US 3881317 A
A control system for a hydrostatic transmission including means for selectively introducing control fluid under pressure to a control piston and means for metering-out fluid from the control piston in response to the working fluid pressure in the transmission. The control system also includes means for biasing the control piston in a neutral position.
Description (OCR text may contain errors)
0 United States Patent [1 1 [111 3,881,317 Swoager May 6, 1975  CONTROL SYSTEM FOR VARIABLE 3,214,911 11/1965 Kempson 60/450 X DISPLACEMENT HYDRAULIC PUMP 3,236,049 2/1966 Reinke 60/444 3,477,225 11/1969 Cr der et a1 60/444  Inventor: Jon R. Swoager, Imperial, Pa. y
1 3 Assigneei Autonzmfiml Equipment, -i Primary Examiner-Edgar W. Geoghegan Impenal, Attorney, Agent, or Firm-Robert D. Yeager  Filed: Oct. 25, 1973  Appl. No.: 409,746 57 ABSTRACT A control system for a hydrostatic transmission includ- C 1 ijffggig ing means for selectively introducing control fluid  Fie'ld 444 445 under pressure to a control piston and means for me- 6 tering-out fluid from the control piston in response to the working fluid pressure in the transmission. The control system also includes means for biasing the  SS' ZF E ZF control piston in a neutral position.
3,167,907 2/ 1965 6 Claims, 1 Drawing Figure Kempson 60/444 X FROM REPLENISHING PUMP CONTROL SYSTEM FOR VARIABLE DISPLACEMENT HYDRAULIC PUMP BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to a control system for a hydrostatic transmission; more particularly to a neutrallybiased stroking control for a variable displacement hydraulic pump.
2. Description of the Prior Art The operation of a conventional hydrostatic transmission embodying a variable displacement pump is well known. High pressure fluid is pumped through one leg of a closed loop system to a hydraulic motor to cause rotation of the motor in one direction. The expended oil is returned from the motor to the pump through the low pressure leg of the closed loop. The cam angle of the pump is infinitely variable for varying the speed of rotation of the motor; as the cam angle is moved toward the neutral (or null) position, the stroke of the pistons in the pump is shortened, the flow of fluid from the pump to the motor through the high pressure leg is reduced, and the motor slows down. When the pump cam angle reaches null there is no piston stroke in the pump, no flow of fluid to the motor and thus no output rotation of the motor.
As the pump cam angle is reversed (over center), the pistons begin to stroke but now force fluid to the motor through the leg of the closed loop which was formerly the return (low pressure) leg. This reversal of fluid flow causes rotation of the motor in the opposite direction. Pump cam angle can now be varied as before to control the flow rate of fluid to the motor as desired.
Practically all control systems for variable displacement pumps in hydrostatic drives utilize control oil taken from the closed loop replenishing pump to actuate the means for positioning the pump cam plate. Ordinarily, such actuating means takes the form of a double acting piston having a rod connected to the pump cam plate. To move the pump cam plate, it is necessary to apply control oil to one side or the other of the piston. Such application of control oil in the known systems is normally controlled by a metering-in system; that is, controlling the flow of control oil to the control piston. If the transmission signals for movement of the control piston, (eg by moving the control lever, by the occurrence of overpressure in the system, etc.) while the metering-in control is preventing the flow of control oil to the control piston, then the piston can be temporarily starved for control oil and cause the transmission to be placed in an unstable condition.
Also in the use of hydrostatic transmissions, it is frequently desirable to place the pump cam plate in the neutral position (i.e., stop rotation of the motor) and be assured that the neutral position will be maintained. For example, where hydrostatic transmissions are used in propelling underground mining equipment, the operator may often wish to stop the equipment and move to a position between the equipment and a wall of the mine. If the neutral position of the pump is not maintained, the pump will begin stroking and cause movement of the equipment which could result in a serious accident. The components required in achieving and maintaining a positive neutral position are not offered by the known pump control systems. The known systems rely upon the application of control oil to the conthe FIGURE. A pair of conduits 6a and 6b form a path trol piston or upon the high pressure forces in the closed loop to place the pump off stroke".
SUMMARY OF THE INVENTION The present invention overcomes the disadvantages and objections associated with known control systems for hydrostatic drives. Not only is a supply of control oil to the control piston assured with the present invention, but also a means for positive centering of the control system is afforded.
The present invention provides in a control system for a hydrostatic transmission having a pump for supplying fluid under pressure to a motor through a closed circuit, the circuit being in selective communication with a fluid drain and including means for replenishing the fluid and the control system including a control piston operably connected to the cam plate of the pump, the improvement comprising: first valve means having an inlet and an outlet for selectively introducing control fluid under pressure to one or the other side of the control piston; flow control means in communication with the outlet of the first valve means for directing the control fluid to the selected side of the control piston and providing an outlet for a portion of the control fluid; and second valve means having an inlet connected to the flow control means outlet and an outlet connected to the fluid drain for selectively proportioning the flow of the control fluid through the second valve means outlet to the fluid drain in response to the fluid pressure present in the closed circuit. The present invention further comprises means connected to the control piston for biasing the control piston in a neutral position. Preferably, the biasing means comprises at least one spring compressively loaded to overcome a predetermined value of the control fluid pressure.
In a preferred form, the second valve means of the present invention comprises a variable orifice valve which is operably connected with and responsive to the position of the pump cam plate. Also in the preferred form, the flow control means of the present invention comprises a two-position shuttle valve having a central outlet connected to the inlet of the second valve means.
Other details, objects and advantages of the invention will become apparent from a consideration of the following detailed description thereof taken with the accompanying drawing.
BRIEF DESCRIPTION OF THE DRAWING The FIGURE is a schematic of a closed circuit hydrostatic drive embodying the present invention.
DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to the FIGURE, there is shown a closed loop hydrostatic drive with the loop itself designated generally by the reference numeral 2. The loop includes a variable displacement pump 4 which is sized to meet horsepower requirements. Pump 4 may have a flow reversing capability as does the one illustrated in for fluid under pressure to flow from pump 4 to drive a hydraulic motor 8. Motor 8 may also be either of the fixed or variable displacement type, and, as in the motor illustrated, may be bidirectional. The system for replenishing fluid to loop 2 is not shown but may be of the type described in US. Pat. No. 3,744,244 which is incorporated by reference herein.
As shown in the FIGURE, both pump 4 and motor 8 are reversible; thus the direction of fluid flow in loop 2 may be alternately clockwise or counterclockwise. If, for example, the state of pump 4 is such that conduit 6a is on the pressure side of pump 4, conduit 6a is said to be the high pressure leg and'conduit 6b would be termed the low pressure leg. In this condition, fluid will flow from pump 4 through conduit 6a for passage through motor 8 and thence return to pump 4 through conduit 6b. When the state of pump 4 is reversed, fluid flow is opposite to that just described and conduit 6b becomes the high pressure leg and conduit 6a becomes the low pressure leg. When the state of pump 4 is brought to the null position (i.e., the cam plate, depicted schematically in the FIGURE by the arrow 10, is moved to a degree angle), no fluid flow occurs in the loop and thus motor 8 does not rotate.
The position of cam plate 10 within pump 4 is governed by the position of control piston 12 slidably disposed within control cylinder 14. That is, when piston 12 moves, it carries with it attached piston rod 16 which in turn is mechanically linked at 18 with cam plate 10. Attached to the opposite side of control piston 16 is visual indicating rod 20. Visual indicating rod 20 is particularly useful in maintenance and troubleshooting operations because it provides a visual indication of the position and movements of control piston 12 during the various stages of operation of the transmission.
Operably connected to control piston 12 by any well known means is a biasing means (shown schematically in the FIGURE as springs 22 and 24). Springs 22 and 24 operate directly upon control piston 12 and thus upon cam plate 10. Preferably, springs 22 and 24 take the form of a single coiled spring suitably mounted for compression upon movement of control piston 12 in either direction away from center. By using a single spring, the problem of matching a pair of springs is avoided. Springs 22 and 24 are sized to overcome all dynamic forces capable of being produced in pump4 so that upon the loss of fluid pressure against piston 12 (by occurrences discussed hereinafter), springs 22 and 24 force piston 12 to the center of cylinder 14 and hold piston 12 in that (neutral) position. When piston 12 is in the center or neutral position within cylinder 14, cam plate 10 is in the null or 0 position and is held there notwithstanding dynamic forces exerted against it which may otherwise put the pump on stroke.
Centering springs 22 and 24 offer another control advantage. Since they are biased to center piston 12 within cylinder 14, movement of piston 12 in either direction requires the exertion of pressure on one side or the other of piston 12 through the use of hydraulic fluid. A mere differential in fluid pressures on opposite sides of piston 12 (assuming equal piston areas) will not be sufficient to cause movement of piston 12. That is, the fluid pressure on one side of piston 12 must be sufficient to overcome the compressive force exerted by the appropriate one of springs 22 or 24. The threshold level of fluid pressure required to cause initial movement of piston 12 against the spring force may be varied according to the particular transmission characteristics and requirements. Therefore, the present invention permits control of the position of piston 12 (and thus cam plate 10) by controlling the fluid pressure on the appropriate side of piston 12.
Having established that the position of piston 12 may be determined by the fluid pressure within one side or the other of cylinder 14, the means for controlling that fluid pressure will now be discussed. Control oil for use in cylinder 14 is furnished by a replenishing pump (not shown) which normally accompanies a hydrostatic transmission. The flow of control oil from the replenishing pump through line 26 is in the direction of the arrow shown in the FIGURE and is controlled by threeposition valve 28. In the position shown in the FIG- URE, valve 28 is preventing flow of control oil to control cylinder 14. In the same centered position, valve 28 is provided with an open center for equalized fluid pressures beyond valve 28. This position is the normal off position. Valve 28, in addition to controlling direction of flow of the control oil, may control the pressure and flow rate of the control oil by any well known means. Accordingly, valve 28 may be a proportional valve, directional valve, pressure control valve or the like. In most applications, valve 28 will be operated remotely from the hydrostatic transmission itself.
Control oil passes through valve 28 to either of conduits 30a or 30b and thence into hydraulic remote position actuator 32 (hereinafter HRPA for the sake of brevity). I-IRPA 32 includes through passages 34a and 34b in communication with conduits 30a and 30b, respectively. Connecting passages 34a and 34b within I-IRPA 32 is conduit 36 having a two-position shuttle valve 38 disposed therein. Valve 38 has a ball 40 having left and right positions depending upon the differential fluid pressures in conduit 36. Conduit 42 connects with conduit 36 at the center of shuttle valve 38 to provide fluid passage to drain through valve 44 soon to be described. Conduits 46a and 46b connect with passages 34a and 34b, respectively, to provide fluid passage to the left and right sides, respectively, of cylinder 14.
Thus, if control oil is passing through valve 28 into conduit 30a and passage 34a, ball 40 of shuttle valve 38 will be displaced to the right thereby exposing conduit 42 to fluid flow from conduit 30a. Under ordinary operating conditions, however, valve 44 is closed and therefore fluid passing through conduit 30a will flow through conduit 46a, into the left side of cylinder 14, and exert pressure on the left side of piston 12 to move it against the pressure of spring 24. As piston 12 moves from left to right, fluid in the right side of cylinder 14 is forced out of that side of cylinder 14, through conduit 46b and passage 34b of HRPA 32, and to drain through valve 28. Since the ball 40 of shuttle valve 38 is displaced to the right, the fluid leaving the right side of cylinder 14 must take the path just described.
Valve 44 is a variable orifice flow control valve which is responsive to fluctuations in pressure in loop 2. The purpose of valve 44 is to maintain the input horsepower of the transmission according to predetermined, optimum horsepower curves for the particular transmission involved, according to principles well-known in the art. Valve 44 also provides an additional important function; namely, that of preventing dither of control piston 12 by metering-out control oil from the pressure side of piston 12. As alluded to above, the prior art control systems meter-in control oil to the pressure side of the control piston and thus invariably experience dither 0f the control piston. Dither is a physically destructive force in any hydraulic control system and also lends to instability in the operation of the equipment being powered by the hydrostatic transmission.
The pressure value sensed in loop 2 by valve 44 is the pressure in the high pressure leg. This sensing operation is accomplished by means of pilot lines 48 and 50, connected at one end to conduits 6a and 6b, respectively, and at the other to opposed ports of high pressure shuttle valve 52. Shuttle valve 52 is a two-position ball valve with a central port from which pilot line 54 feeds pressure logic from the appropriate high pressure leg of loop 2 to operating means for valve 44. The variable orifice of valve 44 is adjusted to progressively open as pressure in the high pressure leg increases to permit increasingly greater flow of control fluid to drain. The adjustment of valve 44 is determined by the value of input horsepower desired to be maintained according to principles well known in the art. If the pres- 7 sure in the high pressure leg reaches a predetermined maximum, valve 44 is fully open.
The opening and closing of the variable orifice of valve 44 is also controlled by another input. The parallel solid lines 56 on the FIGURE are intended to depict a mechanical feedback linkage connecting cam plate 10 with valve 44. This arrangement is a well known mechanical servo such as, for example, illustrated in US. Pat. No. 3,l64,960. The operation of this servo is according to well known principles such that when cam plate 10 reaches a position dictated by the pressure logic being received by the operating means for valve 44, the mechanical linkage 56 signals valve 44 accordingly and the orifice of valve 44 closes.
The operation of the present invention will now be described. Assuming the transmission is in a shut-down condition, control piston 12 is being firmly held in a neutral position by springs 22 and 24; thus, cam plate 10 is similarly held in the null position. This is an important feature of the present invention not found in the prior art systems because, upon start-up of the driv ing means (not shown) for pump 4, no sudden movement of the equipment powered by the transmission can occur. After start-up of the driving means for pump 4, pump 4 may be placed on stroke by sliding movement of remote control valve 28 to either the extreme left or the extreme right positions, depending upon the desired direction of rotation of motor 8. Assume for this example that valve 28 is moved to the right thereby placing the control oil supply line 26 in communication with conduit 30a. Since valve 28 is a pressure and/or flow regulated valve, control oil flows into conduit 30a at the pressure and/or flow selected by the operator. Ball 40 of HRPA 32 is shifted to the right exposing conduit 42 to fluid flow from conduit 30a, but since the pressure in loop 2 is below the predetermined minimum value established for valve 44, valve 44 remains closed. Fluid under pressure in conduit 30a accordingly passes through passage 34a of HRPA 32, through conduit 46a and into the left side of control cylinder 14. As the fluid pressure in the left side of cylinder 14 increases to the selected value, the pressure acts upon control piston 12 and displaces it to the right against the compressive force of spring 24. Fluid in the right side of cylinder 14 is thereby displaced and flows to drain via conduit 46b, passage 34b of HRPA 32, and conduit 30b. Movement to the right of control piston 12 causes corresponding movement of cam plate 10 away from the null position and places pump 4 on stroke. The position of control piston 12 is maintained by the pressure of control fluid in the left side of cylinder 14.
Any fluctuation in resistance met by motor 8 while operating in the stable state just described is immediately reflected in the pressure in the high pressure leg of loop 2 and transmitted via either of pilot lines 48 or 50 through shuttle valve 52 and pilot line 54 to the operating means for valve 44. If, for example, the fluctuation is an increase in resistance, the pressure in the high pressure leg will increase. In order to maintain a constant input horsepower at this increased pressure level, the fluid flow produced by pump 4 must be decreased. Accordingly, the higher pressure sensed by the operating means for valve 44 causes the orifice of valve 44 to open, permitting control fluid in conduit 30a to be dumped todrain via conduit 36, conduit 42, and the orifice of valve 44. This dumping action causes a consequent reduction in fluid pressure in the left side of cylinder 14 and the compressive force of spring 24 thereby causes movement of control piston 12 to the left against the reduced fluid pressure. Corresponding movement of cam plate 10 toward the null position operates to reduce the flow of fluid from the pressure side of pump 4. The following action of the mechanical feedback linkage 56 signals the operating means for valve 44 that cam plate 10 has reached the desired position to provide the correct reduced value of fluid flow and thus operates to close the orifice in valve 44. Accordingly, the sequence just described results in the maintenance of the desired level of input horsepower by automatically reducing flow in response to the increase in pressure experienced in loop 2. A decrease in pressure in the loop causes the reverse action of the components described above to achieve a similar result.
As will be appreciated by those skilled in the art, the movements detailed above occur in rapid sequence so that compensation is made for even small, rapid pressure fluctuations. However, the metering-out feature provided by valve 44 permits the adjustments to be made smoothly and without evidence of the hunt and seek characteristics typical of the prior art control systems.
If, during normal operation of the transmission, a malfunction occurs which causes rapid loss of control fluid pressure, the present invention provides a fail safe mechanism to cause pump 4 to go off stroke and remain there. This feature is of significant value from a safety standpoint and is not found in the prior art transmissions. Suppose, for example, a fluid line in the transmission were to break. The replenishing pump, which furnishes control oil under pressure, is immediately vented to the atmosphere and control oil pressure will fall sharply. This means that there is no fluid under pressure available to cause control piston 12 to center within cylinder 14 and thereby place pump 4 off stroke. Nevertheless, the pressure of springs 22 and 24, one of which being in a compressed state, operate to return piston 12 to the center of cylinder 14 and hold it in place against any dynamic forces generated in the system. Thus, springs 22 and 24 provide an affirmative centering action in the event of system malfunctions and cause pump 4 to go off stroke.
What is claimed is:
1. In a control system for a hydraulic transmission having a pump for supplying fluid under pressure to a motor through a closed circuit, said control system being in selective communication with a fluid drain and including a control piston operably connected to the cam plate of said pump, the improvement comprising:
first valve means having an inlet and an outlet for selectively introducing control fluid under pressure to one or the other side of said control piston; flow control means in communication with said outlet of said first valve means for directing said control fluid to said selected side of said control piston and providing an outlet for a portion of said control fluid; and
second valve means having an inlet connected to said flow control means outlet and an outlet connected to said fluid drain for selectively proportioning the flow of said control fluid through said second valve means outlet to said fluid drain in response to said fluid pressure present in said closed circuit. 2. The improvement recited in claim 1 which further comprises:
means connected to said control piston for biasing said control piston in a neutral position.
3. The improvement recited in claim 2 wherein:
said biasing means comprises at least one spring compressively loaded to overcome a predetermined positive value of said control fluid pressure.
4. The improvement recited in claim 1 wherein:
said second valve means comprises a variable orifice valve.
5. The improvement recited in claim 4 wherein: said variable orifice valve is operably connected with and responsive to the position of said cam plate.
6. The improvement recited in claim 1 wherein: said flow control means comprises a two-position shuttle valve having a central outlet connected to said inlet of said second valve means.