Flexible Cable Hydraulic Control Means

Abstract

A hydraulic control system for controlling remotely positioned hydraulic cylinders, the system including a number of hydraulically actuated control valves which selectively communicate the hydraulic cylinders to a pressure source, a proportional force amplifier for each of the control valves for selectively opening and closing the control valves, and a feed back assembly to control the amount of movement of the hydraulic cylinders in accordance with the amount of movement of the input lever.

Parent Case Text

RELATED APPLICATION

This application is a continuation in part of my earlier filed copending application Ser. No. 156,620 filed on June 25, 1971.

Description

BACKGROUND OF THE INVENTION

In hydraulically actuated articulated booms commonly employed in heavy equipment such as earth working equipment, the boom sections are moved by double acting hydraulic cylinders. These cylinders are connected to and span the joints between the articulated boom sections and cause relative movement as generally understood in the art. The hydraulic cylinders are actuated directly by a series of mechanical linkages leading from control levers or handles in the control cab to the valves. The mechanical input force required to operate these valves is generally quite substantial due to the high fluid flow rates required to operate the hydraulic cylinders.

High flow directional control valves of the type contemplated herein are also subject to steady state flow forces which are proportional to net axial change of momentum of the fluid passing through the valve. These forces are well known and can be accurately calculated. 100 lb. forces are common in 90 GPM valves. These forces make the valve difficult to control particularly when "metering" the flow of fluid to the cylinder. When the valve is being closed there is resistance to movement due to friction. Then the flow forces build up and aid in the closing of the valve to the point where the operator must resist these forces through the linkage system in order to control the handle. If there is any backlash through the linkage system, due to this change in force, the valve can become uncontrollable at these flow forces.

SUMMARY OF THE INVENTION

The control system of the present invention combines a feed back assembly with a proportional force amplifier to control the movement of the hydraulic cylinders for the boom sections. The feed back assembly senses the movements of the various boom sections and through flexible cables transmits these movements back to the porportional force amplifier in an equal but opposite direction to the input forces from the control handle. Predetermined amounts of movement are thereby provided for the boom sections in response to predetermined movements of the control handle.

Further the proportional force amplifier operates in accordance with the principles disclosed in my copending application entitled "PROPORTIONAL FORCE AMPLIFIER," Ser. No. 196,495, filed Nov. 8, 1971. The proportional force amplifier is a power assist mechanism which is connected to the control valve and responds to manual mechanical input displacement to cause a displacement in the control valve in the same direction in a one-to-one proportion to the input but at a greatly increased force. In this invention the amount of force required to operate the control valve is approximately 100 times that required to operate the pilot valve. Additionally, the amount of movement of the main valve is proportioned to the amount of movement of the pilot valve in a one-to-one ratio.

These and other objects of the invention will become more apparent to those skilled in the art by reference to the following detailed description when viewed in light of the accompanying drawings wherein:

FIG. 1 is a diagrammatic view of the control system of this invention as applied to an articulated boom;

FIG. 2 is a cross-sectional view of the control valve of this invention with feedback means;

FIG. 3 is a cross-sectional view of the control valve of this invention without feedback means; and

FIG. 4 is a cross-sectional view of a modified form of the control valve of this invention;

FIG. 5 is an enlarged view of the pilot spool valve for the control valve shown in FIG. 4;

FIG. 6 is a section view taken on line 6--6 of FIG. 5 showing the flow path through the pilot spool valve;

FIG. 7 is a cross-section taken on line 7--7 of FIG. 6 showing one of th lands.

DESCRIPTION OF THE INVENTION

Referring now to the drawings wherein like numerals indicate like parts, the control system 10 of this invention is shown applied to an articulated boom 12. The boom is attached at its inner end to a rotatable support diagrammatically illustrated at 14, which support is adapted to be positioned on a mobile unit such as track or wheeled vehicle. As is conventional in apparatus of this type, the boom consists of a first boom section 16 pivotally connected to the rotatable support 14 by pivot pin 18 for movement in a vertical plane. The second boom section 20 is pivotally attached to the first boom section 16 by pivot pin 22 for movement in a vertical plane. The bucket 24 is pivotally attached to the outer end of the second boom section 20 by pivot pin 26.

The pivotal movement between the first boom section 16 and the support 14 is controlled by means of a first hydraulic piston and cylinder assembly or jack 28 which includes a cylinder 30 and piston rod 32. The base of the cylinder 30 is pivotally connected to the support 14 by pivot pin 34 and the outer end of the piston rod 32 is pivotally connected to an intermediate point along the first boom section by pivot pin 36. The hydraulic jack 28 is double-acting as are the jacks to be described hereinafter and includes inlet-outlet hydraulic conduits 28a and 28b.

The pivotal movement between the first and second boom section 16 and 20 respectively is controlled by means of a second hydraulic piston and cylinder assembly as jack 38 which includes a cylinder 40 and a piston rod 42. The base of the cylinder 40 is pivotally connected to the first boom section by means of pivot pin 44 and the outer end of the piston rod 42 is pivotally connected to the second boom section 20 by means of pivot pin 46. The jack 38 is provided with inlet-outlet hydraulic conduits 38a and 38b.

The pivotal movement between the bucket 24 and the second boom section 20 is controlled by means of a third hydraulic piston and cylinder or jack 48 which includes a cylinder 50 and piston rod 52. The base of the cylinder 50 is pivotally connected to the second section 20 intermediate its length by pivot pin 54. The outer end of the piston rod 52 is pivotally connected to a bucket extension 53 by pivot pin 56. The jack 48 includes inlet-outlet hydraulic conduits 48a and 48b.

The direction of pivotal movement between the support 14 and the first boom section 16 is indicated by the arrow X; between the first boom section 16 and second boom section 20 by the arrow Y; and between the bucket 24 and the second boom section 20 by the arrow Z. The apparatus described above is conventional and well known in the art. This invention is directed toward but not limited to a control system for actuating the aforementioned hydraulic cylinders and thereby causing relative pivotal movement between the articulated sections.

CONTROL SYSTEM

In accordance with the invention, the hydraulic jacks 28, 38 and 48 are controlled by means of the control system 10 which generally includes a manual input control box 60, feed back assemblies 75 and control valves A, B and C. The manual input control box 60 includes a universally movable handle or lever 62 which enters the box 60 through a resilient or flexible grommet 64. Movements of the handle 62 are transmitted to the feed back assemblies 75 by means of flexible lightweight, push-pull cables 66a, 66b and 66c, which move back and forth in response to the appropriate movement of the handle 62 through suitable mechanical means in the box 60, which mechanical means are well known in the art and do not form a part of this invention. As will hereinafter be described,by pushing or pulling one or a combination of the cables, the hydraulic jacks 28, 38 and 48 will be actuated under the control of the valves A, B and C.

More particularly, the feed back assemblies 75 each include a yoke 68a, 68b and 68c respectively, pivotally connected to actuator rods 72a, 72b and 72c which actuate valves A, B and C respectively. The cables 66a, 66b and 66c are connected through fixed guides 70a, 70b and 70c to one end of the corresponding yoke by pins 67a, 67b and 67c, respectively. Valve A communicates with hydraulic jack 48 through inlet-outlet conduit 48a and 48b on one end, on the other it communicates with a source of pressure, such as a pump through conduit 74a and with the reservoir through conduit 76a. Valve B communicates with the jack 38 via inlet-outlet conduits 38a and 38b and with pressure and tank through conduits 78b and 80b. Valve C likewise communicates with hydraulic jack 28 via inlet-outlet conduits 28a and 28b, and pressure and tank via conduits 82c and 84c.

Means are provided for sensing the movement of the boom sections 16 and 20 and the bucket 24 and transmitting such movements to the feed back assemblies 75. Such means is in the form of feed back cables 90a, 90b and 90c connected at one end to yokes 68a, 68b and 68c respectively, by means of pins 91a, 91b and 91c. Feed back cable 90c is connected at its other end to pivot pin 18 via clamping collar 92. The clamping collar 92 is fixed to the pin 18 and rotates therewith. The pin 18, in turn, rotates with the first boom section 16 such that angular movement of the first boom section 16 will be translated back to the corresponding yoke 68c by means of the flexible feed back cable 90c. Feed back cables 90a and 90b are connected to pivot pins 26 and 22 respectively by means of collars 96 and 94. As with collar 92, collars 94 and 96 rotate with the second boom section 20 and bucket 24 respectively.

To this point it can be seen generally that by actuation of the handle 62 one or more of the control valves A, B or C will be opened to actuate the corresponding hydraulic jack 48, 38 or 28. Arrows X, Y and Z are shown adjacent to the control handle 62 to generally indicate the direction of the handle must be moved to cause the respective boom sections 16, 18 and the bucket 24 to move in the indicated directions X, Y and Z. The amount of movement of the boom sections 16 and 18 and the bucket 24 is controlled by means of the push-pull cables 66a, 66b and 66c to correspond to the amount of movement of the handle 62, i.e., each movmeent of the handle 62 will produce a predetermined amout of movement in the respective boom section or bucket.

In this regard and referring to FIG. 2 the yoke 68a for the bucket 24 is shown. Assuming that the control handle 62 is moved in the Z direction and the cable 66a is pulled to the left, the yoke 68a will pivot clockwise about pivot point 91a pulling actuator rod 72a outward from the control valve A. Opening of valve A will pressurize jack 48 producing a pivoted motion in bucket 24 about pivot pin 26. The motion of the bucket 24 is sensed by collar 96 on pivot pin 26 and transferred back to the yoke 68a by cable 90a. The cable 90a should move to right in FIG. 2 pivoting the yoke 68a clockwise about pivot point 67a, to push the actuator rod 72a into the control valve A to close the valve and stop the movement of bucket 24.

CONTROL VALVE

Since valves A, B and C are identical, only one, valve A, will be described in detail. As indicated above the yoke 68a is pivotally attached to the actuator rod 72a by means of pivot pin 73a. The control valve includes a valve housing 102 having a tubular extension 104 extending from one end thereof for slidably receiving the actuator rod 72a. The housing 102 includes a central bore 106 which slidably receives a spool 108 having a first land 110, a second land 112 and a third land 114 axially spaced thereon and defining annular chambers 116 and 118 therebetween with the interior walls of the bore 106. Inlet-outlet conduits 48a and 48b communicate with the chambers 116 and 118 respectively via ports 120 and 122. The axial bore 106 is counterbored at axially spaced points to define chambers 124, 126 and 128 with lands 110, 112 and 114. Reservoir conduit 76a communicates with chamber 128 via port 130, and with chamber 124 via passageway 132 and port 134.

A chamber 138 is defined between the end of the first land 110 of the spool 108 and the end wall 140 of the bore 106. Pressure conduit 74a communicates with annular chamber 126 via port 142 and with chamber 138 via chamber 126 passageway 144 and port 146.

In the position shown in FIG. 2, the spool 108 is in the null or balanced position. It can be seen that when the spool is moved to the left, inlet-outlet conduit 48b is communicated to pressure and inlet-outlet conduit 48a is communicated to tank and vice versa when the spool moves to the right. As mentioned, however, oftentimes, the movement of the spool is quite difficult due to the flow forces and centering spring forces of the system. Hence a pilot valve or servo valve 150 is slidably received in an axially extending bore 152 in the spool valve 108, whereby, by actuation of the pilot valve 150 as will hereinafter be described, the spool valve 108 is shifted hydraulically and proportionally less amount of manual input force is required.

More particularly, the main valves A, B and C are actuated by means of proportional force amplifiers as disclosed in my copending application Ser. No. 196,495. In this regard, each amplifier includes a pilot valve 150 having lands 154 and 156 and is provided with an axial passageway 158 which communicates chambers 162, 166 and 172 with each other. Chamber 162 is defined by the end 164 of the bore 152 and the land 154. Chamber 166 is defined by the two lands 154 and 156 and chamber 172 is defined by the end wall 174 of the bore 106 and the annular end surface 168 of the spool 108. The area of the end surface 168 is approximately one half the area of the surface 178 defining the one end wall of constantly pressured chamber 138.

Ports 180 and 182 communicate the pressure conduit 74a and the tank conduit 26a respectively to the bore 152 and are normally blocked by lands 154 and 156 when the pilot valve is in the null or balanced position. The facing edges of the lands 154 and 156 are spaced apart from each other a distance corresponding to the distance between the edges of the apertures of ports 180 and 182. The edges of the lands can be used effectively to meter the fluid flow through the ports. The pilot valve 150 is connected at one end to rod 186 which in turn is connected to actuator rod 72a. The rod 186 passes through an opening 188 in the end wall of the spool 108.

The operation of the pilot control valve 150 is as follows. Referring to FIG. 2, assume that by suitable actuation of the control handle 62, the cable 66a is moved to the left. The yoke will move clockwise to the left about the point of connection 91a between the feed back cable 90a and the yoke 68a which point provides a fixed pivot. Upon such movement of the cable to the left, the pilot valve 150 will likewise be pulled to the left a distance determined by the amount of movement of the control handle 62. The land 154 will uncover the pressure port 180 and fluid under pressure will enter chamber 172 via port 170 and axial passageway 158. As mentioned earlier, chamber 138 is in constant communication with the pressure source via passageway 144, and, due to the fact that the area of surface 168 is approximately twice that of the area of surface 178, the spool 108 will shift to the left because of this differential in surface areas. The spool 108 by shifting to the left will communicate the pressure passageway 74a to uncovered annular areas 126 and 118 and permit fluid under pressure to enter the inlet-outlet conduit 48b, while at the same time inlet-outlet conduit 48a is communicated with tank passageway 76a via chambers 116 and 124 and passageway 132. Operating upon the principles described in my earlier mentioned copending application Ser. No. 196,495, the spool 108 will continue to move to the left relative to the pilot valve until the pressure port 180 is again completely covered by the land 154 on the pilot valve 150.

The spool 108 will move relative to the pilot valve 150 and will move the same distance; that is, it will move proportionately in a one-to-one ratio. The hydraulic jack 48 will contract, causing feed back cable 90a to move to the right in FIG. 2 exactly the same distance that the input cable was caused to move to the left initially. The point of connection 67a of the input cable will serve as a fixed pivot point, and the yoke will move clockwise thereabout causing the pilot valve 150 to move in the reverse direction, to the right. Prior to this latter movement, it is to be understood that the pressure source will be in open communication with the conduit leading to the hydraulic jack.

When the pilot valve 150 is shifted back to the right relative to the spool 108 by the feed back means, the land 156 will uncover port 182 thereby communicating control chamber 172 with tank pressure and causing a loss of pressure in chamber 172. As mentioned, since the chamber 138 is under constant static pressure, the spool will be caused to move to the right relative to the spool valve 108 until both the pressure port and tank port 180 and 182 are again covered by lands 154 and 156 at which time the spool valve 150 will be back in the null and balanced position. The principle behind this operation will be more fully understood by referring to my earlier mentioned copending application. It is to be emphasized that the force ratio between the mechanical input and the hydraulic force acting on the spool valve is approximately 1 to 100 such that a small input force acting to shift the pilot valve results in a proportionately greater force being applied to the spool valve.

In order to pressurize inlet-outlet conduit 48a while exhausting conduit 48b, the pilot valve is shifted to the right with point 91a again acting as a fixed pivot. Control chamber 172 is communicated to tank via uncovered port 182 thereby dropping the pressure therein. The pressure in constantly pressurized chamber 138 will be sufficient to shift the spool 108 to the right relative to the pilot valve until the port 182 is again covered, during which time pressure will be transmitted to the hydraulic jack 48 via chambers 126 and 116, port 120 and conduit 48a. Conduit 48b will be communicated to exhaust via line 122, chambers 118 and 128 and line 130. The feed back cable 90a will cause the spool 108 to shift back to the null and balanced position in the manner heretofore described.

In the event of a loss of hydraulic pressure, the spool 108 can be shifted manually to release fluid pressure from the hydraulic jack 48. This is accomplished by means of an enlarged flange 101 attached to actuator rod 186 which is disposed within a chamber 103 in the spool 108. The chamber has end walls 105 and 107 which are engaged by the enlarged flange upon movement of the yoke 68a to manually shift the spool 108.

If desired, the feed means may be eliminated as shown in the embodiment of FIG. 3. In this embodiment, only the input cable 66a is attached to the pilot valve 150. In the embodiment of FIG. 1 and FIG. 2, the valve will seek the null and balanced position for any position of the control handle. That is, if the control handle is pushed to the right in the direction of arrow X 1 inch and is held there, the corresponding element will swing to its angular movement a predetermined distance proportional to the movement of the control handle and will stop automatically without having to move the handle back to neutral since the feed back cable will cause the valves to return to the null and balanced position. In the embodiment FIG. 3, the handle itself must be returned to the neutral position before the valve will return to the null and balanced position.

ALTERNATE PROPORTIONAL FORCE AMPLIFIER

A modified form of amplifier used to actuate the control valve of this invention is shown in FIG. 4. The conrol valve comprises a conventional four-way valve, generally indicated by the numeral 200 having a spool valve 202. In the embodiment of FIGS. 2 and 3 the pilot valve is mounted within a central bore in the spool valve. In the modified form of FIG. 4 the pilot valve is mounted exteriorly of the spool valve and outside of the spool valve housing.

In this regard, an extension 204 is attached to one end of the spool valve 202, is axially aligned therewith and is slidably received in a bore in a housing 208 mounted on the valve 200. The housing 208 includes an inlet port or passage 248 connected to a source of fluid pressure and a counterbore 210 at the open end of the bore 206. The extension 204 is shown threadably engaged with one end of the spool valve 202. However, it is to be understood that the extension 204 may be an integral part of the spool valve 202 or otherwise attached thereto. Means are provided for sealing the extension within the counterbore 210. Such means comprises a seal ring 205 having external O-ring seals 207 and an internal O-ring seal 209.

The extension 204 includes a double faced actuator piston 212 which is positioned for sliding movement within the counterbore 210 and divides the counterbore 210 into chambers 214 and 216. The total cross sectional area of surfaces 218 and 220 on the piston 212 adjacent chamber 216 is approximately half the cross sectional area of the surface 222 on the piston 212 adjacent to chamber 214. The extenstion 204 is provided with an axial bore 236 which is connected to the chamber 216 through a passage 250 and to the chamber 214 through a passage 258 and a port 259, the port 259 being closed by a plug 261. The piston 212 is sealed in the counterbore 210 by means of an O-ring seal 213.

The pressure of the fluid acting on piston 212 is controlled by means of a tubular pilot spool valve 224 having lands 226, 227 and 228 and a central passage 235. The spool valve 224 is slidably received in an axial bore 232 in an insert 234 which is positioned within the axial bore 236 in extension 204. The insert 234 is retained in the bore 236 by means of a plug 242 threadably received in the outer end of the bore 236 in extension 204 and is sealed therein by means of O-ring seals 233. The insert 234 is provided with a flange 244 which abuts an inwardly directed shoulder 245 in the bore 236. Annular chambers 246 and 247 are provided between the insert 234 and the extension 204. The chamber 246 is in communication with inlet pressure passage 250 and the chamber 247 is in communication with port 259 and passage 258.

The pilot spool valve 224 is positioned within the bore 232 in the insert with the lands 226 and 227 spaced to define an annular pressure chamber 254 and the lands 227 and 228 spaced to define a relief chamber 255. The pressure chamber 254 is connected to the chamber 246 by means of a port 252. The relief chamber 255 is connected to the open end of the bore 236 by means of flats 229 on land 228. The chambers 254 and 255 are selectively connected to the chamber 247 through a port 252 by means of the land 227 as described below.

The pilot spool valve 224 as seen in FIGS. 5, 6 and 7 is provided with a counterbore 239 at each end of passage 235 and a cross bore 237 adjacent to land 228. Means are provided for connecting the pilot valve 224 to an actuator rod 240 in the form of a rod 238 which extends through the passage 235 and is connected to the end of the spool valve 224 by means of a plug 257. In this regard, the plug 257 is threadably connected to the rod 238 and is positioned within counterbore 239. Side loads imparted to the pilot valve 224 by the movement of the rod 238 are compensated for by means of a spring 241 positioned between the plug 257 and a shoulder 243 between the bore 235 and the counterbore 239. Since the side loads generally occur when the spool valve 224 is pulled by the rod 238, the spring 241 is provided only on one side of the plug 257. Movement in the other direction is achieved by the mechanical engagement of the plug 257 with an end cap 260 threadably received in the end of the counterbore 239.

The spool valve 224 is pressure balanced within the passage 232 in order to eliminate any variation in forces acting on the ends of the valve 224. This is accomplished by means of the passage 235 which extends through the center of the spool valve and the cross bore 237. Since both ends of the spool valve are now connected the pressure acting on each end of the spool valve will be essentially equal except for the cross sectional area of rod 238 which is negligible. Further the passage 235 is connected to tank pressure through passage 262 in the spool valve 202, the annular chamber 264 and passage 266 in the control valve 200.

Means are provided for mechanically actuating the spool valve 202 in the event of a loss of power. Such means is in the form of a flange 270 provided on the actuator member 240 and positioned in a counterbore 272 in the end of the plug 242. The flange 270 is retained in the counterbore 272 by means of a retainer ring 274. On movement of the actuator rod to the right, the flange will engage the end wall 276 and move the plug 242 to the right. On movement of the actuator member 240 to the left, the flange 270 will engage the retainer ring 274 and move the plug 242 to the left.

Means are provided for sealing the extension 204 within the bore 206 of housing 208. Such means is in the form of an O-ring seal 211 in the bore 206 and an O-ring seal 231 positioned to sealingly engage rod 238. The seal 231 is retained in the end of plug 242 by means of a plate 230.

In operation of the modified valve of FIG. 4 fluid under pressure is introduced into chamber 216 via passageway 248. The fluid passes through passageway 250, annular chamber 246 and passageway 252 to annular chamber 254 defined by the reduced portion of the spool valve 224 between the lands 226 and 227. Chamber 216 is in constant communication with the pressure source and is therefore under constant static pressure, which tends to urge the piston and thereby the extension and spool valve to the left. The valve is shown in its null and balanced position with the forces tending to move the piston either to the ritht or left being equalized.

The force tending to urge the piston to the left is counterbalanced by the lower pressure in the chamber 214 acting on the larger cross sectional area 222. Fluid flow to the chamber 214 to vary the pressure therein is controlled by the pilot valve 224. As shown, land 227 covers port 256 which leads to port 259, passageway 258 and chamber 214. When the pilot valve 224 is shifted to the right fluid under pressure enters the port 256, annular chamber 247, port 259, passageway 258 and chamber 214 to raise the pressure in the chamber 214 causing the piston 212 and extension 204 to move to the right relative to the pilot valve 224 until the port 256 is again covered by the land 227. Of course, the movement of the piston 212 and extension 204 to the right causes the spool valve 202 to move to the right to uncover the pressure and exhaust ports therein. In order to move the spool valve 202 to the left, the pressure in the chamber 214 is reduced by moving the pilot valve 224 to the left and communicating the chamber 214 with the tank T via passage 258, port 259, chamber 247, port 256, chamber 255, passageway 262, annular chamber 264 and passagewaY 266. As in the embodiments of FIGS. 1 through 3, the pilot valve of the embodiment of FIG. 4 may or may not be provided with a feed back means.

Means are provided for preventing the second land 227 from catching on the edges of the port 256. Such means is in the form of the third land 228 which acts as a support for the spool valve 224. On movement of the valve 224 in the bore 232, the land 228 will clear the edges of the port 256.

Claims

I claim:

1. A hydraulic control system for controlling the flow of hydraulic fluid from a fluid pressure source to a plurality of hydraulic piston and cylinder assemblies from a remote point comprising,

main valves for selectively connecting each of the cylinder assmeblies to the fluid pressure source,

a yoke pivotally connected to each of said valves,

first means including a manual input lever operably connected for moving each of said main valves to an open position, said first means including a fixed guide and a flexible cable passing through said fixed guide and being attached to one end of said yoke, and

second means connected to said piston and cylinder assemblies for moving each of said main valves in the reverse direction to the direction of movement imparted by said manual input lever a distance equal to the distance of movement of the lever to close the valves, said second means including a second fixed guide and a second flexible cable passing through said second cable and being attached to the other end of said yoke, whereby the amount of movement of the piston and cylinder assemblies is directly related to the amount of movement of the lever.

2. The control system of claim 1 wherein each of said main valves includes a spool valve slidably received in a housing having inlet-outlet ports communicating with one of said piston and cylinder assemblies, and pressure and tank ports, metering lands on said spool valve normally closing said ports, said spool valve having opposed first and second end surfaces defining first and second chambers respectively with the ends of said housing, the area of said first end surface being half that of said second end surface, said first chamber being in constant communication with said pressure source, a pilot valve coaxial with and slidably received in an axial bore in said spool valve and operably connected to said first and second means, first and second passageways communicating with second chamber with said pressure source and with a reservoir at tank pressure respectively, said pilot valve extending across said passageways and selectively communicating said second chamber with said pressure source and reservoir to cause movement of one of said piston and cylinder assemblies in response to movement of said first and second cables.

3. The control system of claim 2 wherein said pilot valve has third and fourth lands normally closing both said first and second passageways respectively when said spool is in the balanced or null position and wherein the axial spacing between said metering lands is equal to the axial spacing between said passageways, said lands defining a third chamber therebetween, a third passageway constantly communicating said second chamber with said third chamber, and whereby said pilot valve is shifted axially to uncover one of said passageways in response to movement of said first means thereby changing the pressure in said second chamber relative to said constantly pressurized first chamber and causing movement of said spool valve relative to said pilot valve until said first and second passageways are again covered by the lands of said pilot valve.

4. The system according to claim 1 including

a proportional force amplifier operatively connected to said main valve,

said amplifier including a pilot valve,

said yoke being connected to said pilot valve.

5. The system according to claim 1 including,

a proportional force amplifier operatively connected to said main valve, said amplifier including a housing having a cylindrical chamber, a double faced piston mounted for axial movement in said chamber and having a central passage, said piston separating said chamber into a pressure chamber and a control chamber,

first and second passages in said piston connecting said pressure and control chambers to said central passage, and a pilot valve mounted for axial movement in said central piston passage for selectively connecting said pressure chamber to said control chamber.

6. The system according to claim 5 wherein said pilot valve includes a central bore in communication with each end of said valve whereby the fluid pressure action on each end of the valve is equal.

7. A hydraulic control system for controlling the flow of fluid under pressure to a plurality of hydraulic piston and cylinder assemblies from a remote point comprising,

a normally closed main valve for each of said assemblies, each main valve including a housing having a central bore and a spool valve slidably received in said bore in saId housing,

a proportional force amplifier connected to control said spool valve,

a universally movable manual input lever connected to actuate said amplifier and open said main valve to actuate the corresponding piston and cylinder assembly, and

means connected to said amplifier for returning said spool valve to the closed position, said returning means being connected to respond to the movement of the corresponding piston and cylinder assembly.

8. The control system of claim 7 wherein said spool valve includes first and second ends defining first and second chambers with the end walls of the bore in said housing, said first end being of approximately twice the cross sectional area of said second end, the fluid pressure source being in constant communication with said second chamber to provide static pressure therein, and said amplifier including a pilot valve selectively communicating said first chamber to the pressure source and reservoir to vary the pressure therein and to thereby cause the movement of said spool valve, the direction of movement being a function of differential pressures in said chambers and the difference in areas of said surfaces.

9. The control system of claim 7 wherein said amplifier includes a double-faced actuator piston connected to said spool valve, a housing having a bore for receiving said piston, said bore being divided into first and second chambers by said piston, the faces of said piston being of different cross sectional areas such that the area of the face presented to said first chamber is approximately twice that of the face presented to said second chamber, a fluid pressure source in constant communication with said second chamber to provide static pressure therein, a pilot valve in said piston selectively communicating said first chamber to the pressure source to vary the pressure therein and to thereby cause the movement of said actuator piston and said main valve, the direction of movement being a function of the differential pressures in said chambers and the difference in areas of said piston faces.

10. The control system of claim 9 wherein said piston has an axial bore therein and said pilot valve is slidably received in said axial bore, a reservoir in communication with said axial bore, said piston has passageways for communicating said first chamber with said pressure source and reservoir through said pilot valve.

11. The system according to claim 7 including,

a yoke connected to said amplifier,

said lever being connected to one end of said yoke and said returning means being connected to the other end of said yoke.