GB2303407A

GB2303407A - Rotary Actuator

Info

Publication number: GB2303407A
Application number: GB9514504A
Authority: GB
Inventors: John Rodgers
Original assignee: IMPAQ Ltd
Current assignee: IMPAQ Ltd
Priority date: 1995-07-15
Filing date: 1995-07-15
Publication date: 1997-02-19
Also published as: GB9514504D0

Description

ROTARY ACTUATOR Field of the Invention This invention relates to a rotary actuator, for example for causing rotation of a first arm relative to a second arm in a robot or remote handling device, Background to the Invention Conventionally, remote handling arms are rotated relative to each other by means of ram-operated lever arrangements, vane-type rotary actuators or rack-andpinion systems. The lever and rack-and-pinion systems suffer from the disadvantages of lack of precise control because of the backlash inherent in such arrangements, limited arc of rotation, and relatively low torque, while the vane-type actuators also do not develop high torque, and are limited to less than one complete rotation, typically only about 280 degrees for a single vane actuator.

An alternative arrangement has been proposed, in which an endless chain passes over a pair of sprocket wheels, one of which provides the rotary output via its mounting shaft or axle, while the chain is driven by being attached to a piston within a large cylinder, a small piston being attached to the chain on the return run of the chain simply to provide a seal. When pressure is applied, the force gen erated by the main piston greatly exceeds that generated by the small piston, and so the larger piston is moved, causing rotation of the output shaft. This arrangement permits a high level of torque, as well as permitting a number of complete rotations of the output shaft, for example up to five.

This arrangement has a number of disadvantages. Firstly, the lack of chain tensioning means that there will be some backlash in the operation of the device, and therefore precision of positioning will be difficult to achieve. Since the device is a sealed unit, the output shaft requires high-pressure seals which can withstand the rotary motion of the shaft as well as the high and varying internal pressure.

Relatively large volumes of hydraulic fluid (or air) are required to move the piston, in view of its size relative to that of the complete actuator.

Summary of the Invention According to the present invention, there is provided a rotary actuator comprising a drive wheel connected to an output shaft for rotary motion, a flexible power transmission element passing over the drive wheel and connected on one side of the wheel to a linear motor such that, when the motor applies tension to the end of the flexible power transmission element the output shaft is caused to rotate in a first direction, and on the other side of the wheel to means for applying tension in the flexible power transmission element such that, when the motor releases the tension in the element, the output shaft is caused to rotate in the opposite direction.

In the present specification, the term "linear motor" encompasses particularly hydraulic and pneumatic rams and linear electric motors, and more generally any other device which can convert energy directly into linear motion.

The means for applying tension is suitably a second pneumatic or hydraulic ram, preferably coupled to the first such that, fluid exhausted from the first passes to the corresponding side of the piston in the second ram, and vice versa. In an alternative arrangement, the means for applying tension comprises a tension spring, for example a helical steel or other metal spring, or a fluid spring which can store energy when the ram pulls on the flexible power transmission element, the stored energy being used to return the element towards its rest position when the ram is released.

The flexible power transmission element is preferably a power transmission chain, and the wheel a sprocket wheel whose teeth engage in the chain. Alternatively, the element may be a toothed belt engaging corresponding teeth around the wheel. The belt may be formed of reinforced rubber or the like where the loading is low, but a chain is preferred for high loads.

It will be seen that, with the actuator of the invention, there is no necessity for a fluid-pressurised container and relatively large quantities of fluid to be pumped; the rams can be relatively small. Where additional power is required, it is possible to operate with two or more rams or sets of rams, each with its respective flexible drive element, positioned side-by-side driving parallel sprocket wheels mounted on a common shaft.

The actuator of the invention will have a wide range of uses from small precision manipulator arms for industrial robots and the like to lifting, digging and excavating machines, where the possibility of enclosing the operating rams within the arm affords a wider range of movements than with conventional externallymounted rams acting on levers, and offers protection to the rams from dirt, which can cause wear of moving surfaces and seals, and from the risk of physical damage as a result of collision with other machinery, for example. Since the actuator of the invention can generate rotation at a constant torque, a manipulator arm incorporating the actuator can operate effectively through a much greater working arc than conventional arms controlled by external rams and the like.

The speed of movements can be greater than with conventional manipulator arms, while maintaining precision and high torque if required. Manipulator arms incorporating the actuator of the invention can be made smaller than arms driven by conventional actuators, and because the actuator can be fitted within the arm, it would be possible for the arm to fold up into a very compact volume for storage, and for access to and removal from confined spaces.

Because the linear force is transmitted through a relatively simple and inexpensive flexible power transmission element, for example a chain, under tensile load, it is possible to position the sprocket wheel and axle some considerable distance from the linear motor or motors (e.g. hydraulic rams). This arrangement offers the advantage of being able to site the rams or the like in a less hostile environment than that to which the axle may be subjected, for example extremes of temperature, moisture, dirt, radiation or other corrosive or aggressive atmospheres. A further advantage is the possibility of allowing the masses of the sprocket wheel assembly and the rams or other linear motor means to be separated to move the centre of gravity of the arm further away from the axle centre-line than would be possible with other types of rotary actuator.In certain applications, this might reduce or eliminate the need for additional mass to counterbalance the arm in use.

Brief Description of the Drawings In the drawings, which are part-sectional diagrammatic representations of exemplary embodiments of the invention: Figure 1 shows a first embodiment using two hydraulic rams; Figure 2 shows a second embodiment using one ram and one spring; and Figure 3 shows the embodiment of Figure 1 in use as a suspension system for a vehicle wheel or the like.

Detailed Description of the Illustrated Embodiments Referring first to Figure 1, a remote manipulator arm is shown in simple diagrammatic form with a first part 1 comprising the rotary actuator and a second part 2 which can be rotated relative thereto. Typically in practice, the second part 2 would have a manipulator device mounted on its free end, or a further part rotatable relative to the second part, with the manipulator device mounted on the free end of the further part, so as to afford a greater degree of control in positioning of the manipulator device. Such an arrangement might be used, for example, on a submarine vehicle to permit an operator within the vehicle to carry out main- tenance or other operations under water.

The first part 1 has an enclosed body 3 in which a sprocket wheel 4 is mounted on a shaft 5 which extends, by way of a sealed bearing, outside the body to mount the second part 2. Also within the body 3, and at the opposite end thereof from the sprocket wheel 4, a pair of hydraulic rams 6 and 7, each attached to a respective free end of a length of drive chain 8 which passes over the sprocket wheel 4 and engages with the teeth around its periphery so that movement of the chain is translated into rotation of the sprocket wheel, and hence the second part 2 of the arm. Each ram 6 and 7 has a supply inlet 9, 10 for pressurised hydraulic fluid at the side of the piston 11,12 therein nearest to the chain 8, while the fluid inlets 13,14 at the opposite ends of the rams are joined together and supplied with make-up fluid via a common supply 15 and a non-return valve 16.Hydraulic fluid is supplied under pressure in a closed circuit via one or other of the supply inlets 9 and 10 causing the respective ram 6 or 7 to pull on the end of the chain 8, thus causing the sprocket wheel 4, and hence the second part 2 of the arm, to rotate clockwise or anti-clockwise relative to the view of Figure 1. As one ram 6 or 7 is pressurised, so fluid from the other side of its piston 11 or 12 is forced into the other ram 7 or 6, forcing its piston 12 or 11 outwardly as the piston 11 or 12 in the first ram moves inwardly, the fluid in front of the second piston 12 or 11 being returned to the closed circuit against a back pressure which ensures that the chain 8 is held under tension, thus ensuring precise rotational control.

The embodiment shown in Figure 2 substitutes a spring 20 for one of the rams. In this embodiment, the ram 6 is contracted to pull on the chain to cause clockwise rotation of the sprocket wheel 4 and thus the second part 2 of the arm, tension in the chain 8 being maintained by the expansion of the spring, which stores some of the energy expended by the ram 6. When the hydraulic pressure on the ram is released in a controlled manner, the spring 20 contracts again, pulling the chain 8 in the opposite direction, expanding the ram 6 and rotating the sprocket wheel anti-clockwise, and with it the second part 2 of the arm. The hydraulic supply to the ram can again be in a simple closed circuit; if desired, the fluid may also be supplied to the other side of the piston 11 so as to assist the spring in providing the opposite rotation.

It will be appreciated that the spring may be a gas spring, or a metal spring, especially a helical spring. As with the embodiment of Figure 1, the torque may be increased by providing two or more chains operating on parallel sprocket wheels on the same shaft and moved by a respective ram and spring set.

While the second part 2 of the arm is shown in the drawings as mounted on one end of the shaft 5, such an arrangement affording the greatest possible angle of rotation of one part relative to the other, it would also be possible to use other arrangements, such as a pair of the components 2, mounted on opposed ends of the shaft 5, or the end of the part 2 split into a fork or clevis to be attached to both ends of the shaft 5. These alternative arrangements would be more limited in the angle of rotation achievable, but would ensure that the loading on the shaft is more balanced laterally thereof.

Figure 3 illustrates another possible application for the rotary actuator of the invention, as part of a machine or vehicle suspension system, in which a wheel 30 is mounted on the second part 2 of the arm. By varying the angle of the second part 2 to the first part 1, height adjustment or ground clearance could be varied, or a partially or totally retractable undercarriage provided. In the embodiment illustrated, using the arrangement as described with reference to Figure 1, the use of a suitable control system for the hydraulic supply could provide an active suspension system. As an alternative, if the arrangement of Figure 2 were used instead, a passive suspension system could be formed. The wheel 30 may be driven or non-driven.

If the embodiment of Figure 3 were to be used in a machine system, the wheel 30 might also be a roller, sprocket wheel or other rotary part, controlling, for example, web tension in a web forming or processing machine, such as in papermaking, printing, or plastics film production and processing.

While the invention is particularly described with reference to the illus- trated embodiments as incorporating hydraulic rams, it will be understood that other linear motors may be used instead of, or as well as, rams, according to the desired applications for the actuator.

Claims

1) A rotary actuator comprising a drive wheel connected to an output shaft for rotary motion, a flexible power transmission element passing over the drive wheel and connected on one side of the wheel to a linear motor such that, when the motor applies tension to the end of the flexible power transmission element the output shaft is caused to rotate in a first direction, and on the other side of the wheel to means for applying tension in the flexible power transmission element such that, when the motor releases the tension in the element, the output shaft is caused to rotate in the opposite direction.

2) A rotary actuator according to Claim 1, wherein the linear motor is a hydraulic or pneumatic ram, or an electric motor.

3) A rotary actuator according to Claims 1 or 2, wherein the means for applying tension is a second pneumatic or hydraulic ram.

4) A rotary actuator according to Claim 3, wherein the second pneumatic or hydraulic ram is coupled to the first, such that fluid exhausted from the first passes to the corresponding side of the piston in the second ram, and vice versa.

5) A rotary actuator according to Claim 1 or 2, wherein the means for applying tension comprises a tension spring, which can store energy when the ram pulls on the flexible power transmission element, the stored energy being used to return the element towards its rest position when the ram is released.

6) A rotary actuator according to Claim 5, wherein the tension spring is a helical steel or other metal spring, or a fluid spring.

7) A rotary actuator according to any preceding claim, wherein the flexible power transmission element is a power transmission chain, and the wheel a sprocket wheel whose teeth engage in the chain.

8) A rotary actuator according to any of Claims 1 to 6, wherein the flexible power transmission element is a toothed belt engaging corresponding teeth around the wheel.

9) A rotary actuator according to Claim 8, wherein the belt is formed of reinforced rubber or the like.

10) A plurality of rotary actuators as claimed in any preceding claim, arranged side-by-side, the output shaft of each actuator being connected to or integral with the output shaft of the next adjacent actuator.

11) A rotary actuator substantially as herein described with reference to, or as shown in, the drawings.