Abstract:
A method and apparatus wherein a single dual element galvanomagnetic sensor, herein exemplified by a single dual element magnetoresistive sensor ( 16 ′), is utilized to sense crankshaft position and rotational speed from the passage of single tooth edges ( 12 ′) of a target wheel ( 10 ′) by continuous adaptive matching of both MR output signals (V MR1′ , V MR2′ ) during sensor operation. Over a slot ( 28 ′) or tooth ( 26 ′), both MR output signals should be equal, and if not, they are matched by adjusting the current of one of the current sources ( 30′, 32′ ) driving the MRs, performed over a slot or a tooth. Due to higher magnetic sensitivity at smaller air gaps, matching by current adjustment over a tooth is preferred. In a preferred embodiment of the present invention, one MR is driven by a constant current source ( 30′ ) while the other MR is driven by a voltage controlled current source ( 32′ ). The output voltages of the MRs approximate the tooth pattern of the target wheel; they are high over a tooth and low over a slot. Hence, the signals over the teeth can be simply acquired by means of a peak detector ( 36   a,    36   b ), one for each MR. The signals from the peak detectors are subtracted from each other in a comparator ( 36   c ), and then used as a control voltage (V C ) for a voltage controlled current source changing its preset nominal current to a value that minimizes the mismatch of MR signals. If matching of MR signals over slots would be desired, valley detectors would be substituted for the peak detectors.

Description:
TECHNICAL FIELD 
     The present invention relates to a method of sensing precise angular position and speed of rotation of a rotating object and more particularly to a method for sensing crankshaft or camshaft rotational position and speed of rotation wherein a sensor, preferably but not exclusively a single dual element magnetoresistive (MR) sensor, is utilized to sense crankshaft position and rotational speed from the passage of single tooth edges of an encoder or target wheel. 
     BACKGROUND OF THE INVENTION 
     It is well known in the art that the resistance modulation of Hall elements or magnetoresistors can be employed in position and speed sensors with respect to moving magnetic materials or objects (see for example U.S. Pat. Nos. 4,835,467, 4,926,122, and 4,939,456). In such applications, the magnetoresistor (MR) is biased with a magnetic field and electrically excited, typically, with a constant current source or a constant voltage source. A magnetic (i.e., ferromagnetic) object rotating relative, and in close proximity, to the MR, such as a toothed wheel, produces a varying magnetic flux density through the MR, which, in turn, varies the resistance of the MR. The MR will have a higher magnetic flux density and a higher resistance when a tooth of the rotating target wheel is adjacent to the MR than when a slot of the rotating target wheel is adjacent to the MR. The use of a constant current excitation source provides an output voltage from the MR that varies as the resistance of the MR varies. 
     Accurate engine crank position information is needed for ignition timing and OBDII mandated misfire detection. Increasingly more sophisticated spark timing and emission controls introduced the need for crankshaft sensors capable of providing precise position information during cranking. Various combinations of magnetoresistors and single and dual track toothed or slotted wheels (also known as encoder wheels and target wheels) have been used to obtain this information (see for example U.S. Pat. Nos. 5,570,016, 5,714,883, 5,731,702, and 5,754,042). 
     The crank position information is encoded on a rotating target wheel in the form of teeth and slots. The edges of the teeth define predetermined crank positions. The sensor is required to detect these edges accurately and repeatably over a range of air gaps and temperatures. Virtually all such sensors are of the magnetic type, either variable reluctance or galvanomagnetic (e.g., Hall generators or magnetoresistors). Galvanomagnetic sensors are becoming progressively most preferred due to their capability of greater encoding flexibility and speed independent output signals. 
     Furthermore, temperature and the size of the air gap affect the output signal of a magnetic sensing element. Consequently, operation over wide temperature and air gap size ranges requires some form of compensation for the resultant signal drift, both in amplitude and offset. The most common approach is the use of two matched sensing elements operating in a differential mode thereby providing a common mode rejection. 
     High accuracy and repeatability magnetic position sensors employ two matched sensing elements such as magnetoresistors (MR) or Hall generators. They are spaced a few mm apart from each other, either in the axial direction (dual track target wheels) or along the target periphery (sequential sensors). The primary purpose of using two matched sensing elements is common mode signal rejection, since the sensing elements are equally affected by temperature and air gap. Having perfectly matched sensor elements, however, is not sufficient. The uniformity of the bias magnet, packaging tolerances, and inaccuracies of sensor installation can introduce unknown offsets to the output signals of the sensing elements. Presently, selection of matched MR pairs, a tight process control during all phases of sensor manufacture with a final testing of each sensor, is employed to build sensors meeting the required specifications. Unfortunately, this approach increases the final cost of the sensor. 
     Angular position information is contained in the location of target wheel tooth edges (i.e., tooth/slot transitions), and at these locations the output signals of the MRs are by design unequal so that their differential signal is nonzero. Over a slot or tooth, both MR output signals should be equal so that their differential signal is zero but, frequently, the MRs are not well matched resulting in a nonzero differential signal causing an erroneous output signal and switching leading to an incorrect crank position and speed of rotation. 
     An example of such a sensor is the sequential crankshaft sensor used on several of General Motors Corporation trucks. This sensor employs two InSb magnetoresistor elements located radially proximate to the target wheel, one being slightly displaced with respect to the other in the direction of target wheel rotation. FIG. 1 is a schematic representation of an exemplar automotive environment of use according to this prior art scheme, wherein a target wheel  10  is rotating, such as for example in unison with a crankshaft, a drive shaft or a cam shaft, and the rotative position thereof is to be sensed. Rotative position of the target wheel  10  is determined by sensing the passage of a tooth edge  12 , either a rising tooth edge  12   a  or a falling tooth edge  12   b , using a single dual MR differential sequential sensor  14 . A tooth edge  12  is considered rising or falling depending upon the direction of rotation of the target wheel  10  with respect to the magnetoresistive sensors MR 1  and MR 2 . MR 1  is considered leading and MR 2  is considered lagging if the target wheel  10  is rotating in a clockwise (CW) direction whereas if the target wheel is rotating in a counterclockwise (CCW) direction then MR 1  is considered lagging whereas MR 2  is considered leading. For purposes of example, the target wheel  10  will be assumed to be rotating in a CW direction in the views. 
     The single dual MR differential sequential sensor  14  employs two magnetoresistor elements, MR 1  and MR 2 , which are biased by a permanent magnet  16 , wherein the magnetic flux  18  and  20  emanating therefrom are represented by the dashed arrows. The magnetic flux  18  and  20  pass from the permanent magnet  16  through the magnetoresistors MR 1  and MR 2  and through the air gaps  22  and  24  to the target wheel  10 . The target wheel  10  is made of a magnetic material having teeth  26  and spacings  28  therebetween. The spacing L between MR 1  and MR 2  is generally such that the trigger points for the rising and falling edges of the output signal V OUT  are dependent on the leading MR only, as will be later described. 
     Power V IN  is supplied to CURRENT SOURCE1  30  and CURRENT SOURCE2  32  through voltage source  34 . Power is also supplied to a comparator  36  (with hysteresis) through voltage source  34 , but is not shown. CURRENT SOURCE1  30  supplies current to MR 1  thereby providing for an output voltage V MR1  from MR 1 . CURRENT SOURCE2  32  supplies current to MR 2  thereby providing for an output voltage V MR2  from MR 2 . Output voltages V MR1 , and V MR2  are input into the comparator  36  whose output voltage V OUT  is an indication of the position of rotation of the target wheel  10 . It is to be understood that all voltages are measured with respect to ground unless otherwise indicated herein, and that CURRENT SOURCE1  30  is matched to CURRENT SOURCE2  32 . 
     In a first example, wherein the two MR elements are matched, as shown in FIG. 2A, the lagging MR element, in this case MR 2 , provides a delayed signal in every respect identical to the signal from the leading MR, in this case MR 1 . The differential signal V D =V MR1 −V MR2 , shown in FIG. 2B is electronically generated within the comparator  36  and is then used by the comparator to reconstruct the signal V OUT  (shown in FIG. 2C) emulating the profile of the target wheel  10 . Upon a closer inspection of FIGS. 2A,  2 B and  2 C, it becomes evident that the rising edges  42  and the falling edges  44  of the sensor output signal V OUT  are determined only by first points  46  corresponding to the rising edges and second points  48  corresponding to the falling edges where the signal from the leading MR, in this example MR 1 , crosses a first threshold voltage  50  corresponding to the first points and a second threshold voltage  52  corresponding to the second points wherein the first and second threshold voltages are determined by the hysteresis applied to the comparator  36 . The lagging MR, in this example MR 2 , has no part in the generation of the rising edges  42  or the falling edges  44  of the output signal V OUT . The lagging MR, MR 2 , in this example, has the same offset voltage  54  as the leading MR, MR 1 , thereby leading to a zero voltage difference in the differential signal V D =V MR1 −V MR2  whenever MR 1  and MR 2  are not adjacent to or in close proximity to a rising tooth edge  12   a  or a falling tooth edge  12   b  of the target wheel  10  due to the matching of the MRs as depicted by signal line  54   a  in FIG.  2 B. 
     As previously stated, over a slot or tooth, both MR output signals should be equal so that their differential signal is zero but, frequently, the MRs are not well matched resulting in a nonzero differential signal causing switching errors and an erroneous output signal leading to incorrect crank positions and speeds of rotation. Mismatch of the MRs can occur due to offset differences resulting in different bias voltages for each MR or due to gain (sensitivity) differences resulting in different signal amplitudes for each MR or due to a combination of offset and gain differences between the MRs. 
     FIG. 3A is a second example of a schematic representation of an exemplar automotive environment of use according to this prior art scheme wherein the two MR elements of the sensor, configured as in FIG. 1, are mismatched due to a gain error wherein MR 1  has a lower voltage V′ 1H  over a tooth  26  than the voltage V′ 2H  over the same respective tooth produced by MR 2  whereas both MRs have the same voltage V 12  over a slot  28  of the target wheel  10 . 
     In this second example, as shown in FIG. 3A, the lagging MR element, in this case MR 2 , provides a delayed signal having a voltage gain offset V′ 2H −V′ 1H  over a tooth  26 . The differential signal V′ D =V′ MR1 −V′ MR2 , shown in FIG. 3B is electronically generated within the comparator  36  and is then used by the comparator to reconstruct the signal V′ OUT  (shown in FIG. 3C) which should emulate the profile of the target wheel  10 . Upon a closer inspection of FIGS. 3A,  3 B and  3 C, it becomes evident that the rising edges  42 ′ of the sensor output signal V′ OUT  are determined only by first points  46 ′ corresponding to the rising edges where the signal from the leading MR, in this example MR 1 , crosses a first threshold voltage  50 ′ corresponding to the first points wherein the first threshold voltage is determined by the positive hysteresis +ΔV′ applied to the comparator  36  whereas the falling edges  44 ′ of the sensor output signal V′ OUT  are determined only by second points  48 ′ corresponding to the falling edges where the signal from the lagging MR, in this example MR 2 , crosses a second threshold voltage  52 ′ corresponding to the second points wherein the second threshold voltage is determined by the negative hysteresis −ΔV′ applied to the comparator. 
     However, in this particular example, due to the voltage gain error there is a nonzero voltage difference in the differential signal V′ D =V′ MR1 −V′ MR2  whenever MR 1  and MR 2  are adjacent to or in close proximity to a tooth  26  of the target wheel  10  due to the mismatching of the MRs as depicted in FIG. 3B by voltage level V′ DL  wherein this example V′ DL  is at a lower voltage level than the negative hysteresis −ΔV′ applied to the comparator  36 . As shown in FIGS. 3B and 3C, this results in switching errors causing an erroneous output voltage V′ OUT  which does not emulate the profile of the target wheel  10 . 
     FIG. 4A is a third example of a schematic representation of an exemplar automotive environment of use according to this prior art scheme wherein the two MR elements of the sensor, configured as in FIG. 1, are mismatched due to an offset error wherein MR 1  has a higher bias voltage V″ MR1  over a tooth  26 , represented by V″ 1H , and a slot  28 , represented by V″ 1L , than the bias voltage V″ MR2  over the same respective teeth, V″ 2H , and slots, V″ 2L , produced by MR 2 . 
     In this third example, as shown in FIG. 4A, the lagging MR element, in this case MR 2 , provides a delayed signal in every respect identical to the signal from the leading MR, in this case MR 1 , except for the bias voltage offset V″ 1H −V″ 2H  over a tooth  26  and V″ 1L −V″ 2L  over a slot  28 . The differential signal V″ D =V″ MR1 −V″ MR2 , shown in FIG. 4B is electronically generated within the comparator  36  and is then used by the comparator to reconstruct the signal V″ OUT  (shown in FIG. 4C) which should emulate the profile of the target wheel  10 . Upon a closer inspection of FIGS. 4A,  4 B and  4 C, it becomes evident that the rising edges  42 ″ of the sensor output signal V″ OUT  are determined only by first points  46 ″ corresponding to the rising edges where the signal from the leading MR, in this example MR 1 , crosses a first threshold voltage  50 ″ corresponding to the first points wherein the first threshold voltage is determined by the positive hysteresis +ΔV″ applied to the comparator  36  whereas the falling edges  44 ″ of the sensor output signal V″ OUT  are determined only by second points  48 ″ corresponding to the falling edges where the signal from the lagging MR, in this example MR 2 , crosses a second threshold voltage  52 ″ corresponding to the second points wherein the second threshold voltage is determined by the negative hysteresis −ΔV″ applied to the comparator. 
     However, in this particular example, the lagging MR, MR 2 , has an offset voltage  54   b ″ which is not the same as the offset voltage  54   a ″ of the leading MR, MR 1 , thereby leading to a nonzero voltage difference in the differential signal V″ D =V″ MR1 −V″ MR2  whenever MR 1  and MR 2  are not adjacent to or in close proximity to a rising tooth edge  12   a  or a falling tooth edge  12   b  of the target wheel  10  due to the mismatching of the MRs as depicted in FIG. 4B by voltage level V″ DH  wherein this example V″ DH  is at a higher voltage level than the positive hysteresis +ΔV″ applied to the comparator  36 . As shown in FIGS. 4B and 4C, this results in switching errors causing an erroneous output voltage V″ OUT  which does not emulate the profile of the target wheel  10 . 
     What is needed is a method and apparatus wherein continuous adaptive matching of both MR output signals during sensor operation of a single dual element sensor, preferably, but not exclusively, a single dual element magnetoresistive sensor, is utilized to sense crankshaft position and rotational speed from the passage of single tooth edges of an encoder or target wheel. 
     SUMMARY OF THE INVENTION 
     The present invention provides a method and apparatus wherein a single dual element galvanomagnetic sensor, herein exemplified by a single dual element magnetoresistive sensor, is utilized to sense crankshaft position and rotational speed from the passage of single tooth edges of an encoder or target wheel by continuous adaptive matching of both MR output signals during sensor operation. This eliminates the need for precise MR die matching, lowering their cost. Furthermore, the yield of good sensors would be significantly higher. And finally, the installed sensor would be less affected by variations during its service. 
     Over a slot or tooth, both MR output signals should be equal but, frequently, they are not well matched. However, during this time, they could be matched by adjusting the current of at least one of the current sources driving the MRs, which could be done over a slot or tooth via comparison of the voltage extrema, be that a maximum using peak detectors, or a minimum using valley detectors. Due to higher magnetic sensitivity at smaller air gaps, matching by current adjustment over a tooth is preferred. 
     In a preferred embodiment of the present invention, one MR is driven by a constant current source while the other MR is driven by a voltage controlled current source. The output voltages of the MRs approximate the tooth pattern of the target wheel; they are high over a tooth and low over a slot. Hence, the signals over the teeth can be simply acquired by means of a peak detector, one for each MR. The signals from the peak detectors are subtracted from each other and, optionally, amplified. This signal is now used as a control voltage which is fed into a voltage controlled current source changing its preset nominal current to a value that minimizes the mismatch of MR signals. If, for whatever reason, matching of MR signals over slots would be desired, valley detectors would be substituted for the peak detectors. The actual sensor output signal is derived using a comparator with hysteresis. 
     Accordingly, it is an object of the present invention to provide a digital output voltage for detecting angular position of a rotating target wheel utilizing a single dual element sensor by continuous adaptive matching of the MR signals over a tooth or a slot of a target wheel. 
     This, and additional objects, advantages, features, and benefits of the present invention will become apparent from the following specification. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic representation of an exemplar automotive environment of use of a prior art dual MR position sensor. 
     FIG. 2A depicts a plot of MR voltages for a CW rotation of the target wheel according to the prior art wherein the MR elements are matched. 
     FIG. 2B depicts a plot of the differential voltage V D =V MR1 −V MR2  of FIG. 2A according to the prior art wherein the MR elements are matched. 
     FIG. 2C depicts a plot of the output voltage V OUT  of the comparator according to the prior art wherein the MR elements are matched. 
     FIG. 3A depicts a plot of MR voltages for a CW rotation of the target wheel according to the prior art wherein the MR elements have a gain mismatch over a tooth. 
     FIG. 3B depicts a plot of the differential voltage V′ D =V′ MR1 −V′ MR2  of FIG. 3A according to the prior art wherein the MR elements have a gain mismatch over a tooth. 
     FIG. 3C depicts a plot of the output voltage V′ OUT  of the comparator according to the prior art wherein the MR elements have a gain mismatch over a tooth. 
     FIG. 4A depicts a plot of MR voltages for a CW rotation of the target wheel according to the prior art wherein the MR elements have an offset mismatch. 
     FIG. 4B depicts a plot of the differential voltage V″ D =V″ MR1 −V″ MR2  of FIG. 4A according to the prior art wherein the MR elements have an offset mismatch. 
     FIG. 4C depicts a plot of the output voltage V″ OUT  of the comparator according to the prior art wherein the MR elements have an offset mismatch. 
     FIG. 5A depicts an example of a preferred environment of use of a dual MR position sensor according to the present invention. 
     FIG. 5B depicts an example of the signal conditioning circuit of FIG.  5 A. 
     FIG. 5C depicts an example of a valley detector. 
     FIG. 6A depicts a plot of the MR voltage for a CW rotation of the target wheel according to the present invention wherein the MR elements, having a gain mismatch are matched over the tooth. 
     FIG. 6B depicts a plot of the differential voltage V′″ D =V′″ MR1 −V′″ MR2  of FIG. 6A according to the present invention wherein the MR elements, having a gain mismatch are matched over the tooth. 
     FIG. 6C depicts a plot of the output voltage V″″ OUT  of the comparator according to the present invention wherein the MR elements, having a gain mismatch are matched over the tooth. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIG. 5A is a schematic representation of an exemplar automotive environment of use of the present invention, wherein a target wheel  10 ′ is rotating, such as for example in unison with a crankshaft, a drive shaft or a cam shaft, and the rotative position and/or speed thereof is to be sensed. Rotative position of the target wheel  10 ′ is determined by sensing the passage of a tooth edge  12 ′, either a rising tooth edge  12   a ′ or a falling tooth edge  12   b ′, using a single dual MR differential sequential sensor  14 ′. A tooth edge  12 ′ is considered rising or falling depending upon the direction of rotation of the target wheel  10 ′ with respect to the magnetoresistive sensors MR 1 ′ and MR 2 ′. MR 1 ′ is considered leading and MR 2 ′ is considered lagging if the target wheel  10 ′ is rotating in a clockwise (CW) direction. Whereas, if the target wheel is rotating in a counterclockwise (CCW) direction then MR 1 ′ is considered lagging and MR 2 ′ is considered leading. For purposes of example, the target wheel  10 ′ will be assumed to be rotating in a CW direction in the views. 
     The single dual MR differential sequential sensor  14 ′ employs two magnetoresistor elements, MR 1 ′ and MR 2 ′, which are biased by a permanent magnet  16 ′, wherein the magnetic flux  18 ′ and  20 ′ emanating therefrom is represented by the dashed arrows. The magnetic flux  18 ′ and  20 ′ passes from the permanent magnet  16 ′ through the magnetoresistors MR 1 ′ and MR 2 ′ and through the air gaps  22 ′ and  24 ′ to the target wheel  10 ′. The target wheel  10 ′ is made of a magnetic material having teeth  26 ′ and spacings  28 ′ therebetween. The spacing L′ between MR 1 ′ and MR 2 ′ is generally such that the trigger points for the rising and falling edges of the output signal V OUT′  are dependent on the leading MR only as will be later described. 
     Power V′ IN  is supplied to CURRENT SOURCE1  30 ′ and CURRENT SOURCE2  32 ′ through voltage source  34 ′. Power is also supplied to a signal conditioning circuit  36 ′ through voltage source  34 ′ (the connection therefor not being shown). CURRENT SOURCE1  30 ′ supplies current to MR 1 ′ thereby providing for an output voltage V MR1′ . from MR 1 ′. CURRENT SOURCE2  32 ′ supplies current to MR 2 ′ thereby providing for an output voltage V MR2′  from MR 2 ′. Output voltages V MR1′  and V MR2′  are input into the signal conditioning circuit  36 ′ whose output voltage V OUT′  is an indication of the position of rotation of the target wheel  10 ′. It is to be understood that all voltages are measured with respect to ground unless otherwise indicated herein, and that CURRENT SOURCE1′  30 ′ is constant current source whereas CURRENT SOURCE2′  32 ′ is a voltage controlled current source. The implementation of CURRENT SOURCE1  30 ′ and CURRENT SOURCE2  32 ′ are well known in the art. 
     FIG. 5B depicts the signal conditioning circuit  36 ′ consisting of DETECTOR1  36   a , DETECTOR2  36   b , differential amplifier  36   c  (with gain), and comparator  36   d . To match MR 1 ′ and MR 2 ′ over a tooth  26 ′ of the target wheel  10 ′, DETECTOR1  36   a  and DETECTOR2  36   b  would both be peak detectors wherein DETECTOR1 would detect the maximum voltage of MR 1 ′, V MR1′ , over a tooth and DETECTOR2 would detect the maximum voltage of MR 2 ′, V MR2′ , over a tooth. Whereas, to match MR 1 ′ and MR 2 ′ over a slot  28 ′ of the target wheel, DETECTOR1 and DETECTOR2 would both be valley detectors wherein DETECTOR1 would detect the minimum voltage of MR 1 ′, V MR1′ ., over a slot and DETECTOR2 would detect the minimum voltage of MR 2 ′, V MR2′ , over a slot. The outputs of DETECTOR1  36   a  and DETECTOR2  36   b  are fed into differential amplifier  36   c  producing a differential peak voltage V C . The differential peak voltage V C  is applied to the voltage controlled current source CURRENT SOURCE2′  32 ′ as a control voltage which changes the preset nominal current of CURRENT SOURCE2′ to a value that minimizes the mismatch of MR 1 ′ and MR 2 ′ over a tooth  26 ′ or slot  28 ′ according to the actualization of DETECTOR1  36   a  and DETECTOR2  36   b  as peak detectors or valley detectors as previously described. 
     Initially, CURRENT SOURCE2′  32 ′ is preset for initial turn-on to have a nominal current which, for example, may be the same value as the current of CURRENT SOURCE1′  30 ′. In FIGS. 5A and 5B, a positive differential control voltage V C  indicates that the voltage of MR 1 ′, V MR1′ , over a tooth  26 ′ is greater than the voltage of MR 2 ′, V MR2′ , over a tooth if DETECTOR1  36   a  and DETECTOR2  36   b  are peak detectors or that the voltage of MR 1 ′, V MR1′ , over a slot  28 ′ is greater than the voltage of MR 2 ′, V MR2′ , over a slot if DETECTOR1  36   a  and DETECTOR2  36   b  are valley detectors. In this case, V C  would increase the current of CURRENT SOURCE2′  32 ′ to a new value proportional to the magnitude of V C  thereby increasing the voltage of MR 2 ′, V MR2′ , toward the value of the voltage of MR 1 ′, V MR1′ , over a tooth  26 ′ or slot  28 ′ thus tending to match MR 1 ′ and MR 2 ′. On the other hand, a negative differential control voltage V C  indicates that the voltage of MR 1 ′, V MR1′ , over a tooth  26 ′ is less than the voltage of MR 2 ′, V MR2′ , over a tooth if DETECTOR1  36   a  and DETECTOR2  36   b  are peak detectors or that the voltage of MR 1 ′, V MR1′ , over a slot  28 ′ is less than the voltage of MR 2 ′, V MR2′ , over a slot if DETECTOR1  36   a  and DETECTOR2  36   b  are valley detectors. In this case, V C  would decrease the current of CURRENT SOURCE2′  32 ′ to a new value proportional to the magnitude of V C  thereby decreasing the voltage of MR 2 ′, V MR2′ , toward the value of the voltage of MR 1 ′, V MR1′ , over a tooth  26 ′ or slot  28 ′ thus tending to match MR 1 ′ and MR 2 ′. 
     V MR1′  and V MR2′  are also input to comparator  36   d  (with hysteresis) producing the output voltage V OUT′  which emulates the profile of the target wheel  10 ′. The operation of comparator  36   d  is commensurate to the operation of comparator  36  in FIG.  1  and has been previously described. 
     In a first example of the present invention, wherein MR 1 ′ and MR 2 ′ are matched, the voltage V MR1′  over a tooth  26 ′ has the same value as the voltage V MR2′  over the same respective tooth and the voltage V MR1′  over a slot  28 ′ has the same value as the voltage V MR2′  over the same respective slot. 
     Using peak detectors for DETECTOR1  36   a  and DETECTOR2  36   b  to match MR 2 ′ to MR 1 ′ over a tooth  26 ′ generates equal peak values for the voltages on signal line  36   a ′ and  36   b ′ which are input to the differential amplifier  36   c  producing a differential control voltage V C  of zero thereby maintaining the current of CURRENT SOURCE2′  32 ′ at its preset nominal value whereby MR 1 ′ and MR 2 ′ analogously emulate the actions of MR 1  and MR 2  in FIGS. 2A,  2 B, and  2 C of the prior art as previously described. 
     On the other hand, using valley detectors for DETECTOR1  36   a  and DETECTOR2  36   b  to match MR 2 ′ to MR 1 ′ over a slot  28 ′ generates equal valley values for the voltages on signal line  36   a ′ and  36   b ′ which are input to the differential amplifier  36   c  producing a differential control voltage V C  of zero thereby maintaining the current of CURRENT SOURCE2′  32 ′ at its preset nominal value whereby MR 1 ′ and MR 2 ′ analogously emulate the actions of MR 1  and MR 2  in FIGS. 2A,  2 B, and  2 C of the prior art as previously described. 
     In a second example of the present invention, the two MR elements of the sensor, configured as in FIG. 5A, are mismatched due to an offset error, MR 1 ′ has a higher bias voltage V MR1′  over a tooth  26 ′ and a slot  28 ′ than the respective bias voltage V MR2′  over the same respective teeth and slots produced by MR 2 ′, as analogously depicted in FIG. 4A of the prior art. 
     Using peak detectors for DETECTOR1  36   a  and DETECTOR2  36   b  to match MR 2 ′ to MR 1 ′ over a tooth  26 ′ generates a higher peak value for the voltage on signal line  36   a ′ than the peak value for the voltage on signal line  36   b ′ which are input to the differential amplifier  36   c  producing a positive differential control voltage V C  thereby increasing the current of CURRENT SOURCE2′  32 ′ to a new value proportional to the magnitude of V C  thus increasing the voltage of MR 2 ′, V MR2′ , toward the value of the voltage of MR 1 ′, V MR1′ , over a tooth  26 ′ and thereby matching MR 1 ′ and MR 2 ′. When MR 1 ′ and MR 2 ′ are matched, the peak detectors DETECTOR1  36   a  and DETECTOR2  36   b  generate equal peak values for the voltages on signal line  36   a ′ and  36   b ′ which are input to the differential amplifier  36   c  producing a differential control voltage V C  of zero thereby maintaining the current of CURRENT SOURCE2′  32 ′ at its preset nominal value whereby MR 1 ′ and MR 2 ′ analogously emulate the actions of MR 1  and MR 2  in FIGS. 2A,  2 B, and  2 C of the prior art as previously described. 
     On the other hand, using valley detectors for DETECTOR1  36   a  and DETECTOR2  36   b  to match MR 2 ′ to MR 1 ′ over a slot  26 ′ generates a higher valley value for the voltage on signal line  36   a ′ than the valley value for the voltage on signal line  36   b ′ which are input to the differential amplifier  36   c  producing a positive differential control voltage V C  thereby increasing the current of CURRENT SOURCE2′  32 ′ to a new value proportional to the magnitude of V C , thus increasing the voltage of MR 2 ′, V MR2′ , toward the value of the voltage of MR 1 ′, V MR1′ , over a slot  28 ′ and thereby matching MR 1 ′ and MR 2 ′. When MR 1 ′ and MR 2 ′ are matched, the valley detectors DETECTOR1  36   a  and DETECTOR2  36   b  generate equal valley values for the voltages on signal line  36   a ′ and  36   b ′ which are input to the differential amplifier  36   c  producing a differential control voltage V C  of zero thereby maintaining the current of CURRENT SOURCE2′  32 ′ at its preset nominal value whereby MR 1 ′ and MR 2 ′ analogously emulate the actions of MR 1  and MR 2  in FIGS. 2A,  2 B, and  2 C of the prior art as previously described. 
     In a third example of the present invention, wherein the two MR elements of the sensor, configured as in FIG. 5A, are mismatched due to a gain error, MR 1 ′ has a lower voltage over a tooth  26 ′ than the voltage over the same respective tooth produced by MR 2 ′ whereas both MRs have the same voltage over a slot  28 ′ of the target wheel  10 ′, as analogously depicted in FIG. 3A of the prior art. 
     Using peak detectors for DETECTOR1  36   a  and DETECTOR2  36   b  to match MR 2 ′ to MR 1 ′ over a tooth  26 ′ generates a lower peak value for the voltage on signal line  36   a ′ than the peak value for the voltage on signal line  36   b ′ which are input to the differential amplifier  36   c  producing a negative differential control voltage V C , thereby decreasing the current of CURRENT SOURCE2′  32 ′ to a new value proportional to the magnitude of V C  and thus decreasing the voltage of MR 2 ′, V MR2′ , toward the value of the voltage of MR 1 ′, V MR1′ , over a tooth  26 ′ so as to thereby match MR 1 ′ and MR 2 ′ over a tooth. When MR 1 ′ and MR 2 ′ are matched over a tooth  26 ′ as depicted in FIG. 6A, the differential control voltage V C  is zero, thereby maintaining the current of CURRENT SOURCE2′  32 ′ at its former value set by V C . However, in this case, as depicted in FIG. 6A, MR 1 ′ and MR 2 ′ may not be matched over a slot  28 ′ resulting in a voltage offset V OF  in the differential signal V′″ D =V′″ MR1 −V′″ MR2  as shown in FIG.  6 B. 
     The differential signal V′″ D =V′″ MR1 −V′″ MR2 , shown in FIG. 6B is electronically generated within the comparator  36   d  and is then used by the comparator to reconstruct the signal V′″ OUT  (shown in FIG. 6C) emulating the profile of the target wheel  10 ′. Upon a closer inspection of FIGS. 6A,  6 B and  6 C, it becomes evident that the rising edges  42 ′″ and the falling edges  44 ′″ of the sensor output signal V′″ OUT  are determined only by first points  46 ′″ corresponding to the rising edges and second points  48 ′″ corresponding to the falling edges where the signal from the leading MR, in this example MR 1 ′, crosses a first threshold voltage  50 ′″ corresponding to the first points and a second threshold voltage  52 ′″ corresponding to the second points wherein the first and second threshold voltages are determined by the hysteresis applied to the comparator  36   d . Both MRs in this example have the same voltage V′″ 12H  (shown in FIG. 6A) over a tooth  26 ′ thereby leading to a zero voltage difference in the differential signal V′″ D =V′″ MR1 −V′″ MR2  whenever MR 1 ′ and MR 2 ′ are adjacent to or in close proximity to a tooth of the target wheel  10 ′ due to the matching of the MRs over a tooth as shown in FIG.  6 B and previously described. Over a slot  28 ′, the voltage of MR 1 ′, V′″ 1L , is not the same as the voltage of MR 2 ′, V′″ 2L  thereby leading to a voltage offset V OF  in the differential signal V′″ D =V′″ MR1 −V′″ MR2  whenever MR 1 ′ and MR 2 ′ are adjacent to or in close proximity to a slot of the target wheel  10 ′ as depicted in FIG.  6 B. However, decreasing the current of CURRENT SOURCE2′  32 ′ decreases the voltage of MR 2 ′, V MR2′ , such that, as shown in FIG. 6B, the magnitude of the voltage offset V OF  is less than the magnitude of the hysteresis applied to the comparator  36   d , thereby not affecting the determination of the rising edges  42 ′″ or the falling edges  44 ′″ of the sensor output signal V′″ OUT  as previously described. Thus, the sensor output signal V′″ OUT  as depicted in FIG. 6C emulates the profile of the target wheel  10 ′. 
     These examples are not to be considered limiting, as other mismatches involving offset and gain errors are possible such as those in which DETECTOR1  36   a  and DETECTOR2  36   b  are valley detectors instead of peak detectors. 
     The implementation of the valley detector is well known in the art, but, perhaps, not as familiar as the peak detector, and is hereby presented for illustrative purposes. FIG. 5C depicts an example of the valley detector  37 . V MR′  is representative of V MR1′  or V MR2′  whereas V DET  is representative of the output of DETECTOR1  36   a  or DETECTOR2  36   b  and capacitor  37   c  is initially charged to V′ IN    34 ′ thereby setting the value of V DET  to V′ IN . When V MR′  is less than V DET , the output V′ C  of the comparator  37   a  goes low and forward biases the diode  37   b . The capacitor  37   c  will now quickly discharge through the diode  37   b  and the comparator  37   a  (due to a very small discharge time constant) until V DET  becomes greater than V MR′ . Now, the output V′ C  of the comparator  37   a  goes high and reverse biases the diode  37   b . The capacitor  37   c  will now start charging through the resistor  37   d . The charging time (i.e., RC time constant) is adjusted such that the capacitor  37   c  charges very slowly compared to a tooth-slot time cycle, thereby essentially maintaining the voltage of the previous input V MR′  as the present output voltage V DET . However, if a subsequent input V MR′  has a lower voltage than the present output V DET  then V MR′  is less than V DET  and the capacitor  37   c  will quickly discharge to the new value of the input V MR′  as the new value of the output V DET . 
     It is to be understood that while a magnetoresistor (MR) was exemplified in the foregoing detailed description of a preferred embodiment of the present invention, other analogous sensing elements, such as hall elements my be utilized, the class of such sensors being inclusively denoted as galvanomagnetic elements. 
     To those skilled in the art to which this invention appertains, the above described preferred embodiment may be subject to change or modification. Such change or modification can be carried out without departing from the scope of the invention, which is intended to be limited only by the scope of the appended claims.