Abstract:
A method of measuring flux density and run out to accommodate rotors of different diameters, evaluate intrinsic properties of magnet material and the magnetization process. Circular run out measurement capability is also used to evaluate bearing journal “ovality.” The method includes the use of a scan tool, or a DLA Rotor Flux Density Scan Fixture, which evaluates the electromagnetic field strength (gauss), combined with surface run out and presents the data in a scalable pictorial format. The scan tool includes a probe which measures a magnetic field strength and circular run out of the perimeter of the magnet. Simultaneously, a non-contact measurement sensor is used to measure the rotor surface for subtle variations. The resulting sine wave gauss data and the surface dimension data are manipulated into a scalable “radar” plot. The radar plot correlates magnetic pole field strength and surface circular run out variation to the index position.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of U.S. Provisional Application No. 61/579,211 filed Dec. 22, 2011. The disclosure of the above application is incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION 
     The invention relates generally to permanent magnet stepper motors and digital linear actuators, and more particularly, to the calibration of the axial magnetization of a cylindrical magnet used in stepper motors and digital linear actuators during development. 
     BACKGROUND OF THE INVENTION 
     Digital linear actuators (DLAs) and stepper motors typically incorporate a rotor in the form of a magnetic cylinder. During calibration of the rotor, the flux density is measured (in Gauss), to determine the capacity of the rotor. Typical methods used to determine the flux density involve spinning the rotor at a predetermined high velocity (such as 1800 rpm), in a generated voltage fixture device required for screw-in family rotors. A separate tool is required for other types of rotors. These are only applicable for measuring flux density during the production process because the fixture is not suitable to fit nonproduction stepper motor rotors, or rotors which have bearings attached. 
     Another diagnostic method involves spinning the rotor assembly at high velocity, such as 1800 rpm, while assembled within the DLA. This method is an intrusive test method and degrades functionality of the DLA. The rotational speed required to develop generated voltage from the motor is sufficient to predicate degradation of internal bearings or lubricants. Insertion of the threaded shaft adaptor may damage the internal lead screw thread. The flux density in millitesla (mT) is plotted as a sine wave as shown in  FIG. 1 . While the plot shown in  FIG. 1  shows the different flux density measurements, the plot in  FIG. 1  is difficult to interpret, and provides no correlation between the flux density, and variations in the dimensions of the rotor. 
     Therefore, there exists a need for a method of measuring the flux density of a rotor used for a DLA or stepper motor during the development stages, which measures not only flux density, but also is capable of measuring surface irregularities of the rotor. 
     SUMMARY OF THE INVENTION 
     The present invention is a method for measuring the flux density and run out of a rotor used as part of a digital linear actuator (DLA). 
     The method of measuring flux density and run out according to the present invention has flexibility to accommodate rotors of different diameters, evaluate intrinsic properties of magnet material and the magnetization process. Circular run out measurement capability is also used to evaluate bearing journal “ovality.” The method includes the use of a scan tool, or a DLA Rotor Flux Density Scan Fixture, which evaluates the electromagnetic field strength (gauss) combined with surface run out and presents the data in a scalable pictorial format. The cylindrical rotor magnet is affixed in the scan tool, and the scan tool simultaneously measures magnetic field strength and circular run out of the perimeter or outer surface of the magnet. In this embodiment, the scan tool includes a fixture, such as a taper press fit mandrel of appropriate size, and the rotor is mounted in the mandrel. The mandrel is nonintrusive and does not damage the internal features of the rotor. 
     A rotary encoder provides the scalar reference for all measured values. The magnetic field strength (gauss) is measured at a distance from the outer surface of the rotor, defined as a radial “air-gap.” Simultaneously, a laser is used to measure the rotor surface for subtle variation. The resulting sine wave gauss data and the surface radial dimension data are manipulated into a scalable “radar” plot. The radar plot correlates magnetic pole field strength and surface circular run out variation to the index position. Two planar positions are evaluated to correlate front and rear coils typically found in can stack stepper motors. The DLA Rotor Flux Density Scan Fixture is adaptable and accommodates equivalent size DLA rotors for benchmarking purposes. The DLA Rotor Flux Density Scan Fixture may determine bearing journal surface run out or “ovality” in conjunction with the rotor magnetic and run out properties. 
     In one embodiment, the present invention is a method for measuring the magnetic flux and run out of a surface, comprising the steps of providing a digital linear actuator (DLA), and providing a rotor, the rotor being part of the DLA. The method also includes providing a magnet connected to and circumscribing the rotor, the magnet having an outer surface, providing a non-contact measurement sensor for measuring the run out on the outer surface of the magnet, and providing a gauss probe for measuring the magnet flux density on the outer surface of the magnet. The run out and the magnet flux density are simultaneously measured, and plotted on a plot or graph to correlate the measurements of the run out and the magnet flux density. 
     A tool, such as a scan tool, is provided for measuring the magnetic flux and the run out, the gauss probe and the non-contact measurement sensor being part of the tool. The tool includes an indexing rotary head, a shaft connected to and driven by the indexing rotary head, and a fixture connected to the shaft. Also connected to the indexing rotary head is an angular encoder for measuring the rotational position of the shaft. The rotor is mounted in the fixture, and the indexing rotary head is rotated such that rotational force is transferred through the shaft and the fixture, rotating the rotor in the fixture. As the rotor is rotated, the flux density of the rotor is measured using the gauss probe, the run out of the rotor is measured using the non-contact measurement sensor, and the total amount of rotation of the shaft is measured by the angular encoder. 
     The run out is measured in two locations on the magnet. More specifically, the run out is measured on a first section and a second section. The first section is a first central median bisecting plane on the outer surface, and is located in proximity to an end of the magnet. The second section is a second central median bisecting plane on the outer surface, but is located in proximity to the opposite end of the magnet as the first section. In one embodiment, the plot is a radar plot having a first flux density plot, indicating the magnetic flux density measured from the first section, and a first run out plot, indicating the run out measured from the first section. The plot also includes a second flux density plot, indicating the magnetic flux density measured from the second section, and a second run out plot, indicating the run out measured from the second section. The first run out plot and the second run out plot are plotted with the first flux density plot and the second flux density plot, such that a correlation is made between variations in run out and flux density measurements. 
     The radar plot includes a plurality of petals, each of the plurality of petals includes a portion of the first flux density plot and a portion of the second flux density plot. A portion of the plurality of petals represents one or more north poles, and another portion of the plurality of petals represents one or more south poles. The first run out plot and the second run out plot are plotted on the radar plot to provide a correlation between the run out and the flux density of the one or more north poles and the one or more south poles. 
     Further areas of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples, while indicating the preferred embodiment of the invention, are intended for purposes of illustration only and are not intended to limit the scope of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention will become more fully understood from the detailed description and the accompanying drawings, wherein: 
         FIG. 1  is a plot having the flux density measurements of a rotor, according to prior art; 
         FIG. 2A  is a sectional side view of a digital linear actuator having a rotor which includes a magnet, and the flux density and run out of the magnet are measured according to the present invention; 
         FIG. 2B  is a perspective view of a rotor having a magnet, and the flux density and run out of the magnet are measured according to the present invention; 
         FIG. 3A  is a partial sectional view of a portion of a digital linear actuator having a rotor which includes a magnet and the flux density and run out of the magnet are measured, according to the present invention; 
         FIG. 3B  is a pictogram of a radial surface of a magnet and portions of a first stator assembly and a second stator assembly, where the flux density and run out of the magnet are centrally measured, according to the present invention; 
         FIG. 4  is a plot having flux density measurements and run out measurements, according to the present invention; 
         FIG. 5  is a radar plot having flux density measurements and run out measurements, according to the present invention; and 
         FIG. 6  is a perspective view of a tool used to measure the flux density and run out of a rotor, according to the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The following description of the preferred embodiment(s) is merely exemplary in nature and is in no way intended to limit the invention, its application, or uses. 
     Referring to  FIGS. 2A-6  generally, and with specific reference to  FIGS. 2A-3B , and digital linear actuator (DLA) incorporating the method of the present invention is shown generally at  10 . The DLA  10  includes a housing  12  connected to a connector  14 . The connector  14  includes a groove  16  which has an O-ring  18  for providing a sealed connection between the housing  12  and the connector  14 . 
     The connector  14  also includes a set of terminals  20  which are connected to a set of wires  22 . At least one of the wires  22  is in electrical communication with a first coil  24 , and at least another of the wires  22  is in electrical communication with a second coil  26 . The first coil  24  is located in a first stator assembly  28 , and the second coil  26  is located in a second stator assembly  30 . Connected to the first stator assembly  28  is an inner housing  32 , and the inner housing  32  partially surrounds a first end, shown generally at  34 , of a rotor, generally shown at  36 . The first end  34  also has a bearing surface  38  which is supported by a bearing  40 . There is also a bearing locator  42  connected to the inner housing  32 , best seen in  FIG. 2A , which is also used for correctly locating the bearing  40 . 
     The rotor  36  from the DLA  10  is a magnetic cylinder. More specifically, the rotor  36  also includes a cylindrical magnet  44  mounted on a body portion  46  of the rotor  36 . The body portion  46  also include two flanges  48 , 50 . The magnet  44  is mounted to the body portion  46  between the two flanges  48 , 50 . Protruding from the flange  50  is an outer portion  52 , which partially extends into a front bearing housing  54 . The front bearing housing  54  is connected to the second stator assembly  30 , as shown in  FIG. 2A , and has an elongated portion  56  which is at least partially surrounded by a spring  58 . The outer portion  52  is part of the second end  70  of the rotor  36 , and extends into and contacts an inner surface  72  of an inner cavity, shown generally at  74 , of the elongated portion  56 . The elongated portion  56  also has a guide aperture  60  which guides an enlarged diameter portion  64  of an actuation shaft  62 , and the guide aperture  60  prevents the shaft  62  from rotating, the function of which will be described later. The actuation shaft  62  also includes a threaded portion  66  which is engaged with a threaded portion  68  formed as part of the body portion  46  of the rotor  36 . 
     The spring  58  extends away from the front bearing housing  54  and is partially surrounded by a sleeve  76 . The sleeve  76  is connected to a first portion  78  of a valve. The first portion  78  is mounted on the end of the actuation shaft  62  through the use of a threaded connection  80 . The first portion  78  of the valve also includes an extension  82  which is surrounded by the spring  58 , the extension also functions as a guide for the spring  58 . 
     The spring  58  functions to bias the valve toward a closed position. The spring  58  also functions to limit axial play in the bearing  40 , limit backlash between the threaded portions  66 , 68 , and provide dynamic force balance during the travel of the actuation shaft  62 . The valve, and more specifically the first portion  78 , is moved through the use of the rotor  36  and the stator assemblies  28 , 30 . The first stator assembly  28  includes a plurality of field cup teeth and pole plate teeth. Shown in the Figures are a first outer half field cup tooth  84 , a second outer half field cup tooth  86 , and a third outer half field cup tooth  88  of the first stator assembly  28 , as well as a first inner half pole plate tooth  90 , and a second inner half pole plate tooth  92 . Also shown in the Figures are a first inner half pole plate tooth  94 , a second inner half pole plate tooth  96 , and a third inner half pole plate tooth  98  of the second stator assembly  30 , as well as a first outer half field cup tooth  100 , and a second outer half field cup tooth  102  of the second stator assembly  30 . While each of the stator assemblies  28 , 30  are shown having the various pole plate teeth and field cup teeth, the stator assemblies  28 , 30  must have an equal number of teeth to create an equal number of north-south pairs to function properly. It is within the scope of the invention that more or less pole plate teeth and field cup teeth may be used, depending upon the application. 
     The cylindrical magnet  44  includes a plurality of alternating pole pairs, which extend along the length of the magnet  44 . More specifically, the cylindrical magnet  44  includes at least a first south pole  104 , a second south pole  106 , and a third south pole  108 . The magnet  44  also includes at least a first north pole  110 , a second north pole  112 , and a third north pole  114 . While three south poles  104 , 106 , 108  and three north poles  110 , 112 , 114  are shown in  FIG. 3B ,  FIG. 3B  is only a partial view of the magnet  44 , and there are actually six north poles and six south poles, for a total of twelve poles, which creates six pole pairs. It is also within the scope of the invention that more or less poles may be used, depending upon the size of the magnet  44 , and the application. The number of pole pairs affects the step increment of the magnet  44 . In this embodiment, since one full rotation of the magnet is 360°, and there are twelve pole teeth per stator, the stator assembly step increment is 30°. However, since the two stator assemblies  28 , 30  are radial offset by half of the pole tooth distance, the stepper motor step increment 15°. In other embodiments, such as the magnet  44  having 10 pole pairs, the step increment would be 9°. In yet another alternate embodiment, the magnet  44  includes two pole pairs, creating a step increment of 45°. In other embodiments of the invention, more or less pole pairs may be used to create the desired step increment. 
     The cylindrical rotor  36  rotates about an axis, and the rotor  36  used in stepper motors is magnetized longitudinally along the axis. Magnetization of the rotor  36  is achieved using a multiple N-S pole pair configuration, such as the configuration of the north poles  110 , 112 , 114  and south poles  104 , 106 , 108  shown in  FIG. 3B . Spacing and quantity of the pole pairs are motor step angle dependant, as mentioned above. The DLA  10  has two coil fields (shown as the two coils  24 , 26  in  FIGS. 2A and 3A ) which are stacked along the length of the rotor  36 . A central median bisecting plane is targeted for gauss analysis in the area of each coil  24 , 26 . 
     In operation, current is transferred through the terminals  20 , and then through the wires  22  to each of the coils  24 , 26 . The current in the coils  24 , 26  creates an electromagnetic field in the field cup teeth  84 , 86 , 88 , 100 , 102  and the pole plate teeth  90 , 92 , 94 , 96 , 98  of each of the stator assemblies  28 , 30 . This electromagnetic field also interacts with the poles  104 , 106 , 108 , 110 , 112 , 114  of the magnet  44 , causing the magnet  44 , and therefore the rotor  36  to rotate into the nearest phase magnetic balance. 
     As mentioned above, the connection between the guide aperture  60  and the enlarge diameter portion  64  of the actuation shaft  62  prevents the actuation shaft  62  from rotating. The rotation of the threaded portion  68  of the rotor  36  relative to the threaded portion  66  of the actuation shaft  62  translates the rotational motion of the rotor  36  to linear motion of the actuation shaft  62 . The connection between the threaded portion  68  of the rotor  36  and the threaded portion  66  of the actuation shaft  62  moves the actuation shaft  62  linearly as the rotor  36  is rotated. 
     The movement of the actuation shaft  62  linearly moves the first portion  78  of the valve, and extends the first portion  78  away from the front bearing housing  54  when the rotor  36  is rotated in a first direction, or counter clockwise direction. When the current is applied to one of the coils, such as the first coil  24 , is reversed, the electromagnetic field in the field cup teeth  84 , 86 , 88  and the pole plate teeth  90 , 92  of the stator assembly  28  is reversed, causing the magnet  44  and therefore the rotor  36  to rotate in the opposite or retract direction, which in this embodiment is a second direction, or clockwise direction. This again causes the actuation shaft  62  to move linearly, and the first portion  78  of the valve moves toward the front bearing housing  54  when the rotor  36  is rotated in the clockwise direction. 
     The operation of the DLA  10  is affected by the electromagnetic field generated in the coils  24 , 26  and the teeth  84 , 86 , 88 , 90 , 92 , 94 , 96 , 98 , 100 , 102 . The operation of the DLA  10  is also affected by the flux density of the magnet  44 . During the design of the DLA  10 , the flux density of the magnet  44  is measured, as well as the run out on the surface of the magnet  44 . The magnet  44  has a length  148 , and the run out is measured at a first section  118 , which is a planar circumference located at a distance away from an end  118 A of thirty percent of the length  148  of the magnet  44 , and a second section  116 , which is also a planar circumference located at a distance away from another end  116 A of thirty percent of the length  148  of the magnet  44 , shown in  FIG. 2B . More specifically, the flux density is measured along a radial location that is away from each end  116 A, 118 A, which in this embodiment, is a distance of thirty percent of the overall length of the magnet  44  inboard from the first end  116 A, and a distance of thirty percent of the overall length of the magnet  44  inboard from the second end  118 A. The measurement is essentially taken along a central median bisecting plane of the stator assemblies  28 , 30 , and each central median bisecting plane is located at the first section  118  and the second section  116 . 
     Measurement of the flux density is accomplished by rotating the rotor  36  and measuring the magnetic field strength variation between neighboring poles  104 , 100 , 106 , 112 , 108 , 114 , at a defined radial “air gap” distance. The flux density is measured by a gauss probe, and the air gap is a predetermined radial distance away from the outer surface of the rotor  36 . In one embodiment, the air gap is about 0.010 inches in radial distance; however, it is within the scope of the invention that other radial distances may be used to obtain the measured flux density. The Gauss measurement of the magnetic flux is specific to the air gap. This radial distance corresponds physically to the DLA internal “air gap” between rotor  36  and the teeth  84 , 86 , 88 , 90 , 92 , 94 , 96 , 98 , 100 , 102  of the stator assemblies  28 , 30 . The magnetic flux exits the north poles  110 , 112 , 114  and enters in the south poles  104 , 106 , 108 . Stray flux field or Gauss eddy current losses originate from the ends of the cylindrical magnet  44 . The radial Gauss at the ends of the rotor  36  are influenced by eddy current loss. Gauss measurements are therefore not performed too close to the rotor cylinder ends. 
     Additionally, the circular runout is measured by the use of a non-contact measurement sensor. The surface of the rotor  36 , and more specifically, the magnet  44 , is measured to establish the magnitude of variation. Significant variation of the surface of the magnet  44 , such as circular run out, may influence the flux density reading as the dimension of the radial air gap changes. The measurement of the surface of the magnet  44  is not only used to evaluate the planar circular run out of the magnet  44  relative to the core axis, but also any bearing journal surfaces, such as the surface  52 A of the outer portion  52 , shown in  FIG. 2B . In an alternate embodiment, an axial fixture may be used to position the non-contact measurement sensor to measure the total runout of the surface of the rotor  36 , and not just a circular runout measurement of a planar section. 
     The method of the present invention incorporates simultaneously plotting the flux density reading (as a sine wave plot) and the run out variation during one full revolution. In one embodiment, a plot according to the present invention is shown in  FIG. 4 . The flux density and run out are plotted together to provide a correlation between surface variation, and variation in flux density. There is a plurality of peaks  120  and a plurality of valleys  122 . The peaks  120  represent the maximum flux density of each of the north poles  110 , 112 , 114 , and the valleys  122  represent the maximum flux density at each of the south poles  104 , 106 , 108 . There are two plots which show the measured flux density in  FIG. 4 , a first line  124  representing the flux density measured in the first section  118 , and a second line  126  representing the flux density measured in the second section  116 . Furthermore, there is a third line  128  representing the run out measured in the first section  118 , and a fourth line  130  representing the run out measured in the second section  116 . 
     The overlay of the flux density and run out shows a correlation between the variations in the surface of the magnet  44  and the measured flux density. 
     In another embodiment, instead of plotting the flux density as a sine wave plot, and overlaying the run out plot with the flux density plot, as shown in  FIG. 4 , the flux density may be plotted as a polar graph, or radar plot, as shown in  FIG. 5 . Instead of having a plurality of peaks  120  and valleys  122 ,  FIG. 5  includes a plurality of “petals.” Each petal  150  provides an indication of one of the magnet poles  104 , 110 , 106 , 112 , 108 , 114  along a radial indexed location of the magnet  44 . 
     These geometric shapes, or petals  150 , are magnetic flux gauss measurements that software visualizes into absolute vector radar plots. The radar plot in  FIG. 5  is a pictorial which presents the data in an efficient manner. The radar plots are derived from the magnetic flux gauss measurement at the radial air gap from around the outer surface of the magnet  44  of the rotor  36 . The magnetic flux field gauss values are recorded as a sine wave as the test part is rotated during measurement. The magnetic flux field gauss strength determines plot amplitude. The magnetic flux field gauss polarity is determined by rotor magnetization (north and south). The sine wave that is a recorded is plotted as shown in  FIG. 4  to represent the measurement of the flux density. However, in  FIG. 5 , the positive and negative polarity sine waves create “lemniscates” shapes in the radar plot. As this creates a confusing image, the sine wave is translated into absolute values. In the resulting radar plot shown in  FIG. 5 , each petal  150  visually represents the neighboring pole (i.e. north, south, and north) which is easier to relate to the physical magnet  44  of the rotor  36 . Each “petal”  150  represents a single magnetic pole. 
     In  FIG. 5 , the plots of the run out measurements are also plotted along a radial indexed location, correlating to the location of the flux density. Each value on the plot shown in  FIG. 5  is represented as a vector distance from the origin centre of the plot. Values are plotted corresponding to the index position during measurement rotation of the rotor  36 . 
     More specifically, the flux density measured in the first section  118  is shown by the first line  124 , and the flux density measured in the second section  116  is shown by the second line  126 . The first line  124  and the second line  126  shown in  FIG. 5  represent the same flux density as shown by the lines  124 , 126  in  FIG. 4 , with the difference being how the flux density is plotted. Additionally, each petal  150  also represents the alternating north and south poles on the magnet  44 , and are plotted in magnetic sector increments of thirty degrees, beginning at zero degrees, and the pole peak is observed at fifteen degrees. In this embodiment, since there are six pole pairs, or twelve poles, and one rotation of the rotor  36  is 360°, the plotted magnetic sector increments are thirty degrees. More specifically, the north poles are shown generally at  150 A, and the south poles are shown generally at  150 B. The petal  150 B representing the flux density measurement for the first south pole  104  is located at fifteen degrees, the petal  150 A representing the flux density measurement for the first north pole  110  is located at forty-five degrees, the petal  150 B representing the flux density measurement for the second south pole  106  is located at seventy-five degrees, and the petal  150 A representing the flux density measurement for the second north pole  112  is located at one-hundred-five degrees, and this pattern continues as shown in  FIG. 5 , and all of the petals  150  are plotted. Since there are six pole pairs, there are twelve petals  150  plotted. As mentioned above, while there are only three south poles  104 , 106 , 108  and three north poles  110 , 112 , 114  shown in  FIG. 3B , there are actually six north poles and six south poles. In this embodiment, plots are shown representing the flux density for the six south poles and the six north poles of the magnet  44  in an alternating manner, but it is within the scope of the invention that more or less pole pairs may be included as part of the magnet  44 . 
     Also shown in  FIG. 5  is the radial plot of the run out, shown by the third line  128  and the fourth line  130 . These lines  128 , 130  are the same lines  128 , 130  as shown in  FIG. 4 , but are plotted radially, and form two circles. It can be seen in  FIG. 5  that the measurement of the run out along the outer surface of the magnet  44  corresponds to the measurements of the flux density taken at the same location along the outer surface of the magnet  44 . The plot in  FIG. 5  provides a correlation between the measured magnetic flux density and the run out variation at corresponding locations on the surface of the magnet  44 . This provides information that may be used during the development of the rotor  36 , such that the surface finish of the magnet  44  may be changed to reduce or eliminate run out, and provide a consistent air gap and the desired magnetic flux density. 
     When looking at  FIG. 5 , and the two circles formed by the third line  128  and the fourth line  130 , these two circles (the third line  128  representing the run out measured in the first section  118 , and the fourth line representing the run out measured in the second section  116 ) are visualized surface plots of the cylindrical magnet  44  which is part of the rotor  36 , and represent circular run out. Ideally, the two circles constructed by the third line  128  and the fourth line  130  form overlapping circles having a consistent radius. However, when looking at the plot shown in  FIG. 5 , if the two circles (created by the lines  128 , 130 ) do not overlap, it is an indicator that the cylindrical shape of the magnet  44  is inconsistent. When one circle plot is observed inside the other circle plot (the circle created by line  128  is inside of the circle created by line  130 , or vice versa), it would be indicative of the rotor magnet  44  having a conical feature, or being conically shaped. When the circle plots created by the lines  128 , 130  are observed such that the two lines  128 , 130  separate for an arc segment it is an indication that the shape of the surface of the magnet  44  is inconsistent along an axial length. 
     Each “petal”  150  represents a pole on the magnet rotor (and is a portion of the first line  124  and the second line  126 ). The vector magnitude of each petal  150  represents the magnetic flux gauss field strength. The vector distance each petal  150  extends from the origin center can be interpreted as the strength of the magnet (a bigger petal means a stronger Gauss measurement). Differences in the size of the petals  150 , amplitude and width, indicates a variation of the Gauss magnet field strength. If the two petal plot groups (the first group of petals  150  created by the first line  124  representing the flux density measured in the first section  118 , and the second group of petals  150  created by the second line  126  representing the flux density measured in the second section  116 ) do not overlap, it is an indicator that magnetization is inconsistent. 
     However, assuming magnet uniformity and homogeneity, variation between the amplitudes of one pair of petals  150  indicates material variation in the magnet  44  or surface imperfection on the surface of the magnet  44 . Imperfections on the surface of the magnet  44  are usually limited to variation of a single petal  150 . Cross checking against the rotor surface plot variation would confirm this surface variation. 
     Variation between the amplitudes can indicate magnetic flux gauss field strength changing along the length of the rotor. More specifically, if the plot of one petal  150  is observed fully inside the other another petal  150 , it would be indicative of a field strength variation along the length of the magnet  44  of the rotor  36 . Cross checking against the rotor surface plot variation would confirm any surface imperfections, causing air gap variation in Gauss measurements. 
     If the group of petals  150  created by the first line  124  is observed fully inside the group of petals  150  created by the second line  126 , or vice versa, this indicates a field strength variation along the length of the magnet  44  of the rotor  36 . Cross checking against any plot variation shown by the third line  128  and the fourth line  130  would confirm a conical outer surface of the magnet  44 . 
     If both the group of petals  150  created by the first line  124  and the group of petals  150  created by the second line  126  appear to be shifted in only one vector direction, this indicates a field strength variation along the length of the magnet  44 . Cross checking against any plot variation shown by the third line  128  and the fourth line  130  would confirm rotor center misalignment during the magnetization process. 
     If both the group of petals  150  created by the first line  124  and the group of petals  150  created by the second line  126  appear to have an angular phase shift, this indicates a non-axial askew magnetization or axial misalignment during the magnetization process. Cross checking against any plot variation shown by the third line  128  and the fourth line  130  would confirm a defect with the magnetization equipment process. 
     Referring to  FIG. 6 , an example of a tool, shown generally at  132  used for measuring flux density and run out is shown. The tool  132  includes an indexing rotary head, shown generally at  134 , which is connected to an angular encoder, shown generally at  136 , and the angular encoder  136  is connected to a shaft, shown generally at  138 . The shaft  138  is connected to a fixture, shown generally at  140 , which in this embodiment is a collet axial fixture  140 , and the rotor  36  is mounted in the fixture  140 . The rotor  36  is shown as a test specimen, shown generally at  142  in  FIG. 6 . The tool  132  also includes an adjustable non-contact measurement sensor, shown generally at  144 , and an adjustable gauss probe, shown generally at  146 . In this embodiment, the non-contact measurement sensor  144  is a laser probe, but it is within the scope of the invention that other types of non-contact measurement sensors may be used. 
     In operation, the indexing rotary head  134  rotates and drives the shaft  138  and the test specimen  142  located in the fixture  140 . As the shaft  138  rotates, the angular encoder  136  measures the angular position of the shaft  138 . As the test specimen  142  is rotated, the non-contact measurement sensor  144  measures the surface run out on the surface of the magnet  44 , and the gauss probe  146  measures the flux density. Measurement of the flux density and surface run out cannot be performed simultaneous at the same point. Therefore, the measurements are performed ninety degrees apart, as shown in  FIG. 6 , and software compensates for the different radial locations in measurement. Plots the data such that the magnetic flux and run out are correlated, as shown in  FIG. 5 . Although in this embodiment the measurements are performed ninety degrees apart, it is within the scope of the invention that measurements may be taken in other locations as well. 
     In an alternate embodiment, the measurements of flux density and run out may be incorporated into a production process to measure production rotors  36 , and provide a correlation between run out and flux density, such that any variation may be compensated for during assembly of the DLA  10 . 
     The description of the invention is merely exemplary in nature and, thus, variations that do not depart from the gist of the invention are intended to be within the scope of the invention. Such variations are not to be regarded as a departure from the spirit and scope of the invention.