GB2207702A - Pneumatic or hydraulic actuator mechanism (an artificial muscle) - Google Patents

Abstract

An actuator mechanism consists of a tube of loosely woven flexible braiding 7, with connecting leads 1 at each end, enclosing a rubber inner tube or balloon 6 and means of inflation 3 through one of the leads. The diagonal weave pattern of the braiding 7 has the property of being able to change the orientation of the threads allowing it to be stretched or compressed with ease. Inflation of the balloon 6 (either pneumatically or hydraulically), causes an increase in the diameter of the braiding which in turn results in a contraction in its length. Both the appearance and the operation of the actuator can be compared to the biological model offered by the muscle. <IMAGE>

Description

PNEUMATIC OR HYDRAULIC ACTUATOR MECHANISM (AN ARTIFICIAL MUSCLE) The present invention relates to a pneumatic or hydraulic powered actuator mechanism.

As is well known, the use of both pneumatic and hydraulic powered cylinders are so common as to constitute one of the basic building blocks of modern engineering.

Actuators based upon the use of metal bellows rather than cylinders are also well known.

The present invention differs from the above in that the actuator mechanism consists of an inflatable inner tube or balloon enclosed within a tube of loosely woven flexible braiding. The diagonal weave pattern of the braiding has the property of being able to change the orientation of the threads allowing it to be stretched or compressed with ease.

Inflation of the balloon (either pneumatically or hydraulically), causes an increase in the diameter of the braiding which in turn results in a contraction in its length. Attaching some form of lead to each end of the braiding results in an actuator, which can be connected to and used to power mechanisms. Unlike conventional cylinder or bellow actuators, this device is soft, flexible and very light in weight. Both its appearance and its action can better be likened to the biological model of the muscle.

The central inflating region which contracts corresponds to the "muscle", with the leads at each end playing the role of the "tendon". This parallel suggests one of the possible applications of this actuator in manufacturing artificial limbs of a more natural function and appearance than those currently available. The inherent simplicity and low cost of manufacture of this mechanism should ensure a wide range of possible application areas for its usage.

The invention will now be described in more detail by way of example with reference to the drawings in which: Figures la to ld illustrate the stages in the construction of a working model of the mechanism made from readily obtainable materials.

Figures 2a and 2b show this model in the deflated (extended or rest state) and inflated (contracted or powered state).

Figure 3 shows an expanded view of a suitable weave pattern for the braiding.

Figure 4 shows the corresponding mathematical extraction of a unit weave cell.

Figure 5 gives the dimensions for a section of braiding forming the actuator.

Figure 6 illustrates a number of design refinements possible with purpose made components Figures 7a and 7b illustrate expanded views of two alternative forms of braiding based upon a netted construction.

Figure 1 shows the stages in the construction of a working model of the actuator. At each end there is a section of strong plastic tubing 1 which forms the connecting leads or "tendons". Short sections of a larger diameter but tightly fitting tube 2 are forced onto the facing ends. In order to facilitate inflation, a valve mechanism 3 is fitted to the outer end of one tube using a sealing compound 4 to ensure an air tight fit. A suitable valve mechanism can be taken from a bicycle inner tube, a bicycle pump then being used to inflate the model. A balloon 6 is fitted over the end of the tube with the valve and a plug 5 is fitted to the facing end of the opposite tube. A length of tubular braiding 7 is placed over the tubes as shown.

The properties of a braiding suitable for use in the actuator are that it should be strong, with a loose, flexible, symmetrical weave and with low frictional losses.

Figure 3 shows an enlarged view of a suitable weave pattern.

Such a material has the property that it can be stretched or compressed with ease over a considerable range, the change in length being accomplished simply by a change in the orientation of the threads of the weave. This weave orientation is shown in figure 4 by the angle e. As the angle e is increased there is a reduction in the length of the braiding and a corresponding increase in its diameter, and vice versa. A suitable source of braiding is that used in electronics as a cable restraint, (cables are placed within the braiding which is then stretched to hold them firmly in position). This electrical braiding is woven from individual continuous strands of a strong artificial fibre such as nylon.

The ends of the braiding 7 are stretched out to fit over the ends of the tubes 1,2 and then bound tightly into place using a strong thread 8 such as fishing line. The ends are further strengthened with a protective covering of electrical heat shrink sleeving 9.

The balloon 6 should be chosen to be of about the same length as the separation between the ends of the tubes 1 when in the deflated state. When inflated, the balloon 6 is protected from rupture to a large degree by the surrounding braiding 7, in much the same way that a thin inner tube is supported by a much stronger tyre. In this way the balloon can withstand far higher pressures than would otherwise be the case. For the same reason the plug 5 is important to give support to the end of the balloon 6, without which it would be easily ruptured.

Figure 2a shows the finished model held under tension in its deflated or unpowered state. Figure 2b shows the change that occurs when the model has been inflated by means of a bicycle pump. The diameter of the braiding has increased, and there is a corresponding reduction of about 4 percent in its length.

The role of the balloon 6 is simply to provide an airtight membrane that allows the inflation of the "muscle".

The actual elastic properties of the balloon can to a good approximation be ignored in the analysis of the actuator.

In the same way, any small changes in the length of the strands of the braiding 7 upon inflation (resulting from elastic stretching) can be ignored. The mechanism for the contraction of the actuator results from the change in the orientation of the fibres of the weave.

Figure 3 shows an expanded view of a section of weave pattern, in this particular example, each cell is made up of three parallel strands of thread arranged symmetrically about its axis. A unit weave cell 1 is indicated by the dashed line. Figure 4 shows a geometric abstraction of this weave cell, where: s = length of sides of parallelogram of unit weave cell.

e = angle between threads and the axis of the braiding.

T = the tension -in each set of threads.

Ignoring the end effects caused by the rounding of the braiding where it connects to the tendons, figure 5 shows a central cylindrical section of braiding forming part of an actuator where: n = number of weave cells running the length of the section.

m = number of weave cells around circumference of braiding.

1 = length of the section.

L = Maximum length of section, where e = 0 degrees.

d = diameter of the braiding.

D = Maximum diameter of the braiding, where e = 9e degrees.

c = circumference of the braiding.

C = Maximun circumference of the braiding, where e = 9e degrees.

a = cross-sectional area of the braiding.

A = Maximum cross-sectional area of the braiding, where e = 9 degrees.

P = inflation pressure.

F = external force acting on actuator.

v = volume of section.

In practice, due to the finite diameter of the threads forming the braiding, the possible range of the weave angle e will always be less than the 0 to 9 degrees implied above. The practical limits on e are more likely to be about 10 to 80 degrees. Given this approximation, the following analysis will continue to consider the idealised perfectly flexible braiding where e can range from 0 to 9.

The following equations can now be derived from basic geometry.

L = 2ns 1 = 2ns cos(e) = L cos(e) C = 2ms c = 2ms sin(e) = C sin(e) D = 2ms /# = C / d = 2ms sin(e) /# = D sin(e) = c A = # D/ 4 = (ms) /# a = v d / 4 = A sin (#) 2 v = al = AL sin (9) cos(e) On inflation, with no external forces applied, the braiding will tend to adjust the weave angle e to give maximum volume. This condition occurs where the derivative of v with respect to 8 becomes zero.From the above equation: dv = AL [ 2 cos (6) sin(6) - sin (6)] = 0 d6 Which reduces to: tan (e) = 2 or 6 = 54.7 degrees (1) The maximum volume therefore occurs for a weave angle 6 of about 55 degrees.

Equations relating the dimensions of the actuator to the various forces acting upon it will now be derived. In equilibrium, the net force acting on any part of the system must be zero. Consider first the forces'acting over a cross section perpendicular to the axis of the braiding, we get: F + Pa = 2mT cos(6) (2) Now consider the forces acting over a section taken through the axis of the braiding, we get: Pld = 2 [ 2nT sin(6) 3 (3) Eliminating T from equations 2 and 3, and substituting using the above relationships gives:: 2 2 F = PA [ 2 cos (e) - sin (e) ] (4) or 2 F = PA F 3 cos (e) -1 ] For the case where F=, this gives a solution for 6 of about 55 degrees, the same as with equation 1 above. Unlike the case of a conventional cylindrical actuator, where for a given pressure the force F is constant throughout the length of the stroke, here the initial force is high, but falls to zero as 6 approaches 55 degrees. This sets a lower limit on the length of the actuator of about 57.7 percent of its maximum length.

So far the concept has been explained in terms of a model constructed from off the shelf components. In production situations a number of refinements become practicable. Figure 6 shows an actuator in which the connecting leads have been formed directly as part of the braiding by varying the properties of the weave along its length during manufacture. At either end the weave is tight, resulting in a small diameter firm connecting lead or tendon 11. In the central region is the loosely woven muscle 12 as before, connecting to the tendons on either side by transition zones 13 where the weave gradually changes from one form to the other. In this way an actuator can by shaped into a more natural form. The simple balloon has now been replaced by a purpose formed rubber (or similar elastic material) inner tube 14 with a profile matched to that of the braiding.As before, one end 14 serves as a means of inflation, the other end 15 being sealed to form a closed chamber. In some situations it may be desirable for both ends to be left open, in this way several actuators can be connected in series.

In order to attach the actuator to some mechanism it is desirable to have built in connectors. This can be done by directly moulding connectors to the ends of the actuator.

The form of such connectors can be chosen to match the needs of the application. Such mouldings should also incorporate means of connection to the pneumatic or hydraulic supply.

Frictional losses within the braiding, and between the braiding and the rubber inner tube can be minimised by the use of a suitable lubricant. In order to contain such a lubricant it is desirable to enclose the braiding 11,12,13 within a thin outer elastic membrane shaped to fit the actuator.

Other effects can be achieved by varying the properties of the weave of the braiding. For example, where the braiding is woven with an asymmetric weave ie. where the two sets of fibres are inclined at different angles to the axis of the actuator, then upon inflation, the actuator will develop a twisting action. By having a weave pattern that lacks cylindrical symmetry, for example where the front is woven more loosely than the back, then upon inflation, the actuator will develop a bending action. Similar effects can also be achieved by variations in the pattern of the weave itself. In this way the properties of an actuator can be optimised to match a particular application.

In certain applications, for example artificial limbs, it may be desirable to give the actuator a more natural appearance in the deflated state. This can be done by padding out the rubber inner tube with a shaped piece of soft, elastic, open pored foam. This porous foam will change the shape of the actuator without unduly interfering with the operation of inflation and deflation. The padding will however result in the actuator being in a partially contracted state even when no forces are applied to it.

This can be considered as introducing an offset into the actuators force/length relationship and may be used to advantage in some situations.

Additional effects can be achieved by combining individual actuators so far described, into loosely coupled bundles, to produce shapes and structures not possible with a single element. Such macro actuator assemblies could be powered by one or more sources of pneumatic or hydraulic power. With such arrangements it becomes possible to emulate many of the structural models observed within the biological world.

Flexible braidings can also be made using other techniques and this actuator principle is not restricted to the use of the woven construction so far described. The important feature required of the braiding is the flexibility of the weave cell, characterised by a diamond shaped appearance, that allows for large changes in the orientation of the threads. For example, braidings of a tubular netting construction can be used, figures 7a and 7b show expanded sections of two of the simplest such forms.

As before, the actuator can be shaped by changing the characteristics of the netting along its length during manufacture. Creasing the fibres into shape will also help to maintain its form during use. Another possible construction technique for making a flexible braiding is a knitted tube, the simplest stitch pattern however lacks the diamond shaped cell structure discussed above and will result in a less efficient actuator. Braiding can also be made using metal wire as well as fibre.

Claims (9)

1. Apparatus for an actuator mechanism, comprising of a tube of loosely woven braiding with connecting leads at either end enclosing an inner tube or balloon and means of inflation, such that upon inflating by either pneumatic or hydraulic means causes a change in the orientation of the weave of the braiding, resulting in a contraction in the length of the mechanism.

2. Apparatus according to claim 1, characterised in that both the inflatable region and the connecting leads are constructed from a continuous length of braiding shaped by varying the characteristics of the weave during manufacture.

3. Apparatus according to claims 1 and 2, characterised in that the enclosed inner tube is moulded to a shape that matches the profile-of the braiding.

4. Apparatus according to claims 1 to 3, characterised in that a lubricant is used to reduce frictional losses within the braiding and between the braiding and the rubber lining, and the whole enclosed in a thin outer elastic membrane shaped to fit the actuator.

5. Apparatus according to any of claims 1 to 4, characterised in that the ends of the connecting tubes are fitted with purpose moulded connectors and means of inflation.

6. Apparatus according to claims 1 to 5, characterised in that upon inflation, twisting and or bending actions occur as a result of introducing asymmetries or variations into the weave of the braiding.

7. Apparatus according to any of claims 1 to 6, characterised in that the tubular braiding uses a netting rather than a woven construction.

8. Apparatus according to any of claims 1 to 7, characterised in that the actuator can be given a more biological like appearance by padding out the inner tube with a shaped, soft, elastic, large open pored foam material.

9. Apparatus according to any of claims 1 to 8, characterised in that macro actuator structures can be constructed from loosely coupled bundles of smaller individual actuator elements, powered by one or more sources of pneumatic or hydraulic power.