Abstract:
In order to control the passive motion of the pneumatically driven displacer in a cryogenic cooler system of the split cycle type, the refrigerant gas is forced to flow through a narrow passage within the pneumatic pillow in a manner that the gas during that passage is in viscous friction with the wall of the passage.

Description:
FIELD AND BACKGROUND OF INVENTION 
     The present invention relates to cryogenic coolers, and more particularly to the passive movement control of the displacer in coolers of the split cycle type. 
     Cryogenic coolers are employed in a number of fields, e.g. electro-optics, electronics, vacuum techniques etc. Constructions of such coolers comprise two basic constituents: a compression--and an expansion unit. Some of these coolers operate on a closed circuit scheme and the passage of the gas used occurs from compression stage to expansion without use of valves. A typical example is the Stirling cycle and those designed similar to it. 
     There is a well known group of coolers of split build, i.e. the compression--and the expansion stage being strictly distinct and being connected only by gas conducting conduits. In this type of coolers the operation of the expansion unit is by pneumatic, or electric or motor drive. The principle of the function of the expansion unit is based on change of volume of an expansion cell which occurs periodically and under phase difference in relation to pressure pulses acting on the said cell. The creation of a proper phase differential between the volume and pressure pulses is imperative. 
     In pneumatically driven split cycle coolers the change of volume of the expansion space is obtained by means of a pneumatic drive which is actuated by the same source of pressure pulses which acts on the expansion space, it is necessary to provide means ensuring the creation of the necessary phase differential. This differential is in the range of 80°-120°, so as to ensure maximal efficiency. 
     STATE OF ART 
     The means conventionally used in obtaining the said phase differential are: 
     (a) introduction of a source of friction of mechanical kind intermediate the expansion unit body and the displacer which by its movement causes the change of the expansion space. In this case the movement of the displacer is delayed until the pressure pulse which is being built up is great enough to overcome the static friction. Such a means usually consists of a packing between displacer and the body of the expansion unit. The innate disadvantage of such means resides therein that the coefficient of friction of a packing is not constant and is apt to change in the course of time, due to wear and tear, which are likely to cause changes in the phase differential and consequently reduce the potential of the cooler, 
     (b) the employ of hydraulic friction means, by pressure drop which is reached in the regenerator. Such pressure drop creates a drag which causes retardation of the movement of the displacer. Where pressure pulses are of sinusoidal character and the movement of the displacer is solely subject to pneumatic forces it is usually possible to bring about a phase difference of 90°, as would be required for a proper performance. The disadvantage of this method resides therein that the regenerator has to be designed in such a manner that the hydraulic resistance would permit attaining an optimal movement, without laying stress on getting maximal thermic exploitation of the regenerator. Such a regenerator would be much more compact as would be required by thermic considerations only, and would be exposed to stoppage to a greater extent. 
     To overcome the above mentioned disadvantage the use of a gap seal rather than a conventional seal has been suggested. The disadvantage of this method resides therein that on the one hand the resistance of the regenerator must be increased in order to keep the phase difference, on the other hand, the said increased resistance is doing ill the working conditions of the gap seal. 
     OBJECT OF THE INVENTION 
     It is the object of this invention to provide a method and a means for controlling the passive motion of the pneumatically driven displacers for creation of the necessary phase differential. 
     SHORT SUMMARY OF THE DISCLOSURE 
     It has now been found that the necessary phase differential would be attainable by making use of viscous friction of the gas during its passage from one side of the pneumatic pillow to its other side and vice versa by forcing the gas through a constrictive passage wherein the viscous damping is created. In order to achieve this--without encountering the above mentioned disadvantages--the displacer used in a system according to the invention is physically connected with a solid body which divides the pneumatic pillow into two chambers, communicating with one another via an extremely narrow gap. 
     In a preferred and practical embodiment, the said body is cup shaped. 
     Another way or a better effect of such an arrangement may be achieved by placing the displacer and the body connected with it in a magnetic field created by permanent or by electric magnets in which case the said body consists of electrically high conductive material. In such case the movement of the displacer and the said conducting body create periodical changes of magnetic flux in the conductive body and the creation of current within it. Now, if the conductive body is in the shape of a cup--eddy currents will come into existence followed by mechanical drag which relatively correspond with the created current and the change of magnetic flux in the conductor. The mechanical drag obtained is in direct relation with the speed of movement of the displacer. In the case of movement of a displacer of sinusoidal periodical character the speed is at a lag of 90° as against the movement which means that without further means the coveted differential comes into existance, provided that the magnetic damper is adequately designed. 
    
    
     SHORT DESCRIPTION OF DRAWINGS 
     The invention will now be described in detail with reference to the annexed drawings wherein: 
     FIG. 1 is a cross section of the expansion unit, while 
     FIG. 2 is the same unit in a second embodiment. 
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     Turning first to FIG. 1 the expansion unit 1 is composed of the following constituents: An outer casing 2, the displacer 3 onto which is connected a solid body 4, which divides the pneumatic pillow or volume into two chambers 5 and 6, and regenerator 7 which can move within the space 8 of the expansion chamber. At the opposite side of the displacer are positioned two helical springs 9&#39; and 9&#34; (which--however--could be dispensed with). The unit as a whole is connected with a compressor (not shown) by means of a gas conduit 10. 
     The arrangement functions in the following manner: Gas arriving from the compressor by way of conduit 10 passes in the conventional way into the inner space of the regenerator 7. As soon as pressure overcomes the bias of the displacer 3--which is connected with the body 4--it starts moving as indicated by arrow X. The gas trapped within the pneumatic pillow in chamber 5 is forced to pass through the constrictive passage 11 wherein the viscous damping is created and vice versa. 
     It is quite apparent that the greater the viscous friction the greater phase delay is achieved, thus according to a preferred embodiment the said body 4 is larger and cup shaped as can be seen in FIG. 2. In this embodiment the viscous friction is created all along passages 11 and 12. 
     In yet another embodiment, two permanent magnets 13 and 14 (FIG. 2) are positioned within the outer casing 2, and cup 4 consists of electrically highly conductive material. This arrangement functions basically in the same manner as indicated above, the conductive body 4 moves within a magnetic field MF created by the permanent magnets 13 and 14 causes a change in the magnetic period flux in the body 4 and creates current within it. Due to the shape of the said body 4 an eddy circuit is created, due to which--in turn--a mechanical drag becomes evident which is proportional to the electric current which had been created and the speed of magnetic flux change in the conductive body 4. 
     The above function of the device according to FIG. 2 is based on the use of permanent magnets or of electromagnets and may be put into practice in various ways, all of which would be within the scope of the invention: 
     1. The magnetic field may be positioned at the exterior or within the vibrating conductor 4, i.e. the position of the magents may be different. 
     2. The conductor 4 might be static, while the magnetic field vibrates. 
     3. The conductor 4 may be of the shape described and shown in the drawing, but could be a coil the resistance of which is variable, whereby the degree of drag would vary. 
     4. The spring 9&#39; and 9&#34; serving only to: (a) suspend inertial forces, (b) centering the motion of the average but could also be dispensed with. 
     5. Means could be provided to &#34;soften&#34; the movement of the displacer, e.g. perforations for equalisation of pressure at the ends of the strokes, energy absorbing means at those ends and the like. 
     It will be seen that in a device as described the inertial force must be well balanced relative to the returning spring and between the magnetic retarding force and the pneumatic drive. A proper designing warrants a full control of the drag at the angle of performance and degree of amplitude of movements.