BACKGROUND OF THE INVENTION 
     1. Technical Field 
     The present invention relates generally to cable assemblies. More particularly, the present invention relates to a cable assembly whose condition is monitorable so that the cable assembly may be replaced prior to the failure of one or more critical conductors. The present invention also relates to a system and method for monitoring the condition of a cable assembly. 
     2. Discussion 
     Cable assemblies are commonly employed in industrial and telecommunications operations to transmit electrical power and/or signals between two devices. In situations where the two devices operate in stationary positions and are not exposed to environmental agents such as sunlight or chemicals that cause their insulating covers to degrade, the cable assembly will last indefinitely. However, where one device moves relative to the other, the cable assembly is susceptible to fatigue failures wherein one or more of the conductors that make up the cable assembly break due to repeated bending. Similarly, where the cable assembly is exposed to environmental agents which degrade the cable assembly&#39;s insulating material, the insulating material can fail causing the conductors that make up the cable assembly to come into electrical contact with one another or with an electrical ground. 
     Failures of these types are often catastrophic, completely disabling the devices to which they are coupled. Complicating matters is that these types of failures are typically difficult to predict and rather time consuming to repair. Repair may consist of the wholesale replacement of the cable assembly, or it may comprise the removal and replacement of one or more areas that are suspected of being damaged. 
     To combat down-time associated with failures in cable assemblies, some users of cable assemblies simply incorporate additional (spare) conductors into the cable assembly which are then be used to replace one of the actively used conductors upon the occurrence of a fault. This solution, however, has several drawbacks. One drawback relates to fact that the additional cables can be relatively expensive and as such, the incorporation of spare cables further increase the cost of the cable assembly. Another drawback relates to the ease with which the additional cables are festooned between the devices and the impact of additional cable weight on the person or mechanism that must position the device or devices that are coupled to the cable assembly. Yet another drawback concerns the occurrence of a failure related down-time. Failures in an active conductor will still result in some down-time, albeit a smaller amount of down-time than had the cable assembly not included a spare. 
     Another approach has been to predict a lifespan of the cable assembly and simply replace the cable assembly after it has been in use for a duration equal its lifespan, regardless of its actual condition. This approach, however, has not been completely successful due difficulties in calculating the lifespan of the cable assembly. Furthermore, this approach is rather costly, as cable assemblies are replaced regardless of their actual condition. 
     SUMMARY OF THE INVENTION 
     It is one object of the present invention to provide a cable assembly which may be readily monitored so that its condition can be determined prior to the failure of a conductor which is necessary for the operation of the devices to which the cable assembly is connected. 
     It is another object of the present invention to provide a cable assembly having a sacrificial conductor which permits the condition of the cable assembly to be monitored. 
     It is a further object of the present invention to provide a cable condition monitoring system for generating an alarm signal in response to the detection of a fault in a sacrificial conductors in the cable assembly. 
     It is yet another object of the present invention to provide a method for monitoring the condition of a cable assembly. 
     In one preferred form, the present invention provides a cable assembly having a critical conductor and a sacrificial conductor. The critical conductor has a fatigue life of a predetermined first length and the sacrificial conductor has a fatigue life of a predetermined second length which is shorter than the predetermined first length such that exposure of the cable assembly to repeated flexure causes the sacrificial conductor to fail before the critical conductor fails. A cable condition monitoring system for generating an alarm signal in response to the detection of a fault in the sacrificial conductors and a method for monitoring the condition of a cable assembly are also provided. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Additional advantages and features of the present invention will become apparent from the subsequent description and the appended claims, taken in conjunction with the accompanying drawings wherein: 
     FIG. 1A is a perspective view of a cable assembly constructed in accordance with a preferred embodiment of the present invention, the cable assembly being shown in operative association with automated fastening device; 
     FIG. 1B is an enlarged side elevational view of a portion of the automated fastening device of FIG. 1A; 
     FIG. 2 is a cross-sectional view of the cable assembly taken along the line  2 — 2  of FIG. 1A; 
     FIG. 3 is a schematic illustration of the automated fastening device of FIG. 1A, showing the cable assembly at various times during the operation of the automated fastening device; 
     FIG. 4 is a schematic illustration of a portion of the automated fastening device illustrating the system for monitoring the condition of a cable assembly; 
     FIG. 5 is a schematic illustration similar to that of FIG. 4 but illustrating the system after a defect has been detected; 
     FIG. 6 is a cross-sectional view similar to FIG. 2 but illustrating a second embodiment of the present invention; 
     FIG. 7 is a cross-sectional view similar to FIG. 2 but illustrating a third embodiment of the present invention; and 
     FIG. 8 is a cross-sectional view similar to FIG. 2 but illustrating a fourth embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     With reference to FIGS. 1A and 1B of the drawings, a cable assembly constructed in accordance with the teachings of the present invention is generally indicated by reference numeral  10 . Cable assembly  10  is shown in operative association with an automated fastening device  12 . Automated fastening device  12  conventionally includes a control panel  14  and a tool assembly  16 . Control panel  14  conventionally includes a plurality of power amplifiers  20 , a plurality of servo power supplies  22  and plurality of spindle modules  24 . Tool assembly  16  conventionally includes a tool structure  30 , a plurality of spindle assemblies  32  and a subpanel  34  for remotely controlling tool assembly  16 . In the example provided, each of the spindle assemblies  32  includes a motor assembly  38  having a DC or AC electric motor and a gearbox, a resolver  40  and a torque transducer  42 . Subpanel  34  conventionally includes a plurality of control buttons  44  which permit a technician to remotely operate tool assembly  16  and a plurality of status lights  46  for indicating the status of a fastening operation. 
     Cable assembly  10  electrically couples control panel  14  and tool assembly  16 , providing a means for powering tool assembly  16  as well as for transmitting fastening data to spindle modules  24 . The operation of automated fastening device  12  is beyond the scope of this invention and as such, need not be discussed in detail. Briefly, actuation of the control buttons  44  causes power amplifiers  20  and servo power supplies  22  to cooperatively supply electrical power to their associated motor assemblies  38 . Each resolver  40  is operable for monitoring the rotational position of the rotor of its respective motor assembly  38  and generating a resolver signal in response thereto. The resolver signal is received by an associated one of the spindle modules  24  and an associated one of the servo power supplies  22 . Spindle modules  24  employ the resolver signals to monitor the angle through which an output spindle  50  of their respective motor assembly  38  has rotated. Servo power supplies  22  employ the resolver signals to control the switching of electrical power to their respective motor assembly  38  to change the magnetic field produced by the stator of the motor assembly  38  so that the rotor may rotate properly. 
     Tightening of a fastener generates a torque reaction that is transmitted through spindle assembly  32  to torque transducer  42 . Torque transducer  42  produces a transducer signal that is proportional to the corresponding torque reaction that is applied against it. Spindle modules  24  employ the transducer signal to monitor the torque that is output from their respective spindle assembly  32 . 
     With reference to FIG. 2, cable assembly  10  is shown to include a plurality of critical conductors  60 , a conductive shield  62  and a plurality of sacrificial conductors  64 . In the particular embodiment illustrated, the plurality of critical conductors  60  includes a plurality of power cables  68 , a plurality of resolver cables  70  and a plurality of transducer cables  72 . Each of the power cables  68  electrically couples a motor assembly to an associated one of the servo power supplies  22 . Each of the resolver cables  70  electronically couples a resolver to an associated one of the spindle modules  24  and an associated one of the servo power supplies  22 . Each one of the transducer cables  72  electronically couples one of the torque transducers  42  with an associated one of the spindle modules  24 . Conductive shield  62  encircles the plurality of critical conductors  60  and conventionally inhibits the transmission of electrical noise therethrough. Each of the plurality of sacrificial conductors  64  is formed from an appropriate wire (e.g., 20 ga. insulated solid copper wire) and disposed in a filler material  76 . The plurality of sacrificial conductors  64  are circumferentially spaced about conductive shield  62 . An outer jacket  78 , formed from an appropriate insulating material such as polyurethane or polyvinyl chloride encircles the plurality of sacrificial  110  conductors  64  and filler material  76 . 
     A failure in a critical conductor  60  will likely interrupt the transmission of electrical power, the resolver signal or the transducer signal between tool assembly  16  and control panel  14 , thus preventing the associated spindle assembly  32  from operating. In modern high-volume assembly processes where production rates can be greater than 60 pieces per hour, interruptions of these types essentially render tool assembly  16  inoperative, even when only one spindle assembly  32  is effected. 
     A major cause of failures in the critical conductors  60  stems from fatigue that results from the repeated operation of tool assembly  16 . In FIG. 3, cable assembly  10  is schematically shown festooned (i.e., supported at several intervals) between control panel  14  and tool assembly  16 . When tool assembly  16  is not in use it is maintained in a raised condition as designated by reference letter “R” so as to be out of the way of the assembly technicians. When tool assembly  16  is operated, it is lowered onto a component as indicated by reference letter “L” and spindle assemblies  32  are activated. Raising and lowering of tool assembly  16  causes section designated by reference letter “F” of cable assembly  10  to bend. The repeated bending of section F during the operation of tool assembly  16  work hardens the electrical wires that make up the conductors (i.e., critical conductors  60  and sacrificial conductors  64 ) in cable assembly  10 . Eventually, one or more of the conductors in cable assembly  10  will fatigue and break, causing an interruption, which could disable tool assembly  16  if the conductor is one of the critical conductors  60 . 
     To avoid failures in the critical conductors  60  which would disable tool assembly  16 , sacrificial conductors  64  are monitored, either continuously or periodically, to predict the occurrence of a catastrophic failure in the critical conductors  60 . Sacrificial conductors  64  are selected to have a predetermined fatigue life that is shorter in duration than the fatigue life of any of the critical conductors  60 . Sacrificial conductors  64  are preferably incorporated into cable assembly  10  such that they are exposed to relatively higher levels of strain during the operation of tool assembly  16  than any of the critical conductors  60 . Configuration in this manner ensures that one or more of the sacrificial conductors  64  will fail from fatigue prior to the failure of a critical conductor  60 . Accordingly, monitoring of the condition of the sacrificial conductors  64  permits a failure of a sacrificial conductor  64  to be noted well before a fatigue failure of one of the critical conductors  60 . This permits cable assembly  10  to be serviced (e.g., replaced) at a time which is convenient and which does not impact the productive use of tool assembly  16 . 
     In FIG. 4 a fault detection device  80  for monitoring the condition of the sacrificial conductors  64  is schematically illustrated to include a power supply  82 , a fault monitor  84  and an alarm device  86 . Power supply  82  is operable for providing a predetermined electrical signal to the plurality of sacrificial conductors  64 . Fault monitor  84  monitors the transmission of the electrical signal through the sacrificial conductors  64  and responsively generates an alarm signal in response to the detection of a fault. In the particular embodiment illustrated, power supply  82  is operable for providing an electrical signal having a continuous direct current voltage which is transmitted to a first end of sacrificial conductor  64 ′. The sacrificial conductors  64  are coupled together in series through a plurality of electrical jumpers  88 , thereby creating a continuous electrical path through which electrical signal travels. Sacrificial conductor  64 ″ is coupled to fault monitor  84 , which is illustrated to be a control relay or solid state device  90  having a switching element  92 . 
     A first leg  94  of switching element  92  is electrically coupled to a source of power (not specifically shown) and a second leg  96  of switching element  92  is coupled to one electrical terminal of alarm device  86 . While alarm device  86  is illustrated to be an indicator light  98 , those skilled in the art will understand that alarm device  86  may also or alternatively include an audio alarm device or a digital output which is received by a programmable logic controller or a computer which generates an appropriate predetermined response, including the generation of an e-mail message. 
     Control relay  90  is configured such that upon receipt of the electric signal that is transmitted through the sacrificial conductors  64 , switching element  92  is positioned into an open condition as shown in FIG.  4 . When one of the sacrificial conductors  64  succumbs to fatigue and breaks, however, switching element  92  reverts to its normal condition open, closed or both, closed being shown in FIG. 5, generating an alarm signal which is transmitted to alarm device  86  so that a predetermined alarm indicative of the need to preventatively service cable assembly  10  is generated. In the particular embodiment illustrated, the predetermined alarm is the illumination of indicator light  98 . 
     In FIG. 6, a cable assembly constructed in accordance with a second embodiment of the present invention is generally indicated by reference numeral  10   a.  Cable assembly  10   a  is shown to include a single critical conductor  60   a  and two sacrificial conductors  64   a.  Critical conductor  60   a  is shown to be a relatively large cable having an appropriate insulating material  110  which encircles it. Sacrificial conductor  64   a  includes a relatively small cable and a separate insulating material. Sacrificial conductor  64   a  is disposed within insulating material  110 . Configuration of cable assembly  10   a  in this manner is particularly well adapted for welding applications, such as in automated spot resistance welding tools where a relatively large amount of electrical power is transmitted through critical conductor  60   a.    
     The embodiment illustrated in FIG. 7 is similar to that of FIG. 6, except that cable assembly  10   c  is shown to include several sacrificial conductors  64   c  that are disposed within insulating material  110  and circumferentially spaced about critical conductor  60   c.  In this embodiment, the cable assembly  10   c,  is shown to include a plurality of sacrificial conductors  64   c  that are disposed radially outward of the plurality of critical conductor  60   c.  This embodiment provides for failure of sacrificial conductors due to flexing fatigue, environmental fatigue, exposure to chemicals and abrasion or chaffing that could be caused from use in a cable track. 
     FIG. 7 could also be viewed as a cross section of an individual conductor, such as used in aircraft. The critical conductor is made up of many smaller non-insulated conductors bundled together to create one large conductor. For this embodiment the sacrificial conductors should be sized appropriately so as to allow them to fail when the insulation in which they are disposed is breached or chaffed (i.e. rubbing on the airframe). A common cause is vibration and +and −“G” loads associated with takeoff and landing, turbulence and maneuvering. 
     The embodiment illustrated in FIG. 8 shows cable assembly  10   d  to include twelve critical conductors  60   d  and five sacrificial conductors  64   d.  Each of the critical conductors  60   d  is shown to include a first insulation  120 , which is highly resistant to an insulation damaging substance, such as a contaminant, to which cable assembly  10   d  will be exposed during its use. Each of the sacrificial conductors  64   d  is shown to have a second insulation  122 , which is less resistant to the insulation damaging substance. In the particular embodiment illustrated, the insulation damaging substance is oil, first insulation  120  is polyurethane and second insulation  122  is polyvinyl chloride. The critical conductors  60   d  and sacrificial conductors  64   d  are encircled with an insulating material  124 , preferably the material which forms the first insulation  120 . 
     Configuration of cable assembly  10   d  in this manner creates a barrier (i.e., insulating material  124 ) which prevents the critical conductors  60   d  and sacrificial conductors  64   d  from being exposed to oil. Prolonged exposure to oil will eventually cause insulating material  124  to fail, thereby exposing the critical conductors  60   d  and sacrificial conductors  64   d  to oil. As sacrificial conductors  64   d  are insulated with a material that is less resistant to oil than the material which is employed to insulate the critical conductors  60   d,  the second insulation  122  will fail prior to the first insulation  120 . Accordingly, continuous or periodic monitoring of sacrificial conductors  64   d  will detect faults wherein one or more of the sacrificial conductors  64   d  is conducting the electric signal to an electrical ground, or causing a physical failure of the sacrificial conductor or a change in impedance or capacitance or other change in measurable quality. 
     Accordingly, monitoring of the condition of the sacrificial conductors  64   d  permits a failure of a second insulation  122  to be noted well before a failure of the first insulation  120  on one of the critical conductors  60   d.  This permits cable assembly  10   d  to be serviced prior to a failure in the first insulation  120  which causes a critical conductor  60   d  to conduct electric current to an electrical ground, or other critical conductors or sacrificial conductors. As mentioned above, monitoring of the condition of the sacrificial conductors  64   d  permits cable assembly  10   d  to be serviced at a time which is convenient and which does not impact the productive use of tool assembly  16 . 
     While the invention has been described in the specification and illustrated in the drawings with reference to a preferred embodiment, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention as defined in the claims. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment illustrated by the drawings and described in the specification as the best mode presently contemplated for carrying out this invention, but that the invention will include any embodiments falling within the description of the appended claims.
A cable assembly having a critical conductor and a sacrificial conductor and a critical conductor which has a fatigue life of a predetermined first length and the sacrificial conductor has a fatigue life of a predetermined second length shorter than the predetermined first length such that exposure of the cable assembly to repeated flexure or deteriorating substance causes the sacrificial conductor to fail before the critical conductor fails. Additionally, sacrificial conductors to be insulated with an insulation material less suited for the application so that the sacrificial conductors insulation will fail prior to the failure of the critical conductors. Positioning of sacrificial conductors around the circumference so failure from abrasion and chaffing will occur prior to critical conductors. A cable condition monitoring system for generating an alarm signal in response to the detection of a fault in the sacrificial conductors and a method for monitoring the condition of a cable assembly are also provided.