Patent Publication Number: US-3875849-A

Title: Electro-hydraulic proportional servo actuator

Description:
United States Patent Patel SERVO ACTUATOR [75] inventor:  
 [73] Assignee:  
 Wis.  
 [22] Filed: June 11, 1973 [2]] Appl. No.: 368.822  
 [63] Continuation-in-part of Scr. No. 288.126. Sept, 11.  
 1972. abandoned.  
 [52] US. Cl. 91/367; 91/374; 91/417; 91/459 [51] Int. Cl..... FlSb 9/10; Fl5b 13/16; F1513 15/17 [581 Field of Search 91/382. 374. 367. 459  
 [561 References Cited UNlTED STATES PATENTS 1.585.529 5/1926 Boving 91/374 2.018.977 10/1935 Spellman et a1. 91/374 2.373.621 4/1945 Wales 1 1 1 91/374 2.941.601 6/1961) Best 91/374 2.945.449 7/1960 Febure et a1.... 91/374 2.969.808 1/1961 Horlaeher 91/374 3.044.451 7/1962 Morrison 1. 91/367 3.739.813 6/1973 Worden 91/459 Related U.S. Application Data ELECTRO-HYDRAULIC PROPORTIONAL Kishor J. Patel, Hales Corners. Wis.  
 Applied Power lnc., Milwaukee.  
 Apr. 8, 1975 1571 ABSTRACT Two electro-hydraulic proportional servo actuators are disclosed and each comprises an electro magnetic device such as torque motor or proportional solenoid energizable to operate a three-way hydraulic servo valve which. in turn. effects proportional amplified 1in ear movement of a spring centered hydraulic ram in the actuator. The ram comprises two piston areas. one larger than the other. and the smaller piston area is always exposed to the full fluid pressure of the system. The servo valve operates to supply or relieve pilot fluid pressure on the larger piston area to effect ram movement because of the pressure differential. in one actuator in accordance with the invention. feedback means are connected between portions of the threeway valve and the ram to hold the ram in desired position. In the said one actuator manually operable override means are provided to enable ram operation if the electro magnetic device fails. In another actuator in accordance with the invention. the three-way valve is directly associated with the ram for feedback purposes and separate feedback means and manually operable over-ride means between the servo valve and ram are eliminated.  
 17 Claims, 10 Drawing Figures PATENTEDAPR 81975 saw 5 or g 05 am 9 a 5 mwwm EN 2 P L Em mg :m J... L/ L ummm 8 m am \1 @8 EN PATENTEUAPR ma 3, 8758 I19 sum s [15 g FIG. 10  
 ELECTRO-HYDRAULIC PROPORTIONAL SERVO ACTUATOR This application is a continuation-in-part of my copending US. application Ser. No. 288,126 filed Sept. II, 1972 now abandoned.  
 BACKGROUND OF THE INVENTION 1. Field of Use This invention relates generally to electro-hydraulic proportional servo actuators having a ram proportionally movable to effect a function in response to an input signal from an electrical solenoid or other input device.  
 2. Description of the Prior Art A wide variety of hydraulic servo valves and servo actuators are known in the prior art. Many such devices comprise a signal input stage (including pilot valve means and responsive to mechanical or electrical input signals), an amplifier stage, and a power output stage including a ram which is movable to perform some function. Usually, the pilot valve is manually or solenoid operable. In some instances, two solenoids are required to effect opposite linear movement of the ram. Some prior art devices employ feedback control means to regulate pilot fluid pressure and such means sometimes comprise a cam follower which rides on an inclined surface of the ram and is responsive to ram movement to provide feedback control for the pilot valve.  
 SUMMARY OF THE PRESENT INVENTION An electro-hydraulic proportional servo actuator comprises a proportional solenoid, a three-way hydraulic servo valve, and a hydraulic ram linearly movable in opposite directions from a spring-centered position in a first bore in the actuator housing in response to solenoid and servo valve operation to effect control functions. The ram cooperates with its bore to define a high pressure fluid chamber and a pilot pressure fluid chamber. Ram piston area in the pilot pressure fluid chamber is, for example, twice that of the ram piston area in the other chamber. The high pressure fluid chamber is always pressurized with full system pressure. Pilot fluid flow to and from the pilot pressure chamber is controlled by the three-way servo valve comprising a spring-centered hollow servo spool which is slideable in a spring-centered hollow servo sleeve which, in turn, is slideable in a second bore in the housing. The sleeve and spool each have metering orifices therein which are out of registry (closed) when the solenoid is deenergized and cooperate to define a fluid metering aperture when the solenoid is energized to move the spool. Fluid flow to or from the pilot pressure chamber causes the ram to move because of the pressure differential on the ram. Feedback means connected between the sleeve and ram and responsive to ram movement, move the sleeve in the same direction as the spool to then close the fluid metering aperture and hold the ram in the desired position. The feedback means comprise a first cam follower on the sleeve which, in one embodiment, bears directly against an inclined surface on the ram and, in another embodiment, bears against a first inclined surface on a slideably movable second cam follower which, in turn, bears against a first inclined surface on a slideably movable second cam follower which, in turn, bears against the aforesaid or second inclined surface on the ram. Manually operable over-ride means are provided to enable ram operation if the solenoid fails. In the aforesaid other embodiment the override means comprises a manually operable push-pull rod for moving the inclined surface of the second cam follower to cause servo sleeve movement. In another version of the aforesaid other embodiment the override means comprises a manually operable linkage connected to the servo spool to effect movement thereof. In still another version of the aforesaid other embodi ment the over-ride means comprise a manually operable push-pull rod connected to the armature of the pro portional solenoid.  
  An electro-hydraulic proportional servo actuator in accordance with the invention has serveral advantages over prior art devices. For example, the actuator can be operated with pneumatic or mechanical input devices instead of a solenoid. Also, by changing the angles of the inclined surfaces on the ram and cam follower, different amplification of the ram stroke can be obtained. In the event of a malfunction, the ram centers itself by spring action. In the event of electrical malfunction in some of the disclosed embodiments, the ram is manually movable by means of manually operable over-ride means of a configuration adapted to suit various applications. The solenoid is attached to the actuator housing so that it is adjustably movable to null position to overcome the effects of manufacturing tolerances. Furthermore, the actuator could be made integral with a hydraulic valve or other device with it is intended to operate.  
  These and other objects and advantages will appear hereinafter as this disclosure progresses, reference being bad to the accompanying drawings.  
 DRAWINGS FIG. 1 is a schematic diagram showing an electrohydraulic proportional servo actuator in accordance with the invention and electrical and hydraulic circuitry therefor;  
  FIG. 2 is an enlarged cross-sectional view of the actuator shown in FIG. 1&#39;,  
  FIG. 3 is an enlarged elevational view of the end of the actuator shown in FIGS. 1 and 2;  
  FIG. 4 is a graph showing the stroke of the actuator ram plotted against input current to the actuator solenoid;  
  FIG. 5 is an enlarged view ofa portion of the actuator shown in FIG. 2 and shows the servo valve spool moved to a desired position;  
  FIG. 6 is a view similar to FIG. 5 but showing the servo valve sleeve moved by the feedback means to a valve closed position whereby the cam is held in the desired position;  
  FIG. 7 is a cross-sectional view of the servo valve means shown in FIG. 2 and showing another embodiment of manual over-ride means connected thereto;  
  FIG. 8 is a side elevational view of another type of proportional solenoid having still another embodiment of manual over-ride means connected to the solenoid armature;  
  FIG. 9 is an enlarged cross-sectional view of another embodiment of an actuator in accordance with the invention and shows, in schematic form, electrical and hyraulic circuiting therefor; and  
  FIG. 10 is a cross-sectional view taken on line l0 10 of FIG. 9.  
 DESCRIPTION OF PREFERRED EMBODIMENTS First Embodiment Referring to FIG. 1, the numeral designates an electrohydraulic proportional servo actuator in accordance with the present invention. Actuator 10 comprises a housing 11 into which a proportional solenoid 12 extends and from which a power ramor spool 70 extends. In operation, as hereinafter described in detail, operation of a rheostat 29 effects proportional linear movement of ram 70 of actuator 10 to effect some control function, such as operation of a valve (not shown).  
  Housing 11 is also provided with hydraulic fluid input and output ports 15 and 16, respectively. Input port 15 of actuator 10 is connected by a hydraulic fluid pressure line 17 to the discharge end of a conventional motor driven hydraulic fluid pump 18 and the pump is connected by a hydraulic fluid supply line 19 to a fluid reservoir 20. Output port 16 of actuator 10 is connected by a hydraulic fluid return line 21 to reservoir 20. A conventional fluid pressure relief valve 22 is connected between fluid line 17 and reservoir 20.  
  Solenoid 12 is provided with electrical connection terminals 25 and 26. Terminal 25 of solenoid 12 is connected by an electrical conductor 27 to the movable contact 28 of a rheostat 29 which has the end terminals and 36 of its resistance element 30 connected across the positive and negative terminals 32 and 33, respectively, of an electrical power source such as a battery 34. Terminal 26 of solenoid 12 is connected by an electrical conductor 40 to the movable contacts 41 and 42 of limit switches 43 and 44, respectively. The stationary contacts 46 and 47 of the switches 43 and 44, respectively, are connected to the terminals 32 and 33, respectively, of battery 34.  
  Referring to FIG. 2, actuator 10 is shown in detail and is to be understood generally to comprise three stages. The first stage comprises solenoid 12, a serve spool 50 slideably mounted in a servo sleeve 51, which, in turn, is slideable in a chamber, bore or passage 52 in housing 1 1, a servo spool biasing spring 53, and a servo sleeve biasing spring 54.  
  The second stage comprises a cam follower sleeve 57 slideable in a chamber, bore or passage 58 in housing 11, and a follower poppet 59 slideable in a bore or passage 60 in sleeve 57. Bore 58 is transverse i.e., at right angles to, bore 52 and interconnected thereto and em closed at its upper end by an externally threaded gland 61 which has a rod hole 62 therethrough. The second stage further comprises a slideable push-pull rod 64 which extends through hole 62 in gland 61 and has an operating handle 65 at its outer end, a snap ring 66, a follower spring 67, and a retainer spring 68.  
  The third stage comprises power spool or ram which is slideable in a chamber, bore or passage 71 in housing 11 and bore 71 is transverse, i.e., at right angles to, bore 58 and interconnected thereto. One end of bore 71 is closed by a threaded plug 72 and the other end is closed by externally threaded gland 73 which has a ram accommodating hole 74 therethrough.  
  Solenoid 12 is cylindrical in form and has external threads 75 which engage complementary internal threads 76 in a mounting hole 77 in housing 11. Thus, solenoid 12 is adjustable rotatable to move it inwardly or outwardly so that its armature 78, which is axially movable forward or backward from a centered position, can be located at the null point.  
  Armature 78 of solenoid 12 bears against servo spool 50 but is not physically connected thereto. Servo spool 50 has a central passage which is widened at one end to provide a shoulder 86 and another end passage to drain fluid. One end of servo spool biasing spring 53 bears against shoulder 86 and the other end of spring 53 bears against the rear end of a cam follower member 87 which has its shank press-fitted in a central passage 88 in servo sleeve 51. Member 87 is movable with servo sleeve 51. Member 87 bears against a conical cam surface 103 on sleeve 57.  
  Servo sleeve 51 has three annular grooves and ports 90, 91 and 92. Servo spool 50 has one metering land 106 and annular grooves 94 and 93 and a port 95.  
  Fluid supply port 15 in housing 11 is connected by a passage 96 to bore 52 and by passages 97 and 98 to bore 71. Fluid return port 16 in housing 11 is connected by passage 99 and 100 to bore 52. Bore 52 is connected by a passage to bore 71. As FIGS. 2, 5 and 6 show, the relatively movable servo spool 50 and servo sleeve 51 cooperate. as hereinafter explained, so that the land 106 on servo spool 50 controls fluid flow from passage 96 to passage 105 and from passage 105 through passages 85 and 95, to passages 99 and 100.  
  Ram 70 cooperates with bore 71 to define two separate chambers 110 and 111 to which the passages 98 and 105, respectively, are connected. Piston ring seals 112 and 113 on ram 70 prevent fluid flow from cham bers 110 and 111, respectively. Ram 70 is adapted for sliding axial movement in opposite directions in bore 71 and is manintained in a centered position by a biasing spring 114 disposed between a shoulder 115 on the ram and the end of gland 73 and by a biasing spring 117 disposed between a shoulder 118 on the ram and the end of plug 72. The cylindrical surface or area 120 of ram 70 disposed in chamber 110 against which pressurized fluid acts is, for example, about one-half of the cylindrical surface 121 and flat end surface 122 (hereinafter referred to as area 123) which is disposed in chamber 111 and against which the pilot fluid acts.  
  Ram 70 is provided intermediately of its shoulders 115 and 118 with a conical section which provides a cam surface 125 against which cam follower poppet 59 is biased.  
 OPERATION OF THE FIRST EMBODIMENT Actuator 10 operates in the following manner. Assume that the actuator 10 is in the condition shown in FIG. 2 and that the rheostat 29 is in the centered position shown in FIG. 1. In this condition, the land 106 on servo spool 50 prevents fluid flow to or from chamber 111. Assume that it is desired to move ram 70 inwardly to housing 11 for some predetermined distance to a new position. This is accomplished by moving movable contact 28 of rheostat 29 in the appropriate direction (and assuming that the appropriate limit switch 43 and 44 is closed) so as to energize solenoid 12. When solenoid 12 is energized, its armature 78 moves from the position shown in FIG. 2 to, for example, the position shown in FIG. 5 and in the process effects corresponding movement of servo spool 50. Such movement of servo valve spool 50 causes port 93 to align with port 90 in sleeve 51 and fluid is able to flow from chamber 111 to fluid return port 16. Fluid tends to flow in this direction because full system fluid pressure is being maintained in chamber 110 and ram 70 tends to move leftward (with respect to FIG. 2). The movement of ram 70 is transmitted from cam surface 125 of ram 70 to cam follower S9 and from the cam surface 103 of the cam follower sleeve 57 to cam follower 87 on the servo sleeve 51, as FIG. 6 shows. It is to be noted that as ram 70 moves leftward with respect to FIGS. 2, 5 and 6, cam follower 59 and sleeve 57 move upward thereby enabling servo sleeve 51 to move rightward (i.e., in the same direction as spool 50). The effect of such movement of sleeve 51 is to cause land 106 to close off port 90 in servo sleeve 51 and prevent further escape of fluid from chamber 11]. Consequently, ram 70 will be maintained in the new position to which it has been moved.  
  It is apparent from the foregoing that the servo spool 50 and the servo sleeve 51 and their associated passages are functioning as a three position hydraulic valve. Furthermore, the cam surfaces and cam followers are functioning as a mechanical feedback system to maintain proper spool movement and positioning.  
  To move ram 70 in a direction opposite (i.e., rightward with respect to FIGS. 2, 5 and 6), solenoid 12 is energized in such a manner so as to cause its armature 78 to move leftward. As this occurs, biasing spring 86 moves spool 50 leftward so that the land 106 effects fluid flow from input port 15, through passage 96, through port 91 in sleeve 51, through the groove 94 in spool 50, and through passage 105 to chamber 111. Since the surface area 123 of ram 70 in chamber 111 is substantially larger than the other surface area 120 of the ram, the pressure differential is such that ram 70 tends to move toward the right with respect to FIGS. 2, 5 and 6. As this occurs, cam follower 59 and cam follower sleeve 57 move downwardly and, as cam follower 57 moves downwardly, cam follower 87 and its attached servo sleeve 51 tend to move toward the left with respect to FIGS. 2, 5 and 6. As this occurs, fluid flow to chamber 111 is again blocked and ram 70 maintained in the new position to which it has been moved.  
  In the event of some electrical malfunction that prevents function of actuator by means of solenoid 12, it is possible to effect movement of ram 70 by means of push-pull rod 64. For example, pushing on handle 65 effects downward movement of rod 64 and retainer spring 68 compresses. This increase in load causes cam follower sleeve 57 to move down and this in turn causes servo sleeve 51 to move leftward with respect to FIG. 2 so that fluid is able to escape from chamber 111, through passage 105, through port 90 in sleeve 51, through port 93 in spool 50, through passages 85 and 95, and through passages 99 and 100 to return port 16. As a consequence, rod 70 tends to move leftward with respect to FIG. 2 and such movement is possible because cam follower 59 is spring biased by spring 68 and is able to move upwardly, despite downward movement of cam follower sleeve 57.  
  Rightward movement of ram 70 can be effected by pulling on handle 65 to effect upward movement of rod 64. It is apparent that push-pull rod 64 and its associated elements, in addition to serving as a mechanical feedback system, also serves as a mechanical over-ride means.  
  In the invention as disclosed herein, movement of servo spool 50 is effected by a proportional solenoid 12. However, it is to be understood that other means, such as pneumatic or hydraulic, or mechanical unit could effect the desired movement of spool 50.  
  It is also to be noted that in practice for example, only a very slight movement of servo spool 50 is necessary to effect substantially greater movement of ram 70. For example, in a working embodiment maximum travel of armature 78 of solenoid 12 and of servo spool 50 was on the order of plus or minus 20/1000 of an inch and such movement effected, for example, maximum ram travel on the order of plus or minus V2 inch. In such an embodiment the area 123 on spool was on the order of twice that of area 120. It is to be understood that these proportions could be varied to alter the performance of actuator 10 and that the slopes and shapes of the various cam surfaces could also be changed from that shown to effect differences in ram travel.  
 OVER-RIDE MEANS Referring now to FIGS. 2, 7 and 8, three different embodiments of manual over-ride means are provided. FIG. 2 shows a first embodiment of the manual override means, hereinbefore described.  
  In FIG. 7, the second embodiment of the over-ride means is shown as comprising an arrangement wherein spring 54 bears against a washer which is held in place by a snap ring 131 attached to valve housing 11. Means are provided to effect direct manual movement of valve spool 50 and comprise a linkage or rod 132 which has one end rigidly connected by a set screw 133 to the end of spool 50 adjacent solenoid 12. The other end of linkage 132 is connected by a set screw 134 to an over-ride piston 135 which is axially movable in opposite directions in an over-ride cylinder 136. Centering springs 137 and 138 are located between the ends of piston 135 and the ends of cylinder 136 to maintain the piston centered. A push-pull rod 140 has one end connected to piston 135 and its other end is connected, for example, to a manual operating handle 141. In the second embodiment of the over-ride means, the push rod 64 and the spring 68, shown in FIG. 2, are understood to be eliminated and the poppet 59 is integral with or directly connected to the follower sleeve 57. The embodiment of the invention shown in FIG. 7 operates as follows. Movement of handle 141 ultimately effects movement of servo spool 50 to port fluid to or from chamber 111 to effect operation of ram 70, as hereinbefore described.  
  In FIG. 8, the third embodiment of the over-ride means is shown as comprising an arrangement wherein a solenoid 12A is provided which differs from solenoid l2, hereinbefore described, in that solenoid 12A comprises an armature 78A which extends from both ends of the solenoid housing 144. The inner end of armature 78A bears against and operates servo spool 50 in the same manner as armature 78 shown in FIG. 2. However, armature 78A is also movable manually by means of a push-pull rod 145 which is directly connected, as by threads 146, to the outer end of armature 78A. Rod 145 is physically supported for axial movement in a rod housing 147 which is secured to solenoid housing 144, for example. A biasing spring 148 is disposed between housing 144 and a shoulder on rod 145. In the third embodiment of the over-ride means, the push-rod 64 and spring 68, shown in FIG. 2, are also eliminated and the poppet 59 is integral with or directly connected to the follower sleeve 57. The embodiment shown in FIG. 8 operates as follows Movement of push-pull rod 145 effects corresponding movement of armature 78A and corresponding movement of servo spool 50 to effect movement of ram 70 as hereinbefore explained.  
  The over-ride means shown in FIGS. 7 and 8 are understood to be applicable to the embodiment of the invention hereinafter described in connection with FIGS. 9 and 10.  
 SECOND EMBODIMENT Referring to FIGS. 9 and 10, the numeral 210 designates another embodiment of an electro-hydraulic proportional servo actuator in accordance with the present invention. Actuator 210 comprises a housing 211 into which a proportional solenoid 212 extends and from which a power ram or spool 270 extends. In operation, as hereinafter described in detail, operation of a rheostat 229 effects a proportional linear movement of ram 270 of actuator 210 to effect a control function, such as operation of a valve 213.  
  As FIGS. 9 and 10 show, housing 211 of actuator 210 is rigidly connected to housing 304 of valve 213 by a plurality of bolts 214 which extend through holes 300 in actuator housing 211 and take into threaded openings 302 in valve housing 304. Valve housing 304 is provided with a bore 306 which is axially aligned and in communication with bore 271 in actuator housing 211. One end of bore 306 is closed by a threaded plug 308. An axially movable valve spool 310 is mounted in bore 306 and one end of spool 310 bears against an end or ram 270 in actuator housing 211 and is movable therewith. A biasing spring 2114 is mounted between plug 308 and a shoulder 312 near the end of spool 310. Spring 2114 maintains spool 310 in contact with ram 270 and also cooperates with spring 2117 on ram 270 to maintain the ram and the valve spool centered in their respective passages.  
  Housing 304 of valve 213 is provided with three fluid input ports 316, 317 and 318. Ports 316 and 318 connected to bore 306 and are connected by fluid supply lines 217A and 2178 from line 217 to the discharge (i.e., pressure) port ofa conventional motor driven hydraulic fluid pump 218. A conventional fluid pressure relief valve 222 is connected between fluid line 217 and reservoir 220. Port 317 is connected to the pressure port of pump 218 by fluid supply lines 217 and 217C and supplies pressurized fluid through a series of interconnected passages 320A. 320C and 320D in valve housing 304 to a chamber 310A in bore 306 between plug 308 and shoulder 312 on spool 310. Internal passage 320A is connected to and supplies pressurized fluid to input port 215 of housing 211 of actuator 210. Valve housing 304 is also provided with three fluid re turn ports 322, 324 and 326 which connect to bore 306 and which are connected by fluid return lines 322A, 324A and 326A, respectively, to reservoirs 220. Valve housing 304 is further provided with two ports 330 and 332 which are connected to bore 306 and are connected by fluid lines 334 and 336, respectively, to chamber 338 and 340, respectively, on opposite sides of a piston 342 in hydrostatic cylinder 344.  
  Generally considered, axial movement of ram 270 effected by operation of solenoid 212, as hereinafter explained, effects corresponding axial movement of valve spool 310. If valve spool 310 is moved rightward (with respect to FIGS. 9 and 10), pressurized fluid flows from pump 218, through lines 217 and 21713, through port 318 in valve housing 304, through a groove 3188 in valve spool 310, through port 332,  
 through line 336 to chamber 340 of cylinder 344 causing piston 342 to move leftward (with respect to FIG. 9). Fluid exhausted from chamber 338 of cylinder 344 as the piston moves left flows through line 334, into port 330, through groove 32213 in valve spool 310, through return line 322A to reservoir 220. If valve spool 310 is moved leftward (with respect to FIGS. 9 and 10), pressurized fluid flows from pump 218, through lines 217 and 217A, through port 316 in valve housing 304, through groove 3163 in valve spool 310, through port 330, through line 334 to chamber 338 of cylinder 344 causing piston 342 to move rightward (with respect to FIG. 9). Fluid exhausted from chamber 340 of cylinder 344 as the piston moves right flows through line 336, into port 332, through groove 3628 in valve spool 310, through return line 326A to reservoir 220.  
  Housing 211 is provided with hydraulic fluid input and output ports 215 and 216, respectively. Input port 215 of actuator 210 is connected to the discharge end of pump 218, as hereinbefore explained, and the pump is connected by a hydraulic fluid intake line 219 to a fluid reservoir 220. Output port 216 of actuator 210 is connected through passage 306 in valve housing 304 and by hydraulic fluid return line 322A to reservoir 220, as hereinbefore explained.  
  Solenoid 212 is provided with electrical connection terminals 225 and 226. Terminal 225 of solenoid 212 is connected by an electrical conductor 227 to the movable contact 228 of a rheostat 229 which has the end terminals 235 and 236 of its resistance element 230 connected across the positive and negative terminals 232 and 233, respectively, of an electrical power source such as a battery 234. Terminals 226 of solenoid 212 is connected by an electrical conductor 240 to the movable contacts 241 and 242 of limit switches 243 and 244, respectively. The stationary contacts 246 and 247 of the switches 243 and 244, respectively, are connected to the terminals 232 and 233, respectively, of battery 234.  
  Referring to FIG. 9, actuator 210 is shown in detail and is to be understood generally to comprise two stages. The first stage comprises solenoid 212, a servo spool 250 slideably mounted in a servo sleeve 251, which, in turn, is slideable in a chamber, bore or passage 252 in housing 211, a servo spool biasing spring 253, and a servo sleeve biasing spring 254.  
  The second chamber comprises power spool or ram 270 which is slideable in a chamber, bore or passage 271 in housing 211 and bore 271 is transverse, i.e., at right angles to, bore 252 and interconnected thereto. One end of bore 271 is closed and the other end communicates with bore 306 in valve 213.  
  Solenoid 212 (which is identical to solenoid 12 herein before described) is cylindrical in form and has external threads 275 which engage complementary interior threads 276 in a mounting hole 277 in housing 211. Thus, solenoid 212 is adjustably rotatably to move it inwardly or outwardly so that its armature 278, which is axially movable upward or downward from a centered position, can be located at the null point.  
  Armature 278 of solenoid 212 bears against servo spool 250 but is not physically connected thereto. Servo spool 250 has a central passage 285 which is widened at one end to provide a shoulder 286 and another end passage 295 to drain fluid. One end of servo spool biasing spring 253 bears against shoulder 286 and the other end of spring 253 bears against the rear end of a cam follower member 287 which has its shank pressfitted in a central passage 288 in servo sleeve 251. Member 287 is movable with servo sleeve 251. Member 287 bears against a conical cam surface 2125 on ram 270.  
  Servo sleeve 251 has three annular grooves and ports 290, 291 and 292. Servo spool 250 has one metering land 2106 and annular grooves 294 and 293.  
  Fluid supply port 215 in housing 211 is connected directly to bore 252 and by passage 2105 to bore 271. Fluid return port 216 in housing 211 is connected between bore 252 and reservoir 220 as hereinbefore explained. The relatively movable servo spool 250 and servo sleeve 2S1 cooperate, as hereinafter explained, so that the land 2106 on servo spool 250 controls fluid flow from passage 294 to passage 2105 and from passage 2105 to a chamber 2111 in housing 211. Return fluid flow from chamber 2111, as hereinafter explained, is through passage 2105 and through passages 285, 292 and 293, to passage or port 216.  
  Ram 270 cooperates with bore 271 and spool 310 cooperates with bore 306 to define chambers 2111 and 2110, respectively. A piston ring seal 2113 on ram 270 prevents fluid flow from chamber 2111. Ram 270 and spool 310 are biased into engagement with each other and adapted for simultaneous sliding axial movement in opposite directions in their bores and are maintained in a centered position by a biasing spring 2114 in chamber 2110 disposed between a shoulder 312 and the plug 308 and by a biasing spring 2117 in chamber 2111 disposed between a shoulder 2118 on ram 270 and the end of bore 271. The circular surface area of spool 310 disposed in chamber 2110 against which pressurized fluid acts is, for example, about one-half of the circular surface area of ram 270 which is disposed in chamber 2111 and against which the pilot fluid acts.  
  Ram 270 is provided intermediately of its ends with a conical section which provides a cam surface 2125 against which cam follower 287 is biased. This serves as a feedback means or a feedback connection whereby the movement or position of ram 270 is sensed by cam follower 287.  
 OPERATION OF THE SECOND EMBODIMENT Actuator 210 operates in the following manner. Assume that the actuator 210 is in the condition shows in FIGS. 9 and 10 and that the rheostat 229 is in the centered position shown in FIG. 9. In this condition the land 2106 on servo spool 250 prevents fluid flow to or from chamber 2111. Assume that it is desired to move ram 270 and spool 310 leftward (with respect to FIG. 9) for some predetermined distance to a new position and thus operate valve 213. This is accomplished by moving movable contact 228 or rheostat 229 in the appropriate direction (and assuming that the appropriate limit switch 243 or 244 is closed) so as to energize solenoid 212. When solenoid 212 is energized, its armature 278 moves outwardly from the position shown in FIG. 9 and in the process effects corresponding movement of servo valve spool 250. Such movement of servo valve spool 250 causes port 293 to align with port 290 in sleeve 251 and fluid is able to flow from chamber 2111 to fluid return port 216 as ram 270 moves leftward. Fluid tends to flow in this direction because full system fluid pressure is being maintained in chamber 2110, whereas pilot fluid pressure in chamber 2111 is reduced. Thus, ram 270 and spool 310 tend to move leftward (with respect to FIG. 9) under the influence of the pressure in chamber 2110. The movement of ram 270 is transmitted from cam surface 2125 of ram 270 to cam follower 287 on the servo sleeve 251, as FIG. 9 shows. It is to be noted that as ram 270 moves leftward with respect to FIG. 9, servo sleeve 25] moves downwardly. The effect of such movement of sleeve 251 is to cause land 2106 to close off port 290 in servo sleeve 251 and prevent further escape of fluid from chamber 2111. Consequently, ram 270 will be maintained in the new position to which it has been moved.  
  It is apparent from the foregoing that the servo spool 250 and the servo sleeve 251 and their associated passages are functioning as a three position hydraulic valve. Furthermore, the cam surface 2125 and cam follower 287 are functioning as a mechanical feedback system to maintain proper spool movement and positioning.  
  To move ram 270 in a direction opposite (i.e., rightward with respect to FIG. 9), solenoid 212 is energized in such a manner so as to cause its armature 278 to move upward. As this occurs, biasing spring 286 moves spool 250 upward so that the land 2106 effects fluid flow from input port 215, through port 291 in sleeve 251, through the groove 294 in spool 250, and through passage 2105 to chamber 2111. Since the surface area of ram 270 in chamber 2111 is twice the surface area of the valve spool 310 in chamber 2110, the pressure differential is such that spool 310 and ram 270 tend to move together toward the right with respect to FIG. 9. As this occurs, cam follower 287 and its attached servo sleeve 251 tend to move upwardly with with respect to FIG. 9. As this occurs, fluid flow to chamber 211] is again blocked and ram 270 is maintained in the new position to which it has been moved.  
  Generally considered, axial movement of ram 270 and valve spool 310 is effected by operation of solenoid 212, as hereinbefore explained. If valve spool 310 is moved rightward (with respect to FIGS. 9 and 10), pressurized fluid flows from pump 218, through lines 217 and 217B, through port 318 in valve housing 304 through groove 318B in valve spool 310, through port 332, through line 336 to chamber 340 of cylinder 344 causing piston 342 to move leftward (with respect to FIG. 9). Fluid exhausted from chamber 338 of cylinder 344 as the piston moves left flows through line 344, into port 330, through groove 32213 in valve spool 310, through return line 322A to reservoir 220. If valve spool 310 is moved leftward (with respect to FIGS. 9 and 10), pressurized fluid flows from pump 218, through lines 217 and 217A, through port 317 in valve housing 304, through groove 316B in valve spool 310, through port 330, through line 334 to chamber 338 of cylinder 344 causing piston 342 to move rightward (with respect to FIG. 9). Fluid exhausted from chamber 340 of cylinder 344 as the piston moves right, flows through line 336, into port 332, through groove 326B in valve spool 310, through port 326, through line return 326A to reservoir 220.  
  In the invention as disclosed herein, movement of servo spool 250 is effected by a proportional solenoid 212. However, it is to be understood that other means, such as a pneumatic or hydraulic, or mechanical unit could effect the desired movement of spool 250.  
  It is also to be noted that in practice for example, only a very slight movement of servo spool 250 is necessary to effect substantially greater movement of ram 270 and spool 310. For example, in a working embodiment maximum travel of armature 278 of solenoid 212 and of servo spool 250 was on the order of plus or minus 20/1000 of an inch and such movement effected, for example, maximum ram and spool travel on the order of plus or minus A: inch. in such an embodiment the effective piston area on ram 270 was on the order of twice that of the effective piston area on spool 310. It is to be understood that these proportions could be varied to alter the performance of actuator 210 and that the slopes and shapes of the various cam surfaces could also be changed from that shown to effect differences in ram travel.  
 RESUME An electro hydraulic proportional servo actuator (or 210) comprises a housing 11 (or 211), a proportional solenoid 12 (or 212), a three-way hydraulic servo valve, and a hydraulic ram 70 (or 270) linearly movable in opposite directions from a spring centered position in a first bore 71 (or 271) in actuator housing 11 (or 211) in response to solenoid and servo valve operation to effect control functions. In one embodiment the ram 70 cooperates with its bore 71 to define a high pressure fluid chamber 110 and a pilot pressure fluid chamber 111. in the other embodiment the valve spool 310 cooperates with its bore 306 to define a high pressure fluid chamber 2110 and ram 270 cooperates with its bore 271 to define a pilot pressure fluid chamber 2111. Ram piston area in the pilot pressure fluid chamber 111 (or 2111) is, for example, twice that of the ram piston area 120 (or the spool piston area in chamber 2110) which is always pressurized with full system pressure. Pilot fluid flow to and from the pilot pressure chamber 111 (or 2111) is controlled by the three-way servo valve comprising a spring-centered hollow servo spool 50 (or 250) which is slideable in a second bore 52 (or 252) in housing 11 (or 211). The sleeve 51 (or 251 and spool 50 (or 250) each have metering orifices 90, 91 and 93 (or 290, 291 and 293), respectfully, therein which are out of registry (closed) when solenoid 12 (or 212) is de-energized and cooperate to define a fluid metering aperture when solenoid 12 (or 212) is energized so that its armature 78 (or 278) moves spool 50 (or 250). Fluid flow to or from the pilot pressure chamber 111 (or 2111) causes ram 70 (or 270) and spool 310 to move because of the pressure differential on the ram A feedback means or connection between the sleeve 51 (or 251) and ram 70 (or 270) is responsive to ram movement, so as to move sleeve 51 (or 251) in the same direction as spool 50 (or 250) to then close the fluid metering aperture and hold the ram 70 (or 270) in the desired position. The feedback means or connection in one embodiment comprise a first cam follower 87 on sleeve 51 which bears against a first inclined surface 103 on a slideable movable second cam follower 57 which, in turn, bears against a second inclined surface 125 on ram 70. In another embodiment the feedback means or connection comprises first cam follower 287 which bears directly against an inclined surface 2125 on ram 270. In the event of malfunction, ram 70 (or 270) centers itself by the action of springs 114 and 117 (or 2114 and 2117). In the event of electrical malfunction, ram 70 is manually movable by means of manual over-ride means such as, in one embodiment, a push-pull rod 64 connected to the second cam follower 57. The solenoid 12 (or 212) is attached to the actuator housing 11 (or 21 l) by threads (or 275) and 76 (or 276) so that it is adjustably movable axially to null position to overcome the effects of manufacturing tolerances.  
  in the embodiment shown in FIGS. 9 and 10, actuator 210 is directly associated with a valve 213 which operates or controls a hydraulic cylinder 344.  
 1 claim:  
 1. in a proportional servo actuator:  
 a ram mounted in a bore for sliding axial movement in opposite directions from a centered position in said actuator and cooperating with said bore to define two separate chambers,  
 the portion of said ram in one chamber having a smaller fluid pressure area than the portion of said ram in the other chamber,  
 means to continuously supply fluid to said one chamber,  
 servo valve means in said actuator to regulate fluid flow to and from said other chamber,  
 said servo valve means comprising a hollow sleeve mounted in a bore in said actuator for sliding axial movement in opposite directions from a centered position and having a metering orifice means therein,  
 said servo valve means further comprising a spool mounted in said sleeve for sliding axial movement in opposite directions from a centered position and having a metering orifice means therein,  
 both metering orifice means cooperating when said sleeve and spool are centered to control fluid pressure in said other chamber and thereby maintain said ram stationary,  
 said spool being movable relative to said sleeve to cause both said metering orifice means to define an aperture which effects a change in fluid pressure in said other chamber and to thereby effect movement of said ram to a desired position,  
 and feedback means connected between said ram and said sleeve and responsive to ram movement to effect movement of said sleeve relative to said spool to cause said orifice means to control fluid pressure in said other chamber to hold said ram in said desired position, said feedback means comprising:  
 a cam surface on said ram,  
 first cam follower means slidably mounted in a bore in said actuator and engageable with said cam surface on said ram, said first cam follower means hav ing a cam surface thereon,  
 and second carn follower means on said sleeve engageable with said cam surface on said cam follower means.  
 2. An actuator according to claim 1 including a pro portional solenoid for moving said spool.  
  3. An actuator according to claim 2 including override means to effect operation of said servo valve means, said override means being connected to the armature of said solenoid.  
  4. An actuator according to claim 1 including override means to effect operation of said servo spool, said override means comprising a push-pull rod connected to said cam follower means.  
  5. An actuator according to claim 1 including override means to effect operation of said servo spool, said override means being connected directly to said servo spool.  
  6. An actuator according to claim 1 including override means to effect operation of said servo valve means.  
  7. An actuator according to claim 6 wherein said override means are connected to said feedback means.  
  8. An actuator according to claim 6 wherein said override means are connected to said servo valve means.  
  9. An actuator according to claim 1 including an electromagnetic device for moving said spool.  
 10. In a proportional servo actuator having a housing:  
 a ram slidably mounted in a first bore in said housing and cooperating therewith to define first and second pressure chambers,  
 the portion of said ram in said first chamber comprising a fluid pressure application area smaller than the fluid pressure application area of the portion of said ram in said second chamber,  
 said ram being movable in opposite directions from a centered position and having a first cam surface thereon,  
 biasing means tending to maintain said ram in centered position,  
 a fluid source for continuously supplying fluid to said first chamber,  
 servo valve means for regulating the flow of fluid to and from said second chamber,  
 said servo valve means comprising:  
 a second bore in said housing,  
 a hollow servo sleeve slidably mounted in said second bore and having a first metering orifice means therein,  
 a hollow servo spool slidably mounted in said servo sleeve and having a second metering orifice means therein,  
 said spool being axially movable in opposite directions from a centered position,  
 means for effecting sliding movement of said servo spool to cause the second metering orifice means therein to communicate with the first metering orifice means to enable fluid flow to or from said second chamber,  
 and means for effecting sliding movement of said servo sleeve in response to movement of said ram to achieve feedback control, said means comprising first cam follower means on said sleeve,  
 second cam follower means slidably mounted in a third bore in said housing for engagement with said first cam surface on said ram and having a second cam surface thereon for engagement by said first cam follower means of said sleeve.  
  11. An actuator according to claim 10 wherein said means for effecting sliding movement of said servo spool comprises a solenoid having an axially movable armature.  
  12. An actuator according to claim 10 wherein said second cam follower means comprises a hollow follower sleeve and a relatively movable cam follower member slidably mounted within said sleeve, and further including override means to effect operation of said servo spool, said override means comprising a push-pull rod extending into said follower sleeve, a first biasing spring disposed between said rod and said cam follower member, means to effect engagement of said rod with said sleeve during pulling movement of said rod, and a second biasing spring disposed between said follower sleeve and said actuator housing.  
  13. An actuator according to claim 10 wherein said means for effecting sliding movement of said servo spool comprises an electromagnetic device having an axially movable armature.  
 14. In a proportional servo actuator:  
 housing means,  
 a ram movable in opposite directions in said housing means, said ram having two differential sized piston areas thereon,  
 means for constantly supplying pressurized fluid to the smaller one of said piston areas,  
 servo means in said housing operable to selectively increase or decrease fluid pressure on the larger of said piston areas,  
 said servo valve means comprising a hollow sleeve mounted in a bore in said housing of sliding axial movement in opposite directions from a centered position and having a first metering orifice therein,  
 said servo valve means further comprising a spool mounted in said sleeve for sliding axial movement in opposite directions from a centered position and having a second metering orifice therein,  
 both metering orifices cooperating when said sleeve and spool are centered in a null position to maintain such pressure on the larger of said piston areas as is necessary to maintain said ram stationary,  
 means including an electromagnetic device mounted on said housing and having a movable armature for moving said spool relative to said sleeve to cause said metering orifices to cooperate and define an aperture which allows fluid pressure on said larger piston area to change and to thereby effect movement of said ram to a desired position,  
 said electromagnetic device being adjustably mounted on said housing to enable said electromagnetic device to be moved axially with respect to said housing to place said armature and said spool in null position,  
 and a feedback connection between said ram and said sleeve and responsive to ram movement to effect movement of said sleeve relative to said spool to cause said meeting orifices to cooperate and maintain such pressure on the larger of said piston areas as is necessary to hold said ram in said desired position, said feedback connection comprising a cam surface on said ram and cam follower means on said sleeve responsive to axial movement of said cam surface on said ram to effect movement of said sleeve.  
  15. An actuator according to claim 14 including override means to effect operation of said servo valve means.  
  16. An actuator according to claim 15 wherein said override means are connected to said servo valve means.  
  17. An actuator according to claim 14 including override means to effect operation of said servo spool, said override means being connected directly to said servo spool.