FIELD OF THE INVENTION 
     The present invention relates to magnetic rotary sensing systems, and more particularly to improved accuracy in rotary sensing when employing a toothed wheel. This patent application is related to co-pending patent application titled MAGNETIC ROTARY POSITION SENSING, filed herewith. 
     BACKGROUND OF THE INVENTION 
     For rotating members, in particular rotating shafts, it is sometimes necessary to receive accurate rotational information, which may be rotational position, velocity, and acceleration information. Various sensing systems have been developed to accomplish this task. 
     One sensing system in particular that works well in relatively harsh environments, such as with a crankshaft of an internal combustion engine, is a toothed sensor wheel. For this particular sensing system, the wheel is ferromagnetic and an inductive (magnetic field) sensor is located near the wheel periphery. As the wheel rotates, the teeth pass by the sensor, changing the magnetic field. The information is then communicated to a processor via a generally sinusoidal voltage signal from the sensor. This works generally well since it is non-contact—there are no rubbing parts to wear out, dirt and oil won&#39;t generally interfere with the signal, and the temperature effects are minimal. Generally the sensor wheel will have a series of teeth that are the same size and evenly spaced circumferentially about the wheel, with one of the teeth missing. The missing tooth location will provide a gap for indexing, to determine the absolute rotational position. This information can then be used for generally controlling engine operating parameters, such as ignition timing, fuel injector timing, etc. 
     While the information provided by the sensor system is sufficient for conventional internal combustion engines, the need arises to increase the accuracy of readings for this type of system in order to obtain more precise engine operation information. An example of such an instance is the desire to use a toothed crankshaft sensor wheel to detect engine cylinder misfires. It must be very precise because the slight acceleration of the crankshaft due to a cylinder firing must be determined. For this type of calculation, as little as 10 microseconds error may be too much to obtain the desired accuracy. 
     In general the toothed wheel sensor system produces a sinusoidal signal that has periodic zero crossings (i.e. where the voltage is zero). These zero crossings are subsequently used for determining the rotational information needed for misfire detection. The sinusoidal signal is sent to a processor for generation of a square wave from which edges are time stamped for further digital signal processing as part of a misfire monitor. 
     An accuracy concern arises however around the location of the missing tooth. For these inductive sensors, the missing tooth location provides for a different rate of change in magnetic flux linkage than do the other teeth on the wheel, so that residual stored energy will occur due to the loss of this flux coupling at the location of the missing tooth. The additional energy is stored in the inductor of the sensor and decays based on the particular sensor and input circuit characteristics. This residual energy will then result in higher voltages, affecting the signal for a few teeth past the gap as the excess energy decays, inherently causing a time delay in the zero crossing of the signal and hence increases the variation in the edge placement for the square waves which are subsequently generated. This, then, results in inaccurate time stamp data at these locations. The need arises then for compensation in the signal due to the energy storage in the inductive sensor. 
     One method of correction employed is to take the signal from the sensor as is, with the error, and employ software in a signal processor to manipulate the signal in order to compensate for the error. However, the accuracy can be less than satisfactory since the correction is based on operation at a given operating speed to minimize the software complexity, and as the rotational speed varies from the given speed, the accuracy of the error correction is reduced. 
     Thus, it is desirable to assure accuracy in the signal initially sent from the inductive sensor, (i.e. reduce the error at the source), and avoid the need for the error compensation in the software of the signal processor in order to obtain accurate rotational acceleration data from a sensor wheel. 
     SUMMARY OF THE INVENTION 
     In its embodiments, the present invention contemplates a method of sensing rotational position of a shaft with a sensor wheel rotationally fixed thereto having n minus 1 teeth spaced about a periphery of the wheel and forming a gap. The method comprises the steps of: rotating the shaft in a predetermined direction about a center of rotation; creating a magnetic field proximate to the periphery of the wheel with a sensor; sensing the variations in magnetic flux in the magnetic field as the teeth pass; altering the magnetic flux in one of the gap and the teeth rotationally trailing the gap to account for the loss of flux coupling at the gap; creating a periodic signal in response to the variations in flux; and transmitting the periodic signal to a processor. 
     Accordingly an object of the present invention is to provide a method for generating accurate timing of signals produced by a toothed wheel, having a missing tooth configuration, and an inductive sensor, for generating an accurate signal for producing rotational position and acceleration information. 
     An advantage of the present invention is improved accuracy of the sensor signal without the need for electrical filtering of the signal by the signal processor. 
     An additional advantage of the present invention is that the signal correction is affective for all rotational speeds. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a view of a magnetic field sensor and a portion of a toothed sensor wheel connected to a rotary shaft in accordance with the present invention; 
     FIG. 2 is a schematic illustration, on an enlarged scale, of a portion of the toothed wheel shown in encircled area  2  in FIG. 1; 
     FIG. 3 is a view similar to FIG. 2, but illustrating a different embodiment of the present invention; 
     FIG. 4 is a view similar to FIG. 2, but illustrating another embodiment of the present invention; and 
     FIG. 5 is a view similar to FIG. 2, but illustrating still another embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIGS. 1 and 2 illustrate a sensor wheel  20  rotationally affixed to a rotating shaft  22 . For purposes of this discussion, the shaft will be assumed to be a crankshaft employed in an internal combustion engine. The sensor wheel  20  includes a set of thirty six minus one teeth  24 . By this it is meant that the tooth size and spacing is such that the leading edge of each tooth in the set  24  is ten degrees spaced from the leading edge (or trailing edge) of each of its adjacent teeth, with one tooth missing, thus making one gap  26  that has twenty degrees from the leading edge of one tooth to the next. While a configuration of 36-1 teeth is shown, there can be other numbers of teeth on the wheel, as desired for the particular application. The sensor wheel  20  is made of a ferromagnetic type of material so that the teeth will influence any particular magnetic field that they are passing through. 
     Mounted adjacent to the outer periphery of the wheel  20  is an inductive (magnetic field) sensor  30 . This sensor  30  communicates with a processor  31  that will receive the data and convert it to rotational information, such as rotational position, velocity, and acceleration. The magnetic field sensor  30  may, for example, be a Hall Effect sensor, a variable reluctance sensor, or a magnetoresistive or magnetorestrictive sensor. 
     In accordance with the present invention there are specific teeth within the set  24  that may have varied geometries, generally located adjacent the gap  26 . For the discussion herein, six teeth will be specifically discussed and will be labeled with element numbers  32 - 42 . Also, the valleys, i.e. bottom lands, around these six teeth will be labeled with element numbers  44 - 52  for discussion herein. The rotation direction of the wheel  20  is indicated by arrow  56 . 
     In general, all of the teeth  24  about the wheel  20  have the same height, so that the lands are at the same radial distance from the center of rotation  28  of the wheel  20 . This center of rotation  28  is also the axis about which the shaft  22  rotates. Thus lands  44 ,  46 ,  50 , and  52  are at the same radial distance. But there is an exception for the land  48  in the gap  26 . The land  48  is radially farther from the center of rotation  28  than it normally would be, illustrated by phantom line  48   a . What this does is provide extra ferromagnetic material to affect the magnetic flux in the area of the missing tooth without giving a false reading of a tooth in that location. The resulting effect will be discussed below in relation to the sensor system operation. 
     The sensor system operates generally at all times while the shaft  22  is rotating. As the shaft  22  rotates, the sensor wheel  20  rotates with it, causing the teeth  24  to pass by the inductive sensor  30 . As each tooth passes, it alters the magnetic flux path relative to the gap just proceeding that tooth. This variation in magnetic flux is detected by the sensor  30 , and transmitted as a signal to the processor  31 . Because of the alternating between tooth and gap, a generally sinusoidal signal is created. 
     The sinusoidally varying signal, then, that is sent to the processor  31  will be manipulated thereby. The times at which the sinusoidal signal has zero crossings (i.e. where zero voltage occurs in the sinusoidal signal) is used to determine rotary information. This signal provides at least two different types of information from the same sensor  30  and sensing wheel  20 . The first type is that used for the timing in general. The timing between the zero crossings of the sinusoidal signal is monitored by the processor  31 , and when a relatively long time between crossings occurs, the processor will recognize this as the gap  26  for the missing tooth, thus giving absolute rotational position of the crankshaft  22  (i.e., the crank angle). This is the conventional use for the sensing system. 
     The second type of information from the signal is the timing between the zero crossings, employed by the microprocessor  31  to determine the rotational velocity. By time stamping the positions, the rotational velocity can be determined. Further, by knowing the timing of the zero crossings, changes in time between zero crossings is used by the processor  31  to determine rotational acceleration. The rotational acceleration information is needed in order to employ this sensing system as a cylinder misfire detector. By reading the rotational acceleration information, the processor  31  will be able to note whether each cylinder in the engine is firing properly, because as each cylinder fires during a combustion event, the crankshaft  22  will experience a slight acceleration. If the acceleration that should occur for a given cylinder combustion event is missing, then the processor  31  will recognize this and indicate such to, for example, a main engine control computer, not shown. 
     With such precise sensing needed, the concern then arises around the missing tooth location. A first preferred step to take is to only read the sensor signal for every other tooth, i.e. every twenty degrees, which will generally provide enough data points, while avoiding attempts to read the signal at the missing tooth location. 
     The sensor wheel  20  of the present invention also corrects for error around the missing tooth that would otherwise be due to the residual energy stored in the sensor  30 . The difference in the radial location of the third land  48  relative to the other lands changes the geometry, and thus the magnetic flux path around the missing tooth location. The radial location of the land  48  is adjusted outward relative to the other lands so that the flux path around the missing tooth location is changed to equalize the induced emf (voltage) for fixed time intervals to match that for the rest of the sensor wheel  20 . Put another way, this geometry will generally achieve a constant length flux path through the same permeability. The exact amount of adjustment depends upon several factors, including the shape and spacing of the teeth and the type of magnetic sensor employed (i.e. the electromagnetic characteristics of the sensor employed). The concern is not with the change in the shape of the sinusoidal signal, just with the timing of the zero crossings. 
     The radial location of the land provides a means for substantially eliminating the variation in the zero crossing timing due to the increased energy storage in the inductive sensor  30  at the gap  26 , which in turn, reduces variation in the edge placement for the square waves generated from these zero crossings by the processor  31 . This results in improved accuracy around the missing tooth location, thus resulting in improved misfire detection at this angle of crankshaft rotation. 
     FIG. 3 illustrates six teeth of an alternate embodiment of the sensor wheel  120 . For this embodiment, similar elements are similarly designated with the first embodiment, while changed elements are designated with a 100-series number. The depth of the land  48  in the gap  26  is the same for this embodiment as for the first embodiment. But the lands  144 ,  146 ,  150 ,  152  etc. between the other teeth  24  are now radially equal to the land  48 . Otherwise, the teeth  32 ,  34 ,  38 ,  40  and  42  are the same as in the first embodiment. The tooth  136 , though, is now narrower, with material removed from its leading edge  60 . What was the full width tooth is indicated by phantom line  136   a.    
     The tooth  136  corrects for the error in the zero crossings around the missing tooth by changing the flux path immediately after the gap  126 . The flux path is changed in two ways, first the leading face  60  of the tooth is cut back creating a slight delay in time before the tooth  136  affects the sensor signal, and it is a narrower tooth, also affecting the flux path. These changes to the tooth  136  consequently change the timing of the zero crossing for the signal, correcting for the error introduced by the missing tooth location. 
     FIG. 4 illustrates another alternate embodiment of the present invention showing six teeth of the sensor wheel  220 . For this embodiment, similar elements are similarly designated with the first embodiment, while changed elements are designated with a 200-series number. The radial distance to the lands  244 ,  246 ,  250 ,  252  are the same as the land  48  in the gap  26 ; otherwise the teeth  32 ,  38 ,  40 ,  42  are the same. But the two teeth  234 ,  236  immediately trailing the gap  26  are varied in order to correct the timing of the zero crossings in the signal. 
     Tooth  236  is wider than the other teeth and has a trailing face  62  that is rotationally rearward of the location for the rest of the teeth. What the normal tooth width would be is indicated in phantom by trailing face  236   a . Also, tooth  234  is wider than the other teeth, but narrower than tooth  236 , with a trailing edge  64  behind the typical tooth, as indicated in phantom by face  234   a . By pulling these trailing faces  62 ,  64  rotationally rearward, the zero crossings for the teeth trailing the gap  26  are corrected. This embodiment shows that one may wish to alter geometry of more than one tooth in order to assure the desired accuracy in the zero crossings since the residual effect in the sensor may not correct itself until several teeth have passed. 
     FIG. 5 illustrates yet another alternate embodiment of the present invention, similar to FIG. 2, illustrating six teeth of a sensor wheel  320 . For this embodiment, those elements that are altered from the first embodiment will be designated with a 300-series number. For this sensor wheel  320 , the sensor teeth  332 ,  334 ,  338 ,  340 ,  342  have angled leading and trailing faces  66 ,  68 , respectively, while the lands  344 - 352  are all at the same radial distance. This embodiment illustrates a sensor wheel  320  wherein all of the teeth have sloped faces. 
     In order to correct the flux path to obtain the desired zero crossings, the slope of the leading  72  and trailing  70  face for the tooth  336  on the trailing side of the gap  326  are modified. For the leading face  72 , it has a greater slope to provide for a smaller tooth and delayed timing for the sensor detecting the leading edges of the tooth  336 . For the trailing face  70 , it has a lesser slope to provide for larger tooth  336 . These changes to the tooth geometry about the gap  326  will correct for the zero crossing error in the signal due to the missing tooth location. 
     Of course, while the different embodiments have illustrated individual aspects of a sensor wheel that can be changed to correct the magnetic characteristics in order to obtain the proper timing of zero crossings in the signal, a combination of these variations in tooth geometry and lands can be used to make this correction, if so desired. Also, for example, the wheel may be formed from different materials, where the material adjacent the gap is different than in the rest of the sensor wheel, thereby correcting the magnetic characteristics in order to obtain the desired timing. Thus, while certain embodiments of the present invention have been described in detail, those familiar with the art to which this invention relates will recognize various alternative designs and embodiments for practicing the invention as defined by the following claims.
A method for more accurately sensing the rotational position of a shaft having a toothed sensor wheel with a missing tooth is disclosed. The method includes employing an inductive sensor to produce a signal in response to the passing of the teeth on the sensor wheel. The sensor signal is altered by the geometry of the wheel about a gap formed by the missing tooth in order to correct for residual stored energy in the sensor at this location, and thus correct the signal for the timing measured by a processor in communication with the sensor.