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BACKGROUND OF THE INVENTION 
       [0001]    This invention relates generally to rotary machines and more particularly, to methods and apparatus for monitoring rotary machines. 
         [0002]    Some known wells, such as oil wells, are formed by drilling a borehole within a natural formation below the surface of the Earth. Such formations may be found below land-based surfaces and/or submerged surfaces. Some known drilling methods use powered rotating equipment to induce torque to a drill pipe that subsequently rotates a drill bit. The rotating drill bit bores into the formation and generates cuttings of the formation to form a drilling well while appropriate fluids that facilitate transporting the cuttings to the surface are circulated within the well. The drill pipe is lowered and raised within the drilling well by a support cable extending from a drawworks drum. When rotating, the drawworks drum extends and retracts the cable to cause the drill pipe to be lowered and raised, respectively. A pre-determined rate and amount of drill bit movement within the drilling well is influenced by a number of variables that include, but are not limited to a hardness of the formations being drilled and/or a need to withdraw the drill pipe from the well to replace the drill bit. Facilitation of the drilling activities is at least partially attained by determining a depth of the drill bit within the well. The drill bit depth is typically attained by monitoring the length of drill pipe inserted into the drilling well, as well as the rate and direction of movement of the drill pipe. 
         [0003]    To facilitate determining such drill bit depth, some known drilling assemblies include drill bit measurement devices including encoders that measure the rotation of the drawworks drum. The encoders transmit data to a monitoring system that correlates rotation of the drawworks drum to a drill pipe depth. However, because some known encoders require an external power source to supply a power level above 0.25 watts and voltages above 24 volts DC, such encoders may not be suitable for use in areas wherein an ignitable environment may exist. 
       BRIEF DESCRIPTION OF THE INVENTION 
       [0004]    In one aspect, a method of determining the amount of travel of a rotating component that includes a rotor shaft is provided. The method includes providing a self-contained magnetically-powered encoder that includes at least one encoder rotor that extends outward from a sealed housing such that a clearance gap is defined between the rotor and housing. The method also includes rotatably coupling the encoder to the rotor shaft. The method further includes measuring a first position of the encoder rotor and determining a first linear position measurement of the rotor shaft based on the encoder rotor. The method also includes rotating the rotor shaft to a second position and determining a direction of rotation and a second linear position measurement of the rotor shaft using the encoder. 
         [0005]    In another aspect, an encoder for use with a rotary machine including at least one moveable member is provided. The encoder includes at least one sensor configured to activate via magnetic flux. The encoder is configured to dissipate electrical signals with a power amplitude that is less than approximately one microwatt. 
         [0006]    In a further aspect, a measurement system for a drilling assembly including at least one rotatable member is provided. The system includes an encoder including at least one sensor configured to activate via magnetic flux. The encoder is configured to dissipate electrical signals with a power amplitude that is less than approximately one-third of one microwatt. The system also includes at least one processor coupled in electronic data communication with the encoder via at least one input channel. The at least one processor is configured to receive and process at least one encoder output signal. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0007]      FIG. 1  is a schematic view of an exemplary well drilling rig; 
           [0008]      FIG. 2  is a schematic view of an exemplary encoder that may be used with the drilling rig shown in  FIG. 1 ; 
           [0009]      FIG. 3  is a side view of the encoder shown in  FIG. 2 ; 
           [0010]      FIG. 4  is an electrical schematic of an exemplary drill pipe position measurement system that may be used with the drilling rig shown in  FIG. 1 ; and 
           [0011]      FIG. 5  is an exemplary graphical representation of waveforms that may be produced using the encoder shown in  FIG. 2 . 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0012]      FIG. 1  is a schematic view of an exemplary well drilling rig  100 . In the exemplary embodiment, rig  100  is a rotary well top drive drilling rig  100 . Alternatively, rig  100  may be any drilling apparatus in which the invention described herein may be embedded. Rig  100  includes a platform  102  onto which a support structure, or derrick  104 , is coupled. A crown block  106  is suspended from derrick  104 . Rig  100  also includes a drawworks  108  that includes a drum  110  that is powered by a power source (not shown in  FIG. 1 ) that may include, but is not limited to, an electric drive motor. Alternatively, the power source may be any device that enables rig  100  to function as described herein. Specifically, in the exemplary embodiment, the power source is coupled to a drawworks drive shaft  112  that is rotatably coupled to drum  110 . 
         [0013]    A cable  114  is wound around drum  110  and extends from drum  110  to crown block  106 . Cable  114  is coupled to crown block  106 , in a manner similar to a pulley system that facilitates a pre-determined mechanical advantage thereby facilitating support of a traveling block  116  by crown block  106 . Traveling block  116  supports a rotary drive apparatus  118  via a suspension member  120 . In the exemplary embodiment, member  120  may include, but is not limited to being a hook and swivel assembly. Alternatively, member  120  is any device that enables rig  100  to function as described herein. Apparatus  118  is powered by a power source (not shown in  FIG. 1 ). For example, in the exemplary embodiment, apparatus  118  is an electric motor-driven top drive  118 . 
         [0014]    Top drive  118  is rotatably coupled to a kelly  122 . In the exemplary embodiment, kelly  122  is, but is not limited to being, a square or hexagonal member. Alternatively, kelly  122  may have any configuration that enables rig  100  to function as described herein. Kelly  122  is rotatably coupled to a drill pipe  124  and is configured to transfer torque from top drive  118  to drill pipe  124 . A guide member  123  facilitates radial support of kelly  122 . Drill pipe  124  is rotatably coupled to at least one drill bit  126  used to form a borehole or well  128 . Alternative embodiments of drilling rig  100  may include a swivel joint in the place of top drive  118  and a power-driven square or hexagonal bushing in the place of guide member  123 . 
         [0015]    Rig  100  also includes a drill pipe position measurement system  150  that includes at least one encoder  152  that is rotatably coupled to drive shaft  112  and that is electrically coupled to an interface device  154  via an encoder cable  156 . In the exemplary embodiment, encoder cable  156  is an insulated and shielded copper cable and device  154  is a Safe Area Interface (SAI) device  154  that is commercially available from General Electric Energy, Twinsburg, Ohio. Interface device  154  is positioned a distance from platform  102  within an environment that facilitates housing for a plurality of electronic apparatus (not shown in  FIG. 1 ) included within device  154 . Positioning device  154  in a remote location a predetermined distance from platform  102  also facilitates mitigating the potential for introducing inadvertent electrical arcing in the vicinity of well  128 . Interface device  154  is electrically coupled to a data processing assembly  158  that is coupled to an operator interface terminal (OIT)  160  via a plurality of electronic cables  162 . In the exemplary embodiment, electronic cables  162  are serial and/or universal serial bus (USB) cables. Also, in the exemplary embodiment, assembly  158  and OIT  160  are coupled as a portable laptop computer. Alternatively, assembly  158  and OTT  160  are separate units. 
         [0016]    Device  154  and data processing assembly  158  both include at least one processor and a memory (neither shown in  FIG. 1 ). As used herein, the term computer is not limited to just those integrated circuits referred to in the art as a computer, but broadly refers to a processor, a microcontroller, a microcomputer, a programmable logic controller (PLC), an application specific integrated circuit, and other programmable circuits, and these terms are used interchangeably herein. In the exemplary embodiment, memory may include, but is not limited to, a computer-readable medium, such as a random access memory (RAM). Alternatively, a floppy disk, a compact disc—read only memory (CD-ROM), a magneto-optical disk (MOD), and/or a digital versatile disc (DVD) may also be used. Also, in the exemplary embodiment, additional input channels may be coupled to computer peripherals associated with OIT  160 , such as, but not limited to, a mouse and/or a keyboard. Alternatively, other computer peripherals may also be used including, for example, a scanner. Furthermore, in the exemplary embodiment, additional output channels may be coupled to additional data displays, printers, plotters and/or operational control mechanisms. 
         [0017]    Processors for interface device  154  and assembly  158  process information, including signals received from encoder  152  and device  154 . RAM devices store and transfer information and instructions to be executed by the processor. RAM devices can also be used to store and provide temporary variables, static (i.e., non-changing) information and instructions, and/or other intermediate information to the processors during execution of instructions by the processors. Instructions that may be executed include, but are not limited to including, resident conversion, calibration and/or comparator algorithms. The execution of sequences of instructions is not limited to any specific combination of hardware circuitry or software instructions. 
         [0018]    During operation of rig  100 , drill pipe  124  and drill bit  126  are suspended within well  128 . Top drive  118  transfers torque and rotational movement to kelly  122  which transfers the torque and rotational movement to drill pipe  124  and drill bit  126 . A downward force is also induced onto drill bit  126  by the weight of components positioned above bit  126  and this force facilitates penetration of the formation being drilled. Traveling block  116  is positioned via multiple loops of cable  114  coupled between traveling block  116  and crown block  106 . To modulate the downward force induced to drill bit  126 , drawworks drum  110  is rotated to withdraw or extend a portion of cable  114 . The withdrawal and extension of cable  114  causes traveling block  116  to be raised or lowered such that the downward force induced on drill bit  126  is subsequently decreased or increased. Subsurface formation cuttings (not shown in  FIG. 1 ) loosened by drill bit  126  are transported to the surface by circulation of fluids through drill bit  126  and are removed via a material removal sub-system (not shown in  FIG. 1 ). As material is removed from well  128  and the depth of well  128  is increased, drill pipe  124  is lowered into well  128  to permit drill bit  126  to bore deeper. Specifically as drill pipe  124  is lowered, drum  110  is rotated to extend a portion of cable  114 . The length of cable  114  extended may be correlated to a depth of drill pipe  124  and to a number of rotations of drum  110 . Occasionally, as a depth of well  128  increases, additional sections of drill pipe  124  may need to be added to rig  100 . 
         [0019]      FIG. 2  is a schematic view of exemplary encoder  152  that may be used with well drilling rig  100  (shown in  FIG. 1 ).  FIG. 3  is a side view of encoder  152 . Encoder  152  includes a housing  164  that defines an encoder internal cavity  166  therein. Housing  164  seals cavity  166  from the external environment of encoder  152  and facilitates protection from dust and water. 
         [0020]    Encoder  152  also includes a rotor  168  that is rotatably coupled to drawworks drive shaft  112  (shown in  FIG. 1 ). Rotor  168  extends through housing  164  via a seal assembly (not shown) that facilitates mitigating interaction between the external environment and cavity  166 . Rotor  168  rotates about an axis of rotation  169 . Housing  164  and rotor  168  are originated such that a radially outermost surface  170  of rotor  168  and a radially innermost surface  172  of housing  164  define a gap  174  that facilitates preventing contact between rotor  168  and housing  164  during operation of encoder  152 . 
         [0021]    Encoder  152  also includes a plurality of permanent magnets  176  that are oriented generally radially within rotor  168  such that a radially outermost portion of each magnet  176  is substantially flush with rotor surface  170 . During rotation of rotor  168 , magnets  176  generate a magnetic flux with a predetermined magnetic strength and orientation. In the exemplary embodiment, five magnets  176  are positioned substantially circumferentially equidistant from each other. Alternatively, any number of magnets  176  with any circumferential separation that enables encoder  152  to function as described herein may be used. One magnetic cycle is defined as the rotational travel of rotor  168  from a first magnet  176  to a circumferentially adjacent next magnet  176 . 
         [0022]    Encoder  152  further includes two magnetic reed switches  178  and  179  that are securely coupled to a switch holder  180  secured to housing  164 . In the exemplary embodiment, switches  178  and  179  are approximately 18° apart to facilitate operation of encoder  152 . Alternatively, switches  178  and  179  may be positioned with any degree of circumferential separation that enables encoder  152  to function as described herein. Switches  178  and  179  each have a predetermined sensitivity selected to substantially cooperate with the magnetic flux of magnets  176 . In the exemplary embodiment, switches  178  and  179  are circumferentially separated at a distance that is approximately equivalent to one-quarter of a magnetic cycle and at least partially defines the relationship between a first magnetic pulse and a second magnetic pulse as magnets  176  rotate past switches  178  and  179 . Moreover, in the exemplary embodiment, five magnets  176  and two switches  178  and  179  facilitate attaining a predetermined resolution of travel of drill pipe  124 . A pair of common power supply conduits  182  and  184  are electrically coupled with switches  178  and  179 , respectively. Conduits  182  and  184  are electrically coupled with a power supply (not shown in  FIGS. 2 and 3 ) positioned within interface device  154  (shown in  FIG. 1 ). Moreover, a common ground conduit  183  is electrically coupled with switches  178  and  179  on the ends of switches  178  and  179  that are opposite to the connections of conduits  182  and  184 . Conduits  182 ,  183  and  184  are enclosed within encoder cable  156 . In the exemplary embodiment, conduits  182 ,  183  and  184  are copper wire. Alternatively, conduits  182 ,  183  and  184  may be any electrically conductive devices that enable system  150  to function as described herein. Conduit  183 , switch  178 , and conduit  182  at least partially define a first encoder channel  186  and conduit  183 , switch  179  and conduit  184  at least partially define a second encoder channel  188 . 
         [0023]    Encoder  152  facilitates reliability of system  150 , and hence, drilling rig  100 , due to the relatively small number of moving parts of system  150  exposed to field conditions are mitigated and are fully contained within encoder  152 . Specifically, only rotor  168  and switches  178  and  179  utilize operational movement to affect the performance of encoder  152  as described herein. In the event of malfunction, encoder  152  may be easily and quickly replaced while mitigating disruption of drilling operations. Moreover, encoder  152  may be sized such that redundant encoders  152  may be coupled to shaft  112  and/or replacement encoders  152  storage requirements are mitigated. 
         [0024]    During operation, drawworks drum  110  (shown in  FIG. 1 ) retrieves or extends cable  114  (shown in  FIG. 1 ) as a function of drill pipe depth within well  128  (shown in  FIG. 1 ). As drum  110  is rotated by drawworks drive shaft  112 , encoder rotor  168  is rotated in the same direction. For example, as rotor  168  is rotated in the clockwise direction (as illustrated by the arrow) a magnet  176  successively approaches, rotates by, and recedes from switch  178 . Magnet  176  generates a magnetic flux with a predetermined magnetic strength and orientation such that as each magnet  176  approaches switch  178 , at a predetermined circumferential distance away from switch  178 , during the approach, switch  178  closes. Upon closing, switch  178  completes an electric circuit within first channel  186  such that an electric signal may be channeled from device  154  via conduit  182  through switch  178  and back to device  154  via conduit  183 . Switch  178  remains closed until magnet  176  has receded a predetermined circumferential distance from switch  178 . Magnets  176 , device  154 , and the components of second channel  188  including switch  179 , conduit  183  and conduit  184  operate together in a similar manner. The action of each of magnets  176  closing switch  178  defines a first negative magnetic pulse edge and the action of each magnet  176  closing switch  179  defines a second negative magnetic pulse edge. This action and subsequent actions associated with interaction of each magnet  176  and switches  178  and  179  are discussed further below. 
         [0025]      FIG. 4  is an electrical schematic of exemplary drill pipe position measurement system  150  that may be used with drilling rig  100  (shown in  FIG. 1 ). System  150  includes at least one encoder  152  that is electrically coupled to interface device  154  via encoder cable  156 . Interface device  154  is electrically coupled to data processing assembly  158  that is coupled to an operator interface terminal (OIT)  160  via a plurality of electronic cables  162 . Encoder  152  includes two magnetic reed switches  178  and  179 . Common power supply conduits  182  and  184  are electrically coupled with switches  178  and  179 , respectively. Moreover, common ground conduit  183  is electrically coupled with switches  178  and  179  on the ends of switches  178  and  179  that are opposite to the connections of conduits  182  and  184 . Conduits  182 ,  183  and  184  are enclosed within encoder cable  156 . 
         [0026]    Conduit  183 , switch  178 , and conduit  182  at least partially define first encoder channel  186 . Channel  186  further includes a 5 volt direct current (VDC) power supply  190 . Channel  186  also includes a 25,000 ohm current-limiting resistor  191  electrically coupled to power supply  190  and a power supply signal conduit  192  electrically coupled to conduit  182  downstream of resistor  191 . Channel  186  further includes a processor  193  electrically coupled to conduit  192 . Channel  186  also includes an electrical grounding device  194  electrically coupled to conduit  183 , power supply  190  and a ground conduit  195  electrically coupled to processor  193 . Conduit  195  is also electrically coupled to conduit  183  upstream of grounding device  194 . Resistor  191 , conduit  192 , processor  193 , grounding device  194  and ground conduit  195  are positioned within interface device  154 . Therefore, first channel  186  is defined by power supply  190 , resistor  191 , conduit  182 , conduit  192 , switch  178 , conduit  183 , grounding device  194 , conduit  195  and processor  193 . Processor  193  is coupled in electronic data communication with assembly  158  via conduit  162 . 
         [0027]    Similarly, conduit  183 , switch  179 , and conduit  184  at least partially define second encoder channel  188 . Channel  188  further includes power supply  190 , a 25,000 ohm current-limiting resistor  196  electrically coupled to power supply  190  and a power supply signal conduit  197  electrically coupled to conduit  184  downstream of resistor  196 . Channel  188  also includes processor  193  electrically coupled to conduit  197 . Channel  188  further includes electrical grounding device  194  and ground conduit  195 . Resistor  196  and conduit  197  are positioned within interface device  154 . Therefore, second channel  188  is defined by power supply  190 , resistor  196 , conduit  184 , conduit  197 , switch  179 , conduit  183 , grounding device  194 , conduit  195  and processor  193 . 
         [0028]      FIG. 5  is an exemplary graphical representation  200  of a plurality of waveforms that may be produced using encoder  152  (shown in  FIGS. 2 and 3 ) and system  150  (shown in  FIG. 4 ). Ordinate  202  (Y-axis) represents an amplitude of an output signal voltage from switches  178  and  179  (both shown in  FIGS. 2 and 4 ) in voltage units. Abscissa  204  (X-axis) represents time units. Switch  178  facilitates channeling a first channel output signal  206  via first channel  186  (shown in  FIGS. 2 and 4 ) and switch  179  facilitates channeling a second channel output signal  208  via second channel  188  (shown in  FIGS. 2 and 4 ). Signals  206  and  208  are substantially square-waved signals and are illustrated as slightly offset from each other in amplitude for clarity. 
         [0029]    Signals channeled within first channel  186  are received by processor  193  via conduits  192  and  195  and together form a first channel signal  206 . An approximately five VDC voltage differential is applied to switch  178  via power supply  190 , resistor  191 , conduits  183  and  182 , and grounding device  194  (all shown in  FIG. 4 ). Grounding device  194  facilitates substantially all signals channeled through conduit  195  to have a voltage amplitude of approximately zero VDC throughout operation of system  150 . When switch  178  is in an open condition electric current flow through first channel  186  is substantially zero. Moreover, a signal that has a voltage amplitude of approximately five VDC is channeled through conduit  192 . Signal  206  includes a first channel “switch  178  open” output portion  220  that represents a period of time switch  178  is open, as well as an associated value of a voltage differential between conduits  192  and  195 . Portion  220  graphically represents this voltage differential. 
         [0030]    Similarly, signals channeled within second channel  188  are received by processor  193  via conduits  197  and  195  and together form a second channel signal  208 . An approximately five VDC voltage differential is applied to switch  179  via power supply  190 , resistor  196 , conduits  183  and  184 , and grounding device  194  (all shown in  FIG. 4 ). Grounding device  194  facilitates substantially all signals channeled through conduit  195  to have a voltage amplitude of approximately zero VDC throughout operation of system  150 . When switch  179  is in an open condition electric current flow through second channel  188  is substantially zero. Moreover, a signal that has a voltage amplitude of approximately five VDC is channeled through conduit  197 . Signal  208  includes a second channel “switch  179  open” output portion  222  that represents a period of time switch  179  is open, as well as an associated value of a voltage differential between conduits  197  and  195 . Portion  222  graphically represents this voltage differential. In the exemplary embodiment, portions  220  and  222  of signals  206  and  208 , respectively, are substantially similar. 
         [0031]    Signal  206  also includes a first negative magnetic pulse edge  210  and a first positive magnetic pulse edge  212 . Edge  210  is generated as each magnet&#39;s magnetic flux exceeds a sensitivity threshold of switch  178  as magnets  176  approach switch  178  and close switch  178 . Edge  212  is generated as the magnetic flux in the vicinity of switch  178  weakens as each magnet  176  recedes away from switch  178  and switch  178  is opened. A “switch  178  closed” portion  211  of signal  206  is defined and extends between edges  210  and  212 . Portion  211  is equivalent to the duration of time that the strength of the magnetic flux in the proximity of switch  178  exceeds the sensitivity threshold of switch  178  and an associated voltage differential across switch  178 . When switch  178  is closed, an electric current is permitted to be channeled through first channel  186 , including switch  178 , from power supply  190  to grounding device  194  thereby decreasing the voltage amplitude of the signal channeled through conduit  192  to substantially zero. Therefore, the voltage differential between conduits  192  and  195  is substantially zero. 
         [0032]    Similarly, output signal  208  also includes a second negative magnetic pulse edge  214  and a second positive magnetic pulse edge  216 . Also, similarly, a “switch  179  closed” portion  215  of signal  208  is defined and extends between edges  214  and  216 . When switch  179  is closed, an electric current is permitted to be channeled through second channel  188 , including switch  179 , from power supply  190  to grounding device  194  thereby decreasing the voltage amplitude of the signal channeled through conduit  197  to substantially zero. Therefore, the voltage differential between conduits  197  and  195  is substantially zero. In the exemplary embodiment, portions  211  and  215  of signals  206  and  208 , respectively, are substantially similar. 
         [0033]    One magnetic cycle is defined as the rotational travel of rotor  168  from a first magnet  176  to a next magnet  176 . One magnetic cycle is defined in  FIG. 4  as 360°, i.e., 360° is substantially equivalent to the time duration between edge  210  and the next generation event of edge  210 . Subsequently, 90° is substantially equivalent to the time duration between edge  210  and edge  214 . Also, 90° is equivalent to the time duration between edge  214  and edge  212 , and the time duration between edge  212  and edge  216 . Moreover, 90° is substantially equivalent to the time duration between edge  216  and the next generation event of edge  210 . This sequence of events is substantially replicated for each magnetic cycle. In the exemplary embodiment, encoder  152  includes five magnets  176  and each 360° rotation of encoder rotor  168  (shown in  FIGS. 2 and 3 ) generates five magnetic cycles. Therefore, each magnetic cycle is substantially equivalent to 72° of rotation of rotor  168  and each quadrant of the 360° magnetic cycle, i.e., 90° of the magnetic cycle is substantially equivalent to 18° of rotation of rotor  168 . 
         [0034]    Signal  206  leads output signal  208  as encoder  152  rotates in a clockwise direction. In contrast, signal  208  leading output signal  206  indicates encoder  152  is rotating in a counter-clockwise rotation. In the exemplary embodiment, the amplitude of voltage output signals  206  and  208  during portions  220  and  222 , respectively, is approximately five volts DC and substantially zero amperes current is channeled through switches  178  and  179 . In contrast, the amplitude of voltage output signals  206  and  208  from switches  178  and  179 , respectively, during portions  211  and  215  is approximately zero volts DC. Moreover, during periods when portions  211  and  215  overlap, less than one-third of one microwatt of power is dissipated by system  150 . 
         [0035]    The exemplary magnitudes of voltage, current and power associated with system  150 , including encoder  152 , as described herein facilitate reducing potential for inadvertent electrical arcing associated with encoder  152  having sufficient energies to induce ignition of predetermined materials and compounds. Moreover, in the exemplary embodiment, encoder  152  is not electrically coupled to any significant external power sources, i.e., power sources that are configured to transmit more than one microwatt of power. As such, encoder  152  may be used in applications wherein an intrinsically safe device is required, such as, but not limited to, Class I, Division 1 conditions. Such conditions may exist within facilities that include, but are not limited to, chemical plants, grain elevators, and natural gas transfer stations. Alternatively, any values of voltage, average power, peak power, average current and peak current that facilitates operation of encoder  152  as described herein may be used. 
         [0036]    Referring again to  FIG. 1 , during operation of rig  100  as cable  114  is extended from and refracted towards drum  110  to vary a depth of drill pipe  124 , encoder  152 , that is rotatably coupled to drawworks shaft  112 , facilitates channeling output signals  206  and  208  that are transmitted to interface device  154  via conduits  182  and  184 , respectively. Encoder  152  is an incremental encoder  152  in that it measures relative depth from a starting depth and measures depth changes upward or downward from that starting depth. A preliminary set of data that corresponds to an initial starting depth is manually input into system  150 . Device  154  and data processing assembly  158  receive a first set of signals  206  and  208  and assembly  158  uses at least one resident conversion algorithm to determine a first distance of drill pipe  124 . As shaft  112  rotates to change the depth of drill pipe  124  to a second position, a second set of signals  206  and  208  are channeled to device  154  that uses at least one resident conversion algorithm to determine the number and polarity of magnetic cycles. The number and polarity of magnetic cycles as determined by device  154  is transmitted to data processing assembly  158  wherein a plurality of conversion algorithms are executed to determine a distance of movement of drill pipe  124 , a direction of movement, and a rate of movement. Examples of conversion algorithms may include, but are not limited to, integration algorithms to convert the number and polarity of magnetic cycles that are representative of the distance and direction of movement of drill pipe  124 , to values that may be interpreted by an operator. The processed signals are subsequently transmitted to OIT  160 . 
         [0037]    The methods and apparatus for monitoring a rotary machine shaft as described herein facilitate operation and monitoring of a rotary drilling rig. More specifically, the rotary encoder described herein facilitates an efficient and effective drill pipe depth measurement scheme. Also, the rotary encoder facilitates operation of a passive operating system with self-contained low-power components and no external power requirements, and is intrinsically safe in hazardous environments. Further, the rotary encoder also facilitates enhancing drilling rig reliability, and reducing maintenance costs and drilling rig outages. Moreover, the rotary encoder also facilitates operation of facilities that include, but are not limited to, chemical plants, grain elevators, and natural gas transfer stations. 
         [0038]    Exemplary embodiments of rotary encoders as associated with drill pipe depth measurement schemes are described above in detail. The methods, apparatus and systems are not limited to the specific embodiments described herein nor to the specific illustrated drilling rig. 
         [0039]    While the invention has been described in terms of various specific embodiments, those skilled in the art will recognize that the invention can be practiced with modification within the spirit and scope of the claims.

Summary:
A method of determining the amount of travel of a rotating component that includes a rotor shaft includes providing a self-contained magnetically-powered encoder. The encoder includes an encoder rotor that extends outward from a sealed housing such that a clearance gap is defined between the rotor and housing. The encoder also includes at least one sensor configured to activate via magnetic flux and is configured to dissipate electrical signals with a power amplitude less than approximately one microwatt. The method also includes rotatably coupling the encoder to the rotor shaft. The method further includes measuring a first position of the encoder rotor and determining a first linear position measurement of the rotor shaft based on the encoder rotor. The method also includes rotating the rotor shaft to a second position and determining a direction of rotation and a second linear position measurement of the rotor shaft using the encoder.