Patent Publication Number: US-3874269-A

Title: Hydraulic actuator controls

Description:
United States Patent [191 [111 3,874,269 Walters Apr. 1, 1975 [54] HYDRAULIC ACTUATOR CONTROLS 691,449 4/1940 Germany 91/433 1,426,471 11/1968 Germany 91/461 [75] Inventor: Ronald Bernard Walters, Wembley,  
 England [73] Assignee: Sperry Rand Limited, London,  
 England [22] Filed: Nov. 1, 1973 [21] Appl. No.: 412,024  
 [30] Foreign Application Priority Data Nov. 8, 1972 United Kingdom 51471/72 [52] US. Cl 91/433, 91/451, 91/459, 137/106, l37/625.63, 137/625.64 [51] Int. Cl. F15b 13/044 [58] Field of Search 91/433, 451, 459, 461; 137/625.63, 106, 625.64  
 [56] References Cited UNlTED STATES PATENTS 2,944,530 7/1960 Sevcrinsen 91/275 X 3,003,474 10/1961 Giles ct a1 137/625.63 X 3,038,498 6/1962 Seavey 91/433 X 3,225,663 12/1965 Pelisson 137/106 X 3,555,969 l/1971 Shah 137/625.62 X 3,555,970 1/1971 Borgeson l37/625.62 X 3,763,746 10/1973 Walters 91/433 FOREIGN PATENTS OR APPLICATIONS 609,808 10/1948 United Kingdom 91/433 Primary ExaminerAlan Cohan Assistant Examiner-Gerald A. Michalsky Attorney, Agent, or F irm- Barnes, Kisselle, Raisch &amp; Choate [57] ABSTRACT A device for controlling a double acting unsymmetrical hydraulic actuator or two single-acting actuators has a main valve for selectively connecting two service lines to supply and return lines. The pressures at opposite sides of a&#34; flow sensor inserted in one of the supply and return lines are applied via a shuttle valve to feedback chambers of a flow pilot valve operated by a transducer to which an electrical signal is applied. The output ports of the flow pilot valve are connected to control chambers of the main valve and the pressures in these control chambers are also used to actuate the shuttle valve.  
 A pressure pilot valve can be connected in series with the flow pilot valve, the actuator pressures being applied via a second shuttle valve to feedback chambers of the flow pilot valve in opposition to a second electrical signal applied to a second transducer operating the pressure pilot valve. The second shuttle valve is also operated by the pressures in the main stage control chambers.  
 14 Claims, 5 Drawing Figures PILOT L L59 1L DRAIN PRESSURE 73 r&#34; REDUCE-IR SUPPLY MAINSTAGE VALVE PATENTEBAPR l 1975 SHEET 3 BF 5 PILOT DRAIN T l l PILO T VALVE I I .SHUTTL 30 TANK 1 SUPPLY VALV E I I i I\&#39; 3 %6 ss\s0 63 52 E f 49 I PRES SURE R EDUCE R SENSOR MAIN STAGE 37 VALVE 20 PATENTEU APR I 1 9 5 i&#39; ig L TSUPPLY TANK HYDRAULIC ACTUATOR CONTROLS This invention relates to hydraulic actuator controls.  
  United States of America Patent applications Nos. 337,107, l79,997 now U.S. Pat. No. 3,763,746 and 371,861 having a common assignee with the present application, described a device for controlling the flow of fluid to a hydraulic actuator responsivcly to an elec trieal input signal comprising a fluid pressure operated main control valve for regulating the fluid flow to the actuator, a pilot valve for controlling the main control valve, said pilot valve including a spool for regulating the fluid pressure for operating the main control valve, a transducer for producing a first force dependent upon an electrical input signal and for applying said force to said spool, an opposed piston arrangement associated with said spool, a flow sensor for producing a pressure difference dependent upon the rate of fluid flow to the actuator, and feedback means for applying said pres sure difference to said opposed piston arrangement to apply to said spool a second force opposed to said first force.  
 The flow sensor conveniently comprises a housing having at opposite ends thereof fluid connections connected in the flow path of the hydraulic fluid regulated by said main valve for operating said actuator, a movable member in said housing and dividing said housing into two chambers communicating respectively with said fluid connections, said housing and said movable member having cooperating surfaces thereon to define a fluid path interconnecting said chambers and having a restricted flow cross section, said flow cross section being variable dependently upon the position of said movable member in said housing, and spring means biassing said movable member to a position in which said flow cross section is at a minimum, said movable member being movable against said spring means responsively to fluid pressure difference between said chambers in a direction to increase said flow cross section. Preferably, said cooperating surfaces are so shaped that the pressure difference between said chambers is substantially directly proportional to the rate of fluid flow through said variable cross section, and the first force produced by the transducer is directly proportional to the electrical current supplied to the transducer.  
  The above-mentioned Patent applications illustrate the flow sensing means as being disposed in one of two lines leading from the main control valve to a doubleacting hydraulic actuator.  
 It is usual for the opposite sides ofa double-acting ac- I tuator to be connected to tank via non-return valves which permit hydraulic fluid to be sucked directly into the actuator when the load thereon is overrunning the actuator and so avoid cavitation, especially in the case of a non-symmetrical actuator. This results in that not all the fluid flowing to the actuator necessarily flows through the flow sensor whereby the flow sensor can no longer produce a pressure difference determined by the velocity of the load.  
  In the case wherein the device is used for controlling two single-acting hydraulic actuators instead of a double-acting actuator, in the, direction of main control valve operation in which the line by which fluid is being supplied to one actuator is that not containing the flow sensor, the flow sensor cannot measure the rate of fluid flow to said one actuator.  
  According to the present invention the flow sensing means is inserted in one of the hydraulic fluid supply and return lines connected to the main control valve and a shuttle valve is provided in the feedback means and is operable dependently upon the direction of movement of the actuator or actuators for determining in which direction the pressure difference produced by the flow sensing means is applied to the opposed piston arrangement.  
  In the case wherein the device is used to control a symmetrical or unsymmetrical double-acting actuator, the flow sensor is conveniently in the return line as that side of the actuator from which fluid is being returned is always at at least a slight positive pressure, even with an overrunning load.  
  In the case wherein the device is used to control two single-acting actuators, the flow sensor is conveniently in the supplyline as all the fluid supplied to one or other of the actuators flows through the supply line.  
  The device can be modified so that the flow sensing means is in one of the lines leading to a double-acting load, as in the above-mentioned Patent applications, if the shuttle valve is locked in one of its end positions.  
  The device of the invention may be additionally adapted for controlling the pressure of fluid applied to the hydraulic actuator responsively to an electrical input signal and for this purpose can further comprise an additional pilot valve for controlling the main control valve, said additional pilot valve including a pressure pilot spool for controlling the fluid pressure for op erating the main control valve, an additional transducer for producing a first force dependent upon an electrical input signal and for applying said force to said pressure pilot spool, a piston arrangement associated with said pressure pilot spool and preferably having equal opposed piston areas, pressure sensing means for producing a pressure dependent upon the fluid pressure applied to the actuator, preferably a pressure differential dependent upon the fluid pressure difference across the actuator, and additional feedback means for applying the last-mentioned pressure or pressure difference to the last-mentioned piston arrangement to apply to said pressure pilot spool a second force opposed to said first force, the first-mentioned and additional pilot valves being arranged in series, such that the first pilot valve is operative for flow control when the additional transducer is set to override the feedback from the pressure sensing means, and the additional pilot valve is operative for pressure control when the first-mentioned transducer is set to override the feedback from the flow sensing means, at least below a predetermined maximum pressure.  
  Preferably, the pressure sensing device is responsive to the direction of movement of the actuator and is adapted to apply the fluid pressures at the opposite sides of the actuator to the opposed piston areas in accordance with the direction of actuator movement, irrespective of the direction of the pressure difference. For this purpose, the pressure sensing device can comprise a shuttle valve whose housing has inlet ports connected to the hydraulic actuator and outlet ports connected to the opposed piston areas and contains a valve piston movable responsively to pressures in chambers connected to the fluid supply for operating the main control valve to connect the inlet ports alternately to the outlet ports such that one of the outlet ports is always subjected to the operating pressure at the side of the double-acting actuator being supplied with fluid and the other is always subjected to the operating. pressure at the side of the double-acting actuator from which fluid is being withdrawn.  
  The invention is further described, by way of example, with reference to the accompanying drawings, in which:  
  FIG. 1 is a flow diagram of a fluid flow control device in accordance with the invention with a flow sensor fit ted in the tank return line;  
  FIG. 2 is a similar flow diagram, but showing the flow sensor fitted in the fluid supply line;  
  FIG. 3 is a flow diagram showing a modification in which the flow sensor is fitted in one of the load lines;  
  FIG. 4 is a diagrammatic elevation of another embodiment of control device; and  
  FIG. 5 is a flow diagram of the control device of FIG. 4.  
  The drawings show devices for controlling the flow of fluid to at least one hydraulic actuator. The devices are of modular construction and like parts are denoted by like reference numerals in the five FIGS. of the drawings.  
  The device of FIGS. 1 to 3 comprises a main control valve and a pilot valve 19 for controlling the main valve 20. The main and pilot valves are arranged in separate valve blocks which are bolted together with the relevant fluid ports in communication with one another to provide the desired fluid connections as described hereinafter.  
  The main control valve 20 has a valve spool 21 provided with lands 22, 23 and 24 for controlling communication between two inlet/outlet ports 25 and 26 and three load ports 27, 28 and 29. A first load line 30 is connected to the ports 27 and 29 whereas a second load line 31 is connected to the central load port 28. In the arrangement of FIG. 1 a supply line 32 is connected to the port 26 and a return line 33 is connected to the port 25. The spool 21 is biassed towards its central neutral position by return springs 34 and 35 which are conveniently disposed in control chambers 36 and 37 at opposite ends of the spool 21. The valve spool 21 is displaced from its neutral position by the application of a pressure difference between the chambers 36 and 37 by means of the pilot valve 19.  
  The pilot valve 19 has a valve spool 40 which is provided with three lands 41, 42 and 43 controlling the fluid communication between a central inlet port 44 and drain ports 45 and 46 on the one hand, and control ports 47 and 48 on the other hand. The inlet port 44 is connected by a line 49 to the outlet of a pressure reducing valve 50 which serves to maintain a constant pressure in the line 49. The inlet to the pressure reducing valve 50 is connected to a supply line 51 which can, if  
 desired, be connected to the same external supply as the line 32. The pressure reducing valve 50 can be arranged in the same valve block as the pilot valve 19. The pilot valve spool 40 can be displaced from its neutral position by means of a linear force motor 52 which is adapted to produce a force directly proportional to the electrical current applied thereto. The armature of the force motor is supported on diaphragms which act as centering springs for the pilot valve spool 40. The control ports 47 and 48 are connected by respective control lines 53 and 54 to the control chambers 36 and 37 of the main control valve 20.  
  The pilot valve 19 has annular feedback chambers 55 and 56 at the sides of the lines 41 and 43 facing the respective ends of the spool 40. The chambers 57 and 58 at the ends of the spool 40 are connected to a drain line 59 as are the drain ports 45 and 46. Feedback pressures are applied to the feedback chambers 55 and 56 by means of a shuttle valve 60 which may be disposed in the same valve block as the pilot valve 19. The shuttle valve 60 has a shuttle 61 with three lands 62, 63 and 64 for controlling fluid connection between inlet ports 65 and 66 and outlet ports 67, 68 and 69. The central port 68 is connected by a line 70 to the feedback chamber 55 and the outlet ports 67 and 69 are connected by a line 71 to the feedback chamber 56. Chambers 72 and 73 at opposite ends of the shuttle 61 are connected to the control lines 53 and 54, respectively, so that when the pressure in the line 53 is higher than that in the line 54, the shuttle 61 is held in its righthand position as illustrated in which the inlet port 66 is connected via the port 68 to the lefthand feedback chamber 55, and the inlet port 65 is connected via the port 67 to the righthand feedback chamber 56. When the pressure in the line 54 is higher than that in the line 53, the shuttle 61 is moved to the left to connect the port 66 via the port 69 to the righthand feedback chamber 56 and the port 65 to the feedback chamber 55.  
  The pressure difference applied to the feedback chambers 55 and 56 is produced by a flow sensor 74 which is disposed in the line 33 connected to the inlet/- outlet port 25 of the main control valve 20. The flow sensor comprises a housing which is connected by fluid connections at its opposite ends in the line 33. A movable member in the housing divides the housing into two chambers communicating respectively with the fluid connections. Cooperating surfaces in the housing and on the movable member define a fluid path interconnecting said chambers. The flow cross section of the fluid path is variable dependently upon the position of the movable member which is itself biassed by a spring to a position in which the fluid path has a minimum cross section. Said cooperating surfaces are so designed that the pressure drop between the two chambers of the flow sensor is directly proportional to the rate of fluid flow through the flow sensor. The two chambers of the flow sensor are connected by lines 75 and 76 to the inlet ports 65 and 66 of the shuttle valve.  
  The flow sensor 74 is preferably constructed in the same manner as the flow sensor described in the aforementioned United States of America Patent application Ser. No. 179,997. The porting and land construction of the pilot valve 19 and the main control valve 20 are preferably as described in the aforementioned United States of America, Patent applications Ser. Nos. 179,997 and 371,861.  
  The device is shown in FIG. 1 as used for controlling a double-acting unsymmetrical hydraulic actuator 80, but is equally suitable to control a symmetrical actuator or a hydraulic motor. In other words, the opposite effective areas of the piston 81 of the actuator are unequal due to the piston rod 82 passing out of one end only of the actuator cylinder. The lefthand chamber 83 of the actuator is connected to the line 30 and the righthand chamber 84 of smaller effective area is connected to the line 31. The chambers 83 and 84 are connected by respective non-return valves 85 and 86 to a tank line 87. Under those conditions in which the load on the actuator is overrunning the actuator there may be a tendency for a vacuum to be formed in one of the chambers 83 and 84. The non-return valve 85 or 86 enables fluid to be sucked from the tank into the respective chamber to prevent cavitation. For example, if the actuator piston 81 is moving to the right and the force exerted by the load is also acting to the right, the pressure in the chamber 84 will be higher than that in the chamber 83, and due to the unsymmetry of the actuator the quantity of fluid flowing into the chamber 83 will be substantially in excess of that flowing out of the chamber 84. Due to the controlled throttling effected by the main control valve 20, sufficient fluid is prevented from flowing into the chamber 83 via the control valve. The quantity required to fill the chamber 83 is made up by fluid sucked from the tank through the non-return valve 85.  
  In the devices described in the above-mentioned Patent applications the flow sensor is arranged in one of the lines between the main control valve and the actuator. If the flow sensor 74 were arranged in the line 30 leading to the larger area chamber 83 the pressure drop across the flow sensor would not be a measure of the velocity of the load since the quantity of fluid flowing through the flow sensor would not be equal to the quantity of fluid flowing into the chamber 83 when the load is overrunning the actuator. In FIG. 1 the line 32 is made the supply line and the line 33 containing the flow sensor 74 is made the return line so that all of the fluid displaced from the double-acting actuator, whether or not it is being overrun by the load, flows through the flow sensor. This enables the flow sensor to produce a pressure difference dependent upon the velocity of the load.  
  In operation of the actuator 80 of FIG. 1, let it be supposed that the force motor 52 is energised to displace the pilot valve spool 40 to the right as illustrated. This connects the control chamber 36 of the main control valve to the pilot supply and connects the control chamber 37 to drain. The spool 21 of the main control valve is thereby also displaced to the right as illustrated. The supply line 32 is thereby connected via the port 29 to the lefthand chamber 83 of the actuator 80 so that the load moves to the right. Fluid displaced from the chamber 84 flows via the ports 28 and through the return line 33 and thereby through the flow sensor 74 to produce a pressure difference between the lines 75 and 76 and thereby between the ports 65 and 66 of the shuttle valve 60. It will be noted that since the flow sensor is in the return line the pressure at the port 65 is always higher than at the port 66, whichever way the load is being moved. The pressure difference between the ports 65 and 66 is applied by the shuttle valve 60 to the feedback chambers and 56 and shuttle valve serves to apply this pressure difference in a direction dependent upon the direction of movement of the load. Thus, when the load is being moved to the right, the pressure in the main valve control chamber 36 is necessarily higher than that in the chamber 37, as otherwise the spool 21 would not be displaced to the right as illustrated. This means that the shuttle valve 60 is necessarily in its right-hand illustrated position so that the higher pressure port is connected to the feedback chamber 56, and the lower pressure port 66 is connected to the feedback chamber 55. The pressure difference between the chambers 56 and 55 produces a net force acting on the valve spool 40 to the left, i.e., in a direction to oppose the electrically dependent force produced by the force motor 52, thereby tending to return the pilot valve to its null position. In the steady state the pilot valve is returned to its null position with the main control valve suitably displaced to produce a load velocity at which the pressure drop across the flow sensor balances the force applied by the force motor. The velocity of the load is, therefore, in the steady state solely dependent upon the electrical current supplied to the force motor 52.  
  For operation of the load to the left, the force motor 52 is energised in the opposite direction to apply a force acting to the left on the spool 40 of the pilot valve. The main control spool 21 is moved to the left and the shuttle valve 60 changes over so that the pressure difference across the flow sensor 74 results in a feedback force being applied to the pilot spool 40 in the righthand direction to oppose the electrically produced force and the steady state is reached described above.  
  In FIG. 2 the device is used for controlling two singleacting actuators and 91. The actuator 90 is connected to the line 30 and the actuator 91 is connected to the line 31. The shuttle valve 92 differs from the shuttle valve 60 only in that its shuttle 93 has four lands 94, 95, 96 and 97 such that in the righthand position of the shuttle 93 the port 66 is connected via the port 69 to the righthand feedback chamber 56 of the pilot valve 19 and the port 65 is connected via the port 68 to the lefthand feedback chamber 55. When the shuttle 93 is moved to the left due to the pressure in the control line 54 being higher than that in the control line 53, the port 66 is connected via the port 68 to the feedback chamber 55 via the port 67 to the feedback chamber 56.  
  If the flow sensor 74 were arranged in one of the load lines 30 or 31 described in the above-mentioned Patent applications, then the hydraulic fluid flowing to one of the actuators 90 and 91 in one direction of operation of the main control valve 20 would flow through the flow sensor, but in the other direction of operation the hydraulic fluid flowing to the other actuator would not flow through the flow sensor. In this case, the fluid displaced from the first actuator would flow to drain through the flow sensor, but this might not be related to the velocity of the second actuator. To avoid this difficulty the flow sensor 74 is arranged as shown in FIG. 2 in the line 33 leading to the port 25 of the main control valve 20, and in this case the line 33 is made the supply line and the line 32 from the port 26 is made the return line leading to tank. Thus, in each case the fluid flowing to the actuator 90 or 91 flows through the flow sensor 74. The shuttle 93 replaces the shuttle 61 in the shuttle valve 92 to reverse the operation of the shuttle valve because of the reversal of the supply and tank connections to the lines 32 and 33 as compared with FIG. 1. The device is otherwise identical to that of FIG. 1 and its operation is as described with reference to FIG. 1.  
  A comparison of FIGS. 1 and 2 shows that, apart from the above-mentioned change to the shuttle of the shuttle valve, none of the components or the lines interconnecting the components of the device has been changed. The only changes are in the external connections to the device, i.e., to the inlet/outlet lines 32 and 33 and&#39;to the load lines 30 and 31. Thus, the device can be put to different uses without altering the modular construction of the device. It is relatively simple to choose the appropriate shuttle for the shuttle valve.  
  FIG. 3 illustrates how the same device may be adapted for arrangement of the flow sensor 74 in one of the load lines as described in the above-mentioned Patent applications, again without altering the modular construction of the device. In FIG. 3 the lines 32 and 33 are made the load lines and are connected to opposite ends of a double-acting actuator 100. The line 31 is made the supply line and the line is connected to tank. The shuttle valve 60 is rendered ineffective by blocking its spool 61 in its righthand position as illustrated by means of a plug 101. The line 51 for the pilot supply can be connected internally in this case to the line 31, such connection being blocked by means of a suitable plug (not shown) in the arrangements of FIGS. 1 and 2.  
  The device as illustrated in the drawings is only suitable for flow control. It can be modified also as described in the above-mentioned Patent Applications to provide optionally for flow control or pressure control, or flow control with maximum pressure override. In such a case a second pilot valve is connected in series with the pilot valve 19. The second pilot valve is a three-port valve having an inlet port, an outlet port and a drain port. The inlet port is connected to the outlet from the pressure reducing valve and its outlet port is connected to the port 44 of the first pilot valve 19. The second pilot valve is controlled by a second force motor and has opposed feedback chambers effectively identical to the feedback chambers and 56 of the pilot valve 19. However, these feedback chambers are connected by means of a second shuttle valve to the load lines 30 and 31 of FIG. 1 and 2 or to the load lines 32 and 33 of FIG. 3. Thus, the force applied to the spool of the second pilot valve by the second force motor is opposed by a feedback force produced by the pressure difference across the actuator. The shuttle valve is operated by the pressure difference between the control chambers 36 and 37 of the main control valve 20 like the shuttle valve 60. For operation in the so-ealled flow control mode which has been previously described the second pilot valve is energised to fully connect its inlet port to its outlet port so that the pressure from the pressure reducing valve 50 is applied to the pilot valve 19 for flow control. The extent ofenergisation of the second flow motor can be so chosen that the second pilot valve begins to throttle the fluid flow and thereby override the flow control when the pressure difference across the load reaches a point at which it can overcome the force applied by the second force motor. For operation in the so-called pressure control mode the pilot valve 19 is used as a directional control valve, and for this purpose the force motor 52 is fully energised in one direction or the like.  
  The second pilot valve is preferably arranged in its own valve block and the second shuttle valve can be arranged in the same valve block as the second pilot valve.  
  FIGS. 4 and 5 of the drawings illustrate another embodiment of flow control device to provide optionally for flow control or pressure control. For flow control the parts are effectively the same as those described with reference to and as illustrated in FIG. I and are, therefore, denoted with the same reference numerals. As shown in FIG. 4, the main valve 20 is housed in a main block 110 which also contains the flow sensor 74. The main valve block 110 is mounted on a port valve plate 111 provided with four ports 14], 142, 143 and 144 for the fluid flow to be controlled, as will be described with reference to FIG. 5. Mounted on the main valve block is a pressure pilot shuttle block 112 containing a shuttle valve 113 and a load shunt shuttle valve 114. Mounted on the block 112 is a flow pilot shuttle block 115 containing the flow pilot shuttle valve 60 and the pressure reducing valve 50. Mounted on the valve block 115 is a block 116 containing the flow control pilot valve 19, and a pressure control pilot block 117 containing a pressure control pilot valve 118. A housing 119 for the force motor 52 is secured to the block 116 and a housing 120 for a linear force motor 121 is secured to the block 117.  
  As shown in FIG. 5, the pressure control pilot valve 118 has a spool 122 which is connected to the force motor 121. The spool 122 has four lands of which the two middle lands cooperate with an inlet port 123, an outlet port 124 and a drain port 125. The inlet port 123 is connected to the line 49 from the pressure reducing valve 50. The outlet port 124 is connected by a line 126 to the inlet port 44 of the flow control pilot valve 19 and the drain port is connected to a drain line 127 which is connected to tank independently of the main flow so that any back pressures which may be developed in the main flow return line will not be fed back to the outlet side of the pilot valves. The outlet port 124 is also connected via a fixed restriction 128 to tank. The outer lands of the spool 122 have through bores in which needle-like plungers 129 and 130 are freely slidable but sealingly guided. The outer ends of the spool are connected to tank and the effect of the needle-like plungers 129 and 130 is to unbalance the areas of the lands exposed to control chambers I31 and 132 between the inner and outer lands. A pressure difference applied to the chambers I31 and 132 can thereby oppose the force of the force motor 121. In the pressure control mode this pressure difference can be very high which is the reason for making the area unbalance of the lands very small, whereby the net force due to the hydraulic pressure difference is comparable with the force produced by the force motor 121.  
  The pressure pilot shuttle valve 113 is identical to the flow pilot shuttle valve. It has inlet ports 65a and 66a connected by feedback lines 75a and 76a to the load lines 31 and 30, respectively. It also has a central outlet port 68a connected by a line 70a to the feedback chamber 132 of the pressure control pilot valve 118 and outlet ports 67a and 69a connected by a line 71a to the feedback chamber 131. Chambers 72a and 73a at opposite ends of the shuttle 61a of the shuttle valve 113 are connected to the lines 53 and 54, respectively.  
  The load shunt shuttle valve 114 comprises a shuttle 133 having lands 134 and 135. A chamber 136 at the lefthand end of the shuttle 133 is connected via the line 126 to the outlet port 124 of the pressure pilot valve 118. A chamber 137 at the other end of the shuttle 133 is connected to the drain line 127. A spring 138 disposed in the chamber 137 biasses the shuttle 133 in a direction opposite to the force due to the pressure difference between the chambers 136 and 137. The land 134 controls a by-pass port 139 connected to the line 7011 from the outlet port 68a of the pressure pilot shuttle valve 113. Drillings 140 connect the space between the lands 134 and to the chamber 137.  
  During normal operation in the pressure control mode or the flow control mode the pressure in the line 126 is sufficient to displace the spool 133 of the load shunt shuttle valve 114 against the spring 138 so closing the by-pass port 139. To operate the device in the flow control mode as described with reference to FIG. 1 of the drawings, the force motor 121 is energised to displace the spool 122 to the right against whatever feedback pressure difference might be applied between the chambers 131 and 132, whereby the inlet port 123 is fully connected to the outlet port 124. Some of the lluid passing through the valve 118 flows to drain through the throttle 128 but the pressure in the line 126 leading to the inlet port 44 of the flow pilot valve 19 remains constant at substantially the outlet pressure of the pressure reducing valve 50.  
  To operate the device in the pressure control mode the force motor 52 is energised to override the feedback pressure difference between the chambers and 56 and in a direction to determine the direction in which the pressure difference is applied. If it is supposed that the force motor 52 displaces the pilot valve spool 40 to the right of the illustrated position, the inlet port 44 is connected to the outlet port 47, whereby the line 126 is connected to the lefthand control chamber 36 of the main valve 20, and the drain port 46 is connected to the outlet port 48 to connect the righthand control chamber 37 to drain. The shuttle of the pressure pilot shuttle valve 113 is displaced to the right by the pressure difference between the lines 53 and 54 to interconnect the ports 66a and 68a whereby to connect the service line 30 via the feedback line 76a to the control chamber 132 of the pressure pilot valve 118, and to interconnect the ports a and 67a whereby to con nect the service line 31 via the feedback line a to the control chamber 131 of the pressure pilot valve 118. The main spool 21 is displaced to the right so that the service line 30 is controllably connected via the ports 26 and 29 to the supply line 32 and the service line 31 is controllably connected via the ports 28 and 25 to the return line 33. Thus, the line 30 is at a higher pressure than the line 31 and the resulting pressure difference between the chambers 132 and 131 urges the spool 122 of the pressure pilot valve 118 to the left in opposition to the force produced by energisation of the force motor 121. One ofthe inner lands of the spool 122 controls the inlet port 123 and the drain port 125 normally remains closed by the inner land. The fluid from the line 49 is throttled at the inlet port 123 and flows via the outlet port 124 into the line 126 where the How is divided, part of this fluid flowing to the flow pilot valve 19 and the other part flowing through the fixed throttle 128 to tank. The pressure in the line 126 is thereby dependent upon the extent of throttling of the fluid flowing through the inlet port 123 and in turn controls the pressure difference applied to the main spool 21. In the steady state the force produced by the force motor is balanced by the feedback pressure difference between the chambers 131 and 132. The energisation of the force motor thereby determines the pressure difference across the actuator (FIG. 4) and thereby the force applied by or to the actuator. It will be noted that the same steady state is reached whether the actuator is moving against the load or the load is overrunning the actuator.  
  In the event ofa shock load being applied to the actuator the pressure difference between the chambers 132 and 131 is suddenly increased to displace the spool 122 so far as to uncover the drain port 125. The port may be an annular port and serves under such conditions to dump the pressure in the line 126 to drain, effectively by-passing the fixed throttle 128, in order to rapidly centre the main valve spool 21. As soon as the shock load has passed the drain port 125 is once again closed.  
  When operating in the pressure control mode with the load stationary, or almost stationary, the main spool 21 will tend to adopt central position which the service lines 30 and 31 are disconnected from the supply and return lines 32 and 33. The spool 21 would thereby no longer control any flow so that it could not control the pressures at the opposite sides of the actuator. Under such conditions, the pressure in the line 126 will have become virtually zero or very low. This enables the spring 138 to displace the shuttle 133 of the load shunt shuttle valve 114 to the left so opening the by-pass port 139. This connects the service line 30 via the line 76a, the interconnected ports 66a and 68a of the pressure pilot shuttle valve, the line 700, the drillings 140 and the chamber 137 to the return line 127, thereby bleeding some hydraulic fluid from the high pressure side of the actuator. To replenish such bled fluid the main spool 21 must be displaced from its neutral position, which enables the main valve 20 to continue to control the pressures at the opposite sides of the actuator.  
  When operating in the flow control mode, the energization of the force motor 121 determines the maximum pressure difference which may occur across the actuator. When this maximum pressure difference is reached the feedback pressure difference between the chambers 131 and 132 begins to overcome the force of the force motor 121 and to thereby reduce the pressure available in the line 126. A maximum pressure override is thereby provided when opening in the flow control mode.  
  The port plate 111 of FIGS. 4 and 5 contains four ports 141, 144, 142 and 143 connected respectively to the lines 30, 33 (via the flow sensor 74), 31 and 32. In the use of the device as described above, the ports 141 and 142 are service ports and the ports 144 and 143 are tank and supply ports, respectively. If it is desired to operate the flow sensor 74 in the: supply line rather than the return line, the port 144 is made the supply port and the port 143 the return port, and the shuttles 61 and 6111 of the shuttle valves 60 and 113 are replaced by four-land shuttles identical to the shuttle 93 shown in FIG. 2 of the drawings in order to reverse the connections from the feedback lines to the pilot valves.  
  It will be seen that the same hardware, apart from the shuttles 61 and 610, can be used irrespective of whether the flow sensor is to be in the return line, the supply line or one of the service lines. The only changes required, apart from the shuttles, are in the external connections to the port plate 111.  
  If the pressure control mode is not required the pressure pilot shuttle block 112, the pressure pilot block 117 and the force motor 121 are omitted, the flow pilot shuttle block 115 being secured to the main valve block 110 and the outlet line 49 from the pressure reducing valve 50 being connected to the port 44 of the flow pilot valve 19.  
 1 claim:  
  1. In combination: double-acting hydraulic actuating means having opposite sides and a device for controlling said hydraulic actuator means respectively to an electrical input signal, said device comprising supply and return lines, two actuator service lines connected to said opposite sides of said actuator means, a fluid pressure operated main valve, supply and return lines connected to said main valve for the supply of hydraulic fluid thereto and the return of hydraulic fluid therefrom, two actuator service lines connected between said main valve and said opposite sides of said actuator means, respectively, said valve having means to selectively and controllably connect said two actuator service lines to said supply and return lines, a flow pilot valve for controlling the main valve, said pilot valve including a spool for regulating the fluid pressure for operating the main valve, transducer means for producing a first force dependent upon said electrical input signal and for applying said first force to said spool, opposed piston means operative upon said spool, a flow sensor disposed in one of said supply and return lines for producing a pressure difference dependent upon the rate of fluid flow therein, and feedback means including shuttle valve means operable dependently upon the direction of fluid flow in said service lines for applying the pressuree difference produced across the flow sensor to the opposed piston means in a direction to apply to said spool a second force opposed to said first force.  
  2. The combination according to claim 1 in which said shuttle valve means has at least one operating chamber connected to the fluid pressure regulated by the pilot valve and for operating the main valve,  
  3. The combination according to claim 1 in which said main valve includes spring means biasing it to a neutral central position and control chambers for displacing it therefrom, said control chambers being connected to fluid pressures controlled by said pilot valve.  
  4. The combination according to claim 3 in which said shuttle valve means comprises a shuttle and opposed chambers, said shuttle being movable by fluid pressures applied to said opposed chambers and said opposed chambers being connected respectively to said control chambers of said main valve.  
  5. The combination according to claim 1 in which said opposed piston means comprises feedback chambers and equal opposed piston areas in said feedback chambers, said feedback means connecting said feedback chambers to said flow sensor via said shuttle valve means and said shuttle valve means being operable to apply the pressure drop across the flow sensor to said opposed piston areas in a direction to oppose the transducer force, whichever direction the pilot spool is displaced by the latter.  
  6. The combination according to claim 1 in which said pressure drop produced by said flow sensor is directly proportional to the rate of fluid flow therethrough and said first force produced by said transducer means is directly proportional to said electrical input signal.  
  7. The combination according to claim 1 in which said device further comprises a pressure pilot valve in series with said flow pilot valve, said pressure pilot valve including a spool for regulating the fluid pressure for operating the main valve, second transducer means for producing a force dependent upon a second electrical signal and for applying the last-mentioned force to the pressure pilot spool, second piston means operative upon said pressure pilot spool, and pressure feedback means including second shuttle valve means operable dependently upon the direction of fluid flow in said service lines for applying the pressure in at least one of the service lines to said second piston means in a direction to apply to said pressure pilot spool a further force opposed to the second transducer force.  
  8. The combination according to claim 7 in which said second shuttle valve means is operable by the fluid pressure regulated by the pilot valves.  
  9. The combination according to claim 7 in which said flow pilot valve has an inlet port connected to an outlet port of said pressure pilot valve.  
  10. The combination according to claim 7 in which said device further comprises load shunt valve means connected at least indirectly to said service lines, said load shunt valve means having restriction means and being adapted to connect said restriction means in parallel with the actuator means when the main valve is at least substantially in its neutral position and the actuator means is being controlled by said second electrical input signal with one of said service lines at a higher pressure than the other.  
  11. The combination according to claim 10 in which said flow pilot valve has an inlet port connected to an outlet port of said pressure pilot valve and in which said load shunt valve means is responsive to the pressure at said outlet port of said pressure pilot valve and is ope rative to connect said restriction means in parallel with the actuator means when the pressure at said outlet port of said pressure pilot valve falls to a relatively low value.  
  12. The combination according to claim 10 in which said load shunt valve means is operative to connect the service line at the higher pressure to drain via said second shuttle valve means and via said restriction means.  
  13. The combination according to claim 7 in which said second piston means comprises feedback chambers and equal opposed piston areas in said feedback chambers, said pressure feedback means connecting said feedback chambers via said second shuttle valve means to said service lines, said second shuttle valve means being operable to apply the pressures in said service lines to the last-mentioned opposed piston mean to oppose the second transducer force, whichever the direction of fluid flow in the service lines.  
  14. The combination according to claim 7 in which the force produced by said second transducer is directly proportional to the magnitude of said second electrical input signal.