GB2328981A - Improved component of mechanical actuator - Google Patents

Abstract

A mechanical actuator comprises an artificial muscle, the actuator having an expansible vessel 3 and means for effecting flow of a fluid under pressure into and out of the vessel. The actuator comprises means 11 for attaching an end of the vessel to a structure to be acted on. The actuator comprises a sheath 2 for generating a force between the ends of the vessel in response to expansion of vessel 3 and the ends of the vessel are terminated by termination means 4 and means 5 for retaining said termination means. In accordance with the invention termination means 4 are configured with inclined sides to facilitate tightening of the termination means against the retaining means. In this way, the mechanical actuator is able to act upon larger loads with improved reliability.

Description

IMPROVED COMPONENT OF MECHANICAL ACTUATOR The present invention relates to an artificial muscle and a mechanical actuator incorporating the muscle.

The present applicant's UK patent 2255961, dated 13 March 1992 discloses and claims a mechanical actuator including an artificial muscle or "traction element".

This muscle comprises an expansible vessel such as a tube, closed at each end, with means to introduce a fluid, such as air, under pressure, into the tube. The tube is made of expansible material such as rubber. Surrounding the tube is a braided sheath. As the fluid enters the tube, the increased pressure causes it to expand. This in turn causes the braided sheath to expand radially. The nature of the braiding is such that when it is expanded radially it is constrained to contract longitudinally. Thus a tensile force is generated between the points of attachment of the ends of the braided sheath. In the device termed a "Shadow Digit" disclosed in the inventor's patent referred to above, this force is used to cause relative movement of two pivotally interconnected beams.

In use, the air muscle is attached at one end to a fixed point and at the other end to the piece to be moved.

This needs to be done in such a way that the air muscle is extended, prior to the introduction of fluid under pressure.

When fluid is introduced under pressure into the expansible vessel or tube, the walls of the vessel are stretched and expand. The expansion of the expansible vessel would occur both radially and longitudinally, but the attachment of the muscle under tension prevents longitudinal expansion, since the vessel is already stretched in that direction. Hence, only radial expansion can occur.

The braided sheath converts the radial expansion into a longitudinal contraction. It may also be used as the means of attachment to the structure which is to be acted on. It may be looped back and secured by a contractible ring or other suitable tie to the end of the expansible vessel. Certain other materials which have the property of contracting axially when dilated radially may also be used instead of the braided sheath. For example a material moulded to form a diamondshaped lattice might be used. With these criteria, the selection of the sheath material is a simple matter for those skilled in the art We use a braided sheath for example only, and refer to all such coatings as "the sheath".

The present invention is an alternative means of terminating such devices.

An improvement in strength is claimed; that is to say, the device can be subjected to a much greater strain without suffering failure.

The method of terminating the device by folding back the outer sheath and securing it with some retaining device, for example a ring, has been found to be insufficient as the strength of the actuator increases. The retaining ring secures the looped-back sheath to the termination of the actuator. When the load is applied to the actuator, it is transmitted wholly through the looped-back sheath, across the ring, to the terminator of the actuator. Under circumstances of great strain, this loop can be pulled free, causing the muscle to fail.

The present invention addresses this problem. The load on the actuator is transmitted directly to the terminators which seal and form the ends of the inner expansive vessel, rather than to the sheath which surrounds the inner expansive vessel. The sheath is still attached to the terminator.

Attaching the load directly to the terminator has previously been found ineffective. In previous versions, the strain on the terminator had caused the terminator to become separated from the sheath or the inner expansible vessel, with consequent loss of function. However, the present inventor has found that if the terminator is of the shape and size described below it cannot be pulled out. As a result the air muscle is able to withstand a strain several times greater than the previous variation. This leads to an increased usefulness and life expectancy of the device.

The novel terminator gives rise to several improvements: Less braiding is needed.

. The ring does not need to be adjustable. Adjustable rings (such as the clips known as Jubilee clips) have several disadvantages; they are likely to be more expensive than simple rings; part of the clip may protrude inwards and may easily cause damage to the sheath or expansive vessel; under strain, the adjustment mechanism may fail, allowing the ring to expand.

Assembly of such actuators may be simpler, as compared to the looped sheath.

Repair of such actuators may be simpler, as compared to the looped sheath.

The significant factor in the present invention is the shape of the terminator which seals the end of the expansive vessel and its size relative to the ring which holds the end of the muscle together. Because the attachment point on the air muscle is to this terminator, the force that the air muscle generates tends to pull the terminator longitudinally out of the tube and sheath. If the terminator is given a more conical cross-section, however, with the large end of the cone on the inside, and if this large end is sufficiently larger than a ring which closes the end of the muscle, and provided that the angle of rake of the conical terminator is sufficiently great, then the terminator cannot be pulled through the ring (which is relatively inextensible). Provided that the ring stays in place, increased strain on the terminator is sustained without failure.

In order that the invention may be well understood, an example of it, which is given by way of example only, will now be described with reference to the accompanying drawings, in which: Figure 1 is a view of a basic air muscle with loopedback sheath: and Figure 2 is an enlarged view of the same components at one end of the air muscle only, showing one example of the way in which it may be attached to the piece to be moved: and Figure 3 is a diagram to show how the air muscle needs to be fully stretched before fluid is introduced into it: and Figure 4 is a cross sectional view in greater detail of one end of the air muscle: and Figure 5 is a cross sectional diagram of the conical terminator indicating the angle of rake: and Figure 6 is a cross sectional diagram of the conical terminator indicating an alternative shape: and Figure 7 is a cross sectional view of a proposed end cap : and Figure 8 is a cross sectional view of a proposed end cap, shown in position at the end of the air muscle.

In more detail: Figure 1 shows the general structure of the traction element or artificial muscle of the applicant's previous Patent already referred to.

Figure 2 is an enlarged view of the same components at one end of the air muscle only showing one example of the way in which it may be attached to the piece to be moved.

Modifications to this muscle, by way of alteration to the method of terminating the muscle and attaching it to the work, form the substance of the present patent application.

Figure 3 is a diagram to show how the air muscle needs to be extended longitudinally before fluid is introduced.

The introduction of said fluid under pressure then causes the longitudinal contraction shown.

Figure 4 is a cross section of an example of the end of an air muscle according to the present invention. A braided sheath 2 covers an expansible vessel 3 to form the main body and effective part of the air muscle. The expansible vessel 3 is closed and made air-tight by the insertion of a terminator 4. The braiding 2 and the expansible vessel 3 are held tightly against the terminator 4 by a ring 5. The presence of this ring 5 clamping the materials 2 and 3 against the terminator 4 prevents air from escaping from the expansible vessel 3.

When air is introduced under pressure into the expansible vessel 3 (by means not shown), the latter expands. This expansion is limited by the braiding 2, which translates the radial expansion into longitudinal contraction. This force of this contraction - indicated by the two outline arrows 6 - is applied to the work 7 to which the muscle is attached, by means such as the threaded rod 11 shown here. The resistive force of this work 7 is indicated by the single outline arrow 8.

Because the work 7 is attached to the terminator 4 (and not, for example, to the braided sheath 2) the force generated by the muscle is likely to pull the terminator 4 out of the expansive vessel 3, causing the muscle to fail.

The ring 5 might be tightened to clamp the braided sheath 2 and the expansible vessel 3 onto the terminator 4, but if the sides of the terminator 4 are parallel or nearly parallel and sufficient force is generated, the terminator may be pulled out.

However, if the sides of the terminator are not parallel, but sufficiently inclined: that is to say, the terminator is conical in shape, with the large diameter inside the muscle: and if this diameter (indicated by the arrow 9) is sufficiently greater than the inside diameter of the ring 5 (indicated by the arrow 10), and the ring 5 is inextensible, then the terminator 4 cannot be pulled out, unless the ring 5 is moved or removed.

As the muscle pulls harder, or is pulled harder, the terminator 4 is pulled into the ring further. As it is pulled in further, the increasing diameter of the terminator squeezes the expansible vessel 3 and the braided sheath 2 harder and harder against the ring 5: so, the harder the pull, generally speaking, the tighter the fixture.

The ring 5 may expand slightly, and indeed it has been found that a copper ring of particular dimensions expands enough to allow it to deform slightly, becoming slightly conical in shape: this has the added effect of spreading the area of contact slightly, which reduces the likelihood of the ring 5 cutting into the braided sheath 2 or the inner expansible vessel 3.

Figure 5 is a diagram showing an example of a possible shape of the conical terminator and in particular the angle referred to in this patent application as the Angle of Rake 12 If the angle of rake 12 of the conical terminator 4 is too small, and the force acting on the fixture is sufficiently great, and the material of which the ring is made is sufficiently soft and thin, then the terminator may, after all, be pulled through the ring, causing the device to fail.

However, the present inventor has found that the appropriate combination of material strength and dimension will allow the presently described arrangement to withstand very large strains.

For example, if the angle of rake 12 is greater than 4 degrees, and if the ring 5 is made of copper and is more than 0.9mm in thickness, and if the larger diameter of the terminator 4 (indicated by the arrow 9) is at least 10 per cent greater than the inside diameter of the ring 5 (indicated by the arrow 10), then the arrangement can be subjected to pulls of at least 4000N without failure.

Figure 6 illustrates a variation on this arrangement which is to vary the angle of rake of the sides of the terminator. For example, the angle of rake may become steeper at the large end, or have a lip 13, in order to make sure that the largest diameter of the terminator 4 (indicated in Figure 4 by the arrow 9) is sufficiently greater than the inside diameter of the ring 5 (indicated in Figure 4 by the arrow 10) to ensure that it cannot be pulled through.

Preferably the maximum angle of rake 12 of the coneshaped terminator 4 should not, in the present invention, exceed 15 degrees. If it does exceed 15 degrees it may in certain circumstances work the ring 5 loose, by pushing it through the two layers (expansive vessel 3 and the braided sheath 2). That is to say, the terminator 4 may slip out of the muscle pushing the ring (through the materials 2 and 3) with it.

The ring 5 may be worked loose from the end of the muscle by other circumstances, such as being knocked against, or a combination of high pressure in the muscle and low load applied to the muscle.

Figures 7 & 8 illustrate means by which this eventuality may be avoided. A cap 14 is shown in cross section. It may be made of any rigid material including a plastic such as Nylon 66 or Delrin. It may be held in place by various means such as a nut 15 screwed onto the threaded rod 11 which may be used to attach the air muscle to the work piece 7 as shown in Figure 4. In Figure 8 this cap 14 is shown to be in contact with the ring 5, in such a way that it prevents the ring 5 from moving off the end of the air muscle (that is to say, upwards in the drawing, Figure 8).

The cap 14 may preferably be shaped so as to fit firmly against the top of the ring 5, by means of a lip 16.

This lip 16 ensures that the ring 5 is centred correctly on the cap 14. As shown in this example the lip 16 fits inside the ring 5, but it may alternatively fit outside the ring 5, or both inside and out. The cap 14 may be attached to the ring 5 by any other means, and the cap 14 and the ring 5 may be joined permanently or formed from a single piece of material.

The cap 14 is held in place by means such as a nut 15 which in this example is threaded onto the threaded rod 11 which is itself screwed firmly into the conical terminator 4. Thus the cap 14 is fastened to the conical terminator 4, in such a way that the ring 5 (as seen in Figure 8) is held securely in place.

In order to fill the air muscle with fluid, a tube 18 is shown in Figure 8. An aperture in the cap 14 may be provided to allow a such a tube to pass through it and be able to introduce fluid into the expansible vessel 3, through a hole 17 in the conical terminator 4. In the example shown in Figure 8, the tube 18 fits tightly into the hole 17. Other arrangements and fittings may be used to effect the flow of fluid into and out of the expansible vessel 3 to cause the muscle to operate.

Claims (32)

Claims:

1. A mechanical actuator comprising: an expansible vessel having a first end and a second end; means for effecting flow of a fluid under pressure into and out of said vessel; means for attaching a first end of said actuator to a structure to be acted on; means for attaching a second end of said actuator; means for generating a force between said first and second ends of said actuator, and fixtures for terminating said ends of said vessel, wherein a said fixture comprises termination means and means for retaining said termination means, a said termination means being configured with inclined sides to facilitate tightening of said termination means against said retaining means.

2. A mechanical actuator according to Claim 1, wherein said force generating means comprises a braided sheath surrounding said expansible vessel.

3. A mechanical actuator according to Claim 2, wherein said sheath is attached to each said termination means.

4. A mechanical actuator according to Claim 1, wherein said termination means has a diameter, inside said vessel, which is larger than that of a said retaining means through which said termination means is pulled.

5. A mechanical actuator according to Claim 1, wherein said retaining means comprises a ring.

6. A mechanical actuator according to Claim 1 or Claim 5, wherein said retaining means is not adjustable.

7. A mechanical actuator according to Claim 1, 5 or 6, wherein a said retaining means is substantially inextensible.

8. A mechanical actuator according to Claim 1, wherein said termination means is configurable to squeeze a portion of the wall of said expansible vessel substantially against said retaining means.

9. A mechanical actuator according to Claims 1 and 2, wherein said termination means is configurable to squeeze a portion of the wall of said sheath substantially against said retaining means.

10. A mechanical actuator according to any of Claims 1, 3, 4, 8 or 9 wherein said termination means is substantially conical in shape having its larger diameter end positioned substantially inside said vessel.

II. A mechanical actuator according to Claims 1 wherein said inclined sides comprise an angle of rake which is not greater than 15 degrees.

12. A mechanical actuator according to any preceding claim, wherein said termination means comprises sides having a varying angle of rake.

13. A mechanical actuator according to Claim 10, wherein said termination means has an angle of rake which comprises a lip at said larger diameter end.

14. A mechanical actuator according to Claim 1, wherein said termination means has a diameter inside said vessel which is at least 10% greater than the inside diameter of said retaining means.

15. A mechanical actuator according to Claim 1, wherein for a said end of said actuator, said termination means comprises a shaped cap configured to fit firmly against the top of said retaining means.

16. A mechanical actuator according to Claim 15, wherein said cap comprises a rigid material.

17. A mechanical actuator according to Claim 15, wherein for a said end, said cap is held in place by connection to said attachment means.

18. A mechanical actuator according to Claim 1 or Claim 17, wherein said attachment means comprises a threaded rod.

19. A mechanical actuator according to Claim 15, wherein a said shaped cap comprises shaping to create a lip.

20. A mechanical actuator according to Claim 1 and 19, wherein a said lip is configured to fit inside said retaining means.

21. A mechanical actuator according to Claim 1 and 19, wherein a said lip is configured to fit outside said retaining means.

22. A mechanical actuator according to Claim 1 and 19, wherein a said lip is configured to fit both inside and outside said retaining means.

23. A mechanical actuator according to Claim 15, wherein a said cap and a said retaining means are permanently joined.

24. A mechanical actuator according to Claim 15, wherein said cap and said retaining means are formed from a single piece of material.

25. A mechanical actuator according to Claim 15, wherein said cap is held in place by connection to said terminator.

26. A mechanical actuator according to Claim 1, wherein said force comprises a tensile force.

27. A mechanical actuator according to Claim 1, wherein said force generating means comprises means for converting a radial expansion of said vessel into a longitudinal contraction.

28. A mechanical actuator according to any of Claims 1, 26 or 27, wherein said force is transmitted directly to said termination means.

29. A mechanical actuator according to Claim 1, wherein said termination means forms and seals a said end of said inner expansive vessel.

30. A mechanical actuator according to Claim 1, wherein said means for attaching said second end of said vessel comprises attachment of said second end to a fixed point.

31. A mechanical actuator according to Claim 1, wherein said fluid comprises air.

32. A mechanical actuator as herein described with reference to the accompanying drawings.

Amendments to the claims have been filed as follows Claims: 1 A mechanical actuator comprising: an expansible vessel having a first and a second end; means for allowing flow of a fluid under pressure into and out of said vessel; means for attaching a first end of said actuator to a structure to be acted on; means for attaching a second end of said actuator to a structure to be acted on; means for converting radial expansion force of said expansible vessel into longitudinal contraction force, said means comprising a sheath surrounding the expansible vessel; fixtures for terminating and sealing said expansible vessel, said sheath being connected to said fixtures such that the tensile force produced by the actuator is transmitted from said attaching means to said sheath through said fixtures, said fixtures comprising: a terminator whose sides are not parallel and substantially conical with the larger end of said terminator being inside the vessel such that as greater tensile force is applied to the actuator the terminator moves with respect to a retaining ring which clamps said sheath and said expansible vessel to the sides of said terminator, said ring having inside diameter less than the diameter of the larger end of said terminator so that said terminator cannot be pulled through said ring.

2 A mechanical actuator according to Claim 1, wherein said sheath comprises woven or braided strands of high-tensile strength fibre surrounding said expansible vessel.

3 A mechanical actuator according to Claims 1 or 2, wherein said retaining ring is not adjustable.

4 A mechanical actuator according to Claims 1 2 or 3, wherein said retaining ring is substantially inextensible.

5 A mechanical actuator according to Claims 1,2,3 or 4 wherein said terminator is substantially conical in shape having its larger diameter end positioned substantially inside said vessel.

6 A mechanical actuator according to Claims 1,2,3,4 or 5 wherein said sides of said terminator comprise an angle of rake which is not greater than 15 degrees.

7 A mechanical actuator according to any preceding claim, wherein said sides of said terminator have a varying angle of rake.

8 A mechanical actuator according to any preceding Claim, wherein said terminator has a diameter inside said vessel which is at least 1% greater than the inside diameter of said retaining ring.

9 A mechanical actuator according to Claim 1, wherein for a said end of said actuator, said terminating fixture also contains a shaped cap configured to fit firmly against the top of said retaining ring to resist its longitudinal movement.

10 A mechanical actuator according to Claim 9, wherein said cap comprises a rigid material.

11 A mechanical actuator according to Claim 9, wherein for a said end, said cap is held in place by connection to said attachment means.

12 A mechanical actuator according to Claim 1 or Claim 11, wherein said attachment means comprises a threaded rod.

13 A mechanical actuator according to Claims 9, 10, 11 or 12, wherein a said shaped cap comprises shaping to create a lip.

14 A mechanical actuator according to Claim 13, wherein a said lip is configured to fit inside said retaining ring.

15 A mechanical actuator according to Claim 13, wherein a said lip is configured to fit outside said retaining ring.

16 A mechanical actuator according to Claim 13, wherein a said lip is configured to fit both inside and outside said retaining ring.

17 A mechanical actuator according to Claim 9, wherein a said cap and a said retaining ring are permanently joined.

18 A mechanical actuator according to Claim 9, wherein said cap and said retaining ring are formed from a single piece of material.

19 A mechanical actuator according to Claim 9, wherein said cap is held in place by connection to said terminator.

20 A mechanical actuator according to Claim 1, wherein said fluid comprises air.

21 A mechanical actuator as herein described with reference to the accompanying drawings.