Patent Publication Number: US-4549578-A

Title: Coded fluid control system

Description:
FIELD OF THE INVENTION 
     This invention relates generally to control systems for hydraulic and pneumatic elements. More particularly, this invention relates to hydraulic control systems for controlling the operation of a plurality of valves as, for example, in one or more subsea oil and gas wells. 
     BACKGROUND OF THE INVENTION 
     Some of the most significant problems in hydraulic and pneumatic control and power systems involve the transmission of fluid control signals and fluid power over appreciable distances. This is primarily the result of two inherent limitations of such systems relative to electrical control and power systems: the rate of fluid signal transmission is relatively low; and, the cross sectional area of the conduit or passage which transmits the fluid signal or working fluid is relatively great. These two factors are competing; the rate at which a conduit can transmit power and control signals improves as the cross sectional area is increased. 
     These constraints are not a major concern in many common applications of hydraulic and pneumatic control and power systems, such as in earth moving equipment, where fluid transmission distances are small and size and weight limitations are not a significant consideration. However, there are many applications in which these two limitations of fluid control and power systems impose significant economic and technical disadvantages. Perhaps the most challenging problems in this area are presented by hydraulic control systems for subsea control valves and other equipment situated at relatively great distances from a surface facility from which the valves are controlled. 
     In offshore oil and gas wells it is common to locate the plurality of control valves required for each well in a subsea tree situated at the seafloor. It is necessary to provide for control of these valves from a surface location, such as an offshore production platform. It has been found that hydraulic control systems are well suited for this purpose. 
     Existing hydraulic control systems for subsea wells fall into two basic classes, direct acting and indirect acting. In direct acting systems, a discrete hydraulic control line is provided for each hydraulic actuator. Application of hydraulic pressure to a selected control line serves to actuate the corresponding actuator. In the most basic of direct acting systems, a control line is connected directly to the valve actuator, with the hydraulic fluid in the control line serving as the working fluid to operate the subsea valve. In a refinement of the direct acting system, each control line operates a pilot valve. Actuation of the pilot valve connects the valve actuator to a separate source of pressurized working fluid which operates the valve actuator. Though simple and reliable, direct acting hydraulic control systems suffer the disadvantage of requiring a separate control line extending from the ocean surface to the submerged well for each control valve. Where there are a significant number of wells, a large number of control lines is required, yielding a control umbilical of large diameter. The cost of an umbilical having a great number of individual control lines can be substantial. Further, the use of a relatively large diameter umbilical poses especially great problems in deep water wells because of the high loading imposed on the umbilical, relative to a smaller diameter umbilical, in the course of installation. Also, providing means to resist the current and wave induced drag on the relatively large diameter umbilical can pose serious technical problems. Further, direct acting systems are often economically impractical for satellite wells which are located a great distance from the offshore platform from which they are controlled. Not only does the great length of the umbilical impose a significant expense, but anchoring the large diameter umbilical to restrain it from movement caused by current can also add significant cost. 
     In indirect acting hydraulic control systems, one or more hydraulic control lines are used to transmit coded signals to the wellhead. Typically, these control lines serve only to transmit signals and do not transmit working fluid for operation of the subsea valves. The coded signal transmitted by the control lines is received by a subsea switching valve. The switching valve addresses the subsea valve corresponding to the signal transmitted by the control lines. The use of coded signals avoids the need for a dedicated control line for each valve. This yields a decrease, relative to direct acting systems, in the number of hydraulic lines extending from the surface to the subsea well. 
     Pressure sequenced control is one of the most common types of indirect acting control systems. Indirect acting systems employ a number of pilot valves, each of which is adapted to operate in a set range of pressure levels. These pilot valves are connected in parallel to a single control line. By application of a selected pressure level to the control line, all of the pilot valves which are set to operate at that pressure level will operate. To ensure accurate operation of such systems, the pilot valve set points must be separated by about 2.8 MPa (400 psi). This limits the number of functions that can be controlled by any one control line. Details of one form of pressure sequenced control system are discussed in U.S. Pat. No. 3,993,100, issued Nov. 23, 1976. 
     A second type of indirect acting hydraulic control system which has been utilized in the control of subsea wells is disclosed in U.S. Pat. No. 4,356,841, issued Nov. 2, 1982. In this system, a control line extends from a surface control station to a subsea switching valve. The switching valve is adapted to assume a plurality of positions, in each of which it connects a corresponding one of a set of pilot valves to a source of fluid pressure. The switching valve operates through its sequence of positions in response to receiving pressure pulses via the control line. By applying a selected number of sequenced pulses to the control line, a corresponding valve actuator is operated. A disadvantage of this system is that where the length of the control line is great, as is often the case in subsea applications, an appreciable delay must be allowed following each pulse to ensure that the individual pulses remain discrete at the switching valve. This can impose significant delays in the operation of the subsea valves. This system is further disadvantageous in that the subsea valves must be operated in a set sequence. 
     It would be advantageous to provide a control system for subsea wells and other equipment requiring the control of multiple fluid actuated elements from a remote location in which only a relatively small number of control lines are required, which permits the fluid actuated elements to be operated in any desired sequence, and which is not dependent on the use of sequenced code signals. 
     SUMMARY OF THE INVENTION 
     In one aspect of the present invention, a selector valve is provided which is useful in the control of a system having a number of hydraulically or pneumatically actuated elements. The selector valve has an input pressure port and a plurality of outlet pressure ports. The input pressure port is adapted to receive pressurized control fluid. The hydraulically or pneumatically actuated elements are controlled by receipt of pressurized fluid through a control conduit associated with each such element. Each control conduit is selectively supplied with pressurized control fluid from a corresponding one of the outlet pressure ports. The selector valve also includes a plurality of code input ports. Each such code input port receives a fluid pressure signal of either a high or low pressure level. The static combination of the fluid pressure signals applied to each code input port at a given time establishes a binary select code. Each binary select code serves to designate to one or more of the outlet pressure ports. In response to receiving a given select code, the selector valve establishes fluid communication between the input pressure port and the one or more outlet pressure ports designated by the applied select code. 
     Many advantages of the present invention derive from the use of a binary select code in which the individual bits comprising the select code are transmitted simultaneously, each through a separate fluid conduit. A great number of fluid actuated elements can be addressed by a relatively small number of code transmission conduits. Further, because the various bits of the code are transmitted simultaneously, control is achieved quickly relative to other types of indirect acting fluid control systems. Also, the present invention does not require that the fluid actuated elements be addressed in any desired sequence. Further advantages will become evident upon reading the following detailed description and appended claims. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a better understanding of the present invention, reference may be had to accompanying drawings, in which: 
     FIG. 1 shows, in diagrammatic form, a basic embodiment of the present invention adapted for controlling four valves in a single well; 
     FIG. 2 shows, in diagrammatic form, a somewhat more sophisticated embodiment of the present invention in which multiple valves in each of a plurality of wells can be individually controlled; 
     FIG. 3 shows a side view of the selector valve, a portion of the gearbox housing being broken away to show details of the gear and spindle assembly of the selector valve; 
     FIG. 4 shows a top view of the linear actuator and gearbox with the housings of these elements cut away along a horizontal bisector of the actuator to reveal the inner components; 
     FIG. 5 shows an enlarged side view of the actuator endcap; 
     FIG. 6 shows an enlarged side view of the first actuator piston; 
     FIG. 7 shows a top view corresponding to FIGS. 5 and 6 with the actuator endcap and first piston in an assembled, fully retracted condition; and 
     FIG. 8 shows a side view in axial cross section of a multiple position valve suitable for use in the present invention. 
     These drawings are not intended as a definition of the invention but are provided solely for the purpose of illustrating preferred embodiments of the invention described below. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     A somewhat generalized embodiment of the present invention is diagrammatically illustrated in FIG. 1. The coded control system 10 of the present invention operates a plurality of fluid actuated elements 12a-d. As shown in FIG. 1, the fluid activated elements 12a-d are single acting, spring return hydraulic cylinders such as might be used as valve operators in the control of the various valves 13a-d of a subsea oil well. However, those skilled in the art will recognize that the present invention has a broad range of applications in the field of hydraulic and pneumatic control and is not limited solely to use in oil and gas producing operations. To the extent that the following description is specific to the control of subsea well equipment, this is by way of illustration rather than limitation. Further, though the present invention is equally applicable to hydraulic and pneumatic systems, for the purpose of simplicity in explanation, the following discussion will assume a hydraulic system. 
     The control system 10 includes four piloted control valves 14a-d, each corresponding to one of the fluid activated elements 12a-d. Preferably, the piloted control valves 14a-d are latching valves. Accordingly, upon activation of one of the control valves 14a-d by the application of a pilot signal, the corresponding one of the fluid activated elements 12a-d is maintained in the selected condition witout the need for maintaining the pilot signal. The state of the corresponding one of the fluid activated elements 12a-d can be changed only by application of the opposite pilot signal. This is especially advantageous in well control, where the state of certain control valves, for example, the surface controlled subsea safety valve, may remain unchanged for protracted periods. 
     A pressurized hydraulic fluid supply 16 is connected through a supply line 18 to each of the piloted control valves 14a-d. Similarly, a return line 20 connects the hydraulic return of each piloted control valve 14a-d to a fluid return reservoir 22. In FIGS. 1 and 2, all hydraulic return conduits are indicated as dashed lines. A supply accumulator 21 and a surge accumulator 23 are provided, respectively, for the supply and return lines 18,20. 
     Operation of the piloted control valves 14a-d is controlled by a selector valve 24. The selector valve 24 has eight output ports 26a-h, grouped in four pairs. Each of these pairs provides the two control functions, via corresponding control conduits 27a-h, for a corresponding one of the control valves 14a-d. Input pressure from the hydraulic pressure supply 16 is received by the selector valve 24 at a manifolded inlet pressure port 28. The selector valve 24 is also provided with a plurality of code input ports 30a-c. These ports 30a-c. These ports 30a-c receive the hydraulic signal (&#34;select code&#34;) used to control the selector valve 24. In response to receipt of a select code, the selector valve 24 adjusts itself to place the one or more of the outlet ports designated by the applied select code in fluid communication with the inlet pressure port 28. 
     In the preferred embodiment, a binary signal is provided to the code input ports 30a-c through code input lines 32a-c. Each binary digit (bit) of the select code is applied to a corresponding code input port 30a-c. In the embodiment shown in FIG. 1, a three bit binary select code is used, hence only three code input ports 30a-c arre provided. However, as will become apparent, in other applications a greater or lesser number of code input ports may be required. 
     The binary select code is based on two pressure states, a relatively low pressure state and a relatively high pressure state. In the preferred embodiment of the selector valve 24, there is a 0.5 MPa (70 psi) difference between the low and high pressure signals and the low pressure signal is maintained approximately equal to the ambient subsea pressure at the subsea well. In some applications, such as in designs in which it is important to minimize bubble formation in the control fluid, it may be desirable to establish the low pressure signal at a pressure significantly above ambient. Accordingly, it is to be understood that in the present description of the coded control system 10, the terms &#34;low pressure&#34; and &#34;high pressure&#34; are to be interpreted relative to one another and do not indicate absolute pressure. For example, the low and high pressure states in some embodiments could be, respectively, 10 MPa (1450 psi) and 13 MPa (1900 psi). 
     In the present invention, the select code is a binary signal transmitted by the pressure of the fluid in the code input lines 32a-c. A relatively high pressure signal represents one of the two binary digits (bits), while the low pressure signal represents the other of the bits. For example, if the low pressure signal is set at 1 MPa and the high pressure signal set at 4 MPa, a signal in lines 32c-a of 4 MPa, 1 MPa, 1 MPa corresponds to a binary code of 1-0-0 (as will be made clear subsequently, line 32a transmits the least significant bit). Each of the eight possible permutations of the three bit select code corresponds to a unique one of the eight output ports 26a-h. The selector valve 24 incorporates means, described below in conjunction with FIGS. 3-8, for establishing fluid communication between the inlet pressure port 28 and that one or more of the output pressure ports 26a-h corresponding to the applied select code. Further details of the construction and operation of the selector valve 24 are set forth subsequently in this description. 
     The use of separate, parallel signal paths for the transmission of the individual bits of the select code yields numerous advantages. This method permits rapid control of the subsea valves 13a-d, relative to serial bit transmission as would be required were only a single code transmission conduit utilized. This is especially important in the control of satellite wells, where signal transmission distances may be several thousand meters. Further, the use of a separate conduit for each bit of the select code simplifies monitoring of the applied select code. It is necessary only to monitor the instantaneous pressure of each code input line to know the applied select code and, hence, the position of the selector valve 24. This monitoring can be performed with pressure gauges or transducers (not shown) situated in the code input lines 32a-c positioned proximate the surface location at which the select code is generated. This avoids the need for subsea monitoring of the distribution valve, as is necessary in certain prior art systems. 
     Application of hydraulic pressure to the inlet pressure port 28 is controlled by an enable valve 36, situated in a hydraulic line 38 joining the inlet pressure port 28 and the pressure supply line 18. Preferably, the enable valve 36 is a single-acting, non-latching, spring returned control valve. Upon actuation by the application of pressure through an enable valve control line 40, the enable valve 36 places the selector valve input pressure port 28 in fluid communication with the pressurized hydraulic fluid source 16. Thus, upon actuation of the enable valve 36, hydraulic pressure is applied to that one of the control valve output ports 26a-h which corresponds to the then-existing select code. This serves to actuate the appropriate one of the control valves 14a-d, thereby achieving the control valve function selected by application of the select code. When fluid pressure is removed from the enable valve control line 40, the enable valve 36 assumes its non-actuated state. In the non-actuated state the selector valve inlet pressure port 28 is vented to the return line 20. The application of pressure to the enable valve control line 40 is controlled by an enable control 42, which may be a valve situated in a hydraulic line joining the hydraulic pressure supply 16 and the enable valve control line 40. 
     Means 44 are provided for applying the select code to the code input lines 32a-c. The select code applying means 44 includes a hydraulic binary encoder 46 which is supplied with pressurized fluid from the fluid pressure source 16. The binary encoder 46 applies pressurized hydraulic fluid to appropriate ones of the code input lines 32a-c to establish the desired select code. In the preferred embodiment, in which the &#34;low&#34; pressure signal is approximately equal to the ambient pressure, the binary encoder 46 can be as simple as a set of manually operated valves. Each of these manually operated valves corresponds to one of the code input lines 32a-c, and receives an input supply of pressurized fluid from the fluid pressure source 16. To operate a selected one of the fluid actuated elements 14, the operator would manually adjust each of the valves of the binary encoder 46 to the appropriate &#34;on&#34; or &#34;off&#34; position to deliver the select code corresponding to the desired function of the coded control system 10. 
     Alternatively, the binary encoder 46 can be controlled from a separate selector input control 48. The selector input control 48 includes a keyboard, rotary dial or other means of receiving an operator input regarding the desired control function. The selector input control 48 controls the valves of the binary encoder 46, causing them to adjust to the on-off positions necessary to yield the select code corresponding to the desired control function. Control of the binary encoder 46 by the selector input control 48 can be achieved in numerous manners well familiar to those skilled in the art of hydraulic control. 
     To operate the control system 10, the binary encoder 46 is adjusted to apply to the code input lines 32a-c that select code which corresponds to the desired function the selected one of the fluid actuated elements 12a-d. This causes the selector valve 24 to place the selector valve input port 28 in fluid communication with that one of the selector valve outlet ports 26a-h corresponding to the desired function. After the selector valve 24 has established the input port-outlet port fluid communication path corresponding to the applied selector code, the enable control 42 is actuated. This opens the enable valve 36, causing pressurized hydraulic fluid to be applied to the selector valve input port 28. Pressure is then applied through the selected one of the outlet ports 26a-h to the corresponding control line 27 of the appropriate one of the piloted control valves 14a-d. Because the piloted control valves 14a-d are all latched valves, the enable signal and the selector code need be applied only long enough to establish a latched condition. Once this has occurred, the signals applied to the selector valve 24 and the enable valve 36 can be removed. 
     As will be apparent to those skilled in the art, any of the selector valve outlet ports 26a-h could control a plurality of fluid actuated elements 12a-d, rather than just a single fluid actuated element as described above. In such an embodiment, a plurality of control conduits 27 would extend from one outlet port, each such conduit providing the control signal for a corresponding fluid actuated element 12. In this manner, the application of a single select code can be used to obtain multiple control functions in a subsea well system. 
     Further, as will become apparent in view of the subsequent discussion concerning the precise embodiment of the selector valve 24, simultaneous application of the constituent bits of the multi-bit select code is not critical to the operation of the present invention. The individual bits can be applied simultaneously or in any sequence. However, it is preferable that the enable control 42 not be actuated until each bit of the select code is applied. 
     The present control system 10 is well suited for applications in which the location at which the control signals are applied is spaced a significant distance from the fluid actuated elements 12a-d. In the preferred embodiment, the selector valve 24, enable valve 36, piloted control valves 14a-d and accumulators 21,23 are all positioned proximate the fluid actuated elements 12a-d controlled by the control system 10. The enable control 42 and the selector code applying means 44 are positioned at a remote operating station 52 which, in the case of a subsea well, could be located on an offshore platform or drill ship. The fluid lines 18, 20, 32a-c, 40 joining the remote operating station 52 and the subsea well control pod containing the fluid actuated elements 12a-d are routed through an umbilical 50. The umbilical 50 provides the necessary protection and support to the individual hydraulic lines. 
     The use of a binary code system in the present invention lends great economy to the number of hydraulic lines extending between the operating station and the subsea well. For the embodiment shown in FIG. 1, a total of six lines are required to provide all necessary control signals and working fluid supply/return for four well control valves 12a-d. Because the control system 10 is based on a binary select code, each added code input line 32 doubles the number of well control valves 12 which the control system 10 will accommodate. Thus, for the type of control system 10 shown in FIG. 1, a total of ten hydraulic lines extending from surface to a subsea well location can control and power sixty-four individual wellhead control valves 12. Further, the present invention does not require that any hydraulic line transmit more than a single signal to obtain any desired control function. Thus, there is no need for sequenced valve control signals. Accordingly, the fluid actuated elements 12a-d can be controlled rapidly and in any desired sequence. 
     Shown in FIG. 2 is a somewhat more sophisticated embodiment of the present invention, adapted for controlling the operations of a cluster of four subsea wells (not shown). The wells each have four fluid actuated elements for controlling various functions of the well. FIG. 2 depicts the fluid actuated elements as valve actuators for a series of subsea well valves 13a-d. Each fluid actuated element is controlled by a corresponding pilot valve. For the sake of simplicity, in FIG. 2 only the fluid actuated elements and pilot valves for well number 1 are shown, being represented, respectively, by the reference numerals 12a-d and 14a-d. It will be understood in the following discussion that corresponding components exist for each of the remaining three wells. It will be further understood that the number of wells controlled, and the number of fluid actuated elements for each well can differ from that shown in FIG. 2 by making changes to the control system 10&#39; of FIG. 2 which will become evident in view of the following discussion. 
     The pilot valves 14a-d of well number 1 are controlled by a function selector valve 24a. The function selector valve 24a is generally similar in operation to the selector valve 24 described for the embodiment shown in FIG. 1. However, the function selector valve 24a of the embodiment illustrated in FIG. 2 has dual input pressure ports 28a and 28b. Further, the output pressure ports 26a-h are divided into two sets, 26a-d and 26e-h. The function selector valve 24a receives a two-bit function select code input at code input ports 30a,b and hence can assume only four unique output conditions, each position corresponding to one of the four pilot control valves 14a-d. As will be described subsequently in greater detail, the function select code input to the function selector valve 24a of each well is applied through a common set of two input lines 32a,b. The flow paths intermediate the input and output pressure ports 28a,b and 26a-h are established such that each of the four possible permutations of the code input will place the first input port 28a in fluid communication with a unique one of the first set of output ports 26a-d  and will simultaneously place the second input port 28b in fluid communication with a unique one of the second set of output ports 26e-h. The first set of output ports 26a-d transmit the signals which cause the control valves 14a-d to activate the control elements 12a-d. The second set of output ports 26e-h transmit the corresponding deactivate signals. 
     Inputs to the input pressure ports 28a,b of the function selector valve 24a of each of the wells are controlled by a single well selector valve 60. The well selector valve 60 is preferably identical in construction to the function selector valve 24a. A well selector valve encoder 62 transmits a two-bit well select code through two well select code input lines 63a,b to well selector valve code input ports 64a,b. Each of the four possible permutations of the two-bit well select code corresponds to one of the four wells. In common with the function selector valve 24a, the well selector valve 60 has a first input pressure port 66a corresponding to a first set of four output pressure ports 68a-d and a second input pressure port 66b corresponds to a second set of four output pressure ports 68e-h. As indicated in FIG. 2, each port of the first set of well selector valve output pressure ports 68a-d is paired with a corresponding one of the second set of well selector valve output pressure ports 68e-h to form four well control signal pairs. Each such signal pair provides the control input to the input pressure ports 28a,b of a corresponding one of the four function selector valves 24a. For example, in FIG. 2 ports 68d,h provide the two inputs, respectively, to input ports 28a,b of the well number one function selector valve 24a. Similarly, ports 68c and g provide the two inputs to the input ports of the well number two function selector valve (not shown), etc. 
     As previously stated, each of the four wells has its own function selector valve 24a. The function code to each function selector valve 24a is received from means 44a for applying the function selector code, which is situated at the remote operating station. The function code is transmitted through code input lines 32a and b, the four function selector valves 24a each being connected in parallel to these lines 32a,b. Thus, the four function selector valves 24a-d operate in coordinated fashion, always addressing corresponding pilot control valves 14 for each of the wells. 
     The well selector valve input pressure ports 66a,b are connected to the hydraulic pressure supply 16 through separate enable valves 36a,b. Preferably the enable valves 36a,b are single acting, spring return pilot valves. These valves 36a,b are controlled by enable open and enable close controls 42a,b in the same manner as is used for the enable valve 36 of the embodiment shown in FIG. 1. 
     An emergency shut down (&#34;ESD&#34;) feature for each well is provided by piloted, spring return shut down valves 70a-d. Each of these shut down valves 70a-d is situated in series with and upstream of the hydraulic pressure supply line to the corresponding well. An emergency shut down control 72a-d for each well provides pilot pressure to the corresponding shut down valve 70a-d. By maintaining pilot pressure from each of the emergency shut down controls 72a-d to the corresponding shut down valve 70a-d, the hydraulic pressure input for each of the piloted control valves 14a-d remains in fluid communication with the hydraulic pressure supply 16. However, operating the emergency shut down control 72a-d for any well will serve to remove pilot pressure from the corresponding one of the shut down valves 70a-d. This causes the pressure supply line to all pilot control valves of the affected well to be vented to hydraulic return 22. This results in the fluid actuated elements 12 of that well assuming a de-energized (closed) position, shutting in the well. 
     Alternately, a single emergency shut down control (not shown) can be provided for all wells. In such an embodiment, a single shut down valve is placed in the hydraulic supply line 18 at a position upstream of the wells. This shut down valve is controlled by a single emergency shut down control. 
     Operation of the control system 10&#39; of FIG. 2 is similar to that of FIG. 1. The well selector valve encoder 62 is operated to apply the select code corresponding to the well to be controlled. The function selector valve encoder 44a is operated to apply the select code corresponding to the specific valve to be controlled. Following the application of the select codes, the enable open control 42a or enable close control 42b is actuated, depending on whether it is desired to open or close the selected valve. Once the selected valve assumes the desired state, all control signals may be removed. 
     A preferred embodiment of the selector valve 24 is shown in FIG. 3. The selector valve 24 of FIG. 3 is adapted to receive a four-bit code input at input ports 30a-30d and in response thereto to provide an establish fluid communication between a first input pressure port 28a and a selected one of sixteen first output pressure ports 26a-p, and between a second input pressure port 28b and a selected one of sixteen second output pressure ports 26&#39;a-p. It will be recognized that the selector valve 24 described below and shown in FIG. 3 can assume sixteen unique positions. Having dual sets of input-output pressure paths, the selector valve 24 of FIG. 3 can control up to 32 separate functions, enough for sixteen double acting control valves 14. This is a significantly greater number than is required by the basic control systems 10 of FIGS. 1 and 2. As will be readily apparent, selected ones of the input, output and code input ports 26, 28, 30 of the selector valve 24 can be plugged to adapt it for such simpler service. Alternately, the selector valve 24 can be scaled down or up in control capacity by altering the number of stages in the actuator and the type of shear seal valve employed, as will become apparent in view of the following discussion. 
     The selector valve 24 includes an actuator 80 and a flowpath control element 82. Preferably, the flowpath control element 82 is a multiple position valve adapted for establishing fluid communication between each of the input pressure ports 28a,b and a selected one of the plurality of output pressure ports 26a-p, 26&#39;a-p. The actuator 80 is adapted for receiving the select code and setting the flowpath control element 82 to the position necessary to achieve the corresponding input pressure port-output pressure port communication. The selector valve 24 includes means 84 for interfacing the actuator 80 and multiple position valve 82 such that each select code corresponds to a selected output port from each of the output port sets 26a-p, 26&#39;a-p. In the embodiment shown in FIG. 3, the actuator 80 is a four-bit, 16 position linear actuator; the multiple position valve 82 is a two port input--2×16 port output rotary shear seal valve; and the interfacing means 84 is a rack and gearbox assembly. 
     A preferred embodiment of the actuator 80 is shown in FIG. 4. The actuator 80 includes a generally cylindrical housing 86 containing four interconnected pistons 88a-d, each corresponding to one of the code input ports 30a-d. An endcap 90 of the actuator housing 86, best shown in FIG. 5, has a guide portion 92 projecting into the cylinder defined by the housing 86. The endcap guide portion 92 has an oval slot 94 extending transversely therethrough. 
     Referring to FIG. 6, the first piston 88a has a clevis portion 96a adapted to surround the endcap guide portion 92. A pin 98a extends across the clevis portion 96a through the oval slot 94 of the endcap guide portion 92. Thus, the longitudinal dimension of the endcap oval slot 94 defines the limits of extension and retraction of the first piston 88a within the actuator housing 86. The interface between the first piston 88a and the endcap 90 is detailed in FIG. 7, showing the first piston 88a in the fully retracted position. 
     The first piston 88a also has a guide portion 100a extending in a direction away from the clevis portion 96a. Intermediate the first piston clevis and guide portions 96a,100a is a central sealing portion 102a adapted for receiving O-rings or other sealing elements 104. The central sealing portion 102a establishes a radial sealed interface between the two ends of the first piston 88a. 
     As shown in FIG. 4, the second, third and fourth pistons 88b-d are generally similar in construction to the first piston 88a, each having a clevis portion with a pin extending through the oval slot 106 of the preceeding piston 88a-c. The last piston 88d need not have a guide portion. The last piston 88d serves as the output member of the actuator 80. 
     Each piston 88a-d is adapted for longitudinal motion in the actuator housing 86, mechanical restraint being applied only at the clevis portion 96--guide portion 100 interface between the pistons 88a-d and at the endcap 90--first piston 88a interface. The latter interface serves as the anchor point for the entire piston assembly. 
     As shown in FIG. 4, each of the code input ports 30a-d is associated with a corresponding one of the pistons 88a-d. Application of a pressure signal at the first code input port 30a causes the first piston 88a to move in a direction away from the endcap 90 until the first piston clevis pin 98a contacts that end of the endcap guide portion oval slot 94 nearest the first piston 88a. This position represents full extension of the first piston 88a. Similarly, application of a pressure signal to the second code input port 30b will cause the second piston 88b to move in a direction away from the first piston 88a until the second piston clevis pin contacts that end of the first piston oval slot 106a nearest the second piston 88b, this representing full extension of the second piston 88b. The third and fourth pistons 88c,d operate in the same manner. 
     The clevis portions 96, guide portions 100 and oval slot portions 106 of each piston 88a-d have longitudinal dimensions arranged such that maximum travel of the fourth piston 88d relative to the third piston 88c is twice that of the third piston 88c relative to the second piston 88b, etc. This is indicated in FIG. 4. 
     Means are provided for biasing the fourth piston 88d in a direction toward the endcap 90 in response to the absence of a high pressure code signal at port 30d. As shown in FIG. 4, a second generally cylindrical housing 110 is connected to the first housing 86 at that end of the first housing opposite the endcap 90. The two housings 86, 110 are axially aligned. The second housing 110 defines a cylinder 112 of smaller diameter than the first housing 96. A return piston 114 is positioned in the second housing cylinder 112 and adapted for axial movement therein. The return piston 114 and fourth piston 88d are mechanically linked by a gear rack 148. 
     In the operation of the fourth piston biasing means, the second housing cylinder 112 is maintained at a constant pressure equal to that of the high pressure signal used in the select code. This pressure is applied through a port 118 in an endcap 116 of the second housing 110. Accordingly, in the absence of a high pressure signal to any of the actuator pistons 88a-d, return piston 114 moves in direction toward the endcap 90, biasing each actuator piston 88a-d in the direction of the endcap 90. If, however, any single piston 88a-d is actuated by the application of pressure at its corresponding code input port 30a-d, that piston and any pistons relatively nearer the return piston 114 will move toward the return piston 114 by an amount equal to full travel of the activated actuator piston. The return piston 114 will always be displaced by activation of any of the actuator pistons because it has a smaller diameter than do the actuator pistons 88a-d, and hence will exert a smaller force in response to the application of equivalent hydraulic pressure. The actuator pistons nearer the endcap 90 than the activated actuator piston remain biased in the retracted position as a result of the pressure imbalance between the high pressure of the activated actuator piston and the low pressure of the non-activated actuator pistons. 
     The same relations hold for activation of a plurality of the actuator pistons 88a-d. Consider, for example, the application of a pressure signal only to code input ports 30d and 30b. The fourth actuator piston 88d will be fully extended from the third actuator piston 88c because the force acting on the fourth actuator piston 88d overcomes that acting on the return piston 114. In similar fashion, the second actuator piston 88b will be fully extended relative to the first actuator piston 88a. The first and third actuator pistons 88a,c will remain in their unextended state due to the force caused by the pressure imbalance return piston 114, which biases toward the endcap 90 all actuator cylinders to which a pressure signal is not applied. 
     As previously detailed, the ratios of independent travel of the first through fourth actuator pistons 88a-d is 1:2:4:8. Accordingly, the displacement of the actuator output member (the fourth actuator piston 88d) is controlled in a binary manner by code input ports 30a-d. Thus, the signal 1-1-1-1 (a &#34;1&#34; representing the high pressure signal and &#34;0&#34; representing the low pressure signal) at ports 30d-a causes the fourth piston 88d to travel 15 units, while 0-0-0-0 represents a total actuator travel of 0 units. The remaining 14 permutations of the 4-bit binary code provide the intermediate 14 actuator displacements. 
     The multiple position valve 82 is preferably a two port input--2×16 port output rotary shear seal valve, the construction and operation of which is generally familiar to those skilled in the art. A preferred embodiment of such a multiple position valve 82 adapted for use in the present invention is shown in FIG. 8. The multiple position valve 82 has three primary components: a rotor 120, the angular position of which is controlled by the actuator 80; a rotor housing 122 which contains the input pressure ports 28a,b; and, an output port housing 124. The rotor housing 122 and output port housing 124 form the valve body of the multiple position valve 82. The rotor 120 serves as a moveable passageway element establishing fluid communication between the input pressure ports 28a,b and selected ones of the outlet pressure ports 26a-h. 
     It will be appreciated that other embodiments of the multiple position valve 82 could also be employed. The multiple position valve 82 could, for example, be a linear valve. All that is required is that there be some form of valve block or housing assembly containing the input port(s) and output port, and a fluid passageway element moveable to establish different fluid communication paths from the input port(s) 28 to selected ones of the outlet ports 26. 
     The rotor 120 has a central shaft portion 126, a fluid distribution end portion 128, and a drive portion 130 at which the rotor 120 is driven by the actuator 80. Circumferential grooves 132a,b in the surface of the central shaft portion 126 are aligned, respectively, with the two input pressure ports 28a,b. Rotor conduits 134a,b extend along the central shaft portion 126 in a generally axial direction and serve to place the input pressure ports 28a,b in fluid communication with fluid distribution ports 136a,b in the fluid distribution end portion 128. The fluid distribution ports 136a,b are spaced substantially 180° apart on the rotor 120 and are positioned at different radial distances from the axis of rotation of the rotor 120. 
     Positioned intermediate the fluid distribution end portion 128 and the output port housing 124 is a rotor housing endcap 138. The endcap 138 has two sets of 16 axial passages 140, the sets being arranged to form two concentric circles. The radii of these circles correspond, respectively, to the radial offset of the two rotor fluid distribution ports 136a,b. Within each of the circles, the axial passages 140 are equiangularly spaced. When the rotor 120 is positioned such that the first fluid distribution port 136a is aligned with one endcap axial passage 140 from the first set, the second fluid distribution port 136b is aligned with a corresponding axial passage 140 from the second set. Each 22.5° rotation of the rotor 120 will bring the fluid distribution ports 136a,b into alignment with a different pair of axial passages 140. Shear seals 142 are positioned in the fluid distribution ports 136a,b to maintain a sealed fluid interface between the fluid distribution ports 136a,b and the axial passages 140 with which they are aligned. 
     The output port housing 124 is secured in fixed relationship to the rotor housing 122. Two sets of 16 output pressure conduits 144, 144&#39; in the output port housing 124 place each endcap axial passage 140 in fluid communication with a corresponding one of the two sets of 16 output pressure ports 26, 26&#39;. 
     As shown in FIGS. 3 and 4, the multiple position valve 82 and actuator 80 are joined one to the other at a gear housing 146. A gear rack 148 extends through the gear housing 146 between the return piston 114 and the fourth actuator piston 88d and serves to transmit to the fourth piston 88d the restoring force applied by the return piston 114. The multiple position valve 82 is oriented relative to the gear housing 146 such that the rotor portion 130 terminates adjacent the gear rack 148, with the longitudinal axis of the rotor 120 being perpendicular to the longitudinal axis of the gear rack 148. A gear 150 affixed to the rotor interface end portion 130 is driven by the gear rack 148. Accordingly, motion of the actuator output member (the fourth piston 88d) causes the gear rack 148 to drive the rotor 120. The gear pitches of the gear rack 148 and gear 150 are established such that each of the 15 incremental units of motion possible by the fourth actuator piston 88d drives the rotor 120 through substantially 22.5° 
     Prior to assembly of the actuator to the multiple position valve 82, the actuator 80 is set to a known position (e.q. 0-0-0-0; that is, no input to any of the actuator pistons 88a-d such that the actuator output member is in the fully retracted position). The multiple position valve 82 is adjusted to provide the input port 28-output port 26 fluid communication path corresponding to this actuator position. The multiple position valve 82 is then affixed to the actuator 82 maintaining this relationship. 
     The best known mode of practicing the present invention has been described above. However, it is to be understood that this description is illustrative only and that other means and techniques can be employed without departing from the scope of the invention as set forth in the appended claims.