GB2435308A

GB2435308A - Braided sheath air muscle with substantially fixed perimeter bladder

Info

Publication number: GB2435308A
Application number: GB0603325A
Authority: GB
Inventors: Richard Martin Greenhill
Original assignee: Shadow Robot Co Ltd
Current assignee: Shadow Robot Co Ltd
Priority date: 2006-02-18
Filing date: 2006-02-18
Publication date: 2007-08-22
Also published as: GB0603325D0

Abstract

An artificial muscle comprises a fluid impermeable bladder 13 within a braided sheath 11 such that radial bladder inflation causes muscle axial contraction. To minimise bladder wear when rubbing against the sheath the bladder may be made of non-stretching material or a rubber but the uninflated bladder perimeter dimension is close to that of the inflated sheath and is creased, folded, crumpled and/or corrugated in the non-inflated state and devoid of irregularity when fully inflated. An extension sensor is provided comprising an LED (73 Fig. 1) and photo sensor (75) in telescoping tubes. The sheath and bladder ends are trapped between a tapered plastics bung (15) and a collar (19) and retained by wedging action. One endpiece has a fluid admission and mounting adapter 29 and the other has an axle (45) and pulleys for gaining mechanical advantage from a tendon (55).

Description

Artificial Muscles This invention relates to a novel artificial muscle

and to a mechanical system incorporating such a muscle.

The artificial muscle is of the kind comprising a braided sheath and, within the sheath, a bladder, the muscle being operable under the admission and expulsion of a fluid, commonly air under pressure, to and from the bladder, between, at one end of its range of movement, a first end-state, in which both the bladder and the braided sheath are radially-contracted and longitudinally-extended substantially to their fullest working extent, and, at the other end of its range of movement, a second end-state, in which both the bladder and the braided sheath are radially-extended and longitudinally-contracted substantially to their fullest opposite working extents. Such an artificial muscle is, hereinafter, referred to as being "of the kind described".

Applicant's UK Patent # 2255961, dated 13 March 1992, discloses a mechanical actuator incorporating an artificial muscle of the kind described. This muscle, in common with others of like kind, have certain advantages over actuators of other forms. They suffer, however, from a major draw-back, having, as they do, a shorter average working life than is required for many purposes.

As a result of pressure cycling of the bladder a small crack appears in the bladder wall, and under continued pressure cycling of the bladder the small crack develops into a larger fissure with the inevitable result, failure of the muscle.

In comparative trials conducted, using in each case, for the tubular bladder suitably end-stopped, it has been demonstrated that the mean-time between failures of artificial muscles in accordance with the invention will be very substantially greater than that encountered even with the most successful of prior art artificial muscles of the kind described.

Prior art muscles of the kind described have been shown to fail as a result of the development of a progressively deteriorating fissure, initially a small crack, in the bladder wall, after, with the most successful types of such muscles, approximately 120,000 pressure cycles, during each of which the bladder was inflated and deflated between, substantially, the first and second end-states, empty' and full', respectively.

It is an object of the present invention to provide muscles of the kind described and which has a mean-time between failures, resulting from such rupture of the bellows, that is significantly greater than that previously experienced with muscles of the kind described. So, for example, an artificial muscle in accordance with the present invention has been subjected to something in excess of 800,000 corresponding pressure cycles, and has yet to fail. From this it may be gathered that, even were the trial muscle in accordance with the invention to fail at this time, as a result of a pressure cycling induced crack, its longevity would yet surpass than that of the prior art muscle by a factor of around 6.

According to the invention, the foregoing object is achieved by providing that, in an artificial muscle of the kind described, said bladder, when in said first end-state, has a cross-sectional boundary dimension that is closer in value to that of the braided sheath when in said second end-state, than to that of the braided sheath when in said first end-state.

A mechanical system incorporating an artificial muscle in accordance with the invention is hereinafter described with reference to the accompanying drawings, in which: Fig.1 shows, schematically, the mechanical system, the bladder of the artificial muscle thereof being in the substantially fully developed condition, the first end-state, depicted in cross-section, in Fig 5; Fig.2 is a corresponding diagram to that of Fig.1, but with the bladder of the artificial muscle thereof in the substantially fully contracted condition, the second end-state, shown in cross-section, in Fig.6; Figs.3 and 4 show end and side views, respectively, of a part of the actuator of Figs.1 and 2; Fig.5 shows, diagrammatically, a cross-section though the bladder when in the substantially fully expanded condition shown in Fig.1; Fig.6 shows, schematically, a cross-section through the bladder when in the substantially radially fully contracted condition shown in Fig.2; and, Fig.7 shows a variant of a feature of Figs.1 and 2.

In prior art arrangements employing artificial muscles of the kind described, typically as disclosed in Applicant's UK Patent No 2255961, previously referred to, the muscle comprises an expansible vessel, most commonly an elongate tubular bladder, provided with means for admitting air under pressure into, and for expelling such air from, the tube. The bladder has, in the prior art, been of an elastically expansible material, almost invariably rubber.

Surrounding the tubular bladder there is a braided sheath of, for example, a polyester material. Expansion of the bladder caused by the admission of air (or other suitable fluid) under pressure, results in radial expansion of the braided sheath, also. Such is the nature of the braided sheath, that radial expansion thereof gives rise to a longitudinal contraction, also, in the sheath. The braided sheath being anchored at its ends to first and second relatively displaceable parts, respectively, of a mechanical system, the tensile force developed in the sheath under such longitudinal contraction, gives rise to a proportionate displacement of one sheath-anchorage part with respect to the other.

As already remarked, muscles of the kind described have a number of advantages over certain other types of actuator. They suffer, however, from the major disadvantage in having a shorter working life than is required for many applications.

The most common cause of failure in the tubular bladder arises as a result of the development, remarked upon earlier, of a small fissure, a crack, in the bladder wall, in the course of its repetitive expansion and contraction, a fissure the appearance of which is caused, or contributed to, it is thought, by abrasive action of threads of the braided sheath by which the tube is embraced.

The strands of the sheath are subject to slight movement, both circumferentially and longitudinally with respect to the bladder, under pressure-induced changes in the shape and dimensions of the tubular bladder. Small movements between the sheath and the bladder cause the braided sheath to bite into the bladder wall. In the course of time, this causes wear of the tube wall in small areas, creating, it has been observed, an, initially, small crack in the bladder wall.

When, moreover, the tubular bladder is repetitively stretched, relaxed, and stretched again, in the course of continued pressure cycling, the initially small fissure is liable to extend relatively rapidly, forming an enlarging hole with consequent loss of function of the muscle.

If the bladder is not stretched, or is stretched to a limited extent only, the crack may be contained in size. If, moreover, the wall thickness of the tubular bladder is sufficiently large, the abrasive action of the braiding on the bladder may stabilize. This appears to occur (provided that the tube is subjected to not more than stretching to a limited extent), as a result of a bedding-down of the braiding into the bladder. The bladder outer surface is worn away by the more prominent parts of the sheath so that more and more of the sheath is brought to contact with the braiding. The local pressure reduces as the contact area increases, spreading the destructive effect over a larger and larger area. At the same time, fine particles of elastic material abraded from the bladder act as a lubricating and buffering layer between the bladder and the braided sheath.

In addition, the fact that the bladder has to an increased extent, taken up a shape substantially complementary to that of the braiding, tends to inhibit relative movement between the two muscle components.

Whilst this may inhibit the action of the muscle to a small extent, it may help to stabilize degradation of the bladder and, hence, to increase its life.

Notwithstanding these factors, stabilization of wear of the bladder under the abrasion of the braided sheath in the course of repeated pressure cycling of the bladder, is of little consequence.

All too soon, with prior art muscles of the kind described, at least one small fissure eventually develops in the bladder wall, enlarging under continuing stretchings and contractions, with inevitable failure of the bladder, and, hence, of the muscle.

From a consideration of the matters addressed above, it would seem to be desirable to reduce, to eliminate, even, radial expansion of the bladder. Sadly, it is this very action, generating, as it does, the radial expansion of the braided sheath, leading, in turn, to the longitudinal shortening of the braiding and, hence, muscle actuation.

The paradox may be resolved, under the present invention, by providing that rather than have the bladder relaxed, unstressed that is, only when empty, and under stress when extended, to provide that the bladder, ideally, resides in a relaxed, substantially unstressed, condition, throughout, the bladder being, when empty, creased, folded, crumpled, and/or corrugated, and, when full, devoid of such irregularity of form. Since there may be little radial stretching of the bladder as it is filled, the bladder could, ideally, be composed of a substantially unstretchable material.

Features, specifically the change in cross-sectional form of the bladder of an artificial muscle, being a muscle which is in accordance with the invention, and which satisfies the desiderata stated in the last preceding paragraph are to be observed in Figs.5 and 6.

A mechanical system S comprises (Figs.1 and 2) an artificial muscle M, being a muscle in accordance with the invention, having a braided sheath 11, of a polyester material and, within the sheath 11, a cylindrical tubular bladder 13.

As shown, the bladder 13 is closed at one end 13a by closure means 15, the closure means 15 comprising a lightly tapered bung 17 of a hard plastic material, and a collar 19. The end 13a of the tubular bladder 13 and the corresponding end ha of the surrounding braided sheath 11 are trapped between the plastic bung 15 and collar 19, as a result of wedge action therebetween, such wedge action serving, inter alia, to exclude the opportunity for the escape of air from the bladder at the end 13a.

The other end 13b of the tubular bladder 13 is, similarly, closed by closure means 21 again comprising a lightly tapered plastic bung 23 and a collar 25, the end 13b of the tubular bladder 13 and the end lib of the surrounding sheath 11 extending through the annular space between these and being trapped, as before, between the bung closur member 23 and the collar 25.

In distinction from the closure means 15, the bung 23 of the closure means 21 has a threaded axial passage 27 therethrough and an apertured lug 29 has a threaded cylindrical extension 31 in screw engagement with the bung 25 within the threaded passage 27. The extension 31 has an axial passage 33a leading to a sideways extending threaded passage section 33b. Air is admitted, under pressure, to, and expelled from the tubular bladder 13 by way of a pipe (not shown) in end-screwed engagement with the lug 29 within the side passage section 33b, such pipe leading to air ingress/air egress control valve means (not shown), for controlling the admission and expulsion of air to and from the bladder 13.

In Fig.5 the bladder 13 is depicted as being in the first working end-state, that is to say, at the end of the air admission stroke, both the bladder 13 and the braided sheath 11 being radially fully developed, and the braided sheath 11 being in its longitudinally-contracted state.

In Fig.6, the tubular bladder 13 is in the working second end-state, at the end of the air exhaustion stroke, that is to say, both the bladder 13 and the braided sheath 11 being radially fully contracted, and the braided sheath 11 being, now, in its longitudinally-extended, state.

As diagrammatically represented in Fig.6, the rectilinearly tubular bladder 13 is, notwithstanding its lesser cross-sectional area, of a cross-sectional boundary dimension that is substantially the same as that in Fig.5. The bladder 13, when in the form shown in Fig.6, rather than being in intimate contact over substantially the whole of its surface area with the surrounding contracted braided sheath 11, is, in order to accommodate the excess size' cross-sectional boundary dimension of the bladder 13, forced to adopt a longitudinally-corrugated, though relaxed, i.e substantially unstressed, form.

In the course of the development in volume of the bladder 13 from the first end-state of the muscle, depicted in Fig.6, to the muscle second end-state, corresponding to the radially developed, or full' bladder condition, shown in Fig.5, the surface of the bladder 13 is progressively recovered from its corrugated form shown in Fig.6 until, towards the attainment of the muscle second end-state, the bladder surface, relieved at this stage of corrugations, is then in full surface contact with the sheath braiding.

The representations of Figs.5 and 6 represent the cross-section of a non-stretchable bladder, the cross-sectional boundary dimension of the bladder being the same in both working end-states. In practice, where the material employed for the bladder is of intermediate elasticity, rubber for example, the cross-sectional boundary dimension of a bladder, in accordance with the invention, might be expected to differ to some extent between the two muscle end-state conditions depicted in Figs.2 and 6 and Figs.1 and 5, respectively.

The physical difference between the prior art muscle and that of the present invention is to be found in the choice of size for the tubular bladder. It has been found, surprisingly, in the interests of muscle longevity, it is desirable that, for a given braided sheath, that the bladder employed should, even in its fully relaxed state, be large, the larger the better, perhaps, by comparison with that previously thought to be optimum in prior art muscles of the kind described.

It might be asserted, that the closer that the ratio of the cross-sectional boundary dimension of the bladder when at the first and second end-states of the muscle approximates to unity, the better the prospect for the longevity of the bladder under pressure cycling.

Without purporting to describe the physics that might be involved, it seems not unreasonable to suggest that the increase in longevity experienced with artificial muscles in accordance with the present invention, arises from the fact that whilst the bladder 13, as with other artificial muscles of the kind described, produces radial expansion, accompanied by longitudinal contraction in the surrounding sheath 11. The unfolding of the bladder 13 from the corrugated to the smooth cylindrical form resulting, as it does, in progressive enlargement of the area of contact between the bladder 13 and the sheath 11, gives rise to their contact over the full surface area of the bladder 13, such contact being, however, achieved only towards the end of the development of the bladder 13 to its first end-state.

It is hypothesized that surface contact between the bladder 13 and braid 11 over the entire surface of the bladder 13, throughout the full working life of the bladder, may be the agent that is responsible, under pressure cycling of the bladder, for the early development in the bladder, of the small localized fissure or fissures and the subsequent enlargement thereof with inevitable consequence.

Be that as it may, the increase in bladder longevity encountered using artificial muscles in accordance with the invention is a demonstrable fact.

Returning to the mechanical system of Figs.1 and 2, the lug 29 is tied to system reference frame F, at a muscle fixed anchorage position 35, by means of a loop 37 of wire. The bung 17 has, at its outer end, two slots 39, 41, respectively, and a transverse passage 43 which houses a fixed axle 45.

On the axle 45, trapped within the slot 39, there is a first pulley wheel 47. Rotatable on a second axle 49, being an axle mounted on the system frame F at a second fixed position 51, there is a second pulley wheel 53. A load bearing filament 55, a wire or length of cord, is secures to the axle 45, to extend from the slot 41 around the pulley wheel 53, then around the pulley wheel 47 to a part (not shown), movably connected to the system frame F. Since muscles of the kind described necessarily exert a force only under traction, i.e. with the bladder 13 expanded and sheath 11 contracted, the mechanical system incorporates means (not shown) for restoring the movable part to its former position when, as a result of depletion of air from the bladder, the sheath returns to the extended state. The aforesaid means might be spring means, it might be another air muscle.

The pulley arrangement for the transmission of motion from the muscle to the movable part is effective to amplify the movement of the said movable part by, in the example, a factor of 3, at a cost of a diminution by a corresponding factor in the load applied to the movable part.

Whilst the arrangement described may be suitable for many purposes, the admission and expulsion of air at an end of the bladder 13, a passage through a lug, as 29, for example, it is thought to be preferable that the location for such admission and expulsion of air be at a axially stationary site along the bladder 13, itself. Such feature is illustrated in Fig.7 from which it will be seen that adaptor means 57 for the admission and expulsion of air to the bladder 13 comprises: a nipple 59, and first and second internally threaded washers 61, 63, respectively.

The nipple 59 has an enlarged internally threaded end portion 65 and an externally threaded stem portion 67 which extends through an aperture formed through the wall of the bladder 13 and through a corresponding opening through the braided sheath 11. The bladder wall is sandwiched between the washers 61, 63, at a position along the bladder 13 midway between its ends, this being the position of substantially zero axial movement of the braided sheath 11 under expansions and contractions of the bladder 13. Compression of the bladder wall between the washers 61, 63, under screw action of the nipple 59, gives rise to an intimate contact of the periphery of the aperture through the bladder wall with the stem portion 67 of the nipple.

Regardless of the means for admission and expulsion of air to and from the bladder 13, in order to measure, for control purposes, the length of the bladder 13, within the bladder there is a length sensing arrangement which comprises: first and second telescopic plastic tubes 69, 71, respectively, a LED 73, and a photo-sensor 75, suitably, a photo-diode.

The inner one 69 of said tubes, is cantilevered from the bang 17, and the outer tube 71 from the bung 23. Within the tube 69, at the free end thereof, there is a plug 77 having an axial passage holding the LED 73, and, within the tube 71, at the free end thereof, there is a plug 79 holding the photo-sensor 67. The interior surface of the outer tube 71 has a covering 81 that is substantially non-ref lective both to visible and to i.r radiation. Electrical leads 83, 85, respectively, lead, the one 83 from the photo-sensor 67, the other 85 from the LED 73 to a power source (not shown) by way of an aperture through the bung 23.

The braided sheath 11 being trapped at its ends ha, hib, the lenght of the sheath may be determined by the relative intensity of radiation, visible or i.r., as the case may be, emitted by the LED 73 and incident on the photo-sensor 75, such intensity of radiation incident on the photo-sensor 75 being related to the distance separating the sensor from the LED 73.

Claims

CLAIMS

1. An artificial muscle of the kind described in which said bladder, when in said first end-state, has a cross-sectional boundary dimension that is closer in value to that of the braided sheath when in said second end-state, than to that of the braided sheath when in said first end-state.

2. An artificial muscle of the kind described in which the bladder comprises: a cylindrical tube; a bung serving to close said tubular bladder at one end thereof; and adaptor means having a passage through which air may be admitted to and expelled from the bladder, the adaptor means being received by the tubular bladder at the other end thereof, and being such as to form a closure member against the ingress and egress of air otherwise by way of said passage.

3. An artificial muscle as claimed in claim 1 in which said bladder and the surrounding braided sheath have openings therethrough in registration at a position substantially midway along the braided sheath, being the position of minimal axial movement of the sheath in the course of operation of the muscle; and conduit means communicating with the interior of the bladder through said openings in the sheath and the bladder, being conduit means by way of which air is to be admitted to and extracted from said bladder.

4. An artificial muscle as claimed in claim 1, 2 or 3 and which comprises means adapted arranged and constructed to sense the length of the braided sheath.

5. A mechanical system which comprises: an artificial muscle as claimed in any preceding claim; means by which the muscle is anchored at one of its ends at a certain position defined in the system reference frame, a part movably connected to the reference frame at another position thereof; between the other end of the muscle and said movable part, pulley system serving to communicate, with amplified magnitude, contraction of the braided sheath, under admission of air to the bladder of the muscle.

6. An artificial muscle substantially as herein before described with reference to Figs.1, 2, 5 and 6, or as varied in accordance with Fig.7.

7. A mechanical system substantially as hereinbef ore described with reference to Fig.1, 2, 5, and 6, or as varied in accordance with Fig.7.