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CROSS REFERENCE TO RELATED APPLICATION 
     This application claims priority to U.S. Provisional Application Ser. No. 60/032,932 filed Dec. 9, 1996. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to a surface controlled subsurface safety valve (SCSSV) for a sub-terranean well and, more particularly, to a safety valve utilizing an electrical actuation mechanism controlled from the surface or by a downhole intelligent controller. 
     2. Prior Art 
     Employment of a downhole safety valve is well known for subterranean oil and gas wells. Such valves, which can comprise a plug or pocket type, a sleeve valve, a flapper valve or ball valve, are normally positioned downhole to close the bore of the tubing string leading from one or more production zones to the well surface. Safety valves of this type are normally biased to a fail safe condition whereby any significant reduction in the opening force acting upon a valve will allow a pre-energized arrangement such as a spring to close the valve. 
     Commonly, downhole safety valves are actuated by hydraulics. A hydraulic system is connected to a piston arrangement; pressure from the surface via a small diameter control line is directed upon the piston which, in turn, moves a flow tube past a flapper valve thereby opening the flapper valve. In this position the flapper is essentially locked in the open position by the presence of the flow tube. A spring is generally placed in contact with the flow tube and with a non-moveable housing so that when the flow tube is urged downhole it is against the bias of the spring, thus energizing the spring for closing of the valve should the opening impetus caused by the hydraulic pressure from above be lost or reduced. While these valves are highly effective in the field, they do have drawbacks. One such drawback is that since these valves are installed many thousands of feet below the earth&#39;s surface, thus necessitating many thousands of feet of hydraulic control line, the hydrostatic pressure of the control line is sufficient to render the closing of the safety valve a slow process. In order to close the valve, the spring must lift the hydraulic column all of the fluid contained in the piston cylinder back into the control line by forcing, for instance, six thousand feet of fluid in that control line uphole. This requires a strong closure spring to lift the hydraulic fluid column. Necessarily, the control line is susceptible to damage during the running process and the joints in the control line may develop leaks over time. Such a leak would indicate a reduction in the opening impetus and the fail safe feature of the valve would close the same. Loss of integrity of the control line, in general, requires that the entire tubing string be pulled from the well and necessary repairs made. 
     More recently electric actuation of downhole safety valves has become of interest. Electrically operated safety valves are becoming increasingly popular with the introduction of intelligent downhole systems such as those disclosed in U.S. patent application Ser. No. 08/386,504 now U.S. Pat. No. 5,706,896 assigned to the assignee hereof and incorporated herein by reference. Downhole intelligence allows an electrically actuated downhole safety valve to receive commands from the surface or downhole and thereby operate either completely automatically or with input if desired. 
     One of the drawbacks associated with the use of solenoid actuated downhole safety valves arises from the use of the solenoid itself to directly open the valve. Opening valves of larger sizes requires a reasonably long throw. Solenoids, however, generally operate on throws shorter than that necessary. In many cases the throw of the solenoid is not sufficient to completely open the safety valve. This impedes flow of production fluid and risks damage to the safety valve due to the bending moment on the pivot point of the valve caused by production flow. To remedy the drawback, either a larger solenoid or various levering arrangements have been employed with some success. There is still a need, however, for improved methods of electrically actuating the downhole safety valve. 
     SUMMARY OF THE INVENTION 
     The above-discussed and other drawbacks and deficiencies of the prior art are overcome or alleviated by the electrically actuated downhole safety valve of the invention. 
     The invention employs a rotational motor and lead screw to interact with an engageable yoke which moves the flow tube. Extending around the I.D. of the housing is an engageable and disengageable yoke similar to a half nut which when engaged is moveable along the lead screw. The yoke is also attached to a flow tube which is disposed within the I.D. of the yoke such that upon engagement of the yoke with the lead screw and the movement imparted to the yoke is also imparted to the flow tube. As the flow tube is urged toward and through the flapper valve, it is aligned with and connects with the downhole production tube. 
     In order to actuate the yoke of the invention, a solenoid is connected to a camming rod which urges the yoke apart at one point thereof and together at a point diametrically opposed to the first point. This allows engagement of the yoke with the lead screw at the second point. As will be understood by one of skill in the art in order to provide such movement the yoke is divided in two parts (half yokes) and mounted on guide rods. Therefore, separation of the half yokes at one end will produce the movement of the other ends of the half yokes closer together and thus into communication with the lead screw. 
     In another aspect of the invention the friction commonly associated with a half nut for a lead screw is avoided by providing follower screws mounted on bearings and connected to the engagement side of the yoke. Since frictional characteristics are dramatically reduced the effective power requirement of the rotational motor need not be as high as it otherwise might have to be. Several embodiments of actuation mechanisms and particular assemblies are discussed in detail in the pages following. 
     The yoke and lead screw design of the present invention replace the hydraulic actuation of the prior art shown in FIG. 1 while other aspects of the safety valve remain the same as the prior art. 
     The above-discussed and other features and advantages of the present invention will be appreciated and understood by those skilled in the art from the following detailed description and drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Referring now to the drawings wherein like elements are numbered alike in the several FIGURES: 
     FIG. 1 is a view of the prior art hydraulic safety valve; 
     FIGS. 2A and 2B are a cross-section of one embodiment of the invention wherein the valve closed; 
     FIGS. 3A and 3B is a quarter section of the invention wherein the safety valve is open; 
     FIG. 4 is a cross-section view from FIG. 2A taken along section lines  4 — 4 ; 
     FIG. 4A is a view of the invention taken along section line  4   a  in FIG. 4 without rockers with twin follow screws but with merely with a single follower screw on each half yoke; 
     FIG. 5 is a perspective view of one half yoke of the invention; 
     FIG. 6 is an alternate embodiment of the bottom section of FIG.  4  and illustrated in the disengaged position; 
     FIG. 7 is another alternate embodiment of the bottom section of FIG.  4  and illustrated in the disengaged position; 
     FIG. 8 is a cross-sectional end view of yet another alternative embodiment in a disengaged position; 
     FIG. 9 is a cross-sectional top view of the embodiment shown in FIG. 8 in a disengaged position; 
     FIG. 10 is a cross-sectional side view of the embodiment shown in FIG. 8 in a disengaged position; 
     FIG. 11 is a cross-sectional end view of the embodiment shown in FIG. 8 in an engaged position; 
     FIG. 12 is a cross-sectional top view of the embodiment shown in FIG.  8  in an engaged position; 
     FIG. 13 is a cross-sectional side view of the embodiment shown in FIG. 8 in an engaged position; 
     FIGS. 14-18 are an elongated view of an alternate embodiment of the invention; 
     FIGS. 19-23 illustrates the embodiment of FIGS. 14-18 in another position; 
     FIG. 24 is a view of the ramp and ramp follower of the invention in the retracted position; 
     FIG. 25 is a view of the ramp and the ramp follower of the invention in the deployed position; 
     FIG. 26 is a cross-sectional view of the embodiment of FIGS. 14-18 taken along section line  26 — 26  in FIG. 21; and 
     FIG. 27 is a perspective view of the cage of the embodiment of FIGS.  14 - 18 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to FIG. 1, which is a representation of a prior art hydraulically actuated safety valve available from Baker Oil Tools, Broken Arrow, Okla., housing  10  surrounds the production pipe including an axially moveable flow tube  12  and fixed flow pipe  14 . Actuation of the system, as will be recognized by one of skill in the art is by a hydraulic control line (not shown) attached to the safety valve assembly at connector  16  providing hydraulic fluid to chamber  18  which provides the impetus to piston  20  to move downhole taking with it flow tube  12  and compressing power spring  22  in the process. Flapper valve  24  is urged into the open position by the downhole movement of flow tube  12  and is, in fact, forced completely out of the path of flow tube  12  such that end  26  of flow tube  12  will seat in shoulder  28  of the fixed pipe  14 . This provides a smooth flow bore from below to above the safety valve. Upon a loss of pressure in chamber  18 , spring  22  will urge piston  20  back uphole along with flow tube  12  and will thus allow flapper valve  24  to close due to flow and, in addition, the urging of flapper spring  30 . 
     Due to the inherent drawbacks of the hydraulically actuated safety valve noted above, the invention replaces the hydraulic actuation system and piston with a rotational motor and a novel mechanism to translate the rotational movement of the lead screw to the flow tube. In other respects, the safety valve of the invention is similar to the prior art. 
     It should be noted that in addition to the inherent drawbacks of the hydraulically actuated systems in the prior art, another driving force behind the interest in moving to electrically operated systems is the increasingly ubiquitous use of downhole intelligent systems and surface to downhole communication systems. Although one embodiment of the invention employs a hard wired electrical supply line to the surface, downhole intelligence allows for control of this line downhole and additionally allows for the use of alternate downhole power supply systems and therefore can avoid some of the inherent drawbacks of going to the surface. 
     Referring to FIGS. 2A and 2B, it will be appreciated that the valve is in the fail-safe, closed position. In this position, as will be understood, the yoke  32  of the invention is illustrated at an uphole end of the lead screw  34 . In order to actuate the safety valve the yoke and flow tube will be moved downhole. The instruction to actuate the safety valve may be provided from a surface controller or a downhole controller  90  shown in FIG.  2 A. Upon actuation of the rotational motor  36 , turning gears  38  and consequently gear  40 , lead screw  34  is turned. When yoke  32  is actuated by a solenoid and cam, to place follower screws  42  in contact with lead screw  34 , yoke  32  will be urged downhole along with flow tube  12  to compress spring  22  and open flapper valve  24  as in the prior art. Viewing FIGS. 2A and 2B and  3 A and  3 B sequentially will provide an illustration of the tool prior to actuation (valve closed) and after actuation (valve open). Upon any loss of power, the solenoid will allow the cam  50  to rotate to the resting position and disengage yoke  32 . Immediately upon yoke  32  disengaging screw  34 , there is nothing restricting extension of spring  22  back toward its resting length. Spring  22  will thus move flow tube  12  and yoke  32  uphole. As the flow tube  12  is withdrawn from the position where it interferes with the closing of flapper valve  24 , the valve will shut due to fluid flowing there past and to the flapper spring  30 . This is the fail-safe position of the device of the invention and is illustrated in FIGS. 2A and 2B. The fail-safe position is that of the unactuated position as well. 
     Referring more specifically to the yoke of the invention, attention is directed to FIG. 4 which is a cross-section taken from FIG. 2A wherein a plan view of the yoke illustrates the operation thereof. Yoke  32  is actually broken in two sections which will be referred to as yoke halves  32   a  and  32   b . Each section is mounted on a guide rod  44  at about the mid-point between the operative ends of yoke halves  32   a  and  32   b . For purposes of discussion, the orientation of parts in the FIGURE, such as top, bottom and sides will be referred to. It will be understood that apart from the discussion of the various parts in relation to one another, these terms do not have any meaning. As one of skill in the art will readily appreciate, “bottom” in the drawing is not actually a bottom of the device but merely illustrated at the bottom of the drawing. Referring to the top of drawing FIG. 4, lead screw  34  is illustrated in cross-section and has been contacted by follower screws  42  on rocker arms  46  which pivot on pins  48  mounted in the top of each yoke half  32   a  and  32   b . It can be ascertained from the drawing that the top of yokes  32   a  and  32   b  are moved toward one another and into contact with lead screw  34  by moving the bottom of each of yoke half  32   a  and  32   b  apart from one another. Moving the bottom ends of yoke half  32   a  and  32   b  apart pivots each yoke half on respective guide rods  44  to effect the desired result at the top of the drawing. In the most preferred embodiment of the invention an oval cam  50  is mounted between yoke extensions  52  of each yoke half  32   a  and  32   b . Cam  50  is actuated by an electrically powered solenoid (not shown) which turns the cam a sufficient amount to bring roller screws  42  into engagement with lead screw  34 . The cam is also spring loaded such that upon a failure of power to the solenoid, the cam will spring back to its disengaged position allowing follower screws  42  to disengage from lead screw  34 . At this instant, the action of spring  22 , as described above, is effected. 
     Referring more particularly to the circumscribed section  4   a  in FIG.  4  and to FIG. 5 one preferred embodiment is illustrated wherein four follower screws  42  are employed. In FIG. 4A, an alternate embodiment is illustrated wherein only two follower screws are employed. This arrangement replaces a half nut which would otherwise be employed to follow a lead screw design. The half nut, however, adds friction which requires additional rotational power to turn the lead screw and increases the wear on components of the device such that earlier replacement is required. In the invention, either a four follower screw arrangement or a two follower screw arrangement is employed, as desired, wherein the follower screws are threaded complimentarily to the pitch of the lead screw and are mounted on bearings either at the tops of yoke halves  32   a  and  32   b  as illustrated in FIG. 4A or in the rockers  46  as illustrated in FIG.  4 . The follower screws provide very little friction on a lead screw both because they contact the lead screw at discrete portions and not all the way around ( as does a half nut) and because they are not static but roll on the bearings with the movement of lead screw  34 . With this arrangement virtually all significant frictional forces are alleviated. Power of the rotational motor  36  and longevity of the arrangement of the invention are reduced and increased, respectively. It will be understood that disengagement of the follower screws  42  from lead screw  34  is accomplished by moving them the distance of the thickness of the threads and slightly beyond to completely disengage from the lead screw. Movement of the yoke halves more than necessary is contraindicated. 
     Referring to the bottom of FIG. 4, the cam  50  is illustrated in the engaged position and disengagement spring  54  is illustrated in the extended position. Upon movement of the cam  50  pursuant to a loss of power in the solenoid, spring  54  will draw the bottom ends of each yoke half  32   a  and  32   b  toward one another and consequently pull the top ends of yoke halves  32   a  and  32   b  away from one another, thus, disengaging the follower screws from the lead screw. As shown in FIG. 5, each yoke half  32   a  and  32   b  ride on guide rods  102  and push on a bearing  103  positioned around the guide rod  102 . Below the yoke  32 , the flow tube  12  has an increased outer diameter to form a shoulder. The outer diameter at the flow tube shoulder is greater than the outer diameter of the yoke  32 . As the yoke  32  moves, it engages the shoulder  140  (shown in FIG. 10) of the flow tube  12  and consequently moves the flow tube  12 . 
     Referring to FIGS. 6 and 7, alternate embodiments of the cam at the bottom of drawing  4  are illustrated. Movement of each of the cams  51  and  53  respectively, most preferably, is sufficient to cause engagement of the follower screws with lead screw  34  within a few degrees of movement. One of ordinary skill in the art then will clearly understand the alternative embodiments illustrated in FIGS. 6 and 7 from a brief perusal thereof in connection with the understanding of the invention gained from this disclosure. 
     Another alternative embodiment is shown in FIGS. 8-13. The embodiment shown in FIGS. 8-13 operates in a similar manner as the embodiments previously described in that a yoke engages a lead screw and pushes the flow tube to open a flapper valve. FIG. 8 is a cross-sectional end view of the alternative embodiment in a disengaged position. The embodiment shown in FIG. 8 includes two yoke halves  100   a  and  100   b . At one end of each yoke half are follower screws  42  mounted to rocker arm  46  as described previously. The other end of each yoke half rides on a split rod  104  through a roller assembly  142 . As will be described below, the split rods  104  force each yoke half  100   a  and  100   b  towards the lead screw  34  so that the follower screws  42  engage the lead screw  34 . Each yoke half includes openings for receiving guide rails  106  that engage bushings  108 . Each opening in the yoke  100  that receives a guide rail  106  has a dimension greater than the O.D. of each guide rail  106  so that the yoke  100  can move radially with respect to the guide rails. At each end of each bushing  108 , a slot  110  is formed. Tap holes  112  are aligned with each slot  110  and are shown in FIG. 8 on yoke half  100   a . Yoke half  100   b  shows bolts  116  that pass through the slot  110  and connect to the tapped holes  112 . The bolt  116  is dimensioned so slot  110  freely moves around bolt  116 . This configuration allows the yoke halves  100   a  and  100   b  to move radially and axially relative to the guide rails  106 . 
     Springs  114  are connected to the bushing  108  around one of the guide rails  106  and to one of the bolts  116 . The bolt that receives the spring  114  may have an enlarged height to facilitate connecting the spring  114  to bolt  116 . Springs  114 , shown in yoke half  100   b , pull the yoke halves  100  away from the lead screw  34  in the event that the power to the electric safety valve is interrupted. It is understood that yoke half  100   a  is completed in the same manner as yoke half  100   b  and is shown partially completed to provide a detailed description. FIG. 9 is a cross-sectional top view of the embodiment shown in FIG. 8 showing both yoke halves  100   a  and  100   b  fully completed. 
     FIG. 10 is a cross-sectional side view of the embodiment shown in FIG. 8 in the disengaged position. The split rod  104  is made up of two portions  104   a  and  104   b . Rod portion  104   a  includes recess  126  and rod portion  104   b  includes protrusions  124  that engage recesses  126 . When the protrusions  124  and placed in recesses  126 , the inside surfaces of rod portions  104   a  and  104   b  are flush and proximate to each other. A solenoid  120  includes a solenoid arm  122  (shown in FIG. 13) the shifts rod portion  104   b  relative to rod portion  104   a . This causes the protrusions  124  to ride out of the recesses  126  and contact the interior surface of rod portion  104   a . The rod portions  104   a  and  104   b  are spread apart. When power to solenoid  120  is discontinued, the solenoid arm  122  retracts into the solenoid  120 . Spring  128  positioned at one end of rod portion  104   b  shifts rod portion  104   b  so that protrusions  124  are positioned in recesses  126 . The spring  128  returns that yoke to the disengaged (fail-safe) position should power to the solenoid be interrupted. The protrusions  124  include a surface  130  that contacts a shoulder  132  formed in recess  126  to limit the travel of rod portion  104   b  relative to rod portion  104   a.    
     FIG. 11 is a cross-sectional end view of the embodiment shown in FIG. 8 in the engaged position. The rod portions  104   a  and  104   b  are spread apart thereby forcing the yokes halves  100   a  and  100   b  towards the lead screw  34 . The follower screws  42  engage lead screw  34 . The yoke  100  may now be moved by motor  36  thereby moving flow tube  12  as previously described. As shown in FIG. 12, the yoke  100  has traveled along the guide rails  106  through the interaction of lead screw  34  and follower screws  42 . 
     FIG. 13 is a cross-sectional side view of the embodiment of FIG. 8 in an engaged position. The solenoid  120  has been energized causing solenoid arm  122  to shift rod portion  104   b  relative to rod portion  104   a . As rod portion  104   b  move relative to rod portion  104   a , protrusions  124  move out of recesses  126  and contact the interior surface of rod portion  104   a . This spreads the two rod portions apart, forcing the yoke halves  100   a  and  100   b  towards the lead screw  34 . If power to the solenoid is interrupted, spring  128  shifts rod portion  104   b  so that protrusion  124  engage recesses  126  and the rod portions are in proximity of each other. This allows the yoke halves  100  to pull away from the lead screw  34  and assume a disengaged position as shown in FIG.  10 . Springs  114  also tend to pull the yoke halves  100  away from the lead screw  34  to ensure disengagement. As mentioned above with respect to the previous embodiments, the motor  36  may be controlled from the surface or by a downhole controller  90 . 
     In yet another embodiment referring generally to FIGS. 14-27 of the present invention, yoke halves  200 a and  200 b are actuated by a single ramp  202  instead of the two split rods  104  as in the prior embodiment. A single ramp  202  is made functional regarding spreading by supplying two ramp followers  204   a  and  204   b . Referring to FIGS. 24 and 25, the ramp and ramp followers of this embodiment are illustrated in the closed and spread positions, respectively. The ramp and ramp followers are preferably electric discharge machined from a single billet of material. This provides for a perfect match ensuring desired tolerances and reduces waste. Ramp surfaces  206  are preferably at about 10° of incline so that a full stroke causes ramp followers  204   a  and  204   b  to spread by about 30 to 50 and preferably about 40 thousandths (0.040) inch. Since this is approximately equivalent to the thread depth on the lead screw  34  and follower screws  42  it is all the movement that is necessary. It should be noted that a preferred thread angle for both the lead screw and the follower screws is 60°. This provides for both the depth of the thread desired and for yoke return as discussed hereunder. 
     Referring to FIG. 26, outboard of ramp followers  204   a / 204   b  are rollers  208  comprising axel  210  and rotator  212 . Axels  210  are mounted one in each yoke half  200   a / 200   b  at ends thereof opposite follower screws  42  which have been described herein before. Rotators  212  allow for low friction movement through the entire stroke of the safety valve of the invention. Also visible in the cross section view illustrated in FIG. 26, are the arms  214   a / 214   b  of cage  216 . Cage  216  stabilizes yoke halves  200   a / 200   b  and directs their movement inwardly at the rocker arms  46 . A machined surface  218  on the inside diameter of the cage is smooth and helps to translate the outward movement caused by the ramp followers to circumferential movement more smoothly. As will be appreciated, ramp  202  and ramp followers  204   a / 204   b  spread in a direction essentially straight out. Since yoke halves  200   a / 200   b  cannot move straight out due to interference of the (preferably smooth machined surfaces) cage arms  214   a / 214   b , the movement is translated into circumferential movement to bring follower screws  42  into engagement with lead screw  34  in a movement direction opposite that of the ramp and ramp followers. 
     As is appreciated from Drawing FIG. 26, each yoke half  200   a / 200   b  is radially larger at its circumferential ends than at its circumferential center. This is to provide grooves  213   a / 213   b  in which the yoke halves may receive cage arms  214   a / 214   b . This arrangement both adds substantial structural rigidity to the system, as well as minimizes diameter of the yoke system. An added benefit is that the arrangement facilitates assembly of the tool by allowing the yoke assembly to be assembled outside of the housing where access to the several parts is easier and then fit into the housing as a unit. 
     FIG. 27 provides a perspective view of cage  216  to provide a better understanding to one of skill in the art of how an independent unit can be constructed which then can be installed in the housing. Ring  224  of cage  216  is structurally rigid and so holds arms  214   a / 214   b  rigidly. Assembly within the cage of the yoke halves  200   a / 200   b  with a ramp  202  and ramp followers  204   a / 204   b  is a simple matter. Once the parts are combined, they are easily installed in the housing of the tool. 
     The circumferential length of grooves  213   a / 213   b  is slightly larger than the circumferential length of arms  214   a / 214   b  so that each yoke half may be moved into engagement and out of engagement with lead screw  34 . It should be appreciated that in FIG. 26 the left yoke half  200   a  is illustrated in the engaged position and the right yoke half  200   b  is illustrated in the disengaged position. It will be understood that the invention does not preferably operate in this configuration but is illustrated in this way for clarity of understanding. Reviewing FIG. 26, will reveal a gap  220  at the upper part of the FIGURE for the engaged yoke half and a gap  220  at the lower part of the FIGURE for the disengaged yoke half. The gaps illustrate the amount of travel which preferably is in the range of about 30 to about 50 thousandths of an inch. In a preferred arrangement, the gap  220  further includes a leaf spring having sufficient force of preferably about 25 pounds of force stored therein to assist in disengaging the yoke halves from lead screw  34 . The end of the leaf spring  223  is illustrated in the FIGURE and will teach the location and orientation of the spring to one of ordinary skill in the art. The spring is located with its long axis parallel to the axis of the tool and the concave/convex sides of the spring are oriented one facing the cage arm and one facing the yoke half. It is not material which one faces which way. Although the leaf spring  223  is preferred, another preferred arrangement does not employ the leaf spring  223 . In this embodiment the flank angle on the lead screw  34  and follower screws  42  of preferably 60° provides a significant yoke disengagement force and is capable of providing sufficient disengagement for the safety valve power spring to close the flapper by bringing the yoke back uphole. Thus in this embodiment the flank angle selected for the lead screw and follower screws is important to the invention. 
     Referring to FIGS. 14-23 the above discussed features of the invention are illustrated in communication and cooperation with the rest of the safety valve of the embodiment. As will be appreciated by one of skill in the art, the drawings illustrate the tool with the uphole end on the left of the drawing. 
     The terms “uphole” and “downhole” as used herein refer to relative positions of features of the preferred embodiment which could be reversed in some cases as desired. Beginning with FIG. 14, an electronics sub  230  having an electronics cover  232  thereon and in sealed relationship therewith define an atmospheric chamber  234  which houses an electronics package  236  and protects it from damage downhole and while running the tool. Electronics cover  232  includes a standard known seal  238  to help maintain atmospheric pressure within atmospheric chamber  234 . Cover  232  is connected with sub  230  at the downhole end preferably by a threaded connection  240  and sealed at the uphole end by a soft seal. Since electronics package  236  requires information flow, a port  244  is provided in electronics sub  230  to run electrical connectors (not shown) to other electrical systems of the invention. Connection port  244  is known to the industry. 
     Electronics sub  230  is connected preferably by a threaded connection  246  to tool housing  250 . Housing  250  encloses an annular space  252  preferably filled with a suitable, art recognized, dielectric fluid (not shown) to protect an annular solenoid  254 , motor and reducer  256 , lead screw  258 , yoke halves  200   a / 200   b  (and associated parts) and cage  216 . The dielectric fluid protects and lubricates these parts to avoid scale and other buildup that would otherwise occur in the downhole environment if the parts were exposed to wellbore fluids. 
     In order to have a dielectric fluid be maintained separately from wellbore fluid the annular space must obviously be bounded radially inwardly by another structure. This structure is flow tube  12  which is similar to the first embodiment of the invention. Flow tube  12  is sealed at the uphole end of the tool housing by a dynamic seal  260  located in electronics sub  230  near annular space  252 . At the downhole end of the annular space  252  the flow tube is sealed with another dynamic seal  262  of the same diameter as seal  260  in a compensator piston  264 . Maintaining the diameter of the seals equivalent prevents the flow tube from becoming an annular piston itself. As will be understood, another dynamic seal is then needed due to movement capability of the piston  264 . This is dynamic seal  268  which is mounted in compensator housing  270  discussed hereunder. The two seals  262  and  268  allow longitudinal movement of both the flow tube  12  and the compensator piston independently of one another, as indeed they do move in this way, while still maintaining a fluid seal for the dielectric fluid in space  252 . 
     The uphole end  272  of compensator piston  264  is exposed to the dielectric fluid in space  252  while the downhole end  274  of piston  264  is exposed to ambient pressure. This arrangement allows the pressure and temperature differential downhole from surface pressure and temperature to be compensated for in the enclosed dielectric fluid space. More specifically, as the pressure downhole increases the piston  264  is forced to move toward space  252  and increases the pressure thereof to equal ambient pressure. As the temperature in the downhole environment increases with depth of the tool, the piston is able to move in the other direction to allow for expansion of the dielectric fluid in the closed system. As one of skill in the art will readily understand the uphole and downhole ends of the piston and the location of the piston may be varied without departing from the scope of the invention. 
     Returning to the operable parts of the invention contained within the dielectric fluid, reference is made to FIGS.  15 , 16 , 20  and  21 . Solenoid  254  is preferably annularly shaped to extend around flow tube  12  in annular space  252 . Solenoid  254  is operably connected to ramp  202  and functions to push ramp  202  downhole thereby urging ramp followers  204   a / 204   b  outwardly as discussed above. The throw distance of solenoid  254  is preferably about 0.2 inch but is related to the angle of the ramped surfaces  206  and the total spreading desired. A higher angle of the ramp surfaces will require less throw from solenoid  254  to engage follower screws  42  with lead screw  34  as discussed; a lesser angle will require more throw. Preferably, the angle is about 10° which then corresponds to the preferred throw distance noted above. 
     Solenoid  254  is in operable communication with spring mandrel  280  and may actually be attached thereto or may simply bear upon the uphole end thereof since solenoid  254  does not provide any tensile loading but rather only provides compressive loading on spring mandrel  280 . Movement of mandrel  280  in the uphole direction is caused by spring  282  bearing upon spring collar  284  of spring mandrel  280  and an uphole surface  225  of cage ring  224  which is fixedly located within tool housing  250 . Upon energization of solenoid  254 , spring mandrel  280  is urged downhole to activate ramp  202  in the manner discussed hereinabove. Upon deenergization of Solenoid  254 , spring  282  urges the ramp  202  into its rest position. This action is made possible by a fixed connection  288  between mandrel  280  and ramp  202  which may be threaded or any other fixed connection. Cage ring  224  allows the connection through a hole  286  bored therein. Whether deenergization of solenoid  254  is intentional, accidental or in response to a signal received by electronics package  236  from the surface or a controller at the surface or downhole, the result is the same. Upon deenergization, spring  282  urges ramp  202  uphole removing support for yoke halves  200   a / 200   b  . The yoke is thus moved away from the lead screw  34 , disengaged therefrom and allows the power spring of the safety valve discussed in the first embodiment of the invention to move the flow tube uphole and close the safety valve flapper. 
     Referring again to FIG. 15, and to a different portion of annular space  252 , a motor and reducer (and resolver if a brushless motor is employed; resolver is not necessary with a brush-type motor) assembly  256  is illustrated in space  252 . Motor/reducer/resolver is mounted fixedly to cage ring  224 . Cage ring  224  provides a through hole  290  so the motor  256  may access and be operably attached to lead screw  258  via motor shaft  292 . Motor/reducer/resolver  256  is energized when desired or programmed by electronics package  236  to turn lead screw  258 . Lead screw  258  is supported as its downhole end  300  by a pilot hole  301  in cage end  271  which is bolted to cage arms  214   a / 214   b  and is threadedly connected to compensator housing  270  by threaded connection  273  . A further hole  275  is provided to accept nose  277  of ramp  202 . 
     In addition to turning lead screw  258 , motor assembly  256  also includes an electronically activated brake to hold lead screw  258  in a particular position after having turned the predetermined number of times. The brake is necessary because in order to make the safety valve fail safe, the power spring  22  is selected to be strong enough to cause lead screw  258  to back drive when the motor is not turning. The selected strength takes into account the drag of the motor and reducer turning backward, friction of the flow tube dynamic seals  260  and  262 , scale and paraffin buildup in the components of the device and all of the weight of the moving parts of the device. Determining the strength of the spring needed to overcome the noted parts is a matter known to one of ordinary skill in the art. Because of this, if the brake is not supplied the safety valve will not remain open. It should be noted that the entire motor assembly including the electronically activated brake is available commercially from Astro Instruments Corporation, Deerfield, Fla. Compensator housing  270  is then threadedly connected to valve housing  302  which is as it was in the prior art and generally as discussed above. One of skill in the art will appreciate that all of the components within valve housing  302  and illustrated in FIGS.  17 , 18 ,  22  and  23  are known to the art as a prior art safety valve which is commercially available from Baker Oil Tools, Broken Arrow, Okla. These parts are illustrated in the FIGURES noted only for the sake of completeness. 
     An additional feature provided to prevent damage to the motor  256  (see FIG. 16) in the event it does not turn off when intended is dampener spring  294 . Spring  294  is disposed upon lead screw  258  at a downhole end thereof and functions to create a progressively greater electrical draw on motor  256  if the yoke  200  has traveled too far downhole. This is simply due to progressive resistance on the yoke as the spring is compressed. The electronics package  236  is preferably equipped to sense current draw and shut down the motor if the draw gets higher than a predetermined point. 
     One of skill in the art will appreciate that downhole of those sections discussed, and as illustrated in FIGS.  17 , 18 ,  22  and  23 , the valve structure illustrated is a prior art safety valve that is commercially available from Baker Oil Tools, Broken Arrow, Okla. except for the preferred arrangement of flow tube  12 . In this embodiment the tube is in two pieces  12   a  and  12   b  for ease of assembly of the tool. Spring stop  13  includes threaded connection  11  and snap ring  9  to connect tubes  12   a  and  12   b . The assembly of these items in this manner is known to one of skill in the art. 
     While preferred embodiments have been shown and described, various modifications and substitutions may be made thereto without departing from the spirit and scope of the invention. Accordingly, it is to be understood that the present invention has been described by way of illustration and not limitation.

Summary:
An electric safety valve actuator for a surface controlled subsurface safety valve. The actuator includes a motor drive and driver selectively coupled with a drive yoke connected to an otherwise conventional flow tube of a downhole safety valve.