Combined Engine Speed And Pressure Responsive Control For Variable Displacement Pumps

Abstract

There is disclosed a hydraulic power system having a plurality of variable displacement pumps for supplying pressurized fluid for manipulation of an implement, and control means operative to automatically adjust pump displacement to the output horsepower of a prime mover driving the pumps of the system. The control system includes a first valve means responsive to the speed of the prime mover to transmit a fluid pressure signal to servo-control means for adjusting the displacement of the pumps. Second valve means is responsive to the output pressure of the variable displacement pumps for overriding the speed responsive means during transient overload conditions.

Description

BACKGROUND OF THE INVENTION

The present invention is directed to hydraulic control systems, and pertains more particularly to a hydraulic control system having a plurality of variable displacement pumps and means for automatically adjusting the displacement of the pumps.

Many hydraulically actuated implements, such as the hydraulic excavator, normally employ a plurality of variable displacement pumps for supplying the high pressure hydraulic fluid necessary to power the many motors employed in manipulating the excavator bucket. An example of the hydraulic circuitry for such an excavator is illustrated in Application Ser. No. 207,027, filed Dec. 13, 1971 by John W. Bridwell et al., and assigned to the assignee of the present application.

Numerous different circuits are employed for such systems, and may employ a totally different circuit, or separate circuit for each motor or may employ various means for combining the flow from a plurality of pumps to a single one or more of the hydraulic motors. The hydraulic pump is all of these systems is normally driven by a single prime mover such as an internal combustion engine. One difficulty with these arrangements is that each separate motor and each separate pump may be capable of handling the entire horsepower output of the engine. Thus, if two or more of the motors or pumps are loaded, the engine may stall.

Many attempts to solve this problem have suggested means for adjusting the output of the pumps in response either to the load of the engine or the load imposed on the implement itself.

One proposed system for overcoming these problems is disclosed in U.S. Application Ser. No. 68,317, filed Aug. 31, 1970 by Cyril W. Habiger et al., and assigned to the assignee of the present invention. That system is known as an underspeed valve, responsive to the speed of the engine which drives the pumps for adjusting the output of the pumps. The theory of operation of that system is that a load imposed on the engine will cause the engine to lug down. The control means, in response to the slowing of the engine, will automatically adjust the outputs of the pump downward to compensate for the loss in speed of the engine to make the horsepower output of the pump correspond to that of the engine. One problem with such systems is that the inertia of the engine delays a lugging or reduction in the engine speed so that the underspeed valve does not respond immediately to a peak pressure condition.

Take, for example, the situation of an excavator having a circuit as that above described wherein fluid from both pumps may be combined for raising the boom. Under such a situation, the high flow rate of fluid to the boom cylinder results in a rapid build-up of pressure which, with full displacement of the two variable displacement pumps, exceeds the available horsepower from the engine. The inertia of the engine delays the lugging condition so that the underspeed valve does not respond immediately.

Since the boom and the weight carried by the boom are accelerating rapidly as the pressure peak occurs in the circuit, the boom would tend to overrun the pump, so that pressure in the system begins to reduce. Concurrently therewith, the engine speed begins to reduce, which generates a pressure differential across the underspeed orifice, so that the underspeed valve responds to initiate a reduction in the pump displacement. This pump reduction displacement aggravates the reducing pressure situation by allowing a reduction in boom speed.

Due to the natural frequency of the boom system and the varying pump displacements resulting from the response of the underspeed control being approximately 90.degree. out-of-phase with the pressure condition in the system, a rythmic oscillation will be generated wherein the boom lunges and then hesitates. Since the engine speed will increase as the displacement of the pump is reduced and the pressure in the system reduces, the pump will return to full displacement and another surge in the boom will occur. The 90.degree. out-of-phase response of the underspeed horsepower control contributes to the natural frequency of this machine to perpetuate this erratic boom movement.

Another approach to solving the horsepower overload situation is illustrated in Application Ser. No. 127,738, filed by William F. Busby et al., entitled "Summing Valve Arrangement" and assigned to the assignee of the present application. The summing valve arrangement provides means which is responsive to the sum of the pressures imposed by the load on the variable displacement pumps for adjusting a control valve which controls the displacement of the various pumps. This type of arrangement has a disadvantage in that it is responsive mainly to the load pressure of the pumps, and not to the load on the engine. Such a system becomes more sensitive to load on the pump, and less sensitive to the actual load on the engine.

SUMMARY OF THE INVENTION

It is the primary object of the present invention to provide horsepower controlling means operative to overcome the above problems of the prior art.

Another object of the present invention is to provide horsepower controlling means for hydraulic systems including a plurality of variable displacement pumps that is operative in response both to the output of the pumps and the output of the prime mover driving the pumps, to appropriately adjust horsepower or displacement of the pumps to correspond to the optimum horsepower.

In accordance with the present invention, there is provided control means for hydraulic control systems having a plurality of variable displacement pumps that is operative to vary the pump displacement in accordance with load imposed on the engine and load imposed on the pumps.

BRIEF DESCRIPTION OF THE DRAWING

The Drawing is a schematic illustration of a hydraulic system embodying the principles of the present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENT

Referring now to the Drawing, a horsepower limiting pump displacement control system embodying the principles of the present invention, is schematically illustrated in operative combination with a multiple pump implement control circuit, only a portion of which is illustrated. The implement control circuit is of the type that would be utilized in an implement such as a hydraulic excavator wherein a plurality of hydraulic motors are used in combination, individually, and/or simultaneously, for the manipulation of an implement.

The implement control system comprises a plurality of variable displacement pumps 10 and 11, and a fixed displacement pump 12, all driven by suitable prime mover means such as an internal combustion engine 13. Each pump draws fluid from a suitable reservoir or sump 14 and supplies it under pressure by way of respective supply conduits 15, 16 and 17.

Each of the variable displacement pumps 10 and 11 is provided with suitable servo-control means of conventional design 18 and 19, which is responsive to a pressure signal to vary the displacement of the pumps 10 and 11.

The control system for supplying pressurized control fluid for actuating the servo-control means 18 and 19 comprises a fixed displacement pump 20, directly driven by the prime mover means 13 drawing fluid from the reservoir 14, and supplying it to the underspeed pump displacement control, generally indicated at 21. The underspeed control means 21 is of the type disclosed in Application Ser. No. 68,317 referred to above.

The operation of the underspeed valve 21 is such that it responds to a reduction in the speed of pump 20, as a result of a speed reduction in the engine due to overload or for any other reason. The output fluid flow from the pump 20 flows through an orifice 22 which creates a pressure differential acting on opposite ends of a valve 23, which differential pressure is communicated by way of lines 24 and 25. The pressure differential is a function of the fluid flow, which is a function of engine speed.

A predetermined change in a pressure differential across orifice 22 allows valve 21 to be shifted to an open position to communicate pressure from a control pump 20 by way of a line 26 across valve 23 where it is communicated by way of a line 27 past a shuttle valve 28 where it communicates by way of branch lines 29 and 30 with the servo-control means 18 and 19 of the respective pumps 10 and 11. As soon as the load on the engine is relieved and engine speed returns to its normal setting, increased flow through orifice 22 restores the pressure differential across the valve 23 so that it returns to the closed position as shown and thus permits the pumps 10 and 11 to return to a full displacement condition.

In accordance with the present invention, control pressure from or fluid from pump 20 continues through the underspeed valve 21 and flows along a supply line 31 to a summing valve indicated generally by the reference numeral 32. The summing valve is of the type illustrated and disclosed in Application Ser. No. 127,738 mentioned above. The pressure summing control means 32 comprises a valve member or spool 33, which is normally biased to the cut-off position but is responsive to force developed by pressure from each of th lines 15, 16 and 17 by way of lines 34, 35 and 36 to pistons 37, 38 and 39 acting on the spool 33. When the sum of the pressures developed on pistons 37, 38 and 39 approaches or reaches a certain predetermined value, spool 33 is shifted to provide communication between line 31 and, by way of shuttle valve 28, to the lines 29 and 30, to the control means 18 and 19 to shift the valves and to stroke the pumps 11 and 10 to a reduced displacement position. The pressure sensed by way of lines 35, 36 and 34 will be a function of the load imposed on the pumps 10, 11 and 12. A relief valve 40 in line 31 functions to protect the line from overpressurization, and maintains a nearly constant pressure level downstream of orifice 22. The pressure of pump 12 can be ignored where it is not a significant portion of the total load, or where timing is such that the load thereof will not affect the overall load.

The entire system is adjusted so that the pump displacements are primarily under the control of the underspeed control means 21 until such time as sudden peak pressure conditions occur in the system, at which time the pressure responsive or pressure summing control means 32 takes over, communicating a signal to the pump displacement control means 18 and 19, and reduces the displacement thereof to accommodate the load conditions.

Under these peak pressure conditions, the summing valve will immediately sense the pressure and respond quickly to reduce the displacement of the pumps 10 and 11 to control or prevent too much horsepower drain on the engine. The summing valve is set to respond at a higher pressure and therefore a greater horsepower than does the underspeed control 21. Accordingly, if the pressure condition and horsepower drain continue for some extended period of time, the underspeed valve will take control at a slightly lower horsepower drain and the signal will then be directed from the underspeed valve means 21 by way of line 27, shuttle valve 28, to maintain control of the pump displacement.

From the above description it can be seen that there is provided a novel combination of speed responsive control means and pressure responsive control means cooperatively connected in combination to control the displacement of a plurality of variable displacement pumps to appropriately accommodate the horsepower capacity of an engine. This combination includes a summing valve responsive to the sum of the outputs of the loaded pumps and an underspeed valve responsive to the output of a pump directly driven by the engine to control the output of the plurality of variable displacement pumps in accordance therewith.

While this invention has been described by means of a particular embodiment, it is to be understood that numerous changes and arrangement in parts may be made without departing from the spirit and scope of the present invention as defined in the appended claims.

Claims

We claim:

1. A hydraulic power system for an excavator or the like, said system comprising:

a plurality of variable displacement hydraulic pumps for supplying power fluid for said system;

prime mover means operatively connected to drive said pumps;

means including a fixed displacement pump for providing a source of pilot control fluid and underspeed valve means responsive to the output of said fixed displacement pump to generate a control signal that is a function of the speed of said prime mover;

servo-control means responsive to said pilot control fluid to adjust the displacement of said variable displacement pumps as a function thereof; and,

pressure responsive summing valve means responsive to the sum of the pressure of said power fluid from said plurality of pumps to override said pilot control signal under overload conditions, said underspeed valve means and said summing means being communicated by fluid communication means with said servo-control means.

2. A control system in combination with a prime mover and a plurality of variable displacement pumps operatively driven by said prime mover and operative to supply pressurized fluid for operation of a plurality of hydraulic motors for powering an implement, said control system comprising:

a common source of control fluid for supplying said plurality of pumps;

pressure responsive servo-control means in selective communication with said source and operative to adjust the displacement of said pumps to correspond to the power output of said prime mover;

means responsive to the speed of said prime mover to normally control said communication to direct said control fluid via a first path to said servo-control means to adjust the output of said variable displacement pumps;

means responsive to the sum of the pressure output of said variable displacement pumps to direct said control fluid via a second path to override said speed responsive means in response to pre-determined overload conditions; and,

pressure responsive shuttle valve means disposed between said paths and said servo-control means and selectively responsive to the higher of the pressures in said paths to direct fluid therefrom to said servo-control means.

3. The combination of horsepower controlling system for controlling the horsepower output of a plurality of variable displacement pumps driven by a single prime mover, and system comprising:

a source of pilot pressure including a fixed displacement pump driven by said prime mover for controlling placement of said pumps in relation to the speed output of said prime mover;

underspeed control valve means responsive to the speed of said prime mover for normally controlling the communication of said pilot control pressure with said pump displacement means; and,

summing valve means comprising a valve having a predetermined bias in the direction to cut off said communication and,

a plurality of pressure responsive means responsive to the sum of the pressure output of said variable displacement pumps to bias said valve in a direction to provide open communication between said source of control pressure and said pump displacement controlling means.

4. The system of claim 3 wherein said pressure responsive means comprises a plurality of pistons disposed in cylinders;

said cylinders being in communication with the output of said pump at one end of said piston; and,

the other end of said piston is in operative engagement with said valve means.

5. A horsepower controlling system, said system comprising in combination:

a prime mover;

a variable displacement pump driven by said prime mover and including pressure responsive means for varying the displacement of said pump;

A common source of actuating pressure for actuating said pressure responsive means;

first and second valves each having a bore in communication with said common source of actuating pressure and with said pressure responsive means;

a spool disposed in each of said bores and operative to control said communication;

means including a fixed displacement pump in combination with said first valve to define an underspeed valve responsive to the speed of said prime mover to thereby normally communicate said actuating pressure with said actuation means to alter the displacement of said pump in accordance with the speed of said prime mover; and,

means responsive to the output pressure of said variable displacement pump to actuate the second of said valves to thereby communicate said actuating pressure with said actuating means to alter the displacement of said pump when the load of said output exceeds a predetermined amount.

6. The invention of claim 5 wherein said source of actuating pressure is a fixed displacement pump driven by said prime mover.

7. The combination of claim 5 comprising a plurality of variable displacement pumps driven by a common prime mover.

8. The combination of claim 5 comprising a shuttle valve operatively connected between said first and second valves and the pressure responsive means of said variable displacement pump.