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Valve system with rotary stepping actuator to actuate a valve stem. An actuator drive shaft is supported for stepwise rotation in clockwise and counterclockwise directions to impart rotation to a valve stem nut to axially move a valve stem to open and close valve flow trim. The drive shaft forms per

Valve system with rotary stepping actuator to actuate a valve stem. An actuator drive shaft is supported for stepwise rotation in clockwise and counterclockwise directions to impart rotation to a valve stem nut to axially move a valve stem to open and close valve flow trim. The drive shaft forms peripheral first and second slotted circular drive paths co-axially spaced from each other. The drive paths include outwardly-opening, circumferentially-spaced slots which are matched in number, size and spacing, but the slots in the first drive path are rotationally offset from the slots in the second drive path by a set fraction, for example one half of the slot spacing. Actuation assemblies provide separate forward and reverse actuation cycles to the drive shaft to incrementally rotate the drive shaft in predetermined angular increments (steps) set by the slot spacing. Due to the offset, a counter step in a direction counter to the direction of a directly preceding step rotates the drive shaft by an amount less than the predetermined angular increment, as determined by the set fraction. If the set fraction is one half, the counter step rotates the drive shaft by a half step, while the next following step in the same counter direction is a full step in the predetermined angular increment.

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1. A valve system comprising: (i) a valve body configured with an inlet and an outlet and having flow trim configured to be moved axially by an externally threaded valve stem between a closed position, wherein flow through the valve body is restricted, and an open position, wherein fluid may enter t

1. A valve system comprising: (i) a valve body configured with an inlet and an outlet and having flow trim configured to be moved axially by an externally threaded valve stem between a closed position, wherein flow through the valve body is restricted, and an open position, wherein fluid may enter the valve body through the inlet, pass through the flow trim at reduced pressure, and continue through the outlet;(ii) a stem/bonnet assembly connected to the valve body and including a bonnet disengagably connected with, and closing, an upper end of the valve body, the threaded valve stem extending through the bonnet, and a stem nut coaxial with the threaded valve stem and having internal threads cooperatively engaged with the externally threaded valve stem;(iii) an actuator housing coupled to the stem/bonnet assembly and forming an entry port for sealed entry and rotational mounting of the stem nut;(iv) an actuator drive shaft supported in the actuator housing for stepwise rotation in clockwise and counterclockwise directions, the drive shaft being co-axially aligned with, and configured to be rotatably coupled directly or indirectly to, the stem nut to impart rotation to the stem nut, the drive shaft forming at a periphery a first circular drive path and a second circular drive path co-axially spaced from the first drive path, each of the first and second drive paths comprising a number of outwardly-opening, circumferentially-spaced openings, the openings in the first and second drive paths being matched in number, size and spacing, and the openings in the first drive path being rotationally offset from the openings in the second drive path, based on midpoint to midpoint spacing of the openings, by a set fraction of the opening spacing;(v) a first actuation assembly mounted in the actuator housing adjacent the first drive path of the drive shaft and moveable from a retracted position through a forward actuation cycle, and operative to releasably engage one of the openings in the first drive path to incrementally rotate the drive shaft in a forward step through a predetermined angular increment set by the opening spacing in the counterclockwise direction when moved through the forward actuation cycle;(vi) a second actuation assembly mounted in the actuator housing adjacent the second drive path of the drive shaft and moveable from a retracted position through a reverse actuation cycle, and operative to releasably engage one of the openings in the second drive path to incrementally rotate the drive shaft in a reverse step through the predetermined angular increment in the clockwise direction when moved through the reverse actuation cycle;(vii) an actuation control system coupled with the actuator housing and configured to supply a discrete quantity of pressurized fluid to stepwise and separately drive the first and second actuation assemblies to impart stepwise counterclockwise and clockwise rotational movement to the drive shaft in the forward and reverse actuation cycles to move the flow trim in the forward and reverse steps between the open and closed positions; and(viii) the drive shaft and the first and second actuation assemblies being adapted to incrementally rotate the drive shaft in a counter step in a direction counter to the direction of a directly preceding step by an amount less than the predetermined angular increment, said amount being determined by the set fraction of the offset, such that the counter step in the counter direction rotates the drive shaft by said amount determined by the set fraction, while a next step in the same counter direction is a full step in the predetermined angular increment. 2. The valve system of claim 1, wherein the drive shaft and the first and second actuation assemblies are adapted to delay engaging the opening in the first and second drive paths in the counter step by the set fraction. 3. The valve system of claim 2, wherein said amount determined by the set fraction of the offset is about one minus the set fraction. 4. The valve system of claim 2, wherein the openings are slots extending axially along the rotational axis. 5. The valve system of claim 4, wherein the set fraction is about one half of the slot spacing, such that the counter step rotates the drive shaft by said amount which is about one half of the predetermined angular increment set by the slot spacing such that the counter step is a half step. 6. The valve system of claim 5, wherein the number of slots in each of the first and second drive paths is between 5 and 20 so that the predetermined angular increment imparted in the full forward step and the full reverse step is between about 18 and 72° and the offset results in the half step being between about 9 and 36°. 7. The valve system of claim 5, wherein the number of slots in each of the first and second drive paths is 10 so that the predetermined angular increment imparted in the full forward step and the full reverse step is about 36° and the offset results in the half step being between about 18°. 8. The valve system of claim 2, wherein: the actuation control system comprises two hydraulic cylinders connected to the actuator housing, each cylinder having a supply of hydraulic fluid and a piston adapted to extend into the actuator housing; andthe first and second actuation assemblies each comprise:a drive collar mounted co-axially around either the first drive path or the second drive path for rotational counterclockwise and clockwise movement in a radial arc about the first or second drive path;the openings in the drive shaft being slots extending axially along the rotational axis;a ratchet pawl pivotally connected with the drive collar and spring biased to be pivotally movable between an engaged position within one of the slots of the first or second drive paths and a disengaged position released from the slot with each clockwise and counterclockwise rotation of the drive collar; anda connecting arm oriented tangentially to the drive collar and pivotally connected between the piston of one of the hydraulic cylinders and a compression spring, the connecting arm having a central portion pivotally connected to the drive collar, so that supply of the hydraulic fluid to extend the piston causes the connecting arm to impart radial rotational movement to the drive collar in a driven step in either the clockwise or counterclockwise direction to move the ratchet pawl into the engaged position with one of the slots of the drive shaft, and retraction of the piston with the compression spring causes the connecting arm to impart radial rotational movement to the drive collar in a return step in an opposite clockwise or counterclockwise direction to move the ratchet pawl into the disengaged position released from the slot of the drive shaft. 9. The valve system of claim 8, wherein the ratchet pawl is connected and positioned in the drive collar relative to the drive shaft such that, in the counter step the ratchet pawl remains out of engagement with the slot, and instead engages the periphery of the drive shaft to delay engaging the slot by the set fraction of the predetermined angular increment set by the slot spacing. 10. The valve system of claim 9, wherein the hydraulic cylinder, piston, connecting arm, and ratchet pawl associated with each of the first drive path and second drive path are oriented to engage the drive shaft from diametrically opposite positions to stepwise rotate the drive shaft in opposite clockwise and counterclockwise directions. 11. The valve system of claim 10, wherein: the valve is a choke valve;the drive shaft is rotationally coupled to, and co-axially aligned with, the stem nut having internal threads; andthe stem nut is rotationally coupled with the externally threaded valve stem such that the internal threads of the stem nut cooperatively engage the externally threaded valve stem, such that axial rotation of the stem nut produces axial movement of the threaded valve stem to move the flow trim between closed and open positions in accordance with clockwise and counterclockwise rotation of the stem nut. 12. The valve system of claim 11, wherein: the drive collar is formed with diametrically opposed slotted portions, the connecting arm having a pivotal connection to the drive collar in one slotted portion and the ratchet pawl having a pivotal connection to the drive collar in the diametrically opposed slotted portion;the ratchet pawl has a central portion which makes the pivotal connection to the drive collar, a front portion forward of the pivotal connection end and a back portion rearward of the pivotal connection, a top portion facing opposite the drive shaft and a bottom portion facing the drive shaft, the front portion forming a notched portion sized to fit into one of the slots of the drive shaft and having a front drive face and an opposed rear face; andeach slot, when viewed in cross section perpendicular to the rotational axis of the drive shaft, has a front wall, a rear wall and a connecting bottom wall, such that during the drive step of the drive collar the front wall of the slot is engaged by the front drive face of the ratchet pawl to impart stepwise rotational movement to the drive shaft, and during the return step of the drive collar an outer lip of the rear wall of the slot contacts the rear face of the ratchet pawl to move the ratchet pawl into the disengaged position released from the slot so that no movement is imparted to the drive shaft. 13. The valve system of claim 12, wherein the rear face of the notched portion of the ratchet pawl is tapered toward the slot to assist in releasing the ratchet pawl from the slot. 14. The valve system of claim 12, further comprising a locking pin mounted in the actuator housing arranged to contact a back face formed on the back portion of the ratchet pawl to hold the ratchet pawl in the disengaged position against the spring bias of the ratchet pawl. 15. The valve system of claim 12, wherein the drive collar has a pawl limit pin at the periphery parallel to the axis of rotation of the drive collar to contact the top portion of the ratchet pawl to limit pivotal movement of the pawl in the disengaged position. 16. The valve system of claim 12, wherein the drive collar comprises a torsional spring at the periphery to press against the top portion of the ratchet pawl to bias the ratchet pawl into engagement with the slot of the drive shaft. 17. The valve system of claim 12, wherein each slot is generally rectangular shaped in a cross section perpendicular to the rotational axis and the outer lip of the rear wall of the slot has a cut away portion to assist in release of the ratchet pawl and to limit wear of the slot and of the ratchet pawl. 18. The valve system of claim 2, wherein the valve body, valve stem and flow trim components are configured in a choke valve comprising: a hollow valve body assembly having an inlet bore and an outlet bore substantially at right angles, and forming a main bore which is an extension of the outlet bore and which communicates with the inlet bore;the flow trim positioned in the main bore, the flow trim comprising a stationary tubular cage having a side wall, and an external cylindrical flow collar adapted for sliding movement along the side wall of the cage, the side wall of the cage forming an internal bore aligned with the outlet bore and having a ported portion between its ends formed with one or more flow ports, the external flow collar being adapted for movement between a closed position, wherein the one or more flow ports are fully covered by the external flow collar, and an open position, wherein each of the flow ports is fully or partially uncovered by the external flow collar, whereby fluid may enter the valve through the inlet bore, pass through the one or more flow ports at reduced pressure and continue through the outlet bore;the bonnet disengagably connected with, and closing, an upper end of the valve body; andthe externally threaded valve stem configured to bias the external flow collar over the one or more flow ports. 19. The valve system of claim 18, wherein the one or more flow ports are arranged to include one or more pairs of diametrically opposed main flow ports. 20. The valve system of claim 19, wherein there is one pair of diametrically opposed main flow ports. 21. The valve system of claim 18, which further comprises a tubular sleeve positioned in the main bore across the inlet bore and forming at least one side port communicating with the inlet bore, the flow trim being positioned within the tubular sleeve, and wherein the bonnet closes the upper end of both the valve body and the tubular sleeve. 22. The valve system of claim 21, wherein the at least one side port is aligned with the inlet bore, and one pair of the one or more pairs of the diametrically opposed main flow ports is arranged such that a line through a midpoint of the diametrically opposed main flow ports is parallel to a centre axis of the inlet bore. 23. The valve system of claim 22, further comprising: at least a pair of diametrically opposed secondary flow ports formed in the side wall of the cage, the secondary flow ports having a smaller diameter than a diameter of the main flow ports and each pair of secondary flow ports being positioned with an axis which is offset by 90° from the axis of one of the pairs of main flow ports and closer to the body outlet than are the main flow ports. 24. The valve system of claim 21, wherein: the inlet bore and the outlet bore are arranged in a T-shape to provide a body side inlet, a body outlet and an insert chamber at the intersection of the body side inlet and the body outlet; andthe tubular sleeve and the flow trim are arranged as a removable insert assembly positioned in the insert chamber, the insert assembly comprising:a) the tubular sleeve being adapted as a tubular cartridge having a side wall forming an internal bore and the at least one side port communicating with the body side inlet, whereby fluid may enter through the at least one side port from the body side inlet;b) the flow trim being positioned in the cartridge internal bore, the flow trim comprising the tubular cage aligned with the body outlet, and the external flow collar slidable along the side wall of the cage, the cage side wall forming the one or more pairs of diametrically opposed main flow ports located to overlap the intersection of the axes of the body side inlet and the body outlet, and aligned with the at least one side port of the tubular cartridge to communicate with the side port, whereby fluid from the body side inlet may enter the cage bore at reduced pressure and pass through the body outlet;c) the bonnet being disengagably connected with, and closing, the upper end of the tubular cartridge and the body; andd) the externally threaded valve stem being configured to bias the flow collar over the main flow ports. 25. The valve system of claim 24, further comprising: at least a pair of diametrically opposed secondary flow ports formed in the side wall of the cage, the secondary flow ports having a smaller diameter than a diameter of the main flow ports and each pair of secondary flow ports being positioned with an axis which is offset by 90° from the axis of one of the pairs of main flow ports and the secondary flow ports being arranged closer to the body outlet than are the main flow ports. 26. A drive shaft component as defined in claim 1. 27. A rotary stepping actuator comprising the components (iii)-(viii) of claim 1. 28. A method of opening and closing the flow trim of a valve with a rotary stepping actuator by imparting stepwise rotation in predetermined angular increments in clockwise and counterclockwise directions to a drive shaft rotationally coupled to the flow trim through a stem nut and a valve stem in a manner to impart axial movement to the flow trim with each stepwise rotation of the drive shaft, the method comprising: supplying pressurized fluid to stepwise and separately drive first and second actuation assemblies to impart stepwise counterclockwise and clockwise rotational movement to the drive shaft such that the drive shaft is stepwise rotated by a predetermined angular increment for each forward step and each reverse step to move the flow trim axially in steps between an open position and a closed position;incrementally rotating the drive shaft in a counter step in a direction counter to the direction of a directly preceding forward or reverse step by an amount less than the predetermined angular increment; andincrementally rotating the drive shaft in a next step in the same counter direction by the predetermined angular increment. 29. The method of claim 28, wherein the counter step incrementally rotates the drive shaft by about half of the predetermined angular increment.