Abstract:
The method of reducing error in rotor speed measurement includes synchronously measuring a rotor having a target including at least one geometric imperfection. Time intervals for the passing of each tooth of a rotor are stored in a circular buffer memory array. Speed is always determined by extracting the time for a complete revolution, so that geometric imperfections and asymmetry of the rotating target do not influence the speed determination, which is always representing the average speed over the latest complete revolution.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This PCT application is a national stage application under 35 U.S.C. §371(c) of prior filed, co-pending PCT application serial number PCT/US2014/064521, filed on Nov. 7, 2014, which claims priority to U.S. Patent Application Ser. No. 61/902,474, titled “Method for Reducing Error in Rotor Speed Measurements” filed Nov. 11, 2013. The above-listed applications are herein incorporated by reference. 
     
    
     BACKGROUND 
       [0002]    Present embodiments relate generally to measuring systems for determining rotor speed. More particularly, but not by way of limitation, present embodiments relate to methods of reducing error in rotor speed measurements. 
         [0003]    Rotor speed may be utilized to make various determinations in operating characteristics of many types of rotating structures. For example, brake assemblies, engines, turbines, propeller shafts, fans, conveyors or any other rotating structure. The term “rotor” should be understood as a broadly defined rotating mechanical structure. Rotor speed is typically indicated in revolutions per minute (RPM), radians per second or hertz. 
         [0004]    Generally, two methods of determining rotor speed or RPM are utilized. A frequency measurement system is utilized for fast rotating devices such as motors and turbines that typically rotate in thousands of revolutions per minute. Alternatively, period measurement system is more commonly utilized for structures having shafts that rotate at lesser speeds. 
         [0005]    Sensors are normally utilized to determine rotor speed and may be embodied by shaft encoders, rotary pulse generators, proximity sensors or photoelectric sensors. In conjunction with the sensor, a rotor may include a target with one or more features which are measured during rotation of the rotor. These targets may have unintended geometric imperfections or intentional geometric inconsistencies which correspond to a location or condition of the rotor, such as top dead center of the rotor. For example, some targets may have a rib, tooth or other projection which is sized, shaped or spaced differently than other features of the target. Accordingly, these geometric imperfections introduce error into the measuring process which may result in propagation of such error through subsequent calculations based on the measuring process. 
         [0006]    Accordingly, it would be desirable to develop methods in order to provide a more accurate system of measuring in order to reduce errors associated with known methods of measuring rotor speed. 
         [0007]    The information included in this Background section of the specification, including any references cited herein and any description or discussion thereof, is included for technical reference purposes only and is not to be regarded subject matter by which the scope of the instant embodiments are to be bound. 
       SUMMARY 
       [0008]    Present embodiments of the method of reducing error in rotor speed measurement include synchronously measuring a rotor having a target which may include one or more geometric imperfections. A sensor is utilized to create a periodic waveform and used by a measuring system which will create an array of a preselected number of periods. When the preselected number of times is reached, new times are added and old times are removed from the array. An average speed is determined based on a subset of the array of periods. The average may be for the entire list or some portion of the list or some multiple corresponding to a multiple of complete revolutions. 
         [0009]    According to some embodiments, a method of measuring rotor speed comprises positioning a sensor opposite a target, the target including a plurality of features, measuring a period corresponding to time between each of the plurality of features on the target passing the sensor, establishing an array of the periods, the array including up to a preselected number of the periods, removing old periods from the array when new periods are added and the preselected number of periods is reached, and, calculating an average period from a subset of the periods in the array, the average period corresponding to one of a complete revolution or a multiple of complete revolutions and, calculating rotational speed of a rotor from the average period. 
         [0010]    According to some embodiments, a method of measuring rotor speed comprises positioning a sensor opposite a target, the target including a plurality of features wherein the features include at least one geometric imperfection, measuring a period corresponding to time between each of the plurality of features on the target passing the sensor, establishing an array of the periods, the array including up to a preselected number of the periods, removing old periods from the array when new periods are added and the preselected number of periods is reached, calculating an average period from a subset of the periods in the array, the average period corresponding to one of a complete revolution or a multiple of complete revolutions, and, calculating rotational speed of a rotor from the average period to compensate for the at least one geometric imperfection. 
         [0011]    All of the above outlined features are to be understood as exemplary only and many more features and objectives of the method may be gleaned from the disclosure herein. This Summary is provided to introduce a selection of concepts in a simplified form that are further described below in the Detailed Description. This Summary is not intended to identify key features or essential features of the claimed subject matter, nor is it intended to be used to limit the scope of the claimed subject matter. A more extensive presentation of features, details, utilities, and advantages of the present invention is provided in the following written description of various embodiments of the invention, illustrated in the accompanying drawings, and defined in the appended claims. Therefore, no limiting interpretation of this summary is to be understood without further reading of the entire specification, claims, and drawings included herewith. 
     
    
     
       BRIEF DESCRIPTION OF THE ILLUSTRATIONS 
         [0012]    The above-mentioned and other features and advantages of this disclosure, and the manner of attaining them, will become more apparent and the method of reducing error will be better understood by reference to the following description of embodiments taken in conjunction with the accompanying drawings, wherein: 
           [0013]      FIG. 1  is a side view of an exemplary measuring system; 
           [0014]      FIG. 2  is a side view of an exemplary target having at least one geometric imperfection; 
           [0015]      FIG. 3  is a waveform representation of the measured imperfection of the embodiment of  FIG. 2  plotted on a timeline; 
           [0016]      FIG. 4  is an alternative target with a geometric imperfection; 
           [0017]      FIG. 5  is a waveform representation of the measured imperfection of the embodiment of  FIG. 4  plotted on a timeline; 
           [0018]      FIG. 6  is an alternative target embodiment with a geometric imperfection; 
           [0019]      FIG. 7  is a waveform representation of the measured imperfection of the embodiment of  FIG. 6  plotted on a timeline; 
           [0020]      FIG. 8  is a visual relationship of an analog waveform, digital conversion and mathematic representation of a method to eliminate rotor synchronous error; 
           [0021]      FIG. 9  is a flow chart depicting a method of reducing speed error in a synchronous manner; and, 
           [0022]      FIG. 10  is a schematic flow chart of the method of reducing or eliminating speed error in a synchronous manner. 
       
    
    
     DETAILED DESCRIPTION 
       [0023]    Reference now will be made in detail to embodiments provided, one or more examples of which are illustrated in the drawings. Each example is provided by way of explanation, not limitation of the disclosed embodiments. In fact, it will be apparent to those skilled in the art that various modifications and variations can be made in the present embodiments without departing from the scope or spirit of the disclosure. For instance, features illustrated or described as part of one embodiment can be used with another embodiment to still yield further embodiments. Thus, it is intended that the present invention covers such modifications and variations as come within the scope of the appended claims and their equivalents. 
         [0024]    Referring to  FIGS. 1-10 , various methods of reducing speed measurement error for a rotor are provided. Instant embodiments utilize a method which is rotor synchronous to reduce error and improve accuracy of speed measurements. By rotor synchronous, it is meant that the speed measurement is directly related to a rotation of the target. According to some examples, the method may be utilized in an engine, turbine, wheel or brake assembly, conveyor or other rotating mechanism which may benefit from the instant embodiments. 
         [0025]    Referring initially to  FIG. 1 , a view of an exemplary measuring system  10  is depicted. The system includes a target  12  having a plurality of features  14  positioned thereabout. The target  12  is generally circular from which the features  14  extend. The features  14  may be teeth, projections, ribs, protuberances or other structures which may be detected. Further, embodiments may include one or more slots or other internal features as opposed to, or in addition to, the external features described above. Additionally, magnets may be embedded in the rotor. A speed sensor  16  is opposite the target  12  and produces a waveform with a frequency that is proportional to the speed of a rotor  18  which may be embodied by a shaft, for example, upon which the target  12  is mounted by detecting each of the features  14 . The target  12  may be integrally formed with the rotor  18  or alternatively, may be formed separately and connected in a variety of known fashions such as a key, keyway, set screw, interference fit or other known methods of connecting such rotating structures. Additionally, the target  12  may also be physically separate from the rotor  18  while still rotating at a speed proportional to the rotor  18 . For example, the target  12  may be connected to the rotor  18  through a series of gears. The speed sensor  16  may be a variable reluctance sensor, hall effect sensor, shaft encoder, proximity sensor, or photoelectric sensor, for example. For systems using magnetic sensors, the features  14  may be teeth, projections, ribs, protuberances, notches, slots, magnets, or other structures which may be detected. Further, embodiments may include one or more combinations of the above as well as slots or other internal features as opposed to external features. For systems using optical sensors, the features  14  may be alternating colors, changes in reflectivity, for example. However, other sensors and targets may be utilized and this list is not considered to be limiting. Finally, a computer  17  is shown in electrical communication, shown by broken line, to the sensor  16 . The computer  17  may perform various computations and store values provided by the sensor  16 , as described further herein. 
         [0026]    Referring now to  FIG. 2 , a side view of a second exemplary target  112  is depicted. The target  112  includes a plurality of features  114  wherein the target  112  or at least one of the features  114  includes at least one geometric imperfection, for example tooth spacing. According to the instant embodiment, each of the features  114  includes an arcuate spacing  115  between features. This normal spacing  115  represents an angular distance and may be equivalent to 360 degrees divided by the number of features “N”. However, the structure also comprises at least one non-symmetric spacing  117  which is greater than the normal spacing  115 . Additionally, the depicted embodiment target  112  further comprises a second non-symmetric spacing  119  which is less than the normal spacing arcuate distance of spacing  115 . The spacings  117  and  119  are not equal to 360 degrees divided by the number of teeth N. All of the spacings  115 ,  117  and  119  will, however, have an average spacing  115  when summed over one or more revolutions and divided by the number of features considered. 
         [0027]    When the target  112  is rotating, the spacings  115 ,  117 ,  119  all correspond to periods of time  122 ,  126 ,  124  ( FIG. 3 ). The measured arcuate spacing  115  is measured from the center of a first feature  114  to the center of an adjacent feature  114 . Alternatively, the measurement may be taken from one position of a feature to an equivalent position on an adjacent feature, for example one tooth base, or from a position between features to an equivalent position between an adjacent pair of features. 
         [0028]    Referring now to  FIG. 3 , a visual representation of the sensor waveform cycle associated with a rotation of target  112  is depicted and plotted on a timeline  120 . The time period  122  corresponds to a single feature of target  112 . The visual representation is of a waveform which is formed by analog processing of a signal created by sensor  16 , for example. A plurality of periods  122  are designated along the timeline  120  corresponding to measured periods of time  122  associated with feature  114  of targets  112 . Each time period, for example, may be measured between center points of features  114 . Additionally, a second period  124  is noted which corresponds to the measurement of the spacing  119  which is less than the normal spacing  115  and less than the normal period  122 . A third period  126  is designated and corresponds to the spacing  117  which is greater than the normal spacing  115 . Subsequently, a normal period of time  122  is depicted completing the measured instance of  FIG. 3  along timeline  120 . Thus, where the spacing  119  is less than spacing  115 , the corresponding period of time  124  will be less than the normal period of time measurement  122 . Oppositely, where spacing  117  is greater than the normal spacing  115 , the associated time period  126  is greater than the normal time period  122 . These periods  122 ,  124 ,  126  are also depicted in  FIG. 2  for purposes of representing the relationship on the target  112 . 
         [0029]    Referring now to  FIG. 4 , a further alternative embodiment of a target  212  is depicted. The embodiment depicts an additional asymmetric feature type, target feature height. It should be clear to one skilled in the art that any of the feature types discussed in this disclosure may be used either alone or in combination with these or other embodiments of symmetric or asymmetric features. The target  212  includes a plurality of features  214  with a first radial height  216 , which is depicted by a first broken line. The target  212  further comprises a second feature  218  which is of a second radial height  220 . These features  214 ,  218  depict a further geometric imperfection which may be utilized to determine a specific rotor position or condition, such as top dead center. Although the taller feature  214  is considered normal, it alternatively may be that the shorter feature may be the normally sized feature and the taller feature is the asymmetric feature. The low feature  218  causes a reduced amplitude in the speed waveform depicted in  FIG. 5 . 
         [0030]    With reference additionally now to  FIG. 5 , a waveform is depicted corresponding to a rotation of the target  212  of  FIG. 4 . Specifically, the timeline  222  is shown with a measured pictorial representation of an exemplary waveform created by one revolution of the target  212 . The periods  224  depict measured instances of time between features  214  of normal radial height  216 . However, the periods  226  and  228  depict the measured instances of time between features  214  and  218  of the second radial height  220  and corresponding reduced waveform amplitude. These periods  224 ,  226 ,  228  are also shown in  FIG. 4  for ease of reference. The periods  226  and  228  may be different from period  224  due to the interaction of the target and sensor, similar to how periods  124  and  126  are different than period  122  in  FIG. 3 . Alternatively, spacings  224 ,  226 ,  228  may be equal on timeline  222  but the measurement may interpret them as unequal spacings due to interactions between the reduced amplitude  223  and imperfections in the measurement system. 
         [0031]    Referring now to  FIG. 6  a further geometric imperfection is shown in target  312  including a true center  315  and an actual (asymmetric) center  313  wherein the actual center  313  of target  312  is not positioned directly in the true center  315  of target  312 . The target  312  has a true center point  315  which is spaced from the actual center  313 . The actual center  313  causes an off-center rotation of the target  312  and such speed waveform is depicted in  FIG. 7 . On the waveform and along the timeline  320 , the waveform amplitude increases as the features  314  move toward the sensor and the waveform amplitude decreases as the features  314  move away from the sensor. 
         [0032]    In summary, the rotor speed is assumed to be proportional to the period between features on the target wheel. However, error which is desired to be removed from the measurement may be introduced by imperfections in the target geometry or measurement of the features, for example, teeth on the target wheel. Such imperfections may include spacing, concentricity, variations in radial height of the features or other imperfections. Measurement system error may be caused by interactions between imperfections in the measurement system and the target geometry. Such imperfections include, but are not limited to, non-zero detection threshold which causes period measurement error if the speed waveform amplitude is not constant due to inconsistent radial height and target features or a nonconcentric target. Any combination of the target and/or measurement system imperfections shown in  FIGS. 2, 4, 5, 6, and 7  may be combined in rotor and associated measurement systems. While imperfections may be necessary for various embodiments of measure, it would be desirable to eliminate the related measurement error. Additionally, it should be understood that the geometric imperfections previously described may be formed intentionally or unintentionally and the methods described herein provide a means for compensating for such imperfections in order to calculate accurate information, such as speed measurement. 
         [0033]    Referring now to  FIG. 8 , a series of signals and period measurements are depicted and related to a mathematical representation of the method of reducing rotor speed measurement. The signals and period measurements correspond to one another in the vertical direction of the figure. The analog signal represented by a waveform  500  shows a time period  502  corresponding to a single revolution of a target, for example one of the targets previously described, such as target  212  having a geometric imperfection. The analog signal waveform  500  includes a plurality of waves or signals  504  of a first wave amplitude and a first and last wave or signal  506  of a second wave amplitude representing the imperfection such as, for example, a target feature of smaller radial height. 
         [0034]    Beneath the analog signal waveform  500  is a digital signal waveform  520 . The analog waveform or signal  500  is converted to the digital signal with the use of a logic device (not shown) that has a non-zero detection threshold. By non-zero detection threshold, it is meant that the logic device will introduce error in the period measurements due to inconsistent slope of the analog signal near-zero crossings. For example, the zero crossing slope of signal  506  causes error in periods in  534 ,  536 . The analog signal waveform  500  may have ideal zero crossing to zero crossing spacing but the logic device will produce one shorter period and one longer period in the digital waveform  520  that corresponds to the change in amplitude of the analog waveform  500 . The digital signals are of equivalent shape having a first signal  522  which corresponds to the first feature signals  504 . The digital waveform  520  further comprises a second digital signal  524  corresponding to wave  506  of waveform  500 . As will be understood by comparing the waveforms  500 ,  520 , the variations in waveform size correspond to the imperfections in the target wheel features. 
         [0035]    The digital signal  520  is next converted into a period measurement  530  wherein a plurality of time periods  532  are measured and correspond to target features of normal size. However, where a target feature varies in size, shape or spacing, for example, the digital signal  524  and analog signal  500  vary from the normal or standard feature size or spacing. As depicted, the time period measurement  530  includes a longer period of time  534  which may correspond to a longer spacing between the features, for example. Adjacent to this first longer time period  534  is a shortened time period  536 . In general, these may be at least one shortened and one lengthened time period measurement for each asymmetric feature. 
         [0036]    The relationship between the individual periods of time shown in period  530  is described in equation  550 . A bracket  551  is shown between the period measurement  530  and the equation  550 , which depicts a group of periods corresponding to one rotation. As shown in the mathematical representation, the shortened time period  536  and the lengthened time period  534  which correspond to the geometric imperfections are accounted for in an average and the equation  550  accounts for a single rotation by utilizing a time period associated with each target feature. As previously indicated however, the number of features may be a multiple associated with the number of revolutions utilized to calculate the average. Thus, the calculation is not asynchronous wherein the average is not related to a specific rotation but instead, is synchronous with the rotation of the target. In this way, the imperfections are accounted for by averaging periods over at least one revolution of the target. While time period  502  indicates one revolution of the target as indexed from one wave  506  of reduced amplitude, the revolution of the target may be indexed to any feature on the target. 
         [0037]    Referring now to  FIG. 9 , the method  600  of calculating rotor speed is depicted in a flow chart form. According to a first step  602 , a sensor is positioned opposite the target. Next, a feature period is measured at step  604 . The feature period is a measure of time from a single point of one feature to a same second point at an adjacent feature. If this is the first measurement, an array is created and the period measurement is added to the array. Alternatively, the time period measurement is added to an existing array. After the period is measured at step  604 , the new period measurement is added to an array at step  606 . The array includes a preselected number of entries which is equal to or greater than one complete rotation or a multiple of complete rotations. When the preselected number of entries is reached, the oldest measurement period is removed from the array. Thereafter, as additional entries are made, older entries are removed from the array as indicated by broken line. 
         [0038]    Next, at step  608 , the average period measurement is calculated over one or some multiple of complete revolutions in order to account for any target synchronous errors due to target or measurement system imperfections. After this determination is made, a frequency calculation occurs at step  610  and subsequently, the average rotor speed for one or more revolutions is determined at step  612 . 
         [0039]    With reference now to  FIG. 10 , a graphical representation of the method array  600  is shown. At the left side of the figure, an array  620  is depicted. At the top of the array new entries go into the array  620  wherein P 0  represents the newest period measurement added to the array at step  606 . Near the bottom of the array P x  is shown exiting the array  620  as an additional new time period P 0  enters. Once this is completed, at the next step  608  an average period is calculated wherein the time periods are averaged by adding the periods corresponding to each feature in the array corresponding to one revolution or some multiple of complete revolutions and the total is divided by the number of features corresponding to the one or more complete revolutions of the target. 
         [0040]    Following step  608 , the frequency is determined at step  610  wherein an inverse of the average time period is taken to determine the frequency. Next, at step  612 , the rotor speed R is determined by utilizing the frequency determined at step  610 . The calculations in steps  610  and  612  of this example show the calculations necessary to produce rotor speed in RPM for a system with the target mechanically attached to the rotor. Similar calculations may be performed to produce the rotor speed in alternate units, such as hertz. Additionally, a constant may be applied to the calculated rotor speed for systems where the target is physically separate from the rotor and rotating at a speed proportional to the rotor speed. 
         [0041]    While multiple inventive embodiments have been described and illustrated herein, those of ordinary skill in the art will readily envision a variety of other means and/or structures for performing the function and/or obtaining the results and/or one or more of the advantages described herein, and each of such variations and/or modifications is deemed to be within the scope of the invent of embodiments described herein. More generally, those skilled in the art will readily appreciate that all parameters, dimensions, materials, and configurations described herein are meant to be exemplary and that the actual parameters, dimensions, materials, and/or configurations will depend upon the specific application or applications for which the inventive teachings is/are used. Those skilled in the art will recognize, or be able to ascertain using no more than routine experimentation, many equivalents to the specific inventive embodiments described herein. It is, therefore, to be understood that the foregoing embodiments are presented by way of example only and that, within the scope of the appended claims and equivalents thereto, inventive embodiments may be practiced otherwise than as specifically described and claimed. Inventive embodiments of the present disclosure are directed to each individual feature, system, article, material, kit, and/or method described herein. In addition, any combination of two or more such features, systems, articles, materials, kits, and/or methods, if such features, systems, articles, materials, kits, and/or methods are not mutually inconsistent, is included within the inventive scope of the present disclosure. 
         [0042]    Examples are used to disclose the embodiments, including the best mode, and also to enable any person skilled in the art to practice the apparatus and/or method, including making and using any devices or systems and performing any incorporated methods. These examples are not intended to be exhaustive or to limit the disclosure to the precise steps and/or forms disclosed, and many modifications and variations are possible in light of the above teaching. Features described herein may be combined in any combination. Steps of a method described herein may be performed in any sequence that is physically possible. 
         [0043]    All definitions, as defined and used herein, should be understood to control over dictionary definitions, definitions in documents incorporated by reference, and/or ordinary meanings of the defined terms. The indefinite articles “a” and “an,” as used herein in the specification and in the claims, unless clearly indicated to the contrary, should be understood to mean “at least one.” The phrase “and/or,” as used herein in the specification and in the claims, should be understood to mean “either or both” of the elements so conjoined, i.e., elements that are conjunctively present in some cases and disjunctively present in other cases. Furthermore, references to one embodiment are not intended to be interpreted as excluding the existence of additional embodiments that may also incorporate the recited feature. 
         [0044]    It should also be understood that, unless clearly indicated to the contrary, in any methods claimed herein that include more than one step or act, the order of the steps or acts of the method is not necessarily limited to the order in which the steps or acts of the method are recited.