WO2011054394A1

WO2011054394A1 - Artificial cluster muscle

Info

Publication number: WO2011054394A1
Application number: PCT/EP2009/064756
Authority: WO
Inventors: Basarab Cojocaru
Original assignee: Abb Research Ltd
Priority date: 2009-11-06
Filing date: 2009-11-06
Publication date: 2011-05-12
Also published as: EP2350462A1

Abstract

A linear actuator 200 comprising a plurality of actuation cells 204 arranged between a first and a second attachment points 201, 202, each actuation cell 204 being configured to shorten when the volume of the actuation cell 204 is increased.

Description

Artificial cluster muscle TECHNICAL FIELD

The present invention relates to a linear actuator which consists of a plurality of actuation cells arranged in a cluster. The present invention further relates to a linear actuating member that responds to an electrical potential or electrical field.

BACKGROUND ART

There are several principles and designs for actuators accomplishing a linear movement. One such principle which is not so widely used is presented in figures la and lb showing a linear actuator 100. According to figure la an elongated structure 101 enclosing a cavity 102 is given a low

elasticity in the longitudinal direction while in the radial direction the structure is flexible. When the volume of the cavity 102 is increased, the elongated structure 101 changes its shape towards that shown in figure lb. The dimension in the longitudinal direction shortens and a pulling effect occurs between the end points 103 and 104 of the elongated structure 101. The obtained contraction distance 105 can be used for actuating purposes.

Conventionally such an elongated structure is constructed of a fluid tight membrane surrounded by a network of flexible but non-stretchable armoring members. The volume increase is obtained by filling a cavity inside of the membrane with a fluid. Such conventional actuators are known e.g. from US 4,733,603 and DE20216749. Furthermore, WO2003/067097

discloses in figure 12 an actuator with several expansion areas of a single internal cavity, and in figure 14 a plurality of actuators arranged to accomplish complex movements of an artificial hand. One drawback with the actuators described hereinbefore is a long response time which is a consequence of a relatively large fluid volume needed for obtaining a relatively short stroke. Another drawback is that the radial dimension of the actuators in the pressurized state is relatively large. Yet another drawback is that the actuators need to be provided with fluid conduits, which sets quite robust practical limits to the minimum dimensions of the actuators.

SUMMARY OF THE INVENTION One object of the invention is to provide a linear actuator which is fast and compact.

This object is achieved by the linear actuator according to appended claim 1 and the actuation cell according to

appended claim 15. The invention is based on the realization that by replacing one big actuator with a cluster of small actuation cells, the overall dimensions of the actuator are decreased while a more precise and flexible stroke control is achieved. The actuation cells can be clustered both in serial direction and in parallel direction, and different advantages are achieved in both ways .

The contraction force of an actuation cell is a function of the fluid pressure inside of the cell cavity and the cavity wall area. A substantial increased cell membrane area is achieved by replacing one big cell with a plurality of small ones. This implies that an actuator using as small as possible parallel clustered cells instead of one big cell results in a substantial higher contraction force for a device of a given transversal cross-section and internal pressure. On the other hand, the actuator's total

contraction distance is the sum of the contraction distances of many actuation cells arranged in series. As a plurality of small actuation cells in series needs, for accomplishing the same stroke, a smaller fluid volume and a smaller change in diameter than one big cell, arranging the actuation cells in series leads to a faster and more compact actuator, although this happens at the expense of the contraction force.

Therefore, by arranging actuation cells both in series and in parallel, and by controlling the volumes of individual actuation cells independently from one another, the

resulting actuator can be both fast and powerful

selectively.

The invention is further based on the realization that by selecting an appropriate actuating medium inside of the actuation cells, electrical activation means which lead to simple and fast control can be used. Particularly

advantageous is an actuating medium that alters its density in response to an activation signal. Such actuating medium does not need to enter and exit the cell cavity in order to alter the volume but an initial actuating medium inside of the cell cavity is capable of swelling and shrinking. According to a first aspect of the invention, there is provided a linear actuator comprising a first attachment point, a second attachment point, a linear direction defined by a straight line between the first and the second

attachment points, a plurality of actuation cells arranged between the first and the second attachment points, each actuation cell being configured to shorten in the linear direction when the volume of the actuation cell is

increased, the volumes of at least two actuation cells are configured to be alterable independently from each other. When the volumes of the actuation cells can be altered independently from each other, the stroke of the actuator can be controlled flexibly.

According to one embodiment of the invention, the plurality of actuation cells is arranged in series. With serial arranged actuation cells a long stroke with a fast response can be achieved.

According to one embodiment of the invention, the plurality of actuation cells is arranged in parallel. With parallel arranged actuation cells a strong contraction force can be achieved .

According to one embodiment of the invention, the volume of each actuation cell is configured to be alterable

independently from other actuation cells. With independently alterable actuation cell volumes a flexibly controllable actuator can be achieved.

According to one embodiment of the invention, the volumes of the actuation cells are alterable in groups, each group comprising a plurality of actuation cells arranged in series or a plurality of actuation cells arranged in parallel. By this measure the actuator can be adapted for different applications that either require a fast response or a strong contraction force.

According to one embodiment of the invention, each of the actuation cells comprises an initial volume of actuating medium, the volume being alterable in response to an

electrical potential. When using such an actuating medium the activation can be realized with simple and fast control means . According to one embodiment of the invention, the actuating medium comprises carbon nanotube aerogel material. The characteristics of carbon nanotube aerogel make it very suitable for being used as an actuating medium.

electrical field. Also an activation principle based on an electrical field allows a use of simple and fast control means .

According to one embodiment of the invention, the actuating medium comprises a conductive polymer material. There is a variety of conductive polymer materials that are responsive to electrical field and therefore suitable for being used as an actuating medium.

According to one embodiment of the invention, the initial volume is enclosed by an elongated structure which is configured to shorten when the volume is increased. The elongated structure transfers the volume change of each cell into a linear displacement.

According to one embodiment of the invention, each of the actuation cells comprises a dielectric elastomer actuator enclosed by an elongated structure which is configured to shorten when the dielectric elastomer actuator is activated. A dielectric elastomer actuator represents an additional principle for accomplishing a volume change of a cell.

According to one embodiment of the invention, the elongated structure comprises a plurality of armoring members. The elongated structure needs to be armoured in the pulling direction in order to make it relatively rigid in this direction and relatively flexible in bending direction.

According to one embodiment of the invention, the elongated structure comprises a membrane reinforced with fibres. A reinforced membrane has the advantage that it is capable of preventing the actuating medium from escaping and

transforming the volume change into a linear displacement at the same time. According to one embodiment of the invention, the membrane comprises perforations. Perforations have the function of preventing pressure difference from arising between the interior and the exterior of the cells.

According to a second aspect of the invention, there is provided an actuation cell comprising an initial volume of actuating medium, the volume being alterable in response to an electrical potential or electrical field, wherein the volume is enclosed by an elongated structure which is configured to shorten when the volume is increased. When an actuation cell is provided with an initial volume of actuating medium capable of altering its volume in response to an electrical signal, the control of the actuating cell can be realized with very simple and fast control means without the need for medium to enter and exit the cell cavity . BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be explained in greater detail with reference to the accompanying drawings, wherein figure 1 shows a contraction principle fundamental to the invention, figure 2 shows a cluster of actuation cells arranged both in series and in parallel, figure 3 shows a cluster of actuation cells arranged in series and comprising an initial volume of actuating medium, and figure 4 shows three different principles of activating actuation cells.

DESCRIPTION OF PREFERRED EMBODIMENTS

Referring to figure 2, a linear actuator 200 according to one embodiment of the invention comprises three groups of actuation cells 204 in series 206, each group comprising 18 actuation cells 204 in parallel 207. The actuator 200 has attachment points 201, 202 in opposite ends, and the

distance 205 between the attachment points 201, 202 is shortened by activating the actuation cells 204 such that they change their shape. Each actuation cell 204 comprises a cell cavity 208 enclosed by an elongated structure 209 which is relatively rigid in pulling direction and relatively flexible in bending direction, the elongated structure 209 being configured to shorten when the volume of the cell cavity 208 is increased. The activation principle depends on the actuating medium 210 used inside of the cell cavity 208. In a preferred embodiment the actuating medium 210 changes its volume in response to an electrical activation signal.

Further referring to figure 2, each actuation cell 204 may comprise an independent activating means (not shown) and/or a group of actuation cells 204 may be comprise a common activating means. Obviously, a most flexibly controllable actuator 200 is achieved when each actuation cell 204 is activated independently, but depending on the application it may be sufficient to be able to activate the cells 204 in smaller groups or all the cells 204 together. Typically such smaller groups would consist of a plurality of cells 204 arranged in parallel 207 or in series 206. Activating one parallel 207 group would mean a maximum contraction force but a limited stroke, while activating one serial 206 group would mean a stroke up to the maximum stroke with a limited contraction force. Depending on how the independent and common activating means are realized, many combinations of activating parallel 207 and serial 206 actuation cell groups are possible.

As described above, every actuation cell 204 comprises an activating means connected to it. In the case of fluid filled actuation cells the activating means comprises a fluid conduit connected to the cavity 208 inside of each actuation cell 204. The activation of fluid filled actuation cells is controlled by valves which admit the fluid flow in and out of the cell cavities 208. The stroke is controlled simply by adjusting the fluid volume inside of the cell cavity 208, and any stroke between the maximum and zero can be accomplished i.e. the stroke is continuous.

Fluid filled actuation cells have some serious disadvantages which prohibit their wider use. Because of the fluid flow in and out of the actuation cells 204 during the activation the speed of the actuation movement is very limited.

Furthermore, when the number of actuation cells 204 is large, it is very burdensome to provide each cell 204 with a fluid conduit. Also the practical limits of the minimum actuation cell dimensions are quite robust following from the need of providing the cells 204 with fluid terminals.

Referring to figures 3a and 3b, a group of actuation cells 304 arranged in a series 306 is shown. The cell cavities 308 comprise carbon nanotube aerogel, and the activating means comprises electrical conductors 311 connected to the cell cavity 308 of each cell 304. Carbon nanotube aerogel is a material that is capable of increasing its volume by about 220 % when an electrical potential is applied on it. This volume change is relatively strong and has a short response time, only some milliseconds, and therefore this material is very suitable for being used as an actuating medium 210, 310.

The carbon nanotube aerogel material is comprised mostly of air. The nanotubes inside the material have a special nanoscale and microscale structure which causes them to push away from each other when a voltage is applied. Once a carbon nanotube aerogel material inside of an actuation cell 304 is excited, no energy is needed for maintaining the activated state of the actuation cell 304. The actuation cell 304 is returned to non-activated state simply by connecting the actuating medium 210, 310 back to ground potential .

The activation of the cells 304 is controlled by a voltage source 312 capable of selectively providing the cell

cavities 308 with an electrical potential. Instead of pumping activating medium 210, 310 in and out of the cell cavities 308, the volumes of the cavities 308 are changed by altering the density of the initial volume of activating medium 210, 310. Even if the achieved volume change may depend on the magnitude of the applied voltage, the nature of the control with carbon nanotube aerogel is rather on/off than continuous. This means that each actuation cell 304 has essentially only two states - activated and non-activated. In figure 3a one of the actuation cells 304 is activated while the remaining cells 304 are not.

A linear actuator 200 comprising solely on/off cells 304 can only realize actuation strokes in increments, which is a clear drawback. This drawback can be compensated, however, by making the actuation cells 304 very small in size, so that the increments also are small. The on/off cells 304 may also be combined with fluid filled actuation cells or another type of linear actuators capable of realizing a continuous stroke and arranged in series with the on/off cells 304. When such a continuous actuator covers one stroke increment of the on/off cells 304, the combination is capable of ultimately realizing continuous strokes.

Referring to figure 3b, the elongated structure 309

comprises a thin, elastic silicone tube reinforced in longitudinal direction with low elasticity carbon

microfibers 313. The elongated structure 309 may comprise perforations 314 in order to avoid vacuum inside of the cell cavity 308 upon the activation of the cell 304. The

manufacture of such series 306 of actuation cells 304 can easily be implemented in a continuous process wherein the silicone tube is extruded and filled with the carbon

nanotube aerogel material in appropriate sequences. The neighbouring cell cavities 308 can be electrically and mechanically isolated by pinching the silicone tube between the sequences. The dimensions of such actuation cells 304 can be made very small, in order of millimetres or less, and it is easy to obtain very large groups of cells 304 in series and in parallel.

The large number of actuation cells 304 exposes an

additional advantage in comparison with the prior art actuators: the subject actuator 200 is very reliable since it maintains its functionality even when some of the

actuation cells 304 fail. The performance of the actuator 200 will slowly degrade as individual cells 304 fail, but no abrupt performance drop occurs.

The actuating medium 210, 310 is not limited to carbon nanotube aerogel. There is for example a variety of

conductive polymer materials which are capable of swelling and contracting in response to an electrical field. The actuation is caused by the displacement of ions inside the polymer when the polymer is surrounded by an appropriate electrolyte. Furthermore, there is a variety of dielectric elastomer actuators in which actuation is caused by

electrostatic forces acting on an elastomer film sandwiched between two electrodes. When a voltage is applied between the electrodes, the electrostatic force squeezes the

elastomer film forcing its film plane dimensions to expand.

According to figures 4a, 4b and 4c, a group of actuation cells 404 arranged in a series 406 is shown. Each of the cells 404a, 404b and 404c comprises a different actuating principle. The cell cavity 408 of the first cell 404a comprises carbon nanotube aerogel as an actuating medium 410 and one contactor 416a for making electrical contact with the actuating medium 410. The cell 404a is activated according to the principle explained above in connection with figure 3.

The cell cavity 408 of the second cell 404b comprises a polyacrylonitrile (PAN) material as an actuating medium 410, and the cell 404b is surrounded by an electrolyte 415.

Applying a voltage between the two electrodes 416b, 417b, one inside of the cell cavity 408 connected to the PAN material and the other placed in the electrolyte 415, will induce an ion migration. An increased ion quantity in the PAN material will induce a swelling of the cell cavity 408 while a reduced ion quantity will induce a shrinkage of the same. The volume change is reversed by changing the polarity of the electrode potentials. Obviously, the elongated structure 409 inside of which the cell cavities 408 are enclosed has to be permeable to admit free flow of ions.

The cell cavity 408 of the third cell 404c comprises a dielectric elastomer actuator. Applying a voltage between the two electrodes 416c and 417c causes an electrostatic pressure arising from the Coulomb forces acting between the electrodes 416c, 417c, and as a consequence the elastomeric film 418c changes its shape from that shown in figure 4b towards that shown in figure 4c. The elastomer film moves back to its original position when the electrodes 416c, 417c are short-circuited. One cell 404c may comprise a plurality of dielectric elastomer actuators.

The invention is not limited to the embodiments shown above, but the person skilled in the art may, of course, modify them in a plurality of ways within the scope of the

invention as defined by the claims. Therefore, for example, the actuating medium is not limited to any of the materials mentioned above, but any suitable material may be used.

Claims

1. A linear actuator (200) comprising,

a first attachment point (201),

a second attachment point (202),

a linear direction (203) defined by a straight line between the first and the second attachment points (201, 202) ,

a plurality of actuation cells (204, 304) arranged between the first and the second attachment points (201, 202), each actuation cell (204, 304, 404) being

configured to shorten in the linear direction (203) when the volume of the actuation cell (204, 304, 404) is increased,

characterized in that the volumes of at least two actuation cells (204, 304, 404) are configured to be alterable independently from each other.

2. A linear actuator (200) according to claim 1, wherein the plurality of actuation cells (204, 304, 404) is arranged in series (206, 306).

3. A linear actuator (200) according to any of the

preceding claims, wherein the plurality of actuation cells (204, 304, 404) is arranged in parallel (207) .

4. A linear actuator (200) according to any of the

preceding claims, wherein the volume of each actuation cell (204, 304, 404) is configured to be alterable independently from other actuation cells (204, 304, 404) .

5. A linear actuator (200) according to any of the

preceding claims, wherein the volumes of the actuation cells (204, 304, 404) are alterable in groups, each group comprising a plurality of actuation cells (204, 304, 404) arranged in series (206, 306) or a plurality of actuation cells (204, 304, 404) arranged in parallel (207) .

6. A linear actuator (200) according to any of the

preceding claims, wherein each of the actuation cells (204, 304, 404) comprises an initial volume of actuating medium (210, 310), the volume being alterable in

response to an electrical potential.

7. A linear actuator (200) according to claim 6, wherein the actuating medium (210, 310) comprises carbon

nanotube aerogel material.

8. A linear actuator (200) according to any of claims 1 to 5, wherein each of the actuation cells (204, 304, 404) comprises an initial volume of actuating medium (210, 310), the volume being alterable in response to an electrical field.

9. A linear actuator (200) according to claim 8, wherein the actuating medium (210, 310) comprises a conductive polymer material.

10. A linear actuator (200) according to any of claims 6 to 9, wherein the initial volume is enclosed by an

elongated structure (209, 309) which is configured to shorten when the volume is increased.

11. A linear actuator (200) according to any of claims 1 to 5, wherein each of the actuation cells (204, 304, 404) comprises a dielectric elastomer actuator enclosed by an elongated structure (209, 309) which is configured to shorten when the dielectric elastomer actuator is activated .

12. A linear actuator (200) according to claim 10 or 11, wherein the elongated structure (209, 309) comprises a plurality of armoring members (313) .

13. A linear actuator (200) according to any of claims 10 to 12, wherein the elongated structure (209, 309) comprises a membrane reinforced with fibres (313) .

14. A linear actuator (200) according to claim 13, wherein the membrane comprises perforations (314).

15. An actuation cell (204, 304, 404) comprising

an initial volume of actuating medium (210, 310), the volume being alterable in response to an electrical potential or electrical field,

characterized in that the volume is enclosed by an elongated structure (209, 309) which is configured to shorten when the volume is increased.