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BACKGROUND 
       [0001]    Controlled steering or directional drilling techniques are commonly used in the oil, water, and gas industry to reach resources that are not located directly below a wellhead. The advantages of directional drilling are well known and include the ability to reach reservoirs where vertical access is difficult or not possible (e.g. where an oilfield is located under a city, a body of water, or a difficult to drill formation) and the ability to group multiple wellheads on a single platform (e.g. for offshore drilling). 
         [0002]    Directional drilling devices often utilize a plurality of steering devices arranged in a circle on the exterior surface of a drill string. These steering devices need to be cyclically actuated to achieve steering in desired direction. Conventional control systems for steering devices are unnecessarily complicated and often include a valve for each steering device (e.g., three valves are required to control three steering devices). Accordingly, there is a need for simpler control systems. 
       SUMMARY OF THE INVENTION 
       [0003]    Aspects of the invention provide control systems and methods for directional drilling. 
         [0004]    One aspect of the invention provides a system for controlling a first module, a second module, and a third module. The system includes: an inlet configured to receive fluid from a fluid source; a first double-stage valve; and a second double-stage valve. The first double-stage valve is actuatable to a first position wherein fluid from the inlet flows through the first double-stage valve to the second double-stage valve and a second position wherein fluid from the inlet flows through the first double-stage valve to the third module. The second double-stage valve is actuatable to a first position wherein fluid flows from the first double-stage valve to the first module and a second position wherein fluid flows from the first double-stage valve to the second module. 
         [0005]    This aspect can have several embodiments. In one embodiment, the first module, the second module, and the third module are bias pads. In another embodiment, the system is received within a drill string. The fluid source can be pressurized drilling fluid within the drill string. In another embodiment, the system can include an exhaust in communication with the first double-stage valve and the second double-stage valve. 
         [0006]    The first double-stage valve can include: a first stage in fluid communication with the inlet and in selective communication with the second double-stage valve; a second stage in fluid communication with the inlet and in selective communication with the third module; and a shaft received within the first double-stage valve. The shaft can include: a first valve body received within the first stage and a second valve body received within the second stage. 
         [0007]    During actuation of the first double-stage valve to the first position, the shaft can be positioned such that the first valve body is positioned to permit fluid communication between the inlet and the second double-stage valve and the second valve body is positioned to interrupt fluid communication between the inlet and the third module. 
         [0008]    During actuation of the first double-stage valve to the second position, the shaft can be positioned such that: the first valve body is positioned to interrupt fluid communication between the inlet and the second double-stage valve; and the second valve body is positioned to permit fluid communication between the inlet and the third module. 
         [0009]    The second double-stage valve can include: a first stage having a first chamber in fluid communication with the first double-stage valve and in selective communication with the first module; a second stage having a first chamber in fluid communication with the first double-stage valve and in selective communication with the second module; and a shaft received within the second double-stage valve. The shaft can include a first valve body received within the first stage and a second valve body received within the second stage. 
         [0010]    During actuation of the second double-stage valve to the first position, the shaft can be positioned such that the first valve body is positioned to permit fluid communication between the first double-stage valve and the first module and the second valve body is positioned to interrupt fluid communication between the first double-stage valve and the second module. 
         [0011]    During actuation of the second double-stage valve to the first position, the shaft can be positioned such that: the first valve body is positioned to interrupt fluid communication between the first double-stage valve and the first module and the second valve body is positioned to permit fluid communication between the first double-stage valve and the second module. 
         [0012]    In another embodiment, the first stage of the second double-stage valve further includes a second chamber in fluid communication with the inlet and in selective fluid communication with the first chamber of the first stage of the second double-stage valve; the second stage of the second double-stage valve further includes a second chamber in fluid communication with the inlet and in selective fluid communication with the first chamber of the second stage of the second double-stage valve; and the shaft further includes a third valve body received within the first chamber of the first stage of the second double-stage valve, a fourth valve body received within the second chamber of the first stage of the second double-stage valve, a fifth valve body received within the second chamber of the second stage of the second double-stage valve, and a sixth valve body received within the first chamber of the second stage of the second double-stage valve. 
         [0013]    During actuation of the second double-stage valve to the first position, the shaft can be positioned such that the third valve body is positioned to interrupt fluid communication between the second chamber of the first stage and the first chamber of the first stage and the fifth valve body is positioned to interrupt fluid communication between the second chamber of the second stage and the first chamber of the second stage. 
         [0014]    During actuation of the second double-stage valve to the second position, the shaft is positioned such that the fourth valve body is positioned to interrupt fluid communication between the second chamber of the first stage and the first chamber of the first stage and the sixth valve body is positioned to interrupt fluid communication between the second chamber of the second stage and the first chamber of the second stage. 
         [0015]    In another embodiment, the first double-stage valve can include: a first stage having a first chamber in fluid communication with the inlet, a second chamber in fluid communication with the second double-stage valve and in selective fluid communication with the first chamber, and a third chamber coupled in fluid communication with the exhaust and in selective fluid communication with the second chamber; a second stage having a first chamber in fluid communication with the inlet, a second chamber in fluid communication with the third module and in selective fluid communication with the first chamber, and a third chamber coupled in fluid communication with the exhaust and in selective fluid communication with the second chamber; and a shaft received within the first double-stage valve. The shaft can include: a first valve body received within the third chamber of the first stage; a second valve body received within the first chamber of the first stage; a third valve body received within the first chamber of the second stage; and a fourth valve body received within the third chamber of the second stage. 
         [0016]    During actuation of the first double-stage valve to the first position the shaft can be positioned such that: the first valve body is positioned to interrupt fluid communication between the third chamber of the first stage and the second chamber of the first stage; the second valve body is positioned to permit fluid communication between the first chamber of the first stage and the second chamber of the first stage; the third valve body is positioned to interrupt fluid communication between the first chamber of the second stage and the second chamber of the second stage; and the fourth valve body is positioned to permit fluid communication between the third chamber of the second stage and the third chamber of the second stage. 
         [0017]    During actuation of the first double-stage valve to the second position the shaft is positioned such that: the first valve body is positioned to permit fluid communication between the third chamber of the first stage and the second chamber of the first stage; the second valve body is positioned to interrupt fluid communication between the first chamber of the first stage and the second chamber of the first stage; the third valve body is positioned to permit fluid communication between the first chamber of the second stage and the second chamber of the second stage; and the fourth valve body is positioned to interrupt fluid communication between the third chamber of the second stage and the third chamber of the second stage. 
         [0018]    In another embodiment, the second double-stage valve includes: a first stage having a first chamber in fluid communication with the first double-stage valve, a second chamber in communication with the first module and in selective fluid communication with the first chamber, and a third chamber in fluid communication the exhaust and in selective fluid communication with the second chamber; a second stage having a first chamber in fluid communication with the first double-stage valve, a second chamber in communication with the second module and in selective fluid communication with the first chamber, and a third chamber in fluid communication the exhaust and in selective fluid communication with the second chamber; and a shaft received within the first double-stage valve. The shaft can include: a first valve body received within the third chamber of the first stage; a second valve body received within the first chamber of the first stage; a third valve body received within the first chamber of the second stage; and a fourth valve body received within the third chamber of the second stage. 
         [0019]    During actuation of the second double-stage valve to the first position, the shaft can be positioned such that: the first valve body is positioned to interrupt fluid communication between the second chamber of the first stage and the third chamber of the first stage; the second valve body is positioned to permit fluid communication between the first chamber of the first stage and the second chamber of the first stage; the third valve body is positioned to interrupt fluid communication between the first chamber of the second stage and the second chamber of the second stage; and the fourth valve body is positioned to permit fluid communication between the second chamber of the second stage and the third chamber of the second stage. 
         [0020]    During actuation of the second double-stage valve to the second position, the shaft can be positioned such that: the first valve body is positioned to permit fluid communication between the second chamber of the first stage and the third chamber of the first stage; the second valve body is positioned to interrupt fluid communication between the first chamber of the first stage and the second chamber of the first stage; the third valve body is positioned to permit fluid communication between the first chamber of the second stage and the second chamber of the second stage; and the fourth valve body is positioned to interrupt fluid communication between the second chamber of the second stage and the third chamber of the second stage. 
         [0021]    In another embodiment, the first stage of the second double-stage valve further includes a fourth chamber in communication with the inlet and in selective communication with the first chamber of the first stage of the second double-stage valve; the second stage of the second double-stage valve further includes a fourth chamber in communication with the inlet and in selective communication with the first chamber of the second stage of the second double-stage valve; and the shaft further includes a fifth valve body received within the first chamber of the first stage of the second double-stage valve, a sixth valve body received within the first chamber of the fourth stage of the second double-stage valve, a seventh valve body received within the fourth chamber of the second stage of the second double-stage valve, and an eighth valve body received within the first chamber of the second stage of the second double-stage valve. 
         [0022]    During actuation of the second double-stage valve to the first position, the shaft can be positioned such that the fifth valve body is positioned to interrupt fluid communication between the fourth chamber of the first stage and the first chamber of the first stage and the seventh valve body is positioned to interrupt fluid communication between the fourth chamber of the second stage and the first chamber of the second stage. 
         [0023]    During actuation of the second double-stage valve to the second position, the shaft can be positioned such that the sixth valve body is positioned to interrupt fluid communication between the fourth chamber of the first stage and the first chamber of the first stage and the eighth valve body is positioned to interrupt fluid communication between the fourth chamber of the second stage and the first chamber of the second stage. 
         [0024]    Another aspect of the invention provides a system for controlling a first module, a second module, a third module, and a fourth module. The system includes: an inlet coupled to a fluid source; a first double-stage valve; a second double-stage valve; and a third double-stage valve. The first double-stage valve is actuatable to: a first position wherein fluid from the inlet flows through the first double-stage valve to the second double-stage valves and a second position wherein fluid from the inlet flows through the first double-stage valve to the third double-stage valves. The second double-stage valve is actuatable to a first position wherein fluid from the first double-stage valve flows through the second double-stage valve to the first module and a second position wherein fluid from the first double-stage valve flows through the second double-stage valve to the second module. The third double-stage valve is actuatable to a first position wherein fluid from the first double-stage valve flows through the third double-stage valve to the third module and a second position wherein fluid from the first double-stage valve flows through the third double-stage valve to the fourth module. 
         [0025]    Another aspect of the invention provides a method for drilling a curved hole within a wellbore. The method includes: providing a drill string including a first steering module, a second steering module, a third steering module, an inlet configured to receive fluid from a fluid source, a first double-stage valve, and a second double-stage valve; rotating the drill string and actuating the first and second double-stage valves to permit fluid flow to the first module, second module, and third module to steer the drill string, thereby drilling a curved hole within a wellbore. The first double-stage valve can be actuatable to a first position wherein fluid from the inlet flows through the first double-stage valve to the second double-stage valve and a second position wherein fluid from the inlet flows through the first double-stage valve to the third module. The second double-stage valve can be actuatable to a first position wherein fluid flows from the first double-stage valve to the first module and a second position wherein fluid flows from the first double-stage valve to the second module. 
         [0026]    In one embodiment, fluid flows to the first module, second module, and third module in a cyclic pattern. 
     
    
     
       DESCRIPTION OF THE DRAWINGS 
         [0027]    For a fuller understanding of the nature and desired objects of the present invention, reference is made to the following detailed description taken in conjunction with the accompanying drawing figures wherein like reference characters denote corresponding parts throughout the several views and wherein: 
           [0028]      FIG. 1  illustrates a wellsite system in which the present invention can be employed; 
           [0029]      FIGS. 2A-2C  illustrates the structure and operation of a control system for selectively permitting flow from an inlet to a first module, a second module, and a third module according to one embodiment of the invention; 
           [0030]      FIG. 3  illustrates an embodiment of the invention without fourth chambers; 
           [0031]      FIG. 4  illustrates an embodiment of the invention that does not process exhaust from the modules; 
           [0032]      FIGS. 5A-5D  depict the structure and operation of a control system for selectively permitting flow from an inlet to a first module, a second module, a third module, and a fourth module according to one embodiment of the invention; and 
           [0033]      FIG. 6  depicts a method of directional drilling according to one embodiment of the invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0034]    Aspects of the invention provide control systems and methods for directional drilling. Various embodiments of the invention can be used in wellsite systems. 
       Wellsite System 
       [0035]      FIG. 1  illustrates a wellsite system in which the present invention can be employed. The wellsite can be onshore or offshore. In this exemplary system, a borehole  11  is formed in subsurface formations by rotary drilling in a manner that is well known. Embodiments of the invention can also use directional drilling, as will be described hereinafter. 
         [0036]    A drill string  12  is suspended within the borehole  11  and has a bottom hole assembly (BHA)  100  which includes a drill bit  105  at its lower end. The surface system includes platform and derrick assembly  10  positioned over the borehole  11 , the assembly  10  including a rotary table  16 , kelly  17 , hook  18  and rotary swivel  19 . The drill string  12  is rotated by the rotary table  16 , energized by means not shown, which engages the kelly  17  at the upper end of the drill string. The drill string  12  is suspended from a hook  18 , attached to a traveling block (also not shown), through the kelly  17  and a rotary swivel  19  which permits rotation of the drill string relative to the hook. As is well known, a top drive system could alternatively be used. 
         [0037]    In the example of this embodiment, the surface system further includes drilling fluid or mud  26  stored in a pit  27  formed at the well site. A pump  29  delivers the drilling fluid  26  to the interior of the drill string  12  via a port in the swivel  19 , causing the drilling fluid to flow downwardly through the drill string  12  as indicated by the directional arrow  8 . The drilling fluid exits the drill string  12  via ports in the drill bit  105 , and then circulates upwardly through the annulus region between the outside of the drill string and the wall of the borehole, as indicated by the directional arrows  9 . In this well known manner, the drilling fluid lubricates the drill bit  105  and carries formation cuttings up to the surface as it is returned to the pit  27  for recirculation. 
         [0038]    The bottom hole assembly  100  of the illustrated embodiment includes a logging-while-drilling (LWD) module  120 , a measuring-while-drilling (MWD) module  130 , a roto-steerable system and motor, and drill bit  105 . 
         [0039]    The LWD module  120  is housed in a special type of drill collar, as is known in the art, and can contain one or a plurality of known types of logging tools. It will also be understood that more than one LWD and/or MWD module can be employed, e.g. as represented at  120 A. (References, throughout, to a module at the position of  120  can alternatively mean a module at the position of  120 A as well.) The LWD module includes capabilities for measuring, processing, and storing information, as well as for communicating with the surface equipment. In the present embodiment, the LWD module includes a pressure measuring device. 
         [0040]    The MWD module  130  is also housed in a special type of drill collar, as is known in the art, and can contain one or more devices for measuring characteristics of the drill string and drill bit. The MWD tool further includes an apparatus (not shown) for generating electrical power to the downhole system. This may typically include a mud turbine generator (also known as a “mud motor”) powered by the flow of the drilling fluid, it being understood that other power and/or battery systems may be employed. In the present embodiment, the MWD module includes one or more of the following types of measuring devices: a weight-on-bit measuring device, a torque measuring device, a vibration measuring device, a shock measuring device, a stick slip measuring device, a direction measuring device, and an inclination measuring device. 
         [0041]    A particularly advantageous use of the system hereof is in conjunction with controlled steering or “directional drilling.” In this embodiment, a roto-steerable subsystem  150  ( FIG. 1 ) is provided. Directional drilling is the intentional deviation of the wellbore from the path it would naturally take. In other words, directional drilling is the steering of the drill string so that it travels in a desired direction. 
         [0042]    Directional drilling is, for example, advantageous in offshore drilling because it enables many wells to be drilled from a single platform. Directional drilling also enables horizontal drilling through a reservoir. Horizontal drilling enables a longer length of the wellbore to traverse the reservoir, which increases the production rate from the well. 
         [0043]    A directional drilling system may also be used in vertical drilling operation as well. Often the drill bit will veer off of a planned drilling trajectory because of the unpredictable nature of the formations being penetrated or the varying forces that the drill bit experiences. When such a deviation occurs, a directional drilling system may be used to put the drill bit back on course. 
         [0044]    A known method of directional drilling includes the use of a rotary steerable system (“RSS”). In an RSS, the drill string is rotated from the surface, and downhole devices cause the drill bit to drill in the desired direction. Rotating the drill string greatly reduces the occurrences of the drill string getting hung up or stuck during drilling. Rotary steerable drilling systems for drilling deviated boreholes into the earth may be generally classified as either “point-the-bit” systems or “push-the-bit” systems. 
         [0045]    In the point-the-bit system, the axis of rotation of the drill bit is deviated from the local axis of the bottom hole assembly in the general direction of the new hole. The hole is propagated in accordance with the customary three-point geometry defined by upper and lower stabilizer touch points and the drill bit. The angle of deviation of the drill bit axis coupled with a finite distance between the drill bit and lower stabilizer results in the non-collinear condition required for a curve to be generated. There are many ways in which this may be achieved including a fixed bend at a point in the bottom hole assembly close to the lower stabilizer or a flexure of the drill bit drive shaft distributed between the upper and lower stabilizer. In its idealized form, the drill bit is not required to cut sideways because the bit axis is continually rotated in the direction of the curved hole. Examples of point-the-bit type rotary steerable systems, and how they operate are described in U.S. Patent Application Publication Nos. 2002/0011359; 2001/0052428 and U.S. Pat. Nos. 6,394,193; 6,364,034; 6,244,361; 6,158,529; 6,092,610; and 5,113,953. 
         [0046]    In the push-the-bit rotary steerable system there is usually no specially identified mechanism to deviate the bit axis from the local bottom hole assembly axis; instead, the requisite non-collinear condition is achieved by causing either or both of the upper or lower stabilizers to apply an eccentric force or displacement in a direction that is preferentially orientated with respect to the direction of hole propagation. Again, there are many ways in which this may be achieved, including non-rotating (with respect to the hole) eccentric stabilizers (displacement based approaches) and eccentric actuators that apply force to the drill bit in the desired steering direction. Again, steering is achieved by creating non co-linearity between the drill bit and at least two other touch points. In its idealized form, the drill bit is required to cut side ways in order to generate a curved hole. Examples of push-the-bit type rotary steerable systems and how they operate are described in U.S. Pat. Nos. 5,265,682; 5,553,678; 5,803,185; 6,089,332; 5,695,015; 5,685,379; 5,706,905; 5,553,679; 5,673,763; 5,520,255; 5,603,385; 5,582,259; 5,778,992; and 5,971,085. 
       Control Devices for Three-Module Systems 
       [0047]    Referring now to  FIGS. 2A-2C , a control system  200  according to one embodiment of the invention for selectively permitting flow from an inlet  202  to a first module  204 , a second module  206 , and a third module  208  is depicted. Control system  200  includes a first double-stage valve  210  and a second double-stage valve  212 . The first double-stage valve  210  includes a first stage  214  and a second stage  216 . The second double-stage valve  212  includes a first stage  218  and a second stage  220 . 
         [0048]    The first stage  214  of the first double-stage valve  210  can include a first chamber  222 , a second chamber  224  in selective fluid communication with the first chamber  222 , and a third chamber  226  in selective fluid communication with the second chamber  224 . The second stage  216  of the first double-stage valve  210  includes a first chamber  228 , a second chamber  230  in selective fluid communication with the first chamber  228 , and a third chamber  232  in selective fluid communication with the second chamber  230 . 
         [0049]    The first double-stage valve  210  can include shaft  234  received within both stages  214 ,  216 . The shaft  234  can include a first valve body  236  received within the third chamber  226  of the first stage  214 , a second valve body  238  received within the first chamber  222  of the first stage  214 , a third valve body  240  received within the first chamber  228  of the second stage  216 , and a fourth valve body  242  received within the third chamber  232  of the second stage  216 . 
         [0050]    The first stage  218  of the second double-stage valve  212  can include a first chamber  244 , a second chamber  246  in selective fluid communication with the first chamber  244 , a third chamber  248  in selective fluid communication with the second chamber  246 , and a fourth chamber  250  in selective fluid communication with the first chamber  244 . The second stage  220  of the second double-stage valve  212  can include a first chamber  252 , a second chamber  254  in selective fluid communication with the first chamber  252 , a third chamber  256  in selective fluid communication with the second chamber  254 , and a fourth chamber  258  in selective fluid communication with the first chamber  252 . 
         [0051]    The second double-stage valve  212  can include shaft  260  received within both stages  218 ,  220 . The shaft  260  can include a first valve body  262  received within the third chamber  248  of the first stage  218 , a second valve body  264  received within the first chamber  244  of the first stage  218 , a third valve body  266  received within the first chamber  252  of the second stage  220 , a fourth valve body  268  received within the third chamber  256  of the second stage  220 , a fifth valve body  270  received within the first chamber  244  of the first stage  218 , a sixth valve body  272  received within the fourth chamber  250  of the first stage  218 , a seventh valve body  274  received within the fourth chamber  258  of the second stage  220 , and an eighth valve body  276  received within the first chamber  252  of the second stage  220 . 
         [0052]    Fourth chambers  250 ,  258  ensure that high pressure is maintained in first chambers  246 ,  252  when the first valve  210  is actuated to the second position, thereby ensuring fast actuation of first module  204  and second module  206 . Additionally or alternatively, fourth chambers  250 ,  258  could hold pressure-balance elements to seal the actuating device (not depicted) of valve  212  from the working fluid (e.g., mud) received from inlet  202 . In such an embodiment, the actuator could be filled with oil at a pressure substantially equal to the pressure within fourth chambers  250 ,  258 , thereby minimizing stress on sealing elements (e.g., bellows, rubber boots, and the like) between the actuator and the fourth chambers  250 ,  258 . 
         [0053]    In  FIG. 2A , both the first double-stage valve  210  and the second double-stage valve  212  are in first positions. Fluid flows from inlet  202  through the first chamber  222  and second chamber  224  of the first stage  214  of the first double-stage valve  210  to the first chamber  244  and the second chamber  246  of the first stage  218  of the second double-stage valve  212  to the first module  204 . Third module  208  is concurrently vented to exhaust  278 . 
         [0054]    In  FIG. 2B , both the first double-stage valve  210  is in the first position and the second double-stage valve  212  is the second position. Fluid flows from inlet  202  through the first chamber  222  and second chamber  224  of the first stage  214  of the first double-stage valve  210  to the first chamber  252  and the second chamber  254  of the second stage  220  of the second double-stage valve  212  to the second module  206 . First module  204  and third module  208  are concurrently vented to exhaust  278 . 
         [0055]    In  FIG. 2C , both the first double-stage valve  210  is in the second position and the second double-stage valve  212  is in the first position. Fluid flows from inlet  202  through the first chamber  228  and second chamber  230  of the second stage  216  of the first double-stage valve  210  to the third module  208 . First module  204  and second module  206  are concurrently vented to exhaust  278 . 
         [0056]    Valves  210 ,  212  can be actuated by a variety of devices. For example, a pinion can interface with a plurality of rack gear teeth on shafts  234 ,  260 . Alternatively, shafts  234 ,  260  can extend beyond the wall of valves  210 ,  212  and interface with an external actuator. A variety of valve actuators are described in publications such as T. Christopher Dickenson,  Valves, Piping  &amp;  Pipelines Handbook  138-45 ( 3   d  ed. 1999); and Peter Smith,  Valve Selection Handbook  (5th ed. 2004). 
         [0057]    The actuation of valves  210 ,  212  can be effected by a control device (not depicted) to maintain the proper angular position of the bottom hole assembly relative to the subsurface formation. In some embodiments, the control device is mounted on a bearing that allows the control device to rotate freely about the axis of the bottom hole assembly. The control device, according to some embodiments, contains sensory equipment such as a direction and inclination (D&amp;I) sensor, rotational speed sensor, accelerometers (e.g., three-axis accelerometers), and/or magnetometer sensors to detect the inclination and azimuth of the bottom hole assembly. The control device can further communicate with sensors disposed within elements of the bottom hole assembly such that said sensors can provide formation characteristics or drilling dynamics data to control unit. Formation characteristics can include information about adjacent geologic formation gather from ultrasound or nuclear imaging devices such as those discussed in U.S. Patent Publication No. 2007/0154341, the contents of which is hereby incorporated by reference herein. Drilling dynamics data may include measurements of the vibration, acceleration, velocity, and temperature of the bottom hole assembly. 
         [0058]    In some embodiments, control device is programmed above ground to following a desired inclination and direction. The progress of the bottom hole assembly can be measured using MWD systems and transmitted above-ground via a sequences of pulses in the drilling fluid, via an acoustic or wireless transmission method, or via a wired connection. If the desired path is changed, new instructions can be transmitted as required. Mud communication systems are described in U.S. Patent Publication No. 2006/0131030, herein incorporated by reference. Suitable systems are available under the POWERPULSE™ trademark from Schlumberger Technology Corporation of Sugar Land, Tex. 
         [0059]    Referring to  FIG. 3 , each stage  218 ,  220  of second double-stage valve  212  be fabricated without a fourth chamber  250 ,  258 . Such an embodiment can be advantageous due to the simpler valve design and because only a single valve type (i.e., a double-stage, six-chamber valve) is needed in inventory. (The elements in  FIG. 3  correspond to like-labeled elements in  FIG. 2  and the related description herein.) In such an embodiment, the actuator of the second valve  312  can be coupled with a dynamic oil compensator, which communicates with second chamber  324  of first valve  310 . 
         [0060]    Referring to  FIG. 4 , an embodiment of the invention  400  that does not process exhaust from modules  404 ,  406 ,  408  is provided. In such an embodiment, modules  404 ,  406 ,  408  can include an exhaust port from which exhaust can be vented. As will be appreciated from  FIG. 4 , chambers  422 ,  428 ,  444 ,  450 ,  458 , and  452  generally correspond to first chambers  222 ,  228 ,  244 ,  250 ,  258 , and  252 , respectively, in  FIG. 2 . Likewise, chambers  450  and  458  can be omitted as discussed above in the context of  FIG. 4 . 
       Control Devices for Four-Module Systems 
       [0061]    Referring now to  FIGS. 5A-5D , a control system  500  for selectively permitting flow from an inlet  502  to a first module  504 , a second module  506 , a third module  508 , and a fourth module  510  is depicted. System  500  includes a first valve  512 , a second valve  514 , and a third valve  516 . Valves  512 ,  514 ,  516  can be the same or similar to the valves described herein. 
         [0062]    For example, valve  512  can have chambers  518  and  520 . Shaft  522  can be received within valve  512  and can include valve body  524  received within chamber  518  and valve body  526  received within chamber  520 . 
         [0063]    Valve  514  can include chambers  528 ,  530 ,  532 , and  534 . Shaft  536  can be received within valve  514  and can include discs  538  and  540  received within chamber  528 , valve body  542  received within chamber  530 , valve body  544  received within chamber  532 , and discs  546  and  548  received within chamber  534 . 
         [0064]    Valve  516  can include chambers  550 ,  552 ,  554 , and  556 . Shaft  558  can be received within valve  516  and can include discs  560  and  562  received within chamber  550 , valve body  564  received within chamber  552 , valve body  560  received within chamber  554 , and discs  562  and  564  received within chamber  556 . 
         [0065]    In  FIG. 5A , valves  512  and  514  are both actuated to the first positions to permit flow to the first module  504 . In  FIG. 5B , valve  512  is actuated to the first position and valve  514  is actuated to the second position to permit fluid flow to the second module  506 . In  FIG. 5C , valve  512  is actuated to the second position and valve  516  is actuated to the first position to permit fluid flow to the third module  508 . In  FIG. 5D , valve  512  is actuated to the second position and valve  516  is actuated to the second position to permit fluid flow to the fourth module  510 . 
         [0066]    As will be appreciated by one of skill in the art, the principles of the invention can be applied to control systems having any number of modules. For example, system  500  could be modified to control five modules by placing additional valve in place of any of the modules  504 ,  506 ,  508 ,  510  and coupling two modules to the additional valve. 
         [0067]    Thus, to control n modules (n being an integer greater than 1), a system can be fabricated having n−1 valves. 
         [0000]    Integration within Drill Strings 
         [0068]    The systems described herein can be installed within drill strings, bottom hole assemblies, and the like. In such an embodiment, the inlet  202  can be in fluid communication with the interior of the drill string. The systems can be used to control any hydraulic or pneumatic devices such as bias pads, motors, and the like. 
       Methods of Directional Drilling 
       [0069]    Referring now to  FIG. 6 , a method of directional drilling  600  is provided. In step S 602 , a drill string is provided including a n steering modules, and n−1 valves. Exemplary arrangements of valves and steering modules are described herein. In step S 604 , the drill string is rotated. In step S 606 , the valves are actuated to control fluid flow to the steering modules. 
       INCORPORATION BY REFERENCE 
       [0070]    All patents, published patent applications, and other references disclosed herein are hereby expressly incorporated by reference in their entireties by reference. 
       EQUIVALENTS 
       [0071]    Those skilled in the art will recognize, or be able to ascertain using no more than routine experimentation, many equivalents of the specific embodiments of the invention described herein. Such equivalents are intended to be encompassed by the following claims.

Summary:
Aspects of the invention provide control systems and methods for directional drilling. One aspect of the invention provides a system for controlling a first module, a second module, and a third module. The system includes: an inlet configured to receive fluid from a fluid source; a first double-stage valve; and a second double-stage valve. The first double-stage valve is actuatable to a first position wherein fluid from the inlet flows through the first double-stage valve to the second double-stage valve and a second position wherein fluid from the inlet flows through the first double-stage valve to the third module. The second double-stage valve is actuatable to a first position wherein fluid flows from the first double-stage valve to the first module and a second position wherein fluid flows from the first double-stage valve to the second module.