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A system for damping vibration in a drill string can include a magnetorheological fluid valve assembly having a supply of a magnetorheological fluid. A remanent magnetic field is induced in the valve during operation that can be used to provide the magnetic field for operating the valve so as to eli

A system for damping vibration in a drill string can include a magnetorheological fluid valve assembly having a supply of a magnetorheological fluid. A remanent magnetic field is induced in the valve during operation that can be used to provide the magnetic field for operating the valve so as to eliminate the need to energize the coils except temporarily when changing the amount of damping required. The current to be supplied to the coil for inducing a desired magnetic field in the valve is determined based on the limiting hysteresis curve of the valve and the history of the magnetization of the value using a binary search methodology. The history of the magnetization of the valve is expressed as a series of sets of current and it resulting magnetization at which the current experienced a reversal compared to prior values of the current.

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1. A method of damping vibration in a downhole portion of a drill string drilling into an earthen formation, comprising the steps of: a) rotating at least the drill string to form a borehole into the earthen formation;b) causing a magnetorheological (MR) fluid to flow through a passage in an MR valv

1. A method of damping vibration in a downhole portion of a drill string drilling into an earthen formation, comprising the steps of: a) rotating at least the drill string to form a borehole into the earthen formation;b) causing a magnetorheological (MR) fluid to flow through a passage in an MR valve, the MR valve having at least one coil, the MR valve having associated therewith a limiting hysteresis loop relating a strength of the magnetic field in said MR valve to a current supplied to said coil;c) supplying a varying current to said coil so as to subject said MR fluid in said MR valve to a varying magnetic field created by said coil;d) determining a magnetization history of said MR valve as said current supplied to said coil varies, the magnetization history being based on a measurement of the varying current and a determination of the strength of said magnetic field created by said varying current, said strength of said magnetic field based on information representative of said limiting hysteresis loop associated with said MR valve;e) determining the current to be supplied to said coil that will result in a desired magnetic field using said magnetization history of said MR valve determined in step d); andf) supplying said current determined in step e) to said coil so as to substantially obtain said desired magnetic field to dampen vibration of the downhole portion of the drill string, wherein said magnetization history of said MR valve determined in step d) comprises a first stack of first sets of data points, each of said first sets of data points comprising a first data point that is representative of a current that was supplied to said coil and a second data point that is representative of the magnetic field that resulted from the supply of said current, and wherein determining said current to be supplied to said coil in step e) further comprises the steps of:g) copying said first stack of first data points so as to create a second stack of data points;h) adding one or more second sets of data points to said second stack of data points, each of said second sets of data points added to said second stack of data points comprising a selected test current and the magnetization expected to result if said test current were supplied to said coil; andi) performing a binary search of said data points in said second stack after said one or more second sets of data points have been added to said second stack so as to determine the current to be supplied to said coil that will result in said desired magnetic field. 2. The method of damping vibration according to claim 1, wherein the current that was supplied to said coil of which each of said first data points is representative of the current at which the change in current supplied to said coil reversed direction. 3. The method of damping vibration according to claim 1, further comprising the steps of: j) supplying a further current to said coil after step f) that is different from said current supplied to said coil in step f);k) updating said magnetization history of said MR valve determined in step d) so as to include the current supplied to said coil in step f) only if the current supplied to said coil in step j) represented a reversal in a direction of the change in current supplied to said coil when compared to direction of the change in the current supplied to said coil that resulted in said current supplied to said coil in step f). 4. The method of damping vibration according to claim 1, further comprising the step of: j) updating said magnetization history of said MR valve determined in step d) based on the current supplied to said coil in step f). 5. The method of damping vibration according to claim 1, wherein said information representative of said limiting hysteresis loop used in step d) comprises information representative of the magnetic field created in said MR valve versus the current supplied to said coil as said current is increased to the saturation current and then decreased to zero. 6. The method of damping vibration according to claim 1, wherein the step of supplying the varying current to said coil in step c) creates a remanent magnetization in at least one component of said MR valve, and wherein the current supplied to said coil in step f) results in reducing said remanent magnetization. 7. The method of damping vibration according to claim 6, wherein said current supplied to said coil in step f) that results in reducing said remanent magnetization is not an alternating current. 8. The method of damping vibration according to claim 6, wherein the current supplied to said coil in step f) results in substantially eliminating said remanent magnetization. 9. The method of damping vibration according to claim 6, wherein the current supplied to said coil in step f) results in reducing but not substantially eliminating said remanent magnetization. 10. A method of damping vibration in a downhole portion of a drill string, said drill string comprising a magnetorheological (MR) valve containing an MR fluid subjected to a magnetic field created by at least one coil, said MR fluid flowing through a passage formed in said MR valve, the method comprising the steps of: a) supplying current to said at least one coil of said MR valve for a first period of time so as to create a first magnetic field that alters the viscosity of said MR fluid, said first magnetic field being sufficient to induce a first remanent magnetization in at least one component of said MR valve proximate said passage;b) substantially de-energizing said at least one coil for a second period of time following said first period of time so as to operate said MR valve using said first remanent magnetization in said at least one component of said MR valve to create a second magnetic field that alters the viscosity of said MR fluid;c) at least partially demagnetizing said at least one component of said MR valve so as to reduce said first remanent magnetization of said at least one component of said MR valve to a second remanent magnetization, said at least partially demagnetizing step comprising the steps of: (1) determining a magnetization history of said MR valve as said current supplied to said at least one coil varies by measuring said varying current and calculating a strength of said magnetic field created by said varying current, said strength of said magnetic field determined using information representative of a limiting hysteresis loop associated with said MR valve;(2) determining the current to be supplied to said coil that will result in at least partially demagnetizing said at least one component using said magnetization history of said MR valve determined in step c)(1);(3) supplying said current determined in step c)(2) to said coil so as to at least partially demagnetize said at least one component; andd) operating said MR valve for a third period of time after said at least partial demagnetization in step c). 11. The method of damping vibration according to claim 10 wherein the step of supplying the current includes damping a vibration in the drill string. 12. The method of damping vibration according to claim 11, further comprising the step of causing the drill string to rotate so to form a borehole into an earthen formation. 13. The method of damping vibration according to claim 10, wherein the step of supplying the current includes supplying a first current to the at least one coil, and the method further comprises supplying a second current to the at least one coil that is different from the first current. 14. The method of damping vibration according to claim 13, wherein the step of determining the magnetization history of said MR valve further comprises: measuring the first current that was supplied to said at least one coil;measuring the second current supplied to said at least one coil; anddetermining the variance among the first current and the second current, wherein the strength of the magnetic field is based on the determined variance among the first current and the second current. 15. A magnetorheological (MR) valve assembly for damping vibration of a drill bit for drilling into an earthen formation, comprising: at least one coil to which current is supplied and an MR fluid that flows through a passage formed in said MR valve proximate said coil, the current supplied to said coil varying so as to subject said MR fluid in said MR valve to a varying magnetic field created by said coil;a computer memory including stored thereon information representative of a limiting hysteresis loop relating the strength of the magnetic field in said MR valve to the current supplied to said coil;a computer processor configured to determine: i) a magnetization history of the MR valve as the current supplied to the coil varies, the magnetization history of said MR valve based on a measurement of the varying current and a determination of the strength of said magnetic field created by the varying current, the strength of the magnetic field based on the information representative of the limiting hysteresis loop stored in the computer memory, and ii) the current to be supplied to the at least one coil that will result in a desired magnetic field using said magnetization history of said MR valve,wherein said magnetization history of said MR valve comprises a first stack of first sets of data points, each said first set of data points comprising a first data point that is representative of a current that was supplied to said coil and a second data point that is representative of the magnetic field that resulted from the supply of said current;wherein said computer processor is configured to, when executing instructions to determine the current to be supplied to the at least one coil: A) copy said first stack of first data points so as to create a second stack of data points;B) add one or more second sets of data points to said second stack of data points, each of said second sets of data points added to said second stack comprising a selected test current and the magnetization expected to result if said test current were supplied to said coil; andC) perform a binary search of said data points in said second stack after said one or more second sets of data points have been added to said second stack so as to determine the current to be supplied to said coil that will result in said desired magnetic field. 16. The MR valve assembly according to claim 15, wherein the current that was supplied to said coil of which each of said first data points is representative is the current at which the change in current supplied to said coil reversed direction. 17. The MR valve assembly according to claim 15, wherein said MR valve assembly further comprises: a first member capable of being mechanically coupled to said drill bit so that said first member is subjected to vibration from said drill bit;a second member, said first member mounted so as to move relative to said second member, said first and second members defining a first chamber and a second chamber for holding said magnetorheological fluid, said passage through which said MR fluid flows disposed between said first and second members and placing said first and second chambers in fluid communication,wherein at least a portion of one of said first and second members is made from a material having a relative magnetic permeability of at least about 7000, andat least a portion of the other of said first and second members being capable of having induced therein a remanent magnetic field in response to said magnetic field generated by said at least one coil that is sufficient to operate said MR valve when said coil is de-energized, said portion of said other of said first and second members in which said remanent magnetic field is induced being made from a material having a maximum remanent magnetization of at least about 12,000 Gauss. 18. The MR valve assembly according to claim 15, further comprising: a first member capable of being mechanically coupled to said drill bit so that said first member is subjected to vibration from said drill bit;a supply of magnetorheological fluid; anda second member, said first member mounted so as to move relative to said second member, the first and second members defining a first chamber, a second chamber, and a passage placing said first and second chambers in fluid communication with each other, the first and second chambers configured to hold the magnetorheological fluid,wherein at least a portion of one of said first and second members being capable of having induced therein a remanent magnetic field in response to said magnetic field generated by said at least one coil that is sufficient to operate said MR valve when said at least one coil is de-energized. 19. The magnetorheological (MR) valve assembly of claim 18, wherein the measurement of the varying current is a variance in current among a first current supplied to said coil and a second current supplied to said coil, and wherein the strength of said magnetic field is based on the variance in the current among the first and second current supplied to the at least one coil. 20. The MR valve assembly according to claim 18, wherein at least a portion of one of the first and second members is made from a material having a relative magnetic permeability of at least about 7000, and at least a portion of the other of said first and second members is made from a material having a maximum remanent magnetization of at least about 12,000 Gauss. 21. A magnetorheological (MR) valve assembly for damping vibration of a drill bit for drilling into an earthen formation, comprising: a) at least one coil to which current is supplied and an MR fluid that flows through a passage formed in said MR valve proximate said coil, the current supplied to said coil varying so as to subject said MR fluid in said MR valve to a varying magnetic field created by said coil;b) memory means in which is stored information representative of a limiting hysteresis loop relating a strength of the magnetic field in said MR valve to the current supplied to said coil;c) history determining means for determining the magnetization history of said MR valve as said current supplied to said coil varies by measuring said varying current and calculating the strength of said magnetic field created by said varying current, said strength of said magnetic field determined using said information representative of said limiting hysteresis loop stored in said memory means; wherein the determined magnetization history of said MR valve includes a first stack of first sets of data points, each said first set of data points comprising a first data point that is representative of a current that was supplied to said coil and a second data point that is representative of the magnetic field that resulted from the supply of said current, andd) current determining means for determining the current to be supplied to said coil that will result in a desired magnetic field using said magnetization history of said MR valve that also includes a means to: 1) copy said first stack of first data points so as to create a second stack of data points;2) add one or more second sets of data points to said second stack of data points, each of said second sets of data points added to said second stack comprising a selected test current and the magnetization expected to result if said test current were supplied to said coil; and3) perform a binary search of said data points in said second stack after said one or more second sets of data points have been added to said second stack so as to determine the current to be supplied to said coil that will result in said desired magnetic field. 22. The MR valve assembly according to claim 21, wherein the current that was supplied to said coil of which each of said first data points is representative is the current at which the change in current supplied to said coil reversed direction. 23. The MR valve assembly according to claim 21, wherein said MR valve assembly further comprises: a first member capable of being mechanically coupled to a drill bit so that said first member is subjected to vibration from said drill bit; anda second member, said first member mounted so as to move relative to said second member, said first and second members defining a first chamber and a second chamber for holding said magnetorheological fluid, said passage through which said MR fluid flows disposed between said first and second members and placing said first and second chambers in fluid communication,wherein at least a portion of one of said first and second members are made from a material having a relative magnetic permeability of at least about 7000, and at least a portion of the other of said first and second members being capable of having induced therein a remanent magnetic field in response to said magnetic field generated by said at least one coil that is sufficient to operate said MR valve when said coil is de-energized, said portion of said other of said first and second members in which said remanent magnetic field is induced being made from a material having a maximum remanent magnetization of at least about 12,000 Gauss. 24. A method of damping vibration in a downhole portion of a drill string drilling into an earthen formation, comprising the steps of: a) rotating at least the drill string to form a borehole into the earthen formation;b) causing a magnetorheological (MR) fluid to flow through a passage in an MR valve, the MR valve having at least one coil, the MR valve having associated therewith a limiting hysteresis loop relating the strength of the magnetic field in said valve to the current supplied to said coil;c) supplying a varying current to said coil so as to subject said MR fluid in said MR valve to a varying magnetic field created by said coil;d) determining the magnetization history of said MR valve as said current supplied to said coil varies, the magnetization history being based on a measurement of the varying current and a determination of the strength of said magnetic field created by said varying current, said strength of said magnetic field based on information representative of said limiting hysteresis loop associated with said MR valve;e) determining the current to be supplied to said coil that will result in a desired magnetic field using said magnetization history of said MR valve determined in step (c);f) supplying said current determined in step e) to said coil so as to substantially obtain said desired magnetic field to dampen vibration of the downhole portion of the drill string, wherein said magnetization history of said MR valve determined in step d) comprises first sets of data points, each of said first sets of data points comprising a first data point that is representative of a current that was supplied to said coil and a second data point that is representative of the magnetic field that resulted from the supply of said current, and wherein determining said current to be supplied to said coil comprises performing a binary search of said first sets of data points.g) supplying a further current to said coil after step f) that is different from said current supplied to said coil in step f); andh) updating said magnetization history of said MR valve determined in step d) so as to include the current supplied to said coil in step f) only if the current supplied to said coil in step g) represented a reversal in the direction of the change in current supplied to said coil when compared to direction of the change in the current supplied to said coil that resulted in said current supplied to said coil in step f). 25. The method of damping vibration according to claim 24, further comprising the step of: i) updating said magnetization history of said MR valve determined in step d) based on the current supplied to said coil in step f). 26. The method of damping vibration according to claim 24, wherein said information representative of said limiting hysteresis loop used in step d) comprises information representative of the magnetic field created in said MR valve versus the current supplied to said coil as said current is increased to the saturation current and then decreased to zero. 27. The method of damping vibration according to claim 24, wherein the step of supplying a varying current to said coil in step c) creates a remanent magnetization in at least one component of said MR valve, and wherein the current supplied to said coil in step f) results in reducing said remanent magnetization.