WO2012073172A1

WO2012073172A1 - Actuator with fail-safe position

Info

Publication number: WO2012073172A1
Application number: PCT/IB2011/055319
Authority: WO
Inventors: Gaby Jaccoby
Original assignee: Habonim Industrial Valves and Actuators Ltd
Current assignee: Habonim Industrial Valves and Actuators Ltd
Priority date: 2010-11-30
Filing date: 2011-11-28
Publication date: 2012-06-07
Anticipated expiration: 2013-05-30

Abstract

An actuator comprising: a first piston moved between first and second positions by pressurized fluid; and a second piston moved by the pressurized fluid to arm a safety system that returns the first piston from the second position to the first position when pressure in the pressurized fluid drops below a threshold pressure.

Description

ACTUATOR WITH FAIL-SAFE POSITION

RELATED APPLICATIONS

[0001] The present application claims the benefit under 35 U.S.C. 119(e) of U.S. Provisional Application 61/418,061 filed on 30 November, 2010, the disclosure of which is incorporated herein by reference.

TECHNICAL FIELD

[0002] Embodiments of the invention relate to fluid operated actuators.

BACKGROUND

[0003] Various types of fluid operated actuators are known in the art. They generally comprise a piston seated in a cylinder chamber and a valve for introducing and releasing a fluid, such as a gas or liquid, under pressure into the cylinder chamber to generate a force that moves the piston in the cylinder chamber. A coupling element, such as a piston rod, or a rack of a rack and pinion transmission, couples motion of the piston in the cylinder chamber to a load to apply force to and thereby control motion of the load. By way of example, pneumatic actuators operated by pressurized air are commonly used to open and close flow valves that control fluid flow in a pipe system, and are coupled to the valves by rack and pinion transmissions.

[0004] A fluid operated actuator often comprises a safety system that returns a piston comprised in the actuator to an initial, "safe" position in its cylinder chamber if pressure in an operating fluid that operates the actuator decreases below a predetermined safe pressure threshold. The initial, safe position of the actuator piston is generally a position for which a load coupled to the piston is considered to be in a corresponding initial, "benign", position of the load.

[0005] Typically, the safety system comprises an elastic element such as a coil spring or torsion spring. The piston compresses the elastic element, hereinafter referred to as a "return spring", to arm the safety system when the actuator operates to move the piston and load away from their initial positions to respective "working positions". If operating fluid pressure fails, that is drops below the safety pressure threshold, the armed safety systems "triggers" and the compressed return spring forces the piston and load back from their respective working positions to return to their "safe", initial positions. The spring is also relied upon to return the piston to its initial position under normal operating conditions, for which pressure in the operating fluid does not fail. Under normal operating conditions, the piston is returned to its initial position by releasing fluid from the cylinder chamber to remove pressure that maintains the piston in the working position and the return spring compressed so that the return spring can expand and force the piston back to its initial position. A fluid operated actuator that comprises a spring for returning a piston to an initial position under normal operating conditions and also in case of pressure failure is conventionally referred to as a spring return actuator.

SUMMARY

[0006] An embodiment of the invention provides a fluid operated actuator, referred to as a "split action actuator", comprising a first piston for controlling motion of a load to which the actuator is coupled, and a second piston for arming a safety system. The safety system returns the first piston to an initial position if pressure in an operating fluid provided to the actuator decreases below a safety pressure threshold. In an embodiment of the invention, substantially no force applied to move the first piston, hereinafter referred to as a "load piston", and the load, operates to move the second piston, hereinafter referred to as a "safety piston", to arm the safety system. As a result, for a given force applied to a load, the load piston operates at a higher efficiency than load pistons in conventional fluid operated actuators. In conventional fluid operated actuators a load piston operates simultaneously to move both a load and to arm a safety system, when the actuator operates to move a load away from an initial position to a working position.

[0007] In an embodiment of the invention, a split action actuator comprises tandem cylinder chambers, referred to as "safety" and "load" cylinder chambers that house the safety and load pistons respectively. The safety piston is connected to a "plunger" that extends into the load cylinder chamber. When pressurized operating fluid is introduced into the load cylinder chamber to move the load piston and a load connected thereto to working positions, pressurized operating fluid is introduced into the safety cylinder chamber. The pressurized fluid in the safety cylinder chamber moves the safety piston from an unarmed position, at which the safety system is unarmed, and partially extracts the plunger from the load cylinder chamber. Motion of the safety piston also compresses a return spring to a compressed state, referred to as an "armed state", to arm the safety system. If operating fluid pressure drops below a safety pressure threshold, the compressed return spring applies force to the safety piston that pushes the plunger back into the load cylinder chamber to contact and force the load piston back to its initial position.

[0008] In some embodiments of the invention, an operating fluid in a split action actuator is a gas. Optionally, the operating fluid is a liquid.

[0009] This Summary is provided to introduce a selection of concepts in a simplified form that are further described below in the Detailed Description. This Summary is not intended to identify key features or essential features of the claimed subject matter, nor is it intended to be used to limit the scope of the claimed subject matter.

BRIEF DESCRIPTION OF FIGURES

[0010] Non-limiting examples of embodiments of the invention are described below with reference to figures attached hereto that are listed following this paragraph. Identical structures, elements or parts that appear in more than one figure are generally labeled with a same numeral in all the figures in which they appear. Dimensions of components and features shown in the figures are chosen for convenience and clarity of presentation and are not necessarily shown to scale.

[0011] Figs. 1A-1C schematically show cutaway perspective views of a split action fluid operated actuator that illustrate its operation, in accordance with an embodiment of the invention; and

[0012] Figs. 2A and 2B schematically show variations of a split action actuator in accordance with embodiments of the invention.

DETAILED DESCRIPTION

[0013] Aspects of embodiments of the invention are discussed below with respect to Figs.

1A-1C, which schematically show a split action actuator comprising a safety system that is armed by a safety piston separate from a load piston, in accordance with an embodiment of the invention. By way of example, the split action actuator is a pneumatic actuator. Fig. 2A schematically shows a variation of an actuator shown in Figs. 1A-1C comprising a rack and pinion transmission configured to rotate a shaft, such as a shaft of a quarter-turn valve. Fig. 2B schematically shows a pneumatic actuator comprising two load pistons and associated safety pistons, in accordance with an embodiment of the invention.

[0014] In the discussion unless otherwise stated, adjectives such as "substantially" and "about" modifying a condition or relationship characteristic of a feature or features of an embodiment of the invention, are understood to mean that the condition or characteristic is defined to within tolerances that are acceptable for operation of the embodiment for an application for which it is intended.

[0015] Fig. 1A schematically shows a partially cutaway perspective view of a split action actuator 20, in accordance with an embodiment of the invention.

[0016] Split action actuator 20 comprises a cylinder housing 22 formed having a load cylinder chamber 40 in tandem with a safety cylinder chamber 50, in accordance with an embodiment of the invention. A septum wall 24 separates the two cylinder chambers.

[0017] Load cylinder chamber 40 houses a load piston 42 having a groove 44 formed in its rim for seating a sealing element (not shown), such as an o-ring, or piston ring, which seals the piston to a wall 45 of the cylinder chamber. The load piston is attached to a transmission that couples motion of the piston to a load that split action actuator 20 controls. Optionally, the transmission comprises a drive rod 46 that extends out from cylinder housing 22 through a clearance hole 25 in a wall 26 of the cylinder housing. Optionally, drive rod 46 has a circular cross section and is sealed to the clearance hole against gas leakage by an o-ring 47 optionally seated in a groove 27 formed in the wall of through hole 25. Drive rod 46 couples load cylinder 42 to a load (not shown) whose motion split action actuator 20 controls.

[0018] In Fig. 1A load piston 42 is located in an initial position optionally adjacent to and in contact with wall 26 of load cylinder chamber 40. Gas under pressure is introduced into load cylinder chamber 40 to generate pressure in load cylinder chamber 40 that drives the load piston away from its initial position to a working position in the cylinder chamber via an inlet adapter 29 that communicates with an inlet channel 30 optionally formed in wall 26. An exhaust channel 31 optionally formed in cylinder housing 22 allows gas to freely enter and leave load cylinder chamber 40 so that pressure in the load cylinder chamber on a side of load piston 42 opposite that facing inlet channel 30 does not interfere with operation of the load piston.

[0019] Safety cylinder chamber 50 houses a safety piston 52 having a groove 53 formed in its rim for receiving an o-ring or piston ring (not shown) that seals the safety piston to a wall 54 of the safety cylinder chamber. A return spring 60 seats in safety cylinder chamber 50 between a wall 28 of the safety cylinder chamber and safety piston 52. In Fig. 1A safety piston 52 is positioned in a "non-armed" state for which the piston is adjacent to and optionally contacts septum wall 24, and return spring 60 is in a relatively non-compressed state in which it is extended to a maximum in safety cylinder chamber 50.

[0020] A plunger 55 is connected to the safety piston on a side of the safety piston opposite to a side facing the return spring. The plunger extends into load cylinder chamber 40 through a clearance hole 33 formed in septum wall 24 of cylinder housing 22. An o-ring 34, seated in a groove 35 in the septum wall, seals plunger 55 to the wall of hole 33 and prevents leakage of gas between safety cylinder chamber 50 and load cylinder chamber 40. Plunger 55 is connected to a touch plate 56, which contacts load piston 42 when, as shown in Fig. 1A, the load piston is in its initial position and safety piston 52 is in a non-armed position.

[0021] Pressurized operating gas is introduced into safety cylinder chamber 50 to move the safety piston away from the non-armed position and compress return spring 60 optionally via an inlet adapter 36 that communicates with an inlet channel 37 formed in septum wall 24 of cylinder housing 22. An exhaust channel 38 optionally formed in cylinder housing 22 allows gas to freely enter and leave safety cylinder chamber 50 so that pressure in the safety cylinder chamber on a side of safety piston 52 opposite that facing inlet channel 37 does not interfere with operation of the safety piston.

[0022] Fig. IB schematically shows split action actuator 20 shortly after it is controlled to move a load to which it is attached, in accordance with an embodiment of the invention. Pressurized operating gas is first introduced into safety cylinder chamber 50 via inlet channel 37 to force safety piston 52 away from its non-armed position, and begin extracting plunger 55 from load cylinder chamber 40 and compressing return spring 60. Upon initiating motion of safety piston 52, touch plate 56 moves away from load piston 42 and removes any force generated by return spring 60 that the touch plate applies to the load piston.

[0023] Shortly after being freed from force generated by return spring 60 pressurized operating gas is introduced into load cylinder chamber 40 via inlet channel 30 to force the load piston away from its initial position and toward a working position. Pressurized operating gas is continuously flowed into both safety and load cylinder chambers 50 and 40 at rates sufficient to prevent touch plate 56 from applying force to load piston 42, until the safety piston reaches a final armed position and return spring 60 is in a fully armed, compressed state. Shortly thereafter, load piston 42 reaches its working position, at which working position it optionally again contacts touch plate 56.

[0024] In some embodiments of the invention, pressurized gas is not introduced into load cylinder chamber 40 to move the load piston and a load to which it is coupled until safety piston 52 reaches a final armed position and return spring 60 is in a fully armed compressed state.

[0025] Fig. 1C schematically shows split action actuator 20 after safety piston 52 has reached an armed position, return spring 60 is compressed to a fully armed state and load piston 42 is in a working position, in accordance with an embodiment of the invention. Optionally, as shown in Fig. 1C the armed position of safety position is a position for which load piston 42 contacts touch plate 56 and the touch plate contacts septum wall 24.

[0026] As long as pressure in the operating gas in safety cylinder chamber 50 remains above a threshold "safety" pressure for which pressure on the safety piston is sufficient to generate a force that maintains return spring 60 compressed, the return spring remains in the armed state. If the pressure drops below the safety pressure, return spring 60 forces safety and load pistons 52 and 42 back respectively to their unarmed and initial positions shown by way of example in Fig. 1A.

[0027] It is noted that a load piston comprised in a split action actuator in accordance with an embodiment of the invention, such as actuator 20, operates at a greater efficiency than a load piston in a conventional fluid actuator. By way of a simplified example, assume that an actuator comprising a load piston that operates to simultaneously move a load and arm a return spring is required to apply a force "FL" to move a load between initial and working positions.

Assume further that it is desired that the return spring return the load to its initial position if pressure in a fluid that operates the actuator drops below a safety pressure "Ρ§". Let the return spring, when fully compressed to its armed state, exert a return force "FR" to return the load to its initial position. Then, upon operating fluid pressure dropping to below P$, at least initially,

FR satisfies a relation FR > (FL + AP$), where A is a cross section of the load piston on which the pressurized operating fluid operates. To compress the return spring to its armed state, and also move the load, the load piston must be able to provide an operating force "FQ" that satisfies a relation FQ > (2FL + AP$).

[0028] On the other hand, a load piston in a split action actuator in accordance with an embodiment of the invention, such as load piston 42 in split action actuator 20 shown in Figs. 1A-1C, does not operate to compress a return spring and can therefore function satisfactorily by providing an operating force "F*Q" for which F*Q > FL. The operating force provided by the load piston comprised in the split action actuator in accordance with an embodiment of the invention is constrained by a substantially lower minimum threshold than a load piston in a conventional fluid operated actuator.

[0029] For a same force to be provided to a load by a fluid operated actuator, the lower minimum operating force threshold generally enables a split action actuator to operate at lower operating pressures and/or to have a smaller cross section load piston than a conventional fluid operated actuator. For example, for a same operating fluid pressure, a split action actuator in accordance with an embodiment of the invention having a same cross section as a conventional spring return actuator provides at least twice a force as the conventional actuator, that is F*Q > 2FQ. If the force is required to generate a torque, for example to rotate a shaft of a valve to open and/or close the valve, for a same torque arm, the split action actuator in accordance with an embodiment of the invention, provides at least twice the torque as the conventional spring return actuator.

[0030] Fig. 2A schematically shows a partially cutaway perspective view of a split action actuator 120 comprising a rack and pinion transmission 122 for coupling the actuator to a load, in accordance with an embodiment of the invention. Rack and pinion transmission 122 comprises a rack 123 attached to load piston 42 via rod 46, and a pinion 124 that meshes with the rack. Split action actuator 120 may be used to rotate a shaft 125, such as a shaft of a fluid flow valve that is rotated to open and close the valve, by connecting pinion 124 to the shaft.

[0031] Fig. 2B schematically shows a partially cutaway perspective view of a composite, "double cylinder head" split action actuator 220 comprising a pair of optionally identical split action actuators 222 coupled to a rack and pinion transmission 230, in accordance with an embodiment of the invention. Split action actuators 222 are optionally similar to split action actuators 20 shown in Figs. 1A-1C and each comprises a safety piston 52, and a load piston 42 coupled by a rack 232 to a pinion 234 comprised in rack and pinion transmission 230.

[0032] Practice of the invention is of course not limited to comprising two sets of load and safety pistons and may comprise more than two such sets. For example, a split action actuator in accordance with an embodiment of the invention, comprise four cylinder housings each comprising a load and safety piston. [0033] In the description and claims of the present application, each of the verbs, "comprise" "include" and "have", and conjugates thereof, are used to indicate that the object or objects of the verb are not necessarily a complete listing of components, elements or parts of the subject or subjects of the verb.

[0034] Descriptions of embodiments of the invention in the present application are provided by way of example and are not intended to limit the scope of the invention. The described embodiments comprise different features, not all of which are required in all embodiments of the invention. Some embodiments utilize only some of the features or possible combinations of the features. Variations of embodiments of the invention that are described, and embodiments of the invention comprising different combinations of features noted in the described embodiments, will occur to persons of the art. The scope of the invention is limited only by the claims.

Claims

1. An actuator comprising:

a first piston moved between first and second positions by pressurized fluid; and

a second piston moved by the pressurized fluid to arm a safety system that returns the first piston from the second position to the first position when pressure provided by the pressurized fluid drops below a threshold pressure.

2. An actuator according to claim 1 wherein the first and second pistons are respectively housed in first and second tandem cylinder chambers.

3. An actuator according to claim 2 wherein the second cylinder chamber comprises an elastic element that the second piston compresses when it arms the safety system.

4. An actuator according to claim 3 wherein the elastic element comprises a coil spring.

5. An actuator according to claim 3 or claim 4 wherein the elastic element provides force to return the first piston to the first position when pressure in the pressurized fluid drops below a threshold pressure.

6. An actuator according to any of the preceding claims and comprising a component connected to the second piston that extends into the first cylinder chamber and pushes the first piston to return to the first position when pressure provided by the pressurized fluid drops below a threshold pressure.

7. An actuator according to any of the preceding claims and comprising a transmission that couples the first piston to a load to apply force to the load.

8. An actuator in accordance with any of the preceding claims wherein the pressurized fluid is a gas.

9. A compound actuator comprising a plurality of actuators in accordance with any of the preceding claims coupled to a same transmission.