GB2178109A

GB2178109A - Actuator

Info

Publication number: GB2178109A
Application number: GB08518870A
Authority: GB
Inventors: David William Arnold
Original assignee: Individual
Current assignee: Individual
Priority date: 1985-07-25
Filing date: 1985-07-25
Publication date: 1987-02-04
Also published as: GB8518870D0

Abstract

An actuator has a rod 12 attached to a piston 14 slidable in a cylinder 10. The rod 12 is tubular, and acts as the cylinder of a hydraulic piston/cylinder, the piston 24 of which is fixed to the cylinder 10. Both cylinders are preferably double-acting. The actuator is operated by pneumatic pressure in the cylinder 10. The hydraulic cylinder acts to damp movement of the rod 12, since liquid is displaced from one side of the piston to the other through throttle valves 36, 38. The throttle valves 36, 38 may be closable to lock the actuator rod 12 in position, and they may be arranged to remain closed if and for as long as the pneumatic pressure is insufficient to support the load on the rod. A reservoir 34 limited by a diaphragm can communicate with the hydraulic cylinder. A fluid operated spool valve 50 may be provided at the cylinder head end to control the operation of the pneumatic piston/cylinder. <IMAGE>

Description

SPECIFICATION Actuator Field of the invention This invention relates to actuators which are used to translate a signal of some form into physical movement which can be used to actuate a piece of apparatus.

Background to the invention It is well known to produce a pneumatically powered actuator based on a piston and cylinder, or a housed diaphragm, or a rotary vane in a suitable housing.

It is further well known to combine pneumatic actuators firstly with hydraulic dampers for controlling actuation velocity, or secondly with so-called positioner units for controlling actuator position. The latter typically comprises a spring-loaded diaphragm, coupled to a pneumatic spool valve, to provide high gain, proportional feedback control of actuator position.

A further addition which is sometimes employed is a locking mechanism to secure the actuator in an apprpriate position regardless of external loads and internal air pressures.

Such combined units, however, have a number of drawbacks, as follows: i) they are bulky ii) they often have exposed, external linkages and interconnections which are vulnerable to damage iii) locking mechanisms are not fail safe, in so far as they can be unlocked irrespective of an imbalance between the external load and the internal air pressure v) it is difficult to adjust the actuator velocity remotely.

Summary of the invention According to the invention, there is provided an actuator having an actuator rod which can be extended from the actuator housing, wherein a first, pneumatically operated, piston/cylinder arrangement works between the rod and the housing and a second, hydraulically operated, piston/cylinder arrangement is concentric with the first piston/cylinder arrangement and works between the actuator rod and a part fixed to the housing.

The operating movement of the actuator rod relative to the housing is preferably under the control of the pneumatically operated piston/ cylinder arrangement, whilst the hydraulically operated piston/cylinder acts to control the actuating movement of the rod.

By having movement of the rod controlled by both pneumatically and hydraulically operated piston/cylinder arrangements, good control is possible of the speed and position of the rod.

The pneumatically operated piston/cylinder is preferably double-acting so that it can be positively biased in either direction. The hydraulic piston/cylinder is preferably also double-acting.

A piston attached to an end of the actuator rod preferably slides within a cylinder formed by an outer wall of the actuator housing. The actuator rod itself may be tubular, and the hydraulically operated piston can slide within the hollow rod, the piston being secured to a piston rod fastened within the housing.

The hydraulic fluid paths to the opposite sides of the hydraulic piston conveniently extend parallel to the axis of the rod.

The hydraulic piston and pneumatic piston are (in the retracted position of the actuator) conveniently at opposite ends of the housing.

The hydraulic fluid flow paths to the opposite sides- of the hydraulic piston may both feed into a common reservoir, possibly through an adjustable throttle valve, so that the hydraulic piston/cylinder arrangement acts as a damper for actuator rod movement.

The hydraulic throttle valves can be capable of being closed completely, so that the actuator is locked.

The throttle valves can be pneumatically controlled. A branch of the hydraulic circuit can also extend to one side of each throttle valve, so that in the event of an imbalance between the external load and the internal air pressure, the resulting hydraulic pressure will act to prevent the opening of the throttle valves, thereby holding the device in a safe position until and unless the air pressure is adequate to support the load.

In an alternative arrangement, a springloaded diaphragm and a spool valve assembly is incorporated in place of a flexible reservoir and is connected with a second pneumatic control signal to detect and control actuator position.

Brief description of the drawings The invention will now be further described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a section through an actuator in accordance with the invention; Figure 2 is a section through a hydraulic control circuit for the actuator of Figure 1; and Figure 3 is a section through an alternative actuator in accordance with the invention.

Description of preferred embodiments Referring to Figure 1, the illustrated actuator comprises a cylindrical housing 10 with an actuator rod 12 slidably received therein. A piston 14 is mounted on the lower end of the rod 12 and slides in the housing 10. The interior of the housing thus forms a cylinder for the piston, and the cylinder space is closed by O-rings positioned at suitable points.

A first pneumatic port 16 is positioned above the piston 14, and second port 18 is positioned below the piston. Thus application of pneumatic pressure through the port 16 will cause the rod 12 to retract into the housing, whereas application of pressure to the port 18 will cause the rod to extend from the housing.

The actuator rod 12 has a head 20 for contacting the piece of apparatus (not shown) which is to be actuated thereby.

In order to control the movement of the actuator rod, the rod 12 is of tubular configuration and a tubular piston rod 22 extends axially up the middle of the hollow actuator rod. A second piston 24 is mounted on the end of the piston rod 22 and slides -within the hollow interior of the actuator rod 12.

A first hydraulic port 26 communicates with the interior of the hollow piston rod 22, and thus feeds hydraulic fluid up the rod 22 and out into the closed space above the piston 24. A second hydraulic port 28 communicates with the interior of a sleeve 30 which surrounds the piston rod 22 and has exit holes 32 at its top end.

Hydraulic fluid entering through the port 25 therefore passes into the space below the piston 24. This space is closed at the lower end by the piston 14 which seals against the outside of the sleeve 30.

Figure 2 shows a control circuit for the hydraulic fluid passing between the ports 26 and 28. This is a closed circuit. A common, variable-volume reservoir 34 is positioned at the bottom of the housing. During actuator operation, the actuator rod 12 moves relative to the hydraulic piston 24, and to allow this movement it is necessary for hydraulic fluid to be transfered from one side of the piston 24 to the other side thereof. This transfer is effected through the reservoir 34 with the direction of fluid flow in the ports 26 and 28 depending on the direction of movement of the actuator rod 12.

Because of this closed circuit movement of hydraulic fluid, the hydraulic piston acts to damp the movement of the actuator rod 12.

Throttle valves 36 and 38 are placed between the ports 26 and 28 and the reservoir 34, so that the degree of damping can be varied by adjusting the opening of the throttle valves.

The throttle valves 36 and 38 are springloaded towards a closed position and are opened by pneumatically operated plungers 40 and 42 respectively. When the valves 36 and 38 are closed, no hydraulic fluid can flow through the circuit, and therefore the actuator is locked in position. Such locking can occur at any point of the travel of the actuator rod 12.

It is desirable to incorporate a fail-safe mechanism so that if the load applied on the head 20 of the actuator is greater than can be supported by the pneumatic pressure entering through the port 18, there will be a feedback of hydraulic fluid through one or other auxiliary port 44 or 46 which will assist the springloading of the throttle valves (or one of them) being closed thus locking the actuator until either the pneumatic pressure supporting the load is increased, or the load is decreased.

An alternative embodiment is shown in Figure 3, but parts corresponding to the parts shown in Figures 1 and 2 bear the same reference numerals.

In this embodiment, the hydraulic fluid circuit is not shown, but ports 26 and 28 are available for connection to a hydraulic circuit for the purposes of locking, controlling the rate of movement of the rod 12 and/or telemotoring.

To control the position at which the actuator rod comes to a stop, a spool valve 50 controlled by diaphragms 52 and 54 and by a spring 56 are provided. The spring 56 extends up the middle of the piston rod 22, and is supported against the head of the actuator rod 12. The spring is guided on a guide pin 58.

In operation, the main air pressure source is connected to an inlet 60. In one end position of the spool valve 50, the port 60 is con-nected to an outlet port 62 connected by a line 64 to the port 16. In the opposite end position of the spool valve 50, the main air pressure is connected to a port 66 connected through a line 68 to the port 18. Whichever of the ports 62 and 66 is not connected to the inlet port 60, is connected to a vent 70 or 72.

The diaphragms 52 and 54 at the opposite ends of the spool 50 are biased by control pressure entering through ports 74 and 76.

Figure 3 shows the actuator in an end position where both the air ports 62 and 66 are closed. If a control pressure is applied through a port 78, to the underside of the diaphragm 52, the spool 50 will be lifted and the main air pressure will pass from the inlet 60 through the outlet 62, the line 64 to port 16.

The actuator rod 12 will then be retracted. As the rod is retracted, the pressure exerted by the spring 56 on the diaphragm 52 will increase and at a certain point (determined by the spring rate of the spring 56 and the pressure in the port 78) the spring 56 will return the spool to its original position.

The ports 74 and 76 are both connected to the same hydraulic balance line which provides positioning independent of varying hydraulic lock pressures. The areas of the diaphragms 52 and 54 exposed to the pressure of this hydraulic balance line are equal.

This embodiment provides the advantages of conventional positioning cylinders, without the disadvantages of bulky and exposed vulnerable linkages.

Claims

1. An actuator having an actuator rod which can be extended from an actuator housing, wherein a first, pneumatically operated, piston/cylinder arrangement works between the rod and the housing and a second, hydrauli cally operated, piston/cylinder arrangement is concentric with the first piston/cylinder arrangement and works between the actuator rod and a part fixed to the housing.

2. An actuator according to claim 1, in which the pneumatically operated piston/cylinder is double-acting.

3. An actuator according to claim 1 and 2, in which the hydraulic piston/cylinder is double-acting.

4. An actuator according to any one of claims 1 to 3, in which the piston of the pneumatically operated piston/cylinder is attached to an end of the actuator rod and slides within a cylinder formed by an outer wall of the actuator housing.

5. An actuator according to any one of the preceding claims, in which the actuator rod is tubular, and the hydraulically operated piston is slidable within the hollow rod, the piston being secured to a piston rod fastened within the housing.

6. An actuator according to any one of the preceding claims, in which the hydraulic fluid paths to the opposite sides of the hydraulic piston extend parallel to the axis of the rod.

7. An actuator according to any one of the preceding claims, in which the hydraulic piston and pneumatic piston are at opposite ends of the housing in the retracted position of the actuator.

8. An actuator according to any one of the preceding claims, in which the hydraulic fluid flow paths to the opposite sides of the hydraulic piston both feed into a common reservoir, so that the hydraulic piston/cylinder arrangement acts as a damper for actuator rod movement.

9. An actuator according to claim 8, in which the hydraulic fluid flow paths include adjustable throttle valves.

10. An actuator according to claim 9, in which the hydraulic throttle valves are capable of being closed completely to lock the actuator.

11. An actuator according to claim 10, in which the throttle valves are pneumatically controlled.

12. An actuator according to claim 11, in which a branch of the hydraulic circuit extends to one side of each throttle valve, so that in the event of an imbalance between the external load on the actuator rod and the internal air pressure, the resulting hydraulic pressure will act to prevent the opening of the throttle valves, thereby holding the device in a safe position until and unless the pneumatic pressure is adequate to support the load.

13. An actuator according to any one of the preceding claims, in which a spring-loaded diaphragm and a spool valve assembly are provided to control the operation of the pneumatic piston/cylinder and are connected to a pneumatic control signal to control actuator position.

14. An actuator according to any one of the preceding claims, in which the pneumatically operated piston/cylinder arrangement is adapted to cause movement of the actuator rod relative to the housing, and the hydraulically operated piston/cylinder arrangement is adapted to control movement of the rod.

15. An actuator substantially as herein described with reference to the accompanying drawings.

16. A method of operating an actuator according to any one of the preceding claims, in which the movement of the actuator rod relative to the housing is caused by the pneumatically operated piston/cylinder arrangement, whilst the hydraulically operated piston/cylinder acts to control the movement of the rod.