GB2328086A - Rotary signal coupler - Google Patents

Abstract

A device for providing coupling between two relatively rotatable components comprises at least one first substantially annular transmission line 14, 15 secured to a support 6 and having a gap 16 therein which forms a electrical discontinuity. A second substantially annular transmission line (not shown on figure 1) is secured to a member 7 which is rotatable relative to the support 6. The second track also has a gap 16 which forms an electrical discontinuity. Both transmission lines are coaxial with the axis of rotation of the rotatable member 7 and are adjacent to provide signal coupling therebetween. A capacitor 19 is connected across the gap of at least one of the lines. The lines may either face each other along the axis (see figure 1), or one line may be radially outside the other (see figure 2).

Description

ROTARY SIGNAL COUPLER This invention relates to a rotary signal coupler, that is to say a device for providing signal coupling between two components which are rotatable relative to each other.

Published International patent application WO 91/13832 describes a strain measuring method and apparatus particularly suitable for measuring the torque applied to a shaft. The described method and apparatus make use of a surface acoustic wave (SAW) device mounted on the shaft.

Such devices require the passage of high frequency, typically radio frequency (RF), signals between the SAW device and its associated drive/measuring circuitry. If the shaft to which the SAW device is attached rotates only though a small angular range the SAW device may be hard wired to its associated drive/measuring circuitry. There are, however, many applications of the torque measuring technique described in WO 91/13832 which are not succeptable to hard wiring between the SAW device and its associated drive/measuring circuitry, and such applications require the use of a rotary signal coupling device in order to effect the required connection.

Published International patent application WO 96/37921 discloses a rotary signal coupling device which may be used to provide the required coupling to a SAW device at RF frequencies. The described device includes a pair of transmission lines, each comprising an electrically conductive track and an associated ground plane. The tracks are each substantially circular, but each defines a gap so that each track forms with its associated ground plane a transmission line. The tracks are arranged coaxially about the shaft carrying the SAW device, one track and its associated ground plane being secured to the shaft whilst the other track and its associated ground plane is secured to a bearing through which the shaft passes. The tracks are separated by a thin sheet of dielectric material, or by a small air gap. One end of the track secured to the bearing is connected to the drive/measuring circuitry and one end of the track which is secured to the shaft is connected to the SAW device. The ends of the tracks opposite to their respective connections to the drive circuitry and the SAW device may be earthed or may be left open circuit.

The above described rotary signal coupler has a characteristic impedance which is substantially constant over a wide frequency range. However, the device has been found to be unsatisfactory in that both the phase variation and attenuation of signals passing though the coupler have been found to be dependent upon the relative rotational positions of the fixed and movable parts of the coupler.

These phase variations and attenuation variations are highly undesirable since they significantly complicate the interpretation of the signals derived from the SAW device.

We have now found that if a capacitor is coupled across the gap between one or both of the tracks the problem of variable phase and variable attenuation is significantly reduced. Whilst introduction of capacitance in this manner results in some tuning of the coupler so that the frequency range over which it has substantially constant impedance reduced, in practice a device can be produced which still has an acceptable frequency range, but which has the advantages of reduced phase variation and reduced attenuation variation produced by the present invention.

Accordingly, the present invention provides a rotary signal coupler comprising: a first substantially annular track secured to a support, the first track having a gap therein which forms an electrical discontinuity in the first track; a second substantially annular track secured to a member which is rotable relative to the support, the second track having a gap therein which forms an electrical discontinuity in the second track, the first and second tracks being coaxial with the axis of rotation of the rotatable member and being adjacent each other to provide signal coupling there between; a first terminal connected to the first track; a second terminal connected to the second track; and a capacitor connected across the gap of at least one of the tracks.

Preferably, respective capacitors are connected across the gaps of both tracks.

Preferably, each terminal is connected to one end of its associated track. Preferably the other end of each track is earthed.

Preferably each track is secured to one side of a respective support and the other side of each support is provided with a screen. The screens are preferably earthed.

Preferably one end of each track is earthed to its associated screen. The capacitance can conveniently be provided between the opposite end of the track and the ground plane.

The preferred embodiment of the invention has been found not only to provide relative insensitivity to phase variation and attenuation as the parts of the coupler rotate relative to each other, but has also been found to provide a coupler which is relatively insensitive to axial and radial movements of the coupler components relative to each other.

In particular, in one embodiment of the invention where the tracks are located in substantially the same axial zone of the rotary member, and are formed radially nested one within the other, the resultant coupler is relatively insensitive to axial movement of the inner track relative to the outer track.

The above and further features and advantages of the invention will become clear from the following description of preferred embodiments thereof given by way of example only, reference being had to the accompanying drawings therein: Figure 1 illustrates schematically a first embodiment of the invention and Figure 2 illustrates schematically a second embodiment of the invention.

Referring firstly to Figure 1, the illustrated coupler 1 is shown schematically for providing signal coupling between a coax cable 2 and a coax cable 3. In the illustrated embodiment the coax cable 2 is connected to a driver/measuring circuit and the coax cable 3 is connected to a SAW device 4 mounted on a shaft 5. The coupling accordingly facilitates signal connection between the driver/measuring circuit and the SAW device for the purpose of measuring torque applied to the shaft 5.

The coupler 1 comprises a first part 6 which is secured to a fixed support by appropriate means and a second part 7 which is secured to the shaft 5. The parts 6,7 face each other and, in practice, are separated either by a small air gap or by a thin sheet of insulating material. The separation of the parts 6,7 has been exaggerated in the drawing so that the structure of the part 6 may be seen clearly. In practice, the parts 6,7 are likely to be separated by a small amount, typically 1 to 5mm.

The member 6 comprises a sheet 8 of insulating material which supports, on the side thereof remote from the part 7, a metal screen 9. Similarly, the part 7 comprises a sheet 10 of insulating material which supports, on the side thereof remote from the part 6, a metal screen 11. The screen 9 will, in many applications, be earthed, e.g. by way of connection to the screen 12 of the coax cable 2. The screen 11 will, in general, be electrically connected to the shaft 5, e.g. by way of the screen 13 of the coax cable 3.

The shaft 5 will in general be earthed and accordingly the screens 8 and 11 are electrically connected.

The first part 6 has formed thereon two annular tracks 14,15. In a basic embodiment of the invention only one track will be present, but in more complicated embodiments several additional tracks may be present. Additional tracks may be used for signal coupling to additional devices. For example if two separate SAW devices are secured to the shaft two separate tracks would be used to provide coupling to them. It is to be understood, accordingly, that the present invention is not limited to any particular number of annular tracks and, in its broadest embodiment, comprises only a single annular track.

The tracks 14,15 may be of any suitable material, for example copper foil.

The tracks 14,15 are in the form of complete circles except for a gap 16 which forms an electrical discontinuity in each track. One end of the track 14 is connected to the core 17 of the coax cable 2. It is to be understood that if additional tracks, for example the track 15, are used, they will have associated therewith appropriate cable connections. For the purposes of illustration, only the outer track 14 is shown connected to a cable. In the illustrated embodiment of the invention the end of the track 14 opposite to the connection to the core 17 is connected to the screen 12 of the coaxial cable and to the screen 8. It is to be understood, however, that in some embodiments of the invention alternative arrangements may be desirable.

For example, the end of the track remote from the connection to the coaxial cable may be left open circuit (except for the additional capacitor referred to below).

The face of the part 7 adjacent the part 6 has formed thereon tracks which mirror those of the part 6, as described above. One end of the outer track of the part 7 is connected to the core 18 of the coax cable 3, and the opposite end of that track is connected to the screen 13 of the coax cable 3 and to the screen 11 of the part 7. As with the track 14 of the part 6, track of the part 7 may be left open circuit at the end opposite the connection to the core 18.

Each track 14 has connected across the gap 16 a capacitor. The capacitor may be a fixed capacitor or a variable capacitor permitting some adjustment to the capacitance of the circuit. The size of the capacitor will be selected so that the circuit is tuned to a slightly broader bandwidth than the frequency band expected to be encountered in use of the apparatus. The object is to tune the circuit -to a small extent, but to leave the bandwidth broad enough to provide substantially constant coupling characteristics over the entire expected use frequency range. In one typical embodiment of the invention in which the screens 8,11 are earthed, the supports 8,10 have a thickness of 1.5mm, the outer track 14 has an inside diameter of 40mm and a radial extent of 5mm, the value of the capacitor 19 will be approximately 30PF. This figure is similar to the capacitance between the track and the screen 8,11. Such a device would typically have substantially uniform transmission characteristics over the frequency range 185 - 215 MHz.

It has been found that the device described above exhibits a substantially lower phase variation with rotation and a substantially lower variation in impedance with rotation than prior art devices in which the capacitor 19 is not present.

It is envisaged that each track of each pair of mating tracks will be furnished with a capacitor of substantially the same value. It is to be understood, however, that the invention is not limited to this arrangement and useful embodiments of the invention may be constructed in which only one of each pair of associated tracks is furnished with the capacitor 19. Further, although as illustrated the capacitor 19 is physically positioned on the parts 6,7 it is possible for the capacitor to be physically located remote from the part and electrically connected across the gap 16 by, for example, being electrically connected between the core and the screen of the associated coax cable.

Referring now to Figure 2, a second embodiment of the invention is shown. In this embodiment, the first part 20 of the coupler is in the form of a ring which surrounds the second part 21 which is in the form of a ring which surrounds and is secured to the shaft 22. The rings 20,21 are located in the same axial zone relative to the axis for rotation of the shaft 22 and are separated by a small gap 23 which may be an air gap or may be filled with an insulating material. The size of the gap has been exaggerated in the drawing for the purposes of illustration.

The part 20 comprises a ring 24 insulating material which is mounted inside a metal ring 25. An annular track 26 formed, for example, of a metal foil is formed on the inside of the insulating ring 24. The track is substantially circular except for a gap 27 which forms an electrical discontinuity in the track. The core 28 of an incoming coax cable is connected to one end of the track 26 adjacent the gap whilst the screen 29 of the incoming cable is connected to the other end of the track and to the metal ring 20. A capacitor 30 is connected across the gap 27.

The other part of the coupler comprises a metal ring 31 which is secured to and electrically connected with the shaft 22. The ring 31 is surrounded by an insulating ring 32 on the surface of which an annular track 33 is provided by way of a strip of metal foil. The track 33 is continuous except for a gap 34 which forms an electrical discontinuity in the track. A SAW device 35 is connected across the gap 34, and one end of the track 33 is connected to the ring 31.

Although the physical arrangement of the tracks of the embodiments of Figure 2 is different from that of the embodiment of Figure 1, the electrical operation of the system is as described above with reference to Figure 1.

In addition to providing relative insensitivity to phase variation and impedance variation upon relative rotation of the coupler parts, the couplers described above have been found to be relatively insensitive to misalignment, particularly radial mis-alignment in the case of the Figure 1 embodiment and axial mis-alignment in the case of the Figure 2 embodiment.

Claims (1)

1. CLAIM:

   1. A rotary signal coupler comprising: a first substantially annular track secured to a support, the first track having a gap therein which forms an electrical discontinuity in the first track; a second substantially annular track secured to a member which is rotatable relative to the support, the second track having a gap therein which forms an electrical discontinuity in the second track, the first and second tracks being coaxial with the axis of rotation of the rotatable member and being adjacent each other to provide signal coupling therebetween; a first terminal connected to the first track; a second terminal connected to the second track; and a capacitor connected across the gap of at least one of the tracks