Abstract:
Averaged guided wave inspection method for a throttle cable is disclosed. Access to the engine end of the throttle cable is obtained and a transducer is applied to an end of the ribbon cable. The transducer generates an ultrasonic guided wave in the cable. The ultrasonic guided wave propagates down the entire length of the cable and reflects back from any discontinuity in the cross section of the ribbon cable. By determining the time needed for the reflected wave to travel back to the receiver, the location of any defect along the length of the cable can be determined. By moving the ribbon cable and transducer to different positions with respect to the sheath of the throttle cable, repeating the prior steps and averaging, unwanted noise caused by external influences is eliminated.

Description:
This invention was made in part with government support under Contract No. F04606-98-D-0002 awarded by the United States Air Force. The United States Government has certain rights in this invention. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     Applicant&#39;s invention relates to an averaged guided wave inspection technology for ribbon cable. 
     2. Background Information 
     There are throttle cables in many aircraft, particularly the AO/A-10 Thunderbolt, that have the potential for failure during routine flying activities. The failure of the throttle cable can result in loss of throttle control and can lead to critical failure or loss of the aircraft. The throttle cable consists of a steel ribbon cable approximately 0.05 inch by 0.200 inch in cross-sectional dimension and ranging from approximately 26 to 32 feet in length that is contained in a stainless steel sheath casing and supported by a large number of stationary ball bearings inside the sheath. This ribbon cable directly connects the engine throttle to the throttle control in the cockpit. The stationary ball bearings allow the ribbon inside the throttle cable to move freely through the sheath. The throttle cable is strung between the cockpit and engines through the fuselage and in the process must go through several bends. An analysis of the failure modes seems to indicate that cycling the ribbon cable through these bends can lead to fatigue cracking and ultimately to failure of the ribbon. At the present time, during normal maintenance, these cables are given a force test to ensure that the ribbon cable moves freely inside the sheath. However, if the cable passes the force test there is no assurance that the ribbon does not have a defect or that the whole cable is not defective. Presently, there is no way to inspect the ribbon without the costly process of removing the entire throttle cable from the aircraft. Therefore, there is a need to develop a nondestructive evaluation technique that would allow inspection of these cables to detect any abnormalities or defects in the ribbon before failure. Since the ribbon cable is completely inaccessible to any probe except for a few inches at the end of the cable where it attaches the engine, the inspection technique must inspect the entire ribbon length from the accessible end. 
     The present invention accomplishes this goal and provides a nondestructive technique to inspect the throttle cables of aircraft, particularly A-10 aircraft. This inspection technique inspects the ribbon cable from its accessible end and provides complete inspection of the entire length of the ribbon by utilizing an averaged guided wave technology. In addition, this technology has application for inspection of cable in other items as well. 
     An ultrasonic guided wave approach was chosen that would allow an ultrasonic transducer to be placed on the accessible end of the ribbon cable and generate a guided wave that would travel down the entire length of the ribbon. This guided wave was capable of detecting small defects that would show up in the plot of reflected signal strength as a function of time (called an A-scan). However, the initial evaluation of the guided wave approach showed that the contact points between the ball bearings at the bend regions and throughout the length of the throttle cable caused reflections of guided waves as well. Since the ball bearings were spaced approximately every ⅝ inch down the length of the cable, a large number of reflected signals in the A-scan are due to the contact between ball bearings at the bend regions and the ribbon cable were observed. This caused false defect calls in addition to the masking of real defects in the bend regions. Unfortunately, the false calls were not random and simple averaging of the A-scans did not eliminate them. 
     However, it was found that one way to reduce the effect of these signals was to average the guided wave data collected while the ribbon was being moved back and forth through the cable sheath. This would mean that the temporal location of the reflection of the ball bearing contact with the ribbon in one waveform would be different than in the following waveforms because the ball bearings were fixed in the sheath and, as the ribbon moved inside the sheath, its relative position with respect to the ball bearings would be continually changing. In addition, the relative position of any defect in the ribbon cable stays fixed between the guided wave transducer and the defect. In these circumstances, if guided wave A-scan data is collected and averaged as the ribbon cable is being moved back and forth inside the cable sheath, then signals from the defect will continue to remain constant throughout the averaging process, but signals from the random electronic noise and signals from the ball bearing contact will be diminished because those signals are always changing in time with respect to the transducer&#39;s position. 
     SUMMARY OF THE INVENTION 
     Applicant&#39;s primary object for the present invention is to provide a method of inspecting throttle cables. 
     It is a further object of the present invention that the inspection system of the present invention gain access to the end of the throttle cable at the engine end and apply a piezoelectric transducer to the ribbon cable to generate an ultrasonic guided wave in the cable. 
     An additional object of the present invention is when the transducer is attached and coupled to the end of the ribbon cable, it will produce a low frequency guided wave that will propagate down the entire length of the ribbon and reflect back from any discontinuity in the cross section of the ribbon cable. 
     A further object of the present invention is to provide an averaging technique that allows defects to be distinguishable from other discontinuities. 
     Yet another object of the present invention is that in this technique, the ribbon cable with the transducer fixed on one end of it is moved to a number of positions in the cable sheath and upon generation, propagation, reflection, and reception of the wave, a waveform of the received reflections versus time is recorded to create a primary A-scan data set. 
     An additional object of the present invention is that the waveform received from the previous objective is recorded as the ribbon is moved inside the sheath cable. 
     Still another object of the present invention is for the primary and secondary A-scan data sets to then be averaged to obtain information on the position of any defects in the cable ribbon as distinguished from the ball bearing contact points. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic view of the throttle cable inspection system components. 
     FIG. 2 is a cut away perspective view of throttle cable. 
     FIG. 3 is an internal cross section of the throttle cable. 
     FIG. 4 a  is a close up view of the engine end of throttle cable incorporating a long sleeve. 
     FIG. 4 b  is a close up view of the engine end of throttle cable with long sleeve removed. 
     FIG. 4 c  is a close up view of the engine end of throttle cable with the short sleeve being added. 
     FIG. 4 d  is a close up view of the engine end of throttle cable incorporating a short sleeve. 
     FIG. 5 is a perspective view of the transducer attached to the cable ribbon. 
     FIG. 6 is graph of the detection of notch # 1  in test cable # 3  (Cable laid straight with no clamps) as the notch size is increased from 5% to 50% of cable cross section. 
     FIG. 7 is a graph of the waveforms from test cable # 3  with cable ribbon being moved (a) to different locations inside the sheath and (b) continuously inside the sheath, both during tests. 
     FIG. 8 is a schematic view of the averaged waveforms when cable ribbon is moved inside the sheath. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The inspection system of the present invention gains access to the end of the throttle cable at the engine end and applies a piezoelectric transducer to the ribbon cable to generate an ultrasonic guided wave in the ribbon. When the transducer is attached and coupled to the end of the ribbon cable, it will produce a low frequency elastic wave that will propagate down the entire length of the ribbon and reflect back from any discontinuity in the cross section of the ribbon cable. By determining the time needed for the reflected wave to travel back to the receiver, the location of the defect along the length of the throttle cable can be determined. 
     The present inspection system is based mainly on the principle of guided waves. Guided waves are dispersive waves (velocity is a function of frequency) generated when the dimension of the solid body is of the same order or smaller than the acoustic wavelength. Guided waves have energy flow mainly along the direction of the guiding configuration. Such guided waves can be considered elastic perturbations propagating in a solid layer with free boundaries. On the other hand, bulk waves travel along the propagation direction, regardless of the geometry of the propagating medium. Displacements of guided waves are both in the direction of wave propagation and perpendicular to the direction of propagation. 
     Referring to FIG. 1, the throttle cable inspection system components are shown. Ultrasonic guided waves are introduced into the ribbon cable  24  by a 0.5 inch, 500 kHz transducer  34  with a band width of approximately 200 to 800 kHz that is driven by a very low frequency-ultrasonic test system (VLF-UT). This system consists of software running on a portable computer  48  and hardware to generate the tone burst. An industry-standard architecture (ISA) function generator card  54  situated within the portable computer  48  generates the tone burst, more particularly a gaussian signal, of the required frequency. The tone burst is transmitted to the analog-to-digital (A/D) board  56  where it is digitized. A modified tone burst is transmitted over cable  50 . Cable  50  connects the portable computer  48  to a high voltage amplifier  46  at AMP IN. The high voltage amplifier  46  is used to amplify the generated signal of the ISA function generator card  54 . 
     The amplified signal then proceeds out of the high voltage amplifier  46  through AMP OUT and into cable  40 . Cable  40  terminates in AMP IN of a diplexer  38 . Diplexer  38  separates the transmitting signal from the received signal. Power is provided to diplexer  38  from amplifier  46  through power cable  44 . The amplified signal then proceeds from the diplexer  38 , out TRANSDUCER OUT, and into coaxial cable  32 . Coaxial cable  32  connects to transducer  34 . When transducer  34  is attached to ribbon cable  24 , the amplified tone burst signal generates ultrasonic vibration at the active face of the transducer  34  where it has attached to the ribbon cable  24 . The vibration of transducer  34  is transmitted to the ribbon cable  24 , proceeds through the length of throttle cable  25 , and is then reflected from any discontinuity in the ribbon and the end of the ribbon. The reflected ultrasonic vibration is returned to transducer  34 , and is changed to electrical signal and this signal travels back into coaxial cable  32  and into TRANSDUCER OUT of diplexer  38 . The reflected signal then passes out SIGNAL OUT into cable  42  and into SIGNAL IN of amplifier  46 . The amplifier  46  amplifies the reflected signal. This amplified, reflected signal then passes through SIGNAL OUT of amplifier  46  into cable  52  and into the A/D board  56  of the portable computer  48 . The A/D board  56  in the portable computer  48  captures the amplified, reflected signals from the transducer  34 . Finally; the amplified, received signal from the transducer  34  is displayed on the screen of the portable computer  48  and stored in memory of the portable computer  48 . When the signal is received, the function generator  54  generates another signal in accordance with programmed instructions and the process is repeated. 
     Throttle cable  25  is shown in more detail in the internal cross section in FIG.  3  and the cut away perspective in FIG.  2 . The throttle cable  25  directly connects the engines in the rear of the aircraft to the throttle control levers in the cockpit at the front of the aircraft. The throttle cable  25  is approximately 26 to 32 feet long. The ribbon cable  24  is approximately 0.05 inches by 0.200 inches in cross section dimension and is normally made from stainless steel. A protective sheath casing  14  encircles the ribbon cable  24 . A protective sleeve  12  covers the sheath casing  14 . The ribbon cable  24  is supported by a large number of stationary ball bearings  18  on the inside of the sheath casing  14  that hold the ribbon cable  24  and enable the ribbon cable  24  to move freely through the sheath casing  12 . 
     The stationary ball bearings  18  are located between spacer grooves  21  and ribbon grooves  23  and may move along the grooves  21  and  23 . Ball bearings  18  are responsible for many of the false signals that can be obtained in the non-averaged guided wave signal. Outer  20  and inner  22  spacers space the ball bearings  18  apart. cut-away view of the throttle cable  25  is shown in FIG.  2 . The cable  25  is shown with a protective sleeve  12  and sheath casing  14 . Housed within sheath casing  14  are outer spacers  20  and inner spacers  22  which hold ball bearings  18  in place. Located centrally is ribbon cable  24  which is held in its center locations by ball bearings  18 . 
     In order to inspect the ribbon cable  24  for defects, the transducer  34  must be attached to the engine end of the ribbon cable  24 , which has only about one inch of exposed ribbon cable  24 . To increase the exposed part of the ribbon cable  24 , a long sleeve telescopic spacer  58  at the end of the throttle cable  25  is removed. The telescopic spacer  58  retains the ball bearings  18  in position. FIGS. 4 a  and  4   b  are close up views of the engine end of throttle cable  25  with a long sleeve telescopic spacer  58  both attached and detached, respectively. The long sleeve telescopic spacer  58  was replaced with a short sleeve telescopic spacer  60  which is illustrated in more detail in FIGS. 4 c  and  4   d . This shorter sleeve can still hold the ball bearings  18  in position, but also gives a larger exposed space on the ribbon cable  24 . The remaining portions of the engine end of throttle cable  25  include knob  26  which hooks into the aircraft engine (not shown). Adjacent knob  26  is enlarged part  30  which attaches to the engine control. Ribbon cable  24  is formed with knob  26  and enlarged part  30 , but begins after enlarged part  30 . Nut  28  attaches sheath casing  14  to protective sleeve  12  at the end of cable  25 . 
     Once a sufficient amount of the ribbon cable  24  is exposed, the transducer  34  is attached to the exposed part of the ribbon cable  24  to perform the inspection. The transducer  34  attached to the ribbon cable  24  is shown in FIG. 5. A shear wave couplant and a clamp are used to attach transducer  34  to ribbon cable  24 . Shear wave couplant is a viscous liquid introduced between the transducer  34  and the ribbon  24  before transducer  34  is attached to cable  32 . Shear wave couplant allows the ultrasound from transducer  34  to be induced into the ribbon cable  24 . 
     After the transducer  34  is attached to the ribbon cable  24 , all of the electronic parts are adjusted to generate a tone burst of 200 kHz. When the transducer  34  generates guided waves into a straight ribbon cable  24  with no defects, a reflection from the end of ribbon cable  24  is observed. A reverberation of initial tone burst exists at the beginning of the waveform which can prevent detection of defects at the beginning of the ribbon cable  24  which is typically within 14 feet from the engine side. However, most cable failures have occurred near the cockpit end of cable  25  which exists at 18 to 29 feet from the engine side. 
     In order to test the effectiveness of the present method, defects in the throttle cable  25  were simulated by making saw-cut notches in ribbon cable  24 . The notches were made in several stages to determine the minimum size notch detected. All initial notches were made with a 0.018 inch thick diamond saw, and they originated from the side of the ribbon cable  24  and grew inward. Notches were intentionally introduced around critical points, particularly bend locations. 
     FIG. 6 shows the progression in size of the straight notch in test cable # 3  when it is laid straight with no bends and no clamp. When saw-cut notches were made in test cable # 3 , the first notch was made at approximately 27 feet from the engine end of throttle cable  25  as shown at line  62 . The distance as measured from the engine end is indicated in feet on the y-axis. The notch depth was increased in several stages from 5 to 50 percent of ribbon cable  24  cross section as shown on the x-axis  64  to determine the minimum detectable defect size. As shown, the notch can be detected when its size exceeds 20 percent of the ribbon cable&#39;s  24  cross-section as shown beginning at  66 . 
     Next, experiments were performed to determine the ability of the present procedure to detect defects when the ribbon cable  24  is placed in a sheath in its actual state within the aircraft, that is, when the throttle cable  25  goes through several bends. FIG. 7 is a graph of the waveforms from test cable # 3  when the ribbon cable  24  is moved within sheath  14 . Test cable # 3  is given two notches and it is placed through four bends. The first waveform  90  shows the reflection when cable  25  is laid straight with no bends. Next, the routing of the cable  25  is shown at  92 . Two readings of the waveform with the bends were plotted at  94  and  96 . The next three waveforms,  98 ,  100  and  102 , show the ribbon cable  24  position in relation to sheath  14 . Waveform  98  shows ribbon cable  24  all the way inside sheath  14 , waveform  100  shows ribbon cable  24  out of the sheath  14  halfway, and waveform  102  shows ribbon cable  24  out as far as possible. The averaging of waveforms  98 ,  100 , and  102  will reduce the reflections from bends. However, the reflections from notches are unchanged. The averaged waveforms  104  and  106  were obtained while the ribbon cable  24  was continuously moving in and out of sheath casing  14 . 
     The averaging technique of the present invention allows defects, such as notches, to be distinguishable from the discontinuities, such as reflections from ball bearings at bends. 
     FIG. 8 is a schematic view of signals from the reflections from notches and bends when the ribbon cable  24  is moved within the sheath casing  14  for two cases. In the first case,  88 , the position of the transducer is fixed with respect to the sheath casing  14 , and the ribbon cable  24  slides under it. In the second case,  89 , the transducer is attached to the ribbon  24 , and its position relative to the sheath casing  14  is changed. The throttle cable  25  goes through two bends  70 . End  72  can be seen in the reflected signals. A notch  74  is placed between bends  70 . For the physical ribbon position  88  of FIG. 8, the ribbon cable  24  within the throttle cable  25  is moved, but the transducer  34  remains fixed with respect to the sheath casing  14 . If the ribbon cable  24  is pushed all the way in as indicated at  76 , then a signal will be returned for both bends  70 , the notch  74  and the end  72 . If the ribbon cable  24  is moved midway out as indicated at  78 , the bends  70  will be located at the same place; however, the notch  74  and the end  72  have moved closer to the transducer  34 . When the ribbon cable  24  is moved as far out as possible as indicated in  80 , again the bends  70  will be located at the same place, but the notch  74  and end  72  will be moved still closer to the transducer  34 . 
     For the case when the transducer is fixed to the ribbon cable  24  shown in FIG. 8, the transducer  34  and ribbon cable  24  are moved in unison. When the ribbon cable  24  is pushed all the way in as indicated at  82 , the signals for the bends  70 , notch  74 , and end  72  exist as in  76 . However, when the ribbon cable  24  and transducer  34  are moved midway out, the bends  70  will appear farther from the transducer  34  as shown in signal  84 . However, notch  74  and end  72  will appear as though they have not moved, since the relative position between the transducer and the notch  74  and end  72  are fixed. In signal  86 , the transducer  34  and ribbon cable  24  are pulled out as far as possible. Again notch  74  and end  72  will appear as though they have not moved for the same reasons as noted for signal  84 . 
     In the actual scope readings  89  where the transducer is attached to the ribbon cable  24 , the movement of the ribbon cable  24  inside the sheath casing  14  and averaging the waveform over a period of time, will cause the reflections from bends to be greatly reduced. The last waveform  68  shows the average of the waveforms when the ribbon cable  24  is continuously moved in and out indicating both the notch  74  and the end  72 , but not the bends  70 . The characteristics inside the ribbon cable  24 , such as the notch  74  and end  72 , remain stationary to the transducer  34  and therefore when they are averaged, they will not change. In experimental conditions, the ribbon cable  24  was placed in several different positions by an actuator (not shown) and the signals for these various positions were averaged. Computer  48  (see FIG. 1) is programmed to deliver a number of signals and receive those signals. Ultimately the averaging is done by computer  48 . The entire test can be performed by one person. 
     Although the invention has been described with reference to specific embodiments, this description is not meant to be construed in a limited sense. Various modifications of the disclosed embodiments, as well as alternative embodiments of the inventions will become apparent to persons skilled in the art upon the reference to the description of the invention. It is, therefore, contemplated that the appended claims will cover such modifications that fall within the scope of the invention.