Abstract:
A structure is described that includes a first faying surface, a second faying surface for creating an electrical bond with the first faying surface, and a sensor operatively placed proximate the first faying surface and the second faying surface. The sensor includes a current port for injecting a fixed current through the electrical bond, a voltage port for sensing a voltage across the electrical bond induced by the fixed current, a processing device programmed to determine a resistance of the electrical bond based on the fixed current and sensed voltage, and a wireless interface for transmitting at least one of the sensed voltage and the determined resistance to an external device.

Description:
BACKGROUND 
     The field of the disclosure relates generally to the inspection of electrical bonds associated with a platform, and more specifically, to methods and systems for automated measurement of electrical bonds. 
     Currently, an exhaustive and time consuming inspection technique is utilized by mechanics in the inspection of electrical bonds that requires the mechanics to make physical contact with structural and system electrical bonds. In certain manufacturing environments, for example an aircraft production environment, such inspection is required at hundreds of points. Currently, such inspections are estimated to take several days to complete, using a hand held ohm-meter. 
     More specifically, to accomplish the inspection as currently conducted, the mechanic carries a handheld instrument that includes two probes and a visual read out. By making physical contact with the bond in question using the two ohm-meter probes, a resistance associated with the bond is measured in ohms and displayed on a display associated with the ohm-meter. For each resistance measurement, the mechanic manually records the measurement the value on paper, before moving on to inspect the next bond. If there are any obstructions, such as coverings, fairings, insulation, or panels, the mechanic must first remove these so that physical contact can be made with the bond using the ohm-meter probes. 
     Because of the manual recording of data and direct contact requirement, the inspection process takes a long time to complete and is therefore costly. Removal of obstructions to access the part adds to the inspection time. Human error can also be introduced into the inspection process due to the manual recording of resistance measurements. 
     BRIEF DESCRIPTION 
     In one aspect, a structure is provided that includes a first faying surface, a second faying surface for creating an electrical bond with the first faying surface, and a sensor operatively placed proximate the first faying surface and the second faying surface. The sensor includes current ports for injecting a fixed current through the electrical bond, voltage ports for sensing a voltage across the electrical bond induced by the fixed current, a processing device programmed to determine a resistance of the electrical bond based on the fixed current and sensed voltage, and a wireless interface for transmitting at least one of the sensed voltage and determined resistance to an external device. 
     In another aspect, a method for configuring a structure for the testing of electrical bonds between two faying surfaces associated with the structure, is provided. The method includes operatively placing a sensor between a first faying surface and a second faying surface, the sensor including a current port for injecting a fixed current through the electrical bond and a voltage port for sensing a voltage across the electrical bond induced by the fixed current, and configuring the sensor to transmit at least one of the sensed voltage and a resistance calculated from the sensed voltage and fixed current upon receipt of an interrogation signal from an external source. 
     In still another aspect, a system for testing the integrity of electrical bonds between two faying surfaces is provided. The system includes a sensor operatively placed proximate the two faying surfaces comprising a current port for injecting a fixed current through the electrical bond, a voltage port for sensing a voltage across the electrical bond induced by the fixed current, and a wireless interface for transmitting at least one of the sensed voltage and determined resistance, the sensor configured to inject the fixed current upon receipt of a specific RF signal. The system further includes an interrogation device configured to output the specific RF signal and receive the transmission of the at least one of the sensed voltage and determined resistance from the sensor. 
     The features, functions, and advantages that have been discussed can be achieved independently in various embodiments or may be combined in yet other embodiments further details of which can be seen with reference to the following description and drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a flow diagram of an aircraft production and service methodology. 
         FIG. 2  is a block diagram of an aircraft. 
         FIG. 3  is a top view of a thin metallic washer, having a main body and a sensor attached thereto. 
         FIG. 4  is a cross-sectional view of the washer of  FIG. 3  mounted between two faying surfaces. 
         FIG. 5  is a block diagram of the sensor of  FIGS. 3 and 4  further illustrating components of an application specific integrated circuit. 
         FIG. 6  is a schematic diagram of a micro-ohm meter sensor incorporating a Kelvin double bridge circuit. 
         FIG. 7  is a schematic block diagram of one embodiment of a sensor system. 
         FIG. 8  is a circuit diagram of a current pulse generator utilized in the sensor system of  FIG. 7 . 
     
    
    
     DETAILED DESCRIPTION 
     In at least one aspect, the described embodiments relate to a sensor that enables fast and automated inspection of electrical bonds. In certain manufacturing and repair environments, utilization of such a sensor may reduce the time required to perform certain inspections by hundreds of hours. The sensor allows for wireless interrogation, for example by a mechanic, of the sensor from a distance. Interpretation of the data received from the interrogation allows for automatic assessment of the health of the bond. The embodiments provide for the interrogation and assessment without the need for direct contact of the bond and without any disassembly to remove obstructions between an interrogation device and the device to be interrogated. As further described below, in one embodiment the sensor combines a Kelvin double bridge circuit with wireless sensor and radio technology allowing for a relatively easy interrogation. In another embodiment, high precision instrumentation amplifiers are utilized along with supporting electronics to detect the low voltage associated with micro-ohm bonds, without the need for a Kelvin bridge. 
     Another embodiment of a sensor system described below includes an RF rectifier, a digital microcontroller, a current pulse generator, a precision instrumentation amplifier, and supporting electronics including power supply and wireless communication circuits. As described herein, one purpose of such a system is to measure the resistance of an aircraft bond joint or other electrical bond. In this embodiment, a known DC excitation current is passed through the bond joint and the voltage across the joint is sensed and quantified. The resistance of the joint is computed from the known excitation current and measured voltage. In embodiments, the sensor system is powered utilizing harvested RF energy. Therefore, the energy consumed by the sensor system should be minimized. Embodiments for minimizing energy consumption are also described. Specifically, energy consumption is minimized by carefully controlling the duty cycle of the sensor circuits, and limiting the duration of the excitation current to the shortest time possible. 
     Referring more particularly to the drawings, embodiments of the disclosure may be described in the context of aircraft manufacturing and service method  100  as shown in  FIG. 1  and an aircraft  200  as shown in  FIG. 2 . During pre-production, aircraft manufacturing and service method  100  may include specification and design  102  of aircraft  200  and material procurement  104 . 
     During production, component and subassembly manufacturing  106  and system integration  108  of aircraft  200  takes place. Thereafter, aircraft  200  may go through certification and delivery  110  in order to be placed in service  112 . While in service by a customer, aircraft  200  is scheduled for routine maintenance and service  114  (which may also include modification, reconfiguration, refurbishment, and so on). 
     Each of the processes of aircraft manufacturing and service method  100  may be performed or carried out by a system integrator, a third party, and/or an operator (e.g., a customer). For the purposes of this description, a system integrator may include, without limitation, any number of aircraft manufacturers and major-system subcontractors; a third party may include, for example, without limitation, any number of venders, subcontractors, and suppliers; and an operator may be an airline, leasing company, military entity, service organization, and so on. 
     As shown in  FIG. 2 , aircraft  200  produced by aircraft manufacturing and service method  100  may include airframe  202  with a plurality of systems  204  and interior  206 . Examples of systems  204  include one or more of propulsion system  208 , electrical system  210 , hydraulic system  212 , and environmental system  214 . Any number of other systems may be included in this example. Although an aerospace example is shown, the principles of the disclosure may be applied to other industries, such as the automotive industry. 
     Apparatus and methods embodied herein may be employed during any one or more of the stages of aircraft manufacturing and service method  100 . For example, without limitation, components or subassemblies corresponding to component and subassembly manufacturing  106  may be fabricated or manufactured in a manner similar to components or subassemblies produced while aircraft  200  is in service. 
     Also, one or more apparatus embodiments, method embodiments, or a combination thereof may be utilized during component and subassembly manufacturing  106  and system integration  108 , for example, without limitation, by substantially expediting assembly of or reducing the cost of aircraft  200 . Similarly, one or more of apparatus embodiments, method embodiments, or a combination thereof may be utilized while aircraft  200  is in service, for example, without limitation, to maintenance and service  114  may be used during system integration  108  and/or maintenance and service  114  to determine whether parts may be connected and/or mated to each other. 
     The description of the different advantageous embodiments has been presented for purposes of illustration and description, and is not intended to be exhaustive or limited to the embodiments in the form disclosed. Many modifications and variations will be apparent to those of ordinary skill in the art. Further, different advantageous embodiments may provide different advantages as compared to other advantageous embodiments. The embodiment or embodiments selected are chosen and described in order to best explain the principles of the embodiments, the practical application, and to enable others of ordinary skill in the art to understand the disclosure for various embodiments with various modifications as are suited to the particular use contemplated. 
       FIG. 3  is a top view of a smartwasher  300  according to one embodiment. Smartwasher  300  is generally a thin metallic washer, having a main body  302 , the smartwasher  300  including a sensor  310  attached thereto as further described. In the illustrated embodiment, smartwasher  300  is utilized for a faying surface bond. In one embodiment, sensor  310  incorporates an application specific integrated circuit (ASIC)  312  that includes a sensor, a transceiver, a power source and data storage. In embodiments, the power source incorporates one or more of RF energy harvesting, as described below, thermal gradient energy harvesting and piezoelectric energy harvesting. In the illustrated embodiment, a plurality of dipole legs  320 ,  322  form an antenna and extend from the ASIC  312  along a flexible dielectric  330  that extends from the main body  302 . Flexible dielectric  330  includes two sections, an antenna carrier section  332  that is substantially adjacent a portion of a perimeter defined by body  302 , and an attachment portion  334  which includes an upper member  336  and a lower member  338  (not shown in  FIG. 3 ) which operate as a form of clip to attach the flexible dielectric  300  to the main body  302 . A plurality of ports  340  are provided on each of the lower member and upper member  336  and are sometimes referred to herein as voltage ports and current ports. 
     In the illustrated embodiment, the flexible dielectric  330  serves as the sensor body for sensor  310  which houses all electronic components including antenna, the circuits described herein, and the voltage and current ports described below. The flexible dielectric  330  may be in any form and is attached to the body  302  of smartwasher  300  for convenience. The washer body  302  may not be utilized in all locations of a structure. Other embodiments are contemplated where a portion of an existing washer is set aside for current and voltage ports, and a protruding section (generally a dielectric) is used to house the electronics, to keep the electronics from being damaged between faying surfaces, and to prevent the antenna from being grounded out by the electrical bond between the faying surfaces. By connecting a flexible dielectric containing the circuits described herein to a washer, resulting in the “smartwasher” described herein, a step in assembly is eliminated since the installation where use of a smartwasher is contemplated generally utilizes a washer. Other installations my not utilize a washer. In such embodiments, a flexible or thin rigid dielectric may be installed between two faying surfaces. On this dielectric, the voltage and current ports contact the faying surfaces, and the described antenna and circuit will be placed on a portion of the flexible or thin rigid dielectric that protrudes away from the faying surfaces. 
       FIG. 4  is a cross-sectional view of smart washer  300  mounted between two faying surfaces  400  and  402 . Faying surface  400  is, for example, a portion of an airframe  410 . Faying surface  402  is a portion of a bonding lug  420 . In one embodiment, an electrical bond between faying surfaces  400  and  402  is desired to form a current return network. In a specific embodiment, the current return network is formed within an aircraft formed utilizing metallic components embedded throughout an otherwise composite airframe. 
     The voltage and current ports  340  are marked individually in  FIG. 4  as contacts  440 ,  442 ,  444 , and  446  that are situated on the upper member  336  and a lower member  338  to make contact with the respective faying surface  400  and  402 . That contact is secured due to the bolts  450  and nuts  452  used to attach bonding lug  420  to airframe  410 . As further explained herein, the measurement of the electrical bond is performed by determining a voltage across the voltage port (contacts  440  and  442 ) and determining a current that passes through current port (contacts  444  and  446 ). In alternative embodiments, multiple contacts may be associated with each port. 
       FIG. 5  is a block diagram  600  of sensor  310  that also further illustrates components of one embodiment of ASIC  312 . Starting at antenna  602  (such as dipoles  320 ,  322 ), it provides an interface to radio  604  which, as described herein, operates as a transmitter and receiver. A portion of the power received at antenna  602  may be utilized to provide power to the remainder of ASIC  312  using a power module  606 . Power module  606 , in embodiments, includes a power storage capability. 
     The radio  604  is communicatively coupled to microcontroller  608  which is further coupled to a memory/data storage area  610 . The microcontroller  608  is further coupled to a transducer  612 , such as the Kelvin double bridge circuit or high precision instrumentation amplifier circuits mentioned above, which include a mechanical interface  614  to the faying surface. As described elsewhere herein, the mechanical interface  614  includes voltage and current ports and may be considered to include the dielectric to which the other components are coupled. The mechanical interface  614  may also be considered to include, for example, the capability for attachment to a washer, as described above. In one embodiment, microcontroller  608  may incorporate an analog to digital converter (ADC) utilized to measure the voltage at the voltage ports as further described herein. Generally, mechanical interface  614 , and the components coupled thereto, provide a capability for determining the electrical resistance of the electrical bond between the airframe  410  and bonding lug  420 . 
     More specifically,  FIG. 6  is a schematic diagram  700  of one embodiment of micro-ohm meter sensor  620 , particularly a Kelvin double bridge circuit  710  that may form the transducer  612  within ASIC  312  except for the bond resistance  720  that is associated with the bond in between airframe  410  and bonding lug  420 . A Kelvin bridge can be used to detect very low resistances. Referring to schematic diagram  700 , Rx represents the micro-ohm bond of the aircraft to be measured. Rs is a reference resistor that is comparable in value to the value or expected range of Rx. R 3  and R 4  are variable resistors, while R 1  and R 2  are fixed resistors. In use, R 3  and R 4  are adjusted until voltage (at G) is zero. At this point the balance condition exists, and the equation Rx/Rs=R 4 /R 2 =R 3 /R 1  is satisfied, at which point Rx can be determined. To use the Kelvin bridge within transducer  612 , those skilled in the art will understand that additional supporting circuitry is incorporated. The ports  440 ,  441 ,  444 , and  446  are shown as nodes within the schematic  700 . Utilization of the Kelvin double bridge circuit  710  allows for the injection of a fixed current into an electrical bond as well as we measurement of the voltage across the bond generated by the fixed current and the resistance of the bond. 
     With reference to  FIG. 7 , a block diagram of a sensor system  750  is depicted. Sensor system  750 , in the illustrated embodiment, is powered by energy harvested from wireless signals. As such, it does not utilize batteries which is advantageous for reasons described herein. Particularly, radio frequency (RF) energy is received by one or more receive antennas  752 . A portion of the received RF energy is converted to DC power by an RF rectifier circuit  754 . In this embodiment, the DC output signal  756  from the rectifier circuit  754  is referred to as VDET. This DC power is used to power the sensor system  750 . 
     A portion of the DC energy is stored in one or more energy storage capacitors  758 . This stored energy is used to generate a pulse of excitation current  760  in an exciter circuit  762 . A smaller portion of the DC energy  756  supplied from the rectifier  754  is connected through a diode  764  to another capacitor  766  which supplies other circuits within sensor system  750 . In this embodiment, the output of the diode  764  is referred to as VDD. A digital microcontroller  770  is powered directly from VDD. DC power is supplied from VDD through a PNP transistor switch  772  to create VCC. Operation of the transistor switch  772  is controlled by software in the microcontroller  770 . The exciter  762 , instrumentation amplifier  780 , and charge pump circuits  782  are powered from VCC. 
     The charge pump circuits  782  generate a negative supply voltage referred to in the diagram as VEE. VEE is used to supply power to operational amplifiers (not shown) in the exciter  762  and to the instrument amplifier circuits. In this embodiment, when RF energy is received at the antenna  752 , storage capacitors  758  and  766  begin to charge. When sufficient charge has built up on VCC, the microcontroller  770  starts up and monitors the voltage labeled as VDET. When sufficient charge has built up VDET, software in the microcontroller  770  executes a sensor measurement. To start a measurement, the microcontroller  770  turns the transistor switch  772  on, which turns on VCC and VEE. The instrumentation amplifier  780  and exciter circuits  762  contain high precision operational amplifiers for excellent DC measurement accuracy. These operational amplifiers have internal circuits that enhance DC accuracy but require some time at power up to achieve this accuracy. In order to achieve the desired accuracy, these circuits must be powered up for several hundred microseconds before a measurement is taken. In the illustrated embodiment, VCC is turned on for 640 microseconds before a measurement is started. 
       FIG. 8  is a schematic  800  of the exciter circuit  762 . The exciter circuit  762  includes an operational amplifier  802 , a Darlington transistor  804 , and a current sense resistor  806  configured in the form of a non-inverting voltage-to-current amplifier. In one embodiment, input signal PULSE is a nominal 1.2V. PULSE is divided using by a potentiometer  808  to 0.25V and applied to the + input  810  of the operational amplifier  802 . Exciter current  812  is supplied to the device under test (DUT)  814  through the NPN Darlington transistor  804 , which is controlled by the operational amplifier  802 . The exciter current  812  passes through the DUT  814  and then through the sense resistor  806  to ground. DC feedback from the SENSE− terminal through a resistor  820  to the − input  822  of the operational amplifier  802  forces the voltage at SENSE− to equal the voltage at the + input  810  of the operational amplifier  802 . 
     The voltage across the sense resistor  806  is therefore 0.25V. The current through the sense resistor  806 , in the illustrated embodiment is therefore 0.25V/0.05 Ohms=5 Amperes. The current into the − input  822  of the operational amplifier  802  is virtually zero, and the excitation current  812  is therefore about five Amperes. The exciter circuit  762  is controlled by the PULSE signal  830  and the MEAS_EN signal  832 . The MEAS_EN signal  832  is a gating signal that prevents the exciter  762  from injecting any current into the DUT  814  unless this signal is in a logic high state. This is necessary because during the amplifier startup time there are transient signals in the exciter  762  that would otherwise cause some of the stored energy in the VDET capacitor  758  (shown in  FIG. 7 ) to discharge through the DUT  814 , reducing the energy available for an excitation pulse  830 . 
     When MEAS_EN signal  832  is at a logic low level, an NPN transistor  840  connected through a resistor  842  to the base of the Darlington transistor  804  is turned on. The voltage at the base of the Darlington transistor  804  is pulled close to ground and the Darlington transistor  804  cannot turn on. When the MEAS_EN signal  832  is in a logic high state, the exciter  762  generates a current pulse controlled by the PULSE signal  830 . At about 640 microseconds after VCC is turned on, the PULSE and MEAS_EN signals  830 ,  832  are turned on, generating an excitation current of five Amperes. The DUT  814  is a resistance to be measured with a four terminal connection. Two connections are for the exciter current, EXCITE+, and EXCITE− (corresponding to the current ports), and the other two are for the voltage measurement SENSE+, and SENSE− (corresponding to the voltage ports). 
     SENSE+ and SENSE− are connected to the inputs of instrumentation amplifier  780  (shown in  FIG. 7 ). The voltage between the SENSE+ and SENSE− connections is DC amplified and applied to the input of an analog-to-digital converter (ADC)  790  built into the microcontroller  770  (Both shown in  FIG. 7 ). After allowing 150 microseconds for settling time in the exciter  762  and instrumentation amplifier  780  circuits, the microcontroller  770  reads the voltage from the ADC  790  and the measurement is complete. VCC, MEAS_EN  832 , and PULSE  830  signals are turned off. The duration of the VCC on time is 800 microseconds, and the duration of the exciter pulse is 160 microseconds. In the embodiment shown, the sensor data is transmitted over a wireless link, via transmitter  792  and transmit antenna  794  to an external device such as a computer. 
     The above describe embodiments are therefore a portion of a system for measuring electrical bonds. In a typical fabrication scenario, sensors are acquired from stock, for example, in the form of smartwashers  300  or another embodiment as mentioned above. Whatever physical embodiment is utilized for a particular application, the sensors are essentially identical. When installed, each sensor  310  is assigned a unique identifier that includes, for example, an aircraft tail number and a location of the electrical bond on the aircraft. A reader is utilized in this programming, and as is easily imagined, a multitude of other applications exist outside of aircraft fabrication. 
     Once the device, such as a smartwasher  300  carrying sensor  310  is installed and deployed within a platform, in field data acquisition is performed, for example, using a reader that is operable to transmit an RF signal for powering the sensor  310  and retrieving data therefrom. A maintenance action decision is made based on the determined resistance in the electrical bond, for example, the reader is equipped with a processing device that is programmed to direct a maintenance action and record the event into a maintenance management system with which the reader communicates. An onboard maintenance management system is also contemplated. In such embodiments, sensor  310  includes an energy harvesting source that replenishes over time, and the processing device, such as ASIC  312 , is programmed to take measurements at scheduled intervals and transmit those measurements to the onboard maintenance management system, for example, on the aircraft. 
     With the described embodiments, an entire electrical bond network can be scanned in a few hours. All electrical bonds that are not within the required tolerance are automatically flagged as dictated by the data management system. Using such generated data, which includes location data, a visual map of the entire bond network can be generated thereby providing quick access to displays of various data, status and progress of scanning. Examples of status and progress of scanning may include: progress of the inspection, completed scans vs. pending scans, date and time of the inspection, value at last inspection, history of all inspections, history of sensor, and next scheduled inspection. 
     This written description uses examples to disclose various embodiments, which include the best mode, to enable any person skilled in the art to practice those embodiments, including making and using any devices or systems and performing any incorporated methods. The patentable scope is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims if they have structural elements that do not differ from the literal language of the claims, or if they include equivalent structural elements with insubstantial differences from the literal languages of the claims.