Abstract:
A control valve and a method of controlling fluid flow include an input device which provides an input for moving a primary valve member an amount which is a function of the input, thereby opening flow pathways through the valve. The control valve is connected to a mechanical feedback mechanism which moves a feedback valve member an amount which is a function of the movement of a device to which the fluid flow is directed, such as a hydraulic actuator. Movement of the actuator to a desired position causes the second valve member to be moved to such a position that, in combination with the first valve member, the flow pathways through the valve are closed. The actuator is thereby moved to and maintained at the desired position without the need for the electronics feedback sensor used in prior art systems to sense actuator position.

Description:
This application claims priority from U.S. Provisional Application No. 60/135,204, filed May 21, 1999. 
    
    
     TECHNICAL FIELD 
     The invention relates to control valves for fluid power actuators and methods for controlling flow to such actuators. More particularly, the invention relates to control valves and methods for controlling flow that utilize feedback. 
     BACKGROUND OF THE INVENTION 
     In many circumstances it is desirable to control movement of a hydraulic actuator over a range of movement, for example by partially extending an actuator and holding it in place. Such partial extension may be accomplished by initiating hydraulic fluid flow to the actuator through a control valve, and by using information from an electronic sensor which senses the actuator position to determine when to shut off flow to the actuator. 
     However, electronic sensors are unsuitable for certain environments, such as where the actuator and the control valve will be subjected to high temperatures. Accordingly it will be appreciated that a means of accomplishing such partial actuation without use of electronic sensors would be desirable. 
     SUMMARY OF THE INVENTION 
     A control valve and a method of controlling fluid flow include an input device which provides an input for moving a primary valve member an amount which is a function of the input, thereby opening flow pathways through the valve. The control valve is connected to a mechanical feedback mechanism which moves a feedback valve member an amount which is a function of the movement of a device to which the fluid flow is directed, such as a hydraulic actuator. Movement of the actuator to a desired position causes the second valve member to be moved to such a position that, in combination with the first valve member, the flow pathways through the valve are closed. The actuator is thereby moved to and maintained at the desired position without the need for the electronic feedback sensor used in prior art systems to sense actuator position. 
     According to an aspect of the invention, a single-stage fluid flow cartridge control valve includes a cage having openings therethrough; a first valve member internally slidable within the cage; a second valve member internally slidable within the first valve member; and an input mechanism coupled to one of the valve members for moving the one of the valve members; wherein movement of the one of the valve members selectively opens fluid flow pathways between pairs of the openings, and movement of the other of the valve members selectively closes the fluid flow pathways. In a fluid actuator assembly, the other of the valve members is mechanically coupled to an actuator to which fluid is controllably supplied by the control valve. 
     According to another aspect of the invention, a fluid flow control valve includes a cage having openings therethrough; a first valve member internally slidable within the cage; a second valve member internally slidable within the first valve member, the second valve member having a bore therein and holes therethrough in communication with the bore; and an input mechanism coupled to one of the valve members for moving the one of the valve members; wherein movement of the one of the valve members selectively opens fluid flow pathways between pairs of the openings and movement of the other of the valve members selectively closes the fluid flow pathways, and wherein the holes and the bore are part of a fluid flow pathway between non-adjacent openings. Again, in a fluid actuator assembly, the other of the valve members is mechanically coupled to an actuator to which fluid is controllably supplied by the control valve. 
     According to a further aspect of the invention, a method of positioning a hydraulic actuator in response to an input signal includes opening flow pathways in a control valve by moving a main spool of the control valve a distance which is a function of the input signal; sending pressurized fluid to one side of the actuator, and draining fluid from the other side of the actuator, through the pathways; and closing the pathways after the actuator has reached a desired position by moving a feedback follower or spool which is mechanically coupled to the actuator. 
     According to a still further aspect of the invention, an actuator assembly includes an actuator for moving an external member, a control valve which controllably provides fluid to effect movement of the actuator, and a mechanical feedback device which provides actuator position feedback to the control valve. 
     In a preferred embodiment of the invention, the feedback valve member is internally slideable in and guided by a cage, while the primary or main valve member is internally slideable in the feedback valve member. This arrangement advantageously reduces or eliminates potential binding problems that might arise from side loads being applied to the feedback valve member by the feedback mechanism coupling the feedback valve member to the actuator. Further in accordance with a preferred embodiment, the input device or mechanism is an electric solenoid having the plunger thereof connected, preferably coaxially, to the primary or main valve member. 
     To the accomplishment of the foregoing and related ends, the invention comprises the features hereinafter fully described and particularly pointed out in the claims. The following description and the annexed drawings set forth in detail certain illustrative embodiments of the invention. These embodiments are indicative, however, of but a few of the various ways in which the principles of the invention may be employed. Other objects, advantages and novel features of the invention will become apparent from the following detailed description of the invention when considered in conjunction with the drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the annexed drawings: 
     FIG. 1 is a schematic illustration of an actuator assembly using a control valve with mechanical feedback in accordance with the present invention; 
     FIG. 2 is a cross-sectional view of the control valve of FIG. 1; 
     FIGS. 3A-3C are cross-sectional views showing different operational positions of the control valve, some parts of which have been removed or modified for clarity of illustration; 
     FIG. 4 is a cross-sectional view of another embodiment of control valve according to the present invention; 
     FIG. 5 is a cross-sectional view of yet another embodiment of control valve according to the present invention; 
     FIGS. 6A and 6B are an end view and a cross-sectional view, respectively, of an alternate embodiment plunger; and 
     FIG. 7 is a cross-sectional view of a further embodiment of the present, invention. 
    
    
     DETAILED DESCRIPTION 
     Referring now in detail and initially to FIG. 1, an actuator assembly according to the invention is indicated generally at  10 . The assembly  10  comprises a fluid power actuator  12 , a control valve  14  for selectively providing fluid pressure to move the actuator  12 , and a feedback mechanism  16  for providing feedback to the control valve  14  regarding the position of the actuator  12 . In the illustrated embodiment, the fluid power actuator  12  is a hydraulic actuator, but the principles of the invention may be applied to other fluid actuators, e.g., pneumatic actuators. The position of the actuator  12  is controlled by the control valve which preferably is a solenoid-type valve that receives electrical control inputs from electrical control circuitry (not shown). Accordingly, the control valve  14  has a valve portion  18  and a solenoid portion  20 . 
     The valve portion  18  of the control valve  14  fits into a manifold  22  which has a pressure port  23  for connection to a high pressure fluid supply and a return or drain port  24  for connection to a low pressure fluid return or drain. In an exemplary embodiment, the length of the portion of the control valve that is inserted into the manifold is approximately 2.5 inches. The manifold  22  also has connections for fluid lines  26  and  28  which run between the manifold  22  and opposite sides of a piston  30  of the actuator  12 . By connecting one of the fluid lines  26  and  28  to high pressure and the other of the lines to low pressure, the piston  30  is thereby moved (the fluid actuator is extended or retracted) to do useful work. 
     The feedback mechanism  16  provides mechanical feedback to the control valve  14  regarding the position of the piston  30 . The illustrated feedback mechanism  16  includes a rack  34  on a rod  36  which is connected to the piston  30 . A pinion  38  meshes with the rack  34  and thus translation of the rod  36  is converted to rotational motion of the pinion  38 . The pinion  38  is connected to an eccentric cam  40  which rotates along with the pinion. The eccentric cam  40  is in contact with the control valve  14 , so that rotation of the eccentric cam  40  causes displacement of a control valve contact surface  42  which is in contact therewith. 
     As explained in greater detail below, the control valve  14  receives an input signal which shifts internal parts of the control valve so as to provide high pressure fluid through one of the fluid lines  26 ,  28 , with the other of the fluid lines  26 ,  28  connected to return. Movement of the piston  30  moves other internal parts of the control valve  14  via the feedback mechanism  16 . After the piston  30  has moved a given amount, the given amount being a function of the input signal magnitude, the internal parts of the control valve  14  align so as to block further flow of fluid to the actuator  12 , thus stopping further movement of the piston within the actuator. 
     Details of the control valve  14  are shown in FIG.  2 . The solenoid portion  20  includes an input section  46  which receives an input such as an electrical signal. The input from the input section  46  is then used in energizing a coil  50  which is at least partially within a housing  52 . Preferably the current used to energize the coil  50  is a function of the strength of the input signal, and may be proportional to the input signal. For example, the input signal may be a variable current which is used to energize the coil  50 . 
     A tube  56  is located within the housing  52 , surrounded by the coil  50 . The tube  56  is held in a fixed position within the housing  52  using a tube flange  58  at one end of the tube which is pulled against an adapter  60  which is part of the housing  52 . This pulling is accomplished by means of a nut  62  which mates with an externally-threaded opposite end  64  of the tube  56 , the nut  62  being tightened against end plate  66  of the housing  52 . 
     A plunger  70  is slidable within the tube  56 . The plunger  70  has a conically-shaped end  72  which corresponds in shape to a conical interior surface  74  of the tube  56 . At the conically-shaped end  72  a stop  76  is coupled to the plunger  70 , the stop  76  fitting into a narrow plunger bore  78 . The stop  76  has a stop recess  80  at its distal end for receiving a spring  82 . The spring  82  pushes the stop  76  into and against the plunger  70 , and urges the plunger  70  rightward as shown in FIG.  2 . The spring force may be adjusted using an adjustment mechanism  84 , in which an externally-threaded adjuster  86  is positioned within a nut  88  to increase or decrease the compression of the spring  82 . 
     An O-ring  90  provides sealing between the adjustment mechanism  84  and the interior of the tube  56 . The O-ring is of a conventional design, and is made of conventional materials compatible with the fluid used and able to withstand the environment to which the control valve is to be exposed. For example, the O-ring material may and should be selected to be able to withstand temperature extremes to which the control valve will be subjected. 
     The plunger  70  is preferably made of a ferromagnetic material such as steel. Generally, the other parts of the control valve  14  are made out of steel, although it will be appreciated that other rigid metallic or non-metallic materials which are suitable for use may alternatively be employed. 
     Current in the coil  50  induces a magnetic field which pulls the plunger  70  against the force of the spring  72  (leftward in FIG.  2 ). As is preferred, the magnetic field, and thus the magnetic force on the plunger  70 , is linearly proportional to the current in the coil  50 . The spring force in the spring  72  is (to a first approximation) a linear function of the amount of compression. Therefore, beyond a certain minimum current in the coil  50  which is required to initiate movement of the plunger  70 , displacement of the plunger  70  increases linearly with increasing current in the solenoid. Those skilled in the art will appreciate that a non-linear response may be provided, if desired, by modifying the solenoid coil, plunger, and/or spring. 
     The stop  76  prevents the plunger  70  from coming into contact with the interior surface  74  of the tube  56 . Such contact can lead to latching, a magnetic coupling of the tube  56  and the plunger  70 . Further, the stop  76  has a stop bore  92  therethrough which allows free flow between the narrow plunger bore  78  and a gap  94  between the conically-shaped end  72  and the conical interior surface  74 . This equalizes pressure on both sides of the plunger  70  and prevents pressure changes in the gap  94  due to movement of the plunger  70 ; unequal pressures or pressure changes might affect the operating characteristics of the valve. 
     At its end  103  opposite the stop  76 , the plunger  70  has a plunger bore  102 . Fitted in the bore  102  is a narrow end  98  of a primary or main valve member  100 , the main valve member being a part of the valve portion  18 . The narrow end  98  is connected to the plunger  70  by a roll pin  104 . 
     As is preferred, the main valve member  100  is in the form of a main spool. The main spool  100  is internally slideable in a feedback valve sleeve or spool  106  which functions as a feedback valve member or follower of the illustrated control valve  14 . The feedback valve spool  106  is internally slideable in a cage  110  that is fixedly connected to the adapter  60 . The connection between the cage  110  and the adapter may include, for example, a threaded connection. An O-ring  112  provides sealing between the cage  110  and the adapter  60 . 
     It is noted here that the control valve  14  preferably is provided in the form of a cartridge that may be installed as a unit in the manifold  22  or other housing. Also, although not preferred, the solenoid portion  20  may be replaced by other input mechanisms suitable for moving the main valve member  100  of the valve portion  18  in response to a command prompt. 
     The cage  110  provides the connection between the control valve cartridge  14  and the manifold  22 . The cage  110  has series of holes  114   a - 114   d  corresponding to the locations of the passages  115   a - 115   d  in the manifold  22 . The passages  115   a - 115   d  are respectively connected to the ports  23 ,  24 ,  26 , and  28 . The holes  114  and associated annular grooves allow passage of fluid through the cage  110  as appropriate. Each of the series of holes  114  has one or more holes circumferentially spaced around the cage  110 . A hole  116  is used to provide pressure equalization on the plunger  70 , as will be explained further below. 
     The cage  110  has annular sealing ribs or protrusions  118  between adjacent pairs of the holes  114   a - 114   d . Each of the sealing ribs  118  has an O-ring seal to prevent fluid from passing directly from one passage in the manifold  22  to another. Additional sealing ribs  120  are provided in the cage  110  to prevent leakage of fluid outside of the manifold  22 . The sealing ribs  118  and  120  preferably have different diameters that correspond to stepped ledges in the manifold  22 . This “stepped” cage and manifold are used to avoid the risk that the O-rings of the sealing ribs  118  and  120  will be cut by the edges of the passages  115   a - 115   d  in the manifold  22 . 
     The cage  110  has a circumferential groove along its interior surface for holding a retaining ring  124  therein. Washers  126  are located on either side of the retaining ring  124 . The retaining ring  124  and the washers  126  provide a fixed stop that limits motion of the plunger  70 . In addition, the retaining ring  124  and the washers  126  fix the location of one end of a spring  130 , the other end of which presses on an end surface  132  of the feedback spool  106 . 
     The feedback spool  106  has a series of openings  134   a - 134   d  and associated annular grooves which communicate with respective of the holes  114   a - 114   d  in the cage  110 . The openings  134  are preferably somewhat longer than the holes  114  in order to maintain a fluid path between respective openings  134  and holes  114  as the feedback spool  106  axially moves relative to the cage  110 . The openings  134  may be, for example, a series of circumferentially-spaced holes about the feedback spool  106  at axial locations corresponding to the holes  114 . 
     An external sliding surface  136  of the feedback spool  106  fits closely against its counterpart internal surface  138  of the cage  110  to prevent flow between the feedback spool  106  and the cage  110 . A close fit between the surfaces  136  and  138  provides a sufficiently good seal to prevent external leakage or undesired internal flow between passages  115   a - 115   d  of the manifold  22 . The close fit also allows the cage to carry any side loads applied to the feedback spool that might otherwise cause cocking and possible binding of the feedback spool  106  or the main spool  100  which slides in the feedback spool. 
     The feedback spool  106  has a closed cam follower end  140  which protrudes from the remainder of the control valve  14 . The contact surface  42  of the closed end  140  is designed to contact the feedback mechanism  16  such as the eccentric cam  40  (FIG.  1 ). The contact surface preferably is flat but it will be appreciated that the contact surface may have a curved or other non-flat shape if desired. 
     The feedback spool  106  has attached thereto, at an annular groove, a retaining ring  144 . The retaining ring  144  has an outside diameter greater than the inside diameter of the cage  110 . This limits the travel of the feedback spool  106  and thereby limits the amount by which the closed end  140  protrudes from the remainder of the control valve  14 . 
     Still referring to FIG. 2, the main spool  100  is hollow, having a narrow (small diameter) spool bore  148  in its narrow spool end  98  and a wide spool bore  150  in its wide spool end  154 . The bores  148  and  150  are connected to each other and thus provide a passage for fluid to flow through the main spool  100 , as well as providing a passageway for fluid to flow between either end of the main spool  100  and spool holes  158  in the main spool  100 . 
     The holes  158  communicate with a passage  115   a  in the manifold  22  which is maintained at relatively constant pressure, such as at a system drain (return) pressure, via the openings  134   a  in the feedback spool  106  and the cage holes  114   a  in the cage  110 . Thus the gap  94  between the conically-shaped end  72  and the conical interior surface  74  is maintained at that same pressure, since the gap  94  and the spool holes  158  are linked via the stop bore  92 , the plunger bores  78  and  102 , and the spool bores  148  and  150 . The opposite end  103  of the plunger  70  is also maintained at the same pressure, since a volume  164  is communication with the opposite end  103  of the plunger  70  via central apertures in the retaining ring  124  and the washers  126 , and the volume  164  is also in communication with the passage  115   a  via the holes  116  in the cage  110 . Thus both sides of the plunger  70  are maintained at the same pressure, so that movement of the plunger does not cause pressure changes on one or both sides thereof that might affect the operating characteristics of the valve  14 , and further to pressure balance the plunger. 
     It will be appreciated that the valve may alternatively be configured for using any of the passages in the manifold as the source of the pressure for equalizing pressure on both sides of the plunger, and that the pressure source for the equalization need not provide constant pressure. 
     The main spool  100  has recessed regions (annular grooves)  166   a  and  166   b  and cover portions (annular lands)  170   a  and  170   b . The recessed regions  166   a  and  166   b , depending on the relative orientation of the main spool  100  and the cage  110 , can provide a flow pathway or passageway linking adjacent of the openings  134   a - 134   d  in the feedback spool  106 . The recessed regions  166   a  and  166   b  need not necessarily be recessed fully about the circumference of the main spool  100 , but may for example be grooves or channels in a region which is otherwise not recessed. 
     The cover portions  170   a  and  170   b  are sufficiently axially long enough to cover the respective openings  134   b  and  134   d  of the feedback spool  106 . Thus when the main spool  100  and the feedback spool  106  are positioned such that the cover portions  170   a  and  170   b  block flow through the openings  134   b  and  134   d , there is no flow of fluid to or from the actuator  12 , and the position of the actuator  12  is maintained. This no-flow condition is referred to as a “null” condition of the valve  14 . Such a null condition is the default condition when no input signal is applied to the control valve. A null condition also occurs when the cover portions  170   a  and  170   b  and the openings  134   b  and  134   d  are aligned due to displacement of the feedback spool  106  by the feedback mechanism  16  when the desired position of the piston  30  is achieved, as explained in greater detail below. 
     It will be appreciated that alternatively the control valve may provide flow when no current or other input is provided, rather than being in a null condition. 
     Preferably the cover portions  170   a  and  170   b  are only slightly larger than their respective openings  134   b  and  134   d . The greater the overlap between the cover portions  170   a  and  170   b  and the areas around the respective openings  134   b  and  134   d , the slower the response of the control valve  14  to an input signal. More overlap means more motion of the main spool  100  is required to initiate flow. 
     FIGS. 3A-3C illustrate operation of the fluid control valve cartridge  14 . In FIG. 3A the control valve  14  is shown with no current applied to solenoid portion  20 , and with the actuator  12  fully retracted. The valve  14  is in a null position, with cover portions  170   a  and  170   b  overlapping respective openings  134   b  and  134   d , and blocking flow through the control valve  14 . The actuator being fully retracted corresponds to the eccentric cam  40  oriented so that surface  42  of feedback spool  106  protrudes a maximum amount from the remainder of the valve  14 , with retaining ring  144  against its stop on cage  110 . 
     FIG. 3B shows the configuration of the control valve  14  when an input current has been applied and the actuator  12  is extending. The magnetic field produced by the current through the coil  50  causes plunger  70  to move leftward, further compressing spring  72 . The main spool  100  likewise moves to the left. This causes the cover portions  170   a  and  170   b  to move at least partially off of the openings  134   b  and  134   d , providing flow passageways within the valve  14  for fluid to flow to and from the actuator  12 . 
     Fluid from high pressure passage  115   c  in the manifold  22  flows through hole  114 c in the cage  110 , through openings  134 c in the feedback spool  106 , along recessed region  166   b  of the main spool  100 , through openings  134   b  and holes  114   b  to passage  115   b  which is linked to port of the actuator for extending the actuator. This path is indicated by arrows  174  in FIG.  3 B. 
     Fluid from the other port of the actuator enters passage  115   d  of the manifold, passes through holes  114   d  and openings  134   d  into bore  150  in open end  154  of the main spool  100 , along the bore  150  and through spool holes  158 , openings  134   a , and holes  114   a  into drain line (low pressure) passage  115   a . This path is indicated by arrows  176 . 
     In response to the movement of the actuator the eccentric cam  40 , part of the feedback mechanism  16 , rotates counterclockwise about an axis  180 . This rotation of the eccentric cam  40  pushes the feedback spool  106  leftward, thereby causing the cover portions  170   a  and  170   b  to gradually cover the openings  134   b  and  134   d . Eventually, when the actuator has reached the desired position, the movement of the feedback spool  106  by the feedback mechanism  16  causes the valve  14  to again reach a null condition, as shown in FIG.  3 C. 
     In FIG. 3C it is seen that the feedback spool  106  has moved leftward, with the retaining ring  144  off its stop on the cage  110 . The cover portions  170   a  and  170   b  fully cover the openings  134   b  and  134   d , preventing any further flow to or from the actuator, and locking the actuator in its desired position. 
     As the eccentric cam  40  rotates, friction forces between the cam  40  and the contact surface  42  of the feedback spool  106  will exert a lateral force on the feedback spool  106 . In addition rotation of the eccentric cam  40  causes a contact point  182  between the cam  40  and the contact surface  42  to move away from the centerline of the feedback spool  106 , which also leads to a lateral force on the feedback spool  106 . 
     Since the feedback spool  106  is between the main spool  100  and the cage  110 , these lateral forces do not tend to trap the main spool or cause it to bind, as might happen if the main spool was between the cage  110  and the feedback spool. However, it will be appreciated that the feedback spool might alternatively be slidable within the main spool, rather than vice versa, if the risk of binding or added wear was considered acceptable. 
     It will be appreciated that the actuator  12  may be retracted in whole or in part by reversing the steps outlined above. Making reference to the null extended condition of the valve  14  shown in FIG. 3C, reducing or removing the input current would cause the magnetic field produced by the coil  50  to be reduced or eliminated, which would cause the spring  82  to reposition the plunger  70  and the main spool  100  rightward, with the main spool  100  sliding within the feedback spool  106 . 
     Movement of the main spool  100  causes the cover portions  170   a  and  170   b  to move off of the openings  134   b  and  134   d , with the passages  115   a  and  115   b  connected together via a flow passageway which includes the recessed region  166   a , and the passages  115   c  and  115   d  connected together via a flow passageway which includes the recessed region  166   b.    
     As the actuator retracts the eccentric cam  40  rotates clockwise due to the action of the feedback mechanism  16 . This rotation of the eccentric cam  40  allows the feedback spool  106  to move rightward under the action of the spring  130 . This rightward movement of the feedback spool  106  causes the cover portions  170   a ,  170   b  to gradually cover the openings  134   b ,  134   d , at which point the actuator  12  has reached its desired position and the valve  14  is again in a null configuration, with no further flow to or from the actuator. 
     In an exemplary application, the above-described control valve may be used as part of a system for adjusting vanes of a turbocharger via a hydraulic actuator. Turbocharger temperatures can reach 1200° F., and an electronic feedback system for such an actuator would be unable to withstand the thermal environment created by close proximity to the turbocharger. 
     It will be appreciated that the embodiments described heretofore are merely exemplary, and that numerous variations that would occur to one skilled in the art are embraced by the invention. For example, numerous parts are described above as involving narrower and wider portions and/or bores, but it will be appreciated that relative widths of the portions and/or the bores may be reversed or otherwise altered. 
     Further, it will be appreciated that many variations of the configuration of the ports in the manifold are possible, although it is preferable that the pressure/drain passages alternate with the passages for the hydraulic lines to the actuator. 
     While the embodiments described above have been generally related to a control valve for a hydraulic actuator, a control valve of the present invention may also be usable with a pneumatic system for delivering a pressurized gas in order to do work. 
     The invention may be used with a wide variety of work-performing devices in place of the actuator described above, as long as the work-performing device is able to provide movement that can be used for the feedback mechanism. 
     The feedback mechanism may include a wide variety of mechanical couplings and/or linkages, for instance belts, pulleys, levers, many varieties of gears, etc. The feedback mechanism may have a linear or nonlinear feedback between movement of the actuator or other device and movement of the feedback follower. The feedback mechanism may provide feedback which moves the cam follower substantially the same distance that the actuator moves. 
     A mechanical input device may be substituted for the solenoid portion, if desired, with the design altered as necessary. 
     It will be understood that a variety of known resilient biasing devices may be used in place of the coil springs shown in the illustrated embodiments. 
     What follows below are descriptions of some alternate embodiment cartridge control valves of the present invention, description of some similar features being omitted below for the sake of brevity. 
     FIG. 4 shows a control valve  214  which has a solenoid portion  215  with a housing  218  which has a folded portion  220  for holding a washer  222  in place at one end. The solenoid portion  215  also has a tube  226  which is crimped onto a pole piece  228 , with an O-ring  230  sealing the connection between the tube  226  and the pole piece  228 . This collection of parts substitutes for the tube  56  of the control valve  14 . 
     Plunger  240  has a T-shaped slot  242  for receiving a T-shaped protrusion  244  on one end of a main spool  250 . The plunger  240  has a central bore  254  therethrough, the bore  254  being in communication with the slot  242 . A pin  258  is located in the bore  254 . A spring  260  between the pin  258  and the protrusion  244  provides biasing for the location of the plunger  240  and the main spool  250 . 
     Referring to FIG. 5, an alternate embodiment cartridge control valve  414  has a cam follower  420  which slides within a main spool or sleeve  422 . A spring  430  between the cam follower  420  and the main spool or sleeve  422  provides a force which biases the cam follower  420  to protrude from the remainder of the control valve  414 . 
     FIGS. 6A and 6B show an alternate embodiment plunger  470  which has grooves  472  in an axial direction along its external surface. The grooves  472  allow the pressures on both sides of the plunger  470  to be maintained equal without the necessity of boring a hole or otherwise providing a flow passage through the plunger. 
     Referring to FIG. 7, a feedback control system  610  is shown in which a fluid actuator  612  has an integral feedback member  614  directly in contact with a contact surface  618  of a control valve  620 , the control valve  620  being a valve of the type described above. The actuator  612  and the control valve  620  may both be housed in a manifold  624 , with fluid connections between the actuator  612  and the control valve  620  being passages  626  and  628  in the manifold  624 . The manifold has a vent  630  which is in communication with a volume  632  in which the feedback member  614  and the control valve  620  meet. 
     An input signal to the control valve  620  causes the passages  626  and  628  to be connected to pressure and drain (return) passages  640  and  642  in the manifold  624  such that pressure is applied to extend or retract the actuator  612 . Movement of the actuator  612  causes movement of the feedback member  614 , which in turn moves the contact surface  618  which is part of a feedback follower or spool. In a manner similar to that described above in connection with FIGS. 3A-3C, the control valve  620  reaches a null state when the desired actuator position is reached. 
     It will be appreciated that the feedback member may alternatively be a separate part that is attached or otherwise connected to the fluid actuator. It will further be appreciated that the actuator and the control valve may be housed in different manifolds, or that fluid lines may used in connecting the actuator and the control valve, if desired. 
     Although the invention has been shown and described with respect to a certain preferred embodiment or embodiments, it is obvious that equivalent alterations and modifications will occur to others skilled in the art upon the reading and understanding of this specification and the annexed drawings. In particular regard to the various functions performed by the above described elements (components, assemblies, devices, compositions, etc.), the terms (including a reference to a “means”) used to describe such elements are intended to correspond, unless otherwise indicated, to any element which performs the specified function of the described element (i.e., that is functionally equivalent), even though not structurally equivalent to the disclosed structure which performs the function in the herein illustrated exemplary embodiment or embodiments of the invention. In addition, while a particular feature of the invention may have been described above with respect to only one or more of several illustrated embodiments, such feature may be combined with one or more other features of the other embodiments, as may be desired and advantageous for any given or particular application.