GB2301662A - Joule-Thomson cooling arrangement for electronic components - Google Patents

Description

1 2301662 Sensor Assembly with Joule-Thomson Cooler and Electronic

Components The invention relates to a sensor assembly with a JouleThomson cooler and electronic components.

Such sensors are, in particular, infrared-sensitive sensors for seekers which track an infrared emitting target. Such seekers are used in seeker heads of target-tracking missiles.

EP-A-0,604,790 describes a sensor assembly with a cooled sensor. The sensor is arranged within the vacuum chamber of a Dewar vessel. The Dewar vessel has inner and outer housing portions. Both housing portions are pot-shaped. The outer housing portion extends around the inner housing portion Both housing portions limit the vacuum chamber of the Dewar vessel. The "bottom" of the inner housing portion carries the sensor on its vacuum-side outer surface.

The inner housing portion encloses a Joule-Thomson cooler in its inner chamber. Such a Joule-Thomson cooler comprises a heat exchanger in the form of a helical forward-flow conduit and an expansion nozzle at the end of the forwardflow conduit near the "bottom" of the Dewar vessel. Cooling gas under pressure is supplied through the forward-flow conduit and emerges from the expansion nozzle. Thereby, the cocrling gas is expanded and cooled down. Then the expanded and cooled-down cooling gas f lows towards the open end of it cools the forward flowing, a counter-flow mode. The thus further cooled down, when it the Dewar vessel. Thereby, pressurized cooling gas in pre-cooled cooling gas is 2 emerges from the nozzle. Eventually, this permits very low temperatures to be reached. The cooling gas, in this process, cools the sensor correspondingly. After the cooling gas has passed through the heat exchanger, it emerges from the "warm" open end of the Dewar vessel.

In the EP-A-0,604,790, the Dewar vessel has a flange-like socket. The socket consists of a first socket portion integral with the outer housing portion and a second socket portion integral with the inner housing portion. A annular disc-shaped printed circuit board is sealingly held between the first and second socket portions and limits a vacuum chamber. Electronic components including integrated circuits are mounted on the printed circuit board. Arranging electronic components directly at the "warm" end of the Dewar vessel permits near-sensor signal processing of the sensor signals and reduces susceptibility to electromagnetic interferences.

Such electronic components, however, develop heat. In conventional seeker heads, this heat is difficult to dissipate. Heat dissipation would have to be effected through the ball bearings of gimbals in which the infrared detector of a seeker is usually mounted in the missile, this dissipation involving long path length for the heat to be dissipated and poor heat transmission. In addition, the seeker itself becomes hot, in operation, whereby no heat dissipation by radiation takes place. This can have various disadvantageous consequences.

The' reliability if the components is reduced. There is the risk of overheating of components. This may result in system failure of the seeker.

3 Furthermore, the development of heat at the entrance of the Joule-Thomson cooler adversely affects the operation of the Joule-Thomson cooler. The gas flow which is required for cooling the sensor down to a predetermined temperature is thereby increased. This limits the flight time, if the pressurized cooling gas is provided by a bottle of pressurized gas. In an extreme case, the temperaturedependent power limit of the Joule-Thomson cooler can be exceeded. Also this results in deterioration of the cooling power an, possibly, in system failure.

It is the object of the invention, to improve the heat dissipation from electronic components with cooled sensors of the type mentioned in the beginning.

According to the invention this object is achieved by a sensor assembly comprising a sensor cooled by a JouleThomson cooler, from which expanded cooling gas emerges, electronic components associated with the sensor and component heat exchanger means in heat-conductive contact with said electronic components, said component heat exchanger means being arranged to be flown-through by said expanded cooling gas.

The invention is based on the discovery, that the expanded cooling gas flowing back after passage through the heat exchanger of the JouleThomson cooler and pre-cooling of the forward-flowing, pressurized cooling gas is still cool enough to absorb and dissipate a noticeable quantity of heat from the electronic components, if the heat exchanger means provided at the electronic components are designed appropriately. The the expanded, "waste" cooling gas, which otherwise would emerge unutilized into atmosphere, is still used for cooling the electronic components. In this connection, it is advantageous, that the quantity of 4 cooling gas required for the cooling of the sensor becomes the larger the higher the temperature of the missile is. Then a correspondingly larger quantity of "waste" cooling gas is available for the cooling of the electronic components.

Modifications of the invention are subject matter of the dependent claims.

An embodiment of the invention is described in greater detail with reference to the accompanying drawings.

Fig. 1 Fig. 2 is a sectional view of a sensor in the form of an IR-detector with a Joule-Thomson cooler, a Dewar vessel with flange-like socket and cooled electronic components.

is a schematic-perspective illustration and shows the construction of a heat exchanger for cooling electronic components.

Referring to Fig.1, numeral 10 designates a pot-shaped Dewar vessel. The Dewar vessel 10 has an outer portion 12 and an inner portion 14. The outer portion 12 extends, at a distance, around the inner portion 14. A vacuum chamber 16 is def ined therebetween. A sensor 18 in the form of an IRdetector is mounted within the vacuum chamber 16 on the vacuum-side surface of a "bottom" 20 of the pot-shaped inner portion 14. Furthermore, a cold stop 22 is attached to the bottom 20. The cold stop 22 is cooled together with thiY sensor 18 and shields the sensor 18 from IR-radiation which would impinge directly upon the sensor 18 from the environment, for example from hot wall portions of the missile. The outer portion 12 forms a window 24 in the region of the "bottom".

A bell-shaped rim portion 26 of the outer portion 12 is sealingly retained in an aperture 28 of a mounting flange 30. An annular disc 34 engages a shoulder 32 of the aperture 28. On its inner side, the annular disc 34 is sealingly attached to the cylindrical, peripheral surface of the inner portion 14. The annular disc 34 forms the lower limit of the vacuum chamber, as viewed in Fig.l.

A Joule-Thomson cooler 14 is located in the inner portion and thus in the Dewar vessel. The Joule-Thomson cooler 36 comprises a heat exchanger 38 with a helical tube 40 for the supply of pressurized cooling gas. The cooling gas is supplied to the tube 40 through a cooling gas conduit 42. The cooling gas emerges from the end of the tube 40 through an eypansion nozzle A4 directly opposite the bottom 20. During the expansion, the cooling gas is cooled down. The cooling gas may, for example, be air, nitrogen or argon. Then the expanded and cooled-down cooling gas flows, in counter-flow mode, from the top to the bottom in Fig.1 through the heat exchanger 38 opposite to the supplied pressurized cooling gas, which flows from the bottom to the top in Fig.l. Thereby the pressurized cooling gas is precooled. After the pre-cooled cooling gas has been expanded, this will, in turn, adopt a still lower temperature. This is continued, whereby very low temperatures can be reached an, for example, air can be liquefied. Thereby, also the bottom 20 with the sensor 18 and the cold stop 22 are cooled down correspondingly.

A printed circuit board 46 with electronic components 48 and 50 is mounted on the mounting f lange 30 at the "warm enC of the sensor assembly, the electronic components, in operation, developping dissipation heat. The printed circuit board is covered by a cap 52 attached to the 6 mounting flange 30. The heat from the electronic components 48 and 50 is difficult to dissipate, as has been described hereinbefore. This may cause the disadvantageous effects also described.

For this reason, the electronic components 48 and 50 are mounted on heat exchangers 54 and 56, respectively. The heat exchangers 54 and 56 are flown-through by "waste" cooling gas which emerges from the Joule-Thomson cooler 36 at the warm end and, in accordance with the prior art, would emerge into atmosphere. Here, this waste cooling gas is collected in a chamber 58 and is directed into the heat exchangers 54 and 56 through conduits 60 and 62, respectively.

The component heat exchanger means comprise miniaturized component heat exchangers 54,56 of well heat conducting material which include narrow cooling passages 76 made by micro-system technique. The heat exchangers 54 and 56 are constructed as illustrated in Fig.2.

Each heat exchanger 54 or 56 consists of a plurality of thin silicon plates 64, 66, 68, 70, 72 and 74. A multitude of parallel channels 76 are machined in the surfaces of each of the silicon plates 64, 66, 68, 70, 72 and 74. As can be seen from Fig.2, the silicon plates are stacked and interconnected in such a way, that the grooves in each of the silicon plates 64, 66, 68, 70, 72 and 74 extend crosswise to the grooves of the respective adjacent silicon plates. The grooves 76 in the silicon plates 64, 68 and 72 extend from right to left in Fig.2. The grooves in the silicon plates 66, 70 and 74 placed therebetween extend from the front to the rear. When the silicon plates 64, 66, 68, 70, 72 and 74. are interconnected, a system of mutually crossed passages for generating a crossed flow is 7 generated. The plates 64 to 74. may also be arranged such that they form a counter-flow heat exchanger.

The cooling gas leaves the heat exchanger of the JouleThomson cooler with a temperature which is slightly, by 8 to 10C, lower than the ambiant temperature. As far as the thermal characteristics are concerned, the usual cooling gases, air, nitrogen or argon, are not very good as coolant for a heat exchanger. They have only low density, limited heat capacity and poor thermal conductivity. These poor characteristics are compensated by skillful design of the heat exchanger.

To this end, the heat exchanger 54 or 56 has to have small and compact dimensions, matching with the respective electronic component 48 or 50, respectively, to be cooled. 'The le-at exchErger surface 1-.--s to be large E.s compared to the heat exchanger volume. The material of the heat exchanger 54 or 56 has to have high heat conductivity. Thereby, there will be only a small temperature difference between the electronic component 48 or 50 and the heat exchanger 54 or 56, respectively. The pressure drop of the cooling gas when flowing through the heat exchanger has to be low. There must be a small heat transmission coefficient between the surface of the heat exchanger 54 or 56 and the cooling gas. The large surface of the passages flown through yields good heat transmission.

The dimensions of the bases of the heat exchangers 54 and 56 are substantially identical with those of the electronic components 48 and 50, respectively, to be cooled. In the embodiment shown in Fig.2, the heat exchangers 54 and 56 have a basis of 12 mm x 25 mm, and a total thickness of 2 mm. The individual silicon plates 66, 68, 70, 72 and 74 can be interconnected by welding or cementing.

8 About sixteen layers of silicon plates are provided. About one hundred grooves 76 or channels are formed in each silicon plate. The ribs between the grooves 76 have a width of.025 millimeters. The total length of all passages is fourty meters. The heat exchanger surface amounts to a total of 14,000 mm'. The heat exchanger volume is Boo MM3. Thus the ratio of heat exchanger surface to heat exchanger volume becomes 17.5 mm-1. By optionally connecting the passages in parallel or in series, the hydraulic diameter of the heat exchanger may be adapted optimally to the prevailing conditions.

The cooling gas is highly pure, as it is already supplied in highly pure form for the preceding cooling process.

Thereby, the passages can be made very narrow without having to be afraid of clogging of the passages. At higher temperatures, higher cooling power at the heat exchangers is required. At higher temperatures, however, also the cooling gas flow is higher which is demanded by the Joule Thomson cooler. With a Joule-Thomson cooler having a cooling power of about 400 mW and air of 300 to 100 bar as cooling gas at an ambient temperature of 70'C, the gas flow required for the Joule-Thomson cooler is so large, that a heat flow of about 350 to 650 mW can be dissipated thereby from the endangered electronic components. Then, the temperature of the electronic components is maximally 20'C above ambient temperature.

The heat exchangers described permit relatively large heat transmission numbers of (x = 1.0 kW/m2K to be reached with relatively small pressure drops of 100 to 250 mbar.

9

Claims (10)

1. Claims

   A sensor assembly comprising a sensor (18) cooled by a Joule-Thomson cooler (36), from which expanded cooling gas emerges, electronic components (48,50) associated with the sensor (18) and component heat exchanger means (54,56) in heat-conductive contact with said electronic components (48,50), said component heat exchanger means (54,56) being arranged to be flown-through by said expanded cooling gas.
2. A sensor assembly as claimed in claim 1, characterized in that (a) the Joule-Thomson cooler (36) is arranged in a potshaped Dewar vessel (10), the Dewar being closed at one end by an end portion (20), the JouleThomson cooler being arranged to cool said end portion by expanding cooling gas, the expanded cooling gas being guided to flow in counterflow mode through a pre-cooling heat exchanger (38) for pre-cooling the cooling gas supplied under pressure and to an open end of the pot-shaped Dewar vessel (10), (b) the sensor (18) to be cooled is mounted on said end portion (20) of the pot-shaped Dewar vessel (10), (c) a socket is formed at the open end of the Dewar vessel (10), said electronic components (48,50) being mounted on said socket, and (d) means are provided for collecting the cooling gas emerging from said pre-cooling heat exchanger an directing this cooling gas through said component heat exchanger means (54,56).
3. 3.

   A sensor assembly as claimed in claim 1 or 2, characterized in that the component heat exchanger means comprise miniaturized component heat exchangers (54,56) of well heat conducting material which include narrow cooling passages (76) made by micro-system technique.
4. A sensor assembly as claimed in claim 1, characterized in that the component heat exchangers (54,56) consist of silicon.
5. 5. A sensor assembly as claimed in claim 3 or 4, characterized in that the component heat exchangers are blocks the dimensions of which substantially match those of a respective associated electronic component (48,50).
6. 6. A sensor assembly as claimed in anyone of the claims 3 to 5, characterized in that each of the the component heat exchangers is composed of stacked plates (64,66,68,70,72,74), each of said plates being provided with parallel grooves (76).
7. 7. A sensor assembly as claimed in claim 6, characterized in that said plates (64,66,68,70,72,74) are stacked in such a way that the grooves of adjacent plates extend crosswise to each other.
8. 8. A sensor assembly as claimed in anyone of the claims 3 to 7, characterized in that each of the component heat exchangers (54,56) is mounted between an associated electronic component (48,50) and a base plate (46).
9. A sensor assembly as claimed in anyone of the claims 3 to 7, characterized in that each of the component heat exchangers is mounted on top of an associated electronic component.
10. 10. A sensor substantially as hereinbefore described with reference to and as shown in the accompanying drawings.