BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to the control of fluid pumps in the interest of providing constant pressure fluid sources. More specifically, this invention is directed to the operation of a balanced variable displacement vane pump as a pressure compensated device. Accordingly, the general objects of the present invention are to provide novel and improved methods and apparatus of such character.
2. Description of the Prior Art
There are a substantial number of hydraulic system applications that require the delivery of constant pressure to a load device such as an actuator. Hydraulic control systems, and particularly those designed for use in aircraft and other vehicles, are typical examples of the need for constant pressure sources.
An operative balanced variable displacement vane pump has recently been invented. This pump is described in U.S. Pat. No. 3,547,562. The disclosure of this patent is incorporated herein by reference. The pump of U.S. Pat. No. 3,547,562, and other variable displacement pumps, have previously been used as flow compensated devices for applications such as the pumping of low lubricity fluids; a typical fluid being fuel to be burned in a gas turbine engine. However, variable displacement pumps, and particularly the balanced vane pump of U.S. Pat. No. 3,547,562, have not previously been operated as pressure compensated devices to provide constant pressure drop across the devices.
SUMMARY OF THE INVENTION
The present invention comprises methods and apparatus which permit the operation of a balanced variable displacement vane pump as a pressure compensated rather than a flow compensated device. In accordance with the present invention a balanced variable displacement vane pump is operated with pump speed and output pressure maintained as fixed operating parameters and flow is used as the varied parameter to maintain a constant pressure rise across the pump.
In accordance with the invention control is exercised over the movable seal blocks of a variable displacement pump; the position of the seal blocks defining pump displacement. Within a pump speed range which meets hydraulic load flow requirements, and with output pressure the parameter to be maintained constant, the system controls pump displacement to regulate flow so as to maintain the desired output pressure. The system initiates control from the maximum flow position only and at the direction of an increase in the operating pressure. Once the pump output pressure exceeds the desired value of operating pressure the system adjusts to a lower flow point; i.e., towards minimum flow; thereby dropping the pressure back to the design point. The reverse operation occurs for a drop in operating pressure.
BRIEF DESCRIPTION OF THE DRAWING
The present invention may be better understood and its numerous objects and advantages will become apparent to those skilled in the art by reference to the accompanying drawing wherein like reference numerals refer to like elements in both figures and in which:
FIG. 1 is a schematic view of a servo controlled hydraulic vane pump system in accordance with the present invention; and
FIG. 2 is a schematic view of a hydraulic vane pump system employing a differential pressure relief valve in accordance with the present invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
To briefly describe the details and operation of the balanced variable displacement vane pump of referenced U.S. Pat. No. 3,547,562, mechanically the pump is of the cartridge type. Referring to FIG. 1, the movable cam or seal blocks 10 and 12, rotor assembly 14 and bearings (not shown) are contained in a cylindrical bore in the main housing 16. Hydraulically the device is a double acting pump. The variable displacement function is performed by controlling the position of the vane cam blocks 10 and 12 with respect to the axis of rotation. The position of the cam blocks determines the net vane radial displacement as desired between the limits of full stroke and zero stroke. To guide the vanes between the cam blocks when they are at a reduced flow position; i.e., with the blocks in their full outward position with respect to the rotor; tongue and groove bridges are incorporated between the cam blocks. The contour of the vane track assures that these bridges will always be tangential to the track. The pump is inherently 100 percent balanced because the left and right cam blocks are identical with seal areas symmetrically positioned. There will be four seal areas between the alternately located pairs of discharge and inlet ports and these areas are located so that the bridges are in areas of no pressure differential. This arrangement results in two identical volumetric pumping arcs. The volumetric displacement of these arcs is determined by the allowable vane accelerations in the interest of insuring constant vane contact with the vane cam blocks and therefore stable pump operation.
In operation, discharge pressure is introduced under the vanes to provide pressure loading thereof against the cam blocks. This loading is accomplished by grooves in the pump side plates, not shown, which also seal each end of the rotor and cams. Inleting and discharging of the pump is done radially through the cam blocks.
Continuing with the description of the embodiment of FIG. 1, in accordance with the invention the movement of cam blocks 10 and 12 is controlled directly by respective spring loaded hydraulic pistons 18 and 20 via piston rods 18' and 20'. A force tending to move the cam blocks apart is always present during operation of the pump due to dynamic and pressure forces acting on the cam block inner faces. In the interest of minimizing these forces, and also reducing system complexity, the pump discharge is routed through passages 19 and 21 respectively in pistons 18 and 20. The outwardly disposed faces of the pistons, located within the cylinders provided therefore in the pump housing, are thus exposed to pump discharge pressure. Accordingly, excessive cam block loads are balanced and the pressure force on the outwardly disposed faces of the pistons, coupled with the force of springs 22 and 24, provides the actuating force to return the cam blocks to the maximum flow position shown in FIG. 1. The internal cam block force loading along with the discharge pressure is used to actuate the cam blocks toward the minimum flow position. The latter actuations; i.e., movement of the cam blocks away from the rotors; is accomplished in the manner to be described below by bleeding the hydraulic pistons.
A first embodiment of the valving and flow path arrangement required to achieve the above briefly described mode of operation, in the interest of maintaining constant pressure rise across the pump, is shown in FIG. 1. In FIG. 1 a four land spool or servo valve, indicated generally at 26, is employed. Valve 26 utilizes two lands 28 and 30 for actuating the pump hydraulic pistons and two lands 32 and 34 for sensing the pertinent pump pressures in the interest of providing accurate control of the pump.
In describing the operation of the embodiment of FIG. 1 it will be presumed that the pump is operative with rotor 14 being driven at a constant speed. Fluid from a reservoir, not shown, is delivered to the pump inlet ports in the manner described in referenced U.S. Pat. No. 3,547,562. Fluid at a selected discharge pressure is supplied to the load via discharge conduit means 40; the fluid being delivered through the pump discharge ports in the manner described in U.S. Pat. No. 3,547,562 and the flowing through the passages 19 and 21 provided therefore in the cam block actuator pistons 18 and 20.
It will further be assumed, for purposes of explanation only, that the pump is not deliberately oversized and that the maximum flow position of the cam blocks, as depicted in FIG. 1, is commensurate with the desired constant output pressure. Under these conditions, the spool valve 26 will be in the balanced position as also shown in FIG. 1. Balancing of spool valve 26 is achieved by applying the pump inlet pressure to a first end of the valve; i.e., to the face of valve portion defining land 32; and the pump discharge pressure to the opposite end of the valve; i.e., to the face of the portion defining land 34. Since the pump discharge pressure exceeds the inlet pressure, and in the interest of simplifying valve construction and avoiding the use of differential area piston, a compensating spring 36 acting in opposition to the discharge pressure is located in the cylinder of valve 26 between the first end thereof and the end of the valve exposed to inlet pressure. The conduit 41 through which inlet pressure is delivered to valve 26 is connected to the fluid source for the pump, indicated schematically at 42, and fluid is delivered from source 42 to the pump inlet ports in the manner described in U.S. Pat. No. 3,547,562. The means by which the pump discharge pressure is applied to valve 26 is indicated schematically as conduit 44 which includes a restriction 48 for dropping the sensing pressure.
Control pressure for moving pistons 18 and their associated cam blocks outwardly in the reduced flow direction is derived from the pump discharge pressure. For this purpose a conduit 46 is connected between the pump discharge and a centrally disposed port 49 in valve 26. Valve 26 is also provided with a pair of drain ports which are connected, via conduits 50 and 52, to a sump or reservoir.
Presuming that the pump is operating with discharge pressure at the desired level, valve 26 will be in the balanced condition as shown. As previously noted, for purposes of illustration only it may be presumed that the desired output pressure is commensurate with maximum flow and thus cam blocks 10 and 12 will initially be in their fully inward position as shown. If the pump discharge pressure should exceed the desired level, valve 26 will become unbalanced and its slide will be moved to the left. The movement of valve 26 to the left will permit application of control pressure, as derived from conduit 46, to the inwardly disposed sides of pistons 18 and 20; the control pressure passing through valve 26 and the control port connected to conduit 38 which has been opened by movement of the valve to the left. Simultaneously, the pressure behind pistons 18 and 20 will be blead via conduits 54 to the sump through the valve 26 and drain conduit 52. Accordingly, cam blocks 10 and 12 will move away from rotor 14 under the influence of the pressure acting on the inwardly disposed sides of pistons 18 and 20 and the dynamic and pressure forces acting on cam blocks 10 and 12. The outward movement of cam blocks 10 and 12 will cause the output flow from the pump to decrease thereby causing the pump discharge pressure to decrease. When pump discharge pressure decreases to the preselected level, the slide of valve 26 will move to the right and return to the balanced position shown. Re-balancing of valve 26 will again isolate conduits 54 from the drain 52 and conduit 38 from the source of control pressure. The cam blocks will now have assumed a new position commensurate with a new flow and the desired output pressure.
Should output pressure now fall below the desired level the slide in valve 26 will move to the right thereby connecting conduit 38 to drain 50 and applying the control pressure to conduits 54. The cam blocks will accordingly move inwardly and the flow rate will increase whereby the pump output pressure will increase to the desired level.
Referring now to FIG. 2, a second embodiment of the invention is disclosed wherein a differential relief valve, indicated generally at 60, is employed for control purposes. The pump discharge pressure is applied to one side of relief valve 60 via sensing conduit 62. The pump inlet or supply pressure is applied to the opposite side of valve 60 via conduit 64 which is connected to source 42; the source being connected to the pump inlet ports as described in reference U.S. Pat. No. 3,547,562. A biasing spring 66 cooperates with the pressure sensed via conduit 64 to maintain valve 60 in the position shown in FIG. 2 with pump outlet pressure at the desired level. Control ports in the pump housing at the outwardly disposed sides of pistons 18 and 20 are connected, via conduits 68, to valve 60 as shown. Conduit 68 is provided with a restriction 70 for the purpose of providing a drop in the discharge pressure used for sensing purposes. The decrease in the valve of the sensing or control pressure permits a reduction in the sizing of relief valve 60 and thus restriction 70 serves the same purpose as restriction 48 in the FIG. 1 embodiment.
The control system of FIG. 2 is depicted in the balanced condition with the pressure rise across the pump being at the desired value as preset by selection of spring 66. In the balanced condition of valve 60 the conduits 68 are isolated from a discharge port 72 in valve 60; discharge port 72 being connected to a suitable sump. If the pressure differential across the pump exceeds the set point the piston of valve 60 will move upwardly thus establishing communication between conduits 68 and drain 72 and bleeding the areas behind pistons 18 and 20. The pistons and cam blocks will thus move away, under the influence of the internal cam block force loading, from the rotor and the pump output flow rate will decrease. As a result of the decrease in flow rate the pressure across the pump will drop thereby permitting the piston of valve 60 to return, under the influence of spring 66 and the inlet pressure, to the position shown in FIG. 2. At this time the pump will be stabilized with the cam blocks in a new operating position commensurate with a new flow rate and the set discharge pressure will be reattained.
If discharge pressure falls below the set point the pump will self-compensate itself by movement of the cam blocks towards the maximum flow position. Self-compensation results through the reduction of the pressure forces acting outwardly on the cam blocks whereby the springs 22, 24 move the blocks toward the rotor.
While a preferred embodiment has been shown and described, various modifications and substitutions may be made without departing from the spirit and scope of this invention. Accordingly, it is to be understood that this invention has been described by way of illustration and not limitation.