Patent Application
20190063794
This invention relates to a suspension device utilized in low temperature environments, and more particularly, one for use with an adiabatic demagnetization refrigerator (ADR).
Multi stage adiabatic demagnetization refrigerators (ADRs) are needed to provide detectors with a low operating temperature (generally less than 100 mK). In order to accomplish this, the salt pills used in an ADR need to be thermally isolated from the warmer surrounding components. This thermal isolation is generally accomplished by suspending the salt pills or ADR stages with supports made from materials with low thermal conductivity, such as T300 or Kevlar®. On Astro-H/SXS, a 3-stage ADR was used to provide the 48.5 mK operating temperature to the detector army. The lateral suspension device (LSD) was developed to provide the lateral structure support to each ADR stage salt pill while providing the salt pills with the appropriate thermal isolation from the warmer components of the ADR. Prior implementations over-constrained the salt pill, using more than the minimum number of Kevlar® supports, resulting in higher conducted heat loads than is possible with this LSD.
An LSD can be used to stabilize at least one end of the salt pill against lateral loads and to connect the salt pill to the magnets or other housing components utilized in an ADR apparatus. The LSD provides the end of the salt pill to which it is attached with stability against lateral loads as well as good thermal isolation of the salt pill with respect to the ADR so as to minimize parasitic heating.
In one embodiment, a suspension device for use with a refrigerator is provided. The suspension device includes a ring-shaped support base, at least three tension assemblies, each comprising pulleys and tensioning elements, further comprising a plurality of bands of material that is entrained about the pulleys and held in a taut state by the tensioning elements, wherein each tension assembly is urged by its tensioning element in a generally radially outward direction with respect to the support base.
In another embodiment, an apparatus for signal detection is provided. The apparatus for signal detection includes an adiabatic demagnetization refrigerator, including a quantity of salt that is contained within a salt housing having first and second ends, the first end being a high temperature side and the second end being a low temperature side; a laterally constraining suspension connecting the high temperature side of the housing to the refrigerator to stabilize that side of the salt housing against lateral movements of the salt housing with respect to the refrigerator, wherein the salt housing is centrally disposed in the laterally constraining suspension and connected thereto via a plurality of bands made of a material having a low coefficient of thermal conductivity and high strength; a gimbal suspension securing the low temperature side of the salt housing against axial movement; and a detector in thermal communication with the low temperature side of the salt housing.
In yet another embodiment, an apparatus for signal detection is provided. The apparatus for signal detection includes an adiabatic demagnetization refrigerator, including a quantity of salt that is contained within a housing having first and second ends connected to the salt in such a way as to limit the thermal gradient between the salt and the housing; a laterally constraining suspension connecting one of die first or second ends of the housing to a surrounding magnet to stabilize that end of the housing against lateral movements of the housing with respect to the magnet, wherein the housing is centrally disposed in the laterally constraining suspension and connected thereto via a plurality of bands made of a material having a low coefficient of thermal conductivity and high strength; a gimbal suspension securing the other of the first or second ends of the housing not connected to the magnet against axial and lateral movement and a detector in thermal communication with the housing.
These and other features, aspects, and advantages of the present invention will become better understood when the following detailed description is read with reference to the accompanying drawings in which like characters represent like parts throughout the drawings, wherein:
One embodiment is an LSD that utilizes a plurality of bands of an aramid polymer rope or yarn to provide lateral support to adiabatic demagnetization refrigerator (ADR) salt pills. The aramid polymer rope or yarn may be Kevlar® rope or yarn (including, for example Kevlar® 49 195 Denier or Kevlar® 29 195 Denier where Denier is defined to be the mass in grams for 9000 meters of fiber. Herein, it represents the number of individual fibers in a yarn, which specifies both the strength of the yarn and the thermal conductance at a given temperature) and may be one loop (e.g., a triangular loop) of a plurality of hands (including, for example, 4, 8, 16) to provide the lateral support to an ADR salt pill. An LSD is able to provide foe appropriate lateral support to the salt pills while enabling the ADR to minimize thermal heal loads because of the aramid polymer rope's or yarn's (for example, Kevlar®'s) low thermal conductivity. As a result, the LSD is able to provide the appropriate lateral support to the salt pills while enabling the ADR to achieve optimal thermal performance.
The LSD may include a base ring for attachment to the ADR stage magnet mandrel, where a plurality of tensioner arms (for example, at least 3) attach. Each of the tensioner arms may have a pulley fastened on top that is used to round each aimer of the loop of the plurality of bands of an aramid polymer rope or yarn, for example, 8 bands of Kevlar® (called an LSD Kevlar® loop). For example, where there are 3 tensioner arms, each of the tensioner arms may have a pulley fastened on top that is used to round each comer of the triangular loop of the plurality of bands of an aramid polymer rope or yarn, for example, 8 bands of Kevlar® (called the LSD Kevlar® loop). The tensioner arms may be each fastened to a base at their rotation points and a lateral constraint point with a stack of Bellville washers and screw that is properly torqued to provide the appropriate preload (for example, 25-30 lbs) to die plurality of bands of an aramid polymer rope or yarn, for example the LSD Kevlar® loop. A central clamp assembly may be used that grips the salt pill and may be epoxied onto each of the sides of the loop, for example, the three sides of an LSD Kevlar® loop triangle via tapped holes to secure the epoxy. This allows the lateral load of the salt pill to be transferred to the LSD Kevlar® Loop. The central clamp may be split for ease of attachment to the salt pill, with the clamping force to the salt pill acting to prevent rotation (i.e. providing control of the rotational degree of freedom).
To assemble an embodiment, first, a salt pill is placed within a holder. The holder is attached to the plurality of bands of an aramid polymer rope or yarn, which are held in a taught configuration by a series of three spring loaded tension assemblies. The aramid polymer rope or yarn may be made of a material having a low coefficient of thermal conductivity and high strength. An aramid fiber sold under the trademark Kevlar® may be suitable for this purpose, for example, Kevlar® 49 195 Denier or Kevlar® 29 195 Denier. The tension assemblies may be arrayed equally spaced about a ring-shaped base, and may include a generally T-shaped tension arm spring loaded to provide a tensioning force in a generally outwardly radial direction and a pulley about which the plurality of bands of rope or yarn is entrained. Should foe plurality of bands of rope or yarn slacken during foe cooling process, the spring force provided within the tension assemblies directs the pulley in a generally outwardly direction, thereby taking up any slack, or a substantial portion thereof, and maintaining the tension and stiffness of the system.
In this embodiment, an outer ring support base 110 serves as a mount for three equally spaced apart (e.g., about 120° apart) and identical spring-loaded tension assemblies 130, which collectively entrain a plurality of bands (including, for example, 8 bands formed from a loop) of an aramid polymer rope or yarn (including, for example Kevlar® 49 195 Denier or Kevlar® 29 195 Denier) 102 that is made of a strong material having a low coefficient of thermal conductivity. It has been found that rope or yarn constructed of aramid polymer fiber sold under the trademark Kevlar® may possess the suitable thermal and other mechanical properties suitable for this purpose. However, other fibers having similar properties of low hear conductivity and high strength may also be used.
The Kevlar bands may be mechanically loaded (often by having them suspend a weight) prior to assembly to pre-stretch them and minimize the need for re-tensioning after assembly. During assembly, the Kevlar fiber bundles may be wetted with isopropyl or other suitable alcohol to allow the fibers to slip past each other (especially when the loop consists of a single larger loop that is folded over on itself (below)) as the loop assumes foe triangular shape under tension.
The plurality of bands may be formed by taking a single strand of an aramid polymer rope or yarn (including, for example Kevlar® 49 195 Denier or Kevlar® 29 195 Denier) and joining or tying the ends together (e.g., using a knot or other acceptable form of attaching foe ends together) to form a loop. The plurality of bands can be formed by a multiple of configurations including, for example, 8 bands formal from a single loop, 2 loops used together with each loop forming 4 bands or 8 single loops. Where a single loop is used to form multiple bands therefrom, once the loop is formed, the loop is twisted and the opposite ends of the twisted loop are folded and brought together to result in twice the number of bands than were present before the loop was twisted and folded (for example, a single strand loop twisted and folded results in a 2 band loop, a 2 band loop twisted and folded results in a 4 band loop, a 4 band loop twisted and folded results in a 8 band loop).
The initial loop before the twist and fold process described above are sized so that the final loops size provides the desired amount of appropriate preload (for example 25-30 lbs) in cooperation with the tensioner arms to which the final loops are fastened when both are connected to the base.
The number of bands (for example, 8) of the aramid polymer lope or yarn and the appropriate preload (for example, about 25 - about 30 lbs, preferably about 30 lbs) may vary depending on the amount of strength, lateral stability and thermal conductivity desired between the salt pill and the magnet and other elements of the ADR apparatus. The number of bands of the aramid polymer rope or yarn should be at least 2, and can include an integer number 2 or greater that meets both the thermal and mechanical requirement of the LSD device up to the maximum allowed by the size of the pulleys providing tension (the fillers are preferably contained within the pulley to prevent excess strain in any one fiber which can cause breakage; another practical limit is that with too many fiber bundles, the fibers may not slip past each other and distribute the tension equally, leading to breakage). The amount of preload of the bands of the aramid polymer rope or yarn should be that needed to prevent any portion of the bands from going slack under mechanical loads, as this can cause high mechanical shock loads, and less than about half the breaking strength of the band (Kevlar is unique in that it may tend to creep over time and will eventually break if the load exceeds about half the breaking strength, which is defined as the load which will cause immediate breakage).
The ring support base 110 may be made of a metal such as aluminum, magnesium, or other metal or metal alloy (typically non-ferromagnetic). Where the apparatus is to be launched into space and weight is at a premium, light weight metals and alloys may be preferred. The material chosen should not be a material that undergoes a transition to the superconducting state at its intended operating temperature, as this will lead to undesirable interactions with foe magnetic field used to drive the magnetic cooling cycle.
In the embodiment of
During cooling, die plurality of bands aramid polymer rope or yarn 102 may become lax due to radial forces between the magnetic material in the salt pill and the magnetic field produced by the electromagnet that will tend to pull the salt pill off axis. Compensation for this laxity may be provided by foe three spring-loaded tension assemblies 130, which provide for a generally outwardly directed force that helps keep foe plurality of bands aramid polymer rope or yarn 102 taut. In a preferred embodiment, three tension assemblies are employed, as this provides the optimal degree of mechanical stability (three points suffice to determine a plane). However, in some settings it may be that more than three tension assemblies may be mechanically optimal.
In this embodiment, each tension assembly 130 may include a generally T-shaped tension arm 132 (see
Bracket 162 may be attached to tension arm 132 via threaded bolts 164, which pass through bracket through holes 165 to screw into foe tapped holes 144 located in foe central portion 134 of foe tension arm 132. Bracket 162 also axially bounds a pulley 170 having a lower protruding portion 172 and an upper protruding portion 174. The upper protruding portion 174 may fit into a through hole 169 in bracket 162, and foe lower protruding portion 172 may fit into the tapped hole 136 of the tension aim 132. Washers 167 and 168 may also be provided as shown in
At the right portion 135 of tension arm 132, a Bellville washer spring assembly may be provided. A threaded bolt 182 may pass through a fitting 184 into tapped hole 142, and provides a stem onto which are placed the Bellville washers 188. Alternatively, other mechanical elements providing a compressive force, such as a compression spring, may be used in place of a Bellville washer. Therefore, the spring, which can be adjusted by adding nestable Bellville washers to one another and/or by adjustments to the bolt 382, generates a compression force that directs the tension assembly 130 to pivot generally radially outwardly about the cylindrical stem 122, and thereby take up any slack, or a substantial portion thereof, that should develop in the plurality of bands aramid polymer rope or yarn during cooling.
The sail pill 7 can be held within a split ring salt pill collar 104 (
When the suspension device is assembled, it is pretensioned (for example, 25-30 lbs, preferably 30 lbs) as noted above to provide a level of spring force via the Bellville washer assembly sufficient to keep the plurality of bands aramid polymer rope or yarn 102 taut during its subsequent cool-down and use within the ADR.
In another embodiment,
A pair of compression springs may be employed so as to bias the tension assembly 230 in a generally radially outwardly direction. For example, an assembly of compression spring 280, washer and threaded bolt 282 may be provided at each side of tension arm 232 to bias the tension arm in a radially outward direction in order lo remove the slack, or a substantial portion thereof, in the plurality of bands aramid polymer rope or yarn 204. The level of bias can be adjusted by adjustments to the bolts 282 or to the stiffness of the springs employed.
A split ring salt pill collar similar to the one depicted in
Each embodiment provides an LSD that is a mechanically stable and thermally isolated mounting for a salt pill in a low temperature apparatus such as in an ADR and allows for the salt pill to be maintained in a correct position with respect to the ADR, even when the device is subjected to temperatures within a few degrees of absolute zero.
Kevlar® 49 is an organic fiber unique in its high tensile strength, tensile modulus, toughness and thermal stability. Kevlar® 49 also has low-thermal conductivity at low temperatures (0.04 W/m·K). Kevlar® 49 that has been tested is 195 Denier, which includes 134 filaments each having a diameter equal to 0.00047 inch). Kevlar® 29 is a similar organic fiber with a different tensile strength, tensile modulus, toughness and thermal stability. Kevlar® 29 also has low thermal conductivity at low temperatures and in some embodiments may be a preferred substitute for Kevlar® 49.
The Kevlar® 49 195 Denier material properties at room temperature are summarized in Table 1 (material properties for 195 Denier are not provided by DuPont, but should scale appropriately by number of filaments).
Loop specimens of Kevlar® 49 195 Denier have been tensile tested at room temperature (293 K) and cryogenic temperature (77 K). The specimens tested were Kevlar® loops (8 bands of Kevlar® 49 195 Denier per loop). Isopropyl alcohol (IPA) was applied to the Kevlar® loops before they were set up for tensile testing. This application allows the numerous fibers that make up each strand of Kevlar® to reduce fiber-to-fiber friction, allowing the fibers to slide past each other as tension is applied and take load equally. Once the specimens were set up, they were preloaded to 2 lbs. At this point, IPA was applied again to equally distribute the load. Once the load dropped to around 1.6-1.7 lbs, the specimens were preloaded to 25 lbs (the initial preload of the Kevlar® loops in the ADR model). IPA was applied again. These loops were designed with a center diameter of 0.2 inches, which matched the ADR pulleys that hold the Kevlar® loops. Furthermore, these loops were made with the same material as the pulleys (6061-T6 aluminum) and were polished to prevent any failure of the loops specimens due to a rough surface damaging the fibers. For the room temperature tests, the load relaxed to about 24.4-24.6 lbs and the specimens were pulled to failure thereafter. For the 77 K tests, the load was relaxed to a similar load, and liquid nitrogen was poured in a surrounding Dewar. The shrinking of the stainless steel parts pulling apart the Kevlar® caused the load to increase to around 40 lbs each time during the cool down. At this point, the Kevlar® specimens were pulled to failure. For every Kevlar® test, the strain rate was 0.05 in./in./min. (ASTM Standard E8 approved).
Tables 2 and 3 illustrate the mechanical properties of Kevlar® 49 195 Denier loop specimens at room temperature and at 77 K. Loops of Kevlar® 49 195 Denier that are knotted have a higher tensile modulus, yield strength, break strength, and ultimate tensile strength at 77 K than at room temperature. At 77 K these loops had slightly more elongation. Overall, it is concluded that the elongation does not change much with cold temperature.
This written description uses examples as part of the disclosure, including the best mode, and also to enable any person skilled in the art to practice the disclosed implementations, 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.
