GB2266148A - Self erecting structure - Google Patents

Abstract

A structure (1) which unfolds to form an elongate arm on immersion in a liquid comprises a number of sealed tubular sections (2-5) which are hinged together at their ends (6-9). The sections are designed to be substantially weightless when in the liquid. The structure is so arranged that the gravity and buoyancy forces acting on each section cause rotation of the section, so that the structure unfolds automatically on entry into the liquid. The arm may be used to support, for example, one or more acoustic hydrophones of a sonobuoy. <IMAGE>

Description

Self-Erecting Structure This invention relates to a self-erecting structure which can be stowed into a compact volume but which expands automatically underwater to form a larger structure. More particularly, the invention relates to a device for supporting a horizontal array of hydrophones such as are used in radio sonobuoys for the detection and localisation of submarines.

Underwater acoustic sensors for the detection of submarines or other acoustic sources sometimes incorporate distributed arrays of multiple hydrophones in preference to a single hydrophone. If the spacing between elements is sufficiently large in relation to the acoustic wavelength detected, the use of multiple sensor elements can increase the ratio of the target signal to the background noise, and additionally, if the elements of the array are distributed horizontally, the array can be used in determining the target bearing angle. The required array size increases as the acoustic frequency of interest decreases. Acoustic sensors of this type, particularly radio sonobuoys, have to be packaged, prior to use, in a container suitable for compact storage and launch from an aircraft. Various array configurations may be required.One form of horizontal array which is currently used is a 'star' pattern comprising a number of straight arms disposed radially around a central hub.

Known array structures have employed expanding tubular telescopic arms which are extended by means of a weight whilst oriented vertically, and then hinge outwards to form the required star shape. These telescopic structures necessarily use open tubular members the weight of which, when immersed, is a significant proportion of their dry weight, depending upon the density of the materials used in their construction. The size of such structures is therefore limited by the strength and weight of the structural members. Other designs have employed telescopic or folding groups of tubes supported by an arrangement of cords, and again using gravity forces to deploy the array structure. In this case, the use of supporting cords increases the complexity of the assembly and can reduce the reliability of deployment because of the possibility of tangling or snagging of the cords.Typically, a large proportion of the size and weight of the structure is required for the array support structure and for the mechanism for deploying the structure when it is operational. Furthermore, the ability to operate at low frequency is limited by the maximum size of array structure which can be accommodated in the packaged sonobuoy.

It is an object of the present invention to provide an improved self-erecting structure.

According to the invention there is provided a structure which unfolds to form an elongate arm on immersion in a liquid, the structure comprising a plurality of interconnected tubular sections which are sealed against ingress of the liquid and which are substantially weightless when immersed in the liquid.

An embodiment of the invention will now be described, by way of example, with reference to the accompanying drawings, in which Figures 1(a) - 1(e) are schematic side views of a single array arm, showing successive steps in the unfolding of arm sections; Figure 2 illustrates, schematically, one arm of the array fully extended and showing possible positions of trim weights and hydrophones; Figure 3 is a pictorial view of a detail of two pivot joints of the arm; and Figure 4 is a schematic pictorial view of an example of a full array of three radial self-erecting neutrally-buoyant arms as might be used for a sonobuoy.

Referring to Figure 1, one arm 1 of the self-erecting structure comprises four tubular members 2, 3, 4 and 5. Initially the arm is folded compactly as shown in Figure 1(a). A particular application for such an arm is to support acoustic hydrophones, or another payload, at intervals along its length. These are omitted from Figure 1 to clarify the operation of the self-erecting structure.

Each member 2-5 of the arm 1 is constructed from a hollow tubular section, preferably using a material of low density but high stiffness, such as a carbon fibre/epoxy composite. Pivot joints 6-9, which interconnect the ends of the tubular members, are bonded into the tubes to prevent water leaking into the tube bores when immersed.

me wall thickness of the tubular members 2 to 5 is such that after making due allowance for the immersed weight of attached components, including hydrophones and any trim weights, as described below, and the pivot joints 6 to 9, the net buoyancy force on each arm is approximately zero. Because of variations of water density with temperature, salinity and depth,is not possible to ensure that the resultant buoyancy force is precisely zero, but it will be apparent from the description below that zero buoyancy is not essential for the correct operation of the invention.

Figures 1(b) to 1(e) show stages in the sequence of unfolding of the arm, and Figure 2 shows the arm in its final fully-extended position illustrating the forces acting on each arm due to its weight and buoyancy. Figure 2 also shows trim weights 10-13 attached one to each tubular member and hydrophones 14-17 as might be fitted if the self-erecting arm were part of a sonobuoy.

It may be assumed, in order to illustrate the main principle by which the arm automatically extends and subsequently remains in the extended position, that the weight of each hydrophone is equal to the weight of water which it displaces, so that the immersed weight is zero. It is also assumed that the centre of gravity and the centre of buoyancy of each tubular member of the arm, excluding the trim weight, are co-located at the geometrical centre of each member. It is further assumed that each member, when immersed, is positively buoyant (i.e. tending to float) by an upward force B and that the immersed weight of each trim weight 10-13 is precisely equal to the upward force B on the member. These assumptions are not essential to the correct functioning of the arm but help to illustrate the principle of operation.

From Figure 2 it can be seen that the trim weight 10 attached to the outer tubular member 2 is offset from the centre of the tube by a distance D1 so as to produce a moment of force of magnitude B x D1 acting in an anti-clockwise direction. In the position shown, the arm is prevented from rotating further anti-clockwise by a pivot stop in the joint 6 which prevents angular motion through more than 180 from the initial folded position. If the member 2 is rotated in a clockwise direction and released, the moment of force due to the offset trim weight will cause the tube to rotate in an anti-clockwise direction to restore its position to that shown in Figure 2.

Considering, now, the moment of force acting about the pivot joint 7 of the member 3. The trim weight 11 fitted to the member 3 is in this case offset by a distance D2, which is greater than D1, so that there is a net clockwise moment of force about that pivot equal to BX(D2-D1). This moment maintains the member 3 in the position shown against its pivot stop, and restores it to that position if it is deflected in an anti-clockwise direction. Similarly, the member 4 is maintained in position against its pivot stop by a trim weight 12 offset by a distance D3 such that D3 is greater than D2-D1, whereby an anti-clockwise moment equal to BX(D3-D2+D1) is produced about its pivot joint 8. Similarly, the member 5 is maintained in position by positioning its trim weight 13 such that the total clockwise moment about the pivot joint 9 is of positive value.

By the arrangement described above, each arm is maintained neutrally buoyant, but experiences a turning moment in the appropriate direction to extend the arm as a result of the position offset between the centre of buoyancy and the centre of gravity of the arm member It is equally possible to provide a self-erecting arm member by maintaining the centre of gravity and the centre of buoyancy of each arm member in the same position, but adjusting the magnitude of the trim weight such that the arm members are alternately positively buoyant and negatively buoyant. Equally, it is possible to adjust both the position and the magnitude of the trim weight so that the correct sense and magnitude of turning moment is achieved by a combination of net buoyancy (in water weight) and centre of gravity to centre of buoyancy offset.In the case of the innermost arm section 5, the position and the magnitude of the trim weight can be selected such that the extending force moment is substantial. In practical operation, the density of the water in which the device is immersed will vary with temperature, depth and salinity and it would not be possible to select a trim weight such that each member has precisely zero weight when immersed. This condition is not essential for the self-extending arm to operate correctly, since the dimensions and weights of the arm members can be selected such that the moments of force which initially extend the arm and thereafter maintain the arm in a fully-extended attitude are of the correct sense when the arm is immersed in water having a range of possible density values.

Similarly, the dimensions and weights of the arm members can be selected such that the self-erecting force moments are sufficient to support the immersed weight of the acoustic sensors and electrical wires connected to the acoustic sensors.

A self-erecting arm as described has been constructed using four tubular carbon fibre/epoxy members of 7mm outside diameter and having a total length of 3.3 metres and capable of supporting five hydrophones in a representative range of water densities.

The mode of operation described above, whereby the arm automatically extends itself, is not effective when the assembly as so far described is folded as shown in Figure 1(a), with the axes of the tubular members vertical. In this position, the centres of gravity and buoyancy are directly above or below the pivot axis of the respective joint and there is therefore no horizontal moment of force to initiate unfolding. It is therefore necessary to provide an initial force to rotate each tubular member 2-5 through a small angle, after which the gravity and buoyancy force moments will be sufficient to cause rotation of each tubular member and extension of the complete arm assembly as described above.Referring to Figure 3, which shows the construction of the pivot joints 6 and 8 at the upper end of the folded arm, a small compression spring 18 is located in a recess 19 in one of the two components 20,21 of each pivot joint assembly, in such a way that it bears on the other component of the pivot joint assembly, thereby producing a force which tends to initiate opening.

From the mode of operation explained above, it will be apparent that the arm, if not constrained, will tend to extend itself rather than remain in the stowed position illustrated in Figure 1(a).

It is therefore necessary to provide means to hold the arm in the stowed position prior to operation. This can be achieved, for example, by containment of the complete array assembly in a cylindrical can (not shown) which is allowed to fall away under the influence of its own weight when the array assembly has descended to its required operating depth. If the array forms part of a sonobuoy, the cylindrical can will be the outer body of the sonobuoy.

It is also necessary to provide means to control the sequence of unfolding such that the outer member 2 starts to unfold initially, followed by the second member 3, and so on, in the sequence illustrated in Figure 1. Otherwise, it would be possible for the arm to come to rest in a stable final position other than the required fully-extended state, for example with one or more tubular members 'doubled-back' towards the inner pivot. Figure 3 illustrates how this sequence of unfolding is achieved in the preferred embodiment.

A pin 22 is fixed through the component 20 of each pivot joint 6 and 8. The axis of the pin 22 is parallel to the pivot axis of the respective joint and perpendicularly intersects the longitudinal axis of the respective member a small distance out from the pivot axis.

The pin 22 is longer than the thickness of the component 20 and is positioned so that it protrudes from both sides of the component.

The pin bears on shaped cam plates 23,24 at each side of the folded arm assembly, so that the upper (outermost) end of the member 3 is held in place by the cam plates until such time as the member 2 has rotated through a predetermined angle, which is controlled by the dimensions of the cam plates. From practical experiments conducted on such an arm, it has been found that the arm assembly will unfold reliably and adopt the stable fully-extended position if the cam plates are shaped so as to release the pivot joint 6 when the tube 2 'nas rotated through an initial angle of 90 , as illustrated in Figure 1(b).At this point, the member 3 starts to rotate outwards with its lower pivot joint 7 restrained by a similar pin and cam plates (not shown) which form part of a lower pivot joint cluster. h'hen the tube 3 has rotated through an angle of 90 as shown in Figure 1(c), the lower cam plates release the pivot joint 7 so that the tube 4 can start to rotate outwards. Since a pin 22 is provided at the pivot of each tubular member 2-5, the unfolding is controlled by the upper and lower pairs of cam plates so that it follows the sequence illustrated in Figure 1.

Figure 3 also illustrates how, in the preferred embodiment, the rotation of the pivot joints between adjacent pairs of tubular members is limited to 180 . Each pivot joint comprises a clevis 20, which is the end fitting of the outer of the two tubular members, and a block 27 which is the end fitting of the inner of the two tubular members. Each block 21 has a tongue 25 protruding from one side which fits into the clevis 20. A pivot pin 26 extends through the tongue and the clevis. An end portion 27 of the block 21 extends beyond the tongue 24 such that after the outer tubular member 2 has rotated through 180 relative to the inner member 3, the edges of the clevis contad the end portion 27 of the block 21 and prevents further rotation.

In a practical utilisation of the self-erecting structure, for example as means to support an array of hydrophones, it is likely that the complete structure will comprise a plurality of the arm assemblies described above. Figure 4 illustrates a possible embodiment wherein three self-erecting arms 1,28,29 are disposed radially around a central support 30 which will be suspended from a suitable float (not shown) at the sea surface, as would be the case if the array were part of a sonobuoy.In the application illustrated, the structure so formed is employed to carry an array of hydrophones which are distributed along the arms (four hydrophones per arm). In this example, the tubular inner hub 30 supports the inner arm pivots 9 at its lower end and the upper cam plates 23, for controlling the release sequence of the upper pivot assemblies 6,8 at its upper end.

In a typical sonobuoy application, the bore of the tubular hub 30 could be used to stow other components including, possibly, the suspension cable, any required sea anchor or drogue assembly, a compass assembly and electronic units such as a signal multiplexer.

Electrical wires connecting the hydrophones to a unit in the hub could be fine copper wires with a suitable insulation and might be adhesively bonded to each tubular member 2-5, except at the tube ends where a short length of each wire would be left unattached to accommodate the rotation of the pivot joints 6-9.

The invention may therefore be used to provide a compactly stowable device which when released underwater will expand to form a large distributed structure suitable for supporting sensor elements, such as hydrophones, or for any other purpose. The structure expands automatically when released underwater without the need for any external mechanism. The structure is preferably nearly neutrally buoyant when immersed in water so that the possible size of the deployed structure is not limited by its own self weight. It can be used in a self-expanding sensor array structure which expands in a folding rather than in a telescopic manner, thereby avoiding the requirement for lengthwise expanding groups of electrical wires to the sensor elements.

In the illustrated embodiment each arm comprises four folded sections but any suitable number of sections may be used.

Claims (18)

1. A structure which unfolds to form an elongate arm on immersion in a liquid, the structure comprising a plurality of interconnected tubular sections which are sealed against ingress of the liquid and which are substantially weightless when immersed in the liquid.

2. A structure as claimed in Claim 1, comprising a support member; a first elongate buoyant arm section pivotally connected adjacent one of its ends to said support member; and a second elongate buoyant arm section pivotally connected adjacent one of its ends to the other end of said first arm section; wherein the centre of gravity and the centre of buoyancy of each arm section are spaced apart along the arm section, the relative positions of the centre of gravity and the centre of buoyancy being reversed on one of said arm sections as compared with the other of said arm sections, whereby when the arm sections are immersed in the liquid they are rotated in relatively opposite directions by a respective turning moment applied to the arm section due to the mass of the arm section and the buoyancy force acting on the arm section.

3. A structure as claimed in Claim 2, wherein each arm section includes weight means to set the position of the centre of gravity at a predetermined distance from the centre of buoyancy.

4. A structure as claimed in Claim 2 or Claim 3, wherein the distance of the centre of gravity from the centre of buoyancy for said first arm section is different from that for said second arm section.

5. A structure as claimed in any one of Claims 2-4, wherein the pivotal connection of the arm sections includes means to limit the overall rotation of the two arm sections relative to each other to substantially 1800.

6. A structure as claimed in any one of Claims 2-5, wherein the pivotal connection of the arm sections includes resilient means to provide an initial relative rotation of the arm sections.

7. A structure as claimed in any one of Claims 2-6, including a further arm section, or group of pivotally interconnected arm sections, pivotally connected to the other end of said second arm section.

8. A structure as claimed in Claim 7, wherein the relative positions of the centre of gravity and the centre of buoyancy are reversed at each alternate arm section.

9. A structure as claimed in any one of Claims 2-8, including means to ensure a predetermined sequence of rotation of the arm sections.

10. A structure as claimed in Claim 9, wherein the means to ensure the predetermined sequence of rotation comprises cam means associated with the pivotal connections.

11. A structure as claimed in any one of Claims 2-10, wherein the arm carries at least one acoustic sensor.

12. A structure as claimed in any one of Claims 2-11, wherein each arm section comprises a tubular member having closed ends.

13. A structure as claimed in any one of Claims 2-12, wherein each arm section is formed of a carbon fibre/epoxy material.

14. A structure which unfolds to form an elongate arm on immersion in a liquid, substantially as hereinbefore described with reference to Figures 1, 2 and 3 of the accompanying drawings.

15. An array comprising a plurality of structures as claimed in any preceding claim attached to a central support, whereby the arms when unfolded radiate from said support.

16. An array as claimed in Claim 15, wherein the support comprises a tube, which in use is orientated with its longitudinal axis substantially vertical.

17. An array as claimed in Claim 16, wherein the tube constitutes a storage container.

18. A hydrophone array substantially as hereinbefore described with reference to the accompanying drawings.