Patent Publication Number: US-9851225-B2

Title: Nonvolatile multitum rotation sensor with magnetic particle following a spiral track

Description:
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
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     CROSS REFERENCE TO RELATED APPLICATION 
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     BACKGROUND OF THE INVENTION 
     The present invention relates to rotation sensors and in particular to multi-turn rotation sensor that can output absolute angular positions over a rotational range of greater than 360 degrees. 
     Rotation sensors, including encoders, resolvers and the like, are electromechanical devices providing an electrical output indicating the position of a rotatable shaft. A common type of rotation sensor uses a disk-shaped rotor having an optically readable pattern marked on its surface, the pattern forming alternating opaque and transmissive frames. These frames are illuminated from one side by a lamp and light traveling from the lamp through the opaque and transmissive frames of the rotor and then through similar frames in a stationary stator, to be detected by one or more stationary photodetectors. Rotation of the shaft moves the rotor which in turn causes a fluctuation in the light transmitted through the rotor and stator thus producing a signal that may be decoded into a digital indication of shaft movement. 
     Rotation sensors may be classified as absolute rotation sensors or incremental rotation sensors. Incremental rotation sensors provide only an indication of the change in position of the rotation sensor shaft. In incremental rotation sensors, the rotor normally contains a uniform periodic pattern whose movement past a photodetector creates an index signal indicative of the amount that the shaft has rotated. A separate track may also provide a zero signal for a particular angular position. In some cases, one or more photodetectors arranged with an offset of 90 degrees (“quadrature”) provide an indication of the direction of rotation as well as amount of rotation of the shaft, as is understood in the art. 
     Absolute rotation sensors, in contrast to incremental rotation sensors, produce a unique value (typically a digital code word) for each rotation sensor position. The rotor of an absolute rotation sensor may carry a series of concentric tracks whose opaque and transmissive segments, examined along a line of radius, reveal a binary or Grey code value indicative of shaft position. Each track provides the value of one bit and is read by a separate photodetector to produce an output digital word. 
     Often it is desired to have an absolute measure of rotary position over multiple turns (that is, a measurement that spans an angular range of greater than 360 degrees). This can be done using an absolute single-turn rotation sensor by adding an electronic counter that counts up each time the value from the rotation sensor “rolls over” from its maximum value to zero and down when the rotation sensor rolls over from zero to the maximum value. Precise angular position over multiple turns may be done by adding the output from the absolute single-turn rotation sensor to the value of the counter times 360 degrees. 
     The use of an electronic counter can allow the absolute angular position to be lost in the event of a power failure which causes the electronic counter to reset. 
     The problem of creating a “non-volatile” multi-turn absolute rotation sensor, can be addressed replacing the electronic counter with a mechanical counter, for example, using a gear train where successive gears are each attached to simple absolute rotation sensors that provide successive bits in a count value. For example, each gear in the gear train may provide a 2:1 reduction and may connect with a single bit absolute rotation sensor. Each rotation sensor then provides a separate binary digit of a count value. 
     Two alternative approaches use either a battery or electricity developed by Wiegand wires to write to a nonvolatile memory. 
     The addition of mechanical gear systems, multiple rotation sensors, batteries, or power generation systems greatly increases the cost, complexity and potential for failure of the resulting rotation sensor. 
     SUMMARY OF THE INVENTION 
     The present invention provides an absolute rotation sensor that avoid the disadvantages of prior art systems by using an interacting radial guide and spiral guide which together move a marker element along the radial guide. A sensor system determines the location of the marker within the radial guide to provide a nonvolatile indication of single or multi-turns of the shaft. In some embodiments, a resolution less than a full turn can be obtained in distinction from prior art systems. 
     Specifically, one embodiment of the invention provides an absolute rotation sensor having a housing supporting a shaft rotatable along an axis. The housing also holds a radial guide slidably supporting a marker element to move along a radial path perpendicular to the axis and a spiral guide corralling the marker element to follow a spiral path about the axis. At least one of the radial guide and spiral guide are attached to the shaft so that the spiral guide rotates about the axis with respect to the radial guide causing the marker element to move progressively along a radial path with rotation of the shaft. A sensor system identifies the location of the marker element along the radial guide to output at least a number of turns of the shaft according to a position of the marker element along the radial guide. 
     It is thus a feature of at least one embodiment of the invention to provide a nonvolatile, multi-turn, rotation sensor that eliminates the cost and complexity of gearing combining multiple rotation sensors. 
     The marker element may be a ferromagnetic material, for example, selected from the group consisting of: a ferromagnetic bead, a droplet of ferrofluid, and a droplet of ferrofluid surrounding a magnetized bead. 
     It is thus a feature of at least one embodiment of the invention to provide a marker element that has extremely low friction. 
     The radial guide may be a groove constraining the marker element therein. 
     It is thus a feature of at least one embodiment of the invention to provide a radial guide that may be readily fabricated at small scales using integrated circuit fabrication techniques or the like. 
     The spiral guide may be a magnetic material attracting the marker element within the radial guide. 
     It is thus a feature of at least one embodiment of the invention to permit isolation of the marker element within the radial guide to reduce contamination or mechanical wear, and to permit the use of ferrofluidic elements. 
     The sensor system uses may use a variety of sensors, for example, selected from the group consisting of: optical, resistive, capacitive, magnetic, and inductive sensors and including non contact electrical field sensors. 
     It is thus a feature of at least one embodiment of the invention to provide a system that may flexibly adapt to a variety of different sensor types. 
     The sensor system may provide multiple discrete sensors and output a different count value for a number of turns according to the closest discrete sensor. 
     It is thus a feature of at least one embodiment of the invention, to provide a simple sensing system providing integer rotation count values. 
     Alternatively or in addition, the sensor system may provide multiple discrete sensors and output an interpolation between values from the discrete sensors to provide an interpolated angle value between the angle positions of the discrete sensors. 
     It is thus a feature of at least one embodiment of the invention to provide an rotation sensor that may discriminate among different angles within a turn of the shaft as well as among different angles of different numbers of turns of the shaft. 
     Alternatively, the sensor system may provide continuous output indicating the position of the marker element along the radial track. 
     It is thus a feature of at least one embodiment of the invention to provide a rotation sensor that may discriminate among different angles within a turn of the shaft with a continuous output. 
     The rotation sensor may further include a single-turn or absolute rotation sensor attached to the shaft for providing an indication of a plurality of angular positions within a range of 360 degrees of shaft rotation. 
     It is thus a feature of at least one embodiment of the invention to provide a system that may be used to augment single-turn absolute rotation sensors. 
     The spiral track may be mounted to rotate with the shaft. 
     It is thus a feature of at least one embodiment of the invention to simplify integration of the radial guide with sensors by moving the permanent magnet material to the spiral guide. 
     The spiral path of the spiral guide may have a radial extent no less than a radial extent of the radial path. 
     It is thus a feature of at least one embodiment of the invention to provide a system that may accommodate over-travel by ensuring that the marker element can be recaptured by the spiral element. 
     These particular objects and advantages may apply to only some embodiments falling within the claims and thus do not define the scope of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a simplified perspective view of an rotation sensor according to one embodiment of the present invention, with the rotation sensor also shown in exploded view with the housing removed to reveal a rotating spiral guide and fixed radial guide holding a marker element; 
         FIG. 2  is a simplified top plan view of the spiral guide in phantom revealing the radial guide beneath it and showing a detail of a sensor structure on the radial guide for determining location of the magnetic element; 
         FIGS. 3 a  and 3 b    are fragmentary views of alternative embodiments of the sensors of the sensor system of  FIG. 2 ; 
         FIG. 4  is an exploded perspective view of the rotation sensor  FIG. 1  attached to an absolute rotation sensor and showing signals from each; 
         FIG. 5  is a graphical representation of a method of calculating intra-turn angle from the discrete sensors of  FIGS. 2 and 3 ; and 
         FIG. 6  is a figure similar to that of the sensor system of  FIG. 2  showing a sensor providing a continuous output for intra-turn measurement. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring now to  FIG. 1 , a rotation sensor  10  per the present invention may provide a housing  12  through which a shaft  14  may project. The remainder of the shaft  14  may extend through the housing  12  and be supported by the housing  12  to rotate about an axis  16  with respect to the housing  12 . Electrical conductors  18  may pass out of the housing  12  to provide electrical signals indicating an absolute rotary position of the shaft  14  with respect to the housing  12 . 
     Within the housing  12 , the shaft  14  may attach to a first spiral guide disk  20  extending in a plane generally perpendicular to the axis  16 . The spiral guide disk  20  may support on one face a helical magnetic spiral  22  of a magnetized permanent magnet material. Generally, the magnetic spiral  22  follows the path of an Archimedean spiral and extends multiple turns around the axis  16 , having an expanding radius as a function of angle. The magnetic spiral  22 , through magnetic attraction, will provide a spiral guide for a marker element as will be described below. 
     Positioned adjacent to the spiral guide disk  20  is a radial guide  24  also extending in a plane generally perpendicular to the axis  16  but fixed with respect to the housing  12 . The radial guide  24  may hold a marker element  25  to slide along the radial guide  24  while being constrained against movement in a circumferential direction. The marker element  25  may be, for example, a magnetic particle  26  including but not limited to a solid ferrous material, a ferrofluid, or a solid permanent magnet coated with ferrofluid material. 
     The shaft  14  may be supported at opposite ends by rotary bearings  28  as is generally understood in the art. 
     Referring now to  FIG. 2 , as held in the radial guide  24 , the marker element  25  will be attracted to a given turn  30  of the magnetic spiral  22  and will be held radially centered along that turn by a magnetic attraction. Accordingly, rotation of the spiral guide disk  20 , and hence the magnetic spiral  22 , will cause the marker element  25  to move radially along the radial guide  24  to retain its centered position on each turn  30 . With successive rotations of the spiral guide disk  20 , the magnetic marker element  25  is passed smoothly and continuously among turns  30  of smaller radius (for a clockwise magnetic spiral  22  and clockwise rotation of the shaft  14  as depicted in  FIG. 2 ) or to turns  30  of larger radius (for a clockwise magnetic spiral  22  and counterclockwise rotation of the shaft  14 ). As the magnetic marker element  25  moves along the radial guide  24 , it will pass through multiple sensing locations  36  so that its position may be detected. 
     Referring still to  FIG. 2 , in one embodiment the radial position of the marker element  25  along the radial guide  24  may be determined by placement of electrodes  50   a  and  50   b  on opposite sides of the radial guide  24  at each sensing location  36  to flank the marker element  25  when it is at the given sensing location  36 . In one embodiment, electrodes  50   a  and  50   b  may electrically contact the marker element  25  to measure a change of resistance between the electrodes  50   a  and  50   b  with the presence and absence of the marker element  25 . Noncontact electrical measurement, however, may alternatively be obtained by placing electrodes  50   a  and  50   b  in close proximity to the marker element  25  at each of the sensing locations  36  so that they may measure change in electrical qualities of the circuit formed with electrodes  50   a  and  50   b  with the presence and absence of the magnetic marker element  25 . For example, a sinusoidal voltage from a voltage source  52  may be imposed across the electrodes  50   a  and  50   b  and changes in an AC impedance (inductance or capacitance) in a circuit so formed can be measured such as will change, according to the presence or absence of the magnetic marker element  25 , indicating the presence or absence of the marker element  25 . Likewise, a magnetic hysteresis caused by the presence of the marker element  25  in the environment of a changing magnetic field, for example, generated when electrodes  50   a  and  50   b  provide coil forms generating a magnetic field, may be detected. 
     In one embodiment, each of the electrodes  50   a  be commonly driven and each of the electrodes  50   b  separately measured by being connected through a multiplexer  54  controlled by a microprocessor  58  to selectively connect one electrode  50   b  at a time to a sensing circuit  56 . The sensing circuit  56  may measure changes in voltage or current and may provide an input to the microprocessor  58 , for example, via an analog-to-digital converter. Analysis of the signals from the sensing circuit  56  may thus be used to determine a location of the magnetic marker element  25  along the radial guide  24 . This location, determined by the closest sensing location  36  (e.g., closest electrodes  50   a  and  50   b ) indicates the absolute number of rotations of the shaft  14  and may be output from the microprocessor  58  as an angular output signal through conductors  18 . 
     Specified or dedicated electrodes  50  may be placed at end-of-travel positions  37  to provide a signal indicating the marker element has arrived at the end of the magnetic spiral  22  signaling a likelihood that the travel range of the rotation sensor  10  has been exceeded and thus that the absolute number of rotations output from the microprocessor  58  cannot be trusted. 
     Referring to  FIG. 3 a   , it will be appreciated that alternative sensing systems may be employed for detection of the position of the magnetic marker element  25  including those having a photoemitter  60  that may project light upward into the sensing location  36  to be reflected downward to a photodetector  62  when a magnetic marker element  25  is in a sensing location  36 . The photodetector  62  may be connected to multiplexer  54  to permit measurement of the reflected light such as indicates presence or absence of the marker element  25 . Similarly as shown in  FIG. 3 b   , the photoemitter  60  and photodetector  62  may be placed on opposite sides of the marker element  25  when the marker element  25  is in the sensing location  36  to detect the marker element  25  when it blocks transmitted light energy from the photoemitter  60 . 
     Referring now to  FIG. 4  the rotation sensor  10  of the present invention, measuring multiple turns of the shaft  14 , may be advantageously combined on a single shaft  14  with an absolute, single-turn rotation sensor  64  providing high-resolution measurement of a single shaft rotation. As will be understood to those of ordinary skill in the art, the single-turn rotation sensor  64  may be adjusted to provide an output signal  66  having a value of zero when the marker element  25  is at one extreme position along the radial guide  24 , for example, the inner limit of the radial guide  24  representing a full clockwise limit of the rotation sensor  10 . The output signal  66  will climb to a peak value after one full counterclockwise revolution of the shaft  14  before dropping back to the zero value again at a rollover angle  70  generally corresponding to the angular position of zero. The single-turn rotation sensor  64  may thus provide multiple output values that uniquely identify multiple angular positions within one rotation of the shaft  14  but may not indicate how many rotations of the shaft have occurred. 
     An output from the rotation sensor  10 , in this case, can augment the signal  66  from the single-turn rotation sensor  64  to provide an indication of how many rotations of the shaft  14  have occurred. The rotation sensor  10  provides for a generally rising turn-count signal  74  whose magnitude indicates total number of turns. As noted above, the turn-count signal  74  may be determined by a microprocessor  58  (shown in  FIG. 2 ) polling the sensing locations  36  to determine the closest sensing location  36  to the given magnetic marker element  25 . For this purpose, the pitch of the magnetic spiral  22  may ideally match the pitch of the spacings of the sensing locations  36 , and the rollover angle  70  of the signal  66  may be adjusted so that the magnetic marker element  25  is aligned in a sensing location  36  for maximum sensor signal at the rollover angles  70 . Each rising count value of count value signal  74  represents an additional 360 degrees of travel which may be added to the output signal  66  to provide a high-resolution multi-turn angular measurement. 
     Referring now to  FIG. 5 , the present rotation sensor  10  may also be used without an absolute single-turn rotation sensor  64  while still providing intra-turn angular discrimination. This additional discrimination is obtained by providing an output signal  74  that indicates the position of the marker element  25  not only at the sensing locations  36  but in between the sensing locations  36 . This may be accomplished with a discrete sensor system described above by noting the signal strength from each sensor (e.g., pairs of electrodes  50   a  and  50   b ). For example, a first electrode  50   b  may provide a varying signal strength  80   a , and a second electrode  50   b  may provide a second varying signal strength  80   b  as the marker element  25  moves radially along the radial guide  24  past the adjacent sensing locations  36  of the first and second electrodes  50   b . In one embodiment, signals  80   a  and  80   b  from the two closest sensing locations  36  to the marker element  25  are identified and a ratio of the signals  80  associated with these sensing locations determined as indicated by divider  82 . This ratio may be applied to a lookup table  84  relating that ratio to an empirically determined intra-turn angle of the shaft a forming signal  86 . This value, analogous to signal  66  in  FIG. 4 , may be summed to the signal  74 , determined as described above, indicating the closest sensing location  36 , the signal  74  increasing by 360 degrees with each turn count. The sum can then be output as signal  74 ′ on conductors  18  to provide a high-resolution multi-turn rotation sensor output value. 
     Referring now to  FIG. 6 , intra-turn angular resolution can also be obtained, for example, by constraining an electrically conductive marker element  25  between two resistive tracks  90   a  and  90   b  which provide walls of the radial guide  24 . Current flowing from resistive track  90   a  from a voltage source  92  through the marker element  25  to be received by resistive track  90   b  will experience a varying resistance depending on the position of the marker element  25  along the radial guide  24 . This resistance may be measured by sensing circuit  56  to provide a continuous measurement of rotational position with a finer resolution than single turns. The further the marker element  25  moves inward along the radial guide  24  guide, the more resistance will be measured. This technique may be combined with discrete position sensors discussed above and it will be appreciated that there are other methods to provide continuous position measurement as well. 
     It is noted that there need only be relative rotation between the magnetic spiral  22  and the radial guide  24 , and therefore that either can be fixed with respect to the housing  12  or attached to the shaft  14 . The invention contemplates other embodiments, for example, in which the magnetic spiral  22  is replaced with a spiraling channel holding the marker element  25  and the radial guide  24  is a magnetic material attracting the marker element  25  to travel along the radial guide  24  as constrained by the spiral channel. 
     It will be understood that the present invention is applicable to a wide variety of applications and can replace conventional encoders, resolvers, eddy current sensors, and even operate in the context of sensorless motor to replace a two pole resolver over a limited angular range. 
     The word “magnetic” may indicate either a ferromagnetic material that does not generate its own magnetic field or a material generating a magnetic field in the manner of a permanent magnet accordingly and should be interpreted according to context. 
     Certain terminology is used herein for purposes of reference only, and thus is not intended to be limiting. For example, terms such as “upper”, “lower”, “above”, and “below” refer to directions in the drawings to which reference is made. Terms such as “front”, “back”, “rear”, “bottom” and “side”, describe the orientation of portions of the component within a consistent but arbitrary frame of reference which is made clear by reference to the text and the associated drawings describing the component under discussion. Such terminology may include the words specifically mentioned above, derivatives thereof, and words of similar import. Similarly, the terms “first”, “second” and other such numerical terms referring to structures do not imply a sequence or order unless clearly indicated by the context. 
     When introducing elements or features of the present disclosure and the exemplary embodiments, the articles “a”, “an” “the” and “said” are intended to mean that there are one or more of such elements or features. The terms “comprising”, “including” and “having” are intended to be inclusive and mean that there may be additional elements or features other than those specifically noted. It is further to be understood that the method steps, processes, and operations described herein are not to be construed as necessarily requiring their performance in the particular order discussed or illustrated, unless specifically identified as an order of performance. It is also to be understood that additional or alternative steps may be employed. 
     References to “a microprocessor” and “a processor” or “the microprocessor” and “the processor,” can be understood to include one or more microprocessors that can communicate in a stand-alone and/or a distributed environment(s), and can thus be configured to communicate via wired or wireless communications with other processors, where such one or more processor can be configured to operate on one or more processor-controlled devices that can be similar or different devices. Furthermore, references to memory, unless otherwise specified, can include one or more processor-readable and accessible memory elements and/or components that can be internal to the processor-controlled device, external to the processor-controlled device, and can be accessed via a wired or wireless network. 
     It is specifically intended that the present invention not be limited to the embodiments and illustrations contained herein and the claims should be understood to include modified forms of those embodiments including portions of the embodiments and combinations of elements of different embodiments as come within the scope of the following claims. All of the publications described herein, including patents and non-patent publications, are hereby incorporated herein by reference in their entireties.