US3677005A

US3677005A - Hydraulic power unit

Info

Publication number: US3677005A
Application number: US76928A
Authority: US
Inventors: Raymond J Estlick
Original assignee: Raytheon Co
Current assignee: Raytheon Co
Priority date: 1970-09-30
Filing date: 1970-09-30
Publication date: 1972-07-18
Anticipated expiration: 1989-07-18

Abstract

A hydraulic power unit from which a hydraulic fluid may be discharged under pressure to the wing control actuators of a guided missile, the actual pressure at which such fluid is discharged during operation being changed in accordance with the requirements of the wing control actuators. In one of two embodiments disclosed the power unit includes two plenum chambers for the hydraulic fluid, means for independently varying the pressure on the hydraulic fluid in each one of such chambers using a source of substantially constant pressure for such purpose, and means for connecting the wing control actuators to either one of the plenum chambers as required during the flight of the guided missile.

Description

United States Patent Estlick [52] U.S. Cl ..60/54.6 A, 60/54.6 P, 92/6 R, 92/62 [51] Int. Cl ..Fl5b 7/00,F01b31/00, FOlb 7/00 Field of Search ..60/54.6 P, 54.5 R, 10.5, 54.6 S, 60/545 A, 54.6 A, 54.5 HA, 54.6 HA; 92/6, 52,62,

[56] References Cited UNITED STATES PATENTS Alping 2.92/52 Brent et a1. ..60/54.6 A

CONTROL ACTUATOR GUIDANCE 51 July 18,1972

Oswalt ..60/54.6 A Lagerquist ..60/54.6 A

[57] ABSTRACT A hydraulic power unit from which a hydraulic fluid may be discharged under pressure to the wing control actuators of a guided missile, the actual pressure at which such fluid is discharged during operation being changed in accordance with the requirements of the wing control actuators. In one of two embodiments disclosed the power unit includes two plenum chambers for the hydraulic fluid, means for independently varying the pressure on the hydraulic fluid in each one of such chambers using a source of substantially constant pressure for such purpose, and means for connecting the wing control actuators to either one of the plenum chambers as required during the flight of the guided missile.

12 Claims, 2 Drawing Figures GATE VALVE VALVE SIG CONTROLL PATiNn-inauuemz 77,0 5

SHEET 1 [IF 2 WING CONTROL ACTUATOR GUIDANCE SIGNALS VALV E CONTROL INVENTO/P RAYMOND J- ESTL/CK PATENTED JUL18|972 3.671005 SHEET 2 OF 2 FIG. 2

lNVE/VTOR RAYMOND J. ESTL/CK HYDRAULIC POWER UNIT BACKGROUND OF THE INVENTION This invention relates generally to hydraulic power units and particularly to power units wherein high pressure storage gas is used to discharge a hydraulic fluid stored in a plenum chamber within such unit, such fluid being used to provide hydraulic power to a load such as the wing control actuator for a guided missile. In such an application, the missile typically is maneuvered in accordance with the aerodynamic response of the missile to the angular deviation of wings mounted on the missile, the desired angular deviation being controlled by a hydraulic wing actuator servo responsive to the guidance command signals. In the usual case the guidance commands require that the missile make relatively violent maneuvers at the launch and terminal phases of its flight and relatively moderate maneuvers during the midcourse phase of its flight. The large maneuvers during the launch phase are required to correct for initial launching angle errors while the large maneuvers during the terminal phase are required to enable the missile to outmaneuver the target just prior to intercept. Known hydraulic wing actuator servos employ a high pressure stored gas operating on a movable wall of a plenum chamber to discharge a hydraulic fluid to operate such actuators. As the stored fluid is ejected from the plenum chamber, the stored gas expands to maintain pressurization of the fluid. In maintaining such high level pressurization throughout the missiles flight it has been found that an excessive amount of the potential energy of the stored gas is expended unnecessarily by response of the wing actuator servo to various noise disturbances superimposed on the guidance command signals during the midcourse phase of flight. Therefore, adequate gas pressure may not be available for the large maneuvers generally required of the missile during its terminal phase of flight.

SUMMARY OF THE INVENTION It is therefore an object of the invention to provide a hydraulic power unit wherein potential energy of the gas stored within such unit is utilized in a most efficient manner.

It is an object of the invention to provide a hydraulic power unit wherein the duty cycle of such unit is matched to the duty cycle of a hydraulic device coupled to such unit.

It is an object of the invention to provide a hydraulic power unit wherein the fluid contained within such unit is supplied to a hydraulic device, such fluid being supplied at a pressure level commensurate with the pressure level requirement of such device, thereby to utilize the potential energy of the stored gas in the most efficient manner.

These and other objects of the invention are accomplished by providing a hydraulic power unit for supplying a hydraulic fluid stored in a plenum chamber to a hydraulic device coupled thereto, the pressure on the hydraulic fluid being matched to the requirements of the hydraulic device, thereby conserving the potential energy of the stored gas. The desired matching is accomplished by sequentially varying the effective area of a piston operating on the hydraulic fluid in the plenum chamber without changing the area on which the stored gas operates.

BRIEF DESCRIPTION OF THE DRAWINGS The aforementioned objects and other features of the invention are explained in the following description taken in connection with the accompanying drawings wherein:

FIG. 1 is an isometric view of a preferred embodiment of the invention, partially cut away and somewhat distorted, to shown the features of the invention; and

FIG. 2 is an alternative embodiment of the invention, greatly simplified, to illustrate the principles of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to FIG. 1 it may be seen that a wing control actuator 10, which may, for example, be a conventional double acting hydraulic ram, receives hydraulic fluid (not numbered) through either

lines

12 or 14, depending on the condition of servo valve 16. The latter element is responsive to control signals from a valve controller 18 to permit operation of the wing control actuator 10 in either direction. The hydraulic fluid supplied to the wing control actuator 10 is vented through line 20 and a gate valve 22 after a desired maneuver is completed. Gate valve 22 may be a conventional solenoid valve so that it may be opened and closed in response to a signal from valve controller 18 to control venting. The servo valve 16 is connected as shown to a distribution chamber 24, such chamber being supplied hydraulic fluid from hydraulic power unit 26. For reasons to become clear,

gate valves

28 and 30 provide the control means for enabling hydraulic fluid to enter distribution chamber 24 selectively from either port 32 or port 34.

The hydraulic power unit 26 includes a casing 36, such casingenclosing a gas cartridge 38, slidably mounted within the casing 36, the outer surface of such cartridge slidably supporting piston 40.

Pistons

40, 42 are annular in shape, piston 40 extending as indicated between the outer surface of gas cartridge 38 and the inner wall (not numbered) of casing 36, and piston 42 extending as indicated between the outer surface 44 of end cap 46 and the inner wall of casing 36. Portions of

pistons

40, 42, inner wall of casing 36, outer surface 44 of end cap 46 and gas cartridge 38 define, as shown, the walls of a pair of

plenum chambers

48, 50 for hydraulic fluid.

Plenum chambers

48, 50 are filled with hydraulic fluid via

fill valves

52 and 54. Gas cartridge 38 is tilled with gas, preferably helium, via fill valve 56. To initiate the launch phase of operation a signal is transmitted to gate valve 28 by valve controller 18. Such transmitted signal opens gate valve 28 whereby gas contained within gas cartridge 38 flows through pressure regulator valve 57 and into annular space 58, thereby forcing

pistons

40, 42 to move apart. It is noted, however, that because gate valve 30 is closed during the launch and midcourse phases of operation, piston 42 moves but slightly, if at all, due to the incompressible nature of the hydraulic fluid in plenum chamber 50. It is obvious, however, that the hydraulic fluid in plenum chamber 50 is pressurized. The piston 40 during its initial movement similarly causes the hydraulic fluid in plenum chamber 48 to be pressurized and such fluid is discharged via port 32 through distribution chamber 24 and through either line 12 or 14 (depending on the condition of servo valve 16) to wing control actuator 10. It follows then that hydraulic fluid under pressure is forced from plenum chamber 48 as piston 40 continues to move. When piston 40 engages with gas cartridge 38 by means of rim 60, both then move together, continuing to force hydraulic fluid out of plenum chamber 48 until piston 40 and gas cartridge 38 rest on casing 36. A moments thought will make it clear, however, that after piston 40 engages gas cartridge 38 the pressure on the hydraulic fluid drops to a lower level than it was prior to such engagement. The reason for the drop in pressure is that: l the force (F) exerted on piston 40 by the gas from gas cartridge 38 is the same before and after engagement of piston 40 with gas cartridge 38; (2) prior to such engagement the pressure on the hydraulic fluid in plenum chamber 48 is P, F/A (where A is the pressurizing area of piston 40); and (3) after such engagement the pressure on such hydraulic fluid is P F/A (where A A plus the pressurizing area of gas cartridge 38)..

The tenninal phase of operation is initiated by the valve controller transmitting a signal to gate Valve 30. Such signal opens gate valve 30, thereby releasing hydraulic fluid from plenum chamber 50 to port 34 via line 62. Since the force (F) on piston 42 is nearly equal to that on piston 40, and since the pressurizing area (A of piston 42 is smaller than the pressurizing area A of the engaged piston 40/gas cartridge 38, the

desirable leaks of hydraulic fluid or gas.

Referring now to FIG. 2, the here contemplated hydraulic power unit includes a gas cartridge 76, a piston 74, a casing 78 and an end cap 80. Gas cartridge 76 has a gas flow annulus 90 formed therein and a longitudinal rod 94 affixed thereto. Such rod has a notch 96 formed therein as shown. Casing 78 has an aperture (not numbered) formed therein for receiving longitudinal rod 94. Piston 74, mounted between gas cartridge 76 and casing 78, is in slidable engagement therewith and has a rearward extending sleeve 86 with a notch 88 formed therein. Gas cartridge 76 and piston 74 are sealed by end cap 80, such end cap being threadably engaged with casing 78. A sleeve locking mechanism 100, mounted to gas cartridge 76, comprises a spring loaded piston 102, a fluid receiving chamber 104 and an input port 106. A cartridge locking mechanism 108, mounted to casing 78, comprises a piston 110, an input port 112, a fluid receiving chamber 114, an output port 116 and an exhaust port 118. A tube 120 connects output port 116 to input port 106. A distribution chamber 122, formed within casing 78, is used to couple the hydraulic power unit to a load (not shown), such chamber having included therein a spring loaded check valve (not numbered).

Having assembled the casing 78, piston 74, gas cartridge 76 and end cap 80, a stop cap 126, the diameter of such cap being larger than the aperture (not numbered) is threaded to rod 94. A plenum chamber 98 is formed within the hydraulic power unit, such chamber being defined by portions of piston 74, casing 78 and gas cartridge 76 as shown. Gas, preferably helium, is introduced into gas cartridge 76 by a gas fill valve (not shown). Hydraulic fluid is introduced into plenum chamber 98 by a conventional valve (not shown). In operation, as the stored gas within gas cartridge 76 is introduced into the gas flow annulus 90 by a conventional gas release and regulator valves (not shown) such gas passes through hole 89, forcing piston 74 to slide and thereby discharge hydraulic fluid contained within plenum chamber 98 through distribution chamber 122. The pressure level of the fluid discharged from plenum chamber 98 is determined by the force (F) of the gas on piston 74 and the pressurizing area (A,) of such piston. Piston 74 continues to slide on gas cartridge 76 until notch 88 passes over piston 102 of sleeve holding mechanism 100. The spring loaded piston 102 engages piston 74 to thereby lock the gas cartridge to the piston 74. As piston 74 and gas cartridge 76 slide together in response to the force of the gas, the pressure of the hydraulic fluid passing through distribution chamber 102 drops to a lower pressure level. This lower pressure level hydraulic fluid flow continues until such time as slot 96 passes under cartridge locking mechanism 108 so that piston 110 is forced into slot 96 by hydraulic fluid entering fluid receiving chamber 114 via port 112. The gas cartridge 76 is thereby locked to casing 78 while essentially simultaneously piston 102 is disengaged from notch 88 by hydraulic fluid flowing via port 116 and tube 120 into fluid receiving chamber 104. After such disengagement gas cartridge 76 continues to slide in response to force of the gas; however, since the effective piston area pressurizing the hydraulic fluid in phenum chamber 98 is reduced, such fluid is delivered through distribution chamber 122 at a high pressure level. Gas cartridge 76 is restrained from reacting to this high pressure level fluid by piston 110.

O-ring seals 128 are appropriately incorporated into the various components of the hydraulic power unit to prevent undesirable leaks of fluid or gas. The exhaust port 118 of bottle lockinG mechanism 108 is provided to exhaust hydraulic oil and thereby prevent hydraulic lock in such mechanism.

It will be obvious to one of ordinary skill in the art that the manner of varying the size of the walls of the plenum chamber may be by other means than the simple piston arrangements discussed and that the means for applying a force to a portion of the walls of the plenum chamber may be by means other than gas, as for examPle a spring mechanism Therefore, the form of the invention described above should be considered as illustrated and not as limiting the scope of the following claims.

What is claimed is:

1. A hydraulic power unit for discharging hydraulic fluid at varying pressure levels to a hydraulic device coupled to such power unit, comprising:

a. a plenum chamber for storing hydraulic fluid to be discharged, such plenum chamber being contractile and having a discharge port, such discharge port providing at least a part of the coupling between the plenum chamber and the hydraulic device;

b. means for contracting the plenum chamber by applying a substantially constant force to a portion of a wall partially defining such chamber whereby hydraulic fluid is discharged at a first pressure level from the plenum chamber to the hydraulic device through the discharge port; and

c. engagement means to connect said portion with the remainder of said wall to vary the size of said portion of the wall of the plenum chamber as such plenum chamber contracts whereby hydraulic fluid is discharged at a varying pressure level from the plenum chamber to the hydraulic device through the discharge port.

2. A hydraulic power unit for discharging hydraulic fluid at varying pressure levels to a hydraulic device coupled to such power unit, comprising:

a. a plenum chamber for storing hydraulic fluid to be discharged, such plenum chamber being contractile, partially defined by first and second slidable pistons and having a discharge port, such discharge port providing at least a part of the coupling between the plenum chamber and the hydraulic device;

b. means for sliding, with substantially constant force in a direction to contract the size of the plenum chamber, the first slidable piston through a first and second distance;

c. means for connecting the first slidable piston to the second slidable piston during the second distance.

3. A hydraulic power unit for discharging hydraulic fluid at varying pressure levels to a hydraulic device coupled to such power unit, comprising:

a. a plenum chamber for storing hydraulic fluid to be discharged, such plenum chamber being contractile, partially defined by a wall and having a discharge port formed therein, such discharge port providing at least a part of the coupling between the plenum chamber and the hydraulic device;

b. a plurality of pistons forming such wall of the plenum chamber;

c. meanS for contracting the plenum chamber by applying a substantially constant force to one of such pistons whereby hydraulic fluid is discharged at a first pressure level from the plenum chamber to the hydraulic device through the discharge port; and

d. means for interconnecting selected ones of such plurality of pistons as such plenum chamber contracts whereby hydraulic fluid is discharged at corresponding selectively varied pressure levels from the plenum chamber to the hydraulic device through the discharge port.

4. A hydraulic power unit for discharging hydraulic fluid at varying pressure levels to a hydraulic device coupled to such power unit, comprising:

b. first and second slidable pistons forming a wall of the plenum chamber;

c. means for sliding the first slidable piston with substantially constant force in a direction to contract the size of the plenum chamber a first, second and third portion;

d, means for connecting the first slidable piston to the second slidable piston during the second portion of the contraction of the plenum chamber; and,

. means for disconnecting the first slidable piston from the level from the plenum chamber to the hydraulic device through the discharge port; and

. engagement means to connect said portion with the remainder of such wall to vary the size of said portion of the wall of the plenum chamber as such plenum chamber contracts whereby h draulic fluid is discharged at a varymg pressure level rom the plenum chamber to the hydraulic device through the discharge port.

. The hydraulic power unit recited in claim 7 wherein: the wall is comprised of a first and second piston, each such piston being slidable within such housing;

. the contracting means includes means for sliding the first piston, with substantially constant force in a direction to contract the size of the plenum chamber, through a first and second distance; and

5. A hydraulic power unit for discharging hydraulic fluid at varying pressure levels to a hydraulic device coupled to such power unit, comprising:

a. a first plenum chamber for storing a first portion of c. the engagement means includes means for connecting the first piston to the second piston as such first piston slides from the first distance to the second distance.

hydraulic fluid to be discharged, such plenum chamber being contractile and having a first discharge port, such first discharge port providing at least a part of the coupling between the plenum chamber and the hydraulic device;

. means for contracting the first plenum chamber by applying a substantially constant force to a portion of the wall of the plenum chamber;

. means for varying the size of said portion of the wall of the first plenum chamber;

. a second plenum chamber for storing a second portion of means for contracting the second plenum chamber by applying a substantially constant force to a portion of the wall of the second plenum chamber; and

. means connected to the first discharge port of the first plenum chamber, the second discharge port of the second plenum chamber, and the hydraulic device for selecting which one of such is coupled to the hydraulic device.

9. The hydraulic power unit recited in claim 7 wherein:

a. the wall is comprised of a plurality of pistons;

b. the contracting means includes means for applying a substantially constant force to one of such pistons whereby hydraulic fluid is discharged at a first pressure level from the chamber to the hydraulic device through the discharge port; and

c. the engagement means includes means for interconnecting selected ones of such plurality of pistons as such plenum chamber contracts whereby hydraulic fluid is discharged at corresponding varied pressure levels from the plenum chamber to the hydraulic device through the discharge port.

10. The hydraulic power unit recited in claim 8 wherein the area of the wall defined by the first piston is smaller than the area of the wall defined by the second piston.

11. A hydraulic power unit for discharging hydraulic fluid at a varying pressure level to a hydraulic device coupled to such power unit, comprising:

a. a housing, such housing having formed therein a first and second plenum chamber; i. such first plenum chamber having stored therein a first portion of hydraulic fluid to be discharged, such first plenum chamber being contractile, partially defined by a first wall and having a first discharge port, such first discharge port providing at least a part of the coupling between the first plenum chamber and the hydraulic device; and

6. A method for discharging hydraulic fluid at varying pressure levels through a discharge port formed within a contractile fluid storing plenum chamber, the steps comprising:

a. contracting the plenum chamber by applying a substantially constant force to a portion of a wall partially defining the plenum chamber whereby hydraulic fluid is discharged at a first pressure level from the plenum chamber through the discharge port; and

b. increasing the size of the wall of the plenum chamber operating on the hydraulic fluid as such plenum chamber contracts and maintaining the substantially constant force on the portion of the plenum chamber whereby hydraulic means for contracting the first plenum chamber by applying a substantially constant force to a portion of the first fluid is discharged at varying pressure levels from the plenum chamber through the discharge port. 7. A hydraulic power unit for discharging fluid at varying pressure levels to a hydraulic device coupled to such power unit, comprising:

a. a housing, such housing having formed therein a plenum chamber for storing hydraulic fluid to be discharged, such wall of the plenum chamber;

c. means for varying the size of said portion of the first wall of the first plenum chamber;

(1. means for contracting the second plenum chamber by applying a substantially constant force to a portion of the second wall of the second plenum chamber; and,

e. means connected to the first discharge port of the first plenum chamber being contractile and defined by a portion of such housing and a wall slidable within said housing, such plenum chamber having a discharge port for providing at least a portion of the coupling between the plenum chamber, the second discharge port of the second plenum chamber, and the hydraulic device for selecting which one of such first and second plenum chambers is coupled to the hydraulic device. 12. The method recited in claim 6 wherein the size of the wall is increased.

plenum chamber and the hydraulic device; b. means for contracting the plenum chamber by applying a substantially constant force to a portion of such wall whereby hydraulic fluid is discharged at a first pressure

Claims

1. A hydraulic power unit for discharging hydraulic fluid at varying pressure levels to a hydraulic device coupled to such power unit, comprising: a. a plenum chamber for storing hydraulic fluid to be discharged, such plenum chamber being contractile and having a discharge port, such discharge port providing at least a part of the coupling between the plenum chamber and the hydraulic device; b. means for contracting the plenum chamber by applying a substantially constant force to a portion of a wall partially defining such chamber whereby hydraulic fluid is discharged at a first pressure level from the plenum chamber to the hydraulic device through the discharge port; and c. engagement means to connect said portion with the remainder of said wall to vary the size of said portion of the wall of the plenum chamber as such plenum chamber contracts whereby hydraulic fluid is discharged at a varying pressure level from the plenum chamber to the hydraulic device through the discharge port.

2. A hydraulic power unit for discharging hydraulic fluid at varying pressure levels to a hydraulic device coupled to such power unit, comprising: a. a plenum chamber for storing hydraulic fluid to be discharged, such plenum chamber being contractile, partially defined by first and second slidable pistons and having a discharge port, such discharge port providing at least a part of the coupling between the plenum chamber and the hydraulic device; b. means for sliding, with substantially constant force in a direction to contract the Size of the plenum chamber, the first slidable piston through a first and second distance; c. means for connecting the first slidable piston to the second slidable piston during the second distance.

3. A hydraulic power unit for discharging hydraulic fluid at varying pressure levels to a hydraulic device coupled to such power unit, comprising: a. a plenum chamber for storing hydraulic fluid to be discharged, such plenum chamber being contractile, partially defined by a wall and having a discharge port formed therein, such discharge port providing at least a part of the coupling between the plenum chamber and the hydraulic device; b. a plurality of pistons forming such wall of the plenum chamber; c. meanS for contracting the plenum chamber by applying a substantially constant force to one of such pistons whereby hydraulic fluid is discharged at a first pressure level from the plenum chamber to the hydraulic device through the discharge port; and d. means for interconnecting selected ones of such plurality of pistons as such plenum chamber contracts whereby hydraulic fluid is discharged at corresponding selectively varied pressure levels from the plenum chamber to the hydraulic device through the discharge port.

4. A hydraulic power unit for discharging hydraulic fluid at varying pressure levels to a hydraulic device coupled to such power unit, comprising: a. a plenum chamber for storing hydraulic fluid to be discharged, such plenum chamber being contractile and having a discharge port, such discharge port providing at least a part of the coupling between the plenum chamber and the hydraulic device; b. first and second slidable pistons forming a wall of the plenum chamber; c. means for sliding the first slidable piston with substantially constant force in a direction to contract the size of the plenum chamber a first, second and third portion; d. means for connecting the first slidable piston to the second slidable piston during the second portion of the contraction of the plenum chamber; and, e. means for disconnecting the first slidable piston from the second slidable piston during the third portion of the contraction of the plenum chamber, whereby the hydraulic fluid stored within the plenum chamber is discharged at a first pressure level during the first and third portion of contraction of the plenum chamber and at a second pressure level during the second portion of contraction of the plenum chamber through the discharge port.

5. A hydraulic power unit for discharging hydraulic fluid at varying pressure levels to a hydraulic device coupled to such power unit, comprising: a. a first plenum chamber for storing a first portion of hydraulic fluid to be discharged, such plenum chamber being contractile and having a first discharge port, such first discharge port providing at least a part of the coupling between the plenum chamber and the hydraulic device; b. means for contracting the first plenum chamber by applying a substantially constant force to a portion of the wall of the plenum chamber; c. means for varying the size of said portion of the wall of the first plenum chamber; d. a second plenum chamber for storing a second portion of hydraulic fluid to be discharged, such second plenum chamber being contractile and having a second discharge port, such second discharge port providing at least a part of the coupling between the second plenum chamber and the hydraulic device; e. means for contracting the second plenum chamber by applying a substantially constant force to a portion of the wall of the second plenum chamber; and f. means connected to the first discharge port of the first plenum chamber, the second discharge port of the second plenum chamber, and the hydraulic device for selecting which one of such is coupled to the hydraulic device.

6. A method for discharging hydraulic fluid at varying pressure levels through a discharge port formed within a contractile fluid stoRing plenum chamber, the steps comprising: a. contracting the plenum chamber by applying a substantially constant force to a portion of a wall partially defining the plenum chamber whereby hydraulic fluid is discharged at a first pressure level from the plenum chamber through the discharge port; and b. increasing the size of the wall of the plenum chamber operating on the hydraulic fluid as such plenum chamber contracts and maintaining the substantially constant force on the portion of the plenum chamber whereby hydraulic fluid is discharged at varying pressure levels from the plenum chamber through the discharge port.

7. A hydraulic power unit for discharging fluid at varying pressure levels to a hydraulic device coupled to such power unit, comprising: a. a housing, such housing having formed therein a plenum chamber for storing hydraulic fluid to be discharged, such plenum chamber being contractile and defined by a portion of such housing and a wall slidable within said housing, such plenum chamber having a discharge port for providing at least a portion of the coupling between the plenum chamber and the hydraulic device; b. means for contracting the plenum chamber by applying a substantially constant force to a portion of such wall whereby hydraulic fluid is discharged at a first pressure level from the plenum chamber to the hydraulic device through the discharge port; and c. engagement means to connect said portion with the remainder of such wall to vary the size of said portion of the wall of the plenum chamber as such plenum chamber contracts whereby hydraulic fluid is discharged at a varying pressure level from the plenum chamber to the hydraulic device through the discharge port.

8. The hydraulic power unit recited in claim 7 wherein: a. the wall is comprised of a first and second piston, each such piston being slidable within such housing; b. the contracting means includes means for sliding the first piston, with substantially constant force in a direction to contract the size of the plenum chamber, through a first and second distance; and c. the engagement means includes means for connecting the first piston to the second piston as such first piston slides from the first distance to the second distance.

9. The hydraulic power unit recited in claim 7 wherein: a. the wall is comprised of a plurality of pistons; b. the contracting means includes means for applying a substantially constant force to one of such pistons whereby hydraulic fluid is discharged at a first pressure level from the chamber to the hydraulic device through the discharge port; and c. the engagement means includes means for interconnecting selected ones of such plurality of pistons as such plenum chamber contracts whereby hydraulic fluid is discharged at corresponding varied pressure levels from the plenum chamber to the hydraulic device through the discharge port.

11. A hydraulic power unit for discharging hydraulic fluid at a varying pressure level to a hydraulic device coupled to such power unit, comprising: a. a housing, such housing having formed therein a first and second plenum chamber; i. such first plenum chamber having stored therein a first portion of hydraulic fluid to be discharged, such first plenum chamber being contractile, partially defined by a first wall and having a first discharge port, such first discharge port providing at least a part of the coupling between the first plenum chamber and the hydraulic device; and ii. such second plenum chamber having stored therein a second portion of hydraulic fluid to be discharged, such second plenum chamber being contractile, partially defined by a second wall and having a second discharge port providing at least a part of the coupling between the second plenum chamber and the hydraulic device; b. meanS for contracting the first plenum chamber by applying a substantially constant force to a portion of the first wall of the plenum chamber; c. means for varying the size of said portion of the first wall of the first plenum chamber; d. means for contracting the second plenum chamber by applying a substantially constant force to a portion of the second wall of the second plenum chamber; and, e. means connected to the first discharge port of the first plenum chamber, the second discharge port of the second plenum chamber, and the hydraulic device for selecting which one of such first and second plenum chambers is coupled to the hydraulic device.

12. The method recited in claim 6 wherein the size of the wall is increased.