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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 first member, and a second member capable of moving in relation to first member in response to vibration of the drill bit. The first and second memb

A system for damping vibration in a drill string can include a magnetorheological fluid valve assembly having a supply of a magnetorheological fluid, a first member, and a second member capable of moving in relation to first member in response to vibration of the drill bit. The first and second members define a first and a second chamber for holding the fluid. Fluid can flow between the first and second chambers in response to the movement of the second member in relation to the first member. The valve assembly can also include a coil for inducing a magnetic field that alters the resistance of the magnetorheological fluid to flow between the first and second chambers, thereby increasing the damping provided by the valve. A remnant magnetic field is induced in one or more components of the magnetorheological fluid 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 during operation except temporarily when changing the amount of damping required, thereby eliminating the need for a turbine alternator power the magnetorheological fluid valve. A demagnetization cycle can be used to reduce the remnant magnetic field when necessary.

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1. In a damping system for damping vibration in a down hole portion of a drill string, said damping system 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, a

1. In a damping system for damping vibration in a down hole portion of a drill string, said damping system 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, a method of operating said MR valve comprising the steps of: a. energizing said 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 remnant magnetization in at least one component of said MR valve proximate said passage, said first remnant magnetization being at least about 12,000 Gauss;b. substantially de-energizing said coil for a second period of time following said first period of time so as to operate said MR valve using said first remnant 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. subjecting said at least one component of said MR valve to a demagnetization cycle over a third period of time so as to reduce said first remnant magnetization of said at least one component of said MR valve to a second remnant magnetization;d. operating said MR valve for a fourth period of time after said demagnetization cycle in step (c). 2. The method according to claim 1, wherein the magnetic field associated with said first remnant magnetization is sufficient to magnetically saturate said MR fluid. 3. The method according to claim 1, wherein said at least one component of said MR valve in which said first remnant magnetization is induced is made from a material having a maximum remnant magnetization of at least about 13,000 Gauss. 4. The method according to claim 1, wherein said at least one component of said MR valve in which said first remnant magnetization is induced is made from a material having a coercivity of at least about 10 Oe. 5. The method according to claim 4, wherein said at least one component of said MR valve in which said first remnant magnetization is induced is made from a material having a coercivity of not more than about 20 Oe. 6. The method according to claim 1, wherein said at least one component of said MR valve in which said first remnant magnetization is induced is made from a material having a coercivity of not more than about 20 Oe. 7. The method according to claim 1, wherein said demagnetization step comprises energizing said at least one coil in steps of decreasing current and alternating polarity. 8. The method according to claim 1, wherein said demagnetization step comprises the steps of: e. supplying a dc current;f. converting said dc current into steps of decreasing current and alternating polarity; andg. directing said decreasing current of alternating polarity to said at least one coil. 9. The method according to claim 1, wherein said second remnant magnetization is approximately zero. 10. The method according to claim 1, wherein step (d) of operating said MR valve for said fourth period of time after said demagnetization cycle comprises operating said MR valve using said second remnant magnetization in said at least one component of said MR valve to create a third magnetic field that alters the viscosity of said MR fluid. 11. The method according to claim 1, wherein step (d) of operating said MR valve for said fourth period of time after said demagnetization cycle comprises energizing said coil of said MR valve at a point in time during said fourth period of time so as to create a magnetic field that alters the viscosity of said MR fluid. 12. The method according to claim 1, wherein step (d) of operating said MR valve for said fourth period of time after said demagnetization cycle comprises measuring the strength of the magnetic field in said MR valve. 13. The method according to claim 12, further comprising the step of re-energizing said coil based on said measured strength of the magnetic field in said MR valve. 14. The method according to claim 12, further comprising the steps of: e. comparing said measured strength of the magnetic field in said valve to a prescribed value, andf. re-energizing said coil when the difference between said measured and prescribed values exceeds a predetermined amount. 15. The method according to claim 1, wherein said coil is energized in step (a) by supplying current from a battery to said coil. 16. The method according to claim 1, wherein said at least one component of said MR valve proximate said passage in which said first remnant magnetization is induced comprises a holder for said coil. 17. The method according to claim 1, wherein said at least one component of said MR valve proximate said passage in which said first remnant magnetization is induced is a first component, and wherein said MR valve further comprises a second component disposed proximate said passage but on a side of said passage that is opposite to said first component, at least a portion of said second component being made from a material having a relative magnetic permeability of at least about 7000. 18. The method according to claim 1, wherein the step of energizing said coil comprises supplying current to said coil from a battery located in said down hole portion of a drill string. 19. In a damping system for damping vibration in a down hole portion of a drill string, said damping system 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, a method of operating said MR valve comprising the steps of: a. energizing said 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 remnant magnetization in at least one component of said MR valve proximate said passage;b. substantially de-energizing said coil for a second period of time following said first period of time so as to operate said MR valve using said remnant 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. measuring the strength of the magnetic field in said MR valve resulting from said remnant magnetization;d. re-energizing said coil based on said measured the strength of the magnetic field in said MR valve. 20. The method according to claim 19, further comprising the step of: e. comparing said measured strength of the magnetic field in said valve to a prescribed value, andwherein said step of re-energizing said coil comprises re-energizing said coil when the difference between said measured and prescribed values exceeds a predetermined amount. 21. The method according to claim 19, wherein the step of energizing said coil comprises supplying current to said coil from a battery located in said down hole portion of said drill string. 22. A magnetorheological (MR) valve assembly for damping vibration of a drill bit for drilling into an earthen formation, comprising: a. 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;b. a supply of magnetorheological fluid;c. 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, a passage disposed between said first and second members placing said first and second chambers in fluid communication;d. at least one coil proximate said passage so that said magnetorheological fluid can be subjected to a magnetic field generated by said at least one coil when said coil is energized;e. at least a portion of one of said first and second members made from a material having a relative magnetic permeability of at least about 7000 and a coercivity of less than 1.0 Oe;f. at least a portion of the other of said first and second members being capable of having induced therein a remnant 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 remnant magnetic field is induced being made from a material having a maximum remnant magnetization of at least about 12,000 Gauss and a coercivity of at least about 10 Oe and not more than about 20 Oe. 23. The valve assembly according to claim 22, further comprising means for demagnetizing said portion of said other of said first and second members in which said remnant magnetic field is capable of being induced so as to reduce said induced remnant magnetic field. 24. The valve assembly according to claim 23, wherein said means for demagnetizing said portion of said other of said first and second members comprises means for generating a current in said coil that alternates polarity and decreases in amplitude in a stepwise fashion. 25. The valve assembly according to claim 23, further comprising a power supply for supplying a dc current, and wherein said means for demagnetizing said portion of said other of said first and second members comprises circuitry for converting said dc current into a current that alternates polarity and decreases in amplitude in a stepwise fashion. 26. The valve assembly according to claim 23, wherein said means for demagnetizing said other of said first and second members in which said remnant magnetic field is capable of being induced comprises means for reducing said induced remnant magnetic field to essentially zero. 27. The valve assembly according to claim 22, further comprising a sensor for measuring the magnetic field in said MR valve and means for energizing said coil based on said measured value of said magnetic field. 28. The valve assembly according to claim 22, wherein said portion of said other of said first and second members in which said remnant magnetic field is capable of being induced is made from a material having a maximum remnant magnetization of at least about 13,000 Gauss. 29. The valve assembly according to claim 22, wherein said portion of said other of said first and second members in which said remnant magnetic field is capable of being induced comprises a holder for holding said at least one coil. 30. The valve assembly according to claim 29, wherein said one of said first and second members made from a material having a relative magnetic permeability of at least about 7000 and a coercivity of less than 1.0 is a shaft. 31. The valve assembly according to claim 22, further comprising a battery for supplying power to said coil. 32. The valve assembly according to claim 22, further comprising a sensor for measuring the value of said remnant magnetic field. 33. The valve assembly according to claim 32, further comprising means for energizing said coil based on said value of said remnant magnetic field measured by said sensor. 34. The valve assembly according to claim 32, wherein said sensor for measuring the value of said remnant magnetic field comprises a Hall effect sensor. 35. The valve assembly according to claim 33, wherein means for energizing said coil based on said measured value of said remnant magnetic field comprises a microprocessor programmed with software that compares the measured value of said remnant magnetic field to a specified value.