Patent Publication Number: US-9413049-B2

Title: Rotary joint including first and second annular parts defining annular waveguides configured to rotate about an axis of rotation

Description:
FIELD OF THE INVENTION 
     The invention is in the field of rotary electrical connections. 
     DESCRIPTION OF THE RELATED ART 
     Traditional slip rings are electromechanical technology that enables the transmission of power and electrical signals from a stationary to a rotating structure. This transmission of power/data is made possible through electrical contact connections made by such devices as stationary brushes, cylindrical pins, or a sphere pressing against rotating circular conductors. This pressure electrical contact has reliability and wear issues in use. Contactless rotary joints also exist through capacitive and inductive coupling or by fiber optic signal transmission. Traditional rotary joints, like these, utilize rotational symmetry about the center axis and require the input, output ports and critical signal path be placed at the center axis of rotation to maintain constant phase and amplitude transmission independent of rotation. It would be advantageous to have rotary electrical contacts that avoid these shortcomings. 
     SUMMARY OF THE INVENTION 
     A rotary joint includes a contactless annular electrical connection. 
     According to an aspect of the invention, a rotary joint includes: a first part; and a second part that rotates relative to the first part about an axis of rotation. The first part has a first electrical connection annular portion. The second part has a second electrical connection annular portion. The electrical connection annular portions make contactless electrical connection with one another. The electrical connection annular portions together define and surround a core region, in which an electrical connection between the electrical connection annular portions is not made. The core region includes the axis of rotation. 
     According to another aspect of the invention, a method of passing an electrical signal across a rotary joint includes the steps of: inputting an incoming electrical signal into a first feed that splits the signal; generating in the first feed a transverse electromagnetic (TEM) wave, wherein the TEM wave propagates in an axial direction through a first annular waveguide structure that is coupled to the first feed; passing the TEM wave across an axial gap, from the first annular waveguide structure to a second annular waveguide structure that is able to rotate relative to the first annular waveguide structure about an axis of rotation of the rotary joint that does not pass through the annular waveguide structures; and generating an outgoing electrical signal from the TEM wave in a second feed that is operatively coupled to the second annular waveguide structure. 
     To the accomplishment of the foregoing and related ends, the invention comprises the features hereinafter fully described and particularly pointed out in the claims. The following description and the annexed drawings set forth in detail certain illustrative embodiments of the invention. These embodiments are indicative, however, of but a few of the various ways in which the principles of the invention may be employed. Other objects, advantages and novel features of the invention will become apparent from the following detailed description of the invention when considered in conjunction with the drawings. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       The annexed drawings, which are not necessarily to scale, show various aspects of the invention. 
         FIG. 1  is a schematic view of parts of a rotary joint in accordance with an embodiment of the invention. 
         FIG. 2  is a plan view showing a layout of one of the feeds of the rotary joint of  FIG. 1 . 
         FIG. 3  is a side cross-sectional view of the electrical connection of the rotary joint of  FIG. 1 . 
         FIG. 4  is a side cross-sectional view of part of an electrical connection of an alternate embodiment rotary joint. 
         FIG. 5  is a plan view showing details of a feed of the rotary joint of  FIG. 1 . 
         FIG. 6  is a side view of the feed of  FIG. 5 . 
         FIG. 7  is a partially schematic view showing passage of an optical signal through a core of the annular electrical connection of the rotary joint of  FIG. 1 . 
         FIG. 8  is a block diagram illustrating a multiplexing interface usable to transmit multiple signals simultaneously across one or more rotary joints. 
     
    
    
     DETAILED DESCRIPTION 
     A rotary joint includes a contactless electrical connection that has an annular shape, not extending into a central region surrounded and defined by the annular contactless electrical connection. The annular shape of the electrical connection portions allows other uses for the central region, such as for passing an optical signal through the rotary joint. Feeds are coupled to annular waveguide structures in both halves of the rotary joint, for input and output of signals. The feeds may provide connections to the annular waveguide structures at regularly-spaced circumferential intervals around the waveguide structures. The intervals may be at about every half-wavelength of the incoming (and outgoing) signals, or may be at any of a variety of other suitable spacing. The annular waveguide structures propagate signals in an axial direction, parallel to the axis of rotation of the rotary joint. The signals propagate contactlessly (non-electrically-conductively) across a gap in the axial direction between the two annular waveguides. 
       FIG. 1  shows some parts of a rotary joint  10  that includes a first part  12  that rotates relative to a second part  14 , about an axis of rotation  16  of the rotary joint  10 . The parts  12  and  14  include respective annular waveguide structures  22  and  32 , and respective feeds  24  and  34 . The feeds  24  and  34  are used for feeding signals to and receiving signals from the waveguide structures  22  and  32 . The annular waveguide structure  22  and the feed  24  together make a first electrical connection annular portion  26 , and the annular waveguide structure  32  and the feed  34  together make a second electrical connection annular portion  36 . The electrical connection portions  26  and  36  together constitute an electrical connection  40  that is part of the rotary joint  10 . As described in greater detail below, the structure of the electrical connection  40  sets up a transverse electromagnetic (TEM) wave that propagates in the axial direction, being able to propagate across a gap between the annular waveguide structures  22  and  32 . The electrical connection between the portions  26  and  36  is therefore contactless in that the primary way that electrical signals are transferred from the parts  12  and  14  is not through electrical conduction by contact of electrically-conducting materials of the two parts  12  and  14 . 
     The electrical connection  40  is annular in shape, with the annular portions  26  and  36  together defining and surrounding a core region  42 , in which an electrical connection between the annular portions  26  and  36  is not made. The core region  42  includes the axis of rotation  16 . 
     The parts  12  and  14  may include additional components that are not related to the electrical connection  40 . For example, the parts  12  and  14  may include parts of casings, or other components. 
       FIG. 2  shows the layout for the feed  24 . The feed  34  (not shown in  FIG. 2 ) has a similar layout, and both of the feeds  24  and  34  may have substantially identical layouts. The feed  24  is a splitter, with an input or output  43  in the form of a single electrical signal, and branching feeds that distribute that signal to multiple locations (connection points) equally circumferentially spread about the annular waveguide structure  22  ( FIG. 1 ). The feed  24  branches out to 256 connections to the annular waveguide, with connections being separated approximately half a wavelength apart from one another, based on the smallest wavelength in the range of signal wavelengths that the joint  10  is intended to pass. For example, the rotary joint  10  may be configured to pass electrical signals having a frequency centered around 20-24 GHz, although the electrical connection may be configured to pass signals of many other frequencies and wavelengths. More broadly, the signals may be in the Ka band, which has been defined as the frequencies of 26.5-40 GHz, with wavelengths from slightly over one centimeter down to 0.75 centimeters. The feeds  24  and  34  may be printed circuit boards or other suitable electrical splitter structures. The feeds  24  and  34  may have a different number of connections, and/or a different spacing of connections, than those in the illustrated embodiment. 
     The half-wavelength spacing in the illustrated embodiment is not intended to be limiting. Other suitable wavelengths may be used, such as a spacing larger than the half-wavelength spacing of the illustrated embodiment. 
     The branching of the feeds  24  and  34  means that the size of the individual cells (the distance between adjacent connections, the finest branches of the feeds  24  and  34 ) may be small compared to the circumference of the annular waveguide structures  22  and  32 . This means that the curvature may have negligible effects, and that the annular waveguide structures behave to a good approximation as infinite parallel-plate waveguides. 
     Referring now in addition to  FIG. 3 , additional details of the electrical connection  40  are shown. One or more bearings  44  bridge the axial gap  50  between the waveguide structures  22  and  32 , allowing relative rotation between the waveguides  22  and  32 . The bearings  44  may be suitable ball bearings, and may be made of any of a variety of suitable materials. There may be some electrical conduction through the bearings  44 , but any electrical conduction is not the primary way that electrical signals are passed between the waveguide structures. 
     Electrical connectors  46  and  48  may be parts of the feeds  24  and  34 , respectively, to route electrical signals into and out of the feeds  24  and  34 . The electrical connectors  46  and  48  may be coaxial connectors or other suitable kinds of electrical connectors. 
     The signals travel between the waveguide structures  22  and  32  along respective annular gaps  52  and  54  in the waveguide structures  22  and  32 . The waveguide structure  22  has annular notches  56  and  58 , and the waveguide structure  32  has annular notches  62  and  64 . The notches  56 ,  58 ,  62 , and  64  extend outward from the axial gap  50  between the waveguide structures  22  and  32 , into part of the depth of the material of the waveguide structures  22  and  32 . The notches  56  and  58  are on opposite respective sides of the annular gap  52 , with the notch  56  being an inner notch and the notch  58  being an outer notch. The notches  62  and  64  are similarly on opposite respective sides of the annular gap  54 . The notches  56 ,  58 ,  62 , and  64  act as choke points or radio frequency (RF) chokes to prevent leakage of the signal radially inward or outward from the axial gap  50 . The RF chokes also may operate to prevent power leakage out of or into the electrical connection  40 , and/or may aid in complying with requirements related to electromagnetic compatibility (EMC) and/or electromagnetic interference (EMI). 
     The waveguide structures  22  and  32  may be made of a suitable electrically conductive material, for example aluminum. Alternatively the waveguide structures may be made of an electrically-nonconductive material that is coated by an electrical conductor. 
       FIG. 4  shows an alternate configuration, with dual inner notches  66  and  68 , and dual outer notches  72  and  74 , in a single waveguide structure  32 ′, on opposite sides of an annular gap  54 ′. Features of the waveguide structures  22 ′ and  32 ′ may be combined with those of the waveguide structures  22  and  32  ( FIG. 3 ). The other waveguide structure  22 ′ has no notches around its annular gap  52 ′. As an example of the dimensions involved, the waveguide structure  22 ′ may have a height of 6.35 mm (250 mils), and the waveguide structure  32 ′ may have a height of 8.9 mm (350 mils). The annular gaps  52 ′ and  54 ′ may have a width of 1.27 mm (50 mils). The notches  66 ,  68 ,  72 , and  74  may each have a width of 1.27 mm (50 mils), and a depth of 1.9 mm (150 mils). The notches  66  and  68  may be separated by 1.9 mm (150 mils), and the notches  72  and  74  may be separated by the same distance. The axial gap may be about 0.076 mm (3 mils). The feeds  24 ′  24  and  34 ′  34  may have a thickness of 2.1 mm (81 mils), which is about one-quarter of the wavelength of electrical signals expected to be passed using the electrical connection  40 ′. As described in greater detail below, such a thickness for the feeds  24 ′ and  34 ′ helps provide better signal strength by reflecting the signal in the feeds  24 ′ and  34 ′ (incoming or outgoing) roughly in phase, to improve the signal strength. The notches  66 ,  68 ,  72 , and  74  also may be sized in relation to the wavelength of electrical circuits to be passed in the electrical connection  40 ′. The dimensions given above are those for a single specific embodiment. A wide variety of other dimensions are possible in other embodiments. 
     With reference now in addition to  FIGS. 5 and 6 , the feed structure  24  of the waveguide structure  22  ( FIG. 6 ) includes a series of striplines or microstrip lines, such as a stripline  90 , that are located between a pair of ground planes  92  and  94  ( FIG. 6 ) on opposite top and bottom sides of the feed structure  24 . The striplines may be located within a printed circuit board, with no additional cavities (such as machined cavities) needed within the waveguide structure. The striplines in the illustrated embodiment are one example of a variety of transmission lines that may be used. The ground planes  92  and  94  are connected with one another through a series of conductive ground vias  98 . The stripline  90  may have a configuration as shown in  FIGS. 5 and 6 , with an impedance transformation provided by segments of conductive material with various suitable widths, on both sides of a final split  102  in the stripline  90 , into fingers  104  and  106  ( FIG. 5 ). The cell size, the distance between the adjacent fingers, may be about half a wavelength, for example 3.5 mm (138 mils). The stripline  90  extends across a long slot  108  in the bottom ground plane  94 . Conductive signal vias  110  ( FIG. 6 ) may be used to electrically couple the stripline  90  to the bottom ground plane  94 , with the signal vias  110  making connection at connection points  112  ( FIG. 6 ) for incoming or outgoing signals. The long slot  108  may have a width of 0.18 mm (7 mils), to give one example value. The configuration of the feed  24  produces a TEM wave in the long slot  108 . It is this TEM wave that propagates in an axial direction to carry the signal from across the rotary joint  10 . The ground plane  92  reflects and reinforces the TEM wave produced in the slot  108 . The reinforcing may includes reinforcing in phase, with the ground plane  92  one-quarter wavelength of the electrical signal away from the ground-plane  90 . 
     The stripline  90  may be closer to the bottom ground plane  94  than to the top ground plane  92 . In one example embodiment, the stripline  90  may be 0.13 mm (5.1 mils) away from the bottom ground plane  94 , and may be about 1.9 mm (75 mils) away from the top ground plane  92 . These distances are only examples, and many other distances are possible. 
     The feeds  24  and  34  do not remain aligned as the rotary joint parts  12  and  14  rotate relative to one another. There may be a misalignment of cells of the feeds  24  and  34  and the annular waveguide structures  22  and  32  by as much as half a cell width (a quarter wavelength) of misalignment. However, this misalignment has been found to have no appreciable effect on the ability of the electrical connection  40  to accurately pass electrical signals. 
     In one embodiment, the electrical connection  40  ( FIG. 1 ) passes electrical signals with a loss of 20-30 dB. Signal strength may be boosted by use of suitable amplifiers upstream and/or downstream of the rotary joint  10  ( FIG. 1 ), if needed or desired. 
     With reference now in addition to  FIG. 7 , one advantage that the rotary joint  10  has is that the central core region  42  surrounded by the electrical connection  40  may be used for other purposes, for example for transmitting optical signals  140  from an optical element (transmitter)  142  on one side of the rotary joint  10  to an optical element (receiver)  144  on the other side of the rotary joint  10 . The optical transmitter  142  and the optical receiver  144  may be any of a variety of suitable optical elements for transmitting, receiving, and/or otherwise manipulating optical signals sent in either or both directions through the core region  42 . The optical transmitter  142  and/or the optical receiver  144  may be directly connected to or otherwise part of the rotary joint  10 , or alternatively may be separate from the rotary joint  10 . The passage of optical signals through the core region  42  is only one example of use of the core region  42  for purposes other than passage of electrical signals. Many other uses of the core region  42  are possible, for example with another electrical connection made in the core region  42 . For example, the core region  42  may alternatively be used for hydraulic power transfer, for cooling air transfer, for RF power transfer, for passage of signals for examining a specimen (as in a scanner, such as a computed tomography (CT) scanner or a magnetic resonate imaging (MRI) scanner), or for passage of mechanical devices, such as in a drill rig, to list just a few examples of alternative uses. 
     The rotary joint  10  may be used to pass multiple signals simultaneously. Multiple signals may be collected on either side of the rotary joint, and multiplexed into a single serial digital data stream. An RF carrier signal may be modulated with the serial digital data stream, to be transported through the rotary joint  10  and through other similar rotary joints that are connected in series with the rotary joint  10 . Multiple transmitters and receivers may share the same RF conduit through any combination of time, frequency, and/or code division multiplexing. 
     With reference to  FIG. 8 , the multiplexing and demultiplexing may be accomplished in an interface  200 . The various parts of the interface  200  that are described below may be embodied in hardware and/or software, as appropriate. The interface  200  may include a gigabit media access controller (GMAC)  202  that is able configure and interpret signals for transmission across the rotary joint  10 . The GMAC  202  outputs to a command stack  204 , which in turn sends signals as appropriate to a multiplexer  208 . Multiple signals may be multiplexed at the multiplexer  208 , which receives input from a time of day (TOD) clock master  210  that in turn receives a pulse per second (PPS) signal from a global positioning system (GPS)  214 . The GPS  214  also provides the pulse signal to a master oscillator  220 , which provides a 10 MHz (or other suitable frequency) signal to both the TOD clock master  210  and a frequency generator  224 . 
     The frequency generator  224  generates a transmit carrier signal (Tx carrier) and a receive carrier signal (Rx carrier), used in transmitting and receiving RF signals at a transmitter  230  and a receiver  232 . The carrier signals may be at 22 GHz and 24 GHz, to give non-limiting example values. 
     Output from the multiplexer  208  is passed through a serializer/deserializer (SerDes)  240 , which has a data link layer (DLL)  244 . The serializer/deserializer  240  also receives input from the receiver  232 , and passes data to a response stack  246 , which is coupled to the GMAC  202  at the downstream end of the response stack  246 . 
     The transmitter  230  and the receiver  232  are coupled to a triplexer  250 , which is configured to send signals to and receive signals from the rotary joint  10 . The triplexer  250  also sends received signals through a low noise amplifier (LNA)  254 , to provide baseband sensor data  256  to the GMAC  202 . 
     As noted above, the interface  200  may be used to provide multiplexed signals using any suitable combination of time, frequency, and/or code division multiplexing. Signals can be passed through multiple of the rotary joints  10 , without a need to demultiplex the signals after each of the rotary joints  10 . The multiplexed signal may be interacted with along the way, for example with control multiplexer and add/drop (CMAD) interfaces  300 , many details of which are shown in  FIG. 8  but are not described herein because such details are conventional in the art and thus are not necessary to understand the invention. The interfaces  300  use components similar to those of the interface  200  to transmit and receive the multiplexed signals passing along and through some or all of the rotary joints  10 . The CMAD interfaces  300  may include hardware interfaces  310  for interacting with hardware that may receive signals from the multiplexed signal, and/or that may send signals to be transmitted as part of the multiplexed signal. 
     The rotary joint  10  provides many advantages over prior rotary electrical connections. The electrical connection is contactless, which means that there is no wear and tear from a need to have electrical contact maintained as the parts are rotated relative to one another. In addition the rotary joint  10  can operate with a full 360-degree rotation, which cannot be achieved by coaxial cables, for example. Further, as noted earlier, by keeping the central core region open, sending of optical signals can be accomplished along the axis of rotation. Near-constant phase and amplitude performance can be maintained independent of rotation. 
     The rotary joint  10  may be used in any of a variety of situations. One example of use is to send signals for rotating motors for positioning an optical sensor, such as in a pod on an airplane. Many other uses for the rotary joint  10  are possible. 
     Although the invention has been shown and described with respect to a certain preferred embodiment or embodiments, it is obvious that equivalent alterations and modifications will occur to others skilled in the art upon the reading and understanding of this specification and the annexed drawings. In particular regard to the various functions performed by the above described elements (components, assemblies, devices, compositions, etc.), the terms (including a reference to a “means”) used to describe such elements are intended to correspond, unless otherwise indicated, to any element which performs the specified function of the described element (i.e., that is functionally equivalent), even though not structurally equivalent to the disclosed structure which performs the function in the herein illustrated exemplary embodiment or embodiments of the invention. In addition, while a particular feature of the invention may have been described above with respect to only one or more of several illustrated embodiments, such feature may be combined with one or more other features of the other embodiments, as may be desired and advantageous for any given or particular application.