Method for automatically calibrating a camshaft sensor for a motor vehicle engine and associated sensor

Abstract

A method for automatically calibrating an engine camshaft sensor, the sensor measuring variations in magnetic field value and delivering an electrical signal having a high state after the passage of the values of the magnetic field above the switching threshold on a rising edge and a low state after the passage of the values of the magnetic field below the switching threshold on a falling edge. After the passage of the values of the magnetic field above the switching threshold and measuring a new maximum value, the electrical signal remains in a high state as long as the magnetic field values are higher than a hysteresis threshold, which is dependent on the amplitude of the magnetic field calculated with the measured new maximum value; after the passage of the values of the magnetic field below the hysteresis threshold, a new switching threshold is calculated according to the new maximum value.

Claims

1. A method for automatically calibrating a camshaft sensor for a motor vehicle engine, said motor vehicle engine comprising: at least one camshaft, a toothed coded target associated with the camshaft, and a magnetic field sensor placed near the target to detect magnetic field variations induced by passage of the teeth of the target in proximity to the sensor, said sensor measuring values of the magnetic field and delivering an electrical signal indicative of teeth and troughs of the target according to a predetermined switching threshold dependent on an amplitude of the measured magnetic field and applied to rising edges and falling edges of the variations in the values of the magnetic field, the electrical signal having a high state after the passage of the values of the magnetic field above the switching threshold on a rising edge and a low state after the passage of the values of the magnetic field below the switching threshold on a falling edge, said method comprising continuously measuring the value of the magnetic field, wherein: after the passage of the values of the magnetic field above the switching threshold on the rising edge and measuring a new maximum value of the magnetic field, the electrical signal remains in a high state for as long as the values of the magnetic field are higher than a hysteresis threshold, which is dependent on the amplitude of the magnetic field calculated with the measured new maximum value; after the passage of the values of the magnetic field below the hysteresis threshold, a new switching threshold is calculated according to the new maximum value.

2. The automatic calibration method as claimed in claim 1, wherein after the passage of the values of the magnetic field above the switching threshold on the rising edge and for as long as the values of the magnetic field are higher than the hysteresis threshold, the value of the switching threshold is decreased.

3. The automatic calibration method as claimed in claim 2, wherein after the passage of the values of the magnetic field above the switching threshold on the rising edge and for as long as the values of the magnetic field are higher than the hysteresis threshold, the value of the switching threshold is close to zero.

4. The automatic calibration method as claimed in claim 1, wherein the electrical signal is controlled so as to remain in a high state after the passage of the values of the magnetic field above the switching threshold on the rising edge and for as long as the values of the magnetic field are higher than a hysteresis threshold, which is dependent on the amplitude of the magnetic field calculated with the measured new maximum value.

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 section depicting a camshaft sensor and its associated target;

(3) FIG. 2 illustrates an example of curves of the variation in the values of the magnetic field B perceived by a sensor associated with a target and the corresponding electrical signal Se, according to the prior art;

(4) FIG. 3 illustrates an example of curves of the variation in the values of the magnetic field B perceived by a sensor associated with a target and the corresponding electrical signal Se, according to an aspect of the invention.

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 of 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, the sensor 10 perceives a series of variations in values of 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, for example, the teeth D.sub.1, D.sub.2 are not of the same height h1, h2 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 passes by varies for each of the two teeth and adopts the respective values Bmax1, Bmax2. 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. FIG. 2 shows the passage of two teeth D.sub.1, D.sub.2, the two first (D.sub.1, D.sub.2) corresponding to the geometry of the target 14 shown in FIG. 1 (the passage of the third tooth D.sub.3 is not shown in FIG. 2).

(10) It is known practice to detect the passage of a tooth edge 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 edge of the second tooth S1 is calculated, for example, using the following mathematical formula:
S1=0.75*(Bmax1Bmin1)

(12) Then, after the new maximum value of the magnetic field B upon passage of the second tooth Bmax2 has been crossed, a new switching threshold S2 is calculated, for example, for the falling edge of the second tooth D.sub.2:
S2=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 and validated.

(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 and validated maximum and minimum values.

(15) For example, in order to calculate the switching threshold for the rising edge of the second tooth S1, it is a matter of considering the last recorded maximum value 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 edge of the second tooth S2, 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) However, as explained above, in order to ensure that the last extremum measured, for example Bmax2, is indeed an extremum, a hysteresis H threshold S.sub.H is calculated on the passage of this extremum (Bmax2, cf. FIG. 2), and the new value of the switching threshold S2 is calculated according to this new extremum Bmax2 only when the hysteresis threshold S.sub.H has been crossed.

(18) As a consequence, when the new maximum value Bmax2 is smaller than the last maximum value measured Bmax1, the magnetic signal B passes two successive switching thresholds S1 and S2, which results in a parasitic pulse I on the electrical signal Se.

(19) In order to alleviate this disadvantage, the method for automatically calibrating the camshaft sensor according to an aspect of the invention proposes the following steps.

(20) After the passage of the values of the magnetic field B above the switching threshold S1 on the rising edge and measuring a new maximum value Bmax2 of the magnetic field B, the electrical signal Se remains in a high state for as long as the values of the magnetic field B are higher than the hysteresis threshold S.sub.H, which is dependent on the amplitude of the magnetic field B calculated with the measured new maximum value Bmax2.

(21) After the passage of the values of the magnetic field B below the hysteresis threshold S.sub.H, a new switching threshold S2 is calculated according to the new maximum value Bmax2.

(22) The electrical signal Se therefore passes to a high state I as soon as the values of the magnetic field B pass above the switching threshold S1 on the rising edge and the electrical signal Se remains in the high state I for as long as a new maximum value Bmax2 has not been validated by the passage of the values of the magnetic field B below the hysteresis threshold S.sub.H, which is calculated according to the new maximum value Bmax2.

(23) Once the hysteresis threshold S.sub.H has been crossed, the new switching threshold S2 is calculated with the validated last maximum value Bmax2.

(24) In one preferred embodiment, after the passage of the values of the magnetic field B above the switching threshold S1 on the rising edge, the value of the switching threshold S1 is decreased until it crosses the hysteresis threshold S.sub.H. In other words, the value of the switching threshold S1 is set lower than its previously calculated value.

(25) In another embodiment, the electrical signal Se is controlled so as to remain in a high state I as soon as the values of the magnetic field B pass above the switching threshold S1 on the rising edge and for as long as a new maximum value Bmax2 has not been validated by the passage of the values of the magnetic field B below the hysteresis threshold S.sub.H, and to do so regardless of the value of the switching threshold S1 on the falling edge.

(26) This is illustrated in FIG. 3, at the top of FIG. 3, showing the magnetic signal B on the passage of the second tooth D2.

(27) After passing P0 the switching threshold S1, the threshold value S1 is decreased and is equal, in this example, to a value close to zero, until passing P2 the hysteresis threshold S.sub.H.

(28) After passing P2 the hysteresis threshold S.sub.H, the new maximum value Bmax2 of the magnetic field is validated and the new switching threshold S2 is calculated according to this new maximum value Bmax2.

(29) At the bottom of FIG. 3, the electrical signal Se at the output of the sensor 10 is illustrated, the magnetic signal B crossing only a switching threshold, more specifically the new switching threshold S2, on the falling edge, which signal passes from a high state I indicative of the tooth D2 to a low state II indicative of the trough s.sub.2 at the angle 2 corresponding to passing the new switching threshold S2.

(30) The electrical signal Se therefore does not exhibit a parasitic pulse; like in the prior art.

(31) For this, the camshaft sensor 10 further comprises means M1 for calculating a hysteresis threshold (cf. FIG. 1) and means M2 for monitoring the switching threshold S1 so as to decrease the value of the switching threshold S1 after its crossing on a rising edge.

(32) The sensor 10 further comprises means M3 for controlling the electrical signal Se in order to control the electrical signal Se so that it is in the high state after passing above the switching threshold S1 and for as long as the hysteresis threshold S.sub.H has not been crossed.

(33) The means M1 for calculating a hysteresis threshold, the monitoring means M2 and the control means M3 are for example software means incorporated in the sensor 10 (cf. FIG. 1).

(34) The method for automatically calibrating a camshaft sensor 10 according to an aspect of the invention therefore makes it possible to avoid parasitic pulses on the electrical signal during the successive passages of teeth having different maximum magnetic field values, in this instance when a tooth has a maximum value that is lower than that of the preceding tooth.

(35) An aspect of the invention has been described and illustrated in the case in which the maximum value varies from tooth to tooth, specifically the out-of-roundness defect of the target affects the maximum value of the magnetic field above all, but aspects of the invention may also be applied to the variation in the minimum value of two successive teeth, in this instance when a minimum value of a trough is higher than the minimum value of the preceding trough.