WO1988004835A1

WO1988004835A1 - Hollow, noncontacting rotary joint

Info

Publication number: WO1988004835A1
Application number: PCT/US1987/003146
Authority: WO
Inventors: James S. Ajioka; Robert T. Clark
Original assignee: Hughes Aircraft Company
Priority date: 1986-12-23
Filing date: 1987-11-30
Publication date: 1988-06-30

Abstract

Disclosed is an around the mast rotary joint capable of multichannel operation. There are two hollow coaxial sections (18, 20), each of which operates in the TEM or quasi-TEM mode. Each section comprises a coaxial feed line (16, 14) connected to a microstrip corporate feed network which is cylindrical (24, 40). Coupled to the microstrip corporate feed is a low impedance cylindrical interface section (22, 40) which couples the power to the other coaxial section. In addition to evenly feeding the interface section, the microstrip network includes impedance transformers and matching means to match the relatively high impedance of the coaxial feed line to the relatively low impedance of the interface section. The two sections are coupled together with a noncontacting choke (54).

Description

HOLLOW, NONCONTACTING ROTARY JOINT

BACKGROUND OF THE INVENTION

The invention relate? generally to rotary joints for conducting electromagnetic energy and more particularly, to hollow rotary joints.

Where additional radio frequency (RF) channels are needed at an existing rotary joint and a common axis of rotation with the existing rotary joint channels must be maintained for these additional channels, an "around the mast" type of rotary joint is one! technique which may be used. Examples of such a situation may be a need for auxiliary channels for IFF or fori a sidelobe suppressing antenna that must rotate on the same pedestal as the main radar or communication antenna. ' The inside area of an around the mast type of rotary joint is hollow so as to not interfere with the normal operation of the existing rotary joint and mounting structure. It is typically fitted around the existing rotary joint and contains the additional channel or channels needed.

In addition to being usable with existing rotary joints, the typical around the mast rotary joint is also usable with other objects through which the axis of rotation must be located, such as a ship's mast. In the case of a ship's mast, the antenna mounting structure may be fixed to the mast while the antenna rotates around the mast.

Prior around the mast type rotary joint structures include the contacting type where devices such as brushes located on one member of the rotary joint are forced into sliding contact with slip rings located on the opposing member of the rotary joint. The contacting type of technique is undesirable in certain applications due its lower reliability. Wear caused by the constant friction between moving contacting parts, galling, noise generation, RF losses, and the potential for arcing all contribute to its undesirability. Even with the development of special lubricants and better finishes on the contacting materials,_, unacceptable failure rates still exist.

Noncontacting rotary joints do not experience the wear of the moving electrical surfaces experienced by the contacting type joints. Such joints, for example, the joint found in U.S. Patent 3,633_Λ130 to J.S. Ajioka, have been used successfully, however, they have not been applied to the around the mast type of rotary joint.

It is an object of the invention to provide an improved, noncontacting, hollow rotary joint.

It is also an object of the invention to provide a hollow rotary joint which operates in the TEM mode.

It is also an object of the invention to provide a hollow rotary joint which is relatively simple in structure, is reliable, and is relatively inexpensive to construct.

It is also an object of the invention to provide a hollow rotary joint which uses relatively low impedance coaxial circuits at the motional interface to diminish the possibility of arcing while permitting the conductance of relatively high rf currents.

It is also an object of the invention to provide a hollow rotary joint having a hollow portion large enough so that it may be positioned around another rotary joint or other object.

It is also an object of the invention to provide an improved, noncontacting, hollow rotary joint which ^" is multichannel.

It is also an object of the invention to provide a hollow rotary joint having a hollow portion large enough so that it may be positioned around an existing rotary joint for adding more RF channels to the system in which the existing rotary joint is used.

SUMMARY OF THE INVENTION

The foregoing objects and other objects are attained by the invention wherein there is provided a hollow, noncontacting rotary joint having two noncontacting low impedance coaxial sections at the motional interface operating in the TEM coaxial mode and which are coupled together by a wide frequency bandwidth, noncontacting, coaxial choke joint. Each coaxial section includes an interface section comprising an outer and an inner hollow cylindrical conducter separated from each other by a substrate having a relatively low dielectric constant. These hollow, cylindrical conductors are closely spaced in relation to each other and form an interface section having low impedance. Each interface section is fed by a microstrip type of corporate feed network which operates in the TEM mode or quasi TEM mode and evenly distributes power to the interface section. The microstrip corporate feed also comprises a hollow electrically conductive cylindrical section that forms the ground plane for the microstrip corporate feed network. This cylindrical microstrip network and its associated cylindrical ground plane section are coaxial with the low impedance interface section. In the preferred embodiment, the microstrip/ground plane section uses the same low dielectric constant substrate as the interface section.

The microstrip corporate feed incorporates impedance transformers that impedance match the coaxial feed line feeding the microstrip to the low impedance interface section. The coaxial feed line is typically at a much higher impedance than the interface section and impedance transformers are used for efficient power transfer.

The microstrip corporate feeds of both coaxial sections are fed by standard fifty ohm coaxial lines.

The choke used at the motional interface is a noncontacting type of device and couples the two low impedance interface sections together for efficient energy transfer while sealing against the leakage of RF energy. To obtain greater frequency bandwidth, a double choke design may be used.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more detailed description of the invention, reference is now made to the following description of the preferred embodiment taken in conjunction with the accompanying drawings wherein:

FIG. 1 presents a hollow rotary joint in accordance with the invention fitted around an existing rotary joint. A choke is shown in a cutaway view;

FIG. 2 presents a perspective view of part of a rotary joint in accordance with the invention showing microstrip corporate feeds each coupled between respective coaxial line feeds and low impedance interface sections located at the motional interface. The choke joint has been excluded for clarity;

FIG. 3 presents a developed view of one-half of the rotary joint of FIG. 2 showing the microstrip corporate feed with various transformers and power dividers and its connection to the low impedance interface section. The choke has also been excluded for clarity.

FIG. 4 is a cross-sectional, side view of a rotary joint in accordance with the invention showing a typical double choke design at the motional interface.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

In the following description, like reference numerals will be used to refer to like elements in the different figures of the drawings. Referring now to the drawings with more particularity, in FIG. 1 there is shown an existing rotary joint 10 about which is located an added hollow rotary joint 12 in accordance with the invention. The added rotary joint 12 has a large enough hollow area to permit normal operation of the existing rotary joint 10 and normal operation of the added rotary joint 12. By this means, channels may be added to an existing system since the added rotary joint 12 has the same axis of rotation as the existing joint 10. Where the coaxial feed lines 14 and 16 are kept compact enough or are located differently, such as on the inside surfaces of the added rotary joint 12, more hollow rotary joints can be added in the same manner as the rotary joint 12 was added in FIG. 1, or more may be stacked along a common axis with the existing rotary joints.

In addition to the application of adding the hollow rotary joint 12 about an existing rotary joint 10 as shown in FIG. 1, the added rotary joint 12 may also be fitted around a stationary object such as a mast or may be fitted around other objects which are rotating at different rates.

In FIG. 2, there is shown a partial view of a hollow, noncontacting rotary joint 12 in accordance with the invention. One coaxial member 18 is considered a stationary member or stator while the other coaxial member 20 is considered the moving member or rotor. This description of the two coaxial members, however, is not intended to be restrictive but only illustrative. Relative motion between them is contemplated and referring to one member as non-rotating or stationary, i.e., the stator, is for purposes of convenience only. It may in fact be rotating relative to its environment.

As is generally apparent by a cursory reference to FIG. 2, the rotor 20 has a similar configuration as the stator 18. This also is for purposes of convenience of description only and is not intended to be restrictive of the invention. FIG. 3 presents a developed or unfolded view of part of FIG. 2. The following description will refer to both figures.

As shown, stator 18 comprises a cylindrical section having two main parts, an interface section 22 and a corporate feed section 24. The interface section 22 comprises a hollow inner conductor 26. This interface section 22 is in the form of a coaxial line due to its having a cylindrical inner conductor 26, a cylindrical outer conductor 28, and a dielectric substrate 30 separating the two. In this embodiment, the impedance of the interface section 22 is affected by the dielectric constant of the substrate 30 between the outer conductor 28 and the inner conductor 26. In the preferred embodiment, the impedance of the interface section 22 is relatively low.

As shown in FIGS. 2 and 3, the microstrip corporate feed 24 is designed to evenly couple the coaxial feed line 16 to the low impedance interface section 22. Although the FIGS. herein show the corporate feed branching into eight smaller branches for coupling to the interface section 22, this is not meant to be restrictive of the invention. More or fewer branches may be employed depending upon a variety of considerations such as the size of the hollow rotary joint, the difference in impedances between the interface section 22 and the coaxial line feed 16, the standing wave ratio which may be tolerated, and other considerations.

As is shown, the microstrip corporate feed 24 comprises a series of power dividers/combiners to result in equal line lengths from the point of contact with the coaxial feed line 16 to the plurality of positions around the interface section 22. The corporate feed 24 is shown in microstrip. However, the corporate feed 24 may also be formed in other ways, such as stripline. The microstrip corporate feed 24 comprises transformers and matching devices for transforming the relatively high characteristic impedance of the coaxial feed line 16 (for example 50 ohms) to the relatively low characteristic impedance of the parallel plates of interface section 22 (for example 10 ohms) . Although step transformers are shown, other types, such as tapers, may also be usable. Such designs of microstrip power dividers/combiners and stepped transformers are well known in the art and are not described further herein. References related to such microstrip transmission lines include Matthaei, Young, and Jones, Microwave Filters, Impedance Matching Networks, and Coupling Structures, Artech, 1980.

The ground plane for the microstrip feed 24 is shown in FIG. 2 as the inner conductor 26 which is also the inner conductor for the low impedance interface section 22. By using a common conductor, the hollow rotary joint in accordance with the invention may be more compact and easier to construct.

The dielectric substrate between the microstrip corporate feed 24 and its ground plane 26 is also the dielectric substrate 30 used by the interface section 22. Once again, this common usage facilitates construction and allows more compactness.

As shown in FIG. 2, the spacing between centerlines of the microstrip 34 and 36 contacting the interface section 22 is approximately one-half wavelength of the center of the band frequency. The width of the outer conductor 28 of the interface section 22 between the motional interface 38 and the microstrip (24) is great enough to attenuate higher order modes as required. By using this spacing, operation in the TEM coaxial mode is supported. The thickness of the microstrip is selected in accordance with considerations well known to those skilled in the art.

The corporate feed 24 is fed with a coaxial line feed 16. In the embodiment shown, the coaxial line feed 16 is a threaded connector, the outer conductor of which is contacting the microstrip corporate feed 24. The inner conductor of the connector is disposed through the microstrip (24) and dielectric substrate (30) and contacts the ground plane 26. Although the coaxial line feed 16 is shown as being placed on the outside of the stator 18 in the FIGS., it may in certain applications be placed on the inside. In those applications, the outer conductor of the connector would be in electrical contact with the ground plane 26 and the inner conductor of the connector would be disposed through the ground plane 26 and the dielectric substrate 30, and would be in electrical contact with the microstrip corporate feed 24.

As shown in FIG. 2, the rotor 20 is of substantially the same configuration as the stator 18. The rotor 20 also has a low impedance interface section 40, a corporate feed 42 which is also shown as a microstrip structure, and a coaxial line feed 14. Although not shown, the interface section 40 and the microstrip corporate feed 42 both share the same dielectric substrate and the same inner conductor as in the case of the stator 18. In both the stator 18 and the rotor 20, the microstrip corporate feed and the interface section may be a single piece formed by well known techniques such as etching.

As shown in FIGS. 2 and 4, there is a gap 46 between the stator 18 and the rotor 20. The size of this gap is set in dependence upon the frequency bandwidth desired but is also influenced by other factors such as mechanical requirements which include bearing mounting and wear compensation. In one case, a gap of 3.175 mm (0.125 in.) was found to be usable. Methods for mounting the stationary member 18 in relation to the rotor member 20 are well known in the art. One method is diagrammatically shown in FIG. 4. A first clamp 48 is attached to the rotor member 20 and a second clamp 50 is attached to the stationary member 18. These clamps each include members which surround and are coaxial with the rotor 20 and stator 18 respectively, and are joined together by a bearing 52. The bearing 52 acts to hold the two clamps 48, 50 in position while allowing them to 10

rotate in relation to each other. Further methods for mounting the two members in relation to each other are not discussed in any further detail herein since various acceptable methods are well known to those skilled in the art. For a reference which discusses various techniques, refer to G. L. Ragan, Microwave Transmission Circuits, Radiation Laboratory Series, McGraw-Hill, 1948, pgs. 409-416.

The stator 18 and rotor 20 are shown in FIGS. 1 and 4 as being coupled together by means of a choke 54 mounted on the interface section 22 of the stator 18. The choke 54 is a standard one-quarter wavelength choke and in the preferred embodiment, is formed as part of the interface section 22 of the stator 18. To increase the frequency bandwidth of the coupling, a second choke 56 may be added as shown in FIG.4, and is also one-quarter wavelength in length. This second choke 56 may be formed as part of the interface section 40 of the rotor 20 or may be formed separately and attached by means known to those skilled in the art.

As is well known to those skilled in the art, the chokes seal against the leakage of RF energy while facilitating the efficient transfer of energy between the stator 18 and the rotor 20. The chokes 54, 56 shown have wide frequency bandwidths. Both are annular and are disposed completely around the circumferences of the stator 18 and the rotor 20. The length of the portion of each choke which is parallel to the outer conductor of its respective interface section is approximately one- quarter wavelength.

Although the above chokes 54, 56 have been specifically shoxn and described, this is intended for descriptive purposes only of an embodiment of the invention and is not meant to restrict the invention to any particular choke type. Other choke designs may 11

function in the invention. More detail on choke designs is given in G. L. Ragan, Microwave Transmission Circuits, Radiation Laboratory Series, McGraw-Hill, 1948, pgs. 100-114.

Thus there has been and shown and described a new and useful rotary joint comprising two members having a noncontacting motional interface, which are hollow to permit locating the joint around existing rotary joints, and which operate in the TEM mode. Due to the use of low impedance, coaxial type interface sections 22, 40, the rotary joint constructed in accordance with the invention is capable of conducting relatively large amounts of power. The use of microstrip corporate feeds 24, 42 provides a smoother transition between the coaxial feed lines 16, 14 and the respective interface sections 22, 40 than prior techniques using devices such as probes which are more difficult to match. As is known to those skilled in the art, probes are generally more costly, more complex to match, and fabrication is relatively difficult. The use of microstrip provides a transmission line more easily manufactured, and having much fewer critical points than probe techniques. By forming the microstrip corporate feed 24 and low impedance interface section 22 as a solid piece, fabrication is easier, more consistent and higher RF current levels may be conducted due to the absence of interface discontinuities.

A further feature of a rotary joint made in accordance with the invention lies in its ability to handle multiple modes. If a multiple beam type feed 12

network, such as a Butler matrix feed were used as the corporate feed 24, 42, quasi TEM modes with circumferential phase variation of:

where: JL = base of natural logarithms m = mode = circumferential angle in radians

would be generated in the low impedance interface sections 22, 40. An example of such a feed network is presented in U.S. Patent 3,290, 682 entitled "Multiple Beam Forming Antenna Apparatus." All modes have no amplitude variation so there will be no amplitude fluctuation during rotation of the rotary joint. Ideally the modes are mathematically orthogonal to each other and will not couple. Hence, the rotary Joint in accordance with the 'invention will be multichannel, i.e., a channel for each mode. Each mode will have an input port and an output port. In a radar antenna application, it has been found that a multichannel rotary joint in accordance with the invention has its isolation between channels limited by the isolation of the hybrid junctions in the multiple beam feed network. An isolation of -20 dB is achievable over a 10% frequency band.

Although the invention has been described in detail, it is anticipated that modif cations and variations may occur to those skilled in the art which do not depart from the inventive concepts. It is intended that the invention be limited only by the scope of the claims, not by the description, and so the invention will include such modifications and variations unless the claims limit the invention otherwise.

Claims

13 CLAIMSWhat is claimed is:

1. A hollow rotary joint for conducting electromagnetic across a motional interface, characterized by: first (22) and second (40) hollow coaxial interface sections, each interface section having a relatively low impedance and comprising an outer electrically conductive surface (28) and a hollow, inner, electrically conductive surface (26) separated from each other by a dielectric substrate (30) , each interface section being capable of supporting the TEM mode; first (16) and second (14) line feed means for respectively feeding the first (22) and second (40) interface sections; first (24) and second (42) corporate feed means for respectively providing substantially even feeding in the TEM mode between the first (22) and second (40) interface sections and the respective first (16) and second (14) line feed means; and mounting means (48, 50) for mounting the first (22) ^ar*d second (40) interface sections coaxially and adjacent each other for relative motion between the two and for conducting energy between the two.

2. The hollow rotary joint of Claim 1 further characterized in that the first (24) and second (42) corporate feed means each comprise a ground plane, a dielectric substrate, and microstrip transmission lines disposed on the dielectric substrate opposite the ground plane thereby establishing a microstrip corporate feed, the dielectric substrate of the corporate feed means being continuous with the dielectric substrate of the respective interface section and the ground plane of the microstrip corporate feed means being continuous with one of the electrically conductive surfaces of the respective 14 interface section, the microstrip corporate feed connecting the respective line feed means with the respective interface section.

3. The hollow rotary joint of Claim 2 further characterized in that the spacing between the microstrip transmission lines, the ground plane, and the dielectric substrate of each corporate feed means is such that the TEM mode is supported.

4. The hollow rotary joint of Claim 2 further characterized in that the first (16) and second (14) line feed means each comprise a feed line coupling to the respective corporate feed means at a single point.

5. The hollow rotary joint of Claim 4 further characterized in that each line feed means has two electrical conductors, one of which is connected to the microstrip transmission lines of the respective corporate feed means and the second of which is connected to the ground plane of the respective corporate feed means.

6. The hollow rotary joint of Claim 1 further characterized in that the microstrip corporate feed comprises matching means for substantially matching the impedance of the interface section to the impedance of the line feed means.

7. The hollow rotary joint of Claim- 1 further characterized by choke means for coupling the first interface section (22) to the second interface section (40).

8. The hollow rotary joint of Claim 7 further characterized in that the choke means comprises two chokes, one (54) disposed on the first interface section (22) and the second choke (56) disposed on the second 15 interface section (40) , both chokes providing coupling between the first and second interface sections.

9. The hollow rotary joint of Claim 1 further characterized in that each interface section (22, 40) is at least one-quarter wavelength wide.

10. The hollow rotary joint of Claim 1 further characterized in that each corporate feed means (24, 42) provides a plurality of spaced apart points of contact with the respective interface section (22, 40) and the spacing between adjacent points of contact of the corporate feed means with the interface section is approximately one-half wavelength.