GB2475073A

GB2475073A - Artificial muscle with former allowing large chamber diameter with low volume of fluid

Info

Publication number: GB2475073A
Application number: GB0919344A
Authority: GB
Inventors: Dennis Majoe
Original assignee: Individual
Current assignee: Individual
Priority date: 2009-11-05
Filing date: 2009-11-05
Publication date: 2011-05-11
Also published as: GB0919344D0

Abstract

An artificial muscle comprises a variable volume flexible chamber 44, a system for varying the volume of and/or pressure in the chamber to inflate and deflate the chamber, and a tendon 18 for transmitting force and/or movement between the chamber and a load. In its deflated state, the chamber is wrapped at least partly around an elongate former 12. In its deflated state, the chamber can therefore have a substantial diameter but the volume of working fluid in the chamber can be very small or, in the limit, zero. The former might be a solid body contained within the chamber. Alternatively, the former is preferably outside the interior of the chamber. When the chamber is deflated, the tendon tension tends to pull the chamber neatly around the former.

Description

TITLE

Artificial muscles

DESCRIPTION

This invention relates to artificial muscles which can be used as robotic actuators and which convert the pressure of a liquid or gas into motive forces More particularly, this invention relates to an artificial muscle comprising a variable volume flexible chamber, means for varying the volume of and/or pressure in the chamber to inflate and deflate the chamber, and a tendon for transmitting force and/or movement between the chamber and a load. Desirably, the muscle should be made from durable plastics material and be light in weight and economic to manufacture.

One form of artificial muscle of this type, known as a McKibben muscle, uses expansion in the chamber to create contractive motive forces. However, this type of actuator has smaller contractive stroke range than is possible with other embodiments. In addition it relies on chamber wall expansion and external sheathing which results in energy losses due to friction.

Patent documents U53570814 and GB1 184330 describes another form of artificial muscle of this type in which a generally cylindrical chamber with a fold in its wall is made to expand and an external interleaved tendon arrangement converts this expansion into a contractive force. Patent document W02009/077785A2 describes a further form of artificial muscle of this type, in which the chamber is driven by a vapour pressure source.

One problem with the muscles described in U53570814, GB1 184330 and W02009/077785A2 is that diameter of the chamber when deflated should not be allowed to become too small. To a first order approximation, the tendon tension per unit length of cylinder is equal to the product of the radius of the chamber and the pressure in the chamber. In the extreme, if the diameter of the chamber were infinitesimally small when deflated, an infinite pressure would be required in order to exert a finite tension in the tendon. However, if the chamber diameter when deflated is non-zero, then its volume too is also non-zero. Assuming the working fluid acts as a perfect gas, then in order to double the pressure in the chamber while its volume remains constant (i.e. to double the tendon tension without any contraction of the tendon) it is required to introduce into the chamber a further mass of gas equal to the initial mass.

A primary aim of the invention, or at least of specific embodiments of it, is to reduce the amount of working fluid needed to achieve a particular pressure. To this end, the artificial muscle of the invention is characterised in that, in its deflated state, the chamber is wrapped at least partly around or contains an elongate former. When in its deflated state, the chamber can therefore have a substantial diameter but the volume of working fluid in the chamber can be very small or, in the limit, zero.

The former might be a solid body contained within the chamber. However, a further problem with the muscles described in US3570814, GB1184330 and W02009/077785A2 is that the chamber wall needs to fold into the chamber upon deflation of the chamber (as shown in Figure 5A of the drawings), and to achieve this neatly and quickly can present difficulties. If this is not done well, the chamber wall will suffer mechanical stress, and friction will also result in loss of energy. If a solid former were contained within the chamber, it could increase the difficulty with which the chamber wall folds into the chamber.

A secondary aim of the invention, or at least of specific embodiments of it, is to enable the chamber to deflate neatly. To this end, the former is preferably outside the interior of the chamber. More specifically, as will be appreciated from the following detailed description of the invention, when the chamber is deflated, the tendon tension then tends to pull the chamber neatly around the former.

The chamber preferably has a pair of flexible walls connected along one edge to each other and to the tendon. The walls are preferably connected adjacent an opposite edge to the former. The walls are preferably also connected adjacent said opposite edge to a second tendon, with the first-mentioned tendon, the chamber and the second tendon encircling the former.

When in its deflated state, the chamber preferably substantially completely encircles the former to provide the chamber with a large capacity when inflated.

The former is preferably substantially cylindrical. The former is also preferably hollow so as to reduce its weight and provide space, for example, for control valves for the chamber.

A specific embodiment of the present invention will now be described, purely by way of example, with reference to the accompanying drawings, in which: Figures 1A-E are isometric views of the components of an artificial muscle; Figures 2A-F are isometric views illustrating construction of the artificial muscle; Figure 3 is a sectioned end view, on a larger scale, of the muscle; Figures 4A-E are schematic end views of the muscle at five stages of its contraction; Figures 5A & B are schematic views of an artificial muscle suggested in patent document W02009/077785A2 at two stages of its contraction.

Referring to Figures 1A-E, an artificial muscle comprises: a lightweight hollow plastic cylindrical former 12 (Figure 1A) having a radius RF and length LF; a pair of rectangular sheets 14,16 (Figure 1B) of thin flexible inextendable plastics material each having a length Ls slightly longer than 3ItRF and a width W slightly shorter than the length LF of the former 12; a first tendon 18 (Figure 1C) of thin flexible inextendable plastics material and formed at one end with five fingers 20 having a pitch P slightly less than one-fifth of the W of the sheets 14,16; a second tendon 22 (Figure 1 D) of thin flexible inextendable plastics material and formed at one end with four fingers 24 having the same pitch P; and a small diameter flexible plastics tube 26 (Figure 1E).

Referring to Figures 2A-F, in order to construct the muscle, one end of the tube 26 is heat-welded to the sheet 14 adjacent one end 28 of the sheet 14, and a hole 30 is formed through the sheet 14 into communication with the tube 26, and the tips of the four fingers 24 of the second tendon 22 are heat-welded onto one side of the sheet 14 about one third of its length from its opposite end 32 (Figure 2A). The other sheet 16 is then placed over the opposite side of the sheet 14 (Figure 2B), and the sheets 14,16 are heat-welded to each other along a weld line 34 at the ends 28 of the sheets 14,16, along a weld line 36 at the ends 32 of the sheets 14,16, along a weld line 38 about one third of their length from their ends 32, and along weld lines 40,42 along the edges of the sheets 14,16 between the weld lines 34,38 (Figure 2C). The area bounded by the weld lines 34,38,40,42 therefore forms a chamber 44 communicating the tube 26 but otherwise sealed, and the area between the weld lines 36,38 forms a sleeve 46 into which the former 12 is inserted (Figure 2D). The four fingers 24 of the second tendon 22 are then interdigitated with the five fingers 20 of the first tendon 18 as shown in Figure 2E, and the ends of the five fingers 20 are heat welded along the weld line 34 to the ends 28 of the sheets 14,16 (Figure 2F).

Referring to Figure 3 which is a schematic end view of the muscle 10 with the chamber 44 deflated and omitting to show the plastic tube 26, the sleeve 46 therefore encircles the former 12. The chamber 44 encircles the sleeve 46 extending clockwise from the weld line 38 with the sleeve 46 to the weld line 34 with the first tendon 18, which will now be referred to as the right tendon 18 and which extends to the right from the weld line 34. The second tendon 22, which will now be referred to as the left tendon, extends to the left from the weld line 38. The portion of the sheet 14 forming the chamber 44 provides the outer chamber wall 48, and the portion of the sheet 16 forming the chamber 44 provides the inner chamber wall 50. The fingers 22,24 of the tendons 18,22 and their interdigitation enable the chamber 44 to inflate and deflate to its full extent.

In use, the chamber 44 is pressurised using the flexible tube 26, for example using apparatus as described in patent document W02009/077785A2, the content of which is incorporated herein by reference. Alternatively, the tube 26 may be supplied via a control valve with air from a compressed air bottle or a compressor.

The muscle 10 is shown even more schematically in Figures 4A-E.

In Figure 4A, the chamber is deflated and extends one turn around the former from point a to point e. The left tendon 22 extends from point a to its end at point b. The right tendon 18 extends from point e to its end at point c. The points a and e are coincident.

In Figure 4B, the chamber 44 is shown slightly inflated. The inner chamber wall 50 extends from point a (the weld line 38) clockwise around the former 12 to a point d were it folds back tightly and then extends back, lifted from the former 12 to point e (the weld line 34).

The outer chamber wall 48 extends from point a, lifted from the former 12, to point e. The left tendon 22 extends from point a slightly anticlockwise around the former 12 and then straight to its end at point b. The right tendon 18 extends from point e over the folded back portion of the inner chamber wall 50, then part way around the former 12, and then straight to its end at point c.

Figures 4C and 4D show the chamber 44 inflated even further. It will be seen that the inner chamber wall 50 folds back with a more gentle curve from point d where it is tangential to the former to point f where it is tangential to the right tendon 18.

Figure 4E shows the maximum inflation of the chamber 44, where the radii of curvature of the outer and inner chamber walls 48,50 are equal. It should be noted that the artificial muscle 10 can only achieve this configuration if there is no tension in the tendons 18,22.

It will see seen from Figures 4B to 4D that, to a first order approximation, the chamber 44 has: (i) a convex wall portion a-d of fixed radius equal to the radius RF of the former 12; (ii) a concave wall portion a-e-f of variable radius Ro; and (iii) a concave wall portion d-f of varying radius Ri. The total length of these wall portions a-d, a-e-f, d-f is equal to twice the circumference of the former 12, i.e. 4ItRF. The other constraints affecting the shape and size of the chamber 44 are that: the wall portion d-f is tangential to the former 12 at point d and is tangential both to the right tendon 18 and to the wall portion a-e-f at the point f; and the difference between the radius of curvature Ro of the wall portion a-f and the radius of curvature RI of the wall portion d-f is equal to the tension T in the tendons 18,22 per unit length of the chamber 44 divided by the pressure P in the chamber 44, i.e. (Ro -Ri) = TIP. In the extreme case of Figure 4E where the chamber 44 is filly inflated, the length of the wall portion a-d reduces to zero. In the extreme case of Figure 4A where the chamber 44 is completely deflated, the length of the wall portion d-f reduces to zero. As the chamber 44 reduces in size to this latter extreme, the pressure in the chamber 44 maintains the concave shape of the wall portion d-f but with the radius Ri descreasing, and the tension in the right tendon 18 pulls the chamber 44 towards its deflated state, so that the inner chamber wall 50 rolls smoothly and tidily onto the former 12.

From a comparison of Figures 4A and 4E, it will be noted that the straight portion of the left tendon 22 moves little relative to the former 12. The left tendon 22 is therefore more suited for being anchored to a skeletal structure of a robot, with the right tendon 18 being connected to a load limb. Indeed, the left tendon 22 may be omitted, and the former 12 may be secured directly to the skeletal structure, in which case the sleeve 46 should be fixed to the former 12.

It should be noted that in the case where the radius RMAX of the filly inflated chamber 44 (Figure 4E) is equal to twice the radius RF of the former, the maximum contraction CMAX of the tendons between their ends b,c is equal to approximately 11.0 times the radius RF of the former, given by CMAxIRF = 3m + -cos1(113). By comparison, and referring to Figures 5A and SB which schematically show the arrangement suggested in patent document W020091077785A2, with the maximum radius RMAX of the chamber 54 (Figure SB) being twice its minimum radius RMIN (Figure SA), the maximum contraction CMAX of the tendons between their ends b,c is equal to only approximately 6.3 times the minimum chamber radius RMIN, given by CMAx/RMIN = 2m.

The embodiment of the invention of Figures 4A-E also compares favourably with the prior art of Figures SA and SB with regard to speed of reaction and consumption of working fluid at least for small contractions of the tendons 18,22 (as shown in Figure 4B) from the relaxed states shown in Figure 4A. For small contractions C of the tendons, the arrangement of Figure 4B approximates to being a cylinder of radius R having a cylindrical void of radius RF or RMIN. By contrast, the arrangement of Figures SA and SB is merely a cylinder of radius R. In either case, for small contractions C of the tendons, the contraction C is equal to 2m(R -RMIN), where in the case of Figures SA and SB, RMIN is the minimum radius of the chamber 54. For small contractions C of the tendons, the volume per unit length V of the chamber 44 of Figure 4B approximates to V = ir(R2 -RMIN2). However, the volume per unit length V of the chamber 54 of Figures SA and SB approximates to V = mR2, assuming that the volume of the folded portion 56 chamber wall inside the chamber 54 can be neglected. Assuming that the working fluid is an ideal gas and that its expansion and contraction are isothermal, the mass M of gas in the chamber 44,54 per unit length is, from the equation of state for an ideal gas, proportional to its pressure P and the volume V of the chamber 44,54 per unit length, i.e. M = kPV, where k is a constant. The tendon tension T per unit length of the chamber 44,54 is given by T = PR.

Combining the above relationships to eliminate R results in the mass M of gas per unit length of chamber in the chamber 44 of Figure 4B being given by M = C + -(2KRMIN)2 By contrast, in the case of Figures SA and SB, the mass 2K C+22tRMJN) M of gas in the chamber 54 is given by M = (c + 2ItRMIN).

At small contractions of the tendons of Figure 4B, the change in gas mass M per unit length of the chamber 44 with respect to changes in tendon tension T per unit length at a 7 \ I k ________ constant small contraction C is given by I I = C + 22tRMJN -. As the 2K C+2IERMJN contraction C tends towards zero, this partial derivative also tends towards zero and in the limit I I = 0. In other words, for small contractions C, in order for the arrangement of oT)co Figures 4A to 4E to maintain that contraction C with variations in tendon tension, only a very small mass of gas needs to be introduced into or released from the chamber 44. This enables the muscle of Figures 4A to 4E to react very quickly in such situations and makes the muscle particularly useful in position control with varying loads.

By contrast, at small contractions of the tendons of Figure 5A and SB, the change in gas mass M per unit length with respect to changes in tendon tension T per unit length at a constant (dM k contraction C is given by i = -(c + 2IERMIN). As the contraction C tends towards 2K zero, this partial derivative tends towards a finite amount, and in the limit 9 = kRMJN.

In other words, for small contractions C, in order for the arrangement of Figures SA and SB to maintain that contraction C with variations in tendon tension, a significant mass of gas needs to be introduced into or released from the chamber 44. This causes the muscle of Figures 5A and SB to react more slowly in such situations compared with the muscle of Figures 4A-E.

Furthermore, in the case where gas that is released from the chambers 44,54 is released to the atmosphere and wasted, it will be appreciated that, in the situation discussed above, the muscle of Figures SA and SB will have a significantly higher gas consumption than the muscle of Figures 4A-E.

It might be considered that the disadvantages described above of the muscle of Figures 5A and SB could be reduced by reducing the minimum radius RMIN of the chamber 54, in the limit to zero so that 9 = kRMJN also reduces to zero. However, it should be recalled that the tendon tension T per unit length of the chamber 54 is given by T = PR. Accordingly the pressure P required to produce a finite tendon tension T per unit length of chamber when at the minimum chamber radius RMIN is given by P = T/RMIN. As RMIN tends towards zero, the required pressure P becomes very large, and in the limit is infinite. A similar analysis explains why it is so difficult, when inflating a sausage-shaped party balloon, to get the inflation started.

It should be noted that the embodiment of the invention has been described above purely by way of example and that many modifications and developments may be made thereto within the scope of the present invention.

Claims

CLAIMS1. An artificial muscle comprising a variable volume flexible chamber, means for varying the volume of and/or pressure in the chamber to inflate and deflate the chamber, and a tendon for transmitting force and/or movement between the chamber and a load, wherein, when in its deflated state, the chamber is wrapped at least partly around or contains an elongate former.
2. An artificial muscle as claimed in claim 1, wherein the former is outside the interior of the chamber.
3. An artificial muscle as claimed in claim 2, wherein the chamber has a pair of flexible walls connected along one edge to each other and to the tendon.
4. An artificial muscle as claimed in claim 3, wherein the walls are connected adjacent an opposite edge to the former.
5. An artificial muscle as claimed in claim 4, wherein the walls are connected adjacent said opposite edge to a second tendon, with the first-mentioned tendon, the chamber and the second tendon encircling the former.
6. An artificial muscle as claimed in any preceding claim, wherein when in its deflated state, the chamber substantially completely encircles the former.
7. An artificial muscle as claimed in any preceding claim, wherein the former is substantially cylindrical.
8. An artificial muscle as claimed in any preceding claim, wherein the former is hollow.
9. An artificial muscle, substantially as described with reference to Figures 1A to 4E of the drawings.