Apparatus for Controlling the Reflectance of an Object Surface

1. Field of the Invention

The present invention relates generally to an apparatus for selectively varying the reflectance of electromagnetic energy from an object surface and, more particularly, such apparatus of mechanical electrical construction which operates over a relatively wide reflectance dynamic range, has low power requirements, and higher switching speed compared to other known reflectance control measures.

2. Description of Related Art

There are many situations in which the surface of an object or craft may experience a substantial exposure to radiation in the thermal band (3-18 microns), for example, requiring considerable dissipation in order to protect the object from deleterious results or even total destruction. It is particularly true of a space craft which may be exposed on one surface to a very high amount of thermal band radiation while adjacent surfaces remain potentially radiation free, the resulting high temperature differential of the space craft posing additional danger. Accordingly, it is desirable to have some way for selectively controlling reflectance from the surface of a craft or other object in order to manage the amount of heat induced into the object and, in that way, maintain the temperature within appropriate limits.

In the past, devices operating on a chemical and electrical basis, referred to generally as electrochromics, have been used for reflectance control. These devices have been found to experience chemical breakdown on exposure to ultraviolet radiation making them substantially inoperative for the desired reflectance variation purposes. Also, electrochromics devices are life cycle limited, have limited operational dynamic ranges and relatively slow switching cycle times, all of which leave them less than fully satisfactory for practical use.

In accordance with the present invention apparatus is provided for mounting onto a surface of an object for altering the reflectance of the radiation away from the object. The device includes a plurality of identically shaped plates of generally rectangular configuration which are arranged with an end of each mounted onto the surface of the object and extending away therefrom in a generally mutually parallel condition to each other and forming a plurality of chambers or cavities opening outwardly away from the object surface. A reflective modulator plate has an appropriate plurality of slots therein to permit placing onto the plates and sliding therealong producing a corresponding change in depth of the cavities. An actuator (e.g., piezoelectric element) is electrically controllable to selectively move the modulator plate away or toward the object surface. A cover constructed of a suitably radiation transparent material depending upon the radiation range to be controlled may optionally be placed on the upper and outer ends of the plates and secured in a suitable manner to the actuator so as to form a unitary construction and protect the cavities where needed.

In operation to increase radiation reflectance away from the object, the actuator is energized to move the modulator plate in such a direction as to alter the depth of the black-body cavities formed by the plates between the transparent cover and the modulator plate. Appropriate modification in this regard provides a corresponding modification of the reflectance of the incoming radiation and thus achieves substantially optimal radiation reflectance away from the object surface so that object absorption of thermal radiation is reduced accordingly.

FIG. 1 is an isometric view of the apparatus of the present invention for effecting reflectance modification;

FIG. 2 is an enlarged, fragmentary view of a reflective modulator plate;

FIG. 3 is a perspective, partially exploded view of the invention; and

FIG. 4 is a graph of emittances obtained over a range of L/W ratios and for selective initial surface reflectances.

With reference now to the drawing, the reflectance modulating apparatus of the present invention is enumerated generally as 10 and is seen to be secured onto the outer surface 12 of an object 14 to have the reflectance of incident radiation thereon modified in a desirable manner. Although this control of surface reflectance of the object, and thus the control of the amount of radiation that would be retained by the object, can apply to a number of different practical circumstances, it will be described herein as being applied to the surface of a space craft, for example. Also, although the range of radiation to be modulated can vary considerably depending primarily upon dimensional aspects of the apparatus, the apparatus will be described herein as applying specifically to reflectance control of radiation in the thermal band (i.e., 3-18 microns) and, therefore, for the end result of producing efficient thermal system management for a space craft.

A reflective modulator plate 16 particularly effective for reflecting infrared radiation, which is desired to be controlled, is a generally rectangular element provided with a plurality of identical straight-line slots 18 extending through the plate and which form a slot matrix enclosing a plurality of square, identically dimensioned wall portions 20.

An equal plurality of rectangular side wall plates 22 of identical geometry have such dimensions as to enable sliding receipt through a slot 18 forming overall a plurality of square cross-section cavities 24 opening outwardly away from the modulator plate 16. An adjustable amount of cavity volume is obtained on each side of the modulator plate depending upon the location of the modulator plate as it is moved along the side wall plates 22; however, it is those cavities extending outwardly away from the plate 16 and away from the object 14 that we are particularly concerned with here.

A cover 26 may be optionally securely located over the outermost ends of the side wall plates 22 enclosing the cavities 24. This cover must be highly transparent to the range of radiation sought to be controlled. A suitable material from which this cover can be made to use in controlling infrared radiation would be zinc selenide.

Post actuators 28 support at least a pair of opposite corners of the modulator plate 16 with respect to the object surface 12 and are affixed to both so that the entire assembly of the apparatus 10 is unitarily mounted onto the object 14. In a way to be described, the actuators operate to move the modulator plate toward and away from the object surface 12 and, in that way, produce a controlled amount of cavity depth extending outwardly beyond the modulator plate toward incoming radiation (arrow, FIG. 1). This action mechanically changes the depth of the black body cavities 24 and, thus, produces controllable reflectance of incoming radiation. Although other devices may be found satisfactory for this purpose, best results are believed obtained by constructing the post actuators 28 from a piezoelectric material which upon being appropriately electrically stimulated extends the posts (or retracts) them producing the desired mechanical movement of the modulator plate to maximize reflectance or reduce it, as desired.

It can be shown for the cavity construction described that the effective emittance .epsilon..sub.o, or reflectance which equals 1-.epsilon..sub.o, is generally represented by the following equation: ##EQU1## where, .epsilon.=emittance of modulator plate 16 and side wall plates 22

.alpha.=L/W;

L=cavity edge

W=cavity depth

FIG. 4 depicts effective emittances that are obtained for different values of .alpha.=L/W with selected surface emittances of the plate 16 and side wall plates 22.

Although other materials may be found suitable for making plate 16 and side wall plates 22, preferably they are constructed of silicon and formed to proper shape by micromachining techniques. To improve emitrance (or reflectivity) the outer surface of the modulator plate is coated with a metallic layer (e.g., gold). Moreover, with a fixed predetermined square dimension L, W will have to be adjustable in the range 6-10 L for a wide dynamic range of operation.

The side wall plates 22 are coated with a good radiation absorber (e.g., black). It has been found that where the reflectance of the modulator plate is >0.9 (i.e., .epsilon.<0.1) that excellent results are achieved when the side wall plates have a reflectivity <0.1.

In use, assuming that the object (e.g., space craft) is not being subjected to dangerously high infrared energy, it may be desirable that the object receive incident radiation without benefit of maximum reflection. This condition will occur when the reflective modulator plate is located closer to the object outer surface 12 which condition is achieved by appropriate energization of the post actuators 28. On a circumstance being presented where there is a high incidence of infrared radiation, the modulator plate 16 is actuated by appropriately energizing the piezoelectric post actuators to move toward the IR transparent cover 26 which increases the reflectance through the action of the black body cavities to an optimal point resulting in a substantially reduced amount of infrared energy actually reaching the object itself.

The described embodiment related to the controlled reflectance of thermal radiation. However, the black body cavities 24 on being appropriately dimensioned can be readily applied to controlling other ranges of radiation.

Also, although the invention has been described in connection with a preferred embodiment, it is to be understood that those skilled in the appertaining arts may contemplate changes within the spirit of the invention and within the scope of the appended claims.