Method of controlling a fluid-powered actuator

Abstract: A method of controlling the velocity of a fluid-powered actuator (73) having one member (90) movable relative to another member (91) and separating opposing chambers (92 and 93), comprises the steps of causing a supply flow of fluid from a source (Ps) thereof to the expanding chamber (e.g. 92) whenever it is desired to cause relative movement between said members, causing a bypass flow (99) of fluid from the contracting chamber (93) to the expanding chamber (92) only when a load acting on said one member (90) aids the desired direction of relative movement between said members, summing said supply and bypass flows in the expanding chamber, and controlling the magnitude of such summed flows.

Description: This invention relates to the operation of servo mechanism for connection with a source of pressurized fluid at a supply pressure. In particular, the invention relates to a method of controlling the velocity of a fluid-powered actuator having one member movable relative to another member and separating opposing chambers. Many types of servo valves have, of course, been developed heretofore. These servo valves have typically included a spool slidably mounted within the bore of a body for controlling the flow of fluid between supply and return openings and one or more control openings. An electro-mechanical driver, such as a torque motor, was operatively arranged to move the spool relative to the body for creating a desired pressure at a control opening, or for creating a desired differential pressure between two control openings. The magnitude of such spool displacement was proportional to an error signal, which was the algebraic sum of a command signal (reflecting the desired spool position) and a negative feedback signal (reflecting the actual spool position). The polarity of the error signal determined the direction of spool movement relative to the associated body. Such a servo valve was typically associated with a conventional double-acting fluid-powered cylinder. The servo valve was used to control the flow of fluid to and from the opposing actuator chambers. If it was desired to move the actuator rod against an "opposing" load, the servo valve was operated such that fluid could flow from the fluid source to the expanding actuator chamber, and could flow from the contracting actuator chamber to return. However, if the actuator rod was moved in the same direction as an "aiding" load, the pressure in the actuator contracting chamber would be greater than the pressure in the actuator expanding chamber. The conventional servo valve controlled the position of the actuator rod by metering the flow of fluid from the contracting chamber. At the same time, however, fresh fluid from the source was admitted to the expanding chamber. This arrangement was inefficient because fresh pressurized fluid did no useful work in controlling displacement of the actuator rod when the applied load was aiding". US-A-4080872 discloses a hydraulic system in which the velocity of a double acting fluid pressure actuator having one member movable relative to another and separating opposing chambers is controlled by causing a supply flow of fluid from a source thereof to the expanding chamber whenever it is desired to cause relative movement between said members, and causing a bypass flow of fluid from the contracting chamber to the expanding chamber to increase the velocity of the actuator when moving in one direction in a no load condition. In many applications, it is sometimes desired to move an actuator rod in the same direction as an "aiding" load. For example, if a servo mechanism is used to move an airfoil surface, the load on the actuator rod may be "opposing" if it is desired to move the load in one direction, but "aiding" if it is desired to move the load in the opposite direction. Similarly, in an active vehicle suspension system, the load may be "opposing" on a bound stroke, but "aiding" on a rebound stoke, or vice versa. The problem to be solved by the present invention is to provide a method of operation which will render a servo mechanism more energy efficient than existing servo mechanisms when the load is "aiding" the actuator displacement. According to the present invention this problem is overcome by the provision of a method of controlling the velocity of a fluid-powered actuator having one member movable relative to another member and separating opposing chambers, wherein a supply flow of fluid is caused to flow from a source thereof to the expanding chamber whenever it is desired to cause relative movement between said members, and a bypass flow of fluid can be caused to flow from the contracting chamber to the expanding chamber, characterised by the steps of sensing the polarity of a load acting on said one member with respect to a desired direction of relative movement between said members, causing said bypass flow of fluid from the contracting chamber to the expanding chamber whenever and only when said load acting on said one member aids the desired direction of relative movement between said members, summing said supply and bypass flows in the expanding chamber, and controlling the magnitude of such summed flows. One example of an improved valve enabling the invention to be carried out is shown in Figures 3a-3c. The improved valve (eg 119) is associated with a source of fluid and includes a body (eg 120,136) provided with a bore (eg 138). The body has first and second supply openings (eg sleeve slots 140,141) and first and second return openings (eg sleeve slots 139,142) joining the bore. Each of the supply openings communicates with the source. A valve spool (eg 121) is mounted in the bore for longitudinal sliding movement relative thereto. The spool has first, second and third lobes, (eg 143,144,145) arranged such that when the spool is in a null position relative to the body, the first lobe (eg 143) will cover the first return opening (eg 139), the second lobe (eg 144) will cover the first and second supply openings (eg 140,141), and the third lobe (eg 145) will cover the second return opening (eg 142). The spool is movable off this null position in either axial direction to communicate a first space between the first and second lobes with one of the first return and supply openings (eg 139,140) and to communicate a second space between the second and third lobes with the opposite one of the second supply and return openings (eg 141,142). A driver (eg 122) is operatively arranged to move the spool to a desired position relative to the body. A first passageway (eg 132 or 163) continuously communicates one of the first supply and return openings with the first space at all operative positions between the spool and body. This first passageway has a first check valve (eg 133 or 164) operatively associated therewith to permit only unidirectional fluid flow through the first passageway. A second passageway (eg 134 or 165) continuously communicates the like one of the second supply and return openings with the second space at all operative positions between the spool and body. This second passageway also has a second check valve (eg 135 or 166) operatively associated therewith to permit only unidirectional fluid flow through the second passageway. When the spool is moved to an off-null position, if the pressure in one of the spaces is greater than the pressure in the other of the spaces, fluid may flow from such other space to return, but if the pressure in such other space is greater than the pressure in such one space, fluid may flow from such other space to such one space. Another example of an improved valve enabling the invention to be carried out is shown in Figure 4. Again, the valve (eg 172) is adapted to be associated with a source of pressurized fluid, and includes a body (eg 173,174) provided with a bore (eg 189). The body has first and second supply openings (eg sleeve slots 192,193), first and second bypass openings (eg sleeve slots 191,194), and first and second return openings (eg 190,195). Each of the supply openings communicates with the fluid source. The valve also includes a valve spool (eg 175) mounted in the bore (eg 189) for longitudinal sliding movement relative thereto. The spool has first, second, third, fourth and fifth lobes (eg 196,198,199,200,201). These lobes are arranged such that when the valve is in a null position relative to the body, the first lobe (eg 196 will cover the first return opening (eg 190), the second lobe (eg 198) will cover the first bypass opening (eg 191), the third lobe (eg 199) will cover the first and second supply openings (eg 192,193), the fourth lobe (eg 200) will cover the second bypass opening (eg 194), and the fifth lobe (eg 201) will cover the second return opening (eg 195). The spool has a first annular space between the first and second lobes, a second annular space between the second and third lobes, a third annular space between the third and fourth lobes, and a fourth annular space between the fourth and fifth lobes. The spool is movable off-null position in either direction to selectively communicate one of the first and fourth spaces with an uncovered one of the return openings, to communicate one of the second and third spaces with an uncovered one of the supply openings, and to communicate the other of the second and third spaces with an uncovered one of the bypass openings. A driver (eg 176) is operatively arranged to move the spool to a desired position relative to the body. A first passageway (eg 212,209,211) continuously communicates the first bypass opening (eg 191) with the third and fourth spaces at all operative positions between the spool and body. This first passageway has a check valve (eg 213) operatively arranged to permit flow between the third and fourth spaces, but to prevent flow from the first bypass opening. A second passageway (eg 214,208,210) continuously communicates the second bypass opening (eg 194) with the first and second spaces at all operative positions between the spool and body. The second passageway has a check valve (eg 215) operatively arranged to permit flow between the first and second spaces but to prevent flow from the second bypass opening. If the pressure in one of the second and third spaces exceeds the supply pressure when the spool is off-null, fluid may flow from one of the second and third spaces to the other of the second and third spaces through one of the first and second passageways. In any of these embodiments, an optional throttling valve may be used to cut off a flow to return and/or a flow from supply, when the load is "aiding". Moreover, such throttling valve may perform the function of check valves. Another embodiment of the invention is shown in Figures 8 - 11, and provides an improved servo mechanism which is adapted to be associated with a fluid source and a fluid return. Such improved servo mechanism includes a fluid-powered actuator (eg 123,178 or 308) having opposing first and second chambers (eg 149,150,204,205 or 313,314); an electrohydraulic servo valve (eg 119,172A or 309) operatively arranged to control the flow of fluid with respect to the two actuator chambers; and a throttling valve (eg 216A,216B,216C or 310) operatively arranged between the source and return and the servo valve. When an external load opposes the desired direction of actuator movement, the servo valve may be selectively operated so as to communicate the source with the higher pressure actuator chamber and to communicate the lower pressure actuator chamber with the return. However, when an external actuator load aids the desired direction of actuator movement, fluid in the higher pressure actuator chamber will be constrained to flow through the servo valve to the lower pressure chamber, while the flow from supply and/or to return is cut off. Various servo mechanisms embodying the invention will now be described, by way of example, with reference to the accompanying diagrammatic drawings: Fig. 1a is a schematic view of a prior art energy-conserving servo mechanism which had a four-way valve and a two-way valve operating on a mutually-exclusive basis; Fig. 1b is a block diagram of the servo mechanism shown in Fig. 1a; Fig. 2a is a schematic view of servo mechanism embodying the invention and having two three-way electrohydraulic servo valves operable in continuous cooperation with one another to control the displacement of an actuator rod, and showing the displaced condition of the valve spools when it is desired to move an "opposing" load; Fig. 2b is a schematic view of the improved servo mechanism shown in Fig. 2a, but showing the displaced condition of the valve spools when it is desired to move the actuator rod in the same direction as an "aiding" load; Fig. 2c is a block diagram of the servo mechanism shown in Figs. 2a and 2b; Fig. 2d is a schematic view of a modified form of the servo mechanism shown in Figs. 2a and 2b, wherein the opposing actuator chambers communicate through the common fluid return; Fig. 3a is a schematic view of a first form of an improved electrohydraulic servo valve for use in controlling an actuator rod, this view showing the bypass conduits as communicating the sleeve supply slots with the spaces between the spool lobes; Fig. 3b is a block diagram of the improved valve shown in Fig. 3a; Fig. 3c is a modified form of the improved valve shown in Fig. 3a, this view showing the bypass conduits as being provided between the sleeve return slots and the spaces between the spool lobes; Fig. 4 is a schematic view of another form of an improved valve, which may be operatively employed to control the position of an actuator rod when either an "aiding" or "opposing" load is applied thereto; Fig. 5 is a schematic view of a spring-centered throttling valve, which may be used in association with the improved valve shown in Fig. 3a, this view showing the throttling valve spool in a centered position relative to its body; Fig. 6 is a schematic view of the throttling valve shown in Fig. 5, but showing how this valve may be associated with the improved valve shown in Fig. 4 to cut off the flow to return; Fig. 7 is a schematic view of a modified throttling valve which incorporates check valve functions; Fig. 8 is a schematic view of the modified throttling valve shown in Fig. 7, in association with the servo valve and actuator shown in Fig. 3a, to cut off the flow to return in the case of an "aiding" load; Fig. 9 is a schematic view of a further modified throttling valve in association with the servo valve and actuator shown in Fig. 3c, to cut off the flow from supply in the case of an "aiding" load; Fig. 10 is a schematic view of a modified form of the servo valve shown in Fig. 4 in association with a further modified throttling valve, which functions to completely cut off fluid flow from the supply and to the return in the case of an "aiding" load; and Fig. 11 is a schematic view of yet another form of an improved servo mechanism which functions to cut off completely fluid flow from the source and to the return in the case of an "aiding" load. At the outset, it should be clearly understood that like reference numerals are intended to identify the same structural elements, portions or surfaces consistently throughout the several drawing figures, as such elements, portions or surfaces may be further described or explained by the entire written specification, of which this detailed description is an integral part. As used in the following description, the terms "horizontal", "vertical", "left", "right", "up" and "down", as well as adjectival and adverbial derivatives thereof (eg "horizontally", "rightwardly", "upwardly", etc), simply refer to the orientation of the illustrated structure as the particular drawing figure faces the reader. Similarly, the terms "inwardly" and "outwardly" refer to the orientation of a surface relative to its axis of elongation. Fig. 1a shows a form of an energy-conserving servo mechanism, generally indicated at 10, which is presently believed to either be individually "old" or "obvious" over the prior art. This servo mechanism broadly included a four-way electro hydraulic servo valve 11, a two-way electrohydraulic servo valve 12, and a double-acting fluidpowered actuator 13. Valve 11 has a body 14 provided with a horizontally-elongated bore 15. Three axially-spaced annular grooves extended radially into the body from bore surface 15. The leftward groove 16 communicates with a fluid return, such as a sump (not shown), at a return pressure R via a conduit 18, the middle groove 19 communicates with a source (not shown) of pressurized fluid at a supply pressure P s via a conduit 20, and the rightward groove 21 communicates with the return via a conduit 22. A tubular sleeve 23 is mounted fast within the body bore, and has an inwardly-facing cylindrical surface or bore 24. This sleeve is provided with four circumferentially-spaced radial through-slots at each of four axially-spaced locations therealong. The first slots, one of which is indicated at 25, are aligned with body groove 16; the second slots, one of which is indicated at 26, registers with body groove 19; the third slots, one of which is indicated at 28, also registers with body groove 19; and the fourth slots, one of which is indicated at 29, are aligned with body groove 21. While only those slots which are arranged at the 6:00 o'clock positions have been numbered in Fig. 1a, it will be understood that the other slots of each circumferentially-spaced group are located at the 9:00 o'clock positions (not shown), at the 12:00 o'clock positions, and at the 3:00 o'clock positions (which are seen in phantom elevation and cross-hatched for clarity). It should also be understood that the body and sleeve are formed separately only for manufacturing convenience, and are subsequently assembled together. Hence, the sleeve should be regarded as integral with the body. The valve also has a three-lobed valve spool 30 mounted within the sleeve for horizontal sliding movement therealong. When the spool is in a null position relative to the body (as shown in Fig. 1a), the spool left lobe 31 is arranged to cover the sleeve left return slots 25, the spool middle lobe 32 is arranged to cover the sleeve left and right supply slots 26,28, and the spool right lobe 33 is arranged to cover the sleeve right return slots 29. An electro-mechanical driver, indicated at 34, is arranged to selectively displace the spool either leftwardly or rightwardly, as desired from the null position to a desired position relative to the body. The magnitude of such spool displacement is proportional to the magnitude of the error signal supplied to driver 34, while the direction of spool movement is governed by the polarity of the error signal. As the spool begins to move relative to the sleeve from such null position, the partially-uncovered sleeve slots would form ports or orifices through which fluid can pass. Thus, if the spool is displaced leftwardly off null, the space between lobes 31,32 communicate with left return slots 25 through the uncovered left return ports, while the space between lobes 32,33 communicates with right supply slots 28 through the uncovered right supply ports. On the other hand, if the spool is displaced rightwardly off null, the space between lobes 31,32 communicates with left supply slots 26 through the uncovered left supply ports, while the space between lobes 32,33 communicates with right return slots 29 through the uncovered right return ports. These various sleeve supply and return ports are all identical in exposed area, at any operative position of the spool relative to the sleeve, and therefore have the same gain. Hence, the size of the orifices defined by these supply and return ports is simply proportional to the magnitude of the spool displacement from the null position. Because of this, the pressure drops across the supply and return ports are the same at any displaced spool position. The two-way servo valve 12 also has a body 35 provided with a horizontally-elongated bore 36. An annular groove 38 extends radially into the body from bore surface 36. A tubular sleeve 39 is mounted fast within the body bore, and has an inwardly-facing cylindrical surface 40. This sleeve is provided with four circumferentially-spaced radial through-slots at each of two axially-spaced locations therealong. The leftward sleeve slots, one of which is indicated at 41, as well as the rightward sleeve slots, one of which is indicated at 42, are both aligned with body groove 38. While only those slots which are arranged in the 6:00 o'clock positions have been numbered in Fig. 1a, it will be understood that the other slots of each circumferentially-spaced group are located at the 9:00 o'clock positions (not shown), at the 12:00 o'clock positions, and the 3:00 o'clock positions (which are seen in phantom elevation and cross-hatched for clarity). Here again, the body and sleeve are formed separately only for manufacturing convenience, and are subsequently assembled together. Servo valve 12 also has a three-lobed valve spool 43 mounted within sleeve 39 for longitudinal sliding movement therealong. The spool has left, middle and right lobes, 44,45,46, respectively. An electro-mechanical driver, generally indicated at 48, is arranged to selectively displace spool 43 either leftwardly or rightwardly, as desired, from the illustrated null position to a desired position relative to the body. If spool 43 is in a null position relative to the body (as shown in Fig. 1a), the spool middle lobe 45 is arranged to cover each of sleeve left and right slots 41,42. However, when the spool begins to move relative to the sleeve from such null position, the partially uncovered slots form ports or orifices through which fluid can pass. Thus, if the spool is displaced leftwardly off null, right sleeve slots 42 would communicate with the space between lobes 45,46. Alternatively, if the spool is displaced rightwardly off null, left sleeve slots 41 communicate with the space between lobes 44,45. These various sleeve slots 41,42 are identically configured, and the size of the ports or orifices defined by the partially-uncovered slots is again simply proportional to the magnitude of such spool displacement from the null position, which, in turn, is proportional to the magnitude of the error signal supplied to driver 48. The direction of spool movement is again dictated by the polarity of the error signal. The actuator 13 is a conventional double-acting fluid-powered actuator and had a piston 49 slidably mounted within a cylinder 50. A rod 51 is fixed to the piston and sealingly penetrates both ends of the cylinder. The piston subdivides the cylinder into a left chamber 52 and a right chamber 53. A conduit 54 communicates the space between first valve lobes 31,32 with actuator left chamber 52, while a conduit 55 communicates the space between first valve lobes 32,33 with actuator right chamber 53. Conduit 56 communicates conduit 54 with second valve body groove 38, and conduit 58 communicates conduit 55 with the spaces between second valve lobes 44,45 and 45,46. The pressure differential between conduits 54 and 55 is sensed by a pressure sensor 59. Fig. 1b is a block diagram of the servo mechanism schematically shown in Fig. 1a. An electrical command signal, reflective of the desired position of actuator rod 51, is supplied to a summing point 60, which also receives a negative feedback signal from a feedback transducer 61, reflective of the actual position of the actuator rod. The algebraic sum of the command and feedback signals is supplied as an error signal (⁺e) to a polarity sensor 62, to a multiplier 63, and to a multiplier 64. Sensor 62 supplies the polarity of the error signal to a logic unit 65, which also receives from polarity sensor 66, the polarity of the differential pressure in conduits 54,55, as sensed by pressure sensor 59. By comparing the polarities of the signals supplied by polarity sensors 62,66, the logic unit can determine whether the load exerted on actuator rod 51 is "opposing" or "aiding" with respect to the direction of desired rod movement. For example, if it is desired to move actuator rod 51 rightwardly, polarity sensor 62 determines the polarity of the error signal and supplies such information to the logic unit. If, at the same time, the force exerted on the actuator rod is opposite to such desired direction of rod movement, an "opposing" load is sensed by the pressure in conduit 54 being greater than the pressure in conduit 55. On the other hand, if the force exerted on the actuator rod is in the same direction as the desired direction of rod mdovement, an "aiding" load is sensed by the pressure in conduit 55 being greater than the pressure in conduit 54. The polarity of the sensed pressure differential is supplied to the logic unit by polarity sensor 66. Thus, logic unit 65 could determine whether the load is "opposing" or "aiding". If the load is "opposing", logic unit 65 causes multiplier 63 to produce a multiplicand of "1", and causes multiplier 64 to produce a multiplicand of "0". The product of the error signal (±e), the unit multiplicand (1) provided by multiplier 63, and the gain (K₁₁) of servoamplifier 68, is supplied to the driver of valve 11, which displaced the spool relative to the body and causes the actuator 11, which displaced the spool relative to the body and causes the actuator rod 51 to move in the appropriate direction until the error signal is reduced to zero. At the same time, logic unit 65 causes multiplier 64 to produce a "0" multiplicand. The product of the error signal (±e) the zero multiplicand (0) provided by multiplier 64, and the gain (K₁₂) of servoamplifier 69, is zero. Hence, the spool of valve 12 would return to its null position, blocking communication between conduits 56,58. Thus, when an "opposing" load is sensed, active flow control to and from the actuator is provided by four-way valve 11, while two-way valve 12 is returned to null and therefore rendered inactive. On the other hand, if the logic unit sensed that the load is "aiding", it causes multiplier 63 to produce a multiplicand of "0", while multiplier 64 is caused to provide a multiplicand of "1". Since the produce of the error signal (±e), the zero multiplicand (0) produced by multiplier 63, and the gain (K₁₁) of servoamplifier 68, is zero, valve 11 returns to its null position, However, the product of the error signal (±e), the unit multiplicand (1) provided by multiplier 64, and the gain (K₁₂) of servoamplifier 69, is some positive or negative value, and valve 12 operates to control fluid flow to and from the expanding and contracting chambers of the actuator. Thus, if an "aiding" load is sensed, active flow control from and to the actuator is provided by two-way valve 12, while four-way valve 11 is returned to null and therefore rendered inactive. In this regard, it should be noted that in the case of an "aiding" load, fluid from the higher pressure actuator chamber flows through the two-way valve to the lower pressure actuator chamber. The significance of the foregoing is that while the combination of a four-way servo valve and a two-way servo valve provides operable control over the actuator rod, the four-way valve provides flow control only when the load is "opposing", and the two-way valve becomes active only when the load is "aiding". In effect, the operation of the two valves is mutually-exclusive. Referring now to Fig. 2a, an improved servo mechanism embodying the invention, generally indicated at 70, is shown as broadly including a leftward first valve 71, a rightward second valve 72, and a conventional double-acting fluid-powered actuator 73. Each of valves 71,72 is a three-=way electrohydraulic servo valve. Since these two valves are structurally identical to one another, albeit the second valve is arranged as a mirror image of the first, only the first valve will be specifically described. However, the corresponding parts, portions or surfaces of the second valve are indicated by the prime of the same reference numeral used to describe such parts, portions or surfaces with respect to the first valve. Valve 71 has a body 74 provided with a horizontally-elongated bore 75. Two axially-spaced annular grooves extend radially into the body from bore surface 75. The leftward groove 76 communicates with a fluid return (not shown) at a return pressure R via a conduit 78. The rightward groove 79 communicates with a sump or other source (not shown) of pressurized fluid at a supply pressure P s via conduit 80 and a check valve (not shown). A tubular sleeve 81 is mounted fast within the body bore, and has an inwardly-facing cylindrical surface or bore 82. This sleeve is provided with four circumferentially-spaced radial through-slots at each of two axially-spaced locations therealong. The leftward first slots, one of which is indicated at 83, are aligned with leftward body groove 76, while the rightward second slots, one of which is indicated at 84, are aligned with the rightward body groove 79. While only those slots which are arranged in the 6:00 o'clock positions have been numbered, it will be understood that the other slots of each circumferentially-spaced group are arranged at the 9:00 o'clock positions (not shown), at the 12:00 o'clock positions, and at the 3:00 o'clock positions (which are again seen in phantom elevation and cross-hatched for clarity). Here again, the body and sleeve are formed separately only for manufacturing convenience and are subsequently assembled together. Hence, for all practical purposes, the sleeve should be regarded as a part of the body. Valve 71 also has a two-lobed valve spool 85 mounted in the sleeve for horizontal sliding movement therealong. When the spool is in a null position (not shown) relative to the sleeve, the spool left lobe 86 is arranged to cover left return slots 83, and the spool right lobe 88 is arranged to cover right supply slots 84. However, in Fig. 2a, spool 85 is shown as having been displaced rightwardly from such null position by a distance of +x so as to continue to cover return slots 83 while partially uncovering supply slots 84. These uncovered supply slots form ports or orifices through which fluid may flow from supply conduit 80 to the annular space between spool lobes 86,88. Alternatively, if the spool were to be displaced leftwardly from such null position (not shown), spool lobe 88 would continue to cover right supply slots 84, while the left return slots 83 would continue cover right supply slots 84, while the left return slots 83 would be partially uncovered. These partially-uncovered return slots would form ports or orifices through which fluid could flow from the space between spool lobes 86,88 to return conduit 78. An electro-mechanical driver, indicated at 89, is operatively arranged to selectively displace spool 85 either leftwardly or rightwardly to a desired position relative to the associated sleeve. Because the circumferential widths (w) of slots 83,84 are the same, the sizes of the ports or orifices defined by the partially-uncovered slots are proportional to the magnitude of such spool displacement, which, in turn, is proportional to the magnitude of the error signal supplied to driver 89. In other words, all ports have the same gain. The direction of spool displacement is governed by the polarity of the error signal. As previously indicated, the second valve 72 is structurally identical to the first valve, but is arranged as a mirror image of same. However, whereas the first valve spool is shown as having been displaced rightwardly from its null position by a distance of +x so as to partially uncover the first valve supply slots 84, the second valve spool 85' is shown as having been displaced rightwardly from its null position by a distance of -x so as to partially uncover return slots 83'. These partially-uncovered return slots form ports or orifices through which fluid may flow from the space between right spool lobes 86',88' and right return conduit 78'. Moreover, because the various sleeve slots in each valve have the same circumferential width (w) as the corresponding slots of the first valve, and because both spools have been displaced from this null positions by the same axial distance (ie ⁺x), the cross-sectional area of the first valve supply ports is substantially equal to the cross-sectional area of the second valve return ports. In other words, the ports of the second valve have the same gain as the ports of the first valve. This would also hold if the first valve 71 were to De displaced leftwardly from its null position by a distance of -x, and the second valve 72 were to be similarly displaced leftwardly from its null position by a distance of +x, as shown in Fig. 2d. The significance of this is that, for like displacements of the spools, the pressure drops across the supply and return ports will be substantially the same at all operative positions of the spools relative to their associated sleeves. The actuator 73 is again shown as having a piston 90 slidably mounted within a cylinder 91. This piston subdivides the cylinder into a leftward chamber 92, and a rightward chamber 93. A rod 94 is mounted fast to the piston and penetrates both ends of the cylinder. A conduit 95 continuously communicates the space between first valve lobes 86,88 with actuator left chamber 92, and another conduit 96 continuously communicates the space between second valve lobes 86',88' with the actuator right chamber 93. A pressure sensor 98 is operatively arranged to sense the differential pressure in conduits 95,96. Still another conduit continuously communicates one of the supply and return slots of the first valve with the like one of the supply and return slots of the second valve. In the specific embodiment shown in Figs. 2a and 2b, this conduit is indicated at 99, and is shown as communicating the supply conduits 80,80' of the respective valves. Conduit 80' has a check valve (not shown) to prevent reverse flow to the sump. In the modified embodiment shown in Fig. 2d, this conduit is indicated at 110, and communicates the return conduits 78,78' of the two valves. Fig. 2c is a block diagram of the valve shown in Figs. 2a and 2b. An electrical command signal, reflective of the desired position of the actuator rod, is supplied to a summing point 100, which also receives a negative feedback signal, reflective of the actual position of the rod, from a feedback transducer 101. The algebraic sum of the command and feedback signals is supplied as an error signal (±e) to a multiplier 102, to a polarity sensor 103, and to a multiplier 104. The polarity of the error signal is supplied by polarity sensor 103 to a logic unit 105. The polarity of the load pressure differential between conduits 95,96 is determined by a polarity sensor 106, and this signal is also supplied to the logic unit. The logic unit compares the polarities of the signals supplied by sensors 103,106, and causes one multiplier to produce a multiplicand of either +1 or -1, and causes the other multiplier to produce a multiplicand of -1 or +1, as appropriate. The polarity of the signal supplied by polarity sensor 103, indicates the desired direction of rod movement. If the load is "opposing", the logic unit causes the appropriate one of the multipliers to produce a +1 multiplicand, and causes the other of the multipliers to produce a -1 multiplicand. The desired direction of actuator movement, and the direction of the "opposing" load, will determine which of the multipliers is caused to generate the positive multiplicand and which is caused to generate the negative multiplicand. In any event, the product of the error signal (+e), the multiplicand (+1 or -1) provided by multiplier 102, and the gain (K₁) of the first valve servoamplifier 108, is supplied to the first valve driver, which displaces the first spool in the appropriate direction by a proportional distance to uncover one of the first valve supply and return slots. At the same time, the product of the error signal (±e), the multiplicand (-1 or +1) provided by multiplier 104, and the gain (K₂) of the second valve servoamplifier 109, is supplied to the second valve driver, which displaces the second spool in the opposite direction by the same proportional distance to uncover the opposite one of the second valve supply and return slots. Since the various supply and return slots of both servovalves are of the same circumferential width, the gains of the two valves are identical. Because the same error signal (±e) is supplied to both multipliers, the displacements of the first and second spools will be substantially the same for both "aiding" and "opposing" loads. However, if the load is "opposing", the signs of the unit multiplicands respectively provided by multipliers 102,104 will be such that one of the spools will be moved off null by a distance of +x, while the other spool will be moved off null by the same distance of -x. This is shown in Fig. 2a. On the other hand, if the logic unit determines that the load is "aiding", it will cause both multipliers to produce multiplicands of the same polarity (ie either +1 or -1, as appropriate) such that both spools will be displaced off null by the same distance in the same direction (ie either +x or -x and -x). The effect of this is to cause the first valve spool to uncover one of its supply and return slots, and to cause the second valve spool to uncover the like one of its supply and return slots, as shown in Fig. 2b. In the embodiment shown in Figs. 2a and 2b, the two supply slots 84,84' are in continuous communication through conduit 99. Thus, if both supply slots are uncovered when an "aiding" load is sensed, as shown in Fig. 2b, the expanding actuator chamber will effectively be at supply pressure and the contracting actuator chamber will be at some pressure greater than supply pressure. Hence, fluid may flow from the higher pressure right actuator chamber 93 through conduit 96, the second valve supply ports, conduit 99, the first valve supply ports, and conduit 95, and enter the lower pressure left actuator chamber 92, without drawing fresh pressurized fluid from the source. Thus, energy is conserved when an "aiding" load is applied to the actuator, by permitting fluid to flow from the higher pressure contracting actuator chamber through conduit 99 to the lower pressure expanding actuator chamber, rather than by supplying fresh pressurized fluid to such expanding chamber. Fig. 2d illustrates a reverse configuration in which conduit 99 has been replaced by a conduit 110, which continuously communicates the return slots 83,83' of the two valves, If desired, a further conduit 111 may communicate conduit 110 with a sump 112. Another conduit 113 may communicate the sump with conduit 95. This conduit is shown as containing a check valve 114, which permits flow from the sump to conduit 95, but prevents flow in the opposite direction. Another conduit 115 may communicate conduits 113,96, and may also include a check valve 116 to permit only unidirectional flow from the sump to conduit 96. These additional conduits 113,115, and their associated check valves, function to prevent cavitation as fluid is drawn into the expanding actuator chamber. Thus, if a rightward "aiding" load is applied to the actuator rod and both spools are displaced off null by a distance of +x so as to uncover the respective return slots, as shown in Fig. 2d, fluid may flow from the higher pressure contracting right actuator chamber through conduit 96, the second valve return port, right return conduit 78', conduit 110, left return conduit 78, the first valve return port, conduit 95, and enter the expanding lower pressure left actuator chamber. In this regard, conduit 110 may simply be a common fluid return such that, as the actuator piston moves rightwardly, fluid from either the return, or the sump, or both, is drawn into the expanding actuator chamber without admitting fresh supply pressure to same. Another feature of this first embodiment (ie Figs. 2a and 2b), and its reverse configuration (ie Fig. 2d), lies in the manner by which flow control is achieved. In the prior art embodiment shown in Figs. 1a and 1b, the two valves operated on a mutually-exclusive basis such that flow control was afforded by whichever valve was operable at the time. However, in the cooperative operation of the first embodiment (Figs. 2a and 2b) and the modification (Fig. 2d) thereof, fluid is constrained to flow through one orifice provided by the first valve and through a second orifice provided by the second valve, when the load is either "opposing" or "aiding". This feature readily lends itself to application of optional fall-safe logic, such as indicated in phantom at 118 in Fig. 2c. This logic may compare the actual position of each valve spool with the magnitude of the signal supplied to its associated driver. A discrepancy between such compared ratios can then be used to sense a failure of one valve or the other, and to cause an appropriate signal to be supplied to disable the system before the errant valve assumes a hard-over position. While valves 71,72 may be operated cooperatively such that their respective spools 85,85' are displaced proportionally off-null in opposite positional ranges (ie +x and -x, or vice versa) when the load is "opposing", but in the same positional range (ie +x and +x, or -x and -x) when the load is "aiding", this mode of operation may be varied. For example, when the apparatus is in the condition shown in Fig. 2d, the spool of left valve 71 may be returned to its null position. If this occurs, fluid may circulate from the actuator contracting chamber 93 through conduit 96, the return port of valve 72, conduits 78',110,111, sump 112, conduit 113, check valve 114, and conduit 95, to enter the actuator expanding chamber 92. The structure shown in Figs. 2a and 2b could be readily modified to provide a similar mode of operation. To do this, suitable conduits (not shown) would be provided so as to communicate conduit 95 with conduit 99, and to communicate conduit 96 with conduit 99. Each of these additional conduits would incorporate a check valve arranged to permit unidirectional flow from conduit 99 to the associated one of conduits 95,96. Thus, such modified structure could be operated such that, when an "aiding" load was applied to the actuator, whichever valve was associated with the higher-pressure actuator contracting chamber, would be returned to null while the supply port of the other valve remained open. Therefore, the valves may be operated cooperatively by having the spools be displaced by the same distance in the appropriate directions, or may be operated independently of one another. Such cooperative operation requires that the recirculating fluid flow sequentially through the like ports of both valves, whereas such independent operation requires that the recirculating fluid pass through a single port of one one valve. Referring now to Fig. 3a, an improved valve, generally indicated at 119, is shown as broadly including a body 120, a three-lobed spool 121 movable relative to the body, and a driver 122 operatively arranged to selectively displace the spool relative to the body. Valve 119 is shown as being operatively associated with a double-acting fluid-powered actuator 123. The body 120 is shown as being provided with a horizontally-elongated bore 124. Three axially-spaced annular grooves extend radially into the body from bore surface 124. The leftward groove 125 communicates with a fluid return (not shown) at a return pressure R via a conduit 126. The middle groove 128 communicates with a source (not shown) of pressurized fluid at a supply pressure P s via a conduit 129. The rightward groove 130 communicates with the fluid return via a conduit 131. A first bypass passageway 132 is provided in the body to communicate supply conduit 129 with bore surface 124 at a position between body recesses 125,128, and contains a check valve 133 operatively arranged to permit fluid to flow from the bore to the supply conduit, but not reversely. Similarly, a second bypass passageway 134 is provided in the body to communicate the supply conduit 129 with bore surface 124 at a position between body grooves 128,130. This second bypass passageway also includes a check valve 135 which is operatively arranged to permit fluid flow from the bore to the supply conduit, but not reversely. To avoid obfuscating the improvement, bypass conduits 132,134 have been schematically shown as being lines. A tubular sleeve, generally indicated at 136, is mounted fast within the body bore, and has an inwardly-facing cylindrical surface or bore 138. Sleeve 136 is shown as having four circumferentially-spaced radial through-slots arranged at each of four axially-spaced locations therealong. The leftward first slots, one of which is indicated at 139, are aligned with left body groove 125. The next-rightward second slots, one of which is indicated at 140, register with middle body groove 128, the next-rightward third slots, one of which is indicated at 141, also communicate with the middle body groove 128. The rightward fourth slots, one of which is indicated at 142, are aligned with right body groove 130. While only those slots which are arranged in the 6:00 o'clock positions have been numbered, it will be appreciated that the other slots of each circumferentially-spaced group are arranged at the 9:00 o'clock positions (not shown) at the 12:00 o'clock positions, and at the 3:00 o'clock positions (which are seen in phantom elevation and cross-hatched for clarity). Here again, the body and sleeve are formed separately only for manufacturing convenience, but are subsequently assembled together. Hence, the sleeve should, for practical purposes, be regarded as a part of the body. The axial length of each of slots 139,140,141,142 is substantially the same. Each of return slots 139,142 is of substantially equal circumferential width (w r ). Similarly, each of supply slots 140,141 is of substantially equal circumferential width (w p ). However, the supply slots 140,141 are shown as occupying substantially greater arcuate distances than return slots 139,142. Hence, the gain of the supply slots is substantially greater than the gain of the return slots. Indeed, each of supply slots 140,141 may have a circumferential width on the order of four times the circumferential width of return slots 139,142. Thus, if each of return slots 139,142 occupies an included angle of, say, 15 degrees, then each of supply slots 140,141 may occupy an included angle of about, say, 60 degrees. The exact ratio between the circumferential widths of these supply and return slots is not deemed critical, and may be readily varied. However, it is presently preferred that the circumferential width (w p ) occupied by the supply slots be substantially greater than the circumferential width (w r ) occupied by the return slots (as shown in Fig. 3a), or vice versa (as shown in Fig. 3c). These circumferential slot widths determine the gain of the valve per unit of spool displacement off-null. The sleeve is shown as further including a first passageway which communicates the body first bypass passageway 132 with sleeve bore 138, and a second passageway which communicates the body second passageway 134 with the sleeve bore 138. Valve spool 121 is mounted within the sleeve for longitudinal sliding movement along bore 138. When the spool is in a null position relative to the sleeve (as shown in Fig. 3a), the spool left lobe 143 covers left return slots 139, the spool middle lobe 144 covers each of the left and right supply slots 140,141, and the spool right lobe 145 covers right return slots 142. The driver 122, is operatively arranged to move the spool either leftwardly or rightwardly, as desired, to a selected position relative to the body. If the spool moves rightwardly from the null position shown in Fig. 3a, the left lobe 143 will continue to cover left return slots 139, the middle lobe 144 will partially uncover left supply slots 140, thereby forming left supply ports or orifices which communicate the supply conduit 129 with the annular space between the spool left and middle lobes, but will continue to cover right supply slots 141; and the right lobe 145 will partially uncover right return slots 142, thereby forming right return ports or orifices which communicate the annular space between the spool middle and right lobes with return conduit 131. On the other hand, if the spool is moved leftwardly from the illustrated null position, the spool left lobe 143 will partially uncover left return slots 139, thereby forming left return ports or orifices which communicate the annular space between the spool left and right lobes with return conduit 126; the spool middle lobe 144 will continue to cover left supply slots 140 but will partially uncover right supply slots 141, thereby forming right supply ports or orifices which communicate supply conduit 129 with the space between the spool middle and right lobes; and the spool right lobe 145 will continue to cover right return slots 142. Thus, depending on whether the spool is moved leftwardly or rightwardly from such null position, one of the return slots will be partially uncovered and one of the supply slots will be partially uncovered. However, because of the different circumferential widths occupied by the respective supply and return slots, the cross-sectional area (ie gain) of the uncovered supply ports per unit of spool displacement, will be substantially greater than the cross-sectional area of the uncovered return ports. The actuator 123 is shown as having a piston 146 slidably mounted within a cylinder 148, and as separating the cylinder into a leftward first chamber 149 and a rightward second chamber 150. A rod 151 is fixed to the piston and sealingly penetrates both ends of the cylinder. A first conduit 152 continuously communicates the annular space between the spool left and middle lobes with the actuator left chamber 149, and a second conduit 153 continuously communicates the annular space between the spool middle and right lobes with the actuator right chamber 150. A pressure sensor 154 is operatively arranged to sense both the polarity and the magnitude of the differential load pressure between conduits 152,153. Fig. 3b is a block diagram of the valve shown in Fig. 3a. An electrical command signal, reflective of the desired position of actuator rod 151, is supplied to a summing point 155, which also receives a negative feedback signal, reflective of the actual position of the actuator rod, from a feedback transducer 156. The algebraic sum of the command and feedback signals is supplied as an error signal (⁺e) to a multiplier 158 and to a polarity sensor 159, which, in turn, supplies the polarity of the error signal to a logic unit 160. Pressure sensor 154 is arranged to sense the magnitude and polarity of the pressure differential in conduits 152,153, and supplies the magnitude and polarity of such sensed differential to the logic unit. The polarity of such sensed pressure differential is determined by a polarity sensor 161, which supplies such sensed polarity to the logic unit. The logic unit compares the signals produced by polarity sensors 159,161, determines whether the load applied to the actuator rod is "opposing" or "aiding" with respect to the desired direction of rod movement, and causes multiplier 158 to produce an appropriate multiplicand. The product of the error signal (⁺e), the multiplicand provided by multiplier 158, and the gain (K) of servoamplifier 162, is supplied to driver 122 which moves the sleeve appropriately relative to the body. If desired, the valve of the multiplicand may be such that the product of the multiplicand and the gain of the valve will be a constant. Again, the polarity of the error signal determines the direction of spool displacement. If the load applied to the actuator rod is "opposing", valve 119 operates in the conventional manner. For example, if it is desired to move a leftward "opposing" load rightwardly, the driver would displace the spool rightwardly relative to the body. In this displaced condition of the spool, supply pressure from conduiit 129 would be admitted through the relatively-large aggregate area of the left supply ports to the space between the left and middle spool lobes, and would flow through conduit 152 to enter the actuator left chamber 149. Conversely, fluid in actuator right chamber 150 could flow to return via conduit 153, the space between the spool middle and right lobes, and the relatively-small area of the right return ports. Since the aggregate orifice area of either of the return ports is substantially smaller than the aggregate orifice area of either of the supply ports, actuator displacement would be effectively controlled by flow through the smaller-area return orifices. On the other hand, if it is desired to move the actuator rod in the same direction as an "aiding" load, the valve spool is moved in the appropriate direction to communicate the actuator expanding chamber with the fluid source, and to communicate the actuator contracting chamber with the fluid return. However, because the aggregate orifice area of the supply ports is on the order of four times the aggregate orifice area of the return ports, the actuator expanding chamber will be substantially at the supply pressure P s . However, by definition, the "aiding" load will create a positive pressure differential between the retracting and expanding actuator chambers. Hence, if the actuator expanding chamber is at supply pressure, the pressure in the actuator contracting chamber will be at some pressure greater than the supply pressure. This pressure differential will cause the appropriate one of the bypass conduit check valves to open, and fluid will flow from the actuator contracting chamber through the appropriate bypass conduit and through the supply ports to the actuator expanding chamber. While some fluid bleeds to return through the smaller-area return ports, most of the fluid passes through the larger-area supply ports, which therefore provides effective control over displacement of the actuator rod. Since most of the actuator total flow goes through the check valve and bypass conduit, and only a small portion goes out the return port to be recirculated by the supply pump to the high pressure supply port, substantial energy is conserved relative to that dissipated by a conventional four-way servovalve. Thus, in the embodiment shown in Fig. 3a, when the improved valve is operated to displace an "opposing" load, effective control over the actuator rod is provided by the smaller-area return ports. However, when it is desired to move an "aiding" load, fluid flow is effectively controlled by the larger-area supply ports. Fig. 3c illustrates an alternative form in which the bypass conduits 132,134, and their associated check valves 133,135 have been removed, and the positions of the relatively-large and relatively-small sleeve slots have been reversed. In this embodiment, a first bypass passageway 163 communicates the space between the spool left and middle lobes with return conduit 126. This first bypass passageway has a check valve 164 therein to prevent fluid from flowing from the space between the spool left and middle lobes to return, but to permit reverse flow from the return to such space. The embodiment shown in Fig. 3c is also provided with a second bypass passageway 165 which communicates the space between the spool middle and right lobes with return conduit 131. Similarly, this second bypass passageway also incorporates a check valve 166, which is arranged to permit flow from the return to the space between the spool middle and right lobes, but not reversely. An optional third conduit 167 continuously communicates sleeve slots 168,171. As previously indicated, the positions of the relatively-large and relatively-small sleeve slots provided through the sleeve, have been reversed. Hence, when the spool is in its null position, as shown in Fig. 3c, the left spool lobe is arranged to cover wide left return slots 168, the spool middle lobe is arranged to cover narrow left and right supply slots 169,170 and the spool right lobe is arranged to cover wide right return slots 171. Here again, the circumferential width (w r ) of each of return slots 168,171 may be on the order of four times the circumferential width (w p ) of each of supply slots 169,170. If it is desired to move the actuator rod, say, rightwardly against an "opposing" load, driver 122 displaces spool 121 rightwardly relative to the body. The effect of this is to partially uncover the smaller-area left supply ports, and to permit flow therethrough into the actuator left chamber 149. At the same time, the actuator right chamber will communicate with return via the larger-area right return ports. Because of the difference in the aggregate cross-sectional areas of these ports, the right side of the actuator will be substantially at the return pressure, and control over actuator displacement will be effectively provided by the smaller-area supply ports, Of course, return conduits 126,131 may communicate with a common return (not shown), such as a sump. However, if a rightward "aiding" load is applied to the actuator rod, the pressure in actuator right chamber 150 will increase, and the pressure in actuator left chamber 149 will decrease. If it is desired to move the actuator rod in the same direction as the "aiding" load, spool 121 is shifted rightwardly relative to the body. When this occurs, the higher pressure in actuator right chamber 150 is permitted to flow to return through the larger-area right return ports, which effectively control the flow. At the same time, some fluid will flow from the source through the smaller-area left supply ports to enter the actuator left chamber. However, additional fluid may also flow from return conduit 126 through bypass conduit 163 to enter actuator left chamber 149. Thus, as in the previously-described case, the energy-wasting flow through the pump is a small fraction of the recirculating flow. Referring now to Fig. 4, another embodiment of an improved valve, generally indicated at 172, is shown as broadly including a body 173, a sleeve 174 mounted within the body, a valve spool 175, and an electro-mechanical driver 176. Valve 172 is shown as being operatively associated with a conventional double-acting fluid-powered actuator 178. Here again, the sleeve is mounted fast to the body. Body 173 is shown as having a horizontally-elongated bore 179, from which five axially-spaced annular grooves extend radially into the body. The leftwardmost body groove 180 communicates with a fluid return (not shown) at a return pressure R via a passageway 181. The next-rightwardmost body groove is indicated at 182. The next-rightward body groove 183 communicates with a source (not shown) of pressurized fluid at a supply pressure P s via a conduit 184. The next-rightward body groove is indicated at 185. The rightward body groove 186 communicates with the fluid return via a passagway 188. The tubular sleeve 174 is mounted fast within the body bore, and has an inwardly-facing cylindrical surface or bore 189. The sleeve is provided with four circumferentially-spaced radial through-slots at each of six axially-spaced locations therealong. The leftwardmost first sleeve slots, one of which is indicated at 190, are aligned with the first body groove 180. The second sleeve slots, one of which is indicated at 191, are aligned with second body groove 182. The third sleeve slots, one of which is indicated at 192, are aligned with the left margin of body groove 183. The fourth sleeve slots, one of which is indicated at 193, are aligned with the right margin of body groove 183. The fifth sleeve slots, one of which is indicated at 194, are aligned with body groove 185. Finally, the rightwardmost sixth sleeve slots, one of which is indicated at 195, are aligned with right body groove 186. While only those slots which are arranged in the 6:00 o'clock positions have been numbered, it will be understood that the other slots of each circumferentially-spaced group are arranged in the 9:00 o'clock positions (not shown), at the 12:00 o'clock positions, and at the 3:00 o'clock positions (which are again shown in phantom elevation and cross-hatched for clarity). Each of supply and return slots 190,192,193,195 is of substantially equal circumferential width (w), while each of bypass slots 191,194 is of substantially greater circumferential width (W). The ratio of such widths (W/w) may be on the order of four to one. However, while the exact ratio between these circumferential widths is not deemed to be particularly critical, and may be readily varied, it is presently preferred that the bypass slots be of substantially greater width than the supply and return slots. As before, the sleeve is mounted fast to and should be regarded as a part of the body. The valve spool 175 is shown as having five lobes. When the spool is in its null position, as shown in Fig. 4, the leftward first lobe 196 covers left return slots 190, the next-rightward second lobe 198 will cover left bypass slots 191, the next-rightward third or middle lobe 199 will cover left and right supply slots 192,193, the next-rightward fourth lobe 200 will cover right bypass slots 194, and the rightward-most fifth lobe 201 will cover right return slots 195. Thus, if the spool is moved leftwardly off-null, the left lobe 196 will partially uncover slots 190 to form left return ports, the spool third lobe 199 will partially uncover right supply slots 193 to form right supply ports and the fourth lobe 200 will partially uncover bypass slots 194 to form right bypass ports. At the same time, left bypass slots 191, left supply slots 192, and right return slots 195, will each remain covered. Conversely, if the spool were to be moved rightwardly off-null, the spool second lobe 198 would partially uncover bypass slots 191 to form left bypass ports, the spool middle lobe 199 would partially uncover left supply slots 192 to form left supply ports, and spool rightward lobe 201 would partially uncover return slots 195 to form right return ports. At the same time, left return slots 196, right supply slots 193 and right bypass slots 194, will each remain covered. The actuator 178 is again shown as having a piston 202 slidably mounted within a cylinder 203, and as subdividing the cylinder into a leftward chamber 204 and a rightward chamber 205. A rod 206 is fixed to the piston and sealingly penetrates both end walls of the cylinder. A conduit 208 communicates the sleeve inner surface 189 between slots 191,192 with actuator left chamber 204. Another conduit 209 communicates the sleeve inner surface 189 between slots 193,194 with actuator right chamber 205. A third conduit 210 communicates the sleeve surface 189 between slots 190,191 with conduit 208, and a fourth conduit 211 communicates the sleeve surface 189 between slots 194,195 with conduit 209. A first bypass conduit 212 communicates conduit 209 with left bypass slots 191. This conduit is shown as containing a check valve 213 arranged to prevent flow from bypass slots 191 toward conduit 209, but to permit fluid from flowing reversely. A second bypass conduit 214 communicates conduit 208 with right bypass slots 194. This conduit is also shown as including a check valve 215 operatively arranged to prevent fluid flow from slots 194 toward conduit 208, but to permit flow in the reverse direction. Conduits 210,208 continously communicate sleeve surface 189 between first and second slots 190,191 with another point on the sleeve surface between second and third slots 191,192. Similarly, conduits 209,211 continuously communicate the sleeve surface between fourth and fifth slots 193,194 with another point on the sleeve surface between the fifth and sixth slots 194,195. If it is desired to move the actuator rod rightwardly against an "opposing" load, driver 176 is operated to displace the spool rightwardly relative to the sleeve and body. When this happens, the left return slots, the right supply slots and the right bypass slots, will each remain covered. However, the left bypass slots, the left supply slots and the right return slots, will all be partially uncovered by the same axial distance. In this condition, fluid may flow from the source through the left supply ports and may enter actuator left chamber 204. At the same time, fluid may flow from the actuator right chamber 205 through the right return ports to the return. The lower pressure in actuator right chamber 205 will be applied to the right side of check valve 213. However, the higher pressure in conduit 208 will be applied to the left side of this check valve through conduit 210, left bypass ports, and conduit 212. Hence, check valve 213 will remain closed. Thus, in moving an "opposing" load, actuator displacement is controlled by both of the appropriate supply and return ports, which are of the same aggregate cross-sectional orifice area and therefore have the same gain. On the other hand, if a rightward "aiding" load is applied to the actuator rod, the pressure in actuator right chamber 205 will increase, and the pressure in actuator left chamber 204 will decrease. If it is now desired to move the actuator rod in the same direction as the "aiding" load, the driver is again operated to displace the spool rightwardly relative to the sleeve and body. In such displaced condition of the spool, the actuator left chamber 204 communicates with the source through the relatively-small left supply ports, and the actuator right chamber 205 communicates with the return through the relatively-small right return ports. The lower pressure in conduit 208 is applied to the left side of check valve 213, while the higher pressure in conduit 209 will be applied to the right side thereof. Hence, check valve 213 will open to permit the majority of flow to be from the actuator right chamber through bypass conduit 212 to the actuator left chamber, without drawing fresh pressurized fluid from the source. Specifically, the fluid in higher-pressure actuator right chamber 205 may pass through check valve 213, the larger-area left bypass ports 191, and conduit 210, and enter the lower-pressure actuator left chamber 204. Since, the aggregate cross-sectional area of the bypass ports is substantially greater than the aggregate cross-sectional area of the supply and return ports, effective control over actuator displacement will be provided by the bypass ports. In other words, as the actuator rod 206 begin to move in the same direction as the "aiding" load, the error signal is reduced. This causes the rightwardly-displaced spool to move leftwardly toward the null position, thereby decreasing the size of the supply and return ports. At the same time, the orifice defined by the left bypass ports is still relatively large, and this orifice provides effective control over the flow between the two actuator chambers. While the embodiments shown in Figs. 3a,3c and 4 offer the advantage of substantial energy conservation over conventional servo valves, it is appreciated that these embodiments may be further improved by selectively cutting off the unnecessary flow into or out of the valves in the case of an "aiding" load. To this end, the valve, shown in Fig. 3a may be associated with a simple yet highly effective, spring-centered cut-off or throttling valve, such as indicated at 216 in Fig. 5. As best shown in Fig. 5a, valve 216 has a body 218 provided with a horizontally-elongated bore 219 and a pair of axially-spaced left and right abutment stops 220,221. A leftward annular groove 222 extends radially into the body from bore 219, and communicates with servo valve left return conduit 126. A rightward annular groove 223 extends radially into the body from bore 219, and communicates with servo valve right return conduit 131. Conduit 224 communicates bore surface 219 between grooves 222,223 with a common fluid return at a return pressure R. A two-lobed valve spool 225 is slidably mounted within the body bore, and is biased to the centered position shown in Fig. 5 by left and right centering springs 226,228, which are arranged in the left and right spool end chambers, respectively. The pressure P₁ in actuator left chamber 149 is supplied through a conduit 229, which may communicate with conduit 152, to act on the left end face of the spool. The pressure P₂ in the actuator right chamber 150 is supplied through conduit 230, which may communicate with conduit 153, to act on the right end face of the spool. If P₁ = P₂ , springs 226,228 will center the spool relative to the body, as shown in Fig. 5. In this condition, the left and right spool lobes will partially uncover grooves 222,223, and both of the return inlet conduits 126,131 will communicate with common return outlet conduit 224. However, if P₂ > P₁, the spool will be driven leftwardly against abutment stop 220. In this condition, the left return conduit 126 will be fully uncovered, and will continue to communicate with common outlet 224, while the right spool lobe will cover right body groove 223, thereby blocking communication between right return conduit 131 and common outlet 224. Alternatively if P₁ > P₂, the spool will be driven rightwardly against abutment stop 221. In this condition, groove 223 will be fully uncovered and right return conduit 131 will communicate with common outlet 224. However, the spool left lobe will cover groove 222, thereby blocking communication between conduits 126,224. Thus, when an "opposing" load is applied to actuator rod 151, the polarity of the pressure differential between actuator chambers 149,150 will displace the throttling piston in the appropriate direction to continue to communicate the appropriate servo valve return port with the common return outlet 224. However, if the actuator load is "aiding", the opposite polarity of the pressure differential between chambers 149,150 will displace the throttling piston so as to block flow from the uncovered dmain valve return port to the common return. In this manner, the throttling valve may be used to selectively cut off the flow to return when an "aiding" load is applied to the actuator in Fig. 3a. Persons skilled in this art will also appreciate that throttling valve 216 may be used in association with the third embodiment shown in Fig. 4 to cut off the flow to return. As shown in Fig. 6, the throttling valve may be associated with servo valve 172 such that the servo valve left return conduit 181 communicates with body groove 222, and the servovalve right return conduit 188 communicates with body groove 223. A conduit 231, which communicates with actuator left chamber 204, supplies actuator pressure P₁ to the throttling valve left spool end chamber. Another conduit 232, which communicates with actuator right chamber 205, supplies actuator pressure P₂ to the throttling valve right spool end chamber. Thus, the pressures in the two actuator chambers are applied to the opposite ends of the throttling valve spool end chambers, and a differential between such pressures will selectively drive spool 225 either leftwardly or rightwardly, as appropriate, to completely cut off flow to return when the load is "aiding". However, the throttling valve will continuously communicate the appropriate return conduit, 181 or 188, with common return 224 when the load is "opposing". If desired, the throttling valve 216 may be modified to further provide the function of check valves. Referring now to Fig. 7, such a modified throttling valve, generally indicated at 216A, is shown as being substantially similar to the throttling valve 216, described supra. However, the modified form is further provided with left and right annular grooves 233,234 which extend into body 218 from bore surface 219 between left abutment surface 220 and groove 222, and between groove 223 and right abutment surface 221, respectively. Left groove 233 is positioned such that when spool 225 is in its centered or null position, the left marginal end portion of the spool left lobe will just cover groove 233. Conversely, groove 234 is positioned such that when the spool is centered, as shown in Fig. 7, the right marginal end portion of the spool right lobe will just cover groove 234. Hence, if the throttling valve spool is shifted leftwardly relative to the body, groove 233 will remain covered, but groove 234 will communicate with the right spool end chamber. Conversely, if the spool is shifted rightwardly off-null, groove 234 will remain covered, but groove 233 will communicate with the left spool end chamber. As best shown in Fig. 8, the modified throttling valve 216A may be operatively associated with the four-way servo valve 119 and actuator 123 shown in Fig. 3a. The first and second bypass conduits 132,134, and their associated check valves 133,135, have been eliminated. Servo valve return conduits 126,131 communicate with throttling valve grooves 222,223, respectively. Supply pressure P s is admitted via branch conduit 129 and manifold 128 to servo valve supply slots 140,141, and also passes via conduit 235 and branch conduit 236 to throttling valve grooves 233,234. A conduit 238 continuously communicates the space between servo valve left and middle lobes 143,144 with the throttling valve spool left end chamber. Another conduit 239 continuously communicates the space between servo valve middle and right lobes 144,145 with the throttling valve spool right end chamber. Thus, left and right spool end chambers are at actuator pressures P₁,P₂, respectively. The throttling valve 216A is therefore operatively arranged to sense the polarity of the applied load in terms of the differential between actuator pressures P₁ and P₂. On the other hand, the direction of movement of servo valve spool 121 represents the desired direction of load movement. If P₁ = P₂ , the throttling valve spool will be in its centered or null position, as shown in Fig. 8, at which both of the return conduits 126,131 will communicate with common return line 224. However, flow through conduit 236 will be locked by the throttling valve spool left and right lobes, which cover body grooves 233,234 respectively. If P₁ > P₂, the throttling valve spool will be driven rightwardly against abutment surface 221, thereby fully uncovering return groove 223, covering return groove 222, and uncovering groove 233 so as to permit flow from conduit 238 to conduit 236. Conversely, if P₂ > P₁, the throttling valve will be driven leftwardly against abutment surface 220, thereby uncovering groove 222, covering groove 223, and uncovering groove 234 so as to permit flow from conduit 239 to conduit 236. Conduit 236 communicates with the fluid source and with servo valve supply conduit 129. If a rightward load is applied to the actuator rod 151, when spool 121 is in its null position, the pressure P₂ in the actuator right chamber 150 will be greater than the pressure P₁ in the actuator left chamber 149. This differential P₂ > P₁, will displace the throttling valve spool leftwardly to cut off flow in servo valve return line 131. If it is desired to move this load leftwardly, so that the load is "opposing" with respect to the desired direction of actuator movement, servo valve spool 121 is moved leftwardly off null. As this occurs, fluid at P s from the source is admitted to the servo valve through supply conduit 129, passes through the uncovered right supply port, to expand actuator right chamber 150. At the same time, return fluid from the contracting left chamber 149 passes through the servo valve left return port, conduit 126, the throttling valve, to common return 224. However, if it were alternatively desired to move this same load rightwardly, so that the load would be "aiding" with respect to the desired direction of actuator movement, the servo valve spool would be shifted rightwardly off-null. Fluid from supply would quickly pressurize the left actuator chamber 149 to the magnitude of P s , and the right actuator chamber would therefore be at some pressure greater than P s because of the applied load. Fluid would flow from the contracting right actuator chamber 150 through conduits 153,239,236,235,129,128, the servo valve left supply port, and conduit 152, to enter the expanding left actuator chamber. Thus, in this embodiment, the modified throttling valve 216A performs the function of check valves 133,135, which have been eliminated. At the same time, the throttling valve completely cuts off the flow to return in the case of an "aiding" load, thereby conserving energy, while permitting normal operation of the servo valve when the load is "opposing". If desired, a further modified form of this throttling valve may be associated with the valve shown in Fig. 3c, to completely cut off the flow from the source when the load is "aiding" with respect to the desired direction of actuator movement. Referring now to Fig. 9, this embodiment of the throttling valve 216B is associated with servo valve 119 and actuator 123. The first and second bypass conduits 163,165, and their associated check valves 164,166, have been eliminated. Moreover, servo valve body middle groove 128 has been modified so that sleeve supply slots 169,170 do not communicate with one another. Throttling valve 216B has a body 240 provided with a horizontally-elongated bore 241. Four axially-spaced annular grooves 242,243,244,245 extend radially into the body. A spring-centered valve spool 246, having four lobes 248,249,250,251, is slidably mounted within the bore and is biased to a null position by left and right springs 252,253. In this null position, as shown in Fig. 9, first lobe 248 covers groove 242, second lobe 249 partially covers groove 243, third lobe 250 partially covers lobe 244, and fourth lobe 251 covers groove 245. A conduit 254 communicates a suitable fluid source (not shown) at a supply pressure P s , with the body bore between grooves 243,244. Groove 243 communicates with servo valve right supply groove 170 via conduit 255, and groove 244 communicates with servo valve left supply groove 169 via conduit 256. Groove 242 communicates with return via conduit 258, and with servo valve left return groove 168 via conduit 259. Groove 245 communicates with return via conduit 260, and with servo valve right return groove 171 via conduit 261. Conduit 262 communicates the space between servo valve lobes 143,144 within the throttling valve spool left end chamber. A conduit 263 communicates conduit 262 with the space between throttling valve spool lobes 248,249. Conduit 264 communicates the space between servo valve lobes 144,145 with the throttling valve spool right end chamber. Conduit 265 communicates conduit 264 with the space between throttling valve lobes 250,251. Thus, the pressure P₁ in actuator left chamber 149 is provided to the throttling valve spool left chamber, and the space between lobes 248,249. The pressure P₂ in actuator right chamber 150 is provided to throttling valve spool right end chamber and to the space between lobes 250,251. Here again, the polarity of the actual load pressure differential, P₁-P₂, is used to displace spool 242 either leftwardly or rightwardly, as appropriate between abutment stops 266,268. The position of servo valve spool 121 relative to its body determines the desired direction of actuator movement. Assume that a rightward load is applied to actuator rod 151. Since P₂ > P₁, the throttling valve spool 242 will be driven leftwardly against abutment stop 266. When this happens, lobe 249 will completely uncover groove 243, but lobe 250 will cover lobe 244, thereby preventing flow through conduit 256 to return. If it is desired to move the actuator rod leftwardly, so that the load "opposes" the desired direction of actuator movement, servo valve spool 121 is shifted leftwardly. Pressurized fluid from the source passes through conduits 254,255, the servo valve right supply port, and conduit 153 to enter actuator right chamber 150. Conversely, fluid in actuator left chamber 149 flows to return through communicating conduits 152,259 and 258. However, if it is desired to move the load rightwardly, so that the load will "aid" the desired direction of movement, the servo valve spool is displaced rightwardly off-null. However, since P₂ > P₁, flow through supply conduit 256 is blocked by the leftwardly-displaced throttling valve spool. However, fluid in the contracting right actuator chamber may flow to return via conduits 153,261, annular groove 245, and conduit 260. At the same time, fluid may be drawn into the expanding actuator chamber 149 from return via conduits 258,263,262 and 152. Thus, while this embodiment does contemplate some fluid bleed to return, the throttling valve blocks the flow of fresh pressurized fluid at supply pressure P s from entering the expanding actuator chamber. If desired, a modified form of the servo valve shown in Fig. 4 may be associated with a further modified throttling valve to completely cut off fluid flow from supply and to return in the case of an "aiding" load. Referring now to Fig. 10, the modified servo valve 172A is shown as being associated with actuator 178 and with modified throttling valve 216C. Servo valve 172A has a body 173 provided with a horizontally-elongated cylindrical bore 179. Six axially-spaced annular grooves 269,270,271,272,273,274 extend radially into the body from bore surface 179. Tubular sleeve 174 is arranged within the bore, and is mounted fast to the body. Sleeve slots 190,191,192,193,194,195 communicate with body grooves 269,270,271,272,273,274, respectively. The servo valve also includes a five-lobed spool, generally indicated at 175, the position of which relative to the sleeve is controlled by driver 176. When the spool is in the null position, as shown in Fig. 10, leftwardmost lobe 196 covers sleeve slots 190, lobe 198 covers sleeve slots 191, lobe 199 covers sleeve slots 192,193, lobe 200 covers sleeve slots 194, and lobe 201 covers sleeve slots 195. The spool 175 and sleeve 174 are as previously described. However, the servo valve body is modified in that grooves 271,272 have replaced common body groove 184 of the Fig. 4 embodiment. Also, the arrangement and connections of the various conduits have been changed, as described below. Conduit 208 communicates the actuator left chamber 209 with sleeve bore 189 between spool lobes 198,199, and conduit 210 communicates conduit 208 with sleeve bore 189 between spool lobes 196,198. Conversely, conduit 204 communicates actuator right chamber 205 with sleeve bore 189 between spool lobes 199,200, and conduit 211 communicates conduit 209 with sleeve bore 189 between spool lobes 200,201. However, in the Fig. 10 modification, bypass conduits 212,214, together with their associated check valves 213,215, respectively, have been eliminated. The modified throttling valve 216C includes a body 275 provided with a horizontally-elongated cylindrical bore 276. Six axially-spaced annular grooves 278,279,280,281,282,283 extend radially into the body from bore surface 276. A four-lobed valve spool, generally indicated at 284, is arranged within bore 276 for sliding movement therealong, and is biased to a centered or null lposition by opposing springs 285,286 in the throttling valve spool and chambers. When the spool is in its null position, as shown in Fig. 10, the left margin of leftwardmost lobe 288 covers body groove 278, the right margin of lobe 288 partially covers body groove 279, the right margin of next-rightward lobe 289 partially uncovers body groove 280, the left margin of next-rightward lobe 290 partially uncovers body groove 281, the left margin of rightwardmost lobe 291 partially covers body groove 282, and the right margin of lobe 291 covers body groove 283. Thus, spool 284 is mounted for sliding movement along bore 276 between left and right abutment stops 292,293. When the spool is shifted leftwardly off-null to abut left stop 292, grooves 279,280,283 will be uncovered, while grooves 278,281,282 will be covered. Conversely, when the spool is shifted rightwardly and abuts right stop 293, grooves 278,281,282 will be uncovered, and grooves 279,280,283 will be covered. A conduit 294 admits pressurized fluid at supply pressure at P s from a source (not shown) to throttling bore 276 between grooves 280,281. Conduit 295 communicates bore surface 276 between grooves 279,280 with a fluid return at a return pressure R. Similarly, conduit 296 communicates bore surface 276 between body grooves 281,282 with the return. Conduit 298 communicates servo valve bore surface 189 between slots 190,191 with the throttling valve spool left end chamber. Conduit 299 communicates servo valve body groove 269 with throttling body groove 279. Conduit 300 communicates throttling body groove 278 with servo valve body groove 273. Conduit 301 communicates throttling body groove 280 with servo valve body groove 272. Conduit 302 communicates throttling body groove 281 with servo valve body groove 271. Conduit 303 communicates throttling body groove 283 with servo valve body groove 270. Conduit 304 communicates throttling body groove 282 with servo valve body groove 274. Conduit 305 communicates the throttling valve spool right end chamber with servo valve sleeve bore 189 between sleeve slots 194,195. Thus, for example, assume that a rightward load is applied to actuator rod 206. Hence, the pressure P₂ in actuator right chamber 205 will be greater than the pressure P₁ in actuator left chamber 204. Since P₂ > P₁, the throttling valve spool will be shifted leftwardly to abut stop 292. If it is desired to move this load leftwardly, so that the load is "opposing" with respect to the desired direction of actuator movement, driver 176 is operated to shift servo valve spool 175 to the left. Thus, pressurized fluid may flow from the source through conduits 294,301, the now-uncovered servo valve right supply ports, and conduit 209, to enter the expanding actuator right chamber 205. At the same time, fluid may flow from contracting actuator left chamber 204 through conduits 208,210, the now-uncovered servovalve left return ports, and conduits 299,295, to return. If it is alternatively desired to move this load rightwardly, so that the load will be "aiding" with respect to the desired direction of actuator movement, driver 176 is operated so as to shift servo valve spool 175 rightwardly off-null. Since P₂ > P₁, throttling valve spool 284 will still abut left stop 292. However, fluid in the contracting actuator right chamber 205 will be constrained to flow through conduits 209,211,305,303,210 and 208, to enter the expanding actuator left chamber 204. At the same time, the throttling valve will completely cut off fluid flow from the supply or to the return. Hence, no fresh pressurized fluid from the source will be admitted to the actuator when it is desired to move the actuator rod in the same direction as an "aiding" load. Referring now to Fig. 11, a simplified servo mechanism, generally indicated at 306, is shown as broadly including a double-acting fluid-powered actuator 308, an electrohydraulic servo valve 309, and a throttling valve 310. Actuator 308 is shown as having a piston 311 slidably mounted within a cylinder 312, and as separating a leftward or first actuator chamber 313 from a rightward or second actuator chamber 314. The pressures within the first and second actuator chambers are again indicated as being P₁,P₂, respectively. Servo valve 309 has a body 315 provided with a horizontally-elongated cylindrical bore 316. Four axially-spaced annular grooves 318,319,320,321 extend radially into body 315 from bore surface 316. A three-lobed valve spool 322 is slidably mounted within the bore. Spool 322 may be selectively displaced in the appropriate axial direction relative to body 315, either leftwardly or rightwardly, as desired, by an electro mechanical driver 323. When spool 322 is in its null position relative to the body, the right margin of left lobe 324 just covers first body groove 318, the left and right margins of middle lobe 325 just cover second and third body grooves 319,320, and the left margin of right lobe 326 just covers fourth body groove 321. Thus, if spool 322 is shifted leftwardly off-null, body grooves 318,320 will be uncovered, while body grooves 319,321 will remain covered. Conversely, if spool 322 is shifted rightwardly off-null, grooves 319,321 will be uncovered, while body grooves 318,320 will remain covered. The throttling valve 310 is shown as having a body 328 provided with a horizontally-elongated cylindrical bore 329. Five axially-spaced annular grooves 330,331,332,333,334 extend radially into the body from bore surface 329. A five-lobed valve spool, generally indicated at 335, is mounted within this bore for longitudinal sliding movement therealong between left and right abutment stops 336,338 respectively. When the throttling valve spool is in its null position, as shown in Fig. 11, the left margin of leftwardmost lobe 339 just covers body groove 330; the right margin of lobe 339 partially covers body groove 331; the next-rightward lobe 340 is positioned between body grooves 331,332; next-rightward lobe 341 is centered with respect to, but only partially covers, body groove 332, next-rightward lobe 342 is positioned between body grooves 332,333; the left margin of rightwardmost lobe 343 partially covers body groove 333; and the right margin of lobe 343 just covers body groove 334. The spool 335 is biased toward a centered or null position by opposing left and right centering springs 344,345 in the left and right spool end chambers, respectively. In the illustrated null position, the spool end faces are spaced equally from the proximate abutment stops 336,338. Thus, if the spool is shifted leftwardly so as to abut stop 336, body grooves 330,333 will be covered, body grooves 331,334 will be uncovered, and middle body groove 332 will only communicate with the space between lobes 341,342. Conversely, if the spool is shifted rightwardly so as to abut stop 338, body grooves 331,334 will be covered, body grooves 330,333 will be uncovered, and middle body groove 332 will only communicate with the space between lobes 340,341. Pressurized fluid at supply pressure P₂ is continuously supplied to the throttling valve bore between lobes 339,340 by conduit 346, and is also continuously supplied to this bore between lobes 342,343 by a conduit 348. Another conduit 349 communicates middle body groove 332 with a fluid return at a return pressure R. Conduit 350 communicates throttling groove 331 with servovalve groove 318. Branch conduit 351 communicates conduit 350 with throttling groove 330. Conduit 352 communicates throttling groove 333 with servo valve groove 321. Branch conduit 353 communicates conduit 352 with throttling groove 334. Conduits 354,355 communicate the spaces between throttling valve spool lobes 340,341 and 341,342 with servovalve grooves 319 and 320, respectively. Conduits 356,358 communicate the spaces between servovalve spool lobes 324,325 and 325,326 with actuator chambers 313 and 314, respectively. Conduit 359 communicates conduit 358 with the throttling valve left spool end chamber, and conduit 360 communicates conduit 356 with the throttling valve right spool end chamber. Thus, the pressure P₁ in actuator left chamber 313 is supplied to the throttling valve right spool end chamber, while the pressure P₂ in actuator right chamber 314 is supplied to the throttling valve left spool end chamber. Assume, for example, that a rightward load is applied to actuator rod 317. Hence, by definition, P₂ > P₁, and throttling valve spool 335 will be shifted rightwardly to abut stop 338. If it is now desired to move this load leftwardly, so that the load opposes the desired direction of actuator movement, driver 323 is operated to displace servo valve spool 322 rightwardly off-null. Hence, pressurized fluid may flow through conduits 348,352,358 to enter the actuator right chamber 314. At the same time, fluid may flow from the actuator left chamber 313 through conduits 356,354,349 to return. On the other hand, if it is alternatively desired to move this load rightwardly, so that the load aids the desired direction of actuator movement, driver 323 shifts the servo valve spool leftwardly off-null. In this situation, fluid will be constrained to flow from the higher pressure actuator chamber 314 through conduits 358,359,351,350,356 to the lower pressure actuator chamber 313. At the same time, flow from supply and to return is completely cut off by the throttling valve. Thus, the throttling valve allows the servovalve to provide conventional control over actuator displacement in the case of an "opposing" load. However, in the case of an "aiding" load, the throttling valve selectively isolates the servo mechanism from supply and return, while the servo valve provides continued control as fluid is directed to flow from the higher pressure actuator chamber to the lower pressure actuator chamber. Thus, energy is conserved because fresh pressurized fluid from the source will be prevented from entering the expanding actuator chamber in the case of an "aiding" load.

Claims: A method of controlling the velocity of a fluid-powered actuator (73;123;178;308) having one member (90;146;202;311) movable relative to another member (91;148;203;312) and separating opposing chambers (92, 93;149,150;204,205;313,314), wherein a supply flow of fluid is caused to flow from a source (Ps) thereof to the expanding chamber whenever it is desired to cause relative movement between said members (90,91;146,148;202,203;311,312), and a bypass flow of fluid can be caused to flow from the contracting chamber to the expanding chamber, characterised by the steps of sensing the polarity of a load acting on said one member (90;146;202;311) with respect to a desired direction of relative movement between said members (90,91;146,148;202,203;311,312), causing said bypass flow of fluid from the contracting chamber to the expanding chamber whenever and only when said load acting on said one member (90;146;202;311) aids the desired direction of relative movement between said members (90,91;146,148;202,203;311,312), summing said supply and bypass flows in the expanding chamber, and controlling the magnitude of such summed flows. The method as set forth in claim 1 wherein said one member (90;146;202;311) has surfaces of equal area facing into said chambers (92,93;149,150;204,205;313,314). The method as set forth in claim 1 wherein said supply flow passes through a variable-orifice supply port (192,193), and said bypass flow passes through a variable-orifice bypass port (191,194). The method as set forth in claim 3 wherein said supply and bypass ports (192,193,191,194) are opened simultaneously. The method as set forth in claim 3 wherein the orifice areas of said supply and bypass ports (192,193,191,194) are controlled simultaneously. The method as set forth in claim 3 wherein said supply and bypass ports (192,193,191,194) have different gains. The method as set forth in claim 6 wherein the gain of said bypass port (191,194) is substantially greater than the gain of said supply port (192,193). The method as set forth in claim 1 wherein the step of controlling the magnitude of such summed flows, includes the further steps of controlling the magnitude of said supply flow, and controlling the magnitude of said bypass flow. The method as set forth in claim 8 wherein said supply flow is controlled such that said bypass flow is augmented by said supply flow so that such summed flows are just sufficient to achieve the desired velocity of said actuator. The method as set forth in claim 9 wherein the magnitude of said bypass flow is maximized and the magnitude of said supply flow is minimized. The method as set forth in claim 3 and further comprising the step of causing a return flow of fluid from the contracting chamber to a fluid return (R) whenever it is desired to cause relative movement between said members (90,91;146,148;202,203;311,312). The method as set forth in claim 11 wherein said return flow passes through a variable-orifice return port (190,195). The method as set forth in claim 12 wherein said supply and return ports (192,193,190,195) are opened simultaneously. The method as set forth in claim 12 wherein the orifice areas of said supply and return ports (192,193,190,195) are controlled simultaneously. The method as set forth in claim 12 wherein the gains of said supply and return ports (192,193,190,195) are substantially the same. The method as set forth in claim 1 wherein the step of causing said bypass flow includes the further step of opening a check valve (116;133,135;164,166;213,215) to permit said bypass flow only when said load aids the desired direction of relative movement between said members (90,91;146,148;202,203).

Inventors: GARNJOST KENNETH D, GARNJOST, KENNETH D.
Applicants: MOOG INC, MOOG INC.
Application year: 1987
Main classification: F15B11
IPC codes: F15B21, F15B13