GB2280315A - Rotatable electric transformer coupling - Google Patents

Abstract

An electrical coupling (10) incorporates a primary element (36, 40) electromagnetically coupled and rotatably movable relative to a secondary element (38, 42). Electrical signals are transmitted to the primary element (36, 40) where magnetic flux is generated in a ferrite core. The flux is coupled to the ferrite core of the secondary element (38, 42) and an output signal induced in the cable winding of the secondary element (38, 42). Several primary-secondary element pairs can be stacked together to provide a number of channels between the system parts, and for simultaneous transmission of signals in different directions. Each primary-secondary element pair may be located within metal housings. The primary and secondary elements may have an axis perpendicular to axes of tubes which can rotate about the said axis. <IMAGE>

Description

ELECTRICAL COUPLING This invention relates to an electrical coupling. More particularly it relates to an electrical coupling incorporating an electrical conductor which is movable relative to a second electrical conductor coupled thereto.

Many detection systems, such as infra-red scanners or radar systems, include a rotating sensor or antenna which sweep an area under observation. Such systems also require signal processing equipment to analyse the signal received by the rotating sensor or antenna. The signal processing equipment is usually mounted below the rotating sensor or antenna in a stationary housing. This leads to a need for an electrical coupling between rotary and stationary elements.

In radar systems, microwave radiation is generated with frequencies above 1 GXz by the use of a solid state device or by a valve such as a magnetron. The solid state device or valve may be located in a stationary housing, or on a rotating antenna in the case of a solid-state device. In some radar systems it is therefore necessary for radiation to be conveyed to a rotating antenna for transmission. In all systems employing a rotating antenna the received radar signals are conveyed to signal processing equipment for analysis, usually after conversion to an intermediate frequency in the range 10 MHz to 1 GHz. It is also necessary to couple other electrical signals from a stationary housing to a rotating antenna, such as digital clock signals, control signals or calibration signals.

Several devices are currently used in radar systems for the transmission of microwave radiation and intermediate frequency electrical signals from a stationary housing to a rotating antenna and vice versa. Slip rings are one such device. They consist of a set of metal rings and a corresponding set of connectors, such as brushes, which contact the surfaces of the respective rings. The brushes and rings can rotate relative to each other. The rings are arranged to rotate with an antenna and the connectors are stationary. The signal from the antenna ws transmitted to the set of rotating rings, and passes to the connectors in contact therewith, and thence to signal processing equipment. This device has several disadvantages. The rings and connectors are in contact whilst rotating relative to each other, and they suffer from mechanical wear.

Consequently, metal slivers can be produced by abrasion with the risk of short circuits in the slip ring assembly. Mechanical wear of the rings and connectors also causes signal loss in transmission, and their contact can also generate sparking causing noise in the signal. Because of the effects of wear on the noise performance the slip rings need to be replaced regularly.

Rotary joints are used for the transmission of microwave radiation between a stationary housing and rotary antenna, and they can also be used for transmission of signals at intermediate frequencies. They comprise a rotor and a stator. The rotor is rotatable relative to the stator about a common longitudinal axis. The rotor and stator can rotate relative to each other with or without physical contact. They define an electromagnetic coupling junction. Signals from the radar antenna are transmitted through the rotor by a waveguide or coaxial line to the electromagnetic junction. The signal is then coupled across the electromagnetic junction to the stator and transmitted through the stator by a waveguide or co-axial line to the signal processing equipment.

Rotary joints with contacting junctions between the rotor and stator are used in applications requiring a broadband frequency response. In these devices there is a low impedance contact at the rotational interface between the rotor and stator. However, such joints are not reliable at high power levels or at high relative rotational speeds between the rotor and stator. Furthermore, wear at the contacting junction can cause oxidation of the contacting surfaces. This can result in noise being imposed on the signal transmitted between the elements.

In rotary joints where there is no physical contact between the rotor and stator the signal is transmitted from one to the other by two quarter-wave transmission lines of different impedances. The signal then propagates along a transmission line formed by an overlap between co-operating parts of the rotor and stator.

In systems where more than one signal is transmitted from the radar antenna to the signal processing equipment, several microwave rotary joints are stacked vertically and rotate around a common vertical axis.

Each rotary joint couples a respective signal between the radar antenna and the signal processing equipment.

Rotary joints are unusual devices in the microwave field because they are also dynamic devices. Consequently, mechanical aspects of the design of rotary joints must be considered in addition to the microwave design features. Axial and radial alignment of the rotary joints are also particularly important in applications where the rotary joints are assembled into a stack arrangement. As both mechanical and microwave aspects of the rotary joint have to be accurately designed the rotary joint is a precision component, and is therefore an expensive item. In applications where stacking of the joints is necessary the cost is proportionally higher.

Radar systems currently in use typically employ a combination of slip rings and rotary joints for the transfer of microwave radiation and intermediate frequency signals to and from the rotating part of the radar system.

It is an object of the invention to provide an alternative form of device for transmitting signals between two members which move relative to one another.

The present invention provides an electrical signal coupling arranged as a transformer and including a primary element electromagnetically coupled to a secondary element, and wherein one of the elements is movable relative to the other such that electromagnetic coupling is preserved during movement.

The invention provides the advantage that signals can be transmitted by electromagnetic coupling between two members with relative rotary moti-n.

Tests on an embodiment of the invention incorporated in a radar system indicate that transmission losses are low over a wide bandwidth in the intermediate frequency range. The invention has the further advantage that it can be implemented simply and can be made from cheap, commercially-available materials. The invention is capable of being manufactured for a fraction of the cost of current alternatives, and is therefore suitable for use in low-cost high volume production radar systems such as those used on small marine vessels.

The primary and secondary elements may be spatially separated. This has the advantage that there is little scope for damage to the elements due to wear. The invention may include several pairs of primary and secondary elements arranged in a stack. This enables several electrical signals to be transmitted simultaneously between relatively movable parts of a system.

The invention may include a plurality of pairs of primary and secondary elements, each pair being located within a housing. Each housing has an individual shaft about which one of the elements of each pair is rotatable. The relative spacing of the primary and secondary elements can be adjusted to compensate for any relative movement of the elements along the shaft.In a stacked version each housing is connected to its neighbouring housings by a flexible coupling.

In order that the invention might be more fully understood, embodiments thereof will now be described, by way of example only, with reference to the accompanying drawings in which: Figure 1 is a perspective view of an electrical coupling device of the invention; Figure 2 is a plan view of a core of primary and secondary elements incorporated in the device in Figure 1; Figure 3 is a view along the arrows AA of the section along the line III-III in Figure 2; Figure 4 is a sectional view of a housing containing primary and secondary elements; Figure 5 is a sectional view of a device incorporating the housings shown in Figure 4; Figure 6 is a sectional view of an angled device of the invention; Figure 7 is a plan view of the device of Figure 6.

Referring to Figure 1, there is shown a perspective view of an electrical coupling device of the invention indicated generally by 10. The coupling 10 incorporates a hollow cylindrical outer tube 12 within which is an inner shaft 14. The tube 12 and the shaft 14 share a common central longitudinal axis 16 extending in the z-direction indicated by an arrow 17. The tube 12 has end plates 18 and 20 with respective central apertures 22 and 24. The shaft 14 extends in the z-direction 17 through the apertures 22 and 24. The diameter of the apertures 22 and 24 is significantly greater than the diameter of the shaft 14. The plates 18 and 20 have respective ball bearing assemblies 26 and 28 mounted on their respective inner surfaces 30 and 32. The bearing assemblies 26 and 28 cooperate with the shaft 12 which passes through them.They enable the shaft 14 to rotate relative to the tube 12 with substantially no friction at its points of support. The shaft 14 is rotatable as indicated by an arrow 34.

Four toroidal ferrite cores 36, 38, 40 and 42 with like physical dimensions are situated inside the tube 12. Referring now also to Figures 2 and 3 a core of the kind 36, 38, 40 or 42 is shown in more detail, and is indicated generally by 200. The core 200 is circularly symmetric about the central axis 16 of the tube 12 and shaft 14. The diameter of the core 200 is smaller than the diameter of the tube 12. The core 200 has a raised circular outer rim 202 and a raised annular central section 204.

The central section 204 has a circular aperture 206 which extends throughout the thickness of the core 200. The raised outer rim 202 and the raised central section 204 define an annular groove 208 between them.

The core 200 has a rear surface 210 and an open front surface indicated generally by 212. The rear surface 210 has a bore 214 directly under the groove 208. The bore 214 extends through the thickness of the core 200 to connect the groove 208 to the rear surface 210. One end of a co-axial cable 216 is inserted into the bore 214 at the rear surface 210. One end of a copper wire 218 is attached to an outer conductor of the co-axial cable 214 and is wound twice around the raised central section 204 to form a two-turn coil in the groove 208. The other end of the wire 218 is attached to an inner conductor of the cable 216.

Referring again to Figure 1, the cores 36 to 42 have central circular apertures 44 to 50 respectively. They have a common central axis which is the axis 16 shared by the tube 12 and the shaft 14. The shaft 14 thus passes through the central circular apertures 44 to 50 of the cores 36 to 42 respectively. The diameter of the circular apertures 44 to 50 is greater than the diameter of the shaft 14.

The cores 36 and 40 have flat rear surfaces 52 and 54 respectively, and grooved front surfaces 56 and 58 respectively. Similarly, cores 38 and 42 have flat rear surfaces 60 and 62 respectively and grooved front surfaces 64 and 66 respectively. The rear surfaces 52 and 54 are closer to the end plate 20 than the front surfaces 56 and 58. Beginning with the core 36 nearest the end plate 20, the sequence of cores is 36, 38, 40 and 42. The rear surfaces 60 and 62 are more remote from the end plate 20 than the front surfaces 64 and 66. Thus the grooved front surfaces 56 and 58 of the cores 36 and 40 respectively directly face the like surfaces 64 and 66 of the cores 38 and 42 respectively. The cores 36 to 42 have respective outer rims 68, 70, 72 and 74. The rims 70 and 74 are 0.25 mm distant from the rims 68 and 72 respectively in the positive z-direction 17.

The cores 38 and 42 are secured to the tube 12 by cylindrical bushes 76 and 78 respectively which surround the cores 38 and 42 at respective outer edges 80 and 82. The bushes 76 and 78 prevent rotational movement of the cores 38 and 42 with respect to the tube 12, and also prevent longitudinal movement of the cores 38 and 42 along the z-axis.

The cores 36 and 40 are secured to the shaft 14 by bushes 84 and 86 respectively. The bushes 84 and 86 are located in the apertures 44 and 48 of the respective cores 36 and 40. They prevent rotational movement of the cores relative to the shaft 14, and hence rotation of the shaft 14 in the direction of the arrow 34 therefore causes a corresponding rotation of the cores 36 and 40 in the directions of the arrows 88 and 90 respectively. The bushes 84 and 86 also prevent movement of the cores 36 and 40 with respect to the shaft 14 in the z-direction 17.

The hollow shaft 14 has an opening 92. Two co-axial cables 94 and 96 are located inside the shaft 14, and project through the opening 92. The cable 94 also projects through an aperture 98 in the shaft 14 and into the core 36 in an aperture 100. A copper wire (not shown) with ends attached to the inner and outer conductor of the cable 94 is wound twice around the raised central section (not shown) of the core 36. Similarly, the opposing end of the cable 96 projects through an aperture 102 in the hollow shaft 14 and into the core 40 via an aperture 104. A copper wire (not shown) with ends attached to the inner and outer conductors of the cable 96 is wound twice around the raised central section (not shown) of the core 40.

The hollow tube 12 has two apertures 106 and 108 respectively. Two co-axial cables 110 and 112 project into the tube 12 via the apertures 106 and 108 respectively, and are inserted into the cores 38 and 42 via apertures (not shown) in the rear surfaces 60 and 62. The cables 110 and 112 are each connected to copper wires (not shown), the ends of which are attached to the respective inner and outer conductors of the co-axial cables 110 and 112. The wires are each wound twice around the respective central sections (not shown) of the cores 38 and 42.

The operation of the coupling 10 will now be described. This embodiment is a two-level stack device in which electrical signals can be transmitted between a rotating part of a radar system and a stationary housing in two directions simultaneously.

Electrical signals from the rotating part of a radar system are transmitted along the cable 94 to the coupling 10. The signals are transmitted along the cable 94 through the shaft 14, and through shaft aperture 98 to the aperture 100 of core 36. The wire (not shown) attached to the cable 94 is wound twice around the raised annular central section (not shown) of core 36 and the signal is transmitted to the winding around the raised central section of core 36. The current in the winding of core 36 sets up a magnetic flux in the ferrite material of the core 36. The magnetic flux in core 36 couples into the core 38. The winding in the core 38 intercepts the magnetic flux coupled into core 38.

For a signal transmitted to the core 36 there is an associated primary current il, induced in the primary winding, and a voltage V1 induced in the primary winding which is proportional to the inductance. As the signal changes with time, and hence V1 changes, the current il also changes. Changes in il cause a change in the magnetic flux in core 36, and hence causes a change in the flux coupled from core 36 to 38 which is intercepted by the winding of core 38. The time-varying current i1 thus induces an electromotive force (e.m.f.) in the circuit connected to the windings of core 38. This e.m.f. is denoted by V2, where dil V2 - M di1 in which M is the mutual inductance between the circuits connected to the primary and secondary elements.

The inductance of the primary winding determines the primary voltage V1, and the primary inductance is dependent on the number of terms of the winding. Similarly, if the winding of the secondary intercepts all of the magnetic flux in the core 38 the induced voltage V2 depends upon the number of turns of the winding around core 38. If the number of turns of winding around the core 36 is denoted by n1 and the number of turns around the core 38 by n2, then V1 nl TZ n2 For n1 equal to n2, as in the current embodiment where n1 and n2 are both equal to 2, then V1 is equal to V2. The signal transmitted to the stationary part of the radar system from the coupling 10 along the cable 110 will therefore be substantially the same as the signal transmitted to the coupling 10 along the cable 94.

Tests on a coupling device of the invention show that transmission loss between cores is low across a wide bandwidth. Transmission loss was also found to be little changed when the cores were rotated.

Similarly, signals from the stationary part of a radar system are transmitted to the rotating part of the system through the coupling 10.

The input electrical signals are transmitted along the cable 112 to the core 42, which is fixed. The signals induce a magnetic flux in the core 42 and this is coupled into the rotating core 40. An output signal is generated in the cable 96 which forms the winding of core 40, as previously described for the coupling between cores 36 and 38. The output signal is transmitted along cable 96 through the core aperture 104 into the shaft aperture 102, and along the shaft 14 to the rotating part of the radar system.

Referring to Figures 4 and 5, there is shown a further embodiment of the invention. Figure 4 shows a fixed core 310 and a rotating core 312 located within a cylindrical metal housing 314. The housing 314 has an end plate 316. The fixed core 310 is securely attached to the end plate 316 with glue. The end plate 316 is secured to the body of the housing 314 by two screws 318 and 320. The end plate 316 can be removed by unscrewing screws 318 to 320 to obtain access to the interior cavity of the housing 314 if required. The rotating core 312 is securely attached to a hollow rotating shaft 322 by a plastic bush 324. The shaft 322 is threaded on its external surface. The housing 314 and the shaft 322 share a common central longitudinal axis 323. Each of the cores 310 and 312 has a two-turn winding 324 and 326 respectively.Windings 324 and 326 are formed from copper wires connected at their respective ends to the inner and outer conductors of the co-axial cables 328 and 330 respectively.

Cable 328 enters the core 310 via an aperture 332 in housing end plate 316 and aperture 334 in core 310. Cable 330 is threaded along the inside of hollow shaft 322 and fed through an aperture 336 therein to the interior cavity of the housing 314. It then passes through an aperture 338 in the core 312 to form the winding 326. The shaft 322 is capable of substantially frictionless rotation within the housing 314 by virtue of bearing elements 340 and 342. The shaft 322 has ends 344 and 346, to which are attached respective coupling members 348 and 350. Each of the coupling members 348 and 350 is capable of receiving a bridge member (not shown) for connection of the coupling members 348 and 350 to like coupling members of neighbouring housings. A threaded nut 352 is located on the shaft 322 between the coupling member 350 and the housing 314.A spring 354 is located on the shaft 322 between the coupling member 348 and the housing 314. The spring 354 biasses the housing 314 towards movement along the axis 323 in the positive x-direction, as indicated by axes 356, and the nut 352 prevents movement of the housing 314 in the positive xdirection. The combination of the nut 352 and the spring 354 enables the separation of the cores 310 and 312 to be adjusted. Looking towards the nut 352 from the region of the coupling member 350, rotation of the nut 352 in a clockwise direction around the threaded shaft 322 causes the nut 352 to move along the shaft 322 in the negative x-direction indicated by axes 356. As the housing 314 is biassed against the nut 352 by the spring 354, movement of the nut 352 in the negative x-direction causes movement of the housing 314 with respect to the shaft 322 in the same direction.

The core 310 is secured to the end-plate 316 of the housing 314, and thus movement of the housing 314 in the negative x-direction causes movement of the core 310 in the negative x-direction. The core 312 is secured to the shaft 322 and is not affected by movement of the housing 314 with respect to the shaft 322. Consequently, the movement of the core 310 in the negative x-direction causes an increased separation between the cores 310 and 312.

Conversely, rotation of the nut 352 in an anticlockwise direction around the shelf 322 causes the nut 352 to move along the shaft 322 in the positive x-direction. As the spring 354 biasses the housing 314 in the positive x-direction against the nut 352 the housing 314, and the core 310 move in the positive x-direction along the shaft 322. The core 310 thus moves towards the core 312 and the separation between the cores 310 and 312 is reduced.

This embodiment is particularly advantageous in that it enables the spacing between the elements to be adjusted separately to optimise the electromagnetic coupling between the elements. Tests on a device of this embodiment indicate that transmission losses are low over a wide bandwidth in the intermediate frequency range. Between 10 MHz and 200 MHz transmission losses are only 2 dB and at the mid-point of the range the transmission loss is only 1 dB. The tests also indicate that there is virtually no variation in coupling during the relative rotation of the primary and secondary elements.

A stacked coupling device in which the core pairs are located in housings is shown in cross-section in Figure 5. The details of each of the housings has been omitted, these being identical to the housing described previously and shown in Figure 4. Each of the housings 410, 412 and 414 has a shaft 416, 418 and 420 respectively. The housings 410 to 414 are located within a hollow tube 422. The housings 410 to 414, shafts 416 to 420 and tube 422 share a common longitudinal axis 424. Housing 410 has a coupling 426, which is connected to a coupling 428 of the housing 418.

Similarly, housing 418 has a second coupling 430, which is connected to a coupling 432 of housing 420.

This embodiment has the advantage that several electrical signals can be transmitted simultaneously between parts of a system moving relative to one another. The signals can also be transmitted in opposing directions.

The separation between the elements in each of the housings can be adjusted separately, enabling the electromagnetic coupling between the elements to be optimised for each housing.

Figures 6 and 7 show a further embodiment of the invention. It incorporates a hollow cylindrical tube 510 which contains a hollow shaft 512. The hollow tube 510 has a central longitudinal axis 514. The shaft 512 is offset to one side of the axis 514. A ferrite core 516 is attached at an end of the hollow shaft 512. The core 516 is circularly symmetric about an axis 518, which is shown in the sectional plan view of the embodiment in Figure 7. The axes 514 and 518 are mutually perpendicular.

A co-axial cable 520 is located within the shaft 512. The cable 520 emerges from the shaft 512 via an aperture 522 and enters the core 516 via aperture 524. The copper wire is attached at one end to the outer conductor of the cable 520, and at its other end to the inner conductor of the cable 520. The wire forms a two-turn winding (not shown) within the core 516.

A second hollow cylindrical tube 526 contains a hollow shaft 528. The hollow tube 526 has a central longitudinal axis 530. The shaft 528 is offset to one side of the axis 530. A ferrite core 532 is attached at one end of the shaft 528. The core 532 is circularly symmetric about the axis 518. Axes 518 and 530 are mutually perpendicular. A co-axial cable 534 is threaded through the hollow shaft 528, emerging from the shaft 528 through an aperture 536 and into the core 532 via aperture 538. A copper wire is attached at one end to the outer conductor of the cable 534, and at its other end to the inner conductor of the cable 534. The wire forms a two-turn winding (not shown) in the core 528.

The cores 516 and 532, and the axes 514 and 530 are capable of relative rotation about the axis 518. As axes 514 and 530 can rotate about the axis 518 the tubes 510 and 516 and shafts 512 and 528 are also capable of rotation about the axis 518.

As cores 516 and 532 are both symmetric about the same axis 518 the cores 516 and 532 directly face each other. Signals transmitted along the cable 520 to the core 516 are coupled across to the core 532 and an output signal is transmitted from the core 532 along cable 536.

The tubes 510 and 526 are capable of relative rotation about axis 518, and this embodiment is therefore particularly suited to robotic applications.

For instance, tubes 510 and 526 may form segments of a jointed robotic arm.

Claims (25)

1. An electrical coupling arranged as a transformer and including a primary element electromagnetically coupled to a secondary element, and wherein one of the elements is movable relative to the other such that electromagnetic coupling is preserved during movement.

2. An electrical coupling as claimed in Claim 1 including a shaft located in and rotatably movable relative to a tube, wherein one of the elements is mounted on the shaft and the other is mounted inside the tube.

3. An electrical coupling as claimed in Claim 2 wherein both elements are annular and the shaft extends through respective central apertures therein.

4. An electrical coupling as claimed in Claim 2 or 3 wherein the shaft is hollow and contains means for conveying electrical signals, and the conveying means is connected at one end to a primary or secondary element mounted on the shaft.

5. An electrical coupling as claimed in Claim 4 including a plurality of conveying means arranged to convey signals in opposing directions along the shaft simultaneously.

6. An electrical coupling as claimed in Claim 1 or 5 including a plurality of primary elements, each primary element being electromagnetically coupled to a corresponding secondary element.

7. An electrical coupling as claimed in Claim 1 including a tube having a central longitudinal axis and one element rotatably movable relative to the other about an axis common to both elements, wherein the tube ax - and element axis are substantially perpendicular and the tube is rotatable about the element axis.

8. An electrical coupling as claimed in Claim 1 including a plurality of primary and secondary element pairs, each pair being located within a respective housing, and wherein each housing has a respective shaft is connected to other shafts of neighbouring housings by flexible connections.

9. An electrical coupling as claimed in Claim 8 incorporating means for varying the separation between a primary element and a secondary element to alter the electromagnetic coupling between the elements.

10. An electrical coupling as claimed in Claim 9 wherein the separation varying means incorporates a biassing means arranged to bias the housing and a means for opposing the bias.

11. An electrical coupling as claimed in Claim 10 wherein the biassing means is a spring and the biassing opposing means is a threaded nut.

12. An electrical coupling as claimed in any preceding claim incorporating a primary element separated from a secondary element by a distance of less than 0.5 mm.

13. An electrical coupling as claimed in Claim 12 incorporating a primary element separated from a secondary element by a distance of 0.25 mm.

14. An electrical coupling as claimed in any preceding claim wherein each element comprises a core of ferrite material.

15. An electrical coupling as claimed in any preceding claim arranged to couple electrical signals at a frequency in the range 10 Hz to 1 GHz.

16. An electrical coupling as claimed in Claim 16 arranged to couple electrical signals at a frequency in the range 1 MHz to 400 MHz.

17. An electrical coupling as claimed in Claim 17 arranged to couple electrical signals at a frequency in the range 1 MHz to 200 MHz.

18. An electrical coupling as claimed in Claim 18 arranged to couple electrical signals at a frequency in the range 10 MHz to 100 MHz.

19. An electrical coupling as claimed in any preceding claim mounted in a radar system and arranged to couple radar antenna signals.

20. An electrical coupling as claimed in any preceding claim mounted in an infra-red scanner system and arranged to couple infra-red scanner signals.

21. An electrical coupling as claimed in any preceding claim mounted in a robotic system and arranged to couple robotic system signals.

22. A method of coupling electrical signals in a transformer arrangement including arranging for electromagnetic coupling between a primary element and a secondary element wherein one of the elements is movable relative to the other such that electromagnetic coupling is preserved during movement.

23. An electrical coupling substantially as described herein with reference to and illustrated by Figure 1.

24. An electrical coupling substantially as described herein with reference to and illustrated by Figures 4 and 5.

25. An electrical coupling substantially as described herein with reference to and illustrated by Figures 6 and 7.