Abstract:
The invention is a system for determining the presence of ice on the surface of a structure. The system includes a guard layer mounted to the surface. A non-conductive layer is mounted on top of the ground plane. First and second electrodes made of a resistive material mounted on the non-conductive layer. First and second electrical leads having first ends of attached to first and second electrodes, respectively. The first and second leads having impedance equal to the impedance of the first ends thereof with the impedance of the first and second leads decreasing toward the second ends thereof.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The invention relates to the field of ice detection systems for aircraft and, in particular, to an ice detection system for low observable aircraft. 
   2. Description of Related Art 
   Aircraft icing can occur under certain atmospheric conditions. The icing primarily forms on the leading edge of the wings. Such ice accretion, if allowed to build up, can cause a loss of lift, which can, in extreme cases, cause the aircraft to crash. Thus modern commercial aircraft incorporated anti-icing devices. For example, large aircraft incorporate hot air ducts along the leading edges of wings. Hot bleed air from the compressor stages of the turbine engines are fed through these ducts, melting the ice. Smaller aircraft use inflatable boots that can be pulsed to expand and contract, breaking up the ice. Other systems involve the use of electromechanical actuators that flex the outer skin of the wings breaking up the ice. On most small aircraft, de-icing systems are not employed. Therefore, the pilot is required to fly the aircraft out of the “ice forming” environment. All aircraft having de-icing systems must have ice formation sensors strategically placed to sense the ice forming so that the de-icing system can be actuated in a timely manner. 
   Even on aircraft that do not have de-icing systems, detection systems are often incorporated. The most obvious method is visual examination by the flight crew. While the pilot can usually see the wings on small general aviation aircraft, on larger aircraft the wings are not always visible from the flight station. At night, visual examination may not be possible. Thus an ice detection system will give the pilot warning and allow him or her to fly the aircraft out of the area. If the aircraft is unmanned, the remote operator will have the same capability. 
   There are numerous types of ice detection systems available, for example, U.S. Pat. No. 6,052,056 “Substance Detection System” by J. D. Burns, et al. In this system a modulated light source is directed to an optical sensor located in an area where ice will tend to accumulate, such as an aerodynamic surface or engine inlet. The sensor transmits light back to a detector that is proportional to the amount of ice on the surface. 
   Another approach is to use ultrasonic transducer guided waves, which are applied to the aircraft&#39;s skin. The waves interact with the reflective geometry of the skin and a portion of the waves scatter back to the transducer. By monitoring the amplitude, frequency change, etc. of the returned waves, ice build up on the skin can be identified. Magnetostrictive ice detection is also available. Sensors vibrate ultrasonically at a set frequency. As ice accretes on a probe, the vibrational frequency decreases, which is detected signaling the to remove the ice. In another system a detector operates periodically by heating a temperature element to a constant temperature. A microprocessor measures the element&#39;s rate of temperature increase by comparing the time it takes the element to pass through two reference temperatures. Since the melting process absorbs considerable energy, the temperature increases at a slower rate when ice has accumulated. 
   Another approach is to use capacitance probes mounted on the external surface. Examples of these can be found in U.S. Pat. No.: 4,766,369 “Ice Detector System” by L. M. Weinstein; U.S. Pat. No. 45,569,850 “Ice Detector” by R. L. Raunchorst, 111; and 5,854,672 “Apparatus And Method For Determining The Existence Of Ice Or Water On a Surface From The Capacitance Between Electrodes On Said Surface” by J. J. Gerardi, et al. In these devices, capacitance probes, generally spaced conductive electrodes encapsulated in a non-conductive substrate, are mounted on a surface where ice will tend to accumulate. The accumulating ice, of course, will change the capacitance of the probe, which can be sensed, by a capacitance measuring circuit. 
   The problem with existing capacitance probe type ice detection systems is that they do not lend themselves to use on low radar observable aircraft. The lead wires to the probes and the probes themselves tend to increase the radar signature on the aircraft. The conductive probes and lead wires scatter the incoming radar signals and have radar cross-sections that are much to large for low observable aircraft applications. 
   Thus, it is a primary object of the subject invention to provide an ice detection system for an aircraft. 
   It is a still further object of the subject invention to provide an ice detection system for an aircraft having a low radar cross-section. 
   An additional object of the subject invention is to provide an ice detection system using capacitance probes for an aircraft having a low radar cross-section. 
   SUMMARY OF THE INVENTION 
   The invention is a system for determining the presence of ice on an external surface subject to ice accretion thereon of an aircraft designed with a low radar cross-section. For example, the leading edges of the wings, tails, or engine inlets are the most likely areas where ice detection systems are required. For example, the leading edge of a wing, tail, or engine inlet of a low radar cross-section aircraft typically includes a non-metallic outer skin made of a dielectric material that is filled with a structural bulk absorber. The bulk absorber is typically either a “loaded” foam or coated honeycomb core. Thus the system for determining the presence of ice on an external surface of a structure includes a guard layer, made of a layer of resistive material, for mounting on the external surface. A non-conductive layer is mounted on top of the guard layer. First and second electrodes made of a resistive material are mounted on the non-conductive layer. Preferably, the electrodes are parallel to each other. Preferably, the resistive material of the first and second electrodes has a nominal resistance of 400 ohms per linear inch of length. 
   The system further includes first and second electrical leads having first and second ends, with the first ends of the leads attached to the first and second electrodes, respectively. The leads extend parallel to each other in opposite directions from each other. The first ends of first and second leads have an impedance equal to the impedance of the first and second electrodes with the resistance decreasing toward the second ends thereof to around a nominal value of 50 ohms per square. The non-conductive layer and guard layer extend under the entire length of the first and second leads. Preferably, the resistance of the guard layer tapers from a nominal value of 750 ohms per square under the electrodes to around 50 ohms per square at the second ends of the leads. 
   Preferably, a layer of semi-conductive material is bonded over the first and second electrodes and a coating of dielectric material covers the non-conductive layer surrounding the first and second electrodes and the layer of semi-conductive material and extends to the second ends of the first and second leads. 
   Thus the non-conductive layer, guard layer, the first and second electrodes with the semi-conductive coating there over, first and second leads, and the coating of dielectric material form an assembly. To insure a smooth external surface, the external surface includes a recess and the assembly is mounted in the recess by an adhesive. Additional dielectric material fills any remaining portions of the recess. The second ends of the leads are electrically connected to a control module, which measures any capacitance change in the electrodes and sends a signal to the pilot of the aircraft. 
   In a preferred version an adhesive layer is applied to the guard layer and over which is mounted a strippable cover. Thus, the system is self-contained and can be stored with ease until installation is required. To install, one need only peel the cover off and insert the system into the recess on the airfoil leading edge. Of course, the system must be sized for the particular surface and the design values of the resistive sheet, electrode and strips will also vary from application to application. 
   The novel features which are believed to be characteristic of the invention, both as to its organization and method of operation, together with further objects and advantages thereof, will be better understood from the following description in connection with the accompanying drawings in which the presently preferred embodiment of the invention is illustrated by way of example. It is to be expressly understood, however, that the drawings are for purposes of illustration and description only and are not intended as a definition of the limits of the invention. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a partial perspective view of the leading edge portion of the wing of an aircraft having a low radar cross-section. 
       FIG. 2  is a partial view of the system prior to installation in to the wing of the aircraft. 
       FIG. 3  is a partial cross-sectional view of  FIG. 1  taken along the line  3 — 3 . 
       FIG. 4  is a partial cross-sectional view of  FIG. 1  taken along the line  4 — 4  shown in FIG.  1 . 
       FIG. 5  is a partial cross-sectional view of  FIG. 1  taken along the line  5 — 5  shown in FIG.  1 . 
       FIG. 6  is a cross-sectional view of an alternate design of the system. 
   

   DESCRIPTION OF PREFERRED EMBODIMENT 
   In  FIGS. 1 and 3 , an illustration of a leading edge portion of a wing  10  of an aircraft designed to have a low radar cross-sectional area is shown. The leading edge portion  11  includes an outer skin  12  made of a dielectric material having an external surface  13 , an internal surface  14 , a leading edge  17 , and the aft ends  18 A and  18   b , which are joined to a bulkhead  19 . The skin  12  is typically a composite material such as Kevlar® filaments in a resin matrix, manufactured by the manufactured by the E.I. DuPont de Nemours &amp; Company, Delaware. The loading edge portion  11  is filled with a bulk absorber  20 , which can be a loaded foam core or coated honeycomb core bonded to the skin  12 . The foam or honeycomb core would also be made of a dielectric material. A small recess  21  is located on surface  14  and extends from the end  18 A, across the leading edge  37  and to end  18 B. The recess  21  has width indicated by numeral  22  of approximately two inches and a depth, indicated by numeral  23 , or approximately 0.030 inch. However, it must be noted that actual required dimensions will vary from application to application. 
   Still referring to  FIGS. 1 and 3 , and additionally to  FIGS. 2 and 4 , the detector system, indicated by numeral  24  has a length  26  that is a little less than the overall length of the recess  21 . The width  28  of the detector assembly  24  is a little less than the width  22  of the recess  21 . The detector system  24 , includes a non-conductive substrate or layer  32 , typically, a polycarbonate sheet, having first and second sides  33 A and  33 B extending along the entire length  26 . A guard layer  34  made of a resistive material, such as a graphite or carbon impregnated urethane ink, is silk screened on side  33 A of the layer  32  along the entire length. The resistive value of the guard layer  34  will be subsequently discussed. A pair of resistive wire electrodes  36 A and  36 B, which act as the capacitance probe, made of a resistive material such as graphite or carbon impregnated urethane ink, are silk-screened on the opposite side  33 B of the layer  32 . The electrodes should have a nominal resistance of 400 ohms per linear inch of length with an overall length, indicated by numeral  37 , of a bout two inches. The electrodes  36 A and  36 B should be separated by a distance, indicated by numeral  38 , of about 0.1 inch. A semi-conductive urethane cover  39  is placed over the electrodes  36 A and  36 B to provide protection, but only over the two-inch length  37 . 
   The electrodes  36 A and  36 B are connected to electrical leads in the form of resistive strips  40 A and  40 B, respectively. The strips  40 A and  40 B can also comprise loaded urethane ink silk screened onto side  33 B of the layer  32 . At the ends  42 A and  43 A of the strips  40 A and  40 B, respectively, the resistance of the strips should be equal to the electrodes  36 A and  36 B. Thereafter, the resistance should decrease such that it approaches a nominal value of 50 ohms per square at ends  42 B and  43 B. This could be accomplished by silk-screening the strips  40 A and  40 B in steps using higher “loaded” ink as ends  42 A and  43 B are approached. The resistive value of the guard layer  34  should be a nominal 750 ohms per square under the electrodes  36 A and  36 B and taper in value to a nominal 50 ohms per square at the second ends  42 B and  43 B of the strips  40 A and  40 B, respectively. The guard layer  34  isolates the electrodes  36 A and  36 B from spurious electrical signals (noise) generated with the aircraft. Thereafter, the entire side  33 B of the layer  32  is covered with a non-conductive coating  44 , for example urethane sealant or the like, such that the entire surface  32  is covered to a level equal to the semi-conductive urethane cover  39 . Thus the strips  40 A and  40 B are also protected from rain erosion and the like. The completed system  24  is bonded in the recess  21  by an adhesive coating  46  applied to the resistive layer  34  on the surface  33 A of the layer  32 . 
   Referring to  FIG. 5 , the end  20 A of the skin  14  is joined to the bulkhead  19  by fasteners  48 . The end  43 B of the strip  40 B is joined to a wire  48 , that extends through a hole  50  in the skin  12  and is connected to a coax connector half  52 A joined to the internal surface  17  of the skin. The guard layer  34  in connected to ground by wire  49  that also extends through the hole  50  and connects to the exterior to the coax connector half  52 A. An electronic module  54  that is used to sense capacitance changes in the electrodes  36 A and  36 B is connected via coax cable  56  and second connector half  52 B to connector half  52 A. While not illustrated, strip  40 A is connected to the module  54  in a similar manner. Referring to  FIGS. 3-5 , the final step is to fill any gaps between the system  24  and recess  21  walls with a dielectric filler  58  such as urethane to eliminate any gaps. 
   In  FIG. 6 , an alternate design for the system, designated by numeral  31 ′ is illustrated. Here all features are identical except that a layer of pressure sensitive adhesive  60  is applied directly to the resistive layer  34  and a strippable cover  62  applied there over. The advantage with this design is that the system  24 ′ can be stored until ready for installation. The cover  62  can then be removed and, the system  24 ′ installed in the recess  21 , eliminating the need to apply an adhesive to the recess. 
   Again referring to  FIGS. 2-5 , manufacturing of the system can be easily accomplished by first silk-screening a coating a first side  33 A of thin flexible polycarbonate sheet  32  with the resistive coating  34 . Thereafter, electrodes  36 A and  36 B and strips  40 A and  40 B are silk screened on the second side  33 B. Inert plugs (not shown) having the size and shape of the semi-conductive coatings  38  are placed over the electrodes. The wires  48  are attached to the ends  42 B and  43 B of the strips  40 A and  40 B. The second side  33 B of the sheet is then coated with the non-conductive urethane cover  44 . The inert plugs are removed and the holes filled with the semi-conductive urethane covers  39 . Again referring to  FIG. 6 , If so desired, the adhesive coating  60  and removable cover  62  can be easily added. 
   In review, a capacitive ice detector senses the presence of an ice coating through an increase in measured capacitance. A coating of ice increases the dielectric constant in the region near the electrodes and raises the capacitance thereof. To have high sensitivity to ice formation the ice detector needs to be on the outer surface of the aircraft, adjacent to the ice coating. If the icing detector has direct current coupling with the ice, the resistance of the ice can be measured. The semi-conductive urethane cover  38  performs this function. High moisture content ice or wet ice has a low resistance compared to a low moisture content ice or dry ice. Hence, wet and dry ice can be discriminated through resistance measurement. 
   To have high measurement sensitivity to ice formation, the strips  40 A and  40 B that connect the electronic module  54  to electrodes  36 A and  36 B must have a capacitance that is much smaller than the capacitance of the electrodes. The capacitance of electrodes  36 A and  36 B have a nominal value of 1 picofarad. The capacitance of the strips  40 A and  40 B needs to be, at a minimum, a factor of 4, and preferably a factor of 10, less than the capacitance of electrodes or less than 0.25 picofarad. The capacitance of the strips  40 A and  40 B is reduced by physical separation and through the use of electrostatic shields. The conventional non-low observable method of electrostatic shielding is accomplished with the use of coaxial cables. The coaxial cable outer conductor provides the electrostatic shielding. However, the coaxial cable outer conductors will have a radar cross section that is much too large for low observable applications. The subject invention uses the guard layer,  34 , for the electrostatic shield to reduce the capacitance between the feed lines,  40 A and  40 B. The guard layer is a tapered resistive film. The resistance values and taper function depends upon the electrical properties of the materials in the edge absorber design. The guard layer resistive values will be developed so the guard layer is an integral part of the edge absorber design. This will minimize the radar cross section of the guard layer. 
   In review, the capacitance and resistance measurement electronics use an alternating current signal with a nominal frequency of 10 kHz. With no ice on the capacitance probe, the measured impedance is in excess of 10 megohms. When ice coats the capacitance probe, the impedance is reduced up to a factor of five. The series resistance of the strips  40 A and  40 B and the resistive loss of guard layer  34  is much smaller than the measured impedance of the capacitance probe. Hence, the strips  40 A and  40 B and the guard layer  34  will not degrade the operation of the measurement electronics. Since the measurement impedance of the ice free capacitance probe is very high, the resistance between the strips  40 A and  40 B, the capacitance probe electrodes  36 A and  36 B need to be greater than a factor of three larger than the measured impedance. Therefore, the materials that surround the strips  40 A and  40 B and the electrodes  36 A and  36 B need to be a very high resistivity insulator. 
   In conclusion, it has been demonstrated that the above system can be manufactured and easily installed on an aircraft surface subject to icing. It has particular application to use on low observable aircraft that require a reduced radar cross-section where conventional capacitance measuring systems would produce an unacceptable radar signature due to the scattering of incoming radar signals caused by the conductive probes and lead wires. The electrodes  36 A and  36 B and strips  40 A and  40 B have a combined resistance on the order of a few thousand ohms. Actual effects of ice are on the order of a million ohms. Thus measurement of ice accumulation is readily made. 
   While the invention has been described with reference to particular embodiment, it should be understood that the embodiments is merely illustrative as there are numerous variations and modifications, which may be made by those skilled in the art. Thus, the invention is to be construed as being limited only by the spirit and scope of the appended claims. 
   INDUSTRIAL APPLICATION 
   The invention has application in the aircraft industry and, in particular, to a company making ice detection systems for aircraft.