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CROSS-REFERENCE TO RELATED APPLICATIONS 
     Not applicable. 
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     Not applicable. 
     BACKGROUND OF INVENTION 
     1. Field of the Invention 
     The present invention relates generally to logging tool conveyance methods for highly deviated or horizontal wells. More specifically, the invention relates to downhole tractor tools that may be used to convey other logging tools in a well. 
     2. Background Art 
     The invention is a device that selectively grips or releases the well wall. It can also position the tractor tool at the center of the well bore. 
     Once a well is drilled, it is common to log certain sections of it with electrical instruments. These instruments are sometimes referred to as “wireline” instruments, as they communicate with the logging unit at the surface of the well through an electrical wire or cable with which they are deployed. In vertical wells, often the instruments are simply lowered down the well on the logging cable. In horizontal or highly deviated wells, however, gravity is frequently insufficient to move the instruments to the depths to be logged. In these situations, it is necessary to use alternative conveyance methods. One such method is based on the use of downhole tractor tools that run on power supplied through the logging cable and pull or push other logging tools along the well. 
     Downhole tractors use various means to generate the traction necessary to convey logging tools. Some designs employ powered wheels that are forced against the well wall by hydraulic or mechanical actuators. Others use hydraulically actuated linkages to anchor part of the tool against the well wall and then use linear actuators to move the rest of the tool with respect to the anchored part. A common feature of all the above systems is that they use “active” grips to generate the radial forces that push the wheels or linkages against the well wall. The term “active” means that the devices that generate the radial forces use power for their operation. The availability of power downhole is limited by the necessity to communicate through a long logging cable. Since part of the power is used for actuating the grip, tractors employing active grips tend to have less power available for moving the tool string along the well. Thus, an active grip is likely to decrease the overall efficiency of the tractor tool. Active grips have another disadvantage. This is the relative complexity of the device and, hence, it&#39;s lower reliability. A more efficient and reliable gripping device can be constructed by using a passive grip that does not require power for the generation of high radial forces. In one such design, the gripping action is achieved through sets of arcuate-shaped cams that pivot on a common axis located at the center of the tool. This gripping system allows the tractor tool to achieve superior efficiency. However, by virtue of the physics of their operation, the cams allow tractoring in only one (downhole) direction. Another limitation of this system is the relatively narrow range of well bore sizes in which these cams can operate. In addition, the cams cannot centralize the tool by themselves. This requires the usage of dedicated centralizers, which increase the tractor tool length. 
     Downhole tractor tools that use various methods of operation to convey logging tools along a well have been previously disclosed and are commercially available. 
     U.S. Pat. No. 6,179,055 discloses a conveyance apparatus for conveying at least one logging tool through an earth formation traversed by a horizontal or highly deviated borehole. The conveyance apparatus comprises a pair of arcuate-shaped cams pivotally mounted to a support member, a spring member for biasing the arcuate surface of each cam into contact with the borehole wall, and actuators operatively connected to each cam. A logging tool is attached to the conveyance apparatus. When either actuator is activated in a first direction, the cam connected to the activated actuator is linearly displaced forward and the arcuate surface of the cam slides along the borehole wall. When either actuator is activated in a second direction, the activated actuator pulls the connected cam backwards and the spring member thereby urges the arcuate surface of the cam to lock against the borehole wall. Once the cam is locked, further movement of the actuator propels both the conveyance apparatus and the logging tool forward along the highly deviated or horizontal borehole. 
     U.S. Pat. No. 6,089,323 discloses a tractor system which, in certain embodiments, includes a body connected to an item, first setting means on the body for selectively and releasably anchoring the system in a bore, first movement means having a top and a bottom, the first movement means on the body for moving the body and the item, the first movement means having a first power stroke, and the tractor system for moving the item through the bore at a speed of at least 10 feet per minute. 
     U.S. Pat. No. 6,082,461 discloses a tractor system for moving an item through a wellbore with a central mandrel interconnected with the item, first setting means about the central mandrel for selectively and releasably anchoring the system in a wellbore, the central mandrel having a top, and a bottom, and a first power thread therein, the first setting means having a first follower pin for engaging the first power thread to power the first setting means to set the first setting means against an inner wall of the bore. In one aspect, the tractor system is for moving the item through the bore at a speed of at least 10 feet per minute. In one aspect, the tractor system has second setting means on the central mandrel for selectively and releasably anchoring the system in the bore, the second setting means spaced apart from the first setting means, and the central mandrel having a second power thread therein and a second retract thread therein, the second retract thread in communication with the second power thread, and the second setting means having a second follower pin for engaging the second power thread to power the second setting means to set the second setting means against the inner wall of the bore. 
     U.S. Pat. No. 5,954,131 discloses a conveyance apparatus for conveying at least one logging tool through an earth formation traversed by a horizontal or highly deviated borehole. The conveyance apparatus comprises a pair of arcuate-shaped cams pivotally mounted to a support member, means for biasing the arcuate surface of each cam into contact with the borehole wall, and actuators operatively connected to each cam. A logging tool is attached to the conveyance apparatus. When either actuator is activated in a first direction, the cam connected to the activated actuator is linearly displaced forward and the arcuate surface of the cam slides along the borehole wall. When either actuator is activated in a second direction, the activated actuator pulls the connected cam backwards and the biasing means thereby urges the arcuate surface of the cam to lock against the borehole wall. Once the cam is locked, further movement of the actuator propels both the conveyance apparatus and the logging tool forward along the highly deviated or horizontal borehole. 
     U.S. Pat. No. 5,184,676 discloses a self-propelled powered apparatus for traveling along a tubular member that includes power driven wheels for propelling the apparatus, a biasing means for biasing the driven wheels into contact with the inner surface of the tubular member, and a retracting means for retracting the driven wheels from the driving position so that the apparatus can be withdrawn from the tubular member. The retracting means also include means to automatically retract the driven wheels from the driving position when the power to the apparatus is cut-off. 
     SUMMARY OF INVENTION 
     One embodiment of the invention comprises a linkage apparatus for selectively gripping and releasing the inside walls of a conduit, the apparatus comprising: a first arm; a bi-directional gripping cam rotatably attached to the arm; and an extension and locking device adapted to selectively radially extend the arm from a tool housing to an inside wall of a conduit and adapted to selectively lock the arm in an extended position. 
     Another embodiment of the invention comprises an apparatus for selectively gripping and releasing the inside wall of a conduit, the apparatus comprising: a plurality of linkages, each linkage comprising a first arm having a first end and a second end; a second arm having a first end and a second end, the second end of the first arm pivotably attached to the second end of the second arm, and a bi-directional gripping cam rotatably attached to at least one of the second end of the first arm and the second end of the second arm; a grip body, the first end of the first arm pivotably attached to the grip body; a hub, adapted to slide relative to the grip body, the first end of the second arm pivotably attached to the hub; and an extension and locking device adapted to selectively radially extend the linkages from the grip body and adapted to selectively lock the linkages in an extended position. 
     Another embodiment of the invention comprises a method for conveying a tool body through a conduit, comprising: moving a bi-directional gripping cam into contact with an inner wall of a conduit; laterally locking a position of the cam; and moving the tool body axially with respect to the cam in a first direction. 
     Advantages of the invention include one or more of the following: 
     A device that acts as a tool centralizer; 
     A device that selectively grips or releases the inside walls of a circular conduit such as a well or a pipe; 
     A device with an extended operational range of well bore sizes; 
     A device having double-sided cams that can grip in both the downhole and uphole directions; 
     A device that provides superior efficiency and reliability; and 
     A device having a passive grip system; 
     Other aspects and advantages of the invention will be apparent from the following description and the appended claims. 
    
    
     BRIEF DESCRIPTION OF DRAWINGS 
     FIG. 1 is an cross-sectional view of the overall architecture of a downhole tractor conveyance system. 
     FIG. 2 is a three dimensional perspective view of the invention. 
     FIG. 3 is a magnified perspective view of one of the linkages of the invention. 
     FIG. 4 is an exploded view of the elements of the linkage shown in FIG.  3 . 
     FIGS. 5A and 5C are side views of the double-sided cam geometry, FIG. 5B is a perspective view of same. 
     FIGS. 6A,  6 B, and  6 C are side views that demonstrate the gripping action of the cam. 
     FIGS. 7A through 7H are side views that illustrate the process of cam reversal. 
     FIGS. 8A,  8 B, and  8 C are longitudinal cross-sectional views of a hydraulic embodiment of the invention. 
     FIGS. 9A and 9B are longitudinal cross-sectional views of a hydraulic a embodiment of the invention in different states of operation. 
     FIG. 10A is a top view of the invention in its fully open state. 
     FIG. 10B is a sectional view of a hydraulic embodiment of the invention in a fully closed state taken along the section line A—A of FIG.  9 A. 
     FIG. 11A through 11E are longitudinal cross-sectional views of a hydraulic embodiment of the invention that schematically show the major operational processes. 
     FIGS. 12A,  12 B, and  12 C are longitudinal cross-sectional views of an electro-mechanical embodiment of the invention that schematically show the major operational processes. 
    
    
     DETAILED DESCRIPTION 
     The present invention proposes an improved passive grip system. It may be used to centralize a logging or other well tool, allow bi-directional motion, and/or have a much wider operational range of well bore sizes than prior art systems. The invention is a combination of gripping cams and a centralizer with lockable geometry. It may be used to perform two major functions. The first is to act as a tool centralizer. The second is to selectively grip or release the inside walls of a conduit such as a well or a pipe. In one embodiment, the invention may be used as a part of a downhole tractor conveyance system. Its major elements may include a grip body, double-sided cams, cam springs, centralizer arms, wheels, hub, centralizer opening/closing device, and/or a locking device. The arms and the hub may be combined into linkages that can expand or contract radially as the hub slides with respect to the grip body in the axial direction. These linkages provide extended operational range, centralizing action, and when the hub is locked in place, support for the cams when they grip. The centralizer opening/closing device may selectively bias the linkages towards the well walls or close the arms back into the grip body. The cams are mounted at the tips of the linkages that come in contact with the well wall. The cams may be used to provide the gripping action. Since the cams are double-sided they can be used to grip in both the downhole and uphole directions. Cam springs may be provided to keep the cams in contact with the conduit wall. The wheels reduce the friction between the arms and the conduit wall when the device does not grip. The function of the locking device is to selectively lock or unlock the hub and thus the geometry of the centralizer. All these elements may be mounted onto the grip body. 
     The invention may be combined with a linear actuator, rails, a compensator, and an electronics block to form a tractor tool sonde. The grip body can slide back and forth on the rails of the sonde. One of the linear actuator&#39;s functions may be to reciprocate the grip body with respect to the rest of the sonde. The compensator provides pressure compensation of internal volumes and the fluid necessary for the operation of the grip. The electronics block may drive and control the electric motor of the linear actuator and the locking device. Two or more sondes may be used in a complete tractor tool to enable continuous motion of the tractor. In addition, the tractor tool may contains an electronics cartridge and a logging head that connects the tool to the logging cable. It may also contain additional auxiliary devices. The tractor tool may be attached to other logging tools that it can convey along the well. 
     In one embodiment, the invention, further referred to as grip, may be a part of a downhole tractor conveyance system. One possible embodiment of the tractor system in a tool string is schematically shown in FIG.  1 . The tool string shown in the figure comprises a logging head  4  that connects the tool string to the logging cable  2 , auxiliary equipment  6 , electronics cartridge  8 , two tractor mechanical sondes  10 , and multiple logging tools  12 . The electronics cartridge  8  and the two mechanical sondes  10  comprise the downhole tractor conveyance system. The electronics cartridge  8  is responsible for communication with surface equipment and other tools in the tool string, supply of power to the logging tools, and control of the mechanical sondes  10 . In another embodiment, the elements of the tractor system are not connected to each other and may have logging tools  12  between them as shown in FIG.  1 . 
     In another embodiment, the grip, which is denoted with the reference number  20 , may be a part of a mechanical sonde  10 . Other elements of the mechanical sonde can include an electronics section  14 , linear actuator section  16 , rail section  18 , compensator section  22 , and lower head  24 . The grip  20  slides back and forth inside the rail section  18  and is connected to the linear actuator section  16  and compensator section  22  through push rods  26  and  28 . The grip  20  and the linear actuator  16 , rail  18 , and compensator  22  sections are oil-filled, while the electronics section  14  and the lower head  24  are typically air-filled. Bulkheads  30  and  48  separate the oil and air-filled sections of the tool and provide electrical communications between these sections. The role of the linear actuator  16  is to reciprocate the grip  20  along the rails  18 . In this embodiment, the major elements of the linear actuator  16  are a motor  32 , a gearbox  34 , a ball screw  36 , and a ball nut  38 . The ball nut  38  is attached to push rod  26 . The motor  32  is the prime source of mechanical power for the tool. The power and control circuits for the motor can be located in the electronics section  14 . The ball screw  36  and the ball nut  38  convert the rotary motion at the output shaft of the gearbox  34  into linear motion. As the motor  32  rotates back and forth, the ball nut  38  reciprocates along the ball screw  36 . This reciprocating motion is transmitted to the grip  20  through the push rod  26 . The push rod  26  also contains a cocking piston  42 , which acts as a source of high pressure when activating the grip  20 . A compensator-side push rod  28  is mainly responsible for electrical and hydraulic communications between the grip  20  and the rest of the tool. This is schematically shown by the wire  44 . Note that the grip  20  is exposed to well bore fluid. The push rods  26  and  28  have to repeatedly exit the oil-filled sections of the tool, get into the well bore fluids and then reenter the tool. Dynamic seals  40  and  46  prevent any entry of well fluids into the tool. The function of the compensator  22  is to provide pressure compensation, and hydraulic fluid necessary for the operation of the grip  20 . The compensator  22  is piston-type, which major elements are a piston  50 , spring  52  and dynamic seals  54 . Except for the grip  20 , all other elements of the mechanical sonde have been previously disclosed and are commercially available in embodiments similar to those shown in FIG.  1 . These devices are discussed here because their presence is helpful in explaining the operation of the invention. 
     In general, the invention comprises a grip body, double-sided cams, wheels, biasing springs, centralizer linkages, a hub, a centralizer opening/closing device and a locking device. A three dimensional view of the one possible embodiment of the invention is shown in FIG. 2 where the grip body is denoted by the reference number  60 . Three sets of linkages  62  are attached to the grip body  60  and to a hub  64 , which can slide with respect to the grip body  60 . The grip body  60  is attached to the other parts of the tool (not shown) with push rods  26  and  28 . A magnified view of one of the linkages  62  is shown in FIG.  3 . The linkages  62  are comprised of a first arm  66 , a second arm  67 , and pins  68 , which attach the first arm  66  and the second arm  67  to the grip body  60  and to the hub  64 . The cams  70  and the wheels  72  are mounted on a common axle  74 , which also joins the two arms  66 . One possible arrangement of the elements that are located at the tip of the linkage  62  is shown in FIG.  4 . The wheels  72  can rotate freely on the axle  74 . The cams  70  also can rotate on the axle  74  but are oriented in an outward pointing direction by biasing springs (not shown in the figure) located in slots  76  cut in the arms  66 . The wheels  72  and the cams  70  are separated by spacers  78 , which prevent direct frictional interaction between the wheels  72  and the cams  70 . The axle  74  is secured in place by a retaining ring  79 . 
     The shape of the cams  70  is an important feature of the invention. The shape is used to provide both gripping action and bi-directionality. A bi-directional gripping cam is shown in FIGS. 5A,  5 B, and  5 C. FIG. 5A is a front view, while FIG. 5B represents a three-dimensional view of the cam. The geometry of the cam is characterized by a constant contact angle, designated by the letter α in FIGS. 5A and 5C. The contact angle is defined as the angle between a line connecting the center of the cam pivot with the point of contact between the cam surface and a tangential plane, and the normal to that plane that passes through the cam axle. The advantage of this cam is that the contact angle does not change with the location of the contact point on the cam surface, which ensures consistent gripping force. Although the constant-angle is the geometry for the embodiment shown in FIG. 4, other geometries such as eccentric wheels (shown in FIG. 5C) or cams with variable contact angle may also be constructed to provide similar functionality. 
     The combination of the double-sided cam  70  with the wheels  72  is an important feature of the invention. Its different ways of interaction with the well wall determine the most important functions of the invention, including its ability to act as a centralizer, its ability to grip the well wall, and its ability to reverse direction. The interaction of the cam  70  and the wheels  72  with the well wall is explained in FIGS. 6A,  6 B, and  6 C. FIG. 6B represents a static contact between the cam/wheel system and the well wall  150 . The contact is described as static because no axial forces F C    152  is applied to the centerline) are applied to the axle  74 . A radial centralizing force F C    152  is applied to the axle  74  by a centralizing device, which is not shown in the figure and which is discussed in detail later. In addition, a much smaller force F S    154  is applied to the cam surface, which is the resultant of the action of two cam springs (shown at  157  in FIGS.  11 A-E). The function of the cam springs  157  is to keep cam  70  in constant contact with the well wall  150 . The centralizing force F C  gives rise to a reaction force F N    156  in the point of contact between the wheel  72  and the wall  150 . The cam  70  also contacts the wall  150  but in a different contact point. As explained in FIG. 5A, this contact point is always at an angle α from the normal direction. The force at the point where the cam  70  contacts the wall is denoted by F RS    158 . Note that this force is much smaller than F C    152  because force F S  exerted by the cam spring  157  is much weaker than the force F C  exerted by the centralizing device. Thus, in this situation, the wheel  72  carries the majority of the radial load. 
     Now consider the application on axle  74  of an axial force F R    160  pointing to the right. This situation is shown in FIG.  6 C. The axial force creates a tendency of the whole system to move to the right and gives rise to frictional forces at both contact points on the wheel  72  and the cam  70 . Under the influence of the axial force F R    160 , the wheel  72  starts to roll on the well wall  150 , as indicated by the arrow  164 . Since rolling contacts are characterized by very small coefficients of friction, the frictional drag due to the interaction between the wheel and the well wall is negligible. For this reason it is not shown in FIG.  7 C. The other contact point is between the cam  70  and the well wall  150 . It is characterized by sliding friction and, hence, a much larger coefficient of friction. This contact, however, does not generate much frictional drag either. The reason is that the frictional force F FR    162  tends to rotate the cam in the clockwise direction and thus out of contact with the well wall  150 . Thus, the spring force F S    154  and the frictional force F FR    162  act against each other, which results in minimal frictional drag. Another reason for the small magnitude of F FR  is that the radial force F S  that generates it is quite small. In summary, the motion of the cam/wheels system to the right generates very little frictional interaction between the tip of the linkage  62  (FIG. 4) and the well wall  150 . This results in practically free rolling of the grip with respect to the well wall  150  when pushed to the right. Also note that during this rolling motion, the axle  74  stays at a substantially constant distance from the well wall. 
     Application of an axial force F P    166  in the opposite direction (pointing to the left) is shown in FIG.  6 A. As the direction of motion changes, so are the friction forces at all contact points. The friction force, which in FIG. 6C tended to rotate the cam  70  in the clockwise direction and, thus, away from the wall  150 , now forces the cam to rotate in the counterclockwise direction, as indicated by the arrow  172 . The geometry of the cam  70  is such (see FIG. 5) that when it rotates on its axle, its contact radius (defined as the distance between the contact point and the axis of the cam axle) either increases or decreases. In this case it increases. Thus, as the cam  70  rotates, it becomes wedged against the well wall  150  by the frictional force F FP    176  at the contact point. Also, its contact radius becomes larger than the radius of the wheels  72  and the wheels  72  come out of contact with the well wall. Note that this action also requires that the axle  74  move away from the well wall, as indicated by the change in distance denoted by Δh  170 . This change in distance usually involves an increase in the magnitude of the radial force. In FIG. 6A, this is shown by the addition of the force F L  to the existing centralizing force F C    168 . After the wheels lift off from the wall surface, the whole radial load is carried by the cam  70 . This, in turn, leads to higher normal contact forces and, consequently, higher friction. Higher friction forces wedge the cam harder against the wall, which leads to even higher frictional forces, and so on. This is a self-actuating process, which can result in an extremely high radial contact force. This is especially true if the axle  74  is prevented from moving away from the well wall by some mechanical locking device (not shown). In the latter case, the rolling of the cam  70  with respect to the well wall stops and the only possibility for relative motion between the cam and the well wall is through sliding friction. A moderate coefficient of friction at the contact point between the cam  70  and the well wall  150  combined with the very large force F N    174  can generate high enough frictional force F FP    176  to prevent any relative sliding between the cam  70  and the well wall  150 . In this situation, the grip ( 20  in FIG. 1) grips the well wall and becomes anchored in place. 
     FIGS. 7A through 7H show the reversal of the cam  70 , which then allows change in the direction of tractoring. The cam reversal process is similar to the process of gripping the casing that was explained with regards to FIG.  6 A. However, in this case, the vertical displacement of axle  74  is not constrained. In the position of the cam/wheel system shown in FIG. 7A, the system can move freely to the left and grip if forced to the right. In its initial stage, the cam reversal process follows the events explained in FIG.  6 A. An axial force F R    160  is applied to the cam axle  74 . A reaction friction force μF RS    162  is then generated by the tendency of the cam  70  to slide with respect to the well wall  150 . The forces F R  and μF RS  rotate the cam  70  in the direction indicated by the arrow  164 . The rotation of the cam  70  in the clockwise direction tends to increase the contact radius of the cam, which pushes axle  74  upward. Since the wheels&#39; radius is smaller than the contact radius of the cam  70 , the wheels  72  come out of contact with the well wall. These events are shown in FIG. 7B, wherein the axial force on the axle  74  is denoted by F P    166 . This indicates the increase in axial force necessary to push the axle  74  upwards and to roll the cam towards increasing its contact radius. The next phase in the rotation of the cam is shown in FIG.  7 C. This figure is the mirror image of FIG.  6 A. As explained with respect to FIG. 6A, the rotation of the cam  70  will stop and the cam will grip the casing if axle  74  is locked in place radially. In contrast, in FIG. 7C, the axle  74  remains unlocked and the rotation of cam  70  continues. This process leads to the situation shown in FIG.  7 D. In this position, cam  70  makes contact at its largest contact radius and is at the turning point of flipping over. FIG. 7E shows the moment just after flipping the cam beyond its largest radius. Note that the axial force has dropped substantially in value and is again indicated by F R    160 . From this point on forces F C , F N , and F R  all act to continue the rotation of the cam, which for this reason proceeds very quickly. Consecutive positions of the cam are shown in FIGS. 7F and 7G. Finally the can comes to the position shown in FIG. 7H, which is exactly the same as that shown in FIG.  6 C. From this point on, the cam/wheel assembly moves with very little resistance with respect to the well wall  150 , as explained with respect to FIG.  6 C. This completes the reversal of the cam  70 . Note that the cam/wheel system now moves freely to the right and grips when an attempt is made to move it to the left as long as the radial position of the axle  74  is locked or fixed. This is exactly the opposite of the position shown in FIG.  7 A. Thus, the reversal of the cam  70  has the effect of changing the direction of tractoring. 
     In addition to the elements explained above, the grip ( 20  in FIG. 1) also includes a centralizer opening/closing device and a locking device. There are a number of possible embodiments for these devices, including but not limited to a fully hydraulic system, an electromechanical system, and combinations of these systems. The embodiment of a fully hydraulic system for the centralizer opening/closing device and the locking device is presented in detail in FIGS. 8-11. The embodiment of an electromechanical system is schematically presented in FIG.  12 . 
     The top portion of the hydraulic embodiment of the grip is shown in FIG.  8 A. FIG. 8B is a continuation of FIG. 8A, and FIG. 8C is a continuation of FIG.  8 B. The grip body  60  is connected to other parts of the tractor tool (not shown in FIG. 8) through push rods  26  on the top and  28  on the bottom. As explained earlier, the push rods are used to reciprocate the grip in the rail section ( 18  in FIG. 1) and to provide electrical and hydraulic communications. 
     The embodiment of the grip shown in FIG. 8 can be subdivided into several major sections depending on their functionality. These major sections from top to bottom are drive rod attachment  80 , opening/closing hydraulic block  90 , high pressure accumulator  100 , linkages section  110 , grip actuator  120 , locking hydraulic block  130 , and compensator rod attachment  140 . These elements are discussed in more detail below. 
     The forces involved in reciprocating the grip along the rails are equal to the pull that the tractor tool creates and can be substantial. Therefore, special attention should be paid to the attachment of the push rods  26  and  28  to the grip body  60 . The drive section attachment consists of a split clamp  83  and an end cap  82 , which is attached to the grip body  60  with bolts  84 . Passage  81  in the push rod  26  is used for fluid communication between the grip and a cocking piston (not shown in FIG.  8 ), which will be explained later. Static seals  85  are used to seal off external well fluids from the internal volumes of the tool. The invention also includes several identical fill ports  86 , which are used for initial filling of the tool with oil, for pressure measurements, and inspection. 
     The opening/closing hydraulic block  90  includes a hydraulic block body  96 , a solenoid valve  92 , check valves  98  and a contact assembly  94 . The latter is used to supply electrical power to the solenoid valve  92 , which can be selectively opened or closed by the control circuits located in the electronics block ( 14  in FIG.  1 ). The function of the check valves  98  is to direct the fluid flow in the proper chamber of the grip. A more detailed description of the role of the various hydraulic components is provided later with respect to FIG.  11 . 
     The third major section presented in FIG. 8 is the high-pressure accumulator  100 . It is located inside chamber  108  of grip body  60 . The major elements of the high-pressure accumulator are a floating piston  103  and a spring  106 . High-pressure dynamic seals  102  mounted on the piston  103  separate the high- pressure region  101  on the top of the piston from the low-pressure region  105  at the bottom. In addition, a pressure relief valve  104  is mounted inside the piston  103 . The role of the valve  104  is to set the maximum pressure of the high-pressure accumulator  100 . 
     The next section of the grip is the linkages section  110 . In the embodiment shown, this section houses three identical linkages  62  (described earlier in FIGS. 3-6) as well as the centralizer hub  64 . In other embodiments the linkages section  110  may have 2, 4, 5, or 6 linkages. The hub  64  is connected to the piston rod  118  with a bolt  116 , ensuring that the motion of the piston rod  118  is transmitted to the hub  64 . Other elements of this section are the auxiliary wheels  112  that pivot on hubs  114 . These wheels  112  are used to assist the opening of the arms in small-diameter well bore sizes. Features of the grip body  60  in this section include special cuts  115  and slots  117  that provide space for the linkages when the grip is fully closed. The closing of the linkages  62  into the grip body  60  can be better understood by examining FIG. 9, which will be discussed later. Also shown in FIG. 8 are internal passages  107 , which are used for hydraulic communication, as well as for passage of electrical wires. The hydraulic connections are discussed in more detail in FIG.  11 . 
     The function of the grip actuator  120  is to force the hub  64  to slide with respect to the grip body  60 , thus, opening or closing linkages  62  into the grip body  60 . Another function of the actuator  120  is to react the large axial forces that may be created by the cams  70  and then transmitted through the linkages  62  and the hub  64  to the actuator rod  118 . The actuator  120  is similar to a single-acting hydraulic cylinder. It consists of a piston  125  that is attached to the actuator rod  118 . The piston  125  slides inside bore  128  in the grip body  60 . The piston  125  separates the cylinder chamber  128  into a low-pressure region  124  on top of the piston  125  and a high-pressure region  127  at the bottom. High-pressure dynamic seals  126  prevent fluid communication between the low  124  and high  127  pressure regions. In addition, dynamic seals  122  mounted in a seal cartridge  121  seal around the surface of the actuator rod  118  and prevent external fluid from entering the cylinder chamber  128 . When the pressure in region  127  exceeds the pressure in region  124 , the piston  125  is pushed upward. This motion is transmitted through the actuator rod  118  to the hub  64 , which, in turn, drives linkages  62  out of the grip body  60 . When the pressure on both sides of the piston  125  is the same, spring  123  pushes piston  125  downward, resulting in closing linkages  62  into the grip body  60 . 
     The pressure in the actuator  120  is controlled by the locking hydraulic block  130 . Its function is to open or close the ports that connect chamber  128  to the rest of the grip. When these ports are closed, the fluid volume inside the actuator  120  is trapped. Since this fluid is practically incompressible (in one embodiment, oil), the effect of trapping the fluid is to lock the hub  64  in place and, thus, the geometry of linkages  62 . Similar to the hydraulic block  90 , discussed previously, the locking hydraulic block  130  consists of a body  132 , solenoid valve  134  and a contact assembly  136  that provides electric power to the solenoid valve. The contact assembly is connected to other electrical contacts  141  with the wire  138 , which runs along a hole  139  in the grip body  60 . 
     The last major section of the grip is the compensator-side push rod attachment  140 , which joins the push rod  28  to the grip body  60 . This attachment is very similar to the drive rod attachment  80 . It consists of a clamp  143  and an end cap  144  that is bolted to the grip body  60  with screws  145 . The attachment  140  also has static seals  142  that isolate the internal volumes of the grip from external fluids. The compensator-side push rod attachment  140  also provides oil communication with the tractor tool low-pressure compensator ( 24  in FIG. 1) through an internal channel  148 . The major difference between rod attachments  80  and  140  is the presence of electrical contacts  142  in attachment  140 . These contacts are used to supply power to solenoid valves  92  and  134 . These contacts are also connected with the electronics block ( 14  in FIG. 1) by wires  146  that run in the channel  148 . 
     In FIG. 8, linkages  62  are shown in a filly open position. This corresponds to the topmost position of the hub  64  and the piston  125 . As mentioned earlier, one of the advantages of a grip according to various embodiments of the invention is its capability to cover a large range of well bore sizes. To achieve this, linkages  62  can fold completely into the grip body  60 . Linkages  62  are also capable of assuming any intermediate position between their fully open and fully closed states. This is demonstrated in FIGS. 9A and 9B. FIG. 9A shows the same elements of the grip that were described in FIG. 7B with linkages  62  in the fully closed position. FIG. 9B, on the other hand, shows linkages  62  in an intermediate position. Note that in FIG. 9A, the arms  66  are completely retracted into the grip body cuts  115 . Even the cams  70  are retracted below the outline of the grip body  60 . Also note that the hub  64  is in contact with the seal cartridge  121  and the actuator rod  118  is completely inside the cylinder chamber  128 . In FIG. 9B, the actuator rod is extended upward by the distance denoted by “STROKE” in FIG.  9 B. The hub  64  has moved the same distance. This has forced linkages  62  to move out of cuts  115  in the grip body  60  and to expand outwardly in the radial direction. Further upward movement of the actuator rod  118  will cause the linkages  62  to extend even further out. This process of outward expansion can continue until the rod  118  exhausts its stroke or the spring  123  is compressed solid. 
     In the front cross-sectional view of the grip shown FIG. 9A, it is difficult to appreciate the amount of radial expansion that can be achieved by the grip. This is more clearly shown in FIG.  10 . FIG. 10A represents a top view of the grip in its fully open state. FIG. 10B, on the other hand, shows a cross section through the middle of the grip (denoted by  10 B— 10 B in FIG. 9A) when it is fully closed. FIG. 10A shows that the grip&#39;s radial dimensions can reach several times the envelope of the grip body  60 . FIG. 10A also presents a different view for the elements of the linkages  62  that were explained in FIGS. 3 and 4. Also note the three-lobe shape of the grip body  60 . This shape is required because the grip has to slide inside the rail section ( 18  in FIG.  1 ). The space  149  between the lobes and the circle  147  defined by the outlines of the grip body is occupied by the rails, on which the grip slides. FIG. 10B also shows how the cams  70 , wheels  72 , axles  74 , and the other elements located at the tips of the linkages  62  fit inside the grip body  60 . Note that when the linkages are fully closed the cams  70  meet at the centerline of the grip body  60 . The cross section in FIG. 10B also shows three of the oil and wire communication passages  107  that are machined into the grip body  60 . 
     The principle of operation of the embodiment of the invention that was shown in FIGS. 8-10 is explained in FIGS. 11A through 11C. This figure shows a simplified representation of the embodiment of the invention. The simplification is done for the sake of clarity when explaining the principle of operation. In FIG. 11, only one of the linkages  62  is shown because all linkages operate in a substantially identical manner. Similarly, only one of the rails of rail section  18  is shown. FIGS. 11A through 11C also depict the hydraulic communications between different sections of the grip. The numerical notations used in FIGS. 11A through 11C are the same as those in the figures explained earlier. 
     FIG. 11A shows the invention in its initial non-powered state. In this state, linkages  62  are fully closed into the grip body  60 . This state corresponds to the cross sectional view of the grip shown in FIG.  10 B. If the tractor tool is located in a horizontal section of a well, and if the grip is closed, the tractor tool body lies at the bottom of the well bore. Note that in FIG. 11A both solenoid valves  92  and  134  are not powered and open. Solenoid valve  134  allows hydraulic communication between chambers  101  of the high-pressure accumulator ( 100  in FIG. 8B) and  128  of the grip actuator ( 120  in FIG.  8 B). The other solenoid valve  92  and check valves  95 ,  97 ,  98 , and  99  allow communication between chamber  101 , the cocking piston chamber  180  and through push rod  28  the compensating section of the tool ( 22  in FIG.  1 ). Thus, all internal volumes of the grip are at the same pressure, which is equal to the pressure generated by the tractor tool compensator ( 22  in FIG.  1 ). In this situation, piston  102  is kept in its topmost position by spring  106  and piston  125  is pushed down by spring  123 . The hub  64  is also all the way down and the actuator rod  118  is fully retracted into the grip body  60 . Through piston  125 , actuator rod  118 , and hub  64 , spring  123  exerts closing force on linkages  62  and keeps them retracted into the grip body  60 . Thus, the linkages  62  do not extend beyond the outlines of the grip body  60 , which corresponds to the situation shown in FIG.  9 A. 
     FIG. 11B demonstrates one function of the grip, which is to centralize the tractor tool in the well bore. This centralization is achieved by pushing linkages  62  out of the grip body in the radial direction until they lift the tool off the well wall and position it at the center of the bore. This process begins by powering solenoid valve  92 , which is indicated by arrow  186 . Next, the grip ( 20  in FIG. 1) is pulled up by the linear actuator section ( 16  in FIG.  1 ). Initially, cocking piston  42  travels with the grip and is kept in its topmost position by cocking spring  182 . As the grip moves upwards, cocking piston  42  comes in contact with the end of the ball screw  36 , which prevents further upward motion of piston  42 . Since the motion of the grip  60  continues, the volume of chamber  180  in push rod  26  decreases. The pressure of the fluid trapped in this chamber increases, which is indicated by arrow  192 . The fluid used in the grip is substantially incompressible (in one embodiment, oil), hence, it forces its way out of the chamber. Since solenoid  92  is closed, the only possible way for the fluid to escape is through check valve  97  into chamber  101 . From chamber  101 , the high pressure fluid goes into passage  123  and through solenoid valve  134 , chamber  128 . The high pressure in chamber  101  pushes piston  102  down, compressing spring  106 . At the same time, the pressure in camber  128  pushes piston  125  up. The pressure exerted on piston  125  creates the axial force  190  designated by FA in the figure. The latter is transmitted through linkages  62  creating the radial centralizing force  152 , designated by F C  in FIGS. 6A,  6 B,  6 C,  7 A through  7 H,  11 A,  11 B, and  11 C. As the pressure in chamber  180  increases, the centralizing force F C  becomes high enough to overcome the weight of the tool and lifts the tool off the well wall. Due to the radial symmetry of linkages  62  (see FIG. 2) and due to the fact that they all are attached to the same hub  64 , the tool body moves towards the center of the well bore. When the tool is positioned at the center of the well bore, the pumping of fluid through rod  26  is stops. In this state, the grip  20  is ready to perform its function of a tool centralizer. Note, that although the grip  20  exerts radial forces that centralize the tool, the geometry of the linkages is not locked. This is demonstrated in FIG.  11 C. When the tool is pulled through a restriction by force F R    160 , linkages  62  must contract radially. This requires the hub  64 , actuator rod  118 , and piston  125  to move down. This reduces the volume of chamber  128  and fluid must flow out of it. This is possible because solenoid valve  134  is still open. Through passage  129  the extra fluid goes to chamber  101  pushing piston  102  down. Thus, the flexibility of the centralizer and the capability of the invention to adjust to changes in well bore size are ensured by the high-pressure accumulator ( 100  in FIG.  8 ). The processes just described are reversed if the grip moves from a smaller to a larger well bore. In this case fluid flows from the high-pressure accumulator (camber  101 ) to the grip actuator chamber  128 . Under all these circumstances, the grip continues to exert radial centralizing forces on the well wall. 
     The gripping function of the grip  20  is shown n FIG.  11 D. In this case, the drive rod exerts a pull force FP  166  in the upward direction, which is opposite to the direction of F R    160  in FIG.  11 C. The solenoid valve  134  is now energized and closed, which is indicated by the arrow  194 . By closing solenoid valve  134 , the only passage out of chamber  128  is blocked and the fluid inside chamber  128  becomes trapped. Due to force F P    166 , there is a tendency of the grip  20  to move upwards. This creates a friction force at the interface of the cam  70  and the well wall  150 , which tends to rotate the cam  70  in such a way as to enlarge the distance between the wall  150  and axle  74 . This process is the same as that described in FIG.  6 A. The tendency of axle  74  to move to the right requires that hub  64  moves down. However, the movement of hub  64  and hence piston  125  downward is prevented by the fluid that is trapped in chamber  128 . This makes the geometry of linkage  62  rigid, and prevents any further motion of axle  74 . As explained in FIG. 6A these are the conditions that cause the cam  70  to grip the well wall  150  and to become anchored in place. Since cams  70  and, therefore, grip  20  cannot move with respect to the well wall, the whole tool is pulled with respect to the anchored grip by force F P    166 . Anchored grip  20  and pulling of the whole tool with respect to the grip  20  are the events characteristic of the power stroke of the tool. 
     Finally, FIG. 11E describes the closing of linkages  62  back into the grip body  60  when power to solenoid valves  92  and  134  is shut off. In this case, both solenoid valves become open and fluid can flow freely through them. Spring  123  pushes piston  125  down, which results in closing linkages  62  into the grip body  60 . The fluid from chamber  128  flows through solenoid valve  134  and then through passage  129  to chamber  101 . In FIG. 11C, the fluid could not escape from chamber  101  because solenoid valve  92  was closed. Now solenoid valve  92  is open and the fluid from chamber  101  is pushed through it by spring  106 . Next, the fluid passes through check valves  98  and  99  to the cocking piston chamber  180  and through passage  107  and rod  28  to the compensator ( 22 _in FIG._ 1 ). At the end of this process, the grip returns back to the position shown in FIG.  11 A. 
     As indicated earlier, the hydraulic embodiment described in FIGS. 8-11 is only one possible construction of centralizing and locking devices. Another embodiment uses electromechanical devices as shown schematically in FIGS. 12A through 12C. One of the major elements of the electromechanical centralizing and locking devices is ball screw  200 , which is supported by bearings  202  and  218  in the grip body  60 . The ball screw  200  is powered by an electric motor  222 . A first ball nut  210  and second ball nut  214  travel on the ball screw  200 . The first ball nut  210  travels with hub  64 . The first ball nut  210  can rotate with respect to the hub on bearings  208 . The second ball nut  214  is attached to the carrier  216 , which prevents rotation, but allows axial displacement with respect to the grip body  60 . Other important elements are electromechanical brakes  206  and  220  and springs  204  and  212 . Brake  206  selectively locks ball nut  210  with respect to hub  64 . Brake  220  locks the ball screw  200  with respect to the grip body  60 . Spring  204  is the closing spring and its action is similar to spring  123  in FIG.  8 . Spring  212  provides the flexibility necessary for the centralization function of the invention and is functionally equivalent to spring  106  in FIG.  8 . 
     FIG. 12A shows the grip  20  in its non-powered state. The grip body  60  is in contact with the well wall  150 . Both hub  64  and ball nut  214  are pushed all the way down by springs  204  and  212 . FIG. 12A is functionally the same as FIG.  11 A. FIG. 12B shows the centralizing section of the grip  20 . The centralizing action begins by powering motor  222 , which turns ball screw  200 . Ball nut  214  is forced to travel upward until it reaches the position designated by “OPENING STROKE”  224  in FIG.  12 C. At this point, the motor  222  is turned off and brake  220  is activated. Brake  220  prevents ball screw  200  from rotating and, hence, keeps ball nut  214  in a fixed position. This action is equivalent to the action of the cocking piston in FIG.  11 B. Similarly, brake  220  performs the same function as solenoid valve  94  in FIG.  11 B. FIGS. 12B and 12C demonstrate the capability of the invention to accommodate changes in the well bore diameter. This is possible through the action of spring  212 , which either pushes hub  64  up in order to force linkages  64  further out or takes up the extra stroke when the grip goes through restrictions. In FIG. 12B and 12C, this is shown by the difference in displacements ΔS, designated by numbers  226  and  228 . 
     The other major function of the grip, the capability to grip the well wall is provided by linkages  62  and by the capability of the grip to lock the position of hub  64  with respect to the grip body  60 ; the locking is achieved by brake  206 . When activated, brake  206  prevents the rotation of ball nut  210  with respect to the ball screw  200 . Since ball screw  200  cannot rotate due to the action of brake  220 , the prevention of the rotation of ball nut  210  with respect to ball screw  200  is equivalent to locking the position of hub  64 . After the geometry is locked, the gripping action of the cams is the same as that described in FIGS. 6A,  6 B, and  6 C. 
     Having explained the centralizing and locking functions of a grip according to the invention, it is now possible to explain the tractoring action of the whole tool, of which the grip is an essential part. As explained in FIGS. 11A and 12A, when the tractor tool is not operational, the arms and the cams of the grip are retracted into the grip body. When the tool is first powered, the centralizing function of the grip is activated. The grip arms extend from the grip body and position the tool at the center of the well. At this stage, the grip has the flexibility of a conventional biased-arm centralizer. The linkages automatically open or close to follow any variation in well bore size. 
     To begin tractoring, the linear actuator ( 16  in FIG. 1) is activated. It starts reciprocating the grip with respect to the sonde body. If the tool has to tractor in the downhole direction, the radial position of the linkages  62  is kept unlocked during the downward stroke of the linear actuator and is locked during the upward stroke. During the downward stroke, the cams automatically orient themselves (see FIG. 7) in such a way that they can slide freely downhole and grip if an attempt is made to move them uphole. Thus, during the downward stroke the grip is easily pushed downhole by the linear actuator. During the upward stroke, the the radial position of the linkages  62  is locked and, as explained in FIG. 11D, the linkages  62  form a rigid body that keeps the axles of cams at fixed radial positions. The attempt to move the grip uphole creates frictional forces between the cam surfaces and the well wall. These forces tend to rotate the cams on their axles. Since the axles&#39; positions are fixed, the tendency of the cams to rotate creates very strong radial forces on the axles. These forces are passively reacted by the centralizer linkages and by the locking device. The high radial forces create sufficient frictional interaction between the grip and the well wall to anchor the grip in place. Thus, during the upward stroke, the grip is anchored to the well wall and the linear actuator pulls the rest of the tool with respect to the grip in the downward direction. At the end of the upward stroke, the the radial position of the linkages  62  is unlocked and the grip releases the well wall. The grip is free to be moved further downhole during the second downward stroke. The sequence of locking the the radial position of the linkages  62  during the upward stroke and unlocking it during the downward stroke is repeated, which results in an “inchworm-like” downward motion of the tractor tool. With the linear actuators of the two sondes moving in opposite directions, it is possible to convert the inchworm motion of each individual sonde into a continuous motion for the whole tool. 
     To reverse the tractor&#39;s direction of motion from downhole to uphole, it is only necessary to change the locking sequence of the grip solenoid valves in the hydraulic embodiment. If the grip is unlocked during the upward stroke and locked during the downward stroke, the whole tool will travel uphole. It is to be noted that during the first upward stroke, the cams automatically reorient themselves to grip in the proper direction, following the events shown in FIGS. 7A through 7H. 
     The tractoring is achieved by a “ratchet” action of the tractor. When moving in the downhole direction, there are two “strokes” that are combined to produce the motion. In the downward stroke, the grip is unlocked and moves downhole, while the rest of the device is stationary. In the upward stroke, the grip is locked and stationary relative to the hole, while the rest of the device is pulled downhole with the grip acting as an anchor to the hole wall. When moving in the uphole direction, the same two strokes are combined to produce the motion. In the downward stroke, the grip is locked and anchors to the hole wall, while the rest of the device moves uphole. In the upward stroke, the grip is unlocked and moves uphole, while the rest of the device remains stationary. In a first embodiment, there are two grips operating simultaneously in opposite cycles that allows one grip to always be anchored to the wall while the other grip is moving which allows for a simulated continuous movement of the device. In a second embodiment, one grip is provided that moves, and a secondary stationary grip is also provided. In this embodiment, when the movable grip is released and moved, the stationary grip is engaged to hold the device stationary relative to the wall of the hole. When the movable grip reaches the top of its stroke, the movable grip is anchored to the hole and the stationary grip is released so that the device can be pulled up or down the hole while the grip remains stationary. This provides a “inchworm-like” motion. 
     When tractoring is no longer needed, the linkages can be closed back into the grip body by the closing device. 
     While the invention has been described with respect to a limited number of embodiments, those skilled in the art, having benefit of this disclosure, will appreciate that other embodiments can be devised which do not depart from the scope of the invention as disclosed herein. Accordingly, the scope of the invention should be limited only by the attached claims.

Summary:
A linkage apparatus for selectively gripping and releasing the inside walls of a conduit, the apparatus comprising: a first arm; a bi-directional gripping cam rotatably attached to the arm; and an extension and locking device adapted to selectively radially extend the arm from a tool housing to an inside wall of a conduit and adapted to selectively lock the arm in an extended position.