HYDRAULIC CONTROL SYSTEM

Abstract: A hydraulic control system comprising a hydraulic actua­ tor (20) having opposed openings (A, B) adapted to alternately functions as inlets and outlets for moving the element (21) of the actuator (20) in opposite directions and a variable dis­ placement pump (22) with load sensing control for supplying fluid to said actuator. A pilot operated spool type meter-in valve (27) is provided to which the fluid from the pump (22) is supplied and a pilot controller (23) alternately supplies fluid at pilot pressure to the meter-in valve (27) for controlling the dis­ placement of the meter-in valve spool (51) and the direction and velocity of the actuator (20). A pair of lines (32,33) extends from the meter-in valve (27) to the respective openings (A, B) of the actuator (20) and a pilot operated meter-out valve (34, 35) is associated with each line (32,33) of the actuator (20) for controlling the flow out of the actuator (20) when that line (32, 35) to the actuator (20) does not have pressure fluid from the pump (22) applied thereto. Pressure of fluid in the line (32, 33) to the actuator. which does not have pressure fluid from the pump, is applied to the meter-in valve (27) to apply a centering force which aids the pressure compensating flow forces to keep the flow constant.

Description: This invention relates to a hydraulic control system according to the preamble to claim 1, especially for earth moving equipment including excavators and cranes. In a known system of that kind (US-A-4,407,122) the spool passage means are arranged symmetrically with respect to the outlet passages of the valve, when the spool thereof is in its neutral position. Furthermore, the spool passage means remain connected to the outlet passages when the spool is shifted so that the working pressure acts onto the valve spool in a centering direction thereof. It is intended to damp jerky motions of the load. A similar system, yet without pins sliding in the valve spool is known from US-A-4,201,052. In such a hydraulic circuit, pressure compensation, and resultant constant flow, is achieved by utilization of flow forces in conjunction with the spring rate which tend to center the spool of the meter-in valve. The amount of pressure compensation may allow variation in flow when the pressure drop varies from the normal load sensing point. In most cases, this performance is acceptable and the operator would not notice the change in flow in operating the actuator. However, in some cases, particularly motor applications, greater accuracy may be needed. Accordingly, among the objections of the present invention is to provide greater accuracy of pressure compensation at low cost. This is achieved by the invention using the combination of the features in claim 1. In accordance with the invention, pressure of fluid in the returning line from the actuator, which therefore does not have the pressure from the pump, is applied to the meter-in valve to apply a centering force which aids the pressure compensating flow forces to keep the flow constant. More specifically, feedback pins are associated with the spool of the meter-in valve and pressure from the returning line from the actuator is applied to one of the pins to apply a centering force on the spool of the meter-in valve which aids the pressure compensating flow forces to keep the flow constant. Fig. 1 is a diagrammatic view of a prior art hydraulic system. Fig. 2 is a diagrammatic view of a meter-in valve utilized in the system. Fig. 3 is a diagrammatic view of a meter-out valve. Fig. 4 is a diagrammatic view of a port relief valve and meter-out valve. Fig. 5 is a diagrammatic view of a portion of hydraulic system embodying the invention. Fig. 6 are curves of flow versus delivery pressure of a prior art hydraulic system. Fig. 7 are curves of flow versus delivery pressure of a hydraulic system embodying the invention. Referring to Fig. 1, a hydraulic system as shown in US-A-4,201,052 comprises an actuator 20, herein shown as a hydraulic cylinder, having a rod 21, that is moved in opposite directions by hydraulic fluid supplied from a variable displacement pump system 22 which has load sensing control in accordance with conventional construction. The hydraulic system further includes a manually operated pilot controller 23 that directs pilot pressure to a valve system 24 for controlling the direction of movement of the rod 21. Fluid from the pump 22 is directed to pump line 25 and inlet passage 26 to a meter-in valve 27 that functions to direct and control the flow of hydraulic fluid to one or the other outlet passage 32 or 33. The meter-in valve 27 has a spool 51 which is pilot pressure controlled by controller 23 through lines 28, 29 and passages 30, 31. Depending upon the direction of movement and position of the valve spool, hydraulic fluid passes through outlet passages 32, 33 and working lines A, B to one or the other end of the actuator 20. One of these working lines A, B is a supplying line and the other line is a returning line. The hydraulic system further includes at least a meter-out valve 34, 35 which is associated with the returning line A or B for controlling the flow of fluid to a tank passage 36. The hydraulic system further includes spring loaded load drop check valves 37, 38 between a respective outlet passage 32, 33 and the lines A, B and spring loaded anticavitation valves 39, 40 which are adapted to open lines A, B to the tank passage 36. In addition, spring loaded poppet valves 41, 42 are associated with each meter-out valve 34, 35. A bleed line 47 having an orifice 49 extends from passage 36 to meter-out valves 34, 35 and to the pilot control lines 28, 29 through check valves 77. The system also includes a back pressure valve 44 associated with the tank line 36. Back pressure valve 44 functions to minimize cavitation when an overrunning or a lowering load tends to drive the actuator down. A charge pump relief valve 45 is provided to make excess flow above the inlet requirements of the pump 22 and apply it to the back pressure valve 44 to augment the fluid available to the actuator. Referring to Fig. 2, the meter-in valve 27 comprises a bore 50 in which a spool 51 is positioned and in the absence of pilot pressure maintained in a neutral position by springs 52. The spool 51 normally blocks the flow from the inlet passage 26 to the outlet passages 32, 33. When pilot pressure is applied to either passage 30 or 31, the meter-in spool 51 is moved in the direction of the pressure until a force balance exists among the pilot pressure, the spring load and the flow forces. The direction of movement determines which of the passages 32, 33 is provided with fluid under pressure from passage 26. Referring to Fig. 3, each meter-out valve 34, 35 is of identical construction and, for purposes of clarity, only valve 34 is described. The meter-out valve 34 includes a bore 60 in which a poppet 61 is positioned. The poppet 61 includes a passage 62 extending to a chamber 63 within the poppet and one or more passages 64 to the tank passage 36. A stem 65 normally closes the connection between the chamber 63 and passages 64 under the action of a spring 66. The pressure in chamber 63 equalizes with the pressure in line A and the resulting force unbalance keeps poppet 61 seated. The valve further includes a piston 67 surrounding the stem 65 yieldingly urged by a spring 68 to the right as viewed in Fig. 3. The pilot line 28 from the controller 23 extends through a passage 69 to the chamber 70 that acts against the piston 67. When pilot pressure is applied to passage 28, the piston 67 is moved to the left as viewed in Fig. 3 moving the stem 65 to the left permitting chamber 63 to be vented to tank passage 36 via passage 64. The resulting force unbalance causes poppet 61 to move to the left connecting line A to passage 36. It can thus be seen that the same pilot pressure which functions to determine the direction of opening of the meter-in valve also functions to determine and control the opening of the appropriate meter-out valve so that the fluid in the actuator can return to the tank line. Referring to Fig. 4, each of the meter-out valves has associated therewith a spring loaded pilot spool 71 which functions when the load pressure in passage 32 exceeds a predetermined value to open a flow path from the load through a control orifice 62 to the tank passage 36 through an intermediate passage 73. This bleed flow reduces the pressure and closing force on the left end of the poppet valve 61 permitting the valve 61 to move to the left and allowing flow from passage 32 to the tank line 36. In order to prevent overshoot when the pressure rises rapidly, an orifice 72 and associated chamber 72a are provided so that there is a delay in the pressure build-up to the left of poppet valve 71. As a result, poppet valves 71 and 61 will open sooner and thereby control the rate of pressure rise and minizize overshoot. In the case of an energy absorbing load, when the controller 23 is moved to operate the actuator 20 in a predetermined direction, pilot pressure applied through line 28 and passage 30 moves the spool of the meter-in valve to the right causing hydraulic fluid under pressure to flow through passage 33 and line B opening poppet valve 38 and continuing to the right hand inlet of actuator 20. The same pilot pressure is applied to the meter-out valve 34 permitting the flow of fluid out of the left hand end of the actuator 20 to the tank passage 36. When the controller 23 is moved to operate the actuator, for example, for an overrunning or lowering a load, the controller 23 is moved so that pilot pressure is applied to the line 28. The meter-out valve 34 opens before the meter-in valve 27 under the influence of pilot pressure. The load on the actuator forces hydraulic fluid through the left hand opening of the actuator past the meter-out valve 34 to the tank passage 36. At the same time, the poppet valve 40 is opened permitting return of some of the fluid to the other end of the actuator through the right hand opening thereby avoiding cavitation. Thus, the fluid is supplied to the other end of the actuator without opening the meter-in valve 27 and without utilizing fluid from the pump. To achieve a float position, the controller 23 is bypassed and pilot pressure is applied to both pilot pressure lines 28, 29. This is achieved, for example, by the use of solenoid operated valves, not shown, which bypass controller 23 when energized and apply the fluid from pilot pump 76 directly to lines 28, 29 causing both meter-out valves 34 to open and thereby permit both ends of the actuator to be connected to tank pressure. In this situation, the meter-out valves function in a manner that the stem of each is fully shifted permitting fluid to flow back and forth between opposed ends of the cylinder, as described in US-A-4,201,052. Where the pressure in the return line A of the actuator is excessive, the pilot spool 71 functions to permit the poppet valve 61 to open and thereby compensate for the increased pressure as well as permit additional flow to the actuator 20 through opening of the poppet valve 40 extending to the passage which extends to the other end of the actuator. By varying the spring forces and the areas on the meter-in valve 27 and the meter-out valves 34, 35, the timing between these valves can be controlled. Thus, for example, if the timing is adjusted so that the meter-out valve leads the meter-in valve, the meter-in valve will control flow and speed in the case where the actuator is being driven. In such an arrangement with an overhauling load, the load-generated pressure will result in the meter-out valve controlling flow and speed. In such a situation, the anti-cavitation check valves 39, 40 will permit fluid to flow to the supply side of the actuator so that no pump flow is needed to fill the actuator in an overhauling load mode or condition. With this knowledge of independent control of the meter-out and meter-in valves, varying metering arrangements can be made to accommodate the type of loading situation encountered by the particular actuator. Where there are primarily energy absorbing or driving loads, the spring and areas of the meter-out valve can be controlled so that the meter-out valve opens quickly before the meter-in valve opens. In the case of primarily overrunning loads, the meter-out valve can be caused to open gradually but much sooner than the meter-in valve so that the meter-out valve is the primary control. A check valve 77 is provided in a branch 78 of each pilot line 28, 29 adjacent each meter-out valve 34, 35. The valves 77 allow fluid to bleed from the high back pressure in tank passage 36, which fluid is relatively warm, and to circulate through pilot lines 28, 29 back to the controller 23 and the fluid reservoir when no pilot pressure is applied to the pilot lines 28, 29. When pilot pressure is applied to a pilot line, the respective check valve 77 closes isolating the pilot pressure from the back pressure. Provision is made for sensing the maximum load pressure in one of a series of valve systems 24 controlling a plurality of actuators and applying that higher pressure to the load sensitive variable displacement pump 22. Each valve system 24 includes a line 79 extending to a shuttle valve 80 that receives load pressure from an adjacent actuator through line 81. Shuttle valve 80 senses which of the two pressures is greater and shifts to apply the same to a shuttle valve 82 through line 83. A line 84 extends from passage 32 to shuttle valve 82. Shuttle valve 82 senses which of the pressure is greater and shifts to apply the higher pressure to pump 22. Thus, each valve system in succession incorporates shuttle valves 80, 82 which compare the load pressure therein with the load pressure of an adjacent valve system and transmit the higher pressure to the adjacent valve system in succession and finally apply the highest load pressure to pump 22. The provision of the load sensing system and the two load drop check valves 37, 38 provide for venting of the meter-in valve in the neutral position so that no orifices are required in the load sensing lines which would result in a horsepower loss during operation which would permit flow from the load during build up of pressure in the sensing lines. In addition, there will be no cylinder drift if other actuators are in operation. Further, the load drop check valves 37, 38 eliminate the need for close tolerances between the spool 51 and the bore 50. Referring to Fig. 5, the load drop check valves 37, 38 are removed. The valve spool 51 is provided with pins 90a, 90b sliding in axial chambers 91a, 91b in the ends of spool 51. Chambers 91a, 91b are connected to the outlet passages 32, 33 by radial openings 92a, 92b in the spool 27. The radial openings 92a, 92b are arranged close to the walls of the outlet openings 32, 33, when the valve spool 51 is in the neutral position. Provision is made that the inner ends of the pins 90a, 90b do not obstruct the radial openings 92a, 92b. An axial passage 93 interconnects chambers 91a, 91b. Radial bleed holes 94a, 94b are provided in the spool axially outwardly of openings 92a, 92b. When the valve spool 51 is in its neutral position, any load pressure either in line A or B will act through openings 92a or 92b on pins 90a or 90b, pushing them outward, hence uncovering bleed holes 94a, 94b and bleeding the pressure through pilot lines 28, 29 and through controller 23 back to tank. When pilot pressure is, for example, admitted in line 29, the valve spool 51 is shifted to the left and fluid flow from inlet passage 26 is ported through passage 32 to line A. The radial opening 92a will be closed off by the valve bore wall 27a whereas the other radial opening 92b remains connected to passage 33 so that fluid in the chambers 91a, 91b and the axial passage 93 assumes pressure of the passage 33 and, therefore, pressure of the cylinder outlet port. In this situation, both feedback pins 90a, 90b are exposed to pressure in cylinder outlet port 33 or line B. Since the pilot pressure is always higher that the pressure of the returning flow in line B, the feedback pin 90b will be kept in the inner end of chamber 91b. The line B pressure will, however, act upon feedback pin 90a, and push it outwardly to the valve bore end or an end cap. The pressure in line B is proportional to flow for a constant pilot pressure, since the flow passage area of the meter-out valve to tank is constant for that pilot pressure. A centering force proportional to the cross section of pin 90a and to line B pressure is thus exerted on the valve spool 51 which will aid the pressure compensating flow forces to keep the flow constant. In case the flow increases, due to a larger pressure drop across the valve spool, the pressure drop over the meter-out element 35 (or 34 as the case is) will also increase (since the flow passage area remains constant). This increased pressure will act upon the feedback pin 90a in centering direction of the spool 51, thus reducing the flow so that it is substantially constant. Referring to Fig. 6, which is a series of curves of flow versus valve spool pressure drop, of the hydraulic control circuit shown in Fig. 1, it can be seen that the flow is not as constant as in Fig. 7, which are curves of a hydraulic control circuit embodying the invention.

Claims: A hydraulic control system comprising a hydraulic actuator (20) having opposed openings adapted to alternately function as inlets and outlets for moving the element (21) of the actuator in opposite directions; a pump (22) for supplying fluid to said actuator (20); a meter-in valve (27) comprising an inlet passage (26) and a pair of outlet passages (32, 33), a valve spool (51) adapted to be actuated by pilot pressure so as to connect said inlet passage (26) with one (32 or 33) outlet passage, spring means (52) adapted to center said spool (51) in a neutral disconnecting position, said spool (51) having at least one axial chamber (91a), at least one pin (90a) sliding in said at least one axial chamber (91a), and spool passage means (92a, 92b) in said spool (51) providing communication between said at least one axial chamber (91a) and one of said outlet passages (32, 33); working lines (A, B) comprising a fluid supplying and a fluid returning line extending from said outlet passages (32, 33) to said respective openings of said actuator (20); a pilot controller (23) for alternately supplying fluid at pilot pressure to said meter-in valve (27) for controlling the direction of movement and position of said spool (51); and meter-out valve means (34, 35) which are separate from and are pilot operable independently of said meter-in valve means (27) further are associated with the returning line (A, B) for controlling the flow out of said actuator (20), characterized in that said spool passage means (92a, 92b) is arranged so as to be shut off from the outlet passage (32 or 33) which is connected to the inlet passage (26) and remains opened to the outlet passage (33 or 32) which is disconnected from the inlet passage (26), and in that said outlet passage (33 or 32) which is disconnected from the inlet passage (26), is connected to said returning line (A or B) so that the pressure thereof is applied to the valve spool (51) in a centering direction for an area corresponding to the cross-sectional area of said pin (90a, 90b). The hydraulic control system set forth in claim 1 wherein said spool passage means (92a, 92b) comprises radial openings (92a or 92b) each of which is just clear of wall sections (27a), when said spool (51) is in its neutral position. The hydraulic control system set forth in claim 1 or 2 including at least a bleed passage (94a) in said spool (51) and associated with said at least one pin (90a) such that when the meter-in spool (51) is in its neutral position and the pressure acts in the working line (A, B) to force said at least one pin (90a) axially outwardly, the pressure in the working line escapes through said bleed passage (94a). The hydraulic control system set forth in any of claims 1-3 wherein said spool (51) has a second axial chamber (91b) opposed to the first-mentioned axial chamber (91a) and connected thereto by a passage (93). The hydraulic control system set forth in claim 4 including a second pin (90b) in said axial chamber (91b), and a second radial opening (92b) and a second bleed passage (94b) associated with said second pin (90b). The hydraulic control system set forth in any of claims 1-5 wherein each said axial chamber (91a, 91b) has limiting means for limiting the movement of each pin (90a, 90b) axially inwardly. The hydraulic control system set forth in any of claims 1-6 wherein each said outlet passage (32, 33) is directly connected to its respective returning line (A, B).

Inventors: LONNEMO, KURT R., SHAH, NALIN J.
Applicants: VICKERS INCORPORATED
Application year: 1986
Main classification: F15B13
IPC codes: F15B11, F15B13