WO2016051141A1

WO2016051141A1 - Deployable structure

Info

Publication number: WO2016051141A1
Application number: PCT/GB2015/052785
Authority: WO
Inventors: Juan REVELES; Vincent FRAUX
Original assignee: Oxford Space Systems Limited
Priority date: 2014-09-29
Filing date: 2015-09-25
Publication date: 2016-04-07
Also published as: GB201417161D0

Abstract

A deployable structure(20)for attachment to a space structure includes a closed kinematic chain of interconnected struts(21). Adjacent pairs of the struts (21) are connected by a joint (22) such that the closed kinematic chain forms a Bricard linkage-based mechanism. The deployable structure also includes an actuation means (23) for deploying the structure from a stowed configuration to a deployed configuration, and a flexible membrane or semi-rigid surface attached to at least one of the struts(21)and arranged such that when the deployable structure is deployed into its deployed configuration by the actuation means, the flexible membrane or semi-rigid surface is unfurled.

Description

Deployable Structure

This invention relates to deployable motion structures, in particular to a deployable structure for use on a space structure.

Deployable structures such as antennas, solar panels, solar sails, etc., are constructed from a number of interlinked elements which move in a coordinated fashion in order to deploy or stow the structure. This enables such deployable structures to be used as part of larger structures which are launched into space, e.g. satellites or other, e.g. manned, spacecraft.

The aim of the present invention is to provide an improved deployable structure. When viewed from a first aspect the invention provides a deployable structure for attachment to a space structure comprising:

a closed kinematic chain of interconnected struts, wherein adjacent pairs of the struts are connected by a joint such that the closed kinematic chain forms a

Bricard linkage-based mechanism;

an actuation means for deploying the structure from a stowed configuration to a deployed configuration; and

a flexible membrane or semi-rigid surface attached to at least one of the struts and arranged such that when the deployable structure is deployed into its deployed configuration by the actuation means, the flexible membrane or semi-rigid surface is unfurled.

The invention also extends to a space structure, e.g. a satellite or spacecraft, comprising a deployable structure according to the first aspect of the invention, the space structure comprising a main body to which the deployable structure is attached.

The present invention relates to a deployable structure which can be attached to a space structure, e.g. to enable the deployable structure to be deployed in space. The structure is formed from a closed kinematic chain, i.e. a loop, of struts which are connected to each other by a joint between each pair of adjacent struts in order to form a Bricard linkage-based mechanism, e.g. a Bricard linkage. Attached to at least one of the struts is a flexible membrane or semi-rigid surface. The deployable structure also includes actuation means to deploy the structure between a stowed configuration and a deployed configuration, with the flexible membrane or semi-rigid surface being unfurled as the structure is deployed, i.e. the flexibility of the membrane or semi-rigid surface allows the membrane or semi-rigid surface to be folded up when the structure is in its stowed configuration, e.g. during launch of the space structure, with the membrane or semi-rigid surface being unfurled when the structure is deployed, e.g. once the space structure has been positioned in its intended operational, e.g. a stable, orbit.

Thus it will be appreciated that the deployable structure of the present invention is an effective way to provide a surface, i.e. the flexible membrane or semi-rigid surface, which can be attached to a space structure and stowed compactly during launch but then unfurled over a relatively large surface area for use in space, e.g. for a solar panel, a solar sail, a de-orbiting sail or an antenna. Having a flexible membrane or semi-rigid surface attached to the struts minimises the mass of the structure so that it becomes a cost effective way, in terms of the mass of the deployable structure versus the power needed for launch, versus the stowed volume, to deploy such a surface for use in space.

The closed kinematic chain of interconnected struts, together forming a Bricard linkage-based mechanism, e.g. a Bricard linkage, could take any suitable or desired form. In one embodiment the Bricard linkage-based mechanism is arranged as a mobility one structure, i.e. having one degree of freedom or requiring a single input to define the kinematic state of the whole structure. This allows the deployable structure to have a well defined path over which it is deployed, i.e. the paths the struts move along between the stowed and deployed configurations when actuated by the actuation means, and also that actuation of the structure at a single point, e.g. actuation of a joint by the actuation means, defines the movement of the deployable structure, i.e. all the struts and joints in the Bricard linkage-based mechanism. The Bricard linkage-based mechanism is also preferably arranged such that it is a collision free motion structure. The Bricard linkage-based mechanism could form any one of the family of Bricard linkage-based mechanisms as is suitable or desired, i.e. a structure having one or more of the following properties: general line-symmetric, general plane-symmetric, trihedral, line-symmetric octahedral or plane-symmetric octahedral. In a preferred embodiment the Bricard linkage-based mechanism is plane-symmetric (e.g. having three planes of symmetry) and trihedral (e.g. with each pair of adjacent struts, connected by a joint, falling in a different plane). This produces a structure with three-fold rotational symmetry, three planes of symmetry and six revolute variables. In another preferred embodiment, the Bricard linkage-based mechanism is plane symmetric with two-fold symmetry.

The deployable structure could have any number of struts and corresponding joints, as is suitable or desired to form the Bricard linkage-based mechanism. However preferably the number of struts is equal to the number of joints, i.e. each adjacent pair of struts is connected by a joint. In a particularly preferred embodiment the deployable structure comprises six struts and six joints connecting adjacent struts together to form the closed kinematic chain.

The joints could be any suitable or desirable joints. In a preferred embodiment the joints each comprise a revolute joint, i.e. such that the adjacent pairs of struts connected by a revolute joint rotate with respect to each other about an axis through the revolute joint and move in a plane at a pre-defined angle with respect to the axis of the joint. Preferably the pre-defined angle is between 40 to 80 degrees, e.g. between 50 to 70 degrees, e.g. between 55 to 65 degrees, e.g. 60 degrees.

The deployable structure could form any suitable or desirable shape in its stowed configuration. However in a preferred embodiment the deployable structure is arranged such that the struts are substantially parallel to each other in its stowed configuration.

The deployable structure could form any suitable or desirable shape in its deployed configuration. However in a preferred embodiment the deployable structure forms a substantially rectangular or a substantially hexagonal frame in its deployed configuration. The actuation means for deploying the structure from the stowed configuration to the deployed configuration could be provided in any suitable or desired way. As has been discussed above, preferably the Bricard linkage-based mechanism is arranged as a mobility one structure. This means that for such embodiments it is possible to actuate the structure at only a single point, i.e. the deployable structure comprises only a single actuation means for deploying the structure at one point on the structure. However in a preferred embodiment the actuation means for deploying the structure is arranged to actuate the structure at two or more points. This provides redundancy in the structure in case the actuation means at one point fails or the structure jams during deployment, for example.

In a preferred embodiment the actuation means for deploying the structure comprises a motorised actuation mechanism, preferably in combination with a hold down and release mechanism. Alternatively the actuation means may comprise a shape memory alloy or a stored energy element, for example, also in combination with a hold down and release mechanism. The stored energy element may comprise a stored energy composite or one or more springs.

In one embodiment the deployable structure comprises means for retaining the deployable structure in the deployed configuration. This could be provided by the actuation means for deploying the structure, e.g. a motor with a non-backdriveable gear train. However, preferably one or more of the joints comprise a latch to retain the deployable structure in the deployed configuration. In a preferred embodiment, the area of the membrane or semi-rigid surface is such that when the deployable structure is fully deployed, e.g. such that all the struts lie in a single plane and the membrane or semi-rigid surface is extended between the struts, the membrane or semi-rigid surface is pulled taut between the struts to which it is attached. Thus, although the membrane or semi-rigid surface could be attached to only one of the struts, preferably the membrane or semi-rigid surface is attached to at least two of the struts (e.g. in a rectangular configuration), e.g. three of the struts (e.g. in a hexagonal configuration), preferably all the struts, e.g.

attached to six struts. The membrane or semi-rigid surface may be attached to the struts directly or via compliant mounts. The membrane or semi-rigid surface can be made out of any desirable or suitable material. However in a preferred embodiment the membrane is at least partially elastic. Thus, particularly in the embodiment in which the membrane is pulled taut between the struts when the structure is deployed, the elasticity of the membrane, and the compliant mounts where provided, means that some tolerance can be provided by having a membrane with an area that is smaller than the area within the closed kinematic chain defined by the struts, i.e. the membrane will still be pulled taut even if the structure does not fully deploy. As has been discussed, deployable structures for attachment to space structures can perform one or more of a number of different functions, e.g. as an antenna, a solar panel, a solar sail, etc.. Thus in a preferred embodiment the membrane or semi-rigid surface comprises a (flexible) photovoltaic cell, e.g. an array of photovoltaic thin-film solar cells. This allows the deployable structure to function as a solar panel when deployed, e.g. to provide power to the space structure.

In another preferred embodiment the membrane or semi-rigid surface comprises a solar sail. This allows the space structure to be propelled by radiation pressure from the sun as a means of propulsion. In a further preferred embodiment the membrane or semi-rigid surface comprises a de-orbiting sail. This allows, for example, a low earth orbit (LEO) satellite to be removed from its operational orbit at the end of its mission through atmospheric drag, e.g. by virtue of the resistance to motion exerted over the membrane or semi-rigid surface when immersed in a fluid environment, e.g. gas molecules.

Preferably the membrane or semi-rigid surface comprises both a photovoltaic cell and a de-orbiting sail. These could be provided as separate membranes or semirigid surfaces, but preferably the membrane or semi-rigid surface comprising the photovoltaic cell also acts as a de-orbiting sail, e.g. for use at the end of the space structure's mission. In these embodiments, preferably the struts of the deployable structure are solely structural, i.e. to support the membrane or semi-rigid surface.

In an alternative, also preferred, embodiment the Bricard linkage-based mechanism is arranged to act as an antenna, e.g. by being attached to a deployable mast or boom which, in use, it moved to a desired distance away from the spacecraft or space structure to operate as an antenna.

The orientation of the deployable structure when deployed, and therefore the membrane or semi-rigid surface, could be fixed with respect to the space structure to which it is attached. However, preferably the deployable structure comprises means to adjust the orientation of the deployable structure with respect to the main body of the space structure. This allows the deployable structure to be moved between different positions, e.g. to maximise the amount of solar radiation received by the membrane or semi-rigid surface, such that the efficiency of the photovoltaic cell and/or the de-orbiting sail is maximised. These adjustment means could be actuated either before, during and/or after deployment of the deployable structure. The adjustment means could comprise any suitable or desirable device, e.g. a motor or trim mechanism to adjust the orientation of the deployable structure with respect to the main body of the space structure via a joint, which could be combined with the actuation means.

The deployable structure may be connected to the main body of the space structure in any desirable or suitable way. In one embodiment the deployable structure is attached directly to the main body of the space structure or spacecraft, e.g. one of the struts and/or at least one of the joints is attached directly to (i.e. in contact with) the main body of the space structure. In another embodiment the deployable structure is attached remotely to the main body of the space structure, e.g. via a deployable boom. In this embodiment preferably the space structure comprises means to adjust the position of the boom, e.g. to move it towards and away from the main body of the space structure, and/or to adjust its orientation, e.g. to adjust the orientation of the deployable structure with respect to the main body of the space structure (and therefore the means to adjust the position of the boom could also comprise the means to adjust the orientation of the deployable structure). The boom may also be extendable, e.g. telescopic or actuated via tape springs.

The space structure could be any suitable or desirable type of space structure. However in a preferred embodiment the space structure is a satellite, e.g. a microsatellite or a cubesat, e.g. having a volume of approximately 1 m³. In one embodiment the struts are between 50cm and 1 m in length, e.g. approximately 75cm long. This gives the membrane or semi-rigid surface a surface area of approximately 1.5m² when unfurled, e.g. in the embodiment in which the membrane or semi-rigid surface is attached to all of the struts in the Bricard linkage-based mechanism.

The space structure may have only a single deployable structure attached to its main body. However in a preferred embodiment the space structure comprises a plurality of deployable structures in accordance with the present invention, e.g. each attached to a different part or face of the main body of the space structure. Also preferably the space structure comprises one or more photovoltaic cells on one or more of its faces. Thus for a cubesat or microsatellite of volume

approximately 1 m³, i.e. each face having a surface area of approximately 1 m², and with deployable structures in accordance with the present invention attached to each edge of one of the faces, the total surface area presented (i.e. one face and four membranes or semi-rigid surfaces) will be approximately 7m².

Embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings, in which:

Figs. 1a and 1 b show schematically two Bricard linkage-based mechanisms which form part of the deployable structure in accordance with an embodiment of the present invention;

Fig. 2 shows a Bricard linkage-based mechanism which forms part of the deployable structure in accordance with an embodiment of the present invention being deployed from a stowed configuration to a deployed configuration;

Fig. 3 shows a microsatellite in accordance with an embodiment of the present invention, to which on each edge of one face is attached the Bricard linkage-based mechanism of Fig. 2;

Fig. 4 shows the microsatellite of Fig. 3, with a flexible membrane attached to each of the Bricard linkage-based mechanisms; and

Fig. 5 shows a microsatellite in accordance with another embodiment of the present invention, to which is attached the Bricard linkage-based mechanism of Fig.

2.

Fig. 1a shows schematically a Bricard linkage-based mechanism 10 which forms part of the deployable structure in accordance with an embodiment of the present invention. The Bricard linkage-based mechanism comprises six struts 11 , with adjacent struts each being connected by a revolute joint 12 (numbered 1 to 6) to form a closed kinematic loop. In the configuration shown in Fig. 1 a, the Bricard linkage-based mechanism 10 is shown in its deployed configuration where it forms a hexagon.

The Bricard linkage-based mechanism 10 of Fig. 1 a is general plane-symmetric (i.e. it has three planes of symmetry) and trihedral (i.e. with each pair of adjacent struts 1 1 , connected by a revolute joint 12, falling in a different plane). This produces a structure with three-fold rotational symmetry, three planes of symmetry and six revolute variables.

As outlined, the symmetry of the Bricard linkage-based mechanism 10 shown in Fig. 1a imposes the following conditions on the geometric parameters of the Bricard linkage-based mechanism 10.

Θ-Ι = θ₃ = θ₅

3l2 ⁼ 323 ⁼ 334 ⁼ ^45 ⁼ ^56 ⁼ 3β1 ⁼ /

a₁₂ = a₃₄ = a₅₆ = a

R, = 0 (/ = 1 , 2, 3, 4, 5, 6; where the geometric parameters follow the Denavit-Hartenberg convention as follows:

Θ,- is the angle of the revolute joint 12 from strut /^'-1 to strut /^' around the axis of the joint /^'-1 , also known as the "revolute variable";

a,y is the length of the strut separating two joints /^' and y;

an is the angle between the axis of joints /^' and j around the axis defining strut / using the right hand rule, also known as the "twist" of strut /; and

R, is the distance between strut /^'-1 to strut /^' along the axis of joint i, also known as the "offset" of the joint.

The condition that the Bricard linkage forms a closed loop can be expressed by the condition:

[T₆i] [Tse] [T₄₅] [T₃₄] [T₂₃] [T₁₂] = [/] where [/] is the identity matrix and the transformation matrix between two adjacent joints is given by the following expression:

ί 0 0 0

— Ri sin a — cos ¾,· sin θ₍ cos _tj cos Θ sin a.

—R cos Ctn sib n sin — sin n cos cos ££

Fig. 1 b shows a similar Bricard linkage-based mechanism to that shown in Fig. 1 a, except that the length of all of the struts is not equal, i.e. a₂3 = a₅₆ and a₁₂ = a₃₄ = a₄₅ = a₆₁, but a₂₃≠ a₁₂, i.e. such that it forms a plane symmetric structure with two-fold symmetry.

For the Bricard linkage-based mechanism in Fig. 1 b, the following conditions also met by the angles between the struts:

:-·ι

"5

ST

*¾4 ^—

ΐ¾¾ ·¾

For the strut offsets the following conditions are met:

Fig. 2 shows a Bricard linkage-based mechanism 20 which forms part of the deployable structure, in accordance with an embodiment of the present invention, being deployed from a stowed configuration (top left) to a deployed configuration (bottom right), through four intermediate configurations during deployment, with the progression being indicated by the arrows.

The Bricard linkage-based mechanism 20 shown in Fig. 2 has the same geometric structure as the Bricard linkage 10 shown in Fig. 1a, i.e. comprising six struts 21 , with adjacent struts each being connected by a revolute joint 22 to form a closed kinematic loop. In the stowed configuration of the Bricard linkage-based

mechanism 20 (top left), the six struts 21 lie substantially parallel to each other, such that it takes up a small volume, i.e. not much more than the combined volume of the six struts 21 and joints 22. In the deployed configuration of the Bricard linkage-based mechanism 20 (bottom right), the struts 21 all lie in substantially the same plane, forming a hexagon. The intermediate configurations shown in Fig. 2 show the configurations of the Bricard linkage-based mechanism 20 between the stowed configuration and the deployed configuration, i.e. that it takes during deployment.

The Bricard linkage-based mechanism 20 also comprises two drive motors or actuators 23, attached to two of the struts 21 (separated by another of the struts 21), which are operable to actuate the deployment of the Bricard linkage-based mechanism 20, i.e. by rotating the strut 21 to which the motor or actuator 23 is connected with respect to the adjacent strut 21 about the connecting joint 22.

Fig. 3 shows a microsatellite 30 in accordance with an embodiment of the present invention, to which on each edge of one face is attached the Bricard linkage-based mechanism 20 of Fig. 2, i.e. such that four Bricard linkage-based mechanisms 20 are attached to the microsatellite 30. In the same manner as Fig. 2, Fig. 3 illustrates the transition of the Bricard linkage-based mechanism 20 from its stowed configuration to its deployed configuration, through four intermediate configurations. It can be seen from the deployed configuration (bottom right of Fig. 3) that the Bricard linkage-based mechanisms 20 are attached to the microsatellite 30 such that when they are in their deployed configuration, they project outwards from the centre of the face 31 of the microsatellite 30 to which they are attached, such that the plane of the face 31 of the microsatellite 30 is nearly coincident with the plane of the deployed Bricard linkage-based mechanism 20, i.e. there is only a small angle between the two planes.

Fig. 4 shows the microsatellite 30 of Fig. 3, with a flexible membrane 35 attached to each of the Bricard linkage-based mechanisms 20, to form four deployable structures each attached to the microsatellite 30. The flexible membrane 35 is attached to each of the struts 21 of the Bricard linkage-based mechanism 20. The flexible membrane 35 is covered with photovoltaic cells and also acts as a de- orbiting sail for use at the end of its mission.

Operation of the microsatellite 30 and its deployable structures will now be described with reference to Figs. 2, 3 and 4. In use, the Bricard linkage-based mechanisms 20 each have attached to them a flexible membrane 35. The resultant deployable structures are attached to the four edges of one face 31 of the microsatellite 30 and arranged into the stowed configuration. The microsatellite 30 is launched into space, e.g. using a rocket, and the microsatellite 30 is placed into its intended operational orbit.

Once in orbit, the actuation devices (e.g. DC motors) 23 on each of the Bricard linkage-based mechanisms 20 are energised to deploy the deployable structures, i.e. to move them from their stowed configuration to their deployed configuration. As the Bricard linkage-based mechanisms 20 move through the intermediate configurations shown in Figs. 2 and 3, the flexible membranes 35 are unfurled such that when the deployable structure reaches its deployed configuration, i.e. as shown in Fig. 4, the flexible membranes 35 are pulled taut between the struts 21 of the Bricard linkage-based mechanisms 20. The photovoltaic cells on the flexible membranes 35 can then be used to generate electricity to power the microsatellite 30.

Fig. 5 shows a microsatellite 50 in accordance with another embodiment of the present invention, to which is attached the Bricard linkage-based mechanism 20 of Fig. 2. This embodiment differs from the arrangement shown in Figs. 3 and 4 in that instead of one of the struts 51 of the Bricard linkage-based mechanism 20 being attached directly to the microsatellite 50, the Bricard linkage-based mechanism 20 is attached to the microsatellite 50 via a boom 53. The boom 53 connects the Bricard linkage-based mechanism 20 to the microsatellite 50 via one of the joints 52 of the Bricard linkage-based mechanism 20.

The operation of this embodiment is very similar to that of the previous

embodiment, other than that a further actuation device (e.g. a motor, not shown) is provided to move the boom 53 away from the microsatellite 50 during deployment of the deployable structure. Thus in its stowed configuration, the Bricard linkage- based mechanism 20 lies flat against the microsatellite 50, e.g. in a similar position to that shown in Fig.3.

It can be seen from the above that in at least preferred embodiments of the invention, a deployable structure is provided which can be attached to a space structure, e.g. to enable the deployable structure to be deployed in space. This enables a surface, i.e. the flexible membrane or semi-rigid surface, to be provided, which can be attached to a space structure and stowed compactly during launch but then unfurled over a relatively large surface area for use in space, e.g. for a solar panel or a de-orbiting sail. Having a flexible membrane or semi-rigid surface attached to the struts minimises the mass of the structure so that it becomes a cost effective way, in terms of mass versus power needed for launch versus the stowed volume, to deploy a surface for use in space.

Claims

1. A deployable structure for attachment to a space structure comprising: a closed kinematic chain of interconnected struts, wherein adjacent pairs of the struts are connected by a joint such that the closed kinematic chain forms a Bricard linkage-based mechanism;

2. A deployable structure as claimed in claim 1 , wherein the Bricard linkage- based mechanism is arranged such that it is a collision free motion structure.

3. A deployable structure as claimed in claim 1 or 2, wherein the deployable structure comprises six struts and six joints connecting adjacent struts together to form the closed kinematic chain.

4. A deployable structure as claimed in claim 1 , 2 or 3, wherein the joints each comprise a revolute joint.

5. A deployable structure as claimed in any one of the preceding claims, wherein the deployable structure forms a substantially hexagonal or substantially rectangular frame in the deployed configuration.

6. A deployable structure as claimed in any one of the preceding claims, wherein the Bricard linkage-based mechanism is arranged as a mobility one structure.

7. A deployable structure as claimed in any one of the preceding claims, wherein the actuation means for deploying the structure is arranged to actuate the structure at two or more points.

8. A deployable structure as claimed in any one of the preceding claims, wherein the actuation means for deploying the structure comprises a motorised actuation mechanism or a stored energy element.

9. A deployable structure as claimed in any one of the preceding claims, wherein the deployable structure comprises means for retaining the deployable structure in the deployed configuration.

10. A deployable structure as claimed in any one of the preceding claims, wherein the area of the flexible membrane or semi-rigid surface is such that when the deployable structure is fully deployed the membrane or semi-rigid surface is pulled taut between the struts to which it is attached.

1 1. A deployable structure as claimed in any one of the preceding claims, wherein the flexible membrane or semi-rigid surface is attached to at least two of the struts.

12. A deployable structure as claimed in any one of the preceding claims, wherein the flexible membrane or semi-rigid surface comprises a photovoltaic cell.

13. A deployable structure as claimed in claim 12, wherein the flexible membrane or semi-rigid surface is also arranged to act as a de-orbiting sail and/or a solar sail.

14. A deployable structure as claimed in any one of the preceding claims, wherein the flexible membrane or semi-rigid surface is arranged to act as an antenna.

15. A deployable structure as claimed in any one of the preceding claims, further comprising means to adjust the orientation of the deployable structure with respect to the main body of the space structure.

16. A deployable structure as claimed in any one of the preceding claims, wherein one of the struts and/or at least one of the joints is attached directly to the main body of the space structure.

17. A space structure comprising a main body and the deployable structure of any one of the preceding claims, wherein the deployable structure is attached to the main body of the space structure.

18. A space structure as claimed in claim 17, wherein the space structure is a microsatellite or a cubesat.