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

Abstract

A method for automatic calibration of an engine camshaft sensor, the engine including at least one camshaft, a coded toothed target associated with this camshaft and a magnetic field sensor placed in the vicinity of the target to detect magnetic field variations induced by passage of the target's teeth in the vicinity of the sensor, the sensor delivering an electrical signal representative of teeth and gaps of the target depending on a predetermined switching threshold as a function of the magnetic field's amplitude, the method continuously measuring the value of the magnetic field. The method calculating switching thresholds of the leading edges of the teeth over a new turn of the target to improve the precision of detection of the leading edges of the teeth.

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 this camshaft and a magnetic field sensor placed in proximity to the target to detect magnetic field variations induced by a passage of teeth (D.sub.1, D.sub.2, D.sub.3) of the target in proximity to the sensor, said sensor delivering an electrical signal representative of teeth (D.sub.1, D.sub.2, D.sub.3) and troughs (S.sub.1, S.sub.2, S.sub.3) of the target as a function of a predetermined switching threshold (S) that is a function of an amplitude of the magnetic field (B), said method continuously measuring a value of the magnetic field, said method comprising: during a first revolution of the target: Step E1: measuring a maximum value and a minimum value of the magnetic field (B) for each tooth, Step E2: calculating an amplitude of the magnetic field for said teeth, and calculating the switching threshold for each tooth rising edge as a function of the duly calculated amplitude, said method further comprises: Step E3: measuring an absolute minimum value of the magnetic field over the revolution (N1) of the target, Step E4: calculating an average of the maximum values of the magnetic field over the revolution (N1) of the target, Step E5: memorizing the maximum values, the absolute minimum value, and the average, then, on each new revolution (N) of the target: Step E6: if a minimum value of a tooth (i1) is greater than the absolute minimum value of the preceding revolution, then: Step E7: the switching threshold of the rising edge of the next tooth (i) is set equal to
Th(i,N)=(B max(i1,N)B min(i1,N))KB min(i1,N) where Th(i, N) is the switching threshold of the rising edge of the next tooth (i) on each new revolution, B max (i1,N)) is a maximum value of the magnetic field of the tooth (i1) on each new revolution (N), and B min (i1, N) is a minimum value of the magnetic field of the tooth (i1) for the new revolution (N), Otherwise: if a minimum value of a tooth (i1) is equal to the absolute minimum value of the preceding revolution (N1), and: Step E8: if, furthermore, the maximum value of said tooth (i1) is equal to the maximum value of the preceding revolution for the same tooth (i1), such that B max(i1, N)=B max(i1, N1), then: Step E9: the maximum value of said (i1) is removed from the average of the maximum values of the new revolution, such that
Avg(B max,N)=Avg(B max,N1)B max(i1,N) where Avg(B max, N) is an average of the maximum values of the magnetic field over the new revolution (N), and Avg(B max, N1) is an average of the maximum values of the magnetic field over the revolution (N1), otherwise, if the maximum value of the tooth (i1) is different from the maximum value of the preceding revolution for the same tooth, such that B max(i1,N)B max(i1, N1), then: Step E10: the average of the maximum values for the new revolution (N) is equal to the average of the maximum values of the preceding revolution (N1), such that: Avg(B max, N)=Avg (B max, N1) Step E11: the switching threshold of the rising edge for the next tooth (i) is then calculated as a function of the average of the maximum values of the new revolution (N) and of the minimum value of the preceding tooth; such that:
Th(i,N)=(Avg(B max,N)B min(i1,N))KB min(i1,N) with k: a factor lying between 0 and 1, repeating the steps E3 to E11 for each new revolution of the target in order to deliver a signal representative of the teeth (D.sub.1, D.sub.2, D.sub.3) and of troughs (S.sub.1, S.sub.2, S.sub.3) of the target.

2. The automatic calibration method as claimed in claim 1 wherein the first revolution of the target is performed each time the camshaft sensor is powered up.

3. A camshaft sensor for a motor vehicle engine, said motor vehicle engine comprising: at least one camshaft, a toothed coded target associated with this camshaft and a magnetic field sensor placed in proximity to the target to detect magnetic field variations induced by a passage of teeth (D.sub.1, D.sub.2, D.sub.3) of the target in proximity to the sensor, said sensor continuously measuring a value of the magnetic field and delivering an electrical signal representative of the teeth (D.sub.1, D.sub.2, D.sub.3) and troughs (S.sub.1, S.sub.2, S.sub.3) of the target as a function of a predetermined switching threshold (S) that is a function of an amplitude of the magnetic field (B), said camshaft sensor comprising a central processor configured to: measure a maximum value (Bmax1, Bmax2, Bmax3) and a minimum value (Bmin1, Bmin2, Bmin3) of the magnetic field (B) on the passage of a tooth (D.sub.1, D.sub.2, D.sub.3), calculate the amplitude of the magnetic field for each tooth and for calculating the switching threshold, measure an absolute minimum value (B min(N1)) over the target revolution (N1), calculate an average of the maximum values (Avg(B max, N1)) over the target revolution (N1), store the maximum values (Bmax1, Bmax2, Bmax3), the average of the maximum values (Avg(B max, N1)) and the absolute minimum value (B min(N1)) over the target revolution, compare between each minimum value (B min (i1, N)) of the new target revolution and the absolute minimum value (B min(N1)) of the preceding revolution, compare between each maximum value (B max(i1,N)) of the new target revolution and the maximum value of the preceding revolution (B max(i1)) for the same tooth (i1), calculate the average of the maximum values Avg(B max, N) over the new target revolution as a function of the result of the comparison between each maximum value (B max(i1, N)) of the new target revolution and the maximum value of the preceding revolution (B max(i1)) for the same tooth (i1), and calculate a switching threshold (Th(i, N)) as a function of the result of the comparisons.

4. A motor vehicle, comprising a camshaft sensor as claimed in claim 3.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Other features and advantages of aspects 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, representing a camshaft sensor and its associated target,

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

(5) FIG. 4 illustrates the automatic calibration method according to an aspect of the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

(6) According to the embodiment described and represented 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.

(7) A target 14 associated with this sensor 10 takes the form of a metal disk 15 securely fixed 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 represented) 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 spacings constitute, in a manner that is known per se, a coding.

(8) The operation of such a sensor 10 plus target 14 assembly is described hereinafter.

(9) When the target 14 is driven in rotation (arrow F FIG. 1) by the camshaft 16, the sensor 10 perceives a series of variations of the magnetic field B representative of the length l of the teeth D.sub.1, D.sub.2, D.sub.3 passing in front of 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 represented in FIG. 2.

(10) In this FIG. 2, the x axis indicates the angles of the engine cycle varying from 0 to 360 and the y axis indicates the value B of the perceived magnetic field (field normalized as a function of the air gap). As represented 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 geometry defect. Because of this, the maximum field perceived by the sensor 10 upon the passage of each of the teeth D.sub.1, D.sub.2, D.sub.3 varies for each of the three teeth and respectively takes the values B max1, B max2, B max3. Likewise, the minimum field perceived by the sensor 10 upon the passage of each of the teeth D.sub.1, D.sub.2, D.sub.3 varies tooth-by-tooth, and takes the respective values B min1, B min2, B min3. 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 close together, 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 that is narrower and further away from the second tooth D.sub.2. This in fact corresponds to the geometry of the target 14 represented in FIG. 1.

(11) It is known practice to detect the passage of a tooth edge as soon as the perceived magnetic field B 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 (B max1B min1) for example).

(12) The threshold values are embodied in FIG. 2 as dotted lines. After the passage of the first tooth D.sub.1, a switching threshold for the rising edge of the second tooth S2a is calculated according to the following mathematical formula:
S2a=0.75(B max 1B min 1)

(13) Then, after the passage of the maximum value of the magnetic field B on the passage of the second tooth B max2, a new switching threshold Std is calculated for the falling edge of the second tooth D.sub.2:
S2d=0.75(B max 2B min 1)

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

(15) 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.

(16) For example, calculating the switching threshold of the rising edge of the second tooth S2a, here entails considering the last recorded maximum and minimum values of the magnetic field B, that is to say B max1 and B min1, namely the maximum value and the minimum value of the magnetic field B after the passage of the first tooth D.sub.1.

(17) Similarly, in order to calculate the switching threshold for the falling edge of the second tooth S2d, use is made of the last recorded maximum and minimum values, in this case B min1 and B max2, 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.

(18) For the sake of simplification, the last measured maximum and/or minimum values, which are taken into consideration for calculating the switching threshold for said tooth, will be referred to here as the maximum value (Bmaxi) and/or the minimum value (Bmini) which are associated with said tooth D.sub.1. And maximum value is understood to mean the maximum value of the magnetic field and minimum value is understood to mean the minimum value of the magnetic field.

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

(20) In this case, it will 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, B max1, B max2, B max3 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 B max1, the second tooth D.sub.2 with a new maximum value B max2 and the third tooth D.sub.3 with a new maximum value B max3. The same applies for 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 B min1, B min2, B min3, 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 (B min1, B min2, B min3).

(21) In the case represented in FIGS. 2 and 3:
B max 1<B max 1,
B min 1>B min 1,
B max 2>B max 2,
B min 2<B min 2,
B max 3=B max 3,
B min 3<B min 3. An aspect of the invention therefore proposes a method for automatically calibrating a camshaft sensor, that makes it possible to take account of new maximum and minimum values of the magnetic field for each tooth D.sub.1, D.sub.2, D.sub.3, in order to improve the accuracy of the sensor, while being simple to implement and less memory-intensive than the prior art.

(22) An aspect of the invention described hereinbelow applies only to the switching thresholds applied to the rising edges of the teeth.

(23) An aspect of the invention therefore ingeniously makes it possible to correct an out-of-roundness as soon as it appears.

(24) An aspect of the invention proposes an automatic calibration method as described hereinbelow and illustrated in FIG. 4.

(25) In a first target revolution preliminary to the powering-up, for example, of the camshaft sensor 10 there is a first step (step E1) of measurement of a maximum value B max1, B max2, B max3 and of a minimum value B min1, B min2, B min3 of the magnetic field B for each tooth D.sub.1, D.sub.2, D.sub.3: And, in a second step (step E2): an amplitude of the magnetic field for said teeth is calculated, and a switching threshold S2a, S3a is applied for each tooth rising edge that is determined as a function of the duly calculated amplitude. This is known from the prior art.

(26) The method according to an aspect of the invention is noteworthy in that it further comprises the following steps: In a third step (step E3), there is determined, from among the measured minimum values B min1, B min2, B min3, an absolute minimum value B min (N1) of the magnetic field over the revolution (N1) of the target 14 which has just been performed. An average Avg(B max, N1) of the maximum values B max1, B max2, B max3 of the magnetic field over said target 14 revolution is also calculated (step E4). Finally, the maximum values B max1, B max2, B max3 associated with each tooth, the determined absolute minimum value B min(N1) and the calculated average Avg(B max, N1) are memorized (step E5).

(27) An aspect of the invention then proposes that, on each new revolution of the target 14:

(28) If a minimum value B min (i1, N) of a tooth (i1) over the new revolution N is greater than the absolute minimum value B min(N1) of the preceding revolution (step E6), then: the switching threshold Th(i, N) of the rising edge for the next tooth i of the new revolution N is equal to (step E7):
Th(i,N)=(B max(i1,N)B min(i1,N))KB min(i1,N)

(29) Otherwise, if the minimum value B min (i1, N) of a tooth (i1) of the new revolution N is equal to the absolute minimum value B min(N1) of the preceding revolution, and: if the maximum value of the magnetic field for said tooth B max(i1, N) has not changed since the preceding revolution (step E8), more specifically if the maximum value of the magnetic field for the tooth i1, on the revolution N is equal to the maximum value of the magnetic field for the tooth i1, on the revolution N1, that is to say, if
B max(i1,N.sup.o)=B max(I1,N1),then, the average Avg(B max, N) of the maximum values is modified as follows (step E9):
Avg(B max,N)=Avg(B max,N1)B max(I1,N) More specifically, the maximum value of the magnetic field of the tooth i1 measured on the new revolution is removed from the average of the maximum values of the magnetic field determined on the preceding revolution. Otherwise, if the maximum value of the magnetic field for the tooth i1, on the revolution N, is different from the value of the magnetic field for the tooth i1 (step E10), namely if B max(i1,N)>B max(i1,N1) or if B max(i1,N)<B max(i1,N1), in other words if B max(i1,N)B max(i1,N1), then the average of the maximum values of the new revolution is not modified and remains equal to that of the preceding revolution, namely (step E10):
Avg(B max,N)=Avg(B max,N1)

(30) It should be noted that, if the minimum value B min(i1,N) of the tooth i1 is the sole value over the revolution N, which is less than the absolute minimum value B min(N1) of the preceding revolution (N1), then said minimum value B min(i1,N) becomes the absolute minimum value B min(N) of the present revolution N, which will be used in the next revolution N+1.

(31) It should also be noted that the tooth i is the tooth following the tooth i1.

(32) Finally, the switching threshold Th(i,N) of the next tooth i for the new revolution N is calculated according to the formula (step E11):
Th(i,N)=(Avg(B max,N)B min(i1,N))KB min(i1,N)
with k: a factor lying between 0 and 1,
Avg(B max, N): the average of the maximum values of the magnetic field over the new revolution N,
B min(i1,N): the minimum value of the magnetic field on the preceding tooth i1, for the new revolution N.

(33) The method thus repeats the steps 3 to 11 for each new target revolution.

(34) The method will now be explained by applying it to the teeth D.sub.1, D.sub.2, D.sub.3 of FIGS. 2 and 3.

(35) FIG. 2 illustrates the magnetic field variation induced by the passage of three teeth D.sub.1, D.sub.2, D.sub.3 in a first tooth revolution, namely N=1.

(36) FIG. 3 illustrates the magnetic field variation induced by the passage of three teeth D.sub.1, D.sub.2, D.sub.3 in a second tooth revolution, namely N=2, following the first revolution.

(37) By applying the convention of notations of the calibration method according to an aspect of the invention to FIGS. 2 and 3, with i varying from 1 to 3 and N varying from 1 to 2:
B max 1=B max(1,1)
B min 1=B min(1,1)
B max 2=B max(2,1)
B min 2=B min(2,1)
B max 3=B max(3,1)
B min 3=B min(3,1)
B max 1=B max(1,2)
B min 1=B min(1,2)
B max 2=B max(2,2)
B min 2=B min(2,2)
B max 3=B max(3,2)
B min 3=B min(3,2) In FIG. 2, the minimum value of the magnetic field B min(1a) is equal to the minimum value of the magnetic field of the second tooth, namely B min(1)=B min2.

(38) The average of the maximum values of the magnetic field, namely Avg(B max, 1), is equal to:

(39) $\mathrm{Avg}\left(B\max ,1\right)=\frac{\left(B\max \left(1,1\right)+B\max \left(2,1\right)+B\max \left(3,1\right)\right)}{3}$

(40) In FIG. 3, the minimum value of the magnetic field of the first tooth D.sub.1 namely (i1)=1 on the second revolution, namely N=2, is greater than the absolute minimum B min(1) of the magnetic field calculated on the preceding revolution, namely B min1>B min(1); consequently, the switching threshold Th for the rising edge of the second tooth, i=2 on the second revolution, is equal to:
Th(2,2)=(B max(1,2)B min(1,2))KB min(1,2)

(41) The minimum value of the magnetic field of the second tooth B min2 is less than the absolute minimum of the preceding revolution:
B min 2<B min(1)

(42) The maximum value of the magnetic field of the second tooth B max2 is greater than the maximum value of the magnetic field of the same tooth D.sub.2 of the preceding revolution; namely:
B max 2>B max 2

(43) Consequently, the average of the values of the magnetic field for the second revolution is equal to:
Avg(B max,2)=Avg(B max,1)

(44) And the switching threshold for the rising edge of the third tooth Th(3,2) is equal to:
Th(3,2)=(Avg(B max,2)B min(2,2))KB min(2,2)

(45) The minimum value of the magnetic field for the third tooth B min3 of the second revolution is less than the absolute minimum value of the magnetic field of the preceding revolution, namely:
B min 3<B min(1)

(46) And the maximum value of the magnetic field of the third tooth B max3 is equal to the maximum value of the magnetic field of the same tooth of the preceding revolution:
B max 3=B max 3

(47) Consequently, the average of the values of the magnetic field for the third revolution is equal to:
Avg(B max,3)=Avg(B max,2)B max(3,2)

(48) and the switching threshold for the rising edge of the third tooth Th(3,2) is equal to:
Th(1,3)=(Avg(B max,3)B min(3,2))KB min(3,2)

(49) An aspect of the invention is inexpensive, and easy to implement, and it makes it possible to improve the accuracy on the detection of the rising edge of the teeth, above all in the case of an out-of-roundness.