MXPA97000090A - Rotational sensor with axial robu alignment tolerance - Google Patents

Abstract

A rotation sensor for high accuracy angle applications such as sensing the position of the crank shaft of the internal combustion engine includes a pair of complementary target wheels and a dual element magnetoresistive sensor. The target wheels are spaced from one another along the rotational axis by a predetermined distance. The individual magnetoresistive elements are similarly spaced from each other and are influenced by the ferrite effects of the objective wheels. The spacing of the target wheels and the magnetoresistive elements are selected one in relation to the other and in relation to the amount of axial play on the crank axis so that changes in the ferrite effects of the target wheels on the elements due to the axial displacement of the crank shaft has a minimal effect on the accuracy of the percept device

Description

i ROTATIONAL SENSOR WITH AXIAL ROBUSTA ALIGNMENT TOLERANCE Technical Field The present invention relates to a rotation sensor apparatus. More particularly, the invention is directed towards a two-track high accuracy rotational objective wheel sensor.

Background of the Invention The high accuracy rotational sensors are known which use the two-track target wheels and a double element sensor. Each section of such a target wheel is axially adjacent to the other along the rotational axis thereof, and each section has one of two elements radially disposed thereon. Such wheel wheel arrangements are particularly advantageous when used, for example, as part of a rotating member such as an internal combustion engine crank shaft to determine the angular position information therefor. A known target wheel for such a sensor array includes substantially complementary or identical image geometries. That is, a tooth location on one section is adjacent to a slot on the other section. The output signals from the two elements are complementary and provide a high accuracy tooth edge detection with relatively simple differential signal processing.

Referring to Figures IA and IB, the arrangements of the related rotational sensor are shown as being viewed from a point of tangential advantage virtually with respect to a rotation member. In each figure, a dual element magnetoresistive sensor is generally indicated with s the number 11 and comprises the individual magnetoresistive elements MR1 and MR2, and a pressure magnet 13. Although not illustrated separately, a ferromagnetic wedge may be located between the pressure magnet 13 and the individual MR elements. MR sensors are generally balanced having a positive magnitude equivalence correlated so that the equivalent flow density through each individual element produces a virtually equivalent MR response. Such dual element MR sensors are generally well known, an example of such sensors is found in U.S. Patent No. 4,926,122 also assigned to the assignee of the present invention. Also in each figure, a rotation axis is labeled (Ar) and corresponds to an axis of rotation of a member (not shown) such as a motor crank shaft. Coupled to the rotation member is a target wheel 15 forming a pair of tracks Immediately adjacent, a track on each respective side of the plane "P" which is orthogonal to the axis of rotation and which therefore appears as a line in the figure separating the left and right parts of the receptive views. Each objective wheel 15 is characterized by the teeth on each section generally and axially on one side of the discontinuities on the other section with the exception perhaps of small axially adjacent discontinuous sections on the angular interfaces between each tooth on the opposite axial sides of the wheel of objective.

In each figure IA and IB, the objective wheels 15 are illustrated in a rotational position (solid line) in which one tooth and a disengagement to the left of the right of the plane P, respectively, are axially aligned below the respective MR sensor. Alternatively, another position rotational is exemplified by the broken line on one tooth and a discontinuity to the right and left of the plane P, respectively, are axially aligned below the MR sensor.

^ When turning the objective wheel, these two positions essentially alternate, thus arising virtually all resistive changes inverses virtually coincident in angle to the individual MR elements which are perceived and processed to produce a signal indicative of angle. Generally this is achieved with a comparator or differential signal processing means.

In all applications, a certain degree of axial movement of the rotating member will occur in relation to the sensor. The accommodation of such axial movement is required in order to minimize the angular inaccuracy in minor cases and to ensure the operation of the sensor in the extreme case. Such accommodation has been achieved by extending each track of the target wheel and / or by widening the axial spacing between the two sensor elements to ensure acceptable levels of operation. Of course, the space and limitations of placement can greatly restrict the degree to which such accommodations can be implemented.

The two views of the related sensors in Figures IA and IB generally graphically describe the handling Conventional axial play affecting the MR sensor outputs. Figures 2A to 2C, illustrating the individual MR element outputs and the angular information derived therefrom, • they will also be referred to here in the discussion of the handling of the axial play illustrated and described with respect to Figure IA and IB. In each figure IA and IB, it is understood that the respective MR sensor is mounted in the network to a main room; in the case of the example application of the crank axis perception, the motor block provides the preferred mounting provision. Therefore, any axial movement of the rotating member, the motor bell hand crank in the example by hand, is therefore with respect to the engine block and the MR sensor. The target wheel alignment - preferred sensor is one where the line * central 16 located halfway between the individual elements MR, MR1 and MR2 corresponds to the interface or plane P between the two tracks of the objective wheel. Such relative positioning preferably corresponds to the position of the crank axis at the center of its axial tolerance. In other words, assuming a total axial tolerance for the movement of?, The alignment of the crank axis as described will allow maximum movement on each side of the centerline of the sensor 16 # virtually? / 2. In each figure IA and IB, an amount of axial displacement of d is illustrated where d < . ?/2.

The operation of the desired sensor is achieved in any of the arrangements of Figures IA and IB where the MR sensor elements are aligned symmetrically on the target wheel as previously described. The response of the sensor with such ideal alignment is represented * ßß from Figure 2A where the MR elements produce outputs having an inverse symmetry with the respective outputs intersecting in the desired rotation angle © network.

Generally, the width of the target wheel is of specific application and established with such considerations as the mechanical resistance and the location and spaces available on the crank shaft. For axial tolerances that are relatively small with respect to an available width of the objective wheel as illustrated in Figure IA, and also assuming that in that figure d =? / 2 such that the axial displacement is illustrated in a case maximum or worst, the axial spacing between MR1 and MR2 ensures that for all axial displacements the MR elements remain on the respective tracks. However, even with such a provision, the MR outputs will vary from the ideal virtually as shown in Figure 2B. An examination of the outputs exposes a change in the level of the output of MR2 when it is located on a respective discontinuity due to the ferrite effects of the adjacent track tooth. A resulting change in the intersection of the respective outputs occurs at an angle of rotation? . Therefore, the angular error sensor is virtually ± (ered-? D).

For axial tolerances that are relatively large with respect to an available width of the target wheel as illustrated in Figure IB, and continuing with the presumption that d =? / 2 so that the axial displacement is illustrated to a maximum case or worse, the axial spacing between MR1 and MR2 is relatively wide to provide a suitable peak-to-peak output and a maximum axial displacement operating band. The closest placement will result in increased adjacent track ferrite effects and less tolerance to smaller axial displacements. However, even with such a provision, the substantial angular error will result due to the change in the intersection of outputs and the potential loss of the intersection where, as illustrated, the axial deviation is so large as to place an MR element (MR2) closer to one tooth of the adjacent track than the other MR element (MR1) is for the same tooth. In this case, the MR outputs will vary from an ideal virtually as shown in Figure 2C and will result in a total loss of an angular position signal since this is, as it was discovered, a result of the differential processing of the MR outputs. Synthesis of the Invention Therefore, according to a first general aspect of the present invention, a sensor was provided Dual view double element rotational which assures MR outputs to give angular position information over a predetermined range of axial displacements of a rotational member, and preferably over the entire range of axial displacement. According to a second general aspect of the present invention, the dual track dual element rotational sensor virtually eliminates angular sensor errors characteristic of axial shifts in the related sensors.

The first general aspect is provided by a sensor array having a pair of tracks, each of which is characterized by circumferentially alternating high and low permeability areas such as is characteristic of a cogwheel arrangement. The two tracks are arranged one in relation to the other so that the high permeability areas of a track are axially aligned with the areas of low permeability of the other track at all angular positions around it. The two tracks are separated by a predetermined axial distance which, in concert with the spacing between the individual MR elements and the tolerance of known axial deflection of the rotation member, ensures MR outputs to give angular position information by ensuring that the The ferrite effects of the track corresponding to a particular MR element dominate any ferrite effect that the same track can impose on the other MR element.

The second general aspect is provided by the sensor array having a pair of tracks, each of which is characterized by high and low permeability areas circumferentially alternating as is characteristic of a cogwheel arrangement. The two tracks are arranged one in relation to the other so that the areas of high permeability of one track are aligned axially with the areas of low permeability of the other track in all angular positions there. The two tracks are separated by a predetermined axial distance which, in concert with the separation between the individual MR elements and the axial deflection tolerance of # the rotation member, ensure a non-variant angle intersection of MR outputs to give angular position information by ensuring that the ferrite effects of the track corresponding to a particular MR element are substantially equivalent to the ferrite effects of the other track corresponding to the other particular MR element.

Brief Description of the Drawings The present invention will now be described by way of example, with reference to the accompanying drawings in which: Figure 1 is a view of a rotational sensor including a dual element magnetoresistive sensor and dual track target wheels demonstrating the variety of ß rotation sensors to which the present invention relates. Figure 2 is a schematic of several outputs of the dual element magnetoresistive sensor cooperating with the dual track lens wheel geometries as illustrated in Figure 1; Figure 3 is a view of a first embodiment * of a rotational sensor including a first dual track lens wheel geometry; Figure 4 is a schematic of the dual element magnetoresistive sensor outputs cooperating with the dual track lens wheel geometry as shown in Figure 3; F? Figures 5A and 5B are views of a second embodiment of a rotational sensor, including a second dual track lens wheel geometry, in the network and a maximum axial tolerance, respectively; Y, Figure 6 is a schematic of the outputs of the dual element magnetoresistive sensor cooperating with the dual track lens wheel geometry as illustrated in ß Figures 5A and 5B.

Description of the Preferred Modality Referring now to Figure 3 and the description that follows, it will be noted that the general relationships between the various described components of a rotation sensor are identical to those described in relation to the sensors of the art related to FIGS. A and IB. In other words, the sensor is a network mounted on a motor block, the target wheel is mounted to the crank shaft of the motor, and the crank axis of the motor has a predetermined amount of axial play relative to the block of the motor. motor and sensor. A rotation sensor according to a first embodiment of the present invention is generally designated 10 'and includes a dual element magnetoresistive (MR) sensor generally designated with the number 11 cooperating with a rotating dual track target wheel generally designated with the number 25. The rotation of the crank axis, and therefore the rotation of the target wheel, occurs around the rotational axis (Ar). The dual element MR sensor 11 includes the individual MR elements labeled MR1 and MR2 and a pressure magnet 13. The individual MR magnets are viewed as being symmetrically spaced on the opposite axial sides of the center line 16, each MR element spaced by a distance (d) of the center line 16 with a total distance of 2d between the two MR elements.

The target wheel 25 defines two tracks not separately labeled but generally understood to correspond to the respective peripheral areas of alternating high and low permeability on each side of the plane P. In the present illustrated embodiment, the reference is made to a wheel of single objective; however, it will be understood that any objective wheel 25 comprises a pair of target wheels 25A and 25B which may be physically distinct or parts of a unitary structure as illustrated. Both target wheels 25A and 25B can be referred to together as the target wheel 25 or individually as the target wheel 25A or 25B, the symmetry of the target wheel 25 and the track structure around the plane P is assumed. The width of the target wheel 25 or the distance between the respective axially opposite side edges of the two target wheels 25A and 25B, is designated as (). The teeth of a target wheel 25A or 25B are separated from the teeth of the other target wheel 25A or 25B by a designated axial distance (L), again symmetrically around the plane P.

As between the sensor 13 and the objective wheel 25, the objective wheel is illustrated in the deviation? maximum axial on one side of the center line 16 of the sensor 13. Although the discussion that follows relates only to the deviation as generally illustrated, the deviation to the opposite side is equally applicable to the general principles described. The distance between MRl and the closest surface point of a tooth of the corresponding track when it is aligned radially with the sensor is designated as xx, and similarly the distance between MR2 and the closest surface point of a corresponding track tooth when it is aligned radially with the sensor it is designated x1. The distance between MR2 and the closest surface point of a track tooth corresponding to MRl when aligning radially with the sensor is designated as y2.

The rotation sensor 10 'as illustrated, provides the accommodation of even the maximum axial deflection of the crank shaft and of the objective wheel to ensure the operability of the sensor at all axial deviations. That is, the intersection of the respective MR outputs is ensured by selectively choosing the various dimensions D described so that when any tooth of the corresponding track is radially aligned with the sensor each MR element provides an MR output that is greater than the output MR matching of the other MR sensor. In order for this to be achieved, the distance designated xx must be less than the distance 5 designated y2 / o ^ < y2. This ensures the MR output intersection and consequently the angular information via the differential signal processing, as described.

Therefore, a first relationship that must be considered is between the distance d from the center line of the sensor to the MR element and the maximum axial deviation? . Assuming a relation where d =?, The distance L must be more than double the difference between the maximum axial deviation? and the distance d from the center line of the sensor to the element 5 MR, or L > 2 * (? D), and (a) the width of the objective wheel W must be greater than twice the sum of the maximum axial deviation? and the distance d from the centerline of the sensor to the element * MR, or > 2 * (? + D), or (b) the distance L must be greater than four times the maximum axial deviation? minus the width of the objective wheel W, or L > 4?- . Where condition (a) has been filled, condition (b) necessarily follows; however, where a condition (b) is filled, condition (a) may or may not be filled. If on the other hand d > ?, the distance L must be greater than four times the maximum axial deviation? less the width of the objective wheel (the distance between the respective axially opposite side edges), or L > 4? -W.

With a sensor filling the geometric limitations established above to ensure the intersection of MR outputs, the current angle of the intersection will occur at some angle which varies with the real axial displacement. In the illustration of Figure 3, the actual axial displacement d is equal to the maximum axial displacement? and the geometry is such • that d > ? Therefore for the general geometry and the direction of axial displacement as shown in figure 3, the angle of intersection? D relative to the desired angle? Network is as shown in figure 4. The axial deviation to the opposite side of the center line of the sensor will of course change the current intersection angle? Gives the left of the desired angle dred in similar way. An examination of Figures 3 and 4 gives a understanding that the distance xx increases when the axial displacement begins to move MRl beyond the edge of the respective track. As such, the upper peak output MRl is at a relative minimum (solid indicia) when the axial displacement, and hence the distance xx, is at a maximum. For the axial displacements which minimize the distance x-_, for example those which do not move MRl beyond the edge of the respective track, the high peak output MRl approaches a relative maximum (broken sign). Similarly, the low peak output MR2 is at a relative maximum (solid indicia) when the axial displacement is at a maximum and therefore the distance y2 w is at a minimum. For the axial displacements to which the ferrite effects of the track correspond to MRl are insignificant on MR2, for example those ahead to the right in Figure 2, the low peak MR2 approaches a relative minimum (broken sign). Referring now to Figure 5A and 5B, a rotation sensor according to the second embodiment of the present invention is generally designated with the number 10". The portions of the present sensors that are identical to the 20 portions of the previously described sensors are similarly labeled in the present figure and will be understood to be fully described by the previous descriptions.

The magnetoresistive sensor of the dual element (MR) generally designated with the number 11 cooperates with a rotating dual track target wheel generally designated with the number 25 '. Again, reference may be made to a single-objective wheel 25 'or to a pair of target wheels 25' A and 25 'B which may be physically distinct or parts of a unitary structure as illustrated. Both target wheels 25 'A and 25' B can be referred to together as the target wheel 25 'or individually as the target wheel 25' A or 25 'B. In addition to the rotation sensor 10 '' accommodating the axial deviations of the crank shaft and the wheel of - objective to ensure the operability of the sensor, the sensor? it also ensures that the intersection angle of the respective MR outputs does not vary with the axial deviations. This means that the intersection of the respective MR outputs is made invariably at all axial deviations anticipated by selectively choosing the various dimensions described so that the respective distances between each MR element and the radially aligned teeth of the corresponding track are equivalent for all * axial nations. a f »* e that is Xogre, Xa Ran ia designated x1 must be equivalent to the designated distance x2, or X-L = x2. This relationship can be expressed as an additional construction on the MR space as follows: 2d = / 2 + L / 2.

With a sensor filling the geometric limitations established above to ensure the intersection of the MR outputs at equivalent average values, the current intersection angle will occur at the desired angle? Network is as shown in figure 6. Axial deviations to one side or the other of the sensor center line will not cause the angle of intersection deviates from? network. An examination of Figures 5A and 5B gives an understanding that the distances x1 and x2 are minimum equivalents to all axial displacements where the MR elements are above the respective tracks, and increase from the minimum, however, remaining equivalent. one another, when the axial displacement begins to move each MR element beyond the edge of the respective track. The outputs MR corresponding to the general case illustrated in Figure 5A are shown as solid indicia in Figure 6, and the outputs MR corresponding to the general case illustrated in Figure 5B are shown as broken indicia in Figure 6. It is noted that the symmetry output MR remained at extreme axial deviations.

Therefore, of the above, From these general relationships, the designer is able to work within the constraints of the particular request (for example a fixed or maximum target wheel width, an available MR item spacing, etc.) to arrive at an acceptable set of dimensions complementary to ensure the operability of the sensor through axial deviations of the rotating member, where a greater degree of angular accuracy is desired.

It is, however, more desirable to provide a relationship between the components which ensures that even at the maximum axial deviations the MR elements remain directly above the respective track as this will result in consistent output amplitudes over the range of axial deviations. Furthermore, it is more desirable to ensure that only the significant ferrite effects on each MR element come from the teeth of a corresponding track. Therefore, the distance L is understood as being sufficient to ensure that the transverse ferrite track affects the MR sensors when there is a maximum axial displacement that will not have a substantial effect on the respective lower peak output MR. As such, the lower peak outputs MR will remain virtually equivalent across all axial deviations. Additionally, where an implementation is such that the axial deviations can result in MR elements hanging over the respective tracks, it is desirable to put the depth of the groove between the tracks so that any ferrite effects of the same equal those imposed on them. the outer tracks. This will also ensure that the high and low peak outputs MR will remain virtually equivalent across all axial deviations.

The preferred embodiment illustrated in Figures 5A # and 5B can be manufactured with minimum machining operations ÍU by setting the sections generally corresponding to the individual tracks as any single unitary structure using known metal powder forging operations (PMF) or by setting the sections separately in similar PMF operations and then join the sections just like during a sintering operation normally practiced on such produced components of PMF. Of course, the union of the objective wheel to the motor crank shaft can * require that the frames provide the necessary opening through the rotational axis of the objective wheel or additional post-sintering machining operations providing such provision. The low green resistance will usually preclude a significant machining operation before sintering. Alternatively, conventional stamping operations on the sheet supply ferromagnetic to provide the two general track sections and then join them in proper alignment.

Preferably, however, the modality is more • Easily produced as an integral part of the motor crank shaft. The setting of the crank shaft of the motor is preferably produced in a conventional manner with a dual lens wheel section 5 on a part thereof coaxial with the rotational axis. This process is a variation of the crank axle frames where a single track target wheel section is set in an appropriate part of the -, same. Preferably, the setting can take the form of an annular span supported by the crank shaft. A first milling operation is carried out to the full length to the outer diameter of the teeth followed by material removal operations such as by rotating carriers. Generally, the rotary cutters brought from the side of the crank shaft will remove the material to the desired depth to establish the slot in the center of the target wheel the discontinuities. The rotary cutters therefore result in a border at the base of each tooth as shown in Figures 5A and 5B. Such manufacturing processes are generally well known.

The embodiments described and the illustration cover virtually solid ferromagnetic structures. However, the ferromagnetic target wheels may comprise non-magnetic hub members 5 with one or more virtually annular ferromagnetic outer members secured thereto to provide the necessary toothed tracks. Therefore, you must • understand that any target wheel needs only to comprise ferromagnetic material in the parts tending to have a substantial influence on the operation of the sensor.

Although the invention has been described with respect to certain exemplary preferred embodiments, it is understood that certain alterations thereto will be apparent to those with ordinary skill in the art and are intended to be encompassed by the scope of the invention as set forth in the appended claims. *

Claims (10)

Claims •

1. An apparatus for sensing rotation of the member about an axis comprising: the first and second objective wheels, fixedly coupled to said member in an axially spaced vicinity; # a dual-element magnetoresistive sensor having the first and second magnetoresistive elements axially aligned, said first magnetoresistive element corresponding to said first and virtually aligned target wheel and said second magnetoresistive element 15 corresponding to said second target wheel and virtually aligned therewith, each of the first and second magnetoresistive elements having the respective outputs? ß which vary according to the magnetic flux density therethrough; Said first and second objective wheels are characterized by alternating areas of high and low permeability as seen by the dual element magnetoresistive sensor when the member is rotated, each high permeability area of said The first target wheel being virtually aligned with one of said areas of low permeability of said second target wheel, each low permeability area of said second target wheel, each low permeability area of said first target wheel being virtually aligned with one of said high permeability areas of said second target wheel; wherein for the predetermined axial displacements of the rotation member the first target wheel is closer to the first magnetoresistive element than the second magnetoresistive element, and the second The objective is closer to the second magnetoresistive element than to the first magnetoresistive element.

2. An apparatus for sensing the rotation of a member about an axis as claimed in clause 1 characterized in that for the predetermined axial displacements of the rotation member the distance between the first magnetoresistive element and the first target wheel is virtually equivalent at the distance between the second magnetoresistive element and the second objective wheel.

3. An apparatus for sensing the rotation of a member about an axis as claimed in clause 2, characterized in that for the predetermined axial displacements of the rotation member both elements 25 magnetoresistives remain radially aligned with the respective target wheel.

4. An apparatus for sensing the rotation of a crank shaft of an internal combustion engine comprising: an engine crank shaft having a shaft 5 rotation, said crank axis characterized by axial displacements on each side of a network position within said motor; the first and second target wheels coupled = fixedly to said crank axis in an axially or spaced closeness, each target wheel has a respective axial center at the side edges; a dual element magnetoresistive sensor having the first and second magnetoresistive elements axially aligned separated by an axial distance virtually corresponding to the distance between said respective axial centers, said first magnetoresistive element corresponding to said first target wheel and virtually aligned therewith when said crank axis is at the network position said second magnetoresistive element corresponding to said second target wheel and virtually aligned therewith when said crank axis is in the network position, each of the first and second magnetoresistive elements having the respective outputs which 5 vary according to the density of the magnetic flux therethrough; said first and second objective wheels characterized by alternating areas of high and low permeability as seen by the dual element magnetoresistive sensor by rotating the objective wheels with the crank axis each high permeability area of said objective wheel being virtually aligned with one of said low permeability areas of said second target wheel, each low permeability area of said first target wheel being virtually aligned with one of said high permeability areas of said second target wheel; wherein for the axial displacements of the crank axis the first target wheel is closer to the first magnetoresistive element than with respect to the second magnetoresistive element and the second target wheel is closer to the second magnetoresistive element than to the first magnetoresistive element.

5. An apparatus for sensing the rotation of a motor crank shaft 20 about an axis as claimed in clause 4 characterized in that for the axial displacements of the crank shaft the distance between the first magnetoresistive element and the first target wheel is virtually equivalent to the distance between the second magnetoresistive element 25 and the second objective wheel.

6. An apparatus for perceiving the rotation of an axis * motor crank of an axis as claimed in clause 5 characterized in that for the axial displacements of the crank axis both magnetoresistive elements remain radially aligned with respect to the target wheels.

7. An apparatus for sensing the rotation of a motor crank shaft about an axis as claimed in clause 4 characterized in that the axial or maximum displacement from said network position is less than one half of the axial distance separating the elements magnetoresistive and said first and second objective wheels are separated by a distance greater than four times the maximum axial displacement minus the distance between the opposite lateral edges 5 respectively.

8. An apparatus for sensing the rotation of a crank axis about an axis as claimed in clause 4 characterized in that the maximum axis of displacement 0 from said network position is not less than one half of the axial distance separating the elements magnetoresistive, said first and second objective wheels are separated by a distance greater than twice the maximum axial displacement minus the distance separating the magnetoresistive elements, and the distance between the respective axially opposed side edges is greater than twice the maximum axial displacement more the distance overcoming the elements * magnetoresistivos.

9. An apparatus for sensing the rotation of a crank axis about an axis as claimed in clause 4 characterized in that the maximum axial displacement from the network position is not less than one half of the axial distance separating the magnetoresistive elements, said first and second objective wheels are separated by a distance greater than twice the maximum axial displacement minus the distance separating the magnetoresistive elements, and said distance separating the first and second objective wheels is greater than four times the maximum axial displacement less the distance between the respective lateral or axially opposite edges.

10. An apparatus for sensing the rotation of a crank axis about an axis as claimed in clause 9, characterized in that the distance between the respective axially opposite side edges is greater than twice the maximum axial displacement from said network position. less than one half of the axial distance separating the magnetoresistive elements, said first and second objective wheels are separated by a distance greater than four times the maximum axial displacement minus the distance between the respective axially opposite side edges, and twice the | tt The axial distance separating the magnetoresistive elements is virtually equivalent to the sum of the distance between the respective axially opposite side edges and the distance separating the first and second objective wheels. 12. An apparatus for sensing the rotation of a crank shaft of an internal combustion engine comprising: i a motor crank shaft having an axis of rotation, said crank axis is characterized by axial displacements on either side of a network position within said motor; the first and second target wheels coupled to said crank axis in an axially spaced vicinity, each target wheel having a respective axial center and side edges; a dual element magnetoresistive sensor 20 having axially aligned first and second magnetoresistive elements separated by an axial distance virtually corresponding to the distance between said respective axial centers, said first magnetoresistive element corresponding to said first target wheel and virtually aligned therewith when the crank axis is in said network position and said second magnetoresistive element corresponds to the second target wheel and virtually aligned therewith when said crank axis is in the network position, each of the first and second magnetoresistive elements having the respective outputs which vary according to the magnetic flux density therethrough; the first and second objective wheels are characterized by alternating areas of high and low permeability as seen by the dual element magnetoresistive sensor when rotating the objective wheels with the crank axis, each high permeability area of said first objective wheel being virtually aligned with one of said low permeability areas of said second target wheel, each low permeability area of said first target wheel being virtually aligned with one of said high permeability areas of said second target wheel; wherein for the axial displacements of the crank axis the first target wheel is closer to the first magnetoresistive element than to the second magnetoresistive element, the second target wheel is closer to the second magnetoresistive element than to the first magnetoresistive element, and the distance between the first magnetoresistive element and the first objective wheel is virtually equivalent to the distance between the second magnetoresistive element and the second target wheel