Abstract:
A magnetic sensor has magnetically sensitive element located at a side surface, instead of the bottom surface, of a bias magnet, the magnet being located adjacent a magnetic target wheel, wherein the bias magnet is magnetized parallel to the direction of motion of the teeth/slots of the target wheel. The output may be of a single or double frequency. Sampling of output slope can provide information regarding direction of movement of the target wheel. In a second embodiment the bias magnet is magnetized perpendicular to the movement.

Description:
TECHNICAL FIELD  
       [0001]     The present invention relates to magnetosensitive or galvanomagnetic devices (e.g. Hall generators, magnetoresistors, etc.) for use as encoders to determine position and speed.  
       BACKGROUND OF THE INVENTION  
       [0002]     It is well known in the art that magnetic sensors can be employed in position and speed sensors with respect to moving ferromagnetic materials or objects (see for example U.S. Pat. Nos. 4,926,122, and 4,939,456). In such applications, the magnetic sensor 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 magnetic sensor, such as a toothed wheel, produces a varying magnetic flux density through the magnetic sensor.  
         [0003]      FIG. 1A  is an example of a magnetic sensor  50  according to the prior art, wherein the magnetic sensor element  10  is mounted on the bottom surface  12  of a permanent magnet (bias magnet)  14  magnetized in a direction  16  perpendicular to the direction of motion  18  of target wheel  20  having teeth  22  and slots  24 . The total package thickness  26  is determined by the thickness  30  of the magnetic sensor element  10 , incorporating protection and electrical connections for the magnetic sensor element, and the magnet length  32 . The magnet length  32  cannot be small since it determines the magnetic signal strength detected by magnetic sensor element  10  and a total package thickness  26  of 5 millimeters or more is common.  
         [0004]     The resolution of magnetic sensor  50  is related to the number of teeth  22  of target wheel  20 . In some cases, the number of teeth  22  is fixed by external constraints, for instance, when target wheel  20  is a gear used for both mechanical advantage and for position sensing. The number of teeth  22 , in such a case, may not be sufficient to provide the desired resolution.  
         [0005]      FIG. 1B  is a plot  52  of the magnetic flux density detected by the magnetic sensor  50  according to the prior art of  FIG. 1A  as the target wheel  20  passes the magnetic sensor. The larger magnetic flux density  22 ′ represents the passage of a tooth  22  past the sensor  50  whereas the smaller magnetic flux density  24 ′ represents the passage of a slot  24  past the sensor.  
         [0006]     Accordingly, what is needed in the art is a more robust magnetic sensor configuration enabling a smaller total package thickness and a means for increasing the resolution of the magnetic sensor.  
       SUMMARY OF THE INVENTION  
       [0007]     The present invention is a magnetic sensor (magnetic encoder) utilizing magnetosensitive or galvanomagnetic devices, herein referred to as magnetically sensitive (MS) elements, much thinner than prior art magnetic sensors, providing similar functionality and, with appropriate design parameters, can double the resolution of prior art magnetic sensors utilizing the same target wheel. The present invention also has the capability to directly provide direction of rotation information of the target wheel.  
         [0008]     The magnetic sensor senses changes in the magnetic flux density as the target wheel moves relative to the magnet and outputs a signal representing changes in the magnetic flux density. Preferably, the MS element is a Hall effect sensor or device, a semiconductor magnetoresistor (SMR), a permalloy magnetoresistor (PMR), or a giant magnetoresistor (GMR). If a Hall sensor or a semiconductor magnetoresistor is used, it senses a component of the magnetic flux density that is normal to its surface. On the other hand, if a permalloy magnetoresistor or a giant magnetoresistor is used, it senses the component of magnetic flux density which is co-planar, or parallel, to its surface.  
         [0009]     In a first preferred embodiment of the present invention, a magnetic sensor consists of an MS element located at a side surface, instead of the bottom surface, of a stationary permanent magnet, the magnet being located adjacent a magnetic target wheel, wherein the permanent magnet is magnetized parallel to the direction of motion of the surface of a magnetic target wheel. Proper selection of magnetic sensor dimensions enables changes in magnetic flux density upon the passage of one tooth and one slot of the target wheel (one tooth pitch) past the magnetic sensor to be represented as a single or double frequency magnetic sensor output. For the single frequency magnetic sensor output, a single cycle of changes in magnetic flux density consisting of one minimum and one maximum is output by the magnetic sensor upon the passage of one tooth and one slot of the target wheel (one tooth pitch) past the magnetic sensor. Whereas for the double frequency magnetic sensor output, two cycles of changes in magnetic flux density consisting of two minima and two maxima are output by the magnetic sensor upon the passage of one tooth and one slot (one tooth pitch) of the target wheel past the magnetic sensor, thereby increasing the resolution by doubling the frequency of the output signal.  
         [0010]     In a second preferred embodiment of the present invention, a magnetic sensor consists of an MS element located on a side surface, instead of the bottom surface, of a stationary permanent magnet wherein the permanent magnet is magnetized perpendicular to the direction of motion of the surface of a magnetic target wheel.  
         [0011]     Accordingly, it is an object of the present invention to provide a magnetic sensor having a total package thickness much smaller than Prior Art magnetic sensors with similar functionality.  
         [0012]     These and additional objects, features and advantages of the present invention will become clearer from the following specification of a preferred embodiment. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0013]      FIG. 1A  is an example of a prior art magnetic sensor.  
         [0014]      FIG. 1B  is a plot of the detected magnetic flux density according to the prior art magnetic sensor of  FIG. 1A .  
         [0015]      FIG. 2  depicts a first preferred embodiment of a magnetic sensor according to the present invention.  
         [0016]      FIG. 3  is a first graph of detected magnetic flux densities according to the first preferred embodiment of the present invention.  
         [0017]      FIG. 4  is a second graph of detected magnetic flux densities according to the first preferred embodiment of the present invention.  
         [0018]      FIG. 5  is a third graph of detected magnetic flux densities according to the first preferred embodiment of the present invention.  
         [0019]      FIGS. 6A-6E  depict the positions of the magnetic sensor according to the present invention at various points of  FIG. 5   
         [0020]      FIG. 7  depicts a second embodiment of the magnetic sensor according to the present invention. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0021]      FIG. 2  depicts a first preferred embodiment of a magnetic sensor  100  according to the present invention. The magnetic sensor  100  incorporates a magneto sensitive (MS) element  102  adjacent in facing relation to side surface  104  or  104 ′ (shown facingly adjacent to side surface  104  in  FIG. 2 ) of a permanent magnet (bias magnet)  106  magnetized in a direction  108  parallel to the direction of motion  110  of a target wheel  112  having teeth  114  and slots  116 , wherein the MS element  102  is sensitive to the component  124  (the detected magnetic flux density) of magnetic flux density  132  which is parallel to the direction of motion of the target wheel and parallel to the direction of magnetization of the permanent magnet, and wherein the component  134  of the magnetic flux density is perpendicular to component  124 . The location of the MS element  102  facingly adjacent the permanent magnet side surface  104  or  104 ′, rather than between the magnet  106  and target wheel  112 , decreases the overall package thickness  118  compared to the aforedescribed prior art magnetic sensor  50 , since the MS element, connecting wires, leadframe, bonding, and protective layers (not shown) are now removed from the overall thickness. The most preferred placement of the MS element  102  on the permanent magnet side surface  104  or  104 ′ is nearest the bottom surface  136  of permanent magnet  106 , wherein the lower edge  138  of the MS element aligned with the bottom surface of the permanent magnet. Another advantage of the present invention is that the MS element  102  and its connections (not shown), which are the most fragile parts, are located away from the target wheel  112  resulting in a more robust design. Additionally, the magnetic sensor  100  is also more amenable to electronic integration, in that the MS element  102  can more easily be connected or combined with electronic circuitry (not shown). The permanent magnet length  120  determines the magnetic field strength (magnetic flux density)  132  and, thus, the strength of magnetic field components  124 ,  134  (see inset of  FIG. 2 ). Whereas the permanent magnet width  122 , by contrast, is not a significant design constraint for semiconductor sensor elements and can be reduced to obtain an overall very thin package. The permanent magnet width  122  is limited by the mechanical strength necessary for the application.  
         [0022]     It is to be understood that the MS element  102  can be, for example, a Hall effect device, a semiconductor magnetoresistor (SMR), a permalloy magnetoresistor (PMR), or a giant magnetoresistor (GMR). For ease of discussion, the MS elements  102  can be divided into two types: type A elements and type B elements. Type A elements include Hall effect devices and SMRs. On the other hand, type B elements include PMRs and GMRs. It is to be appreciated that the type A elements are sensitive to the component of magnetic flux density, for example  124 , that is perpendicular to their surfaces. On the other hand, type B elements are sensitive to the component of magnetic flux density, for example  124 , that is parallel to their surfaces. A type A MS element  102  is depicted in  FIG. 2  wherein the component of magnetic flux density  124  is perpendicular to the surface  131  of the MS element.  
         [0023]      FIGS. 3 and 4  are examples of first and second graphs of plots  302  through  314  and  402  through  422 , respectively, of magnetic flux densities  124  detected by MS element  102  for one tooth pitch P of 14.5 millimeters according to the first preferred embodiment of the present invention shown at  FIG. 2 , using finite element simulation for various permanent magnet lengths  120 , wherein the MS element is, for example, a type A element. The distance  130  (magnetic air gap) between the bottom surface  136  of the magnet  106  and the top of the teeth  114  is 0.7 millimeters, consisting of a 0.2 millimeter protective layer for the sensor  100  and 0.5 millimeters mechanical clearance between the target wheel  112  and the protective layer (not shown).  
         [0024]     Shown for comparison of the prior art magnetic sensor  50  is plot  302  in  FIG. 3  and plot  402  in  FIG. 4 , wherein the distance  28  (magnetic air gap) in  FIG. 1A  is 1.5 millimeters, consisting of a 1 millimeter protective layer for the sensor  50  and 0.5 millimeters mechanical clearance between the target wheel  20  and the protective layer (not shown). In  FIG. 3 , the tooth width  126  and slot width  128  are the same, 7.25 millimeters, whereas in  FIG. 4  the tooth width is 4 millimeters and the slot width is 10.5 millimeters. The larger magnetic flux density  302 ′ and  402 ′ for the prior art magnetic sensor plot  302  in  FIG. 3  and plot  402  in  FIG. 4 , respectively, represent the passage of a tooth  22  past the magnetic sensor  50  depicted as  22 ′ in  FIG. 1B , whereas the smaller magnetic flux density  302 ″ and  402 ″ for the prior art magnetic sensor plot  302  in  FIG. 3  and plot  402  in  FIG. 4 , respectively, represent the passage of a slot  24  past the magnetic sensor  50  depicted as  24 ′ in  FIG. 1B . In  FIGS. 3 and 4 , the permanent magnet length  120  determines the strength of the magnetic flux density  132  and, thus, the strength of magnetic field components  124 ,  134 .  
         [0025]     Plots  304 ,  306 ,  308 , and  310  in  FIG. 3  represent permanent magnet lengths  120  of 8 mm, 7 mm, 6 mm, and 5 mm, respectively, each showing one minimum and one maximum per tooth pitch P representing single frequency magnetic sensor  100  outputs, wherein the minima and maxima have, approximately, the same variations of magnetic flux densities  124 . Plots  404 ,  406 ,  408 ,  410 ,  412 ,  414 ,  416 , and  418  in  FIG. 4  represent permanent magnet lengths  120  of 13 mm, 12 mm, 11 mm, 10 mm, 9 mm, 8 mm, 6 mm, and 5 mm, respectively, each showing one minimum and one maximum representing single frequency magnetic sensor  100  outputs, wherein the minima and maxima have, approximately, the same variations of magnetic flux densities  124 .  
         [0026]     As can be seen from  FIGS. 3 and 4 , the plots, wherein the minima and maxima have, approximately, the same relatively large variations of magnetic flux densities  124  (the best plots) for single frequency magnetic sensor  100  output, are obtained for a permanent magnet length  120  longer than 4 mm, for example plot  310  in  FIG. 3 , and at least 5 mm, for example plot  418  in  FIG. 4 . Related to the slot width  128  of  FIGS. 3 and 4 , the permanent magnet length  120  must be at least 50% of the slot width and, preferably, between 70% and 100% of the slot width for a single frequency output of magnetic sensor  100 .  
         [0027]     Plots  310 ,  312 , and  314  in  FIG. 3  represent permanent magnet lengths  120  of 5 mm, 4 mm, and 3 mm, respectively, each showing two minima and two maxima per tooth pitch P representing double frequency magnetic sensor  100  outputs for one tooth pitch P, wherein the minima and maxima have, approximately, the same variations of magnetic flux densities  124 . For a permanent magnet length  120  of 5 mm (plot  310 ), the double frequency is clear, but the variation of magnetic flux density  124  between the maxima and minima is small.  
         [0028]     Referring now to  FIG. 4 , the double frequency is somewhat clear for plots  418 ,  420 , and  422  representing permanent magnet lengths  120  of 5 mm, 4 mm, and 3 mm, respectively, but the variation of magnetic flux density  124  between the maxima and minima is small. Hence, as can be seen from  FIGS. 3 and 4 , the plots, wherein the minima and maxima have, approximately, the same relatively large variations of magnetic flux densities  124  (the best plots) for double frequency magnetic sensor  100  output, are obtained for a permanent magnet length  120 , preferably, no longer than 50% of the slot width  128  whereby the tooth width  126  and slot width are, preferably, the same.  
         [0029]     Maxima, for example  304 ′,  312 ′ in  FIG. 3  and  404 ′,  422 ′ in  FIG. 4 , are obtained if any two of the following three conditions are achieved:  
         [0030]     Condition 1: There is a large magnetic flux density  132  due to the proximity of a tooth  114  in front of the MS element  102  (i.e. on the right side surface of the MS element depicted in  FIG. 2 ) resulting in a large magnetic flux component  124  albeit the angle A in  FIG. 2  may be large.  
         [0031]     Condition 2: The angle A is small resulting in a large magnetic flux component  124  of magnetic flux density  132 .  
         [0032]     Condition 3: There is a low reluctance magnetic flux return path due to the proximity of a tooth  114  near the back side surface (see  104 ′ in  FIG. 2 ) of permanent magnet  106  (in other words, the side surface opposite the MS element  102 ).  
         [0033]     Minima, for example  304 ″,  312 ″ in  FIG. 3  and  404 ″,  422 ″ in  FIG. 4 , are obtained if, at most, only one of the previous three conditions is achieved.  
         [0034]     Single frequency maxima, for example  304 ′,  312 ′ in  FIG. 3  and  404 ′,  422 ′ in  FIG. 4 , are obtained with a permanent magnet length  120  about as long as the slot width  128 , as previously described, thereby satisfying conditions 1 and 3 mentioned above, once per tooth pitch P, when the permanent magnet subtends the slot width resulting in a maximum once per tooth pitch. Single frequency minima, for example  304 ″,  312 ″ in  FIG. 3  and  404 ″,  422 ″ in  FIG. 4 , are obtained with a permanent magnet length  120  more than 50% of the slot width  128 , as previously described, thereby satisfying one of the three conditions above once per tooth pitch P when the permanent magnet subtends approximately half the slot width resulting in a minimum once per tooth pitch. Therefore, a permanent magnet length  120  about as long as the slot width  128  results in a single frequency output with one maxima and one minima per tooth pitch P.  
         [0035]      FIG. 5  depicts a finite element plot  500  of magnetic flux density components  124 , according to the configuration of  FIG. 2 , which is analogous to the plots of  FIGS. 3 and 4 , and which represents a double frequency output of the magnetic sensor  100  per tooth pitch P, wherein the tooth width  126  and slot width  128  are equal to 7.25 mm and the permanent magnet length  120  is 3 mm.  
         [0036]      FIGS. 6A through 6E  depict positions of the permanent magnet  106  relative to the toothed wheel at points A through E, respectively, of  FIG. 5 . Referring first to point A of  FIG. 5  and simultaneously to position depicted at  FIG. 6A , Conditions 1 and 3 are realized, resulting in a maximum at point A, but the angle A is large because a tooth  114  is directly below the permanent magnet  106 , therefore condition 2 is not realized. At point B in  FIG. 5  and the position depicted at  FIG. 6B , condition 3 is realized, but neither conditions 1 or  2  are realized, resulting in a minimum at point B. At point C in  FIG. 5  and the position depicted at  FIG. 6C , conditions 2 and 3 are realized resulting in a maximum at point C. Referring now to point D in  FIG. 5  and the position depicted at  FIG. 6D , conditions 1 and 2 are realized thereby maintaining the magnetic sensor  100  output maximum at point D. Referring finally to point E in  FIG. 5  and the position depicted at  FIG. 6E , condition 1 is realized but conditions 2 and 3 are not realized resulting in a minimum at point E. Therefore, a double frequency output results when the permanent magnet length  120  is, preferably, no longer than 50% of the slot width  128  whereby the tooth width  126  and slot width are, preferably, the same.  
         [0037]     In some applications, it is desirable for a position sensor to also detect the direction of wheel rotation. Referring to the curves or plots of  FIGS. 3, 4  and  5 , the slopes of each plot going from minimum to maximum, and from maximum to minimum, are different. These different slopes can be tailored by design to accentuate the contrast between steep and less steep slopes on either side of a maximum. For example, when the wheel is rotating clockwise, the steeper slope can occur when the output is rising to a maximum and, when the wheel is rotating counterclockwise, the steeper slope can occur when the output is falling to a minimum (or vice-versa).  
         [0038]      FIG. 7  depicts a second embodiment of the magnetic sensor  200  according to the present invention. The magnetic sensor  200  incorporates an MS element  202  mounted on the side surface  204  of a permanent magnet (bias magnet)  206  magnetized in a direction  208  perpendicular to the direction of motion  210  of target wheel  212  having teeth  214  and slots  216 . The location of the MS element  202  on the magnet side surface  204 , rather than between the magnet  206  and target wheel  212 , decreases the overall package thickness  218  compared to the prior art magnetic sensor  50  since the MS element, connecting wires, leadframe, bonding, and protective layers (not shown) are now removed from the overall thickness. Another advantage of the present invention is that the MS element  202  and its connections (not shown), which are the most fragile parts, are located away from the target wheel  212  resulting in a more robust design. Magnetic sensor  200  is also more amenable to electronic integration, in that the MS element  202  can more easily be connected or combined with electronic circuitry (not shown). It is to be understood that the MS element  202  can be either a type A element or a type B element sensitive to the component of magnetic flux density  224 . The changes in magnetic flux density detected by the MS element  202  are analogous to that of the prior art magnetic sensor as depicted in  FIG. 1B .  
         [0039]     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.