Patent Application
20120225784
The present invention relates generally to high temperature coiled-wire devices and, more particularly, to high temperature electromagnetic coil assemblies for usage within coiled-wire devices, as well as to methods for the production of high temperature electromagnetic coil assemblies.
There is an ongoing demand in the aerospace industry for low cost electromagnetic coils suitable for usage in coiled-wire devices, such as actuators (e.g., solenoids) and sensors (e.g., linear variable differential transformers), capable of providing prolonged and reliable operation in high temperature environments and, specifically, while subjected to temperatures in excess of 260° C. It is known that low cost electromagnetic coils can be produced utilizing aluminum wire, which is commercially available at minimal cost, which provides excellent conductive properties, and which can be anodized to form an insulative alumina shell over the wire's outer surface. However, the outer alumina shell of anodized aluminum wire is relatively thin and can easily abrade due to contact between neighboring coils during winding. As a result, bare anodized aluminum wire is prone to shorting during the coiling process. Coil-to-coil abrasion can be greatly reduced or eliminated by utilizing anodized aluminum wires having insulative organic-based (e.g., polyimide) coatings to form the electromagnetic coil; however, organic materials rapidly decompose, become brittle, and ultimately fail when subjected to temperatures exceeding approximately 260° C.
A limited number of ceramic insulated wires are commercially available, which can provide continuous operation at temperatures exceeding 260° C.; however, such wires tend to be prohibitively costly for most applications and may contain an undesirably high amount of lead. High temperature wires are also available that employ cores fabricated from non-aluminum metals, such as silver, nickel, and copper. However, wires having non-aluminum cores tend to be considerably more costly than aluminum wire and may be incapable of forming an insulative oxide shell. In addition, wires formed from nickel tend to be less conductive than is aluminum wire and, consequently, add undesired bulk and weight to an electromagnetic coil assembly utilized within avionic applications. Finally, while insulated wires having cores fabricated from a first metal (e.g., copper) and claddings formed from a second meal (e.g., nickel) are also known, such wires are relatively costly, which tend to become less conductive over time due to diffusion of the cladding material into the wire's core, and may exhibit alloying-induced resistance creeping when exposed to elevated temperatures for longer periods of time. Additionally, wires employing metal-clad conductors still require electrically-insulative coatings of the type described above.
Considering the above, there exists an ongoing need to provide embodiments of a electromagnetic coil assembly suitable for usage within high temperature coiled-wire devices (e.g., solenoids, linear variable differential transformers, and three wire position sensors, to list but a few) utilized within avionic applications and other high temperature applications. Ideally, embodiments of such a high temperature electromagnetic coil assembly would be relatively inexpensive to produce, relatively compact and lightweight, and capable of reliable and continual operation when subjected to temperatures in excess of 260° C. It would also be desirable to provide embodiments of a method for fabricating such a high temperature electromagnetic coil assembly. Other desirable features and characteristics of the present invention will become apparent from the subsequent Detailed Description and the appended Claims, taken in conjunction with the accompanying Drawings and the foregoing Background.
Embodiments of a high temperature electromagnetic coil assembly are provided, as are embodiments of a method for fabricating such a high temperature electromagnetic coil assembly. In one embodiment, the method includes the steps of applying a high thermal expansion ceramic coating over an anodized aluminum wire, coiling the coated anodized aluminum wire around a support structure, and curing the high thermal expansion ceramic coating after coiling to produce an electrically insulative, high thermal expansion ceramic body in which the coiled anodized aluminum wire is embedded.
Embodiments of a high temperature electromagnetic coil assembly are further provided. In one embodiment, the high temperature electromagnetic coil assembly includes a support structure, an anodized aluminum wire wound around the support structure, and an electrically-insulative, high thermal expansion body formed around the support structure and in which the anodized aluminum wire is embedded. The electrically-insulative, high thermal expansion body electrically insulates the coils of the anodized aluminum wire to reduce the probability of electrical shorting and to increase the breakdown voltage of the anodized aluminum wire during high temperature operation of the high temperature electromagnetic coil assembly.
At least one example of the present invention will hereinafter be described in conjunction with the following figures, wherein like numerals denote like elements, and:
The following Detailed Description is merely exemplary in nature and is not intended to limit the invention or the application and uses of the invention. Furthermore, there is no intention to be bound by any theory presented in the preceding Background or the following Detailed Description.
Next, at STEP 24 of method 10 (
During STEP 24 of method 10 (
After winding of the anodized aluminum wire and application of the wet-state, HTE ceramic material (STEP 24,
To ensure compatibility with the anodized aluminum wire, and to ensure maintenance of the structural and insulative integrity of the electromagnetic coil assembly through aggressive and repeated thermal cycling, the HTE ceramic material is selected to have several specific properties. These properties include: (i) the ability to produce, upon curing, a ceramic body that provides mechanical isolation, position holding, and electrical insulation between neighboring coils of the anodized aluminum wire through the operative temperature range of the electromagnetic coil assembly; (ii) the ability to produce, upon curing, a ceramic body capable of withstanding significant mechanical stress without structural compromise during thermal cycling; (iii) the ability to prevent significant movement of the anodized aluminum wire coils during wet winding and, in certain embodiments, during subsequent heat treatment (e.g., during melting of low melt glass particles, as described more fully below); (iv) the ability to be applied to the anodized aluminum wire in a wet state during the winding process at temperatures below the melting point of the anodized aluminum wire (again, approximately 660° C.); and (v) the ability to harden (e.g., by curing or firing) into a solid state or near-solid state at temperatures lower than the melting point of the anodized aluminum wire.
In addition to the above-listed criteria, it is also desired for the selected electrically-insulative, HTE ceramic material to produce, upon curing, a ceramic body having a coefficient of thermal expansion falling within a specific range. By definition, the electrically-insulative, HTE ceramic body has a coefficient of thermal expansion (“CTE”) exceeding approximately 10 ppm per ° C. By comparison, the CTE of anodized aluminum wire is approximately 23 ppm per ° C. By selecting the HTE ceramic material to have a CTE exceeding approximately 10 ppm per ° C., and therefore more closely matched to the CTE of the anodized aluminum wire, relative movement and mechanical stress between cured HTE ceramic body and the anodized aluminum wire can be reduced during thermal cycling and the likelihood of structural damage to the ceramic body or to the wire (e.g., breakage due to stretching) can be minimized. Stated differently, by forming the high thermal expansion ceramic body from a material having a coefficient of thermal expansion substantially matched to that of the anodized aluminum wire, thermal mismatch between the ceramic body and the anodized aluminum wire is minimized resulting in a significant reduction in the mechanical stress exerted on the ceramic body and the wire through thermal cycling of the high temperature electromagnetic coil assembly.
The ability of the cured HTE ceramic body to withstand mechanical stress induced by thermal cycling is also enhanced, in certain embodiments, by forming the HTE ceramic body from an inorganic cement having a relatively high porosity, as described more fully below. In a similar regard, it is also desirable to form bobbin 14 and the bobbin's dielectric coating from materials having coefficients of thermal expansion similar to that of anodized aluminum wire. While selecting the electrically-insulative, HTE ceramic body to have a CTE approaching that of the anodized aluminum wire is advantageous, it is generally preferred that the CTE of the HTE ceramic body does not exceed the CTE of the anodized aluminum wire. In this manner, it can be ensured that the HTE ceramic body is subjected to compressive stress, rather than tensile stress, during thermal cycling of the high temperature electromagnetic coil assembly thereby further reducing the likelihood of fracture and spalling of the HTE ceramic body. For the foregoing reasons, the HTE ceramic body is preferably selected to have a coefficient of thermal expansion between approximately 10 and approximately 23 ppm per ° C. and, more preferably, between approximately 16 and approximately 23 ppm per ° C.
In a first group of embodiments, the electrically-insulative, HTE ceramic material applied to the anodized aluminum wire during STEP 24 comprises a mixture of at least a low melt glass and a particulate filler material. As defined herein, the term “low melt glass” denotes a glass or glass mixture having a melting point less than the melting point of the anodized aluminum wire. Low melt glasses having coefficients of thermal expansion exceeding approximately 10 ppm per ° C. include, but are not limited to, leaded borosilicates glasses. Commercially available leaded borosilicate glasses include 5635, 5642, and 5650 series glasses having processing temperatures ranging from approximately 350° C. to approximately 550° C. and available from KOARTAN™ Microelectronic Interconnect Materials, Inc., headquartered in Randolph, N.J. During STEP 24 (
It is desirable to include a particulate filler material in the embodiments wherein the electrically-insulative, HTE ceramic material comprises a low melt glass to prevent relevant movement and physical contact between neighboring coils of the anodized aluminum wire during coiling and firing processes. Although the filler material may comprise any particulate material suitable for this purpose (e.g., zirconium or aluminum powder), binder materials having particles generally characterized by thin, sheet-like shapes (commonly referred to as “platelets” or “laminae”) have been found to better maintain relative positioning between neighboring coils as such particles are less likely to dislodge from between two adjacent turns or layers of the wire's cured outer surface than are spherical particles. Examples of suitable binder materials having thin, sheet-like particles include mica and vermiculite. As indicated above, the low melt glass may be applied to the anodized aluminum wire by brushing immediately prior to the location at which the wire is being coiled around the support structure. Subsequently, during STEP 26 of exemplary method 10 (
In a second group of embodiments, the ceramic body is formed from a high thermal expansion, electrically-insulative, inorganic cement, which may undergo a chemical or thermal curing process to set the inorganic cement into the solid, electrically-insulative body. As one example, a water-activated, silicate-based cement can be utilized, such as the sealing cement bearing Product No. 33S and commercially available from the SAUEREISEN® Cements Company, Inc., headquartered in Pittsburgh, Pa. As was the case previously, the water-activated cement may be continuously applied to the anodized aluminum wire via a brush just ahead of the location at which the wire is wound around the support structure. A relatively thin layer of cement is preferably applied, while ensuring that ample cement is available for filling the space between adjacent coils and winding layers. After winding, the cement may be allowed to air dry or heated to a temperature less than the boiling point of water to evaporate excess water from the cement, and the entire assembly may then be heat treated to thermally cure the cement in the above-described manner (STEP 26,
While, as indicated in
In embodiments wherein the HTE ceramic body is formed from a material that is not susceptible to the ingress of water (e.g., when HTE ceramic body is formed from a non-porous glass), exemplary method 10 may conclude after STEP 26 (
In addition to or in lieu of application of a liquid sealant, a water-tight seal may also be formed over the electrically-insulative HTE ceramic body by packaging the electromagnetic coil assembly within a hermetically-sealed container or canister. For example, as shown in
In many implementations of exemplary method 10 (
In the above-described manner, a high temperature electromagnetic coil assembly can be produced having potted coils (e.g., coils 66 and 68 shown in
The foregoing has thus provided embodiments of methods for producing electromagnetic coil assemblies suitable for usage within high temperature operating environments characterized by temperatures exceeding the threshold at which organic materials breakdown and decompose (approximately 260° C.). The above-described electromagnetic coil assemblies are consequently well-suited for usage in high temperature coiled-wire devices, such as those utilized in avionic applications. As a point of emphasis, embodiments of the electromagnetic coil assembly can be employed in any coiled-wire device exposed to operating temperatures exceeding approximately 260° C. However, by way of non-limiting example, embodiments of the high temperature electromagnetic coil assembly are especially well-suited for usage within actuators (e.g., solenoids) and position sensors (e.g., linear variable differential transformers and three wire position sensors) deployed onboard aircraft. To further emphasize this point, two exemplary coiled-wire devices employing high temperature electromagnetic coil assemblies produced utilizing the above-described method will now be described in conjunction with
Opposite core 88, rod 86 is fixedly coupled to a translating component, such as a piston valve element (not shown), and translates therewith relative to stationary housing 82. As rod 86 translates in this manner, magnetically permeable core 88 slides axially within bore 84 (indicated in
The following testing examples are set-forth to further illustrate non-limiting embodiments of the high temperature electromagnetic coil assembly and methods for the fabrication thereof. The following testing examples are provided for illustrative purposes only and are not intended as an undue limitation on the broad scope of the invention, as set-forth in the appended claims.
A support structure was etched and anodized to create an electrically insulating layer. Utilizing a rotating apparatus, the anodized support structure was then rotated slowly while a thin layer of a water-based cement was applied via a brush. The cement was allowed to air dry. Utilizing a wire winding machine, anodized aluminum wire was wound around the support structure. The water-based cement was continuously applied via the brush just ahead of the location where the wire was laid down. Ample cement was applied to ensure filling of the spaces between winding layers and adjacent wires. The entire structure was then subjected to the cement's curing cycle up to the expected operating temperature of the final device. Anodized aluminum wire from OXINAL® was wound on tubes coated with either wet or dried cement. An overcoat of the cement was also applied.
Three candidate cements were tested for usage as the high thermal expansion ceramic material: (i) a water-based cement bearing product no. “33S” and commercially available from the SAUEREISEN® Cements Company, Inc., headquartered in Pittsburgh, Pa. (“SAUEREISEN®”); (ii) a two-part, non-water based cement bearing product name “Aluseal 2L” and also commercially available from SAUEREISEN®; and (iii) a water-based cement bearing product no. “538N” and commercially available from Aremco™ Products, Inc., headquartered in Valley Cottage, N.Y. Electrical properties (i.e., resistance of the wound wire to detect shorting between windings, resistance between the wire and tube, and the breakdown voltage) were measured for each sample. The samples were also subjected to thermal cycling between −20° C. and 150° C., as well as to room temperatures and elevated temperatures of approximately 400° C. The SAUEREISEN® 33S cement proved to be the best performer, and was thus chosen as the cement to use for further testing. Without being bound by theory, the SAUEREISEN® 33S cement was believed to outperform the other tested cements due, in substantially part, to its relatively high coefficient of thermal expansion (approximately 17 ppm per ° C.).
After the optimum cement was chosen for the application, the cement and wire were combined with a bobbin to make a solenoid. Although the bobbin has two halves for redundancy, only one side was used for the initial trial. The bobbin support structure and walls were coated with a glass and fired. The anodized aluminum wire was then wrapped around the support structure, with cement being continuously applied, until the winding diameter had reached the top of the bobbin walls or a pre-set number of layer/windings was achieved. The structure was then cured. The structure was placed in an air furnace, electrical connections made to the two ends of the wound wire, and a thermocouple inserted into the support structure of the bobbin. A constant current of 0.3 A was applied, first at room temperature, and then the furnace temperature was increased to 320° C. The resultant voltage and bobbin support structure temperature were recorded. Testing demonstrated that thermal and electrical stability was achieved relatively quickly. Thermal and electrical stability remained constant during continuous thermal and electrical exposure of approximately 3000 hours. While the ambient temperature was 350° C., the bobbin temperature was approximately 358° C. due to the power produced from the applied current.
Further testing was performed utilizing a second dual support structure bobbin having two identically-wound halves. The bobbin was electrically connected inside a furnace in the same manner as the single bobbin sample, with each half having its own current supply and support structure thermocouple. As both support structures of the dual support structure bobbin were simultaneously energized, the power output and bobbin temperature was expectedly higher. In particular, the bobbin temperature of each half was recorded at approximately 410° C. When energizing only one side of the dual support structure bobbin over a given period of time, the required operating conditions for the tested device were approximately 320° C. and 0.2 A. As was the case previously, stability was reached rather quickly, and both halves have shown excellent stability and similarity over the duration of a relatively prolonged trial period (approximately 3000 hours).
The foregoing has thus provided embodiments of electromagnetic coil assemblies suitable for usage within high temperature coiled-wire devices of the type utilized within avionic applications and other high temperature applications. As noted above, such high temperature coiled-wire devices include, but are not limited to, solenoids, linear variable differential transformers, and three wire position sensors. Notably, embodiments of the above-described high temperature electromagnetic coil assembly are capable of reliable and continual operation when subjected to temperatures in excess of 260° C. Furthermore, due in substantial part to the usage of anodized aluminum wire, embodiments of the above-described high temperature electromagnetic coil assembly are relatively inexpensive to produce, compact, and lightweight. The foregoing has also described several exemplary embodiments of a method for fabricating such a high temperature electromagnetic coil assembly.
In general, the above-described embodiments of the high temperature electromagnetic coil assembly fabrication method include the steps of: (i) coating an anodized aluminum wire with a high thermal expansion ceramic material, (ii) coiling the coated anodized aluminum wire around a support structure, and (iii) curing the high thermal expansion ceramic coating after coiling to produce an electrically insulative, high thermal expansion ceramic body in which the coiled anodized aluminum wire is embedded. In preferred embodiments, the step of coating is carried-out utilizing a wet winding process wherein the anodized aluminum wire is wound around a support structure while the wire is covered with a wet-state or viscous coating (commonly referred to as a “green state” coating), which contains or is comprised of the high thermal expansion ceramic material. The wet winding process does not necessarily entail application of the wet-state, high thermal expansion ceramic material to the anodized aluminum wire during the winding process. However, in still more preferred embodiments, the step of coating is carried-out utilizing a wet winding process wherein the anodized aluminum wire is wound around a support structure while the high thermal ceramic material is simultaneously or concurrently applied to the wire as a, for example, a pre-cure, wet-state cement or a low melt glass particles carried by a paste, slurry, or other such solution, which can be conveniently applied to the wire by brushing, spraying, or similar technique, as previously described.
The foregoing has also disclosed a method for fabricating a high temperature electromagnetic coil assembly that includes the steps of: (i) applying a wet-state, high thermal expansion ceramic material over a coiled anodized aluminum wire; and (ii) curing the wet-state, high thermal expansion ceramic material to produce an electrically-insulative, high thermal expansion ceramic body in which the coiled anodized aluminum wire is embedded. The wet-state, high thermal expansion ceramic material is selected to produced, when cured, an electrically-insulative, high thermal expansion ceramic body having a coefficient of thermal expansion substantially matched to the coefficient of thermal expansion of the coiled anodized aluminum wire. As utilized herein, the phrase “substantially matched” denotes that a first coefficient of thermal expansion (e.g., the coefficient of thermal expansion of the ceramic body) differs from a second coefficient of thermal expansion (e.g., the coefficient of thermal expansion of the anodized aluminum wire) by no more than 7 ppm per ° C. Advantageously, by forming the high thermal expansion ceramic body from a material having a coefficient of thermal expansion substantially matched to that of the anodized aluminum wire, thermal mismatch between the ceramic body and the anodized aluminum wire is minimized resulting in a significant reduction in the mechanical stress exerted on the ceramic body and the wire through thermal cycling of the high temperature electromagnetic coil assembly.
While multiple exemplary embodiments have been presented in the foregoing Detailed Description, it should be appreciated that a vast number of variations exist. It should also be appreciated that the exemplary embodiment or exemplary embodiments are only examples, and are not intended to limit the scope, applicability, or configuration of the invention in any way. Rather, the foregoing Detailed Description will provide those skilled in the art with a convenient road map for implementing an exemplary embodiment of the invention. It being understood that various changes may be made in the function and arrangement of elements described in an exemplary embodiment without departing from the scope of the invention as set-forth in the appended Claims.
