Abstract:
Apparatus and methods of measuring the electrical resistance of electrically-conductive materials are disclosed. In one aspect, an apparatus includes a substrate, and first, second, third, and fourth elongated conductive members. Each conductive member includes a first portion at least partially disposed on the substrate and a second portion. Each of the first portions is spaced apart from one or more adjacent first portions and is engageable with the electrically-conductive material along a contact length. The apparatus may include a source operatively coupled to the second portions of the first and fourth conductive members, and a meter operatively coupled to the second portions of second and third conductive members.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
   This patent application is related to a concurrently-filed patent application entitled “Apparatus and Methods for Measuring, Resistance of Conductive Layers” and bearing attorney docket number BOEI-1-1173, which application is hereby incorporated by reference. 
   FIELD OF THE INVENTION 
   The present disclosure relates to measuring electrical resistance and, more specifically, to measuring electrical resistance of layers of conductive material. 
   BACKGROUND OF THE INVENTION 
   Due to continuing improvements in materials technology, modern aerospace vehicles include an increasing amount of structural components made of composite materials. Because vehicle components made of non-conducting composite materials may become degraded when subjected to an electrical discharge (e.g. a lightning strike), such components are typically coated with an electrically conductive material, such as conductive paints, anti-static coatings, and the like. 
   Throughout various stages of development of such aerospace vehicles, measurements are often made of electrical resistance of a conductive layer that is disposed on a composite component of the vehicle. One known test device that has been successfully used for this purpose is shown in  FIG. 1 . As shown in  FIG. 1 , the prior art test device  100  includes first and second conductive strips  102 ,  104  disposed on a non-conductive layer  106  that is attached to a non-conductive substrate  110 . In this example, the substrate  110  includes a flexible, compliant layer  111 . Each conductive strip  102 ,  104  is operatively coupled to a conductive lead  112 ,  114  that extends from the test device  100  to a suitable piece of test equipment  120 , such as a digital ohm meter. 
   As further shown in  FIG. 1 , the conductive strips  102 ,  104  pass through the non-conductive layer  106  to an inner side of the non-conductive layer  106  (indicated by dashed lines) prior to passing around a proximal end  113  of the substrate  110 . On the proximal end  113 , first and second auxiliary contact members  115 ,  116  are disposed on the non-conductive layer  106 . Each of the first and second auxiliary contact members  115 ,  116  is electrically coupled to a corresponding one of the first and second conductive strips  102 ,  104 , respectively, by a plated-through hole  117 . 
   In operation, the test device  100  may be used by pressing the first and second conductive strips  102 ,  104  into engagement with a conductive layer  122  (not shown) to be tested. The test equipment  120  then measures the electrical resistance R T  of the conductive layer  122  between the first and second conductive strips  102 ,  104  in ohms per square. Because the first and second conductive strips  102 ,  104  are disposed on the compliant layer  111 , the non-conductive layer  106  and conductive strips  102 ,  104  may flex to conform to the curvature of the conductive layer  122 . In an alternate mode of operation, the first and second auxiliary contact members  115 ,  116  may be pressed into engagement with the conductive layer  122  under test, and the resistance R T  of the conductive layer  122  is then determined by the test equipment  120 . Due to their relatively smaller size, the auxiliary contact members  115 ,  116  may be used on smaller surfaces in comparison with the first and second conductive strips  106 ,  107 . 
   Although desirable results have been achieved using the prior art test device  100 , recent developments in conductive coatings are placing increased demands on such apparatus. For example, in the past, conductive coatings have been characterized by relatively high resistance per square values which were readily capable of accurate measurement using the prior art test device  100 . More modern conductive coatings, however, have relatively smaller resistance per square, thereby posing a greater challenge to such test apparatus. 
   As the resistance of the conductive coating  122  decreases, the additional component of measured resistance attributable to the contact resistance between the surfaces of each conductive strip  102 ,  104  and the conductive coating  122  becomes an ever-increasing percentage of the resistance measured by the test equipment  120 , thereby increasing the uncertainty associated with the measurement. In some cases, the resistance of the conductive coating  122  may even be smaller than the component of contact resistance between the conductive strips  102 ,  104  and the conductive coating  122 , thereby preventing accurate measurement of the resistance of the conductive coating  122  using the prior art test device  100 . Therefore, there is an unmet need in the art for an improved test device capable of accurately measuring the resistance of modern, low resistance conductive coatings on non-conducting composite structures. 
   SUMMARY OF THE INVENTION 
   The present invention is directed to apparatus and methods for measuring the electrical resistance of electrically-conductive materials. Apparatus and methods in accordance with the present invention may advantageously provide improved accuracy of electrical resistance measurements, and may enable the accurate measurement of the resistance of certain conductive materials having relatively small resistance. 
   In one embodiment, an apparatus includes a substrate, and first, second, third, and fourth elongated conductive members. Each conductive member has a first portion at least partially disposed on the substrate and a second portion. Each of the first portions is spaced apart from one or more adjacent first portions of the other elongated conductive members, and is engageable with the electrically-conductive material along a contact length. The first end portions of the second and third conductive members being disposed between the first portions of the first and fourth conductive members and spaced apart by a lateral distance. The apparatus may further include a source operatively coupled to the second portions of the first and fourth conductive members, and a meter operatively coupled to the second portions of second and third conductive members. In operation, the electrical resistance of the electrically-conductive material is determinable from a known value applied by the source and an observed value measured by the meter. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The preferred and alternative embodiments of the present invention are described in detail below with reference to the following drawings. 
       FIG. 1  is an isometric view of a test device for measuring electrical resistance of a conductive layer in accordance with the prior art; 
       FIG. 2  is an isometric view of a test device for measuring electrical resistance of a conductive layer in accordance with an embodiment of the invention; 
       FIG. 3  is a top elevational view of a sensor portion of the test device of  FIG. 2 ; 
       FIG. 4  is an enlarged end elevational view of the test device of  FIG. 2  engaged with a conductive layer during a test in accordance with an embodiment of the invention; 
       FIG. 5  is a circuit diagram for the device of  FIG. 2  engaged with a conductive layer during a test in accordance with an embodiment of the invention; and 
       FIG. 6  is an elevational view of an apparatus for measuring electrical resistance of a conductive layer in accordance with an alternate embodiment of the invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   The present invention relates to apparatus and methods for measuring the electrical resistance of materials, and more specifically, to measuring the resistance of electrically-conductive coatings on aircraft surfaces and the like. Many specific details of certain embodiments of the invention are set forth in the following description and in  FIGS. 2–6  to provide a thorough understanding of such embodiments. One skilled in the art, however, will understand that the present invention may have additional embodiments, or that the present invention may be practiced without several of the details described in the following description. 
     FIG. 2  is an isometric view of a test device  200  for measuring an electrical resistance R T  of a conductive layer  122  in accordance with an embodiment of the invention. In this embodiment, the test device  200  includes a sensor portion  202  coupled to a substrate portion  210 .  FIG. 3  is a top elevational view of the sensor portion  202  of the test device  200  of  FIG. 2 . The sensor portion  202  includes first and second pairs  204 ,  205  of conductive contacts. Each pair  204 ,  205  includes an outer conductive contact  206  and an inner conductive contact  207 . The outer and inner conductive contacts  206 ,  207  are disposed on a non-conductive layer  208  that is, in turn, attached to the substrate portion  210 . In this embodiment, the substrate portion  210  includes a flexible, compliant layer  212  proximate the non-conductive layer  208  of the sensor portion  202 . The conductive contacts  206 ,  207  may be formed of any desired conductive material, including copper, gold, aluminum or other suitable conductive material. 
   As further shown in  FIGS. 2 and 3 , the conductive contacts  206 ,  207  pass through the non-conductive layer  208  to an inner side of the conductive layer  206  (indicated by dashed lines) prior to passing around a proximal end  213  of the substrate  210 . On the proximal end  213 , first and second pairs of auxiliary contacts  214 ,  216  are disposed on the non-conductive layer  208 . Each pair of auxiliary contacts  214 ,  216  includes an outer auxiliary contact  218  and an inner auxiliary contact  219  that are coupled to corresponding outer and inner conductive contacts  206 ,  207 , respectively, by plated-through holes  217 . 
   An article of test equipment  220  is coupled to the conductive contacts  206 ,  207  by a plurality of conductive leads  222 . The test equipment  220  may be any of a variety of widely-known, commercially-available devices used for measuring electrical resistance, including, for example, digital ohm meters offered by Keithley Instruments, Inc. of Cleveland, Ohio, or by AVO Biddle Instruments Corporation of Blue Bell, Pa. 
     FIG. 4  is an enlarged end elevational view of the test device  200  of  FIG. 2  engaged with a conductive layer  230  during a test in accordance with a first mode of operation. The conductive layer  230  is formed on a non-conducting composite material  232 . In this mode of operation, the test device  200  is used by pressing the first and second pairs of conductive contacts  204 ,  205  into engagement with the conductive layer  230  being tested. As described more fully below, the test equipment  220  then measures the electrical resistance R T  of the conductive layer  230  between the inner conductive contacts  207  in ohms per square. Preferably, the non-conducting layer  208  and the conductive contacts  206 ,  207  of the sensor portion  202  are flexible so that, when the sensor portion  202  is disposed on the compliant portion  212  of the substrate  201  ( FIG. 2 ), the sensor portion  202  may flex to conform to the curvature of the conductive layer  230 . 
   In an alternate mode of operation (not shown), the test device  200  is re-positioned so that the first and second pairs  214 ,  216  of auxiliary contacts  218 ,  219  are engaged against the conductive layer  230  being tested. In this mode, the resistance R T  of the conductive layer  230  between the inner auxiliary contacts  219  is measured by the test equipment  220 . 
     FIG. 5  is a circuit diagram  300  for the test device  200  of  FIG. 2  engaged with the conductive layer  230  during a test in accordance with an embodiment of the invention. In this embodiment, the resistance of the outer conductive contact  206   a  of the first pair  204  is represented by a first resistance R 1 , the resistance of the inner conductive contact  207   a  of the first pair  204  is represented by a second resistance R 2 , the resistance of the inner conductive contact  207   b  of the second pair  205  is represented by a third resistance R 3 , and the resistance of the outer conductive contact  207   b  of the second pair  205  is represented by a fourth resistance R 4 . Similarly, as set forth above, the electrical resistance of the conductive layer  230  between the inner conductive contacts  207  is represented by a test resistance R T . A current source  302  is coupled in series between the first and fourth resistances R 1 , R 4 , and a meter  304  (e.g. a voltmeter) is coupled in series between the second and third resistances R 2 , R 3 . In addition, an ammeter that measures the known current I K  (not shown in  FIG. 5 ) may also be coupled in series between the first and fourth resistances R 1 , R 4 , either separately or included with the current source  302 . The source  302  and the meter  304  may be included in the test equipment  220 , or alternately, may be separate components. The circuit diagram  300  shown in  FIG. 5  may be of a variety known as a Kelvin double bridge circuit, or may be any other suitable circuit. 
   In operation, the source  302  applies a known current I K  to the circuit  300  which flows through the first resistance R 1 , the test resistance R T , and the fourth resistance R 4 . The meter  304  measures a characteristic value, such as a test voltage V T , across the test resistance R T . Because only a negligible amount of current passes through the meter  304 , practically no current passes through the second and third resistances R 2 , R 3 , and therefore, the current passing through the test resistance R T  is approximately the known current I K . Using the measured test voltage V T  and the known current I K , the test resistance R T  is determinable using Ohm&#39;s law according to the following Equation 1:
 
 R   T   =V   T   /I   K    (1)
 
   Similarly, when the test device  200  is used in the alternate mode of operation with the first and second pairs of auxiliary contacts  214 ,  216  engaged with the conductive layer  230 , the resistance of the conductive layer  230  is determined in the manner described above. With continued reference to the circuit diagram  300  shown in  FIG. 5 , in the alternate mode of operation, the resistance of the outer auxiliary contact  218   a  of the first pair  214  is represented by the first resistance R 1 , the resistance of the inner auxiliary contact  219   a  of the first pair  214  is represented by the second resistance R 2 , the resistance of the inner auxiliary contact  219   b  of the second pair  216  is represented by the third resistance R 3 , and the resistance of the outer auxiliary contact  218   b  of the second pair  216  is represented by the fourth resistance R 4 . Thus, by applying the known current I K  across the outer auxiliary contacts  218 , the test resistance R T  of the conductive layer  230  may be determined by measuring the test voltage V T  between the inner auxiliary contacts  219 . 
   Referring to  FIGS. 2 and 3 , if the lateral distance w between the inner conductive contacts  207  is equal to the contact length l such that the area of the conductive layer  230  under test that lies between the inner conductive contacts  207  is a square, then the measured value of the test resistance R T  is the equal to the resistance of the conductive layer  230  in ohms per square. This is the true regardless of the actual physical dimension of the lateral distance w and the contact length l so long as these values are equal. 
   On the other hand, if the lateral distance w is not equal to the contact length l, such that the test area of the conductive layer  230  that lies between the inner conductive contacts  207  is not square (e.g. rectangular), then a scaling factor is applied to the test resistance R T  in order to compute the resistance of the conductive layer  230  in the standard unit of measurement, namely, ohms per square. For example, as shown in  FIG. 3 , if the lateral distance w is one-tenth of the contact length l, then the test resistance R T  is multiplied by a scaling factor of 10 in order to compute the resistance of the conductive layer  230  in ohms per square. 
   It may be noted that during testing, current losses may occur within the conductive layer  230  outwardly from the ends of the test area that are not bounded by the conductive contacts  206 ,  207 . These losses may contribute to any uncertainty that may be associated with the measurement of the test resistance R T . Therefore, it may be desirable to design the test device  200  such that the lateral distance w is relatively small in comparison to the length l in order to reduce errors due to current losses. 
   For example, a 1 mm lateral distance of a 1 mm by 1 mm probe may have the same current losses as a 2 ft by 2 ft probe. Similarly, a 1 mm by 10 mm probe may have the same current losses as the 1 mm by 1 mm probe because the lateral distance w is the same width and the current is allowed to flow around two ends, but the error due to the current losses gets divided by 10 when the correction is applied. In a particular aspect of the invention, the geometric correction for a probe with parallel and equal length conductive members is the lateral distance w divided by the contact length l (i.e. w/l). 
   Preferably, the outer and inner conductive contacts  206 ,  207  of each pair  204 ,  205  may be substantially parallel. Similarly, the first and second pairs  204 ,  205  may also be substantially parallel with each other. Such parallelism of the conductive contacts  206 ,  207  may simplify the process of data reduction and may improve measurement accuracy. If the conductive contacts  206 ,  207  are non-parallel, however, the non-parallelism may be measured and accounted for in the data reduction algorithm. Furthermore, the contact lengths l of all of the conductive contacts  206 ,  207  may be equal, as shown in  FIGS. 2 and 3 , or alternately, may be unequal. Embodiments having unequal contact lengths l of the conductive contacts  206 ,  207  may include additional correction factors within the data reduction algorithm, as may be determined by persons of ordinary skill in the art. 
   The test device  200  advantageously provides improved measurement of the electrical resistance of the conductive layer  230 . Because the test resistance R T  is measured between the inner conductive contacts  207 , and because approximately no current (or only a negligible amount of current) passes through the inner conductive contacts  207 , the additional component of measurement uncertainty caused by the resistance associated with the surface-to-surface contact between the conductive contacts and the conductive layer  230  is eliminated from the resulting measurement. The test device  200  may therefore be employed to measure the resistance of conductive coatings having relatively small resistance, including such coatings having a resistance value smaller than the surface-to-surface contact resistance between the conductive contacts and the conductive coating. 
     FIG. 6  is an elevational view of an apparatus  400  in accordance with an alternate embodiment of the invention. In this embodiment, the apparatus  400  includes a non-conductive substrate  402  having a conductive material  404  disposed thereon. First and second conductive members  406 ,  408  are disposed on the conductive material  404  and are separated by a first gap  410 . Similarly, third and fourth conductive members  412 ,  414  are disposed on the conductive material  404  and are separated by a second gap  416 . In the embodiment shown in  FIG. 6 , the conductive members  406 ,  408 ,  412 ,  414  are wrapped over the leading and trailing edges  403 ,  405  of the conductive layer  404  and the non-conductive substrate  402 . 
   In a particular aspect, the first and second conductive members  406 ,  408  are formed by attaching a strip of conductive tape or other suitable conductive material to the conductive coating  404 , and then cutting the strip of conductive tape to form the first gap  410 . The third and fourth conductive members  412 ,  414  and the second gap  416  may be formed in a similar fashion. Alternately, the conductive members  406 ,  408 ,  412 ,  414  may simply be formed using strips of conductive tape (e.g. copper tape) having a layer of conductive adhesive that attaches the conductive tape to the conductive layer  404 . 
   As further shown in  FIG. 6 , the second and third conductive members  408 ,  412  are separated by a lateral distance w, and all of the conductive members  406 ,  408 ,  412 ,  414  contact the conductive material  404  along a contact length l. In this embodiment, the lateral distance w is equal to the contact length l. The apparatus  400  may further include a plurality of conductive leads  418  that couple each of the conductive members  406 ,  408 ,  412 ,  414  to an article of test equipment  420 . As described above, the test equipment  420  may be any acceptable, commercially-available device that includes the components necessary for conducting tests and performing measurements of the resistance of the conductive material  404 . 
   In operation, the test equipment  420  applies a known current I K  (approximately) into the first conductive member  406 , across the conductive material  404 , and out through the fourth conductive member  414 , and measures the resulting test voltage V T  existing between the second and third conductive members  408 ,  412 . In the manner described above, the test resistance R T  of the conductive material  404  is determined from the test voltage V T  and the known current I K . Because the lateral distance w is equal to the contact length l, the area between the second and third conductive members  408 ,  412  is a square and the measured test resistance R T  of the conductive material  404  is measured in the standard unit, ohms per square. In other words, no scaling factor is necessary, and the measurement is the same whether the square is, for example, 1 cm by 1 cm or 3.5 inch by 3.5 inch. 
   The apparatus  400  advantageously provides improved measurement of the electrical resistance of the conductive material  404 . As described above, because the test resistance R T  is measured between the second and third conductive members  408 ,  412 , and because approximately no current (or only a negligible amount of current) passes through these conductive members  408 ,  412 , the measurement uncertainty caused by the surface-to-surface contact resistance between the conductive members  406 ,  408 ,  412 ,  414  and the conductive material  404  is eliminated from the resulting measurement. The apparatus  400  may therefore provide improved measurement accuracy, especially for conductive materials having relatively small electrical resistance. Also, because the conductive material  404  is co-extensive with the contact length l of the conductive members  406 ,  408 ,  412 ,  414 , there are no current losses due to the bounded ends of the test area, thereby reducing or eliminating another component of measurement uncertainty. 
   While the preferred embodiment of the invention has been illustrated and described, as noted above, many changes can be made without departing from the spirit and scope of the invention. Accordingly, the scope of the invention is not limited by the disclosure of the preferred embodiment. Instead, the invention should be determined entirely by reference to the claims that follow.