WO2007059126A2

WO2007059126A2 - Lockable compressible fluid actuator

Info

Publication number: WO2007059126A2
Application number: PCT/US2006/044182
Authority: WO
Inventors: Joshua Coombs
Original assignee: Joshua Coombs
Priority date: 2005-11-12
Filing date: 2006-11-13
Publication date: 2007-05-24
Also published as: WO2007059126A9; WO2007059126A3

Abstract

The load handling system of the preferred embodiment includes a lockable strut, a hydraulic cavity, a reservoir, and a volume modulator. The lockable strut includes a compressible fluid, a hydraulic tube that defines an inner cavity and contains a portion of the compressible fluid, a displacement rod to move into and out of the inner cavity, a cavity piston connected to the displacement rod and extending to the hydraulic tube, thereby separating the inner cavity into a first section and a second section, a passage to allow flow of the compressible fluid between the first section and the second section of the inner cavity, a controllable valve to selectively restrict the flow of the compressible fluid between the first section and the second section of the inner cavity through the passage, an electric control unit to selectively activate the controllable valve, thereby actively substantially locking or unlocking the lockable strut.

Description

LOCKABLE COMPRESSIBLE FLUID ACTUATOR

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims the benefit of US Provisional

Applications numbers 60/735,771 entitled "Compliant Positioning and Isolation System", 60/735,770 entitled "Lockable Compressible Fluid Actuator", and 60/735,413 entitled "Compressible Fluid Actuator", which were all filed on 12 November 2005 and are all incorporated in their entirety by this reference.

TECHNICAL FIELD

[0002] This invention relates generally to the load handling systems field, and more specifically to a load handling system with a lockable strut with compressible fluid.

BACKGROUND

[0003] Conventional linear-displacement actuators are used in load handling systems to manipulate, position, or isolate a load. Applications today are required to (1) respond with high speed, (2) isolate the load from vibration, and (3) hold the actuator and/or the load in a position without expending energy. Some actuators are rigid and offer high speed of response, but adding compliance to these systems is complex and costly. Other actuators can hold a position, but require energy or additional hardware to lock or hold this position for extended periods. Other actuators offer considerable compliance, but do not have high speed of response and are difficult to lock in place due to the excessive compressibility of the gas or fluid used in these systems.

[0004] Thus, there is a need in the load handling systems field for a load handling system meeting the needs for a fast-acting, compliant, lockable actuator. This invention provides such a new and useful a load handling system with a lockable strut with compressible fluid. BRIEF DESCRIPTION OF THE FIGURES

[0005] FIGURE l is a cut away perspective view of the load handling system of the preferred embodiment, shown within a vehicle. [0006] FIGURE 2 is a schematic view of the load handling system of

FIGURE l.

[0007] FIGURE 3A, 3B,and 3C are cross-sectional views of a lockable strut of the first, second, and third variation of the preferred embodiment of the invention.

[0008] FIGURE 4 is a detailed view of the volume modulator of the load handling system of FIGURE 1.

[0009] FIGURES 5A, 5B, 6A, and 6B are schematic views of the different stages of the volume modulator of FIGURE 4.

[0010] FIGURE 7 is a schematic view of the volume modulator of a second preferred embodiment of the load handling system of FIGURE 1. [0011] FIGURES 8A, 8B, 9A, and 9B are schematic views of the different stages of the volume modulator of FIGURE 7.

[0012] FIGURES 10A, 10B, and loC are cut away side views of the load handling system of the preferred embodiment, shown within an alternative environment.

DESCRIPTION OF THE PREFERRED EMBODIMENTS [0013] The following description of the preferred embodiments of the invention is not intended to limit the invention to these preferred embodiments, but rather to enable any person skilled in the art of load handling systems to make and use this invention.

[0014] As shown in FIGURES 1, 2 and 3A, the load handling system 10 of the preferred embodiment includes a lockable strut 14, a hydraulic cavity 16, a reservoir 18, and a volume modulator 20. The lockable strut 14, as shown in FIGURE 3, includes a compressible fluid 12, a hydraulic tube 28 that defines an inner cavity 38, a displacement rod 30, a cavity piston 32 separating the inner cavity 38 into a first section 40 and a second section 42, a passage 44 between the first section 40 and the second section 42, a controllable valve 74 coupled to the passage 44, and an electric control unit. [0015] As shown in FIGURE i, the load handling system 10 may be implemented as a suspension system for a wheeled or tracked land vehicle. In this application, the system provides a compliant suspension control over terrain and improves ride and handling. The benefits of a lockable strut in this application include: holding the vehicle height on key-off to reduce height change due to temperature change; holding the vehicle height and creating a stable platform for material handling, weapons deployment, or implement operation; holding the vehicle height and creating a stable platform for recreational vehicle living; holding a suspension corner up for servicing (or to remove a damaged suspension unit from operation on a multi-wheeled or tracked vehicle); and holding a suspended wheel of a bicycle for smooth road or uphill operation.

[0016] As shown in FIGURES ioA, loB, and loC, the load handling system io may be implemented within a suitable load handling device that uses actuators, such as implement control, material handling, and agricultural tractor implements in order to raise and lock the implement in place for transport, lower implement to engage ground or crops, and to allow compliance to follow the terrain. The system may also be implemented with a telehandler boom in order to raise the boom and lock the boom in place during loading/unloading of the load. With the use of the controllable valve, the user will be able to unlock and allow compliance in the system when traveling over terrain to isolate load. The system may be additionally useful in the construction equipment field. The system will provide compliance to smooth operation, isolate the load while traveling over terrain, and will be able to lock actuator in place to hold a load in position. In this configuration, the system preferably includes a base, an implement that functions to carry a load, and a link suspending the implement from the base and allowing relative movement of the implement and the base. In this configuration, the hydraulic cavity is adapted to cooperate with the compressible fluid to supply a suspending spring force that biases the implement toward a position relative to the base. The implement is preferably a backhoe, a front loader, a bobcat, shovel, a plow, a forklift, or any other suitable implement. [0017] The system may also be implemented in the naval or watercraft field, providing a compliant stabilizer fin or hull surface control to improve the vehicle control and comfort. A user may also lock the actuator to hold the stabilizer in a stowed or deployed position. The system may be implemented in the aircraft field, providing a stabilizer fin or wing surface control to improve the vehicle control and comfort. A user may also lock the actuator to hold the stabilizer in a stowed or deployed position. The system may be further used with doors, lift-gates, or convertible vehicle top openers, providing compliance for smooth operation and to reduce risk of injury. The user can lock actuator to hold the door, gate, or top in place. The system may be useful in the prosthetics field as well, providing active control and allowing for a locked position.

[0018] As shown in FIGURES l, 2, and 3A, the compressible fluid 12 of the preferred embodiment, which cooperates to supply the suspending spring force, is preferably a silicon fluid that compresses about 1.5% volume at 2,000 psi, about 3% volume at 5,000 psi, and about 6% volume at 10,000 psi. Above 2,000 psi, the compressible fluid 12 has a larger compressibility than conventional hydraulic oil. The compressible fluid 12, however, may alternatively be any suitable fluid, with or without a silicon component, which provides a larger compressibility above 2,000 psi than conventional hydraulic oil.

[0019] As shown in FIGURES 3A, 3B, and 3C, the lockable strut 14 of the preferred embodiment includes a hydraulic tube 28 and a displacement rod 30. The hydraulic tube 28 preferably defines an inner cavity 38, which functions to contain a portion of the compressible fluid 12. The displacement rod 30 is adapted to move into the inner cavity 38 upon a compression movement and to move out of the inner cavity 38 upon a rebound movement. As it moves into the inner cavity 38, the displacement rod 30 displaces, and thereby compresses, the compressible fluid 12. In this manner, the movement of the displacement rod 30 into the inner cavity 38 increases the suspending spring force of the lockable strut 14. The result is a strut that performs similar to a conventional steel coil spring strut, with a spring rate that can be tuned per application. As the displacement rod 30 moves out of the inner cavity 38, the compressible fluid 12 decompresses and the suspending spring force of the lockable strut 14 decreases. The displacement rod 30 is preferably cylindrically shaped. Many modifications may be made to change the spring force and the spring rate. For example, increasing the diameter of the displacement rod 30 increases the magnitude of the suspending spring force, while reducing the diameter reduces the magnitude of the suspending spring force. Further, increasing the volume of the hydraulic cavity 16 reduces the spring rate, while reducing the volume of the hydraulic cavity 16 increases the spring rate. The hydraulic tube 28 and the displacement rod 30 are preferably made from conventional steel and with conventional methods, but may alternatively be made from any suitable material and with any suitable method.

[0020] The lockable strut 14 of the preferred embodiment also includes a cavity piston 32 that is coupled to the displacement rod 30 and that preferably extends to the hydraulic tube 28. In this manner, the cavity piston 32 separates the inner cavity 38 into a first section 40 and a second section 42. The cavity piston 32 is preferably securely mounted to the displacement rod 30 by a conventional fastener, but may alternatively be integrally formed with the displacement rod 30 or securely mounted with any suitable device. The cavity piston 32 is preferably made from conventional materials and with conventional methods, but may alternatively be made from other suitable materials and with other suitable methods.

[0021] The lockable strut 14 of a first variation of the preferred embodiments, as shown in FIGURES 3B and 3C, further includes a pressure vessel 48. The pressure vessel 48 cooperates with a modified hydraulic tube 28' to define an outer cavity 50 located between hydraulic tube 28 and the pressure vessel 48. In this embodiment, the hydraulic cavity 16 includes both the inner cavity 38 and outer cavity 50. In another variation (as shown in FIGURE 3C), the hydraulic tube 28 may define a tube opening or second passage 80 to fluidly connect the second section 42 of the inner cavity 38 and the outer cavity 50 which would greatly expand the volume of compressible fluid 12 on the "rebound side" of the cavity piston 32. In yet another variation, the hydraulic tube 28 could be inverted and the passage 80 could be defined on the compression side.

[0022] As shown in FIGURES 3A, 3B, and 3C, the lockahle strut further includes a passage 44 and a controllable valve 74. The passage 44 functions to allow flow of the compressible fluid 12 that is displaced by the cavity piston 32 as it travels through compression and rebound. The controllable valve 74 functions to stop flow and thereby hydraulically lock the cavity piston 32 and lockable strut 14 in position. The lockable strut 14 may further include a damping valve 76 in the passage 44 to variably restrict flow and damp or attenuate cavity piston 32 travel, thereby attenuating load imbalances or disturbances. The lockable strut 14 preferably includes a leak-free seal to maintain actuator position.

[0023] In a first variation of the preferred embodiment, as shown in

FIGURE 3A, the cavity piston 32 defines the passage 44, which preferably extends between the first section 40 and the second section 42 of the inner cavity 38, and the controllable valve 74 is preferably coupled to the passage 44 and adapted to selectively restrict flow of the compressible fluid 12 between the first section 40 and the second section 42 of the inner cavity 38 through the passage 44. By placing a controllable valve 74 in the passage 44, it is possible to prevent the communication between the first section 40 and the second section 42 of the inner cavity 38 and hydraulically lock the cavity piston 32 and therefore the lockable strut 14 and the load in place. The controllable valve 74 is preferably a normally open, electric motor-operated ball valve. When the valve is open, the compressible fluid 12 is allowed to flow through the passage 44. When the valve is electrically rotated closed (to the 90⁰ angle shown), the compressible fluid 12 flow is stopped and a change in pressure across the cavity piston 32 is created, forcing the ball against a seat and sealing it shut. The controllable valve 74 may alternatively be any suitable valve or device to accomplish the locking feature, such as solenoid-operated poppet valves.

[0024] In a second variation (not shown), the lockable strut 14 includes an external pipe connecting the first section 40 to the second section 42 of the inner cavity, which defines the passage 44 and allows flow of the compressible fluid 12 between the first section 40 and the second section 42 of the inner cavity 38. In this variation, the cavity piston may be a sealed, solid piston. The controllable valve 74 of this variation is preferably located in the external pipe, acting to stop flow through the pipe, thereby hydraulically locking the piston

32-

[0025] In a third variation of the preferred embodiment, as shown in

FIGURE 3B, the hydraulic tube 28 defines a passage 44, which functions to fluidly connect the first section 40 of the inner cavity 38 and the outer cavity 50. The hydraulic tube 28 also defines a second passage 80 adapted to allow flow of the compressible fluid between the second section 42 of the inner cavity and the outer cavity 50. By placing a similar electric motor-operated ball valve 74 in passage 44, fluid flow can be stopped between the first section 40 and the outer cavity 50, which is coupled to the second section 42 of the inner cavity 38 through the passage 80. Closing the controllable valve 74 stops fluid flow and hydraulically locks the cavity piston 32 in place, acting similarly to the first variation of the preferred embodiment.

[0026] In a fourth variation of the preferred embodiment, as shown in

FIGURES 3C and 3D, a damping valve 76 can also be added to the lockable strut 14 to enable further shock attenuation. The damping valve may be a passive valve 76 (shown in FIGURE 3C) or an electronically controlled valve 76' (shown in FIGURE 3D). The passive damping valve 76 can be of any damping valve architecture such as a shim stack or deflecting disk valve, or a steel coil spring preloaded valve. The electronically controlled damper valve 76' can be of any solenoid-operated electronically controlled valve architecture such as direct acting or pilot operated, proportional spool or poppet valve. [0027] The electric control unit of the preferred embodiments (not shown) is coupled to the controllable valve 74. The electric control unit functions to selectively activate the controllable valve 74. Activating controllable valve 74 substantially locks or unlocks the lockable strut 14, allowing loads or vehicle suspension systems to be locked in place. The electronic control unit can be based on algorithms and sensing transducers, or based on operator manual switching. For example, after a load handling system has traversed terrain and is ready to transfer its load, the operator can switch the electric control unit to lock the controllable valve 74 to hold the load in place while it is transferred safely. The electric control unit of the preferred embodiment may also be coupled to the controllable valve 76'. In this manner, the electric control unit functions to selectively activate both the controllable valve 74 and the controllable valve 76'. Activating controllable valve 76' varies strut damping forces, allowing the transient performance of the strut to be tuned and optimized to control the load handling systems compliance performance or to enhance the vehicle suspension systems ride and handling. For example, an active electronic control system can command the load control system 10 to stiffen the damping valve 76' when a fast turn is sensed. This response may achieve the desired handling for the load handling system 10.

[0028] As shown in FIGURE 1, the load handling system 10 of the preferred embodiment also includes hydraulic lines 54 that function to communicate the compressible fluid 12 between the individual lockable struts 14 and the volume modulator 20. Together with the inner cavity 38 of the individual lockable struts 14, the hydraulic lines 54 define individual hydraulic cavities 16. Preferably, the compressible fluid 12 flows freely between the volume modulator 20 and the inner cavity 38 of the individual lockable struts 14. Alternatively, the hydraulic cavities 16 may include one or more controllable valves such that the hydraulic cavity 16 is entirely defined by the lockable strut 14 or by the lockable strut 14 and a portion of the hydraulic line

54-

[0029] As shown in FIGURE 2, the reservoir 18 of the preferred embodiment is coupled to the hydraulic line 54 and the volume modulator 20. The reservoir 28 functions to contain a portion of the compressible fluid 12 that has been vented from the hydraulic cavity 16 and that may eventually be pushed into the hydraulic cavity 16. The reservoir 18 is preferably made from conventional materials and with conventional methods, but may alternatively be made from any suitable material and with any suitable method. The load handling system 10 of the preferred embodiment includes a pump 56 adapted to pressurize the compressible fluid 12 within the reservoir 18. In this manner, the reservoir 18 acts as an accumulator 58. By using compressible fluid 12 under a pressure of about 1500 psi within the reservoir 18, the volume modulator 20 consumes less energy to reach a particular pressure within an individual hydraulic cavity 16. In an alternative embodiment, the compressible fluid 12 within the reservoir 18 may be at atmospheric pressure or may be vented to the atmosphere.

[0030] The volume modulator 20 of the preferred embodiment is coupled to the hydraulic line 54 and to the reservoir 18. The volume modulator 20 functions to selectively push the compressible fluid 12 into the hydraulic cavity 16 and to vent the compressible fluid 12 from the hydraulic cavity 16. In the preferred embodiment, the volume modulator 20 is a digital displacement pump/motor as described in U.S. Patent No. 5,259,738 entitled "Fluid-Working Machine" and issued to Salter et al. on 09 November 1993, which is incorporated in its entirety by this reference. In alternative embodiments, the volume modulator 20 may be any suitable device that selectively pushes the compressible fluid 12 into the hydraulic cavity 16 and vents the compressible fluid 12 from the hydraulic cavity 16 at a sufficient rate to actively modulate the suspending spring force.

[0031] As shown in FIGURE 4, the volume modulator 20 of the preferred embodiment defines a modulator cavity 60 and includes a modulator piston 62 adapted to continuously cycle through a compression stroke and an expansion stroke within the modulator cavity 60. The modulator piston 62 is preferably connected to an eccentric 64 that is rotated by a motor 66 (shown in FIGURE 1 and 2). Because of the "active" nature of the modulation of the suspending spring force, the modulator piston 62 cycles through the compression stroke and expansion stroke at a relatively high frequency (approximately 30 Hz) and, thus, the motor preferably rotates at a relatively high rotational velocity (approximately 2000 rpm). [0032] The volume modulator 20 of the preferred embodiment also includes a valve system. The valve system functions to selectively restrict and/or direct the passage of the compressible fluid. In a first variation, the volume modulator 20 of the preferred embodiment also includes a cavity-side valve 68 coupled between the hydraulic line and the volume modulator 20 and a reservoir-side valve 70 coupled between the reservoir and the volume modulator 20. Preferably, the cavity-side valve 68 and the reservoir-side valve 70 are so-called poppet valves that may be actuated at relatively high frequencies. Alternatively, the cavity-side valve 68 and the reservoir-side valve 70 may be any suitable device that selectively restricts the passage of the compressible fluid at an adequate frequency. In a second variation, as shown in FIGURES 7, 8A, 8B, 9A₃ and 9B, the volume modulator 20 of the preferred embodiment also includes a rotary valve 174. The rotary valve variation is further described in U.S. Patent 7,036,835 entitled "Suspension System for a Vehicle" and issued to Coombs et al. on 02 May 2006, which is incorporated in its entirety by this reference.

[0033] As shown in FIGURES 5A and 5B, the cavity-side valve 68, the reservoir-side valve 70, and the modulator piston 62 can cooperate to draw compressible fluid 12 from the reservoir and push the compressible fluid 12 into the hydraulic cavity. In the first stage, as shown in FIGURE 5A, the cavity-side valve 68 is closed and the reservoir-side valve 70 is opened, while the modulator piston 62 increases the volume in the modulator cavity 60 (the expansion stroke). The expansion stroke of the modulator piston 62 draws the compressible fluid 12 into the modulator cavity 60. During the second stage, as shown in FIGURE 5B, the reservoir-side valve 70 is closed and the cavity- side valve 68 is opened, while the modulator piston 62 decreases the volume in the modulator cavity 60 (the compression stroke). The compression stroke of the modulator piston 62 pushes the compressible fluid 12 into the hydraulic cavity, which increases the suspending spring force at that particular lockable strut.

[0034] As shown in FIGURES 6A and 6B, the cavity-side valve 68, the reservoir-side valve 70, and the modulator piston 62 can also cooperate to draw compressible fluid 12 from the hydraulic cavity and vent the compressible fluid 12 into the reservoir. In the first stage, as shown in FIGURE 6A, the cavity-side valve 68 is opened and the reservoir-side valve 70 is closed, while the modulator piston 62 increases the volume in the modulator cavity 60 and draws the compressible fluid 12 into the modulator cavity 60. During the second stage, as shown in FIGURE 6B, the reservoir- side valve 70 is opened and the cavity-side valve 68 is closed, while the modulator piston 62 decreases the volume in the modulator cavity 60 and vents the compressible fluid 12 into the reservoir, which decreases the suspending spring force at that particular lockable strut. [0035] During the operation of load handling system 10, it may be advantageous to neither increase nor decrease the suspending spring force. Since the motor 66, the eccentric 64, and the modulator pistons 62 are continuously moving, the reservoir-side valve 70 and the volume modulator 20 can also cooperate to draw compressible fluid 12 from the reservoir (shown in FIGURE 5A) and vent the compressible fluid 12 back into the reservoir (shown in FIGURE 6B). This process does not modulate the pressure of the hydraulic cavity 16 and does not increase or decrease the suspending spring force.

[0036] Although FIGURES 5A, 5B, 6A₅ and 6B show only one modulator cavity 60 and modulator piston 62, the volume modulator 20 preferably includes a modulator cavity 60, a modulator piston 62, a cavity- side valve 68, and a reservoir-side valve 70 for each lockable strut 14 in the load handling system 10. Preferably, the motor 66 and the eccentric 64 drive the multiple modulator pistons 62, but the individual modulator pistons 62 may alternatively be driven by individual motors and individual eccentrics. Preferably, multiple volume modulators are employed per lockable strut, allowing improved resolution of control and redundancy for fail-safe operation. Additional modulator units can also be employed for control over numerous other types of actuators such as power steering rack and pinion, brake caliper, driveline clutch actuation, rotary actuators, etc. Further, a control unit 72 (shown in FIGURE 1) may individually control the cavity-side valve 68 and reservoir-side valve 70 corresponding to a particular lockable strut 14. The control unit 72 is preferably made from conventional material and with conventional methods, but may alternatively be made from any suitable material and with any suitable method.

[0037] Although omitted for conciseness, the preferred embodiments include every combination and permutation of the various lockable struts, a compressible fluids, hydraulic tubes, displacement rods, cavity pistons, passages, controllable valves, electric control units, hydraulic cavities, reservoirs, and volume modulators.

[0038] As a person skilled in the art of load handling systems will recognize from the previous detailed description and from the figures, modifications and changes can be made to the preferred embodiments of the invention without departing from the scope of this invention.

Claims

CLAIMS I claim:

1. A lockable strut with compressible fluid comprising:

• a compressible fluid;

• a hydraulic tube defining an inner cavity adapted to contain a portion of the compressible fluid;

• a displacement rod adapted to move into and out of the inner cavity;

• a cavity piston, coupled to the displacement rod and extending to the hydraulic tube thereby separating the inner cavity into a first section and a second section;

• a passage adapted to allow flow of the compressible fluid between the first section and the second section of the inner cavity;

• a controllable valve coupled to the passage and adapted to selectively restrict flow of the compressible fluid between the first section and the second section of the inner cavity through the passage.

2. The strut of Claim i wherein the cavity piston defines the passage.

3. The strut of Claim 1 wherein the compressible fluid has a larger compressibility above 2,000 psi than hydraulic oil.

4. The strut of Claim 1 further comprising an electric control unit adapted to selectively activate the controllable valve, thereby actively substantially locking or unlocking the strut.

5. A system comprising:

• the strut of Claim 1;

• a hydraulic cavity at least partially defined by the strut and adapted to contain a portion of the compressible fluid and to cooperate with the compressible fluid to supply a suspending spring force;

• a reservoir adapted to contain a portion of the compressible fluid; and

• a volume modulator connected to the hydraulic cavity and the reservoir and adapted to selectively push the compressible fluid into the hydraulic cavity and vent the compressible fluid from the hydraulic cavity, thereby actively modulating the suspending spring force, wherein the volume modulator defines a modulator cavity receiving compressible fluid and includes a modulator piston adapted to cycle through a compression stroke and an expansion stroke within the modulator cavity for pushing compressible fluid from or drawing compressible fluid into the modulator cavity, and a valve system adapted to selectively restrict the passage of the compressible fluid between the hydraulic cavity and the modulator cavity or restrict the passage of the compressible fluid between the reservoir and the modulator cavity.

6. The system of Claim 5 further comprising an electric control unit adapted to selectively activate the controllable valve, thereby actively substantially locking or unlocking the strut.

7. A suspension system for a vehicle having a wheel contacting a surface under the vehicle and a suspension link suspending the wheel from the vehicle and allowing relative movement of the wheel and the vehicle, the suspension system comprising the system of Claim 5, wherein the hydraulic cavity is adapted to cooperate with the compressible fluid to supply a suspending spring force that biases the wheel toward the surface.

8. The suspension system of Claim 7 further comprising an electric control unit adapted to selectively activate the controllable valve, thereby actively substantially locking or unlocking the strut.

9. An actuator system for a load handling system having a base, an implement adapted to carry a load, and a link suspending the implement from the base and allowing relative movement of the implement and the base, the actuator system comprising the system of Claim 5, wherein the hydraulic cavity is adapted to cooperate with the compressible fluid to supply a suspending spring force that biases the implement toward a position relative to the base.

10. The actuator system of Claim 9 further comprising an electric control unit adapted to selectively activate the controllable valve, thereby actively substantially locking or unlocking the strut.

11. A twin-tube lockable strut with compressible fluid, comprising:

• a compressible fluid;

• a displacement rod adapted to move into and out of the inner cavity;

• a pressure vessel defining an outer cavity located between the pressure vessel and the hydraulic tube and adapted to contain a portion of the compressible fluid;

• a first passage adapted to allow flow of the compressible fluid between the first section of the inner cavity and the outer cavity;

• a second passage adapted to allow flow of the compressible fluid between the second section of the inner cavity and the outer cavity; and

• a controllable valve coupled to one of the first passage and the second passage and adapted to selectively restrict flow of the compressible fluid between the inner cavity and the outer cavity.

12. The strut of Claim 11 wherein the hydraulic tube defines the first passage and defines the second passage.

13. The strut of Claim 11 wherein the compressible fluid has a larger compressibility above 2,000 psi than hydraulic oil.

14- The strut of Claim 11 further comprising an electric control unit adapted to selectively activate the controllable valve, thereby actively substantially locking or unlocking the strut.

15. A system comprising:

• the strut of Claim 11;

• a reservoir adapted to contain a portion of the compressible fluid; and

16. The system of Claim 15 further comprising an electric control unit adapted to selectively activate the controllable valve, thereby actively substantially locking or unlocking the strut.

17. A suspension system for a vehicle having a wheel contacting a surface under the vehicle and a suspension link suspending the wheel from the vehicle and allowing relative movement of the wheel and the vehicle, the suspension system comprising the system of Claim 15, wherein the hydraulic cavity is adapted to cooperate with the compressible fluid to supply a suspending spring force that biases the wheel toward the surface.

18. The suspension system of Claim 17 further comprising an electric control unit adapted to selectively activate the controllable valve, thereby actively substantially locking or unlocking the strut.

19. An actuator system for a load handling system having a base, an implement, and a link coupled to the implement and the base, the actuator system comprising the system of Claim 15, wherein the hydraulic cavity is adapted to cooperate with the compressible fluid to supply a suspending spring force that biases the implement toward a position relative to the base.

20. The actuator system of Claim 19 further comprising an electric control unit adapted to selectively activate the controllable valve, thereby actively substantially locking or unlocking the strut.