Abstract:
A system determining whether the shielding on a shielded signal or power cable has been compromised without the need of detaching the cable. A special-made cable is used with a dedicated shielding surveillance conductor and a process for injecting a known current on the shield of the cable and monitoring a voltage on the shielding surveillance conductor.

Description:
FIELD OF THE INVENTION 
     The present invention generally relates to a cable shielding for mission critical systems. 
     BACKGROUND OF THE INVENTION 
     In the past, it is well known that many defense systems must operate through intense electro-magnetic environments with a very high level of reliability and availability. Damage to cable/connector system shielding can result from personnel pulling on the cable with excessive force, vehicle traffic over the cable, rodents, inadvertent abrasion or cutting during system deployment, exposure to the climatic environment, and intentional actions. 
     Often, long cables must have a high level of shielding to protect the signal and/or conductors from susceptibility to external electro-magnetic fields and threats. Cables are considered long when their length is significant with respect to the wavelength (λ) associated with the high frequency contained in an electro-magnetic threat. Long cables (e.g. λ/20) act as “good antennae”, which may allow the transfer of radiated energy to voltages and currents at signal and power connector pins of equipment. Damage to shielding on long cable/connector systems may thus allow the introduction of damaging voltages and currents at equipment connector pins. Threatening electromagnetic fields may be produced by transmission of radio frequency energy, lightning, static discharge and radio frequency energy from radio transmitters. 
     An example exists with modern aircraft electronics which must continue to operate without incidence to allow continuous safe flight while being exposed to high electro-magnetic fields. The long signal cable/connector systems that link the various avionics and navigation equipment must maintain a high level of shielding to ensure immunity to equipment damage and/or data loss due to exposure to such fields. Consider, for instance, a fly-by-wire system. If there is some damage to the cable/connector system shielding, the flight control system may still perform flawlessly in the benign electro-magnetic environment. But if the aircraft flies near a high intensity source of electro-magnetic radiation, such as a ground-based Voice of America transmitter antenna, data, and hence flight control, could be lost. 
     Common approaches to addressing cable/connector system reliability center on frequent inspection. Other approaches center around “over-design” of electronic equipment such that performance does not rely heavily on cable shielding. This approach has recently become used in commercial air transport aircraft. A prior art copper Ethernet switch is an example where this approach proved expensive with respect to schedule and cost. To address possible cable shield degradation or damage, aircraft manufactures have required Ethernet switches to be tested for RF susceptibility with high-speed Ethernet cable shields disconnected from ground. 
     Another approach to address cable/connector system reliability is to route the shielded cables inside rigid structures to protect them from damage, or providing another or auxiliary layer of shielding should the shielded cable&#39;s shield become damaged. The “rigid structure” may be metallic conduit (pipe) or a shielding compartment (a room with continuous metal on all six sides). 
     Current state of the art for cable shielding verification involves disconnecting the cable from equipment and measuring inductance and/or resistances of the cable/connector system&#39;s shield. Of course, this could be difficult in tight spots, and infallibility of this method is questioned since low inductance and/or low resistance does not always equate to a high level of shielding integrity. These methods may not allow the equation of voltages and currents at equipment connector pins that may result from such electro-magnetic fields. 
     Consequently, there exists a need for improved methods and systems for efficiently improving assurances of continued integrity of mission critical electronics of the type having long cables especially during occasions of exposure to high levels of electro-magnetic fields. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a system and method for efficiently improving the integrity of mission critical electronics. 
     It is a feature of the present invention to utilize an integrated cable/connector shielding surveillance system. 
     It is another feature of the present invention to include a new cable having therein a dedicated surveillance conductor. 
     It is yet another feature of the present invention to utilize a surveillance module at one end of a long cable under surveillance and a termination of the surveillance conductor at the opposing end. 
     It is still another feature of the present invention to include a cable current monitor probe. 
     It is still another feature of the present invention to include a cable RF signal injection probe. 
     It is an advantage of the present invention to improve the ability to accurately detect a cable shield damage or degradation situation and thereby reduce the workload associated with inspection of long cable/connector systems. 
     The present invention is a system and method for monitoring the shielding and cable/connector systems, which invention is designed to satisfy the aforementioned needs, provide the previously stated objects, include the above-listed features, and achieve the already articulated advantages. 
     Accordingly, the present invention is a system and method including a cable containing a dedicated surveillance conductor therein and a surveillance module at one end of the cable, as well as a monitor probe and an injection probe. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a simplified block diagram of an embodiment of the present invention showing a cable/connector system of the present invention. 
         FIG. 2  is a cross-sectional view of the cable taken on line  2 - 2  of  FIG. 1 . 
         FIG. 3  is a more detailed block diagram of the shielding surveillance module of  FIG. 1 . 
         FIG. 4  is flow diagram of the overall method of the present invention. 
         FIG. 5  is a flow diagram of the set-up process shown in  FIG. 4 . 
         FIG. 6A  is a flow diagram of initial stages of the calibration process shown in  FIG. 4  with a transition line  629 . 
         FIG. 6B  is a flow diagram of final stages of the calibration process shown in  FIG. 4  with the transition line  629 . 
         FIG. 7A  is a flow diagram of initial stages of a surveillance process shown in  FIG. 4  with a transition line  727 . 
         FIG. 7B  is a flow diagram of intermediate stages of a surveillance process shown in  FIG. 4  with transition lines  727 ,  745 ,  747 , and  775 . 
         FIG. 7C  is a flow diagram of final stages of a surveillance process shown in  FIG. 4  with transition lines  745 ,  747 ,  775 . 
     
    
    
     DETAILED DESCRIPTION 
     Now referring to  FIG. 1 , there is shown a simplified block diagram of an embodiment of the present invention showing a cable/connector system of the present invention, generally designated  100 , which includes a host electronic equipment item  102 , which could be any type of electronic equipment which gets connected to other pieces of electronic equipment and where a high level of assurance of shield integrity is highly valued, including, but not limited to, avionics equipment and defense communication and control equipment. Host electronic equipment item  102  is shown coupled to interfacing electronic equipment item  104  via shielded connecting cable  106 . It should be noted that whenever the term shielded connecting cable  106  is discussed herein it is intended to include the cable, connectors and cable shield terminations. Shielded connecting cable  106  is a special cable with special characteristics. Shielded connecting cable  106  is coupled in several ways to shielding surveillance module  108 , which is an electronic module that resides inside host electronic equipment item  102  that the shielded connecting cable  106  connects to. Shielding surveillance module  108  produces a signal to drive the shield current injection probe  110  and measures the resulting voltages from the shield current monitor probe  112  and shielding surveillance conductor  114  (preferably having its own independent insulation as shown) at each test frequency. Shielding surveillance module  108  compares the measurements to expected values to determine if the shielding of shielded connecting cable  106  is still acceptable. Shielding surveillance module  108  interfaces to allow loading of probe data and pass/fail data, as well as presenting results. Control and I/O interfaces via  120  may be implemented locally at the host electronic equipment item  102  or may be implemented remotely through a computer network. The shielding surveillance module receives power from the host electronic equipment item, batteries or an external source via  122 . Shield current injection probe  110  could be a current transformer driven by the shielding surveillance module  108  that inductively couples current on to the shield of shielded connecting cable  106  under surveillance or another method (such as directly injecting current) could be used as well. Shield current injection probe  110  may be a portable clamp-on device, or it may be permanently attached/integrated with the shielded connecting cable  106 . Shield current injection probe  110  could be made from a commercially available product and could be calibrated to yield coupling or transfer factors as a function of frequency. Shield current monitor probe  112  is a current transformer that produces an output voltage across a specified impedance that is proportional to the current on the cable shields enclosed by its aperture. Shield current monitor probe  112  may be a portable clamp-on device, or it may be permanently attached/integrated with the shielded connecting cable  106 . Shield current monitor probe  112  could be made from a commercially available probe and could be calibrated to yield coupling factors as a function of frequency. Shielding surveillance conductor  114  could be an insulated conductor that is added inside the shielded volume (under the shield(s) of interest) during manufacture of the shielded connecting cable  106 . It should be understood that the electrically conductive shield contemplated by this invention could contain 1 or more than two layers and would as is well known in the art have some sort of insulating sleeve as an outermost covering. Shielding surveillance conductor  114  is placed under the shields  202  and  204  ( FIG. 2 ) and should be uniformly controlled such that its characteristic impedance with respect to the shields  202  and  204 , which represents the shielding system under surveillance, remains constant along the length of shielded connecting cable  106 . Shielding surveillance conductor  114  can terminate through connector pins to the shielding surveillance module  108  at one end and to the surveillance conductor termination  118  at the interfacing equipment. The termination of the shielding surveillance conductor  114  could be a resistive load between the surveillance conductor and the ground reference of cable shields  202  and  204 . The termination resistance could be equal to the characteristic impedance of the shielding surveillance conductor  114 . Shielding surveillance connectors  116  could be two coaxial connectors added to the host electronic equipment item  102  chassis, with one being for the shield current injection probe  110  and the other for the shield current monitor probe  112 . Control can be provided to shielding surveillance module  108  via equipment to shielding surveillance module control interface  120  while power is provided via equipment to shielding surveillance module power line(s)  122 . 
     While it is believed that shielding surveillance module  108  is best constructed as an internal module hosted by an electronic equipment unit, it should be understood that shielding surveillance module  108  could be external to the host electronic equipment item  102  and, for example, could be battery powered and contained in an application specific connector at the ends of shielded connecting cable  106 . Such specialized connector could have also included therein indicator lights to display the status of the shielding of the shielded connecting cable  106  or otherwise display the results of the surveillance. 
     Now referring to  FIG. 2 , there is shown a cross-sectional view of shielded connecting cable  106  taken on line  2 - 2  of  FIG. 1 . Shielded connecting cable  106  is shown having an item  202  which could be an insulating sleeve or a connecting cable outer shield with an insulating outer sleeve and possibly an insulating inner sleeve as well, disposed around a connecting cable inner shield  204 , which could be electrically conductive metallic shields which are well known in the art to provide shielding from electromagnetic fields of high strengths. Disposed inside the shielded volume of shielded connecting cable  106  are shown three (3) independent shielded wire groups  206 . These are representative, and it should be understood that more or less than three (3) shielded wire groups  206  could be included, as well as other shielded internal cables having more or fewer than four insulated wires for carrying power and signals therein. Also shown is power and low speed signal wires  208  which are representative of the various other insulated, but unshielded wires that may be disposed within the shielded volume provided by connecting cable outer shield  202  and connecting cable inner shield  204 . Shielded connecting cable  106  differs from well-known shielded cables by the inclusion of shielding surveillance conductor  114 . 
     Now referring to  FIG. 3 , there is shown a view of the shielding surveillance module  108  of  FIG. 1  which includes a power converter  302  which receives power from power input  304  and provides DC power to output  306  to module components. For drawing simplicity, the power connections are omitted, but it is understood that a person skilled in the art would readily know how to make the requisite connections. 
     Also shown is the control processor  308 , which is the main processor of the shielding surveillance module  108 . Control processor  308  is built around a microcontroller that provides operator and programmer data input and output interfaces to the host electronic equipment item  102 , direct network connections or via other means. Control processor (CP)  308  also controls the circuits inside the shielding surveillance module  108  and performs the necessary mathematical calculations to set up and control internal circuits, process measurement data and determine surveillance test results. CP is coupled to non-volatile memory  310 , which could contain the operating program for the control processor  308  and data relating to the specific application. Non-volatile memory  310  is controlled, read from and written to via control processor  308 . 
     Modulator/transmitter  312  outputs a frequency shift keyed (FSK) signal with characteristics (e.g. frequency, frequency deviation) as directed by the control processor. Of course, other modulation schemes could be used as well. The output is modulated with message data provided by the control processor. A popular frequency range for testing shields of data and power cables is 30 MHz to 450 MHz, using at least four frequencies per octave. The FSK deviation is set to be relatively small with respect to the test frequency, while being kept as large as possible to increase sensitivity at the receiver. 
     Amplifier  314  is a broadband radio frequency amplifier capable of driving the injection probe with the modulator/transmitter output at a level high enough to allow adequate shielding measurement dynamic range while not being too great as to cause interference, as a result of the injected current on the cable under surveillance, to near-by equipment. The typical output level may be 10 milliwatt. 
     Self-test enabling coupler  316  allows a small portion of the amplifier&#39;s RF output to be sent to the multiplexer (MUX) and receiver if selected, to allow performance of a module self-test. 
     Receiver  320  demodulates the FSK signal received via the monitor probe and surveillance conductor and outputs the data message to the control processor. It also provides an output voltage that is proportional to its RF input. This output is called the Receive Signal Strength Indicator (RSSI). 
     Mux  322  is a switch that allows selection of one of three inputs to be sent to the receiver for signal strength measurement and message data extraction. 
     Programmable attenuator  1   324  allows the level of the input signal from the monitor probe to be reduced to a level that allows maximum accuracy from receiver. Its setting (amount of attenuation provided) is considered by the control processor when computing shield current. 
     Programmable attenuator  2   325  allows the level of the input signal from the shielding surveillance conductor  114  to be reduced to a level that allows maximum accuracy from receiver. Its setting (amount of attenuation provided) is considered by the control processor  308  when computing surveillance conductor voltage. 
     Analog-to-digital converter  326  converts the receiver  320 &#39;s RSSI output voltage to an equivalent digital value that is sent to the control processor  308  for processing. 
     Also shown is clock oscillator  318  which provides a stable frequency base for the control processor  308  and the frequency synthesizer in the modulator/transmitter  312  and receiver  320 . 
     Now referring to  FIG. 4 , there is shown a flowchart of the innovative process of the present invention. 
     Step  402 : The entire process of the present invention is begun here and broken into several independent processes. 
     Step  404 : The set-up process may be run with the shielding surveillance module  108  installed in host electronic equipment item  102  or before installation of the shielding surveillance module  108  into the host electronic equipment item  102 . The details of the set-up process are shown in  FIG. 5  and discussed in the text associated with  FIG. 5 . 
     Step  406 : The next overall step is to install the shielding surveillance module  108 , shield current injection probe  110 , shield current monitor probe  112 , and connect shielded connecting cable  106  and other cables. 
     Step  408  is to run the calibration process which is shown in detail in  FIGS. 6A and 6B  and discussed in the associated text. 
     Step  412  is to determine if a surveillance test is desired. 
     If Step  412  is NO, then WAIT per Step  410 . 
     If Step  412  is YES, then run the surveillance process which is described in  FIGS. 7A-7C . 
     Now referring to  FIG. 5 , there is shown a flowchart of the Set-Up process of the present invention generally labeled  500 , including a starting point  502 . 
     Step  504  is to determine transfer factors for the shield current injection probe  110  and shield current monitor probe  112 . These transfer factors may be obtained from the manufacturer as a function of frequency or by calibration. 
     Step  506  is to determine the required shielding for the shielding surveillance conductor  114 . Shielding is measured in dB, and is defined as 20 log [Shield current in amperes/voltage on surveillance conductor in volts]. Shielding may be a function of frequency. 
     Step  508  is to determine test frequencies. A popular frequency range for testing the outer shields of signal and power cables is 30 MHz to 450 MHz, using at least 4 frequencies per octave. It may be best to not choose frequencies equal to those produced by the signals in the cable if it is desired to test when the cable is actively carrying data traffic. 
     Step  510  is to determine allowable variation in cable shield current at shield current monitor probe  112 . In addition to variation caused by faults in the cable shield, shield-to-connector bonding, and connector-to-chassis bonding, shield current may vary due to changes in equipment electrical bonding and cable position, in particular, if the cable is routed near (within 10 cm of) metallic structures. 
     Step  512  is to load injection probe factors and required shielding data (determined above) at each test frequency into the control processor  308  via the programming and data load port. The programming and data load port  122  may be a port implemented via a connector on the surveillance module  108  that temporarily interfaces to a computer, or it may be an interface with the host electronic equipment item  102 &#39;s processor/controller to allow activity via the host equipment or a computer network. 
     Step  514  is to have control processor  308  compute and store in non-volatile memory  310  attenuator settings and the allowable ranges of inputs from the shield current monitor probe  112  and shielding surveillance conductor  114  at each test frequency. A value check may be performed by the CP  308  to determine if the inputs are reasonable and can allow valid computations. 
     Step  516  is to determine if the data load and computation is OK. If YES, then Step  520  is to indicate READY on ready line. Test Result and Ready output lines may connect to indicator lamps or may send data to the host equipment&#39;s processor/controller, or may send data over a computer network. If NO, then per step  518 , indicate SET-UP UNSUCCESSFUL via the programming and data load port. 
     Step  522  is to Stop. 
     Now referring to  FIGS. 6A and 6B , there is shown the calibration process generally designated  600 , which includes a starting block  602  and Step  604 , which is to check for readiness. If Ready is indicated, then proceed to step  606 ; if NOT Ready is indicated, then go to set-up process ( FIG. 5 ). Step  606  is to set up equipment and cables with shielding surveillance module  108  installed in host electronic equipment item  102 , if not already done. 
     Step  608  is to install shield current injection probe  110  and shield current monitor probe  112 , if not already integral with the assembly of shielded connecting cable  106  or previously installed. 
     Step  610 , Toggle “initiate calibration” line, the action may be performed as a manual toggle/push button switch, remotely via an equipment internal processor/controller, or remotely over a computer network via the control interface  120 . 
     Step  612 : Control processor  308  selects first test frequency and sets up modulator/transmitter  312  and receiver  320 . Setup includes frequency selection and output of continuous test message from the control processor  308  to the modulator/transmitter  312  for Frequency Shift Keying (FSK) modulation. FSK deviation set to be relatively small with respect to selected test frequency, while being kept as large as possible to increase sensitivity of the receiver  320 . 
     Step  614  is to have control processor  308  select MUX input  1  for Self-test mode. 
     Step  616  is to have analog-to-digital converter  326  measure the RSSI voltage and report results to control processor  308 . 
     Step  620  is to have control processor  308  check if RSSI is in tolerance and for correct message data at receiver output. This process is to verify that the module is outputting the correct power and that the message data transmit and receive features are operational. 
     Step  622  is to determine if RSSI and Data are OK. If yes, proceed to Step  628  which is to determine if there is another test frequency. If Step  622  returns a NO, then proceed to step  624 , which is to send message “Self Test Failed” on test result output line. Then stop. 
     If there is another test frequency per step  628 , then proceed to step  618 , which is where the control processor  308  selects next test frequency and loops back to step  616 . 
     If there are no other test frequencies, then proceed along line  629  to  FIG. 6B  and to step  630 , which is to have control processor  308  set programmable attenuator  1   324  to highest value, then to step  632 , which is to have control processor  308  select MUX input  2  to allow measurement of shield current monitor probe  112  output. Then go to step  634 , which is to have control processor  308  select first test frequency and set up modulator/transmitter  312 , then to step  636  is to have analog-to-digital converter  326  measure the RSSI voltage and report the results to control processor  308 . Receive Signal Strength Indicator (RSSI) is a voltage proportional to the power level at the input of the receiver  320 . 
     Step  638  is to decide whether the RSSI output is Midrange. If YES, then proceed to step  642 . If NO, then proceed to step  640 , which is to have control processor  308  change value of programmable attenuator  1   324  step in the direction of the variance and then go back to Step  636 . If the RSSI output is midrange, then per step  642 , the next step is to store programmable attenuator  1   324  setting and measured RSSI for that frequency in non-volatile memory  310 . Then on to step  644 , where it is determined whether there is another test frequency. If YES, then proceed to step  646 , where the control processor  308  selects the next test frequency and sets programmable attenuator to highest value the loops back to Step  636 . If there are no other test frequencies, then proceed to Step  648 , which is to Store “Cal Complete” flag in non-volatile memory  310 , then to proceed to  650 , which is to send message “Calibration Complete” on test result output line, which is part of  120 . Then to Stop—Step  652 . 
     Now referring to  FIGS. 7A ,  7 B and  7 C, there is shown a flowchart of the innovative surveillance process of the present invention generally designated  700 , which includes a start Step  702  and a Step  704  which is to toggle “initiate surveillance” line. The next step is Step  706 , which is to determine if “Cal Complete” flag has been set. If NO, proceed to Calibration process and  FIGS. 6A and 6B . If YES, then proceed to Step  710 , which is to have control processor  308  select first test frequency from non-volatile memory  310  and setup modulator/transmitter  312  and receiver  320 . Step  712  has control processor  308  selecting MUX input  1  for self-test mode. Step  714  has analog-to-digital converter  326  measuring the RSSI voltage and reporting the results to control processor  308 . Step  718  is to have control processor  308  check if RSSI is in tolerance and for correct message data at receiver output. 
     Step  720  is to determine if RSSI and Data are OK. If NO, then per Step  722 , Send Self Test Failed message on Test result output line (part of interface  120 ) and then stop. If YES, then determine if there is another frequency. If YES, then per Step  726 , proceed to Step  716 , where the control processor  308  selects next test frequency and proceeds to Step  714 . If no other test frequency exists, then proceed along line  727  to  FIG. 7B  and to Step  728 , which is to have control processor  308  select first test frequency and set up modulator/transmitter  312  and receiver  320 . Then proceed to Step  730 , which is to have control processor  308  set programmable attenuators  1  and  2 — 324  and  325  respectively, and retrieve calibration data from non-volatile memory  310 . Then on to Step  732 , which is to have control processor  308  select input  2  (monitor probe voltage) on MUX. Then on to Step  734 , which is to have analog-to-digital converter  326  measure the RSSI voltage and report the results to control processor  308 . Step  736  is to compare result from monitor probe voltage to calibration value with tolerance in non-volatile memory  310 . Step  738  is to determine whether the monitor voltage is OK. If YES, then proceed to step  740 ; if NO, then branch off to Step  742 . Step  742  is to check for correct message data at receiver output; if NO, then per Step  746 , proceed to Step  748  and Send message “Cable/connector system shield fault or fault indication due to external noise sources, Check Set-up” on Test result output line. Then to Stop—Step  750 . 
     In order to accurately confirm a shield fault, an out-of-tolerance monitor probe voltage (indicates change in how the current flows on the shield) and/or a surveillance conductor voltage greater than the allowed amount (indicates excessive leakage through the shield), must be correlated to the injected signal at one or more of the test frequencies. The injected signal is modulated with a data message that the receiver should be able to detect if the received signal(s) are above the amount allowed to indicate good shielding. High-level uncorrelated received signals may be due to noise from near-by sources. At that point, the measurement setup and cable should be checked. 
     Step  740  is to check for correct message data at receiver output and per step  744 , determine if receive message data is OK. If YES, then proceed along line  745  to  FIG. 7C  to Step  760 . If NO, then proceed to Step  748 . 
     In Step  746 , if YES, the message data was received OK, then proceed to Step  752 , which is send message “Cable/Connector System Shield Fault” on test result output line and then stop, per Step  754 . 
     Now referring to  FIG. 7C , there is shown Step  760  where the control processor  308  selects input  3  (surveillance Conductor voltage) on MUX. Then to Step  762 , where analog-to-digital converter  326  measures RSSI voltage and reports the result to control processor  308 , then to Step  764 , which is to compare result (from surveillance conductor voltage) to maximum allowed voltage previously stored in non-volatile memory  310 . Then proceed to Step  766 , where you determine whether the surveillance conductor voltage is less than or equal to the allowed maximum. If YES, proceed to step  772 . If NO, then proceed to Step  768 , which is to check for correct message data at receiver output and then to Step  770 , which is to determine whether the receive message data is OK. If YES, proceed along line  747  to  FIG. 7B  to Step  752 . If NO, then proceed to Transfer Point A  771 , which is coupled to Transfer point  749  on  FIG. 7B  and then to Step  748 . 
     Step  772  determines whether another test frequency exists. If YES, then proceed to Step  774 , where the control processor  308  selects next test frequency from non-volatile memory  310  and sets up modulator/transmitter  312  and receiver  320 . Then along line  775  to Step  730  in  FIG. 7B . If NO, then to Step  776 , which is Send message “Cable/connector system shield OK” on test result output line. Then to Stop  778 . 
     It is thought that the method and apparatus of the present invention will be understood from the foregoing description and that it will be apparent that various changes may be made in the form, construct steps, and arrangement of the parts and steps thereof, without departing from the spirit and scope of the invention or sacrificing all of their material advantages. The form herein described is merely a preferred exemplary embodiment thereof.