Deployable structure

A deployable structure includes a structural mechanism consisting of a plurality of rigid links 1 connected together by rotational joints 2 to form an array of Bennett linkages 20. The Bennett linkages 20 are interconnected so that the structural mechanism, including all the Bennett linkages 20, has a single degree of mobility. The structural mechanism has a profile with a curvature that varies during movement to deploy the structure from a state in which its profile is flat to a state in which its profile is curved. This allows is very convenient as the structure may be assembled, stored and/or transported in the flat state, prior to deployment into the curved state. As such the structure has many application including the frame for a tent.

BACKGROUND OF THE INVENTION

The present invention relates to deployable structures which include a structural mechanism which is movable for deployment of the structure. A structural mechanism is a mechanism which is mobile without inducing any strain on its components. The deployable structures of the present invention may be used as structures in a wide range of engineering applications ranging from small structures to large ones, for example in aerospace applications. They are particularly, but not exclusively, applicable as frames for tent-like structures.

In general, a structural mechanism may have any number of degrees of mobility, that is degrees of freedom. The present invention relates to the use of a structural mechanism having a single degree of mobility. In general, the advantage of such a single mobility system is that it is much easier to control and therefore more reliable. Having a single degree of mobility avoids the need for complex operations for deployment. Such structural mechanisms having a single degree of mobility, and over-constrained ones in particular, generally have a relatively high stiffness even without the use of latches. These advantages of easy deployment into a particular state, combined with relatively high stiffness in the resultant deployed state, make structural mechanisms particularly useful. Structural mechanisms have particular use to date as precision aerospace structures.

On the other hand, having a single degree of mobility means that any particular structural mechanism can only be deployed in one way. Thus, the configuration on deployment is limited by the design of the structural mechanism itself. Therefore, the utility of structural mechanisms in general is limited by the knowledge of constructions having particular configurations in their undeployed and deployed states.

A number of structural mechanisms are known based on the use of two-dimensional mechanisms as basic elements which are assembled together. To design such structural mechanisms, a suitable two-dimensional mechanism must be identified, together with a construction technique allowing the basic elements to be assembled whilst retaining mobility of each of the basic elements.

An aim of the present invention is to provide a new and useful form of structural mechanism.

According to the present invention, there is provided a deployable structure including a structural mechanism based on Bennett linkages as a basic element.

A Bennett linkage is a three-dimensional, over-constrained linkage consisting of four rigid links connected together in a loop by rotational joints. Each joint connects two links in the loop and provides for rotation of the connected links about an axis of rotation. Bennett linkages are in themselves known. Indeed they were discovered almost a century ago. They are remarkable in that they consist of only four links and yet are mobile by virtue of the joints having axes of rotation in particular directions which are neither parallel nor concurrent.

On movement of a Bennett linkage, the links move in three dimensions. In particular, if one considers the four links as two opposed pair of connected links, movement of the Bennett linkage causes the opposed pairs of links to relatively rotate about an imaginary line through the opposed joints connecting together the two opposed pairs of links. At the same time, those opposed joints move towards or away from each other. Such relative rotation may be visualised by considering the Bennett linkage observed from a position in line with the two opposed joints which connect the two opposed pairs of links.

Bennett linkages have fascinated and challenged kinematicians. However, research to date on Bennett linkage has been concentrated mainly on the design of small linkages based on a Bennett linkage, for example linkages having five or six links.

SUMMARY OF THE INVENTION

The present invention is a structural mechanism consisting of a plurality of rigid links connected together by rotational joints to form an array of Bennett linkages which are interconnected so that the entire structural mechanism, including all the Bennett linkages, has a single degree of mobility.

The interconnection may be achieved in various ways, but preferably as follows. Respective pairs of Bennett linkages may be interconnected by two connective links of each of the interconnected Bennett linkages being connected together by respective rotational joints so that the two connected links of each of the interconnected Bennett linkages are connected in a loop to form an intermediate Bennett linkage. The interconnected and intermediate Bennett linkages remain mobile provided that certain conditions are met, as described in more detail below. Thus, both interconnected Bennett linkages have a single degree of mobility, and consequently the entire structural mechanism has a single degree of mobility through the interconnection of the individual Bennett linkages.

The Bennett linkages are interconnected so that the structural mechanism has a profile with a curvature that varies during movement of the mechanism. This may be achieved by certain conditions on the intermediate linkage linking the Bennett linkages, which will be described in more detail below.

The interconnection along the direction in which the curvature of the profile of the structural mechanism occurs is now explained, based on a consideration of the movement of a Bennett linkage as causing two opposed pairs of links to rotate relative to one another. The interconnection causes the relative rotation of each Bennett linkage to be additive as one progresses along the structural mechanism. This in turn causes the profile of the structural mechanism to curve.

The curvature of the profile of the structural mechanism may be visualised by considering the interconnection between one pair of links of one of the interconnected Bennett linkages with one of the pairs of links of the other interconnected Bennett linkage. The individual Bennett linkages are arranged such that movement of the structural mechanism causes relative rotation of the two opposed pairs of links of each individual Bennett linkage to occur in the same sense. At the same time, the intermediate linkage may either cause no relative rotation of the pairs of links of the interconnected Bennett linkages or may cause additional rotation in the same sense (or even in the opposite sense but by a different amount). Thus the net effect is for the profile of the structural mechanism to have a changing curvature.

The structural mechanism may consist of a single series of Bennett linkages interconnected together. Alternatively, perpendicular to the direction in which the profile curves, the structural mechanism may comprise rows of Bennett linkages interconnected together. In this case, the Bennett linkages are interconnected so that the rows remain straight. This is achieved by use of an intermediate linkage which maintains each of the Bennett linkages in any individual row in the same orientation in three dimensions.

Consequently, the present invention provides a deployable structure including a structural mechanism which may be deployed in to a state in which its profile is curved. Furthermore, in the direction along the axis of curvature, the profile of the structural mechanism is straight. As the curvature varies during deployment, in another state, which may be taken as the undeployed state, the structural mechanism may have a flat profile.

This provides a very convenient structure which may be assembled, stored and/or transported in a flat state prior to deployment into a curved state. In particular, the deployable structure in accordance with the present invention has the following advantages. As the structural mechanism consists solely of rigid links and rotational joints, it is easy to manufacture and assemble and maintain. The links may be very simple in construction and in fact regular mechanisms may consist of identical links. Similarly, it is possible to use simple joints which are easy to manufacture, because the joints simply provide for relative rotation of the links.

As the structural mechanism may be collapsed flat, it is easily packed prior to deployment. Similarly, the structural mechanism may easily be segmented for packing, due to its simple nature. In its deployed state with a curved profile, the structural mechanism provides an open interior volume on the inside of the curve. Around this internal volume, the structural mechanism may be formed as a single layer of rigid links interconnected together. This allows the structure in its deployed state to provide a frame around a relatively large interior volume. At the same time, the open nature of the structural mechanism causes the weight to be relatively low.

Also, the over-constrained nature of the structural mechanism provides a high degree of stiffness and structural strength. In particular, the structural mechanism can withstand failure of one or more of the links and/or joints.

These advantages make the deployable structure very useful for a wide range of engineering applications over a wide range of scales. The deployable structures may have a small size, limited only by the need to form a rotational joint between the link. Conversely, the structural mechanism may be formed with a much larger size, for example as a structure for a building or in an aerospace application.

A particularly advantageous use of the present invention is as the frame for a tent. In this case, the structural mechanism is used to support flexible material. In use as a tent, the ease and speed of deployment is a particular advantage.

Preferred embodiments of the present invention will now be described by way of non-limitative example with reference to the accompanying drawings.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Deployable structures which constitute the preferred embodiments are constructed as follows. The deployable structures include a structural mechanism consisting of a plurality of rigid links1connected together by rotational joints2, for example the alternative joints2illustrated in cross-section inFIGS. 1A and 1B, respectively.

The rigid links1may have any structural form. By “rigid” it is meant that the links are sufficiently rigid to maintain the single degree of mobility of the structural mechanism. In fact, in any give application, the links1will inevitably have some degree of flexibility. The links1may be simple cylindrical bars as illustrated in FIG.1. However, the links may have any cross-section. In the structural mechanisms described below, the links1are straight, but in principle they may equally be curved.

The joints2connecting the links1provide for relative rotation of the two links1connected by the joint2about an axis of rotation6. In the examples ofFIGS. 1A and 1B, the joints2are formed by a pin3extending through bores4in each of the connected links1. Thus, each link1is constrained to rotate around the pin3. The pin3has enlarged heads5at both ends to retain the pin3in place. For example, the enlarged heads5may be formed by rivetting. In the joint2ofFIG. 1A, the links1are arranged side-by-side, that is they are not co-axial. In the joint2ofFIG. 1B, the links1are arranged to be co-axial by means of one of the links1having a pair of arms7arranged on either side of the other link1, so that the arms7, together with the end of the link1from which they protrude, extend around the end of the other link1. In this case, the bore4is formed in the arms7.

In the examples ofFIGS. 1A and 1B, the axis of rotation6is perpendicular to the links1which are themselves straight, but in general this is not essential, as will be described in more detail below.

The particular forms of the joint2shown inFIGS. 1A and 1Bare merely given by way of example and in principle any joint which provides for relative rotation of the links1may be used. For example, the joint could have a more complicated structure than the joint2ofFIG. 1or could be formed integrally with both the links1.

The structural mechanism consists of a plurality of rigid links1connected by joints2to form an array of interconnected Bennett linkages. To assist in understanding, there will first be described a single Bennett linkage10which is illustrated schematically in perspective view in FIG.2. The Bennett linkage10comprises four links1connected in a loop by joints2. The positions of the joints2are labelled A, B, C, D for ease of reference.

Each joint2connects two links1and provides for relative rotation of those two links1about an axis of rotation6. Considering any individual link1, the axes of rotation6of the joints connecting that link1are skewed relative to each other around an imaginary line perpendicular to the axes of rotation6of both joints1. In the Bennett linkage10ofFIG. 2, that imaginary line is along the links1because each link1extends perpendicular to the axes of rotation6of the two joints2connecting the particular link1(although this is not essential, as described below). The skew of the joints2connected to a given link may be defined by reference to a skew angle for that link1. Herein, the skew angles are measured progressing in the same sense around this Bennett linkage10. Thus, the skew angles for the four links1are illustrated inFIG. 2by the angles αAB, αBC, αCDand αDA, resptively.

The axes of rotation6of the two joints2connecting each respective link1are non-parallel and non-intersecting. That being said, if the axes of rotation6of each of the joints2were parallel, then the linkage would be a plain parallelogram, or a plain-crossed isogram, with a single degree of freedom. Such arrangements may be considered as a special case of a Bennett linkage.

Another point to bear in mind is that since the axes of rotation6do not have a direction, then a link1having a skew angle of θ has joints2with identical axes of rotation6to a link1having a skew angle of (180°−θ). Herein, skew angles will be defined by an angle between 0 and 180°, whereby the sine of the angle will always take a positive value.

The mobility conditions for the Bennett linkage10are as follows. Firstly, the length of opposite links1must be equal, i.e.
{overscore (AB)}={overscore (DC)}=a(1)
{overscore (BC)}={overscore (AD)}=b(2)

Thirdly, the lengths and skew angles of the links1satisfy the following formula, taking the length a of a first pair of opposite links1, the length b of the second pair of opposite links1, the skew angle α for the first pair of opposite links1, and the skew angle β for the second pair of opposite links1:sin⁢⁢αa=sin⁢⁢βb(5)

In general, the length a of the first pair of opposite links1can take any value with respect to the length b of the second pair of opposite links1, provided this condition is met.

However, it is preferable for structural mechanisms in accordance with the present invention for the length of all the links1of each Bennett linkage10to be identical. In this case, the sum of the skew angle α and β for the two pairs of opposite links1is 180°. Incidentally, this is equivalent to the skew angles α and β for the two opposite pairs of bars being of the same magnitude, but opposite signs, using the alternative notation that the skew angles are between −90°and 90°. In other words, in this case, the axes of rotation6of each pair of opposite joints2(i.e. the pair of joints2at positions A and C, or the pair of joints2at positions B and D) always lie in a common plane.

Providing these mobility conditions are met, the linkage can move by the pairs of connected links1simultaneously rotating relative to each other. If the conditions for mobility are not met, then the Bennett linkage10is constrained from moving.

The Bennett linkage moves in three dimensions. Denoting the deployment angles at a joint2, that is the angles between the connected links1(or, in the general case, the angle between the imaginary lines perpendicular to the axis of rotation6of the joint2in question and the axes of rotation6of the adjacent joints2) at each of the positions A, B, C and D as θA, θB, θCand θD, respectively, the following equations hold:
θA=θC=γ  (6)

The movement will now be described with reference toFIGS. 3to6.FIGS. 3 and 4are views of the Bennett linkage10in a flat state andFIGS. 5 and 6which are views of the Bennett linkage10moved into a bent state.FIGS. 3 and 5are similar views of the Bennett linkage10taken from the side along the direction in which two of the joints2at positions A and C are aligned, whereasFIGS. 4 and 6are views of the Bennett linkage10from above.FIGS. 3to6illustrate the case that the Bennett linkage10is equilateral, but fundamentally the same motion is observed if the two pairs of opposite links1are of different lengths.

In the flat state ofFIGS. 3 and 4, the deployment angle γ at the joints2at positions A and C approaches 0°. Thus, the deployment angle φ at the other joint2at positions D and B approaches 180°. In this case, the joints2at positions A and C are close to each other, and the pair of links1connected at position B lie adjacent one another, as do the pair of links1connected at position D. On movement of the Bennett linkage10, the four joints move in the directions illustrated by the arrows X in FIG.2. The deployment angle φ increases whilst the deployment angle γ decreases. Simultaneously, the joints2at positions A and C move away from each other and the joints2at positions B and D move toward each other. The pair of links1connected at position B rotate relative to the pair of links1connected at position D around an imaginary line through the two opposed joints2at positions A and C which connect those two pairs of links1. This relative rotation may be seen clearly by comparing FIG.3and FIG.5. This relative rotation is important in that it is used to generate curvature in the structural mechanism as a whole, as will be described in more detail below.

A feature of all the structural mechanisms described herein is that each link1is straight and perpendicular to the axes of rotation6of the joints2connecting that link1. Thus, each joint2is formed in each respective link1at the intersection of the axis of rotation6of the joint2with an imaginary line perpendicular to the axis of rotation6of that joint2and to the axes of rotation6of adjacent joints2. In this case, the length of the links1between the joints is the shortest distance between the two axes of rotation6. However, this is not essential. The present invention applies equally to Bennett linkages in which the links1do not extend perpendicularly to the axes of rotation6of the joints2connecting that link1.

As a first alternative, the joints2may be formed in each respective link1at the same position relative to the imaginary lines perpendicular to the axes of rotation6, but with the links1being curved in between.

As a second alternative, the joints2may be offset from the intersection of the axis of rotation6of the joint2with the imaginary lines perpendicular to the axis of rotation6of the respective joint2and the axes of rotation6of the adjacent joints2in the Bennett linkage10. In this case, the actual distance between the two joints2connected to a given link1is greater than the shortest distance between the two axes of rotation6of the joints2. However, it can be shown that the Bennett linkage10remains mobile provided that the length of the imaginary line perpendicular to the axes of rotation6of the two joints2connecting any link1remains the same. In other words, the Bennett linkage10remains mobile provided the above mobility conditions are met, taking the length of a link to be the length between the joints2connecting that link1resolved along the imaginary line. This type of Bennett linkage is in itself known and has the advantage that it can provide for more compact packing of the links1. For example this type of Bennett linkage is described in Chen and You, “Deployable Structural Element Based On Bennett Linkages”, 2001 American Society of Mechanical Engineers International Congress, 11-15 Nov. 2001, New York, U.S.A.

There will now be described structural mechanisms which include a plurality of links connected together to form an array of interconnected Bennett linkages. Each of the Bennett linkages in the structural mechanisms described below is a Bennett linkage10as described above with reference to FIG.2. In particular, each of the Bennett linkages meets the mobility conditions described above. However, in the following description, the various Bennett linkages are given different reference numerals for ease of identification.

Some of the figures referred to in the following description are schematic. In the schematic figures, although the structural mechanism can be moved in to a state in which its profile is curved around an axis of curvature, the structural mechanisms are illustrated in plan view developed onto a flat plane. This view most clearly shows the nature of the interconnection between the various Bennett linkages. Also in the schematic figures, the links1are illustrated by the solid lines where the joints2are not themselves illustrated, a joint2is in fact present at every position where links1meet or cross one another.

FIG. 7is a schematic view of a portion of a first structural mechanism comprising an array of interconnected Bennett linkages20.FIG. 7illustrates only a portion of the structural mechanism including four complete Bennett linkages20which are interconnected together. Around those four interconnected Bennett linkages20, parts of eight further Bennett linkages20are illustrated to show how the structural mechanism may be developed further. In fact, the structural mechanism ofFIG. 7may be repeated indefinitely in both horizontal and vertical directions to produce a structural mechanism of any desired size.

The Bennett linkages20are the larger rectangles (or parts of rectangles) shown in FIG.7. The Bennett linkages20overlap to form smaller rectangles, which are in fact further Bennett linkages or plain parallelogram linkages, as will be described further below. Where the interconnected Bennett linkages20cross one another, joints2connect the links1of the interconnected Bennett linkages20. All the joints2formed in any link1of any interconnected Bennett linkage20have axes of rotation6which are perpendicular to a common imaginary line.

All the interconnected Bennett linkages20are arranged in a common orientation.

The various interconnected Bennett linkages20may in general be of different sizes. However, for each of the interconnected Bennett linkages20the ratio of the lengths of the first and second pairs of opposite links, in corresponding positions in each Bennett linkage20, is the same for each of the interconnected Bennett linkages20and the skew angles of each of the interconnected Bennett linkages20is the same for the links in corresponding positions.

The interconnected Bennett linkages20are arranged in a series of rows22. Along each row, the interconnected Bennett linkages20are aligned along the same direction as shown by the dotted lines, which directions are parallel for each row22. Hereinafter, this direction will be referred to as the major direction and will be illustrated by the arrow M. The direction perpendicular to the major direction will be referred to as the minor direction and will be illustrated by the arrow N.

Along the major direction M, the Bennett linkages20are interconnected. The nature of the interconnection is illustrated inFIG. 8which illustrates just two of the interconnected Bennett linkages20for clarity. The two interconnected Bennett linkages20are connected by an intermediate Bennett linkage23. Two connected links24of one of the interconnected Bennett linkages20are each connected by a respective rotational joint25to a respective one of two connected links26of the other of the Bennett linkages20. As a result, the two connected links24,26of each of the interconnected Bennett linkages20are connected in a loop to form the intermediate Bennett linkage23.

The intermediate Bennett linkage23is mobile with both the interconnected Bennett linkages20. To achieve such an interconnection, the intermediate linkage23, as well as meeting the general mobility conditions for a Bennett linkage described above, satisfies the condition that the ratio of the lengths of the first and second pairs of opposite links of the intermediate Bennett linkage23are the same as the ratio for the interconnected Bennett linkages20. In general, to maintain mobility with the interconnected Bennett linkages20, the intermediate Bennett linkage23could have any skew angle α and β for the first and second opposite pairs of links. However, in accordance with the present invention, the intermediate Bennett linkage23is arranged to maintain the two interconnected Bennett linkages20in a straight line. That is to say, the links of the two interconnected Bennett linkages20at corresponding positions are maintained parallel to one another. This is achieved by the intermediate Bennett linkage23having skew angles which are equal to the skew angles of the corresponding links of the interconnected Bennett linkages20.

As a result of the interconnection of the Bennett linkages20in the rows22, the entire structural mechanism is constrained to remain straight in the major direction M during movement of the structural mechanism.

In the minor direction N, the series of rows22of Bennett linkages20are interconnected. The nature of the interconnection is illustrated inFIG. 9which shows just two interconnected Bennett linkages20for clarity. In a similar manner to the interconnection along the major direction21, the two interconnected Bennett linkages20are connected along the minor direction N by an intermediate linkage27. Two connected links28of one of the interconnected Bennett linkages20are each connected by a respective rotational joint29to a respective one of two connected links30of the other of the interconnected Bennett linkages20. Thus, the two connected links28,30of both of the interconnected linkages20are connected in a loop to form an intermediate linkage27.

The intermediate linkage27is mobile with both the interconnected Bennett linkages20. To achieve this, there is a condition on the intermediate linkage27that the ratio of the length of the first and second pairs of opposite links is the same for the intermediate linkage27as for both of the interconnected Bennett linkages20.

The intermediate linkage27which connects Bennett linkages20along the minor direction N has an important difference from the intermediate Bennett linkage23which connects Bennett linkages20along the major direction M. In particular, the intermediate linkage27is arranged to cause the profile of the structural mechanism along the minor direction N to curve. This is achieved as follows.

It is described above how selection of the skew angles of the intermediate Bennett linkage23in the major direction M equal to the skew angles of the interconnected Bennett linkages20causes the Bennett linkages20connected along the major direction M to remain in a straight line. If the intermediate Bennett linkage27along the minor direction N has skew angles with any other value then this will cause the profile of the structural mechanism to have a varying curvature along the minor direction N during movement.

In particular, the arrangement of the intermediate linkage27generates curvature in the profile of the structural mechanism as follows. It is described above with reference toFIGS. 3to6how movement of a Bennett linkage causes rotation of two opposed pairs of connected links relative to one another by an imaginary line through the opposed joints connecting those two opposed pairs of links. In the structural mechanism ofFIG. 7this imaginary line corresponds to the major direction M. Thus, the relative rotation of the opposed pairs of links in the Bennett linkages20of each row22occurs in the same sense. The intermediate linkage27is arranged to cause that relative rotation within the Bennett linkages20of each row22to be additive as one progresses along the minor direction N. This causes the profile of the structural mechanism to curve.

The curvature is a property of the overall profile of the structural mechanism. As the individual links1are rigid, they cannot themselves bend. However, the manner in which they are interconnected causes curvature in the overall profile and causes the curvature to vary during movement of the links1. Thus, if one considers a median line along the minor direction N at a median position with respect to the locally positioned links1, so that the individual links are inclined inwardly and outwardly to give the overall structural mechanism a multi-faceted configuration, then it is that median line which curves.

In general, the intermediate Bennett linkage27along the minor direction N may have any skew angle which is not equal to the corresponding skew angle of the interconnected Bennett linkages20. Some particular skew angles for the intermediate linkage27along the minor direction N will now be described.

A first possibility is for the skew angles of the intermediate linkage27to be 0°. That is to say all the joints of the intermediate linkage27have parallel axes of rotation, whereby the intermediate linkage27is a plane parallelogram. In this case, the two interconnected links28and30of the two interconnected Bennett linkages20all remain in a common plane. Thus, the intermediate linkage27serves to interconnect the Bennett linkages20and to constrain them to have a single degree of mobility together without introducing any curvature along the minor direction N. The curvature along the minor direction N is introduced solely by the relative rotation between the opposed pairs of connected links of each interconnected Bennett linkage20. No additional curvature is introduced by the intermediate linkage27.

To illustrate the varying curvature during movement of the structural mechanism, movement of the portion of the structural mechanism ofFIG. 9is illustrated inFIGS. 10to13. These correspond with the views ofFIGS. 3to6, except that instead of showing the single Bennett linkage10, two interconnected Bennett linkages20and the intermediate linkage27with a skew angle of 0° are shown. As shown inFIGS. 10 and 11, in the flat state, the interconnected Bennett linkages are both substantially aligned along a straight line, thereby giving the structural mechanism a flat profile. As shown inFIGS. 12 and 13, as the structural mechanism moves, the relative rotation between the opposed pairs of connected links of both the interconnected Bennett linkages20occurs in the same sense. Thus, the intermediate linkage27causes the relative rotation to combine additively along the direction N causing the profile of the structural mechanism to increase in curvature.

A second possibility is for the skew angles of the intermediate linkage27to be 180° minus the corresponding skew angles of the interconnected Bennett linkages20. In this case, the intermediate linkage27is a Bennett linkage. It therefore meets the mobility conditions for a Bennett linkage as set out above. The intermediate Bennett linkage27serves to interconnect the Bennett linkages20and to constrain them to have a single degree of mobility together. Curvature along the minor direction N is introduced by the relative rotation between the opposed pairs of connected links of each interconnected Bennett linkage20. Additional curvature along the minor direction N is introduced by the intermediate Bennett linkage27. This results from the skew angles of the intermediate Bennett linkages27. On movement of the structural mechanism, this skew angle causes relative rotation, within the intermediate Bennett linkage27, of the pair of connected links28of the one Bennett linkage20relative to the pair of connected links30of the other Bennett linkage20around the major direction M. This relative rotation in the intermediate Bennett linkage27is in the same sense as the relative rotation of the opposed pairs of connected links within each interconnected Bennett linkage20.

Thus, the second possibility introduces additional curvature along the minor direction N as compared to the first possibility for the intermediate linkage27of having skew angles of 0°.

To illustrate the varying curvature during movement of the structural mechanism having the second type of intermediate linkage27, movement of the portion of the structural mechanism ofFIG. 9is illustrated inFIGS. 14to17. These correspond with the views ofFIGS. 3to6, except that in place of showing the single Bennett linkage10, the two interconnected Bennett linkages20are shown. As shown inFIGS. 14 and 15, in a first state, the interconnected Bennett linkages20are both substantially aligned along a straight line, thereby giving the structural mechanism a flat profile. As shown inFIGS. 12 and 13, as the structural mechanism moves, the relative rotation between the opposed pairs of connected links of both the interconnected Bennett linkages20curve in the same sense. In addition, it can be seen that the movement of the intermediate linkage27causes relative rotation of the pair of connected links28of the one Bennett linkage20with respect to the pair of connected links30of the other Bennett linkage20. Thus, the intermediate Bennett linkage27causes the relative rotation within both the interconnected Bennett linkages20and the intermediate Bennett linkages27to combine additively along the direction N causing the profile of the structural mechanism to increase in curvature.

Thus, the advantage of the second form of intermediate Bennett linkage27is that it increases the degree of curvature of the profile of the structural mechanism along the minor direction N. On the other hand, it does result in the ends of the Bennett linkages20forming the intermediate linkage27protruding outwardly from the rest of the interconnected Bennett linkages20, as can be seen in FIG.17. Such protrusion can create difficulties in some applications, for example when it is desired to cover the structural mechanism with a flexible material. Therefore, the first form of the intermediate linkage27has the advantage of avoiding any such protrusion of the links28and30forming the intermediate linkage27.

In contrast to the structural mechanism illustrated inFIG. 7in which plural Bennett linkages20are arranged along each row22in the major direction M, a structural mechanism in accordance with the present invention may be formed with only a single Bennett linkage20in the major direction M. In such a case, the structural mechanism comprises a series of Bennett linkages along the minor direction N.FIG. 9may be considered as a schematic diagram of such a structural mechanism comprising two Bennett linkages. In general, the structural mechanism could have any number of Bennett linkages20interconnected along the minor direction N.

In the structural mechanisms described above, the interconnected Bennett linkages20are interconnected by intermediate linkages23and27formed inside the Bennet linkages20by overlapping those interconnected Bennett linkages20. However, this is merely one possible form for the intermediate linkages23and27. In general, many other forms of intermediate linkage along both the minor direction N and the major direction M are possible. For example the interconnected Bennett linkages20do not need to overlap. Different forms of intermediate linkage may be mixed.

One simple alternative is to form an intermediate linkage by two connected links of both the interconnected Bennett linkages20by respective rotational joints in the same manner as the intermediate linkages23and27illustrated inFIGS. 8 and 9, but instead extending the link of the interconnected Bennett linkages20outside the interconnected Bennett linkages20as a result, the intermediate linkage is itself formed outside the interconnected Bennett linkages20.FIGS. 18 and 19illustrate such intermediate linkages31and32, respectively, outside the interconnected Bennett linkages20along the major direction M and the minor direction N, respectively. InFIG. 18the intermediate Bennett linkage is labelled31and inFIG. 19the intermediate linkage is labelled32. The above comments made about the intermediate linkages23and27apply equally to the intermediate linkages31and32formed outside the interconnected Bennett linkages30, except that the skew angles of the intermediate linkages31and32formed outside the interconnected Bennett linkages20are 180° minus the skew angles of the intermediate Bennett linkages23and27, respectively, formed inside the interconnected Bennett linkages20. Thus, in the case of the intermediate Bennett linkage31formed along the major direction M, the skew angles which are equal to 180° minus the corresponding skew angles of the interconnected Bennett linkages20. Similarly, the intermediate Bennett linkage32along the minor direction N has skew angles which are, in general, not equal to 180° minus the corresponding skew angles of the interconnected Bennett linkages20.

Another simple form for the intermediate linkage along the minor direction N is for two connected links of one of the interconnected Bennett linkages to also constitute connected links of the other of the interconnected Bennett linkages, whereby the joint connected to those connected links is common to both the interconnected Bennett linkages.FIG. 19may be considered as an example of this if one considers the intermediate Bennett linkage32to be instead one of the interconnected Bennett linkages, so thatFIG. 19consists of three interconnected Bennett linkages20,32,20.

Other, more complicated intermediate linkages are possible. For example,FIG. 20illustrates an intermediate linkage33interconnected to Bennett linkages20. In particular, the entire arrangement of the two interconnected Bennett linkages20and the intermediate linkage33is equivalent to the structural mechanism comprising two rows of two Bennett linkages20connected together in the same manner as illustrated inFIG. 7, but with links removed from two of the diametrically opposed Bennett linkages20, so that the remaining complete Bennett linkages20are interconnected by the remaining portions of the Bennett linkages from which links are removed.

It should also be noted, that a given structural mechanism may be described in more than one manner. For example, the structural mechanism illustrated inFIG. 7has been described as comprising the interconnected Bennett linkages20interconnected by intermediate Bennett linkages23and27. This very same structural mechanism could equally have been described in terms of the intermediate Bennett linkages27along the minor direction N, which are aligned along rows34, and the intermediate Bennett linkages23, which are aligned along the rows22, as being the “interconnected” Bennett linkages. In this case, the “intermediate” Bennett linkages are formed by the interconnected Bennett linkages20.

In the structural mechanisms described above, the skew angles of each Bennett linkage20are the same. However, in general, the skew angles for the different rows22of Bennett linkages20along the minor direction N may be the same or different, provided that it is the same for all Bennett linkages20within any single row22. The curvature of the profile of the structural mechanism along the minor direction N is therefore not necessarily circular. The curvature depends on the selected values of the skew angles and the lengths of the links1of the Bennett linkages20in each row22. In particular, the relative rotation of the opposed pairs of links1and hence the curvature in the profile of the structural mechanism caused by a given Bennett linkage, increases as the magnitude of the skew angle increases. Thus, the degree of curvature may vary as one progresses along the minor direction N.

It will be noted that the structural mechanisms described above are over-constrained. Therefore, it is possible to remove one or more links for joints while retaining a single degree of mobility for the entire structural mechanism as a whole. Structural mechanisms which are based on Bennett linkages, but which have links or joints removed whilst retaining a single degree of mobility are within the scope of the present invention. In addition, this feature provides a particular advantage that failure of individual links or joints does not in itself result in failure of the structure as a whole. Thus, structural mechanisms in accordance with the present invention are particularly reliable.

Also, the over-constrained nature of the structural mechanism means that it is not necessary to interconnect every adjacent pair of Bennett linkages20. For example,FIG. 21illustrates a structural mechanism similar to that illustrated inFIG. 7, except that between some of the rows22of Bennett linkages20along the major direction M, not all pairs of adjacent Bennett linkages20are connected along the minor direction N. For example, between the second and third rows22, only the first and fourth Bennett linkages are connected along the minor direction N.FIG. 21also illustrates how it is possible to vary the size of the individual Bennett linkages20relative to each other.

In addition to the structural mechanisms described above, deployable structures in accordance with the present invention may include additional constructional elements. These may or may not be mobile with the structural mechanism and may or may not introduce additional degrees of freedom or mobility. For example, such additional elements may be additional links connected to the structural mechanism described above by rotational joints to form an additional mechanism having a single degree of mobility with the structural mechanism described above. Alternatively, the additional structural elements may be unrelated to the structural mechanism.

One particularly desirable addition is some mechanism for limiting the movement of the structural mechanism beyond the deployed state to hold the mechanism in the deployed state. For example, such limiting means may be formed by flexible elements wires attached between opposed pairs of links1or joints2which move apart during movement from the undeployed state to the deployed state, so that the flexible elements are held in tension in the deployed state, preventing further movement. Alternatively, the structural mechanism may be held in its deployed state by entirely separate means which are attached to the structural mechanism after deployment.

FIGS. 22to24illustrate deployment of an actual structural mechanism40which has the same form as the structural mechanism illustrated in FIG.7. In particular, in the structural mechanism40ofFIGS. 22to24; each of the interconnected Bennett linkages20is equilateral and of identical size; each of the intermediate linkages23along the major direction M and each of the intermediate Bennett linkages27along the minor direction N are of the same size; and the skew angles of each Bennett linkage in the structural mechanism40are identical and approximately equal to 30° (or 120°).FIG. 22illustrates the structural mechanism40in a flat state in which its profile is straight along both the minor direction N and the major direction M. On movement of the structural mechanism30, the profile along the minor direction N curves with an increasing degree of curvature as illustrated inFIGS. 23 and 24. The profile along the major direction M remains straight. As shown inFIG. 23, the structural mechanism40is movable into a state in which it forms a generally cylindrically arch. As shown inFIG. 24, the structural mechanism40can reach a further state in which the edges of the structural mechanism40along the minor direction N meet one another to give the mechanism40a cylindrical shape. In both cases, the structural mechanism defines an internal volume around which, the structural mechanism forms an open frame as can be seen by comparingFIG. 22with either ofFIGS. 23 and 24.

The structural mechanism in its flat state illustrated inFIG. 22has a very compact form. This property makes the structural mechanism particularly useful as a deployable structure. The flat state of the structural mechanism, for example as illustrated inFIG. 22, may be taken as the undeployed state. Any of the curved states, for example those illustrated inFIGS. 23or24, or other states in between, are used as the deployed state. The structure is deployed by movement of the structural mechanism.

The structural mechanism may be used as a deployable structure in a wide range of engineering applications and may have any size. It may be very small, limited only by the ability to produce a rotational joint. Equally, the structural mechanism may be very large, for example in aerospace applications. In all such applications, the present invention provides the advantages described above.

A particularly useful application is as the frame for a tent in which the structural mechanism, in its deployed state, supports flexible material.FIG. 25illustrates such a tent formed by the structural mechanism40illustrated inFIG. 23covered by a sheet41of flexible material.

Although a tent could be formed simply by draping a sheet of flexible material over the structural mechanism, preferably a flexible material is formed from a number of panels attached together so that the flexible material conforms with the shape of the structural mechanism in its deployed state. The panels of flexible material may be attached together using techniques which are conventional for known tents, for example by sewing the panels together along a seam.

The flexible material may be attached to the frame using any type of fastening. For example, the fastening may be formed simply by ties attached to the flexible material at suitable positions for being tied around links of the structural mechanism. Alternatively, the fastenings may be formed by clips. Indeed, any type of fastening which is conventional for attaching the flexible material to the frame of a known tent may be applied.

The flexible material may be of any suitable form. For example, it may be a fabric made from natural or man-made materials. Alternatively, it may be material which is manufactured as a sheet. In general, any type of flexible material which is known for use in a tent may be applied.

The flexible material may be formed as a single piece or plural pieces. Generally, the flexible material is arranged over the structural mechanism after deployment of the structural mechanism. It would not be desirable for a single sheet of flexible material to be attached over the entire structural mechanism prior to deployment, because the dimensions of the structural mechanism along the minor direction N decrease during deployment of the structural mechanism. Therefore, if the material were attached at two positions separated along the minor direction N in the flat state, then this would result in the formation of flaps of material during deployment. To avoid this problem it is possible to attach the, or each piece of the flexible material to the structural mechanism only at positions aligned along the major direction M. When using plural pieces of flexible material, those pieces will then move together during deployment of the structural mechanism to cover it in the deployed state. Then, the individual pieces of flexible material may be attached together or to the structural mechanism using any suitable form of fastening.

The present invention may be applied to tents of any size. This includes both small tents designed to accommodate one or a few individuals, for example for camping. On the other hand, the size of the tents may extend up to very large structures capable of accommodating large numbers of people or equipment, e.g. helicopters or planes.

When the deployable structure in accordance with the present invention is used as the frame for a tent, it provides all the advantages described above for the present invention in general. For a tent, the ability to easily and rapidly deploy the structure provides particular advantage.