1, h2, h3 and the target 14 exhibits a small defect in its geometry. Because of this, the maximum field perceived by the sensor 10 as each of the teeth D.sub.1, D.sub.2, D.sub.3 passes by varies for each of the three teeth and adopts the respective values Bmax1, Bmax2, Bmax3. Likewise, the minimum field perceived by the sensor 10 as each of the teeth D.sub.1, D.sub.2, D.sub.3 passes by varies from tooth to tooth and adopts the respective values Bmin1, Bmin2, Bmin3. This FIG. 2 shows the passage of three teeth D.sub.1, D.sub.2, D.sub.3, the first two (D.sub.1, D.sub.2) being relatively closely spaced, the first tooth D.sub.1 being wider than the second tooth D.sub.2 and the passage of a third tooth D.sub.3 which is narrower and more distant from the second tooth D.sub.2. This in effect corresponds to the geometry of the target 14 depicted in FIG. 1.

(10) It is known practice to detect the passage of a tooth front as soon as the magnetic field B perceived rises above or drops below a predetermined switching threshold proportional to the amplitude of the field perceived during the passage of a tooth (75% of (Bmax1Bmin1) for example).

(11) The threshold values are embodied in FIG. 2 as dotted lines. After the first tooth D.sub.1 has passed by, a switching threshold for the rising front of the second tooth S2a is calculated using the following mathematical formula:
S2a=0.75*(Bmax1Bmin1)

(12) Then, after the maximum value of the magnetic field B upon passage of the second tooth Bmax2 has been crossed, a new switching threshold S2d is calculated for the falling front of the second tooth D.sub.2:
S2d=0.75*(Bmax2Bmin1)

(13) This process is repeated, for the passage of each tooth, when a new maximum value or new minimum value of the magnetic field B has been measured.

(14) It should be noted that the maximum value and the minimum value of the magnetic field B for each tooth correspond to the last recorded maximum and minimum values.

(15) For example, in order to calculate the switching threshold for the rising front of the second tooth S2a, it is a matter of considering the last recorded maximum values and minimum value of the magnetic field B, that is to say Bmax1 and Bmin1, namely the maximum value and the minimum value of the magnetic field B after the passing of the first tooth D.sub.1.

(16) Similarly, in order to calculate the switching threshold for the falling front of the second tooth S2d, use is made of the last recorded maximum and minimum values, in this instance Bmin1 and Bmax2, namely the minimum value of the magnetic field B associated with the first tooth D.sub.1 and the maximum value of the magnetic field associated with the second tooth D.sub.2.

(17) For the sake of simplification, the last measured maximum or minimum values, which are taken into consideration for calculating the switching threshold for a tooth, will be referred to here as the maximum value (Bmaxi) and/or the minimum value (Bmini) which are associated with said tooth Di.

(18) The target 14 associated with the camshaft 16 may, however, exhibit geometric imperfections which vary over time. In particular, the target 14 may exhibit an airgap which increases over time, or with temperature. In that case, when the target 14 is rotationally driven, during a revolution after the first revolution, the passage of its teeth D.sub.1, D.sub.2, D.sub.3 past the sensor 10 causes variations in the magnetic field B as indicated in FIG. 3. In a similar way to FIG. 2 this curve indicates on the abscissa axis the angles of the engine cycle and, on the ordinate axis, the magnetic field B normalized as a function of the airgap perceived by the sensor 10.

(19) In this case, it may be noted that the new maximum value of the magnetic field B perceived for each of the teeth D.sub.1, D.sub.2, D.sub.3, respectively, Bmax1, Bmax2, Bmax3 is not identical to the maximum value of the magnetic field perceived by each of those same teeth D.sub.1, D.sub.2, D.sub.3 during the first revolution of the target 14 (cf. FIG. 2). Specifically, the first tooth D.sub.1 is detected with a new maximum value Bmax1, the second tooth D.sub.2 with a new maximum value Bmax2 and the third tooth D.sub.3 with a new maximum value Bmax3. The same can be said of the minimum values of the magnetic field B perceived by the sensor 10 upon the passage of each tooth, D.sub.1, D.sub.2, D.sub.3. The new minimum values of the magnetic field B for each of the teeth, D.sub.1, D.sub.2, D.sub.3 respectively Bmin1, Bmin2, Bmin3, are not identical to the minimum values of the magnetic field B, which were measured during the first revolution of the target 14, for the same teeth (Bmin1, Bmin2, Bmin3).

(20) For the instance depicted in FIGS. 2 and 3: Bmax1<Bmax1, Bmin1>Bmin1, Bmax2=Bmax2, Bmin2<<Bmin2, Bmax3<<Bmax3, Bmin3=Bmin3.

(21) As explained previously, it is known practice from the prior art to calculate the switching threshold on each passage of a tooth D.sub.1, D.sub.2, D.sub.3 once a new maximum value (Bmaxi, Bmax2, Bmax3) or a new minimum value (Bmin1, Bmin2, Bmin3) of the magnetic field B has been measured for said tooth D.sub.1, D.sub.2, D.sub.3.

(22) What is meant here by a new maximum value is the maximum value of the magnetic field B of each tooth D.sub.1, D.sub.2, D.sub.3 rather than the absolute maximum value of the magnetic field B over one full revolution of the target 14 for all teeth combined (which means to say, in this example, the maximum value of the magnetic field for all three teeth combined).

(23) Similarly, what is meant by a new minimum value is the minimum value of the magnetic field B of each tooth D.sub.1, D.sub.2, D.sub.3 rather than the absolute minimum value of the magnetic field B over one full revolution of the target 14, which means to say the absolute minimum value of the magnetic field B over one revolution of the target after all three teeth D.sub.1, D.sub.2, D.sub.3 have passed, for all teeth combined.

(24) However, this method of calibration generates disturbances on the output signal from the sensor 10, and these are undesirable.

(25) An aspect of the invention proposes a calibration method that allows the output signal from the sensor 10 to be smoothed, this signal exhibiting fewer disturbances than the signal of the prior art.

(26) To this end, the method of an aspect of the invention proposes, during the first revolution of the target 14, for each tooth D.sub.1, D.sub.2, D.sub.3, measuring the maximum value Bmax1, Bmax2, Bmax3, and the minimum value Bmin1, Bmin2, Bmin3 for each of said teeth D.sub.1, D.sub.2, D.sub.3 (step 1).

(27) Next, the amplitude of the magnetic field for said tooth D.sub.1, D.sub.2, D.sub.3 is calculated as a function of the maximum values Bmax1, Bmax2, Bmax3 and of the minimum values Bmin1, Bmin2, Bmin3 measured for each tooth D.sub.1, D.sub.2, D.sub.3, and the switching threshold is calculated as a function of the amplitude thus calculated (Step 2).

(28) Namely:
Ai=BmaxiBmini
And:
Si=kAi

(29) Where:

(30) Si is the switching threshold,

(31) k is a non-zero factor (equal for example to 0.75),

(32) Ai is the amplitude of the magnetic field for the tooth Di,

(33) Bmaxi is the maximum value for the tooth Di,

(34) Bmini is the minimum value for the tooth Di.

(35) This method for calculating the switching threshold is known from the prior art.

(36) An aspect of the invention then proposes also measuring the absolute minimum value Bmin for the magnetic field B (Step 3) over the first revolution of the target 14. Then, in a fourth step, the maximum values Bmax1, Bmax2, Bmax3, associated with each tooth D.sub.1, D.sub.2, D.sub.3 and the absolute minimum value Bmin of the magnetic field B of the first revolution of the target 14 are stored in memory. (Step 4).

(37) Then, during each subsequent revolution of the target 14, and for each tooth D.sub.1, D.sub.2, D.sub.3 the auto-calibration method of the invention comprises the following steps: during a fifth step (Step 5), for each tooth D.sub.1, D.sub.2, D.sub.3 the new maximum value Bmax1, Bmax2, Bmax3 and the new absolute minimum value Bmin of the magnetic field B are measured and stored in memory. Then, for each tooth D.sub.1, D.sub.2, D.sub.3, the new maximum value Bmax1, Bmax2, Bmax3 is compared against the maximum value Bmax1, Bmax2, Bmax3 stored in memory during the previous revolution of the target, which means to say, in this example, during the first revolution of the target 14 and associated with that same said tooth D.sub.1, D.sub.2, D.sub.3 (Step 6).

(38) If the difference between the new maximum value Bmax1, Bmax2, Bmax3 and the maximum value Bmax1, Bmax2, Bmax3 from the previous revolution of the target 14 is higher, in terms of absolute value, than a percentage of the difference between the maximum value Bmax1, Bmax2, Bmax3 for said tooth and the absolute minimum value Bmin of the magnetic field B measured during the previous revolution of the target 14, or in other words, if:
|BmaxiBmaxi|>K|BmaxiBmin|

(39) Where:

(40) K is a factor comprised between 0.003 and 0.1 (namely between 0.3% and 10%),

(41) Bmaxi is the maximum value of the magnetic field stored in memory during the previous revolution of the target 14 (in this instance the first revolution of the target) for a given tooth Di,

(42) Bmaxi is the new maximum value of the magnetic field during a further revolution of the target 14 for that same given tooth Di,

(43) Bmin is the absolute minimum value of the magnetic field stored in memory during the previous revolution of the target 14 (in this instance the first revolution of the target).

(44) Then, for each tooth D.sub.1, D.sub.2, D.sub.3 the switching threshold is calculated using the new maximum value Bmaxi associated with said tooth D.sub.1, D.sub.2, D.sub.3 and/or (Step 7) (and the absolute minimum value Bmin stored in memory during the previous revolution of the target 14), and the stored values Bmax1, Bmax2, Bmax3 are replaced by the new measured maximum values Bmax1, Bmax2, Bmax3. Else the switching threshold is calculated using the maximum value Bmax1, Bmax2, Bmax3 associated with said tooth D.sub.1, D.sub.2, D.sub.3 and the absolute minimum value Bmin stored in memory during the previous revolution of the target 14 (Step 8), and the stored values (Bmax1, Bmax2, Bmax3) remain unchanged.

(45) Then in the next step (Step 9), steps 5 to 8 are repeated for each new revolution of the target 14.

(46) For the instance depicted in FIG. 3: Bmin over the first revolution of the target (cf. FIG. 2) is equal to Bmin2; Bmin=Bmin2, Bmax1<Bmax1, with |Bmax1Bmax1|<K|Bmax1Bmin|, with the effect that the new switching threshold for the falling front of the first tooth S1d of the revolution following the first revolution of the target, is calculated as a function of the maximum Bmax1 and minimum Bmin3 stored values (for each revolution of the target 14, the tooth that precedes the first tooth D.sub.1 is the third tooth D3) of the magnetic field B of the first revolution of the target 14 which are associated with the first tooth D.sub.1 and is equal to the switching threshold of the first revolution of the target 14 for the falling front of the first tooth S1d; S1d=S1d, and the value Bmax1 stored in memory is not modified; it is not replaced by Bmax1, Bmax2=Bmax2, with the effect that the new switching threshold for the falling front of the second tooth S2d during the revolution following the first revolution of the target 14 is equal to the switching threshold of the falling front of the second tooth S2d of the first revolution of the target 14; S2d=S2d, and the value Bmax2 stored in memory is not modified, and is not replaced by Bmax2, Bmax3<<Bmax3, where |Bmax3Bmax3|>K*|Bmax3Bmin|, and so the switching threshold S3d for the revolution following the first revolution of the target is calculated as a function of Bmin and of Bmax3; the value Bmax3 stored in memory is updated and replaced by Bmax3.

(47) Thus, according to the calibration method according to the invention, the value of the switching threshold is modified on the passage of each tooth D.sub.1, D.sub.2, D.sub.3 only if the new maximum value Bmax1, Bmax2, Bmax3 of the magnetic field B associated with said tooth D.sub.1, D.sub.2, D.sub.3 differs from the maximum value Bmax1, Bmax2, Bmax3, associated with that same said tooth D.sub.1, D.sub.2, D.sub.3 measured during the previous revolution of the target 14. The difference between the two values needs to be higher than a predetermined value, calculated on the basis of the maximum value Bmax1, Bmax2, Bmax3 for each tooth D.sub.1, D.sub.2, D.sub.3 and of the absolute minimum value Bmin of the magnetic field B from the previous revolution of the target 14, in order for the value of switching threshold to be modified.

(48) As for the minimum value, which is used for calculating the switching threshold over a given revolution of the target 14, this is the absolute minimum value Bmin from the previous revolution.

(49) The calibration method of the invention therefore makes it possible to considerably reduce the disturbances on the output signal of the sensor 10.

(50) In addition, because the maximum values of the magnetic field B are compared on each new revolution of the target 14 against the maximum values measured and stored in memory during a previous revolution of the target 14, if out-of-roundness appears slowly (slow drift in the minimum and maximum values), it will be detected and corrected.

(51) The invention also relates to a camshaft sensor 10 comprising: Means for measuring a maximum value (Bmax1, Bmax2, Bmax3) and a minimum value (Bmin1, Bmin2, Bmin3) for the magnetic field (B) for each passage of a tooth (D.sub.1, D.sub.2, D.sub.3), Means for calculating the amplitude of the magnetic field for each tooth and for calculating the switching threshold, Means for measuring an absolute minimum value (Bmin) for the magnetic field over one full revolution of the target (14), Means for memory-storing the maximum value (Bmax1, Bmax2, Bmax3) associated with said tooth (D.sub.1, D.sub.2, D.sub.3), and the absolute minimum value (Bmin) over a full revolution of the target (14), Means for comparing, for each tooth (D.sub.1, D.sub.2, D.sub.3), the absolute value of the difference between the new maximum value (Bmax1, Bmax2, Bmax3) and the maximum value (Bmax1, Bmax2, Bmax3) stored in memory for the previous revolution of the target (14) which are associated with said tooth (D.sub.1, D.sub.2, D.sub.3) and the absolute value of the difference between the maximum value (Bmax1, Bmax2, Bmax3) and the absolute minimum value (Bmin) for the previous revolution of the target (14), Means for calculating a switching threshold (S1d, S2a, S2d, S3a, S3d, S1d, S2a, S2d, S3a, S3d) according to the result of the comparison.

(52) The measurement means, the memory-storage means, the means for calculating the amplitude of the magnetic field B, the comparison means and the means for calculating a switching threshold are, for example, software means incorporated into the sensor 10.

(53) An aspect of the invention therefore provides an ingenious way of reducing disturbances on the output signal of the camshaft sensor while at the same time detecting and correcting out-of-roundness as soon as it appears.