WO1982000719A1

WO1982000719A1 - Energy collector

Info

Publication number: WO1982000719A1
Application number: PCT/AU1981/000114
Authority: WO
Inventors: N Fourikis
Original assignee: N Fourikis
Priority date: 1980-08-13
Filing date: 1981-08-13
Publication date: 1982-03-04
Also published as: EP0057705A1

Abstract

The energy collector which utilises an off-access portion of a paraboloid of revolution (18) may be used to focus electromagnetic radiation and/or solar energy. A main reflector (18) is mounted on a support frame (10) pivotally connected by elevation bearing (11)to a support stand (15). An azimuth support (12)is rotatable about an azimuth bearing (14) and the support frame (10) is mounted on the azimuth frame (12) by legs (33, 34) which have a single point of connection (28, 29) to the support frame (10) and spaced points of connection (26, 27) to the azimuth frame (12). The connections (26, 27) are moveable towards or away from one another to vary the elevation of the support frame (10).

Description

ENERGY COLLECTOR

This invention relates to energy collectors, and more particularly to collectors which may be used to focus electro-magnetic radiation and/or solar energy. The impetus for research and development in antennas, radiometers and transmission equipment operating at millimetre (mm) wavelengths was provided by post-1969 astronomy and the ever-increasing need for communication channels. The present invention is mainly directed to antennas of such equipment and in particular to the design and construction of antennas utilizing off-set paraboloids as main reflectors which are capable of operating at mm wave-lengths. The invention is described in relation to antennas utilizing reflecting surfaces, mainly because of the losses arising from lenses having large bandwidths and reasonable sizes at mm wavelengths. Many pertinent characteristics of reflectors depend on their geometry and thus it is appropriate to consider the characteristics of commonly used reflector geometries before the constraints imposed by operation at mm wavelengths are examined. Symmetrical paraboloid reflectors have been used for focussing electro-magnetic radiation onto the phase centres of suitable feed horns for a long time. Despite their many attractive features which are due to their symmetry, these reflectors have a number of disadvantages. Off-set paraboloids on the other hand possess the following attractive features when compared to their symmetrical counterparts: (a) No obstruction of the incoming radiation results in:

( i) a decre ase of the s idelobe leve l , typically by 10 dB; and (ii) a considerable improvement of the instrumental profile of the telescope. (Obstructed radiation in symmetrical reflectors reaches the radiometer input after multiple reflections; the effect of these multiple reflections on the frequency-versus-antenna temperature instrumental profile ultimately determines the SNR of some astronomical observations. These effects are minimized when off-set reflectors are used. )

(b) The input reflection coefficient of off-set reflectors is up to 20 dB lower than that of their symmetrical counterparts; and

(c) Possible multiple beam operation; several independent antenna beams are realizable conventionally. Again all the feed-horns used, are not obstructing the incoming radiation.

Other features of this telescope geometry are discussed below.

Unconventional multiple antenna beams can be formed when a circular array of N off-set antennas is placed on a fully steerable structure. With this arrangement, M antenna beams are formed at the I.F. of the telescope and where M = N²/2 (N is even) . Despite their many attractive features, off-set reflectors have not been generally used because: (i) the georaetric-optics (g.o.) cross-polarization level of an off-set paraboloid is relatively higher than that of an idealized conventional paraboloid where there is no obstruction of the incoming energy; and (ii) antenna mounting and driving arrangements particularly suitable for off-set paraboloids have not yet been proposed.

Whereas off-set paraboloids can have negligible g.o. cross-polarization, their symmetrical counterparts have cross-polarization levels which depend on the geometry of the blocking structures.

One of the most serious problems encountered in the realization of telescopes operating at mm wavelengths is meeting the required surface tolerance. If the /16 criterion is adopted, then the r.m.s. surface error of the reflector of the telescope operating at, say 1 mm, should be 0.0625 mm. If a dual reflector system is used (and the advantages opf such systems are considerable) the tolerance on the main reflector should be even less. Accurately shaped panels are usually manufactured and then fixed onto a stiff back-up structure, with the aid of adjustable bolts; these operations are costly and time-consuming. It has been shown that the maximum size of telescopes operating at mm wavelengths is limited by the temperature differentials between the component parts. More specifically, the maximum size of the telescope is such that resolutions between 12" and 14" are attainable when commonly used materials are considered. Furthermore, the above resolution is not wavelength dependent. Recent research and development into materials, has resulted in the evolution of composite materials having near zero coefficients of expansion, high rigidity and light weight.

Of the two antenna mounting and driving arrangements used for astonomical applications, the altitude over azimuth mounting and driving arrangement, illustrated in figure 1(a) , is usually preferred when the antenna is to operate at mm wavelengths. This is because the gravitational deflections which tend to deform the reflector's surface are dependent only on the altitude angle. Any corrections applied to the telescope can, therefore, conveniently depend on one parameter only. By contrast, the same deflections are dependent on the altitude and azimuth angles when an equatorial mounting and driving arrangement (as illustrated in figure 1(b)) is utilized. It is often convenient to characterize antenna mounting and driving arrangements in terms of the direction of their primary axis (the orientation of which remains unchanged) for this direction defines the gimbal - lock zone of the telescope. It is, however, more appropriate to characterize the altitude over azimuth mounting and driving arrangement illustrated in figure 1(a) in terms of the height of its elevation axis. For symmetrical paraboloids, the height of the elevation axis is at least equal to the projected radius of the telescope. This constraint is not applicable to the off-set paraboloids, hence the elevation axis of a mounting and driving arrangement particularly suitable for off-set paraboloids can be conveniently close to the ground. The altitude over azimuth antenna mounting and driving arrangement according to the invention has the following features. It provides a three point support for the main reflector (two points are provided by the elevation bearings and the third point is preferably at a distance approximately D/3 from the rim of the main reflector) . By contrast, a symmetrical reflector is supported at one point by a conventional altitude over azimuth mounting and driving arrangement, and the overhand is D/2. The mounting and driving arrangement therefore reduces gravitational deformations. Additionally it is a low cost solution because it is closer to the ground and no counter -weights or elevation gears are required. According to the invention, there is provided an energy collector having a support frame on which is mounted a main reflector adapted to be directed towards the source of energy, the main reflector being an off-axis portion of a paraboloid of revolution and operative to reflect the energy from the source towards the focus of the paraboloid, and, a second reflector between the main reflector and the focus of the paraboloid arranged to reflect the energy from the main reflector to a receiving station which does not overlay the main reflector.

Preferably, the support frame is mounted on an azimuth-over-elevation drive which may be adjusted so that the main reflector is correctly positioned with respect to the source in order to achieve maximum efficiency. The invention may be adapted to operate as a barbeque cooker, in which case the receiving station constitutes a cooking zone which is readily accessible to the cook and which is so separated from the main reflector that hot fats and juices falling therefrom do not impair the efficiency of the main reflector.

In order that the invention may be more readily understood and put into practical effect, reference will now be made to the accompanying drawings in which :-

Figure 2 is a side elevational view of energy collector according to one embodiment of the invention, Figure 3 is a view taken along lines A-A of

Figure 2, Figure 4 is a view of the main reflector taken along lines B-B of Figure 2,

Figure 5 is an end view of the elevation lifting device shown in Figures 2 and 3, Figure 6 is a schematic side elevational view of an electro-magnetic telescope incorporating modified altitude over azimuth antenna mounting and driving arrangement, particularly suitable for off-set paraboloids,

Figure 7 is a view taken along lines A-A of

Figure 6, Figure 8 (a) is a paraboloid surface,

(b) is a flat approximating a small portion of the paraboloid surface,

(c) is an approximation of a small portion of a curve, having a radius of curvature, , by a straight line,

(d) is a schematic arrangement method of supporting the flats to approximate the paraboloid, and,

Figure 9 is a perspective view of a stiff back-up structure for an electro-magnetic telescope operating at mm wavelengths. For the sake of simplicity, the invention will be first described in relation to a solar energy collector which is adapted for use as a cooker. However, it is to be understood that the invention is not limited thereto. As shown in Figure 2, the solar energy collector of this embodiment of the invention includes a main frame 10 which is connected by spaced bearings 11 to an azimuth frame 12 supported by azimuth wheels 13 and rotatable about an azimuth bearing 14 in a central stand 15. The main frame 10 supports a plurality of ribs 16, 17 (see Figure 3) which provide a support for the main reflector 18 which, in this instance, is a 90° sector of a paraboloid of revolution which is offset from the axis of the paraboloid. The main frame 10 is raised and lowered by a lifting arrangement upon rotation of the handle 19 of the shaft 30 which is supported by bearings 25. The elevation lifting device consists of two legs 33 and 34 supported at two ball joints 26 on the azimuth support and at hinge joint 28, 29 on the back of the frame 10. The two joints 26 at which the lifting device is connected onto the azimuth support by nuts 27 which are threaded on a shaft 30. One end of the shaft 30 has a right handed thread 31 while the other end has a left-handed thread 32. The shaft 30 can be moved by handle 19 or with the aid of motor.

The main reflector 18 directs the sun's rays towards the focus F of the paraboloid as shown in Figure 2. A second reflector 20 is positioned between the main reflector 18 and the focus F to direct the reflected rays to a receiving or heating station 21 which may have a base 22 of corrugated metal on which food to be cooked may be placed. An overlying grid may also be provided. The base 22 is adjustably supported by the member 23 of the main from 10 so that the base 22 may be kept horizontal when the main frame is adjusted by the handle 20.

A tube 24 is provided on the main frame as a means of indicating that the main reflector is directed with respect to the sun for maximum efficiency. Initially, the elevation drive is used to minimize the shadow of the tube and then the azimuth position is changed so that the same shadow is further minimized. When the main reflector is correctly pointed toward the sun, a circle of sunlight is seen on the ground. Whilst cooking, the elevation and azimuth of the main reflector are manually adjusted so that the light circle is always visible on the ground. Figure 4 illustrates a section of the main reflector; its collecting area, assuming 100% efficiency, is 3.6m². The reflecting material is aluminium deposited on a 0.004" thiσh MYLAR. The ratio of the focal distance to the diameter of the paraboloid is 0.4 and the ribs 16 and 17 are accurately cut from wood and glued onto the base 10 of the parabola. Although the main reflector is shown as a 90° section, it could be of any convenient shape, such as a 60° sector or even a 120° sector, the dimensions being so adjusted to give the djesired reflecting area.

Turning now to Figures 6 to 9, the invention will be described in relation to an electro-magnetic telescope. Figure 6 is substantially similar to Figure 2 and this like numerals represent like parts. The main reflector 18 is raised and lowered by the lifting mechanism shown schematically at 40. In this instance the second reflector 20 is a curved reflector and the receiving station 21 is a feed-horn.

It is convenient and economical to construct a stiff back-up structure 41 for the main reflector 10 and then attach the reflecting panels onto it with the aid of adjustable bolts. The back-up structure approximates the paraboloid so it can be readily constructed at reasonable costs. The manufacture of curved reflecting panels is usually expensive because many expensive operations are involved in producing doubly-curved panels to the required tolerance and in checking their accuracy. In an alternative approach, the double curved surface of the reflector is approximated by a number of flat or planar members hereinafter called "flats". However, this is inexpensive, for the operations involved in producing and checking the accuracy of flats are relatively straightforward. If a very large number of flats is required however, the proposed alternative can also be expensive. For this reason it is now appropriate to calculate the size of the required flats for telescopes capable of operating at mm wavelengths. Since the surface of a paraboloid illustrated in Figure 8 (a) is doubly curved, it can be approximated by flats the sizes of which are related to the principal radii of curvature. At any point of the parabola the flat 42 will have the shape of a trapezium see Figure 8 (b) ; its height h being determined by the radius of curvature of the parabolic curve and its side W by the radius of curvature of the curve defined by the intersection of the paraboloid -and the plane parallel to the OZ axis and normal to the tangent plane (see Figure 8 (a)).

It can be easily shown that the radii of curvature for the above two curves are:

and

respectively. As any curve can be by approximated at a point A (see Figure 8 (c) ) by a circle, the radius of which is equal to the radius of curvature of the curve at A, it can be shown that

and

where k is the required tolerance. Equations (3) and (4) are deducted by stipulating that h or MC is such that AN = k. The angle

in equations (1) and (2) can be defined as

When the angle

is equal to zero then

= 2F: as increases, both

increase from the value of 2F at different rates. It is clear that the region near the vertex of a paraboloid is a region of maximum curvature hence the area of flats approximating the paraboloid is small (many flats are required) . Eor symmetrical paraboloids the illumination of the near the vertex region is considerable (almost equal to one) whereas the illumination of the same area for an off-set paraboloid is minimum (typically -10 to -15 dB) .

If an off-set and a symmetrical paraboloid have the same F/D then the area of a flat near the extreme rim of the former is twice that of a flat near the rim of the latter. This is a considerable advantage for off-set paraboloids. h and w₁ for an off-set paraboloid equal to 236 and 211 mm respectively when F = r_max = 5000 mm and k = 0.5mm; the minimum values of h and w₁ are 200mm. The dimensions of the above flats are considerable.

Depending on the upper frequency of operation the reflection surface can be realized by utilizing light gauge perforated sheet metal. The method of attaching the reflecting surface onto the back-up structure is important; if 3 or 4 adjustable bolts are for instance required per flat, then the total number of bolts required is unmanageable. An ideal arrangement is one where one bolt adjusts the height of one flat. The left hand side of figure 8 (d) illustrates a method of approximating the ideal arrangement where each adjustable bolt 43 supports the tips of four flats. Thus the flats are securely held on to the back-up structure and some curving of the flats is expected. If the adjustable bolts are normal to the surface of the paraboloid then the expected curving will improve the approximation of the resulting paraboloid to the ideal. In the right hand side of figure 8 (d) the flats are arranged so that the deviations from the ideal paraboloid are not systematic. With this arrangement the magnitude of the sidelobes away from the main beam are minimized.

The same metal/composite material should be used for both the back-up structure and the reflecting surface in order to minimise the differential expansion, between them. For the reasons discussed below mild steel is preferred to aluminium when a low-cost design is required. If the budget of the telescope is considerable, composite materials should be considered. 3oth the Cassegrain and Gregorian versions of off-set paraboloids can be used.

The considerations outlined in relation to the back-up structure for paraboloids capable of operating at mm wavelengths are necessarily of a heuristic nature. This is because there are no unique solutions to the problems of realizing back-up structures for paraboloids, even when the budget, maximum frequency of operation, telescope diameter, F/D ratio and wind loading are known. These general considerations however are necessary before a detailed design is undertaken.

Comparisons between stainless steel and aluminium indicate that the ratio of the modulus of elasticity over density over the coefficient of expansion for the former metal is 2.125 higher than that of the latter. Since mild-steel has similar properties to stainless steel and the former metal is cheaper than the latter, mild steel especially treated against rust is preferred for the back-up structure of the main reflector. An additional advantage of mild steel over aluminium is the ease of welding. The reflecting surface should be chosen to be perforated mild steel also treated against rust.

It is now appropriate to consider the back-up structure in some detail. Normally the back-up structure of a paraboloid consists of a number of radial ribs and a number of inter-rib supports which form concentric circles. At cm wavelengths the maximum allowable deflections are such that both the radial-ribs and their inter-rib supports can have relatively small dimensions, so rigidity is achieved by realizing a space-dome type back-up structure. The back-up structure for reflectors capable of operating at mm wavelengths require ribs having rectangular sections of considerable dimensions [100 mm in depth for instance for a 6 m off-set paraboloid operating at 7 mm and 50 mm wide (normal max. wind loading conditions)]. These ribs 50, illustrated in figure 9, can be rolled to the required parabolic shape approximately; for the inter-rib supports 51, I-sections could be selected (having the same depths) and they could be attached to the ribs 50 at spacings partly dictated by the considerations outlined above (some inter-rib supports 51 can be used to accommodate the adjustable bolts 52 required to fix the reflection surface) . The inter-rib supports may be welded onto the ribs. Such a structure minimizes the need for a space-dome structure, especially when the depth of both the above sections is considerable. As welding introduces distortions, it is convenient to weld end plates 53 onto the I-sections 51 before attaching them on to the ribs 50 with the aid of bolts 52. When the reflector 10 is pointed toward the zenith the width, depth and thickness of the ribs can be calculated by considering the distributed weight of the telescope and assuming that wind loading at this position is negligible (this is especially true when the antenna mounting and driving arrangement of the invention is used) . When the reflector is pointed toward the local horizon, the maximum wind loading becomes the dominant load on the structure. It is instructive to consider whether the proposed mounting and driving arrangement is useable for symmetric paraboloids. Such an arrangement would require the elevation axis to be at the rim of the paraboloid, a position which has the least stiffness. This proposition, though not attractive, does illustrate the point that the elevation axis of the new antenna mounting and driving arrangement is at a point of maximum stiffness only when an off-set paraboloid is used.

Claims

The claims defining the invention are as follows:-

1. An energy collector comprising a support frame on which is mounted a main reflector adapted to be directed towards the source of energy, the main reflector being an off-axis portion of a paraboloid of revolution and operative to reflect the energy from the source towards the focus of the paraboloid, and, a second reflector between the main reflector and the focus of the paraboloid arranged to reflect the energy from the main reflector to a receiving station which does not overlay the main reflector.

2. A collector according to claim 1 wherein the support frame is mounted on an azimuth-over-elevation drive which is adjustable so that the main reflector may be correctly positioned with respect to the source of energy.

3. A collector according to claim 2 wherein the support frame is pivotally mounted on an elevation bearing of a stand for the collector which has an azimuth frame extending therefrom, the azimuth frame being rotatable about an azimuth bearing of the stand and wherein the azimuth-over-elevation drive comprises a pair of legs having a common connection to the support frame and spaced connections to the azimuth frame.

4. A collector according to claim 3 wherein the common connection comprises a hinge joint and the spaced connections comprise ball joints movable toward and away from each other.

5. A collector according to claim 4 wherein each ball joint is mounted on a shaft by means of a nut which engages a threaded portion of the shaft, each threaded portion being oppositely threaded to the other so that rotation of the shaft will either move the nuts towards or away from each other.

6. A collector according to claim 1 wherein the main reflector has a reflecting surface of aluminium foil and the receiving station constitutes a cooking zone which is so separated from the main reflector that hot fats and juices falling therefrom do not impair the efficiency of the main reflector.

7. A collector according to claim 1 wherein the support frame is comprised of a plurality of radial ribs inter-connected by inter-rib supports so spaced that they define a series trapezoidal spaces between adjacent radial ribs.

8. A collector according to claim 7 wherein each trapezoidal space is closed by a reflecting flat member, the height (h) and width (W) of each flat being defined by: h = 2 (2r_lK)^1/2 and W = 2 (2r₂K)^1/2 wherein: r₁ is the radius of curvature of the parabolic curve, r₂ is the radius of curvature of the curve defined by the intersection of the paraboloid and the plane parellel to the OZ axis and normal to the tangent plane, K is the predetermined tolerance.