Abstract:
A rotary signal coupler for providing signal coupling to a Surface Acoustic Wave (SAW) device ( 4 ) mounted on a shaft ( 5 ) includes a first electrically conducting loop ( 21 ) mounted on a disc ( 23 ) and connected to the SAW device ( 4 ), and a second electrically conductive loop ( 22 ) mounted on a disc ( 24 ) and connected to external electronic circuitry. The disc ( 23 ) is fixed relative to the shaft ( 5 ) and the disc ( 24 ) is fixed relative to the structure in which the shaft ( 5 ) rotates so that the loops ( 21, 22 ) are inductively coupled. A grounded screen ( 27 ), which preferably takes the form of a plurality of radially extending fingers is located on the disc ( 23 ) and positioned between the loops ( 21, 22 ) to eliminate capacitive coupling therebetween.

Description:
FIELD OF THE INVENTION 
   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. 
   DESCRIPTION OF RELATED ART 
   Published International patent application WO 91/13832 describes a strain measuring method and apparatus particularly suitable for measuring torque applied to a shaft. The described method and apparatus make use of a surface acoustic wave (SAW) device mounted on the shaft. Use of such a device requires the passage of high frequency, typically radio frequency (RF), signals between the device itself and an associated drive/measuring circuitry. If the shaft to which the SAW device is attached rotates only through 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 susceptible 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. 
   Our prior United Kingdom patent publication GB-A-2328086 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 as to form a transmission line with its associated ground plane. The tracks are arranged coaxially about the shaft carrying the SAW device, one track and its associated ground plane being secured to the shaft while the other track and its associated ground plane is secured to a fixed structure 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 fixed structure 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. 
   In the arrangement of GB-A-2328086 the degree of coupling between the respective transmission lines is determined, for any particular spacing between the respective tracks, by the length of the tracks. For a high degree of coupling, the length of the tracks must be optimized to the frequency of the signal to be coupled. Also, in order to form transmission lines each track must have associated therewith a respective ground plane. These characteristics impose design limitations on the coupling device and, in particular, restricts the degree to which the coupling device may be reduced in size and yet still achieve an acceptable degree of coupling. 
   SUMMARY OF THE INVENTION 
   We have now devised a coupling device which can provide the necessary signal coupling to the SAW device, but in which the degree of coupling provided by the coupling device is determined by the inductance of loops provided within the coupling device, rather than by the length of the transmission lines as in the case of GB-A-2328086. One of the loops may, in fact, be provided by an annular track of a transmission line, this track having associated with it a ground plane. However, even if one of the loops is provided by a transmission line, the other loop can be a simple turn of conducting material, and need not have a ground plane associated therewith. 
   According to one aspect of the present invention a rotary signal coupler for providing signal coupling to a SAW device mounted on a shaft rotatable relative to a fixed structure comprises: a first electrically conductive loop, connected to the SAW device, and fixed relative to the shaft; a second electrically conductive loop, connectable to electronic circuitry, and fixed relative to the fixed structure, the first and second loops being arranged in juxtaposition to provide inductive coupling therebetween and being positioned to remain at a substantially constant mutual spacing as the shaft rotates relative to the fixed structure; and electrically conductive means located between the first loop and the second loop, the electrically conductive means being grounded to eliminate or substantially eliminate electric coupling between the loops. 
   A preferred embodiment of the invention has been found to provide acceptable coupling (−4 dB or better) and a substantially flat response over a frequency range of 100-170 MHz. 
   In the preferred embodiments of the invention each loop is provided on a disc of material, one of the discs being fast with the shaft and the other of the discs being fast with the structure in which the shaft rotates. The discs are arranged face to face with a small air gap or a wafer of insulating material therebetween. One loop is provided on that face of one disc which is adjacent to the other disc, while the other loop is provided on that face of the other disc which is remote from the one disc. The electrically conductive means is provided on that face of the other disc which is adjacent to the one disc. Such an arrangement results in a structure which can readily be manufactured and implemented on a mass production basis. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above and further features and advantages of the present 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 wherein: 
       FIG. 1  illustrates schematically a prior art rotary signal coupler disclosed in GB-A-2 329 086; 
       FIG. 2  illustrates schematically the electric circuitry of a first embodiment of the present invention; 
       FIG. 3  illustrates schematically the mechanical arrangement of the first embodiment; 
       FIG. 4  illustrates schematically concentric tracks provided on the stator/rotor of the first embodiment; 
       FIG. 5  illustrates schematically the radial screen of the first embodiment; 
       FIG. 6  illustrates schematically the electric circuitry of a second embodiment of the present invention; 
       FIG. 7  illustrates schematically the overall response of a coupling system comprising either of the first or second embodiments connected to a SAW device; 
       FIG. 8  illustrates schematically the coupling response of either of the first or second embodiments; 
       FIG. 9  illustrates schematically the coupling response of a third embodiment of the present invention; 
       FIG. 10  illustrates schematically either of the first or second embodiments of the present invention in use in a multi-spindle drilling machine; and 
       FIG. 11  illustrates schematically a fourth embodiment of the present invention. 
   

   DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
   Referring firstly to  FIG. 1 , the illustrated prior art coupler  1  is shown schematically for providing signal coupling between a coax cable  2  and a coax cable  3 . In the illustrated coupler, the coax cable  2  is connected to a drive/measuring circuit (not shown) and the coax cable  3  is connected to a SAW device  4  mounted on a shaft  5 . The coupler accordingly facilitates signal connection between the drive/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 mm. 
   The first part  6  comprises a sheet  8  of insulating material which supports, on the side thereof remote from the second part  7 , a metal screen  9 . Similarly, the second part  7  comprises a sheet  10  of insulating material which supports, on the side thereof remote from the first part  6 , a metal screen  11 . The screen  9  is grounded, e.g. by way of connection to a screen  12  of the coax cable  2 . The screen  11  is electrically connected to the shaft  5 , e.g. by way of a screen  13  of the coax cable  3 . The shaft  5  will in general be grounded 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 arrangement, only one track will be present, but in more complicated arrangements, 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. 
   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 . If an additional track, for example the track  15 , is used, it will have associated therewith appropriate cable connections. For the purposes of illustration, only the outer track  14  is shown connected to a cable. 
   The face of the second part  7  adjacent the first 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 . 
   The respective tracks and their associated ground planes form transmission lines. The degree of coupling between the respective transmission lines is determined by the length of the respective transmission lines which, for circular transmission lines, is proportional to the radius of the tracks. Also, each track must have associated therewith a ground plane to form the required transmission line. These factors impose significant limitations on the design of the coupling. 
   Referring now to  FIGS. 2-11 , various embodiments of the invention are illustrated. 
   In the first embodiment, the improved coupler  20  comprises a first electrically conductive loop  21  connected to a SAW device  4  and a second electrical loop  22  connectable by appropriate cable to drive/measuring circuitry (not shown) for providing an excitation signal to the SAW device  4  and for analyzing the characteristic response of the SAW device  4 . Tuning capacitors  19  are connected across both loops to facilitate tuning the characteristics of the loops. 
   The loops  21 ,  22  are mounted on respective discs  23 , 24 , the disc  23  being fast with the shaft  5  and the disc  24  being fast with a fixed structure in which the shaft  5  is mounted. The discs are, in practice, mounted close to each other and may be separated by a small air gap or by a wafer of insulating material. The separation of the discs has been exaggerated in the drawings in the interests of clarity. 
   The loop  21  is mounted on the face  25  of the disc  23  which is remote from the disc  24 . The loop  22  is mounted on the face  26  of the disc  24  which is adjacent to the disc  23 . An electrical screen  27  is provided between the loops  21 , 22 . The screen  27  is conveniently provided by fixing suitable screening material to the face  28  of the disc  23  which is nearest the disc  24 . The screen  27  may, as illustrated in  FIG. 5 , conveniently take the form of a multiplicity of radial fingers  29  of metal, the fingers being connected together by a common central ring  30  which, in use, is electrically connected to the shaft  5 . The shaft  5  is itself grounded and accordingly the screen  27  forms a grounded screen between the loops  21 , 22  and hence reduces or substantially eliminates capacitive coupling between the loops. 
   If more than one SAW device is mounted on the shaft  5  the discs  23 , 24  may be provided with further loops each of which provides coupling to a respective SAW device. The loops will be arranged concentrically.  FIG. 4  illustrates a disc  23  having two loops  21 A and  21 B suitable for providing connections to two SAW devices. 
   Whilst the loops  21 , 22  are shown schematically in  FIG. 2  as being of different diameters with the loop  21  smaller than the loop  22 , it will be appreciated that in practice the loops may be as illustrated, or may be of equal diameter, or the loop  21  may be larger than the loop  22 . 
   It will be appreciated that because, in the case of the embodiment of the invention described above, the coupling between the SAW device and the driver/measuring circuitry is provided by the inductive coupling of two loops, no “ground plane” is required as was required in the prior art GB-A-2328086. This absence of ground plane may substantially simplify design of couplings. It is to be noted, however, that the face  31  of the disc  24  which is remote from the disc  23  may, if desired, be provided with a grounded screen  32 . Such a grounded screen  32  may be used as a ground plane which, in association with the loop  22 , forms a transmission line. It will be noted that even if the loop  22  and ground plane form a transmission line on the disc  24 , the arrangement is still different from the prior art as represented by GB-A-2328086 since no ground plane is associated with the loop  21  and the screen  27  is interposed between the loops  21  and  22 . 
   In the second embodiment of the invention illustrated in  FIG. 6  one end of the loop  22  is grounded and that end is connected to a grounded screen  32 . Accordingly, in this arrangement the loop  22  and ground plane  32  form a transmission line having an unbalanced input characteristic. 
   The performance of the first and second embodiments is illustrated in  FIGS. 7 and 8 . The overall response of a system comprising either of the first or second embodiments  20 ,  24  together with a SAW device is graphically illustrated in  FIG. 7  whereas the coupler response of the embodiments per se is shown in FIG.  8 . By way of comparison, the coupler response of a third embodiment is shown in FIG.  9 . In the third embodiment (not shown), the radial electric shield  27  is provided on the disc  24  rather than on the disc  23 . The rotor disc  23  provides support on its face  28 , for concentric circular loops and on its face  25 , for a metal screen ground plane. The face  26  of the disc  24  is used to support a radial electric shield  27 . The opposite face ( 31 ) of the disc  24 , is used for mounting the fixed loops. The radial electric shield  27  is thereby located between the concentric loops of the disc  23  and the disc  24 . The ground plane provided on the stator part  6  of the prior art coupler  1  is not present in the third embodiment. The loops of the third embodiment may be connected so as to provide an input and output which are each either balanced or unbalanced. The radial electric shield  27  of the third embodiment may be electrically connected to the loops provided on the disc  24 . 
   In certain circumstances, the use of the third embodiment is inappropriate. For example, where it is necessary to locate the disc  24  in close proximity to a relatively large mass of metal, the effectiveness of the loops provided on the disc  24  is reduced. In such a case, it is preferable to maximize the spacing of the loops from the mass of metal. In this regard, displacement of the loops from the face of the disc  24  adjacent the mass of metal to the opposite face of the disc  24  can be advantageous. As described above, such arrangement is provided by the first and second embodiments of the present invention. 
   An application where the first and second embodiments  20 , 24  are preferred is shown in FIG.  10 . This figure shows a multi-spindle drilling machine  35  comprising a metal body  36  in which a shaft  5  is rotatably supported on bearings  37 , 38 . The shaft  5  is provided at one end with a chuck  39  and bit  40 . At the end of the shaft  5  distal to the bit  30 , the shaft is provided with a drive gear. 
   The shaft  6  has mounted thereon SAW devices  4 . Each SAW device is electrically connected to a loop provided on a disc  41  secured to the shaft  5 . The loops are on the face  42  of the disc  41  which is remote from a disc  43  secured to the body of the drilling machine. Loops connected to appropriate drive/measuring circuitry are provided on the face  44  of the disc  43  which is nearest to the disc  41 . An electrical shield is provided on the face  45  of the disc  41  which is nearest the disc  43 . 
   Several sets of loops may be provided if several SAW devices are required. The face of the disc  43  which is nearest the bearing  38  is provided with a grounded metal covering to act as a ground plane for the loops formed on the face  44 . Accordingly, the loops connected to the drive/measuring circuit may form a transmission line. 
   Referring now to  FIG. 11 , a further embodiment of the invention is shown. In this embodiment a first loop  50  is provided on a first disc  51  while a first radial electric shield  52  and a second loop  53  are provided on a second disc  54  as described in relation to the first and second embodiments. A third loop  55  is provided on a third disc  50  while a second radial electric shield  57  and a fourth loop  58  are provided on a fourth disc  59  in an arrangement which is a mirror image of that described in relation to the first and second embodiments. The first and third discs  51 , 56  are fixedly secured to a support structure  60  and the second and fourth discs  54 , 59  are fixedly secured to the shaft  5  which is itself rotatably mounted to the support structure  60  by means of two bearings  61 , 62 . Thus, by axially spacing the loops (rather than arranging them concentrically), the overall diameter of the coupler shown in  FIG. 11  is less than that of the previously described couplers.