Linear drive system for downhole applications

1. An apparatus for use in a borehole, comprising:

a bottomhole assembly configured to be disposed in the borehole;

a yoke disposed along the bottomhole assembly;

a rotor having at least one set of magnetic elements, wherein a polarity of each magnetic element is arranged to alternate along a long axis of the rotor; and

a cylindrical stator inside and in sliding engagement with the rotor, the stator having a winding flowing current in a direction transverse to a long axis of the stator, the stator having a core formed at least partially of laminations aligned along a plane co-planar with the long axis of the stator,

wherein energizing the winding causes relative sliding between the rotor and the stator, the sliding being principally along the long axis of the rotor; and

wherein the core includes a plurality of circumferentially distributed planar strips, each strip being formed of the laminations.

2. The apparatus of claim 1 , wherein the core is at least partially formed of a mixture of an epoxy base and granular iron filling.

3. The apparatus of claim 1 , wherein the magnetic elements of the same polarity are formed as planar bodies that are circumferentially distributed within the rotor.

4. The apparatus of claim 1 , wherein the stator includes at least one multiphase winding serially arranged along a long axis of the stator, and further comprising:

at least one multiphase driver associated with the at least one multiphase winding;

at least one position sensor estimating a position of the rotor relative to the at least one multiphase winding; and

a controller using information from the at least one position sensor to activate the at least one multiphase driver and sequentially energize the at least one multiphase winding.

5. The apparatus of claim 1 , wherein the stator includes a plurality of multiphase windings serially arranged along a long axis of the stator, and further comprising:

at least one multiphase driver associated with the plurality of multiphase windings;

at least one position sensor estimating a position of the rotor relative to the plurality of multiphase windings; and

a controller using information from the at least one position sensor to activate the at least one multiphase driver and sequentially energize each of the plurality of multiphase windings.

6. The apparatus of claim 1 , further comprising:

at least one sensor estimating a subsurface parameter of interest; and

at least one controller using information from the at least one sensor to control the linear motor.

7. The apparatus of claim 1 , further comprising a bottomhole assembly including at least one pad engaging a borehole wall, wherein the rotor is operatively connected to the at least one pad, and wherein axial displacement of the rotor radially displaces the at least one pad.

8. The apparatus of claim 1 , wherein the rotor includes a bore for receiving the stator.

9. An apparatus for use in a borehole, comprising:

a bottomhole assembly configured to be disposed in the borehole;

a yoke disposed along the bottomhole assembly;

a rotor having at least one set of magnetic elements, wherein a polarity of each magnetic element is arranged to alternate along a long axis of the rotor; and

a cylindrical stator inside and in sliding engagement with the rotor, the stator having a winding flowing current in a direction transverse to a long axis of the stator, the stator having a core formed at least partially of laminations aligned along a plane co-planar with the long axis of the stator,

wherein energizing the winding causes relative sliding between the rotor and the stator, the sliding being principally along the long axis of the rotor, wherein the core includes a plurality of circumferentially distributed planar strips, each strip being formed of the laminations, and further comprising a plurality of circumferentially distributed non-conductive spacers, wherein at least one of the plurality of non-conductive spacers separates at least two of the plurality of planar strips.

10. A method for performing an operation in a borehole, comprising:

forming a well tool with at least one linear motor, wherein the linear motor includes:

a yoke disposed inside the well tool and connected to a displaceable structure associated with the well tool;

a rotor having at least one set of magnetic elements fixed to the yoke, wherein a polarity of each magnetic element is arranged to alternate along a long axis of the rotor, and

a cylindrical stator inside and in sliding engagement with the rotor, the stator having a winding flowing current in a direction transverse to a long axis of the stator, the stator having a core formed at least partially of laminations aligned along a plane co-planar with the long axis of the stator, wherein energizing the winding causes relative sliding between the rotor and the stator, wherein the sliding is principally along the long axis of the rotor, wherein the core includes a plurality of circumferentially distributed planar strips, each strip being formed of the laminations;

conveying the well tool into the borehole; and

operating the well tool by activating the linear motor.

11. The method of claim 10 , wherein the stator includes at least one multiphase winding serially arranged along a long axis of the stator, and at least one phase driver associated with the at least one multiphase windings, and further comprising:

estimating a position of the rotor relative to the at least one multiphase winding; and

sequentially energizing each winding of the at least one multiphase winding with a controller that uses information from the at least one position sensor to activate the at least one phase driver.

12. The method of claim 10 , further comprising: using at least one position sensor.

13. The method of claim 10 , further comprising:

estimating a subsurface parameter of interest using at least one sensor; and

controlling the linear motor with at least one controller that uses information from the at least one sensor.

14. The method of claim 10 , wherein the well tool is a tractor, and further comprising:

moving a bottomhole assembly through the borehole using the tractor.

15. The method of claim 10 , wherein the well tool is a thruster, and further comprising:

applying a weight on a drill bit using the thruster.

16. The method of claim 10 , wherein the well tool is a well intervention tool, and further comprising:

manipulating a subsurface feature using the well intervention tool.

17. The method of claim 10 , wherein the well tool is a pump, and further comprising:

conveying a fluid using the pump.

18. The method of claim 10 , wherein the well tool is a sensor assembly, and further comprising:

adjusting a position of the sensor assembly using the linear motor.

19. The method of claim 10 , wherein the well tool is an underreamer, and further comprising:

extending at least one cutter into cutting engagement with a borehole wall using the linear motor.