Abstract:
An ultrasonic one pass inspection system for determining the presence, location, and size of flaws in laminated structure such as an “I” stiffener in a single inspection cycle. The exemplary system includes an immersion tank, six (6) single probes, 184 transducers, four (4) motor assemblies, two (2) encoder assemblies, a collection tank, and a recirculation assembly. The probes are designed to match the shape of the stiffener. Two (2) motors fore and two (2) motors aft of the immersion tank, produce information related to the position of the stiffener with respect to the position of the transducer. The position encoder is spring-loaded against the stiffener.

Description:
RELATED PATENT APPLICATIONS 
     This is continuation-in-part of U.S. patent application Ser. No. 08/664,899, filed Jun. 17, 1996, abandoned, assigned to The Boeing Company, which is a continuation in part of U.S. patent application Ser. No. 08/086,283, filed Jul. 1, 1993, now U.S. Pat. No. 5,585,564, issued Dec. 17, 1996, assigned to the Boeing Company 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to ultrasonic inspection and, more particularly, to the use of ultrasonic transducers to detect and size flaws in laminated composite “I” stiffeners. 
     BACKGROUND OF THE INVENTION 
     The use of graphite/epoxy materials for building aircraft structures is expanding. For example, graphite/epoxy “I” stiffeners are +being used to give strength and rigidity to the empennage of new aircraft. In order to keep pace with this expanding use of these new materials, new and faster methods for inspection are required. 
     In general, ultrasonic systems, whether multiple or single channel (a pair of transducers, transmitting and receiving) requires repeated passes over the part for 100% ultrasonic inspection of the part. This is a time consuming procedure. 
     The present invention, hereinafter described “feedthrough stiffener inspection system” (FSIS), provides a rapid ultrasonic inspection of the aforementioned “I”-shaped stiffeners. FSIS is a one pass, 100% inspection capable of handling stiffeners, regardless of length. FSIS&#39;s transducers are stationary, which eliminates the need for long cables (a problem characteristic of devices which creep along the stiffener while inspecting or gantry-type robots). Also, because FSIS is stationary, one person can control movement of the part through the system and evaluate the inspection data at the same time. Because FSIS is an immersion ultrasonic technique, its transducer shoes are much more simple and cost effective to design and manufacture than those of other inspections which are relegated to using a bubbler technique. Compared to other immersion techniques, FSIS is a faster technique. Due to its small size, FSIS does not require a large storage area while not in use. 
     In accordance with the present invention, there is provided a feedthrough stiffener inspection system for determining the presence, location, and size of flaws in the radius region and adjacent areas of a structure. A preferred embodiment of the present system comprises: 6 probes with a plurality of transducers; an immersion tank with “I”-shaped windows fore and aft; a water collection tank; a fore and aft motor drive assembly; a fore and aft position encoder assembly; a water recirculation system; and, a plurality of roller tables. 
     In accordance with further features of the present invention, the probe assembly further comprises a plurality of individual shoes configured for complementary engagement with the structure under inspection. The individual shoes are pushed toward each other by spring assemblies so as to clamp the “I” stiffener. The plurality of transducers inspect the “I” stiffener and produce relevant inspection information. 
     In accordance with further features of the present invention, the fore and aft motor assemblies are mounted to a spring-loaded platform which pushes the motor assemblies against the “I” stiffener. A pressure wheel with movable platform is directly opposite the motor drive wheel. The interaction of the two wheels create the translational movement of the “I” stiffener enabling it movement through the immersion system. The fore and aft motor assemblies operate synchronously. 
     In accordance with yet further features of the present invention, the position encoder assemblies are mounted to a spring-loaded platform. The relative motion of the structure moving through the system drives the encoder, thereby producing position information for generating C-scans. The fore and aft position encoder assemblies operate synchronously. As the structure moves through the system, the fore encoder electronics is activated. When the structure disengages from the fore position encoder, the aft encoder is activated until the structure disengages from the aft encoder. 
     In accordance with still further features of the present invention, the recirculation system maintains the water level of the immersion tank so that the “I” stiffener and probes are always immersed. The recirculation system recycles the water from the collection tank back into the immersion tank. 
     In accordance with yet further features of the present invention, conveyor tables fore and aft of the immersion system support the structure as it moves through the inspection station. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The foregoing and other features and advantages of the invention will become more readily appreciated as the same becomes further understood by reference to the following detailed description when taken in conjunction with the accompanying drawings, wherein: 
     FIG. 1 is a block diagram of the feedthrough “I” stiffener inspection system which includes the motor/encoder system, data acquisition system, and ultrasonic system; 
     FIG. 2 is a simplified isometric view of the presently preferred embodiment of the feedthrough stiffener inspection system; 
     FIG. 2A is a cross-sectional view of the “I” stiffener; 
     FIG. 3 is an exploded isometric view of the motor/encoder assembly with stiffener; 
     FIG. 4 is an exploded isometric view illustrating the stiffener and immersion tank; 
     FIG. 5 is a side view illustrating the stiffener before entering the immersion tank; 
     FIG. 6 is a side view illustrating the stiffener in the immersion tank; 
     FIG. 7 is an isometric view of the cap probe; 
     The frontal view of FIG. 7A illustrates the area of inspection coverage; 
     FIG. 8 is an isometric view of the flange probe; 
     The frontal view of FIG. 8A illustrates the area of inspection coverage; 
     FIG. 9 is an isometric view of the web probe; 
     The frontal view of FIG. 9A illustrates the area of inspection coverage; 
     FIG. 10 is an isometric view of the cap radius probe; 
     The frontal view of FIG. 10A illustrates the area of inspection coverage; 
     FIG. 11 is an isometric view of the flange radius probe; 
     The frontal view of FIG. 11A illustrates the area of inspection coverage; 
     FIG. 12 shows positioning of the edge probe transmit and receive transducers relative to the “I” stiffener and ultrasonic signal generated by the edge; 
     FIG. 12A is illustrative of the positioning of both transmit and receive transducers precisely along the edge; 
     FIG. 13 is an isometric view of the four edge probes; 
     FIG. 14 is an isometric view, with parts exploded in relative assembly position of one side of probe; 
     FIGS. 15,  16  and  17  are pictorial representations sequentially showing the stringer passing through the edge probe; and 
     FIG. 18 is a vertical section showing the use of stainless steel reflectors to redirect the sound beam. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Ultrasonic inspection probes are known in the composite or laminated structure art; e.g., such a probe is described in a U.S. Pat. No. 4,848,159 entitled “Ultrasonic Inspection Probe For Laminated Structures,” by Kennedy, et al., assigned to The Boeing Company. The details of probe shoes, including the ultrasonic transducers, the internal conduits, and the biasing spring assemblies shown in U.S. Pat. No. 4,848,159 may be referred to and are incorporated herein by reference. 
     Turning now to FIG. 1, there is illustrated a block diagram of a feedthrough stiffener inspection system (FSIS)  100  in accordance with a preferred embodiment of the present invention. The feedthrough stiffener inspection system includes: two fore motor assemblies  101  and  102 ; two aft motor assemblies  103  and  104 ; a fore encoder assembly  105 ; an aft encoder assembly  106 ; an immersion tank  107 ; a cap probe assembly  108 ; an edge probe assembly  208 ; a flange probe assembly  109 ; a web probe assembly  110 ; a cap radius probe assembly  111 ; and a flange radius probe assembly  112 . 
     The fore motor assemblies,  101  and  102 , and the aft motor assemblies,  103  and  104 , operate cooperatively to move the stiffener through immersion tank  107 . The stiffener direction is indicated by double arrow  113 . Ultrasonic electronics  114  and preamp electronics  115  transmit and receive the ultrasonic signals. Data acquisition system  1   16  receives and analyzes the encoded position information on line  118  and the ultrasonic signal information on line  117  information on line  118  and the ultrasonic signal information on line  117  to determine the presence, position, and size of flaws in the part under inspection. Controller  119  controls the fore and aft motor drives. 
     FIG. 2 is an isometric view of a preferred embodiment of the present ultrasonic inspection system for laminated stiffeners. A stiffener  120  is resting on the conveyor tables  121  which are both fore and aft of the immersion tank  107 . There are a total of six conveyors. The individual rollers  127  are  4  inches long and rotate independently of each other. The “I” stiffener  120  of FIG. 2 has a cap  108 , a flange  109 , an edge  208 , a web  110 , a cap radius  111 , and a flange radius  112 . Stiffener  120  is engaged into motor drive  101  which then begins pulling action. Stiffener  120  then comes in contact with fore encoder  105  which provides the position signals to data acquisition system  116 . Engagement with the second fore motor drive  102  forces stiffener  120  to become aligned with the inspection system. Stiffener  120  enters the immersion tank  107  through fore window  128  shaped like the stiffener  120 . Window  128  serves two purposes: to further align the stiffener  120 , and for reducing the loss of water in immersion tank  107 . The water exiting the immersion tank  107  is collected in stainless steel collection tank  129 . The water is recirculated by recirculation system  130 . The overflow water runs down trough  132  to minimize introduction of air into the water. Air attenuates the ultrasonic signal, thereby producing false readings. 
     Six probes are required to inspect the entire cross-section of stiffener  120 . The five probes are cap  122 , flange  123 , web  124 , cap radius  125 , and flange radius  126 . Each probe is composed of several shoes that are held together by bearings, rods, and springs. A shoe is a collection of transducers mounted in a machined plexiglass block. Each shoe can hold up to  16  transducers. Each pair of transducers covers an inspection width of  0 . 125  inch. With the exception of web probe  124 , the shape of the ultrasonic probe fits the shape of the part surface, ensuring that there is a stable equilibrium position when the probes are pressed against the part. An edge  208  is also utilized for production inspection. Probe  208  is necessary because of visible delaminations along the edges of the stiffener not detected by either the cap or flange probes. Normally these delaminations are not wide but are tight and therefore not visible with the eye. The ultrasonic signal generated by the edge is shown in FIG. 12. A high voltage electrical spike is sent to the transmit transducer of edge probe  208  generating in FIG. 12, if the transducer was positioned so that the ultrasonic beam were half on the stiffener, the resulting A-scan display (appearing on oscilloscope  114  of FIG. 1) will show the signal through the part (1) and through the water (2). The time between the signals is too small for the electronic gate to differentiate. Therefore, the signal through the water is the only signal detected. 
     The stiffener exits the aft window  131  which is the same as fore window  128 . Stiffener  120  engages first aft motor  103 , the aft encoder  106 , and the second aft motor  104 . A maximum of four motors are pushing/pulling the stiffener through the inspection system. When fore encoder  105  and aft encoder  106  are engaged on the stiffener  120 , position information is being generated by fore encoder  105 . As the stiffener  120  disengages from fore encoder  105  and stops rotating, aft encoder  106  is activated and starts generating position information. Conveyor table  121  supports the stiffener  120  as it exits the inspection system. 
     FIG. 3 is an isometric view of the present motor/encoder assembly. Stiffener direction is shown by double arrow  113 . The four motor assemblies are identical. The motor assemblies are attached to a stainless steel base plate. Each motor assembly consists of the following hardware: a fixed mountbase  141 ; an adjustable mountbase  142 ; an inside fixed upright  143 ; an outside adjustable upright  144 ; a motor mount  145 ; a motor  146 ; a grooved drive wheel  147 ; two shafts  148 ; two springs  149 ; a boss  150 ; a pressure wheel  151 ; a sliding boss mount  152 ; and, two split hub clamps  153 . 
     Discussion of motor assembly 
     Fixed mountbase  141  is attached and keyed to baseplate  140 . Inside fixed upright  143  is attached to adjustable mountbase  142  which is attached to fixed mountbase  141 . Outside adjustable upright  144  is attached to inside fixed upright  143 . Outside adjustable upright  144  can be adjusted vertically for maximum contact against stiffener  120 , more specifically, web  124 . Grooved drive wheel  147  is attached to motor  146  and this assembly is mounted on motor mount  145 . The motor mount is attached to outside adjustable upright  144 . Drive wheels  147  are grooved to provide maximum traction against wet stiffener  120 . Attached to fixed mountbase  141  are two shafts  148 . A pressure wheel assembly opposing the motor assembly provides the pinch force necessary to force stiffener  120  through the inspection system. Attach to shafts  148  is sliding boss mount  152 . Boss  151  is attached to sliding boss mount  152 . Pressure wheel  150  is attached to a shaft and slides into boss  151 . Spring  149  and split hub clamp  153  provide the pinch pressure. 
     Discussion of encoder assembly 
     A similar arrangement of mounting to baseplate  140  is employed in the encoder assembly. There is a fixed mountbase  141  and two shafts  148  which extend horizontally for a sliding encoder mountbase  154 . Adjustable mountbase  142  is attached to sliding encoder mountbase  154 . Inside fixed upright  143  is attached to adjustable mountbase  142 . Outside adjustable upright  144  allows encoder  156  and encoder wheel  157  to be adjusted vertically for proper position on stiffener  120 , more specifically, web  124 . Encoder wheel  157  is attached to encoder top mount  155  which is attached to the outside adjustable upright  144 . Spring  149  provides the necessary pressure against web  124  to rotate encoder wheel  157  for generating position information. 
     FIG. 4 illustrates the window assembly which includes the following: a window guide  161 ; a rubber seal  162 ; and a window  163 . The direction of the stiffener  120  is depicted by arrow  160 . Rubber seal  162  is sandwiched between window  163  and window guide  161 . The window assembly is attached to immersion tank  107 . 
     FIG. 5 is a side view of the stiffener just before entering immersion tank  107 . Window guide  161  and window guide  163  are slotted to the shape of stiffener  120  cross-section. Rubber seal  162  contains a slit the shape of stiffener  120 . The width of the slit on rubber seal  162  is very small, therefore reducing the loss of water through the opening. 
     FIG. 6 is a side view of stiffener  120  in immersion tank  107 . Window guide  161  opening contains a 30-degree angle around the entire inside and outside edge. This provides for some misalignment of the stiffener. As stiffener  120  enters immersion tank  107 , the rubber seal comes in contact with stiffener  120 . The interaction between rubber seal  162  and stiffener  120  reduces the loss of water through the opening. 
     Discussion of probe in the immersion tank 
     FIGS. 7 through 11 represent the probes in the immersion tank. FIG. 7 is an isometric view of a cap probe and stiffener with a frontal view shown in FIG.  7 A and area inspected by the cap probe denoted by numeral  201 . 
     FIG. 8 is an isometric view of a flange probe and stiffener with a frontal view shown in FIG.  8 A and area inspected by the flange probe denoted by numeral  203 . 
     FIG. 9 is an isometric view of a Neb probe and stiffener with a frontal view shown in FIG.  9 B and area inspected by the web probe denoted by numeral  205 . 
     FIG. 10 is an isometric view of a cap radius probe and stiffener with a frontal view shown in FIG.  10 B and area inspected by the cap radius probe denoted by numeral  207 . 
     FIG. 11 is an isometric view of a flange radius probe and stiffener with a frontal view shown in FIG.  11 B and area inspected by the flange radius probe denoted by numeral  209 . 
     FIG. 12 shows edge probe  208  location and the aforementioned A-scan display. 
     The aforementioned five probes are required to inspect the entire cross-section of the stiffener. The five probes are cap, flange, web, cap radius, and flange radius. Edge probe  208  herein before discussed is required to complete production inspection. Each ultrasonic probe is composed of several shoes that are held together by bearings, rods, and springs. A shoe is a collection of transducers mounted in a plexiglass block. Each shoe can hold up to 16 transducers. Each through-transmission ultrasonic channel covers an inspection width of 0.125″. With the exception of the web probe, the shape of the ultrasonic probe fits the shape of the part surface, thereby ensuring that there is a stable equilibrium position when probes are pressed against the part. 
     The probes (except the web probe) are attached to a positioner. The positioner has vertical and horizontal adjustments to center the probe for the oncoming stiffener. The positioner allows for the probe to move vertically or horizontally during the inspection. The positioner for each probe is identical and is attached to the bottom of the immersion tank. 
     The cap and flange probes are identical in design and inspect the cap and flange, respectively. Each probe consists of a pair of vertically opposed shoes. Each pair of vertically opposed shoes contains 16 through transmission ultrasonic channels for a total of 32 channels. The probe is designed to maintain a constant distance from the inspection surface. The maximum inspectable cap width is 4″. 
     The web probe consists of a pair of horizontally opposed shoes. The probe is designed to have a total of 16 channels. The shoes are mounted a fixed distance of 2 inches from the stiffener centerline and do not contact the part. The maximum inspectable web height is 2 inches. 
     The cap radius and flange radius probe inspect the cap radius and flange radius. Each probe consists of a pair of vertically opposed shoes containing 7 through-transmission ultrasonic channels. 
     In an exemplary embodiment the system comprises: 
     
       
         
               
               
             
               
               
               
             
           
               
                   
                   
               
               
                   
                 Transducers 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 Cap Probe 
                 64 
               
               
                   
                 Flange Probe 
                 64 
               
               
                   
                 Wet Probe 
                 32 
               
               
                   
                 Cap Radius Probe 
                  8 
               
               
                   
                 Flange Radius Probe 
                  8 
               
               
                   
                 Edge Probe 
                  8---- 
               
               
                   
                   
                 184 Transducers 
               
               
                   
                   
               
             
          
         
       
     
     FIG. 12 hereinbefore discussed was illustrative of the effect of the ultrasonic signal while the following discussion relating to FIGS. 12A through 18 are believed helpful in demonstrating the purpose of the edge probe in the present ultrasonic inspection system. 
     The ultrasonic signal generated by the edge is shown in FIG. 12. A high voltage electrical spike is sent to the transmit transducer generating an ultrasonic signal (normally called the main bang). If the transducer was positioned so that the ultrasonic beam was half on the edge, the resulting A-scan trace will show the signal through the part and through the water. The time between the signals is too small for the electronic gate to differentiate. The electronic gate detects only the largest signal and that is the signal through the water. As shown in FIG. 12A, if both the transmit and receive transducers were located precisely along the edge, a single ultrasonic signal would be displayed. That is the signal through the edge. 
     FIG. 13 shows the relative positioning of the four edge probes  404 ,  405 ,  406  and  407  spring mounted with adjustable means  420  on stationery base  425  while FIG. 14 is an isometric exploded view showing in more detail one side of the probe assembly of FIG.  13 . FIGS. 15,  16  and  17  are pictorial representations showing sequentially the stringer passing through the edge probe assembly. 
     FIG. 18 is a vertical section showing stainless steel reflectors  502  for redirecting sound beam  505  into the edge of stringer  510 . Locator strip  512  enables positioning of transmit transducer  515  and receive transducer  517  precisely along the edge thereby further enabling, as hereinbefore discussed, the display of a single ultrasonic signal.