Abstract:
An electrical signal coupling device and more particularly, a rotary signal coupler suitable for use in transmitting electrical signals between transducers mounted on a shaft and wiring which is fixed relative to the structure in which the shaft is rotatably mounted. The coupling device includes a first part mounted on a rotary shaft and a second part mounted on the shaft in juxtaposition to the first part. The first and second parts include respective first and second conductors for electrically coupling the parts. The coupling device also includes means for maintaining a predetermined and substantially constant annular gap between the first and second parts and means, disposed on the second part, for preventing rotation of the second part as the first part rotates with the shaft.

Description:
RELATED APPLICATION DATA 
     This application is a Continuation of International Application (WIPO) No. PCT/GB00/02009 filed May 25, 2000, that designates the United States and which claims priority from British Application No. 9912201.2, filed May 25, 1999. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to an electrical signal coupling device and more particularly to a rotary signal coupler suitable for use in transmitting electrical signals between transducers mounted on a shaft and wiring which is fixed relative to the structure in which the shaft is rotatably mounted. 
     2. Prior Art 
     The invention is particularly applicable to rotary signal couplers for use in torque measuring equipment for example of the type described in our patent application GB-A-2328086. It is to be understood, however, that the invention is not limited to such applications and the electrical signal coupling device of the present invention may be used in other applications where it is necessary to establish a signal path between fixed wiring and transducers located on a shaft which is rotatable relative to the fixed wiring. 
     A known rotary signal coupler comprises a first part which is mounted on a rotatable shaft and a second part which is mounted on a housing in which the shaft is rotatably mounted. Such an arrangement is illustrated in FIG.  1 . The first part  1  of the coupler includes conductors forming a transmission line which is connected to a SAW transducers  2  which is secured to the surface of a shaft  4 . The first coupling part  1  is mounted on a sleeve  5  which is itself secured to the shaft  4  for rotation therewith. The second part  6  of the coupler comprises conductors which form a transmission line for coupling with the transmission line on the first part  1 . Wires lead from the second part  6  to fixed circuitry which provides signals for exciting the SAW device  2  and analyses the effects of the distortion of the SAW device to provide a measure of the torque applied to the shaft  4 . The second part  6  is secured to a housing  7  in which the shaft  4  is mounted via bearings  8 ,  9 . In the arrangement shown in FIG. 1 a second rotary coupler comprising a first coupling part  1 ′ and a second coupling part  6 ′ is provided to facilitate connection to a second SAW device  3 . 
     The type of arrangement illustrated in FIG. 1 suffers from the disadvantage that as a result of manufacturing tolerances it is difficult to maintain a consistent air gap between the first part  1  (or  1 ′) and the second part  6  (or  6 ′) of the coupler. Further, as the shaft  4  is rotated relative to the housing  7  the spacing between the first and second parts of the couplers is liable to vary as a result of eccentricity in the various components used. The inconsistent air gap between the first and second parts of the couplers, and the variation in the size of this air gap as the shaft rotates, makes it very difficult to interpret the signals derived from the SAW devices and limits the accuracy with which torque can be measured. 
     SUMMARY OF THE INVENTION 
     With a view to obviating the disadvantages outlined above, the present invention provides an electrical signal coupling device comprising a first part mountable on a rotary shaft; a second part mountable on the rotary shaft in juxtaposition to the first part, the first and second parts including respective conductors for electrically coupling the first and second parts; means for maintaining a predetermined and substantially constant annular gap between the first and second parts; and means provided on the second part for preventing rotation of the second part as the first part rotates with the shaft. 
     In the usual case where the coupling device is mounted with a housing which itself mounts the shaft, the housing will be provided with a clearance space surrounding the second part of the coupling device, and the coupling device will be provided with means for engaging the housing to prevent rotation of the second part. With such an arrangement, as the first part of the coupling device rotates with the shaft the second part of the coupling device will be restrained against rotation but will be maintained at a constant gap from the first part. If as a result there is radial or longitudinal movement of the second part relative to the housing this will be accommodated by the clearance space therebetween. Nonetheless, rotation of the second part will be prevented by the rotation prevention means. 
     In one embodiment of the invention the second part of the coupling device is mounted on the first part of the coupling device by means of a plain bearing, a ball-bearing or a roller bearing. In an alternative arrangement the second part is mounted on the shaft by way of a bearing and is positioned to be maintained adjacent the first part by the bearing. Rotation of the shaft will be accommodated by the bearing which mounts the second part of the coupling device. Because this bearing can be located immediately adjacent to the member which supports the first part on the shaft, relative lateral or longitudinal movement of the first and second parts will not occur during rotation of the shaft. 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above and further features and advantages of the invention will become clear from the following description of a preferred embodiment thereof, given by way of example only, reference being had to the accompanying drawings wherein: 
     FIG. 1 illustrates schematically an embodiment of prior art electrical signal coupling device; 
     FIG. 2 illustrates schematically an embodiment of the present invention; 
     FIG. 3 illustrates schematically a second embodiment of the present invention; 
     FIGS.  4 . 1 - 4 . 6  illustrate further embodiments of the present invention; and 
     FIGS. 5-7 show schematically arrangements for providing the required coupling parts. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to FIG. 2 the illustrated electrical coupling device  10  comprises a first part  11  which is mounted on a collar  12  which is itself mounted on a shaft  3 . The first part  11  includes electrical conductors which form a transmission line. These conductors are connected to a SAW transducer  15  which is itself mounted on the surface of the shaft  13 . The first coupling part  11  is surrounded by a second coupling part  16  which includes a transmission line which electrically couples with the transmission line of the first part  11  in use of the device. The second coupling part  16  is mounted in a carrier  17  which is mounted on the first part by means of a ball-bearing  18 . The ball-bearing  18  is formed by an inner race provided in the outer surface of the collar  12 , an outer race formed on the inner surface of the crier  17 , and a multiplicity of balls. The exact form of the bearing is not critical to the present invention and any ball, roller or plain bearing arrangement will suffice. The carrier  17  also carries the second part  16 ′ of a second coupling device, the first part  11 ′ of which is mounted on the collar  12  and is connected to a second SAW device  14 . 
     A cable  27  extends from the transmission lines of the second parts  16 ,  16 ′ to appropriate electronic circuits which provide emerging signals for the SAW devices and analyze the signals produced by the SAW devices to measure the torque applied to the shaft  13 . 
     Because the second parts  16 ,  16 ′ of the coupling devices are mounted on the collar  12  by way of a bearing the second parts  16 ,  16 ′ and the first parts  11 ,  11 ′ are concentric to a high degree of accuracy and remain concentric as one part rotates relative to the other. 
     In order to allow for manufacturing tolerances and possible eccentricity of the shaft  13  relative to the housing  19  in which it is mounted, the carrier  17  of the coupling device is mounted within a clearance  20  formed in the housing  19 . Both radial and axial clearances are provided around the carrier  37  to accommodate component part and assembly variations. A pin  21  secured to the carrier  17  is located in a clearance hole  22  provided in the housing to prevent rotation of the carrier  17 , and thus the second parts  16 ,  16 ′ relative to the housing. This arrangement ensures that no strain is put on the connecting cable  27  but at the same time permits the coupling device is free to move relative to the housing to a limited extent as the shaft rotates. 
     Whilst, in the case of the arrangement illustrated in FIG. 2, the second part  16 ,  16 ′ of the couplings are mounted directly on the first parts  11 ,  11 ′ by means of the ball-bearing  18 , other arrangements are possible within the scope of the present invention. 
     An alternate embodiment of the invention is illustrated in FIG.  3 . In this embodiment the first parts  11 ,  11 ′ of the coupling devices are mounted on an inner carrier  26  which itself is secured to a sleeve  28  by means of a radially extending web  29 . The sleeve  28  is retained on the shaft  4  by a ridge  30  formed integrally with the sleeve which is received in a groove  31  formed in the shaft. The sleeve  28  is retained against rotation relative to the shaft by any suitable means. A window  32  formed in the sleeve  28  provides space for mounting SAW devices, as will be understood by those skilled in the art. The SAW devices are connected to transmission lines formed on the first coupling parts  11 ,  11 ′ by suitable wires (not shown). 
     The second parts  16 ,  16 ′ of the coupling devices are themselves mounted on an outer carrier  33 . The outer carrier  33  is rotated on the inner carrier  36  by means of a caged ball-bearing  34 . Although in some instances the use of a plastic bearing may be desirable, it is believed that in the connection illustrated in FIG. 3 a metal ball-bearing will be acceptable provided that a relatively few balls are provided. Such an arrangement is possible with the use of a caged ball-bearing  34 . 
     Referring to FIG.  4 . 1 - 4 . 6 , various other embodiments of the invention are shown. 
     In the arrangement of FIG. 4.1 the second parts  16 ,  16 ′ of the coupling are mounted in a carrier  23  which is itself mounted on the shaft (not shown) by means of a bearing  24 . The bearing  24  is mounted on the shaft immediately adjacent the sleeve  11 A on which the first parts  11 ,  11 ′ of the coupling devices are mounted. The close proximity of the bearing  24  to the first parts  11 ,  11 ′ of the coupling devices, together with a relatively small size of the carrier  23 , ensures that the second parts  16 ,  16 ′ of the couplings are maintained concentric and at an even spacing from the first parts  11 ,  11 ′. 
     The arrangement of FIG. 4.2 is generally similar to that illustrated in FIG. 2 save that the first parts  11 ,  11 ′ of the couplings are formed mounted on an integral sleeve portion  11 A for mounting on the shaft The second parts  16 ,  16 ′ of the couplings are mounted on the first parts  11 ,  11 ′ by means of a non-conducting ball race  18 . 
     In FIG. 4.3 the second parts  16 ,  16 ′ of the couplings are mounted on the first parts  11 ,  11 ′ by means of a carrier  23  which forms a plain bearing  25  with the sleeve portion  11 A of the first part. 
     In the arrangement of FIG. 4.4 the second parts  16 ,  16 ′ of the couplings are again mounted on the first parts  11 ,  11 ′ by means of a plain bearing  25 . 
     In the case of both FIGS. 4.3 and  4 . 4  the plain bearing arrangements can conveniently be provided by making one or both of the bearing elements of the plastics material. 
     FIGS. 4.5 and  4 . 6  show arrangements suitable for mounting the coupling parts in an axially spaced apart arrangement. Coupling parts mounted this way require a consistent and substantially constant space between the coupling parts as the shaft rotates. This again can conveniently be achieved by mounting the second coupling parts  16 ,  16 ′ on the shaft adjacent the mounting of the first coupling parts  11 ,  11 ′ (FIG. 4.5) or by mounting the second coupling part on the fir coupling part by moans of a ball-bearing  18  (FIG.  4 . 6 ). 
     In use, each of the arrangements of FIG. 4 will utilize means of preventing rotation of the second coupling parts relative to the housing which surrounds them. The arrangement could consist of a steady pin  21  working in an oversized hole  22  as described above with reference to FIG. 2, or any other suitable rotation restraining arrangement. 
     It will be noted that the arrangements of FIGS. 2,  3 ,  4 . 2 ,  4 . 3 ,  4 . 4  and  4 . 6  are particularly advantageous in that the entire coupling device can be assembled as a unit and bench tested before it is applied to the shaft upon which it is required. This is in contrast to prior art arrangements shown in FIG. 1 where the first coupling part is mounted on the shaft and the second coupling part is mounted on the housing with the result that the complete coupling is not formed until after the housing bas been assembled to the shaft during manufacture of the product in which the coupling is employed. 
     Referring now to FIGS. 5-7 one possible construction for the first coupling part  11  and the second coupling parts  16  is shown. Each coupling part comprises a base rings  35  formed from suitable non-conductive material. The base rings  35  may, for example, be plastics injection mouldings. Each base is coated on the radially inner and radially outer surface thereof with a conductive metal layer. The conductive layer may be provided by any suitable means, for example vacuum deposition, electro-plating, screen printing, or by the adhesion to the surface of the base rings  35  of tin metal strips. Each ring includes a slot  36  formed in one axial face thereof. The slots  36  house electrically conductive material which electrically connects the radially inner and radially outer faces of the respective rings. Suitable connections for ground wires  37  arc provided on the radially outer surfaces of both rings. A connection for a signal wire  38  is provided on the outer surface of the first coupling part whilst a N on for a signal wire  39  is provided on the radially inner surface of the second coupling part  16 . The electrically conductive coating on the outer surface of the first coupling part  7  is broken by a gap  40  which is located between the columns for the wires  37  and  38 . The electrically conductive coating on the inner surface of the second coupling part  16  is broken by a gap  41  located between the connection for the wire  39  and the slot  36  of the outer ring  35 . The above described arrangements enable the coupling to be produced at relatively low cost and to have the necessary robust mechanical and electrical characteristics for use in the automotive industry.