EP0675329A1 - Process for thermal regulation of a miniature Joule-Thomson cooler - Google Patents

Abstract

The mini cooler comprises a piezo electric ceramic section (10) which controls a needle valve (5) for the refrigerant fluid. A variable voltage, pseudo periodic, is applied to the piezo electric device, to activate it. The voltage may be modulated in the form of pulses, which may be frequency modulated at constant amplitude, or constant frequency at variable amplitude. Alternatively, a combination of these pulse forms may be used. The piezo electric section may be held between two rigid plates (8,9), against a regulating screw (12), in an enclosure (7). A control rod (6), supported in a tube (2), may move under the piezo electric action, to operate the needle within the valve nozzle (3) orifice (4). <IMAGE>

Description

The present invention relates to a thermal regulation method of a Joule-Thomson type mini-cooler, used in an optronics detector cryostat.

It is known that optronic sensors use, for the most part, detectors which are charge transfer circuits, or CCDs, on which an image focused by an optical objective is formed. These circuits operate at ambient temperature in the visible range, but in order to have the greatest possible sensitivity to radiation, certain detectors such as infrared detectors must operate at a very low ambient temperature. To obtain this result, the detectors are generally mounted in cryostatic devices of the type comprising a finger worn at very low temperature on which are mounted the detector or detectors to be cooled and which is surrounded by a wall forming with said finger a sealed enclosure. The wall is provided on the part facing the detectors with a window made of a material transparent to the radiation to be detected.

The regulation systems of mini-chillers of the Joule-Thomson type used for example for sensors operating at a cryogenic temperature are generally based on the control of a needle, regulating the flow of the refrigerant gas, by a heat-deformable enclosure incorporated in the body of the mini-cooler.

This configuration has the disadvantage of regulating the flow of refrigerant gas not as a function of the temperature of the detector to be cooled but as a function of the temperature of the mini-cooler itself.

This is why we use preferably mini-coolers with flow regulation by piezoelectric ceramic, which can be controlled by a voltage directly linked to the temperature of the CCD sensor.

The disadvantage of regulation by piezoelectric ceramic is the existence of a strong hysteresis, which is easily understood: if the thermal regulation balance is almost reached, the error signal applied to the ceramic is only '' a low DC voltage, insufficient to modify the geometry of the piezoelectric part. There is a threshold phenomenon there, analogous to the threshold voltage of a transistor or to the phenomenon of supercooling of very slowly cooled water. To the hysteresis of the piezoelectric component is added the mechanical hysteresis of the connection between the ceramic and the regulating needle.

The object of the present invention is therefore to propose a regulation method making it possible to eliminate these hysteresis phenomena in mini-coolers with flow regulation by piezoelectric ceramic. To this end, the voltage applied to the piezoelectric ceramic is a pseudo-periodic variable voltage, of variable shape, frequency and amplitude. The regulation system, which is constantly unbalanced and in motion, averages over time and provides more precise regulation than if the setpoint signal is a DC voltage.

More precisely, the invention consists of a method of thermal regulation of a Joule-Thomson mini-cooler, comprising a piezoelectric ceramic which controls an expansion needle of a gripping fluid, this method being characterized in that 'it consists in applying a pseudo-periodic activation voltage to the piezoelectric ceramic.

• figure 1 : schéma d'un mini-refroidisseur Joule-Thomson, auquel est appliqué le procédé selon l'invention,
• figure 2 : cycle d'hystérésis de base, donnant le débit en fonction d'une tension continue, selon l'art connu,
• figure 3 : premier exemple de forme de tension aux bornes de la céramique piézo-électrique, selon l'invention,
• figure 4 : second exemple de forme de tension selon l'invention, illustrant les variations d'amplitude ou de forme,
• figure 5 : troisième exemple de forme de tension selon l'invention, illustrant les variations de fréquence,
• figure 6 : courbe de débit en fonction de la durée des impulsions d'ouverture, selon le procédé de l'invention.

The invention will be better understood from the following description of an exemplary embodiment, supported by the appended figures, which represent:

• FIG. 1: diagram of a Joule-Thomson mini-cooler, to which the method according to the invention is applied,
• FIG. 2: basic hysteresis cycle, giving the flow as a function of a DC voltage, according to known art,
• FIG. 3: first example of the form of voltage across the terminals of the piezoelectric ceramic, according to the invention,
• FIG. 4: second example of a form of tension according to the invention, illustrating the variations in amplitude or in shape,
• FIG. 5: third example of a voltage form according to the invention, illustrating the frequency variations,
• FIG. 6: flow curve as a function of the duration of the opening pulses, according to the method of the invention.

The preliminary recall of the structure of a mini-cooler will make it possible to better understand the process for regulating the temperature of an optronic sensor, according to the invention.

• un mandrin 1 sur lequel est enroulé un tube 2, muni d'ailettes, et parcouru par le fluide frigogène,
• à une première extrémité du mandrin, par exemple une buse 3, munie d'un orifice calibré 4, plus ou moins bouché par un pointeau 5 déplacé par une tige de commande 6,
• à une deuxième extrémité du mandrin, une embase 7 qui protège un translateur piézo-électrique composé de deux plaques indéformables 8 et 9 et d'une céramique piézo-électrique 10. Par sa déformation, la céramique 10 déplace la plaque 8 solidaire de la tige de commande 6. Une tension de commande est appliquée en 11 sur les deux faces de la céramique piézo-électrique, pour la déformer. Une vis de réglage 12 permet une première approche de régulation recherchée.

FIG. 1 represents a configuration of mini-cooler comprising:

• a mandrel 1 on which is wound a tube 2, provided with fins, and traversed by the refrigerant,
• at a first end of the mandrel, for example a nozzle 3, provided with a calibrated orifice 4, more or less blocked by a needle 5 displaced by a control rod 6,
• at a second end of the mandrel, a base 7 which protects a piezoelectric translator composed of two non-deformable plates 8 and 9 and a piezoelectric ceramic 10. By its deformation, the ceramic 10 displaces the plate 8 secured to the rod control 6. A control voltage is applied at 11 on both sides of the piezoelectric ceramic, to deform it. An adjustment screw 12 allows a first regulation approach sought.

The right part of this figure constitutes what is called a cold finger, which is slid into a tubular cryostat, as a thermowell, which carries the CCD sensor circuit, maintained under vacuum and close to the needle-nozzle system 3-5 which is the coldest point.

This structure uses a control rod 6 connecting a needle 5 to a piezoelectric element 10 placed outside the cold finger which is not in direct thermal contact with the cold vapors of refrigerant gas. The ceramic 10 therefore remains at a temperature close to ambient, hence better piezoelectric characteristics.

Depending on the target flow range and operating pressure, various piezoelectric actuators can be envisaged with direct effect (displacement of a few hundred microns) or multiplied (displacement of the order of mm).

• l'hystéresis mécanique (frottement,etc..) de la liaison entre la céramique et le pointeau;
• l'hystérésis du composant piézo-électrique lui-même.

The regulation of the flow can be done by the application of a more or less high voltage across the terminals of the piezoelectric element, but in practice, this method is not very effective to obtain a fine regulation (typically the infrared detectors red require stability of around 0.1 Kelvin) due to:

• mechanical hysteresis (friction, etc.) of the connection between the ceramic and the needle;
• the hysteresis of the piezoelectric component itself.

FIG. 2 represents the hysteresis of the flow rate of a mini-cooler as a function of the voltage applied across the terminals of the piezoelectric ceramic 10: the applied voltage is a DC voltage, or in slots which do not pass through zero.

This figure proves that the continuous regulation of such a system is practically impossible, because the flow of refrigerant gas can be, for the same applied voltage, from simple to triple depending on whether one is on a branch or the other of the hysteresis curve. Typically, to regulate to one tenth of a degree, a mini-chiller must regulate to tenth of a liter / minute.

The object of the present invention is therefore to propose a regulation system making it possible to alleviate these hysteresis phenomena.

It consists in imposing a pseudo-periodic variable control voltage across the ceramic such as that represented in FIG. 3: voltage slots, with return to a zero voltage between two slots. The form of this tension can be arbitrary; in practice, the usual functions of voltage generators can be used: sine, triangular, square signal, etc. Depending on the form and frequency of this voltage, the mobile assembly consisting of ceramic, the mechanical connection with the needle and the needle itself will move at the same frequency, simultaneously causing variations in the flow of refrigerant gas. If the activation frequency is high enough (a few Hz is sufficient), these variations in flow rate will not cause a significant variation in cold power for the detector due to the different thermal inertias (conduction, thermal mass of the interfaces and the detector), and thermodynamics (pressure drop in the exchanger, quantity of liquefied gas present near the detector, etc.) existing in the detector and the cooler itself.

We thus obtain a regulation system where the hysteresis effects described above no longer have any influence, because their effects are averaged over time: the refrigerator and detector assembly acts as a low-pass filter.

• soit en modifiant la forme de la tension d'activation : par exemple on peut utiliser une régulation à modulation de largeur de pulsation ;
• soit en modifiant la fréquence de la tension d'activation, son amplitude restant constante ;
• soit en modifiant l'amplitude de la tension d'activation, sa fréquence restant constante,
• soit en utilisant toute combinaison des trois méthodes ci-dessus.
  Parmi toutes les combinaisons possibles,
• la figure 4 illustre à la fois une modification d'amplitude et une modification de forme des créneaux d'impulsion,
• la figure 5 illustre une modification de largeur et une modification de fréquence des créneaux d'impulsion.

We can then regulate the flow rate and therefore the cold power of the refrigerator in several ways:

• either by modifying the form of the activation voltage: for example, it is possible to use a pulse width modulation regulation;
• either by modifying the frequency of the activation voltage, its amplitude remaining constant;
• either by modifying the amplitude of the activation voltage, its frequency remaining constant,
• either using any combination of the three methods above.
  Among all the possible combinations,
• FIG. 4 illustrates both a modification of amplitude and a modification of the shape of the pulse slots,
• FIG. 5 illustrates a change in width and a change in frequency of the pulse slots.

By way of example, FIG. 6 is an example of a flow curve, in the case of regulation by variation of the width of the pulse applied to the terminals of the ceramic. We note that we then obtain a system without hysteresis which is easily adjustable.

The invention is specified by the following claims:

Claims (5)

1 - Method of thermal regulation of a Joule-Thomson mini-cooler, comprising a piezoelectric ceramic (10) which controls a needle (5) for expansion of a refrigerant, this method being characterized in that it consists applying to the piezoelectric ceramic (10) a pseudo-periodic variable activation voltage. 2 - Method according to claim 1, characterized in that the pseudo-period consists of a shape modulation of the pulsation of the activation voltage. 3 - Method according to claim 1, characterized in that the pseudo-period consists of a modulation of the frequency of the activation voltage, at constant amplitude. 4 - Method according to claim 1, characterized in that the pseudo-period consists of an amplitude modulation of the activation voltage, at constant frequency. 5 - Method according to claim 1, characterized in that the pseudo-period is obtained by a combination of modulations of shapes, frequencies and amplitudes of the activation voltage.

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