Abstract:
A system and method for detecting damage to a structure is provided. The system includes a voltage source and at least one capacitor formed as a layer within the structure and responsive to the voltage source. The system also includes at least one sensor responsive to the capacitor to sense a voltage of the capacitor. A controller responsive to the sensor determines if damage to the structure has occurred based on the variance of the voltage of the capacitor from a known reference value. A method for sensing damage to a structure involves providing a plurality of capacitors and a controller, and coupling the capacitors to at least one surface of the structure. A voltage of the capacitors is sensed using the controller, and the controller calculates a change in the voltage of the capacitors. The method can include signaling a display system if a change in the voltage occurs.

Description:
STATEMENT OF GOVERNMENT RIGHTS  
       [0001]     The invention described herein was made in the performance of work under Subcontract No. 1268619-00 granted by the California Institute of Technology Jet Propulsion Laboratory to The Boeing Company, which was performed under NASA Contract No. NMO0710922 and is subject to the provisions of Section 305 of the National Aeronautics and Space Act of 1958 (72 Stat. 435, 42 U.S.C. 2457.) The U.S. Government has certain rights in this invention. 
     
    
     FIELD  
       [0002]     The present disclosure relates to systems for detecting damage to structures from high-velocity impact, and more particularly to three-dimensional structure damage localization using a layered two-dimensional array of capacitance sensors.  
       BACKGROUND  
       [0003]     The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.  
         [0004]     Generally, in operation, mobile platforms can be subjected to high-velocity impacts from debris. This is especially so with high-speed mobile platforms, such as jet aircraft. These high-velocity impacts can cause damage to the mobile platform, the extent of which may or may not be readily visually detectable by the crew. Currently, damage detection in mobile platforms is labor intensive and many damage detection apparatuses require a portion of the mobile platform to be disassembled, which is time consuming and costly. In addition, some forms of damage detection may not be compatible with mobile platforms that are composed of certain materials. Many presently available damage detection systems also are unable to be used on jet aircraft while in-flight, which can be undesirable. Such systems require the aircraft to remain at a specified location for maintenance testing, which can result in lost service time for the aircraft if it is ultimately determined that an impact occurrence did not result in any structural damage to the aircraft.  
         [0005]     Accordingly, it would be desirable to provide a damage detection system that provides for even more efficient damage detection, and also damage detection that can be used while a mobile is traveling en route to a given destination.  
       SUMMARY  
       [0006]     A system and method for detecting damage to a structure is provided. The system includes a voltage source and at least one capacitor coupled to the structure and responsive to the voltage source. The system also includes at least one sensor responsive to the capacitor to sense a voltage across the capacitor. The system further includes a controller responsive to the sensor to determine if damage to the structure has occurred based on the voltage of the capacitor.  
         [0007]     The present teachings further provide a method for non-invasively sensing damage to a structure. The method includes providing a plurality of capacitors and a controller, and integrating the capacitors into at least one surface of the structure. The method also includes sensing voltages across the capacitors with the controller, and calculating a change in the voltages of the capacitors with the controller. The method includes signaling if a change in the overall voltage occurs.  
         [0008]     The present teachings also provide a structural component. The structural component includes a structural element, and a capacitor integrated into the structural element. The capacitor forms an integral layer of the structural element. The structural element also includes a sensor electrically coupled to the capacitor for sensing a change in capacitance of the capacitor in the event of an anomaly in the integrity of the structural element. The capacitor within the structural element is placed in communication with a controller for monitoring the sensor and providing an output indicating when the integrity of the structural element has been compromised.  
         [0009]     Further areas of applicability will become apparent from the description provided herein. It should be understood that the description and specific examples are intended for purposes of illustration only and are not intended to limit the scope of the present disclosure.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0010]     The drawings described herein are for illustration purposes only and are not intended to limit the scope of the present disclosure in any way.  
         [0011]      FIG. 1  is a perspective view of an exemplary mobile platform employing a layered two-dimensional array of capacitance sensors;  
         [0012]      FIG. 2  is a cross-sectional view of the mobile platform of  FIG. 1 , taken along line  2 - 2  in  FIG. 1 , illustrating, in exploded perspective fashion, the layered two-dimensional array of capacitance sensors arranged within the structure of the mobile platform;  
         [0013]      FIG. 3  is a detailed perspective view of a portion of the layered two-dimensional array of capacitance sensors and a system for monitoring the layered two-dimensional array of capacitance sensors;  
         [0014]      FIG. 4  is a detailed side perspective view of the capacitive sensor grid of  FIG. 3  showing the exemplary layers of the capacitor; and  
         [0015]      FIG. 5  is an exemplary schematic diagram of the capacitive sensor array of  FIG. 2  including the control system. 
     
    
     DETAILED DESCRIPTION  
       [0016]     The following description is merely exemplary in nature and is not intended to limit the present disclosure, application, or uses. It should be understood that throughout the drawings, corresponding reference numerals indicate like or corresponding parts and features.  
         [0017]     Although the following description is related generally to structure damage localization using a layered two-dimensional array of capacitance sensors for a mobile platform, such as an aircraft, ship, spacecraft, train or motor vehicle, it will be understood that the damage localization system, as described and claimed herein, can be used with any appropriate application where it would be useful to be able to monitor and detect for structural anomalies in a structure or component, without having to disassemble the structure or component. Therefore, it will be understood that the following discussion is not intended to limit the scope of the appended claims.  
         [0018]     With reference to  FIG. 1 , an exemplary mobile platform  10  employing a damage localization system  12  is shown. The mobile platform  10 , in this example, is a spacecraft including a series of outer layers  14  separated from a series of inner layers  16  by the damage localization system  12 . With additional reference now to  FIG. 2 , the damage localization system  12  includes at least one or a plurality of capacitive sensor arrays  18  disposed between pairs of structural elements, pairs of protective shielding  20 , or various combinations thereof. The capacitive sensor arrays  18  and protective shielding  20  can generally be sandwiched between the series of outer layers  14  and the series of inner layers  16 . The capacitive sensor arrays  18  are coupled to the protective shielding  20  through any appropriate fastening mechanism, such as mechanical fasteners, adhesive bonding or welding, or could be integrally formed with the protective shielding  20  (not specifically shown). With additional reference to  FIG. 3 , a control system  22  is in communication with each of the capacitive sensor arrays  18  to determine if an anomaly in the integrity of the outer layers  14  and/or protective shielding  20  has occurred, which thereby compromises the integrity of the outer layers  14  and/or protective shielding  20 , as a result of puncture damage from an impact with a meteoroid in space. A display system  24  ( FIG. 5 ) coupled to the control system  22  can notify an operator (not specifically shown) of the mobile platform  10  that such damage has occurred, as will be discussed in greater detail herein in conjunction with  FIG. 5 .  
         [0019]     With continuing reference to  FIGS. 1, 2  and  3 , the capacitive sensor arrays  18  include a capacitor grid  26  including a plurality of capacitors  28  each in communication with a ground  40  and the control system  22 . As each of the capacitive sensor arrays  18  are substantially similar, only a portion of a capacitor grid  26  of one capacitive sensor array  18  will be discussed herein. Generally, the capacitors  28  are parallel plate capacitors having at least a pair of plates, and more preferably three or more parallel plates. In this example, the capacitors  28  include a first layer  30 , a second layer  32 , a third layer  34 , a fourth layer  36  and a fifth layer  38 , as further shown in  FIG. 4 . The first layer  30  can be composed of the same material as the fifth layer  38 , which is a conductive material, such as copper or aluminum. The thickness of the first layer  30  and the fifth layer  38  can range from about 0.01 mil to about 2.0 mils (˜0.000254-0.0508 mm), but more preferably is about 0.5 mil (˜0.0127 mm).  
         [0020]     The second layer  32  and fourth layer  36  are each composed of a dielectric material, and is disposed between the first layer  30  and the fifth layer  38 . Generally, the dielectric material employed is flexible for ease in manufacturing, such as KAPTON® polyimide film manufactured by DuPont. The thickness of each of the second layer  32  and the fourth layer  36  preferably ranges from about 0.01 mil to about 4.0 mils (˜0.000254-0.1016 mm), but is more preferably about 3.0 mils (˜0.0762 mm) thick. The first layer  30 , second layer  32 , fourth layer  36  and fifth layer  38  form a first electrode of the capacitor  28  that is coupled to a resistor  39  which is coupled to ground  40 . The resistor  39  sets the input impedence into the controller  48 .  
         [0021]     The third layer  34  forms a second electrode of the capacitor  28  that is in communication with the control system  22 . The third layer  34  is composed of a conductive material, such as copper or aluminum, and is disposed between the second layer  32  and the fourth layer  36 . The thickness of the third layer  34  can range from about 0.01 mil to about 2.0 mils (˜0.000254-0.0508 mm), but more preferably is about 1.0 mil (˜0.0254 mm). A section  42  of the third layer  34  extends beyond the first, second, fourth and fifth layers  30 ,  32 ,  34 ,  36  and  38  to enable each of the capacitors  28  to be electrically coupled to the control system  22  ( FIG. 4 ). The first, second, third, fourth and fifth layers  30 ,  32 ,  34 ,  36  and  38  are coupled together through any suitable technique, such as spray adhesives, adhesive tape or other bonding techniques (not specifically shown).  
         [0022]     The capacitor grid  26  is arranged with capacitors  28  forming rows R 1 , R 2  . . . Rn and columns C 1 , C 2  . . . Cn. Each of the rows R and columns C form conductive circuit traces that make up a grid for the control system  22 . This arrangement of the capacitors  28  enables each capacitor  28  to provide a discrete input to the control system  22  for the determination of damage to the particular region defined by the row R and column C of the capacitor  28 .  
         [0023]     The control system  22  can determine if damage has occurred to a specific capacitor  28  in the capacitor grid  26 . The control system  22  includes at least one or a plurality of sensor systems  44 , an output sensor system  46 , a controller  48 , and voltage sources  50  that are coupled to the capacitors  28  in a particular row R 1 , R 2  . . . Rn of the capacitor grid  26 , as best shown in  FIGS. 3 and 5 . The control system  22  is identical for each of the capacitive sensor arrays  18 , and thus the control system  22  will be described herein as applied to one portion of the capacitor grid  26  of one capacitive sensor array  18 . It will be understood that the control system  22 , as described herein, could be modified as necessary to interact with various layers of capacitive sensor arrays  18 .  
         [0024]     Each capacitor  28  in the capacitor grid  26  includes an associated sensor system  44  that is in communication with the third layer  34  of each capacitor  28 . Each of the sensor systems  44  is also in communication with the conductive row R and column C associated with the particular capacitor  28 , as best shown in  FIG. 5 . A separate one of the voltage sources  50  is coupled to each sensor system  44 , and can be turned on and off independently by the controller  48 . The sensor system  44  includes a resistor  52 , a diode  54  and a transistor  56 , which, in this example, is an N-channel metal-oxide semiconductor field-effect transistor (MOSFET).  
         [0025]     The resistor  52  is in communication with a particular row R and with the third layer  34  of the capacitor  28  to enable current to flow to the capacitor  28 . The resistor  52  has a resistance of about 10 kilohms; however, this value can be varied as needed to suit an application. The diode  54  is in communication with a particular row R to control the flow of current in the particular row R to one direction only, and to maximize the sensitivity of the row R.  
         [0026]     The MOSFET  56  of a particular sensor system  44  is in communication with the diode  54  and a selected capacitor  28 . The MOSFET  56  has its gate  58  in communication with the capacitor  28 . A drain  59  is in communication with the diode  54 , and a source  60  is in communication with the output sensor system  46  through a selected column C. The MOSFET  56  associated with each capacitor  28  can act as a buffer to isolate the capacitor  28  from the other capacitors  28  in the capacitor grid  26 , thus maximizing the sensitivity of the capacitive sensor array  18 . The gate  58  of the MOSFET  56  receives the voltage from the capacitor  28 . The voltage from the capacitor  28  is used to determine if the mobile platform  10  has been damaged, as will be discussed in greater detail herein. The voltage from the capacitor  28  is used to turn on the MOSFET  56  to enable a current signal to be sent to the output sensor system  46 , and then to the controller  48  through the source  60  of the MOSFET  56 .  
         [0027]     The output sensor system  46  is in communication with each MOSFET  56  on an associated column C through the source  60  of each MOSFET, and in communication with the controller  48 . The output sensor system  46  includes a MOSFET  62  and a biasing resistor  64 . The MOSFET  62  includes a first drain  66  in communication with the column C 1 , C 2  . . . Cn for receiving the voltage across the selected capacitor  28  in the column C 1 , C 2  . . . Cn, and a gate  68  in communication with the resistor  64 , which is in turn in communication with the controller  48 . The resistor  64  has a resistance of about 1 kilohm. The resistor  64  serves to increase the sensitivity and accuracy of the output signal from the MOSFETs  62 . The controller  48  applies a signal to the gate  68  of a selected one of the MOSFETs  62  to turn it on at a predetermined time when its associated capacitor  28  is being checked. All of the other MOSFETs  62  are held in an “off” condition during this time. This enables one selected capacitor  28  at a time to be analyzed by the controller.  
         [0028]     The controller  48  is in communication with each of the output sensor systems  46  of the columns C 1  to Cn. The controller  48  is preferably a microcontroller and may include an analog-to-digital (A/D) converter; however, any suitable controller could be used. The gate  68  of the MOSFET  62  is in communication with the controller  48  so that the controller  48  can enable the current flowing through the circuit line C 1 , and thus the voltage associated with the capacitors  28  in column C, to be measured when its associated voltage source  50  is turned on by the controller  48 .  
         [0029]     The voltage source  50  provides five volts (V) to the selected row R of capacitors  28 . The voltage source  50  can be any temperature compensated voltage reference. After the particular voltage source  50  for a row R has been activated to charge the particular row R of capacitors  28 , the controller  48  queries the output sensor systems  46 , one at a time, to determine the voltage on each of the capacitors  28 . The controller  48  is in communication with the display system  24  to visually inform the operators of the mobile platform  10  if there has been any damage to any of the protective shielding  20  based on the signal from the output sensor systems  46  for each of the particular columns C 1  to Cn.  
         [0030]     The display system  24  can provide operators on the mobile platform  10  with data regarding damage to the mobile platform  10  based on the data received from the controller  48 . The display system  24  includes a display  74  in communication with the controller  48  for receipt of data associated with a particular row R and column C of the capacitor grids  26 . The data from the controller  48  is displayed in a grid format with each box  76  associated with a particular capacitor  28  of a particular capacitive sensor array  18 . A color scale  78  is used to indicate if any of the capacitors  28  are damaged. For illustration purposes only, the color scale  78  can be such that undamaged areas are a lighter color than damaged areas. The amount of damage to a particular capacitor  28  can also be illustrated by the color intensity of the particular box  76 . For example purposes only, the capacitor  28  illustrated as box  76 ′ has a 2.0 mm (˜0.079 inch) hole, while the capacitor  28  illustrated as box  76 ″ has a 1.0 mm (˜0.039 inch) hole, as shown by the darker shade associated with the box  76 ′.  
         [0031]     In order to determine the amount of damage to the capacitor  28 , first the capacitor grid  26  is assembled such that each capacitor  28  in the capacitor grids  26  is in communication with an associated sensor system  44 . Then, the output sensor systems  46  are placed in communication with each column C of the multiple capacitor grids  26 . Next, each voltage source  50  is placed in communication with the particular rows R 1 , R 2  . . . Rn of the multiple capacitor grids  26 . The output sensor systems  46  and the voltage sources  50  are then coupled to the controller  48 . Each of the capacitor grids  26  can then be layered between the protective shielding  20  at the desired intervals.  
         [0032]     In order to detect if damage has occurred to the protective shielding in front of the capacitor grid  26 , the controller  48  queries each of the capacitor grids  26  to determine if any of the capacitors  28  have been damaged. Damage to the capacitors  28  is highly likely to indicate damage to the protective shielding  20  disposed in front of a given capacitor  28 . Damage to the capacitors  28  is determined by an increase in voltage (V) of the capacitor  28 . More specifically, if a capacitor  28  has been damaged, the capacitance of the capacitor  28  will change. The capacitance of the capacitor  28  in an uncompromised or undamaged position is approximately 175 picoFarads (ρF). Generally, the capacitance of the capacitor  28  is given by:  
             C   =         ɛ   r     ⁢     ɛ   0     ⁢   S     d             (   1   )             
 
 wherein S is the surface area, d is the plate spacing, and ε r  is the relative dielectric constant (the dielectric constant for KAPTON® polyimide film is approximately 3.1). 
 
         [0033]     When damage occurs to the capacitor  28 , the puncture will remove some of the surface area S of the capacitor  28 , causing a reduction in the capacitance of the capacitor  28 . However, one suitable way to measure the change in capacitance of the capacitor  28  is by measuring the change in voltage ΔV of the capacitor  28 . First, starting with the capacitor current given by:  
               i   c     =     C   ⁢       Δ   ⁢           ⁢   V       Δ   ⁢           ⁢   t                 (   2   )             
 
 wherein ΔV is the change in voltage in volts (V) and Δt is the change in time t in seconds (sec), and then inserting equation (1) into equation (2), and solving for the change in voltage, ΔV yields:  
               Δ   ⁢           ⁢   V     =         i   c     ⁢   d   ⁢           ⁢   Δ   ⁢           ⁢   t         ɛ   r     ⁢     ɛ   0     ⁢   S               (   3   )             
 
         [0034]     Thus, as the change in voltage is inversely proportional to the surface area S of the capacitor  28 , and as the area S decreases, the voltage V of the capacitor  28  increases with the capacitor current, charging time, and plate spacing held constant. In order to determine the appropriate size of the capacitor  28  for sensing a reduction in surface area S, a small damage area s can be subtracted from the surface area S in equation (3) to arrive at:  
               Δ   ⁢           ⁢   V     =           i   c     ⁢   d   ⁢           ⁢   Δ   ⁢           ⁢   t         ɛ   r     ⁢     ɛ   0         ⁢     (     1       S   0     -   s       )               (   4   )             
 
 wherein S 0  is the surface area of the undamaged capacitor  28  and s is the damaged area. Solving for Δyields:  
             Δ   =       S   0         S   0     -   s               (   5   )             
 
         [0035]     Thus, Δcan be maximized when the undamaged surface area S 0  of the capacitor  28  is not too large relative to the damaged area s of the capacitor  28 . Thus, in order to most effectively measure a small change in voltage of the capacitor  28  (approximately 0.1% change), the ratio of the undamaged surface area S 0  to the damaged area s of the capacitor  28  should be greater than 0.001. Generally then, the capacitors  28  can be about 1 inch by 1 inch (˜2.54×2.54 cm) patches. It will be appreciated, however, that the specific shape and dimensions of the capacitors  28  may vary to suit the needs of specific applications.  
         [0036]     In order to determine if any of the capacitors  28  have been punctured, the controller  48  sends a signal to the particular voltage source  50  to energize a particular row R of capacitors  28  in a particular capacitive sensor array  18 . In the example of  FIG. 5 , the controller  48  has the voltage source  50  energize the row R of capacitors  28  at 5V for 2 microseconds (μs). This causes a voltage to be applied to the gates  58  of each MOSFET  56  as is associated capacitor  28  charges. The controller  48  can then apply a signal to the gate  68  of the MOSFET  62  of a specific one of the output sensor systems  46  to turn it on. As only one of the MOSFETs  62  is energized at a time, each MOSFET  62  can provide the controller  48 , through its associated MOSFET  62 , with a discrete signal. This signal enables the controller  48  to precisely determine the capacitance of a specific one of the capacitors  28  in the capacitor grid  26  of the particular capacitive sensor array  18 , and transmit that information to the display system  24 .  
         [0037]     Based on the voltage detected from the capacitor  28 , the controller  48  transmits a signal to the display system  24  to illuminate the box  76  in the display  74  associated with the particular capacitor  28 . If the voltage V associated with the particular capacitor  28  is substantially equivalent to a voltage expected from an undamaged capacitor  28 , then the controller  48  sends a signal to the display system  24  to illuminate the box  76  associated with the particular capacitor  28  according to the color scale  78  for an undamaged capacitor  28 . For example, the undamaged capacitor  28  can be shown as a light gray color, as indicated in box  76 .  
         [0038]     If, however, the voltage V of the capacitor  28  is greater than the voltage expected with an undamaged capacitor  28 , then the controller  48  sends a signal to the display system  24  to illuminate the box  76  associated with the damaged capacitor  28  as a darker or more intense shade of color, depending on the color scale  78 . For example, the damaged capacitor  28  can be shown as a darker gray box  76 ′. The controller  48  performs the above procedure for each of the capacitive sensor arrays  18  at time intervals defined by the application employing the capacitive sensor arrays  18  to enable the operators of the mobile platform  10  to check each of the capacitive sensor arrays  18  as desired. It will be noted, however, that the controller  48  could query each of the capacitive sensor arrays  18  and display the results on a larger display or on a three-dimensional display (not shown). Thus, based on the shading of the boxes  76  in the display  74 , the operators of the mobile platform  10  can determine if the mobile platform  10  has been damaged.  
         [0039]     While various preferred embodiments have been described, those skilled in the art will recognize modifications or variations which might be made without departing from the inventive concept. The examples illustrate the invention and are not intended to limit it. Therefore, the description and claims should be interpreted liberally with only such limitation as is necessary in view of the pertinent prior art.