FR2978253A1 - ASSEMBLY COMPRISING A MIRROR AND A CARRIER STRUCTURE IN TREILLIS - Google Patents

Abstract

The invention relates to an assembly comprising: - a mirror (1), and - a structure (2) carrying the mirror (1), comprising at least one base (20) comprising: - a first plane (201), - a second plane, and - an internal link (203) located between the first plane (201) and the second plane, characterized in that the first plane (201), the second plane and the internal link (203) respectively comprise only uninterrupted beams , without intermediate piece between beams, to form a rigid structure (2) lattice.

Description

GENERAL TECHNICAL FIELD The present invention relates to an assembly comprising a mirror, and a carrier structure of the mirror itself comprising at least one base comprising a first plane, a second plane, and an internal link located between the first plane and the second plane. STATE OF THE ART Lightened mirrors of large size and optical quality are known for applications at terrestrial telescopes or possibly on artificial satellites or airplanes or inflatable balloons. For these applications, there is known either an assembly comprising the mirror, for example a glass mirror, and a carrier structure of the mirror, the structure generally comprising a base comprising two parallel planes between which is located an internal link; or an assembly comprising the mirror, for example a silicon carbide mirror and ribs perpendicular to the mirror. However, on the one hand the two parallel planes of the base are generally solid and machined plates, and are therefore relatively heavy to support mirrors of more than 3 m in diameter. On the other hand, it is difficult to manufacture and machine silicon-carbide optical quality mirrors more than 1.5 m in diameter. PRESENTATION OF THE INVENTION The invention proposes to overcome at least one of these disadvantages. For this purpose, there is provided according to the invention a set comprising a mirror, and a carrier structure of the mirror, comprising at least one base 30 having a first plane, a second plane, and an internal link located between the first plane and the second. plane, characterized in that the first plane, the second plane and the internal link respectively comprise only uninterrupted beams, without intermediate piece between beams, to form a rigid lattice structure. The invention is advantageously completed by the following features, taken alone or in any of their technically possible combination: the beams comprise longitudinal fibers, and are preferably pultruded elements; the structure comprises a support plane of rods integral with the mirror, the support plane comprising only uninterrupted beams, without intermediate piece between beams, and being parallel to the first plane and the second plane; - The rod support plane is confused with the second plane; the rod support plane is located between the first plane and the second plane, each rod then being linked in the foreground also by a hinge; the assembly comprises a stiffening connection between the rod support plane and the second plane; the mirror is made of a first material, preferably glass, of thermal expansion coefficient a1, the first plane, the second plane, and the intermediate space are made of a second material, preferably a composite material, with a coefficient of thermal expansion. a2, the rod support plane is in a third material, preferably a composite material, of thermal expansion coefficient a3, with h2 = h1 (a2-a1) -1 (a3-a2) where h2 is the height between the foreground (201) and the rod support plane (208) (205); and h1 is the height between the first plane (201) and the mirror (1); - The assembly further comprises blockers adapted to block the rods relative to the rod support plane, when the mirror has a desired shape; the mirror comprises local machining operations on a rear face; and the lattice structure is of tetrahedral pattern.

The invention has many advantages. The invention makes it possible to provide a carrier structure of the mirror, which is rigid and light. The structure is indeed rigid because the constituent elements of the base of the carrier structure of the mirror comprise only uninterrupted beams, without intermediate piece between the beams. There is no articulation between the beams, and therefore no drop in rigidity. Advantageously, the beams comprise longitudinal fibers and therefore have a high longitudinal rigidity, and are for example pultruded. The structure is light because it is lattice-shaped and has no plates. The invention makes it possible to make mirrors of great dimension 15 ultra-light, that is to say having a diameter greater than 2 meters for total mass per unit area less than 25 kg / m2. The assembly according to the invention makes it possible to use a glass mirror, which makes it possible to use the conventional manufacturing and polishing processes. Conventional polishing processes have almost no limitations related to the size or optical quality of the mirror (related to the surface roughness of the mirror) attainable. It is thus possible to manufacture mirrors having a diameter ranging for example beyond 8 meters. The assembly according to the invention makes it possible to use a material with a very low coefficient of thermal expansion on the one hand for the mirror (of the order of 25 10- $), but also on the other hand for the structure: It is indeed possible to optimize the coefficient of thermal expansion of the beams to obtain a very low coefficient of thermal expansion (for example less than 10-6), to avoid deformation of the assembly in case of significant external heat flow. The carrier structure of the mirror also makes it possible to carry out retouches of the frozen form of the mirror by ionic machining, that is to say without contact.

PRESENTATION OF THE FIGURES Other features, objects and advantages of the invention will emerge from the description which follows, which is purely illustrative and nonlimiting, and which should be read with reference to the appended drawings, in which: FIG. sectional view of a possible embodiment of an assembly according to the invention; - Figure 2 schematically shows a sectional view of a second possible embodiment of an assembly according to the invention; Figure 3 is a perspective view of a third possible embodiment of the invention; FIG. 4 is a detailed view of section IV-IV shown in FIG. 3, and FIG. 5 is a sectional view of a mirror according to the invention. In all of the figures, similar elements bear identical numerical references. DETAILED DESCRIPTION The figures show schematically possible embodiments of an assembly according to the invention. The assembly mainly comprises a mirror 1 of optical quality and of large size, and a structure 2 carrying the mirror 1. The mirror 1 comprises a front face 11 and a rear face 12. It is of optical quality, that is, that is to say that the front face 11 is polished with an optical precision, a tolerance of the order of a few tens of nanometers on the roughness of the surface (the roughness is about 25 of the order of 2 nm on a lower spatial scale at 1 mm). The rear face 12 is also polished to avoid internal stresses in the mirror 1, but without necessarily having an optical precision polishing. Alternatively, it may have undergone an acid treatment which eliminates microcracks and stresses. Mirror 1 preferably but not limited to applications to a terrestrial telescope or onboard an artificial satellite, an airplane or an inflatable balloon.

In the figures, the mirror 1 is represented in the form of a plane mirror, but the mirror 1 can also be curved according to the desired applications. The structure 2 comprises a base 20 itself comprising a first plane 201, a second plane 202 and an internal link 203 located between the first plane 201 and the second plane 202, and connecting the first plane 201 and the second plane 202, for imparting rigidity to the base 20. The first plane 201 faces the rear face 12 of the mirror 1. The second plane 202 is parallel to the first plane 201.

The first plane 201, the second plane 202 and the internal link 203 respectively comprise only beams 204, to also give lightness to the base 20, in addition to its rigidity. The structure 2 is in the form of rigid mesh. The structure 2 may have any lattice pattern, for example cubic or star, but is preferably tetrahedral. The pattern can be regular or irregular: the mesh of the pattern can thus be taller than wide to optimize the rigidity of the structure depending on the intended applications. Conventionally, each beam 204 is an element of the structure intended to stiffen it, generally with a small cross section relative to its longitudinal length. The beams 204 may have any cross-section, and be hollow (for example with a straight cross section in the form of a ring, the beam 204 may thus be described as a tube), or - solid (for example with a profiled cross section). external circular or rectangular, the beam 204 can be described as rod).

The beams 204 of the first plane 201, second plane 202, and the inner link 203 are uninterrupted, that is to say that they extend along their length without intermediate part between beams. Indeed, the establishment of beams connected to each other by intermediate parts, forming joints in the first plane 201, the second plane 202 or the third plane 203, would be detrimental to the rigidity of the structure 2 and its thermal stability, due to the expansion of glue joints for example. Each beam 204 thus extends respectively in the first plane 201, the second plane 202 or the internal link 203 at its maximum length to form a desired structure geometry 2, limited by the periphery 2011 of the first plane 201 or the periphery 2021. of the second plane 202, without being interrupted by an intermediate piece. For example, at the periphery 2011 or 2021, each beam 204 extends continuously until a change of direction, without being interrupted by an intermediate part. Similarly, the only junctions 211 between beams 204 are preferably carried out by gluing, without an intermediate part. In addition, to avoid the presence of intermediate parts between beams, the beams 204 are offset relative to each other in space to avoid any intersection between beams. As shown in Figure 4 for example, the beams 204 may however be in contact with each other. The assembly example of FIG. 4 is nonlimiting, and other assemblies of the beams 204 with each other in the structure are also possible.

To fix the ideas, the length of beams necessary to realize a mirror of 2m of diameter is approximately 300m.

The structure 2 comprises a rod support plane 208 205. The rods 205 thus provide the connection between the mirror 1 and the base 20 of the structure 2. For this purpose, the rods 205 are integral with the mirror 1, and the rods 205 extend between the support plane 208 and the rear face 12 of the mirror 1. In order to maintain optimum lightness, the support plane 208 comprises only uninterrupted beams 204, as well as the foreground 201, the second plane 202 and the internal link 203. According to a first embodiment shown in FIG. 1, the plane 208 coincides with the second plane 202.

The assembly also comprises blockers 210 adapted to block the rods 205 relative to the support plane 208 when the mirror has a desired shape. The mirror 1 has indeed a fixed form, that is to say that its shape can not be modified point by point by deformation of the mirror during its operation. On the contrary, once the desired shape obtained for the mirror during its manufacture, its shape is frozen throughout its lifetime. Indeed, during the manufacture of the assembly, the deformable surface of the mirror 1 is deformed by acting on the respective length of the plurality of rods 205 relative to each other and relative to the plane 208, the gap between the modified form obtained and the desired theoretical shape of the mirror 1 is then controlled, and said mirror surface 1 is finally frozen by the blockers 210 when the theoretical shape is reached with a precision less than a given precision threshold.

The carrier structure 2 of the mirror 1 also makes it possible to carry out retouches of the frozen form of the mirror by ionic machining, that is to say without contact. However, it will be understood that the fact of being able to mechanically adjust the shape of the front face 11 of the mirror 1 makes it possible, starting from an initial polishing whose accuracy is relaxed, to carry out only small retouches by ionic machining (ie without retouching by contact process) after assembly with the lattice structure 2. The prior mechanical adjustment minimizes the amplitude of necessary retouching, which is very slow to perform. The separation of the structure from the optical side allows this advantage.

The blockers 210 may be formed by gluing, or by any other fastening means.

The mirror 1 is made of a first material, of thermal expansion coefficient a1.

The first plane 201, the second plane 202 and the internal link 203 are in a second material, thermal expansion coefficient a2. In general, this is preferably a 2 to 2 in order to minimize the bending requirements of the rods 205 during thermal expansions. The rods 205 can thus be chosen a little flexible, but with sufficient residual rigidity so that the mirror 1 does not have a too low mode of vibration in the plane. The first material is preferably optical quality glass, for example of the zerodur brand. The glass can be manufactured and polished to optical precision by methods known to those skilled in the art.

The second material is a material with high rigidity, for example - a material comprising longitudinal fibers, advantageously a very high fiber content in the longitudinal axis of the beams, for example a composite material, for example glass fiber type or very preferably carbon, high modulus or ultra high modulus, in a resin for example epoxy, polyester, or cyanate ester (low coefficient of water expansion), or - a silicon carbide for example. The longitudinal rigidity of the material of the beams 204 is for example greater than 100 GPa, advantageously greater than 150 GPa. For example, but not exclusively, the composite material makes it possible to give the beams 204 a longitudinal Young's modulus greater than 150 GPa, but for example very advantageously greater than 400 GPa, preferably greater than 500 GPa and very preferably greater than 600 GPa. Moreover, the carbon fibers also make it possible to have the relation al ~ a2.

The longitudinal fibers of the material of the beams 204 give them a high rigidity (stiffness) longitudinal. Beams having longitudinal fibers can be produced by any manufacturing method. For example, but without limitation, in order to reinforce the rigidity of the structure 2, the beams 204 are pultruded elements, that is to say elements produced by pultrusion, namely a known technique of forcing, through a die, strands of continuous fibers, for example glass or preferably carbon, previously impregnated with resin, for example epoxy, polyester, or cyanate ester, and to obtain a high unidirectional stiffness in the longitudinal axis. The use of pultrusion makes it possible to exploit optimally the material's rigidity potential: the choice of the right type of fibers and the resin forming the matrix makes it possible to obtain a very high longitudinal rigidity. Pultrusion is also expensive. The rods 205 may for example be also but not limited to composite material.

To further avoid the thermal stresses on the mirror 1, the rods 205 are advantageously secured to the mirror 1 by means of pads 13 in the same material as the mirror 1. The rods 205 and the pads 13 are for example glued together and on the mirror 1.

As shown by the arrows 3 of Figure 1, the assembly allows a thermal decoupling between the mirror 1 and the structure 2, by bending rods 205. The mirror 1 can move slightly in its plane.

However, it may be desirable not to allow this movement of the mirror 1 25 in its plane, to avoid the natural modes of vibration of the mirror for example. Thus, according to a second possible embodiment shown in FIG. 2, the rod support plane 208 is situated between the first plane 201 and the second plane 202.

In this case, each rod 205 is also connected to the first plane 201 by a flexible hinge 209, for example by flexing the beams 204 of the first plane 201 and / or the second plane 202 and / or the internal link 203, the rods 205 being then no longer flexible in the embodiment of Figure 2. The hinge 209 may be performed for example by a cooperation of the rod 205 with a drilling of a beam 204 of the first plane 201, or by mechanical support of the rod 205 on the beams 204 of the first plane 201 Similarly, the structure 2 comprises a stiffening connection 206 between the rod support plane 208 and 205 the second plane 202. In this case, to compensate for the bending of the beams 204 from the first plane 201 and / or the second plane 202 and / or the internal link 203, the rod support plane 208 is made of a third material, different from the second material, and of thermal expansion coefficient "3, and the height h2 between the first plane 201 and the support plane 208 of rods 205; and the height h1 between the first plane 201 and the mirror 1 are linked by the relation: h2 = h1 (a2-a1) -1. (a3-a2) Thus, in the embodiment of FIG. 2, there is no lateral stress at the rods 205 under the mirror 1, and therefore no deformation of the mirror 1. The structure of FIG. 2 also benefits from high rigidity, reinforced in the planes 201 and 202, thanks to the link 206. As shown in FIG. 5, the rear face 12 of the mirror 1 can comprise local machining operations 207, in order to optimize the local rigidity of the mirror. As shown in Figure 3, the structure 2 may comprise more than one base 20, to further increase the rigidity of the structure 2 lattice, the bases 20 then being superimposed on each other. 30

Claims (10)

REVENDICATIONS1. An assembly comprising: - a mirror (1), and - a structure (2) carrying the mirror (1), comprising at least one base (20) comprising: a first plane (201), a second plane (202), and an internal link (203) situated between the first plane (201) and the second plane (202), characterized in that the first plane (201), the second plane (202) and the internal link (203) respectively comprise only beams (204) uninterrupted, without intermediate piece between beams, to form a structure (2) rigid lattice. 15 2. The assembly of claim 1, wherein the beams (204) comprise longitudinal fibers, and are preferably pultruded elements. 20 3. An assembly according to one of claims 1 or 2, wherein the structure (2) comprises a plane (208) of support rods (205) integral with the mirror (1), the plane (208) of support - comprising only beams (204) uninterrupted, without intermediate part between beams, and 25 - being parallel to the first plane (201) and the second plane (202). 4. The assembly of claim 3, wherein the plane (208) of rod support (205) is coincident with the second plane (202). 30 An assembly according to claim 3, wherein the rod support plane (208) (205) is located between the first (201) plane and the second (202) plane, each rod (205) then being bonded to the first (201) plane as well. plane (201) by a hinge (209). 6. An assembly according to claim 5, comprising a connection (206) for stiffening between the plane (208) of rod support (205) and the second plane (202). 7. Assembly according to one of claims 5 or 6, wherein the mirror (1) is a first material, preferably glass, thermal expansion coefficient a1, the first plane (201), the second plane (202) , and the intermediate space (203) are made of a second material, preferably a composite material, of thermal expansion coefficient a2, the rod support plane (208) (205) is made of a third material, preferably a composite material , of thermal expansion coefficient a3, and wherein: h2 = h1 (a2-a1) -1 (a3-a2) where h2 is the height between the first plane (201) and the rod support plane (208) (205); and h1 is the height between the first plane (201) and the mirror (1). 8. Assembly according to one of claims 3 to 7, further comprising blockers (210) adapted to lock the rods (205) relative to the plane (208) of rod support (205), when the mirror has a shape sought. 25 9. Assembly according to one of claims 1 to 8, wherein the mirror comprises machining (207) local on a face (12) rear. 10. Assembly according to one of claims 1 to 9, wherein the structure (2) lattice is tetrahedral pattern. 20 30